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. Copyright © 2013 John Wiley & Sons, Ltd.     

HTLV-1 infection promotes excessive T cell activation and transformation into adult T cell leukemia/lymphoma

Human T cell leukemia virus type 1 (HTLV-1) mainly infects CD4⁺ T cells and induces chronic, persistent infection in infected individuals, with some developing adult T cell leukemia/lymphoma (ATL). HTLV-1 alters cellular differentiation, activation, and survival; however, it is unknown whether and how these changes contribute to the malignant transformation of infected cells. In this study, we used single-cell RNA-sequencing and T cell receptor–sequencing to investigate the differentiation and HTLV-1–mediated transformation of T cells. We analyzed 87,742 PBMCs from 12 infected and 3 uninfected individuals. Using multiple independent bioinformatics methods, we demonstrated the seamless transition of naive T cells into activated T cells, whereby HTLV-1–infected cells in an activated state further transformed into ATL cells, which are characterized as clonally expanded, highly activated T cells. Notably, the greater the activation state of ATL cells, the more they acquire Treg signatures. Intriguingly, the expression of HLA class II genes in HTLV-1–infected cells was uniquely induced by the viral protein Tax and further upregulated in ATL cells. Functional assays revealed that HTLV-1–infected cells upregulated HLA class II molecules and acted as tolerogenic antigen-presenting cells to induce anergy of antigen-specific T cells. In conclusion, our study revealed the in vivo mechanisms of HTLV-1–mediated transformation and immune escape at the single-cell level.     

Implication of the <Emphasis Type="Italic">HTLV- I bZIP Factor</Emphasis> Gene in the Leukemogenesis of Adult T-Cell Leukemia

Adult T-cell leukemia (ATL) is a leukemia derived from CD4+ mature T-cells and induced by human T-cell leukemia virus type I (HTLV-I) infection. Although previous studies have revealed many aspects of its leukemogenesis, enigmas remain about how HTLV-I transforms mature T-cells in infected individuals. Furthermore, an effective therapy for ATL has not yet been established. The critical role of a nonstructural regulatory viral protein, Tax, in transformation has been established through many molecular studies, in vitro cell culture experiments, and transgenic mouse model systems. In addition, other accessory viral proteins have been implicated in ATL pathogenesis. Recent studies of a minus strand viral gene, HTLV-I bZIP factor (HBZ), suggest it plays a role in ATL leukemogenesis. In addition to viral components, genetic and epigenetic events of the host cellular genome must be considered in developing a complete picture of the transformation process. In this review, we summarize the molecular and cellular mechanisms involved in the leukemogenesis induced by HTLV-I; we consider both viral and host cellular factors and focus particularly on the viral gene HBZ.     

Development of T cell lymphoma in HTLV-1 bZIP factor and Tax double transgenic mice

Adult T-cell leukemia (ATL) is an aggressive T-cell malignancy caused by human T-cell leukemia virus type 1 (HTLV-1). ATL cells possess a CD4+ CD25+ phenotype, similar to that of regulatory T cells (Tregs). Tax has been reported to play a crucial role in the leukemogenesis of HTLV-1. The HTLV-1 bZIP factor (HBZ), which is encoded by the minus strand of the viral genomic RNA, is expressed in all ATL cases and induces neoplastic and inflammatory disease in vivo. To test whether HBZ and Tax are both required for T cell malignancy, we generated HBZ/Tax double transgenic mice in which HBZ and Tax are expressed exclusively in CD4+ T cells. Survival was much reduced in HBZ/Tax double-transgenic mice compared with wild type littermates. Transgenic expression of HBZ and Tax induced skin lesions and T-cell lymphoma in mice, resembling diseases observed in HTLV-1 infected individuals. However, Tax single transgenic mice did not develop major health problems. In addition, memory CD4+ T cells and Foxp3+ Treg cells counts were increased in HBZ/Tax double transgenic mice, and their proliferation was enhanced. There was very little difference between HBZ single and HBZ/Tax double transgenic mice. Taken together, these results show that HBZ, in addition to Tax, plays a critical role in T-cell lymphoma arising from HTLV-1 infection.     

A novel animal model of Epstein-Barr virus-associated hemophagocytic lymphohistiocytosis in humanized mice

EBV-associated hemophagocytic lymphohistiocytosis (EBV-HLH) is a rare yet devastating disorder caused by EBV infection in humans. However, the mechanism of this disease has yet to be elucidated because of a lack of appropriate animal models. Here, we used a human CD34(+) cell-transplanted humanized mouse model and reproduced pathologic conditions resembling EBV-HLH in humans. By 10 weeks postinfection, two-thirds of the infected mice died after exhibiting high and persistent viremia, leukocytosis, IFN-γ cytokinenemia, normocytic anemia, and thrombocytopenia. EBV-infected mice also showed systemic organ infiltration by activated CD8(+) T cells and prominent hemophagocytosis in BM, spleen, and liver. Notably, the level of EBV load in plasma correlated directly with both the activation frequency of CD8(+) T cells and the level of IFN-γ in plasma. Moreover, high levels of EBV-encoded small RNA1 were detected in plasma of infected mice, reflecting what has been observed in patients. These findings suggest that our EBV infection model mirrors virologic, hematologic, and immunopathologic aspects of EBV-HLH. Furthermore, in contrast to CD8(+) T cells, we found a significant decrease of natural killer cells, myeloid dendritic cells, and plasmacytoid dendritic cells in the spleens of infected mice, suggesting that the collapse of balanced immunity associates with the progression of EBV-HLH pathogenesis.     

Proteasome inhibitor, bortezomib (Vercade), potently inhibits the growth of adult T-cell leukemia cells

The proteasome is a multicatalytic proteinase complex responsible for the degradation of most intracellular proteins, including crucial to cell cycle regulation, cell growth, and apoptosis. Bortezomib (Vercade; formerly known as PS-341) is a novel dipeptide boronic acid that is the first proteasome inhibitor to have progressed to clinical trials. Below we discuss the mechanism of bortezomib for cancer therapy, review the clinical data, and introduce the potent effect on ATL cells.     

Detection of HTLV-1 by means of HBZ gene in situ hybridization in formalin-fixed and paraffin-embedded tissues

Adult T-cell leukemia/lymphoma (ATLL) is a T-cell malignancy associated with HTLV-1. The HTLV-1 provirus genome has the pX region that encodes tax and HTLV-1 basic leucine zipper factor (HBZ). Previous studies have reported that the tax gene is expressed in few ATLL cases, but the HBZ gene in all ATLL cases. In this study, we used HBZ gene in situ hybridization (HBZ-ISH) for detection of the HBZ gene in formalin-fixed paraffin-embedded tissues. This method showed that all cases (n = 19) were positive for the ATLL cell line (MT-1, MT-2, and MT-4) and ATLL mouse model (HBZ-Tg mice and NOD/SCID/β2-microglobulin(null) mice with ATLL transplanted), and the HBZ gene was also detected in all human ATLL cases (n = 16). The percentage of positive cells in HBZ-ISH was 5-70%. Immunohistochemical staining for Tax protein showed positivity in seven of 11 cases in NOD/SCID/β2-microglobulin(null) mice with ATLL transplanted and in six of eight human ATLL cases, but the percentage of positive cells was very low (range, 1-5%). Although HBZ-ISH is unsuitable to detect HTLV-1 clonality, this method is convenient and can be useful for the histological diagnosis of ATLL in HTLV-1 sero-indeterminate patients.