HTLV-1 orf-I is linked to immune evasion, viral replication and persistence. Examining the orf-I sequence of 160 HTLV-1-infected individuals; we found polymorphism of orf-I that alters the relative amounts of p12 and its cleavage product p8. Three groups were identified on the basis of p12 and p8 expression: predominantly p12, predominantly p8 and balanced expression of p12 and p8. We found a significant association between balanced expression of p12 and p8 with high viral DNA loads, a correlate of disease development. To determine the individual roles of p12 and p8 in viral persistence, we constructed infectious molecular clones expressing p12 and p8 (D26), predominantly p12 (G29S) or predominantly p8 (N26). As we previously showed, cells expressing N26 had a higher level of virus transmission in vitro. However, when inoculated into Rhesus macaques, cells producing N26 virus caused only a partial seroconversion in 3 of 4 animals and only 1 of those animals was HTLV-1 DNA positive by PCR. None of the animals exposed to G29S virus seroconverted or had detectable viral DNA. In contrast, 3 of 4 animals exposed to D26 virus seroconverted and were HTLV-1 positive by PCR. In vitro studies in THP-1 cells suggested that expression of p8 was sufficient for productive infection of monocytes. Since orf-I plays a role in T-cell activation and recognition; we compared the CTL response elicited by CD4+ T-cells infected with the different HTLV-1 clones. Although supernatant p19 levels and viral DNA loads for all four infected lines were similar, a significant difference in Tax-specific HLA.A2-restricted killing was observed. Cells infected with Orf-I-knockout virus (12KO), G29S or N26 were killed by CTLs, whereas cells infected with D26 virus were resistant to CTL killing. These results indicate that efficient viral persistence and spread require the combined functions of p12 and p8.

A pproximately 2 to 3% of human T cell leukemia virus type 1 (HTLV-1)-infected individuals develop adult T-cell leukemia/lymphoma (ATL) and another 2 to 3% develop HTLV-1-associated myelopathy (HAM)/tropical spastic paraparesis (TSP) in their lifetimes (1-4). In addition to HAM/TSP (5, 6), HTLV-1 is also associated with other inflammatory conditions, such as uveitis (6) Sjögren's syndrome (7), bronchoalveolitis and arthritis (8), and polymyositis (9). It is noteworthy that some patients present with more than one of these inflammatory conditions (10). HTLV-1 primarily infects CD4 ϩ and CD8 ϩ effector and memory T cells and regulatory CD4 ϩ CD25 ϩ T cells (11,12). A high viral DNA burden in peripheral blood mononuclear cells (PBMCs) is a risk factor for HAM/TSP (13) and ATL development (14-16), and patients with HAM/TSP have a higher virus level in the cerebrospinal fluid (CSF) than in the peripheral blood (12). The virus level alone is not sufficient to differentiate symptomatic patients from healthy carriers, suggesting the importance of other factors, including the host immune response (16)(17)(18)(19)(20). HAM/TSP patients present diverse immunological alterations, such as increased levels of spontaneous lymphocyte proliferation (21, 22), tax-specific cytotoxic CD8 ϩ T cell expansion, and the production of high levels of inflammatory cytokines (23)(24)(25). Several studies have also suggested that monocytes are involved in immune reg-

The 3′ end of the human T-cell leukemia/lymphoma virus type-1 (HTLV-1) genome contains four overlapping open reading frames (ORF) that encode regulatory proteins. Here, we review current knowledge of HTLV-1 orf-I and orf-II protein products. Singly spliced mRNA from orf-I encodes p12, which can be proteolytically cleaved to generate p8, while differential splicing of mRNA from orf-II results in production of p13 and p30. These proteins have been demonstrated to modulate transcription, apoptosis, host cell activation and proliferation, virus infectivity and transmission, and host immune responses. Though these proteins are not essential for virus replication in vitro, p8, p12, p13, and p30 have an important role in the establishment and maintenance of HTLV-1 infection in vivo.