Tax and Overlapping Rex Sequences Do Not Confer the Distinct Transformation Tropisms of Human T-Cell Leukemia Virus Types 1 and 2

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are distinct complex oncogenic retroviruses. HTLV-1 has been associated with adult T cell leukemia, a malignancy of CD4+ T lymphocytes, and a chronic neurological disorder termed HTLV-1-associated myelopathy/tropical spastic paraparesis (11). HTLV-2 disease association is less clear in that only a few cases of a variant hairy cell leukemia (CD8+ T-cell origin) and several cases of neurological disease have been reported (17, 21, 38). HTLV infects a number of cell types, including T cells, B cells, endothelial cells, glial cells, and monocytes of both human and nonhuman origin (2, 18, 19, 24), but displays its transforming or pathogenic activity only in T cells. It has been shown that HTLV-1 has a preferential tropism for CD4+ T cells in both asymptomatic patients and those with neurological disease (36). More recently, studies indicated that CD8+ T cells are an additional viral reservoir in vivo for HTLV-1 in patients with HTLV-1-associated myelopathy/tropical spastic paraparesis (30). In vitro, it has been shown that Tax-mediated transcription of HTLV-1 is significantly increased in purified CD4+ versus CD8+ T-cell subsets (32). This is consistent with the hypothesis that this enhanced rate of transcription is ultimately responsible for the cell tropism and the leukemogenic potential of HTLV-1. HTLV-2 in vivo tropism is less clear. One in vivo study indicated that HTLV-2 has a preferential tropism for CD8+ T cells (20), whereas others have detected HTLV-2 in both CD4+ and CD8+ T-cell subsets, with a greater proviral burden in CD8+ T cells (26, 34). In contrast to the case for HTLV-1, we have recently shown that purified CD4+ and CD8+ T cells are equally susceptible to HTLV-2 infection and subsequent viral gene expression (46). However, coculture of irradiated HTLV-2 producer cells with peripheral blood mononuclear cells (PBMCs) or purified T-cell subsets resulted in preferential transformation of CD8+ T cells (46). Therefore, we have hypothesized that the distinct biological difference between HTLV-2 and HTLV-1 is attributable to genetic differences between the viruses and is likely responsible for the differing pathogenicities of these two related viruses. In addition to carrying structural and enzymatic genes, gag, pol, and env, HTLV encodes the Tax and Rex trans-regulatory gene products, which are essential for viral replication, and several accessory gene products that have been shown to be important for viral persistence in vivo. The tax and rex genes are carried in separate but partially overlapping reading frames. Tax increases the rate of transcription from the viral long terminal repeat (LTR), whereas Rex acts posttranscriptionally to induce the cytoplasmic expression of the unspliced and incompletely spliced viral RNAs encoding the viral structural and enzymatic proteins (4, 25). Tax also modulates the expression or activity of numerous cellular genes involved in cell growth and differentiation, cell cycle control, and DNA repair (1, 3, 35, 45, 47). Strong evidence indicates that these pleiotropic effects of Tax on cellular processes are critical in HTLV-mediated cellular transformation and oncogenesis (10, 15, 31, 37, 40). The goal of this study was to determine whether Tax is the determinant of the distinct in vitro transformation tropism of HTLV-1 and HTLV-2. We used molecular clones of HTLV-1 (Ach) and HTLV-2 (pH6neo) to construct recombinants in which tax and overlapping rex genes of the two viruses are exchanged. Our results indicate that tax/rex transactivating activities are altered in the different viral genetic backgrounds. However, both recombinants were competent to replicate in and transform primary human T lymphocytes in culture. Although the tax and rex genes are absolutely required for efficient replication and cellular transformation by HTLV, the distinct in vitro transformation tropism of HTLV-1 and HTLV-2 is not encoded by these genes.