Molecular mechanisms of HTLV-1 infection and pathogenesis

Structural insight into the Scribble PDZ domains interaction with the oncogenic Human T-cell lymphotrophic virus-1 (HTLV-1) Tax1 PBM

Scribble (Scrib) is a highly conserved cell polarity regulator that harbours potent tumour suppressor activity and plays an important role in cell migration. Dysregulation of polarity is associated with poor prognosis during viral infections. Human T-cell lymphotrophic virus-1 (HTLV-1) encodes for the oncogenic Tax1 protein, a modulator of the transcription of viral and human proteins that can cause cell cycle dysregulation as well as a loss of genomic integrity. Previous studies established that Scribble interacts with Tax1 via its C-terminal PDZ-binding motif (PBM), leading to aggregation of polarity regulators and subsequent perturbation of host cell adhesion, proliferation, and signalling. Using isothermal titration calorimetry, we now show that all four PDZ domains of Scribble bind to Tax1 PBM. We then determined crystal structures of Scribble PDZ1, PDZ2 and PDZ3 domains bound to Tax1 PBM. Our findings establish a structural basis for Tax1-mediated subversion of Scribble-mediated cell polarity signalling and provide the platform for mechanistic studies to examine Tax1 induced mislocalization of Scribble and the associated changes in cellular architecture and subsequent tumorigenesis.

Viral, genetic, and immune factors in the oncogenesis of adult T-cell leukemia/lymphoma

Adult T-cell leukemia/lymphoma (ATL) is a malignancy of mature CD4 + T cells induced by human T-cell leukemia virus type I (HTLV-1). HTLV-1 maintains life-long infection in the human host by clonal proliferation of infected cells and cell-to-cell spread of the virus. Two viral genes, tax and HTLV-1 bZIP factor (HBZ), promote expansion of infected cells through the important roles they play in acceleration of cell proliferation and protection from cell death. Long-term survival of infected clones in vivo causes genetic mutations and aberrant epigenetic changes to accumulate in host genes, resulting in the emergence of an ATL clone. Recent advances in sequencing technology have revealed the broad picture of genetic and transcriptional abnormalities in ATL cells. ATL cells have hyper-proliferative and anti-apoptotic signatures like those observed in other malignancies, but also notably have traits related to immune evasion. ATL cells exhibit a regulatory T-cell-like immuno-phenotype due to both the function of HBZ and mutation of several host genes, such as CCR4 and CIC. These findings suggest that immune evasion is a critical step in the oncogenesis of ATL, and thus novel therapies that activate anti-ATL/HTLV-1 immunity may be effective in the treatment and prevention of ATL.

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.

Helicobacter pylori-induced DNA double-strand break in the development of gastric cancer

Infection with cagA-positive Helicobacter pylori strains plays an etiological role in the development of gastric cancer. The CagA protein is injected into gastric epithelial cells through a bacterial Type IV secretion system. Inside the host cells, CagA promiscuously associates with multiple host cell proteins including the prooncogenic phosphatase SHP2 that is required for full activation of the RAS-ERK pathway. CagA-SHP2 interaction aberrantly activates SHP2 and thereby deregulates RAS-ERK signaling. Cancer is regarded as a disease of the genome, indicating that H. pylori-mediated gastric carcinogenesis is also associated with genomic alterations in the host cell. Indeed, accumulating evidence has indicated that H. pylori infection provokes DNA double-strand breaks (DSBs) by both CagA-dependent and -independent mechanisms. DSBs are repaired by either error-free homologous recombination (HR) or error-prone non-homologous end joining (NHEJ) or microhomology-mediated end joining (MMEJ). Infection with cagA-positive H. pylori inhibits RAD51 expression while dampening cytoplasmic-to-nuclear translocalization of BRCA1, causing replication fork instability and HR defects (known as "BRCAness"), which collectively provoke genomic hypermutation via non-HR-mediated DSB repair. H. pylori also subverts multiple DNA damage responses including DNA repair systems. Infection with H. pylori additionally inhibits the function of the p53 tumor suppressor, thereby dampening DNA damage-induced apoptosis while promoting proliferation of CagA-delivered cells. Thus, H. pylori cagA-positive strains promote abnormal expansion of cells with BRCAness, which dramatically increases the chance of generating driver gene mutations in the host cells. Once such driver mutations are acquired, H. pylori CagA is no longer required for subsequent gastric carcinogenesis (Hit-and-Run carcinogenesis).

Computed tomography with 6-year follow-up demonstrates the evolution of HTLV-1 related lung injuries: A cohort study

Previous observational studies have demonstrated the development of pulmonary impairments in human T-lymphotropic virus type 1 (HTLV-1) infected individuals. The main observed lesions due to chronic inflammation of viral infection in situ are bronchiectasis and lung-scarring injuries. This lung inflammation may be the causal agent of restrictive and obstructive lung diseases, primarily in tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP-HAM) patients. We conducted a prospective cohort study to compare spirometry and high-resolution computed tomography (HRCT) findings among 28 HTLV-1-carrier patients over the course of 6 years (2014–2019) (male/female: 7/21; mean age: 54.7 ± 9.5, range: 41–68 years). Chest HRCT exams revealed the development and evolution of lung lesions related to TSP-HAM: including centrilobular nodules, parenchymal bands, lung cysts, bronchiectasis, ground-glass opacity, mosaic attenuation, and pleural thickening. Spirometry exams showed maintenance of respiratory function, with few alterations in parameters suggestive of obstructive and restrictive disorders primarily in individuals with lung lesions and TSP-HAM. The findings of the present study indicate that pulmonary disease related to HTLV-1 is a progressive disease, with development of new lung lesions, mainly in individuals with TSP-HAM. To improve clinical management of these individuals, we recommend that individuals diagnosed with PET-MAH undergo pulmonary evaluation.

Immune Privilege: The Microbiome and Uveitis

Immune privilege (IP), a term introduced to explain the unpredicted acceptance of allogeneic grafts by the eye and the brain, is considered a unique property of these tissues. However, immune responses are modified by the tissue in which they occur, most of which possess IP to some degree. The eye therefore displays a spectrum of IP because it comprises several tissues. IP as originally conceived can only apply to the retina as it contains few tissue-resident bone-marrow derived myeloid cells and is immunologically shielded by a sophisticated barrier – an inner vascular and an outer epithelial barrier at the retinal pigment epithelium. The vascular barrier comprises the vascular endothelium and the glia limitans. Immune cells do not cross the blood-retinal barrier (BRB) despite two-way transport of interstitial fluid, governed by tissue oncotic pressure. The BRB, and the blood-brain barrier (BBB) mature in the neonatal period under signals from the expanding microbiome and by 18 months are fully established. However, the adult eye is susceptible to intraocular inflammation (uveitis; frequency ~200/100,000 population). Uveitis involving the retinal parenchyma (posterior uveitis, PU) breaches IP, while IP is essentially irrelevant in inflammation involving the ocular chambers, uveal tract and ocular coats (anterior/intermediate uveitis/sclerouveitis, AU). Infections cause ~50% cases of AU and PU but infection may also underlie the pathogenesis of immune-mediated “non-infectious” uveitis. Dysbiosis accompanies the commonest form, HLA-B27–associated AU, while latent infections underlie BRB breakdown in PU. This review considers the pathogenesis of uveitis in the context of IP, infection, environment, and the microbiome.

HTLV-1 induces T cell malignancy and inflammation by viral antisense factor-mediated modulation of the cytokine signaling

Human T cell leukemia virus type 1 (HTLV-1) is the etiologic agent of a T cell neoplasm and several inflammatory diseases. A viral gene, HTLV-1 bZIP factor (HBZ), induces pathogenic Foxp3-expressing T cells and triggers systemic inflammation and T cell lymphoma in transgenic mice, indicating its significance in HTLV-1-associated diseases. Here we show that, unexpectedly, a proinflammatory cytokine, IL-6, counteracts HBZ-mediated pathogenesis. Loss of IL-6 accelerates inflammation and lymphomagenesis in HBZ transgenic mice. IL-6 innately inhibits regulatory T cell differentiation, suggesting that IL-6 functions as a suppressor against HBZ-associated complications. HBZ up-regulates expression of the immunosuppressive cytokine IL-10. IL-10 promotes T cell proliferation only in the presence of HBZ. As a mechanism of growth promotion by IL-10, HBZ interacts with STAT1 and STAT3 and modulates the IL-10/JAK/STAT signaling pathway. These findings suggest that HTLV-1 promotes the proliferation of infected T cells by hijacking the machinery of regulatory T cell differentiation. IL-10 induced by HBZ likely suppresses the host immune response and concurrently promotes the proliferation of HTLV-1 infected T cells.

Strategies of Human T-Cell Leukemia Virus Type 1 for Persistent Infection: Implications for Leukemogenesis of Adult T-Cell Leukemia-Lymphoma

Human T-cell leukemia virus type 1 (HTLV-1) establishes persistent infection in vivo in two distinct ways: de novo infection and clonal proliferation of infected cells. Two viral genes, Tax and HTLV-1 bZIP factor (HBZ) play critical roles in viral transcription and promotion of T-cell proliferation, respectively. Tax is a potent transactivator not only for viral transcription but also for many cellular oncogenic pathways, such as the NF-κB pathway. HBZ is a suppressor of viral transcription and has the potential to change the immunophenotype of infected cells, conferring an effector regulatory T cell (eTreg)-like signature (CD4+ CD25+ CCR4+ TIGIT+ Foxp3+) and enhancing the proliferation of this subset. Reports that mice transgenic for either gene develop malignant tumors suggest that both Tax and HBZ are involved in leukemogenesis by HTLV-1. However, the immunogenicity of Tax is very high, and its expression is generally suppressed in vivo. Recently, it was found that Tax can be expressed transiently in a small subpopulation of adult T-cell leukemia-lymphoma (ATL) cells and plays a critical role in maintenance of the overall population. HBZ is expressed in almost all infected cells except for the rare Tax-expressing cells, and activates the pathways associated with cell proliferation. These findings indicate that HTLV-1 fine-tunes the expression of viral genes to control the mode of viral propagation. The interplay between Tax and HBZ is the basis of a sophisticated strategy to evade host immune surveillance and increase transmission - and can lead to ATL as a byproduct.

Inhibition of IκBα phosphorylation potentiates regulated cell death induced by azidothymidine in HTLV-1 infected cells

Adult T cell leukemia/lymphoma (ATL) can be susceptible, at least transiently, to treatments with azidothymidine (AZT) plus IFNα and/or arsenic trioxide. However, the real role of AZT in this effect is still unclear. In fact, while reverse transcriptase (RT) inhibition could explain reduction of clonal expansion and of renewal of HTLV-1 infected cells during ATL progression, this effect alone seems insufficient to justify the evident and prompt decrease of the pro-viral load in treated patients. We have previously demonstrated that AZT is endowed with an intrinsic pro-apoptotic potential towards both peripheral blood mononuclear cells from healthy donors or some tumor cell lines, but this cytotoxic potential cannot be fully achieved unless IκBα phosphorylation is inhibited. Since the constitutive activation of NF-kappa B (NF-κB) appears a common biological basis of HTLV-1-infected cells, a pharmacological inhibition of IκBα phosphorylation seems a potential strategy for treating and preventing HTLV-1 related pathologies. In this study, we have demonstrated that a combination treatment with the IκBα phosphorylation inhibitor Bay 11-7085 and AZT induced increased levels of regulated cell death (RCD) by apoptosis compared to the single treatments in HTLV-1 infected cells of different origin. Importantly, levels of RCD were considerably higher in infected cells in comparison with the uninfected ones. Inhibition of NF-κB activation following the combined treatment was confirmed by analysis of both gel-shift and functional activity of the NF-κB complex proteins, p65/p52. Moreover, a transcriptional analysis revealed that the addition of Bay 11-7085 to AZT treatment in HTLV-1-infected cells modified their transcriptional profile, by inducing the upregulation of some pro-apoptotic genes together with the downregulation of some anti-apoptotic genes. Our data suggest that addition of adequate concentrations of IκBα phosphorylation inhibitor to therapeutic regimens including AZT could be a promising strategy in ATL.

Initial human T-cell leukemia virus type 1 infection of the salivary gland epithelial cells requires a biofilm-like structure

The initial phase of the human T cell leukemia virus-1 (HTLV-1) infection of salivary gland epithelial cells (SGECs) was examined. SGECs of patients with Sjögren's syndrome (SS) and non-SS subjects were co-cultured with the HTLV-1-infected cell line HCT-5 or MOLT-4, then immunofluorescence (IF), scanning and transmission electron microscopy (SEM/TEM) were employed. The extracellular matrix and linker proteins galectin-3, agrin, and tetherin were expressed on the surfaces of both HCT-5 and MOLT-4 cells. HTLV-1 Gag-positive spots were observed on adjacent SGECs after 1 h of co-culture with HCT-5. Both in subjects with and those without SS, agrin and tetherin were co-expressed with HTLV-1 Gag on SGECs after co-culture with HCT-5, although no polarization of HTLV-1 Gag and relevant molecules was observed. SEM showed HTLV-1 virions that were found on HCT-5 were observed in the interfaces between HCT-5 cells and SGECs. TEM imaging showed that HTLV-1 virions were transmitted to SGECs at the interface with thin film-like structure, while HTLV-1 virions were released from the surface of HCT-5 cells. No endogenous retroviruses were observed. These results showed that the initial phase of HTLV-1 infection toward SGECs of SS was mediated not by viral synapses, but by biofilm-like components.

[Astute strategies of HTLV-1 with driven viral genes]

Human T-cell leukemia virus type 1 (HTLV-1) is the world's first retrovirus with pathogenicity to cause adult T-cell leukemia-lymphoma (ATL) and chronic inflammatory diseases,such as HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) and HTLV-1 uveitis. As the virological characteristic, HTLV-1 can transmit efficiently only through cell-to-cell contact. Spread of infection and viral persistence is ingeniously driven by several viral genes as exemplified by HTLV-1 bZIP factor (HBZ) and tax. After the infection, the virus promotes proliferation and immortalization of the infected cells with acculturating immunophenotype into effector/memory T cells. In addition, HBZ enhances expression of co-inhibitory receptors on the surface of infected cells, potentially leading to suppression of host immune responses. These viral strategies can also result in unforeseen by-product, the pathogenicity of HTLV-1-associated diseases. In this review, with recent progress of HTLV-1 researches, we focus on astute regulation systems of the viral genes developed by HTLV-1.

Adult T-Cell Leukemia/Lymphoma. Review of the Literature

Adult T-cell Leukemia/Lymphoma (ATLL) is an aggressive neoplasm of T lymphocytes associated with Human T-lymphotropic virus type1 (HTLV-1) infection. HTLV-1 is a public health problem because it is endemic in native groups in Latin America, and its infection leads to several chronic diseases as ATLL. We aimed to review current literature of ATLL in order to consider it as a differential diagnosis in front of patients with compatible symptoms. Prognosis is still poor in aggressive and indolent variants, with survival rates from months to few years. Treatment based on chemotherapy, antiretroviral, and allogenic stem cell transplantation are currently improving survival rates, but with limited results.

Sporadic on/off switching of HTLV-1 Tax expression is crucial to maintain the whole population of virus-induced leukemic cells

Viruses causing chronic infection artfully manipulate infected cells to enable viral persistence in vivo under the pressure of immunity. Human T-cell leukemia virus type 1 (HTLV-1) establishes persistent infection mainly in CD4+ T cells in vivo and induces leukemia in this subset. HTLV-1-encoded Tax is a critical transactivator of viral replication and a potent oncoprotein, but its significance in pathogenesis remains obscure due to its very low level of expression in vivo. Here, we show that Tax is expressed in a minor fraction of leukemic cells at any given time, and importantly, its expression spontaneously switches between on and off states. Live cell imaging revealed that the average duration of one episode of Tax expression is ∼19 hours. Knockdown of Tax rapidly induced apoptosis in most cells, indicating that Tax is critical for maintaining the population, even if its short-term expression is limited to a small subpopulation. Single-cell analysis and computational simulation suggest that transient Tax expression triggers antiapoptotic machinery, and this effect continues even after Tax expression is diminished; this activation of the antiapoptotic machinery is the critical event for maintaining the population. In addition, Tax is induced by various cytotoxic stresses and also promotes HTLV-1 replication. Thus, it seems that Tax protects infected cells from apoptosis and increases the chance of viral transmission at a critical moment. Keeping the expression of Tax minimal but inducible on demand is, therefore, a fundamental strategy of HTLV-1 to promote persistent infection and leukemogenesis.

Oncogenic spiral by infectious pathogens: Cooperation of multiple factors in cancer development

Chronic infection is one of the major causes of cancer, and there are several mechanisms for infection-mediated oncogenesis. Some pathogens encode gene products that behave like oncogenic factors, hijacking cellular pathways to promote the survival and proliferation of infected cells in vivo. Some of these viral oncoproteins trigger a cellular damage defense response leading to senescence; however, other viral factors hinder this suppressive effect, suggesting that cooperation of those viral factors is important for malignant transformation. Coinfection with multiple agents is known to accelerate cancer development in certain cases. For example, parasitic or bacterial infection is a risk factor for adult T-cell leukemia-lymphoma induced by human T-cell leukemia virus type 1, and Epstein-Barr virus and malaria are closely associated with endemic Burkitt lymphoma. Human immunodeficiency virus type 1 infection is accompanied by various types of infection-related cancer. These findings indicate that these oncogenic pathogens can cooperate to overcome host barriers against cancer development. In this review, the authors focus on the collaborative strategies of pathogens for oncogenesis from two different points of view: (i) the cooperation of two or more different factors encoded by a single pathogen; and (ii) the acceleration of oncogenesis by coinfection with multiple agents.

Dual inhibition of the mTORC1 and mTORC2 signaling pathways is a promising therapeutic target for adult T-cell leukemia

Adult T‐cell leukemia (ATL) has a poor prognosis as a result of severe immunosuppression and rapid tumor progression with resistance to conventional chemotherapy. Recent integrated‐genome analysis has revealed mutations in many genes involved in the T‐cell signaling pathway, suggesting that the aberration of this pathway is an important factor in ATL pathogenesis and ATL‐cell proliferation. We screened a siRNA library to examine signaling‐pathway functionality and found that the PI3K/Akt/mTOR pathway is critical to ATL‐cell proliferation. We therefore investigated the effect of mammalian target of rapamycin (mTOR) inhibitors, including the dual inhibitors PP242 and AZD8055 and the mTORC1 inhibitors rapamycin and everolimus, on human T‐cell leukemia virus type 1 (HTLV‐1)‐infected‐cell and ATL‐cell lines. Both dual inhibitors inhibited the proliferation of all tested cell lines by inducing G1‐phase cell‐cycle arrest and subsequent cell apoptosis, whereas the effects of the 2 mTORC1 inhibitors were limited, as they did not induce cell apoptosis. In the ATL‐cell lines and in the primary ATL samples, both dual inhibitors inhibited phosphorylation of AKT at serine‐473, a target of mTORC2, as well as that of S6K, whereas the mTORC1 inhibitors only inhibited mTORC1. Furthermore, AZD8055 more significantly inhibited the in vivo growth of the ATL‐cell xenografts than did everolimus. These results indicate that the PI3K/mTOR pathway is critical to ATL‐cell proliferation and might thus be a new therapeutic target in ATL.

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.

Downregulation of histone methyltransferase EHMT2 in CD4+ T-cells may protect HTLV-1-infected individuals against HAM/TSP development

Approximately 5% of human T-cell leukemia virus type 1 (HTLV-1)-infected individuals will develop one of the HTLV-1-related diseases, such as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) or adult T-cell leukemia. However, the mechanisms responsible for the appearance of symptoms have not been fully clarified. It is believed that viral factors, host genetic and epigenetic mechanisms are implicated in this process. Studies have shown the involvement of histone methyltransferases in retrovirus infection, but no study observed their expression in HTLV-1-infected patients. Among them, euchromatic histone-lysine N-methyltransferase (EHMT)-1 and EHMT-2 were related to retroviral latency in HIV-1 infection. We investigated whether histone methyltransferases EHMT1 and EHMT2 exert any influence on HAM/TSP development by assessing their expression levels in CD4⁺ T-cells from HTLV-1-infected patients. CD4⁺ T-cells were immunomagnetically isolated from peripheral blood mononuclear cells of HTLV-1-infected or non-infected individuals and the expression levels of EHMT1 and EHMT2 were determined by RT-qPCR. We observed that EHMT2 was negatively regulated in HTLV-1 asymptomatic carriers compared to non-infected individuals. No difference was observed for EHMT1. These results suggest that EHMT2 downregulation in CD4⁺ T-cells may be linked to a protection mechanism against the development of HAM/TSP.

Chemopreventive Strategies for Inflammation-Related Carcinogenesis: Current Status and Future Direction

A sustained and chronically-inflamed environment is characterized by the presence of heterogeneous inflammatory cellular components, including neutrophils, macrophages, lymphocytes and fibroblasts. These infiltrated cells produce growth stimulating mediators (inflammatory cytokines and growth factors), chemotactic factors (chemokines) and genotoxic substances (reactive oxygen species and nitrogen oxide) and induce DNA damage and methylation. Therefore, chronic inflammation serves as an intrinsic niche for carcinogenesis and tumor progression. In this article, we summarize the up-to-date findings regarding definitive/possible causes and mechanisms of inflammation-related carcinogenesis derived from experimental and clinical studies. We also propose 10 strategies, as well as candidate agents for the prevention of inflammation-related carcinogenesis.

Tumor necrosis factor alpha inhibitors have no effect on a human T-lymphotropic virus type-I (HTLV-I)-infected cell line from patients with HTLV-I-associated myelopathy

Background

While tumor necrosis factor alpha (TNF-α) inhibitors (TNFi) and other biologics are very effective against autoimmune diseases, they can also cause infectious diseases. Therefore, it is important to clarify whether the TNFi sometimes used to treat patients with rheumatoid arthritis (RA) complicated with human T-lymphotropic virus type-I (HTLV-I) infection have the unintended side effect of promoting HTLV-I proliferation.

Methods

We used the HTLV-I-infected cell line HCT-5, derived from spinal fluid cells of a patient with HTLV-I associated myelopathy, to evaluate the production of cytokines and chemokines, TNF-α receptor (TNFR), the expression of HTLV-I associated genes, the HTLV-I proviral load (PVL), the expression of HTLV-I structural protein, and apoptosis. We used Jurkat cells as a control.

Results

Supernatants of HCT-5 showed time-dependent elevations of IL-6, RANTES and ICAM-1. HCT-5 supernatants treated with infliximab, adalimumab, etanercept (ETN), golimumab and certolizumab pegol showed no significant differences in the levels of these molecules compared to the control. Neither TNFR1 nor TNFR2 expression was altered by any TNFi treatment, relative to phosphate-buffered saline (PBS) treatment, with the exception that TNFR2 was significantly decreased and internalized in HCT-5 cells by ETN treatment. The HTLV-I associated genes Tax and HBZ and the PVL levels were not significantly changed. Immunofluorescence staining of HCT-5 for an HTLV-I-associated protein, GAG, was also not significantly different between any of the TNFi treatments and the PBS treatment. DNA ladders as an index of apoptosis were not detected. Apoptotic cells were not increased by the addition of any TNFi.

Conclusions

In vitro, TNFi did not affect the cytokine profiles, expression of associated genes and proteins, proviral load or apoptosis of HCT-5 cells. The results suggested that TNFi treatment of RA patients complicated with HTLV-I might have no effect on HTLV-I infection.

A Luciferase Functional Quantitative Assay for Measuring NF-ĸB Promoter Transactivation Mediated by HTLV-1 and HTLV-2 Tax Proteins

HTLV-1 and HTLV-2 viruses express Tax transactivator proteins required for viral genome transcription and capable of transforming cells in vivo and in vitro. Although Tax oncogenic potential needs to be further elucidated, it is well established that Tax proteins activate, among others, transcription factors of the NF-ĸB family, which are involved in immune and inflammatory responses, cell growth, apoptosis, stress responses and oncogenesis. Here, we describe a reporter gene assay applied for quantitative analysis of Tax-dependent NF-ĸB activation. The procedure is based on co-transfection of two individual vectors containing the cDNA for firefly and Renilla luciferase enzymes and vectors expressing Tax proteins. The luciferase expression is driven by cis-NF-ĸB promoter regulatory elements responsive to Tax transactivating factor. This assay is particularly useful to investigate Tax influence on NF-ĸB activation mediated by viral or host factors.

WRN-targeted therapy using inhibitors NSC 19630 and NSC 617145 induce apoptosis in HTLV-1-transformed adult T-cell leukemia cells

Background

Human T-cell leukemia virus type 1 (HTLV-1) infection is associated with adult T-cell leukemia/lymphoma (ATLL), a lymphoproliferative malignancy with a dismal prognosis and limited therapeutic options. Recent evidence shows that HTLV-1-transformed cells present defects in both DNA replication and DNA repair, suggesting that these cells might be particularly sensitive to treatment with a small helicase inhibitor. Because the “Werner syndrome ATP-dependent helicase” encoded by the WRN gene plays important roles in both cellular proliferation and DNA repair, we hypothesized that inhibition of WRN activity could be used as a new strategy to target ATLL cells.

Methods

Our analysis demonstrates an apoptotic effect induced by the WRN helicase inhibitor in HTLV-1-transformed cells in vitro and ATL-derived cell lines. Inhibition of cellular proliferation and induction of apoptosis were demonstrated with cell cycle analysis, XTT proliferation assay, clonogenic assay, annexin V staining, and measurement of mitochondrial transmembrane potential.

Results

Targeted inhibition of the WRN helicase induced cell cycle arrest and apoptosis in HTLV-1-transformed leukemia cells. Treatment with NSC 19630 (WRN inhibitor) induces S-phase cell cycle arrest, disruption of the mitochondrial membrane potential, and decreased expression of anti-apoptotic factor Bcl-2. These events were associated with activation of caspase-3-dependent apoptosis in ATL cells. We identified some ATL cells, ATL-55T and LMY1, less sensitive to NSC 19630 but sensitive to another WRN inhibitor, NSC 617145.

Conclusions

WRN is essential for survival of ATL cells. Our studies suggest that targeting the WRN helicase with small inhibitors is a novel promising strategy to target HTLV-1-transformed ATL cells.

TET2 Mutation in Adult T-Cell Leukemia/Lymphoma

Loss-of-function of ten-eleven translocation-2 (TET2) is a common event in myeloid malignancies, and plays pleiotropic roles, including augmenting stem cell self-renewal and skewing hematopoietic cells to the myeloid lineage. TET2 mutation has also been reported in lymphoid malignancies; 5.7～12% of diffuse large B-cell lymphomas and 18～83% of angioimmunoblastic T-cell lymphomas had TET2 mutations. We investigated TET2 mutations in 22 adult T-cell leukemia/lymphoma (ATLL) patients and identified a missense mutation in 3 cases (14%). TET2 mutation occurred in a number of ATLL patients and was likely involved in their leukemogenesis.

Transition of adult T-cell leukemia/lymphoma clones during clinical progression

Adult T-cell leukemia/lymphoma (ATLL) is a peripheral T-cell neoplasm caused by the transformation of HTLV-1-infected T cells. ATLL, especially its aggressive form, is known for its poor prognosis, even with intensive chemotherapy. ATLL cells are considered to be monoclonal; however, multiclonal proliferation or emergence of a new clone over time has been reported based on Southern blot analysis, although direct molecular evidence remains elusive. Furthermore, it is thought that clonal change may be a cause of early drug resistance in ATLL. To directly analyze potential clonal changes in ATLL during its clinical course, we used inverse PCR to detect integration sites in combination with a newly developed method using next-generation sequencing, and compared ATLL cell clonality at different time points. The results of inverse PCR indicated that the major clone was altered in three of 19 patients. Together with results from five patients, using this new method, we found direct evidence of clonal change occurring during the clinical course or in response to chemotherapy in ATLL. These results also highlight the importance of clonality analysis for understanding the mechanisms of ATLL development and drug resistance.

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.

STAT1: A Novel Target of miR-150 and miR-223 Is Involved in the Proliferation of HTLV-I-Transformed and ATL Cells

We have previously reported on the deregulation of cellular microRNAs involved in hematopoiesis and inflammation in human T-cell lymphotropic virus type 1 (HTLV-I)–transformed cells. In this study, we demonstrate that miR-150 and miR-223 specifically target the signal transducer and activator of transcription 1 (STAT1) 3′ untranslated region, reducing STAT1 expression and dampening STAT1-dependent signaling in human T cells. The effects of miR-150 and miR-223 on endogenous STAT1 were confirmed using inducible cell lines. Our studies also showed that miR-150 expression is upregulated by interleukin-2 signaling in adult T cell leukemia/lymphoma (ATL) cells. HTLV-I–transformed and ATL-derived cells have reduced levels of miR150 and miR223 expression, which coincide with increased STAT1 expression and STAT1-dependent signaling. Knockdown of STAT1 by short hairpin RNA demonstrated that the constitutive activation of STAT1 is required for the continuous proliferation of HTLV-I–transformed cells. Our studies further demonstrate that increased expression of STAT1 in ATL cells is associated with higher levels of major histocompatibility complex class I expression. Previous studies have demonstrated that the pressure exerted by natural killer (NK) cells in vivo can edit leukemic tumor cells by forcing an increased expression of major histocompatibility complex class I to escape immune clearance. STAT1-expressing tumor cells produce more aggressive tumors because they cannot be eliminated by NK cells. Our results suggest that therapeutic approaches using combined targeting of STAT1 and MHC class I may be an effective approach to activate NK cell–mediated clearance of ATL tumor cells.

Historical review of the causes of cancer

In the early 1900s, numerous seminal publications reported that high rates of cancer occurred in certain occupations. During this period, work with infectious agents produced only meager results which seemed irrelevant to humans. Then in the 1980s ground breaking evidence began to emerge that a variety of viruses also cause cancer in humans. There is now sufficient evidence of carcinogenicity in humans for human T-cell lymphotrophic virus, human immunodeficiency virus, hepatitis B virus, hepatitis C virus, human papillomavirus, Epstein-Barr virus, and human herpes virus 8 according to the International Agency for Research on Cancer (IARC). Many other causes of cancer have also been identified by the IARC, which include: Sunlight, tobacco, pharmaceuticals, hormones, alcohol, parasites, fungi, bacteria, salted fish, wood dust, and herbs. The World Cancer Research Fund and the American Institute for Cancer Research have determined additional causes of cancer, which include beta carotene, red meat, processed meats, low fibre diets, not breast feeding, obesity, increased adult height and sedentary lifestyles. In brief, a historical review of the discoveries of the causes of human cancer is presented with extended discussions of the difficulties encountered in identifying viral causes of cancer.

HTLV-1 bZIP factor protein targets the Rb/E2F-1 pathway to promote proliferation and apoptosis of primary CD4(+) T cells

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that induces a fatal T-cell malignancy, adult T-cell leukemia (ATL). Among several regulatory/accessory genes in HTLV-1, HTLV-1 bZIP factor (HBZ) is the only viral gene constitutively expressed in infected cells. Our previous study showed that HBZ functions in two different molecular forms, HBZ protein and HBZ RNA. In this study, we show that HBZ protein targets retinoblastoma protein (Rb), which is a critical tumor suppressor in many types of cancers. HBZ protein interacts with the Rb/E2F-1 complex and activates the transcription of E2F-target genes associated with cell cycle progression and apoptosis. Mouse primary CD4(+) T cells transduced with HBZ show accelerated G1/S transition and apoptosis, and importantly, T cells from HBZ transgenic (HBZ-Tg) mice also demonstrate enhanced cell proliferation and apoptosis. To evaluate the functions of HBZ protein alone in vivo, we generated a new transgenic mouse strain that expresses HBZ mRNA altered by silent mutations but encoding intact protein. In these mice, the numbers of effector/memory and Foxp3(+) T cells were increased, and genes associated with proliferation and apoptosis were upregulated. This study shows that HBZ protein promotes cell proliferation and apoptosis in primary CD4(+) T cells through activation of the Rb/E2F pathway, and that HBZ protein also confers onto CD4(+) T-cell immunophenotype similar to those of ATL cells, suggesting that HBZ protein has important roles in dysregulation of CD4(+) T cells infected with HTLV-1.

Permissive Sense and Antisense Transcription from the 5' and 3' Long Terminal Repeats of Human T-Cell Leukemia Virus Type 1

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus, and, as such, its genome becomes chromosomally integrated following infection. The resulting provirus contains identical 5' and 3' peripheral long terminal repeats (LTRs) containing bidirectional promoters. Antisense transcription from the 3' LTR regulates expression of a single gene, hbz, while sense transcription from the 5' LTR controls expression of all other viral genes, including tax. Both the HBZ and Tax proteins are implicated in the development of adult T-cell leukemia (ATL), a T-cell malignancy caused by HTLV-1 infection. However, these proteins appear to harbor opposing molecular functions, indicating that they may act independently and at different time points prior to leukemogenesis. Here, we used bidirectional reporter constructs to test whether transcriptional interference serves as a mechanism that inhibits simultaneous expression of Tax and HBZ. We found that sense transcription did not interfere with antisense transcription from the 3' LTR and vice versa, even with strong transcription emanating from the opposing direction. Therefore, bidirectional transcription across the provirus might not restrict hbz or tax expression. Single-cell analyses revealed that antisense transcription predominates in the absence of Tax, which transactivates viral sense transcription. Interestingly, a population of Tax-expressing cells exhibited antisense but not activated sense transcription. Consistent with the ability of Tax to induce cell cycle arrest, this population was arrested in G(0)/G(1) phase. These results imply that cell cycle arrest inhibits Tax-mediated activation of sense transcription without affecting antisense transcription, which may be important for long-term viral latency.

IMPORTANCE

The chromosomally integrated form of the retrovirus human T-cell leukemia virus type 1 (HTLV-1) contains identical DNA sequences, known as long terminal repeats (LTRs), at its 5' and 3' ends. The LTRs modulate transcription in both forward (sense) and reverse (antisense) directions. We found that sense transcription from the 5' LTR does not interfere with antisense transcription from the 3' LTR, allowing viral genes encoded on opposite DNA strands to be simultaneously transcribed. Two such genes are tax and hbz, and while they are thought to function at different times during the course of infection to promote leukemogenesis of infected T cells, our results indicate that they can be simultaneously transcribed. We also found that the ability of Tax to induce cell cycle arrest inhibits its fundamental function of activating viral sense transcription but does not affect antisense transcription. This regulatory mechanism may be important for long-term HTLV-1 infection.

Human T-cell leukemia virus type 1 (HTLV-1) Tax1 oncoprotein but not HTLV-2 Tax2 induces the expression of OX40 ligand by interacting with p52/p100 and RelB

Human T-cell leukemia virus type 1 (HTLV-1) is a causative retrovirus of adult T-cell leukemia and HTLV-1-associated myelopathy. Unlike HTLV-1, the same group of retrovirus HTLV-2 has not been found to be associated with these diseases. HTLV-1 and HTLV-2 encode transforming proteins Tax1 and Tax2, and a few distinct activities of Tax1 from those of Tax2 have been proposed to contribute to the HTLV-1-specific pathogenesis of disease. One significant difference of Tax1 from Tax2 is the activation of transcription factor NF-κB2/p100/p52. We found that Tax1 but not Tax2 induces the expression of OX40 ligand (OX40L) in a human T-cell line. To induce the OX40L expression, Tax1 but not Tax2 was observed to interact with NF-κB2/p100/p52 and RelB and the distinct interaction activity was mediated by the Tax1 amino acid region of 225-232. In addition, Tax1 but not Tax2 or Tax1/225-232 interacted with p65, p50, and c-Rel; however, the interactions were much less than those noted with NF-κB2/p100/p52 and RelB. OX40L is a T-cell costimulatory molecule of the tumor necrosis factor family, and its signal plays a critical role in establishing adaptive immunity by inducing the polarized differentiation of T-cells to cells such as T helper type 2 and T follicular helper cells. Therefore, the present findings suggest that Tax1 might alter the immune response to HTLV-1 and/or differentiation of HTLV-1-infected T-cells via OX40L induction, thereby acting as a factor mediating the distinct phenotypes and pathogenesis of HTLV-1 from that of HTLV-2.

B'-protein phosphatase 2A is a functional binding partner of delta-retroviral integrase

To establish infection, a retrovirus must insert a DNA copy of its RNA genome into host chromatin. This reaction is catalysed by the virally encoded enzyme integrase (IN) and is facilitated by viral genus-specific host factors. Herein, cellular serine/threonine protein phosphatase 2A (PP2A) is identified as a functional IN binding partner exclusive to δ-retroviruses, including human T cell lymphotropic virus type 1 and 2 (HTLV-1 and HTLV-2) and bovine leukaemia virus (BLV). PP2A is a heterotrimer composed of a scaffold, catalytic and one of any of four families of regulatory subunits, and the interaction is specific to the B' family of the regulatory subunits. B'-PP2A and HTLV-1 IN display nuclear co-localization, and the B' subunit stimulates concerted strand transfer activity of δ-retroviral INs in vitro. The protein-protein interaction interface maps to a patch of highly conserved residues on B', which when mutated render B' incapable of binding to and stimulating HTLV-1 and -2 IN strand transfer activity.

Roles of HTLV-1 basic Zip Factor (HBZ) in Viral Chronicity and Leukemic Transformation. Potential New Therapeutic Approaches to Prevent and Treat HTLV-1-Related Diseases

More than thirty years have passed since human T-cell leukemia virus type 1 (HTLV-1) was described as the first retrovirus to be the causative agent of a human cancer, adult T-cell leukemia (ATL), but the precise mechanism behind HTLV-1 pathogenesis still remains elusive. For more than two decades, the transforming ability of HTLV-1 has been exclusively associated to the viral transactivator Tax. Thirteen year ago, we first reported that the minus strand of HTLV-1 encoded for a basic Zip factor factor (HBZ), and since then several teams have underscored the importance of this antisense viral protein for the maintenance of a chronic infection and the proliferation of infected cells. More recently, we as well as others have demonstrated that HBZ has the potential to transform cells both in vitro and in vivo. In this review, we focus on the latest progress in our understanding of HBZ functions in chronicity and cellular transformation. We will discuss the involvement of this paradigm shift of HTLV-1 research on new therapeutic approaches to treat HTLV-1-related human diseases.

[Molecular pathogenesis of peripheral T cell lymphoma (2): extranodal NK/T cell lymphoma, nasal type, adult T cell leukemia/lymphoma and enteropathy associated T cell lymphoma]

Peripheral T-cell lymphomas (PTCL) belong to the group of non-Hodgkin lymphoma and particularly that of mature T /NK cells lymphoproliferative neoplasms. The 2008 WHO classification describes different PTCL entities with varying prevalence. With the exception of histologic subtype "ALK positive anaplastic large cell lymphoma", PTCL are characterized by a poor prognosis. The mechanisms underlying the pathogenesis of these lymphomas are not yet fully understood, but development of genomic high-throughput analysis techniques now allows to extensively identify the molecular abnormalities present in tumor cells. This review aims to summarize the current knowledge and recent advances about the molecular events occurring at the origin or during the natural history of main entities of PTCL. The first part published in the October issue was focused on the three more frequent entities, i.e. angioimmunoblastic T-cell lymphoma, peripheral T-cell lymphoma, not otherwise specified, and anaplastic large cell lymphoma. The second part presented herein will describe other subtypes less frequent and of poor prognosis : extranodal NK/T-cell lymphoma, nasal type, adult T-cell leukemia/lymphoma, and enteropathy-associated T-cell lymphoma.

Small PARP inhibitor PJ-34 induces cell cycle arrest and apoptosis of adult T-cell leukemia cells

Background

HTLV-I is associated with the development of an aggressive form of lymphocytic leukemia known as adult T-cell leukemia/lymphoma (ATLL). A major obstacle for effective treatment of ATLL resides in the genetic diversity of tumor cells and their ability to acquire resistance to chemotherapy regimens. As a result, most patients relapse and current therapeutic approaches still have limited long-term survival benefits. Hence, the development of novel approaches is greatly needed.

Methods

In this study, we found that a small molecule inhibitor of poly (ADP-ribose) polymerase (PARP), PJ-34, is very effective in activating S/G2M cell cycle checkpoints, resulting in permanent cell cycle arrest and reactivation of p53 transcription functions and caspase-3-dependent apoptosis of HTLV-I-transformed and patient-derived ATLL tumor cells. We also found that HTLV-I-transformed MT-2 cells are resistant to PJ-34 therapy associated with reduced cleaved caspase-3 activation and increased expression of RelA/p65.

Conclusion

Since PJ-34 has been tested in clinical trials for the treatment of solid tumors, our results suggest that some ATLL patients may be good candidates to benefit from PJ-34 therapy.

Inducible nitric oxide synthase mediates DNA double strand breaks in Human T-Cell Leukemia Virus Type 1-induced leukemia/lymphoma

Background

Adult T-cell leukemia/lymphoma (ATLL) is an aggressive and fatal malignancy of CD4⁺ T-lymphocytes infected by the Human T-Cell Virus Type 1 (HTLV-1). The molecular mechanisms of transformation in ATLL have not been fully elucidated. However, genomic instability and cumulative DNA damage during the long period of latency is believed to be essential for HTLV-1 induced leukemogenesis. In addition, constitutive activation of the NF-κB pathway was found to be a critical determinant for transformation. Whether a connection exists between NF-κB activation and accumulation of DNA damage is not clear. We recently found that the HTLV-1 viral oncoprotein, Tax, the activator of the NF-κB pathway, induces DNA double strand breaks (DSBs).

Results

Here, we investigated whether any of the NF-κB target genes are critical in inducing DSBs. Of note, we found that inducible nitric oxide synthase (iNOS) that catalyzes the production of nitric oxide (NO) in macrophages, neutrophils and T-cells is over expressed in HTLV-1 infected and Tax-expressing cells. Interestingly, we show that in HTLV-1 infected cells, iNOS expression is Tax-dependent and specifically requires the activation of the classical NF-κB and JAK/STAT pathways. A dramatic reduction of DSBs was observed when NO production was inhibited, indicating that Tax induces DSBs through the activation of NO synthesis.

Conclusions

Determination of the impact of NO on HTLV-1-induced leukemogenesis opens a new area for treatment or prevention of ATLL and perhaps other cancers in which NO is produced.

Does chronic infection in retroviruses have a sense?

Over recent years, retroviral gene expression has been shown to depend on a promoter that is bidirectional. This promoter activity is likely to occur at either end of the retroviral genome and has important consequences at the level of retroviral gene expression. This review focuses on the recent discovery of retroviral antisense genes termed HBZ [in human T-cell leukemia virus type 1 (HTLV-1)] and ASP (in HIV-1) in terms of their function and the regulation of their expression, both of which are interconnected with the expression and function of other viral proteins. Emphasis is also given to the potential implication of these proteins in the maintenance of chronic infection in infected individuals. In light of recent findings, the discovery of these new genes opens a new avenue for the future treatment of HTLV-1- and HIV-1-infected individuals.

ATL response to arsenic/interferon therapy is triggered by SUMO/PML/RNF4-dependent Tax degradation

Survival of ATL cells depends on continuous Tax expression. Arsenic/interferon combination induces SUMO/PML/RNF4-mediated Tax degradation.

A critical role for IL-17RB signaling in HTLV-1 tax-induced NF-κB activation and T-cell transformation

Human T-cell leukemia virus type 1 (HTLV-1) infection is linked to the development of adult T-cell leukemia (ATL) and the neuroinflammatory disease HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). The HTLV-1 Tax protein functions as a potent viral oncogene that constitutively activates the NF-κB transcription factor to transform T cells; however, the underlying mechanisms remain obscure. Here, using next-generation RNA sequencing we identified the IL-25 receptor subunit IL-17RB as an aberrantly overexpressed gene in HTLV-1 immortalized T cells. Tax induced the expression of IL-17RB in an IκB kinase (IKK) and NF-κB-dependent manner. Remarkably, Tax activation of the canonical NF-κB pathway in T cells was critically dependent on IL-17RB expression. IL-17RB and IL-25 were required for HTLV-1-induced immortalization of primary T cells, and the constitutive NF-κB activation and survival of HTLV-1 transformed T cells. IL-9 was identified as an important downstream target gene of the IL-17RB pathway that drives the proliferation of HTLV-1 transformed cells. Furthermore, IL-17RB was overexpressed in leukemic cells from a subset of ATL patients and also regulated NF-κB activation in some, but not all, Tax-negative ATL cell lines. Together, our results support a model whereby Tax instigates an IL-17RB-NF-κB feed-forward autocrine loop that is obligatory for HTLV-1 leukemogenesis.

HTLV-1 bZIP factor suppresses the centromere protein B (CENP-B)-mediated trimethylation of histone H3K9 through the abrogation of DNA-binding ability of CENP-B

Human T-cell leukaemia virus type-1 (HTLV-1) infection causes adult T-cell leukaemia (ATL). The viral protein HTLV-1 bZIP factor (HBZ) is constitutively expressed in ATL cells, suggesting that HBZ plays a major role in the pathogenesis of HTLV-1-associated disease. Here, we identified centromere protein B (CENP-B) as a novel interacting partner of HBZ. HBZ and CENP-B associate with their central regions in cells. Furthermore, overexpression of HBZ abrogated the DNA-binding activity of CENP-B to the α-satellite DNA region containing the CENP-B box motif, which in turn inhibited the CENP-B-mediated trimethylation of histone H3K9 in T-cells.

Downregulation of proapoptotic Bim augments IL-2-independent T-cell transformation by human T-cell leukemia virus type-1 Tax

Human T-cell leukemia virus type 1 (HTLV-1), an etiological agent of adult T-cell leukemia, immortalizes and transforms primary human T cells in vitro in both an interleukin (IL)-2-dependent and IL-2-independent manner. Expression of the HTLV-1 oncoprotein Tax transforms the growth of the mouse T-cell line CTLL-2 from being IL-2-dependent to IL-2-independent. Withdrawal of IL-2 from normal activated T cells induces apoptosis, which is mediated through the inducible expression of several proapoptotic proteins, including Bim. In this study, we found that Tax protects IL-2-depleted T cells against Bim-induced apoptosis. Withdrawal of IL-2 from CTLL-2 cells induced a prominent increase in the level of Bim protein in CTLL-2 cells, but not in Tax-transformed CTLL-2 cells. This inhibition of Bim in Tax-transformed CTLL-2 cells was mediated by two mechanisms: downregulation of Bim mRNA and posttranscriptional reduction of Bim protein. Transient expression of Tax in CTLL-2 cells also inhibited IL-2 depletion–induced expression of Bim, however, this decrease in Bim protein expression was not due to downregulation of Bim mRNA, thus indicating that Bim mRNA downregulation in Tax-transformed CTLL-2 occurs only after long-term expression of Tax. Transient expression of Tax in CTLL-2 cells also induced Erk activation, however, this was not involved in the reduction of Bim protein. Knockdown of Bim expression in CTLL-2 cells augmented Tax-induced IL-2-independent transformation. HTLV-1 infection of human T cells also reduced their levels of Bim protein, and restoring Bim expression in HTLV-1-infected cells reduced their proliferation by inducing apoptosis. Taken together, these results indicate that Tax-induced downregulation of Bim in HTLV-1-infected T cells promotes their IL-2-independent growth, thereby supporting the persistence of HTLV-1 infection in vivo.

Human T-lymphotropic virus type 1-infected cells secrete exosomes that contain Tax protein

Human T-lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. The HTLV-1 transactivator protein Tax controls many critical cellular pathways, including host cell DNA damage response mechanisms, cell cycle progression, and apoptosis. Extracellular vesicles called exosomes play critical roles during pathogenic viral infections as delivery vehicles for host and viral components, including proteins, mRNA, and microRNA. We hypothesized that exosomes derived from HTLV-1-infected cells contain unique host and viral proteins that may contribute to HTLV-1-induced pathogenesis. We found exosomes derived from infected cells to contain Tax protein and proinflammatory mediators as well as viral mRNA transcripts, including Tax, HBZ, and Env. Furthermore, we observed that exosomes released from HTLV-1-infected Tax-expressing cells contributed to enhanced survival of exosome-recipient cells when treated with Fas antibody. This survival was cFLIP-dependent, with Tax showing induction of NF-κB in exosome-recipient cells. Finally, IL-2-dependent CTLL-2 cells that received Tax-containing exosomes were protected from apoptosis through activation of AKT. Similar experiments with primary cultures showed protection and survival of peripheral blood mononuclear cells even in the absence of phytohemagglutinin/IL-2. Surviving cells contained more phosphorylated Rb, consistent with the role of Tax in regulation of the cell cycle. Collectively, these results suggest that exosomes may play an important role in extracellular delivery of functional HTLV-1 proteins and mRNA to recipient cells.

HTLV-1 HBZ positively regulates the mTOR signaling pathway via inhibition of GADD34 activity in the cytoplasm

Human T-cell leukemia virus type-1 (HTLV-1) infection causes adult T-cell leukemia (ATL). Modulation of the transcriptional control of cellular genes by HTLV-1 is thought to be associated with the development of ATL. The viral protein HTLV-1 basic leucine-zipper factor (HBZ) has been shown to dysregulate the activity of cellular transcription factors. Here, we demonstrate that HBZ is exported from the nucleus to the cytoplasm, where it activates the mammalian target of rapamycin (mTOR) signaling pathway through an association with growth arrest and DNA damage gene 34 (GADD34). The N-terminal region of HBZ interacts with the C-terminal region of GADD34. HBZ contains a functional nuclear export signal (NES) sequence within its N-terminal region and it is exported from the nucleus via the CRM1-dependent pathway. Nuclear export of HBZ is essential for its interaction with GADD34 and increased phosphorylation of S6 kinase, which is an established downstream target of the mTOR pathway. Starvation-induced autophagy is significantly suppressed by the overexpression of HBZ. These findings indicate that HBZ is actively exported to the cytoplasm, where it dysregulates the function of cellular factors.

HTLV-1 Tax oncoprotein inhibits the estrogen-induced-ER α-Mediated BRCA1 expression by interaction with CBP/p300 cofactors

BRCA1 is a multifunctional tumor suppressor, whose expression is activated by the estrogen (E2)-liganded ERα receptor and regulated by certain recruited transcriptional co-activators. Interference with BRCA1 expression and/or functions leads to high risk of breast or/and ovarian cancer. Another multifunctional protein, HTLV-1Tax oncoprotein, is widely regarded as crucial for developing adult T-cell leukemia and other clinical disorders. Tax profile reveals that it can antagonize BRCA1 expression and/or functionality. Therefore, we hypothesize that Tax expression in breast cells can sensitize them to malignant transformation by environmental carcinogens. Here we examined Tax effect on BRCA1 expression by testing its influence on E2-induced expression of BRCA1 promoter-driven luciferase reporter (BRCA1-Luc). We found that E2 strongly stimulated this reporter expression by liganding to ERα, which consequently associated with BRCA1 promoter, while ERα concomitantly recruited CBP/p300 to this complex for co-operative enhancement of BRCA1 expression. Introducing Tax into these cells strongly blocked this E2-ERα-mediated activation of BRCA1 expression. We noted, also, that Tax exerted this inhibition by binding to CBP/p300 without releasing them from their complex with ERα. Chip assay revealed that the binding of Tax to the CBP/p300-ERα complex, prevented its link to AP1 site. Interestingly, we noted that elevating the intracellular pool of CBP or p300 to excessive levels dramatically reduced the Tax-mediated inhibition of BRCA1 expression. Exploring the mechanism of this reduction revealed that the excessive co-factors were sufficient to bind separately the free Tax molecules, thus lowering their amount in the CBP/p300-ERα complex and relieving, thereby, the inhibition of BRCA1 expression.

HTLV-1 and leukemogenesis: virus-cell interactions in the development of adult T-cell leukemia

Human T-cell lymphotropic virus type 1 (HTLV-1) was originally discovered in the early 1980s. It is the first retrovirus to be unambiguously linked causally to a human cancer. HTLV-1 currently infects approximately 20 million people worldwide. In this chapter, we review progress made over the last 30 years in our understanding of HTLV-1 infection, replication, gene expression, and cellular transformation.

Controlling leucine-zipper partner recognition in cells through modification of a–g interactions

By focusing on the a–g interactions, successful design and selection were accomplished to obtain a leucine-zipper segment that discriminates the appropriate partner.

HTLV-1/-2 and HIV-1 co-infections: retroviral interference on host immune status

The human retroviruses HIV-1 and HTLV-1/HTLV-2 share similar routes of transmission but cause significantly different diseases. In this review we have outlined the immune mediated mechanisms by which HTLVs affect HIV-1 disease in co-infected hosts. During co-infection with HIV-1, HTLV-2 modulates the cellular microenvironment favoring its own viability and inhibiting HIV-1 progression. This is achieved when the HTLV-2 proviral load is higher than that of HIV-1, and thanks to the ability of HTLV-2 to: (i) up-regulate viral suppressive CCL3L1 chemokine expression; (ii) overcome HIV-1 capacity to activate the JAK/STAT pathway; (iii) reduce the activation of T and NK cells; (iv) modulate the host miRNA profiles. These alterations of immune functions have been mainly attributed to the effects of the HTLV-2 regulatory protein Tax and suggest that HTLV-2 exerts a protective role against HIV-1 infection. Contrary to HIV-1/HTLV-2, the effect of HIV-1/HTLV-1 co-infection on immunological and pathological conditions is still controversial. There is evidence that indicates a worsening of HIV-1 infection, while other evidence does not show clinically relevant effects in HIV-positive people. Possible differences on innate immune mechanisms and a particularly impact on NK cells are becoming evident. The differences between the two HIV-1/HTLV-1 and HIV-1/HTLV-2 co-infections are highlighted and further discussed.