Human T cell leukemia virus type I (HTLV-I) causes adult T cell leukemia (ATL) in 2–5% of carriers after a long latent period. An HTLV-I encoded protein, Tax, induces proliferation and inhibits apoptosis, resulting in clonal proliferation of infected cells. However, tax gene expression in ATL cells is disrupted by several mechanisms, including genetic changes in the tax gene and DNA methylation/deletion of the 5′ long terminal repeat (LTR). Because Tax is the major target of cytotoxic T-lymphocytes in vivo , loss of Tax expression should enable ATL cells to escape the host immune system. The 5′ LTR of HTLV-I is frequently hypermethylated or deleted in ATL cells, whereas the 3′ LTR remains unmethylated and intact, suggesting the involvement of the 3′ LTR in leukemogenesis. Here we show that a gene encoded by the minus strand of the HTLV-I proviral genome, HTLV-I basic leucine zipper factor ( HBZ ), is transcribed from 3′-LTR in all ATL cells. Suppression of HBZ gene transcription by short interfering RNA inhibits proliferation of ATL cells. In addition, HBZ gene expression promotes proliferation of a human T cell line. Analyses of T cell lines transfected with mutated HBZ genes showed that HBZ promotes T cell proliferation in its RNA form, whereas HBZ protein suppresses Tax-mediated viral transcription through the 5′ LTR. Thus, the single HBZ gene has bimodal functions in two different molecular forms. The growth-promoting activity of HBZ RNA likely plays an important role in oncogenesis by HTLV-I.

To clarify the status of tax gene, we analyzed human T‐cell leukemia virus type‐I (HTLV‐I) associated cell lines and fresh adult T‐cell leukemia (ATL) cells. We compared 2 types of HTLV‐I associated cell lines: one was derived from leukemic cells (leukemic cell line) and the other from nonleukemic cells (nonleukemic cell line). Although all nonleukemic cell lines expressed Tax, it could not be detected in 3 of 5 leukemic cell lines, in which nonsense mutation or deletion (60 bp) of tax genes, and DNA methylation in 5′‐LTR were identified as the responsible changes. We found such genetic changes of the tax gene in 5 of 47 fresh ATL cases (11%). The tax gene transcripts could be detected in 14 of 41 fresh ATL cases (34%) by RT‐PCR. In ATL cases with genetic changes that could not produce Tax protein, the tax gene was frequently transcribed, suggesting that such cells do not need the transcriptional silencing. Although DNA methylation of 5′‐LTR was detected in the fresh ATL cases (19 of 28 cases; 68%), the complete methylation associated with transcriptional silencing was observed only in 4 cases. Since partial methylation could not silence the transcription, and the tax gene transcription was not detected in 27 of 41 cases (66%), the epigenetic change(s) other than DNA methylation is considered to play an important role in the silencing.

Human T-cell leukemia virus type 1 (HTLV-1) is the causal agent of a neoplastic disease of CD4⁺ T cells, adult T-cell leukemia (ATL), and inflammatory diseases including HTLV-1 associated myelopathy/tropical spastic paraparesis, dermatitis, and inflammatory lung diseases. ATL cells, which constitutively express CD25, resemble CD25⁺CD4⁺ regulatory T cells (T_reg). Approximately 60% of ATL cases indeed harbor leukemic cells that express FoxP3, a key transcription factor for T_reg cells. HTLV-1 encodes an antisense transcript, HTLV-1 bZIP factor (HBZ), which is expressed in all ATL cases. In this study, we show that transgenic expression of HBZ in CD4⁺ T cells induced T-cell lymphomas and systemic inflammation in mice, resembling diseases observed in HTLV-1 infected individuals. In HBZ-transgenic mice, CD4⁺Foxp3⁺ T_reg cells and effector/memory CD4⁺ T cells increased in vivo. As a mechanism of increased T_reg cells, HBZ expression directly induced Foxp3 gene transcription in T cells. The increased CD4⁺Foxp3⁺ T_reg cells in HBZ transgenic mice were functionally impaired while their proliferation was enhanced. HBZ could physically interact with Foxp3 and NFAT, thereby impairing the suppressive function of T_reg cells. Thus, the expression of HBZ in CD4⁺ T cells is a key mechanism of HTLV-1-induced neoplastic and inflammatory diseases. Human T-cell leukemia virus type 1 (HTLV-1) is the first retrovirus that is associated with human diseases including an aggressive leukemia derived from CD4⁺ T cells, adult T-cell leukemia (ATL), and chronic inflammatory diseases of the central nervous system, lung, or skin. However, it remains to be elucidated how HTLV-1 induces these diseases. A viral gene, tax, has been considered as a critical player in HTLV-1 pathogenesis, yet Tax expression is frequently lost in ATL cells. Another viral gene, HBZ, is constitutively expressed in both HTLV-1 infected cells and ATL cells. However, it remains unknown how HBZ functions in the HTLV-1-related diseases. We show here that the HBZ induced T-cell lymphoma and chronic inflammation in vivo similar to those in HTLV-1 infected individuals, indicating an important role of HBZ in HTLV-1 associated human diseases. As observed in HTLV-1 infected individuals, effector/memory and regulatory CD4⁺ T cells were increased in the HBZ-transgenic mice. Further, HBZ could interact with host transcription factors, Foxp3 and NFAT, leading to dysregulation of T_reg function. The T_reg dysregulation induced by HBZ is thought to be a critical mechanism of the HTLV-1 pathogenesis. This study sheds light on the HTLV-1 associated pathogenesis and provides an important clue to prevent or treat the human diseases.

Adult T-cell leukemia (ATL) is a highly aggressive T-cell malignancy caused by human T-cell leukemia virus type 1 (HTLV-1). The activation of NF-κB by Tax has been reported to play a crucial role in HTLV-1–induced transformation. The HTLV-1 bZIP factor (HBZ), which is encoded by an mRNA of the opposite polarity of the viral genomic RNA, is involved in both T cell proliferation and suppression of Tax-mediated viral gene transcription, suggesting that HBZ cooperates closely with Tax. In the present study, we observed that HBZ specifically suppressed NF-κB–driven transcription mediated by p65 (the classical pathway) without inhibiting the alternative NF-κB signaling pathway. In an immunoprecipitation assay, HBZ bound to p65 and diminished the DNA binding capacity of p65. In addition, HBZ induced p65 degradation through increasing the expression of the PDLIM2 gene, which encodes a ubiquitin E3 ligase for p65. Finally, HBZ actually repressed the transcription of some classical NF-κB target genes, such as IL-8, IL2RA, IRF4, VCAM-1, and VEGF. Selective suppression of the classical NF-κB pathway by HBZ renders the alternative NF-κB pathway predominant after activation of NF-κB by Tax or other stimuli, which might be critical for oncogenesis.

Viruses causing chronic infection artfully manipulate infected cells to enable viral persistence in vivo under the pressure of immunity. Human T-cell leukemia virus type 1 (HTLV-1) establishes persistent infection mainly in CD4+ T cells in vivo and induces leukemia in this subset. HTLV-1–encoded Tax is a critical transactivator of viral replication and a potent oncoprotein, but its significance in pathogenesis remains obscure due to its very low level of expression in vivo. Here, we show that Tax is expressed in a minor fraction of leukemic cells at any given time, and importantly, its expression spontaneously switches between on and off states. Live cell imaging revealed that the average duration of one episode of Tax expression is ∼19 hours. Knockdown of Tax rapidly induced apoptosis in most cells, indicating that Tax is critical for maintaining the population, even if its short-term expression is limited to a small subpopulation. Single-cell analysis and computational simulation suggest that transient Tax expression triggers antiapoptotic machinery, and this effect continues even after Tax expression is diminished; this activation of the antiapoptotic machinery is the critical event for maintaining the population. In addition, Tax is induced by various cytotoxic stresses and also promotes HTLV-1 replication. Thus, it seems that Tax protects infected cells from apoptosis and increases the chance of viral transmission at a critical moment. Keeping the expression of Tax minimal but inducible on demand is, therefore, a fundamental strategy of HTLV-1 to promote persistent infection and leukemogenesis.

Background: Human T-cell leukemia virus type I (HTLV-I) causes adult T-cell leukemia (ATL) after a long latent period. Among accessory genes encoded by HTLV-I, the tax gene is thought to play a central role in oncogenesis. However, Tax expression is disrupted by several mechanims including genetic changes of the tax gene, deletion/hypermethylation of 5'-LTR. To clarify the role of epigenetic changes, we analyzed DNA methylation and histone modification in the whole HTLV-I provirus genome. Results: The gag, pol and env genes of HTLV-I provirus were more methylated than pX region, whereas methylation of 5'-LTR was variable and 3'-LTR was not methylated at all. In ATL cell lines, complete DNA methylation of 5'-LTR was associated with transcriptional silencing of viral genes. HTLV-I provirus was more methylated in primary ATL cells than in carrier state, indicating the association with disease progression. In seroconvertors, DNA methylation was already observed in internal sequences of provirus just after seroconversion. Taken together, it is speculated that DNA methylation first occurs in the gag, pol and env regions and then extends in the 5' and 3' directions in vivo, and when 5'-LTR becomes methylated, viral transcription is silenced. Analysis of histone modification in the HTLV-I provirus showed that the methylated provirus was associated with hypoacetylation. However, the tax gene transcript could not be detected in fresh ATL cells regardless of hyperacetylated histone H3 in 5'-LTR. The transcription rapidly recovered after in vitro culture in such ATL cells. Conclusion: These results showed that epigenetic changes of provirus facilitated ATL cells to evade host immune system by suppressing viral gene transcription. In addition, this study shows the presence of another reversible mechanism that suppresses the tax gene transcription without DNA methylation and hypoacetylated histone.