HIV-1 cell to cell transfer across an Env-induced, actin-dependent synapse

Epidemiology, pathogenesis, immune evasion mechanism and vaccine development of porcine Deltacoronavirus

Coronaviruses have been identified as pathogens of gastrointestinal and respiratory diseases in humans and various animal species. In recent years, the global spread of new coronaviruses has had profound influences for global public health and economies worldwide. As highly pathogenic zoonotic viruses, coronaviruses have become the focus of current research. Porcine Deltacoronavirus (PDCoV), an enterovirus belonging to the family of coronaviruses, has emerged on a global scale in the past decade and significantly influenced the swine industry. Moreover, PDCoV infects not only pigs but also other species, including humans, chickens and cattles, exhibiting a broad host tropism. This emphasizes the need for in-depth studies on coronaviruses to mitigate their potential threats. In this review, we provided a comprehensive summary of the current studies on PDCoV. We first reviewed the epidemiological investigations on the global prevalence and distribution of PDCoV. Then, we delved into the studies on the pathogenesis of PDCoV to understand the mechanisms how the virus impacts its hosts. Furthermore, we also presented some exploration studies on the immune evasion mechanisms of the virus to enhance the understanding of host-virus interactions. Despite current limitations in vaccine development for PDCoV, we highlighted the inhibitory effects observed with certain substances, which offers a potential direction for future research endeavors. In conclusion, this review summarized the scientific findings in epidemiology, pathogenesis, immune evasion mechanisms and vaccine development of PDCoV. The ongoing exploration of potential vaccine candidates and the insights gained from inhibitory substances have provided a solid foundation for future vaccine development to prevent and control diseases associated with PDCoV.

Ultrasensitive quantification of HIV-1 cell-to-cell transmission in primary human CD4+ T cells measures viral sensitivity to broadly neutralizing antibodies

IMPORTANCE

HIV-1 can efficiently transmit from one cell to another but accurate quantification of this mode of transmission is still challenging. Here, we developed an ultrasensitive assay to measure HIV-1 transmission between cells and to evaluate HIV-1 escape from broadly neutralizing antibodies in primary human T cells. This assay will contribute to understanding the fundamental mechanisms of HIV-1 cell-to-cell transmission, allow evaluation of pre-existing or acquired HIV-1 resistance in clinical trials, and can be adapted to study the biology of other retroviruses.

Virus-Induced Cell Fusion and Syncytia Formation

Most enveloped viruses encode viral fusion proteins to penetrate host cell by membrane fusion. Interestingly, many enveloped viruses can also use viral fusion proteins to induce cell-cell fusion, both in vitro and in vivo, leading to the formation of syncytia or multinucleated giant cells (MGCs). In addition, some non-enveloped viruses encode specialized viral proteins that induce cell-cell fusion to facilitate viral spread. Overall, viruses that can induce cell-cell fusion are nearly ubiquitous in mammals. Virus cell-to-cell spread by inducing cell-cell fusion may overcome entry and post-entry blocks in target cells and allow evasion of neutralizing antibodies. However, molecular mechanisms of virus-induced cell-cell fusion remain largely unknown. Here, I summarize the current understanding of virus-induced cell fusion and syncytia formation.

Relationship of human cytomegalovirus-infected endothelial cells and circulating leukocytes in the pathogenesis of disseminated human cytomegalovirus infection: A narrative review

Among the leucocyte subpopulations circulating in peripheral blood of immune-compromised patients with disseminated Human cytomegalovirus (HCMV) infection, polymorphonuclear leuckocytes (PMNL) and M/M may carry infectious virus. While only in PMNL early HCMV replicative events do occur, monocytes are susceptible to complete virus replication when they enter human organs, where as macrophages become a site of active complete virus replication. In vivo leucocytes and endothelial cells interact continuously, as suggested by several in vitro experimental findings showing the bidirectional HCMV transmission from leucocytes to and from endothelial cells with the critical aid of adhesion molecules. Recently, the neutralising antibody response in sera from subjects with primary HCMV infection was reported to be much higher and earlier than in human embryonic lung fibroblasts (HELF) cells when measured in endothelial cells and epithelial cells, where virus entry is mediated mostly by the pentamer complex gH/gL/pUL128/pUL130/pUL131, whereas it was much lower and delayed when determined in HELF, where virus entry is mediated mostly by the trimer complex gH/gL/gO. Thus, these results suggested that products of UL128L were the molecules primary responsible for the differential neutralising antibody response. This conclusion was confirmed by a series of polyclonal and monoclonal antibodies directed to the components of pUL128L. Very recently, based on two sets of experiments including inhibition and immunoblotting assays, the pentamer complex/trimer complex ratio has been finally identified as the main factor of the neutralising antibody response. This ratio may change with the virus suspension producer and target cell system as well as number of cell culture passages.

Human immunodeficiency virus transmission-Mechanisms underlying the cell-to-cell spread of human immunodeficiency virus

Despite the success of combined antiretroviral therapy in controlling viral load and reducing the risk of human immunodeficiency virus (HIV) transmission, an estimated 1.5 million new infections occurred worldwide in 2021. These new infections are mainly the result of sexual intercourse and thus involve cells present on the genital mucosa, such as dendritic cells (DCs), macrophages (Mø) and CD4+ T lymphocytes. Understanding the mechanisms by which HIV interacts with these cells and how HIV exploits these interactions to establish infection in a new human host is critical to the development of strategies to prevent and control HIV transmission. In this review, we explore how HIV has evolved to manipulate some of the physiological roles of these cells, thereby gaining access to strategic cellular niches that are critical for the spread and pathogenesis of HIV infection. The interaction of HIV with DCs, Mø and CD4+ T lymphocytes, and the role of the intercellular transfer of viral particles through the establishment of the infectious or virological synapses, but also through membrane protrusions such as filopodia and tunnelling nanotubes (TNTs), and cell fusion or cell engulfment processes are presented and discussed.

Identifying physiological tissue niches that support the HIV reservoir in T cells

Successful antiretroviral therapy (ART) can efficiently suppress Human Immunodeficiency Virus-1 (HIV-1) replication to undetectable levels, but rare populations of infected memory CD4+ T cells continue to persist, complicating viral eradication efforts. Memory T cells utilize distinct homing and adhesion molecules to enter, exit, or establish residence at diverse tissue sites, integrating cellular and environmental cues that maintain homeostasis and life-long protection against pathogens. Critical roles for T cell receptor and cytokine signals driving clonal expansion and memory generation during immunity generation are well established, but whether HIV-infected T cells can utilize similar mechanisms for their own long-term survival is unclear. How infected, but transcriptionally silent T cells maintain their recirculation potential through blood and peripheral tissues, or whether they acquire new capabilities to establish unique peripheral tissue niches, is also not well understood. In this review, we will discuss the cellular and molecular cues that are important for memory T cell homeostasis and highlight opportunities for HIV to hijack normal immunological processes to establish long-term viral persistence.

Viral manipulation of cell polarity signalling

Cell polarity refers to the asymmetric distribution of biomacromolecules that enable the correct orientation of a cell in a particular direction. It is thus an essential component for appropriate tissue development and function. Viral infections can lead to dysregulation of polarity. This is associated with a poor prognosis due to viral interference with core cell polarity regulatory scaffolding proteins that often feature PDZ (PSD-95, DLG, and ZO-1) domains including Scrib, Dlg, Pals1, PatJ, Par3 and Par6. PDZ domains are also promiscuous, binding to several different partners through their C-terminal region which contain PDZ-binding motifs (PBM). Numerous viruses encode viral effector proteins that target cell polarity regulators for their benefit and include papillomaviruses, flaviviruses and coronaviruses. A better understanding of the mechanisms of action utilised by viral effector proteins to subvert host cell polarity sigalling will provide avenues for future therapeutic intervention, while at the same time enhance our understanding of cell polarity regulation and its role tissue homeostasis.

Multicolor lifetime imaging and its application to HIV-1 uptake

Simultaneous imaging of nine fluorescent proteins is demonstrated in a single acquisition using fluorescence lifetime imaging microscopy combined with pulsed interleaved excitation of three laser lines. Multicolor imaging employing genetically encodable fluorescent proteins permits spatio-temporal live cell imaging of multiple cues. Here, we show that multicolor lifetime imaging allows visualization of quadruple labelled human immunodeficiency viruses on host cells that in turn are also labelled with genetically encodable fluorescent proteins. This strategy permits to simultaneously visualize different sub-cellular organelles (mitochondria, cytoskeleton, and nucleus) during the process of virus entry with the potential of imaging up to nine different spectral channels in living 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.

Productive HIV-1 infection of tissue macrophages by fusion with infected CD4+ T cells

Macrophages are essential for HIV-1 pathogenesis and represent major viral reservoirs. Therefore, it is critical to understand macrophage infection, especially in tissue macrophages, which are widely infected in vivo, but poorly permissive to cell-free infection. Although cell-to-cell transmission of HIV-1 is a determinant mode of macrophage infection in vivo, how HIV-1 transfers toward macrophages remains elusive. Here, we demonstrate that fusion of infected CD4+ T lymphocytes with human macrophages leads to their efficient and productive infection. Importantly, several tissue macrophage populations undergo this heterotypic cell fusion, including synovial, placental, lung alveolar, and tonsil macrophages. We also find that this mode of infection is modulated by the macrophage polarization state. This fusion process engages a specific short-lived adhesion structure and is controlled by the CD81 tetraspanin, which activates RhoA/ROCK-dependent actomyosin contractility in macrophages. Our study provides important insights into the mechanisms underlying infection of tissue-resident macrophages, and establishment of persistent cellular reservoirs in patients.

Porcine deltacoronavirus resists antibody neutralization through cell-to-cell transmission

Porcine deltacoronavirus (PDCoV) is an emerging enteric coronavirus that has been reported to infect a variety of animals and even humans. Cell-cell fusion has been identified as an alternative pathway for the cell-to-cell transmission of certain viruses, but the ability of PDCoV to exploit this transmission model, and the relevant mechanisms, have not been fully elucidated. Herein, we provide evidence that cell-to-cell transmission is the main mechanism supporting PDCoV spread in cell culture and that this efficient spread model is mediated by spike glycoprotein-driven cell-cell fusion. We found that PDCoV efficiently spread to non-susceptible cells via cell-to-cell transmission, and demonstrated that functional receptor porcine aminopeptidase N and cathepsins in endosomes are involved in the cell-to-cell transmission of PDCoV. Most importantly, compared with non-cell-to-cell infection, the cell-to-cell transmission of PDCoV was resistant to neutralizing antibodies and immune sera that potently neutralized free viruses. Taken together, our study revealed key characteristics of the cell-to-cell transmission of PDCoV and provided new insights into the mechanism of PDCoV infection.

Cell-to-Cell Transmission of HIV-1 and HIV-2 from Infected Macrophages and Dendritic Cells to CD4+ T Lymphocytes

Macrophages (Mø) and dendritic cells (DCs) are key players in human immunodeficiency virus (HIV) infection and pathogenesis. They are essential for the spread of HIV to CD4+ T lymphocytes (TCD4+) during acute infection. In addition, they constitute a persistently infected reservoir in which viral production is maintained for long periods of time during chronic infection. Defining how HIV interacts with these cells remains a critical area of research to elucidate the pathogenic mechanisms of acute spread and sustained chronic infection and transmission. To address this issue, we analyzed a panel of phenotypically distinct HIV-1 and HIV-2 primary isolates for the efficiency with which they are transferred from infected DCs or Mø to TCD4+. Our results show that infected Mø and DCs spread the virus to TCD4+ via cell-free viral particles in addition to other alternative pathways. We demonstrate that the production of infectious viral particles is induced by the co-culture of different cell populations, indicating that the contribution of cell signaling driven by cell-to-cell contact is a trigger for viral replication. The results obtained do not correlate with the phenotypic characteristics of the HIV isolates, namely their co-receptor usage, nor do we find significant differences between HIV-1 and HIV-2 in terms of cis- or trans-infection. The data presented here may help to further elucidate the cell-to-cell spread of HIV and its importance in HIV pathogenesis. Ultimately, this knowledge is critical for new therapeutic and vaccine approaches.

Exposure to Secreted Bacterial Factors Promotes HIV-1 Replication in CD4+ T Cells

Microbial translocation is associated with systemic immune activation in HIV-1 disease. Circulating T cells can encounter microbial products in the bloodstream and lymph nodes, where viral replication takes place. The mechanisms by which bacteria contribute to HIV-associated pathogenesis are not completely deciphered. Here, we examined how bacteria may impact T cell function and viral replication. We established cocultures between a panel of live bacteria and uninfected or HIV-1-infected activated peripheral blood CD4-positive (CD4+) T cells. We show that some bacteria, such as Escherichia coli and Acinetobacter baumannii, sustain lymphocyte activation and enhance HIV-1 replication. Bacteria secrete soluble factors that upregulate CD25 and ICAM-1 cell surface levels and activate NF-κB nuclear translocation. Our data also demonstrate that CD25 polarizes at the virological synapse, suggesting a previously unappreciated role of CD25 during viral replication. These findings highlight how interactions between bacterial factors and T cells may promote T cell activation and HIV-1 replication. IMPORTANCE People living with HIV suffer from chronic immune activation despite effective antiretroviral therapy. Early after infection, HIV-1 actively replicates in the gut, causing the breakage of the intestinal epithelial barrier and microbial translocation. Microbial translocation and chronic immune activation have been proven linked; however, gaps in our knowledge on how bacteria contribute to the development of HIV-related diseases remain. Whether T cells in the peripheral blood react to bacterial products and how this affects viral replication are unknown. We show that some bacteria enriched in people living with HIV activate T cells and favor HIV-1's spread. Bacteria release soluble factors that cause the overexpression of cellular molecules related to their activation state. T cells overexpressing these molecules also replicate HIV-1 more efficiently. These results help us learn more about how HIV-1, T cells, and bacteria interact with each other, as well as the mechanisms behind chronic immune activation.

The cytomegalovirus gB/MF59 vaccine candidate induces antibodies against an antigenic domain controlling cell-to-cell spread

Vaccination against human cytomegalovirus (CMV) infection remains high priority. A recombinant form of a protein essential for CMV entry, glycoprotein B (gB), demonstrated partial protection in a clinical trial (NCT00299260) when delivered with the MF59 adjuvant. Although the antibody titre against gB correlated with protection poor neutralising responses against the 5 known antigenic domains (AD) of gB were evident. Here, we show that vaccination of CMV seronegative patients induces an antibody response against a region of gB we term AD-6. Responses to the polypeptide AD-6 are detected in >70% of vaccine recipients yet in <5% of naturally infected people. An AD-6 antibody binds to gB and to infected cells but not the virion directly. Consistent with this, the AD-6 antibody is non-neutralising but, instead, prevents cell-cell spread of CMV in vitro. The discovery of AD-6 responses has the potential to explain part of the protection mediated by gB vaccines against CMV following transplantation.

Inflammasomes as mediators of inflammation in HIV-1 infection

Human immunodeficiency virus type 1 (HIV-1) infection is a chronic disease without a known cure. The advent of effective antiretroviral therapy (ART) has enabled people with HIV (PWH) to have significantly prolonged life expectancies. As a result, morbidity and mortality associated with HIV-1 infection have declined considerably. However, these individuals experience chronic systemic inflammation whose multifaceted etiology is associated with other numerous comorbidities. Inflammasomes are vital mediators that contribute to inflammatory signaling in HIV-1 infection. Here, we provide an overview of the inflammatory pathway that underlies HIV-1 infection, explicitly highlighting the role of the NLRP3 inflammasome. We also delineate the current literature on inflammasomes and the therapeutic targeting strategies aimed at the NLRP3 inflammasome to moderate HIV-1 infection-associated inflammation. Here we describe the NLRP3 inflammasome as a key pathway in developing novel therapeutic targets to block HIV-1 replication and HIV-1-associated inflammatory signaling. Controlling the inflammatory pathways is critical in alleviating the morbidities and mortality associated with chronic HIV-1 infection in PWH.

Collective fusion activity determines neurotropism of an en bloc transmitted enveloped virus

Measles virus (MeV), which is usually non-neurotropic, sometimes persists in the brain and causes subacute sclerosing panencephalitis (SSPE) several years after acute infection, serving as a model for persistent viral infections. The persisting MeVs have hyperfusogenic mutant fusion (F) proteins that likely enable cell-cell fusion at synapses and "en bloc transmission" between neurons. We here show that during persistence, F protein fusogenicity is generally enhanced by cumulative mutations, yet mutations paradoxically reducing the fusogenicity may be selected alongside the wild-type (non-neurotropic) MeV genome. A mutant F protein having SSPE-derived substitutions exhibits lower fusogenicity than the hyperfusogenic F protein containing some of those substitutions, but by the wild-type F protein coexpression, the fusogenicity of the former F protein is enhanced, while that of the latter is nearly abolished. These findings advance the understanding of the long-term process of MeV neuropathogenicity and provide critical insight into the genotype-phenotype relationships of en bloc transmitted viruses.

Sensitivity to a CD4 mimic of a consensus clone of monkey-adapted CCR5-tropic SHIV-MK38C

By acclimatizing CCR5-tropic tier 1B SHIV-MK1 to rhesus monkeys, a tier 2 SHIV-MK38 strain with neutralization resistance and high replication ability was generated. In this study, we generated SHIV-MK38C, a monkey-infectious consensus molecular clone of SHIV-MK38. Analysis using pseudotype viruses showed that MK38C was tier 1C because it lacked the N169D mutation, which is the most important mutation for neutralization resistance. MK38C harboring the N169D mutation became tier 2. However, the replication ability of SHIV-MK38C with N169D was low; more than 17 weeks elapsed before its detection in monkeys. Tier 1C MK38C was sensitive to a CD4 mimic. Therefore, SHIV-MK38C could be used to evaluate CD4 mimics in vivo.

Analyzing Single Cell Secretions by "Shadow Imaging"

Here, we describe a method, which we term "shadow imaging," to analyze the secretions of individual cells at immune synapses or other cell contacts. Following immune synapse formation and cellular activation on ligand-rich slides, the position of each cell is recorded using a pulsed immunofluorescence stain against the proteins on the ligand-rich slide surface. The pulsed stain does not penetrate the synaptic cleft, resulting in an unlabeled region or "shadow" beneath cells that is retained following cellular detachment. The secreted components, such as perforin, exosomes, or other types of extracellular vesicles, are retained on the slide and can be analyzed on a single-cell basis using immunofluorescence. The ability to identify single cells secreting different combinations of particles, proteins, and vesicles enables us to better understand the heterogeneity in immune cell secretions and can be used as a novel approach for phenotyping cell populations.

Rab11-FIP1C Is Dispensable for HIV-1 Replication in Primary CD4+ T Cells, but Its Role Is Cell Type Dependent in Immortalized Human T-Cell Lines

The HIV-1 envelope glycoprotein (Env) contains a long cytoplasmic tail harboring highly conserved motifs that direct Env trafficking and incorporation into virions and promote efficient virus spread. The cellular trafficking factor Rab11a family interacting protein 1C (FIP1C) has been implicated in the directed trafficking of Env to sites of viral assembly. In this study, we confirm that small interfering RNA (siRNA)-mediated depletion of FIP1C in HeLa cells modestly reduces Env incorporation into virions. To determine whether FIP1C is required for Env incorporation and HIV-1 replication in physiologically relevant cells, CRISPR-Cas9 technology was used to knock out the expression of this protein in several human T-cell lines-Jurkat E6.1, SupT1, and H9-and in primary human CD4+ T cells. FIP1C knockout caused modest reductions in Env incorporation in SupT1 cells but did not inhibit virus replication in SupT1 or Jurkat E6.1 T cells. In H9 cells, FIP1C knockout caused a cell density-dependent defect in virus replication. In primary CD4+ T cells, FIP1C knockout had no effect on HIV-1 replication. Furthermore, human T-cell leukemia virus type 1 (HTLV-1)-transformed cell lines that are permissive for HIV-1 replication do not express FIP1C. Mutation of an aromatic motif in the Env cytoplasmic tail (Y795W) implicated in FIP1C-mediated Env incorporation impaired virus replication independently of FIP1C expression in SupT1, Jurkat E6.1, H9, and primary T cells. Together, these results indicate that while FIP1C may contribute to HIV-1 Env incorporation in some contexts, additional and potentially redundant host factors are likely required for Env incorporation and virus dissemination in T cells. IMPORTANCE The incorporation of the HIV-1 envelope (Env) glycoproteins, gp120 and gp41, into virus particles is critical for virus infectivity. gp41 contains a long cytoplasmic tail that has been proposed to interact with host cell factors, including the trafficking factor Rab11a family interacting protein 1C (FIP1C). To investigate the role of FIP1C in relevant cell types-human T-cell lines and primary CD4+ T cells-we used CRISPR-Cas9 to knock out FIP1C expression and examined the effect on HIV-1 Env incorporation and virus replication. We observed that in two of the T-cell lines examined (Jurkat E6.1 and SupT1) and in primary CD4+ T cells, FIP1C knockout did not disrupt HIV-1 replication, whereas FIP1C knockout reduced Env expression and delayed replication in H9 cells. The results indicate that while FIP1C may contribute to Env incorporation in some cell lines, it is not an essential factor for efficient HIV-1 replication in primary CD4+ T cells.

Nef inhibits HIV transcription and gene expression in astrocytes and HIV transmission from astrocytes to CD4+ T cells

HIV infects astrocytes in a restricted manner but leads to abundant expression of Nef, a major viral factor for HIV replication and disease progression. However, the roles of Nef in HIV gene expression and replication in astrocytes and viral transfer from astrocytes to CD4⁺ T cells remain largely unclear. In this study, we attempted to address these issues by transfecting human primary astrocytes with HIV molecular clones with intact Nef and without Nef (a nonsense Nef mutant) and comparing gene expression and replication in astrocytes and viral transfer from astrocytes to CD4⁺ T cells MT4. First, we found that lack of Nef expression led to increased extracellular virus production from astrocytes and intracellular viral protein and RNA expression in astrocytes. Using a HIV LTR-driven luciferase reporter gene assay, we showed that ectopic Nef expression alone inhibited the HIV LTR promoter activity in astrocytes. Consistent with the previously established function of Nef, we showed that the infectivity of HIV derived from astrocytes with Nef expression was significantly higher than that with no Nef expression. Next, we performed the co-culture assay to determine HIV transfer from astrocytes transfected to MT4. We showed that lack of Nef expression led to significant increase in HIV transfer from astrocytes to MT4 using two HIV clones. We also used Nef-null HIV complemented with Nef in trans in the co-culture assay and demonstrated that Nef expression led to significantly decreased HIV transfer from astrocytes to MT4. Taken together, these findings support a negative role of Nef in HIV replication and pathogenesis in astrocytes.

Cost-effective high-speed, three-dimensional live-cell imaging of HIV-1 transfer at the T cell virological synapse

The availability of cost-effective, highly portable, and easy to use high-resolution live-cell imaging systems could present a significant technological break-through in challenging environments, such as high-level biosafety laboratories or sites where new viral outbreaks are suspected. We describe and demonstrate a cost-effective high-speed fluorescence microscope enabling the live tracking of virus particles across virological synapses that form between infected and uninfected T cells. The dynamics of HIV-1 proteins studied at the cellular level and the formation of virological synapses in living T cells reveals mechanisms by which cell-cell interactions facilitate infection between immune cells. Dual-color 3D fluorescence deconvolution microscopy of HIV-1 particles at frames rates of 100 frames per second allows us to follow the transfer of HIV-1 particles across the T cell virological synapse between living T cells. We also confirm the successful transfer of virus by imaging T cell samples fixed at specific time points during cell-cell virus transfer by super-resolution structured illumination microscopy.

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

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.

Cell-to-cell transmission of HIV-1 from provirus-activated cells to resting naïve and memory human primary CD4 T cells is highly efficient and requires CD4 and F-actin but not chemokine receptors

Latently infected cells harboring replication-competent proviruses represent a major barrier to HIV-1 cure. One major effort to purge these cells has focused on developing the "shock and kill" approach for forcing provirus reactivation to induce cell killing by viral cytopathic effects, host immune responses, or both. We conducted kinetic and mechanistic studies of HIV-1 protein expression, virion production, and cell-to-cell virus transmission during provirus reactivation. Provirus-activated ACH-2 cells stimulated with romidepsin (RMD) or PMA produced Nef early, and then Env and Gag in parallel with the appearance of virions. Env on the surface of provirus-activated cells and cellular F-actin were critical in the formation of virological synapses to mediate cell-to-cell transmission of HIV-1 from provirus-activated cells to uninfected cells. This HIV-1 cell-to-cell transmission was substantially more efficient than transmission seen via cell-free virus spread and required F-actin remodeling and CD4, but not chemokine receptors. Resting human primary CD4⁺ T cells including naïve and memory subpopulations and, especially the memory CD4⁺ T cells, were highly susceptible to HIV-1 infection via cell-to-cell transmission. Cell-to-cell transmission of HIV-1 from provirus-activated cells was profoundly decreased by protease inhibitors (PIs) and neutralizing antibodies (nAbs) that recognize the CD4-binding site (CD4bs) such as VRC01, but not by reverse transcriptase (RT) inhibitor Emtricitabine (FTC). Therefore, our results suggest that PIs with potent blocking abilities should be used in clinical application of the "shock and kill" approach, most likely in combination with CD4bs nAbs, to prevent new HIV-1 infections.

Development of an efficient reproducible cell-cell transmission assay for rapid quantification of SARS-CoV-2 spike interaction with hACE2

Efficient quantitative assays for measurement of viral replication and infectivity are indispensable for future endeavors to develop prophylactic or therapeutic antiviral drugs or vaccines against SARS-CoV-2. We developed a SARS-CoV-2 cell-cell transmission assay that provides a rapid and quantitative readout to assess SARS-CoV-2 spike hACE2 interaction in the absence of pseudotyped particles or live virus. We established two well-behaved stable cell lines, which demonstrated a remarkable correlation with standard cell-free viral pseudotyping for inhibition by convalescent sera, small-molecule drugs, and murine anti-spike monoclonal antibodies. The assay is rapid, reliable, and highly reproducible, without a requirement for any specialized research reagents or laboratory equipment and should be easy to adapt for use in most investigative and clinical settings. It can be effectively used or modified for high-throughput screening for compounds and biologics that interfere with virus-cell binding and entry to complement other neutralization assays currently in use.

Viral and Cellular Factors Contributing to the Hematogenous Dissemination of Human Cytomegalovirus via Polymorphonuclear Leukocytes

Polymorphonuclear leukocytes (PMNs) presumably transmit human cytomegalovirus (HCMV) between endothelial cells in blood vessels and thereby facilitate spread to peripheral organs. We aimed to identify viral components that contribute to PMN-mediated transmission and test the hypothesis that cellular adhesion molecules shield transmission sites from entry inhibitors. Stop codons were introduced into the genome of HCMV strain Merlin to delete pUL74 of the trimeric and pUL128 of the pentameric glycoprotein complex and the tegument proteins pp65 and pp71. Mutants were analyzed regarding virus uptake by PMNs and transfer of infection to endothelial cells. Cellular adhesion molecules were evaluated for their contribution to virus transmission using function-blocking antibodies, and hits were further analyzed regarding shielding against inhibitors of virus entry. The viral proteins pUL128, pp65, and pp71 were required for efficient PMN-mediated transmission, whereas pUL74 was dispensable. On the cellular side, the blocking of the αLβ2-integrin LFA-1 reduced virus transfer by 50% and allowed entry inhibitors to reduce it further by 30%. In conclusion, these data show that PMN-mediated transmission depends on the pentameric complex and an intact tegument and supports the idea of a virological synapse that promotes this dissemination mode both directly and via immune evasion.

Galectin-3 facilitates cell-to-cell HIV-1 transmission by altering the composition of membrane lipid rafts in CD4 T cells

Galectin-3 (GAL3) is a β-galactoside-binding lectin expressed in CD4 T cells infected with human immunodeficiency virus-1 (HIV-1). GAL3 promotes HIV-1 budding by associating with ALIX and Gag p6. GAL3 has been shown to localize in membrane lipid rafts in dendritic cells and positively regulate cell migration. HIV-1 spreads between T cells by forming supramolecular structures (virological synapses [VSs]), whose integrity depends on lipid rafts. Here, we addressed the potential role of GAL3 in cell-to-cell transmission of HIV-1 in CD4 T cells. GAL3 expressed in donor cells was more important for facilitating HIV-1 cell-to-cell transfer than GAL3 expressed in target cells. GAL3 was found to be co-transferred with Gag from HIV-1-positive donor to HIV-1-negative target T cells. HIV-1 infection induced translocation of GAL3 together with Gag to the cell-cell interfaces and colocalize with GM1, where GAL3 facilitated VS formation. GAL3 regulated the coordinated transfer of Gag and flotillin-1 into plasma membrane fractions. Finally, depletion of GAL3 reduced the cholesterol levels in membrane lipid rafts in CD4 T cells. These findings provide evidence that endogenous GAL3 stimulates lipid raft components and facilitates intercellular HIV-1 transfer among CD4 T cells, offering another pathway by which GAL3 regulates HIV-1 infection. These findings may inform the treatment of HIV-1 infection based on targeting GAL3 to modulate lipid rafts.

Revisiting Regulated Cell Death Responses in Viral Infections

The fate of a viral infection in the host begins with various types of cellular responses, such as abortive, productive, latent, and destructive infections. Apoptosis, necroptosis, and pyroptosis are the three major types of regulated cell death mechanisms that play critical roles in viral infection response. Cell shrinkage, nuclear condensation, bleb formation, and retained membrane integrity are all signs of osmotic imbalance-driven cytoplasmic swelling and early membrane damage in necroptosis and pyroptosis. Caspase-driven apoptotic cell demise is considered in many circumstances as an anti-inflammatory, and some pathogens hijack the cell death signaling routes to initiate a targeted attack against the host. In this review, the selected mechanisms by which viruses interfere with cell death were discussed in-depth and were illustrated by compiling the general principles and cellular signaling mechanisms of virus–host-specific molecule interactions.

Development of an HIV reporter virus that identifies latently infected CD4+ T cells

There is no cure for HIV infection, as the virus establishes a latent reservoir, which escapes highly active antiretroviral treatments. One major obstacle is the difficulty identifying cells that harbor latent proviruses. We devised a single-round viral vector that carries a series of versatile reporter molecules that are expressed in an LTR-dependent or LTR-independent manner and make it possible to accurately distinguish productive from latent infection. Using primary human CD4+ T cells, we show that transcriptionally silent proviruses are found in more than 50% of infected cells. The latently infected cells harbor proviruses but lack evidence for multiple spliced transcripts. LTR-silent integrations occurred to variable degrees in all CD4+ T subsets examined, with CD4+ TEM and CD4+ TREG displaying the highest frequency of latent infections. This viral vector permits the interrogation of HIV latency at single-cell resolution, revealing mechanisms of latency establishment and allowing the characterization of effective latency-reversing agents.

A cellular trafficking signal in the SIV envelope protein cytoplasmic domain is strongly selected for in pathogenic infection

The HIV/SIV envelope glycoprotein (Env) cytoplasmic domain contains a highly conserved Tyr-based trafficking signal that mediates both clathrin-dependent endocytosis and polarized sorting. Despite extensive analysis, the role of these functions in viral infection and pathogenesis is unclear. An SIV molecular clone (SIVmac239) in which this signal is inactivated by deletion of Gly-720 and Tyr-721 (SIVmac239ΔGY), replicates acutely to high levels in pigtail macaques (PTM) but is rapidly controlled. However, we previously reported that rhesus macaques and PTM can progress to AIDS following SIVmac239ΔGY infection in association with novel amino acid changes in the Env cytoplasmic domain. These included an R722G flanking the ΔGY deletion and a nine nucleotide deletion encoding amino acids 734-736 (ΔQTH) that overlaps the rev and tat open reading frames. We show that molecular clones containing these mutations reconstitute signals for both endocytosis and polarized sorting. In one PTM, a novel genotype was selected that generated a new signal for polarized sorting but not endocytosis. This genotype, together with the ΔGY mutation, was conserved in association with high viral loads for several months when introduced into naïve PTMs. For the first time, our findings reveal strong selection pressure for Env endocytosis and particularly for polarized sorting during pathogenic SIV infection in vivo.

HIV-1 cell-to-cell spread overcomes the virus entry block of non-macrophage-tropic strains in macrophages

Macrophages (MΦ) are increasingly recognized as HIV-1 target cells involved in the pathogenesis and persistence of infection. Paradoxically, in vitro infection assays suggest that virus isolates are mostly T-cell-tropic and rarely MΦ-tropic. The latter are assumed to emerge under CD4+ T-cell paucity in tissues such as the brain or at late stage when the CD4 T-cell count declines. However, assays to qualify HIV-1 tropism use cell-free viral particles and may not fully reflect the conditions of in vivo MΦ infection through cell-to-cell viral transfer. Here, we investigated the capacity of viruses expressing primary envelope glycoproteins (Envs) with CCR5 and/or CXCR4 usage from different stages of infection, including transmitted/founder Envs, to infect MΦ by a cell-free mode and through cell-to-cell transfer from infected CD4+ T cells. The results show that most viruses were unable to enter MΦ as cell-free particles, in agreement with the current view that non-M-tropic viruses inefficiently use CD4 and/or CCR5 or CXCR4 entry receptors on MΦ. In contrast, all viruses could be effectively cell-to-cell transferred to MΦ from infected CD4+ T cells. We further showed that viral transfer proceeded through Env-dependent cell-cell fusion of infected T cells with MΦ targets, leading to the formation of productively infected multinucleated giant cells. Compared to cell-free infection, infected T-cell/MΦ contacts showed enhanced interactions of R5 M- and non-M-tropic Envs with CD4 and CCR5, resulting in a reduced dependence on receptor expression levels on MΦ for viral entry. Altogether, our results show that virus cell-to-cell transfer overcomes the entry block of isolates initially defined as non-macrophage-tropic, indicating that HIV-1 has a more prevalent tropism for MΦ than initially suggested. This sheds light into the role of this route of virus cell-to-cell transfer to MΦ in CD4+ T cell rich tissues for HIV-1 transmission, dissemination and formation of tissue viral reservoirs.

The HIV Env Glycoprotein Conformational States on Cells and Viruses

The HIV Env glycoprotein is the surface glycoprotein responsible for viral entry into CD4+ immune cells. During infection, Env also serves as a primary target for antibody responses, which are robust but unable to control virus replication. Immune evasion by HIV-1 Env appears to employ complex mechanisms to regulate what antigenic states are presented to the immune system. Immunodominant features appear to be distinct from epitopes that interfere with Env functions in mediating infection. Further, cell-cell transmission studies indicate that vulnerable conformational states are additionally hidden from recognition on infected cells, even though the presence of Env at the cell surface is required for viral infection through the virological synapse. Cell-cell infection studies support that Env on infected cells is presented in distinct conformations from that on virus particles. Here we review data regarding the regulation of conformational states of Env and assess how regulated sorting of Env within the infected cell may underlie mechanisms to distinguish Env on the surface of virus particles versus Env on the surface of infected cells. These mechanisms may allow infected cells to avoid opsonization, providing cell-to-cell infection by HIV with a selective advantage during evolution within an infected individual. Understanding how distinct Env conformations are presented on cells versus viruses may be essential to designing effective vaccine approaches and therapeutic strategies to clear infected cell reservoirs.

HIV-1 Vpr drives a tissue residency-like phenotype during selective infection of resting memory T cells

HIV-1 replicates in CD4+ T cells, leading to AIDS. Determining how HIV-1 shapes its niche to create a permissive environment is central to informing efforts to limit pathogenesis, disturb reservoirs, and achieve a cure. A key roadblock in understanding HIV-T cell interactions is the requirement to activate T cells in vitro to make them permissive to infection. This dramatically alters T cell biology and virus-host interactions. Here we show that HIV-1 cell-to-cell spread permits efficient, productive infection of resting memory T cells without prior activation. Strikingly, we find that HIV-1 infection primes resting T cells to gain characteristics of tissue-resident memory T cells (TRM), including upregulating key surface markers and the transcription factor Blimp-1 and inducing a transcriptional program overlapping the core TRM transcriptional signature. This reprogramming is driven by Vpr and requires Vpr packaging into virions and manipulation of STAT5. Thus, HIV-1 reprograms resting T cells, with implications for viral replication and persistence.

Calcium Contributes to Polarized Targeting of HIV Assembly Machinery by Regulating Complex Stability

Polarized or precision targeting of protein complexes to their destinations is fundamental to cellular homeostasis, but the mechanism underpinning directional protein delivery is poorly understood. Here, we use the uropod targeting HIV synapse as a model system to show that the viral assembly machinery Gag is copolarized with the intracellular calcium (Ca²⁺) gradient and binds specifically with Ca²⁺. Conserved glutamic/aspartic acids flanking endosomal sorting complexes required for transport binding motifs are major Ca²⁺ binding sites. Deletion or mutation of these Ca²⁺ binding residues resulted in altered protein trafficking phenotypes, including (i) changes in the Ca²⁺-Gag distribution relationship during uropod targeting and/or (ii) defects in homo/hetero-oligomerization with Gag. Mutation of Ca²⁺ binding amino acids is associated with enhanced ubiquitination and a decline in virion release via uropod protein complex delivery. Our data that show Ca²⁺-protein binding, via the intracellular Ca²⁺ gradient, represents a mechanism that regulates intracellular protein trafficking.

T cell migration potentiates HIV infection by enhancing viral fusion and integration

T cells actively migrate along reticular networks within lymphoid organs in search for cognate antigen, but how these behaviors impact HIV entry and infection is unclear. Here, we show that migratory T cells in 3D collagen matrix display significantly enhanced infection and integration by cell-free R5-tropic lab adapted and transmitted/founder molecular HIV clones in the absence of exogenous cytokines or cationic polymers. Using two different collagen matrices that either support or restrict T cell migration, we observe high levels of HIV fusion in migratory T cells, whereas non-motile T cells display low viral entry and integration. Motile T cells were less sensitive to combination antiretroviral drugs and were able to freely migrate into regions with high HIV densities, resulting in high infection rates. Together, our studies indicate that the environmental context in which initial HIV-T cell encounters occur modulates HIV-1 entry and integration efficiencies.

Dynamics of HIV-1 Gag Processing as Revealed by Fluorescence Lifetime Imaging Microscopy and Single Virus Tracking

The viral polyprotein Gag plays a central role for HIV-1 assembly, release and maturation. Proteolytic processing of Gag by the viral protease is essential for the structural rearrangements that mark the transition from immature to mature, infectious viruses. The timing and kinetics of Gag processing are not fully understood. Here, fluorescence lifetime imaging microscopy and single virus tracking are used to follow Gag processing in nascent HIV-1 particles in situ. Using a Gag polyprotein labelled internally with eCFP, we show that proteolytic release of the fluorophore from Gag is accompanied by an increase in its fluorescence lifetime. By tracking nascent virus particles in situ and analyzing the intensity and fluorescence lifetime of individual traces, we detect proteolytic cleavage of eCFP from Gag in a subset (6.5%) of viral particles. This suggests that for the majority of VLPs, Gag processing occurs with a delay after particle assembly.

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.

HIV-1 and HTLV-1 Transmission Modes: Mechanisms and Importance for Virus Spread

So far, only two retroviruses, human immunodeficiency virus (HIV) (type 1 and 2) and human T-cell lymphotropic virus type 1 (HTLV-1), have been recognized as pathogenic for humans. Both viruses mainly infect CD4+ T lymphocytes. HIV replication induces the apoptosis of CD4 lymphocytes, leading to the development of acquired immunodeficiency syndrome (AIDS). After a long clinical latency period, HTLV-1 can transform lymphocytes, with subsequent uncontrolled proliferation and the manifestation of a disease called adult T-cell leukemia (ATLL). Certain infected patients develop neurological autoimmune disorder called HTLV-1-associated myelopathy, also known as tropical spastic paraparesis (HAM/TSP). Both viruses are transmitted between individuals via blood transfusion, tissue/organ transplantation, breastfeeding, and sexual intercourse. Within the host, these viruses can spread utilizing either cell-free or cell-to-cell modes of transmission. In this review, we discuss the mechanisms and importance of each mode of transmission for the biology of HIV-1 and HTLV-1.

A Conserved Tryptophan in the Envelope Cytoplasmic Tail Regulates HIV-1 Assembly and Spread

The HIV-1 envelope (Env) is an essential determinant of viral infectivity, tropism and spread between T cells. Lentiviral Env contain an unusually long 150 amino acid cytoplasmic tail (EnvCT), but the function of the EnvCT and many conserved domains within it remain largely uncharacterised. Here, we identified a highly conserved tryptophan motif at position 757 (W757) in the LLP-2 alpha helix of the EnvCT as a key determinant for HIV-1 replication and spread between T cells. Alanine substitution at this position potently inhibited HIV-1 cell–cell spread (the dominant mode of HIV-1 dissemination) by preventing recruitment of Env and Gag to sites of cell–cell contact, inhibiting virological synapse (VS) formation and spreading infection. Single-molecule tracking and super-resolution imaging showed that mutation of W757 dysregulates Env diffusion in the plasma membrane and increases Env mobility. Further analysis of Env function revealed that W757 is also required for Env fusion and infectivity, which together with reduced VS formation, result in a potent defect in viral spread. Notably, W757 lies within a region of the EnvCT recently shown to act as a supporting baseplate for Env. Our data support a model in which W757 plays a key role in regulating Env biology, modulating its temporal and spatial recruitment to virus assembly sites and regulating the inherent fusogenicity of the Env ectodomain, thereby supporting efficient HIV-1 replication and spread.

Embedding of HIV Egress within Cortical F-Actin

F-Actin remodeling is important for the spread of HIV via cell-cell contacts; however, the mechanisms by which HIV corrupts the actin cytoskeleton are poorly understood. Through live cell imaging and focused ion beam scanning electron microscopy (FIB-SEM), we observed F-Actin structures that exhibit strong positive curvature to be enriched for HIV buds. Virion proteomics, gene silencing, and viral mutagenesis supported a Cdc42-IQGAP1-Arp2/3 pathway as the primary intersection of HIV budding, membrane curvature and F-Actin regulation. Whilst HIV egress activated the Cdc42-Arp2/3 filopodial pathway, this came at the expense of cell-free viral release. Importantly, release could be rescued by cell-cell contact, provided Cdc42 and IQGAP1 were present. From these observations, we conclude that a proportion out-going HIV has corrupted a central F-Actin node that enables initial coupling of HIV buds to cortical F-Actin to place HIV at the leading cell edge. Whilst this initially prevents particle release, the maturation of cell-cell contacts signals back to this F-Actin node to enable viral release & subsequent infection of the contacting cell.

HIV-1 trans-Infection Mediated by DCs: The Tip of the Iceberg of Cell-to-Cell Viral Transmission

HIV-1 cell-to-cell transmission is key for an effective viral replication that evades immunity. This highly infectious mechanism is orchestrated by different cellular targets that utilize a wide variety of processes to efficiently transfer HIV-1 particles. Dendritic cells (DCs) are the most potent antigen presenting cells that initiate antiviral immune responses, but are also the cells with highest capacity to transfer HIV-1. This mechanism, known as trans-infection, relies on the capacity of DCs to capture HIV-1 particles via lectin receptors such as the sialic acid-binding I-type lectin Siglec-1/CD169. The discovery of the molecular interaction of Siglec-1 with sialylated lipids exposed on HIV-1 membranes has enlightened how this receptor can bind to several enveloped viruses. The outcome of these interactions can either mount effective immune responses, boost the productive infection of DCs and favour innate sensing, or fuel viral transmission via trans-infection. Here we review these scenarios focusing on HIV-1 and other enveloped viruses such as Ebola virus or SARS-CoV-2.

A Replication-Competent HIV Clone Carrying GFP-Env Reveals Rapid Env Recycling at the HIV-1 T Cell Virological Synapse

HIV-1 infection is enhanced by cell-cell adhesions between infected and uninfected T cells called virological synapses (VS). VS are initiated by the interactions of cell-surface HIV-1 envelope glycoprotein (Env) and CD4 on target cells and act as sites of viral assembly and viral transfer between cells. To study the process that recruits and retains HIV-1 Env at the VS, a replication-competent HIV-1 clone carrying an Env-sfGFP fusion protein was designed to enable live tracking of Env within infected cells. Combined use of surface pulse-labeling of Env and fluorescence recovery after photobleaching (FRAP) studies, enabled the visualization of the targeted accumulation and sustained recycling of Env between endocytic compartments (EC) and the VS. We observed dynamic exchange of Env at the VS, while the viral structural protein, Gag, was largely immobile at the VS. The disparate exchange rates of Gag and Env at the synapse support that the trafficking and/or retention of a majority of Env towards the VS is not maintained by entrapment by a Gag lattice or immobilization by binding to CD4 on the target cell. A FRAP study of an Env endocytosis mutant showed that recycling is not required for accumulation at the VS, but is required for the rapid exchange of Env at the VS. We conclude that the mechanism of Env accumulation at the VS and incorporation into nascent particles involves continuous internalization and targeted secretion rather than irreversible interactions with the budding virus, but that this recycling is largely dispensable for VS formation and viral transfer across the VS.

Tunneling nanotubes and related structures: molecular mechanisms of formation and function

Tunneling nanotubes (TNTs) are F-actin-based, membrane-enclosed tubular connections between animal cells that transport a variety of cellular cargo. Over the last 15 years since their discovery, TNTs have come to be recognized as key players in normal cell communication and organism development, and are also exploited for the spread of various microbial pathogens and major diseases like cancer and neurodegenerative disorders. TNTs have also been proposed as modalities for disseminating therapeutic drugs between cells. Despite the rapidly expanding and wide-ranging relevance of these structures in both health and disease, there is a glaring dearth of molecular mechanistic knowledge regarding the formation and function of these important but enigmatic structures. A series of fundamental steps are essential for the formation of functional nanotubes. The spatiotemporally controlled and directed modulation of cortical actin dynamics would be required to ensure outward F-actin polymerization. Local plasma membrane deformation to impart negative curvature and membrane addition at a rate commensurate with F-actin polymerization would enable outward TNT elongation. Extrinsic tactic cues, along with cognate intrinsic signaling, would be required to guide and stabilize the elongating TNT towards its intended target, followed by membrane fusion to create a functional TNT. Selected cargoes must be transported between connected cells through the action of molecular motors, before the TNT is retracted or destroyed. This review summarizes the current understanding of the molecular mechanisms regulating these steps, also highlighting areas that deserve future attention.

Cell-to-cell transmission of HSV1 in human keratinocytes in the absence of the major entry receptor, nectin1

Herpes simplex virus 1 (HSV1) infects the stratified epithelia of the epidermis, oral or genital mucosa, where the main cell type is the keratinocyte. Here we have used nTERT human keratinocytes to generate a CRISPR-Cas9 knockout (KO) of the primary candidate HSV1 receptor, nectin1, resulting in a cell line that is refractory to HSV1 entry. Nonetheless, a small population of KO cells was able to support infection which was not blocked by a nectin1 antibody and hence was not a consequence of residual nectin1 expression. Strikingly at later times, the population of cells originally resistant to HSV1 infection had also become infected. Appearance of this later population was blocked by inhibition of virus genome replication, or infection with a ΔUL34 virus defective in capsid export to the cytoplasm. Moreover, newly formed GFP-tagged capsids were detected in cells surrounding the initial infected cell, suggesting that virus was spreading following replication in the original susceptible cells. Additional siRNA depletion of the second major HSV1 receptor HVEM, or PTP1B, a cellular factor shown elsewhere to be involved in cell-to-cell transmission, had no effect on virus spread in the absence of nectin1. Neutralizing human serum also failed to block virus transmission in nectin1 KO cells, which was dependent on the receptor binding protein glycoprotein D and the cell-to-cell spread glycoproteins gI and gE, indicating that virus was spreading by direct cell-to-cell transmission. In line with these results, both HSV1 and HSV2 formed plaques on nectin1 KO cells, albeit at a reduced titre, confirming that once the original cell population was infected, the virus could spread into all other cells in the monolayer. We conclude that although nectin1 is required for extracellular entry in to the majority of human keratinocytes, it is dispensable for direct cell-to-cell transmission.

Herpes simplex virus 1 (HSV1) infects the epithelia of the epidermis, oral or genital mucosa to cause cold sores, genital herpes, or more serious outcomes such as keratitis and neonatal herpes. Like many viruses, HSV1 can spread through the extracellular environment or by direct cell-to-cell transmission, with the latter mechanism being important for avoiding antibody responses in the host. Here we have studied HSV1 entry and transmission in the human keratinocyte, the main cell type in the target epithelia, by generating a CRISPR-Cas9 knockout of the primary candidate virus receptor, nectin1. While HSV1 was unable to infect the majority of nectin1 knockout keratinocytes, a small population of these nectin1 KO cells remained susceptible to virus entry, and once infected, the virus was able to spread into the rest of the monolayer. This spread continued in the presence of neutralising serum which blocks extracellular virus, and required glycoprotein D, the main virus receptor-binding protein, and glycoproteins gE and gI which are known to be involved in cell-to-cell spread. Hence, while nectin1 is required for virus entry into the majority of human keratinocyte cells, it is dispensable for cell-to-cell transmission of the virus. These data have implications for the mechanism of HSV1 epithelial spread and pathogenesis.

The Adenosine Pathway and Human Immunodeficiency Virus-Associated Inflammation

Human immunodeficiency virus (HIV) is associated with an increased risk of age-associated comorbidities and mortality compared to people without HIV. This has been attributed to HIV-associated chronic inflammation and immune activation despite viral suppression. The adenosine pathway is an established mechanism by which the body regulates persistent inflammation to limit tissue damage associated with inflammatory conditions. However, HIV infection is associated with derangements in the adenosine pathway that limits its ability to control HIV-associated inflammation. This article reviews the function of purinergic signaling and the role of the adenosine signaling pathway in HIV-associated chronic inflammation. This review also discusses the beneficial and potential detrimental effects of pharmacotherapeutic strategies targeting this pathway among people with HIV.

Endocytic Motif on a Biotin-Tagged HIV-1 Env Modulates the Co-Transfer of Env and Gag during Cell-to-Cell Transmission

During HIV-1 transmission through T cell virological synapses, the recruitment of the envelope (Env) glycoprotein to the site of cell-cell contact is important for adhesion and for packaging onto nascent virus particles which assemble at the site. Live imaging studies in CD4 T cells have captured the rapid recruitment of the viral structural protein Gag to VSs. We explored the role of endocytic trafficking of Env initiated by a membrane proximal tyrosine motif during HIV transfer into target cells and examined the factors that allow Gag and Env to be transferred together across the synapse. To facilitate tracking of Env in live cells, we adapted an Env tagging method and introduced a biotin acceptor peptide (BAP) into the V4 loop of Env gp120, enabling sensitive fluorescent tracking of V4-biotinylated Env. The BAP-tagged and biotinylated HIVs were replication-competent in cell-free and cell-to-cell infection assays. Live cell fluorescent imaging experiments showed rapid internalized cell surface Env on infected cells. Cell-cell transfer experiments conducted with the Env endocytosis mutant (Y712A) showed increased transfer of Env. Paradoxically, this increase in Env transfer was associated with significantly reduced Gag transfer into target cells, when compared to viral transfer associated with WT Env. This Y712A Env mutant also exhibited an altered Gag/biotin Env fluorescence ratio during transfer that correlated with decreased productive cell-to-cell infection. These results may suggest that the internalization of Env into recycling pools plays an important role in the coordinated transfer of Gag and Env across the VS, which optimizes productive infection in target cells.

Mucosal Dendritic Cell Subsets Control HIV-1's Viral Fitness

Dendritic cell (DC) subsets are abundantly present in genital and intestinal mucosal tissue and are among the first innate immune cells that encounter human immunodeficiency virus type 1 (HIV-1) after sexual contact. Although DCs have specific characteristics that greatly enhance HIV-1 transmission, it is becoming evident that most DC subsets also have virus restriction mechanisms that exert selective pressure on the viruses during sexual transmission. In this review we discuss the current concepts of the immediate events following viral exposure at genital mucosal sites that lead to selection of specific HIV-1 variants called transmitted founder (TF) viruses. We highlight the importance of the TF HIV-1 phenotype and the role of different DC subsets in establishing infection. Understanding the biology of HIV-1 transmission will contribute to the design of novel treatment strategies preventing HIV-1 dissemination.

Employing Broadly Neutralizing Antibodies as a Human Immunodeficiency Virus Prophylactic & Therapeutic Application

Despite the discovery that the human immunodeficiency virus 1 (HIV-1) is the pathogen of acquired immunodeficiency syndrome (AIDS) in 1983, there is still no effective anti-HIV-1 vaccine. The major obstacle to the development of HIV-1 vaccine is the extreme diversity of viral genome sequences. Nonetheless, a number of broadly neutralizing antibodies (bNAbs) against HIV-1 have been made and identified in this area. Novel strategies based on using these bNAbs as an efficacious preventive and/or therapeutic intervention have been applied in clinical. In this review, we summarize the recent development of bNAbs and its application in HIV-1 acquisition prevention as well as discuss the innovative approaches being used to try to convey protection within individuals at risk and being treated for HIV-1 infection.

Circumcision as an Intervening Strategy against HIV Acquisition in the Male Genital Tract

Unsafe sex with HIV-infected individuals remains a major route for HIV transmission, and protective strategies, such as the distribution of free condoms and pre-or post-prophylaxis medication, have failed to control the spread of HIV, particularly in resource-limited settings and high HIV prevalence areas. An additional key strategy for HIV prevention is voluntary male circumcision (MC). International health organizations (e.g., the World Health Organization, UNAIDS) have recommended this strategy on a larger scale, however, there is a general lack of public understanding about how MC effectively protects against HIV infection. This review aims to discuss the acquisition of HIV through the male genital tract and explain how and why circumcised men are more protected from HIV infection during sexual activity than uncircumcised men who are at higher risk of HIV acquisition.

The Interplay of HIV and Autophagy in Early Infection

HIV/AIDS is still a global threat despite the notable efforts made by the scientific and health communities to understand viral infection, to design new drugs or to improve existing ones, as well as to develop advanced therapies and vaccine designs for functional cure and viral eradication. The identification and analysis of HIV-1 positive individuals that naturally control viral replication in the absence of antiretroviral treatment has provided clues about cellular processes that could interact with viral proteins and RNA and define subsequent viral replication and clinical progression. This is the case of autophagy, a degradative process that not only maintains cell homeostasis by recycling misfolded/old cellular elements to obtain nutrients, but is also relevant in the innate and adaptive immunity against viruses, such as HIV-1. Several studies suggest that early steps of HIV-1 infection, such as virus binding to CD4 or membrane fusion, allow the virus to modulate autophagy pathways preparing cells to be permissive for viral infection. Confirming this interplay, strategies based on autophagy modulation are able to inhibit early steps of HIV-1 infection. Moreover, autophagy dysregulation in late steps of the HIV-1 replication cycle may promote autophagic cell-death of CD4⁺ T cells or control of HIV-1 latency, likely contributing to disease progression and HIV persistence in infected individuals. In this scenario, understanding the molecular mechanisms underlying HIV/autophagy interplay may contribute to the development of new strategies to control HIV-1 replication. Therefore, the aim of this review is to summarize the knowledge of the interplay between autophagy and the early events of HIV-1 infection, and how autophagy modulation could impair or benefit HIV-1 infection and persistence, impacting viral pathogenesis, immune control of viral replication, and clinical progression of HIV-1 infected patients.