Mingyu Han

Marie Woottum

Rémi Mascarau

Zoï Vahlas

Christel Verollet

Serge Benichou
email: serge.benichou@inserm.fr

Mechanisms of HIV-1 cell-to-cell transfer to myeloid cells

Keywords: iDC, immature dendritic cell, OC, osteoclast, T/F, transmitted/founder, SAMHD1, SAM APOBEC3, apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3, TRIM, TRIpartite Motif protein, IFITM, InterFeron-Induced TransMembrane protein, CNS, central nervous system, MGCs, multinucleated giant cells, TNTs, tunneling nanotubes, Mtb, Mycobaterium tuberculosis, Vpu, viral protein U

In addition to CD4+ T lymphocytes, cells of the myeloid lineage such as macrophages, dendritic cells (DCs), and osteoclasts (OCs) are emerging as important target cells for HIV-1, as they likely participate in all steps of pathogenesis, including sexual transmission and early virus dissemination in both lymphoid and nonlymphoid tissues where they can constitute persistent virus reservoirs. At least in vitro, these myeloid cells are poorly infected by cell-free viral particles. In contrast, intercellular virus transmission through direct cell-to-cell contacts may be a predominant mode of virus propagation in vivo leading to productive infection of these myeloid target cells. HIV-1 cell-to-cell transfer between CD4+ T cells mainly through the formation of the virologic synapse, or from infected macrophages or dendritic cells to CD4+ T cell targets, have been extensively described in vitro. Recent reports demonstrate that myeloid cells can be also productively infected through virus homotypic or heterotypic cell-to-cell transfer between macrophages or from virus-donor-infected CD4+ T cells, respectively. These modes of infection of myeloid target cells lead to very efficient spreading in these poorly susceptible cell types. Thus, the goal of this review is to give an overview of the different mechanisms reported in the literature for cell-to-cell transfer and spreading of HIV-1 in myeloid cells.

K E Y W O R D S

cell-cell fusion, cell-to-cell transfer, HIV-1, myeloid cells

INTRODUCTION

 Macrophages then participate, in addition to latently infected CD4+ T cells, to the for-Abbreviations: DC, dendritic cell

human T-cell lines. In contrast, primary target cells, and especially noncycling myeloid cell targets such as macrophages, are poorly infected, even in vitro, by cell-free viruses. The viral envelope also determines the cellular tropism of the HIV-1 strains that can be classified in 3 types: CXCR4-using T cell-tropic, CCR5-using T cell-tropic (also called nonmacrophage-tropic), and macrophage-tropic strains using either CCR5 or CXCR4, or both, as coreceptors. [START_REF] Joseph | The evolution of HIV-1 entry phenotypes as a guide to changing target cells[END_REF] Although CCR5-using viruses, including sexually transmitted/founder (T/F) viruses, [START_REF] Joseph | Bottlenecks in HIV-1 transmission: insights from the study of founder viruses[END_REF][START_REF] Keele | Identification and characterization of transmitted and early founder virus envelopes in primary HIV-1 infection[END_REF] predominate during early, acute, and asymptomatic phases of infection, CXCR4using variants can emerge at later stage of infection and are mostly associated with advanced disease progression. [START_REF] Chiappetta | HIV-1 tropism test evaluation: assessment and clinical implications[END_REF][START_REF] Lin | Humoral immune pressure selects for HIV-1 CXC-chemokine receptor 4-using variants[END_REF][START_REF] Shepherd | Emergence and persistence of CXCR4-Tropic HIV-1 in a population of men from the multicenter AIDS cohort study[END_REF] The first block to efficient macrophage infection is related to virus entry and specific cellular tropism of the different HIV-1 variants found in infected individuals and characterized as non-M-tropic using cellfree in vitro infection assays. Poor virus replication in these myeloid cells is also largely related to the high expression and/or efficiency of host cell restriction factors that inhibit postentry replication steps of the viral life cycle, such as SAMHD1 (SAM domain and HD domaincontaining protein 1), an enzyme that inhibits the reverse transcription process through depletion of the pool of intracellular nucleotides necessary for efficient synthesis of the viral DNA, but also members of the APOBEC3 (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3), TRIM (TRIpartite Motif protein), and IFITM (InterFeron-Induced TransMembrane protein) families. [START_REF] Colomer-Lluch | Restriction factors: from intrinsic viral restriction to shaping cellular immunity against HIV-1[END_REF] In addition to cell-free virus infection, there is substantial evidence indicating that cell-to-cell transfer of HIV-1 through direct contacts between infected virus-donor cells and recipient targets cells likely represents the dominant mode of virus dissemination in vivo and may allow for productive infection of both lymphoid and myeloid target cells. At least in vitro, this cell-to-cell mode of virus transfer is very efficient for virus dissemination in target cells, [START_REF] Bracq | Mechanisms for cell-to-cell transmission of HIV-1[END_REF] such as myeloid cells, [START_REF] Dupont | Macrophage cell-cell interactions promoting HIV-1 infection[END_REF] poorly susceptible to cell-free virus infection but detected in vivo in many tissues, mainly in the central nervous system (CNS), and also in other tissues, including lymph nodes, spleen, lungs, and digestive and genital tracts. [START_REF] Costiniuk | The lungs as anatomical reservoirs of HIV infection[END_REF][START_REF] Ganor | HIV-1 reservoirs in urethral macrophages of patients under suppressive antiretroviral therapy[END_REF][START_REF] Tan | The HIV-1-containing macrophage compartment: a perfect cellular niche?[END_REF][START_REF] Watters | Macrophages: the neglected barrier to eradication[END_REF] Remarkably, these infected myeloid cells are very often found as multinucleated giant cells (MGCs) in tissues. [START_REF] Raynaud-Messina | Bone degradation machinery of osteoclasts: an HIV-1 target that contributes to bone loss[END_REF][START_REF] Costiniuk | The lungs as anatomical reservoirs of HIV infection[END_REF][START_REF] Tan | The HIV-1-containing macrophage compartment: a perfect cellular niche?[END_REF][START_REF] Watters | Macrophages: the neglected barrier to eradication[END_REF][START_REF] Burdo | Monocyte/macrophages and their role in HIV neuropathogenesis[END_REF][START_REF] Carter | Cell biology of HIV-1 infection of macrophages[END_REF][START_REF] Dargent | HIV-associated multinucleated giant cells in lymphoid tissue of the Waldeyer's ring: a detailed study[END_REF][START_REF] Frankel | Replication of HIV-1 in dendritic cell-derived syncytia at the mucosal surface of the adenoid[END_REF][START_REF] Harbison | Giant cell encephalitis and microglial infection with mucosally transmitted simian-human immunodeficiency virus SHIVSF162P3N in rhesus macaques[END_REF][START_REF] Matsuda | A SIV molecular clone that targets the CNS and induces neuroAIDS in rhesus macaques[END_REF][START_REF] Soulas | Recently infiltrating MAC387+ monocytes/macrophages[END_REF][START_REF] Teo | Circular forms of unintegrated human immunodeficiency virus type 1 DNA and high levels of viral protein expression: association with dementia and multinucleated giant cells in the brains of patients with AIDS[END_REF][START_REF] Vicandi | HIV-1 (p24)positive multinucleated giant cells in HIV-associated lymphoepithelial lesion of the Parotid Gland[END_REF] Although CD4+ T cells do not substantially contribute to infection of the CNS, both parenchymal microglia, the resident macrophage cell lineage of the brain, and infected perivascular monocytes/macrophages migrating through the blood-brain barrier are the main cells infected by HIV-1 in the brain. They participate in the development of HIV-associated neurocognitive disorders, which can persist in infected patients despite effective suppressive antiretroviral therapy. [START_REF] Burdo | Monocyte/macrophages and their role in HIV neuropathogenesis[END_REF] Cell-to-cell virus dissemination may also enable virus to escape elimination by the immune system, including neutralizing antibodies, and antiretroviral drugs. [START_REF] Abela | Cell-cell transmission enables HIV-1 to evade inhibition by potent CD4bs directed antibodies[END_REF][START_REF] Agosto | Highly active antiretroviral therapies are effective against HIV-1 cell-to-cell transmission[END_REF][START_REF] Dufloo | HIV-1 cell-to-cell transmission and broadly neutralizing antibodies[END_REF][START_REF] Gorry | HIV-1 envelope-receptor interactions required for macrophage infection and implications for current HIV-1 cure strategies[END_REF][START_REF] Malbec | Broadly neutralizing antibodies that inhibit HIV-1 cell to cell transmission[END_REF][START_REF] Sigal | Cell-to-cell spread of HIV permits ongoing replication despite antiretroviral therapy[END_REF] HIV-1 cell-to-cell transfer between CD4+ T cells, mainly through the formation of the so-called virologic synapse, [START_REF] Agosto | HIV cell-to-cell transmission: effects on pathogenesis and antiretroviral therapy[END_REF][START_REF] Duncan | High-multiplicity HIV-1 infection and neutralizing antibody evasion mediated by the macrophage-T cell virological synapse[END_REF][START_REF] Labudová | Cell-to-cell transport in viral families: faster than usual[END_REF] or from infected macrophages or DCs to CD4+ T cell targets, have been extensively described in vitro. [START_REF] Bracq | Mechanisms for cell-to-cell transmission of HIV-1[END_REF][START_REF] Groot | Efficient HIV-1 transmission from macrophages to T cells across transient virological synapses[END_REF][START_REF] Hammonds | Siglec-1 initiates formation of the virus-containing compartment and enhances macrophage-to-T cell transmission of HIV-1[END_REF][START_REF] Izquierdo-Useros | HIV-1 capture and transmission by dendritic cells: the role of viral glycolipids and the cellular receptor Siglec-1[END_REF][START_REF] Mcdonald | Recruitment of HIV and its receptors to dendritic cell-T cell junctions[END_REF][START_REF] Moris | Dendritic cells and HIV-specific CD4+ T cells: hIV antigen presentation, T-cell activation, and viral transfer[END_REF][START_REF] Rodriguez-Plata | HIV-1 capture and antigen presentation by dendritic cells: enhanced viral capture does not correlate with better T cell activation[END_REF][START_REF] Dupont | Tunneling nanotubes: intimate communication between myeloid cells[END_REF] In addition, recent reports demonstrate that myeloid cells can be also productively infected through virus cell-tocell transfer for more efficient spreading in these poorly susceptible cell types. As myeloid cells are now emerging as important target cells involved in all steps of HIV-1 pathogenesis and in viral persistence in tissues of infected individuals, [START_REF] Ganor | HIV-1 reservoirs in urethral macrophages of patients under suppressive antiretroviral therapy[END_REF][START_REF] Abbas | Eradication of HIV-1 from the macrophage reservoir: an uncertain goal?[END_REF][START_REF] Douce | Molecular mechanisms of HIV-1 persistence in the monocyte-macrophage lineage[END_REF] even under conditions of antiretroviral treatment, the goal of this review is to discuss the different mechanisms reported in the literature regarding HIV-1 cell-to-cell spread leading to productive infection of myeloid cells. The first part of the review will be related to the mechanisms of homotypic virus cell-tocell transfer between macrophages, whereas the second part will focus on the 2 different mechanisms reported for heterotypic virus cell-tocell dissemination from virus-donor-infected CD4+ T cells to myeloid cell targets, including macrophages, DCs, and OCs.

HOMOTYPIC HIV-1 CELL-TO-CELL TRANSFER BETWEEN MACROPHAGES

Virus transfer between myeloid cells through the formation of tunneling nanotubes

The role in intercellular communication of several types of membrane protrusions, including lamellipodia, pseudopodia, filopodia, as well as various other structures such as tunneling nanotubes, has been widely described both in vitro and in vivo as a major mechanism of transmission of information between different types of cells in many organs and tissues. [START_REF] Casas-Tintó | Their formation, regulation, and roles in signaling and communication in tumorigenesis[END_REF][START_REF] Gurke | The art of cellular communication: tunneling nanotubes bridge the divide[END_REF][START_REF] Ljubojevic | The ways of actin: why tunneling nanotubes are unique cell protrusions[END_REF][START_REF] Rilla | Diverse plasma membrane protrusions act as platforms for extracellular vesicle shedding[END_REF][START_REF] Tiwari | Role of tunneling nanotubes in viral infection, neurodegenerative disease, and cancer[END_REF][START_REF] Wittig | Multi-level communication of human retinal pigment epithelial cells via tunneling nanotubes[END_REF] Among these specialized structures, the formation of membrane nanotubes corresponds to protrusions that extend from the plasma membrane and enable cells to touch over long distances, sometimes over 100 μm, as reported between T cells [START_REF] Sowinski | Membrane nanotubes physically connect T cells over long distances presenting a novel route for HIV-1 transmission[END_REF] but also between myeloid cells, such as macrophages and DCs. [START_REF] Eugenin | Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages[END_REF][START_REF] Hase | M-Sec promotes membrane nanotube formation by interacting with Ral and the exocyst complex[END_REF][START_REF] Önfelt | Structurally distinct membrane nanotubes between human macrophages support long-distance vesicular traffic or surfing of bacteria[END_REF][START_REF] Önfelt | Cutting edge: membrane nanotubes connect immune cells[END_REF][START_REF] Watkins | Functional connectivity between immune cells mediated by tunneling nanotubules[END_REF] These membrane tunneling protrusions are regrouped as 2 main types: close-ended nanotubes and open-ended nanotubes also known as tunneling nanotubes (TNTs). [START_REF] Dupont | Tunneling nanotubes: intimate communication between myeloid cells[END_REF][START_REF] Gurke | The art of cellular communication: tunneling nanotubes bridge the divide[END_REF][START_REF] Önfelt | Structurally distinct membrane nanotubes between human macrophages support long-distance vesicular traffic or surfing of bacteria[END_REF][START_REF] Kolba | Tunneling nanotubemediated intercellular vesicle and protein transfer in the stromaprovided imatinib resistance in chronic myeloid leukemia cells[END_REF][START_REF] Zurzolo | Tunneling nanotubes: reshaping connectivity[END_REF] TNTs are dynamic structures that contain F-actin and do not touch the substrate, meaning that they are hovering above the substrate in the culture medium in contrast to other membrane protrusions such as filopodia, which are attached to the substrate.

They are again subdivided in 2 classes depending of their thickness and cytoskeleton composition: thin TNTs (<0.7 μm) and thick TNTs (>0.7 μm) containing microtubules. [START_REF] Sowinski | Membrane nanotubes physically connect T cells over long distances presenting a novel route for HIV-1 transmission[END_REF][START_REF] Eugenin | Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages[END_REF][START_REF] Hashimoto | Potential role of the formation of tunneling nanotubes in HIV-1 spread in macrophages[END_REF][START_REF] Rustom | Nanotubular highways for intercellular organelle transport[END_REF][START_REF] Souriant | Tuberculosis exacerbates HIV-1 infection through IL-10/STAT3-dependent tunneling nanotube formation in macrophages[END_REF] Although TNTs are physiologically involved in multiple cell-to-cell transfer of many cellular components and macromolecules, and even big cellular organelles such as mitochondria, several studies have demonstrated that both thin and thick TNTs could also transport pathogens, such as bacteria or viruses, including HIV-1 particles leading to efficient virus cell-to-cell transfer between CD4+ T cells or macrophages (Figure 1(A)). [START_REF] Sowinski | Membrane nanotubes physically connect T cells over long distances presenting a novel route for HIV-1 transmission[END_REF][START_REF] Eugenin | Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages[END_REF][START_REF] Hashimoto | Potential role of the formation of tunneling nanotubes in HIV-1 spread in macrophages[END_REF][START_REF] Souriant | Tuberculosis exacerbates HIV-1 infection through IL-10/STAT3-dependent tunneling nanotube formation in macrophages[END_REF][START_REF] Dupont | Tuberculosis-associated IFN-I induces Siglec-1 on tunneling nanotubes and favors HIV-1 spread in macrophages[END_REF][START_REF] Kadiu | Cytoskeletal protein transformation in HIV-1-infected macrophage giant cells[END_REF] In CD4+ T cells, in vitro HIV-1 infection does not seem to trigger or increase the formation of TNTs, but viral particles can be quickly transferred from a CD4+ T cell to another through this intercellular connection. [START_REF] Sowinski | Membrane nanotubes physically connect T cells over long distances presenting a novel route for HIV-1 transmission[END_REF] In contrast, it was reported that the number of both thin and thick TNTs was increased in HIV-1-infected macrophages compared with uninfected cells. [START_REF] Eugenin | Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages[END_REF][START_REF] Souriant | Tuberculosis exacerbates HIV-1 infection through IL-10/STAT3-dependent tunneling nanotube formation in macrophages[END_REF] In an in vivo context using HIV-1infected "humanized mice," TNT-like structures containing the viral p24 capsid protein, as well as the viral mRNA and nonintegrated DNA have been documented in tissues of infected animals. [START_REF] Okafo | Role of tunneling nanotubelike structures during the early events of HIV infection: novel features of tissue compartmentalization and mechanism of HIV spread[END_REF]

D

F I G U R E 1 Intercellular structures and processes involved in cell-to-cell transfer of HIV-1 in myeloid cells. Different pathways for HIV-1 cell-to-cell transfer between virus-donor-infected cells (in green) and target myeloid cells (in purple) have been described through (A) formation of tunneling nanotubes (TNTs), (B) homotypic cell-cell fusion, (C) phagocytosis of infected T cell, and (D) heterotypic cell-cell fusion infection in lymph nodes, spleen, and lungs, whereas only macrophages were able to form TNTs in the brain. Interestingly, treatment of infected mice with the TAK779 CCR5 antagonist decreased TNT formation between CD3+ T cells in spleen and lungs, whereas it was enhanced between macrophages in lymph nodes and brain, suggesting that HIV-1 can continue to spread between macrophages in these tissues in a TNT-dependent manner despite antiretroviral therapy with TAK779.

Thin TNTs would be directly involved in the transport of virus particles, [START_REF] Eugenin | Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages[END_REF] whereas thick TNTs could transport HIV-1 constituents such as viral Gag and Env proteins through intracellular vesicles derived from cellular organelles like the endoplasmic reticulum and the Golgi network or from virus-containing compartments. [START_REF] Eugenin | Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages[END_REF][START_REF] Kadiu | Cytoskeletal protein transformation in HIV-1-infected macrophage giant cells[END_REF] These initial results were then confirmed by other studies describing the presence of these viral constituents into endocytic compartments that might allow efficient viral cell-to-cell transfer through TNTs formed between macrophages. [START_REF] Kadiu | Human immunodeficiency virus type 1 endocytic trafficking through macrophage bridging conduits facilitates spread of infection[END_REF][START_REF] Kadiu | Macrophage bridging conduit trafficking of HIV-1 through the endoplasmic reticulum and Golgi network[END_REF] In addition, the HIV-1-dependent formation of TNTs between adjacent macrophages likely favors subsequent cell-cell fusion for more efficient spreading in this target cell-type through formation of MGCs. [START_REF] Souriant | Tuberculosis exacerbates HIV-1 infection through IL-10/STAT3-dependent tunneling nanotube formation in macrophages[END_REF][START_REF] Vérollet | HIV-1 Nef triggers macrophage fusion in a p61Hck-and protease-dependent manner[END_REF][START_REF] Vérollet | HIV-1 reprograms the migration of macrophages[END_REF] It was reported that the stimulation of TNT formation observed in HIV-1-infected macrophages is dependent and modulated by the viral Nef auxiliary protein, [START_REF] Hashimoto | Potential role of the formation of tunneling nanotubes in HIV-1 spread in macrophages[END_REF][START_REF] Xu | HIV-1 evades virus-specific IgG2 and IgA responses by targeting systemic and intestinal B cells via long-range intercellular conduits[END_REF] as HIV-1-induced TNT formation and cell-cell fusion leading to MGC formation are almost abolished when macrophages are infected by Nef-deficient viruses. [START_REF] Hashimoto | Potential role of the formation of tunneling nanotubes in HIV-1 spread in macrophages[END_REF][START_REF] Vérollet | HIV-1 Nef triggers macrophage fusion in a p61Hck-and protease-dependent manner[END_REF][START_REF] Vérollet | HIV-1 reprograms the migration of macrophages[END_REF] This role of Nef on TNT formation and cell-cell fusion is directly related to the capacity of Nef to modulate cytoskeletal dynamics. [START_REF] Raynaud-Messina | Bone degradation machinery of osteoclasts: an HIV-1 target that contributes to bone loss[END_REF][START_REF] Dupont | Tunneling nanotubes: intimate communication between myeloid cells[END_REF][START_REF] Vérollet | HIV-1 reprograms the migration of macrophages[END_REF][START_REF] Jansens | Bridging the gap: virus long-distance spread via tunneling nanotubes[END_REF][START_REF] Vérollet | HIV-1 Nef alters podosomes and promotes the mesenchymal migration in human macrophages[END_REF][START_REF] Vérollet | HIV-1 infection of T lymphocytes and macrophages affects their migration via Nef[END_REF] Other authors reported that Nef is also able to interact with members of the exocyst complex involved in membrane vesicle trafficking through the M-sec cellular protein. [START_REF] Hashimoto | Potential role of the formation of tunneling nanotubes in HIV-1 spread in macrophages[END_REF][START_REF] Xu | HIV-1 evades virus-specific IgG2 and IgA responses by targeting systemic and intestinal B cells via long-range intercellular conduits[END_REF][START_REF] Imle | Association with PAK2 enables functional interactions of lentiviral Nef proteins with the exocyst complex[END_REF][START_REF] Mukerji | Proteomic analysis of HIV-1 Nef cellular binding partners reveals a role for exocyst complex proteins in mediating enhancement of intercellular nanotube formation[END_REF] This cytosolic protein, expressed in cells of the monocytic lineage, including macrophages, participates in the formation of TNTs. [START_REF] Dupont | Tunneling nanotubes: intimate communication between myeloid cells[END_REF][START_REF] Dufrançais | Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune[END_REF][START_REF] Mccoy-Simandle | Exosomes and nanotubes: control of immune cell communication[END_REF] Interestingly, tuberculosis is a risk factor in HIV-1-infected individuals, and we have revealed that the formation of TNTs was facilitated in HIV-1-infected macrophages in the context of coinfection with Mycobacterium tuberculosis (Mtb), the pathogen causing tuberculosis.

In vitro, many studies showed that tuberculosis environment favors HIV-1 replication in macrophages. [START_REF] Souriant | Tuberculosis exacerbates HIV-1 infection through IL-10/STAT3-dependent tunneling nanotube formation in macrophages[END_REF][START_REF] Dupont | Tuberculosis-associated IFN-I induces Siglec-1 on tunneling nanotubes and favors HIV-1 spread in macrophages[END_REF][START_REF] Bell | Pathogenesis of HIV-1 and Mycobacterium tuberculosis co-infection[END_REF][START_REF] Diedrich | HIV-1/mycobacterium tuberculosis coinfection immunology: how does HIV-1 exacerbate tuberculosis?[END_REF][START_REF] Toossi | Virological and immunological impact of tuberculosis on human immunodeficiency virus type 1 disease[END_REF] Mtb infection induces a proinflammatory environment through the production of TFNα, IL-6, IL-1β, and IL-10 cytokines by HIV-1-infected macrophages that increases NFκB binding to the viral promotor initiating viral transcription in coinfected macrophages. [START_REF] Souriant | Tuberculosis exacerbates HIV-1 infection through IL-10/STAT3-dependent tunneling nanotube formation in macrophages[END_REF][START_REF] Diedrich | HIV-1/mycobacterium tuberculosis coinfection immunology: how does HIV-1 exacerbate tuberculosis?[END_REF][START_REF] Duh | Tumor necrosis factor alpha activates human immunodeficiency virus type 1 through induction of nuclear factor binding to the NF-kappa B sites in the long terminal repeat[END_REF][START_REF] Goletti | The in vitro induction of human immunodeficiency virus (HIV) replication in purified protein derivative-positive HIV-infected persons by recall antigen response to Mycobacterium tuberculosis is the result of a balance of the effects of endogenous interleukin-2 and proinflammatory and antiinflammatory cytokines[END_REF][START_REF] Hoshino | Mechanisms of polymorphonuclear neutrophil-mediated induction of HIV-1 replication in macrophages during pulmonary tuberculosis[END_REF][START_REF] Hoshino | Maximal HIV-1 replication in alveolar macrophages during tuberculosis requires both lymphocyte contact and cytokines[END_REF][START_REF] Lederman | Mycobacterium tuberculosis and its purified protein derivative activate expression of the human immunodeficiency virus[END_REF][START_REF] Zhang | Mycobacterium tuberculosis enhances human immunodeficiency virus-1 replication by transcriptional activation at the long terminal repeat[END_REF] We have also reported that Mtb-associated microenvironments specifically induce formation of both thin and thick TNTs through an IL-10/STAT3 signaling pathway in HIV-1-infected macrophages. [START_REF] Souriant | Tuberculosis exacerbates HIV-1 infection through IL-10/STAT3-dependent tunneling nanotube formation in macrophages[END_REF] Interestingly, the formation of TNTs induced by Mtb in HIV-1-infected macrophages is also dependent of Siglec-1, a receptor mainly expressed by myeloid cells, which has been involved in capture of several pathogens, including HIV-1, by macrophages. [START_REF] Dupont | Tuberculosis-associated IFN-I induces Siglec-1 on tunneling nanotubes and favors HIV-1 spread in macrophages[END_REF][START_REF] Martinez-Picado | Identification of Siglec-1 null individuals infected with HIV-1[END_REF][START_REF] Pino | HIV-1 immune activation induces Siglec-1 expression and enhances viral trans-infection in blood and tissue myeloid cells[END_REF] Exposure of macrophages to Mtb-derived environment results in induction of type 1 IFN-I production leading to up-regulation of Siglec-1 expression on TNTs, which favors their stabilization for a more efficient cell-to-cell viral transfer between macrophages. [START_REF] Dupont | Tuberculosis-associated IFN-I induces Siglec-1 on tunneling nanotubes and favors HIV-1 spread in macrophages[END_REF] As mentioned above, the formation of these TNTs could also drive HIV-1 cell-to-cell transfer by favoring direct intercellular contacts between macrophages leading to cell-cell fusion and formation of infected MGCs. [START_REF] Eugenin | Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages[END_REF][START_REF] Hashimoto | Potential role of the formation of tunneling nanotubes in HIV-1 spread in macrophages[END_REF][START_REF] Vérollet | HIV-1 Nef triggers macrophage fusion in a p61Hck-and protease-dependent manner[END_REF][START_REF] Vérollet | HIV-1 reprograms the migration of macrophages[END_REF]

Virus transfer between macrophages through homotypic cell-cell fusion

When infected by HIV-1, CD4+ T cells and macrophages express the viral envelope glycoproteins (gp120 and gp41) at the cell surface, and the fusogenic capacity of the viral envelope can induce fusion between infected donor cells and neighboring target cells expressing the CD4 receptor and the CCR5 and/or CXCR4 coreceptors to form infected syncytia (or MGCs). [START_REF] Leroy | Virus-mediated cell-cell fusion[END_REF] Although the process of syncytia formation between infected CD4+ T cells has been largely documented in vitro but discussed for its in vivo relevance, [START_REF] Bracq | Mechanisms for cell-to-cell transmission of HIV-1[END_REF] some recent reports indicate that this process of cell-cell fusion could be more relevant for HIV-1 cell-to-cell infection of myeloid cells (Figure 1(B)). Infected multinucleated macrophages can be found in vivo in tissues of HIV-1infected patients and nonhuman primates experimentally infected with the SIV, mainly in the CNS, but also in other lymphoid or nonlymphoid tissues. [START_REF] Dargent | HIV-associated multinucleated giant cells in lymphoid tissue of the Waldeyer's ring: a detailed study[END_REF][START_REF] Harbison | Giant cell encephalitis and microglial infection with mucosally transmitted simian-human immunodeficiency virus SHIVSF162P3N in rhesus macaques[END_REF][START_REF] Soulas | Recently infiltrating MAC387+ monocytes/macrophages[END_REF][START_REF] Geny | Multifocal multinucleated giant cell myelitis in an AIDS patient[END_REF][START_REF] Ryzhova | Simian immunodeficiency virus encephalitis: analysis of envelope sequences from individual brain multinucleated giant cells and tissue samples[END_REF][START_REF] Teo | Circular forms of unintegrated human immunodeficiency virus type 1 DNA and high levels of viral protein expression: association with dementia and multinucleated giant cells in the brains of patients with AIDS[END_REF] In the CNS of HIV-infected patients, the detection of MGCs has even been proposed by some authors as a hallmark of AIDS-associated neurologic disorders that can be observed even in patients under suppressive antiretroviral therapy. [START_REF] Burdo | Monocyte/macrophages and their role in HIV neuropathogenesis[END_REF][START_REF] Dargent | HIV-associated multinucleated giant cells in lymphoid tissue of the Waldeyer's ring: a detailed study[END_REF][START_REF] Harbison | Giant cell encephalitis and microglial infection with mucosally transmitted simian-human immunodeficiency virus SHIVSF162P3N in rhesus macaques[END_REF][START_REF] Vicandi | HIV-1 (p24)positive multinucleated giant cells in HIV-associated lymphoepithelial lesion of the Parotid Gland[END_REF][START_REF] Geny | Multifocal multinucleated giant cell myelitis in an AIDS patient[END_REF][START_REF] Teo | Circular forms of unintegrated human immunodeficiency virus type 1 DNA and high levels of viral protein expression: association with dementia and multinucleated giant cells in the brains of patients with AIDS[END_REF][START_REF] Fischer-Smith | Monocyte/macrophage trafficking in acquired immunodeficiency syndrome encephalitis: lessons from human and nonhuman primate studies[END_REF][100][101][102] In vitro, homotypic cell-cell fusion of HIV-1-infected myeloid cells has been described more specifically between macrophages, but also between OCs, [START_REF] Raynaud-Messina | Bone degradation machinery of osteoclasts: an HIV-1 target that contributes to bone loss[END_REF][START_REF] Kadiu | Cytoskeletal protein transformation in HIV-1-infected macrophage giant cells[END_REF][START_REF] Vérollet | HIV-1 Nef triggers macrophage fusion in a p61Hck-and protease-dependent manner[END_REF][START_REF] Vérollet | HIV-1 reprograms the migration of macrophages[END_REF]103 leading to the formation of long-lived infected MGCs that can produce high levels of infectious viral particles. This homotypic cell-cell fusion process is directly related to interactions between the viral envelope glycoproteins expressed at the surface of the infected virus-donor cell and the CD4 and chemokine coreceptors expressed at the surface of the target cell, but other molecular mechanisms linked to cytoskeleton reorganization could also participate in the modulation of this complex process. [START_REF] Kadiu | Cytoskeletal protein transformation in HIV-1-infected macrophage giant cells[END_REF][START_REF] Vérollet | HIV-1 Nef triggers macrophage fusion in a p61Hck-and protease-dependent manner[END_REF][START_REF] Vérollet | HIV-1 reprograms the migration of macrophages[END_REF] The intrinsic propensity of cells of the myeloid lineage to mediate homotypic cell-cell fusion to form multinucleated myeloid cells both in physiologic and pathologic conditions could also facilitate HIV-1mediated homotypic cell-cell fusion of myeloid cells through engagement of signaling pathways and effectors involved in programming of myeloid cells into a fusion-competent state depending of the cytokine environment. 104,105 Although the formation in bones of functional multinucleated OCs is mainly orchestrated by the receptor activator of NF-kB ligand cytokine, alternative activation of macrophages by IL-4 and IL-13 can also promote homotypic macrophage cell-cell fusion and MGC formation. 105 The formation of these particular MGCs, also called foreign body giant cells, formed in response to organic and inorganic materials, has been studied at the molecular level, [START_REF] Dufrançais | Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune[END_REF] but the contribution of the cellular pathways mobilized by these cytokines in the cell-cell fusion of macrophages, OCs, and DCs observed during HIV-1 infection has not been yet investigated.

Again, it was reported that the viral Nef protein was involved in the modulation of the process to stimulate cell-cell fusion, as MGC formation was significantly decreased when macrophages are infected with Nef-deficient viruses compared with wild-type HIV-1. Furthermore, expression of the Nef protein was sufficient to trigger the formation of MGCs in the macrophage-like RAW264.7 cell line. [START_REF] Vérollet | HIV-1 Nef triggers macrophage fusion in a p61Hck-and protease-dependent manner[END_REF] This mechanism of HIV-1 cell-cell fusion is related to the activation by Nef of Hck, a Src-family tyrosine kinase specifically expressed in phagocytes, through direct interaction between the polyproline motif of Nef and the SH3 domain of Hck. [106][107][108] Even if the 2 p61 and p59 isoforms of Hck are activated by Nef, only activation of the lysosomal p61Hck isoform seems to trigger the formation of MGCs through its catalytic activity and its association with lysosomes containing several proteins including proteases that participate in Nef-induced cellcell fusion between macrophages and MGC formation. [START_REF] Vérollet | HIV-1 Nef triggers macrophage fusion in a p61Hck-and protease-dependent manner[END_REF] Despite their large size, these newly formed infected MGCs migrate faster in dense matrices (i.e., mesenchymal mode of migration) 109 than their mononucleated counterparts, again by a Nef-dependent mechanism. This effect of Nef on HIV-1-infected macrophage migration ability has been evi-denced both in vitro and in vivo in transgenic mice expressing Nef. [START_REF] Vérollet | HIV-1 Nef triggers macrophage fusion in a p61Hck-and protease-dependent manner[END_REF][START_REF] Vérollet | HIV-1 reprograms the migration of macrophages[END_REF] Thus, cell-cell fusion of infected macrophages and MGC formation may represent in vivo a very efficient route for viral dissemination and formation of resident HIV-1 reservoirs in different tissues such as the CNS.

HETEROTYPIC HIV-1 CELL-TO-CELL TRANSFER FROM INFECTED CD4 T CELLS TO MYELOID TARGET CELLS

Virus transfer in macrophages through phagocytosis of HIV-1-infected CD4+ T cells

As innate immune sentinels, one of the biologic function of macrophages is their capacity to detect and eliminate dying cells and infected cells through phagocytosis, a cellular process by which large particles (≥0.5 μm) are engulfed at the plasma membrane. [START_REF] Dupont | Macrophage cell-cell interactions promoting HIV-1 infection[END_REF]110,111 In addition to this function, the team of Quentin Sattentau has proposed a new mechanism for HIV-1 cell-to-cell transfer from infected CD4+ T cells to macrophages through selective capture and engulfment of HIV-1-infected CD4+ T cells (Figure 1(C)), leading to infection of macrophages. 112 This in vitro observation may correlate with results obtained in SIV-infected macaques showing that myeloid cells from lymphoid tissues contain T cell markers and viral DNA originating from infected CD4+ T cells, 113,114 although it is difficult to reconcile this finding with the phagocytosis process as it would involve a progressive degradation of the T-cell material. 115 As expected, this engulfment (or phagocytosis) process is more pronounced and efficient when infected T cells are dead or dying. 112 Only CD4+ T cells infected by CCR5-using macrophage-tropic strains and in a lesser extend CCR5-using T/F viruses, which poorly infect macrophages in cell-free infection assays, 116,117 led to productive infection of the target macrophages. 112 By contrast, no productive infection of the macrophage targets is observed after engulfment of T cells infected with non-macrophage-tropic CXCR4-using viruses, although these infected T cells are efficiently engulfed by target macrophages. This mechanism of engulfment of infected T cells is independent of the interactions between the viral envelope glycoproteins and cellular receptors (i.e., CD4 and CCR5/CXCR4) required for HIV-1 entry, as T cells infected with either wild-type or envelopedeficient viruses were equally captured by macrophages. Similarly, T cells infected with the laboratory-adapted lymphotropic CXCR4-using strain NL4.3 were efficiently captured. However, the subsequent step leading to productive infection of the macrophage target relies on interactions of the viral envelope with the CD4 receptor and the CCR5 coreceptor, as it is inhibited by antibodies or drugs targeting Env and CD4, and to a lesser extent by antagonists of CCR5, suggesting that this second step likely results from fusion between the viral envelope and the cellular membrane of the phagolysosomes.

Together, these results indicate that this process of macrophage infection through engulfment of infected CD4+ T cells is related to a nonconventional mechanism requiring further investigations to understand why engulfment of infected T cells infected by CCR5-using macrophage-tropic and weakly macrophage-tropic T/F viruses lead to the productive infection of the macrophage targets. In contrast, CD4+ T cells infected by nonmacrophage-tropic CXCR4-using viruses are efficiently engulfed but failed to result in productive infection of macrophages targets. Interestingly, it was recently reported that the viral protein U (Vpu), a membrane-associated HIV-1 auxiliary protein that assists viral dissemination, can promote the susceptibility of macrophages to infection through phagocytosis of infected CD4+ T cells. 118 This role is related to the function of Vpu in the cell surface remodeling observed in HIV-1-infected cells through down-regulation of cell surface proteins such as CD4 and the antiviral host restriction factor BST2, 119-122 but also CD47. 123,124 It was shown that CD47 serves as a ligand of the inhibitory receptor signal regulatory proteinalpha (SIRPα) expressed on myeloid cells, and acts as a "don't eat me" signal. 125 By inducing down-regulation of CD47 from the surface of infected CD4+ T cells, Vpu thus promotes capture and phagocytosis of infected T cells by macrophages leading to productive infection of these target cells by a prototypic CCR5-using macrophage-tropic virus, but also by a T/F virus, which was not previously defined as macrophage tropic. 118

Virus transfer in macrophages through heterotypic cell-cell fusion with infected CD4+ T cells

Finally, recent studies from our team have revealed a new route for rapid and massive cell-to-cell transfer and dissemination of HIV-1 from infected CD4+ T cells toward the main target myeloid cells, including macrophages, but also OCs and immature DCs (iDCs). [START_REF] Raynaud-Messina | Bone degradation machinery of osteoclasts: an HIV-1 target that contributes to bone loss[END_REF]126,127 This process of virus cell-to-cell transfer is completed by a 2-step cell-cell fusion mechanism leading to the productive infection of the myeloid target cells. In the first step, infected T cells establish rapid contacts with macrophages, OCs, or iDCs, resulting in cell-cell fusion with these target myeloid cells (Figure 1(D)). The newly formed Gag+ fused cells then acquire the ability to fuse with surrounding noninfected macrophages, iDCs, or OCs, leading to the formation of infected MGCs containing at least 1 nucleus coming from the initial fusion of infected T cells. These MGCs are able to produce high levels of fully infectious new virus particles. As already mentioned, infected macrophage-derived MGCs, but also DC-derived MGCs, can be found in vivo in tissues of HIV-1-infected patients. In the CNS of HIV-1infected patients and SIV-infected monkeys, infected macrophages are very often detected as multinucleated cells. [START_REF] Burdo | Monocyte/macrophages and their role in HIV neuropathogenesis[END_REF][START_REF] Dargent | HIV-associated multinucleated giant cells in lymphoid tissue of the Waldeyer's ring: a detailed study[END_REF][START_REF] Harbison | Giant cell encephalitis and microglial infection with mucosally transmitted simian-human immunodeficiency virus SHIVSF162P3N in rhesus macaques[END_REF][START_REF] Vicandi | HIV-1 (p24)positive multinucleated giant cells in HIV-associated lymphoepithelial lesion of the Parotid Gland[END_REF][START_REF] Geny | Multifocal multinucleated giant cell myelitis in an AIDS patient[END_REF][START_REF] Teo | Circular forms of unintegrated human immunodeficiency virus type 1 DNA and high levels of viral protein expression: association with dementia and multinucleated giant cells in the brains of patients with AIDS[END_REF]100,101 The presence of HIV-1-infected multinucleated cells derived from myeloid cells was also found at the surface of the nasopharyngeal tonsils, adenoids, and parotid glands of HIV-1-infected patients. [START_REF] Frankel | Replication of HIV-1 in dendritic cell-derived syncytia at the mucosal surface of the adenoid[END_REF][START_REF] Vicandi | HIV-1 (p24)positive multinucleated giant cells in HIV-associated lymphoepithelial lesion of the Parotid Gland[END_REF]128,129 This heterotypic and 2-step cell-cell fusion process described in vitro for virus transfer in myeloid cells agrees with the in vivo observations already mentioned showing that T cell markers are detected in myeloid cells from lymphoid tissues of SIV-infected macaques. 113,114 Initially revealed using HIV-1 strains expressing CCR5-using macrophage-tropic viral envelopes, we recently obtained results suggesting that primary viral variants isolated from infected patients, using either CCR5 and/or CXCR4 and characterized by cell-free infection assays as non-macrophage tropic, including some R5-tropic T/F viruses, could be efficiently transferred from infected T cells and then disseminate in macrophages by cell-cell fusion for virus spreading through the formation of productively infected MGCs (SB and MH, results referenced in bioRxiv 130 ). Although the viral tropism of the virus strains, defined by the coreceptor usage, is maintained between cell-free and cell-to-cell infection processes, the initial block of virus entry for efficient replication in macrophages could be overcome in T cell-to-macrophage cell-cell fusion, indicating that HIV-1 has a more prevalent tropism for macrophages than initially suggested. In agreement, a recent report shows that non-macrophage-tropic primary HIV-1 isolates can efficiently infect alveolar macrophages through contact with infected CD4+ T cells. 131 These findings indicate that the cellular tropism of primary HIV-1 isolates needs to be revisited to reflect the capacity of both CCR5-and CXCR4-using non-M-tropic variants, including some T/F viruses, to spread efficiently in myeloid cells via cell-to-cell transfer from infected T cells by cell-cell fusion.

Compared with cell-free infection, our results show an enhancement of interactions of CCR5-using macrophage-and non-macrophagetropic viral envelope glycoproteins with CD4 and CCR5 during T-cell/macrophage contacts. Enhanced envelope/receptor interactions may increase the capacity of HIV-1 envelopes, and particularly non-M-tropic envelopes, to trigger cell-cell fusion of infected T cells with macrophages. This enhancement results in a reduced dependence on receptor expression levels on macrophages for viral entry. As proposed to explain the higher efficiency of virus cell-to-cell transfer between CD4+ T cells across the virologic synapse, multiple mechanisms could be involved in virus cell-to-cell transfer from infected T cells to macrophages. The formation of the virologic synapse is characterized by a redistribution and clustering of HIV-1 receptors and envelope glycoproteins at the surface of the target and virus-donor cells, respectively. 132,133 Although similar processes in the context of infected T cell-macrophage contacts remain to be demonstrated, they could similarly reinforce envelope/receptor interactions and decrease the dependence on receptor expression levels. In addition, redistribution of coreceptors into distinct membrane environments might change their conformational states, thereby influencing envelope binding and virus entry. 134 Although non-macrophage-tropic envelope trimers adopt a conformation distinct from that of macrophage-tropic envelopes with a weaker affinity for CD4, 135-137 a local increase of viral envelope molecules at the T cell/macrophage contact site could overcome this limitation and favors cell-cell fusion. In contrast, cell-free virus particles, known to express limited amounts of Env trimers at their surface, could not overcome this limitation. 138,139 Both cell-cell fusion sequential processes, first between infected T cells and myeloid cells (i.e., macrophages, iDCs, and OCs) and then with surrounding noninfected myeloid cells, are dependent on interactions of the viral envelope glycoproteins expressed on virus-donor cells with the CD4 and chemokine coreceptors expressed on myeloid target cells. Both cell-cell fusion processes are inhibited by anti-CD4 and anti-envelope antibodies, as well as by drugs, such as T20, a specific fusion inhibitor targeting the transmembrane gp41 subunit, and also by CCR5 and CXCR4 antagonists. 113,114 Interestingly, no such cell-cell fusion process was observed between HIV-1-infected T cells and nondifferentiated blood monocytes or mature DCs, 127 even though these other myeloid cells both express CD4 and CCR5. This could be related to the lower expression of the CD4 and CCR5 receptors on the cell surface of these cell types compared with immature DCs and differentiated macrophages. 127,140,141 These cell-cell fusion mechanisms could be also regulated by other cell-surface molecules such as adhesins, integrins or lectins, as shown for homotypic macrophage cell fusion and OC formation. 104 Again, some signaling pathways involved in programming of myeloid cells into a fusioncompetent state could participate in this 2-step cell-cell fusion process for specific HIV-1 cell-to-cell spreading in these myeloid cells. [START_REF] Dufrançais | Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune[END_REF]105 As mentioned above for virus cell-to-cell transfer in myeloid cells through homotypic cell-cell fusion, cellular pathways mobilized by some cytokines, such as IL-4 and IL-13, may contribute to both cell-cell fusion steps allowing virus cell-to-cell transfer and then dissemination in macrophages, iDCs and OCs. Interestingly, it was reported that some cell surface molecules, such as tetraspanin proteins (CD9, CD63, and CD81), may have a modulatory role in cell-cell fusion and formation of syncytia between infected and target CD4+ T cells. 142 By regulating the remodeling of actin cytoskeleton and ERM (Ezrin, Radixin, Moesin) proteins, these tetraspanins could negatively regulate cell-cell fusion between CD4+ T cells for efficient HIV-1 cell-to-cell transfer through formation of the virologic synapse. 122,[143][144][145][146][147] It is not known whether such tetraspanins could differentially modulate the cytoskeleton organization depending on the target cell-type during intercellular interactions between infected CD4+ T cells and macrophages leading to cellcell fusion for HIV-1 transfer.

Interestingly, the 2-step cell-cell fusion mechanism for massive virus cell-to-cell transfer between infected CD4+ T cells and myeloid cell targets bypasses the inhibition to HIV-1 replication related to the high expression of the SAMHD1 host cell restriction factor observed when these myeloid cells are infected with cell-free viruses. [148][149][150][151] In infected MGCs formed upon cell-cell fusion between infected T cells and macrophages or iDCs, SAMHD1 is expressed in its inactive phosphorylated form explaining that MGCs can survive for a long time in vitro while producing large amounts of virus particles, and this could be responsible for high productive infection of these target cells in vivo. 127,[152][153][154][155] Although further investigations are needed to elucidate this intriguing mechanism of SAMHD1 phosphorylation in MGCs, we can speculate that the cyclin/cyclin-dependent kinase complex involved in SAMHD1 phosphorylation could be transferred from activated proliferative infected T cells through the delivery of its cellular content to the myeloid target cell. In addition, the high levels of dNTPs transferred from proliferative-infected T cells could also help to overcome the restriction activity of SAMHD1 expressed in myeloid cells. Therefore, it would be of great interest to validate that the different mechanisms of HIV-1 cell-to-cell transfer described above can be recapitulated with tissue macrophages such as lung alveolar or placental macrophages, [148][149][150][151]164 which are particularly more resistant to infection than monocyte-derived macrophages, using ex vivo human tissue explant models. Finally, further in vivo investigations in animal models, such as SIV-infected monkeys or HIV-1-infected humanized mice, are absolutely required for formal demonstration of the biologic relevance of this mode of intercellular virus transfer for spreading in myeloid cells in HIV-1-infected patients.

  TNT formation was increased in both macrophages and CD3+ lymphocytes early after Tunneling nanotubes (TNTs) A Homotypic cell-cell fusion B Phagocytosis of infected T cell C Heterotypic cell-cell fusion

Finally

  , an attractive hypothesis could be related to the maintenance of transcriptionally active T cell nucleus after the initial heterotypic fusion between infected T cells and myeloid target cells (S. B. and M. W., unpublished observations). Thus, transfer of the T cell nucleus already containing integrated proviral DNA may allow to bypass the reverse transcription step restricted by SAMHD1, leading to efficient viral expression by the newly fused cells formed. Similarly, it would be intriguing to analyze whether virus cell-to-cell transfer by cell-cell fusion could also bypass or overcome other host cell restriction factors acting through inhibition of the early steps of the HIV-1 life cycle (from virus entry until integration of the proviral genome into the host chromosomes), such as members of the APOBEC3 family, UNG2, SERINC, as well as TRIM and IFITM proteins.[156][157][158][159][160][161][162][163] The transfer and maintenance of the T cell nucleus containing integrated proviral DNA may also significantly decrease the sensitivity of infected MGCs to antiretroviral drugs acting in the early steps of the virus life cycle, such as inhibitors of the reverse transcriptase and integrase viral enzymes.Again, these results may represent an original cellular mechanism for SAMHD1-independent cell-to-cell transfer of HIV-1 in myeloid cell targets through the formation of the infected MGCs observed in vivo in tissues of HIV-1-infected patients.

  During the last decade, myeloid cells, including macrophages, DCs, and OCs, have emerged as crucial target cells of HIV-1 and likely participate in all steps of pathogenesis. Infected myeloid cells have been involved in sexual transmission and early virus dissemination in lymphoid and nonlymphoid tissues, such as brain, lung, and bones, where they can participate in the establishment of long-lived virus reservoirs even in infected patients under highly active antiretroviral therapy. Although these myeloid cells are poorly infected in vitro by cell-free viruses, the different mechanisms of virus cell-to-cell transfer recapitulated in this review (Figure1) may represent the in vivo dominant mode of virus dissemination for productive infection of both myeloid and lymphoid cell types. As reviewed here, different mechanisms of homotypic or heterotypic virus cell-to-cell transfer between macrophages or from virusdonor-infected CD4+ T lymphocytes toward myeloid cell targets have been reported, respectively. At least in vitro, these different intercellular routes of infection of myeloid cells, and especially the mechanism related to cell-cell fusion with infected CD4+ T cells, are several orders of magnitude more efficient than infection with cell-free virus particles.These cell-to-cell infection processes may indeed bypass the inhibitory activity of host cell restriction factors, such as SAMHD1, especially in the early steps of the viral life cycle. They may also allow efficient transfer of HIV-1 strains previously defined as non-macrophage tropic, such as T/F viruses involved in sexual transmission and early dissemination in tissues. However, there is still a paucity of studies addressing the importance of HIV-1 cell-to-cell transfer in vivo even if HIV-1 cellto-cell dissemination is believed to contribute to HIV-1 pathogenesis.

ACKNOWLEDGEMENTS

We want to thank Bernard Lagane and Jean-Christophe Pages for constant scientific discussions and advices. Financial support is provided by INSERM, CNRS, and the Universities of Paris and Toulouse. Both teams are also funded by grants from "Agence Nationale de Recherche sur le SIDA et les Hépatites virales" (ANRS) and "Agence Nationale de la Recherche" (ANR). M. H. is supported by the China Scholarship Council, whereas M. W., Z. V., and R. M. are supported by grants from ANRS.

DISCLOSURES

The authors declare no conflict of interest.

AUTHORSHIP