Ph.D Jérôme Galon
email: jerome.galon@crc.jussieu.fr

Daniela Bruni

Tumor immunology and tumor evolution: intertwined histories

Keywords: Tumor microenvironment, immunotherapy, adaptive immunity, T cells, Tumorimmunology, Immunoscore, Immune contexture

Cancer is a complex disease whose outcome depends largely on the cross-talk between the tumor and its microenvironment. Here we review the evolution of the field of tumor immunology and the advances, in lockstep, of our understanding of cancer as a disease. We discuss the involvement of different immune cells at distinct stages of tumor progression and how immune contexture determinants shaping tumor development are being exploited therapeutically. Current clinical stratification schemes focus on the tumor histopathology and the molecular characteristics of the tumor cell. We argue for the importance of revising these stratification systems to include immune parameters, so as to address the immediate need for improved prognostic and/or predictive information to guide clinical decisions.

Introduction

Although the first indications of the involvement of the immune system in cancer control can be traced back to over a century ago, tumor immunology can be regarded as an emerging field, rightly complementing and completing that of oncology. Indeed, the renaissance of tumor-immunology came in the past two decades, with the demonstration of two key concepts that bring to fore the major role of the pre-existing adaptive immunity within tumors: immunosurveillance and immunoediting [START_REF] Schreiber | Cancer immunoediting: integrating immunity's roles in cancer suppression and promotion[END_REF][START_REF] Shankaran | IFNgamma and lymphocytes prevent primary tumour development and shape tumour immunogenicity[END_REF] and the importance of immune contexture [START_REF] Galon | Type, density, and location of immune cells within human colorectal tumors predict clinical outcome[END_REF][START_REF] Mascaux | Immune evasion before tumour invasion in early lung squamous carcinogenesis[END_REF]. The encouraging results obtained with the employment of novel cancer immunotherapies such as immune checkpoint inhibitors (ICIs) revealed the power of the immune system to counteract and possibly defeat the disease. The recognition of the tumor-immunology field was conclusively sealed by the Nobel Prize in Physiology or Medicine 2018, awarded for the discovery that inhibiting the negative immune regulation of T-cells could be exploited as a powerful anti-cancer strategy. These groundbreaking findings were accompanied by numerous studies elucidating how various components of the immune system control or contribute to disease progression, thus revealing their part in the natural history of the tumor, as well as their prognostic value. Indeed, it is now appreciated that innate and adaptive immune cells influence cancer evolution directly and indirectly, including at the precancerous stages.

Here, we discuss the current understanding of the how components of the immune system shape the progression of cancer. The fact that ICIs yield clinical benefit in a limited percentage of patients indicates the need for a deeper understanding of the different actors playing a role in cancer development. Nonetheless, readily available powerful evidence makes it already plausible to encourage the adoption of immune-based parameters to improve patient stratification and guide treatment decisions. Thus, we argue for the importance of brining the current cancer classification system up-to-date, to take into account recent advances in tumor immunology.

The dawn of Tumor Immunology

The field of tumor immunology has deep roots, dating back to the nineteenth century (Figure 1). Rudolf Virchow reported a link between inflammation and cancer in 1863. Virchow hypothesized that cancer is caused by severe irritation in the tissues; it was not until the 1990s that his theory -known as "chronic irritation theory"was substantiated by solid evidence, at least in certain cancer types [START_REF] Balkwill | Inflammation and cancer: back to Virchow?[END_REF]. A few decades later, in 1891, the bone sarcoma surgeon William Bradley Coley started the first systematic study of immunotherapy [START_REF] Coley | The Treatment of Malignant Tumors by Repeated Innoculations of Erysipelas: With a Report of Ten Original Cases[END_REF]. Inspired by tumor regression in a Streptococcus-infected (and seemingly doomed) patient, he first formulated and tested the hypothesis that cancer could be defeated by stimulating the patient"s immune system. Hence, the so-called Coley"s Toxins (a mix of bacteria and bacterial products) were injected in over 1000 patients over the subsequent 40 years, yielding remarkable results. Despite the evidence, many clinicians didn"t trust his results, and his methods were gradually forgotten, replaced by the more promising novel radiotherapeutic (1896) [START_REF] Holsti | Development of clinical radiotherapy since 1896[END_REF] and chemotherapeutic (1942) [START_REF] Fenn | First Use of Intravenous Chemotherapy Cancer Treatment: Rectifying the Record[END_REF] approaches. What couldn"t be suspected at the time is that the efficacy of these seemingly unrelated therapeutic modalities may be due at least in part to their modulatory and potentially activating effects on the immune system [START_REF] Formenti | Combining radiotherapy and cancer immunotherapy: a paradigm shift[END_REF][START_REF] Galluzzi | Immunological Effects of Conventional Chemotherapy and Targeted Anticancer Agents[END_REF]. Around Coley"s time, another important milestone was provided by George Beatson [START_REF] Beatson | On the Treatment of Inoperable Cases of Carcinoma of the Mamma: Suggestions for a New Method of Treatment, with Illustrative Cases[END_REF], who exploited ovarian ablation in an attempt to cure unresectable breast cancer. In doing so, Beatson leveraged on the connection between sex steroid hormones and cancer [START_REF] Beatson | On the Treatment of Inoperable Cases of Carcinoma of the Mamma: Suggestions for a New Method of Treatment, with Illustrative Cases[END_REF], thereby paving to way to hormonal therapy in hormones-sensitive cancers (such as breast, ovarian, endometrial, and prostate cancers). This treatment modality became later a widely-deployed strategy, still in use today [START_REF] Shevach | Adjuvant therapy in high-risk prostate cancer[END_REF][START_REF] Zang | Hormonal therapy in uterine sarcomas[END_REF]. Nonetheless, the underlying link between hormones, inflammation and cancer began to unravel only more recently [START_REF] Gharwan | The role of reproductive hormones in epithelial ovarian carcinogenesis[END_REF][START_REF] Key | Hormones and cancer in humans[END_REF][START_REF] Mantovani | Cancer-related inflammation[END_REF][START_REF] Ozdemir | Sex Hormones and Anticancer Immunity[END_REF].

A similarly hypothesis-driven path was followed by use of the bacille Calmette-Guérin (BCG) in bladder cancer settings. Originally developed by Calmette and Guérin in the early 20 th century as a vaccine for tuberculosis (TB) [START_REF] Calmette | Prophylactic vaccination of the newly born against tuberculosis[END_REF][START_REF] Luca | History of BCG Vaccine[END_REF], the BCG remains to-date the only available vaccine against TB. TB was first linked to low rates of cancer in 1929 by Raymond Pearl [START_REF] Pearl | Cancer and tuberculosis[END_REF], whilst he was carrying out a series of autopsies at Johns Hopkins Hospital. A few decades later, also based on this evidence, Morales and colleagues performed the first intravesical administration of BCG to patients with recurrent superficial bladder tumors [START_REF] Morales | Intracavitary Bacillus Calmette-Guerin in the treatment of superficial bladder tumors[END_REF], which showed indeed promising results. Fibronectin was shown to mediate the attachment of BCG to tumor cells [START_REF] Ratliff | Role of fibronectin in intravesical BCG therapy for superficial bladder cancer[END_REF], and fibronectin-mediated internalization of BCG was a necessary prerequisite for its anti-tumor activity [START_REF] Kavoussi | Fibronectin-mediated Calmette-Guerin bacillus attachment to murine bladder mucosa. Requirement for the expression of an antitumor response[END_REF]. The consequently initiated functional immune response includes an enhancement of antigen presentation and cytotoxic T cell-mediated immunity, as well as an increase in pro-inflammatory mediators occurring following successive treatments [START_REF] Fuge | Immunotherapy for bladder cancer[END_REF]. Despite a lack of understanding of its mode of action at the time, Morales et al. initiated a therapeutic modality that is still used today for treating early-stage bladder cancer, representing the only remnant of the microbial era of immunotherapy efforts.

The advent of cellular and antibody therapies

Apart from these exceptions, the fields of Oncology and Immunology developed relatively independently since the late 19 th century, and only truly merged in the past two decades. This is despite some pioneering hypotheses, such as: that the host defense may prevent neoplastic cells from developing into tumors by Paul Ehrlich in 1909 [START_REF] Ehrlich | Über den jetzigen stand der karzinomforschung [the present state of carcinoma research[END_REF]; that the humoral immune system may recognize newly arising tumors antigens by Lewis Thomas in 1959[START_REF] Thomas | Discussion[END_REF]; the theory of cancer immunosurveillance by Frank MacFarlane Burnet in 1957 [START_REF] Burnet | Cancer; a biological approach. I. The processes of control[END_REF], and related studies [START_REF] Foley | Antigenic properties of methylcholanthrene-induced tumors in mice of the strain of origin[END_REF][START_REF] Graham | The effect of vaccine on cancer patients[END_REF][START_REF] Gross | Intradermal immunization of C3H mice against a sarcoma that originated in an animal of the same line[END_REF].

In 1957, Edward Donnall Thomas treated a leukemia patient, following total-body high irradiation therapy, with bone marrow infusion from his identical twin [START_REF] Thomas | Intravenous infusion of bone marrow in patients receiving radiation and chemotherapy[END_REF].

This represents de facto the first form of clinically useful cellular therapy, or hematopoietic stem cell transplantation (HSCT). Thomas was awarded the 1990 Nobel Prize in Physiology or Medicine for fathering this groundbreaking approach, which significantly boosted survival rates for blood malignancies. Hence, John Kersey to perform the world"s first bone marrow transplant for lymphoma in 1975 [START_REF] Kersey | Blood and marrow transplantation: a perspective from the University of Minnesota[END_REF]: although the technique was admittedly crude and risky, his first patient is still alive. The success of this technique and other allogeneic HSCT is due the ability of donor-derived stem cells to provide allo-immunity, thus enabling a graft-versus-tumor effect to eradicate residual disease and prevent relapse. Indeed, graft versus leukemia reaction (GvLR) remains an important therapy against hematological malignancies [START_REF] Dickinson | Graft-versus-Leukemia Effect Following Hematopoietic Stem Cell Transplantation for Leukemia[END_REF][START_REF] Yeshurun | The impact of the graft-versusleukemia effect on survival in acute lymphoblastic leukemia[END_REF].

Instrumental for paving the way from a tumor-centric to a tumor plus immune vision of cancer were the advances in basic immunology: the discovery of the interferons (IFN) by Isaacs andLindenmann in 1957 (Isaacs andLindenmann, 1957;Isaacs et al., 1957), and that of T cells and their critical role in the adaptive immune responses [START_REF] Miller | Cellular basis of the immunological defects in thymectomized mice[END_REF] were only the beginning of an avalanche of crucial findings [START_REF] Kiessling | Natural" killer cells in the mouse. II. Cytotoxic cells with specificity for mouse Moloney leukemia cells. Characteristics of the killer cell[END_REF][START_REF] Steinman | Identification of a novel cell type in peripheral lymphoid organs of mice. I. Morphology, quantitation, tissue distribution[END_REF][START_REF] Zinkernagel | Restriction of in vitro T cell-mediated cytotoxicity in lymphocytic choriomeningitis within a syngeneic or semiallogeneic system[END_REF]. Subsequently, the first steps into more modern immunotherapies were taken. The use of purified, large-scale produced IFN against chronic myeloid leukemia yielded promising results [START_REF] Talpaz | Leukocyte interferon-induced myeloid cytoreduction in chronic myelogenous leukemia[END_REF], but still insufficient to convince the clinical oncologists of the utility of immunotherapy. Again, a deeper basic knowledge was needed, as well as translational research to bring it from bench to bedside.

First discovered in 1976 as a "T cell growth factor" [START_REF] Morgan | Selective in vitro growth of T lymphocytes from normal human bone marrows[END_REF], interleukin (IL)-2 represents the first soluble immune mediator administered to cancer patients [START_REF] Lotze | In vivo administration of purified human interleukin 2. I. Half-life and immunologic effects of the Jurkat cell line-derived interleukin 2[END_REF]. It subsequently received U.S. Food and Drug Administration (FDA) approval for renal cell cancer in 1992. In 1986, the adoptive transfer of IL-2-expanded tumor-infiltrating lymphocytes (TIL) efficiently cured tumor-bearing mice, providing a rationale for the use of TIL in the treatment of humans with advanced cancer [START_REF] Rosenberg | A new approach to the adoptive immunotherapy of cancer with tumor-infiltrating lymphocytes[END_REF]. First clinical trials employing the adoptive transfer of expanded TIL to patients with metastatic disease were reported in 1987 [START_REF] Topalian | Expansion of human tumor infiltrating lymphocytes for use in immunotherapy trials[END_REF]. Follow-up studies demonstrated the feasibility and safety of using retroviral gene transduction for human gene therapy, with implications for the design of TIL with improved antitumor potency [START_REF] Rosenberg | Gene transfer into humans-immunotherapy of patients with advanced melanoma, using tumor-infiltrating lymphocytes modified by retroviral gene transduction[END_REF]. The origin of one of the most recently developed immunotherapies, chimeric antigen receptor (CAR) T cells, can be traced back to the same time period. In fact, the generation and expression of functional CAR-expressing T cells were shown in 1989 [START_REF] Gross | Expression of immunoglobulin-T-cell receptor chimeric molecules as functional receptors with antibody-type specificity[END_REF], thereby initiating a process ultimately leading to FDA approval of CAR-T cell-based therapies for hematologic malignancies in 2017. The potency of CAR T-cell therapy was shown by several clinical trials, in which even end-stage patients experienced a full recovery of up to 92% in acute lymphocytic leukemia [START_REF] Abramson | High Durable CR Rates and Preliminary Safety Profile for JCAR017 in R/R Aggressive B-NHL (TRANSCEND NHL 001 Study): A Defined Composition CD19-Directed CART Cell Product with Potential for Outpatient Administration[END_REF][START_REF] Miliotou | CAR T-cell Therapy: A New Era in Cancer Immunotherapy[END_REF], and 82% objective response in refractory large B-cell lymphoma [START_REF] Neelapu | Axicabtagene Ciloleucel CAR T-Cell Therapy in Refractory Large B-Cell Lymphoma[END_REF]. Fratricideresistant and allo-tolerant 'off-the-shelf' CAR-T cells have facilitated clinical adoption of this costly therapeutic modality [START_REF] Cooper | An "off-the-shelf" fratricide-resistant CAR-T for the treatment of T cell hematologic malignancies[END_REF][START_REF] Qasim | Molecular remission of infant B-ALL after infusion of universal TALEN gene-edited CAR T cells[END_REF][START_REF] Sommer | Preclinical Evaluation of Allogeneic CAR T Cells Targeting BCMA for the Treatment of Multiple Myeloma[END_REF].

Nevertheless, disease relapse following CAR-T cell therapy is not uncommon [START_REF] Abbasi | Relapses After CAR-T Therapy[END_REF][START_REF] Kim | CAR T Cell Relapse CD19 Negative ALL Cells Express Integrins and Integrin Blockade Can Prolong Survival In Vivo[END_REF][START_REF] Ruella | Induction of resistance to chimeric antigen receptor T cell therapy by transduction of a single leukemic B cell[END_REF], indicating the need to further refine this type of therapy, or to combine it with further treatments [START_REF] Galon | Approaches to treat immune hot, altered and cold tumours with combination immunotherapies[END_REF][START_REF] Liu | Sequential CD19-and CD22-CART Cell Therapies for Relapsed B-Cell Acute Lymphoblastic Leukemia after Allogeneic Hematopoietic Stem Cell Transplantation[END_REF], depending on the specific cases. Mostly successful against hematological malignancies, the use of CAR-T cells in solid tumor settings is hampered by their limited ability to find, enter and survive in the tumor [START_REF] Martinez | CAR T Cells for Solid Tumors: New Strategies for Finding, Infiltrating, and Surviving in the Tumor Microenvironment[END_REF]. These limitations are being gradually overcome by the use of bispecific CAR designs or local administration, and that of combinatorial strategies, such as the combination with checkpoint inhibitors to mitigate T cell exhaustion [START_REF] Derenzo | Genetic Modification Strategies to Enhance CAR T Cell Persistence for Patients With Solid Tumors[END_REF][START_REF] Gargett | GD2-specific CAR T Cells Undergo Potent Activation and Deletion Following Antigen Encounter but can be Protected From Activation-induced Cell Death by PD-1 Blockade[END_REF].

Apart from T cells, other types of immune cells have been employed for adoptive cellular immunotherapies over the years, including dendritic cells (DCs), natural killer (NK) cells, lymphokine-activated killer (LAK) cells, and cytokine-induced killer (CIK) cells. As for T cells, cellular immunotherapies relying on DCs require a first antigenic characterization, and, most commonly, the use of autologous cells, thereby slowing clinical development. In virtue of DCs" ability to most-efficiently prime T cell responses [START_REF] Banchereau | Dendritic cells and the control of immunity[END_REF], and considering that vaccine adjuvants are known to act via the induction of DC maturation, its not surprising that DCs are being studied as tools to effective therapeutic vaccination against cancer [START_REF] Palucka | Dendritic-cell-based therapeutic cancer vaccines[END_REF] [START_REF] Lee | Distinct features of dendritic cell-based immunotherapy as cancer vaccines[END_REF] and approved for Sipuleucel-T in prostate cancer.

Conversely, LAK cells, CIK cells, and NK cells display antigen-independent cytolytic activity against cancer cells, and represent attractive, off-the-shelf immunotherapy options. Since their discovery by [START_REF] Kiessling | Natural" killer cells in the mouse. II. Cytotoxic cells with specificity for mouse Moloney leukemia cells. Characteristics of the killer cell[END_REF] and, independently, by Herberman [START_REF] Herberman | Natural cytotoxic reactivity of mouse lymphoid cells against syngeneic acid allogeneic tumors. I. Distribution of reactivity and specificity[END_REF] in 1975, NK cells have been implicated in anti-cancer surveillance and anti-tumor immunity [START_REF] Chiossone | Natural killer cells and other innate lymphoid cells in cancer[END_REF], and have been exploited for the design of anti-cancer therapies [START_REF] Cheng | Establishment, characterization, and successful adaptive therapy against human tumors of NKG cell, a new human NK cell line[END_REF][START_REF] Souza-Fonseca-Guimaraes | The Emergence of Natural Killer Cells as a Major Target in Cancer Immunotherapy[END_REF].

Instrumental to these developments was the pioneering concept of missing-self recognition, whereby the absence or incomplete expression of host MHC class I molecules in a normal cell would be make it susceptible to NK cells-mediate killing. This hypothesis was formulated by Klaus Kärre in his PhD thesis [START_REF] Kärre | On the immunobiology of natural killer cells[END_REF], then in subsequent proceedings and articles [START_REF] Kärre | Role of target histocompatibility antigens in regulation of natural killer activity: a reevaluation and a hypothesis[END_REF][START_REF] Karre | Selective rejection of H-2deficient lymphoma variants suggests alternative immune defence strategy[END_REF]. Subsequently, the groups of Yokoyama [START_REF] Karlhofer | MHC class I alloantigen specificity of Ly-49+ IL-2-activated natural killer cells[END_REF] and Moretta [START_REF] Moretta | P58 molecules as putative receptors for major histocompatibility complex (MHC) class I molecules in human natural killer (NK) cells. Antip58 antibodies reconstitute lysis of MHC class I-protected cells in NK clones displaying different specificities[END_REF] discovered the first NK inhibitory receptors (killercell immunoglobulin-like receptors, KIRs), restraining NK cells cytolytic activity. Hence, several KIRs as well as functionally opposite killer activation receptors (KARs) have been described [START_REF] Molgora | IL-1R8 is a checkpoint in NK cells regulating anti-tumour and anti-viral activity[END_REF][START_REF] Sivori | Human NK cells: surface receptors, inhibitory checkpoints, and translational applications[END_REF][START_REF] Vely | Function of killer cell inhibitory receptors for MHC class I molecules[END_REF]. Anti-KIR antibodies recently entered clinical trials, due to their ability to unleash NK cell-mediated antitumor responses [START_REF] Benson | A Phase I Trial of the Anti-KIR Antibody IPH2101 and Lenalidomide in Patients with Relapsed/Refractory Multiple Myeloma[END_REF][START_REF] Vey | A phase 1 study of lirilumab (antibody against killer immunoglobulin-like receptor antibody KIR2D; IPH2102) in patients with solid tumors and hematologic malignancies[END_REF]. The first successful use of allogenic mismatched NK cells was reported in 2002 by the group of Velardi in acute myeloid leukemia (AML) patients [START_REF] Ruggeri | Effectiveness of donor natural killer cell alloreactivity in mismatched hematopoietic transplants[END_REF].

First described in 1994, the generation of LAK cells (a mixture of NK and NKT cells) involved culturing of peripheral blood mononuclear cells (PBMCs) with IL-2, and then cluster of differentiation 3 (CD3) antibody (OKT3 [START_REF] Van Wauwe | OKT3: a monoclonal anti-human T lymphocyte antibody with potent mitogenic properties[END_REF]) [START_REF] Escudier | Immunotherapy with Interleukin-2 (Il2) and Lymphokine-Activated Natural-Killer-Cells -Improvement of Clinical-Responses in Metastatic Renal-Cell Carcinoma Patients Previously Treated with Il2[END_REF]. LAK cell-based immunotherapy didn"t elicit satisfactory anti-tumor efficiency in patients, especially as a monotherapy. Novel combinatorial approaches may yield clinical benefit [START_REF] Saito | A combined lymphokine-activated killer (LAK) cell immunotherapy and adenovirus-p53 gene therapy for head and neck squamous cell carcinoma[END_REF], which nonetheless remains to be proven in human settings. CIK cells, have been described over two decades ago [START_REF] Lu | A novel population of expanded human CD3+CD56+ cells derived from T cells with potent in vivo antitumor activity in mice with severe combined immunodeficiency[END_REF]. CIK cells share features of both NK and T cells, such as functional TCR and NK cell molecules. They are expanded from circulating precursors by treatment with interferon gamma (IFNγ), OKT3 and IL-2 [START_REF] Introna | Innovative Clinical Perspectives for CIK Cells in Cancer Patients[END_REF], following which they acquire a potent, MHC-unrestricted cytotoxicity against (mostly) hematological malignancies in a natural killer group 2 member D (NKG2D) receptor-dependent manner. CIK cells anti-tumor efficacy has been demonstrated in various hematopoietic neoplasms, with varying yet encouraging results [START_REF] Introna | Repeated infusions of donor-derived cytokine-induced killer cells in patients relapsing after allogeneic stem cell transplantation: a phase I study[END_REF][START_REF] Introna | Phase II Study of Sequential Infusion of Donor Lymphocyte Infusion and Cytokine-Induced Killer Cells for Patients Relapsed after Allogeneic Hematopoietic Stem Cell Transplantation[END_REF][START_REF] Zhou | Family haploidentical donor-derived cytokine-induced killer cell biotherapy combined with bortezomib in two patients with relapsed multiple myeloma in a non-allogeneic transplant setting[END_REF].

First discovered in 1883 by Élie Metchnikoff [START_REF] Metchnikoff | Untersuchungen ueber die mesodermalen Phagocyten einiger Wirbeltiere[END_REF], a Russian zoologist, macrophages are innate immune cells critically involved in a multitude of steps of tumor initiation and progression. Notably, Metchnikoff and Paul Ehrlich (the latter for his work on mast cells, described below) were jointly awarded the 1908 Nobel Prize in Physiology or Medicine "in recognition of their work on immunity". According to a current paradigm, rooted in pioneering hypotheses formulated since the late 1970s [START_REF] Balkwill | Inflammation and cancer: back to Virchow?[END_REF][START_REF] Bottazzi | Regulation of the macrophage content of neoplasms by chemoattractants[END_REF][START_REF] Mantovani | Tumourassociated macrophages as treatment targets in oncology[END_REF][START_REF] Mills | M-1/M-2 macrophages and the Th1/Th2 paradigm[END_REF][START_REF] North | The concept of the activated macrophage[END_REF], classically-activated M1 macrophages accompany the early stages of cancer initiation and development, where they play an inflammatory, anti-cancer role. With disease progression, a shift towards pro-tumor M2 or M2-like macrophages occurs: these cells are responsible for the tissue remodeling, angiogenesis, and adaptive immune suppression typically contributing to tumor growth and dissemination [START_REF] Mantovani | Tumourassociated macrophages as treatment targets in oncology[END_REF]. Recent reports indicate the possibility to leverage on the presence of checkpoint molecules (such as PD-1 [START_REF] Gordon | PD-1 expression by tumourassociated macrophages inhibits phagocytosis and tumour immunity[END_REF] or the newly-discovered Clever-1 [START_REF] Viitala | Immunotherapeutic Blockade of Macrophage Clever-1 Reactivates the CD8(+) T-cell Response against Immunosuppressive Tumors[END_REF]) impairing the sought phagocytic ability and anti-cancer functional properties of macrophages. Indeed, blocking PD-1 on tumor-associated macrophages (TAMs) resulted in increased phagocytic properties, reduced tumor growth and increased survival in vivo. Similarly, blocking Clever-1 could switch TAMs towards an M1-like phenotype, resulting in the activation of a protective cytotoxic T cell response. Clever-1 targeting also decreased PD-L1 expression both on TAMs and on cancer cells, providing a rationale for dual (Clever-1 and PD-L1) blockade [START_REF] Viitala | Immunotherapeutic Blockade of Macrophage Clever-1 Reactivates the CD8(+) T-cell Response against Immunosuppressive Tumors[END_REF]. The exploitation of macrophage-targeting strategies in humans, if successful, could provide a further powerful anti-tumor weapon.

Another milestone in the field of immunotherapy was reached with the use of the anti-CD20 monoclonal antibody (mAb) rituximab in patients with with recurrent B-cell lymphoma [START_REF] Maloney | Phase I clinical trial using escalating single-dose infusion of chimeric anti-CD20 monoclonal antibody (IDEC-C2B8) in patients with recurrent B-cell lymphoma[END_REF]. Indeed, anti-CD20 mAb became the first mAb approved by the FDA for cancer treatment in 1997.

The first human tumor-associated antigens (TAA) reported was the epithelial mucin MUC-1, recognized by cytotoxic T cells (CTL) isolated and grown from a pancreatic cancer patient [START_REF] Barnd | Specific, major histocompatibility complex-unrestricted recognition of tumor-associated mucins by human cytotoxic T cells[END_REF]. Since, other TAAs were discovered, such as MZ2-E (a product of the MAGE-1 gene) [START_REF] Van Der Bruggen | A gene encoding an antigen recognized by cytolytic T lymphocytes on a human melanoma[END_REF] and MZ2-E (coded by the MAGE-3 gene) [START_REF] Gaugler | Human gene MAGE-3 codes for an antigen recognized on a melanoma by autologous cytolytic T lymphocytes[END_REF], both on human melanoma. Subsequently, the antigen encoded by the BAGE gene was found expressed in varying percentages in several cancer types, including melanoma, infiltrating bladder carcinoma, mammary carcinoma, head and neck squamous cell carcinoma, and non-small cell lung carcinoma (NSCLC) [START_REF] Boel | BAGE: a new gene encoding an antigen recognized on human melanomas by cytolytic T lymphocytes[END_REF]. Like the MAGE genes [START_REF] Gaugler | Human gene MAGE-3 codes for an antigen recognized on a melanoma by autologous cytolytic T lymphocytes[END_REF][START_REF] Van Der Bruggen | A gene encoding an antigen recognized by cytolytic T lymphocytes on a human melanoma[END_REF], BAGE is not expressed in healthy tissues, except for the testis [START_REF] Boel | BAGE: a new gene encoding an antigen recognized on human melanomas by cytolytic T lymphocytes[END_REF]. The existence of TAAs generated tremendous enthusiasm in the field for their possible exploitation as therapeutic targets. However, recent findings indicate no increased benefit associated with tumor antigen-specific vaccines, compared to placebo, in the randomized phase III clinical trials in NSCLC [START_REF] Vansteenkiste | Efficacy of the MAGE-A3 cancer immunotherapeutic as adjuvant therapy in patients with resected MAGE-A3-positive nonsmall-cell lung cancer (MAGRIT): a randomised, double-blind, placebo-controlled, phase 3 trial[END_REF] and melanoma [START_REF] Dreno | MAGE-A3 immunotherapeutic as adjuvant therapy for patients with resected, MAGE-A3-positive, stage III melanoma (DERMA): a randomised, placebo-controlled, phase 3 trial[END_REF]. Despite this setback, it is possible that targeting TAAs could be effective in combinatorial strategies. Indeed, neoantigen vaccine generated intra-tumoral T-cell responses and demonstrated clinical responses in recent studies [START_REF] Hilf | Actively personalized vaccination trial for newly diagnosed glioblastoma[END_REF][START_REF] Keskin | Neoantigen vaccine generates intratumoral T cell responses in phase Ib glioblastoma trial[END_REF][START_REF] Ott | An immunogenic personal neoantigen vaccine for patients with melanoma[END_REF][START_REF] Sahin | Personalized RNA mutanome vaccines mobilize poly-specific therapeutic immunity against cancer[END_REF].

The renaissance: immunosurveillance and immune contexture

The term "immunosurveillance" refers to the physiological process by which the immune system recognizes and destroys transformed cells. Preliminary experimental evidence for tumor immunesurveillance was provided over 60 years ago [START_REF] Klein | Demonstration of resistance against methylcholanthrene-induced sarcomas in the primary autochthonous host[END_REF][START_REF] Prehn | Immunity to methylcholanthrene-induced sarcomas[END_REF], and further supported by evidence on the role of IFN and perforin in tumor surveillance and rejection [START_REF] Dighe | Enhanced in vivo growth and resistance to rejection of tumor cells expressing dominant negative IFN gamma receptors[END_REF][START_REF] Kaplan | Demonstration of an interferon gamma-dependent tumor surveillance system in immunocompetent mice[END_REF][START_REF] Smyth | Perforin-mediated cytotoxicity is critical for surveillance of spontaneous lymphoma[END_REF][START_REF] Van Den Broek | Decreased tumor surveillance in perforindeficient mice[END_REF]. Such was the scenario that welcomed the concept of "tumor immunoediting" formulated by Schreiber and colleagues [START_REF] Shankaran | IFNgamma and lymphocytes prevent primary tumour development and shape tumour immunogenicity[END_REF] based on study in mouse models, which presented a renewed and improved immunosurveillance hypothesis [START_REF] Dunn | Cancer immunoediting: from immunosurveillance to tumor escape[END_REF]. Using WT and Rag2-deficient mice, three outcomes were found to result from the continuous cross-talk between tumor cell and immune system: tumor elimination [START_REF] Shankaran | IFNgamma and lymphocytes prevent primary tumour development and shape tumour immunogenicity[END_REF], equilibrium [START_REF] Koebel | Adaptive immunity maintains occult cancer in an equilibrium state[END_REF], or tumor escape from immune control [START_REF] Matsushita | Cancer exome analysis reveals a T-cell-dependent mechanism of cancer immunoediting[END_REF]. The equilibrium phase is characterized not by tumor dormancy, as previously postulated, but by a continuous proliferation and mutationthe phenomenon of immunoediting-in an attempt to attenuate tumor antigenicity and ultimately escape the immune response [START_REF] Dunn | Cancer immunoediting: from immunosurveillance to tumor escape[END_REF]. The ability of tumor cells to survive immune attack is now a recognized hallmark of cancer. In the escape phase, when the tumor versus immunity battle is seemingly won by the former, the immune system can still provide useful information on patients" prognosis and disease outcome.

The association of prolonged survival of patients with cancer with intratumoral lymphocytic infiltration was noted in the 1920s [START_REF] Maccarty | Relation of differentiation and lymphocytic infiltration to postoperative longevity in gastric carcinoma[END_REF]. This association become clearer in the late 1990s" and the early 2000s, with the first reports showing how specific tumor-infiltrating immune cells types can influence cancer progression and clinical outcome, with a prominent role for cytotoxic and effector memory T cells: the presence of these cells correlated with increased survival of patients with cancer [START_REF] Clemente | Prognostic value of tumor infiltrating lymphocytes in the vertical growth phase of primary cutaneous melanoma[END_REF][START_REF] Naito | CD8+ T cells infiltrated within cancer cell nests as a prognostic factor in human colorectal cancer[END_REF][START_REF] Pages | Effector memory T cells, early metastasis, and survival in colorectal cancer[END_REF][START_REF] Zhang | Intratumoral T cells, recurrence, and survival in epithelial ovarian cancer[END_REF]. Since the original classification of rectal cancer [START_REF] Dukes | The classification of cancer of the rectum[END_REF], since expanded to all solid tumors, cancer staging relies on the Tumor (T), Node (N), Metastasis (M) AJCC/UICC-TNM cancer classification. The importance of the density and location of cytotoxic and memory T cells within the tumor was demonstrated by studies shown that these parameters were superior predictors of outcome to cancer staging using the AJCC/UICC-TNM classification, and that, in fact, tumor progression and invasion were statistically dependent upon the pre-existing T-cells [START_REF] Galon | Type, density, and location of immune cells within human colorectal tumors predict clinical outcome[END_REF]. This evidence constituted the cradle of the concept of "immune contexture [START_REF] Galon | The adaptive immunologic microenvironment in colorectal cancer: a novel perspective[END_REF], highlighting the importance of the quantity and the quality of the immune infiltrates for the survival of patients with cancer. The four major immune parameters of the "immune contexture" are defined by the nature, density, immune functional orientation and location of tumorassociated immune infiltrates. Importantly, tumor progression (T-stage), tumor invasion (Nstage), tumor differentiation (grade), early-metastatic invasion (emboli and vessel invasion) are dependent upon pre-existing anti-tumor immunity [START_REF] Galon | The continuum of cancer immunosurveillance: prognostic, predictive, and mechanistic signatures[END_REF][START_REF] Galon | Type, density, and location of immune cells within human colorectal tumors predict clinical outcome[END_REF][START_REF] Galon | Towards the introduction of the 'Immunoscore' in the classification of malignant tumours[END_REF][START_REF] Mlecnik | Histopathologic-based prognostic factors of colorectal cancers are associated with the state of the local immune reaction[END_REF][START_REF] Pages | In situ cytotoxic and memory T cells predict outcome in patients with early-stage colorectal cancer[END_REF]. Originally applied to colorectal cancer (CRC), the immune contexture concept is nowadays widely applied to most solid cancers [START_REF] Fridman | The immune contexture in human tumours: impact on clinical outcome[END_REF][START_REF] Remark | The nonsmall cell lung cancer immune contexture. A major determinant of tumor characteristics and patient outcome[END_REF][START_REF] Tazzari | Complex Immune Contextures Characterise Malignant Peritoneal Mesothelioma: Loss of Adaptive Immunological Signature in the More Aggressive Histological Types[END_REF][START_REF] Zhao | Orchestration of immune checkpoints in tumor immune contexture and their prognostic significance in esophageal squamous cell carcinoma[END_REF].

Molecular profiling of melanoma metastases revealed the importance of Th1 signature and suggested classifiers of immune responsiveness [START_REF] Wang | Prospective molecular profiling of melanoma metastases suggests classifiers of immune responsiveness[END_REF]. We previously proposed a continuum of cancer immunosurveillance in human and a broader immunological interpretation of three concepts: immune contexture, Immunoscore, and immunologic constant of rejection, which segregated oncogenic processes independently of their tissue origin [START_REF] Galon | The continuum of cancer immunosurveillance: prognostic, predictive, and mechanistic signatures[END_REF]. The strength of the immune contexture proved instrumental for the development of the consensus Immunoscore, an IHC-based assay, defining hot (inflamed) and cold (non-inflamed) tumors and providing prognostic information based on the presence of CD3 + and CD8 + in specific regions (center and invasive margin) of the tumor [START_REF] Galon | Approaches to treat immune hot, altered and cold tumours with combination immunotherapies[END_REF]. A recent worldwide validation confirmed that the consensus Immunoscore holds prognostic value superior to that of the AJCC/UICC-TNM staging system [START_REF] Pages | International validation of the consensus Immunoscore for the classification of colon cancer: a prognostic and accuracy study[END_REF].

Tumor immunology coming-of-age: anti-CTLA-4 and anti-PD-1 as prototype checkpoint inhibitors

Advances in molecular biology between the 1980s and the 1990s enabled the elucidation of the mechanisms underlying T cell cytotoxicity, such as the perforin-granzyme and the FAS-FASL pathway [START_REF] Golstein | An early history of T cell-mediated cytotoxicity[END_REF]. These advances were also instrumental to the discovery of key molecular players in immunity. Brunet and colleagues reported the identification of the immunoglobulin-superfamily member CTLA-4 in mouse, and revealed its inducible nature upon T cell activation [START_REF] Brunet | A new member of the immunoglobulin superfamily--CTLA-4[END_REF]. These findings were soon extended to humans [START_REF] Dariavach | Human Ig superfamily CTLA-4 gene: chromosomal localization and identity of protein sequence between murine and human CTLA-4 cytoplasmic domains[END_REF]. These finding were contemporary to the identification of the co-stimulatory receptor CD28 [START_REF] Aruffo | Molecular cloning of a CD28 cDNA by a high-efficiency COS cell expression system[END_REF]. Although the ability of CTLA-4 to bind the CD28 ligand B7-1 with high affinity was demonstrated in 1991 [START_REF] Linsley | CTLA-4 is a second receptor for the B cell activation antigen B7[END_REF], the role of CTLA-4 remained controversial until 1995, when an antibody blocking the interaction of CTLA-4 with antigen-presenting cell (APC)-derived B7-2 (functionally equivalent to B7-1 [START_REF] Pentcheva-Hoang | B7-1 and B7-2 selectively recruit CTLA-4 and CD28 to the immunological synapse[END_REF][START_REF] Sigal | The role of B7-1 and B7-2 costimulation for the generation of CTL responses in vivo[END_REF]) was shown to inhibit T cell proliferation [START_REF] Krummel | CD28 and CTLA-4 have opposing effects on the response of T cells to stimulation[END_REF]. Of note, at the time, the role of CD28 in preventing T cell anergy had just been revealed [START_REF] Harding | CD28mediated signalling co-stimulates murine T cells and prevents induction of anergy in T-cell clones[END_REF].

Shortly after, the analysis of a CTLA-4-deficient mouse model revealed its crucial role as negative regulator of T lymphocytes activation, hence contributing to T cell homeostasis and protecting from lethal tissue damage [START_REF] Waterhouse | Lymphoproliferative disorders with early lethality in mice deficient in Ctla-4[END_REF] [START_REF] Tivol | Loss of CTLA-4 leads to massive lymphoproliferation and fatal multiorgan tissue destruction, revealing a critical negative regulatory role of CTLA-4[END_REF]. Subsequent studies revealed that CTLA-4 ligation inhibits CD28-dependent IL-2 production [START_REF] Walunas | CTLA-4 ligation blocks CD28dependent T cell activation[END_REF]. Importantly, monoclonal antibodies targeting CTLA-4 reversed its inhibitory effect on T cells [START_REF] Walunas | CTLA-4 can function as a negative regulator of T cell activation[END_REF]. A big breakthrough came with the discovery by Allison and colleagues that administration of anti-CTLA-4 antibodies could mediate tumor rejection in mouse models in vivo, and confer long-lasting immune protection [START_REF] Leach | Enhancement of antitumor immunity by CTLA-4 blockade[END_REF].

A similar pre-clinical path was followed by the other prototype checkpoint inhibitorprogrammed cell death protein 1 (PD-1, CD279). Also belonging to the immunoglobulin gene superfamily, PD-1 emerged during a screen for genes involved in apoptosis carried out by Ishida, Honjo and colleagues [START_REF] Ishida | Induced expression of PD-1, a novel member of the immunoglobulin gene superfamily, upon programmed cell death[END_REF]. PD-1 is expressed by activated T cells, and also by B cells and myeloid cells. It took 8 more years to conclusively demonstrate the immunoinhibitory role of PD-1via ligation of the B7 family member PD-L1 on antigen-presenting cells, as well as non-lymphoid tissue [START_REF] Freeman | Engagement of the PD-1 immunoinhibitory receptor by a novel B7 family member leads to negative regulation of lymphocyte activation[END_REF]. Taken together, these studies demonstrated the existence of fine-tuning mechanisms regulating tolerance and controlling autoimmunity, and conversely, (as we know now) contributing to tumor immune escape. Tumor cells express PD-L1 (CD274, also known as B7 homolog 1 -B7-H1), and an anti-PD-L1 antibody reduced tumorigenesis and tumor invasiveness, and enhanced cytotoxic T cell-mediated tumor cell lysis [START_REF] Iwai | Involvement of PD-L1 on tumor cells in the escape from host immune system and tumor immunotherapy by PD-L1 blockade[END_REF]. PD-L1 is not normally expressed by healthy human cells (with the exception of macrophages), but is highly expressed by tumor cells in response to IFN [START_REF] Dong | Tumor-associated B7-H1 promotes T-cell apoptosis: a potential mechanism of immune evasion[END_REF]. PD-1-PD-L1 binding, with concomitant TCR engagement, results in antigen-specific T cell apoptosis, thus promoting tumor growth in vivo [START_REF] Dong | Tumor-associated B7-H1 promotes T-cell apoptosis: a potential mechanism of immune evasion[END_REF]. PD-L2, another ligand for PD-1 appears to be functionally redundant to PD-L1, and is expressed by tumor cells, as well as by some normal tissues and by APCs upon activation [START_REF] Latchman | PD-L2 is a second ligand for PD-1 and inhibits T cell activation[END_REF].

It was already clear in the early 2000s thatCTLA4 and PD-1 were profoundly different checkpoints. One of the most striking observations was the different phenotypes of the CTLA-4-or PD-1-deficient mouse models, the former lethal within few weeks [START_REF] Tivol | Loss of CTLA-4 leads to massive lymphoproliferation and fatal multiorgan tissue destruction, revealing a critical negative regulatory role of CTLA-4[END_REF][START_REF] Waterhouse | Lymphoproliferative disorders with early lethality in mice deficient in Ctla-4[END_REF], the latter developing lupus-like disease with age [START_REF] Nishimura | Development of lupuslike autoimmune diseases by disruption of the PD-1 gene encoding an ITIM motif-carrying immunoreceptor[END_REF]. Furthermore, the relevant ligands display distinct expression patterns: B7-1 and -2 are found mainly on professional APCs (most typically residing in lymph nodes or spleen), whilst PD-L1 and -L2 are widely expressed, most notably in peripheral tissues [START_REF] Buchbinder | CTLA-4 and PD-1 Pathways: Similarities, Differences, and Implications of Their Inhibition[END_REF][START_REF] Latchman | PD-L2 is a second ligand for PD-1 and inhibits T cell activation[END_REF].

At this point in time, three key concepts came together: the demonstration of the phenomenon of immunosurveillance in mice (1); the demonstration of the importance of a pre-existing favorable immune contexture (2), and the possibility to unleash these tumor-infiltrating T cells induced by these immune checkpoint antibodies (3). These major milestones generated great enthusiasm and brought forth the renewal of the field of tumor-immunology. Indeed, cancer immunotherapy was named "Breakthrough of the Year" by Science in 2013 [START_REF] Couzin-Frankel | Breakthrough of the year 2013. Cancer immunotherapy[END_REF]. The subsequent clinical application of monoclonal antibodies against CTLA-4 and PD-1 ultimately resulted in regulatory approval in a growing number of indications (particularly for PD-1) since 2011. The milestones in tumor immunology discussed here are summarized in a timeline (Figure 1). As of February 2019, these agents have been approved for 43 different cancer indications. The award of the 2018 Nobel Prize in Physiology or Medicine to immunologists James Allison and Tasuku Honjo should be seen not only as their recognition, but also as a global acknowledgment of the field and of the underlying effort of all contributing parties.

Pre-cancerous lesions: a cradle for cancer

It is widely accepted that most solid cancers develop through a multistep physiopathological process: the normal mucosa-to-adenoma-to-carcinoma sequence, or pre-malignant-to-lowgrade-to-high-grade lesions (Figure 2). This sequence was thought to be characterized by predefined steps of sequential genetic mutations [START_REF] Fearon | A genetic model for colorectal tumorigenesis[END_REF][START_REF] Kinzler | Lessons from hereditary colorectal cancer[END_REF]. More recently, the notion that a branched, rather than linear acquisition of specific mutations applied to the growth and dissemination of primary tumors [START_REF] Gerlinger | Intratumor heterogeneity and branched evolution revealed by multiregion sequencing[END_REF][START_REF] Kim | Subclonal Genomic Architectures of Primary and Metastatic Colorectal Cancer Based on Intratumoral Genetic Heterogeneity[END_REF][START_REF] Sottoriva | A Big Bang model of human colorectal tumor growth[END_REF][START_REF] Thirlwell | Clonality assessment and clonal ordering of individual neoplastic crypts shows polyclonality of colorectal adenomas[END_REF] has been widely accepted. This notion, together with the observation that specific, supposedly required key genetic mutations are not systematically found in established tumors [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF], suggest further revisions to the classical model. Such revision is even more strongly encouraged in the light of the most recent findings on the role of the immune system in shaping later stages of cancer evolution [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF]. Whether a similarly branched, immune-influenced modality applies to the stages of pre-cancer development constitutes a plausible hypothesis which nonetheless remains to be established.

Mutations enhancing proliferative signaling by determining the activation of oncogenes and/or the negative regulation of tumor suppression genes, as well as those conferring resistance to cell death and evasion of TGFβ signaling are amongst the most critically involved in the carcinogenic process [START_REF] Hanahan | Hallmarks of cancer: the next generation[END_REF]. These mutations can result from genomic instability, defined as the loss or rearrangement of the chromosomes during cell division, but also be initiated and/or potentiated by factors directly or indirectly causing DNA disruptions and damage. Smoking [START_REF] Murphy | Heterogeneity of Colorectal Cancer Risk Factors by Anatomical Subsite in 10 European Countries: A Multinational Cohort Study[END_REF][START_REF] Siegel | Deaths Due to Cigarette Smoking for 12 Smoking-Related Cancers in the United States[END_REF], exposure to radiation [START_REF] Ozasa | Studies of the mortality of atomic bomb survivors, Report 14, 1950-2003: an overview of cancer and noncancer diseases[END_REF][START_REF] Preston | Solid cancer incidence in atomic bomb survivors: 1958-1998[END_REF][START_REF] Williams | Thyroid carcinoma after Chernobyl latent period, morphology and aggressiveness[END_REF], and oncogenic viruses [START_REF] Boda | Human papilloma virus: Apprehending the link with carcinogenesis and unveiling new research avenues (Review)[END_REF][START_REF] Koeppel | Helicobacter pylori Infection Causes Characteristic DNA Damage Patterns in Human Cells[END_REF][START_REF] Mcbride | Oncogenic human papillomaviruses[END_REF][START_REF] Parfenov | Characterization of HPV and host genome interactions in primary head and neck cancers[END_REF][START_REF] Slebos | Proteomic analysis of oropharyngeal carcinomas reveals novel HPV-associated biological pathways[END_REF] ) are examples of established carcinogens directly causing DNA damage, although indirect effects also contribute to the carcinogenic process. Other factors also play a pro-tumorigenic role, including obesity [START_REF] Calle | Overweight, obesity, and mortality from cancer in a prospectively studied cohort of U.S. adults[END_REF][START_REF] Wade | Obesity BMI and Mortality in UK Biobank: Revised Estimates Using Mendelian Randomization[END_REF], and lack of physical exercise [START_REF] Dietz | Chronic Disease Prevention: Tobacco Avoidance, Physical Activity, and Nutrition for a Healthy Start[END_REF][START_REF] Stenholm | Smoking, physical inactivity and obesity as predictors of healthy and disease-free life expectancy between ages 50 and 75: a multicohort study[END_REF][START_REF] Zauber | Colonoscopic polypectomy and long-term prevention of colorectal-cancer deaths[END_REF].

Linked to most, if not all above-mentioned factors is chronic inflammation, which in fact provides a fertile niche promoting carcinogenesis, and is indeed a recognized hallmark of cancer [START_REF] Colotta | Cancer-related inflammation, the seventh hallmark of cancer: links to genetic instability[END_REF][START_REF] Hanahan | Hallmarks of cancer: the next generation[END_REF]. Apart from setting up a tumorigenic microenvironment, several lines of evidence indicate that chronic inflammation is associated with chromosomal instability [START_REF] Colotta | Cancer-related inflammation, the seventh hallmark of cancer: links to genetic instability[END_REF][START_REF] Kitamura | Chronic lung injury by constitutive expression of activation-induced cytidine deaminase leads to focal mucous cell metaplasia and cancer[END_REF][START_REF] Lin | Chronic inflammation-associated genomic instability paves the way for human esophageal carcinogenesis[END_REF], thereby highlighting the crosstalk among different tumorigenic components. It should be noted that not all chromosomal abnormalities seem to be linked to cancerogenesis [START_REF] Vitre | Chronic centrosome amplification without tumorigenesis[END_REF], further reiterating its complexity and multifactorial nature.

Whereas the mechanism underlying the connections between inflammation and carcinogenesis remain to be elucidated, a growing body of evidence implicates several immune and immune-derived components in the onset and the progression of cancer.

Malignant cells release pro-inflammatory mediators and chemoattractants that promote immune cell infiltration [START_REF] Gao | CCL5-CCR5 interactions modulate metabolic events during tumor onset to promote tumorigenesis[END_REF][START_REF] Johnson | Effect of the premalignant and tumor microenvironment on immune cell cytokine production in head and neck cancer[END_REF][START_REF] Xia | Expressions of CXCL12/CXCR4 in oral premalignant and malignant lesions[END_REF]. Furthermore, malignant cells are able to reprogram resident fibroblasts turning them into cancer-associated fibroblasts (CAFs); in turn, CAFs play a pro-tumorigenic role, both directly (by stimulating the growth of tumor cells and angiogenesis) and indirectly, via the recruitment of immune cells to the tumor microenvironment [START_REF] Ao | Crosstalk between paracrine-acting cytokine and chemokine pathways promotes malignancy in benign human prostatic epithelium[END_REF][START_REF] Erez | Cancer-Associated Fibroblasts Are Activated in Incipient Neoplasia to Orchestrate Tumor-Promoting Inflammation in an NF-kappaB-Dependent Manner[END_REF][START_REF] Vicent | Cross-species functional analysis of cancer-associated fibroblasts identifies a critical role for CLCF1 and IL-6 in non-small cell lung cancer in vivo[END_REF].

Of note, CAFs in mouse pancreatic ductal adenocarcinoma (PDAC) and mammary tumors, but not cervical tumors, are proinflammatory [START_REF] Erez | Cancer-Associated Fibroblasts Are Activated in Incipient Neoplasia to Orchestrate Tumor-Promoting Inflammation in an NF-kappaB-Dependent Manner[END_REF]. The use of a Kras-driven mouse model of PDAC revealed the critical role for the inflammatory mediator signal transducer and activator of transcription 3 (STAT3) in supporting cell proliferation, metaplasia associated inflammation and matrix metalloproteinase 7 (MMP7) expression during tumorigenesis [START_REF] Fukuda | Stat3 and MMP7 contribute to pancreatic ductal adenocarcinoma initiation and progression[END_REF]. Of note, MMP7 and other metalloproteinase are involved in extracellular matrix (ECM) remodeling, which typically accompanies the malignant transformation [START_REF] Das | MMP proteolytic activity regulates cancer invasiveness by modulating integrins[END_REF][START_REF] Heslin | Role of matrix metalloproteinases in colorectal carcinogenesis[END_REF][START_REF] Page-Mccaw | Matrix metalloproteinases and the regulation of tissue remodelling[END_REF].

Neutrophils, macrophages and lymphocytes infiltrate the inflammatory site, release a plethora of soluble mediators including proinflammatory cytokines and growth factors, and determine the production of genotoxic species, such as reactive oxygen species (ROS) and nitrogen oxide (NO), inducing DNA damage and genetic mutations [START_REF] Kanda | Chemopreventive Strategies for Inflammation-Related Carcinogenesis: Current Status and Future Direction[END_REF][START_REF] Okada | Inflammation-related carcinogenesis: current findings in epidemiological trends, causes and mechanisms[END_REF].

Langerhans cells can create a mutagenic environment by metabolically converting chemical carcinogens into active mutagens, thus inducing DNA damage-induced squamous cell carcinoma [START_REF] Modi | Langerhans cells facilitate epithelial DNA damage and squamous cell carcinoma[END_REF]. Lesion-infiltrating bone marrow-derived myeloid cells not only drive the inflammatory process, but also produce ECM remodeling factors, proangiogenic growth factors and vascular-modulating enzymes [START_REF] Jiang | Influence of Immune Myeloid Cells on the Extracellular Matrix During Cancer Metastasis[END_REF], thereby setting up the stage for the developing malignancy. Neutrophils are generally protumorigenic during the initiation and progression of tumors [START_REF] Tazawa | Infiltration of neutrophils is required for acquisition of metastatic phenotype of benign murine fibrosarcoma cells: implication of inflammationassociated carcinogenesis and tumor progression[END_REF], and often a source of ROS [START_REF] Canli | Myeloid Cell-Derived Reactive Oxygen Species Induce Epithelial Mutagenesis[END_REF][START_REF] Nicolas-Avila | Neutrophils in Homeostasis, Immunity, and Cancer[END_REF]. Other neutrophil-derived genotoxic substances, such as defensins, were shown to induce DNA strand breaks in target cells [START_REF] Gera | Human neutrophil peptide defensins induce single strand DNA breaks in target cells[END_REF]. In vivo studies implicated neutrophils in the neoangiogenetic process in early preneoplastic hepatocellular [START_REF] Huo | Transcriptomic profiles of tumor-associated neutrophils reveal prominent roles in enhancing angiogenesis in liver tumorigenesis in zebrafish[END_REF][START_REF] Kuang | Peritumoral neutrophils link inflammatory response to disease progression by fostering angiogenesis in hepatocellular carcinoma[END_REF] and pancreatic [START_REF] Nozawa | Infiltrating neutrophils mediate the initial angiogenic switch in a mouse model of multistage carcinogenesis[END_REF] lesions, thus favoring malignant progression. The observation that systemic depletion of neutrophils inhibits bacteria-triggered mammary tumor development in mice further demonstrates the key role of these cells in tumorigenesis [START_REF] Lakritz | Gut bacteria require neutrophils to promote mammary tumorigenesis[END_REF].

In addition to neutrophils, other ROS-producing myeloid cells, i.e. macrophages, can contribute to carcinogen-independent intestinal tumor initiation and progression [START_REF] Canli | Myeloid Cell-Derived Reactive Oxygen Species Induce Epithelial Mutagenesis[END_REF]. In vivo evidence suggests a prominent role for macrophage-derived inducible nitric oxide synthase (NOS2) in promoting lung squamous cell carcinoma (SCC) [START_REF] Wang | Macrophage inducible nitric oxide synthase circulates inflammation and promotes lung carcinogenesis[END_REF], confirming previous observations [START_REF] Liu | Increased level of exhaled nitric oxide and up-regulation of inducible nitric oxide synthase in patients with primary lung cancer[END_REF][START_REF] Okayama | NOS2 enhances KRAS-induced lung carcinogenesis, inflammation and microRNA-21 expression[END_REF].

Proinflammatory macrophages are amongst the responsible for the low-grade inflammation observed in obese individuals, involving the production of IL-6 and TNF-α, and the derived hepatic carcinogenesis [START_REF] Park | Dietary and genetic obesity promote liver inflammation and tumorigenesis by enhancing IL-6 and TNF expression[END_REF]. However, proinflammatory, M1-like macrophages seem to play a less prominent role in tumor progression than their anti-inflammatory counterparts, often referred to as M2 macrophages [START_REF] Wang | Dynamic changes and functions of macrophages and M1/M2 subpopulations during ulcerative colitis-associated carcinogenesis in an AOM/DSS mouse model[END_REF]. The expression pattern of inflammatory macrophages is unchanged along the precancerous stages preceding CRC, whereas the frequency of M2-like macrophages markedly increased with the pathological stages of progression [START_REF] Wang | Dynamic changes and functions of macrophages and M1/M2 subpopulations during ulcerative colitis-associated carcinogenesis in an AOM/DSS mouse model[END_REF]. M2-like macrophages can promote carcinogenesis by driving an oncogenic program in epithelial cells [START_REF] Morales | Immune chaperone gp96 drives the contributions of macrophages to inflammatory colon tumorigenesis[END_REF]. The cytokine IL-4, in combination with macrophage colony-stimulating factor induces macrophage polarization to an anti-inflammatory phenotye [START_REF] Yang | Tumor-associated macrophages: from basic research to clinical application[END_REF]. The increased expression of the oncogenic [START_REF] Nussinov | Oncogenic KRAS signaling and YAP1/beta-catenin: Similar cell cycle control in tumor initiation[END_REF][START_REF] Tao | Activation of beta-catenin and Yap1 in human hepatoblastoma and induction of hepatocarcinogenesis in mice[END_REF] protein YAP1 (Yes-associated protein 1) found in colorectal adenocarcinoma, compared to normal tissue, can drive IL-4-and IL-13-induced macrophage polarization [START_REF] Huang | Ovatodiolide suppresses colon tumorigenesis and prevents polarization of M2 tumor-associated macrophages through YAP oncogenic pathways[END_REF]. It should be noted that a high degree of plasticity exists among the different macrophage populations, and that the M1 vs M2 distinction represents an oversimplification of a wide phenotypical spectrum. The significance of the heterogeneity within tumor associated macrophages (TAMS) [START_REF] Aras | TAMeless traitors: macrophages in cancer progression and metastasis[END_REF] in different cancer contexts is an important area of investigation.

Eosinophils and mast cells can also contribute to malignant transformation. The number of tissue-infiltrating eosinophils decreases during the progression of colorectal adenomacarcinoma [START_REF] Cho | Eosinophils in Colorectal Neoplasms Associated with Expression of CCL11 and CCL24[END_REF][START_REF] Kiziltas | Does the severity of tissue eosinophilia of colonic neoplasms reflect their malignancy potential?[END_REF][START_REF] Moezzi | Stromal eosinophilia in colonic epithelial neoplasms[END_REF], suggesting a protective role for these cells and/or the establishment of mechanisms of escape. The association of mast cells with tumorigenesis was first reported by Paul Ehrlich (Ehrlich, 1879a, b). Pioneering work in a model of skin carcinogenesis showed that the number of mast cells increased throughout the precancerous stages and then decreased at the onset of cancer [START_REF] Cramer | Mast Cells in Experimental Skin Carcinogenesis[END_REF]. Mast cell activation, presumably degranulation, occurred at the most advanced pre-malignant phases, and high activation intensity was proposed to act as a functional defensive process against the development of skin cancer [START_REF] Cramer | Mast Cells in Experimental Skin Carcinogenesis[END_REF]. Further studies in different contexts have reported both pro-and anti-tumorigenic roles for mast cells. For instance, mast cells promoted premalignant angiogenesis and pro-tumoral stromal remodeling in squamous epithelial carcinogenesis in mice [START_REF] Coussens | Inflammatory mast cells up-regulate angiogenesis during squamous epithelial carcinogenesis[END_REF], and degranulation-dependent (pre)tumor cell proliferation in mammary carcinogenesis in rats [START_REF] Faustino-Rocha | Mast Cells in Mammary Carcinogenesis: Host or Tumor Supporters?[END_REF]. On the other hand, mast cells promoted eosinophils infiltration and tumor cell apoptosis in a model of early-stage intestinal tumorigenesis [START_REF] Sinnamon | A protective role of mast cells in intestinal tumorigenesis[END_REF]. These differences might reflect a cancer-and/or localization-specific role for these cells, as recently suggested [START_REF] Varricchi | Are Mast Cells MASTers in Cancer?[END_REF].

It is estimated that chronic inflammation increases the risk of human cancers involving almost all organs/tissues [START_REF] Kanda | Chemopreventive Strategies for Inflammation-Related Carcinogenesis: Current Status and Future Direction[END_REF]. In support of this observation, very recently, a tumorpromoting inflammatory microenvironment was found to promote skin carcinogenesis in discoid lupus erythematosus (DLE) patients. Reduced CD8 + T cells, combined with enhanced lesion-infiltrating T REG CD4 + T cells, M2 macrophages, mast cells, as well as upregulated STAT3 signaling in keratinocytes were found in cancer-prone DLE patients [START_REF] Zaalberg | Chronic Inflammation Promotes Skin Carcinogenesis in Cancer-Prone Discoid Lupus Erythematosus[END_REF]. Therefore, it is not surprising that strategies and candidate agents aiming at preventing inflammation-related carcinogenesis have been proposed [START_REF] Kanda | Chemopreventive Strategies for Inflammation-Related Carcinogenesis: Current Status and Future Direction[END_REF], including aspirin for CRC prevention [START_REF] Coyle | Aspirin and Colorectal Cancer Prevention and Treatment: Is It for Everyone?[END_REF][START_REF] Garcia-Albeniz | Aspirin for the prevention of colorectal cancer[END_REF]. Of note, not all chronic inflammations lead to carcinogenesis; for instance, some of chronic inflammatory diseases, such as rheumatoid arthritis, are not linked to cancer risk, whilst others are even leading to tumor regression [START_REF] Okada | Inflammation-related carcinogenesis: current findings in epidemiological trends, causes and mechanisms[END_REF]. The crucial genetic and/or immune determinants leading from inflammation to cancer haven"t been fully elucidated yet. Notwithstanding, the notion that immune-based mechanisms control the transformation of benign to malignant lesions (or fail to do so) has been advocated [START_REF] Cui | Immune microenvironmental shift along human colorectal adenoma-carcinoma sequence: is it relevant to tumor development, biomarkers and biotherapeutic targets?[END_REF]. Recently, the examination of the nine morphological stages of lung squamous carcinogenesis identified specific modules of co-expressed genes and evolutionary trajectories of cancer and immune pathways [START_REF] Mascaux | Immune evasion before tumour invasion in early lung squamous carcinogenesis[END_REF]. A continuous increase of proliferation and DNA repair linearly form normal tissue to cancer along the precancerous stages was found. This was accompanied by a transitory increase of metabolism and early immune sensing through activation of resident immune cells, including mast-cells, in low-grade pre-invasive lesions. Subsequently, the activation of the innate and adaptive immune response and the expression of immune checkpoints and suppressive interleukins were detected from high-grade pre-invasive lesions, indicating the establishment of an immune escape phase. The invasive, more advanced stage, was characterized by the activation of the epithelial-mesenchymal transition (EMT). Highgrade lesions were characterized by the highest percentage of immune-related gene expression, including that of activated T cells, neutrophils and M1 macrophages. The immune composition within lesions from the same patients varied along the stages, with a shift from naive to memory T and B cells, gradual increase in mast cell activation. The abundance of naive CD4 + T cells peaked at the early stage of mild dysplasia, then rapidly declined, concomitantly with the increase of activated CD4 + memory T cells in the successive stages. It would be of crucial interest to pinpoint key determinants involved in the failure of the immune system in controlling and possibly removing (pre)-malignant clones. Nevertheless, this study represents a first comprehensive immune assessment at the precancerous stages, and a first step towards a desired identification of biomarkers of (pre)disease progression or recurrence, with the possibility to perform immunotherapy at a pre-cancer stage.

Immunity versus (or pro) cancer in primary tumors

Primary tumors are characterized by a wide array of immunological scenarios [START_REF] Galon | The continuum of cancer immunosurveillance: prognostic, predictive, and mechanistic signatures[END_REF], partly depending on the tissue of origin [START_REF] Oliver | Tissue-Dependent Tumor Microenvironments and Their Impact on Immunotherapy Responses[END_REF][START_REF] Santegoets | The Anatomical Location Shapes the Immune Infiltrate in Tumors of Same Etiology and Affects Survival[END_REF], tumor location [START_REF] Zhang | Immune Landscape of Colorectal Cancer Tumor Microenvironment from Different Primary Tumor Location[END_REF], the underlying spectrum of genetic mutations [START_REF] Giannakis | Genomic Correlates of Immune-Cell Infiltrates in Colorectal Carcinoma[END_REF] and genetic background [START_REF] Lim | Germline genetic polymorphisms influence tumor gene expression and immune cell infiltration[END_REF], epigenetic features [START_REF] Jones | Epigenetic therapy in immune-oncology[END_REF], the composition of the host gut microbiome [START_REF] Cremonesi | Gut microbiota modulate T cell trafficking into human colorectal cancer[END_REF][START_REF] Gopalakrishnan | The Influence of the Gut Microbiome on Cancer, Immunity, and Cancer Immunotherapy[END_REF], and environmental conditions [START_REF] Yi | Influences of large sets of environmental exposures on immune responses in healthy adult men[END_REF]. All these factors contribute to shape the tumor-associated immune landscape, and consequently both the natural progression of the disease, as well as the response to (immuno)therapeutic agents [START_REF] Bindea | The immune landscape of human tumors: Implications for cancer immunotherapy[END_REF][START_REF] Galon | Approaches to treat immune hot, altered and cold tumours with combination immunotherapies[END_REF]. The analysis of the prognostic value of individual cell types enabled to label them as friends or foes, although this clear-cut distinction is not applicable to all cell subtypes, and/or to all cancer types [START_REF] Fridman | The immune contexture in human tumours: impact on clinical outcome[END_REF] (Figure 3). Such prognostic variability might reflect local/tissutal differences, thereby affecting the milieu of the TME, but it might also arise from: a lack of harmonization in marker selection; technical differences; the exclusion of spatial parameters (localization of specific immune cells in specific tumor regions); the need for combinatorial assessments, whereby combinations of immune cells and/or factors (and their topography within the tumor), rather than individual ones, hold superior prognostic power.

In mechanistic terms, what has been clearly established is that CD8 + cytotoxic T cells are the ultimate effectors of tumor rejection, due to their cytolytic capabilities and of course their specificity against tumor cells, conferring long-lasting protection, thus protecting against cancer recurrence. It should be noted that also innate immune cells can control cancer directly by interacting with tumor cells, and/or indirectly by favouring the anti-tumor activities of CD8 + T cells. NK cells, γδ-T cells and macrophages are amongst the documented mediators of direct tumor cell lysis [START_REF] Chan | Identification, molecular characterization, clinical prognosis, and therapeutic targeting of human bladder tumor-initiating cells[END_REF][START_REF] Hayakawa | Cutting edge: tumor rejection mediated by NKG2D receptor-ligand interaction is dependent upon perforin[END_REF][START_REF] Jadus | Macrophages can recognize and kill tumor cells bearing the membrane isoform of macrophage colony-stimulating factor[END_REF][START_REF] Kagi | Cytotoxicity mediated by T cells and natural killer cells is greatly impaired in perforin-deficient mice[END_REF][START_REF] Uno | Antitumor activity of a monoclonal antibody against CD47 in xenograft models of human leukemia[END_REF][START_REF] Wrobel | Lysis of a broad range of epithelial tumour cells by human gamma delta T cells: involvement of NKG2D ligands and T-cell receptor-versus NKG2D-dependent recognition[END_REF][START_REF] Wu | Ex vivo expanded human circulating Vdelta1 gammadeltaT cells exhibit favorable therapeutic potential for colon cancer[END_REF]. Activated NK cells and γδ T-cells are also potent IFNγ producers. IFNγ increases MHC and immunoproteasome expression by tumor cells [START_REF] Cheon | Interferons and their stimulated genes in the tumor microenvironment[END_REF][START_REF] Rouette | Expression of immunoproteasome genes is regulated by cell-intrinsic and -extrinsic factors in human cancers[END_REF], thereby sensitizing tumor cells to CD8 + T-cell killing. More complex is the role of macrophages, which are mostly considered as pro-, rather than anti-tumor, as detailed below. Nonetheless, the expression of a fixed set of germline-encoded receptors, characteristic of innate immune cells, shows that innate cell-mediated cancer surveillance is fundamentally different from that by the adaptive immune system, which instead relies on a unique specificity for tumor antigens. Hence, CD8 + cytotoxic T cells-mediated tumor rejection represents the last step of a complex dynamic process, nicely exemplified with the proposed Cancer-Immunity Cycle [START_REF] Chen | Oncology meets immunology: the cancer-immunity cycle[END_REF]. As a result of the oncogenic transformation, cancer cells express modified antigens (neoantigens) which are released upon tumor cell death. Neoantigens are then captured by antigen-presenting cells (APCs), most typically in humans by conventional type 1 dendritic cells (cDC1) [START_REF] Sanchez-Paulete | Antigen cross-presentation and T-cell crosspriming in cancer immunology and immunotherapy[END_REF], which then drive the priming and activation of T cell responses, possibly within tertiary lymphoid structures (TLS) [START_REF] Zhu | A high density of tertiary lymphoid structure B cells in lung tumors is associated with increased CD4(+) T cell receptor repertoire clonality[END_REF], in the presence of the right set of costimulatory signals and cytokine milieu [START_REF] Gardner | Dendritic Cells and Cancer Immunity[END_REF]. This represent a critical step, as distinct parameters and factors can tip the balance in favor of either anti-tumor effector T cells, or pro-tumor regulatory T (T REG ) cells [START_REF] Chen | Oncology meets immunology: the cancer-immunity cycle[END_REF]. Of note, the neoantigen specificity of both tumor-associated and circulating T REG cells has been demonstrated only very recently [START_REF] Ahmadzadeh | Tumor-infiltrating human CD4(+) regulatory T cells display a distinct TCR repertoire and exhibit tumor and neoantigen reactivity[END_REF].

T REG cells are not the only immune cells shown to favor cancer progression: monocytic MDSCs (myeloid-derived suppressor cells) and M2-like TAMs (both falling within the broad category of monocyte/macrophage lineage cells, MMLCs) are the most notable examples of pro-tumor immune cells by acting at multiple levels (reviewed in [START_REF] Chew | Immune microenvironment in tumor progression: characteristics and challenges for therapy[END_REF][START_REF] Lindau | The immunosuppressive tumour network: myeloid-derived suppressor cells, regulatory T cells and natural killer T cells[END_REF][START_REF] Takeuchi | Roles of regulatory T cells in cancer immunity[END_REF] [START_REF] Aras | TAMeless traitors: macrophages in cancer progression and metastasis[END_REF]). The degree of CD8 + T cell infiltration is affected by that by both TAMs and TREG CELLS.

Tumor cells are a source of colony stimulating factor 1 (CSF-1), which recruits CSF-1 receptor (CSF-1R)-expressing TAMs [START_REF] Dwyer | Promotion of Tumor Invasion by Tumor-Associated Macrophages: The Role of CSF-1-Activated Phosphatidylinositol 3 Kinase and Src Family Kinase Motility Signaling[END_REF], thereby providing at once a survival, proliferation and differentiation stimulus. The in vivo pharmacological inhibition of the CSF-1R signalling pathway [START_REF] Strachan | CSF1R inhibition delays cervical and mammary tumor growth in murine models by attenuating the turnover of tumor-associated macrophages and enhancing infiltration by CD8(+) T cells[END_REF], or the genetic ablation of CSF1 in colorectal cancer cells [START_REF] Abbasi | Relapses After CAR-T Therapy[END_REF] promoted CD8 + T cell infiltration, which was though counteracted by an enhanced influx of TREG CELLS [START_REF] Gyori | Compensation between CSF1R+ macrophages and Foxp3+ Treg cells drives resistance to tumor immunotherapy[END_REF]. Co-depletion of both TAMs and TREG CELLS increased of CD8 + T cell density and activity and reduced tumor growth [START_REF] Gyori | Compensation between CSF1R+ macrophages and Foxp3+ Treg cells drives resistance to tumor immunotherapy[END_REF]. Hepatocellular carcinoma cells can activate tumorassociated monocytes, hence showing increased HLA-DR, CD80, and CD86 levels with concomitant PD-L1 expression, thereby suppressing tumor-specific T cell-mediated immunity [START_REF] Kuang | Activated monocytes in peritumoral stroma of hepatocellular carcinoma foster immune privilege and disease progression through PD-L1[END_REF]. Components of the TME such as mucins promote an immune suppressive phenotype (increased production of IL-10, decreased Th1-recruiting CCL3, suppressed IL-12) in TAMs by binding the mannose receptor on these cells and determining its internalization [START_REF] Allavena | Engagement of the mannose receptor by tumoral mucins activates an immune suppressive phenotype in human tumor-associated macrophages[END_REF]. Indeed, tumors would seem to distinctively reprogram TAMs, as shown by the transcriptional differences observed between monocytes/tissue resident macrophages and TAMs from endometrial and breast cancers [START_REF] Cassetta | Human Tumor-Associated Macrophage and Monocyte Transcriptional Landscapes Reveal Cancer-Specific Reprogramming, Biomarkers, and Therapeutic Targets[END_REF]. Tumor cells (as well as TAMs themselves) are a source of TNF-α, which supports CCL8 production by TAMs; in turn, CCL8 promotes CSF-1 production by cancer cells, thereby supporting TAMs survival and proliferation [START_REF] Cassetta | Human Tumor-Associated Macrophage and Monocyte Transcriptional Landscapes Reveal Cancer-Specific Reprogramming, Biomarkers, and Therapeutic Targets[END_REF]. Furthermore, TAMs can promote angiogenesis (the "angiogenic switch") by producing a plethora of factors (vascular endothelial growth factor A (VEGFA), epidermal growth factor (EGF), basic fibroblast growth factor 2 (FGF2), chemokines CXCL8, CXCL12, TNF-α, semaphorin 4D, adrenomedullin, and thymidine phosphorylase) activating and recruiting endothelial cells [START_REF] Leek | Association of tumour necrosis factor alpha and its receptors with thymidine phosphorylase expression in invasive breast carcinoma[END_REF][START_REF] Riabov | Role of tumor associated macrophages in tumor angiogenesis and lymphangiogenesis[END_REF][START_REF] Sierra | Tumor angiogenesis and progression are enhanced by Sema4D produced by tumor-associated macrophages[END_REF][START_REF] Zhou | Semaphorin 4D cooperates with VEGF to promote angiogenesis and tumor progression[END_REF].

Whilst a general consensus emerged on the pro-tumoral role of TREG cells, MMLCs display a more complex profile. A recent study in early-stage human lung tumors showed that PD-L1expressing TAMs did not generally suppress tumor-specific effector T cell responses, as opposed to PD-L1-expressing tumor cells; on the other hand, tumor monocytes tended to inhibit more the T cell responses [START_REF] Singhal | Human tumor-associated monocytes/macrophages and their regulation of T cell responses in early-stage lung cancer[END_REF]. Apart from potentially expressing PD-L1, TAMs express increasing levels of checkpoint molecules (PD-1 [START_REF] Gordon | PD-1 expression by tumourassociated macrophages inhibits phagocytosis and tumour immunity[END_REF] and Clever-1 [START_REF] Viitala | Immunotherapeutic Blockade of Macrophage Clever-1 Reactivates the CD8(+) T-cell Response against Immunosuppressive Tumors[END_REF]) over the course of primary human cancers, as discussed above. This evidence indirectly shows the good, anti-tumor side of TAMs.

Chemokines and factors keeping the "good" immune cells within the TME are essential to enable an efficient anti-tumor response. The rarity of cDC1 in the TME, and their most frequent absence from early tumor stages was suggested to contribute to cancer progression [START_REF] Bottcher | NK Cells Stimulate Recruitment of cDC1 into the Tumor Microenvironment Promoting Cancer Immune Control[END_REF]. NK cells were recently shown to be important producers of the cDC1 chemoattractants CCL5 and XCL1 in melanoma [START_REF] Bottcher | NK Cells Stimulate Recruitment of cDC1 into the Tumor Microenvironment Promoting Cancer Immune Control[END_REF]; in turn, cDC1s can recruit effector T cells by releasing CXCL9 and CXCL10, which of note can also attract NK cells in a positive feedback loop [START_REF] Fessenden | A team effort: natural killer cells on the first leg of the tumor immunity relay race[END_REF][START_REF] Spranger | Tumor-Residing Batf3 Dendritic Cells Are Required for Effector T Cell Trafficking and Adoptive T Cell Therapy[END_REF][START_REF] Zelenay | Cyclooxygenase-Dependent Tumor Growth through Evasion of Immunity[END_REF]. T helper 1 (T H 1) cells also significantly contribute to the production of CXCL9 and CXCL10; their epigenetic silencing in these cells was associated with decreased intratumoral CD8 + T cells and reduced survival in human ovarian cancer, thus representing an immuneevasion mechanism of tumors [START_REF] Peng | Epigenetic silencing of TH1-type chemokines shapes tumour immunity and immunotherapy[END_REF]. The critical role played by chemokines in creating an optimal, immune hot (highly infiltrated by CD8 + T cells) was shown using multiple approaches [START_REF] Mlecnik | Biomolecular network reconstruction identifies T-cell homing factors associated with survival in colorectal cancer[END_REF], and very recently confirmed by the group of George Coukos [START_REF] Dangaj | Cooperation between Constitutive and Inducible Chemokines Enables T Cell Engraftment and Immune Attack in Solid Tumors[END_REF]. These studies showed an association between CCL5 and CXCL9 co-expression and CD8 + T cell infiltration across human solid tumors. Tumor cells were identified as a constitutive source of CCL5, where it could be epigenetically repressed, whilst CXCL9 was induced in tumor-associated myeloid cells (TAM and DCs) by IFNγ, dependently on tumor antigen recognition by TILs [START_REF] Dangaj | Cooperation between Constitutive and Inducible Chemokines Enables T Cell Engraftment and Immune Attack in Solid Tumors[END_REF].

Novel proteogenomics approaches enabled the identification of further mechanisms regulating T cell infiltration into microsatellite instability-high (MSI-H) colon tumors [START_REF] Vasaikar | Proteogenomic Analysis of Human Colon Cancer Reveals New Therapeutic Opportunities[END_REF]. MSI-H tumors are characterized by an increased mutational and neoantigen load compared to the microsatellite stable (MSS) phenotype, with is typically accompanied by an increase in TILs [START_REF] Smyrk | Tumor-infiltrating lymphocytes are a marker for microsatellite instability in colorectal carcinoma[END_REF]. Whilst no noticeable differences were found at the mRNA level, a marked increase in the protein levels of glycolytic enzymes was found in a subgroup of MSI-H tumors, and then linked to impaired T cell function and trafficking to the TME [START_REF] Vasaikar | Proteogenomic Analysis of Human Colon Cancer Reveals New Therapeutic Opportunities[END_REF].

Once recruited to the TME, CD8 + effector T cells further propagate this loop by producing, amongst others, CCL5 and XCL1 [START_REF] Fessenden | A team effort: natural killer cells on the first leg of the tumor immunity relay race[END_REF]. Of note, a lack of recruitment of immune cells was proposed to be one possible cause for the existence of so-called excluded tumors, characterized by the presence of effector T cells at the tumor borders, but not at its core (Figure 4) [START_REF] Galon | Approaches to treat immune hot, altered and cold tumours with combination immunotherapies[END_REF]. It remains to be established how this chain of events starts, i.e., how NK cells (which are lowly, if at all present within the TME) are recruited in the first place; nonetheless, it is clear that it ends, when successful, with the CD8 + cytotoxic T cells-mediated killing of cancer cells.

The epigenetic modulation of chemokines expression shown above are only some of the numerous immune escape mechanisms developed by cancers to avoid immune recognition, activation and effector functions. The expression of immune checkpoints, which occurs already at the precancerous stages [START_REF] Mascaux | Immune evasion before tumour invasion in early lung squamous carcinogenesis[END_REF], and the development of tolerance are the most notable examples of well-established mechanisms of immune control. Apart from the immune-mediated mechanisms of escape, the tumor itself can directly or evade the anti-tumor immune response. A comprehensive analysis of tumor cell-intrinsic mechanisms of primary and secondary immune evasion have been extensively provided elsewhere [START_REF] Wellenstein | Cancer-Cell-Intrinsic Mechanisms Shaping the Tumor Immune Landscape[END_REF]. More recently, an additional mechanism of tumormediated immune escape has been provided by Bottcher and colleagues. Tumor cells were previously found to be a source of prostaglandin E2 (PGE 2 ) [START_REF] Zelenay | Cyclooxygenase-Dependent Tumor Growth through Evasion of Immunity[END_REF] [START_REF] Jung | IL-1beta-mediated upregulation of HIF-1alpha via an NFkappaB/COX-2 pathway identifies HIF-1 as a critical link between inflammation and oncogenesis[END_REF].Novel evidence showed how tumor-derived PGE 2 reduced the secretion of CCL5 and XCL1 by NK cells; impaired the expression of chemokine receptors on cDC1s; and increased NK cell death [START_REF] Bottcher | NK Cells Stimulate Recruitment of cDC1 into the Tumor Microenvironment Promoting Cancer Immune Control[END_REF]. These novel mechanisms could contribute to explain the previously known pro-tumorigenic role of PGE 2 and other eicosanoids [START_REF] Wang | Eicosanoids and cancer[END_REF], as well as further demonstrating the complexity of the interplay between tumor and tumor-associated immune components. It is the combination and simultaneous crosstalk among all positive and negative factors that ultimately shapes cancer progression (Figure 34).

The tumor microenvironment (TME) is a highly dynamic entity, in terms of both cellular immune components and soluble mediators. The great majority of the studies describing the TME composition reflect a specific point in space and time, which is an evident limitation of studies in humans. Nonetheless, some studies assessed the changes accompanying tumor progression, offering precious insights into the associated everchanging immune landscape.

To our knowledge, we were the first proposing an integrative study of the spatio-temporal dynamics of tumor-associated immune cell-types with immune-cell specific gene modules (immunome) showing how the immune infiltrate composition changes at each tumor-stage [START_REF] Bindea | Spatiotemporal dynamics of intratumoral immune cells reveal the immune landscape in human cancer[END_REF]. Densities of the TLS-associated T follicular helper (T FH ) cells and innate cells (including macrophages, mast cells, neutrophils, plasmacytoid DC) increased,

whereas most T cell densities (activated T-cells, memory T-cells, effector T-cells, CD4 + helper and CD8 + cytotoxic T-cells) decreased along with tumor progression through T-stages

and AJCC/UICC-TNM classification [START_REF] Bindea | Spatiotemporal dynamics of intratumoral immune cells reveal the immune landscape in human cancer[END_REF], as also found in a previous study [START_REF] Mlecnik | Histopathologic-based prognostic factors of colorectal cancers are associated with the state of the local immune reaction[END_REF]. B cells increased at a later stage, showing a dual effect on recurrence and tumor progression. CXCL13 and IL-21 were pivotal factors for the T FH -B cell axis correlating with prolonged-survival and supporting a good immune contexture (T H 1, cytotoxic and memory T-cells). These findings, first observed with gene expression and immunome analyses, were confirmed by multiple approaches, including quantitative immunohistochemistry and mouse models [START_REF] Bindea | Spatiotemporal dynamics of intratumoral immune cells reveal the immune landscape in human cancer[END_REF]. Recently, T FH cells were found to enhance CD8 + T cells effector function in an IL-21-dependent manner in CRC patients [START_REF] Shi | Follicular helper T cells promote the effector functions of CD8(+) T cells via the provision of IL-21, which is downregulated due to PD-1/PD-L1-mediated suppression in colorectal cancer[END_REF]. This potent T FH cells function was impaired in the presence of PD-1/PD-L1-mediated suppression [START_REF] Shi | Follicular helper T cells promote the effector functions of CD8(+) T cells via the provision of IL-21, which is downregulated due to PD-1/PD-L1-mediated suppression in colorectal cancer[END_REF].

The immune landscape of primary tumors: clinical implications

The study of the immune landscapes associated to cancers has deep clinical implications. One clear example is constituted by primary CRCs, which are surgically removed in the majority of cases. To date, there is no parameter in clinical use that can predict CRC recurrence, which indeed happens in 40% of surgically CRC cases within 5 years [START_REF] Augestad | Tailored Treatment of Colorectal Cancer: Surgical, Molecular, and Genetic Considerations[END_REF]. The analysis of the immune infiltrate and Immunoscore of the resected primary tumors were demonstrated to predict tumor recurrence: indeed, a low adaptive immune reaction in the center and invasive margin of the primary tumor was found in recurrent patients. This Immunoscore feature was observed in early-stage tumor (Stage I) as well as in late tumor stages (Stage III or IV) [START_REF] Mlecnik | Histopathologic-based prognostic factors of colorectal cancers are associated with the state of the local immune reaction[END_REF][START_REF] Pages | In situ cytotoxic and memory T cells predict outcome in patients with early-stage colorectal cancer[END_REF][START_REF] Pages | International validation of the consensus Immunoscore for the classification of colon cancer: a prognostic and accuracy study[END_REF]. Thus, immune features of the TME associated with CRC progression and recurrence could prove instrumental to guiding the choice for additional (neo)adjuvant therapy.

The immune landscape of cancer across 33 different cancer types was recently defined, identifying six immune subtypes based on differences in both immune (macrophage or lymphocyte signatures, Th1:Th2 cell ratio, expression of immunomodulatory genes) and seemingly non-immune parameters (intratumoral heterogeneity, aneuploidy, neoantigen load, overall cell proliferation, patients" prognosis) [START_REF] Thorsson | The Immune Landscape of Cancer[END_REF]. This genomic approach linked the presence of specific driver mutations with lower (CTNNB1, NRAS, or IDH1) or higher (BRAF, TP53, or CASP8) leukocytes frequencies across cancer types. Whether the occurrence of these mutations at different stages of the disease has a differential effect on the amount and/or composition of immune tumor-infiltrates (hence on patients" prognosis) remains to be established. A limitation of the study is that it only relied on RNAseq and ExomeSeq data. Nonetheless, this work has the undoubted merit of having highlighted features of the TME that are shared among histologically distinct tumors, suggesting that the choice of therapeutic strategies should rely on immune parameters rather than cancer type. A first regulatory opening in this sense came with the approval by the FDA in May 2017 of the anti-PD-1 pembrolizumab for the treatment of adult and pediatric patients with unresectable or metastatic solid tumors based on a tumor biomarker (MSI-H or mismatch repair deficient, dMMR). Immune biomarkers constitute likely candidates for future regulatory approvals.

Another important consideration having likely repercussions on prognosis and clinical management of cancer patients concerns the spatial localization and relative distribution of immune cells within the TME. The colocalization of TAMs or B cells with cytotoxic T cells was associated with complete pathological response after neo-adjuvant chemotherapy in inflammatory breast cancer patients [START_REF] Van Berckelaer | The spatial localization of immune cells predicts prognosis and response to therapy in inflammatory breast cancer Proceedings of the[END_REF]. Moreover, whilst the number of PDL1 + or CD8 + cells was not prognostic per se, the presence of more PDL1 + cells around CD8 + cells (r= 30 μm) correlated with worse prognosis [START_REF] Van Berckelaer | The spatial localization of immune cells predicts prognosis and response to therapy in inflammatory breast cancer Proceedings of the[END_REF] or with absence of immunoediting [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF].

Multiplex immunohistochemistry (mIHC) enables the simultaneous detection and spatial resolution of multiple immune determinants at once, and its adoption in clinical routine has been advocated [START_REF] Hofman | Multiplexed Immunohistochemistry for Molecular and Immune Profiling in Lung Cancer-Just About Ready for Prime-Time?[END_REF]. A multiplex analysis of pancreatic ductal adenocarcinoma PDAC patients who received neoadjuvant GVAX (a granulocytemacrophage colony-stimulating factor (GM-CSF)-secreting pancreatic tumor vaccine [START_REF] Lutz | Immunotherapy converts nonimmunogenic pancreatic tumors into immunogenic foci of immune regulation[END_REF]) showed how response to therapy correlated with degree of myeloid cell density and percentages of exhausted CD8 + T cells. Alternative approaches to mIHC have been proposed.

For instance, deep learning mapping of TILs from hematoxylin and eosin (H&E) stain images from 13 TCGA tumor types revealed local spatial structures in TIL patterns and their correlation with overall survival [START_REF] Saltz | Spatial Organization and Molecular Correlation of Tumor-Infiltrating Lymphocytes Using Deep Learning on Pathology Images[END_REF]. Tumor and immune and molecular subtypes were characterized by differential TIL densities and spatial structures enrichment, possibly reflecting specific tumor cell aberrations. Local clustering of TILs was a more distinctive (and better prognostic) feature than overall TIL infiltration in pancreatic adenocarcinoma and prostate adenocarcinoma as opposed to other tumor types [START_REF] Saltz | Spatial Organization and Molecular Correlation of Tumor-Infiltrating Lymphocytes Using Deep Learning on Pathology Images[END_REF], indicating the importance of the spatial immune patterns in affecting survival in specific cancer settings. The reason for these tissue-specific differences are not clear; also unclear is the mechanism regulating the formation and evolution of these patterns overtime; future investigation on these aspects is urgently needed, especially in view of the adoption of evermore feature/biomarker-based therapeutic approaches.

Early cancer dissemination and distant metastases

Tumor progression comes most often with the spreading of the primary tumor to other anatomical locations throughout the body. The metastatic cascade is a multi-step process involving incompletely defined pathological changes of the tumor cell, which has to be able to leave the primary tumor, enter in systemic circulation, penetrate and colonize distant sites, often after a phase of dormancy [START_REF] Kienast | Real-time imaging reveals the single steps of brain metastasis formation[END_REF][START_REF] Luzzi | Multistep nature of metastatic inefficiency: dormancy of solitary cells after successful extravasation and limited survival of early micrometastases[END_REF]. The Epithelialmesenchymal transition (EMT) is a widely recognized process leading to tumor cell invasion and metastatic spreading, and distinct EMT programs were recently associated with different invasion modalities [START_REF] Aiello | EMT Subtype Influences Epithelial Plasticity and Mode of Cell Migration[END_REF]. The hypothesis that the both innate and adaptive arms of the immune system play a role in modulating (positively or negatively) one or more steps in the metastatic cascade is being ever-growingly appreciated and substantiated by compelling evidence.

Macrophages and MMLCs in general are amongst the most prominent innate immune cells promoting all steps characterizing metastatic spreading [START_REF] Lewis | The Multifaceted Role of Perivascular Macrophages in Tumors[END_REF][START_REF] Nielsen | Macrophages as Key Drivers of Cancer Progression and Metastasis[END_REF][START_REF] Qian | Macrophage diversity enhances tumor progression and metastasis[END_REF]. Such prominence was shown, for instance, by the observation that the genetic ablation of CSF-1 completely abolished the metastatic process in a mouse model of breast cancer [START_REF] Lin | Colony-stimulating factor 1 promotes progression of mammary tumors to malignancy[END_REF]. In fact, macrophages were directly linked to metastatic dissemination in multiple preclinical models [START_REF] Griesmann | Pharmacological macrophage inhibition decreases metastasis formation in a genetic model of pancreatic cancer[END_REF][START_REF] Headley | Visualization of immediate immune responses to pioneer metastatic cells in the lung[END_REF][START_REF] Linde | Macrophages orchestrate breast cancer early dissemination and metastasis[END_REF].

Several mechanisms underpinning macrophages ability to favor the metastatic process have been shown, involving in many cases their angiogenic and ECM remodeling properties, and cytokine/chemokine production. MMLC-driven remodeling mechanisms facilitate the migration of malignant cells from the primary tumor to nearby or distant sites. Indeed, the presence of venous invasion and liver metastasis were linked to increased expression TAMderived MMP-2 and MMP-9 in pancreatic cancer patients [START_REF] Nagakawa | Histologic features of venous invasion, expression of vascular endothelial growth factor and matrix metalloproteinase-2 and matrix metalloproteinase-9, and the relation with liver metastasis in pancreatic cancer[END_REF]. TAMs, together with tumor cells, are a source of cathepsin S, which is able to proteolytically process the blood-brain barrier junctional adhesion molecule JAM-B, thereby promoting transmigration of breast-derived tumor cells into the brain [START_REF] Sevenich | Analysis of tumour-and stromasupplied proteolytic networks reveals a brain-metastasis-promoting role for cathepsin S[END_REF]. Another pro-metastatic mechanism involves the typically M2 [START_REF] Kodelja | Alternative macrophage activation-associated CC-chemokine-1, a novel structural homologue of macrophage inflammatory protein-1 alpha with a Th2-associated expression pattern[END_REF] chemokine CCL18, produced by TAMs and acting on PITPNM3-expressing cancer cells, where it triggers integrin clustering thus enhancing their adherence to the ECM [START_REF] Chen | CCL18 from tumor-associated macrophages promotes breast cancer metastasis via PITPNM3[END_REF]. This mechanism was shown to promote breast cancer metastasis formation in vivo (Chen et al., 2011). MMLCs seem also to set up a favorable stage for metastatic seeding, by remodeling the pre-metastatic niche and secreting tumor cell-attracting factors. Monocytic MDSCs (mo-MDSCs), a subtype of MDSCs, are recruited in a CCL12-dependent manner to the premetastatic lungs, where they constitute a major source of IL-1β. This cytokine increases E-selectin expression on endothelial cells thus promoting tumor cell arrest [START_REF] Shi | Recruited monocytic myeloid-derived suppressor cells promote the arrest of tumor cells in the premetastatic niche through an IL-1beta-mediated increase in E-selectin expression[END_REF]. Within the metastatic site, macrophages and MMLCs in general display most commonly an immunosuppressive, M2-like phenotype [START_REF] Biswas | Macrophage plasticity and interaction with lymphocyte subsets: cancer as a paradigm[END_REF].

Neutrophils represent another crucial innate immune cell type favoring the metastatic process, as shown in several preclinical models [START_REF] Tuting | CANCER. How neutrophils promote metastasis[END_REF]. In patients, a relationship between increased circulating neutrophils and the generation of distant metastases in has been found several solid tumors including melanoma [START_REF] Bald | Ultravioletradiation-induced inflammation promotes angiotropism and metastasis in melanoma[END_REF] and PDAC [START_REF] Tao | Neutrophils assist the metastasis of circulating tumor cells in pancreatic ductal adenocarcinoma: A new hypothesis and a new predictor for distant metastasis[END_REF]. The proposed hypothesis that neutrophils might interact with circulating tumor cells (CTCs) in the blood and assist these potentially metastatic cells [START_REF] Tao | Neutrophils assist the metastasis of circulating tumor cells in pancreatic ductal adenocarcinoma: A new hypothesis and a new predictor for distant metastasis[END_REF] has been validated recently by the team of Nicola Aceto [START_REF] Szczerba | Neutrophils escort circulating tumour cells to enable cell cycle progression[END_REF]. The same group revealed how the association with neutrophils promotes cell cycle progression in CTCs, increasing their metastatic potential, compared to unescorted CTCs [START_REF] Szczerba | Neutrophils escort circulating tumour cells to enable cell cycle progression[END_REF].

Further evidence showed how neutrophils can also shape a favorable pre-metastatic niche in distant organs. This has been first shown in pre-clinical models of breast cancer [START_REF] Wculek | Neutrophils support lung colonization of metastasisinitiating breast cancer cells[END_REF] and ovarian cancer [START_REF] Lee | Neutrophils facilitate ovarian cancer premetastatic niche formation in the omentum[END_REF]. Neutrophils undergo a specific type of cell death termed NETosis involving the extrusion of chromatin and the generation of neutrophil extracellular traps (NETs) [START_REF] Singel | Neutrophils in the tumor microenvironment: trying to heal the wound that cannot heal[END_REF]. NETs were previously shown to facilitate tumor progression through promotion of thrombosis and angiogenesis [START_REF] Singel | Neutrophils in the tumor microenvironment: trying to heal the wound that cannot heal[END_REF]. NETs were also required for awakening dormant metastatic cancer cells in inflamed murine lungs, via the cleavage of laminin mediated by NET-associated proteases.

Proteolytically remodeled laminin activated integrin α3β1 signaling in the dormant cancer cells, thus inducing their proliferation [START_REF] Albrengues | Neutrophil extracellular traps produced during inflammation awaken dormant cancer cells in mice[END_REF]. Finally, recent evidence showed how omentum-migrating neutrophils at the early ovarian cancer stages promoted metastatic seeding in this organ in a NET-dependent manner [START_REF] Lee | Neutrophils facilitate ovarian cancer premetastatic niche formation in the omentum[END_REF]. Taken together, the presented evidence highlights the multifaceted role of neutrophils in favoring tumor progression, although mechanisms showing antitumor-roles for these innate immune cells also exist [START_REF] Bindea | Spatiotemporal dynamics of intratumoral immune cells reveal the immune landscape in human cancer[END_REF][START_REF] Singel | Neutrophils in the tumor microenvironment: trying to heal the wound that cannot heal[END_REF], possibly suggesting an incomplete understanding of such complex system.

A recently-developed stable intravital two-photon imaging model in mice enabled the direct observation of the arrival of CTCs in the lung, and the subsequent immune dynamics [START_REF] Headley | Visualization of immediate immune responses to pioneer metastatic cells in the lung[END_REF]. Upon arrival in the lung capillaries, CTCs shed microparticles which either remained attached to the lung vasculature, or migrated through it. These particles were then ingested by distinct temporal waves of myeloid cells: neutrophil first, then conventional monocytes, non-alveolar macrophages, patrolling monocytes and cDCs. Successful metastatic development occurred following accumulation of these interstitial myeloid cells, which was favoured by the CCR2-CCL2 axis, previously shown to promote myeloid recruitment in the lung [START_REF] Qian | CCL2 recruits inflammatory monocytes to facilitate breasttumour metastasis[END_REF]. This study also implied a host-protective role for cDCs, competing with the pro-tumor macrophages by engulfing tumor-derived material in the lung and subsequently migrate to the mesenteric lymph nodes to engage with cognate T cells [START_REF] Headley | Visualization of immediate immune responses to pioneer metastatic cells in the lung[END_REF].

Natural killer (NK) cells represent yet another innate immune cell type linked to the modulation of metastatic spreading. NK cells are more prominently known to exert direct cytotoxicity against tumor cells, and they appear to kill more efficiently tumor cells in circulation, rather than those within the TME [START_REF] Larsen | NK cells in the tumor microenvironment[END_REF]. NK cells are scarcely infiltrating solid tumors but can be found at the tumor borders, where EMT takes place [START_REF] Cantoni | NK Cells, Tumor Cell Transition, and Tumor Progression in Solid Malignancies: New Hints for NK-Based Immunotherapy?[END_REF]. Their ability to kill tumor cells undergoing EMT has been demonstrated in lung cancer [START_REF] Chockley | Epithelial-mesenchymal transition leads to NK cell-mediated metastasis-specific immunosurveillance in lung cancer[END_REF], although mechanisms of resistance to NK-mediated cytotoxicity have been also described [START_REF] Terry | New insights into the role of EMT in tumor immune escape[END_REF]. Therefore, by potentially acting on both the putative metastatic precursors (the EMT-undergone malignant cells) and the CTCs, NK cells appear to limit tumor colonization of distant sites.

NK cells inhibited lung colonization by tumor cells following administration of a phospholipid-conjugated toll-like receptor 7 (TLR7) agonist in murine models of breast cancer, melanoma and Lewis lung carcinoma [START_REF] Hosoya | Induction of oligoclonal CD8 T cell responses against pulmonary metastatic cancer by a phospholipid-conjugated TLR7 agonist[END_REF]. Whether this effect is due to an NK-mediated direct effect on metastatic precursors or CTCs remains to be elucidated.

The same study showed the temporal sequence of immune-mediated anti-dissemination effects, whereby NK cells constrain early tumor colonization, followed by a cytotoxic CD8 + T cell anti-metastatic response at the later phases [START_REF] Hosoya | Induction of oligoclonal CD8 T cell responses against pulmonary metastatic cancer by a phospholipid-conjugated TLR7 agonist[END_REF]. Indeed, cytotoxic immune cells and other adaptive immune cells were also found to affect metastatic spreading.

For instance, the presence of high levels of infiltrating memory CD45RO + T cells correlated with the absence of early signs of invasiveness around the tumor (such as venous emboli, lymphatic invasion, and perineural invasion -VELIPI; Figure 2), as well as increased CRC patient"s survival [START_REF] Pages | Effector memory T cells, early metastasis, and survival in colorectal cancer[END_REF]. This also highlighted that a weak pre-existing adaptive intra-tumor immunity gave the license to the tumor to invade vessels, even at very early-stage of disease. A subsequent study on NSCLC correlated the presence of high levels of intraepithelial CD45RO + cells in lymph-node metastases with increased patients" survival [START_REF] Kilvaer | The presence of intraepithelial CD45RO+ cells in resected lymph nodes with metastases from NSCLC patients is an independent predictor of disease-specific survival[END_REF]. Of note, this held true in squamous cell carcinoma, but not in adenocarcinoma patients [START_REF] Kilvaer | The presence of intraepithelial CD45RO+ cells in resected lymph nodes with metastases from NSCLC patients is an independent predictor of disease-specific survival[END_REF]. The long-lasting antitumor capacity of memory T cells (CD45RO + ) was shown two decades ago in mouse models of colon carcinoma metastases [START_REF] Xiang | T cell memory against colon carcinoma is long-lived in the absence of antigen[END_REF]. The presence of enduring human memory T cells [START_REF] Sallusto | Central memory and effector memory T cell subsets: function, generation, and maintenance[END_REF]) may be a key immune hallmark of tumor control, explaining long-term survival in some cancer patients. Recent evidence shows how the interplay between circulating memory T cells and resident memory CD8 + T cells (T RM ) is required to ensure optimal antitumor immunity [START_REF] Enamorado | Enhanced anti-tumour immunity requires the interplay between resident and circulating memory CD8(+) T cells[END_REF].

Apart from the demonstration of anti-metastatic effect in synergy with NK cells described above [START_REF] Hosoya | Induction of oligoclonal CD8 T cell responses against pulmonary metastatic cancer by a phospholipid-conjugated TLR7 agonist[END_REF], further in vivo studies demonstrated the ability of CD8 + cytotoxic T cells to protect against metastatic lesion formation. CD8 + T cells (and NK cells) protected against metastatic spreading in a type I IFN-dependent manner, as demonstrated by the in vivo analysis of bone metastasis from breast carcinoma [START_REF] Bidwell | Silencing of Irf7 pathways in breast cancer cells promotes bone metastasis through immune escape[END_REF]. Furthermore, depletion of CD8 + T cells in a spontaneous melanoma mouse model resulted in an increased formation of lung and reproductive tract metastases [START_REF] Eyles | Tumor cells disseminate early, but immunosurveillance limits metastatic outgrowth, in a mouse model of melanoma[END_REF][START_REF] Lengagne | Distinct role for CD8 T cells toward cutaneous tumors and visceral metastases[END_REF]. In patients with inflammatory breast cancer, a correlation between circulating tumor cells (CTCs; mediators of metastatic dissemination) and defects in adaptive immunity has been demonstrated [START_REF] Mego | Circulating Tumor Cells (CTC) Are Associated with Defects in Adaptive Immunity in Patients with Inflammatory Breast Cancer[END_REF]. Similarly, CTCs levels negatively correlated with circulating T-cell levels in lung cancer patients [START_REF] Sun | Characteristics of circulating tumor cells in organ metastases, prognosis, and T lymphocyte mediated immune response[END_REF].

Increased levels of immune cytotoxicity and lymphatic vessels density were shown to hinder the generation of distant metastases from CRC, whereas tumor cell genomic alterations didn"t seem to affect this process (Mlecnik et al., 2016b). Indeed, a sole immune-related gene expression profiling could distinguish patients with or without distant metastases, indicating the strength of the immune parameters (Mlecnik et al., 2016b). TILs have been shown to directly affect the metastatic landscape in human CRC [START_REF] Halama | Localization and density of immune cells in the invasive margin of human colorectal cancer liver metastases are prognostic for response to chemotherapy[END_REF][START_REF] Kwak | Immunoscore encompassing CD3+ and CD8+ T cell densities in distant metastasis is a robust prognostic marker for advanced colorectal cancer[END_REF][START_REF] Mlecnik | Comprehensive Intrametastatic Immune Quantification and Major Impact of Immunoscore on Survival[END_REF][START_REF] Van Den Eynde | The Link between the Multiverse of Immune Microenvironments in Metastases and the Survival of Colorectal Cancer Patients[END_REF]. We provided ex vivo evidence of the protective role of CD8 + T cells against metastatic spreading, with killing of tumor cells by anti-frameshift mutations neoepitope-targeting CD8 + T-cells (Mlecnik et al., 2016b). We also showed the non-dissemination of immunoedited tumor clones in the presence of CD8 + cytotoxic T-cells and proliferating T -cells in CRC [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF]. Elevated TIL densities as determined by Immunoscore measured on the least infiltrated metastases correlated with longer patients" survival [START_REF] Mlecnik | Comprehensive Intrametastatic Immune Quantification and Major Impact of Immunoscore on Survival[END_REF]; indeed, the phenotype of such metastases was the ones better predicting patients" outcome [START_REF] Van Den Eynde | The Link between the Multiverse of Immune Microenvironments in Metastases and the Survival of Colorectal Cancer Patients[END_REF].

The brain is typically considered as an immune-privileged site. Whether T cells exert an antimetastatic effector response within the brain parenchyma remained a matter of debate until recently [START_REF] Gonzalez | Innate and acquired immune surveillance in the postdissemination phase of metastasis[END_REF]). An analysis of the immune microenvironment across brain metastases from any type of human primary tumor revealed that Immunoscore was profoundly associated with long-term patients" survival [START_REF] Berghoff | Density of tumor-infiltrating lymphocytes correlates with extent of brain edema and overall survival time in patients with brain metastases[END_REF]. This work would support the existence of a T cell-mediated anti-tumor immune responses within brain metastases.

A recent analysis comparing the TCRβ repertoire of NSCLC primary lesions and paired brain metastases showed a minimal overlap in T cell clones between paired lesions [START_REF] Mansfield | Contraction of T cell richness in lung cancer brain metastases[END_REF], showing clonal heterogeneity, as well as the occurrence of effector responses in the brain. Brain metastases displayed higher mutational burden, yet a significant reduction in the number of unique T cell clones, compared to matched primary lesions [START_REF] Mansfield | Contraction of T cell richness in lung cancer brain metastases[END_REF]. This might reflect a more immunosuppressive nature of TME within the brain compared to the lung, and/or the presence of less immunogenic clones in metastases, compared to primary sites. Further evidence of the direct role of TILs in the control of tumor dissemination was recently provided by Angelova et al. by following the metastatic progression of CRC patients, as well as their associated immune landscape, over an 11-year follow-up period [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF]. In addition, this study proved the validity of the concept of immunoediting in humans and, for the first time, its impact on progression and evolution of human cancers [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF]. Immunoediting was previously defined as the concept describing the change of tumor immunogenicity over time under the selective pressure of the immune system [START_REF] Dunn | Cancer immunoediting: from immunosurveillance to tumor escape[END_REF][START_REF] Mittal | New insights into cancer immunoediting and its three component phases--elimination, equilibrium and escape[END_REF]. The immune system contribution to the steps defining tumor evolution has been elegantly shown in mouse models [START_REF] Matsushita | Cancer exome analysis reveals a T-cell-dependent mechanism of cancer immunoediting[END_REF]. The genetic evidence of immunoediting was then shown in humans for missense [START_REF] Rooney | Molecular and genetic properties of tumors associated with local immune cytolytic activity[END_REF], then missense and frameshift mutations (Mlecnik et al., 2016a). Angelova and colleagues first demonstrated in humans that tumor clones for which immunoediting was more prominent tend to disappear, whilst, conversely, the least immunogenic ones (hence the most resistant to immune attack) were persisting in subsequent metastatic sites [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF]. Accordingly, the immunoediting and Immunoscore were found to be the two best predictors of favorable clinical outcome [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF]. Thus, a novel "parallel immune selection model" of tumor evolution in humans was proposed, incorporating the effects of the immune system [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF]. The most currently accepted "branched evolution model" by Gerlinger el al. describe a landscape of multiple co-evolving and co-existing malignant clones featuring distinct mutations and deriving from a common mutated ancestor [START_REF] Gerlinger | Intratumor heterogeneity and branched evolution revealed by multiregion sequencing[END_REF]; similarly to previously proposed models [START_REF] Fearon | A genetic model for colorectal tumorigenesis[END_REF][START_REF] Sottoriva | A Big Bang model of human colorectal tumor growth[END_REF], it is solely focused and based on tumor cell features. Whilst agreeing with the underlying concept of parallel coevolution and co-existence, the "parallel immune selection model" first assigns a central role to the immune system in driving and directing this process.

In vivo evidence shows how other types of adaptive immune cells (CD4 + T helper cells and Treg cells) impact metastatic spreading. CD4 + T cells can have both a pro-and anti-metastatic roles via their effects on other immune cells. By acting on TAMs in an IL-4-dependent manner, CD4 + T cells promote invasion and pulmonary metastasis of mammary carcinomas. The consequent phenotypic shift from inflammatory M1-like TAMs to EGF-expressing M2-like TAMs activated EGFR signalling programs in malignant epithelial cells, and resulted in tumor cell dissemination and outgrowth in the lung [START_REF] Denardo | CD4(+) T cells regulate pulmonary metastasis of mammary carcinomas by enhancing protumor properties of macrophages[END_REF]. Of note, this phenotypic switch in TAMs is well in line with the previously proposed concept of "smouldering" inflammation that characterizes tumor progression [START_REF] Mantovani | Cancer-related inflammation[END_REF]. On the other hand, a mouse model of permanent immunomediated metastatic dormancy showed a critical role for CD4 + T cells (but more prominently for CD8 + T cells) in restraining the awakening of dormant tumor cells and the development of pulmonary metastases [START_REF] Romero | T lymphocytes restrain spontaneous metastases in permanent dormancy[END_REF]. Perhaps a better phenotypic characterization of these cells could clarify these apparently contrasting results. For instance, subgroups of CD4 + FOXP3 -T cells are known to exhibit (as their Foxp3 + counterpart) potent suppressor functions [START_REF] Han | CD69+ CD4+ CD25-T cells, a new subset of regulatory T cells, suppress T cell proliferation through membrane-bound TGFbeta 1[END_REF], and facilitated melanoma metastasis in vivo, possibly via their inhibitory effect on NK cells [START_REF] Wang | Activated mouse CD4(+)Foxp3(-) T cells facilitate melanoma metastasis via Qa-1dependent suppression of NK-cell cytotoxicity[END_REF].

A more general, although not complete [START_REF] Wang | Intratumoral regulatory T cells are associated with suppression of colorectal carcinoma metastasis after resection through overcoming IL-17 producing T cells[END_REF], consensus exists on the prometastatic effect of Treg cells, and an association between Treg cells and incidence of metastasis has been reported in several cancer types, including NSCLC [START_REF] Erfani | Increase of regulatory T cells in metastatic stage and CTLA-4 over expression in lymphocytes of patients with non-small cell lung cancer (NSCLC)[END_REF], breast cancer [START_REF] Metelli | Surface Expression of TGFbeta Docking Receptor GARP Promotes Oncogenesis and Immune Tolerance in Breast Cancer[END_REF] and hepatocellular carcinoma [START_REF] Ye | Hypoxia-Induced Epithelial-to-Mesenchymal Transition in Hepatocellular Carcinoma Induces an Immunosuppressive Tumor Microenvironment to Promote Metastasis[END_REF].

Inhibition of Treg cells is likely to prevent metastasis, given the ability of Treg cells to counteract cancer-killing immune cells from both the adaptive and innate immune systems [START_REF] Chen | Regulatory T cells suppress tumor-specific CD8 T cell cytotoxicity through TGF-beta signals in vivo[END_REF][START_REF] Ghiringhelli | CD4+CD25+ regulatory T cells inhibit natural killer cell functions in a transforming growth factor-beta-dependent manner[END_REF][START_REF] Mcnally | CD4+CD25+ regulatory T cells control CD8+ T-cell effector differentiation by modulating IL-2 homeostasis[END_REF]. Furthermore, Treg cells supported directly mammary cancer metastasis by producing receptor activator of nuclear factor kappa-Β ligand (RANKL). RANKL stimulated pulmonary metastasis of RANKpositive human breast carcinoma cells in vivo [START_REF] Tan | Tumour-infiltrating regulatory T cells stimulate mammary cancer metastasis through RANKL-RANK signalling[END_REF]. A mechanism promoting metastatic development in the lung, involving the induction of Treg cells, and the concomitant limitation of CD4 + T helper cell-and CD8 + T cell-mediated responses has been recently proposed [START_REF] Clever | Oxygen Sensing by T Cells Establishes an Immunologically Tolerant Metastatic Niche[END_REF]. The oxygen-sensing prolyl-hydroxylase (PHD) proteins limit HIF-driven glycolytic programs within T cells, thus inhibiting spontaneous CD4 + T helper differentiation while promoting Treg cell commitment, and restraining local CD8 + T cell responses, thereby establishing the lung as an immunologically-permissive metastatic site [START_REF] Clever | Oxygen Sensing by T Cells Establishes an Immunologically Tolerant Metastatic Niche[END_REF].

Finally, it should be also noted that immune and non-immune (stroma, tumor-cell)-derived chemokines and cytokines, apart from orchestrating the infiltration and function of the immune cells, also modulate the colonization capabilities of tumor cells, as broadly discussed elsewhere [START_REF] Mantovani | Cancer-related inflammation[END_REF][START_REF] Yao | Cytokine Regulation of Metastasis and Tumorigenicity[END_REF].

Concluding Remarks

The realization that the concerted action and interaction among all immune and non-immune parameter is yet used in clinical practice, even if their inclusion in this context has been advocated to better stratify cancer patients. This is despite evidence that the histopathologic parameters of tumor cells, tumor progression and tumor invasion may be determined by the immune components of the TME, and that the lack of accuracy of the TNM staging-based prognosis among patients within the same tumor stage may be due to underlying differences in the nature and density of tumor-infiltrating adaptive immune cells [START_REF] Galon | Towards the introduction of the 'Immunoscore' in the classification of malignant tumours[END_REF][START_REF] Pages | International validation of the consensus Immunoscore for the classification of colon cancer: a prognostic and accuracy study[END_REF]. Other immune-based prognostic signatures, based on machine learning, improving the TNM staging have also been recently proposed [START_REF] Brieu | Augmenting TNM Staging with Machine Learning-based Immune Profiling for Improved Prognosis Prediction in Muscle-Invasive Bladder Cancer Patients[END_REF]. A revised, immune-including classification should take into account a multimodal combination of several parameters and biomarkers, possibly including, but not limited to: parameters of cytotoxic T-cell response and T cell exhaustion; tumor mutational burden; immune gene expression signatures; parameters of immune suppression (presence of immunosuppressive cells, such as Treg cells and MDSCs). Fundamentally, the proposed models to guide tailored treatment options, such as the "cancer immunogram" [START_REF] Blank | CANCER IMMUNOLOGY. The "cancer immunogram[END_REF], rely on the same valuable parameters and biomarkers to be adopted, in our opinion, to upgrade the current cancer classification to the era of cancer immunotherapy. An opening towards the introduction of immune parameters in clinical settings has been achieved following the evergrowing use of immunotherapy (Figure 5) with the adoption of immune-based response parameters, including irAEs (immune-related Adverse Effects) [START_REF] Kottschade | Incidence and Management of Immune-Related Adverse Events in Patients Undergoing Treatment with Immune Checkpoint Inhibitors[END_REF], irRC (immune-related Response Criteria) [START_REF] Wolchok | Guidelines for the evaluation of immune therapy activity in solid tumors: immune-related response criteria[END_REF], irRECIST (immune-related Response Evaluation Criteria In Solid Tumor) [START_REF] Seymour | iRECIST: guidelines for response criteria for use in trials testing immunotherapeutics[END_REF]. As the field progresses, it is likely that more powerful stratification systems will be implemented. Nonetheless, it would be reasonable to apply our current knowledge to better satisfy the immediate need for an improved prognostic and/or predictive information, and ultimately guide clinical decisions. What these past two decades have established is not only the rebirth of tumor immunology, and its rightful position as an academic entity, but more importantly as a gold mine from which to draw rational approaches to potentially defeat cancer. [START_REF] Balkwill | Inflammation and cancer: back to Virchow?[END_REF], the hypothesis that the immune system prevents neoplastic development [START_REF] Ehrlich | Über den jetzigen stand der karzinomforschung [the present state of carcinoma research[END_REF], the prognostic value of immune infiltration [START_REF] Maccarty | Relation of differentiation and lymphocytic infiltration to postoperative longevity in gastric carcinoma[END_REF], the concept of [START_REF] Aponte-Lopez | Mast Cell, the Neglected Member of the Tumor Microenvironment: Role in Breast Cancer[END_REF][START_REF] Delahaye | Alternatively spliced NKp30 isoforms affect the prognosis of gastrointestinal stromal tumors[END_REF][START_REF] Dong | NK-and B-cell infiltration correlates with worse outcome in metastatic ovarian carcinoma[END_REF][START_REF] Dundar | The significance and relationship between mast cells and tumour angiogenesis in non-small cell lung carcinoma[END_REF][START_REF] Dyduch | Mast cells and cancer: enemies or allies?[END_REF][START_REF] Galon | Approaches to treat immune hot, altered and cold tumours with combination immunotherapies[END_REF][START_REF] Gu-Trantien | CXCL13-producing TFH cells link immune suppression and adaptive memory in human breast cancer[END_REF][START_REF] Mussai | Interaction between invariant NKT cells and myeloid-derived suppressor cells in cancer patients: evidence and therapeutic opportunities[END_REF][START_REF] Pasero | Highly effective NK cells are associated with good prognosis in patients with metastatic prostate cancer[END_REF][START_REF] Powell | Neutrophils in the Tumor Microenvironment[END_REF][START_REF] Rajput | Stromal mast cells in invasive breast cancer are a marker of favourable prognosis: a study of 4,444 cases[END_REF][START_REF] Sautes-Fridman | Tertiary Lymphoid Structures in Cancers: Prognostic Value, Regulation, and Manipulation for Therapeutic Intervention[END_REF]Sun et al., 2018;[START_REF] Truxova | Mature dendritic cells correlate with favorable immune infiltrate and improved prognosis in ovarian carcinoma patients[END_REF][START_REF] Wouters | Prognostic Significance of Tumor-Infiltrating B Cells and Plasma Cells in Human Cancer[END_REF][START_REF] Xu | Prognostic value of tumor infiltrating NK cells and macrophages in stage II+III esophageal cancer patients[END_REF][START_REF] Yano | Mast cell infiltration around gastric cancer cells correlates with tumor angiogenesis and metastasis[END_REF].

  components in space and time ultimately determine cancer initiation, development and dissemination, bring along the need to reconsider the way cancers are classified. Conventional classification strategies are based on histopathological features of tumor cells, tumor morphology, tumor cell of origin, deregulation of tumor-cell-associated molecular pathways, mutational status and tumor gene expression signatures, as well as on cancer progression (T) and invasion of nearby tissues, lymph nodes (N) and distant organs (M). The currently used TNM tumor staging is indeed based on these cancer cell-focused characteristics. No immune

FiguresFigure 1 .

 1 FiguresFigure 1. History of tumor-immunology and cancer (immuno)therapy. The main milestones per time of tumor-Immunology are highlighted (top), including the first proposed link between inflammation and cancer by Rudolf Virchow[START_REF] Balkwill | Inflammation and cancer: back to Virchow?[END_REF]

Figure 2 .

 2 Figure 2. History of cancer: from pre-cancerous lesions, to primary tumors, to metastases. The main pathological stages characterizing the premalignant lesions are shown, followed by the development of the primary tumor. The main parameters regulating the progression of primary tumors along the classical, TNM-based tumor stages, are displayed: the immune contexture and adaptive immunity counteracting tumor growth, yet shaping tumor progression via the immunoediting. The ability of the tumor to evade the immune recognition, with the phenomena of immune tolerance, immune suppression and immune escape, favor tumor progression. Hence, these factors contribute to the early metastatic invasion, characterized by VELIPI (venous emboli and lymphatic and perineural invasion), as well as sustaining distant metastatic spreading.

Figure 3 .

 3 Figure 3. Prognostic impact of immune cells in solid cancer. An overview of the main innate and adaptive immune cell types and their prognostic impact in distinct solid cancers in humans, as determined by immunohistochemistry staining of tumors. Adapted from (Aponte-

Figure 4 .

 4 Figure 4. Immune contexture: good and bad immunity.A) The pre-existing natural intratumor immunity can classify cancer patients into four main categories based on Immunoscore: the optimal immunity (High-Immunoscore, hot, inflamed tumors), the altered immunity (Intermediate-Immunoscore, either with an immunosuppressed phenotype, or with an exclusion phenotype (high density of T-cells only at the tumor margin), and the absent immunity (Low-Immunoscore, cold, non-inflamed tumors.B)The immune microenvironment comes in different flavors including adaptive immunity, inflamed, immunosuppressed or with a tolerogenic phenotype. The immune contexture, defined as the type, the density, the immune functional orientation, and location of immune cells within distinct tumor regions shapes the proper pre-existing adaptive immunity, in contrast to immunosuppressed microenvironment. The main anti-cancer (left) and pro-cancer (right) immune cells and soluble factors are illustrated. To shape a proper immune contexture, immune priming, recruitment of T-cells and other immune cells, and T-cell expansion are necessary (right), whereas immune negative feedback-loop mechanism (checkpoint) could lead to immunosuppression, and intrinsic tumor mechanisms could lead to immune exclusion

Figure 5 .

 5 Figure 5. Cancer types responding to anti-PD1/L1 immunotherapy and to CAR-T cell therapy. Abbreviations: ORR, Overall Response Rate; FDA, Food and Drug Administration; EMA, European Medical Association.

Acknowledgments: For their support, we thank INSERM, the National Cancer Institute of France (INCa), the Canceropole Ile de France, La Ligue Contre le Cancer, La Fondation ARC, the Cancer research for personalized medicine (CARPEM), Paris Alliance of Cancer Research Institutes (PACRI), the LabEx Immuno-oncology.

immunosurveillance [START_REF] Burnet | Cancer; a biological approach. I. The processes of control[END_REF], the cloning of CTLA-4 and PD-1 [START_REF] Brunet | A new member of the immunoglobulin superfamily--CTLA-4[END_REF][START_REF] Ishida | Induced expression of PD-1, a novel member of the immunoglobulin gene superfamily, upon programmed cell death[END_REF]and their anti-tumor immunity role [START_REF] Dong | Tumor-associated B7-H1 promotes T-cell apoptosis: a potential mechanism of immune evasion[END_REF][START_REF] Leach | Enhancement of antitumor immunity by CTLA-4 blockade[END_REF], the first cloning of a tumor antigen [START_REF] Van Der Bruggen | A gene encoding an antigen recognized by cytolytic T lymphocytes on a human melanoma[END_REF], the demonstration of immunosurveillance [START_REF] Shankaran | IFNgamma and lymphocytes prevent primary tumour development and shape tumour immunogenicity[END_REF], equilibrium [START_REF] Koebel | Adaptive immunity maintains occult cancer in an equilibrium state[END_REF], and immunoediting [START_REF] Matsushita | Cancer exome analysis reveals a T-cell-dependent mechanism of cancer immunoediting[END_REF] in mouse models, the concept of immune contexture in human [START_REF] Galon | Type, density, and location of immune cells within human colorectal tumors predict clinical outcome[END_REF], the Immunoscore and demonstration that TNM-staging is dependent on T-cells [START_REF] Galon | Type, density, and location of immune cells within human colorectal tumors predict clinical outcome[END_REF], the concept of immunome and the immune landscape in human [START_REF] Bindea | Spatiotemporal dynamics of intratumoral immune cells reveal the immune landscape in human cancer[END_REF], the demonstration of immune-based metastasis dissemination in human (Mlecnik et al., 2016b), the demonstration of an immune-based theory of cancer evolution in human [START_REF] Angelova | Evolution of Metastases in Space and Time under Immune Selection[END_REF], and the worldwide consensus Immunoscore validation [START_REF] Pages | International validation of the consensus Immunoscore for the classification of colon cancer: a prognostic and accuracy study[END_REF]. The main milestones of cancer therapy and immunotherapy are highlighted (bottom), with the Coley"s bacterial toxin [START_REF] Coley | The Treatment of Malignant Tumors by Repeated Innoculations of Erysipelas: With a Report of Ten Original Cases[END_REF]; radiotherapy [START_REF] Holsti | Development of clinical radiotherapy since 1896[END_REF]; hormonal therapy [START_REF] Beatson | On the Treatment of Inoperable Cases of Carcinoma of the Mamma: Suggestions for a New Method of Treatment, with Illustrative Cases[END_REF]; chemotherapy [START_REF] Fenn | First Use of Intravenous Chemotherapy Cancer Treatment: Rectifying the Record[END_REF], IFN and IL-2 FDA approval in 1986 and 1992, respectively; the first targeted therapy (the tyrosine kinase inhibitor Imatinib) approved by the FDA against chronic myeloid leukemia (2001); first FDA-approved therapeutic vaccine against prostate cancer (2010), first FDA-approved checkpoint inhibitor, anti-CTLA-4, for melanoma (2011); FDA approved anti-PD-1 in lung cancer (2013); Immunotherapy defined as "Breakthrough of the Year" by Science (Couzin-Frankel, 2013); first FDA-approved bi-specific CD19-CD3 antibody (Blinatumomab) for the treatment of acute lymphoblastic leukemia (2014); anti-PD-1 approved by the FDA as first-line treatment in lung cancer (2015); FDA approval of an oncolytic virus (talimogene laherparepvec, T-VEC) for use in melanoma patients (2015); the MAGE-3 vaccine failure in a large Phase III study [START_REF] Vansteenkiste | Efficacy of the MAGE-A3 cancer immunotherapeutic as adjuvant therapy in patients with resected MAGE-A3-positive nonsmall-cell lung cancer (MAGRIT): a randomised, double-blind, placebo-controlled, phase 3 trial[END_REF]; the first FDAapproved CAR-T cell therapy (tisagenlecleucel) for , for pediatric and young adult patients with relapsed and/or refractory B-cell precursor acute lymphoblastic leukemia (2017); and the current (as to February 2019) 43 checkpoint cancer immunotherapies overall approved by the FDA.