Splicing-dependent restriction of the HBZ gene by Tax underlies biphasic HTLV-1 infection

HTLV-1 is an oncovirus that encodes a transactivator Tax and a regulatory gene HBZ. HTLV-1 early or infectious replication depends on Tax; during HTLV-1 late infection, HBZ plays a crucial role in driving the proliferation of infected cells and maintaining viral persistence. The biphasic replication pattern of HTLV-1 dictated by Tax and HBZ represents a result of viral host adaptation, but how HTLV-1 coordinates Tax and HBZ expression to facilitate early and late infection remains elusive. Here we reveal that HBZ RNA splicing exhibits distinct patterns in Tax+ and Tax- HTLV-1 infected cells. We demonstrate that Tax interacts with the host spliceosome and inhibits HBZ splicing by competitively binding splicing factors including WDR83 and GPATCH1. As a result, Tax confers a natural constraint on HBZ, counterbalancing its anti-replication effect at HTLV-1 early infection, while unleashing HBZ to drive HTLV-1 mitotic propagation during late infection. The splicing-dependent restriction of HBZ by Tax thus represents a critical interplay central to HTLV-1 persistence.

Progress in the Phenotypic Study of Adult T-cell Leukemia

HTLV‐1 persistent infection and ATLL oncogenesis

AbstractHuman T‐cell leukemia virus type 1 (HTLV‐1) is an oncogenic retrovirus; whereas HTLV‐1 mainly persists in the infected host cell as a provirus, it also causes a malignancy called adult T‐cell leukemia/lymphoma (ATLL) in about 5% of infection. HTLV‐1 replication is in most cases silent in vivo and viral de novo infection rarely occurs; HTLV‐1 rather relies on clonal proliferation of infected T cells for viral propagation as it multiplies the number of the provirus copies. It is mechanistically elusive how leukemic clones emerge during the course of HTLV‐1 infection in vivo and eventually cause the onset of ATLL. This review summarizes our current understanding of HTLV‐1 persistence and oncogenesis, with the incorporation of recent cutting‐edge discoveries obtained by high‐throughput sequencing.

Multifaceted functions and roles of HBZ in HTLV-1 pathogenesis

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

Significance HTLV-1 is a peripheral T-cell tropic virus and induces proliferation of CD4+ T cells, resulting in T-cell malignancy and inflammatory diseases. Recent studies demonstrated that several restriction factors inhibiting HIV are also inhibitory to HTLV-1. We identified two T-cell–specific proteins, TCF1 and LEF1, as HTLV-1 restriction factors that determine the peripheral T-cell tropism of this virus by targeting Tax. They are highly expressed in immature thymocytes and thereby become a natural intrinsic barrier for HTLV-1 replication in the thymus. However, their expression can be down-regulated by Tax, as well as by activation and differentiation of T cells. These findings provide a mechanistic understanding of how HTLV-1 induces T-cell malignancies in the periphery but never in the thymus.

HTLV-1-mediated dysregulation of the Wnt pathways: roles of Tax and HBZ

STLV-1-infected Japanese macaque as a model of HTLV-1 infection

Characterization of simian T-cell leukemia virus type 1 in naturally infected Japanese macaques as a model of HTLV-1 infection

Abstract Background Human T-cell leukemia virus type 1 (HTLV-1) causes chronic infection leading to development of adult T-cell leukemia (ATL) and inflammatory diseases. Non-human primates infected with simian T-cell leukemia virus type 1 (STLV-1) are considered to constitute a suitable animal model for HTLV-1 research. However, the function of the regulatory and accessory genes of STLV-1 has not been analyzed in detail. In this study, STLV-1 in naturally infected Japanese macaques was analyzed. Results We identified spliced transcripts of STLV-1 corresponding to HTLV-1 tax and HTLV-1 bZIP factor (HBZ). STLV-1 Tax activated the NFAT, AP-1 and NF-κB signaling pathways, whereas STLV-1 bZIP factor (SBZ) suppressed them. Conversely, SBZ enhanced TGF-β signaling and induced Foxp3 expression. Furthermore, STLV-1 Tax activated the canonical Wnt pathway while SBZ suppressed it. STLV-1 Tax enhanced the viral promoter activity while SBZ suppressed its activation. Then we addressed the clonal proliferation of STLV-1+ cells by massively sequencing the provirus integration sites. Some clones proliferated distinctively in monkeys with higher STLV-1 proviral loads. Notably, one of the monkeys surveyed in this study developed T-cell lymphoma in the brain; STLV-1 provirus was integrated in the lymphoma cell genome. When anti-CCR4 antibody, mogamulizumab, was administered into STLV-1-infected monkeys, the proviral load decreased dramatically within 2 weeks. We observed that some abundant clones recovered after discontinuation of mogamulizumab administration. Conclusions STLV-1 Tax and SBZ have functions similar to those of their counterparts in HTLV-1. This study demonstrates that Japanese macaques naturally infected with STLV-1 resemble HTLV-1 carriers and are a suitable model for the investigation of persistent HTLV-1 infection and asymptomatic HTLV-1 carrier state. Using these animals, we verified that mogamulizumab, which is currently used as a drug for relapsed ATL, is also effective in reducing the proviral load in asymptomatic individuals.

Noncanonical Wnt5a Is Induced by HTLV-1 bZIP Factor, and Supports Proliferation and Migration of Adult T-Cell Leukemia Cell.

Abstract Abstract 2405 Adult T-cell leukemia (ATL) is a fatal malignancy of CD4-positive T-cells caused by human T-cell leukemia virus type1 (HTLV-1). HTLV-1 bZIP factor (HBZ) is encoded in the minus strand of HTLV-1 provirus, and is consistently expressed in HTLV-1-infected cells. It has been shown that HBZ promotes proliferation of T-cells, and HBZ-expressing transgenic mice develop T-cell lymphomas and systemic inflammations, suggesting that HBZ has the critical roles in pathogenesis of HTLV-1. However, the molecular mechanisms of HBZ-induced oncogenesis have not been clarified yet. The Wnt signaling is a highly conserved cellular signaling pathway in eukaryotes and the Wnt ligands have been found to trigger multiple pathways of Wnt signaling. The most well-studied cascade is the Wnt/β-catenin pathway (also known as the canonical Wnt pathway), which is β-catenin-dependent and mainly controls cell differentiation, proliferation, and apoptosis. Aberrant activation of the canonical Wnt pathway has been linked to many cancers. In contrast, it is known that the noncanonical Wnt pathway antagonizes the canonical pathway, while a representative noncanonical Wnt ligand, Wnt5a, is suggested to have both oncogenic and tumor suppressive properties in several malignancies. The significance of those Wnt pathways in HTLV-1-associated oncogenesis is still obscure. In this study, we found that HBZ markedly suppressed the canonical Wnt/β-catenin pathway. As a mechanism of HBZ-mediated Wnt suppression, we found that HBZ targets LEF1 and TCF1, the key transcription factors of the pathway, and impairs its DNA-binding ability. In contrast, HTLV-1 Tax protein interacted with Dvl and DAPLE, which are the regulators of the canonical Wnt signaling, and activated the pathway. Indeed, the canonical Wnt activation is not observed in ATL cells, and enforced activation of β-catenin in ATL cells inhibited cellular proliferation, implying that activation of the canonical Wnt pathway might have suppressive effect on ATL cells. On the other hand, HBZ induces the transcription of the noncanonical Wnt5a by enhancing its promoter activity. In addition, Wnt5a transcription level in ATL cells is significantly higher than in HTLV-1-uninifected control cells. Knockdown of Wnt5a impairs both proliferation and migration of ATL cells, suggesting that HBZ-induced Wnt5a supports development and progression of ATL. Our results implicate novel roles of HBZ in ATL leukemogenesis through dysregulation of both the canonical and noncanonical Wnt pathways. Disclosures: No relevant conflicts of interest to declare.