Cell-Free versus Cell-to-Cell Infection by Human Immunodeficiency Virus Type 1 and Human T-Lymphotropic Virus Type 1: Exploring the Link among Viral Source, Viral Trafficking, and Viral Replication

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.

Pathogenicity and virulence of human T lymphotropic virus type-1 (HTLV-1) in oncogenesis: adult T-cell leukemia/lymphoma (ATLL)

Adult T-cell leukemia/lymphoma (ATLL) is an aggressive malignancy of CD4⁺ T lymphocytes caused by human T lymphotropic virus type-1 (HTLV-1) infection. HTLV-1 was brought to the World Health Organization (WHO) and researchers to address its impact on global public health, oncogenicity, and deterioration of the host immune system toward autoimmunity. In a minority of the infected population (3-5%), it can induce inflammatory networks toward HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), or hijacking the infected CD4⁺ T lymphocytes into T regulatory subpopulation, stimulating anti-inflammatory signaling networks, and prompting ATLL development. This review critically discusses the complex signaling networks in ATLL pathogenesis during virus-host interactions for better interpretation of oncogenicity and introduces the main candidates in the pathogenesis of ATLL. At least two viral factors, HTLV-1 trans-activator protein (TAX) and HTLV-1 basic leucine zipper factor (HBZ), are implicated in ATLL manifestation, interacting with host responses and deregulating cell signaling in favor of infected cell survival and virus dissemination. Such molecules can be used as potential novel biomarkers for ATLL prognosis or targets for therapy. Moreover, the challenging aspects of HTLV-1 oncogenesis introduced in this review could open new venues for further studies on acute leukemia pathogenesis. These features can aid in the discovery of effective immunotherapies when reversing the gene expression profile toward appropriate immune responses gradually becomes attainable.

Hijacking Host Immunity by the Human T-Cell Leukemia Virus Type-1: Implications for Therapeutic and Preventive Vaccines

Human T-cell Leukemia virus type-1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and other inflammatory diseases. High viral DNA burden (VL) in peripheral blood mononuclear cells is a documented risk factor for ATLL and HAM/TSP, and patients with HAM/TSP have a higher VL in cerebrospinal fluid than in peripheral blood. VL alone is not sufficient to differentiate symptomatic patients from healthy carriers, suggesting the importance of other factors, including host immune response. HTLV-1 infection is life-long; CD4+-infected cells are not eradicated by the immune response because HTLV-1 inhibits the function of dendritic cells, monocytes, Natural Killer cells, and adaptive cytotoxic CD8+ responses. Although the majority of infected CD4+ T-cells adopt a resting phenotype, antigen stimulation may result in bursts of viral expression. The antigen-dependent "on-off" viral expression creates "conditional latency" that when combined with ineffective host responses precludes virus eradication. Epidemiological and clinical data suggest that the continuous attempt of the host immunity to eliminate infected cells results in chronic immune activation that can be further exacerbated by co-morbidities, resulting in the development of severe disease. We review cell and animal model studies that uncovered mechanisms used by HTLV-1 to usurp and/or counteract host immunity.

HIV Infection and Spread between Th17 Cells

HIV mainly targets CD4+ T cells, from which Th17 cells represent a major cell type, permissive, and are capable of supporting intracellular replication at mucosal sites. Th17 cells possess well-described dual roles, while being central to maintaining gut integrity, these may induce inflammation and contribute to autoimmune disorders; however, Th17 cells’ antiviral function in HIV infection is not completely understood. Th17 cells are star players to HIV-1 pathogenesis and a potential target to prevent or decrease HIV transmission. HIV-1 can be spread among permissive cells via direct cell-to-cell and/or cell-free infection. The debate on which mode of transmission is more efficient is still ongoing without a concrete conclusion yet. Most assessments of virus transmission analyzing either cell-to-cell or cell-free modes use in vitro systems; however, the actual interactions and conditions in vivo are not fully understood. The fact that infected breast milk, semen, and vaginal secretions contain a mix of both cell-free viral particles and infected cells presents an argument for the probability of HIV taking advantage of both modes of transmission to spread. Here, we review important insights and recent findings about the role of Th17 cells during HIV pathogenesis in mucosal surfaces, and the mechanisms of HIV-1 infection spread among T cells in tissues.

Anti-HIV Activity of Snake Venom Phospholipase A2s: Updates for New Enzymes and Different Virus Strains

Since the beginning of the HIV epidemic, lasting more than 30 years, the main goal of scientists was to develop effective methods for the prevention and treatment of HIV infection. Modern medicines have reduced the death rate from AIDS by 80%. However, they still have side effects and are very expensive, dictating the need to search for new drugs. Earlier, it was shown that phospholipases A2 (PLA2s) from bee and snake venoms block HIV replication, the effect being independent on catalytic PLA2 activity. However, the antiviral activity of human PLA2s against Lentiviruses depended on catalytic function and was mediated through the destruction of the viral membrane. To clarify the role of phospholipolytic activity in antiviral effects, we analyzed the anti-HIV activity of several snake PLA2s and found that the mechanisms of their antiviral activity were similar to that of mammalian PLA2. Our results indicate that snake PLA2s are capable of inhibiting syncytium formation between chronically HIV-infected cells and healthy CD4-positive cells and block HIV binding to cells. However, only dimeric PLA2s had pronounced virucidal and anti-HIV activity, which depended on their catalytic activity. The ability of snake PLA2s to inactivate the virus may provide an additional barrier to HIV infection. Thus, snake PLA2s might be considered as candidates for lead molecules in anti-HIV drug development.

The Underestimated Role of Platelets in Severe Infection a Narrative Review

Beyond their role in hemostasis, platelets have emerged as key contributors in the immune response; accordingly, the occurrence of thrombocytopenia during sepsis/septic shock is a well-known risk factor of mortality and a marker of disease severity. Recently, some studies elucidated that the response of platelets to infections goes beyond a simple fall in platelets count; indeed, sepsis-induced thrombocytopenia can be associated with—or even anticipated by—several changes, including an altered morphological pattern, receptor expression and aggregation. Of note, alterations in platelet function and morphology can occur even with a normal platelet count and can modify, depending on the nature of the pathogen, the pattern of host response and the severity of the infection. The purpose of this review is to give an overview on the pathophysiological interaction between platelets and pathogens, as well as the clinical consequences of platelet dysregulation. Furthermore, we try to clarify how understanding the nature of platelet dysregulation may help to optimize the therapeutic approach.

M-Sec induced by HTLV-1 mediates an efficient viral transmission

Human T-cell leukemia virus type 1 (HTLV-1) infects target cells primarily through cell-to-cell routes. Here, we provide evidence that cellular protein M-Sec plays a critical role in this process. When purified and briefly cultured, CD4⁺ T cells of HTLV-1 carriers, but not of HTLV-1^- individuals, expressed M-Sec. The viral protein Tax was revealed to mediate M-Sec induction. Knockdown or pharmacological inhibition of M-Sec reduced viral infection in multiple co-culture conditions. Furthermore, M-Sec knockdown reduced the number of proviral copies in the tissues of a mouse model of HTLV-1 infection. Phenotypically, M-Sec knockdown or inhibition reduced not only plasma membrane protrusions and migratory activity of cells, but also large clusters of Gag, a viral structural protein required for the formation of viral particles. Taken together, these results suggest that M-Sec induced by Tax mediates an efficient cell-to-cell viral infection, which is likely due to enhanced membrane protrusions, cell migration, and the clustering of Gag.

In the present study, we identified the cellular protein M-Sec as a host factor necessary for de novo infection of human T-cell leukemia virus type 1 (HTLV-1), the causative retrovirus of an aggressive blood cancer known as adult T-cell leukemia/lymphoma. The inhibition or knockdown of M-Sec in infected cells resulted in a reduced viral infection in several culture models and a mouse model. We recently demonstrated a similar role of M-Sec in macrophages infected with another human retrovirus HIV-1, but it has been generally thought that M-Sec is not related to HTLV-1 infection because of the lack of its expression in CD4⁺ T cells, the major target of HTLV-1. In this study, we revealed that CD4⁺ T cells of HTLV-1 asymptomatic carriers, but not those of HTLV-1^- individuals, expressed M-Sec, and that the viral protein Tax mediated the induction of M-Sec. Thus, M-Sec is a new and useful tool for further understanding the process of HTLV-1 transmission.

Cell Division Control Protein 42 Interacts With Hepatitis E Virus Capsid Protein and Participates in Hepatitis E Virus Infection

Hepatitis E Virus (HEV) causes viral hepatitis in humans worldwide, while a subset of HEV species, avian HEV, causes hepatitis-splenomegaly syndrome in chickens. To date, there are few reports on the host proteins interacting with HEV and being involved in viral infection. Previous pull-down assay combining mass spectrometry indicated that cell division control protein 42 (CDC42), a member belonging to the Rho GTPase family, was pulled down by avian HEV capsid protein. We confirmed the direct interaction between CDC42 and avian and mammalian HEV capsid proteins. The interaction can increase the amount of active guanosine triphosphate binding CDC42 state (GTP-CDC42). Subsequently, we determined that the expression and activity of CDC42 were positively correlated with HEV infection in the host cells. Using the different inhibitors of CDC42 downstream signaling pathways, we found that CDC42-MRCK (a CDC42-binding kinase)-non-myosin IIA (NMIIA) pathway is involved in naked avian and mammalian HEV infection, CDC42-associated p21-activated kinase 1 (PAK1)-NMIIA/Cofilin pathway is involved in quasi-enveloped mammalian HEV infection and CDC42-neural Wiskott-Aldrich syndrome protein-actin-polymerizing protein Arp2/3 pathway (CDC42-(N-)WASP-Arp2/3) pathway participates in naked and quasi-enveloped mammalian HEV infection. Collectively, these results demonstrated for the first time that HEV capsid protein can directly bind to CDC42, and non- and quasi-enveloped HEV use different CDC42 downstream signaling pathways to participate in viral infection. The study provided some new insights to understand the life cycle of HEV in host cells and a new target of drug design for combating HEV infection.

Extracellular vesicles originating from autophagy mediate an antibody-resistant spread of classical swine fever virus in cell culture

Free spread is a classical mode for mammalian virus transmission. However, the efficiency of this transmission approach is generally low as there are structural barriers or immunological surveillances in the extracellular environment under physiological conditions. In this study, we systematically analyzed the spreading of classical swine fever virus (CSFV) using multiple viral replication analysis in combination with antibody neutralization, transwell assay, and electron microscopy, and identified an extracellular vesicle (EV)-mediated spreading of CSFV in cell cultures. In this approach, intact CSFV virions are enclosed within EVs and transferred into uninfected cells with the movement of EVs, leading to an antibody-resistant infection of the virus. Using fractionation assays, immunostaining, and electron microscopy, we characterized the CSFV-containing EVs and demonstrated that the EVs originated from macroautophagy/autophagy. Taken together, our results showed a new spreading mechanism for CSFV and demonstrated that the EVs in CSFV spreading are closely related to autophagy. These findings shed light on the immune evasion mechanisms of CSFV transmission, as well as new functions of cellular vesicles in virus lifecycles.Abbreviations: 3-MA: 3-methyladenine; CCK-8: Cell Counting Kit-8; CSF: classical swine fever; CQ: chloroquine; CSFV: classical swine fever virus; DAPI, 4-,6-diamidino-2-phenylindole; EVs: extracellular vesicles; hpi: h post infection; IEM: immunoelectron microscopy; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MOI: multiplicity of infection; MVs: microvesicles; ND50: half neutralizing dose; PCR: polymerase chain reaction; PBS: phosphate-buffered saline; SEC: size-exclusion chromatography; siRNA: small interfering RNA; TEM: transmission electron microscopy.

Work-Related Human T-lymphotropic Virus 1 and 2 (HTLV-1/2) Infection: A Systematic Review

Human T-lymphotropic virus 1 and 2 (HTLV-1/2) belong to the delta group of retroviruses which may cause a life-long infection in humans, HTLV-1 leading to adult T-cell leukemia/lymphoma and other diseases. Different transmission modes have been described, such as breastfeeding, and, as for other blood-borne pathogens, unsafe sexual activity, intravenous drug usage, and blood transfusion and transplantation. The present systematic review was conducted to identify all peer-reviewed studies concerning the work-related infection by HTLV-1/2. A literature search was conducted from January to May 2021, according to the PRISMA methodology, selecting 29 studies: seven related to health care workers (HCWs), five to non-HCWs, and 17 to sex workers (SWs). The findings showed no clear evidence as to the possibility of HTLV-1/2 occupational transmission in HCWs, according to the limited number and quality of the papers. Moreover, non-HCWs showed a higher prevalence in jobs consistent with a lower socioeconomic status or that could represent a familial cluster, and an increased risk of zoonotic transmission from STLV-1-infected non-human primates has been observed in African hunters. Finally, a general increase of HTLV-1 infection was observed in SWs, whereas only one paper described an increased prevalence for HTLV-2, supporting the urgent need for prevention and control measures, including screening, diagnosis, and treatment of HTLV-1/2, to be offered routinely as part of a comprehensive approach to decrease the impact of sexually transmitted diseases in SWs.

G2-S16 Polyanionic Carbosilane Dendrimer Can Reduce HIV-1 Reservoir Formation by Inhibiting Macrophage Cell to Cell Transmission

Human immunodeficiency virus (HIV-1) is still a major problem, not only in developing countries but is also re-emerging in several developed countries, thus the development of new compounds able to inhibit the virus, either for prophylaxis or treatment, is still needed. Nanotechnology has provided the science community with several new tools for biomedical applications. G2-S16 is a polyanionic carbosilane dendrimer capable of inhibiting HIV-1 in vitro and in vivo by interacting directly with viral particles. One of the main barriers for HIV-1 eradication is the reservoirs created in primoinfection. These reservoirs, mainly in T cells, are untargetable by actual drugs or immune system. Thus, one approach is inhibiting HIV-1 from reaching these reservoir cells. In this context, macrophages play a main role as they can deliver viral particles to T cells establishing reservoirs. We showed that G2-S16 dendrimer is capable of inhibiting the infection from infected macrophages to healthy T CD4/CD8 lymphocytes by eliminating HIV-1 infectivity inside macrophages, so they are not able to carry infectious particles to other body locations, thus preventing the reservoirs from forming.

Virus-Mediated Cell-Cell Fusion

Cell-cell fusion between eukaryotic cells is a general process involved in many physiological and pathological conditions, including infections by bacteria, parasites, and viruses. As obligate intracellular pathogens, viruses use intracellular machineries and pathways for efficient replication in their host target cells. Interestingly, certain viruses, and, more especially, enveloped viruses belonging to different viral families and including human pathogens, can mediate cell-cell fusion between infected cells and neighboring non-infected cells. Depending of the cellular environment and tissue organization, this virus-mediated cell-cell fusion leads to the merge of membrane and cytoplasm contents and formation of multinucleated cells, also called syncytia, that can express high amount of viral antigens in tissues and organs of infected hosts. This ability of some viruses to trigger cell-cell fusion between infected cells as virus-donor cells and surrounding non-infected target cells is mainly related to virus-encoded fusion proteins, known as viral fusogens displaying high fusogenic properties, and expressed at the cell surface of the virus-donor cells. Virus-induced cell-cell fusion is then mediated by interactions of these viral fusion proteins with surface molecules or receptors involved in virus entry and expressed on neighboring non-infected cells. Thus, the goal of this review is to give an overview of the different animal virus families, with a more special focus on human pathogens, that can trigger cell-cell fusion.

Lymphotropic Viruses: Chronic Inflammation and Induction of Cancers

Inflammation induced by transcription factors, including Signal Transducers and Activators of Transcription (STATs) and NF-κB, in response to microbial pathogenic infections and ligand dependent receptors stimulation are critical for controlling infections. However, uncontrolled inflammation induced by these transcription factors could lead to immune dysfunction, persistent infection, inflammatory related diseases and the development of cancers. Although the induction of innate immunity and inflammation in response to viral infection is important to control virus replication, its effects can be modulated by lymphotropic viruses including human T-cell leukemia virus type 1 (HTLV-1), Κaposi's sarcoma herpesvirus (KSHV), and Epstein Barr virus (EBV) during de novo infection as well as latent infection. These lymphotropic viruses persistently activate JAK-STAT and NF-κB pathways. Long-term STAT and NF-κB activation by these viruses leads to the induction of chronic inflammation, which can support the persistence of these viruses and promote virus-mediated cancers. Here, we review how HTLV-1, KSHV and EBV hijack the function of host cell surface molecules (CSMs), which are involved in the regulation of chronic inflammation, innate and adaptive immune responses, cell death and the restoration of tissue homeostasis. Thus, better understanding of CSMs-mediated chronic activation of STATs and NF-κB pathways in lymphotropic virus-infected cells may pave the way for therapeutic intervention in malignancies caused by lymphotropic viruses.

Cell-to-Cell Spread of Dengue Viral RNA in Mosquito Cells

Dengue virus (DENV) is an important mosquito-borne arbovirus that is particularly prevalent in tropical and subtropical areas of the world. The virus is generally ingested with a blood meal, replicates in host tissues, and disseminates into salivary glands for transmission to the next host. Membrane-bound vacuoles carrying DENV particles have been documented in mosquito cells and play a role in the cell-to-cell transmission of DENV2. C189 is one member of the tetraspanin family and generally increases its expression as one component of the vacuoles (C189-VCs) within C6/36 cells infected with DENV2. In the present study, we have further demonstrated via sucrose gradient centrifugation as well as magnetic immune isolation (MI) that the RNA of DENV2 was eventually carried by C189-VCs. In addition, viral RNA was shown to spread from donor to recipient cells in a coculture assay even when 20 mM NH₄Cl was added to inhibit virus replication in the culture. In an alternate assay using the transwell system, viral RNA was only detected in recipient cells in the absence of 40 mM NH₄Cl, suggesting that cell-cell contact is required for the intercellular spread of DENV2. In turn, the formation of viral synapse (VS) derived from aggregates of viral particles was frequently observed at sites of cell contact. Taken together, the formation of C189-VCs in C6/36 cells is induced by DENV2 infection, which may serve as a vehicle for transferring virions and also viral RNA to neighboring cells by cell-to-cell transmission after cell-cell contact. This finding provides insight into the understanding of viral spread between mosquito cells. It may also elucidate the benign persistent infection in mosquito cells and efficient dissemination of DENV infection within a mosquito vector.

p30 protein: a critical regulator of HTLV-1 viral latency and host immunity

The extraordinarily high prevalence of HTLV-1 subtype C (HTLV-1C) in some isolated indigenous communities in Oceania and the severity of the health conditions associated with the virus impress the great need for basic and translational research to prevent and treat HTLV-1 infection. The genome of the virus’s most common subtype, HTLV-1A, encodes structural, enzymatic, and regulatory proteins that contribute to viral persistence and pathogenesis. Among these is the p30 protein encoded by the doubly spliced Tax-orf II mRNA, a nuclear/nucleolar protein with both transcriptional and post-transcriptional activity. The p30 protein inhibits the productive replication cycle via nuclear retention of the mRNA that encodes for both the viral transcriptional trans-activator Tax, and the Rex proteins that regulate the transport of incompletely spliced viral mRNA to the cytoplasm. In myeloid cells, p30 inhibits the PU-1 transcription factor that regulates interferon expression and is a critical mediator of innate and adaptive immunity. Furthermore, p30 alters gene expression, cell cycle progression, and DNA damage responses in T-cells, raising the hypothesis that p30 may directly contribute to T cell transformation. By fine-tuning viral expression while also inhibiting host innate responses, p30 is likely essential for viral infection and persistence. This concept is supported by the finding that macaques, a natural host for the closely genetically related simian T-cell leukemia virus 1 (STLV-1), exposed to an HTLV-1 knockout for p30 expression by a single point mutation do not became infected unless reversion and selection of the wild type HTLV-1 genotype occurs. All together, these data suggest that inhibition of p30 may help to curb and eventually eradicate viral infection by exposing infected cells to an effective host immune response.

Restriction factors in human retrovirus infections and the unprecedented case of CIITA as link of intrinsic and adaptive immunity against HTLV-1

Background

Immunity against pathogens evolved through complex mechanisms that only for sake of simplicity are defined as innate immunity and adaptive immunity. Indeed innate and adaptive immunity are strongly intertwined each other during evolution. The complexity is further increased by intrinsic mechanisms of immunity that rely on the action of intracellular molecules defined as restriction factors (RFs) that, particularly in virus infections, counteract the action of pathogen gene products acting at different steps of virus life cycle.

Main body and conclusion

Here we provide an overview on the nature and the mode of action of restriction factors involved in retrovirus infection, particularly Human T Leukemia/Lymphoma Virus 1 (HTLV-1) infection. As it has been extensively studied by our group, special emphasis is given to the involvement of the MHC class II transactivator CIITA discovered in our laboratory as regulator of adaptive immunity and subsequently as restriction factor against HIV-1 and HTLV-1, a unique example of dual function linking adaptive and intrinsic immunity during evolution. We describe the multiple molecular mechanisms through which CIITA exerts its restriction on retroviruses. Of relevance, we review the unprecedented findings pointing to a concerted action of several restriction factors such as CIITA, TRIM22 and TRIM19/PML in synergizing against retroviral replication. Finally, as CIITA profoundly affects HTLV-1 replication by interacting and inhibiting the function of HTLV-1 Tax-1 molecule, the major viral product associated to the virus oncogenicity, we also put forward the hypothesis of CIITA as counteractor of HTLV-1-mediated cancer initiation.

Cell-to-Cell Spreading of HIV-1 in Myeloid Target Cells Escapes SAMHD1 Restriction

We demonstrate that HIV-1 uses a common two-step cell-to-cell fusion mechanism for massive virus transfer from infected T lymphocytes and dissemination to myeloid target cells, including dendritic cells and macrophages as well as osteoclasts. This cell-to-cell infection process bypasses the restriction imposed by the SAMHD1 host cell restriction factor for HIV-1 replication, leading to the formation of highly virus-productive multinucleated giant cells as observed in vivo in lymphoid and nonlymphoid tissues of HIV-1-infected patients. Since myeloid cells are emerging as important target cells of HIV-1, these results contribute to a better understanding of the role of these myeloid cells in pathogenesis, including cell-associated virus sexual transmission, cell-to-cell virus spreading, and establishment of long-lived viral tissue reservoirs.

Dendritic cells (DCs) and macrophages as well as osteoclasts (OCs) are emerging as target cells of HIV-1 involved in virus transmission, dissemination, and establishment of persistent tissue virus reservoirs. While these myeloid cells are poorly infected by cell-free viruses because of the high expression levels of cellular restriction factors such as SAMHD1, we show here that HIV-1 uses a specific and common cell-to-cell fusion mechanism for virus transfer and dissemination from infected T lymphocytes to the target cells of the myeloid lineage, including immature DCs (iDCs), OCs, and macrophages, but not monocytes and mature DCs. The establishment of contacts with infected T cells leads to heterotypic cell fusion for the fast and massive transfer of viral material into OC and iDC targets, which subsequently triggers homotypic fusion with noninfected neighboring OCs and iDCs for virus dissemination. These two cell-to-cell fusion processes are not restricted by SAMHD1 and allow very efficient spreading of virus in myeloid cells, resulting in the formation of highly virus-productive multinucleated giant cells. These results reveal the cellular mechanism for SAMHD1-independent cell-to-cell spreading of HIV-1 in myeloid cell targets through the formation of the infected multinucleated giant cells observed in vivo in lymphoid and nonlymphoid tissues of HIV-1-infected patients.

Spatiotemporal immunofluorescent evaluation of porcine reproductive and respiratory syndrome virus transmission across the maternal-fetal interface

Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes severe reproductive failure characterized by high fetal morbidity and mortality leading to substantial economic losses to the swine industry. Evaluation of spatiotemporal transmission of PRRSV at the maternal-fetal interface (MFI) is critical for understanding fetal infection. Localization of PRRSV-2 strain NVSL 97-7895 at different regions of the MFI in 20 pregnant gilts at 2, 5, 8, 12 and 14 days post-inoculation (dpi) were analyzed by immunofluorescence (IF). Samples of MFI were collected from 15 inoculated and 5 control gilts and transplacental PRRSV transmission assessed in randomly selected fetuses from each litter. Localization of NVSL 97-7895 antigen immunoreactivity in the MFI was focused in three major areas: endometrial connective tissues (ENDO), the feto-maternal junction (FMJ) and fetal placenta (PLC). NVSL 97-7895 was detected at the FMJ by 2 dpi. At 2, 5 and 8 dpi, NVSL 97-7895 was localized within the ENDO and FMJ, whereas at 12 and 14 dpi, it was mainly localized in the PLC. Using a novel IF strategy for counting and size sorting NVSL 97-7895 viral antigen in situ, results of this study indicate that non-cell-associated mechanisms are involved in PRRSV transmission across the MFI.

HTLV-1 Extracellular Vesicles Promote Cell-to-Cell Contact

Human T-cell leukemia virus-1 (HTLV-1) is a neglected and incurable retrovirus estimated to infect 5 to 10 million worldwide. Specific indigenous Australian populations report infection rates of more than 40%, suggesting a potential evolution of the virus with global implications. HTLV-1 causes adult T-cell leukemia/lymphoma (ATLL), and a neurological disease named HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). Even though HTLV-1 transmission primarily occurs from cell-to-cell, there is still a gap of knowledge regarding the mechanisms of viral spread and disease progression. We have recently shown that Extracellular Vesicles (EVs) ubiquitously produced by cells may be used by HTLV-1 to transport viral proteins and RNA, and elicit adverse effects on recipient uninfected cells. The viral proteins Tax and HBZ are involved in disease progression and impairment of autophagy in infected cells. Here, we show that activation of HTLV-1 via ionizing radiation (IR) causes a significant increase of intracellular Tax, but not EV-associated Tax. Also, lower density EVs from HTLV-1-infected cells, separated by an Iodixanol density gradient, are positive for gp61+++/Tax+++/HBZ+ proteins (HTLV-1 EVs). We found that HTLV-1 EVs are not infectious when tested in multiple cell lines. However, these EVs promote cell-to-cell contact of uninfected cells, a phenotype which was enhanced with IR, potentially promoting viral spread. We treated humanized NOG mice with HTLV-1 EVs prior to infection and observed an increase in viral RNA synthesis in mice compared to control (EVs from uninfected cells). Proviral DNA levels were also quantified in blood, lung, spleen, liver, and brain post-treatment with HTLV-1 EVs, and we observed a consistent increase in viral DNA levels across all tissues, especially the brain. Finally, we show direct implications of EVs in viral spread and disease progression and suggest a two-step model of infection including the release of EVs from donor cells and recruitment of recipient cells as well as an increase in recipient cell-to-cell contact promoting viral spread.

Inhibition of Tunneling Nanotube (TNT) Formation and Human T-cell Leukemia Virus Type 1 (HTLV-1) Transmission by Cytarabine

The human T-cell leukemia virus type 1 (HTLV-1) is highly dependent on cell-to-cell interaction for transmission and productive infection. Cell-to-cell interactions through the virological synapse, biofilm-like structures and cellular conduits have been reported, but the relative contribution of each mechanism on HTLV-1 transmission still remains vastly unknown. The HTLV-1 protein p8 has been found to increase viral transmission and cellular conduits. Here we show that HTLV-1 expressing cells are interconnected by tunneling nanotubes (TNTs) defined as thin structures containing F-actin and lack of tubulin connecting two cells. TNTs connected HTLV-1 expressing cells and uninfected T-cells and monocytes and the viral proteins Tax and Gag localized to these TNTs. The HTLV-1 expressing protein p8 was found to induce TNT formation. Treatment of MT-2 cells with the nucleoside analog cytarabine (cytosine arabinoside, AraC) reduced number of TNTs and furthermore reduced TNT formation induced by the p8 protein. Intercellular transmission of HTLV-1 through TNTs provides a means of escape from recognition by the immune system. Cytarabine could represent a novel anti-HTLV-1 drug interfering with viral transmission.

HIV transmission from infected CD4+ T cells to allogenic T and dendritic cells is inhibited by broadly neutralizing antibodies

OBJECTIVE

In the semen, both free virus and infected cells are able to establish HIV infection during sexual intercourse. An efficient vaccine should therefore inhibit both infectious states. The aim of this study was to analyze the capacity of broadly neutralizing antibodies (bNAbs) to inhibit HIV transmission by the infected cells.

DESIGN/METHODS

We developed an in-vitro model aiming to mimic mucosal HIV transmission via infected cells. PHA-activated CD4+ T cells stained with PKH26 from donor A were infected and co-cultured with CD4+ T cells and dendritic cells from donor B in the presence of bNAbs.

RESULTS

We showed that dendritic cells were the preferential HIV target cells at early time points in this co-culture model. In the context of this co-culture model where infection and transmission occurred simultaneously, bNAbs efficiently inhibited HIV replication as well as HIV transmission from infected cells to allogenic dendritic cells and CD4+ T cells.

CONCLUSION

Overall, our results indicate that dendritic cells, in addition to CD4+ T cells, are key cells that are efficiently infected by HIV and bNAbs are potent inhibitors of infection of both target cells. Future HIV prophylactic vaccine design should develop immune strategies able to prevent the infection of dendritic cells, in addition to the inhibition of CD4+ T-cell infection.

The HSV-1 mechanisms of cell-to-cell spread and fusion are critically dependent on host PTP1B

All herpesviruses have mechanisms for passing through cell junctions, which exclude neutralizing antibodies and offer a clear path to neighboring, uninfected cells. In the case of herpes simplex virus type 1 (HSV-1), direct cell-to-cell transmission takes place between epithelial cells and sensory neurons, where latency is established. The spreading mechanism is poorly understood, but mutations in four different HSV-1 genes can dysregulate it, causing neighboring cells to fuse to produce syncytia. Because the host proteins involved are largely unknown (other than the virus entry receptor), we were intrigued by an earlier discovery that cells infected with wild-type HSV-1 will form syncytia when treated with salubrinal. A biotinylated derivative of this drug was used to pull down cellular complexes, which were analyzed by mass spectrometry. One candidate was a protein tyrosine phosphatase (PTP1B), and although it ultimately proved not to be the target of salubrinal, it was found to be critical for the mechanism of cell-to-cell spread. In particular, a highly specific inhibitor of PTP1B (CAS 765317-72-4) blocked salubrinal-induced fusion, and by itself resulted in a dramatic reduction in the ability of HSV-1 to spread in the presence of neutralizing antibodies. The importance of this phosphatase was confirmed in the absence of drugs by using PTP1B^-/- cells. Importantly, replication assays showed that virus titers were unaffected when PTP1B was inhibited or absent. Only cell-to-cell spread was altered. We also examined the effects of salubrinal and the PTP1B inhibitor on the four Syn mutants of HSV-1, and strikingly different responses were found. That is, both drugs individually enhanced fusion for some mutants and reduced fusion for others. PTP1B is the first host factor identified to be specifically required for cell-to-cell spread, and it may be a therapeutic target for preventing HSV-1 reactivation disease.

It is estimated that 67% of the global population is infected with herpes simplex virus type 1 (HSV-1). This virus resides in sensory neurons in a quiescent state but periodically reactivates, producing virus particles that travel down the axon to infect epithelial cells of the skin, where it can be transmitted to additional people. To avoid neutralizing antibodies, herpesviruses have evolved mechanisms for moving directly from one cell to another through their sites of intimate contact; however, the mechanism of cell-to-cell spread is poorly understood. Studies of HSV-1 mutants have implicated numerous viral proteins, but the necessary cellular factors are unknown except for the one that the virus uses to enter cells. Our experiments have identified a cellular enzyme (PTP1B, a tyrosine phosphatase) that is dispensable for the production of infectious virions but is critically important for the cell-to-cell spreading mechanism. Promising drugs targeting PTP1B have already been tested in early clinical trials for possible treatment of obesity and type-2 diabetes, and thus, our study may have immediate utility for attenuating HSV-1 reactivation disease in immunocompromised patients.

Platelets in Immune Response to Virus and Immunopathology of Viral Infections

Platelets are essential effector cells in hemostasis. Aside from their role in coagulation, platelets are now recognized as major inflammatory cells with key roles in the innate and adaptive arms of the immune system. Activated platelets have key thromboinflammatory functions linking coagulation to immune responses in various infections, including in response to virus. Recent studies have revealed that platelets exhibit several pattern recognition receptors (PRR) including those from the toll-like receptor, NOD-like receptor, and C-type lectin receptor family and are first-line sentinels in detecting and responding to pathogens in the vasculature. Here, we review the main mechanisms of platelets interaction with viruses, including their ability to sustain viral infection and replication, their expression of specialized PRR, and activation of thromboinflammatory responses against viruses. Finally, we discuss the role of platelet-derived mediators and platelet interaction with vascular and immune cells in protective and pathophysiologic responses to dengue, influenza, and human immunodeficiency virus 1 infections.

Tunneling Nanotubes as a Novel Route of Cell-to-Cell Spread of Herpesviruses

Various types of intercellular connections that are essential for communication between cells are often utilized by pathogens. Recently, a new type of cellular connection, consisting of long, thin, actin-rich membrane extensions named tunneling nanotubes (TNTs), has been shown to play an important role in cell-to-cell spread of HIV and influenza virus. In the present report, we show that TNTs are frequently formed by cells infected by an alphaherpesvirus, bovine herpesvirus 1 (BoHV-1). Viral proteins, such as envelope glycoprotein E (gE), capsid protein VP26, and tegument protein Us3, as well as cellular organelles (mitochondria) were detected by immunofluorescence and live-cell imaging of nanotubes formed by bovine primary fibroblasts and oropharynx cells (KOP cells). Time-lapse confocal studies of live cells infected with fluorescently labeled viruses showed that viral particles were transmitted via TNTs. This transfer also occurred in the presence of neutralizing antibodies, which prevented free entry of BoHV-1. We conclude that TNT formation contributes to successful cell-to-cell spread of BoHV-1 and demonstrate for the first time the participation of membrane nanotubes in intercellular transfer of a herpesvirus in live cells.IMPORTANCE Efficient transmission of viral particles between cells is an important factor in successful infection by herpesviruses. Herpesviruses can spread by the free-entry mode or direct cell-to-cell transfer via cell junctions and long extensions of neuronal cells. In this report, we show for the first time that an alphaherpesvirus can also spread between various types of cells using tunneling nanotubes, intercellular connections that are utilized by HIV and other viruses. Live-cell monitoring revealed that viral transmission occurs between the cells of the same type as well as between epithelial cells and fibroblasts. This newly discovered route of herpesviruses spread may contribute to efficient transmission despite the presence of host immune responses, especially after reactivation from latency that developed after primary infection. Long-range communication provided by TNTs may facilitate the spread of herpesviruses between many tissues and organs of an infected organism.

Mechanisms for Cell-to-Cell Transmission of HIV-1

While HIV-1 infection of target cells with cell-free viral particles has been largely documented, intercellular transmission through direct cell-to-cell contact may be a predominant mode of propagation in host. To spread, HIV-1 infects cells of the immune system and takes advantage of their specific particularities and functions. Subversion of intercellular communication allows to improve HIV-1 replication through a multiplicity of intercellular structures and membrane protrusions, like tunneling nanotubes, filopodia, or lamellipodia-like structures involved in the formation of the virological synapse. Other features of immune cells, like the immunological synapse or the phagocytosis of infected cells are hijacked by HIV-1 and used as gateways to infect target cells. Finally, HIV-1 reuses its fusogenic capacity to provoke fusion between infected donor cells and target cells, and to form infected syncytia with high capacity of viral production and improved capacities of motility or survival. All these modes of cell-to-cell transfer are now considered as viral mechanisms to escape immune system and antiretroviral therapies, and could be involved in the establishment of persistent virus reservoirs in different host tissues.

Adult T-Cell Leukemia/Lymphoma

Adult T-cell lymphoma/leukemia (ATL) is a rare T-cell lymphoproliferative neoplasm caused by human T-lymphotrophic virus 1. In its more common, aggressive forms, ATL carries one of the poorest prognoses of the non-Hodgkin lymphomas. The disease has clinical subtypes (ie, acute, lymphoma, chronic, and smoldering forms) defined by the presenting features, and therefore, the clinical course can vary. For the smoldering and lower-risk chronic forms, combinations involving antiviral therapies have shown some success. However, in many patients, the more indolent forms will evolve into the more aggressive subtypes. In the more aggressive acute, lymphoma, and higher-risk chronic forms, the literature supports initial treatment with combination chemotherapy followed by allogeneic transplantation as a potentially curative approach. Recently, mogamulizumab and lenalidomide have shown promise in the treatment of ATL. With better understanding of the molecular drivers of this disease, we hope that the therapeutic landscape will continue to expand.

HTLV-1, the Other Pathogenic Yet Neglected Human Retrovirus: From Transmission to Therapeutic Treatment

Going back to their discovery in the early 1980s, both the Human T-cell Leukemia virus type-1 (HTLV-1) and the Human Immunodeficiency Virus type-1 (HIV-1) greatly fascinated the virology scene, not only because they were the first human retroviruses discovered, but also because they were associated with fatal diseases in the human population. In almost four decades of scientific research, both viruses have had different fates, HTLV-1 being often upstaged by HIV-1. However, although being very close in terms of genome organization, cellular tropism, and viral replication, HIV-1 and HTLV-1 are not completely commutable in terms of treatment, especially because of the opposite fate of the cells they infect: death versus immortalization, respectively. Nowadays, the antiretroviral therapies developed to treat HIV-1 infected individuals and to limit HIV-1 spread among the human population have a poor or no effect on HTLV-1 infected individuals, and thus, do not prevent the development of HTLV-1-associated diseases, which still lack highly efficient treatments. The present review mainly focuses on the course of HTLV-1 infection, from the initial infection of the host to diseases development and associated treatments, but also investigates HIV-1/HTLV-1 co-infection events and their impact on diseases development.

Human T Cell Leukemia Virus Type 1: Persistence and Pathogenesis

Human T cell leukemia virus type 1 (HTLV-1), also known as human T lymphotropic virus type 1, was the first exogenous human retrovirus discovered. Unlike the distantly related lentivirus HIV-1, HTLV-1 causes disease in only 5-10% of infected people, depending on their ethnic origin. But whereas HIV-1 infection and the consequent diseases can be efficiently contained in most cases by antiretroviral drug treatment, there is no satisfactory treatment for the malignant or inflammatory diseases caused by HTLV-1. The purpose of the present article is to review recent advances in the understanding of the mechanisms by which the virus persists in vivo and causes disabling or fatal diseases.

Dendritic cell maturation, but not type I interferon exposure, restricts infection by HTLV-1, and viral transmission to T-cells

Human T lymphotropic Virus type 1 (HTLV-1) is the etiological agent of Adult T cell Leukemia/Lymphoma (ATLL) and HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP). Both CD4+ T-cells and dendritic cells (DCs) infected with HTLV-1 are found in peripheral blood from HTLV-1 carriers. We previously demonstrated that monocyte-derived IL-4 DCs are more susceptible to HTLV-1 infection than autologous primary T-cells, suggesting that DC infection precedes T-cell infection. However, during blood transmission, breast-feeding or sexual transmission, HTLV-1 may encounter different DC subsets present in the blood, the intestinal or genital mucosa respectively. These different contacts may impact HTLV-1 ability to infect DCs and its subsequent transfer to T-cells. Using in vitro monocyte-derived IL-4 DCs, TGF-β DCs and IFN-α DCs that mimic DCs contacting HTLV-1 in vivo, we show here that despite their increased ability to capture HTLV-1 virions, IFN-α DCs restrict HTLV-1 productive infection. Surprisingly, we then demonstrate that it is not due to the antiviral activity of type-I interferon produced by IFN-α DCs, but that it is likely to be linked to a distinct trafficking route of HTLV-1 in IL-4 DCs vs. IFN-α DCs. Finally, we demonstrate that, in contrast to IL-4 DCs, IFN-α DCs are impaired in their capacity to transfer HTLV-1 to CD4 T-cells, both after viral capture and trans-infection and after their productive infection. In conclusion, the nature of the DCs encountered by HTLV-1 upon primo-infection and the viral trafficking route through the vesicular pathway of these cells determine the efficiency of viral transmission to T-cells, which may condition the fate of infection.

Human T-Cell Leukemia Virus Type 1 Infection and Adult T-Cell Leukemia

Human T-cell leukemia virus type 1 (HTLV-1) is the first retrovirus discovered to cause adult T-cell leukemia (ATL), a highly aggressive blood cancer. HTLV-1 research in the past 35 years has been most revealing in the mechanisms of viral oncogenesis. HTLV-1 establishes a lifelong persistent infection in CD4⁺ T lymphocytes. The infection outcome is governed by host immunity. ATL develops in 2-5% of infected individuals 30-50 years after initial exposure. HTLV-1 encodes two oncoproteins Tax and HBZ, which are required for initiation of cellular transformation and maintenance of cell proliferation, respectively. HTLV-1 oncogenesis is driven by a clonal selection and expansion process during which both host and viral factors cooperate to impair genome stability, immune surveillance, and other mechanisms of tumor suppression. A better understanding of HTLV-1 biology and leukemogenesis will reveal new strategies and modalities for ATL prevention and treatment.

Enhanced Stabilization of MCL1 by the Human T-Cell Leukemia Virus Type 1 bZIP Factor Is Modulated by Blocking the Recruitment of Cullin 1 to the SCF Complex

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that is the etiological agent of adult T-cell leukemia (ATL). The HTLV-1 basic leucine zipper factor (HBZ), which is encoded by the minus strand of the provirus, is constitutively expressed in all ATL patient cells and likely contributes to the development and maintenance of ATL. Furthermore, the overexpression of the myeloid cell leukemia 1 (MCL1) protein is frequently observed in hematological cancers as well as several other types of cancers. Here, we found that the expression of HBZ in cells stabilized MCL1 protein expression and suppressed the MCL1-mediated release of cytochrome c from the mitochondria. This effect was mediated by inhibition of the ubiquitin-dependent degradation of MCL1. In a serial binding assay, HBZ interacted with cullin 1 (CUL1) through a head-to-tail interaction. The association between CUL1 and Skp1, which serves as the molecular scaffold for the components of SCF ubiquitin ligase complexes, was markedly repressed in the presence of HBZ. Mechanistic analysis indicated that HBZ abrogated the CUL1 association with Skp1, which in turn promoted the cellular expression of MCL1. This novel function of HBZ likely plays a role in the viral pathogenesis of HTLV-1 and provides important insights into our understanding of the development of ATL.