Gem-induced cytoskeleton remodeling increases cellular migration of HTLV-1-infected cells, formation of infected-to-target T-cell conjugates and viral transmission

Host Subcellular Organelles: Targets of Viral Manipulation

Viruses have evolved sophisticated mechanisms to manipulate host cell processes and utilize intracellular organelles to facilitate their replication. These complex interactions between viruses and cellular organelles allow them to hijack the cellular machinery and impair homeostasis. Moreover, viral infection alters the cell membrane's structure and composition and induces vesicle formation to facilitate intracellular trafficking of viral components. However, the research focus has predominantly been on the immune response elicited by viruses, often overlooking the significant alterations that viruses induce in cellular organelles. Gaining a deeper understanding of these virus-induced cellular changes is crucial for elucidating the full life cycle of viruses and developing potent antiviral therapies. Exploring virus-induced cellular changes could substantially improve our understanding of viral infection mechanisms.

Upregulation of Neuropilin-1 Inhibits HTLV-1 Infection

Infection with human T-cell leukemia virus type 1 (HTLV-1) can produce a spectrum of pathological effects ranging from inflammatory disorders to leukemia. In vivo, HTLV-1 predominantly infects CD4⁺ T-cells. Infectious spread within this population involves the transfer of HTLV-1 virus particles from infected cells to target cells only upon cell-to-cell contact. The viral protein, HBZ, was found to enhance HTLV-1 infection through transcriptional activation of ICAM1 and MYOF, two genes that facilitate viral infection. In this study, we found that HBZ upregulates the transcription of COL4A1, GEM, and NRP1. COL4A1 and GEM are genes involved in viral infection, while NRP1, which encodes neuropilin 1 (Nrp1), serves as an HTLV-1 receptor on target cells but has no reported function on HTLV-1-infected cells. With a focus on Nrp1, cumulative results from chromatin immunoprecipitation assays and analyses of HBZ mutants support a model in which HBZ upregulates NRP1 transcription by augmenting recruitment of Jun proteins to an enhancer downstream of the gene. Results from in vitro infection assays demonstrate that Nrp1 expressed on HTLV-1-infected cells inhibits viral infection. Nrp1 was found to be incorporated into HTLV-1 virions, and deletion of its ectodomain removed the inhibitory effect. These results suggest that inhibition of HTLV-1 infection by Nrp1 is caused by the ectodomain of Nrp1 extended from virus particles, which may inhibit the binding of virus particles to target cells. While HBZ has been found to enhance HTLV-1 infection using cell-based models, there may be certain circumstances in which activation of Nrp1 expression negatively impacts viral infection, which is discussed.

HAM/TSP Pathogenesis: The Transmigration Activity of HTLV-1-Infected T Cells into Tissues

Slowly progressive spastic paraparesis with bladder dysfunction, the main clinical feature of human T-cell leukemia virus-1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP), is induced by chronic inflammation in the spinal cord, mainly the lower thoracic cord. A long-standing bystander mechanism, such as the destruction of surrounding tissues by inflammatory cytokines, etc., induced under the interaction between infiltrated HTLV-1-infected CD4⁺ T cells and HTLV-1-specific CD8⁺ cytotoxic T cells, has been considered implicated for the induction of chronic inflammation. As this bystander mechanism is triggered conceivably by the transmigration of HTLV-1-infected CD4⁺ T cells to the spinal cord, heightened transmigrating activity of HTLV-1-infected CD4⁺ T cells to the spinal cord might play a crucial role as the first responder in the development of HAM/TSP. This review evaluated the functions of HTLV-1-infected CD4⁺ T cells in HAM/TSP patients as the prerequisite for the acquisition of the activity such as adhesion molecule expression changes, small GTPases activation, and expression of mediators involved in basement membrane disruption. The findings suggest that HTLV-1-infected CD4⁺ T cells in HAM/TSP patients have enough potential to facilitate transmigration into the tissues. Future HAM/TSP research should clarify the molecular mechanisms leading to the establishment of HTLV-1-infected CD4⁺ T cells as the first responder in HAM/TSP patients. In addition, a regimen with an inhibitory activity against the transmigration of HTLV-1-infected CD4⁺ T cells into the spinal cord might be recommended as one of the therapeutic strategies against HAM/TSP patients.

Transcriptomic alterations in the sweet orange vasculature correlate with growth repression induced by a variant of citrus tristeza virus

Citrus tristeza virus (CTV, family Closteroviridae) is an economically important pathogen of citrus. CTV resides in the phloem of the infected plants and induces a range of disease phenotypes, including stem pitting and quick decline as well as a number of other deleterious syndromes. To uncover the biological processes underlying the poorly understood damaging symptoms of CTV, we profiled the transcriptome of sweet orange (Citrus sinensis) phloem-rich bark tissues of non-infected, mock-inoculated trees and trees singly infected with two distinct variants of CTV, T36 or T68-1. The T36 and T68-1 variants accumulated in the infected plants at similar titers. With that, young trees infected with T68-1 were markedly repressed in growth, while the growth rate of the trees infected with T36 was comparable to the mock-inoculated trees. Only a small number of differentially expressed genes (DEGs) were identified in the nearly asymptomatic T36-infected trees, whereas almost fourfold the number of DEGs were identified with the growth-restricting T68-1 infection. DEGs were validated using quantitative reverse transcription-PCR. While T36 did not induce many noteworthy changes, T68-1 altered the expression of numerous host mRNAs encoding proteins within significant biological pathways, including immunity and stress response proteins, papain-like cysteine proteases (PLCPs), cell-wall modifying enzymes, vascular development proteins and others. The transcriptomic alterations in the T68-1-infected trees, in particular, the strong and persistent increase in the expression levels of PLCPs, appear to contribute to the observed stem growth repression. On the other hand, analysis of the viral small interfering RNAs revealed that the host RNA silencing-based response to the infection by T36 and that by T68-1 was comparable, and thus, the induction of this antiviral mechanism may not contribute to the difference in the observed symptoms. The DEGs identified in this study promote our understanding of the underlying mechanisms of the yet unexplained growth repression induced by severe CTV isolates in sweet orange trees.

Viral-induced neuroinflammation: Different mechanisms converging to similar exacerbated glial responses

There is increasing evidence that viral infections are the source/origin of various types of encephalitis, encephalomyelitis, and other neurological and cognitive disorders. While the involvement of certain viruses, such as the Nipah virus and measles virus, is known, the mechanisms of neural invasion and the factors that trigger intense immune reactions are not fully understood. Based on recent publications, this review discusses the role of the immune response, interactions between viruses and glial cells, and cytokine mediators in the development of inflammatory diseases in the central nervous system. It also highlights the significant gaps in knowledge regarding these mechanisms.

Dynamics and consequences of the HTLV-1 proviral plus-strand burst

Expression of the transcriptional transactivator protein Tax, encoded on the proviral plus-strand of human T-cell leukaemia virus type 1 (HTLV-1), is crucial for the replication of the virus, but Tax-expressing cells are rarely detected in fresh blood ex vivo. The dynamics and consequences of the proviral plus-strand transcriptional burst remain insufficiently characterised. We combined time-lapse live-cell imaging, single-cell tracking and mathematical modelling to study the dynamics of Tax expression at single-cell resolution in two naturally-infected, non-malignant T-cell clones transduced with a short-lived enhanced green fluorescent protein (d2EGFP) Tax reporter system. Five different patterns of Tax expression were observed during the 30-hour observation period; the distribution of these patterns differed between the two clones. The mean duration of Tax expression in the two clones was 94 and 417 hours respectively, estimated from mathematical modelling of the experimental data. Tax expression was associated with a transient slowing in cell-cycle progression and proliferation, increased apoptosis, and enhanced activation of the DNA damage response pathways. Longer-term follow-up (14 days) revealed an increase in the proportion of proliferating cells and a decrease in the fraction of apoptotic cells as the cells ceased Tax expression, resulting in a greater net expansion of the initially Tax-positive population. Time-lapse live-cell imaging showed enhanced cell-to-cell adhesion among Tax-expressing cells, and decreased cell motility of Tax-expressing cells at the single-cell level. The results demonstrate the within-clone and between-clone heterogeneity in the dynamics and patterns of HTLV-1 plus-strand transcriptional bursts and the balance of positive and negative consequences of the burst for the host cell.

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.

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.

Novel signaling pathways regulate SARS-CoV and SARS-CoV-2 infectious disease

Severe acute respiratory syndrome coronavirus (SARS-CoV)-2 induces severe infection, and it is responsible for a worldwide disease outbreak starting in late 2019. Currently, there are no effective medications against coronavirus. In the present study, we utilized a holistic bioinformatics approach to study gene signatures of SARS-CoV- and SARS-CoV-2-infected Calu-3 lung adenocarcinoma cells. Through the Gene Ontology platform, we determined that several cytokine genes were up-regulated after SARS-CoV-2 infection, including TNF, IL6, CSF2, IFNL1, IL-17C, CXCL10, and CXCL11. Differentially regulated pathways were detected by the Kyoto Encyclopedia of Genes and Genomes, gene ontology, and Hallmark platform, including chemokines, cytokines, cytokine receptors, cytokine metabolism, inflammation, immune responses, and cellular responses to the virus. A Venn diagram was utilized to illustrate common overlapping genes from SARS-CoV- and SARS-CoV-2-infected datasets. An Ingenuity pathway analysis discovered an enrichment of tumor necrosis factor- (TNF-) and interleukin (IL)-17-related signaling in a gene set enrichment analysis. Downstream networks were predicted by the Database for Annotation, Visualization, and Integrated Discovery platform also revealed that TNF and TNF receptor 2 signaling elicited leukocyte recruitment, activation, and survival of host cells after coronavirus infection. Our discovery provides essential evidence for transcript regulation and downstream signaling of SARS-CoV and SARS-CoV-2 infection.

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

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

Collagen IV (COL4A1, COL4A2), a Component of the Viral Biofilm, Is Induced by the HTLV-1 Oncoprotein Tax and Impacts Virus Transmission

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent for Adult T-Cell Leukemia/Lymphoma (ATLL) and HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP). HTLV-1 infects CD4⁺ T-cells via cell-to-cell transmission requiring reorganization of the cytoskeleton and expression of the viral transactivator and oncoprotein Tax. Viruses spread at the virological synapse (VS), a virus-induced specialized cell-cell contact, by polarized budding into synaptic clefts, and by cell surface transfer of viral biofilms (VBs). Since little is known about Tax’s role in formation of the VB, we asked which component of the VB is regulated by Tax and important for HTLV-1 transmission. Collagens are not only structural proteins of the extracellular matrix and basal membrane but also represent an important component of the VB. Here, we report that among the collagens known to be present in VBs, COL4 is specifically upregulated in the presence of HTLV-1 infection. Further, we found that transient expression of Tax is sufficient to induce COL4A1 and COL4A2 transcripts in Jurkat and CCRF-CEM T-cells, while robust induction of COL4 protein requires continuous Tax expression as shown in Tax-transformed T-cell lines. Repression of Tax led to a significant reduction of COL4A1/A2 transcripts and COL4 protein. Mechanistically, luciferase-based promoter studies indicate that Tax activates the COL4A2 and, to a less extent, the COL4A1 promoter. Imaging showing partial co-localization of COL4 with the viral Gag protein in VBs at the VS and transfer of COL4 and Gag to target cells suggests a role of COL4 in VB formation. Strikingly, in chronically infected C91-PL cells, knockout of COL4A2 impaired Gag transfer between infected T-cells and acceptor T-cells, while release of virus-like particles was unaffected. Taken together, we identified COL4 (COL4A1, COL4A2) as a component of the VB and a novel cellular target of Tax with COL4A2 appearing to impact virus transmission. Thus, this study is the first to provide a link between Tax’s activity and VB formation by hijacking COL4 protein functions.

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.

Human T-Cell Leukemia Virus Type 1 (HTLV-1) bZIP Factor Upregulates the Expression of ICAM-1 To Facilitate HTLV-1 Infection

Human T-cell leukemia virus type 1 (HTLV-1) causes multiple pathological effects, ranging from a form of leukemia to a spectrum of inflammation-mediated diseases. These diseases arise from one or several infected CD4⁺ T cells among thousands acquiring proliferation and survival advantages and ultimately becoming pathogenic. Given the low incidence of HTLV-1-associated diseases among carriers, such cellular evolutionary processes appear to occur rarely. Therefore, infectious spread of HTLV-1 within the T-cell population may be one underlying factor influencing disease development. Free HTLV-1 virions are poorly infectious, so infection of T cells relies on direct contact between infected and target cells. Following contact, virions pass to target cells through a virological synapse or cellular conduits or are transferred to target cells within an extracellular matrix. Lymphocyte functioning antigen 1 (LFA-1) on the surface of the target cell engaging with its ligand, ICAM-1, on the surface of the infected cell (effector cell) initiates and stabilizes cell-cell contact for infection. We found that stable expression of an HTLV-1 accessory protein, HTLV-1 bZIP factor (HBZ), in Jurkat T cells increases homotypic aggregation. This phenotype was attributed to elevated ICAM-1 expression in the presence of HBZ. Using a single-cycle replication-dependent luciferase assay, we found that HBZ expression in Jurkat cells (used as effector cells) increases HTLV-1 infection. Despite this effect, HBZ could not replace the critical infection-related functions of the HTLV-1 regulatory protein Tax. However, in HTLV-1-infected T cells, knockdown of HBZ expression did lead to a decrease in infection efficiency. These overall results suggest that HBZ contributes to HTLV-1 infectivity.IMPORTANCE Human T-cell leukemia virus type 1 (HTLV-1) causes a variety of diseases, ranging from a fatal form of leukemia to immune-mediated inflammatory diseases. These diseases occur rarely, arising from one or a small subset of virally infected cells infrequently evolving into a pathogenic state. Thus, the process of HTLV-1 cell-to-cell transmission within the host helps influence the probability of disease development. HTLV-1 primarily infects T cells and initially spreads within this cell population when virally infected T cells dock to uninfected target T cells and then transfer HTLV-1 virus particles to the target cells. Here we found that the viral protein HTLV-1 bZIP factor (HBZ) promotes infectivity. HBZ accomplishes this task by increasing the surface abundance of a cellular adhesion protein known as intercellular adhesion molecule 1 (ICAM-1), which helps initiate and stabilize contact (docking) between infected and target T cells. These results define a novel and unexpected function of HBZ, diverging from its defined functions in cellular survival and proliferation.

Sensing of cell-associated HTLV by plasmacytoid dendritic cells is regulated by dense β-galactoside glycosylation

Human T Lymphotropic virus (HTLV) infection can persist in individuals resulting, at least in part, from viral escape of the innate immunity, including inhibition of type I interferon response in infected T-cells. Plasmacytoid dendritic cells (pDCs) are known to bypass viral escape by their robust type I interferon production. Here, we demonstrated that pDCs produce type I interferons upon physical cell contact with HTLV-infected cells, yet pDC activation inversely correlates with the ability of the HTLV-producing cells to transmit infection. We show that pDCs sense surface associated-HTLV present with glycan-rich structure referred to as biofilm-like structure, which thus represents a newly described viral structure triggering the antiviral response by pDCs. Consistently, heparan sulfate proteoglycans and especially the cell surface pattern of terminal β-galactoside glycosylation, modulate the transmission of the immunostimulatory RNA to pDCs. Altogether, our results uncover a function of virus-containing cell surface-associated glycosylated structures in the activation of innate immunity.

Quantitating the Transfer of the HTLV-1 p8 Protein Between T-Cells by Flow Cytometry

The Human T-cell leukemia virus type 1 (HTLV-1)-encoded accessory protein p8 is cleaved from the precursor protein p12 encoded by the HTLV-1 open reading frame I. Both p12 and p8 are thought to contribute to efficient viral persistence. Mechanistically, p8 induces T-cell conjugates and cellular conduits. The latter are considered to facilitate transfer of p8 to target cells and virus transmission. Transfer of p8 between p8-expressing T-cells and recipient cells has been analyzed by immunofluorescence and live imaging. However, automatic quantitation of p8-transfer between cells has not been studied yet. Here we developed a novel method allowing time saving quantitation of p8 transfer between cells by flow cytometry. After establishing a protocol for the detection of intracellular p8 by flow cytometry and validation of p8 protein expression by western blot and immunofluorescence, we set up a co-culture assay between p8-expressing donor Jurkat T-cells and recipient Jurkat T-cells that had been prestained with a well-retained live cell dye. Upon quantitating the amount of p8 positive recipient cells with regard to the percentage of p8 expressing donor cells, time course experiments confirmed that p8 is rapidly transferred between Jurkat T-cells. We found that p8 enters approximately 5% of recipient T-cells immediately upon co-culture for 5 min. Prolonged co-culture for up to 24 h revealed an increase of relative p8 transfer to approximately 23% of the recipient cells. Immunofluorescence analysis of co-culture experiments and manual quantitation of p8 expression in fluorescence images confirmed the validity of the flow cytometry based assay. Application of the new assay revealed that manipulation of actin polymerization significantly decreased p8 transfer between Jurkat T-cells suggesting an important role of actin dynamics contributing to p8 transfer. Further, transfer of p8 to co-cultured T-cells varies between different donor cell types since p8 transfer could hardly been detected in co-cultures of 293T donor cells with Jurkat acceptor cells. In summary, our novel assay allows automatic and rapid quantitation of p8 transfer to target cells and might thus contribute to a better understanding of cellular processes and dynamics regulating p8 transfer and HTLV-1 transmission.

HTLV-1 viral oncogene HBZ induces osteolytic bone disease in transgenic mice

Adult T-cell leukemia/lymphoma (ATL) is an aggressive T cell malignancy that occurs in HTLV-1 infected patients. Most ATL patients develop osteolytic lesions and hypercalcemia of malignancy, causing severe skeletal related complications and reduced overall survival. The HTLV-1 virus encodes 2 viral oncogenes, Tax and HBZ. Tax, a transcriptional activator, is critical to ATL development, and has been implicated in pathologic osteolysis. HBZ, HTLV-1 basic leucine zipper transcription factor, promotes tumor cell proliferation and disrupts Wnt pathway modulators; however, its role in ATL induced osteolytic bone loss is unknown. To determine if HBZ is sufficient for the development of bone loss, we established a transgenic Granzyme B HBZ (Gzmb-HBZ) mouse model. Lymphoproliferative disease including tumors, enlarged spleens and/or abnormal white cell counts developed in two-thirds of Gzmb-HBZ mice at 18 months. HBZ positive cells were detected in tumors, spleen and bone marrow. Importantly, pathologic bone loss and hypercalcemia were present at 18 months. Bone-acting factors were present in serum and RANKL, PTHrP and DKK1, key mediators of hypercalcemia and bone loss, were upregulated in Gzmb-HBZ T cells. These data demonstrate that Gzmb-HBZ mice model ATL bone disease and express factors that are current therapeutic targets for metastatic and bone resident tumors.

MicroRNA-200c inhibits epithelial-mesenchymal transition, invasion, and migration of lung cancer by targeting HMGB1

MicroRNAs (miRs) play critical roles in cancer development, proliferation, epithelial-mesenchymal transition (EMT), invasion, and migration through regulating the expression of oncogenes and tumour suppressor genes. Previous studies have indicated that miR-200c acts as a tumour suppressor in various cancers by downregulating high-mobility group box 1 (HMGB1) and thereby suppressing EMT and metastasis. In addition, miR-200c was reported to be downregulated and correlated with poor outcomes in non-small cell lung cancer (NSCLC). However, its functional role in HMGB1 regulation in NSCLC is still unclear. This study aimed to clarify whether miR-200c acts as a tumour suppressor in NSCLC by downregulating HMGB1, which is associated with EMT, invasion, cytoskeleton rearrangement, and migration in vitro and in vivo. In order to demonstrate HMGB1 downregulation by miR-200c, the NSCLC cell line A549 was transfected with miR-200c mimic or inhibitor. The mimic significantly reduced HMGB1 expression and suppressed EMT, invasion, and migration, while the inhibitor generated the opposite effects. Additionally, using xenograft mouse models, we confirmed that HMGB1 overexpression increased tumour EMT. In summary, our results demonstrated that miR-200c could suppress EMT, invasion, and migration of NSCLC cells by downregulating HMGB1.

HTLV-1-induced leukotriene B4 secretion by T cells promotes T cell recruitment and virus propagation

The human T-lymphotropic virus type 1 (HTLV-1) is efficiently transmitted through cellular contacts. While the molecular mechanisms of viral cell-to-cell propagation have been extensively studied in vitro, those facilitating the encounter between infected and target cells remain unknown. In this study, we demonstrate that HTLV-1-infected CD4 T cells secrete a potent chemoattractant, leukotriene B4 (LTB4). LTB4 secretion is dependent on Tax-induced transactivation of the pla2g4c gene, which encodes the cytosolic phospholipase A2 gamma. Inhibition of LTB4 secretion or LTB4 receptor knockdown on target cells reduces T-cell recruitment, cellular contact formation and virus propagation in vitro. Finally, blocking the synthesis of LTB4 in a humanized mouse model of HTLV-1 infection significantly reduces proviral load. This results from a decrease in the number of infected clones while their expansion is not impaired. This study shows the critical role of LTB4 secretion in HTLV-1 transmission both in vitro and in vivo.

Genomewide association studies for hematological traits and T lymphocyte subpopulations in a Duroc × Erhualian F resource population

It has been shown that hematological traits can act as important indicators of immune function in both humans and livestock. T lymphocytes are key components of the adaptive immune system, playing a critical role in immune response. To identify genomic regions affecting hematological traits and T lymphocyte subpopulations, we performed both a SNP-based genomewide association study (GWAS) and a haplotype analysis for 20 hematological traits and 8 T cell subpopulations at 3 different time points (d 20, 33, and 35) in a Duroc × Erhualian F intercross population. Bonferroni correction was used to calculate the threshold -values for suggestive and 5% genomewide significance levels. In total, for SNP-based GWAS, we detected 96 significant SNP, including 15 genomewide-significant SNP, associated with 23 hematological traits and 234 significant SNP, including 27 genomewide-significant SNP, associated with 8 T cell subpopulations. Meanwhile, we identified 563 significant SNP, including 7 genomewide-significant SNP, associated with 5 hematological traits and 2,407 significant SNP, including 1,261 genomewide-significant SNP, associated with 8 T cell subpopulations by haplotype analysis. Among the significant regions detected, we propose both the () gene and the () gene on SSC3 as plausible candidate genes associated with CD/CD T lymphocytes at d 20. The findings provide insights into the basis of molecular mechanisms that are involved with immune response in the domestic pig and would aid further identification of causative mutations.

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.

The Tax-Inducible Actin-Bundling Protein Fascin Is Crucial for Release and Cell-to-Cell Transmission of Human T-Cell Leukemia Virus Type 1 (HTLV-1)

The delta-retrovirus Human T-cell leukemia virus type 1 (HTLV-1) preferentially infects CD4⁺ T-cells via cell-to-cell transmission. Viruses are transmitted by polarized budding and by transfer of viral biofilms at the virological synapse (VS). Formation of the VS requires the viral Tax protein and polarization of the host cytoskeleton, however, molecular mechanisms of HTLV-1 cell-to-cell transmission remain incompletely understood. Recently, we could show Tax-dependent upregulation of the actin-bundling protein Fascin (FSCN-1) in HTLV-1-infected T-cells. Here, we report that Fascin contributes to HTLV-1 transmission. Using single-cycle replication-dependent HTLV-1 reporter vectors, we found that repression of endogenous Fascin by short hairpin RNAs and by Fascin-specific nanobodies impaired gag p19 release and cell-to-cell transmission in 293T cells. In Jurkat T-cells, Tax-induced Fascin expression enhanced virus release and Fascin-dependently augmented cell-to-cell transmission to Raji/CD4⁺ B-cells. Repression of Fascin in HTLV-1-infected T-cells diminished virus release and gag p19 transfer to co-cultured T-cells. Spotting the mechanism, flow cytometry and automatic image analysis showed that Tax-induced T-cell conjugate formation occurred Fascin-independently. However, adhesion of HTLV-1-infected MT-2 cells in co-culture with Jurkat T-cells was reduced upon knockdown of Fascin, suggesting that Fascin contributes to dissemination of infected T-cells. Imaging of chronically infected MS-9 T-cells in co-culture with Jurkat T-cells revealed that Fascin’s localization at tight cell-cell contacts is accompanied by gag polarization suggesting that Fascin directly affects the distribution of gag to budding sites, and therefore, indirectly viral transmission. In detail, we found gag clusters that are interspersed with Fascin clusters, suggesting that Fascin makes room for gag in viral biofilms. Moreover, we observed short, Fascin-containing membrane extensions surrounding gag clusters and clutching uninfected T-cells. Finally, we detected Fascin and gag in long-distance cellular protrusions. Taken together, we show for the first time that HTLV-1 usurps the host cell factor Fascin to foster virus release and cell-to-cell transmission.

Human T-cell leukemia virus type 1 (HTLV-1) is the only human retrovirus causing cancer and is transmitted via breast feeding, sexual intercourse, and cell-containing blood products. Efficient infection of CD4⁺ T-cells occurs via polarized budding of virions or via cell surface transfer of viral biofilms at a tight, specialized cell-cell contact, the virological synapse (VS). The viral protein Tax and polarization of the host cell cytoskeleton are crucial for formation of the VS, however, only little is known about the link between Tax and remodeling of the cytoskeleton to foster viral spread. The actin-bundling protein Fascin has evolved as a therapeutic target in several types of cancer. Here, we show that Fascin is also crucial for release and transmission of the tumorvirus HTLV-1. Since Fascin is a transcriptional target gene of Tax in T-cells, our work provides a link between Tax’s activity and virus transmission. Visualization of cell-cell contacts between infected and uninfected T-cells suggests a role of Fascin in viral transmission potentially by facilitating the transport of viral proteins to budding sites. Thus, Fascin is not only crucial for metastasis of tumors, but also for transmission of HTLV-1 and is a new cellular target to counteract HTLV-1.

Dynamic acquisition of HTLV-1 tax protein by mononuclear phagocytes: Role in neurologic disease

Pathology of HTLV-1 associated myelopathy/Tropical spastic paraparesis (HAM/TSP) is believed to be the result of "bystander damage" involving effector CD8 (+) T lymphocytes (CTLs) killing of virus infected cells. But the specific cellular events leading up to tissue injury are still unclear. Here, we developed the Microscopy Imaging of Cytotoxic T lymphocyte assay with Fluorescence emission (MI-CaFé), an optimized visualization analysis to explore the interactions between CTLs and virus infected or viral antigen presenting target cells. Various cell-to-cell formations can be observed and our results demonstrate elevated frequencies of CTL-target cell conjugates in HAM/TSP patient PBMCs compared to control PBMCs. Furthermore, HTLV-1 Tax protein expression can be localized at the cell-cell junctions and also tracked moving from an infected cell to a CD14 (+) mononuclear phagocyte (MP). Activation of CD14 (+) MPs in HAM/TSP patient PBMCs and antigenic presentation of HTLV-1 Tax by MPs can be inferred by their spontaneous cytotoxicity after 18h of in vitro culture. Given that CD4 (+) T lymphocytes are the primary reservoirs of HTLV-1 and MPs are scavenger cells responsible for pathogen clearance, spontaneous cytotoxicity against MPs in HAM/TSP PBMCs suggests a mechanism of chronic inflammation, secondary to low level of persistent virus infection within the central nervous system.

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

Human immunodeficiency virus type 1 (HIV-1) and human T-lymphotropic virus type 1 (HTLV-1) are complex retroviruses mainly infecting CD4(+) T lymphocytes. In addition, antigen-presenting cells such as dendritic cells (DCs) are targeted in vivo by both viruses, although to a lesser extent. Interaction of HIV-1 with DCs plays a key role in viral dissemination from the mucosa to CD4(+) T lymphocytes present in lymphoid organs. While similar mechanisms may occur for HTLV-1 as well, most HTLV-1 data were obtained from T-cell studies, and little is known regarding the trafficking of this virus in DCs. We first compared the efficiency of cell-free versus cell-associated viral sources of both retroviruses at infecting DCs. We showed that both HIV-1 and HTLV-1 cell-free particles are poorly efficient at productively infecting DCs, except when DC-SIGN has been engaged. Furthermore, while SAMHD-1 accounts for restriction of cell-free HIV-1 infection, it is not involved in HTLV-1 restriction. In addition, cell-free viruses lead mainly to a nonproductive DC infection, leading to trans-infection of T-cells, a process important for HIV-1 spread but not for that of HTLV-1. Finally, we show that T-DC cell-to-cell transfer implies viral trafficking in vesicles that may both increase productive infection of DCs ("cis-infection") and allow viral escape from immune surveillance. Altogether, these observations allowed us to draw a model of HTLV-1 and HIV-1 trafficking in DCs.

Molecular Mechanisms of HTLV-1 Cell-to-Cell Transmission

The tumorvirus human T-cell lymphotropic virus type 1 (HTLV-1), a member of the delta-retrovirus family, is transmitted via cell-containing body fluids such as blood products, semen, and breast milk. In vivo, HTLV-1 preferentially infects CD4⁺ T-cells, and to a lesser extent, CD8⁺ T-cells, dendritic cells, and monocytes. Efficient infection of CD4⁺ T-cells requires cell-cell contacts while cell-free virus transmission is inefficient. Two types of cell-cell contacts have been described to be critical for HTLV-1 transmission, tight junctions and cellular conduits. Further, two non-exclusive mechanisms of virus transmission at cell-cell contacts have been proposed: (1) polarized budding of HTLV-1 into synaptic clefts; and (2) cell surface transfer of viral biofilms at virological synapses. In contrast to CD4⁺ T-cells, dendritic cells can be infected cell-free and, to a greater extent, via viral biofilms in vitro. Cell-to-cell transmission of HTLV-1 requires a coordinated action of steps in the virus infectious cycle with events in the cell-cell adhesion process; therefore, virus propagation from cell-to-cell depends on specific interactions between cellular and viral proteins. Here, we review the molecular mechanisms of HTLV-1 transmission with a focus on the HTLV-1-encoded proteins Tax and p8, their impact on host cell factors mediating cell-cell contacts, cytoskeletal remodeling, and thus, virus propagation.

Molecular Studies of HTLV-1 Replication: An Update

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus discovered. Studies on HTLV-1 have been instrumental for our understanding of the molecular pathology of virus-induced cancers. HTLV-1 is the etiological agent of an adult T-cell leukemia (ATL) and can lead to a variety of neurological pathologies, including HTLV-1-associated-myelopathy/tropical spastic paraparesis (HAM/TSP). The ability to treat the aggressive ATL subtypes remains inadequate. HTLV-1 replicates by (1) an infectious cycle involving virus budding and infection of new permissive target cells and (2) mitotic division of cells harboring an integrated provirus. Virus replication initiates host antiviral immunity and the checkpoint control of cell proliferation, but HTLV-1 has evolved elegant strategies to counteract these host defense mechanisms to allow for virus persistence. The study of the molecular biology of HTLV-1 replication has provided crucial information for understanding HTLV-1 replication as well as aspects of viral replication that are shared between HTLV-1 and human immunodeficiency virus type 1 (HIV-1). Here in this review, we discuss the various stages of the virus replication cycle—both foundational knowledge as well as current updates of ongoing research that is important for understanding HTLV-1 molecular pathogenesis as well as in developing novel therapeutic strategies.

How does HTLV-1 cause adult T-cell leukaemia/lymphoma (ATL)?

A typical person infected with the retrovirus human T-lymphotropic virus type 1 (HTLV-1) carries tens of thousands of clones of HTLV-1-infected T lymphocytes, each clone distinguished by a unique integration site of the provirus in the host genome. However, only 5% of infected people develop the malignant disease adult T cell leukaemia/lymphoma, usually more than 50 years after becoming infected. We review the host and viral factors that cause this aggressive disease.

Regulation of the tumor marker Fascin by the viral oncoprotein Tax of human T-cell leukemia virus type 1 (HTLV-1) depends on promoter activation and on a promoter-independent mechanism

Adult T-cell leukemia/lymphoma is a highly infiltrative neoplasia of CD4(+) T-lymphocytes that occurs in about 5% of carriers infected with the deltaretrovirus human T-cell leukemia virus type 1 (HTLV-1). The viral oncoprotein Tax perturbs cellular signaling pathways leading to upregulation of host cell factors, amongst them the actin-bundling protein Fascin, an invasion marker of several types of cancer. However, transcriptional regulation of Fascin by Tax is poorly understood. In this study, we identified a triple mode of transcriptional induction of Fascin by Tax, which requires (1) NF-κB-dependent promoter activation, (2) a Tax-responsive region in the Fascin promoter, and (3) a promoter-independent mechanism sensitive to the Src family kinase inhibitor PP2. Thus, Tax regulates Fascin by a multitude of signals. Beyond, using Tax-expressing and virus-transformed lymphocytes as a model system, our study is the first to identify the invasion marker Fascin as a novel target of PP2, an inhibitor of metastasis.

[Intercellular transmission of HTLV-1: not all mechanisms have been revealed]

HTLV-1 is the etiological agent of Adult T cell Leukemia/Lymphoma (ATLL) and of HTLV-1-Associated Myelopathy/tropical spastic paraparesis (HAM/TSP). It is mainly detected in CD4+ lymphocytes in vivo, but proviral genomes have also been detected although less frequently, in CD8+ T lymphocytes, B lymphocytes, monocytes, macrophages, dendritic cells and other non-lymphoid cells. Virus spread is highly dependent on cell-cell contact. This mode of transmission is correlated with an increased ability of infected cells to migrate, a property linked to cytoskeleton reorganization induced by the viral Tax protein. Cell-to-cell transmission relies on at least three non-exclusive molecular pathways. First, a specialized area, the "virological synapse'' (VS) promotes direct transmission of budding HTLV-1 particles into a synaptic cleft formed between infected and uninfected cells. Second, HTLV-1 particles accumulate at the plasma membrane of infected cells in a biofilm-like extracellular viral assembly that resembles a bacterial biofilm. Viral biofilm is rapidly transmitted to uninfected cells when infected cells contact target cells. Finally, membrane extensions called inter-cellular conduits facilitate HTLV-1 proteins transfer from infected to uninfected target cells, and may stabilize cell-cell contacts. The aim of this review is to summarize the molecular mechanisms of these HTLV-1 transmission pathways.

Pseudorabies virus triggers glycoprotein gE-mediated ERK1/2 activation and ERK1/2-dependent migratory behavior in T cells

The interaction between viruses and immune cells of the host may lead to modulation of intracellular signaling pathways and to subsequent changes in cellular behavior that are of benefit for either virus or host. ERK1/2 (extracellular signal regulated kinase 1/2) signaling represents one of the key cellular signaling axes. Here, using wild-type and gE null virus, recombinant gE, and gE-transfected cells, we show that the gE glycoprotein of the porcine Varicellovirus pseudorabies virus (PRV) triggers ERK1/2 phosphorylation in Jurkat T cells and primary porcine T lymphocytes. PRV-induced ERK1/2 signaling resulted in homotypic T cell aggregation and increased motility of T lymphocytes. Our study reveals a new function of the gE glycoprotein of PRV and suggests that PRV, through activation of ERK1/2 signaling, has a substantial impact on T cell behavior.

IMPORTANCE

Herpesviruses are known to be highly successful in evading the immune system of their hosts, subverting signaling pathways of the host to their own advantage. The ERK1/2 signaling pathway, being involved in many cellular processes, represents a particularly attractive target for viral manipulation. Glycoprotein E (gE) is an important virulence factor of alphaherpesviruses, involved in viral spread. In this study, we show that gE has the previously uncharacterized ability to trigger ERK1/2 phosphorylation in T lymphocytes. We also show that virus-induced ERK1/2 signaling leads to increased migratory behavior of T cells and that migratory T cells can spread the infection to susceptible cells. In conclusion, our results point to a novel function for gE and suggest that virus-induced ERK1/2 activation may trigger PRV-carrying T lymphocytes to migrate and infect other cells susceptible to PRV replication.

Discovery and characterization of auxiliary proteins encoded by type 3 simian T-cell lymphotropic viruses

Human T-cell lymphotropic virus type 1 (HTLV-1) and HTLV-2 encode auxiliary proteins that play important roles in viral replication, viral latency, and immune escape. The presence of auxiliary protein-encoding open reading frames (ORFs) in HTLV-3, the latest HTLV to be discovered, is unknown. Simian T-cell lymphotropic virus type 3 (STLV-3) is almost identical to HTLV-3. Given the lack of HTLV-3-infected cell lines, we took advantage of STLV-3-infected cells and of an STLV-3 molecular clone to search for the presence of auxiliary transcripts. Using reverse transcriptase PCR (RT-PCR), we first uncovered the presence of three unknown viral mRNAs encoding putative proteins of 5, 8, and 9 kDa and confirmed the presence of the previously reported RorfII transcript. The existence of these viral mRNAs was confirmed by using splice site-specific RT-PCR with ex vivo samples. We showed that p5 is distributed throughout the cell and does not colocalize with a specific organelle. The p9 localization is similar to that of HTLV-1 p12 and induced a strong decrease in the calreticulin signal, similarly to HTLV-1 p12. Although p8, RorfII, and Rex-3 share an N-terminal sequence that is predicted to contain a nucleolar localization signal (NoLS), only p8 is found in the nucleolus. The p8 location in the nucleolus is linked to a bipartite NoLS. p8 and, to a lesser extent, p9 repressed viral expression but did not alter Rex-3-dependent mRNA export. Using a transformation assay, we finally showed that none of the STLV-3 auxiliary proteins had the ability to induce colony formation, while both Tax-3 and antisense protein of HTLV-3 (APH-3) promoted cellular transformation. Altogether, these results complete the characterization of the newly described primate T-lymphotropic virus type 3 (PTLV-3).

IMPORTANCE

Together with their simian counterparts, HTLVs form the primate T-lymphotropic viruses. HTLVs arose from interspecies transmission between nonhuman primates and humans. HTLV-1 and HTLV-2 encode auxiliary proteins that play important roles in viral replication, viral latency, and immune escape. The presence of ORFs encoding auxiliary proteins in HTLV-3 or STLV-3 genomes was unknown. Using in silico analyses, ex vivo samples, or in vitro experiments, we have uncovered the presence of 3 previously unknown viral mRNAs encoding putative proteins and confirmed the presence of a previously reported viral transcript. We characterized the intracellular localization of the four proteins. We showed that two of these proteins repress viral expression but that none of them have the ability to induce colony formation. However, both Tax and the antisense protein APH-3 promote cell transformation. Our results allowed us to characterize 4 new retroviral proteins for the first time.