HTLV-I basic leucine zipper factor gene mRNA supports proliferation of adult T cell leukemia cells

Identification of TBK1 and IKKε, the non-canonical IκB kinases, as crucial pro-survival factors in HTLV-1-transformed T lymphocytes

Persistent activation of NF-κB is a prerequisite for development of adult T cell leukemia-lymphoma (ATL) caused by human T cell leukemia virus type 1 (HTLV-1). HTLV-1 genome encodes a viral transforming protein named Tax, which constitutively activates the canonical IκB kinases (IKK), the central regulator of NF-κB signaling. However, the role of the non-canonical IκB kinases, TBK1 and IKKε, in the pathogenesis of HTLV-1-associated leukemia has not been evaluated. We here show that TBK1/IKKε are crucial pro-survival molecules by maintaining persistent activity of Stat3. Consistent with this finding, silencing Stat3 by the specific shRNA or by the chemical inhibitor ruxolitinib results in drastic impediment of leukemia cell growth. We further find that in HTLV-1-transformed T cells expressing Tax, TBK1 co-localizes with the canonical IκB kinases and Tax in the lipid raft microdomains. The wild type Tax, but not the Tax mutant defective in activating the canonical IKK, promotes the lipid raft translocation of TBK1. This phenomenon correlates with Tax activation of both NF-κB and Stat3. Tax does not interact directly with TBK1/IKKε, and it rather engages a molecular crosstalk between the canonical IKKs and TBK1/IKKε. Our data, therefore, demonstrate a key role of TBK1/IKKε in the survival and proliferation of HTLV-1-transformed T cells and implicate a potential therapy targeting TBK1/IKKε and Stat3 in controlling HTLV-1-mediated oncogenesis.

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.

HTLV-1 and the host immune system: how the virus disrupts immune regulation, leading to HTLV-1 associated diseases

Human T-cell leukemia virus type 1 (HTLV-1) was the first retrovirus shown to cause human diseases, such as adult T-cell leukemia (ATL) and HTLV-1 associated myelopathy/tropic spastic paraparesis (HAM/TSP). Despite extensive study for three decades, it remains elusive how HTLV-1 induces these diseases. HTLV-1 mainly infects CD4 T cells, inducing dysregulation of the host immune system. Recent studies have uncovered the mechanisms of differentiation and function of CD4 T cells at the cellular and molecular levels, extending our understanding of the pathological conditions associated with HTLV-1 infection. This review focuses on recent advances in our understanding of the interaction between HTLV-1 and the host immune system, which should provide us a clue to the mechanisms of HTLV-1 mediated pathogenesis.

The diversity of the structure and genomic integration sites of HTLV-1 provirus in MT-2 cell lines

A human T-lymphotropic virus Type 1 (HTLV-1) positive cell line, MT-2, derived from human cord leukocytes co-culturing with adult T cell leukemia/lymphoma (ATL) cells is commonly used in HTLV-1 research; however, the details of provirus integrated in MT-2 genome have not yet been characterized. In this study, five types of HTLV-1 proviral sequences were detected in 11 different sites of the genome in a reference MT-2 cell line. The five types of HTLV-1 proviral sequences were one complete proviral genome, two types of proviruses with deletion of large internal viral sequences (5.3 and 3.9 kB), one provirus with a large deletion (6.2 kB) from 5'LTR to position 6257, and one provirus of LTR only. The provirus with identical deletion of large internal viral sequence (5.3 kB) was found to be integrated into six different sites (chromosomes). A complete provirus and three of four types of defective provirus were consistently detected in two other MT-2 cell lines cultured in different laboratories. Not only Tax/Rex RNA and HBZ RNA, but also the transcriptional product for a specific defective provirus, were detectable in all three MT-2 cell lines. Because it has been reported that defective provirus is frequently detected in ATL cells, these results may be important in understanding the mechanism of HTLV-1 proviral polymorphism, which may be related to leukemogenesis. In addition, the large variation in integrated HTLV-1 proviruses makes it important for researchers to exercise caution in their assessment and interpretation of results using MT-2 cell lines.

The retrovirus HTLV-1 inserts an ectopic CTCF-binding site into the human genome

Human T-lymphotropic virus type 1 (HTLV-1) is a retrovirus that causes malignant and inflammatory diseases in ∼10% of infected people. A typical host has between 10(4) and 10(5) clones of HTLV-1-infected T lymphocytes, each clone distinguished by the genomic integration site of the single-copy HTLV-1 provirus. The HTLV-1 bZIP (HBZ) factor gene is constitutively expressed from the minus strand of the provirus, whereas plus-strand expression, required for viral propagation to uninfected cells, is suppressed or intermittent in vivo, allowing escape from host immune surveillance. It remains unknown what regulates this pattern of proviral transcription and latency. Here, we show that CTCF, a key regulator of chromatin structure and function, binds to the provirus at a sharp border in epigenetic modifications in the pX region of the HTLV-1 provirus in T cells naturally infected with HTLV-1. CTCF is a zinc-finger protein that binds to an insulator region in genomic DNA and plays a fundamental role in controlling higher order chromatin structure and gene expression in vertebrate cells. We show that CTCF bound to HTLV-1 acts as an enhancer blocker, regulates HTLV-1 mRNA splicing, and forms long-distance interactions with flanking host chromatin. CTCF-binding sites (CTCF-BSs) have been propagated throughout the genome by transposons in certain primate lineages, but CTCF binding has not previously been described in present-day exogenous retroviruses. The presence of an ectopic CTCF-BS introduced by the retrovirus in tens of thousands of genomic locations has the potential to cause widespread abnormalities in host cell chromatin structure and gene expression.

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.

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.

Molecular Studies of HTLV-1 Replication: An Update

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

Functional Comparison of HBZ and the Related APH-2 Protein Provides Insight into Human T-Cell Leukemia Virus Type 1 Pathogenesis

Human T-cell leukemia virus type 1 (HTLV-1) and type 2 (HTLV-2) are highly related retroviruses that transform T cells in vitro but have distinct pathological outcomes in vivo. HTLV-1 encodes a protein from the antisense strand of its proviral genome, the HTLV-1 basic leucine zipper factor (HBZ), which inhibits Tax-1-mediated viral transcription and promotes cell proliferation, a high proviral load, and persistence in vivo. In adult T-cell leukemia/lymphoma (ATL) cell lines and patient T cells, hbz is often the only viral gene expressed. The antisense strand of the HTLV-2 proviral genome also encodes a protein termed APH-2. Like HBZ, APH-2 is able to inhibit Tax-2-mediated viral transcription and is detectable in most primary lymphocytes from HTLV-2-infected patients. However, unlike HBZ, the loss of APH-2 in vivo results in increased viral replication and proviral loads, suggesting that HBZ and APH-2 modulate the virus and cellular pathways differently. Herein, we examined the effect of APH-2 on several known HBZ-modulated pathways: NF-κB (p65) transactivation, transforming growth factor β (TGF-β) signaling, and interferon regulatory factor 1 (IRF-1) transactivation. Like HBZ, APH-2 has the ability to inhibit p65 transactivation. Conversely, HBZ and APH-2 have divergent effects on TGF-β signaling and IRF-1 transactivation. Quantitative PCR and protein half-life experiments revealed a substantial disparity between HBZ and APH-2 transcript levels and protein stability, respectively. Taken together, our data further elucidate the functional differences between HBZ and APH-2 and how these differences can have profound effects on the survival of infected cells and, ultimately, pathogenesis.

IMPORTANCE

Human T-cell leukemia virus type 1 (HTLV-1) and type 2 (HTLV-2) are highly related retroviruses that have distinct pathological outcomes in infected hosts. Functional comparisons of HTLV-1 and HTLV-2 proteins provide a better understanding about how HTLV-1 infection is associated with disease and HTLV-2 infection is not. The HTLV genome antisense-strand genes hbz and aph-2 are often the only viral genes expressed in HTLV-infected T cells. Previously, our group found that HTLV-1 HBZ and HTLV-2 APH-2 had distinct effects in vivo and hypothesized that the differences in the interactions of HBZ and APH-2 with important cell signaling pathways dictate whether cells undergo proliferation, apoptosis, or senescence. Ultimately, these functional differences may affect how HTLV-1 causes disease but HTLV-2 generally does not. In the current study, we compared the effects of HBZ and APH-2 on several HTLV-relevant cellular pathways, including the TGF-β signaling, NF-κB activation, and IRF-1 transactivation pathways.

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.

HTLV-1 bZIP Factor Impairs Anti-viral Immunity by Inducing Co-inhibitory Molecule, T Cell Immunoglobulin and ITIM Domain (TIGIT)

Human T-cell leukemia virus type 1 (HTLV-1) infects CD4⁺ T cells and induces proliferation of infected cells in vivo, which leads to the onset of adult T-cell leukemia (ATL) in some infected individuals. The HTLV-1 bZIP factor (HBZ) gene, which is encoded in the minus strand of HTLV-1, plays critical roles in pathogenesis. In this study, RNA-seq and ChIP-seq analyses using HBZ transduced T cells revealed that HBZ upregulates the expression and promoter acetylation levels of a co-inhibitory molecule, T cell immunoglobulin and ITIM domain (TIGIT), in addition to those of regulatory T cells related genes, Foxp3 and Ccr4. TIGIT was expressed on CD4⁺ T cells from HBZ-transgenic (HBZ-Tg) mice, and on ATL cells and HTLV-1 infected CD4⁺ T cells of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) in vivo. Expression of Blimp1 and IL-10 was upregulated in TIGIT⁺CD4⁺ cells of HBZ-Tg mice compared with TIGIT^-CD4⁺ T cells, suggesting the correlation between TIGIT expression and IL-10 production. When CD4⁺ T cells from HBZ-Tg mice were stimulated with TIGIT’s ligand, CD155, their production of the inhibitory cytokine IL-10 was enhanced. Furthermore, dendritic cells from HBZ-Tg mice produced high levels of IL-10 after stimulation. These data suggest that HBZ alters immune system to suppressive state via TIGIT and IL-10. Importantly, TIGIT suppressed T-cell responses to another HTLV-1 virus protein, Tax, in vitro. Blocking of TIGIT and PD-1 slightly increased anti-Tax T-cell activity in some HAM/TSP patients. These results suggest that HBZ-induced TIGIT on HTLV-1 infected cells impairs T-cell responses to viral antigens. This study shows that HBZ-induced TIGIT plays a pivotal role in attenuating host immune responses and shaping a microenvironment favorable to HTLV-1.

HTLV-1 is a T-cell-tropic, latently infectious virus that causes a T-cell malignancy, ATL, and inflammatory diseases. The mechanisms by which HTLV-1 evades the immune response and establishes chronic infection are not yet understood. Recent studies have demonstrated that TIGIT, a co-inhibitory molecule, is expressed on tumor infiltrating T cells and T cells during viral infection, which suppresses the anti-tumor and anti-viral immune responses. Furthermore, blockade of co-inhibitory molecules of TIGIT and programmed cell death-1 (PD-1) disrupts immune checkpoints and enhances anti-tumor activity. We found that TIGIT is upregulated by HBZ, and TIGIT impairs anti-virus immune responses through an immunosuppressive cytokine, IL-10. These findings show that HTLV-1 utilizes a co-inhibitory molecule on infected cells to evade the host immune responses. We also found that blocking of TIGIT and PD-1 on peripheral blood mononuclear cells in HTLV-1 infected patients enhances immune responses to virus. These findings suggest a mechanism by which HTLV-1 shapes a microenvironment favorable to its persistence using induced TIGIT. TIGIT is a potential therapeutic target for ATL and HTLV-1 infected patients.

PRMT5 Is Upregulated in HTLV-1-Mediated T-Cell Transformation and Selective Inhibition Alters Viral Gene Expression and Infected Cell Survival

Human T-cell leukemia virus type-1 (HTLV-1) is a tumorigenic retrovirus responsible for development of adult T-cell leukemia/lymphoma (ATLL). This disease manifests after a long clinical latency period of up to 2-3 decades. Two viral gene products, Tax and HBZ, have transforming properties and play a role in the pathogenic process. Genetic and epigenetic cellular changes also occur in HTLV-1-infected cells, which contribute to transformation and disease development. However, the role of cellular factors in transformation is not completely understood. Herein, we examined the role of protein arginine methyltransferase 5 (PRMT5) on HTLV-1-mediated cellular transformation and viral gene expression. We found PRMT5 expression was upregulated during HTLV-1-mediated T-cell transformation, as well as in established lymphocytic leukemia/lymphoma cell lines and ATLL patient PBMCs. shRNA-mediated reduction in PRMT5 protein levels or its inhibition by a small molecule inhibitor (PRMT5i) in HTLV-1-infected lymphocytes resulted in increased viral gene expression and decreased cellular proliferation. PRMT5i also had selective toxicity in HTLV-1-transformed T-cells. Finally, we demonstrated that PRMT5 and the HTLV-1 p30 protein had an additive inhibitory effect on HTLV-1 gene expression. Our study provides evidence for PRMT5 as a host cell factor important in HTLV-1-mediated T-cell transformation, and a potential target for ATLL treatment.

HTLV-1, Immune Response and Autoimmunity

Human T-lymphotropic virus type-1 (HTLV-1) infection is associated with adult T-cell leukemia/lymphoma (ATL). Tropical spastic paraparesis/HTLV-1-associated myelopathy (PET/HAM) is involved in the development of autoimmune diseases including Rheumatoid Arthritis (RA), Systemic Lupus Erythematosus (SLE), and Sjögren's Syndrome (SS). The development of HTLV-1-driven autoimmunity is hypothesized to rely on molecular mimicry, because virus-like particles can trigger an inflammatory response. However, HTLV-1 modifies the behavior of CD4⁺ T cells on infection and alters their cytokine production. A previous study showed that in patients infected with HTLV-1, the activity of regulatory CD4⁺ T cells and their consequent expression of inflammatory and anti-inflammatory cytokines are altered. In this review, we discuss the mechanisms underlying changes in cytokine release leading to the loss of tolerance and development of autoimmunity.

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.

From Immunodeficiency to Humanization: The Contribution of Mouse Models to Explore HTLV-1 Leukemogenesis

The first discovered human retrovirus, Human T-Lymphotropic Virus type 1 (HTLV-1), is responsible for an aggressive form of T cell leukemia/lymphoma. Mouse models recapitulating the leukemogenesis process have been helpful for understanding the mechanisms underlying the pathogenesis of this retroviral-induced disease. This review will focus on the recent advances in the generation of immunodeficient and human hemato-lymphoid system mice with a particular emphasis on the development of mouse models for HTLV-1-mediated pathogenesis, their present limitations and the challenges yet to be addressed.

HTLV-1 subgroups associated with the risk of HAM/TSP are related to viral and host gene expression in peripheral blood mononuclear cells, independent of the transactivation functions of the viral factors

Among human T cell leukemia virus type 1 (HTLV-1)-infected individuals, the risk of developing HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) across lifetime differs between ethnic groups. There is an association between HTLV-1 tax gene subgroups (subgroup-A or subgroup-B) and the risk of HAM/TSP in the Japanese population. In this study, we investigated the full-length proviral genome sequences of various HTLV-1-infected cell lines and patient samples. The functional differences in the viral transcriptional regulators Tax and HTLV-1 bZIP factor (HBZ) between each subgroup and the relationships between subgroups and the clinical and laboratory characteristics of HAM/TSP patients were evaluated. The results of these analyses indicated the following: (1) distinct nucleotide substitutions corresponding to each subgroup were associated with nucleotide substitutions in viral structural, regulatory, and accessory genes; (2) the HBZ messenger RNA (mRNA) expression in HTLV-1-infected cells was significantly higher in HAM/TSP patients with subgroup-B than in those with subgroup-A; (3) a positive correlation was observed between the expression of HBZ mRNA and its target Foxp3 mRNA in HAM/TSP patients with subgroup-B, but not in patients with subgroup-A; (4) no clear differences were noted in clinical and laboratory characteristics between HAM/TSP patients with subgroup-A and subgroup-B; and (5) no functional differences were observed in Tax and HBZ between each subgroup based on reporter gene assays. Our results indicate that although different HTLV-1 subgroups are characterized by different patterns of viral and host gene expression in HAM/TSP patients via independent mechanisms of direct transcriptional regulation, these differences do not significantly affect the clinical and laboratory characteristics of HAM/TSP patients.

[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.

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

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

Global Dynamics of a Virus Dynamical Model with Cell-to-Cell Transmission and Cure Rate

The cure effect of a virus model with both cell-to-cell transmission and cell-to-virus transmission is studied. By the method of next generation matrix, the basic reproduction number is obtained. The locally asymptotic stability of the virus-free equilibrium and the endemic equilibrium is considered by investigating the characteristic equation of the model. The globally asymptotic stability of the virus-free equilibrium is proved by constructing suitable Lyapunov function, and the sufficient condition for the globally asymptotic stability of the endemic equilibrium is obtained by constructing suitable Lyapunov function and using LaSalle invariance principal.

HTLV-I Tax-Mediated Inactivation of Cell Cycle Checkpoints and DNA Repair Pathways Contribute to Cellular Transformation: "A Random Mutagenesis Model"

To achieve cellular transformation, most oncogenic retroviruses use transduction by proto-oncogene capture or insertional mutagenesis, whereby provirus integration disrupts expression of tumor suppressors or proto-oncogenes. In contrast, the Human T-cell leukemia virus type 1 (HTLV-I) has been classified in a separate class referred to as "transactivating retroviruses". Current views suggest that the viral encoded Tax protein transactivates expression of cellular genes leading to deregulated growth and transformation. However, if Tax-mediated transactivation was indeed sufficient for cellular transformation, a fairly high frequency of infected cells would eventually become transformed. In contrast, the frequency of transformation by HTLV-I is very low, likely less than 5%. This review will discuss the current understanding and recent discoveries highlighting critical functions of Tax in cellular transformation. HTLV-I Tax carries out essential functions in order to override cell cycle checkpoints and deregulate cellular division. In addition, Tax expression is associated with increased DNA damage and genome instability. Since Tax can inhibit multiple DNA repair pathways and stimulate unfaithful DNA repair or bypass checkpoints, these processes allow accumulation of genetic mutations in the host genome. Given this, a "Random Mutagenesis" transformation model seems more suitable to characterize the oncogenic activities of HTLV-I.

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.

Foxp3-dependent transformation of human primary CD4+ T lymphocytes by the retroviral protein tax

The retroviral Tax proteins of human T cell leukemia virus type 1 and 2 (HTLV-1 and -2) are highly homologous viral transactivators. Both viral proteins can immortalize human primary CD4+ memory T cells, but when expressed alone they rarely transform T cells. In the present study, we found that the Tax proteins displayed a differential ability to immortalize human CD4+Foxp3+ T cells with characteristic expression of CTLA-4 and GITR. Because epidermal growth factor receptor (EGFR) was reportedly expressed and activated in a subset of CD4+Foxp3+ T cells, we introduced an activated EGFR into Tax-immortalized CD4+Foxp3+ T cells. We observed that these modified cells were grown independently of exogenous IL-2, correlating with a T cell transformation phenotype. In Tax-immortalized CD4+Foxp3- T cells, ectopic expression of Foxp3 was a prerequisite for Tax transformation of T cells. Accordingly, treatment of the transformed T cells with erlotinib, a selective inhibitor of EGFR, induced degradation of EGFR in lysosome, consequently causing T cell growth inhibition. Further, we identified autophagy as a crucial cellular survival pathway for the transformed T cells. Silencing key autophagy molecules including Beclin1, Atg5 and PI3 kinase class III (PI3KC3) resulted in drastic impairment of T cell growth. Our data, therefore, unveiled a previously unidentified role of Foxp3 in T cell transformation, providing a molecular basis for HTLV-1 transformation of CD4+Foxp3+ T cells.

Interferon-γ Promotes Inflammation and Development of T-Cell Lymphoma in HTLV-1 bZIP Factor Transgenic Mice

Human T-cell leukemia virus type 1 (HTLV-1) is an etiological agent of several inflammatory diseases and a T-cell malignancy, adult T-cell leukemia (ATL). HTLV-1 bZIP factor (HBZ) is the only viral gene that is constitutively expressed in HTLV-1-infected cells, and it has multiple functions on T-cell signaling pathways. HBZ has important roles in HTLV-1-mediated pathogenesis, since HBZ transgenic (HBZ-Tg) mice develop systemic inflammation and T-cell lymphomas, which are similar phenotypes to HTLV-1-associated diseases. We showed previously that in HBZ-Tg mice, HBZ causes unstable Foxp3 expression, leading to an increase in regulatory T cells (Tregs) and the consequent induction of IFN-γ-producing cells, which in turn leads to the development of inflammation in the mice. In this study, we show that the severity of inflammation is correlated with the development of lymphomas in HBZ-Tg mice, suggesting that HBZ-mediated inflammation is closely linked to oncogenesis in CD4+ T cells. In addition, we found that IFN-γ-producing cells enhance HBZ-mediated inflammation, since knocking out IFN-γ significantly reduced the incidence of dermatitis as well as lymphoma. Recent studies show the critical roles of the intestinal microbiota in the development of Tregs in vivo. We found that even germ-free HBZ-Tg mice still had an increased number of Tregs and IFN-γ-producing cells, and developed dermatitis, indicating that an intrinsic activity of HBZ evokes aberrant T-cell differentiation and consequently causes inflammation. These results show that immunomodulation by HBZ is implicated in both inflammation and oncogenesis, and suggest a causal connection between HTLV-1-associated inflammation and ATL.

Peripheral T-cell and NK cell lymphoproliferative disorders: cell of origin, clinical and pathological implications

T-cell lymphoproliferative disorders are a heterogeneous group of neoplasms with distinct clinical-biological properties. The normal cellular counterpart of these processes has been postulated based on functional and immunophenotypic analyses. However, T lymphocytes have been proven to be remarkably capable of modulating their properties, adapting their function in relationship with multiple stimuli and to the microenvironment. This impressive plasticity is determined by the equilibrium among a pool of transcription factors and by DNA chromatin regulators. It is now proven that the acquisition of specific genomic defects leads to the enforcement/activation of distinct pathways, which ultimately alter the preferential activation of defined regulators, forcing the neoplastic cells to acquire features and phenotypes distant from their original fate. Thus, dissecting the landscape of the genetic defects and their functional consequences in T-cell neoplasms is critical not only to pinpoint the origin of these tumors but also to define innovative mechanisms to re-adjust an unbalanced state to which the tumor cells have become addicted and make them vulnerable to therapies and targetable by the immune system. In our review, we briefly describe the pathological and clinical aspects of the T-cell lymphoma subtypes as well as NK-cell lymphomas and then focus on the current understanding of their pathogenesis and the implications on diagnosis and treatment.

The Emerging Role of miRNAs in HTLV-1 Infection and ATLL Pathogenesis

Human T-cell leukemia virus (HTLV)-1 is a human retrovirus and the etiological agent of adult T-cell leukemia/lymphoma (ATLL), a fatal malignancy of CD4/CD25+ T lymphocytes. In recent years, cellular as well as virus-encoded microRNA (miRNA) have been shown to deregulate signaling pathways to favor virus life cycle. HTLV-1 does not encode miRNA, but several studies have demonstrated that cellular miRNA expression is affected in infected cells. Distinct mechanisms such as transcriptional, epigenetic or interference with miRNA processing machinery have been involved. This article reviews the current knowledge of the role of cellular microRNAs in virus infection, replication, immune escape and pathogenesis of HTLV-1.

Modes of Human T Cell Leukemia Virus Type 1 Transmission, Replication and Persistence

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that causes cancer (Adult T cell Leukemia, ATL) and a spectrum of inflammatory diseases (mainly HTLV-associated myelopathy—tropical spastic paraparesis, HAM/TSP). Since virions are particularly unstable, HTLV-1 transmission primarily occurs by transfer of a cell carrying an integrated provirus. After transcription, the viral genomic RNA undergoes reverse transcription and integration into the chromosomal DNA of a cell from the newly infected host. The virus then replicates by either one of two modes: (i) an infectious cycle by virus budding and infection of new targets and (ii) mitotic division of cells harboring an integrated provirus. HTLV-1 replication initiates a series of mechanisms in the host including antiviral immunity and checkpoint control of cell proliferation. HTLV-1 has elaborated strategies to counteract these defense mechanisms allowing continuous persistence in humans.

Localization, quantification and interaction with host factors of endogenous HTLV-1 HBZ protein in infected cells and ATL

BACKGROUND

Human T cell lymphotropic virus type 1 (HTLV-1) is the etiological agent of a severe form of neoplasia designated Adult T cell Leukaemia (ATL). It is widely accepted that the viral transactivator Tax-1 is the major viral product involved in the onset, but not in the maintenance, of neoplastic phenotype, as only 30-40% of ATL cells express Tax-1. It has been recently demonstrated that HBZ (HTLV-1 bZIP factor), a protein encoded by the minus strand of HTLV-1 genome, constantly expressed in infected cells and in ATL tumor cells, is also involved in the pathogenesis of leukaemia. The full role played by HBZ in oncogenesis is not clarified in detail also because of the limited availability of tools to assess quantitative expression, subcellular location and interaction of HBZ with host factors in ATL.

RESULTS

By the use of the first reported monoclonal antibody against HBZ, 4D4-F3, generated in our laboratory it has been possible to carefully assess for the first time the above parameters in HTLV-1 chronically infected cells and, most importantly, in fresh leukemic cells from patients. Endogenous HBZ is expressed in speckle-like structures localized in the nucleus. The calculated number of endogenous HBZ molecules varies between 17.461 and 39.615 molecules per cell, 20- to 50-fold less than the amount expressed in HBZ transfected cells used by most investigators to assess the expression, function and subcellular localization of the viral protein. HBZ interacts in vivo with p300 and JunD and co-localizes only partially, and depending on the amount of expressed HBZ, not only with p300 and JunD but also with CBP and CREB2.

CONCLUSIONS

The possibility to study endogenous HBZ in detail may significantly contribute to a better delineation of the role of HBZ during HTLV-1 infection and cellular transformation.

HTLV-1-associated myelopathy/tropical spastic paraparesis

Human T-lymphotropic virus 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a progressive disease of the CNS that causes weakness or paralysis of the legs, lower back pain and urinary symptoms. HAM/TSP was first described in Jamaica in the nineteenth century, but the aetiology of the condition, infection with the retrovirus HTLV-1, was only identified in the 1980s. HAM/TSP causes chronic disability and, accordingly, imposes a substantial health burden in areas where HTLV-1 infection is endemic. Since the discovery of the cause of HAM/TSP, considerable advances have been made in the understanding of the virology, immunology, cell biology and pathology of HTLV-1 infection and its associated diseases. However, progress has been limited by the lack of accurate animal models of the disease. Moreover, the treatment of HAM/TSP remains highly unsatisfactory: antiretroviral drugs have little impact on the infection and, although potential disease-modifying therapies are widely used, their value is unproved. At present, clinical management is focused on symptomatic treatment and counselling. Here, we summarize current knowledge on the epidemiology, pathogenesis and treatment of HAM/TSP and identify areas in which further research is needed. For an illustrated summary of this Primer, visit: http://go.nature.com/tjZCFM.

Characteristics of Adult T-Cell Leukemia/Lymphoma Patients with Long Survival: Prognostic Significance of Skin Lesions and Possible Beneficial Role of Valproic Acid

We describe the clinical and biological features of ten patients with a survival superior to ten years (long survival), out of 175 patients diagnosed with Adult T-cell Leukemia/Lymphoma (ATL) in Martinique (1983–2013). There were 5 lymphoma and 5 chronic subtypes. Five of them (3 chronic, 2 lymphoma) had been treated with valproic acid (VA) for neurological disorders developed before or after ATL diagnosis, suggesting a beneficial role for VA as a histone deacetylase inhibitor (HDI) in ATL treatment. Total duration of uninterrupted VA treatment ranged from 8 to 37 years. Overall, the 175 incident ATL cases presented with a median survival of 5.43 months. The five VA-treated (VA⁺) patients presented with longer survival compared to VA treatment-free patients (VA⁻). For chronic subtypes, survival periods were of 213 months for 3 VA⁺ patients and of 33 months for 11 VA⁻ patients (p = 0.023). For lymphoma subtypes, survival periods were of 144 months for 2 VA⁺ patients versus 6 months for 49 VA⁻ patients (p = 0.0046). ATL cases with skin lesions, particularly lymphoma subtypes, had a longer survival (13.96 months) compared to those without skin lesions (6.06 months, p = 0.002). Eight out of the 10 patients presenting with long survival had skin lesions.

Tumor Suppressor Inactivation in the Pathogenesis of Adult T-Cell Leukemia

Tumor suppressor functions are essential to control cellular proliferation, to activate the apoptosis or senescence pathway to eliminate unwanted cells, to link DNA damage signals to cell cycle arrest checkpoints, to activate appropriate DNA repair pathways, and to prevent the loss of adhesion to inhibit initiation of metastases. Therefore, tumor suppressor genes are indispensable to maintaining genetic and genomic integrity. Consequently, inactivation of tumor suppressors by somatic mutations or epigenetic mechanisms is frequently associated with tumor initiation and development. In contrast, reactivation of tumor suppressor functions can effectively reverse the transformed phenotype and lead to cell cycle arrest or death of cancerous cells and be used as a therapeutic strategy. Adult T-cell leukemia/lymphoma (ATLL) is an aggressive lymphoproliferative disease associated with infection of CD4 T cells by the Human T-cell Leukemia Virus Type 1 (HTLV-I). HTLV-I-associated T-cell transformation is the result of a multistep oncogenic process in which the virus initially induces chronic T-cell proliferation and alters cellular pathways resulting in the accumulation of genetic defects and the deregulated growth of virally infected cells. This review will focus on the current knowledge of the genetic and epigenetic mechanisms regulating the inactivation of tumor suppressors in the pathogenesis of HTLV-I.

Protective effect of cytotoxic T lymphocytes targeting HTLV-1 bZIP factor

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia-lymphoma (ATL) and inflammatory diseases in a small percentage of infected individuals. Host immune responses, in particular cytotoxic T lymphocytes (CTLs), influence the proliferation and survival of ATL cells and HTLV-1-infected cells. We generated recombinant vaccinia viruses (rVVs) expressing HTLV-1 basic leucine zipper (bZIP) factor (HBZ) or Tax to study the immunogenic potential of these viral proteins. Vaccination with rVV expressing Tax or HBZ induced specific T-cell responses, although multiple boosters were needed for HBZ. HBZ-stimulated T cells killed HBZ peptide-pulsed T cells and CD4(+) T cells from HBZ transgenic (HBZ-Tg) mice. The anti-lymphoma effect of the CTLs targeting HBZ was tested in mice inoculated with a lymphoma cell line derived from an HBZ-Tg mouse. Transfer of splenocytes from HBZ-immunized mice increased the survival of the lymphoma cell-inoculated mice, suggesting that the anti-HBZ CTLs have a protective effect. The rVV could also induce specific T-cell responses to HBZ and Tax in HTLV-1-infected rhesus monkeys. On the basis of the results of rVV-vaccinated mice and macaques, we identified a candidate peptide (HBZ157-176) for vaccine development. Dendritic cells pulsed with this peptide could generate HBZ-specific CTLs from human CD8(+) T cells. This study demonstrates that HBZ could be a target for immunotherapy of patients with ATL.

Multiple Pathways Control the Reactivation of Telomerase in HTLV-I-Associated Leukemia

While telomerase (hTERT) activity is absent from normal somatic cells, reactivation of hTERT expression is a hallmark of cancer cells. Telomerase activity is required for avoiding replicative senescence and supports immortalization of cellular proliferation. Only a minority of cancer cells rely on a telomerase-independent process known as alternative lengthening of telomeres, ALT, to sustain cancer cell proliferation. Multiple genetic, epigenetic, and viral mechanisms have been found to de-regulate telomerase gene expression, thereby increasing the risk of cellular transformation. Here, we review the different strategies used by the Human T-cell leukemia virus type 1, HTLV-I, to activate hTERT expression and stimulate its enzymatic activity in virally infected CD4 T cells. The implications of hTERT reactivation in HTLV-I pathogenesis and disease treatment are discussed.

High IFN-γ/IL-10 expression ratio and increased frequency of persistent human T-cell lymphotropic virus type 1-infected clones are associated with human T-cell lymphotropic virus type 1-associated myelopathy/tropical spastic paraparesis development

BACKGROUND/AIMS

Human T-cell lymphotropic virus type 1 (HTLV-1) is a retrovirus that causes a persistent infection, and only 0.5-5% of infected individuals will develop HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Therefore, we investigated parameters to discriminate HTLV-1 asymptomatic carriers (ACs) with an increased chance to develop HAM/TSP.

METHODS

We evaluated integration patterns of HTLV-1 provirus, the relative expression of HTLV-1 tax and HBZ mRNAs and of IFN-γ and IL-10 mRNAs, in addition to proviral load (PVL) levels.

RESULTS

HAM/TSP patients presented a higher number of large persistent HTLV-1-carrying clones compared to ACs, and the expression of the HTLV-1 tax and HBZ genes by infected cells was detected at low levels and correlated positively with PVL. In addition, HAM/TSP patients and ACs with high PVL expressed higher levels of IFN-γ mRNA in comparison to IL-10, while ACs with low PVL presented an equilibrate IFN-γ/IL-10 ratio.

CONCLUSIONS

The presence of large persistent HTLV-1-infected clones in association with viral gene expression, even at small levels, could stimulate the intense inflammatory response in HTLV-1-infected individuals. This was supported by a high ratio of IFN-γ/IL-10 relative expression in HAM/TSP patients and ACs with high PVL, indicating that these parameters could aid the identification of ACs with a high risk to develop HAM/TSP.

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.

TCF1 and LEF1 act as T-cell intrinsic HTLV-1 antagonists by targeting Tax

Human T-cell leukemia virus type 1 (HTLV-1) is a delta-type retrovirus that induces malignant and inflammatory diseases during its long persistence in vivo. HTLV-1 can infect various kinds of cells; however, HTLV-1 provirus is predominantly found in peripheral CD4 T cells in vivo. Here we find that TCF1 and LEF1, two Wnt transcription factors that are specifically expressed in T cells, inhibit viral replication through antagonizing Tax functions. TCF1 and LEF1 can each interact with Tax and inhibit Tax-dependent viral expression and activation of NF-κB and AP-1. As a result, HTLV-1 replication is suppressed in the presence of either TCF1 or LEF1. On the other hand, T-cell activation suppresses the expression of both TCF1 and LEF1, and this suppression enables Tax to function as an activator. We analyzed the thymus of a simian T-cell leukemia virus type 1 (STLV-1) infected Japanese macaque, and found a negative correlation between proviral load and TCF1/LEF1 expression in various T-cell subsets, supporting the idea that TCF1 and LEF1 negatively regulate HTLV-1 replication and the proliferation of infected cells. Thus, this study identified TCF1 and LEF1 as Tax antagonistic factors in vivo, a fact which may critically influence the peripheral T-cell tropism of this virus.

Acetylation of the c-MYC oncoprotein is required for cooperation with the HTLV-1 p30(II) accessory protein and the induction of oncogenic cellular transformation by p30(II)/c-MYC

The human T-cell leukemia retrovirus type-1 (HTLV-1) p30(II) protein is a multifunctional latency-maintenance factor that negatively regulates viral gene expression and deregulates host signaling pathways involved in aberrant T-cell growth and proliferation. We have previously demonstrated that p30(II) interacts with the c-MYC oncoprotein and enhances c-MYC-dependent transcriptional and oncogenic functions. However, the molecular and biochemical events that mediate the cooperation between p30(II) and c-MYC remain to be completely understood. Herein we demonstrate that p30(II) induces lysine-acetylation of the c-MYC oncoprotein. Acetylation-defective c-MYC Lys→Arg substitution mutants are impaired for oncogenic transformation with p30(II) in c-myc(-/-) HO15.19 fibroblasts. Using dual-chromatin-immunoprecipitations (dual-ChIPs), we further demonstrate that p30(II) is present in c-MYC-containing nucleoprotein complexes in HTLV-1-transformed HuT-102 T-lymphocytes. Moreover, p30(II) inhibits apoptosis in proliferating cells expressing c-MYC under conditions of genotoxic stress. These findings suggest that c-MYC-acetylation is required for the cooperation between p30(II)/c-MYC which could promote proviral replication and contribute to HTLV-1-induced carcinogenesis.

Gordon Wilson Lecture: Infectious Disease Causes of Cancer: Opportunities for Prevention and Treatment

The role of infectious agents in cancer is generally underappreciated. However, approximately 20% of human cancers are caused by infectious agents and as such they rank second only to tobacco as a potentially preventable cause in humans. Specific viruses, parasites, and bacteria have been linked to specific human cancers. The infectious etiology for these specific cancers provides opportunities for prevention and treatment.

Pathology of T-cell lymphomas: diagnosis and biomarker discovery

T-cell lymphomas are a group of predominantly rare hematologic malignancies that tend to recapitulate different stages of T-cell development, in a similar way that B-cell lymphomas do. As opposed to B-cell lymphomas, the understanding of the biology and the classification of T-cell lymphomas are somewhat rudimentary, and numerous entities are still included as 'provisional categories' in the World Health Classification of hematolopoietic malignancies. A relevant and useful classification of these disorders have been difficult to accomplish because of the rarity nature of them, the relative lack of understanding of the molecular pathogenesis, and their morphological and immunophenotypical complexity. Overall, T-cell lymphomas represent only 15 % of all non-Hodgkin lymphomas. This review is focused on addressing the current status of the categories of mature T-cell leukemias and lymphomas (nodal and extranodal) using an approach that incorporates histopathology, immunophenotype, and molecular understanding of the nature of these disorders, using the same philosophy of the most recent revised WHO classification of hematopoietic malignancies.

HTLV-1-infected CD4+ T-cells display alternative exon usages that culminate in adult T-cell leukemia

Background

Reprogramming cellular gene transcription sustains HTLV-1 viral persistence that ultimately leads to the development of adult T-cell leukemia/lymphoma (ATLL). We hypothesized that besides these quantitative transcriptional effects, HTLV-1 qualitatively modifies the pattern of cellular gene expression.

Results

Exon expression analysis shows that patients’ untransformed and malignant HTLV-1⁺ CD4⁺ T-cells exhibit multiple alternate exon usage (AEU) events. These affect either transcriptionally modified or unmodified genes, culminate in ATLL, and unveil new functional pathways involved in cancer and cell cycle. Unsupervised hierarchical clustering of array data permitted to isolate exon expression patterns of 3977 exons that discriminate uninfected, infected, and transformed CD4⁺ T-cells. Furthermore, untransformed infected CD4+ clones and ATLL samples shared 486 exon modifications distributed in 320 genes, thereby indicating a role of AEUs in HTLV-1 leukemogenesis. Exposing cells to splicing modulators revealed that Sudemycin E reduces cell viability of HTLV-1 transformed cells without affecting primary control CD4+ cells and HTLV-1 negative cell lines, suggesting that the huge excess of AEU might provide news targets for treating ATLL.

Conclusions

Taken together, these data reveal that HTLV-1 significantly modifies the structure of cellular transcripts and unmask new putative leukemogenic pathways and possible therapeutic targets.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0119-3) contains supplementary material, which is available to authorized users.

Cytotoxic T lymphocyte lysis of HTLV-1 infected cells is limited by weak HBZ protein expression, but non-specifically enhanced on induction of Tax expression

BACKGROUND

Immunogenetic evidence indicates that cytotoxic T lymphocytes (CTLs) specific for the weak CTL antigen HBZ limit HTLV-1 proviral load in vivo, whereas there is no clear relationship between the proviral load and the frequency of CTLs specific for the immunodominant antigen Tax. In vivo, circulating HTLV-1-infected cells express HBZ mRNA in contrast, Tax expression is typically low or undetectable. To elucidate the virus-suppressing potential of CTLs targeting HBZ, we compared the ability of HBZ- and Tax-specific CTLs to lyse naturally-infected cells, by co-incubating HBZ- and Tax-specific CTL clones with primary CD4(+) T cells from HLA-matched HTLV-1-infected donors. We quantified lysis of infected cells, and tested whether specific virus-induced host cell surface molecules determine the susceptibility of infected cells to CTL-mediated lysis.

RESULTS

Primary infected cells upregulated HLA-A*02, ICAM-1, Fas and TRAIL-R1/2 in concert with Tax expression, forming efficient targets for both HTLV-1-specific CTLs and CTLs specific for an unrelated virus. We detected expression of HBZ mRNA (spliced isoform) in both Tax-expressing and non-expressing infected cells, and the HBZ26-34 epitope was processed and presented by cells transfected with an HBZ expression plasmid. However, when coincubated with primary cells, a high-avidity HBZ-specific CTL clone killed significantly fewer infected cells than were killed by a Tax-specific CTL clone. Finally, incubation with Tax- or HBZ-specific CTLs resulted in a significant decrease in the frequency of cells expressing high levels of HLA-A*02.

CONCLUSIONS

HTLV-1 gene expression in primary CD4(+) T cells non-specifically increases susceptibility to CTL lysis. Despite the presence of HBZ spliced-isoform mRNA, HBZ epitope presentation by primary cells is significantly less efficient than that of Tax.

Human viral oncogenesis: a cancer hallmarks analysis

Approximately 12% of all human cancers are caused by oncoviruses. Human viral oncogenesis is complex, and only a small percentage of the infected individuals develop cancer, often many years to decades after the initial infection. This reflects the multistep nature of viral oncogenesis, host genetic variability, and the fact that viruses contribute to only a portion of the oncogenic events. In this review, the Hallmarks of Cancer framework of Hanahan and Weinberg (2000 and 2011) is used to dissect the viral, host, and environmental cofactors that contribute to the biology of multistep oncogenesis mediated by established human oncoviruses. The viruses discussed include Epstein-Barr virus (EBV), high-risk human papillomaviruses (HPVs), hepatitis B and C viruses (HBV and HCV, respectively), human T cell lymphotropic virus-1 (HTLV-1), and Kaposi's sarcoma herpesvirus (KSHV).

Reevaluation of confirmatory tests for human T-cell leukemia virus Type 1 using a luciferase immunoprecipitation system in blood donors

BACKGROUND

Recently, Japanese Red Cross blood centers have changed the confirmatory test method from an indirect immunofluorescence (IF) technique to Western blotting (WB) for antibodies against human T-cell leukemia virus Type 1 (HTLV-1). In this study, these HTLV-1 tests were assessed using another sensitive method, that is, a luciferase immunoprecipitation system (LIPS), to identify a better confirmatory test for HTLV-1 infection.

STUDY DESIGN AND METHODS

Plasma samples from 54 qualified donors and 114 HTLV-1 screening-positive donors were tested by LIPS for antibodies against HTLV-1 Gag, Tax, Env, and HBZ recombinant proteins. The donors were categorized into six groups, namely, (Group I) qualified donors, screening positive; (Group II) IF positive; (Group III) IF negative; (Group IV) WB positive; (Group V) WB negative; and (Group VI) screening positive in the previous blood donation, but WB-indeterminate during this study period.

RESULTS

In Groups II and IV, all plasma samples tested positive by LIPS for antibodies against Gag and Env proteins. In Group V, all samples tested negative by LIPS, whereas some Group III samples reacted with single or double antigens in LIPS. In Group VI, the LIPS test identified a donor with suspected HTLV-1 infection. The first case of a blood donor with plasma that reacted with HBZ was identified by LIPS.

CONCLUSION

Reevaluation of the current HTLV-1 screening method using the LIPS test showed that both confirmatory tests had similar sensitivity and specificity only when WB indeterminate results were eliminated. LIPS is a promising method for detecting and characterizing HTLV-1 antibodies.

Integration site and clonal expansion in human chronic retroviral infection and gene therapy

Retroviral vectors have been successfully used therapeutically to restore expression of genes in a range of single-gene diseases, including several primary immunodeficiency disorders. Although clinical trials have shown remarkable results, there have also been a number of severe adverse events involving malignant outgrowth of a transformed clonal population. This clonal expansion is influenced by the integration site profile of the viral integrase, the transgene expressed, and the effect of the viral promoters on the neighbouring host genome. Infection with the pathogenic human retrovirus HTLV-1 also causes clonal expansion of cells containing an integrated HTLV-1 provirus. Although the majority of HTLV-1-infected people remain asymptomatic, up to 5% develop an aggressive T cell malignancy. In this review we discuss recent findings on the role of the genomic integration site in determining the clonality and the potential for malignant transformation of cells carrying integrated HTLV-1 or gene therapy vectors, and how these results have contributed to the understanding of HTLV-1 pathogenesis and to improvements in gene therapy vector safety.

Regulation of HTLV-1 tax stability, cellular trafficking and NF-κB activation by the ubiquitin-proteasome pathway

Human T-cell leukemia virus type 1 (HTLV-1) is a complex retrovirus that infects CD4+ T cells and causes adult T-cell leukemia/lymphoma (ATLL) in 3%–5% of infected individuals after a long latent period. HTLV-1 Tax is a trans-activating protein that regulates viral gene expression and also modulates cellular signaling pathways to enhance T-cell proliferation and cell survival. The Tax oncoprotein promotes T-cell transformation, in part via constitutive activation of the NF-κB transcription factor; however, the underlying mechanisms remain unknown. Ubiquitination is a type of post-translational modification that occurs in a three-step enzymatic cascade mediated by E1, E2 and E3 enzymes and regulates protein stability as well as signal transduction, protein trafficking and the DNA damage response. Emerging studies indicate that Tax hijacks the ubiquitin machinery to activate ubiquitin-dependent kinases and downstream NF-κB signaling. Tax interacts with the E2 conjugating enzyme Ubc13 and is conjugated on C-terminal lysine residues with lysine 63-linked polyubiquitin chains. Tax K63-linked polyubiquitination may serve as a platform for signaling complexes since this modification is critical for interactions with NEMO and IKK. In addition to NF-κB signaling, mono- and polyubiquitination of Tax also regulate its subcellular trafficking and stability. Here, we review recent advances in the diverse roles of ubiquitin in Tax function and how Tax usurps the ubiquitin-proteasome pathway to promote oncogenesis.

Human viruses and cancer

The first human tumor virus was discovered in the middle of the last century by Anthony Epstein, Bert Achong and Yvonne Barr in African pediatric patients with Burkitt's lymphoma. To date, seven viruses -EBV, KSHV, high-risk HPV, MCPV, HBV, HCV and HTLV1- have been consistently linked to different types of human cancer, and infections are estimated to account for up to 20% of all cancer cases worldwide. Viral oncogenic mechanisms generally include: generation of genomic instability, increase in the rate of cell proliferation, resistance to apoptosis, alterations in DNA repair mechanisms and cell polarity changes, which often coexist with evasion mechanisms of the antiviral immune response. Viral agents also indirectly contribute to the development of cancer mainly through immunosuppression or chronic inflammation, but also through chronic antigenic stimulation. There is also evidence that viruses can modulate the malignant properties of an established tumor. In the present work, causation criteria for viruses and cancer will be described, as well as the viral agents that comply with these criteria in human tumors, their epidemiological and biological characteristics, the molecular mechanisms by which they induce cellular transformation and their associated cancers.