Human T-cell Leukemia Virus Type 1 Pathogenesis Research Articles

HTLV-1 Tax Tug-of-War: Cellular Senescence and Death or Cellular Transformation.

Human T cell leukemia virus type 1 (HTLV-1) is a retrovirus associated with a lymphoproliferative disease known as adult T cell leukemia/lymphoma (ATLL). HTLV-1 infection efficiently transforms human T cells in vivo and in vitro. The virus does not transduce a proto-oncogene, nor does it integrate into tumor-promoting genomic sites. Instead, HTLV-1 uses a random mutagenesis model, resulting in cellular transformation. Expression of the viral protein Tax is critical for the immortalization of infected cells by targeting specific cellular signaling pathways. However, Tax is highly immunogenic and represents the main target for the elimination of virally infected cells by host cytotoxic T cells (CTLs). In addition, Tax expression in naïve cells induces pro-apoptotic signals and has been associated with the induction of non-replicative cellular senescence. This review will explore these conundrums and discuss the mechanisms used by the Tax viral oncoprotein to influence life-and-death cellular decisions and affect HTLV-1 pathogenesis.

The PRMT5 inhibitor EPZ015666 is effective against HTLV-1-transformed T-cell lines in vitro and in vivo.

Human T-cell leukemia virus type 1 (HTLV-1) is the infectious cause of adult T-cell leukemia/lymphoma (ATL), an extremely aggressive and fatal malignancy of CD4+ T-cells. Due to the chemotherapy-resistance of ATL and the absence of long-term therapy regimens currently available for ATL patients, there is an urgent need to characterize novel therapeutic targets against this disease. Protein arginine methyltransferase 5 (PRMT5) is a type II PRMT enzyme that is directly involved in the pathogenesis of multiple different lymphomas through the transcriptional regulation of relevant oncogenes. Recently, our group identified that PRMT5 is overexpressed in HTLV-1-transformed T-cell lines, during the HTLV-1-mediated T-cell immortalization process, and in ATL patient samples. The objective of this study was to determine the importance of PRMT5 on HTLV-1 infected cell viability, T-cell transformation, and ultimately disease induction. Inhibition of PRMT5 enzymatic activity with a commercially available small molecule inhibitor (EPZ015666) resulted in selective in vitro toxicity of actively proliferating and transformed T-cells. EPZ015666-treatment resulted in a dose-dependent increase in apoptosis in HTLV-1-transformed and ATL-derived cell lines compared to uninfected Jurkat T-cells. Using a co-culture model of infection and immortalization, we found that EPZ015666 is capable of blocking HTLV-1-mediated T-cell immortalization in vitro, indicating that PRMT5 enzymatic activity is essential for the HTLV-1 T-cell transformation process. Administration of EPZ015666 in both NSG xenograft and HTLV-1-infected humanized immune system (HIS) mice significantly improved survival outcomes. The cumulative findings of this study demonstrate that the epigenetic regulator PRMT5 is critical for the survival, transformation, and pathogenesis of HTLV-1, illustrating the value of this cellular enzyme as a potential therapeutic target for the treatment of ATL.

Establishment of a Cynomolgus Macaque Model of Human T-Cell Leukemia Virus Type 1 (HTLV-1) Infection by Direct Inoculation of Adult T-Cell Leukemia Patient-Derived Cell Lines for HTLV-1 Infection.

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). However, the precise mechanisms leading to HTLV-1 chronic infection and the onset of the diseases have remained unclear, and effective vaccines for inhibiting the infection and the progression of pathogenesis have therefore not been developed. The use of a nonhuman primate (NHP) model is thought to be important for revealing the mechanisms of the progressive status and for the development of prevention procedures. In this study, we developed a cynomolgus macaque (CM) model of HTLV-1 infection by direct intravenous inoculation of HTLV-1-producing cells derived from ATL patients. The cell line used for infection, ATL-040, was selected as the most infectious one in our cell line library. CMs inoculated intravenously with 1 × 108 ATL-040 cells per animal became persistently infected with HTLV-1, as shown by the HTLV-1 provirus load (PVL) in peripheral blood mononuclear cells and HTLV-1-specific antibodies (2/2 animals). One CM inoculated intravenously with 1 × 107 ATL-040 cells did not have detectable PVLs despite the fact that anti-HTLV-1 antibodies were maintained for more than 2 years. Furthermore, immunological approaches, including CD8+ T cell depletion prior to infection (3/3 animals) and intrathecal inoculation (3/3 animals), led to increased proviral loads in the cynomolgus monkeys. The present method and the cynomolgus monkey model of HTLV-1 infection will be beneficial for immunological and virological studies on HTLV-1 aiming at the development of anti-HTLV-1 prophylactic vaccines and therapy drugs. IMPORTANCE HTLV-1 was discovered in the 1980s as the causative agent of adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. However, the precise mechanisms leading to HTLV-1 chronic infection and the onset of the diseases still remain unidentified. Thus, no effective vaccines to inhibit the infection and the progressive of pathogenesis have been developed. The use of appropriate animal models is essential for understanding HTLV-1 infection and pathogenesis. In order to establish a new nonhuman primate model for studies on HTLV-1 infection, cynomolgus monkeys were infected with HTLV-1 under a variety of experimental conditions. Our method, using a cell line generated from an ATL patient as a source of HTLV-1, was able to establish HTLV-1 infection in monkeys with a 100% success rate. This cynomolgus macaque model of HTLV-1 infection will contribute to the elucidation of HTLV-1 infection and its associated disease development.

Identification and characterization of a novel enhancer in the HTLV-1 proviral genome

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that causes adult T-cell leukemia/lymphoma (ATL), a cancer of infected CD4+ T-cells. There is both sense and antisense transcription from the integrated provirus. Sense transcription tends to be suppressed, but antisense transcription is constitutively active. Various efforts have been made to elucidate the regulatory mechanism of HTLV-1 provirus for several decades; however, it remains unknown how HTLV-1 antisense transcription is maintained. Here, using proviral DNA-capture sequencing, we found a previously unidentified viral enhancer in the middle of the HTLV-1 provirus. The transcription factors, SRF and ELK-1, play a pivotal role in the activity of this enhancer. Aberrant transcription of genes in the proximity of integration sites was observed in freshly isolated ATL cells. This finding resolves certain long-standing questions concerning HTLV-1 persistence and pathogenesis. We anticipate that the DNA-capture-seq approach can be applied to analyze the regulatory mechanisms of other oncogenic viruses integrated into the host cellular genome.

HTLV-1 Replication and Adult T Cell Leukemia Development.

Human T-cell leukemia virus type 1 (HTLV-1) was discovered in 1980 as the first, and to date, the only retrovirus that causes human cancer. While HTLV-1 infection is generally asymptomatic, 3-5% of infected individuals develop a T cell neoplasm known as adult T cell leukemia/lymphoma (ATL) decades after infection. Since its discovery, HTLV-1 has served as a model for understanding retroviral oncogenesis, transcriptional regulation, cellular signal transduction, and cell-associated viral infection and spread. Much of the initial research was focused on the viral trans-activator/oncoprotein, Tax. Over the past decade, the study of HTLV-1 has entered the genomic era. With the development of newsystems for studying HTLV-1 infection and pathogenesis, the completion of the whole genome, exome andtranscriptome sequencing analyses of ATL, and the discovery of HBZ as another HTLV-1 oncogene, many established concepts about how HTLV-1 infects, persists and causes disease have undergone substantial revision. This chapter seeks to integrate our current understanding of the mechanisms of action of Tax and HBZ with the comprehensive genomic information of ATLtoprovide an overview of how HTLV-1 infects, replicates and causes leukemia.

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.

The Role of Chemokines in the Pathogenesis of HTLV-1.

Human T cell leukemia virus type 1 (HTLV-1) is a human retrovirus that is associated with two main diseases: HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and adult T cell leukemia/lymphoma (ATL). Chemokines are highly specialized groups of cytokines that play important roles in organizing, trafficking, homing, and in the migration of immune cells to the bone marrow, lymphoid organs and sites of infection and inflammation. Aberrant expression or function of chemokines, or their receptors, has been linked to the protection against or susceptibility to specific infectious diseases, as well as increased the risk of autoimmune diseases and malignancy. Chemokines and their receptors participate in pathogenesis of HTLV-1 associated diseases from inflammation in the central nervous system (CNS) which occurs in cases of HAM/TSP to T cell immortalization and tissue infiltration observed in ATL patients. Chemokines represent viable effective prognostic biomarkers for HTLV-1-associated diseases which provide the early identification of high-risk, treatment possibilities and high-yielding clinical trials. This review focuses on the emerging roles of these molecules in the outcome of HTLV-1-associated diseases.

HTLV-1 Dysregulates IL-6 and IL-10-JAK/STAT Signaling and Induces Leukemia/Lymphoma of Mature CD4+ T Cells with Regulatory T-Cell-like Signatures

Human T-cell leukemia virus, type 1 (HTLV-1) causes adult T-cell leukemia-lymphoma (ATL) and chronic inflammatory diseases (e.g., HTLV-1 associated myelopathy, uveitis). Among viral genes encoded in the HTLV-1 provirus, HTLV-1 bZIP factor (HBZ) is the most important gene for its pathogenesis, since HBZ is constantly expressed in all ATL cases, and has a potential to enhance T-cell proliferation. We generated HBZ transgenic (Tg) mice and found that almost all mice developed systemic inflammation, such as dermatitis, and about 40% of mice suffered from T-cell lymphoma. In addition, this mouse demonstrates the similar immunophenotypes to HTLV-1-infected subjects, such as increase of effector/memory and Foxp3-expressing regulatory T cells (Tregs), indicating that it is a good animal model to analyze the molecular mechanisms of pathogenesis by HTLV-1. There is a clear correlation between the severity of inflammation and the incidence of T-cell lymphoma, suggesting that some immune factors are closely involved in oncogenesis by HBZ. Recent studies showed that several pro-inflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), and interleukin-6 (IL-6), participate in not only inflammation but also tumor development. Especially, IL-6 has been reported to play important roles in oncogenesis, tumor progression, angiogenesis and migration. To clarify the roles of IL-6 in HBZ-mediated lymphomagenesis, we crossed HBZ-Tg mice with IL-6 knockout (KO) mice. Contrary to our hypothesis, inflammation was exacerbated, and incidence of T-cell lymphomas was markedly increased in HBZ-Tg/IL-6KO mice. In the pathological analysis of the lymphoma tissues, the percentage of Foxp3+ cells were much higher in HBZ-Tg/IL-6KO mice than in HBZ-Tg mice. Moreover, flow cytometric analysis also showed that CD4+Foxp3+ T cells were increased in spleens from HBZ-Tg/IL-6KO mice compared with HBZ-Tg mice or WT mice. Since IL-6 inhibits TGF-beta-induced Foxp3 expression and subsequently suppresses differentiation to Tregs, our results suggest that depletion of IL-6 induces aberrant differentiation of CD4+ T cells toward Treg-like cells in HBZ-Tg mice and accelerates lymphomagenesis of this subset. RNA-seq and flow cytometric analyses revealed that expression of IL-10 was significantly higher in CD4+ T cells of HBZ-Tg/IL-6KO than HBZ-Tg. Importantly, IL-10 accelerated the proliferation of CD4+ T cells of HBZ-Tg whereas it did not influence control T cells. We also found that HBZ interacted with both STAT1 and STAT3, which are critical transcription factors in IL-10-JAK/STAT signaling pathways, and enhanced their transcriptional activities. Foxp3-expressing T cells and IL-10 are known to be immune-suppressive, but these results indicate that increased Foxp3-expressing T cells and activation of IL-10-JAK/STAT signaling are associated with inflammation and lymphomagenesis by HBZ in vivo. Our results suggest that IL-6 and IL-10 have suppressive and promoting effects on HBZ-induced pathogenesis, respectively. It has been reported that progression of HTLV-1-associated diseases was observed in several HTLV-1 carriers after administration of anti-IL-6R antibody, Tocilizumab. Inhibition of IL-6/IL-6R signaling and consequent activation of Foxp3/IL-10/JAK/STAT axis seem to be important for HTLV-1 pathogenesis. Disclosures Ohshima: Kyowa Kirin Co., Ltd.: Honoraria, Research Funding; Chugai Pharmaceutical Co., Ltd.: Honoraria, Research Funding; Celgene Corp.: Honoraria, Research Funding; NEC Corp.: Research Funding; SRL, Inc.: Consultancy. Matsuoka:Kyowa Kirin Co., Ltd.: Research Funding; Bristol-Myers Squibb Corp.: Research Funding; Chugai Pharmaceutical Co., Ltd.: Honoraria.

A-106 Strategy and pathogenesis of human T-cell leukemia virus type 1

Human T-cell leukemia virus type 1 (HTLV-1) causes not only adult T-cell leukemia-lymphoma (ATL) but also inflammatory diseases. The most important characteristic of HTLV-1 is that this virus can transmit only through cell-to-cell contact. Therefore, HTLV-1 increases the number of infected cells to facilitate its transmission through breast feeding and sexual intercourse. HTLV-1 promotes proliferation of infected cells, inhibits their apoptosis and facilitates escape from the host immune surveillance. The pathogenesis of these diseases is based on these characteristics of this virus. Infected cells evade the host immune responses to survive in vivo. Early studies of HTLV-1 focused on Tax that is the trans-activator of viral gene transcription. Tax can immortalize T cellsin vitroand in vivo. Tax activates NFkB pathway, and functionally inhibits p53 function. It is assumed that Tax is the viral oncogene that causes ATL. However, analyses of HTLV-1 proviruses in clinical samples revealed that Tax expression is frequently inactivated in ATL cells by 3 mechanisms: genetic changes of the taxgene (mainly non-sense mutation), deletion of 5′ long terminal repeat (LTR), the promoter/enhancer of the taxgene, and DNA methylation of 5′LTR. On the other hand, 3′ provirus is maintained in all ATL cases and HTLV-1 carriers. This region contains HTLV-1 bZIP factor (HBZ) coding region. HBZ gene is encoded in the minus strand of the provirus. These finding suggest that HBZ is critical for ATL cells and HTLV-1 infected cells. To study the function of HBZZ, we generated HBZ-transgenic mice that express HBZ in CD4+ T cells. HBZ-Tg mice developed T-cell lymphoma and inflammatory diseases, suggesting that HBZ is critical for HTLV-1 associated diseases. Thereafter, we focus on function of HBZ in biology and pathology of HTLV-1. This strategy is supported by several attributes of this virus. First, HTLV-1 infected T cells possess the specific immunophenotype; CD4, CD45RO, CCR4, CADM1, TIGIT, Foxp3 (approximately 50%) positive T cells. Effector/memory (CD45RO) T cells express higher level of LFA-1, which likely migrate into breast milk and semen. Furthermore, TIGIT suppresses the host immune responses, which facilitates infected cells to escape from immune surveillance. HBZ also induces expression of Foxp3. Foxp3 induces expression of regulatory T (Treg) cell associated molecules. On the other hand, labile Foxp3 expression is associated with inflammation by increased interferon-gproduction. Thus, HTLV-1 bZIP factor (HBZ) is mainly implicated in immunnophenotype of HTLV-1 infected cells and ATL cells. Second, HTLV-1 infected cells need persistent proliferation and survival in vivo. Both HBZ and tax are implicated in proliferation and survival. Third, HTLV-1 infected cells and ATL cells need to minimize expression of immunogenic viral proteins. Tax is a highly immunogenic protein, and a major target of cytotoxic T-lymphocytes (CTLs) in vivo. On the other hand, immunogenicity of HBZ is quite low. Therefore, HTLV-1 infected cells and ATL cells continuously express HBZ. In contrast, Tax is transiently expressed (Mahgoub et al, PNAS 2018). The intermittent expression of Tax plays an important role to maintain ATL cells, and possibly HTLV-1 infected cells. Pathogenesis of HTLV-1 is based on these characteristics of this virus; infiltration, inflammation, persistent proliferation, and anti-apoptosis. Thus, pro-inflammatory infected cells cause inflammatory diseases, and lead to development of leukemia. In this meeting, I am going to show the roles of HBZ and Tax in the pathogenesis, discuss about complex relation between virus and diseases.

HTLV-1 Alters T Cells for Viral Persistence and Transmission.

Human T-cell leukemia virus type 1 (HTLV-1) was the first retrovirus to be discovered as a causative agent of adult T-cell leukemia-lymphoma (ATL) and chronic inflammatory diseases. Two viral factors, Tax and HTLV-1 bZIP factor (HBZ), are thought to be involved in the leukemogenesis of ATL. Tax expression is frequently lost due to DNA methylation in the promoter region, genetic changes to the tax gene, and deletion of the 5′ long terminal repeat (LTR) in approximately half of all ATL cases. On the other hand, HBZ is expressed in all ATL cases. HBZ is known to function in both protein form and mRNA form, and both forms play an important role in the oncogenic process of HTLV-1. HBZ protein has a variety of functions, including the suppression of apoptosis, the promotion of proliferation, and the impairment of anti-viral activity, through the interaction with several host cellular proteins including p300/CBP, Foxp3, and Foxo3a. These functions dramatically modify the transcriptional profiling of host T cells. HBZ mRNA also promotes T cell proliferation and viability. HBZ changes infected T cells to CCR4+TIGIT+CD4+ effector/memory T cells. This unique immunophenotype enables T cells to migrate into various organs and tissues and to survive in vivo. In this review, we summarize how HBZ hijacks the transcriptional networks and immune systems of host T cells to contribute to HTLV-1 pathogenesis on the basis of recent new findings about HBZ and tax.

HTLV-1 HBZ Viral Protein: A Key Player in HTLV-1 Mediated Diseases.

Human T cell leukemia virus type 1 (HTLV-1) is an oncogenic human retrovirus that has infected 10–15 million people worldwide. After a long latency, 3–5% of infected individuals will develop either a severe malignancy of CD4+ T cells, known as Adult T-cell Leukemia (ATL) or a chronic and progressive inflammatory disease of the nervous system designated Tropical Spastic Paraparesis/HTLV-1-Associated Myelopathy (HAM/TSP). The precise mechanism behind HTLV-1 pathogenesis still remains elusive. Two viral regulatory proteins, Tax-1 and HTLV-1 bZIP factor (HBZ) are thought to play a critical role in HTLV-1-associated diseases. Tax-1 is mainly involved in the onset of neoplastic transformation and in elicitation of the host’s inflammatory responses; its expression may be lost during cell clonal proliferation and oncogenesis. Conversely, HBZ remains constantly expressed in all patients with ATL, playing a role in the proliferation and maintenance of leukemic cells. Recent studies have shown that the subcellular distribution of HBZ protein differs in the two pathologies: it is nuclear with a speckled-like pattern in leukemic cells and is cytoplasmic in cells from HAM/TSP patients. Thus, HBZ expression and distribution could be critical in the progression of HTLV-1 infection versus the leukemic state or the inflammatory disease. Here, we reviewed recent findings on the role of HBZ in HTLV-1 related diseases, highlighting the new perspectives open by the possibility of studying the physiologic expression of endogenous protein in primary infected cells.

Multifaceted functions and roles of HBZ in HTLV-1 pathogenesis.

Human T cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus responsible for the development of adult T-cell leukemia (ATL). Although HTLV-1 harbors an oncogene, tax, that transforms T cells in vitro and induces leukemia in transgenic mice, tax expression is frequently disrupted in ATL, making the oncogenesis of ATL a bit mysterious. The HTLV-1 bZIP factor (HBZ) gene was discovered in 2002 and has been found to promote T-cell proliferation and cause lymphoma in transgenic mice. Thus HBZ has become a novel hotspot of HTLV-1 research. This review summarizes the current findings on HBZ with a special focus on its potential links to the oncogenesis of ATL. We propose viewing HBZ as a critical contributing factor in ATL development.

The Role of HBZ in HTLV-1-Induced Oncogenesis.

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL) and chronic inflammatory diseases. HTLV-1 bZIP factor (HBZ) is transcribed as an antisense transcript of the HTLV-1 provirus. Among the HTLV-1-encoded viral genes, HBZ is the only gene that is constitutively expressed in all ATL cases. Recent studies have demonstrated that HBZ plays an essential role in oncogenesis by regulating viral transcription and modulating multiple host factors, as well as cellular signaling pathways, that contribute to the development and continued growth of cancer. In this article, I summarize the current knowledge of the oncogenic function of HBZ in cell proliferation, apoptosis, T-cell differentiation, immune escape, and HTLV-1 pathogenesis.

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.

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.

HTLV-1 bZIP Factor RNA and Protein Impart Distinct Functions on T-cell Proliferation and Survival.

Infection of T cells with human T-cell leukemia virus type-1 (HTLV-1) induces clonal proliferation and is closely associated with the onset of adult T-cell leukemia-lymphoma (ATL) and inflammatory diseases. Although Tax expression is frequently suppressed in HTLV-1-infected cells, the accessory gene, HTLV-1 bZIP factor (HBZ), is continuously expressed and has been implicated in HTLV-1 pathogenesis. Here, we report that transduction of mouse T cells with specific mutants of HBZ that distinguish between its RNA and protein activity results in differential effects on T-cell proliferation and survival. HBZ RNA increased cell number by attenuating apoptosis, whereas HBZ protein induced apoptosis. However, both HBZ RNA and protein promoted S-phase entry of T cells. We further identified that the first 50 bp of the HBZ coding sequence are required for RNA-mediated cell survival. Transcriptional profiling of T cells expressing wild-type HBZ, RNA, or protein revealed that HBZ RNA is associated with genes involved in cell cycle, proliferation, and survival, while HBZ protein is more closely related to immunological properties of T cells. Specifically, HBZ RNA enhances the promoter activity of survivin, an inhibitor of apoptosis, to upregulate its expression. Inhibition of survivin using YM155 resulted in impaired proliferation of several ATL cell lines as well as a T-cell line expressing HBZ RNA. The distinct functions of HBZ RNA and protein may have several implications for the development of strategies to control the proliferation and survival mechanisms associated with HTLV-1 infection and ATL.

Animal model studies on viral infections.

One of the major missions of animal virology is to understand how viruses replicate and cause asymptomatic/symptomatic conditions in individuals (Nomaguchi and Adachi, 2010). It is especially important for virologists who work on viruses pathogenic for humans to elucidate bases underlying the in vivo viral characteristics. Toward this end, animal model studies in some ways are necessary to precisely analyze the in vivo situation, and also are essential for developing countermeasures against virus infections. Since a full variety of viruses with distinct biological properties exist, we virologists should study “the target virus” in a specialized manner, in addition to common theoretical/experimental approaches. The Research Topic entitled “Animal model studies on viral infections” collects articles that describe the studies on numerous virus species for their animal models, or those at various stages toward animal experiments. Articles in this Research Topic were written by experts in various research fields, and can be fairly grouped into a few categories: (i) descriptions/evaluations/new challenges of animal model studies for investigating the biology of viruses; (ii) experimental materials/methods for upcoming animal model studies; (iii) observations important for animal model studies. (i) Reynaud and Horvat (2013) have described the animal models for human herpesvirus 6 to better understand its pathogenic property. Studies on filoviruses, classified as biosafety level-4 and represent a serious world-wide problem today, have been reviewed by Nakayama and Saijo (2013). Mailly et al. (2013) have focused on the quest for appropriate animal models for hepatitis C virus. Clark et al. (2013) have discussed about the use of non-human primates as models for dengue hemorrhagic fever/dengue shock syndrome. Ohsugi (2013) has summarized mouse strains transgenic for the tax gene of human T-cell leukemia virus type 1 (HTLV-1). Also, a bovine model for HTLV-1 pathogenesis has been described by Aida et al. (2013). Challenging new attempts to establish human immunodeficiency virus type 1 (HIV-1)/macaque infection models have been reviewed by Misra et al. (2013), and also by Saito and Akari (2013). Another approach to understand HIV-1 biology in vivo has been described by Matsuyama-Murata et al. (2013). (ii) Kodama et al. (2013) has described a new and simple method to prepare human dendritic cells from peripheral blood mononuclear cells. Doi et al. (2013) have summarized their studies on macaque-tropic HIV-1 clones. Ikeno et al. (2013) has reported a new, sensitive, and quantitative system to monitor measles virus infection in humanized mice. Iwami et al. (2013) have summarized the quantification of viral infection dynamics based on various quantitative analyses. (iii) Tada et al. (2013) have suggested that LEDGF/p75 may be a cellular factor acting as a species-barrier against HIV-1 in mouse cells. Kuwata et al. (2013) have shown that simian immunodeficiency virus may acquire the increased infectivity and resistance to neutralizing antibodies by truncation of its gp41 cytoplasmic tail. Ohsugi et al. (2013) have reported that natural infection status of laboratory mice by murine norovirus. Finally, Kajitani et al. (2013) have described the possible involvement of E1∧E4 protein of human papillomavirus type 18 in its differentiation-dependent life cycle. We are proud to add our “Animal model studies on viral infections” to a series of Research Topic in Frontiers in Microbiology. A wide variety of DNA and RNA viruses are covered by this special issue consisting of original research, review, mini-review, methods, and opinion articles. As we described in the beginning, animal studies are certainly required for understanding virus replicative/pathogenic properties in vivo and for overcoming virally-caused infectious diseases. We human virologists should make every effort to fight against numbers of unique pathogenic viruses.

153 Molecular pathogenesis by HTLV-1 bZIP factor

Human T-cell leukemia virus type 1 (HTLV-1) mainly transmits by cell-to-cell contact. Therefore, this virus induces proliferation of infected cells to facilitate transmission. In HTLV-1 infected individuals, infected cells proliferate in vivo. This proliferation of infected cells leads to the development of adult T-cell leukemia (ATL) and inflammatory diseases including HTLV-1 associated myelopathy (HAM). HTLV-1 bZIP factor (HBZ) is consistently expressed in ATL cells and HTLV-1 infected cells, and plays a critical role in their proliferation. Transgenic expression of HBZ causes T-cell lymphoma and inflammatory diseases, indicating that HBZ is critical for the pathogenesis of HTLV-1.We analyzed the function of HBZ and found that it possessed opposite functions to Tax in many pathways. We found that Tax could activate the classical Wnt pathway while HBZ inhibited it by interacting with TCF1/LEF1. The canonical Wnt pathway is activated in the thymus, and its transcription factors, TCF1/LEF1, suppressed the viral gene transcription by Tax. Conversely, the canonical Wnt pathway is suppressed in the peripheral T cells in which the non-canonical Wnt pathway is activated. HBZ enhanced the expression of Wnt5a, which is a ligand in non-canonical Wnt signaling. Knockdown of Wnt5a suppressed the proliferation and migration of ATL cells. Thus, activation of the non-canonical Wnt pathway by HBZ is linked with proliferation of ATL cells.These findings suggest that HTLV-1 is adapted to, not thymic T cells, but peripheral T cells. HBZ modulates the intracellular environment suitable for HTLV-1 replication and proliferation of infected cells.

HTLV-1 Tax mediated downregulation of miRNAs associated with chromatin remodeling factors in T cells with stably integrated viral promoter

RNA interference (RNAi) is a natural cellular mechanism to silence gene expression and is predominantly mediated by microRNAs (miRNAs) that target messenger RNA. Viruses can manipulate the cellular processes necessary for their replication by targeting the host RNAi machinery. This study explores the effect of human T-cell leukemia virus type 1 (HTLV-1) transactivating protein Tax on the RNAi pathway in the context of a chromosomally integrated viral long terminal repeat (LTR) using a CD4(+) T-cell line, Jurkat. Transcription factor profiling of the HTLV-1 LTR stably integrated T-cell clone transfected with Tax demonstrates increased activation of substrates and factors associated with chromatin remodeling complexes. Using a miRNA microarray and bioinformatics experimental approach, Tax was also shown to downregulate the expression of miRNAs associated with the translational regulation of factors required for chromatin remodeling. These observations were validated with selected miRNAs and an HTLV-1 infected T cells line, MT-2. miR-149 and miR-873 were found to be capable of directly targeting p300 and p/CAF, chromatin remodeling factors known to play critical role in HTLV-1 pathogenesis. Overall, these results are first in line establishing HTLV-1/Tax-miRNA-chromatin concept and open new avenues toward understanding retroviral latency and/or replication in a given cell type.

Virological and immunological mechanisms in the pathogenesis of human T‐cell leukemia virus type 1

Human T-cell leukemia virus type 1 (HTLV-1) was the first retrovirus shown to cause human disease, such as adult T-cell leukemia and HTLV-1 associated myelopathy/tropic spastic paraparesis. HTLV-1 mainly infects CD4 T cells and deregulates their differentiation, function and homeostasis, which should contribute to the pathogenesis of HTLV-1, for example, inducing transformation of infected CD4 T cells and chronic inflammatory diseases. Therefore, not only virological approach but also immunological approach regarding CD4 T cells are required to understand how HTLV-1 causes related human diseases. This review focuses on recent advances in our understanding of the interaction between HTLV-1 and the main host cell, CD4 T cells, which should provide us some clue to the mechanisms of HTLV-1 mediated pathogenesis.