Myriam Lawand

Julie Déchanet-Merville

Marie-Caroline Dieu-Nosjean
email: mc.dieu-nosjean@crc.jussieu.fr

Key Features of Gamma-Delta T-Cell Subsets in Human Diseases and Their immunotherapeutic implications

Keywords: human gamma-delta T cell, cytotoxicity, cytokine, nK receptor, antigen presentation, cancer, infection, autoimmunity

The unique features of gamma-delta (γδ) T cells, related to their antigen recognition capacity, their tissue tropism, and their cytotoxic function, make these cells ideal candidates that could be targeted to induce durable immunity in the context of different pathologies. In this review, we focus on the main characteristics of human γδ T-cell subsets in diseases and the key mechanisms that could be explored to target these cells.

inTRODUCTiOn Gamma-delta (γδ) T cells are an important subset of "unconventional" T lymphocytes as they have the ability to recognize a broad range of antigens without the presence of major histocompatibility complex (MHC) molecules. They can attack target cells directly through their cytotoxic activity or indirectly through the activation of other immune cells. γδ T-cell functional responses are induced upon the recognition of stress antigens, which promotes cytokine production and regulates pathogen clearance, inflammation, and tissue homeostasis in response to stress [START_REF] Bonneville | Gammadelta T cell effector functions: a blend of innate programming and acquired plasticity[END_REF].

However, given that different parameters concerning human γδ T-cell function and phenotype within tissues are still not well understood, it is important to be updated on the current state of knowledge of human γδ T cells. This will permit a better understanding of what could be performed in the future and how to target them in order to improve the management of patients.

For this aim, this review describes the main features of human γδ T-cell subsets in various pathologies and discusses the mechanisms by which they influence the outcome of the immune response that could be targeted for immunotherapy.

PLeiOTROPiC ROLe OF HUMAn γδ T CeLL SUBSeTS

In humans, there are two major subsets of γδ T cells identified by their Vδ chain. Vδ1 T cells are predominant in the thymus and peripheral tissues and recognize various stress-related antigens mostly uncharacterized. Vδ2 T cells constitute the majority of blood γδ T cells [START_REF] Vantourout | Six-of-the-best: unique contributions of γδ T cells to immunology[END_REF]. They always associate to the Vγ9 chain in adults and mainly recognize phosphoantigens, i.e., phosphorylated non-peptidic molecules that are metabolic intermediates of the isoprenoid biosynthesis [START_REF] Constant | Stimulation of human gamma delta T cells by nonpeptidic mycobacterial ligands[END_REF].

Major Cytokines and Cytotoxic Potential

Both human γδ T-cell subsets exhibit a cytotoxic potential that is induced through the expression of cell surface receptors [i.e., γδ TCR (T-cell receptor) and NKG2D (natural killer group 2D)] and is preponderantly mediated by the release of soluble mediators (i.e., perforin and granzymes) [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] Todaro | Efficient killing of human colon cancer stem cells by gammadelta T lymphocytes[END_REF]. They can produce granulysin, which is a potent anti-microbial protein [START_REF] Spada | Selfrecognition of CD1 by gamma/delta T cells: implications for innate immunity[END_REF][START_REF] Costa | Control of Plasmodium falciparum erythrocytic cycle: γδ T cells target the red blood cell-invasive merozoites[END_REF], and express ligands such as CD95L and Tumor necrosis factor-related apoptosis-inducing ligand, which engage several death receptors on target cells. In addition, they can kill their targets indirectly through antibody-dependent cellular cytotoxicity (ADCC) in a CD16-dependent mechanism [START_REF] Couzi | Antibody-dependent anti-cytomegalovirus activity of human γδ T cells expressing CD16 (FcγRIIIa)[END_REF]. Other molecules such as DNAM-1 (DNAX accessory molecule-1), leukocyte function-associated antigen-1, and the co-stimulatory receptor CD27 are also involved in γδ T-cell activation and cytotoxicity [START_REF] Silva-Santos | γδ T cells in cancer[END_REF]. Importantly, cord blood naïve γδ T cells can differentiate into the IL-17 + IFN-γ -Vγ9Vδ2 T cells with a cytotoxic potential in the presence of IL-23 and a TCR signaling [START_REF] Moens | IL-23R and TCR signaling drives the generation of neonatal Vgamma9Vdelta2 T cells expressing high levels of cytotoxic mediators and producing IFN-gamma and IL-17[END_REF]. In contrast, thymic naïve γδ T cells secrete IFN-γ in the presence of IL-2 or IL-15, through the de novo expression of T-bet and eomesodermin, and the release of cytotoxic molecules against leukemia cells [START_REF] Ribot | Human γδ thymocytes are functionally immature and differentiate into cytotoxic type 1 effector T cells upon IL-2/IL-15 signaling[END_REF]. Other studies reported IL-17 + γδ T-lymphocyte differentiation in the presence of IL-7 [START_REF] Michel | Interleukin 7 (IL-7) selectively promotes mouse and human IL-17-producing γδ cells[END_REF] or other activation stimuli [START_REF] Ness-Schwickerath | Cytokine requirements for the differentiation and expansion of IL-17A-and IL-22-producing human Vgamma2Vdelta2 T cells[END_REF] and high inflammatory conditions [START_REF] Caccamo | Differentiation, phenotype, and function of interleukin-17-producing human Vγ9Vδ2 T cells[END_REF][START_REF] Wu | γδT17 cells promote the accumulation and expansion of myeloid-derived suppressor cells in human colorectal cancer[END_REF].

Altogether, human γδ T cells represent key actors of the immunity because of their pro-inflammatory phenotype and cytotoxic potential.

Antigen Presentation Capacity

Human γδ T cells can exhibit an antigen-presenting capacity. Similar to dendritic cells (DCs), blood Vγ9Vδ2 T cells are able to respond to signals from microbes and tumors and prime CD4 + and CD8 + T cells [START_REF] Brandes | Professional antigen-presentation function by human gammadelta T cells[END_REF]. Indeed, γδ T-APCs were also described to cross-present antigens to CD8 + T cells [START_REF] Brandes | Crosspresenting human gammadelta T cells induce robust CD8+ alphabeta T cell responses[END_REF]. The intracellular protein degradation and endosomal acidification are significantly delayed in γδ T cells in comparison to monocyte-derived DCs [START_REF] Meuter | Prolonged antigen survival and cytosolic export in cross-presenting human gammadelta T cells[END_REF]. The antigens are transported across IRAP (Insulin-Regulated AminoPeptidase)-positive early and late endosomes [START_REF] Saveanu | IRAP identifies an endosomal compartment required for MHC class I cross-presentation[END_REF], and their processing consists of an export to the cytosol for degradation by the proteasome before being imported into a MHC-I-loading compartment [START_REF] Meuter | Prolonged antigen survival and cytosolic export in cross-presenting human gammadelta T cells[END_REF]. Moreover, activated γδ T cells are able to phagocytose tumor antigens and apoptotic or live cancer cells possibly through the scavenger receptor CD36 in a C/EBPα (CCAAT/enhancer-binding protein α)-dependent mechanism and mount a tumor antigen-specific CD8 + T-cell response [START_REF] Muto | Myeloid molecular characteristics of human γδ T cells support their acquisition of tumor antigen-presenting capacity[END_REF]. Moreover, γδ T cells can induce DC maturation through TNF-α production [START_REF] Leslie | CD1mediated gamma/delta T cell maturation of dendritic cells[END_REF][START_REF] Ismaili | Human gamma delta T cells induce dendritic cell maturation[END_REF].

Overall, γδ T cells can process a wide range of antigens for presentation and stimulate other immune cells. Therefore, their implication in response to infections or cancer would help to design new strategies in order to improve clinical response of human γδ T cell-based immunotherapy.

Key Receptors in immune Surveillance

Different receptors namely the TCR, co-stimulatory molecules, and NK receptors play a key role in the regulation of γδ T-cellmediated immune responses [reviewed in Ref. [START_REF] Ribeiro | Five layers of receptor signaling in γδ T-cell differentiation and activation[END_REF]]. For instance, the activation of blood Vγ9Vδ2 T cells by anti-NKG2D antibody or its ligand MICA (MHC class I chain-related sequence A) induces TNF-α production and the release of cytolytic granules [START_REF] Rincon-Orozco | Activation of V gamma 9V delta 2 T cells by NKG2D[END_REF]. Moreover, the triggering of NKG2D enhanced their response to microbe-associated antigens [START_REF] Das | MICA engagement by human Vgamma2Vdelta2 T cells enhances their antigen-dependent effector function[END_REF]. In lymphocytic leukemia, a hematologic tumor highly resistant to activated Vγ9Vδ2 T cells, IL-2 or IL-15, and TCR stimulation upregulates the expression of NK receptors NKp44, NKp46, and NKp30 on Vδ1 + T cells, allowing their acquisition of cytotoxicity against leukemia cells [START_REF] Correia | Differentiation of human peripheral blood Vδ1+ T cells expressing the natural cytotoxicity receptor NKp30 for recognition of lymphoid leukemia cells[END_REF]. DNAM-1 engagement can also promote the activation of Vδ2 T cells and ultimately, the killing of tumor cells [START_REF] Toutirais | DNAX accessory molecule-1 (CD226) promotes human hepatocellular carcinoma cell lysis by Vgamma9Vdelta2 T cells[END_REF][START_REF] Gertner-Dardenne | Human Vγ9Vδ2 T cells specifically recognize and kill acute myeloid leukemic blasts[END_REF]. Phosphoantigen stimulation of Vγ9Vδ2 T cells is able to induce TNF-α production through the upregulation of CD16 expression [START_REF] Lafont | Production of TNF-alpha by human V gamma 9V delta 2 T cells via engagement of fc gamma RIIIA, the low affinity type 3 receptor for the Fc portion of IgG, expressed upon TCR activation by nonpeptidic antigen[END_REF]. Its role in mediating ADCC was highlighted using therapeutic antibodies such as anti-CD20 (Rituximab) [START_REF] Braza | γδ T-cell killing of primary follicular lymphoma cells is dramatically potentiated by GA101, a type II glycoengineered anti-CD20 monoclonal antibody[END_REF] and anti-HER2 antibody (Trastuzumab) [START_REF] Capietto | Stimulated γδ T cells increase the in vivo efficacy of trastuzumab in HER-2+ breast cancer[END_REF].

The CD27-CD70 axis can enhance phosphoantigendependent activation, survival, proliferation, and secretion of pro-inflammatory cytokines of Vγ9Vδ2 T cells [START_REF] Debarros | CD70-CD27 interactions provide survival and proliferative signals that regulate T cell receptor-driven activation of human γδ peripheral blood lymphocytes[END_REF]. These results suggest that CD27 can modulate Vδ2 T-cell activation and hence seems to be a major tool that could be manipulated in clinical settings. Of note, CD27 is expressed on Vδ1 + cells and, thus, may also play a role in their effector functions [START_REF] Debarros | CD70-CD27 interactions provide survival and proliferative signals that regulate T cell receptor-driven activation of human γδ peripheral blood lymphocytes[END_REF].

The promotion of a robust NK cell-mediated antitumor cytotoxicity has also been described through CD137 (4-1BB) engagement on blood activated γδ T lymphocytes which in turn induces the upregulation of NKG2D by NK cells, followed by the eradication of tumor cells [START_REF] Maniar | Human gammadelta T lymphocytes induce robust NK cell-mediated antitumor cytotoxicity through CD137 engagement[END_REF].

In contrast, regulatory receptors for self-MHC class I molecules, particularly KIR (Killer cell Immunoglobulin-like Receptor) and LIR (Leukocyte Immunoglobulin-like Receptor) receptors, were reported to negatively regulate γδ T-cell activation [START_REF] Dolstra | TCR gamma delta cytotoxic T lymphocytes expressing the killer cell-inhibitory receptor p58.2 (CD158b) selectively lyse acute myeloid leukemia cells[END_REF][START_REF] Lesport | Inhibition of human gamma delta [corrected] T-cell antitumoral activity through HLA-G: implications for immunotherapy of cancer[END_REF]. This inhibition is due to the presence of intracytoplasmic ITIM (Immunoreceptor Tyrosine-based Inhibitory Motif) motif in the sequence of these receptors which turn off the activation signals upon phosphorylation. The ligation of BTLA (B-and T-Lymphocyte Attenuator), another regulatory receptor strongly expressed by resting Vγ9Vδ2 T cells, attenuates their own proliferation [START_REF] Gertner-Dardenne | The co-receptor BTLA negatively regulates human Vγ9Vδ2 T-cell proliferation: a potential way of immune escape for lymphoma cells[END_REF]. The engagement of PD-1 (programmed cell death-1) expressed on activated γδ T cells downregulates IFN-γ production and their cytotoxic function [START_REF] Iwasaki | Expression and function of PD-1 in human γδ T cells that recognize phosphoantigens[END_REF].

Understanding the role of these mechanisms in γδ T cellimplication in pathological situations needs further investigations that would be important to develop proper strategies targeting these activation and inhibitory receptors. This would ensure an efficient activation of human γδ T cells in immune surveillance against tumors, pathogens, or autoimmunity and ultimately avoid undesired cytotoxicity against the host through a better discrimination between normal and altered tissues.

HUMAn γδ T-CeLL SUBSeTS in CAnCeR, inFeCTiOUS DiSeASeS, AnD AUTOiMMUniTY

Pro-and Antitumor effect of γδ T-Cell Subsets

Tumor-infiltrating γδ T cells have been demonstrated to be the most favorable prognostic immune population among many cancer types [START_REF] Gentles | The prognostic landscape of genes and infiltrating immune cells across human cancers[END_REF], in agreement with their capacity to kill different tumor cells like leukemia, neuroblastoma, and carcinomas [START_REF] Silva-Santos | γδ T cells in cancer[END_REF].

Vδ1 and Vγ9Vδ2 T cells express distinct chemokine receptors [START_REF] Kabelitz | Features and functions of gamma delta T lymphocytes: focus on chemokines and their receptors[END_REF][START_REF] Lança | Protective role of the inflammatory CCR2/CCL2 chemokine pathway through recruitment of type 1 cytotoxic γδ T lymphocytes to tumor beds[END_REF] and cell adhesion molecules (41) (Table 1), suggesting different homing mechanisms in tumors that can be selectively targeted for immunotherapy. Moreover, isolated Vδ1 lymphocytes from human lung tumors can selectively lyse autologous malignant cells ex vivo [START_REF] Zocchi | Selective lysis of the autologous tumor by delta TCS1+ gamma/delta+ tumor-infiltrating lymphocytes from human lung carcinomas[END_REF]. Interestingly, administration of Vγ9Vδ2 T cells at suitable intervals after chemotherapy and zoledronate treatment increase the cytotoxic function and IFN-γ production by γδ T cells followed by a complete lysis of tumor cells in different malignancies [START_REF] Mattarollo | Chemotherapy and zoledronate sensitize solid tumour cells to Vgamma9Vdelta2 T cell cytotoxicity[END_REF]. Different studies also reported the activation of γδ T cells after phosphoantigen or aminobiphosphonate injection, an approach which provides promising clinical activity by improving the cytotoxicity of γδ T cells particularly in presence of tumor-targeting antibody [START_REF] Fisher | γδ T cells for cancer immunotherapy: a systematic review of clinical trials[END_REF]. In melanoma, tumor-infiltrating effector-memory γδ T cells have been shown to control tumor growth through distinct cytotoxic mechanisms (45) (Table 1).

Furthermore, Vδ1 T cells were less susceptible to activation-induced cell death and could persist in the circulation for many years, which is in favor of a durable antitumor immunity (46) (Table 1). Lamb et al. reported an association between the increased frequency of γδ T cells and improved disease-free survival of leukemia patients who received αβ T cell-depleted bone marrow transplants [START_REF] Lamb | Increased frequency of TCR gamma delta + T cells in disease-free survivors following T cell-depleted, partially mismatched, related donor bone marrow transplantation for leukemia[END_REF]. An increased percentage of Vδ2 -T cells following cytomegalovirus (CMV) infection in kidney transplant recipients was also associated with reduced cancer risk (i.e., lymphoma) [START_REF] Couzi | Cytomegalovirus-induced gammadelta T cells associate with reduced cancer risk after kidney transplantation[END_REF], suggesting a protective role of γδ T cells. In the context of chronic myelogenous leukemia (CML), the activation of Vγ9Vδ2 T cells by zoledronate was shown to increase anti-leukemia activities in a NKG2D-dependent manner, notably in CMLresistant patients [START_REF] D' Asaro | V gamma 9V delta 2 T lymphocytes efficiently recognize and kill zoledronate-sensitized, imatinib-sensitive, and imatinib-resistant chronic myelogenous leukemia cells[END_REF]. Indeed, UL16-binding protein 1 (ULBP1) is involved in lymphoma susceptibility to γδ T-cell-mediated cytolysis and the blockade of its receptor NKG2D significantly inhibits lymphoma cell killing. Thus, the authors propose that γδ T cytotoxic function is achieved through a two-step process (a TCR stimulation presumably by endogenous phophoantigens and a tumor cell recognition by NKG2D) and that ULBP1 could be used as a leukemia/lymphoma biomarker in clinical trials [START_REF] Lança | The MHC class Ib protein ULBP1 is a nonredundant determinant of leukemia/lymphoma susceptibility to gammadelta T-cell cytotoxicity[END_REF]. Moreover, since Vδ1 + T cells were shown to express several NK receptors that are correlated with a high cytotoxic potential [START_REF] Correia | Differentiation of human peripheral blood Vδ1+ T cells expressing the natural cytotoxicity receptor NKp30 for recognition of lymphoid leukemia cells[END_REF], they may constitute a potent therapeutic lymphocyte population that could be targeted for the immunotherapy of lymphocytic leukemia patients that are resistant to activated Vγ9Vδ2 T cells.

Nonetheless, a pro-tumor role of Vδ1 T cells has also been reported. Indeed, in colorectal cancer (CRC), these cells are responsible of the chronic inflammation in CRC patients through IL-17 secretion [START_REF] Wu | γδT17 cells promote the accumulation and expansion of myeloid-derived suppressor cells in human colorectal cancer[END_REF]. In addition, some Vδ1 populations could exhibit immunosuppressive properties (i.e., inhibition of DC maturation, suppression of T-cell proliferation and IL-2 secretion), a function that can also be exploited for cancer therapy (47) (Table 1). A high γδ T-cell infiltrate in breast tumors was positively correlated with advanced tumor stages, HER2 expression status, and lymph node metastasis and ultimately associated with poor outcome of the patients [START_REF] Ma | Tumor-infiltrating γδ T lymphocytes predict clinical outcome in human breast cancer[END_REF]. In this study, γδ T cells were considered as the most significant independent prognostic factor among many parameters including clinical grade and, thus, may serve as a valuable biomarker and potential therapeutic target for breast cancer.

Altogether, the natural contribution in tumor immunosurveillance and the effector functions of Vγ9Vδ2 T cells represent major advantages that have to be better exploited alone or in combination with current therapies (i.e., phosphoantigens + monoclonal antibodies). However, repeated injection of phosphoantigens may also lead to the anergy or exhaustion of effector γδ T cells. From an immunotherapeutic standpoint, the rather limited antitumor efficacy of adoptively transferred Vδ2 T cells and active immunotherapy trials using Vδ2 agonists can be the result of a suboptimal recognition of ex vivo tumor cells, presumably due to insufficient phosphoantigen levels. Therefore, it is now critical to better characterize human γδ T-cell subsets and the engaged mechanisms in individual cancers, especially the stage of differentiation, the activation status, and the immune checkpoint (ICP)/ICP-ligand expression, to irreversibly convert them toward an antitumor function for efficient immunotherapy.

γδ T-Cell Subsets in infections

γδ T-cell subsets have been described as potent effector populations against pathogens (Table 2). Indeed, Vγ9Vδ2 T cells can recognize phosphoantigens that are overexpressed in the methyl-erythritol phosphate biochemical pathway (MEP, also called DOXP (1-desoxy-d-xylulose-5-phosphate) pathway), a pathway which is used by many bacteria, fungi and parasites, such as Plasmodium falciparum, Mycobacterium tuberculosis, Toxoplasma gondii, and Listeria monocytogenes [reviewed in Ref. [START_REF] Champagne | γδ T cell receptor ligands and modes of antigen recognition[END_REF][START_REF] Tyler | Human Vγ9/Vδ2 T cells: innate adaptors of the immune system[END_REF]]. Vγ9Vδ2 T cells are thus activated and able to expand in the blood of infected individuals.

During mycobacterial infection, human Vγ9Vδ2 T cells are potently able to inhibit intracellular mycobacteria growth through the secretion of granzyme A and TNF-α [START_REF] Spencer | Granzyme A produced by γ(9)δ(2) T cells induces human macrophages to inhibit growth of an intracellular pathogen[END_REF]. In children with bacterial meningitis, an increased percentage of IL-17 + Vγ9Vδ2 lymphocytes was detected in the peripheral blood and infected tissues but could be reversed following anti-bacterial therapy [START_REF] Caccamo | Differentiation, phenotype, and function of interleukin-17-producing human Vγ9Vδ2 T cells[END_REF].

Among Vδ2 -γδ T cells, different clones were demonstrated to be reactive against CMV-infected cells [START_REF] Halary | Shared reactivity of V{delta}2(neg) {gamma}{delta} T cells against cytomegalovirus-infected cells and tumor intestinal epithelial cells[END_REF], in agreement with their expansion in the blood of CMV-infected patients (57, 68-70) (Table 2). More recently, the Vγ4Vδ5 γδ T-cell clone isolated from a CMV-infected transplant patient was shown to be reactive against CMV-infected endothelial cells as well as epithelial tumors after binding to the endothelial protein C receptor [START_REF] Willcox | Cytomegalovirus and tumor stress surveillance by binding of a human γδ T cell antigen receptor to endothelial protein C receptor[END_REF]. In addition, CD16 expression by γδ T cells induces IFN-γ responses by opsonized CMV virions in a TCR-independent manner and, thus, contributes to the surveillance of CMV reactivation in transplant recipients [START_REF] Couzi | Antibody-dependent anti-cytomegalovirus activity of human γδ T cells expressing CD16 (FcγRIIIa)[END_REF]. In a case of severe combined immunodeficiency patient, specific antibody responses to some infectious agents were reported with a predominance of Vδ2 -γδ T-cell clones reactive against CMV-infected cells, suggesting functional potentials of γδ T cells in providing B cell help [START_REF] Ehl | A variant of SCID with specific immune responses and predominance of gamma delta T cells[END_REF]. Therefore, γδ T cells do not only serve as sentinels in the innate system but can also act as a bridge between innate and adaptive immune responses.

Regarding the Vδ1 T-cell subset, their expansion and activation were also observed in many patients following viral infection such as HCV (hepatitis C virus) [START_REF] Agrati | Vdelta1 T lymphocytes expressing a Th1 phenotype are the major gammadelta T cell subset infiltrating the liver of HCV-infected persons[END_REF][START_REF] Agrati | Gammadelta T cell activation by chronic HIV infection may contribute to intrahepatic vdelta1 compartmentalization and hepatitis C virus disease progression independent of highly active antiretroviral therapy[END_REF] and HIV [START_REF] De Paoli | A subset of gamma delta lymphocytes is increased during HIV-1 infection[END_REF][START_REF] Autran | T cell receptor gamma/delta+ lymphocyte subsets during HIV infection[END_REF]. The specific depletion of Vδ2 T cells is the consequence of the virus infection through gp120-CCR5 interaction [START_REF] Li | HIV envelope-mediated, CCR5/α4β7-dependent killing of CD4-negative γδ T cells which are lost during progression to AIDS[END_REF][START_REF] Glatzel | Patterns of chemokine receptor expression on peripheral blood gamma delta T lymphocytes: strong expression of CCR5 is a selective feature of V delta 2/V gamma 9 gamma delta T cells[END_REF]. Nevertheless, uninfected Vγ9Vδ2 cells are able to produce large amounts of IFN-γ, TNF-α, and chemokines such as CCL4 and CCL5 [reviewed in Ref. [START_REF] Chen | Vγ2Vδ2+ T cells and anti-microbial immune responses[END_REF]], which bind to CCR4 and CCR5 (the HIV co-receptors), respectively [START_REF] Boismenu | Chemokine expression by intraepithelial gamma delta T cells. Implications for the recruitment of inflammatory cells to damaged epithelia[END_REF][START_REF] Qin | Type 1 responses of human Vγ9Vδ2 T cells to influenza A viruses[END_REF]. Interestingly, in vitro stimulation of Vγ9Vδ2 cells with phosphoantigens induces the release of these chemokines, saturates the co-receptors of HIV, and thus prevents HIV entry and interfere with its replication [START_REF] Poccia | Phosphoantigen-reactive Vgamma9Vdelta2 T lymphocytes suppress in vitro human immunodeficiency virus type 1 replication by cell-released antiviral factors including CC chemokines[END_REF]. It also permits the recruitment of more Vγ9Vδ2 cells able to release additional chemokines that would block HIV entry and kill infected cells through direct cytotoxicity or ADCC [START_REF] Poonia | Gamma delta T cells from HIV+ donors can be expanded in vitro by zoledronate/interleukin-2 to become cytotoxic effectors for antibody-dependent cellular cytotoxicity[END_REF].

Freshly isolated Vδ1 T cells from HIV-infected patients were shown to express CD27, CCR7, and CD161, a molecule involved in γδ T-cell transendothelial migration and interestingly to proliferate and produce IFN-γ and IL-17 in response to Candida albicans ex vivo (59) (Table 2). These observations suggest that Vδ1 lymphocytes might play a major role in the control of HIV infection and in the defense against opportunistic

Antibody response and reactivity against CMVinfected cells (in a case of SCID patient)

No reactivity reported Reactivity against CMV-infected cells like in the case of Vδ1 T cells

Opsonization of CMV virions through CD16 (FcγRIIIa) expression and induction of IFN-γ responses

- CD16 (FcγRIIIa) expression like Vδ1 T cells (8)
HIV infection Increased levels of Vδ1 T cells --

Co-expression of IFN-γ and IL-17 (defense against opportunistic infections), CD27 (memory phenotype), CCR7 (homing), and CD161 (transendothelial migration)

- - (59)
Cytotoxicity against HIV-infected CD4 + T cells through NKG2C triggering

- - (60)
Production of CCL3, CCL4, and CCL5 and suppression of HIV replication through NKp30 engagement

- - (61)
-Large production of IFN-γ, TNF-α, and the chemokines CCL4/CCL5: blockade of HIV co-receptors, attraction of more Vδ2 + T cells able to release additional chemokine blocking HIV entry and kill infected cells through direct cytotoxicity or ADCC -

-Signaling through the CCR5-gp120 interaction: depletion of Vδ2 infections. NKG2C was outlined as the major triggering receptor involved in the T-cell-mediated cytotoxicity against HIV-infected CD4 + T cells [START_REF] Fausther-Bovendo | NKG2C is a major triggering receptor involved in the V[delta]1 T cell-mediated cytotoxicity against HIV-infected CD4 T cells[END_REF]. The engagement of NKp30 on Vδ1 T cells induces CCL3, CCL4, and CCL5 production and suppress HIV replication [START_REF] Hudspeth | Engagement of NKp30 on Vδ1 T cells induces the production of CCL3, CCL4, and CCL5 and suppresses HIV-1 replication[END_REF]. Altogether, these results are consistent with an antiviral potential of the Vδ1 T cells, possibly compensating the impairment of the CD4 + T-cell function during HIV infection.

Thus, the identification of stress-induced self antigens as targets expressed by infected cells may lead to the development of new therapeutic tools in infectious diseases. However, γδ T-cell-based therapy may give rise to an uncontrolled inflammation with unwanted tissue destruction. Therefore, exploiting the underlying mechanisms of γδ T-cell functions in infections will permit the modulation of their immunopathological consequences and could be beneficial to target pathogen proliferation. γδ T cells were also reported to play a key role in various chronic inflammatory pathologies (Table 3). For instance, in a rare variant of myositis, referred to as γδ T-cell-mediated myositis, muscle fibers are attacked by CD8-negative T cells expressing the γδ-TCR. A Vγ1.3Vδ2 TCR clone (M88) was able to recognize the aminoacyl-histidyl-tRNA synthetase, an antigen also targeted by autoantibodies [START_REF] Bruder | Target specificity of an autoreactive pathogenic human γδ-T cell receptor in myositis[END_REF], suggesting a link between γδ T cells and antibody-dependent response in autoimmune myositis. In rheumatoid arthritis, stimulation of effector-memory Vγ9Vδ2 T cell with isopentenyl pyrophosphate induces cell surface HLA-DR and co-stimulatory molecule expression and IFN-γ and IL-17 secretion. This consequently activated antigen presentation to CD4 + T cells, sustained T cell activation, and aggravated the disease [START_REF] Hu | Antigen-presenting effects of effector memory Vγ9Vδ2 T cells in rheumatoid arthritis[END_REF].

A subset of CD27 + CD25 high Vδ1 T cells, expressing FoxP3 similarly to regulatory CD4 + T cells, was reported in patients with systemic lupus erythematosus (SLE). Their immunoregulatory activity is mainly cell-to-cell contact dependent. Unfortunately, this population is decreased in the blood of SLE patients, which could be important for the understanding of the pathogenesis of this disease [START_REF] Li | Generation of human regulatory gammadelta T cells by TCRgammadelta stimulation in the presence of TGF-beta and their involvement in the pathogenesis of systemic lupus erythematosus[END_REF].

In psoriasis patients, a homing/redistribution of Vγ9Vδ2 T cells from blood to skin was observed. These cells produced an array of pro-inflammatory cytokines, such as IFN-γ, TNF-α, and IL-17A, which induced the recruitment of blood immune cells, the release of growth factors, and the tissue remodeling. A psoriasis-targeted therapy could normalize the decreased numbers of circulating Vγ9Vδ2 T cells indicating that they represent an important biomarker [START_REF] Laggner | Identification of a novel proinflammatory human skin-homing Vγ9Vδ2 T cell subset with a potential role in psoriasis[END_REF]. Of note, skin-γδ T cells were reported to play an important role in wound healing [START_REF] Toulon | A role for human skin-resident T cells in wound healing[END_REF].

Given that γδ T cells could influence the pathogenesis of chronic inflammatory diseases, these cells will require a balance between their need for inflammatory mediators to function normally and a negative regulation of their unfavorable impact of the chronic inflammation [START_REF] Fay | Chronic inflammation and γδ T cells[END_REF]. Therefore, additional studies should be carried out on human γδ T-cell subsets to develop personalized γδ T-cell-based therapies.

COnCLUSiOn AnD PeRSPeCTiveS

These studies give us more insights about the relative contribution of Vδ1 and Vδ2 T cells, as well as the mechanisms that could be targeted in order to develop efficient γδ T-cell-based therapies. However, more extensive investigation is required to better evaluate the impact of these cells, i.e., their effector function, the subset plasticity and cell lineage. Furthermore, a deeper characterization of γδ T-cell subsets in human is required given that most studies are conducted in mice. This will ultimately permit the manipulation of a specific subset according to a particular microenvironment. It is also important to understand how the novel therapies (e.g., ICP blockade) modulate γδ T-cell function. Finally, one may speculate that γδ T-cell subset-targeting could be beneficial to enhance or inhibit the outcome of adaptive immune responses against cancer, pathogens, or autoantigens, given their involvement in the regulation of other immune cell function.

Frontiers

  in Immunology | www.frontiersin.org June 2017 | Volume 8 | Article 761

TABLe 1 |

 1 Main features of human γδ T-cell subsets in cancers.

	Pathology		Human γδ T-cell subset features		Reference
		vδ1 +	vδ2 +	Other vδ2 -(non-vδ1)
	Solid cancers	Lysis of the autologous tumor when	-	-	(42)
		extracted from TILs and expanded in vitro in
		the presence of IL-2
		-	Cytotoxic function, IFN-γ production, and almost	-	(43)
			a complete lysis of tumor targets in different
			malignancies (after chemotherapy and zoledronate
			treatment)
			Expansion and improved cytotoxicity in the		Reviewed in
			presence of phosphoantigens, amino-		Ref. (44)
			biphosphonates, or a tumor-targeting antibody in
			cancer immunotherapy
		CCR5 expression for migration to tumor	CCR5 and CXCR3 expression (consistent with a	-	(39)
		sites	Th1-like phenotype)
		CCR2 expression for migration to tumor	No CCR2 expression detected	-	(40)
		sites (antitumor effect: production of IFN-γ
		and cytotoxic function)
		Expression of various adhesion molecules:	Only LFA-1, L-selectin, and CD44v6 expression	-	(41)
		LFA-1, VLA-α4 (CD49d), VLA-α5 (CD49e),
		L-selectin (CD62-L), and αEβ7(CD103)
		Ability to kill tumor cells of all melanoma-	Significant cytotoxic activity for only two out of eight	-	(45)
		isolated Vδ1 cell lines	Vδ2 cell lines
		Lower susceptibility to activation-induced	-	-	Reviewed in
		cell death, persistence in the circulation for			Ref. (46)
		many years (durable immunity)
		Major cellular source of IL-17 (pro-tumor	-	-	(15)
		role: chronic inflammation in CRC patients)
		Immunosuppressive and regulatory	-	-	(47)
		properties, such as suppression of dendritic
		cell maturation, T-cell proliferation, and IL-2
		secretion
	Hematological	-	Activation of Vγ9Vδ2 T cells by zoledronate:	-	(48)
	malignancies		cytotoxicity largely dependent on granule exocytosis
			and partly on TRAIL pathways, TCR-mediated, and
			dependent on isoprenoid production by leukemia
			cells
		-	Leukemia/lymphoma cell killing by γδ T cells	-	(49)
			essentially mediated by ULBP1/NKG2D interaction
		Cytotoxicity against lymphoid leukemia cells	-	-	(26)
		associated with the expression of several NK
		receptors (mainly NKp30)
		-	-	Increased percentage of Vδ2 -	(50)
				T cells following CMV infection
				in kidney transplant recipients
				associated with reduced cancer
				risk (among which lymphoma)
	CMV, cytomegalovirus; CRC, colorectal cancer; LFA-1, leukocyte function-associated antigen-1; NKG2D, natural killer group 2D; TIL, tumor-infiltrating lymphocyte; TRAIL, tumor
	necrosis factor-related apoptosis-inducing ligand; ULBP1, UL16-binding protein 1; VLA-α4, very late antigen-4.

3 Lawand et al. Human γδ T-Cell Subsets in Diseases Frontiers in Immunology | www.frontiersin.org June 2017 | Volume 8 | Article 761

TABLe 2 |

 2 Main features of human γδ T-cell subsets in infectious diseases.

	Pathology		Human γδ T-cell subset features		Reference
		vδ1 +	vδ2 +	Other vδ2 -(non-vδ1)
	CMV infection	-	-	Reactivity of different Vδ2 -γδ	(55)
				T cell clones against CMV-infected
				cells
		-	-	Recognition of CMV-targeted
				endothelial cells and epithelial
				tumors (Vγ4Vδ5 clone interaction
				with EPCR)

TABLe 3 |

 3 Main features of human γδ T-cell subsets in chronic inflammatory manifestations.

	Pathology		Human γδ T-cell subset features		Reference
		vδ1 +	vδ2 +	Other
				vδ2 -(non-vδ1)
	Myositis	-	Recognition of AA-RS (also targeted by anti-Jo-1 autoantibodies) by a	-	(77)
			specific clone of Vγ1.3Vδ2 -TCR (link between γδ T and B cells)
	Psoriasis	-	Biomarkers for psoriasis, homing to the skin, production of pro-	-	(78)
			inflammatory cytokines (IFN-γ, TNF-α, and IL-17A), induction of
			immune cell recruitment from the circulation, release of growth factors
			and tissue remodeling
	Rheumatoid arthritis	-	Antigen presentation capacity of effector-memory Vγ9Vδ2 + T cell	-	(79)
			stimulated with IPP to CD4 + T cells, secretion of pro-inflammatory
			cytokines (IFN-γ and IL-17), disease progression
	Systemic lupus	Decrease of CD27 + CD25 high FoxP3 +	-	-	(80)
	erythematosus	immunoregulatory Vδ1 + T cells
	AA-RS, aminoacyl-histidyl-tRNA synthetase; IPP, isopentenyl pyrophosphate.

6 Lawand et al. Human γδ T-Cell Subsets in Diseases Frontiers in Immunology | www.frontiersin.org June 2017 | Volume 8 | Article 761 γδ T-Cell Subsets in Autoimmune and Other Chronic inflammatory Diseases

Frontiers in Immunology | www.frontiersin.org June 2017 | Volume 8 | Article 761

ACKnOwLeDGMenTS

We thank all members of the Laboratory "Cancer, Immune control and Escape" at the UMRS1138 INSERM, Cordeliers Research Center, Paris, France.

FUnDinG

This work was supported by Institut National de la Santé et de la Recherche Médicale (INSERM), University Paris Descartes (Sorbonne Paris Cité), University Pierre et Marie Curie (Sorbonne University), the Labex Immuno-Oncology (LAXE62_9UMS872), and the Institut National du Cancer (INCa-PLBio 2014-2017 GDSTRESS).

AUTHOR COnTRiBUTiOnS

ML wrote/revised the paper. JD-M and M-CD-N revised the paper.

Conflict of Interest Statement:

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.