Comparative studies on molecular mechanisms utilized by HTLV-1 and HTLV-2 in viral replication and induction of T -cell transformation

Human T-cell leukemia virus (HTLV)-1 and HTLV-2 are closely related human retroviruses that have similar genetic organization and biological properties. However, they display distinct pathogenicity. HTLV-1 has been identified as a causal agent for two human diseases, 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. Studies in this thesis focus on the specific strategies utilized by HTLV-1 and/or HTLV-2 to replicate and induce cellular transformation.;HTLV cellular transformation and disease induction are dependent on expression of the viral Tax oncoprotein. HTLV-1 Tax (Tax-1), but not HTLV-2 Tax (Tax-2), contains a PDZ domain binding motif (PBM) that promotes Tax-1 interaction with several cellular PDZ proteins. PDZ is a modular protein interaction domain involved in cell polarity, synapse, and cellular proliferation. In Chapter 2 of this thesis, we investigate the contribution of the Tax-1 PBM in HTLV-induced proliferation and immortalization of primary T-cells in vitro and viral survival in an infectious rabbit animal model. We generated several HTLV-1 and HTLV-2 Tax viral mutants including HTLV-1DeltaPBM, HTLV-2+C22(+PBM), and HTLV-2+C18(DeltaPBM). Tax mutations did not alter the ability to activate the CREB/ATF or NFkappaB signaling pathways. Coculture microtiter proliferation assays revealed that the Tax-1 PBM significantly increases both HTLV-1 and HTLV-2-induced primary T-cell proliferation. In addition, Tax-1 PBM was responsible for the micronuclei induction activity of Tax-1 relative to that of Tax-2. Viral infection and persistence were severely attenuated in rabbits inoculated with HTLV-1DeltaPBM. Our results provide the first direct evidence that PBM-mediated associations between Tax-1 and cellular proteins play a key role in HTLV-induced cell proliferation and genetic instability in vitro and facilitate viral spread and persistence in vivo.;Our studies indicate that constitutive active Rex-2, particularly the phosphomimetic mutants, can increase viral protein expression, contributing to the enhanced primary T-cell proliferation mediated by HTLV-2 infection. However, constitutive active Rex-2 mutants without CTPD impair HTLV-2-induced cellular proliferation and transformation, implying that the CTPD is required for proper functional regulation of Rex-2 in the context of the virus. Interestingly, HTLV-2/RexP152D was impaired in inoculated rabbits as compared to wtHTLV-2 supporting the conclusion that this domain plays an important regulatory role in vivo. This is the first report of the critical role of HTLV Rex regulatory control in viral mediated cellular proliferation and immortalization, and provides insight into viral-host interaction in HTLV-2-infected cells.;Overall, the work described in this thesis employ methods that manipulate viral elements in the context of full-length proviral clones and analyze their function and mechanism of action in a system closely mimicking in vivo HTLV infection. In this way, our studies provide important insight into the molecular pathogenesis of HTLV-1 and HTLV-2. (Abstract shortened by UMI.).