Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) persists despite a vigorous virus-specific host immune response, and causes adult T-cell leukemia and lymphoma in approximately 2% of infected individuals. Here we report that HTLV-1 has evolved a genetic function to restrict its own replication by a novel post-transcriptional mechanism. The HTLV-1-encoded p30(II) is a nuclear-resident protein that binds to, and retains in the nucleus, the doubly spliced mRNA encoding the Tax and Rex proteins. Because Tex and Rex are positive regulators of viral gene expression, their inhibition by p30(II) reduces virion production. p30(II) inhibits virus expression by reducing Tax and Rex protein expression.

Human T-cell leukemia virus type 1 (HTLV-1) establishes a persistent infection in the host despite a vigorous virus-specific immune response. Here we demonstrate that an HTLV-1-encoded protein, p12 I , resides in the endoplasmic reticulum (ER) and Golgi and physically binds to the free human major histocompatibility complex class I heavy chains (MHC-I-Hc) encoded by the HLA-A2, -B7, and -Cw4 alleles. As a result of this interaction, the newly synthesized MHC-I-Hc fails to associate with ␤ 2 -microglobulin and is retrotranslocated to the cytosol, where it is degraded by the proteasome complex. Targeting of the free MHC-I-Hc, and not the MHC-I-Hc-␤ 2 -microglobulin complex, by p12 I represents a novel mechanism of viral interference and disrupts the intracellular trafficking of MHC-I, which results in a significant decrease in surface levels of MHC-I on human T-cells. These findings suggest that the interaction of p12 I with MHC-1-Hc may interfere with antigen presentation in vivo and facilitate escape of HTLV-1-infected cells from immune recognition.

The p12 I protein, encoded by the pX open reading frame I of the human T-lymphotropic virus type 1 (HTLV-1), is a hydrophobic protein that localizes to the endoplasmic reticulum and the Golgi. Although p12 I contains 4 minimal proline-rich, src homology 3-binding motifs (PXXP), a characteristic commonly found in proteins involved in signaling pathways, it has not been known whether p12 I has a role in modulating intracellular signaling pathways. This study demonstrated that p12 I binds to the cytoplasmic domain of the interleukin-2 receptor (IL-2R) ␤ chain that is involved in the recruitment of the Jak1 and Jak3 kinases. As a result of this interaction, p12 I IntroductionHuman T-lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL), 1 and its genome carries genetic information for the structural and enzymatic proteins, the regulatory proteins Tax and Rex, and other open reading frames (orfs) encoding small proteins with largely unknown functions. 2-5 HTLV-1 infects and immortalizes primary human T cells in vitro, and after several months, these cells acquire the ability to grow in the absence of interleukin-2 (IL-2). 1 The switch to IL-2 independence correlates in most cases with acquisition of a constitutive activation of the Jak/signal transducers and activators of transcription (STAT) pathway 6-8 and decreased expression of the src homology 2-containing tyrosine phosphatase 1 protein, 9 which regulates signaling from several hematopoietic surface receptors. 10 HTLV-1 also confers longevity on T cells in vivo, since expansion of T cells with identical integrations for HTLV-1 can be found at several-year intervals in the same infected individuals. 11 These findings raise the question of how CD4 ϩ T cells carrying the HTLV-1 provirus can expand and survive for a long time in vivo.The HTLV-1 p12 I protein, a hydrophobic protein resident in the endoplasmic reticulum (ER) and Golgi 58 that is encoded by the 3Ј end orf I of the viral genome, 12 forms dimers, 13 has weak oncogenic properties, and binds to the p16 subunit of the vacuolar hydrogen adenosine diphosphatase (H ϩ ATPase). 14 Expression of p12 I in infected cells is suggested by the presence of transcripts in cultured 3,5,15 and ex vivo cells from individuals infected with HTLV-1. 5 The orf I is likely expressed in vivo because antibodies (Abs) and cytotoxic T lymphocytes to peptides from the orf I protein have been detected. 16,17 Importantly, ablation of the splice acceptor site for the singly spliced p12 I messenger RNA from a molecular clone of HTLV-1 impaired viral infectivity in a rabbit model in vivo. 18 This may be partly related to the finding that p12 I interferes with major histocompatibility complex class I (MHC I) heavy-chain trafficking and may facilitate escape of HTLV-1-infected cells from the host's immune surveillance. 58 We previously reported that p12 I also binds the IL-2 receptor (IL-2R) ␤ and ␥ c chains and affects their expression on the cell surface. 19 IL-2 is an essential cytokine for the growth and s...

Three protein isoforms are encoded by the human T-cell leukemia/lymphotropic virus type I pX region open reading frames (ORF) I and II through alternative splicing. Both the singly and doubly spliced mRNAs from ORF I encode a single 12-kDa protein (p121), whereas two distinct proteins of 13 kDa (p13"I) and 30 kDa (p30"1) are encoded from the ORF II alternatively spliced mRNA. Because the p12' protein is very hydrophobic and poorly immunogenic, we genetically engineered its cDNA by adding a short stretch of amino acids from the highly immunogenic epitope HAl1 of influenza virus or the AUl epitope of bovine papillomavirus. The HAl epitope was also added to the p13", and p30"1 proteins, albeit rabbit immune sera raised against synthetic peptides were also available. To determine in which cellular compartments these proteins reside, we transfected the tagged and wild-type cDNAs in HeLa/Tat cells and studied their localization by indirect immunofluorescence. The p12' protein was identified in the cellular endomembranes and, particularly, in the perinuclear area. p13" and p30"1 were found in the nuclei and nucleoli of the transfected cells, respectively. The presence of the HAl epitope at the carboxy terminus of p13I" and p30"1 did not interfere with their cellular localization,

Homologous env sequences from 17 human T-leukemia/lymphotropic virus type I (HTLV-I) strains from throughout the world and from 25 simian T-leukemia/lymphotropic virus type I (STLV-I) strains from 12 simian species in Asia and Africa were analyzed in a phylogenetic context as an approach to resolving the natural history of these related retroviruses. STLV-I exhibited greater overall sequence variation between strains (1 to 18% compared with 0 to 9% for HTLV-I), supporting the simian origin of the modern viruses in all species. Three HTLV-I phylogenetic clusters or clades (cosmopolitan, Zaire, and Melanesia) were resolved with phenetic, parsimony, and likelihood analytical procedures. Seven phylogenetic clusters of STLV-I were resolved with the most primitive (deeply rooted) divergence involving several STLV-I strains from Asian primate species. Combined analysis of HTLV-I and STLV-I revealed that neither STLV-I clusters nor HTLV-I clusters recapitulated host species specificity; rather, multiple clades from the same species were closer to clades from other species than to each other. We interpret these evolutionary associations as support for the occurrence of multiple discrete interspecies transmissions of ancestral viruses between primate species (including human) that led to recognizable phylogenetic clades that persist in modern species. Geographic concordance of divergent host species that harbor closely related viruses reinforces that physical feasibility for hypothesized interspecies virus transmission in the past and in the present.

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Collective evidence from in vitro studies indicates that several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function, such as antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation may therefore be of importance, as also suggested by epidemiological data. Thus genetic and environmental factors together with the virus contribute to disease development. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells. The relevance of these laboratory findings is related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Human T-cell lymphotropic/leukemia virus type 1 (HTLV-1) transforms human T cells in vitro, and Tax, a potent transactivator of viral and cellular genes, plays a key role in cell immortalization. Tax activity is mediated by interaction with cellular transcription factors including members of the CREB/ATF family, the NF-κB/c-Rel family, serum response factor, and the coactivators CREB binding protein-p300. Although p53 is usually not mutated in HTLV-1-infected T cells, its half-life is increased and its function is impaired. Here we report that transient coexpression of p53 and Tax results in the suppression of p53 transcriptional activity. Expression of Tax abrogates p53-induced G1 arrest in the Calu-6 cell line and prevents the apoptosis induced by overexpressing p53 in the HeLa/Tat cell line. The Tax mutants M22 and G148V, which selectively activate the CREB/ATF pathway, exert these same biological effects on p53 function. In contrast, the NF-κB-active Tax mutant M47 has no effect on p53 activity in any of these systems. Consistent with the negative effect of Tax on p53, no activity on a p53-responsive promoter was observed upon transfection of HTLV-1-infected T-cell lines. The p53 protein is expressed at high levels in the nucleus, and nuclear extracts of HTLV-1-infected T cells bind constitutively to a DNA oligonucleotide containing the p53 response element, indicating that Tax does not interfere with p53 binding to DNA. Tax is able to suppress the transactivation function of p53 in three different cell lines, and this suppression required Tax-mediated activation of the CREB/ATF, but not the NF-κB/c-Rel, pathway. Tax and the active Tax mutants were able to abrogate the G1 arrest and apoptosis induced by p53, and this effect does not correlate with an altered localization of nuclear p53 or with the disruption of p53-DNA complexes. The suppression of p53 activity by Tax could be important in T-cell immortalization induced by HTLV-1.

The p12 ~ protein, a small hydrophobic protein encoded by the human T cell leukaemia/lymphotropic virus type I pX region, contains a proline-rich region located between two putative transmembrane (TM) domains. The p12 ~ protein is associated with cellular endomembranes, and physically binds to the 16 kDa subunit of the vacuolar H+-ATPase proton pump. To investigate the nature of the 16kDa and p12 ~ interaction and to determine the oncogenic domain of p 12 I, we constructed p121 mutant proteins in which various portions of the TM domains were deleted, as well as a p121 mutant containing a single amino acid substitution. These mutants were tested for binding to the 16 kDa subunit of the vacuolar H+-ATPase in HeLa/Tat cells and for the capability to potentiate transformation by bovine papillomavirus type 1 E5 oncoprotein in mouse C127 cells. The results indicated that both TM domains of the p12 ~ protein were dispensable for its interaction with the 16 kDa protein, whereas partial or complete deletion of the proline-rich region resulted in decreased or no binding of the p12 ~ protein to the 16 kDa subunit. Immunofluorescence analysis of HeLa/Tat cells transfected with the p121 mutants showed that deletion of the proline-rich region did not alter the subcellular localization of these mutant p12 ~ proteins, suggesting direct involvement of the proline-rich domain in binding rather than the failure of these p12' mutants to reach the appropriate cellular compartment. Mapping of 16 kDa subunit mutants in binding with the p121 protein suggested that molecular determinants located between the second and third TM domain of the 16 kDa protein might be involved in this interaction. Finally, most of the p121 mutants lost the ability to potentiate transformation of C127 cells indicating that binding of p121 to the 16 kDa subunit does not directly correlate with oncogenicity.