Human immunodeficiency virus type 1 envelope gp120 induces a stop signal and virological synapse formation in noninfected CD4+ T cells

HIV-1 virological synapse formation enhances infection spread by dysregulating Aurora Kinase B

HIV-1 spreads efficiently through direct cell-to-cell transmission at virological synapses (VSs) formed by interactions between HIV-1 envelope proteins (Env) on the surface of infected cells and CD4 receptors on uninfected target cells. Env-CD4 interactions bring the infected and uninfected cellular membranes into close proximity and induce transport of viral and cellular factors to the VS for efficient virion assembly and HIV-1 transmission. Using novel, cell-specific stable isotope labeling and quantitative mass spectrometric proteomics, we identified extensive changes in the levels and phosphorylation states of proteins in HIV-1 infected producer cells upon mixing with CD4+ target cells under conditions inducing VS formation. These coculture-induced alterations involved multiple cellular pathways including transcription, TCR signaling and, unexpectedly, cell cycle regulation, and were dominated by Env-dependent responses. We confirmed the proteomic results using inhibitors targeting regulatory kinases and phosphatases in selected pathways identified by our proteomic analysis. Strikingly, inhibiting the key mitotic regulator Aurora kinase B (AURKB) in HIV-1 infected cells significantly increased HIV activity in cell-to-cell fusion and transmission but had little effect on cell-free infection. Consistent with this, we found that AURKB regulates the fusogenic activity of HIV-1 Env. In the Jurkat T cell line and primary T cells, HIV-1 Env:CD4 interaction also dramatically induced cell cycle-independent AURKB relocalization to the centromere, and this signaling required the long (150 aa) cytoplasmic C-terminal domain (CTD) of Env. These results imply that cytoplasmic/plasma membrane AURKB restricts HIV-1 envelope fusion, and that this restriction is overcome by Env CTD-induced AURKB relocalization. Taken together, our data reveal a new signaling pathway regulating HIV-1 cell-to-cell transmission and potential new avenues for therapeutic intervention through targeting the Env CTD and AURKB activity.

A Proteomic Analysis of Detergent-Resistant Membranes in HIV Virological Synapse: The Involvement of Vimentin in CD4 Polarization

The cell-cell contact between HIV-1-infected and uninfected cells forms a virological synapse (VS) to allow for efficient HIV-1 transmission. Not only are HIV-1 components polarized and accumulate at cell-cell interfaces, but viral receptors and lipid raft markers are also. To better understand the nature of the HIV-1 VS, detergent-resistant membrane (DRM) fractions were isolated from an infected-uninfected cell coculture and compared to those from non-coculture samples using 2D fluorescence difference gel electrophoresis. Mass spectrometry revealed that ATP-related enzymes (ATP synthase subunit and vacuolar-type proton ATPase), protein translation factors (eukaryotic initiation factor 4A and mitochondrial elongation factor Tu), protein quality-control-related factors (protein disulfide isomerase A3 and 26S protease regulatory subunit), charged multivesicular body protein 4B, and vimentin were recruited to the VS. Membrane flotation centrifugation of the DRM fractions and confocal microscopy confirmed these findings. We further explored how vimentin contributes to the HIV-1 VS and found that vimentin supports HIV-1 transmission through the recruitment of CD4 to the cell-cell interface. Since many of the molecules identified in this study have previously been suggested to be involved in HIV-1 infection, we suggest that a 2D difference gel analysis of DRM-associated proteins may reveal the molecules that play crucial roles in HIV-1 cell-cell transmission.

T-cell trans-synaptic vesicles are distinct and carry greater effector content than constitutive extracellular vesicles

The immunological synapse is a molecular hub that facilitates the delivery of three activation signals, namely antigen, costimulation/corepression and cytokines, from antigen-presenting cells (APC) to T cells. T cells release a fourth class of signaling entities, trans-synaptic vesicles (tSV), to mediate bidirectional communication. Here we present bead-supported lipid bilayers (BSLB) as versatile synthetic APCs to capture, characterize and advance the understanding of tSV biogenesis. Specifically, the integration of juxtacrine signals, such as CD40 and antigen, results in the adaptive tailoring and release of tSV, which differ in size, yields and immune receptor cargo compared with steadily released extracellular vesicles (EVs). Focusing on CD40L+ tSV as model effectors, we show that PD-L1 trans-presentation together with TSG101, ADAM10 and CD81 are key in determining CD40L vesicular release. Lastly, we find greater RNA-binding protein and microRNA content in tSV compared with EVs, supporting the specialized role of tSV as intercellular messengers.

LFA1 and ICAM1 are critical for fusion and spread of murine leukemia virus in vivo

Murine leukemia virus (MLV)-presenting cells form stable intercellular contacts with target cells during infection of lymphoid tissue, indicating a role of cell-cell contacts in retrovirus dissemination. Whether host cell adhesion proteins are required for retrovirus spread in vivo remains unknown. Here, we demonstrate that the lymphocyte-function-associated-antigen-1 (LFA1) and its ligand intercellular-adhesion-molecule-1 (ICAM1) are important for cell-contact-dependent transmission of MLV between leukocytes. Infection experiments in LFA1- and ICAM1-deficient mice demonstrate a defect in MLV spread within lymph nodes. Co-culture of primary leukocytes reveals a specific requirement for ICAM1 on donor cells and LFA1 on target cells for cell-contact-dependent spread through trans- and cis-infection. Importantly, adoptive transfer experiments combined with a newly established MLV-fusion assay confirm that the directed LFA1-ICAM1 interaction is important for retrovirus fusion and transmission in vivo. Taken together, our data provide insights on how retroviruses exploit host proteins and the biology of cell-cell interactions for dissemination.

Microbial Biofilms: Human T-cell Leukemia Virus Type 1 First in Line for Viral Biofilm but Far Behind Bacterial Biofilms

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus associated with adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). To date, it is the unique published example of a virus able to form a biofilm at the surface of infected cells. Deeply studied in bacteria, bacterial biofilms represent multicellular assemblies of bacteria in contact with a surface and shielded by the extracellular matrix (ECM). Microbial lifestyle in biofilms, either viral or bacterial, is opposed structurally and physiologically to an isolated lifestyle, in which viruses or bacteria freely float in their environment. HTLV-1 biofilm formation is believed to be promoted by viral proteins, mainly Tax, through remodeling of the ECM of the infected cells. HTLV-1 biofilm has been linked to cell-to-cell transmission of the virus. However, in comparison to bacterial biofilms, very little is known on kinetics of viral biofilm formation or dissemination, but also on its pathophysiological roles, such as escape from immune detection or therapeutic strategies, as well as promotion of leukemogenesis. The switch between production of cell-free isolated virions and cell-associated viral biofilm, although not fully apprehended yet, remains a key step to understand HTLV-1 infection and pathogenesis.

HIV-Captured DCs Regulate T Cell Migration and Cell-Cell Contact Dynamics to Enhance Viral Spread

Trafficking of cell-associated HIV-1 from the genital mucosa to lymphoid organs represents a critical first step toward systemic infection. Mature DCs capture and transmit HIV-1 to T cells, but insights into DC-to-T cell viral spread dynamics within a 3-dimensional environment is lacking. Using live-cell imaging, we show that mature DCs rapidly compartmentalize HIV-1 within surface-accessible invaginations near the uropod. HIV-1 capture did not interfere with DC migration toward lymph node homing chemo-attractants and their ability to enter lymphatic vessels. However, HIV-captured DCs engaged in prolonged contacts with autologous CD4+ T cells, which led to high T cell infection. Interestingly, we show that surface bound, virion-associated Env induced signal transduction in motile T cells that facilitated prolonged DC:T cell interactions, partially through high-affinity LFA-1 expression. Together, we describe a mechanism by which surface bound HIV-1 particles function as signaling receptors that regulate T cell motility, cell-cell contact dynamics, and productive infection.

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Mature DCs compartmentalize HIV particles near the uropodia via Siglec-1 receptor

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HIV-captured DCs respond to lymph node-homing chemokines and access lymphatics

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Prolonged contacts between HIV-captured DCs and CD4 T cells facilitate virus transfer

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Surface-accessible HIV particles can induce T cell signaling via Env:CD4 engagement

HIV Reactivation in Latently Infected Cells With Virological Synapse-Like Cell Contact

HIV reactivation from latency is induced by cytokines but also by cell contact with other cells. To better understand this, J1.1 cells, a latent HIV-1-infected Jurkat derivative, were cocultured with its parental Jurkat. J1.1 cells became p17MA-positive and produced a high level of HIV p24CA antigen, only when they were cocultured with stimulated Jurkat with cell-to-cell contact. In contrast, very little p24CA was produced when they were cocultured without cell contact. Similar results were obtained when latent ACH-2 and its parental A3.01 cells were cocultured. Confocal microscopy revealed that not only HIV-1 p17MA and gp120Env but also LFA-1, CD81, CD59, and TCR CD3 accumulated at the cell contact site, suggesting formation of the virological synapse-like structure. LFA-1–ICAM-1 interaction was involved in the cell-to-cell contact. When J1.1 was cocultured with TCR-deficient Jurkat, the p17MA-positive rate was significantly lower, although the cell-to-cell contact was not impaired. Quantitative proteomics identified 54 membrane molecules, one of which was MHC class I, that accumulated at the cell contact site. Reactivation from latency was also influenced by the presence of stromal cells. Our study indicated that latent HIV-1 in J1.1/ACH-2 cells was efficiently reactivated by cell-to-cell contact with stimulated parental cells, accompanying the virological synapse-like structure.

HIV Infection Stabilizes Macrophage-T Cell Interactions To Promote Cell-Cell HIV Spread

Macrophages are susceptible to HIV infection and play an important role in viral dissemination through cell-cell contacts with T cells. However, our current understanding of macrophage-to-T cell HIV transmission is derived from studies that do not consider the robust migration and cell-cell interaction dynamics between these cells. Here, we performed live-cell imaging studies in 3-dimensional (3D) collagen that allowed CD4⁺ T cells to migrate and to locate and engage HIV-infected macrophages, modeling the dynamic aspects of the in situ environment in which these contacts frequently occur. We show that HIV⁺ macrophages form stable contacts with CD4⁺ T cells that are facilitated by both gp120-CD4 and LFA-1-ICAM-1 interactions and that prolonged contacts are a prerequisite for efficient viral spread. LFA-1-ICAM-1 adhesive contacts function to restrain highly motile T cells, since their blockade substantially destabilized macrophage-T cell contacts, resulting in abnormal tethering events that reduced cell-cell viral spread. HIV-infected macrophages displayed strikingly elongated podosomal extensions that were dependent on Nef expression but were dispensable for stable cell-cell contact formation. Finally, we observed persistent T cell infection in dynamic monocyte-derived macrophage (MDM)-T cell cocultures in the presence of single high antiretroviral drug concentrations but achieved complete inhibition with combination therapy. Together, our data implicate macrophages as drivers of T cell infection by altering physiological MDM-T cell contact dynamics to access and restrain large numbers of susceptible, motile T cells within lymphoid tissues.IMPORTANCE Once HIV enters the lymphoid organs, exponential viral replication in T cells ensues. Given the densely packed nature of these tissues, where infected and uninfected cells are in nearly constant contact with one another, efficient HIV spread is thought to occur through cell-cell contacts in vivo However, this has not been formally demonstrated. In this study, we performed live-cell imaging studies within a 3-dimensional space to recapitulate the dynamic aspects of the lymphoid microenvironment and asked whether HIV can alter the morphology, migration capacity, and cell-cell contact behaviors between macrophages and T cells. We show that HIV-infected macrophages can engage T cells in stable contacts through binding of virus- and host-derived adhesive molecules and that stable macrophage-T cell contacts were required for high viral spread. Thus, HIV alters physiological macrophage-T cell interactions in order to access and restrain large numbers of susceptible, motile T cells, thereby playing an important role in HIV progression.

Mechanisms of HIV-1 cell-to-cell transmission and the establishment of the latent reservoir

HIV-1 spreads through both the release of cell-free particles and by cell-to-cell transmission. Mounting evidence indicates that cell-to-cell transmission is more efficient than cell-free transmission of particles and likely influences the pathogenesis of HIV-1 infection. This mode of viral transmission also influences the generation and maintenance of the latent reservoir, which represents the main obstacle for curing the infection. In this review we will discuss general cell contact-dependent mechanisms that HIV-1 utilizes for its spread and the evidence pointing to cell-to-cell transmission as a mechanism for the establishment and maintenance of latent infection.

CD4+ T Cell Subsets and Pathways to HIV Latency

Latent infection of CD4⁺ T cells is the main barrier to eradicating HIV-1 infection from infected patients. The cellular and molecular mechanisms involved in the establishment and maintenance of latent infection are directly linked to the transcriptional program of the different CD4⁺ T cell subsets targeted by the virus. In this review, we provide an overview of how T cell activation, T cell differentiation into functional subsets, and the mode of initial viral infection influence HIV proviral transcription and entry into latency.

Extracellular Matrix in Plants and Animals: Hooks and Locks for Viruses

The extracellular matrix (ECM) of animal and plants cells plays important roles in viral diseases. While in animal cells extracellular matrix components can be exploited by viruses for recognition, attachment and entry, the plant cell wall acts as a physical barrier to viral entry and adds a higher level of difficulty to intercellular movement of viruses. Interestingly, both in plant and animal systems, ECM can be strongly remodeled during virus infection, and the understanding of remodeling mechanisms and molecular players offers new perspectives for therapeutic intervention. This review focuses on the different roles played by the ECM in plant and animal hosts during virus infection with special emphasis on the similarities and differences. Possible biotechnological applications aimed at improving viral resistance are discussed.

HIV Envelope gp120 Alters T Cell Receptor Mobilization in the Immunological Synapse of Uninfected CD4 T Cells and Augments T Cell Activation

HIV is transmitted most efficiently from cell to cell, and productive infection occurs mainly in activated CD4 T cells. It is postulated that HIV exploits immunological synapses formed between CD4 T cells and antigen-presenting cells to facilitate the targeting and infection of activated CD4 T cells. This study sought to evaluate how the presence of the HIV envelope (Env) in the CD4 T cell immunological synapse affects synapse formation and intracellular signaling to impact the downstream T cell activation events. CD4 T cells were applied to supported lipid bilayers that were reconstituted with HIV Env gp120, anti-T cell receptor (anti-TCR) monoclonal antibody, and ICAM-1 to represent the surface of HIV Env-bearing antigen-presenting cells. The results showed that the HIV Env did not disrupt immunological synapse formation. Instead, the HIV Env accumulated with TCR at the center of the synapse, altered the kinetics of TCR recruitment to the synapse and affected synapse morphology over time. The HIV Env also prolonged Lck phosphorylation at the synapse and enhanced TCR-induced CD69 upregulation, interleukin-2 secretion, and proliferation to promote virus infection. These results suggest that HIV uses the immunological synapse as a conduit not only for selective virus transmission to activated CD4 T cells but also for boosting the T cell activation state, thereby increasing its likelihood of undergoing productive replication in targeted CD4 T cells.

IMPORTANCE

There are about two million new HIV infections every year. A better understanding of how HIV is transmitted to susceptible cells is critical to devise effective strategies to prevent HIV infection. Activated CD4 T cells are preferentially infected by HIV, although how this is accomplished is not fully understood. This study examined whether HIV co-opts the normal T cell activation process through the so-called immunological synapse. We found that the HIV envelope is recruited to the center of the immunological synapse together with the T cell receptor and enhances the T cell receptor-induced activation of CD4 T cells. Heightened cellular activation promotes the capacity of CD4 T cells to support productive HIV replication. This study provides evidence of the exploitation of the normal immunological synapse and T cell activation process by HIV to boost the activation state of targeted CD4 T cells and promote the infection of these cells.

Modeling Viral Spread

The way in which a viral infection spreads within a host is a complex process that is not well understood. Different viruses, such as human immunodeficiency virus type 1 and hepatitis C virus, have evolved different strategies, including direct cell-to-cell transmission and cell-free transmission, to spread within a host. To what extent these two modes of transmission are exploited in vivo is still unknown. Mathematical modeling has been an essential tool to get a better systematic and quantitative understanding of viral processes that are difficult to discern through strictly experimental approaches. In this review, we discuss recent attempts that combine experimental data and mathematical modeling in order to determine and quantify viral transmission modes. We also discuss the current challenges for a systems-level understanding of viral spread, and we highlight the promises and challenges that novel experimental techniques and data will bring to the field.

HIV-1 Gag, Envelope, and Extracellular Determinants Cooperate To Regulate the Stability and Turnover of Virological Synapses

Retroviruses spread more efficiently when infected and uninfected cells form tight, physical interfaces known as virological synapses (VSs). VS formation is initiated by adhesive interactions between viral Envelope (Env) glycoproteins on the infected cell and CD4 receptor molecules on the uninfected cell. How high-avidity Env-CD4 linkages are resolved over time is unknown. We describe here a tractable two-color, long-term (>24 h) live cell imaging strategy to study VS turnover in the context of a large cell population, quantitatively. We show that Env's conserved cytoplasmic tail (CT) can potently signal the recruitment of Gag capsid proteins to the VS, a process also dependent on residues within Gag's N-terminal matrix (MA) domain. Additionally, we demonstrate that Env's CT and Gag's MA domain both regulate the duration of interactions between viral donor and target cells, as well as the stability of this interaction over time (i.e., its capacity to resolve or form a syncytium). Finally, we report the unexpected finding that modulating extracellular fluid viscosity markedly impacts target T cell trafficking and thus affects the duration, stability, and turnover of virus-induced cell-cell contacts. Combined, these results suggest a stepwise model for viral cell-to-cell transmission wherein (i) Env-receptor interactions anchor target cells to infected cells, (ii) Env signals Gag's recruitment to the cell-cell contact dependent on an intact Env CT and Gag MA, and (iii) Env CT and Gag MA, in conjunction with extracellular forces, combine to regulate VS stability and infectious outcomes.

IMPORTANCE

HIV-1 spreads efficiently at physical, cell-cell interfaces known as virological synapses (VSs). The VS provides for spatiotemporal coupling of virus assembly and entry into new host cells and may transmit signals relevant to pathogenesis. Disrupting this mode of transmission may be critical to the goal of abolishing viral persistence in infected individuals. We describe here a long-term live cell imaging strategy for studying virus-induced effects on cell behavior in the context of a large cell population. We demonstrate cooperative roles for viral Gag capsid proteins and Envelope glycoproteins in regulating VS formation and turnover. We also show that modulating fluid viscosity markedly affects T cell trafficking and VS stability. Thus, extracellular factors also play an important role in modulating the nature of infectious cell-cell interactions. In sum, our study provides new tools and insights relevant to exposing vulnerabilities in how HIV-1 and other viruses spread infection among cells, tissues, and people.

In Vivo HIV-1 Cell-to-Cell Transmission Promotes Multicopy Micro-compartmentalized Infection

HIV-1 infection is enhanced by adhesive structures that form between infected and uninfected T cells called virological synapses (VSs). This mode of transmission results in the frequent co-transmission of multiple copies of HIV-1 across the VS, which can reduce sensitivity to antiretroviral drugs. Studying HIV-1 infection of humanized mice, we measured the frequency of co-transmission and the spatiotemporal organization of infected cells as indicators of cell-to-cell transmission in vivo. When inoculating mice with cells co-infected with two viral genotypes, we observed high levels of co-transmission to target cells. Additionally, micro-anatomical clustering of viral genotypes within lymphoid tissue indicates that viral spread is driven by local processes and not a diffuse viral cloud. Intravital splenic imaging reveals that anchored HIV-infected cells induce arrest of interacting, uninfected CD4(+) T cells to form Env-dependent cell-cell conjugates. These findings suggest that HIV-1 spread between immune cells can be anatomically localized into infectious clusters.

Viruses exploit the tissue physiology of the host to spread in vivo

Viruses are pathogens that strictly depend on their host for propagation. Over years of co-evolution viruses have become experts in exploiting the host cell biology and physiology to ensure efficient replication and spread. Here, we will first summarize the concepts that have emerged from in vitro cell culture studies to understand virus spread. We will then review the results from studies in living animals that reveal how viruses exploit the natural flow of body fluids, specific tissue architecture, and patterns of cell circulation and migration to spread within the host. Understanding tissue physiology will be critical for the design of antiviral strategies that prevent virus dissemination.

Cell-to-Cell Spread of HIV and Viral Pathogenesis

Human immunodeficiency virus type 1 (HIV-1) gives rise to a chronic infection that progressively depletes CD4(+) T lymphocytes. CD4(+) T lymphocytes play a central coordinating role in adaptive cellular and humoral immune responses, and to do so they migrate and interact within lymphoid compartments and at effector sites to mount immune responses. While cell-free virus serves as an excellent prognostic indicator for patient survival, interactions of infected T cells or virus-scavenging immune cells with uninfected T cells can greatly enhance viral spread. HIV can induce interactions between infected and uninfected T cells that are triggered by cell surface expression of viral Env, which serves as a cell adhesion molecule that interacts with CD4 on the target cell, before it acts as the viral membrane fusion protein. These interactions are called virological synapses and promote replication in the face of selective pressure of humoral immune responses and antiretroviral therapy. Other infection-enhancing cell-cell interactions occur between virus-concentrating antigen-presenting cells and recipient T cells, called infectious synapses. The exact roles that these cell-cell interactions play in each stage of infection, from viral acquisition, systemic dissemination, to chronic persistence are still being determined. Infection-promoting immune cell interactions are likely to contribute to viral persistence and enhance the ability of HIV-1 to evade adaptive immune responses.

HIV cell-to-cell transmission: effects on pathogenesis and antiretroviral therapy

HIV spreads more efficiently in vitro when infected cells directly contact uninfected cells to form virological synapses. A hallmark of virological synapses is that viruses can be transmitted at a higher multiplicity of infection (MOI) that, in vitro, results in a higher number of proviruses. Whether HIV also spreads by cell-cell contact in vivo is a matter of debate. Here we discuss recent data that suggest that contact-mediated transmission largely manifests itself in vivo as CD4+ T cell depletion. The assault of a cell by a large number of incoming particles is likely to be efficiently sensed by the innate cellular surveillance to trigger cell death. The large number of particles transferred across virological synapses has also been implicated in reduced efficacy of antiretroviral therapies. Thus, antiretroviral therapies must remain effective against the high MOI observed during cell-to-cell transmission to inhibit both viral replication and the pathogenesis associated with HIV infection.

DC-T cell virological synapses and the skin: novel perspectives in dermatology

Virological synapses (VS) increase cell-to-cell viral transmission and facilitate propagation of human immunodeficiency virus type 1 (HIV-1) and human T-cell leukaemia virus type 1 (HTLV-1). VS formation also plays a more general role in viral replication and dissemination. VS have been observed in vitro and ex vivo between uninfected T cells and T cells infected with HIV-1 or HTLV-1. In addition, dendritic cells (DC) infected with HIV-1 also play an important role in viral transmission to uninfected CD4+ T cells via VS formation. Recent studies revealed that several DC subsets are also infected with HTLV-1. These findings may help explain the rapid dissemination of both viruses within secondary lymphoid tissues in vivo. VS also explain, at least in part, why HIV-1 can propagate in the mucosal sites during sexual transmission. Furthermore, in the case of HTLV-1, VS can potentially explain some of the features of HTLV-1-associated dermatitis as infected T cells in the skin contribute to the pathogenesis of this condition.

Cell-to-cell transfer of HIV infection: implications for HIV viral persistence

A major research priority for HIV eradication is the elucidation of the events involved in HIV reservoir establishment and persistence. Cell-to-cell transmission of HIV represents an important area of study as it allows for the infection of cell types which are not easily infected by HIV, leading to the establishment of long-lived viral reservoirs. This phenomenon enables HIV to escape elimination by the immune system. This process may also enable HIV to escape suppressive effects of anti-retroviral drugs. During cell-to-cell transmission of HIV, a dynamic series of events ensues at the virological synapse that promotes viral dissemination. Cell-to-cell transmission involves various types of cells of the immune system and this mode of transmission has been shown to have an important role in sexual and mother-to-child transmission of HIV and spread of HIV within the central nervous system and gut-associated lymphoid tissues. There is also evidence that cell-to-cell transmission of HIV occurs between thymocytes and renal tubular cells. Herein, following a brief review of the processes involved at the virological synapse, evidence supporting the role for cell-to-cell transmission of HIV in the maintenance of the HIV reservoir will be highlighted. Therapeutic considerations and future directions for this area of research will also be discussed.

Viral exploitation of actin: force-generation and scaffolding functions in viral infection

As a fundamental component of the host cellular cytoskeleton, actin is routinely engaged by infecting viruses. Furthermore, viruses from diverse groups, and infecting diverse hosts, have convergently evolved an array of mechanisms for manipulating the actin cytoskeleton for efficacious infection. An ongoing chorus of research now indicates that the actin cytoskeleton is critical for viral replication at many stages of the viral life cycle, including binding, entry, nuclear localization, genomic transcription and reverse transcription, assembly, and egress/dissemination. Specifically, viruses subvert the force-generating and macromolecular scaffolding properties of the actin cytoskeleton to propel viral surfing, internalization, and migration within the cell. Additionally, viruses utilize the actin cytoskeleton to support and organize assembly sites, and eject budding virions for cell-to-cell transmission. It is the purpose of this review to provide an overview of current research, focusing on the various mechanisms and themes of virus-mediated actin modulation described therein.

CCR5/CD4/CXCR4 oligomerization prevents HIV-1 gp120IIIB binding to the cell surface

CCR5 and CXCR4, the respective cell surface coreceptors of R5 and X4 HIV-1 strains, both form heterodimers with CD4, the principal HIV-1 receptor. Using several resonance energy transfer techniques, we determined that CD4, CXCR4, and CCR5 formed heterotrimers, and that CCR5 coexpression altered the conformation of both CXCR4/CXCR4 homodimers and CD4/CXCR4 heterodimers. As a result, binding of the HIV-1 envelope protein gp120IIIB to the CD4/CXCR4/CCR5 heterooligomer was negligible, and the gp120-induced cytoskeletal rearrangements necessary for HIV-1 entry were prevented. CCR5 reduced HIV-1 envelope-induced CD4/CXCR4-mediated cell-cell fusion. In nucleofected Jurkat CD4 cells and primary human CD4(+) T cells, CCR5 expression led to a reduction in X4 HIV-1 infectivity. These findings can help to understand why X4 HIV-1 strains infection affect T-cell types differently during AIDS development and indicate that receptor oligomerization might be a target for previously unidentified therapeutic approaches for AIDS intervention.

The immunological synapse: the gateway to the HIV reservoir

A major challenge in the development of a cure for human immunodeficiency virus (HIV) has been the incomplete understanding of the basic mechanisms underlying HIV persistence during antiretroviral therapy. It is now realized that the establishment of a latently infected reservoir refractory to immune system recognition has thus far hindered eradication efforts. Recent investigation into the innate immune response has shed light on signaling pathways downstream of the immunological synapse critical for T-cell activation and establishment of T-cell memory. This has led to the understanding that the cell-to-cell contacts observed in an immunological synapse that involve the CD4⁺ T cell and antigen-presenting cell or T-cell–T-cell interactions enhance efficient viral spread and facilitate the induction and maintenance of latency in HIV-infected memory T cells. This review focuses on recent work characterizing the immunological synapse and the signaling pathways involved in T-cell activation and gene regulation in the context of HIV persistence.

The microvesicle component of HIV-1 inocula modulates dendritic cell infection and maturation and enhances adhesion to and activation of T lymphocytes

HIV-1 is taken up by immature monocyte derived dendritic cells (iMDDCs) into tetraspanin rich caves from which the virus can either be transferred to T lymphocytes or enter into endosomes resulting in degradation. HIV-1 binding and fusion with the DC membrane results in low level de novo infection that can also be transferred to T lymphocytes at a later stage. We have previously reported that HIV-1 can induce partial maturation of iMDDCs at both stages of trafficking. Here we show that CD45⁺ microvesicles (MV) which contaminate purified HIV-1 inocula due to similar size and density, affect DC maturation, de novo HIV-1 infection and transfer to T lymphocytes. Comparing iMDDCs infected with CD45-depleted HIV-1BaL or matched non-depleted preparations, the presence of CD45⁺ MVs was shown to enhance DC maturation and ICAM-1 (CD54) expression, which is involved in DC∶T lymphocyte interactions, while restricting HIV-1 infection of MDDCs. Furthermore, in the DC culture HIV-1 infected (p24⁺) MDDCs were more mature than bystander cells. Depletion of MVs from the HIV-1 inoculum markedly inhibited DC∶T lymphocyte clustering and the induction of alloproliferation as well as limiting HIV-1 transfer from DCs to T lymphocytes. The effects of MV depletion on these functions were reversed by the re-addition of purified MVs from activated but not non-activated SUPT1.CCR5-CL.30 or primary T cells. Analysis of the protein complement of these MVs and of these HIV-1 inocula before and after MV depletion showed that Heat Shock Proteins (HSPs) and nef were the likely DC maturation candidates. Recombinant HSP90α and β and nef all induced DC maturation and ICAM-1 expression, greater when combined. These results suggest that MVs contaminating HIV-1 released from infected T lymphocytes may be biologically important, especially in enhancing T cell activation, during uptake by DCs in vitro and in vivo, particularly as MVs have been detected in the circulation of HIV-1 infected subjects.

Mechanisms of enhanced HIV spread through T-cell virological synapses

An elaborate network of cell-cell interactions in the immune system is essential for vertebrates to mount adaptive immune responses against invading pathogens. For lymphotropic viruses such as the human immunodeficiency virus type 1 (HIV-1), these immune cell interactions can also promote the spread of the virus within the host. The main target of HIV-1 infection is the CD4(+) helper T lymphocyte, a cell type that is responsible for coordinating immune responses and modulating effector responses to foreign antigens. As part of their normal immune surveillance duties, these cells migrate actively within lymphoid tissues and can travel from inductive sites to effector sites in search of their cognate antigen. For CD4(+) T cells, there is an ongoing search for a unique peptide antigen presented in the context of class II MHC that can activate a proliferative or tolerogenic response. This iterative and continual probing and interrogation of other cells determine the outcome of immune responses. Recent studies in vitro have revealed that the viral infection program induces cell-cell interactions called virological synapses between infected and uninfected CD4(+) T cells. These long-lived, virally induced adhesive contacts greatly enhance the rate of productive infection and may be central to the spread of the virus in vivo. Here, we review aspects of this efficient mode of cell-to-cell infection and the implications for our understanding of HIV-1 pathogenesis.

The dominant roles of ICAM-1-encoding gene in DNA vaccination against Japanese encephalitis virus are the activation of dendritic cells and enhancement of cellular immunity

We investigated the cellular immune responses elicited by a plasmid DNA vaccine encoding prM-E protein from the Japanese encephalitis (JE) virus (JEV) with or without various forms of intercellular adhesion molecule (ICAM)-1 gene to maximize the immune responses evoked by the JE DNA vaccine. We observed that co-immunization with the construct containing murine ICAM-1 gene (pICAM-1) resulted in a significant increase in the percentage of CD4(+)T cells, high level of JEV-specific cytotoxic T lymphocyte response, and high production of T helper 1 (Th1)-type cytokines in splenic T cells. Furthermore, the co-expression of ICAM-1 and DNA immunogens was found to be more effective in generating T cell-mediated immune responses than those induced by immunization with pJME in combination with pICAM-1. Our results suggested that ICAM-1 enhanced T cell receptor signaling and activated Th1 immune responses in the JEV model system by increasing the induction of CD4(+)Th1 cell subset and activating dendritic cells.

HIV-1 Virological Synapse is not Simply a Copycat of the Immunological Synapse

The virological synapse (VS) is a tight adhesive junction between an HIV-infected cell and an uninfected target cell, across which virus can be efficiently transferred from cell to cell in the absence of cell-cell fusion. The VS has been postulated to resemble, in its morphology, the well-studied immunological synapse (IS). This review article discusses the structural similarities between IS and VS and the shared T cell receptor (TCR) signaling components that are found in the VS. However, the IS and the VS display distinct kinetics in disassembly and intracellular signaling events, possibly leading to different biological outcomes. Hence, HIV-1 exploits molecular components of IS and TCR signaling machinery to trigger unique changes in cellular morphology, migration, and activation that facilitate its transmission and cell-to-cell spread.

HIV: cell binding and entry

The first step of the human immunodeficiency virus (HIV) replication cycle-binding and entry into the host cell-plays a major role in determining viral tropism and the ability of HIV to degrade the human immune system. HIV uses a complex series of steps to deliver its genome into the host cell cytoplasm while simultaneously evading the host immune response. To infect cells, the HIV protein envelope (Env) binds to the primary cellular receptor CD4 and then to a cellular coreceptor. This sequential binding triggers fusion of the viral and host cell membranes, initiating infection. Revealing the mechanism of HIV entry has profound implications for viral tropism, transmission, pathogenesis, and therapeutic intervention. Here, we provide an overview into the mechanism of HIV entry, provide historical context to key discoveries, discuss recent advances, and speculate on future directions in the field.

Dynamic Association between HIV-1 Gag and Membrane Domains

HIV-1 particle assembly is driven by the structural protein Gag. Gag binds to and multimerizes on the inner leaflet of the plasma membrane, eventually resulting in formation of spherical particles. During virus spread among T cells, Gag accumulates to the plasma membrane domain that, together with target cell membrane, forms a cell junction known as the virological synapse. While Gag association with plasma membrane microdomains has been implicated in virus assembly and cell-to-cell transmission, recent studies suggest that, rather than merely accumulating to pre-existing microdomains, Gag plays an active role in reorganizing the microdomains via its multimerization activity. In this paper, we will discuss this emerging view of Gag microdomain interactions. Relationships between Gag multimerization and microdomain association will be further discussed in the context of Gag localization to T-cell uropods and virological synapses.

Different pattern of immunoglobulin gene usage by HIV-1 compared to non-HIV-1 antibodies derived from the same infected subject

A biased usage of immunoglobulin (Ig) genes is observed in human anti-HIV-1 monoclonal antibodies (mAbs) resulting probably from compensation to reduced usage of the VH3 family genes, while the other alternative suggests that this bias usage is due to antigen requirements. If the antigen structure is responsible for the preferential usage of particular Ig genes, it may have certain implications for HIV vaccine development by the targeting of particular Ig gene-encoded B cell receptors to induce neutralizing anti-HIV-1 antibodies. To address this issue, we have produced HIV-1 specific and non-HIV-1 mAbs from an infected individual and analyzed the Ig gene usage. Green-fluorescence labeled virus-like particles (VLP) expressing HIV-1 envelope (Env) proteins of JRFL and BaL and control VLPs (without Env) were used to select single B cells for the production of 68 recombinant mAbs. Ten of these mAbs were HIV-1 Env specific with neutralizing activity against V3 and the CD4 binding site, as well as non-neutralizing mAbs to gp41. The remaining 58 mAbs were non-HIV-1 Env mAbs with undefined specificities. Analysis revealed that biased usage of Ig genes was restricted only to anti-HIV-1 but not to non-HIV-1 mAbs. The VH1 family genes were dominantly used, followed by VH3, VH4, and VH5 among anti-HIV-1 mAbs, while non-HIV-1 specific mAbs preferentially used VH3 family genes, followed by VH4, VH1 and VH5 families in a pattern identical to Abs derived from healthy individuals. This observation suggests that the biased usage of Ig genes by anti-HIV-1 mAbs is driven by structural requirements of the virus antigens rather than by compensation to any depletion of VH3 B cells due to autoreactive mechanisms, according to the gp120 superantigen hypothesis.

Cannabinoid receptor 2-mediated attenuation of CXCR4-tropic HIV infection in primary CD4+ T cells

Agents that activate cannabinoid receptor pathways have been tested as treatments for cachexia, nausea or neuropathic pain in HIV-1/AIDS patients. The cannabinoid receptors (CB(1)R and CB(2)R) and the HIV-1 co-receptors, CCR5 and CXCR4, all signal via Gαi-coupled pathways. We hypothesized that drugs targeting cannabinoid receptors modulate chemokine co-receptor function and regulate HIV-1 infectivity. We found that agonism of CB(2)R, but not CB(1)R, reduced infection in primary CD4+ T cells following cell-free and cell-to-cell transmission of CXCR4-tropic virus. As this change in viral permissiveness was most pronounced in unstimulated T cells, we investigated the effect of CB(2)R agonism on to CXCR4-induced signaling following binding of chemokine or virus to the co-receptor. We found that CB(2)R agonism decreased CXCR4-activation mediated G-protein activity and MAPK phosphorylation. Furthermore, CB(2)R agonism altered the cytoskeletal architecture of resting CD4+ T cells by decreasing F-actin levels. Our findings suggest that CB(2)R activation in CD4+ T cells can inhibit actin reorganization and impair productive infection following cell-free or cell-associated viral acquisition of CXCR4-tropic HIV-1 in resting cells. Therefore, the clinical use of CB(2)R agonists in the treatment of AIDS symptoms may also exert beneficial adjunctive antiviral effects against CXCR4-tropic viruses in late stages of HIV-1 infection.

The direct passage of animal viruses between cells

The paradigm that viruses can move directly, and in some cases covertly, between contacting target cells is now well established for several virus families. The underlying mechanisms of cell-to-cell spread, however, remain to be fully elucidated and may differ substantially depending on the viral exit/entry route and the cellular tropism. Here, two divergent cell-to-cell spread mechanisms are exemplified: firstly by human retroviruses, which rely upon transient adhesive structures that form between polarized immune cells termed virological synapses, and secondly by herpesviruses that depend predominantly on pre-existing stable cellular contacts, but may also form virological synapses. Plant viruses can also spread directly between contacting cells, but are obliged by the rigid host cell wall to move across pore structures termed plasmodesmata. This review will focus primarily on recent advances in our understanding of animal virus cell-to-cell spread using examples from these two virus families, and will conclude by comparing and contrasting the cell-to-cell spread of animal and plant viruses.

How HIV-1 takes advantage of the cytoskeleton during replication and cell-to-cell transmission

Human immunodeficiency virus 1 (HIV-1) infects T cells, macrophages and dendritic cells and can manipulate their cytoskeleton structures at multiple steps during its replication cycle. Based on pharmacological and genetic targeting of cytoskeleton modulators, new imaging approaches and primary cell culture models, important roles for actin and microtubules during entry and cell-to-cell transfer have been established. Virological synapses and actin-containing membrane extensions can mediate HIV-1 transfer from dendritic cells or macrophage cells to T cells and between T cells. We will review the role of the cytoskeleton in HIV-1 entry, cellular trafficking and cell-to-cell transfer between primary cells.

The regulated secretory pathway in CD4(+) T cells contributes to human immunodeficiency virus type-1 cell-to-cell spread at the virological synapse

Direct cell-cell spread of Human Immunodeficiency Virus type-1 (HIV-1) at the virological synapse (VS) is an efficient mode of dissemination between CD4(+) T cells but the mechanisms by which HIV-1 proteins are directed towards intercellular contacts is unclear. We have used confocal microscopy and electron tomography coupled with functional virology and cell biology of primary CD4(+) T cells from normal individuals and patients with Chediak-Higashi Syndrome and report that the HIV-1 VS displays a regulated secretion phenotype that shares features with polarized secretion at the T cell immunological synapse (IS). Cell-cell contact at the VS re-orientates the microtubule organizing center (MTOC) and organelles within the HIV-1-infected T cell towards the engaged target T cell, concomitant with polarization of viral proteins. Directed secretion of proteins at the T cell IS requires specialized organelles termed secretory lysosomes (SL) and we show that the HIV-1 envelope glycoprotein (Env) localizes with CTLA-4 and FasL in SL-related compartments and at the VS. Finally, CD4(+) T cells that are disabled for regulated secretion are less able to support productive cell-to-cell HIV-1 spread. We propose that HIV-1 hijacks the regulated secretory pathway of CD4(+) T cells to enhance its dissemination.

HIV envelope gp120 activates LFA-1 on CD4 T-lymphocytes and increases cell susceptibility to LFA-1-targeting leukotoxin (LtxA)

The cellular adhesion molecule LFA-1 and its ICAM-1 ligand play an important role in promoting HIV-1 infectivity and transmission. These molecules are present on the envelope of HIV-1 virions and are integral components of the HIV virological synapse. However, cellular activation is required to convert LFA-1 to the active conformation that has high affinity binding for ICAM-1. This study evaluates whether such activation can be induced by HIV itself. The data show that HIV-1 gp120 was sufficient to trigger LFA-1 activation in fully quiescent naïve CD4 T cells in a CD4-dependent manner, and these CD4 T cells became more susceptible to killing by LtxA, a bacterial leukotoxin that preferentially targets leukocytes expressing high levels of the active LFA-1. Moreover, virus p24-expressing CD4 T cells in the peripheral blood of HIV-infected subjects were found to have higher levels of surface LFA-1, and LtxA treatment led to significant reduction of the viral DNA burden. These results demonstrate for the first time the ability of HIV to directly induce LFA-1 activation on CD4 T cells. Although LFA-1 activation may enhance HIV infectivity and transmission, it also renders the cells more susceptible to an LFA-1-targeting bacterial toxin, which may be harnessed as a novel therapeutic strategy to deplete virus reservoir in HIV-infected individuals.

HIV-1 envelope, integrins and co-receptor use in mucosal transmission of HIV

It is well established that HIV-1 infection typically involves an interaction between the viral envelope protein gp120/41 and the CD4 molecule followed by a second interaction with a chemokine receptor, usually CCR5 or CXCR4. In the early stages of an HIV-1 infection CCR5 using viruses (R5 viruses) predominate. In some viral subtypes there is a propensity to switch to CXCR4 usage (X4 viruses). The receptor switch occurs in ~ 40% of the infected individuals and is associated with faster disease progression. This holds for subtypes B and D, but occurs less frequently in subtypes A and C. There are several hypotheses to explain the preferential transmission of R5 viruses and the mechanisms that lead to switching of co-receptor usage; however, there is no definitive explanation for either. One important consideration regarding transmission is that signaling by R5 gp120 may facilitate transmission of R5 viruses by inducing a permissive environment for HIV replication. In the case of sexual transmission, infection by HIV requires the virus to breach the mucosal barrier to gain access to the immune cell targets that it infects; however, the immediate events that follow HIV exposure at genital mucosal sites are not well understood. Upon transmission, the HIV quasispecies that is replicating in an infected donor contracts through a “genetic bottleneck”, and often infection results from a single infectious event. Many details surrounding this initial infection remain unresolved. In mucosal tissues, CD4⁺ T cells express high levels of CCR5, and a subset of these CD4⁺/CCR5^high cells express the integrin α₄β₇, the gut homing receptor. CD4⁺/CCR5^high/ α4β7^high T cells are highly susceptible to infection by HIV-1 and are ideal targets for an efficient productive infection at the point of transmission. In this context we have demonstrated that the HIV-1 envelope protein gp120 binds to α₄β₇ on CD4⁺ T cells. On CD4⁺/CCR5^high/ α4β7^high T cells, α₄β₇ is closely associated with CD4 and CCR5. Furthermore, α₄β₇ is ~3 times the size of CD4 on the cell surface, that makes it a prominent receptor for an efficient virus capture. gp120-α₄β₇ interactions mediate the activation of the adhesion-associated integrin LFA-1. LFA-1 facilitates the formation of virological synapses and cell-to-cell spread of HIV-1. gp120 binding to α₄β₇ is mediated by a tripeptide located in the V1/V2 domain of gp120. Of note, the V1/V2 domain of gp120 has been linked to variations in transmission fitness among viral isolates raising the intriguing possibility that gp120-α₄β₇ interactions may be linked to transmission fitness. Although many details remain unresolved, we hypothesize that gp120-α₄β₇ interactions play an important role in the very early events following sexual transmission of HIV and may have important implication in the design of vaccine strategies for the prevention of acquisition of HIV infection

Nucleocapsid promotes localization of HIV-1 gag to uropods that participate in virological synapses between T cells

T cells adopt a polarized morphology in lymphoid organs, where cell-to-cell transmission of HIV-1 is likely frequent. However, despite the importance of understanding virus spread in vivo, little is known about the HIV-1 life cycle, particularly its late phase, in polarized T cells. Polarized T cells form two ends, the leading edge at the front and a protrusion called a uropod at the rear. Using multiple uropod markers, we observed that HIV-1 Gag localizes to the uropod in polarized T cells. Infected T cells formed contacts with uninfected target T cells preferentially via HIV-1 Gag-containing uropods compared to leading edges that lack plasma-membrane-associated Gag. Cell contacts enriched in Gag and CD4, which define the virological synapse (VS), are also enriched in uropod markers. These results indicate that Gag-laden uropods participate in the formation and/or structure of the VS, which likely plays a key role in cell-to-cell transmission of HIV-1. Consistent with this notion, a myosin light chain kinase inhibitor, which disrupts uropods, reduced virus particle transfer from infected T cells to target T cells. Mechanistically, we observed that Gag copatches with antibody-crosslinked uropod markers even in non-polarized cells, suggesting an association of Gag with uropod-specific microdomains that carry Gag to uropods. Finally, we determined that localization of Gag to the uropod depends on higher-order clustering driven by its NC domain. Taken together, these results support a model in which NC-dependent Gag accumulation to uropods establishes a preformed platform that later constitutes T-cell-T-cell contacts at which HIV-1 virus transfer occurs.

CD4⁺ T cells are natural targets of HIV-1. Efficient spread of HIV-1 from infected T cells to uninfected T cells is thought to occur via cell-cell contact structures. One of these structures is a virological synapse where both viral and cellular proteins have been shown to localize specifically. However, the steps leading to the formation of a virological synapse remain unknown. It has been observed that T cells adopt a polarized morphology in lymph nodes where cell-to-cell virus transmission is likely to occur frequently. In this study, we show that in polarized T cells, the primary viral structural protein Gag accumulates to the plasma membrane of a rear end structure called a uropod. We found that Gag multimerization, driven by its nucleocapsid domain, is essential for Gag localization to uropods and that HIV-1-laden uropods mediate contact with target cells and can become part of the virological synapse. Our findings elucidated a series of molecular events leading to formation of HIV-1-transferring cell contacts and support a model in which the uropod acts as a preformed platform that constitutes a virological synapse after cell-cell contact.

CXCR4 Tropic HIV-1 gp120 Inhibition of SDF-1α-Induced Chemotaxis Requires Lck and is Associated with Cofilin Phosphorylation

Objective:

HIV gp120 is a pleiotropic protein present in the plasma and tissues of HIV-infected patients, which affects a variety of homeostatic functions. In this report, we examine the mechanism of how gp120 blocks CD4 T cells from migrating towards SDF-1α.

Methods:

In vitro treatment of primary CD4 T cells with CXCR4 tropic gp120, SDF, and measurement of chemotaxis and cell signaling.

Results:

gp120 signaling through CD4 receptor and Lck are required for its ability to inhibit chemotaxis induced by SDF, as demonstrated by CD4 receptor decoys, Lck inhibitors, as well as cells deficient in Lck, in which Lck expression is restored. Blocking Lck abrogates the ability of CXCR4 tropic gp120 to antagonize SDF-1α-induced chemotaxis. This inhibition is associated with cofilin phosphorylation, thereby providing a potential mechanism.

Conclusion:

We conclude that the ability of gp120 to inhibit SDF-1α-induced chemotaxis is mediated by the CD4 receptor and Lck signaling, potentially by promoting cofilin phosphorylation.

Functional anatomy of T cell activation and synapse formation

T cell activation and function require a structured engagement of antigen-presenting cells. These cell contacts are characterized by two distinct dynamics in vivo: transient contacts resulting from promigratory junctions called immunological kinapses or prolonged contacts from stable junctions called immunological synapses. Kinapses operate in the steady state to allow referencing to self-peptide-MHC (pMHC) and searching for pathogen-derived pMHC. Synapses are induced by T cell receptor (TCR) interactions with agonist pMHC under specific conditions and correlate with robust immune responses that generate effector and memory T cells. High-resolution imaging has revealed that the synapse is highly coordinated, integrating cell adhesion, TCR recognition of pMHC complexes, and an array of activating and inhibitory ligands to promote or prevent T cell signaling. In this review, we examine the molecular components, geometry, and timing underlying kinapses and synapses. We integrate recent molecular and physiological data to provide a synthesis and suggest ways forward.

The roles of tetraspanins in HIV-1 replication

Tetraspanins are small integral membrane proteins that are known to control a variety of cellular processes, including signaling, migration and cell-cell fusion. Research over the past few years established that they are also regulators of various steps in the HIV-1 replication cycle, but the mechanisms through which these proteins either enhance or repress virus spread remain largely unknown.

Virus cell-to-cell transmission

Viral infections spread based on the ability of viruses to overcome multiple barriers and move from cell to cell, tissue to tissue, and person to person and even across species. While there are fundamental differences between these types of transmissions, it has emerged that the ability of viruses to utilize and manipulate cell-cell contact contributes to the success of viral infections. Central to the excitement in the field of virus cell-to-cell transmission is the idea that cell-to-cell spread is more than the sum of the processes of virus release and entry. This implies that virus release and entry are efficiently coordinated to sites of cell-cell contact, resulting in a process that is distinct from its individual components. In this review, we will present support for this model, illustrate the ability of viruses to utilize and manipulate cell adhesion molecules, and discuss the mechanism and driving forces of directional spreading. An understanding of viral cell-to-cell spreading will enhance our ability to intervene in the efficient spreading of viral infections.

Human immunodeficiency virus type 1 envelope gp120-induced partial T-cell receptor signaling creates an F-actin-depleted zone in the virological synapse

Cell-to-cell transmission of human immunodeficiency virus type 1 (HIV-1) occurs via a virological synapse (VS), a tight cell-cell junction formed between HIV-infected cells and target cells in which the HIV-1-infected cell polarizes and releases virions toward the noninfected target cell in a gp120- and intercellular adhesion molecule 1 (ICAM-1)-dependent process. The response of the target cell has been less studied. We utilized supported planar bilayers presenting gp120 and ICAM-1 as a reductionist model for the infected-cell membrane and investigated its effect on the target CD4 T cell. This study shows that HIV-1 gp120 interaction with its receptors is initially organized into microclusters that undergo F-actin-dependent consolidation into a central supramolecular activation complex (cSMAC). Src kinases are active in both gp120 microclusters and in the VS cSMAC. The early T-cell receptor (TCR) signaling machinery is partially activated at the VS, and signaling does not propagate to trigger Ca(2+) elevation or increase CD69 expression. However, these partial TCR signals act locally to create an F-actin-depleted zone. We propose a model in which the F-actin-depleted zone formed within the target CD4 T cell enhances the reception of virions by releasing the physical barrier for HIV-1 entry and facilitating postentry events.

Simultaneous cell-to-cell transmission of human immunodeficiency virus to multiple targets through polysynapses

Human immunodeficiency virus type 1 (HIV-1) efficiently propagates through cell-to-cell contacts, which include virological synapses (VS), filopodia, and nanotubes. Here, we quantified and characterized further these diverse modes of contact in lymphocytes. We report that viral transmission mainly occurs across VS and through "polysynapses," a rosette-like structure formed between one infected cell and multiple adjacent recipients. Polysynapses are characterized by simultaneous HIV clustering and transfer at multiple membrane regions. HIV Gag proteins often adopt a ring-like supramolecular organization at sites of intercellular contacts and colocalize with CD63 tetraspanin and raft components GM1, Thy-1, and CD59. In donor cells engaged in polysynapses, there is no preferential accumulation of Gag proteins at contact sites facing the microtubule organizing center. The LFA-1 adhesion molecule, known to facilitate viral replication, enhances formation of polysynapses. Altogether, our results reveal an underestimated mode of viral transfer through polysynapses. In HIV-infected individuals, these structures, by promoting concomitant infection of multiple targets in the vicinity of infected cells, may facilitate exponential viral growth and escape from immune responses.

T cell antigen receptor signaling and immunological synapse stability require myosin IIA

Immunological synapses are initiated by signaling in discrete T cell antigen receptor microclusters and are important for the differentiation and effector functions of T cells. Synapse formation involves the orchestrated movement of microclusters toward the center of the contact area with the antigen-presenting cell. Microcluster movement is associated with centripetal actin flow, but the function of motor proteins is unknown. Here we show that myosin IIA was necessary for complete assembly and movement of T cell antigen receptor microclusters. In the absence of myosin IIA or its ATPase activity, T cell signaling was interrupted 'downstream' of the kinase Lck and the synapse was destabilized. Thus, T cell antigen receptor signaling and the subsequent formation of immunological synapses are active processes dependent on myosin IIA.

Genetic determinants of HIV-1 infection and progression to AIDS: susceptibility to HIV infection

Interindividual variability in susceptibility to HIV-1 infection, its transmission, disease progression, and response to antiviral therapy has been attributed to host determinants and variability in multiple genes. Although most people exposed to the virus go on to develop full-blown disease at variable intervals, a proportion of them, labeled as long-term nonprogressors or exposed uninfected, possess 'natural resistance' to infection. A better understanding of genetic and immunologic basis of such a natural resistance to infection would bear important implications in designing therapeutic vaccine designs. The genetic variants that could influence susceptibility to HIV-1 and limit AIDS vary in different populations and among individuals. Meta-analyses of large cohort studies have identified numerous 'AIDS restriction genes' that regulate HIV cell entry (particularly chemokine coreceptors and their ligands), acquired and innate immunity (major histocompatibility complex, killer cell immunoglobulin-like receptor, and cytokines), and others [tripartite interaction motif 5 alpha (TRIM5alpha) and apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3G] that influence outcome of HIV infection. Studies carried out in the Indian population with regard to genetic polymorphisms in chemokine receptors have shown that (i) the protective CCR5 Delta32 variant is rare, (ii) CCR5HHE carrying *59402A is associated with increased likelihood of infection and development of AIDS, and (iii) the Indian population generally has low CCL3L1 copy numbers (approximately 2.3). These data have implications in developing screening tests that could identify people at higher or lower risk of infection and rate of disease progression, predict vaccine responsiveness in clinical trials and understand the pathogenic mechanisms.

Intercellular adhesion molecule 1 (ICAM-1), but not ICAM-2 and -3, is important for dendritic cell-mediated human immunodeficiency virus type 1 transmission

Dendritic cells (DCs) play a critical role in cell-to-cell-mediated transmission of human immunodeficiency virus type 1 (HIV-1). Interactions between intercellular adhesion molecules (ICAMs) and their ligands facilitate DC-T-cell contact. The interaction between ICAM-1 on DCs and leukocyte function-associated molecule 1 (LFA-1) on CD4(+) T cells has been proposed to be important for DC-mediated HIV-1 transmission. Given that DCs and T cells express multiple ICAMs and binding ligands, the relative importance of ICAMs in DC-mediated HIV-1 transmission remains to be defined. Here, we examine the role of ICAM-1, -2, and -3 in DC-mediated HIV-1 transmission to various types of target cells including primary CD4(+) T cells. The expression levels of ICAMs and their ligands on immature and mature DCs and various types of HIV-1 target cells were measured by flow cytometry. Blocking ICAM-1 in DCs with specific monoclonal antibodies and small interfering RNA impaired DC-mediated HIV-1 transmission. DC-mediated viral transmission was significantly inhibited when both ICAM-1 on DCs and LFA-1 on CD4(+) T cells were blocked. However, blockade of ICAM-1 on target cells did not significantly inhibit DC-mediated HIV-1 transmission. Ectopic expression and antibody blocking suggest that DC-mediated HIV-1 transmission to primary CD4(+) T cells is independent of ICAM-2 and ICAM-3. Taken together, our data clarified the role of ICAMs in DC-mediated HIV-1 transmission to CD4(+) T cells.