Envelope Is a Major Viral Determinant of the Distinct In Vitro Cellular Transformation Tropism of Human T-Cell Leukemia Virus Type 1 (HTLV-1) and HTLV-2

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are related but distinct pathogenic complex retroviruses. HTLV-1 has been identified as the etiologic agent of adult T-cell leukemia/lymphoma, a malignancy of CD4+ T lymphocytes, as well as of a chronic progressive neurological disorder termed HTLV-1-associated myelopathy/tropical spastic paraparesis (22, 26, 74, 75). In contrast, HTLV-2 has not been conclusively associated with disease; to date only a few cases of variant hairy cell leukemia (CD8+ T-cell origin) and several cases of neurological disease have been reported (28, 34, 60). It has been shown that HTLV-1 and HTLV-2 exhibit distinct in vivo T-cell tropisms. HTLV-1 has a preferential tropism for CD4+ T lymphocytes in asymptomatic patients and those with leukemia and neurological disease (57, 58, 75). However, CD8+ T cells from patients with HTLV-1-associated myelopathy/tropical spastic paraparesis were identified as an additional reservoir for HTLV-1 (49). In contrast, HTLV-2 in vivo tropism is less clear but seems to favor CD8+ T lymphocytes. Proviral sequences were detected predominantly in CD8+ Tlymphocytes from HTLV-2-infected individuals, whereas others have detected HTLV-2 in both CD4+ and CD8+ T-cellsubsets, with a greater proviral burden in CD8+ T cells (32,41). The distinct in vivo T-cell tropism of HTLV-1 and HTLV-2 has been recapitulated in vitro using transformation/immortalization assays in which irradiated 729 producer cells were cocultured with freshly isolated human peripheral blood mononuclear cells (PBMCs). Results from these studies showed that the majority of cells transformed by HTLV-1 in vitro were CD4+ T lymphocytes (55, 59, 76). In addition, Tax-mediated HTLV-1 transcription was increased significantly in purified CD4+ versus CD8+ T-cell subsets, suggesting that an enhanced rate of viral transcription may be responsible for the preferential transformation of CD4+ T cells by HTLV-1 (51). Conversely, purified CD4+ and CD8+ T cells were shown to be equally susceptible to HTLV-2 infection and subsequent viral gene expression. However, coculture of HTLV-2 producer cells with freshly isolated, nonstimulated PBMCs or purified T-cell subsets resulted in preferential transformation of CD8+ T cells (62, 71, 76). Since Tax has been shown to be critical for cellular transformation in vitro and interacts with numerous cellular processes involving cell growth and differentiation, cell cycle regulation, and DNA repair (17, 19, 35, 73, 77), it has been hypothesized that the Tax would encode the viral determinant for transformation tropism. However, recent studies using recombinant HTLVs indicated that Tax and overlapping Rex did not confer the distinct HTLV-1 and HTLV-2 transformation tropism in vitro (76). This suggests that other viral genes or sequences are responsible for the differential ability to transform CD4+ or CD8+ T cells. Here we generated and evaluated recombinant viruses in which the long terminal repeat (LTR) and env sequences were exchanged between HTLV-1 and HTLV-2. Our results indicated that the exchange of LTR sequences did not alter HTLV transformation tropisms. Interestingly, we identified Env as a major viral factor that confers the in vitro HTLV transformation tropism; wild-type HTLV-2 (wtHTLV-2) and HTLV-1 with Env-2 (HTLV-1/Env2) preferentially transformed CD8+ T cells, whereas wtHTLV-1 and HTLV-2/Env1 displayed a preferred transformation tropism for CD4+ T cells. This study provides the first biological evidence that the envelope gene may play an important role in the distinct pathogenesis resulting from HTLV-1 and HTLV-2 infections.