Molecular analysis of HTLV-2 APH-2 in viral transformation, persistence and host immune response

Human T-cell leukemia virus type 1 (HTLV-1) and type 2 (HTLV-2) are two closely related human retroviruses but they display distinct differences in pathogenicity. HTLV-1 causes adult T-cell leukemia (ATL) and HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), whereas HTLV-2 appears much less pathogenic without conclusive disease association. Chapter 1 reviews important aspects of HTLV-1 pathobiology and highlights insightful comparative studies between HTLV-1 and HTLV-2.;Chapter 2 and chapter 3 focus on APH-2, the antisense protein of HTLV-2, which shares functional homology to HTLV-1 HBZ as a negative viral regulator. In chapter 2, we investigated the contribution of APH-2 to HTLV-2-mediated immortalization of primary T-lymphocytes in vitro and HTLV-1 infection in a rabbit animal model. HTLV-2 APH-2 mutant viruses were generated and evaluated for viral gene expression, protein production, and immortalization capacity. In short-term proliferation and long-term immortalization assays, APH-2 mutant viruses were indistinguishable from wild-type HTLV-1 suggesting that APH-2 is dispensable for viral replication and cellular immortalization in culture. Rabbits inoculated with irradiated cells expressing HTLV-2 APH-2 mutant viruses became persistently infected. In addition, these rabbits displayed an increased antibody response to viral gene products and a higher proviral load in PBMCs as compared to wild type HTLV-2 infected animals. These observations indicate that APH-2 is not required for viral survival and persistence in vivo during the early stage of infection, which is contrary to what has been observed for HTLV-1 HBZ. To broaden our knowledge of the contribution of antisense proteins to HTLV biology, in chapter 3 we further examined the role of APH-2 and HBZ in regulating the host immune response. We found that both APH-2 and HBZ can potentially reduce type I interferon (IFN) production by inhibiting IFN regulatory factor 7 (IRF-7)-mediated gene transcription. This result indicates that HTLV-1 and HTLV-2 have evolved a common way to antagonize host immune attack upon viral infection.;Chapter 4 focuses on the cellular tropism of HTLV-1 and HTLV-2 during the early stage after infection. In vivo, CD4+ T cells are the primary target cells for HTLV-1 in ATL patients even though CD8 + T cells serve as a natural reservoir in HAM/TSP patients and asymptomatic carriers. The HTLV-2 proviral burden has been shown to be higher in CD8 + T cells than in CD4+ T cells in infected individuals. Since most individuals are chronically infected at the time of detection, the early T cell preference of HTLV-1 and HTLV-2 in an immunocompetent host is not known. In chapter 4, we utilized the rabbit animal model to measure the early HTLV-1 and HTLV-2 proviral loads and gene expression patterns in purified CD4+ and CD8+ T cells over time. Our data indicate that the viruses do not exhibit cellular preference during the initial infection stage and the preferential transformation tropism is probably due to a selective clonal expansion during the clinical latency period. Collectively, the data presented in this thesis provides insights into the regulation of HTLV gene expression and the mechanism of cellular transformation and host-virus interplay.