Expression of the pX gene of HTLV-I: general splicing mechanism in the HTLV family

HTLV-1 Tax Tug-of-War: Cellular Senescence and Death or Cellular Transformation

Human T cell leukemia virus type 1 (HTLV-1) is a retrovirus associated with a lymphoproliferative disease known as adult T cell leukemia/lymphoma (ATLL). HTLV-1 infection efficiently transforms human T cells in vivo and in vitro. The virus does not transduce a proto-oncogene, nor does it integrate into tumor-promoting genomic sites. Instead, HTLV-1 uses a random mutagenesis model, resulting in cellular transformation. Expression of the viral protein Tax is critical for the immortalization of infected cells by targeting specific cellular signaling pathways. However, Tax is highly immunogenic and represents the main target for the elimination of virally infected cells by host cytotoxic T cells (CTLs). In addition, Tax expression in naïve cells induces pro-apoptotic signals and has been associated with the induction of non-replicative cellular senescence. This review will explore these conundrums and discuss the mechanisms used by the Tax viral oncoprotein to influence life-and-death cellular decisions and affect HTLV-1 pathogenesis.

Human T-cell Lymphotropic Viruses: Review and Prospects for Antiviral Therapy

The human T-cell lymphotropic viruses types I and II (HTLV-I, II) pose challenges to researchers and clinicians who seek to unveil mechanisms of viral transformation and pathogenesis. HTLV-I infection in humans is associated with a wide array of primary and secondary diseases ranging from mild immunosuppression to adult T-cell leukaemia/lymphoma and HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a neurological degenerative syndrome. As retroviruses, HTLV-I and II share similar replicative cycles with human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome. However, in contrast to HIV-I which destroys CD4+ T cells, HTLV-I and II can preferentially transform a CD4+ T-cell subset to an unrestricted growth state.

HTLV-I and II, along with simian T-lymphotropic virus (STLV) and bovine leukaemia virus (BLV), form a phylogenetic group which is distinct from ungulate, non-human primate and human lentiviruses such as visna, simian immunodeficiency virus (SIV), and human immunodeficiency viruses types 1 and 2. The proviral genome of HTLV-I is flanked at the 5′ and 3′ ends by long terminal repeats (LTR) and is further subdivided into structural gag and env genes, a pro gene encoding an aspartyl protease, a pol gene which encodes reverse transcriptase and endonuclease, and the regulatory gene elements tax and rex. Regions within the LTR contain recognition sites for cellular proteins and the tax gene product that collectively promote viral expression. Tax-mediated activation of cellular genes involved in growth and differentiation is suspected to play a dominant role in the leukaemogenic process associated with HTLV-I infection. Differential rex-regulated splicing of viral message gives rise to transcripts encoding the polyprotein precursor gag-pro-pol (unspliced), envelope (single spliced), or tax/rex (doubly spliced). The 100nm HTLV virion contains an electron-dense core surrounding a divalent-single stranded DNA genome. This core is in turn enclosed by concentric shells of matrix protein and an outer lipid bilayer, the latter acquired as the virus buds from the surface of the infected cell. Envelope glycoproteins associated with the outside of this lipid bilayer can interact with viral receptors on cells and mediate virus entry. Antiviral strategies have been directed at inhibiting viral entry into cells (sulphated and non-sulphated polysaccharides, vaccines), blocking of viral replication (AZT, suramin), intracellular immunization (transdominant repression of rex), and elimination of virus infected cells (IL-2 receptor-directed toxins). Serological screening of the blood supply and curtailing breast feeding of children by HTLV-I + mothers have likely had a major impact in preventing HTLV-I infection.

Combined cytolytic effects of a vaccinia virus encoding a single chain trimer of MHC-I with a Tax-epitope and Tax-specific CTLs on HTLV-I-infected cells in a rat model

Adult T cell leukemia (ATL) is a malignant lymphoproliferative disease caused by human T cell leukemia virus type I (HTLV-I). To develop an effective therapy against the disease, we have examined the oncolytic ability of an attenuated vaccinia virus (VV), LC16m8Δ (m8Δ), and an HTLV-I Tax-specific cytotoxic T lymphocyte (CTL) line, 4O1/C8, against an HTLV-I-infected rat T cell line, FPM1. Our results demonstrated that m8Δ was able to replicate in and lyse tumorigenic FPM1 cells but was incompetent to injure 4O1/C8 cells, suggesting the preferential cytolytic activity toward tumor cells. To further enhance the cytolysis of HTLV-I-infected cells, we modified m8Δ and obtained m8Δ/RT1AlSCTax180L, which can express a single chain trimer (SCT) of rat major histocompatibility complex class I with a Tax-epitope. Combined treatment with m8Δ/RT1AlSCTax180L and 4O1/C8 increased the cytolysis of FPM1V.EFGFP/8R cells, a CTL-resistant subclone of FPM1, compared with that using 4O1/C8 and m8Δ presenting an unrelated peptide, suggesting that the activation of 4O1/C8 by m8Δ/RT1AlSCTax180L further enhanced the killing of the tumorigenic HTLV-I-infected cells. Our results indicate that combined therapy of oncolytic VVs with SCTs and HTLV-I-specific CTLs may be effective for eradication of HTLV-I-infected cells, which evade from CTL lysis and potentially develop ATL.

Molecular approach to human leukemia: isolation and characterization of the first human retrovirus HTLV-1 and its impact on tumorigenesis in adult T-cell leukemia

Molecular biology of mouse and chicken retroviruses had identified oncogenes and provided a revolutionary concept in understanding of cancers. A human retrovirus was established during 1980-1982 in linkage with a unique human leukemia, concurrently in Japan and USA. This review covers our efforts on the discovery of new retrovirus, Human T-cell Leukemia Virus Type 1 (HTLV-1), first introducing to a new class of retroviruses with a unique regulatory factors, Tax and Rex. Then it is followed by analyses of molecular interaction of the vial Tax with cellular machineries involved in the pathogenesis of Adult T-cell Leukemia (ATL). And then a probable mechanism of pathogenesis of ATL is proposed including recent findings on HBZ after our efforts.

Activation and detection of HTLV-I Tax-specific CTLs by epitope expressing single-chain trimers of MHC class I in a rat model

Background

Human T cell leukemia virus type I (HTLV-I) causes adult T-cell leukemia (ATL) in infected individuals after a long incubation period. Immunological studies have suggested that insufficient host T cell response to HTLV-I is a potential risk factor for ATL. To understand the relationship between host T cell response and HTLV-I pathogenesis in a rat model system, we have developed an activation and detection system of HTLV-I Tax-specific cytotoxic T lymphocytes (CTLs) by Epitope expressing Single-Chain Trimers (SCTs) of MHC Class I.

Results

We have established expression vectors which encode SCTs of rat MHC-I (RT1.A^l) with Tax180-188 peptide. Human cell lines transfected with the established expression vectors were able to induce IFN-γ and TNF-α production by a Tax180-188-specific CTL line, 4O1/C8. We have further fused the C-terminus of SCTs to EGFP and established cells expressing SCT-EGFP fusion protein on the surface. By co-cultivating the cells with 4O1/C8, we have confirmed that the epitope-specific CTLs acquired SCT-EGFP fusion proteins and that these EGFP-possessed CTLs were detectable by flow cytometric analysis.

Conclusion

We have generated a SCT of rat MHC-I linked to Tax epitope peptide, which can be applicable for the induction of Tax-specific CTLs in rat model systems of HTLV-I infection. We have also established a detection system of Tax-specific CTLs by using cells expressing SCTs fused with EGFP. These systems will be useful tools in understanding the role of HTLV-I specific CTLs in HTLV-I pathogenesis.

Human T Lymphotropic Virus Type 1 protein Tax reduces histone levels

Background

Human T-Lymphotropic Virus Type-1 (HTLV-1) is an oncogenic retrovirus that causes adult T-cell leukemia/lymphoma (ATLL). The virally encoded Tax protein is thought to be necessary and sufficient for T-cell leukemogenesis. Tax promotes inappropriate cellular proliferation, represses multiple DNA repair mechanisms, deregulates cell cycle checkpoints, and induces genomic instability. All of these Tax effects are thought to cooperate in the development of ATLL.

Results

In this study, we demonstrate that histone protein levels are reduced in HTLV-1 infected T-cell lines (HuT102, SLB-1 and C81) relative to uninfected T-cell lines (CEM, Jurkat and Molt4), while the relative amount of DNA per haploid complement is unaffected. In addition, we show that replication-dependent core and linker histone transcript levels are reduced in HTLV-1 infected T-cell lines. Furthermore, we show that Tax expression in Jurkat cells is sufficient for reduction of replication-dependent histone transcript levels.

Conclusion

These results demonstrate that Tax disrupts the proper regulation of replication-dependent histone gene expression. Further, our findings suggest that HTLV-1 infection uncouples replication-dependent histone gene expression and DNA replication, allowing the depletion of histone proteins with cell division. Histone proteins are involved in the regulation of all metabolic processes involving DNA including transcription, replication, repair and recombination. This study provides a previously unidentified mechanism by which Tax may directly induce chromosomal instability and deregulate gene expression through reduced histone levels.

Current topics in prevention of human T-cell leukemia virus type i infection: NF-kappa B inhibitors and APOBEC3

Human T-cell leukemia virus type I (HTLV-I) is the first human retrovirus and causes adult T-cell leukemia/lymphoma (ATL). Constitutive activation of nuclear factor-kappa B (NF-kappa B) in the leukemic cells is essential for their growth and survival. Thus, NF-kappa B inhibitors have been attracting attention as a potential strategy to treat ATL. Recently, the field of retrovirus research has been stimulated by the discovery of an innate host defense factor, APOBEC3, against the retroviruses. HTLV-I is relatively resistant to the antiviral effects of APOBEC3. To clarify the resistance of HTLV-I against APOBEC3 might contribute to the design of effective therapeutic approaches.

Histone deacetylase inhibitors increase virus gene expression but decrease CD8+ cell antiviral function in HTLV-1 infection

The dynamics of human T-lymphotropic virus type-1 (HTLV-1) provirus expression in vivo are unknown. There is much evidence to suggest that HTLV-1 gene expression is restricted: this restricted gene expression may contribute to HTLV-1 persistence by limiting the ability of the HTLV-1–specific CD8+ cell immune response to clear infected cells. In this study, we tested the hypothesis that derepression of HTLV-1 gene expression would allow an increase in CD8+ cell–mediated lysis of HTLV-1–infected cells. Using histone deacetylase enzyme inhibitors (HDIs) to hyperacetylate histones and increase HTLV-1 gene expression, we found that HDIs doubled Tax expression in naturally infected lymphocytes after overnight culture. However, the rate of CD8+ cell–mediated lysis of Tax-expressing cells ex vivo was halved. HDIs appeared to inhibit the CD8+ cell–mediated lytic process itself, indicating a role for the microtubule-associated HDAC6 enzyme. These observations indicate that HDIs may reduce the efficiency of cytotoxic T-cell (CTL) surveillance of HTLV-1 in vivo. The impact of HDIs on HTLV-1 proviral load in vivo cannot be accurately predicted because of the widespread effects of these drugs on cellular processes; we therefore recommend caution in the use of HDIs in nonmalignant cases of HTLV-1 infection.

Quantification of the virus-host interaction in human T lymphotropic virus I infection

BACKGROUND

HTLV-I causes the disabling inflammatory disease HAM/TSP: there is no vaccine, no satisfactory treatment and no means of assessing the risk of disease or prognosis in infected people. Like many immunopathological diseases with a viral etiology the outcome of infection is thought to depend on the virus-host immunology interaction. However the dynamic virus-host interaction is complex and current models of HAM/TSP pathogenesis are conflicting. The CD8+ cell response is thought to be a determinant of both HTLV-I proviral load and disease status but its effects can obscure other factors.

RESULTS

We show here that in the absence of CD8+ cells, CD4+ lymphocytes from HAM/TSP patients expressed HTLV-I protein significantly more readily than lymphocytes from asymptomatic carriers of similar proviral load (P = 0.017). A high rate of viral protein expression was significantly associated with a large increase in the prevalence of HAM/TSP (P = 0.031, 89% of cases correctly classified). Additionally, a high rate of Tax expression and a low CD8+ cell efficiency were independently significantly associated with a high proviral load (P = 0.005, P = 0.003 respectively).

CONCLUSION

These results disentangle the complex relationship between immune surveillance, proviral load, inflammatory disease and viral protein expression and indicate that increased protein expression may play an important role in HAM/TSP pathogenesis. This has important implications for therapy since it suggests that interventions should aim to reduce Tax expression rather than proviral load per se.

Animal models for human T-lymphotropic virus type 1 (HTLV-1) infection and transformation

Over the past 25 years, animal models of human T-lymphotropic virus type 1 (HTLV-1) infection and transformation have provided critical knowledge about viral and host factors in adult T-cell leukemia/lymphoma (ATL). The virus consistently infects rabbits, some non-human primates, and to a lesser extent rats. In addition to providing fundamental concepts in viral transmission and immune responses against HTLV-1 infection, these models have provided new information about the role of viral proteins in carcinogenesis. Mice and rats, in particular immunodeficient strains, are useful models to assess immunologic parameters mediating tumor outgrowth and therapeutic invention strategies against lymphoma. Genetically altered mice including both transgenic and knockout mice offer important models to test the role of specific viral and host genes in the development of HTLV-1-associated lymphoma. Novel approaches in genetic manipulation of both HTLV-1 and animal models are available to address the complex questions that remain about viral-mediated mechanisms of cell transformation and disease. Current progress in the understanding of the molecular events of HTLV-1 infection and transformation suggests that answers to these questions are approachable using animal models of HTLV-1-associated lymphoma.

Discovery of HTLV-1, the first human retrovirus, its unique regulatory mechanisms, and insights into pathogenesis

I briefly review the discovery and characterization of the first human retrovirus, human T-cell leukemia virus type 1, focusing on contributions from Japanese researchers. The unique regulatory mechanisms for the viral regulation with Tax and Rex, etiology of ATL and possible leukemogenic mechanism with Tax are also discussed briefly.

Cell-mediated immune response to human T-lymphotropic virus type I

Human T-lymphotropic virus type I (HTLV-I) is a retrovirus that causes persistent infection in many populations in tropical and subtropical regions. HTLV-I chronically activates the cell-mediated arm of the host adaptive immune response. There has been much debate about the role of the immune response in determining the outcome of HTLV-I infection: most seropositive individuals remain lifelong asymptomatic carriers of the virus, whereas a small proportion-usually those with higher equilibrium proviral loads-develop an inflammatory disease of the central nervous system known as HAM/TSP. Here we discuss the cell-mediated immune response to HTLV-I infection. We summarize recent data on the HTLV-I-specific CD4(+) cell response and explore its potential role in HAM/TSP pathogenesis. We also explore the controversy surrounding the role of the CD8(+) cell response in controlling HTLV-I infection and/or contributing to HAM/TSP disease, highlighting recent studies of T cell gene expression profiles and a newly developed assay of CD8(+) cell functional efficiency. Finally, we introduce a possible role for cellular innate immune effectors in HTLV-I infection.

A human T-cell lymphotropic virus type 1 enhancer of Myc transforming potential stabilizes Myc-TIP60 transcriptional interactions

The human T-cell lymphotropic virus type 1 (HTLV-1) infects and transforms CD4+ lymphocytes and causes adult T-cell leukemia/lymphoma (ATLL), an aggressive lymphoproliferative disease that is often fatal. Here, we demonstrate that the HTLV-1 pX splice-variant p30II markedly enhances the transforming potential of Myc and transcriptionally activates the human cyclin D2 promoter, dependent upon its conserved Myc-responsive E-box enhancer elements, which are associated with increased S-phase entry and multinucleation. Enhancement of c-Myc transforming activity by HTLV-1 p30II is dependent upon the transcriptional coactivators, transforming transcriptional activator protein/p434 and TIP60, and it requires TIP60 histone acetyltransferase (HAT) activity and correlates with the stabilization of HTLV-1 p30II/Myc-TIP60 chromatin-remodeling complexes. The p30II oncoprotein colocalizes and coimmunoprecipitates with Myc-TIP60 complexes in cultured HTLV-1-infected ATLL patient lymphocytes. Amino acid residues 99 to 154 within HTLV-1 p30II interact with the TIP60 HAT, and p30II transcriptionally activates numerous cellular genes in a TIP60-dependent or TIP60-independent manner, as determined by microarray gene expression analyses. Importantly, these results suggest that p30II functions as a novel retroviral modulator of Myc-TIP60-transforming interactions that may contribute to adult T-cell leukemogenesis.

Activation of a cryptic splice site in the tax gene of HTLV-I by a single nucleotide change

We identified a T-to-C mutation 2 nucleotides (nt) upstream from the AG in a GT-AG intron between exons 2 and 3 in the human T-cell leukemia virus type I (HTLV-I) tax mRNA. This mutation resulted in the preferential usage of an alternative splice site, causing a 75-nt elongation of tax mRNA and reduced production of viral antigens. When the clone containing this T-to-C mutation was reverted to the wild-type (T) DNA sequence, normal splicing of tax mRNA ensued and viral production was restored. These results suggest that the nucleotide at the position 2nt upstream from the AG in a GT-AG intron is important for the proper splicing of the HTLV-I tax gene, although it is not considered important for splicing in eukaryotes.

A functional CD8+ cell assay reveals individual variation in CD8+ cell antiviral efficacy and explains differences in human T-lymphotropic virus type 1 proviral load

The CD8+ lymphocyte response is a main component of host immunity, yet it is difficult to quantify its contribution to the control of persistent viruses. Consequently, it remains controversial as to whether CD8+ cells have a biologically significant impact on viral burden and disease progression in infections such as human immunodeficiency virus-1 and human T-lymphotropic virus type I (HTLV-I). Experiments to ascertain the impact of CD8+ cells on viral burden based on CD8+ cell frequency or specificity alone give inconsistent results. Here, an alternative approach was developed that directly quantifies the impact of CD8+ lymphocytes on HTLV-I proviral burden by measuring the rate at which HTLV-I-infected CD4+ cells were cleared by autologous CD8+ cells ex vivo. It was demonstrated that CD8+ cells reduced the lifespan of infected CD4+ cells to 1 day, considerably shorter than the 30 day lifespan of uninfected cells in vivo. Furthermore, it was shown that HTLV-I-infected individuals vary considerably in the rate at which their CD8+ cells clear infected cells, and that this was a significant predictor of their HTLV-I proviral load. Forty to 50 % of between-individual variation in HTLV-I proviral load was explained by variation in the rate at which CD8+ cells cleared infected cells. This novel approach demonstrates that CD8+ cells are a major determinant of HTLV-I proviral load. This assay is applicable to quantifying the CD8+ cell response to other viruses and malignancies and may be of particular importance in assessing vaccines.

Human T cell lymphotropic virus type I (HTLV-I)-specific CD4+ T cells: immunodominance hierarchy and preferential infection with HTLV-I

CD4(+) T cells predominate in early lesions in the CNS in the inflammatory disease human lymphotropic T cell virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP), but the pathogenesis of the disease remains unclear and the HTLV-I-specific CD4(+) T cell response has been little studied. We quantified the IFN-gamma-producing HTLV-I-specific CD4(+) T cells, in patients with HAM/TSP and in asymptomatic carriers with high proviral load, to test two hypotheses: that HAM/TSP patients and asymptomatic HTLV-I carriers with a similar proviral load differ in the immunodominance hierarchy or the total frequency of specific CD4(+) T cells, and that HTLV-I-specific CD4(+) T cells are preferentially infected with HTLV-I. The strongest CD4(+) T cell response in both HAM/TSP patients and asymptomatic carriers was specific to Env. This contrasts with the immunodominance of Tax in the HTLV-I-specific CD8(+) T cell response. The median frequency of HTLV-I-specific IFN-gamma(+) CD4(+) T cells was 25-fold greater in patients with HAM/TSP (p = 0.0023, Mann-Whitney) than in asymptomatic HTLV-I carriers with a similar proviral load. Furthermore, the frequency of CD4(+) T cells infected with HTLV-I (expressing Tax protein) was significantly greater (p = 0.0152, Mann-Whitney) among HTLV-I-specific cells than CMV-specific cells. These data were confirmed by quantitative PCR for HTLV-I DNA. We conclude that the high frequency of specific CD4(+) T cells was associated with the disease HAM/TSP, and did not simply reflect the higher proviral load that is usually found in HAM/TSP patients. Finally, we conclude that HTLV-I-specific CD4(+) T cells are preferentially infected with HTLV-I.

Repression of tax expression is associated both with resistance of human T-cell leukemia virus type 1-infected T cells to killing by tax-specific cytotoxic T lymphocytes and with impaired tumorigenicity in a rat model

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL). Although the viral transactivation factor, Tax, has been known to have apparent transforming ability, the exact function of Tax in ATL development is still not clear. To understand the role of Tax in ATL development, we introduced short-interfering RNAs (siRNAs) against Tax in a rat HTLV-1-infected T-cell line. Our results demonstrated that expression of siRNA targeting Tax successfully downregulated Tax expression. Repression of Tax expression was associated with resistance of the HTLV-1-infected T cells to Tax-specific cytotoxic-T-lymphocyte killing. This may be due to the direct effect of decreased Tax expression, because the Tax siRNA did not alter the expression of MHC-I, CD80, or CD86. Furthermore, T cells with Tax downregulation appeared to lose the ability to develop tumors in T-cell-deficient nude rats, in which the parental HTLV-1-infected cells induce ATL-like lymphoproliferative disease. These results indicated the importance of Tax both for activating host immune response against the virus and for maintaining the growth ability of infected cells in vivo. Our results provide insights into the mechanisms how the host immune system can survey and inhibit the growth of HTLV-1-infected cells during the long latent period before the onset of ATL.

HTLV-1-encoded p30II is a post-transcriptional negative regulator of viral replication

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.

T-Cell Control by Human T-Cell Leukemia/Lymphoma Virus Type 1

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.

High circulating frequencies of tumor necrosis factor alpha- and interleukin-2-secreting human T-lymphotropic virus type 1 (HTLV-1)-specific CD4+ T cells in patients with HTLV-1-associated neurological disease

Significantly higher frequencies of tumor necrosis factor alpha- and interleukin-2-secreting human T-lymphotropic virus type 1 (HTLV-1)-specific CD4(+) T cells were present in the peripheral blood mononuclear cells of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients than in those of asymptomatic carriers with similar provirus loads. The data suggest that HTLV-1-specific CD4(+) T cells play a role in the pathogenesis of HAM/TSP.

A conserved calcineurin-binding motif in human T lymphotropic virus type 1 p12I functions to modulate nuclear factor of activated T cell activation

The PXIXIT calcineurin binding motif or highly related sequences are found in a variety of calcineurin-binding proteins in yeast, mammalian cells, and viruses. The accessory protein p12(I) encoded in the HTLV-1 pX ORF I promotes T cell activation during the early stages of HTLV-1 infection by activating nuclear factor of activated T cells (NFAT) through calcium release from the endoplasmic reticulum. We identified in p12(I), a conserved motif, which is highly homologous with the PXIXIT calcineurin-binding motif of NFAT. Both immunoprecipitation and calmodulin agarose bead pull-down assays indicated that wild type p12(I) and mutants of p12(I) that contained the motif-bound calcineurin. In addition, an alanine substitution p12(I) mutant (p12(I) AXAXAA) had greatly reduced binding affinity for calcineurin. We then tested whether p12(I) binding to calcineurin affected NFAT activity. p12(I) competed with NFAT for calcineurin binding in calmodulin bead pull-down experiments. Furthermore, the p12(I) AXAXAA mutant enhanced NFAT nuclear translocation compared with wild type p12(I) and increased NFAT transcriptional activity 2-fold greater than wild type p12(I). Similar to NFAT, endogenous calcineurin phosphatase activity was increased in Jurkat T cells expressing p12(I) independent of its calcineurin binding property. Thus, the reduced binding of p12(I) to calcineurin allows enhanced nuclear translocation and transcription mediated by NFAT. Herein, we are the first to identify a retroviral protein that binds calcineurin. Our data suggest that HTLV-1 p12(I) modulates NFAT activation to promote early virus infection of T lymphocytes, providing a novel mechanism for retrovirus-mediated cell activation.

Seizing of T Cells by Human T-Cell Leukemia⧸Lymphoma Virus Type 1

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. 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. Viral proteins modulate the downstream effects of antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation are therefore important, as also suggested by epidemiological data. The ability of a given individual to respond to specific antigens is determined genetically. Thus, genetic and environmental factors, together with the virus, contribute to disease development. As in the case of other virus-associated cancers, HTLV-1-induced leukemia/lymphoma can be prevented by avoiding viral infection or by intervention during the asymptomatic phase with approaches able to interrupt the vicious cycle of virus-induced proliferation of a subset of T-cells. 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 in vitro. The relevance of these laboratory findings will be related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Correlation of major histocompatibility complex class I downregulation with resistance of human T-cell leukemia virus type 1-infected T cells to cytotoxic T-lymphocyte killing in a rat model

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL) in infected individuals after a long incubation period. Despite the apparent transforming ability of HTLV-1 under experimental conditions, most HTLV-1 carriers are asymptomatic. These facts suggest that HTLV-1 is controlled by host immunity in most carriers. To understand the interplay between host immunity and HTLV-1-infected cells, in this study, we isolated several HTLV-1 Tax-specific cytotoxic T-lymphocyte (CTL) lines from rats inoculated with Tax-coding DNA and investigated the long-term effects of the CTL on syngeneic HTLV-1-infected T cells. Our results demonstrated that long-term mixed culture of these CTL and the virus-infected T cells led to the emergence of CTL-resistant HTLV-1-infected cells. Although the Tax expression level in these resistant cells was equivalent to that in the parental cells, expression of surface major histocompatibility complex class I (MHC-I) was significantly downregulated in the resistant cells. Downregulation of MHC-I was more apparent in RT1.A(l), which presents a Tax epitope recognized by the CTL established in this study. Moreover, peptide pulsing resulted in killing of the resistant cells by CTL, indicating that resistance was caused by a decreased epitope density on the infected cell surface. This may be one of the mechanisms for persistence of HTLV-1-infected cells that evade CTL lysis and potentially develop ATL.

Multiple viral strategies of HTLV-1 for dysregulation of cell growth control

The human T cell leukemia virus-1 (HTLV-1) is a retrovirus that causes adult T cell leukemia (ATL) and neurological disorder, the tropical spastic paraparesis (HAM/TSP). The pathogenesis apparently results from the pleiotropic function of Tax protein, which is a key regulator of viral replication. Tax exerts (a) trans-activation and -repression of transcription of different sets of cellular genes through binding to groups of transcription factors and coactivators, (b) dysregulation of cell cycle through binding to inhibitors of CDK4/6, and (c) inhibition of some tumor suppressor proteins. These effects on a wide variety of cellular targets seem to cooperate in promoting cell proliferation. This is an effective viral strategy to amplify its proviral genome through replication of infected cells; ultimately it results in cell transformation and leukemogenesis.

HTLV-I Tax related dysfunction of cell cycle regulators and oncogenesis of adult T cell leukemia

HTLV-I is causually related to the oncogenesis of adult T cell leukemia (ATL). However, the precise mechanism of HTLV-I oncogenesis is unclear. HTLV-I Tax protein functions as an activator of various cellular genes, including IL-2, IL-2 receptor-alpha, and c-fos through the activation of nuclear transfer factors such as NF-kappaB and SRF, and also potently activates trascription of viral genes through CREB/ATF sites in the viral LTR. However, Tax activation of HTLV-I infected T cells through the above pathways induces polyclonal proliferation of the cells in vitro; Tax however may function only transiently in the immediate post-infection period following infection in vivo. The long latent period of 60 years from infection to onset of disease suggests other mechanisms for ATL oncogenesis. Recent studies suggest that the malignant transformation of ATL is a multi-hit phenomena, suggesting that discrete genetic events are responsible for ATL oncogenesis. These genetic events could be responsible for the different stages of ATL: smoldering, chronic, lymphoma, and acute type, p16 and p53 genes are important negative regulators of the cell cycle and are often found to be mutated in neoplasms. Recent studies including ours demonstrated a high frequency of alteration of these two genes in primary ATL cells. Furthermore, alteration of the two genes is associated with acute but not chronic type ATL. In addition, p16 gene alteration is linked to the growth rate of ATL cells, suggesting that the alteration of these cell cycle regulatory genes may be related to progression from smoldering or chronic to acute or lymphoma type ATL. Tax may be involved in mutagenesis of these genes through suppression of DNA-beta polymerase gene expression during the process from latent period to acute/lymphoma type. Once transformation occurs, activation of the pathway between Tax and the three nuclear transfer factors, NF-kappaB, SRF, and CREB/ATF, contributes to establish the aggressive manifestations of acute/lymphoma type ATL cells.

Prevention of adult T-cell leukemia-like lymphoproliferative disease in rats by adoptively transferred T cells from a donor immunized with human T-cell leukemia virus type 1 Tax-coding DNA vaccine

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL) in infected individuals after a long incubation period. To dissect the mechanisms of the development of the disease, we have previously established a rat model of ATL-like disease which allows examination of the growth and spread of HTLV-1 infected tumor cells, as well assessment of the effects of immune T cells on the development of the disease. In the present study, we induced HTLV-1 Tax-specific cytotoxic T lymphocyte (CTL) immunity by vaccination with Tax-coding DNA and examined the effects of the DNA vaccine in our rat ATL-like disease model. Our results demonstrated that DNA vaccine with Tax effectively induced Tax-specific CTL activity in F344/N Jcl-rnu/+ (nu/+) rats and that these CTLs were able to lyse HTLV-1 infected syngeneic T cells in vitro. Adoptive transfer of these immune T cells effectively inhibited the in vivo growth of HTLV-1-transformed tumor in F344/N Jcl-rnu/rnu (nu/nu) rats inoculated with a rat HTLV-1 infected T cell line. Vaccination with mutant Tax DNA lacking transforming ability also induced efficient anti-tumor immunity in this model. Our results indicated a promising effect for DNA vaccine with HTLV-1 Tax against HTLV-1 tumor development in vivo.

Clinical and oncological significance of aberrant Fas (APO-1/CD95) isoform expression in adult T-cell leukemia

Fas (APO-1/CD95), a transmembrane death receptor mediating apoptosis, can induce cell deathin vivo andin vitro of not only normal T-cells but also leukemic T-cells. This indicates that dysfunction in T-cell apoptosis may influence the natural history of the T-cell neoplasms, such as adult T-cell leukemia (ATL) caused by the retrovirus HTLV-1. Fas is ubiquitous, and down-regulated or mutated Fas has been widely detected in tumor cells that escape from elimination via Fas-mediated apoptosis. De novo fresh ATL cells and cell lines derived from the de novo cells, however, express Fas abundantly on the cell surface and are susceptible to Fas ligand and agonistic agents. On the other hand, there are two types of Fas gene transcripts, full-length and alternatively splicing truncated forms corresponding to membrane and soluble Fas isoforms, respectively. Focusing on membrane and soluble Fas isoforms and ATL pathology mediated by apoptosis, this paper reviews and discusses our ATL cases and ATL cell lines, which provide useful "experiments of nature" for understanding the role of Fas-mediated apoptosis in tumor biology.

Parathyroid hormone-related peptide and bone: pathological and physiological aspects

Parathyroid hormone-related peptide (PTHrP) was initially discovered as a tumor-derived systemic factor which causes humoral hypercalcemia of malignancy. When overproduced and secreted by tumor cells, PTHrP acts on target organs such as bone and kidney to cause hypercalcemia through its 'PTH-like effects'. The hypercalcemic effects of PTHrP are attributed to its N-terminal portion (1-36) which shows a limited homology with PTH and is able to bind to the common PTH/PTHrP receptor. In contrast to such pathological effects as a humoral factor, PTHrP is now recognized as a locally active cytokine produced by a variety of tissues and cell types. Gene knockout experiments have revealed critical roles for PTHrP in a wide spectrum of physiological processes including chondrogenesis. It also significantly contributes to various pathological processes such as tumor metastasis to bone and bone destruction in arthropathies, acting as a bone-resorbing cytokine. Consistent with its divergent roles, regulation of PTHrP expression as well as its mode of action seems to be much more complex than its hormonal counterpart, PTH. In this article, we will briefly review the recent progress in our understanding of both physiological and pathological aspects of PTHrP biology, with a particular focus on its roles as a bone cytokine.

Green tea polyphenols induce apoptosis in vitro in peripheral blood T lymphocytes of adult T-cell leukemia patients

Green tea polyphenols (TEA) are known to exhibit antioxidative activity as well as tumor-suppressing activity. In order to examine the tumor-suppressing activity of TEA against adult T-cell leukemia (ATL), we cultivated peripheral blood T lymphocytes of ATL patients (ATL PBLs), an HTLV-I-infected T-cell line (KODV) and healthy controls (normal PBLs) for 3 days in the presence of TEA and its main constituent, epigallocatechin-3-gallate (EGCg), to measure cell proliferation and apoptosis, and to quantitate mRNAs of HTLV-I pX and beta-actin genes of the cultured cells. Growth of ATL PBLs was significantly inhibited by 9-27 microg/ml of TEA and EGCg, in contrast to minimal growth inhibition of T cells of normal PBLs. Inhibition of KODV was intermediate between ATL PBLs and normal PBLs. The ATL PBLs and KODV treated with 27 microg/ml of either TEA or EGCg induced apoptotic DNA fragmentation, producing terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL)-positive cells, while the normal PBLs treated with the same concentration of TEA or EGCg produced a negligibly small number of TUNEL-positive cells, in which apoptotic DNA fragmentation was not detectable. Expression of HTLV-I pX mRNA was suppressed more than 90% in ATL PBLs by treatment with 3-27 microg/ml of either TEA or EGCg, while expression of beta-actin mRNA was much less suppressed by treatment with the same concentration of TEA or EGCg. These results indicate that TEA and EGCg inhibit growth of ATL PBLs, as well as HTLV-I-infected T-cells, by suppressing HTLV-I pX gene expression and inducing apoptotic cell death.

Discrepant expression of membrane and soluble isoforms of Fas (CD95/APO-1) in adult T-cell leukaemia: soluble Fas isoform is an independent risk factor for prognosis

The Fas signalling system probably plays a critical role in the natural and chemotherapeutic cell death machinery, suggesting that aberrant Fas expression is involved in growth control of tumours. The membrane isoform (mFas) is a 45 kD cell surface protein containing a single transmembrane region, and induces apoptosis in normal or tumour cells, whereas the soluble isoform (sFas) lacks the transmembrane domain due to alternative splicing of the transcript and is thought to block Fas-mediated apoptosis. To clarify the clinical roles of expression of these two Fas isoforms in adult T-cell leukaemia (ATL), we investigated the levels of the Fas isoforms in 81 patients with ATL. The expression patterns of the Fas isoforms were heterogenous, and there was no significant correlation between mFas and sFas levels: 10/81 cases were negative for mFas and had high serum sFas levels, whereas the remaining 71 cases were positive for mFas and had various levels of expression of the two Fas isoforms. Irrespective of the status of mFas expression in leukaemic cells, the mRNAs encoding these isoforms were always detectable, indicating the potential for protein translation. Although mFas expressed on freshly isolated ATL cells could iduce apoptosis in vitro, positive versus negative mFas status was not associated with any clinical aspects of ATL, whereas the sFas level was strongly correlated with clinical parameters such as serum LDH activity, tumour burden, serum soluble IL-2R level, hypercalcaemia and prognosis. These results suggest that the ratio of Fas isoforms varies, and high expression of the sFas protein and message reflects the malignant behaviour of ATL and is an independent risk factor for the prognosis.

Persistent activation of NF-kappaB by the tax transforming protein of HTLV-1: hijacking cellular IkappaB kinases

Biochemical coupling of transcription factor NF-kappaB to antigen and co-stimulatory receptors is required for the temporal control of T-cell proliferation. In contrast to its transitory activation during normal growth-signal transduction, NF-kappaB is constitutively deployed in T-cells transformed by the type 1 human T-cell leukemia virus (HTLV-1). This viral/host interaction is mediated by the HTLV-1-encoded Tax protein, which has potent oncogenic properties. As reviewed here, Tax activates NF-kappaB primarily via a pathway leading to the chronic phosphorylation and degradation of IkappaBalpha, a cytoplasmic inhibitor of NF-kappaB. To access this pathway, Tax associates stably with a cytokine-inducible IkappaB kinase (IKK), which contains both catalytic (IKKalpha and IKKbeta) and noncatalytic (IKKgamma) subunits. Unlike their transiently induced counterparts in cytokine-treated cells, Tax-associated forms of IKKalpha and IKKbeta are persistently activated in HTLV-1-infected T cells. Acquisition of the deregulated IKK phenotype is contingent on the presence of IKKgamma, which functions as a molecular adaptor in the assembly of pathologic Tax/IkappaB kinase complexes. These findings highlight a key mechanistic role for IKK in the Tax/NF-kappaB signaling axis and define new intracellular targets for the therapeutic control of HTLV-1-associated disease.

Influence of Rex and intronic sequences on expression of spliced mRNAs produced by human T cell leukemia virus type I

Expression of the incompletely spliced HTLV-I mRNAs relies on the viral posttranscriptional activator Rex, whose interaction with the Rex-responsive element (RXRE) overcomes effects of cis-acting repressive sequences (CRSs). Studies based on heterologous reporter plasmids identified an intronic CRS in the 5' LTR and a CRS that overlaps with the RXRE. The present study investigated the effects of these elements in the context of spliced viral mRNAs encoding p21Rex (mRNA 1-3), Tax/Rex (mRNA 1-2-3), and Tof (mRNA 1-2-B). All three mRNAs were inefficiently expressed when transcribed in their mature intronless form, with the p21Rex mRNA showing the weakest expression. In contrast, efficient expression of p21Rex was obtained from a plasmid containing the 5' LTR and 3' portion of the genome that encoded a spliceable RNA. The defective expression of the intronless mRNAs reflected the inhibitory activity of the RXRE and the lack of 5' intronic sequences. Insertion of an intronic 5' LTR segment located upstream of the 5' CRS overcame Rex dependence conferred by the RXRE. The activity of this segment was mapped to the major splice donor and sequences overlapping with, but functionally distinct from, a previously described transcriptional enhancer. The three mRNAs responded differently to Rex and to insertion of the constitutive transport element of simian retrovirus type 1. Taken together, these results suggest that expression of the spliced mRNAs is controlled by the relative influence of positive and negative sequences present on the primary transcript as well as the Rex-RXRE interaction.

NF-kappaB involvement in the activation of primary adult T-cell leukemia cells and its clinical implications

The HTLV-I provirus-encoded Tax protein induces NF-kappaB in Tax-transfected Jurkat T cells or HTLVL-I- infected T cells in vitro. Tax induction of NF-kappaB is presumed to be involved in proliferation and activation of primary leukemia cells in vivo. Recent studies have demonstrated that NF-kappaB activities in human T cells are mediated by at least four c-Rel-related DNA binding proteins - p50, p55, p75 and p85. We examined the significance of NF-kappaB induction in primary adult T cell leukemia cells and the induction kinetics of each of the four NF-kappaB species. Marked NF-kappaB activity was detected using an electrophoretic mobility shift assay (EMSA) in the primary cells of patients with acute disease, but little activity was noted in the cells of chronic patients. NF-kappaB activity was enhanced in a time-dependent manner in acute type cells cultured with mitogen-free medium; there was no induction of activity in chronic type cells. UV crosslinking demonstrated all four species of NFkappaB complex - high levels of p50 and lower levels of p55 and p75, in acute type cells; chronic type cells showed only the p50. As a control, normal resting T cells similarly showed only p50; control cells showed little change in activity when cultured without mitogenic stimulation, analogous to chronic type ATL. Northern blotting revealed enhancement of c-rel (encoding p85) and KBFI (encoding p50 and p55) expression in acute type cells during culture, while there was no significant enhancement of mRNAs in chronic type ATL cells or unstimulated normal T cells. Northern blotting also revealed that Tax is upregulated at the mRNA level in acute- but not chronic-type cells during culture. Expression of c-rel and KBF1 mRNAs in acute type cells appeared to be related to Tax mRNA expression. These results suggest that Tax is capable of inducing nuclear expression of all four NF-kappaB species in primary ATL cells of acute type patients, with marked effects on p55, p75, and p85. Tax induction of NF-kappaB species is regulated, at least in part, at a pretranslational level involving increases in c-rel and KBF1 mRNA.

Epstein-Barr virus involvement in T-cell malignancy: significance in adult T-cell leukemia

Epstein-Barr virus (EBV) was first reported as the causative virus of Burkitt's lymphoma in 1964. Since then, EBV has also been associated with infectious mononucleosis, AIDS and transplant-related B cell lymphomas, and nasopharyngeal cancer. The virus has further been linked with T cell lymphomas, Hodgkin disease, and NK leukemia or LGL leukemia, establishing a concept of a wide spectrum of EBV associated malignant disorders. EBV DNA encodes several proteins such as EBNA1-6, LMP 1, 2 and others. Recent studies have demonstrated that EBNA2, EBNA5, EBNA3A, EBNA 3C are essential for transformation, and that any gene product is not sufficient to transform cells by itself. Further there are different mechanisms of virus-associated transformation or carcinogenesis among EBV-associated malignant disorders. On the other hand, human T lymphotropic virus type I (HTLV-I) is known as a causative virus of adult T cell leukemia (ATL). However, precise molecular mechanisms of leukemogenesis in ATL still remains unclear. Some additional factors to HTLV-I infection are supposed to be involved in complete leukemogenesis. We demonstrated that HTLV-I infected T cells and primary ATL cells express EBV receptor/CD21 on the cell surface. Therefore, it is possible that EBV infection is one of the factors. We further investigated this possibility in 6 HTLV-I infected T cell lines and primary ATL cells from 18 patients with ATL. However, no EBV genome was detected in both T cell lines and primary ATL cells. EBV involved T-cell lymphoma has unique clinical manifestations as compared to non-EBV involved T-cell lymphoma. Therefore, it is still possible that a small group of ATL patients with unique clinical manifestations is associated with EBV.

Reciprocal interactions between human cytomegalovirus and human T cell leukemia-lymphoma virus type I in monocyte-derived macrophages cultured in vitro

Infection of macrophages with human cytomegalovirus (HCMV) has been shown to be nonlytic and exclusively cell associated. Human T cell leukemia-lymphoma virus type I (HTLV-I) is capable of establishing productive infection in macrophages. We studied the interactions between HCMV and HTLV-I in monocyte-derived macrophages cultured in vitro. We found that coinfection of macrophages with HCMV and HTLV-I significantly enhanced HCMV replication, resulting in release of infectious HCMV from dually infected cells. On the other hand, HCMV inhibited HTLV-I replication in macrophages coinfected with both viruses. Reciprocal interactions between HCMV and HTLV-I were mediated by their trans-acting proteins. Results of transfection studies demonstrated that the tax gene product of HTLV-I alone was capable of upregulating HCMV production. In a transient gene expression assay the immediate-early 2 (IE2) protein of HCMV alone could inhibit HTLV-I replication, whereas the IE1 protein, which had no effect by itself, produced a synergistic inhibitory effect together with the IE2 protein. Results from this study suggest that in vivo double infection of macrophages with HCMV and HTLV-I may contribute to the dissemination of HCMV infection in patients suffering from HTLV-I-associated T cell leukemia-lymphoma.

Mutation of CD95 (Fas/Apo-1) Gene in Adult T-Cell Leukemia Cells

CD95 antigen (also known as Fas or Apo-1) and Fas ligand play key roles in apoptosis of cells of the immune system, function as effector molecules of cytotoxic T lymphocytes, and function in the elimination of activated lymphocytes during the downregulation of the immune response. The critical roles of the Fas-Fas ligand system in apoptosis suggest that its inactivation may be involved in malignant transformation. We analyzed the expression of Fas antigen on adult T-cell leukemia (ATL) cells by flow cytometry and found that Fas antigen expression was absent in a case of ATL and markedly decreased in another case among 47 cases examined. Apoptosis could not be induced in the Fas-negative ATL cells by antibody against Fas antigen. Sequencing of reverse transcription-polymerase chain reaction products of the Fas genes in the Fas negative cells showed two types of aberrant transcripts: one had a 5-bp deletion and a 1-bp insertion in exon 2, and the other transcript lacked exon 4. These mutations caused the premature termination of both alleles, resulting in the loss of expression of surface Fas antigen. These aberrant transcripts were not detected in a nonleukemic B-cell line from the same patient. An RNase protection assay of the Fas gene showed mutations in 2 additional cases with Fas-positive ATL cells of 35 cases examined: 1 case lacked exon 4 and the other was a silent mutation. In the Fas antigen-negative case, leukemic cells were resistant to anticancer drugs in vivo, indicating that the loss of expression of Fas antigen may be associated with a poor response to anticancer drugs. Indeed, Fas-negative ATL cells were resistant to adriamycin-induced apoptosis in vitro, which is consistent with the finding that ATL in this case was resistant to chemotherapy. These findings indicate that mutation of the Fas gene may be associated with the progression of ATL and with resistance to anticancer drugs.

Mutation of CD95 (Fas/Apo-1) Gene in Adult T-Cell Leukemia Cells

CD95 antigen (also known as Fas or Apo-1) and Fas ligand play key roles in apoptosis of cells of the immune system, function as effector molecules of cytotoxic T lymphocytes, and function in the elimination of activated lymphocytes during the downregulation of the immune response. The critical roles of the Fas-Fas ligand system in apoptosis suggest that its inactivation may be involved in malignant transformation. We analyzed the expression of Fas antigen on adult T-cell leukemia (ATL) cells by flow cytometry and found that Fas antigen expression was absent in a case of ATL and markedly decreased in another case among 47 cases examined. Apoptosis could not be induced in the Fas-negative ATL cells by antibody against Fas antigen. Sequencing of reverse transcription-polymerase chain reaction products of the Fas genes in the Fas negative cells showed two types of aberrant transcripts: one had a 5-bp deletion and a 1-bp insertion in exon 2, and the other transcript lacked exon 4. These mutations caused the premature termination of both alleles, resulting in the loss of expression of surface Fas antigen. These aberrant transcripts were not detected in a nonleukemic B-cell line from the same patient. An RNase protection assay of the Fas gene showed mutations in 2 additional cases with Fas-positive ATL cells of 35 cases examined: 1 case lacked exon 4 and the other was a silent mutation. In the Fas antigen-negative case, leukemic cells were resistant to anticancer drugs in vivo, indicating that the loss of expression of Fas antigen may be associated with a poor response to anticancer drugs. Indeed, Fas-negative ATL cells were resistant to adriamycin-induced apoptosis in vitro, which is consistent with the finding that ATL in this case was resistant to chemotherapy. These findings indicate that mutation of the Fas gene may be associated with the progression of ATL and with resistance to anticancer drugs.

In vivo retrovirus-mediated herpes simplex virus thymidine kinase gene therapy approach for adult T cell leukemia in a rat model

We have previously demonstrated that human T-lymphotropic virus type I (HTLV-I) tax-expressing human T cell lines are selectively eliminated in the presence of aciclovir, using a retroviral vector carrying the herpes simplex virus thymidine kinase (HSV TK) gene under the control of the long terminal repeat (LTR) of HTLV-I. Based on these findings in vitro, we investigated whether this system could also be effective in vivo, using a rat model. Following infection of the HTLV-I-transformed and tax-expressing rat T cell line TARS-1 with this retrovirus (LNLTK virus), high levels of HSV TK expression were observed and resulted in increased susceptibility to ganciclovir (GCV). Tumors were generated by subcutaneous injection of TARS-1 in newborn syngeneic WKA/H rats. While the tumors derived from infected TARS-1 cells with control virus, as well as uninfected cells, continued to grow in all the rats with or without administration of GCV, those derived from LNLTK-infected cells exhibited dramatic regression upon GCV treatment. These results indicate that the HTLV-I LTR-HSV TK system also causes selective elimination of HTLV-I-transformed, tax-expressing T cells in vivo. Therefore, our present study may provide a rationale for clinical gene therapy against adult T cell leukemia.

In vitro transcription of human T-cell leukemia virus type 1 is RNA polymerase II dependent

The HTLV-1 promoter directs RNA polymerase II transcription of viral genomic RNA in vivo. However, it has been reported that in vitro, a unique RNA polymerase, with characteristics of RNA polymerases II and III, is capable of HTLV-1 transcription (G. Piras, F. Kashanchi, M. F. Radonovich, J. F. Duvall, and J. N. Brady, J. Virol. 68:6170-6179, 1994). To further characterize the polymerase involved in HTLV-1 transcription in vitro, runoff transcription assays were performed with a variety of extracts and RNA polymerase inhibitors. Under all in vitro reaction conditions tested, RNA polymerase II appeared to be the only polymerase capable of correct transcriptional initiation from the HTLV-1 promoter. Synthesis of the specific HTLV-1 RNA transcript showed sensitivities to the RNA polymerase inhibitors tagetitoxin and alpha-amanitin that are consistent with RNA polymerase II transcription. Together, these data indicate that in vitro, as in vivo, the HTLV-1 promoter directs transcription by RNA polymerase II.

Rat primary T cells expressing HTLV-I tax gene transduced by a retroviral vector: in vitro and in vivo characterization

We prepared a recombinant retroviral vector expressing the human T-lymphotropic virus type-I tax gene. Infection of WKA/H rat splenocytes yielded T-cell lines which proliferated continuously in media supplemented with exogenous interleukin-2 (IL-2) after the control cells ceased to grow. The phenotype of these cells closely resembled that of typical adult T-cell leukemia cells and tax-immortalized human T cells; i.e., positive for CD3, CD4 and IL-2 receptor alpha-chain. Chromosomal analysis revealed that about 10% of the tax-transduced T cells had several chromosomal abnormalities. We also performed in vivo characterization of tax-transduced splenocytes by injecting them into newborn syngeneic rats soon after in vitro infection. Maintenance of the injected tax-transduced cell population and in vivo expression of the tax gene was confirmed in the splenocytes of the injected rats by polymerase chain reaction. However, development of obvious disease was not observed in these rats for up to 18 months after inoculation. These results indicate that tax is capable of immortalizing rat mature CD4+ T cells in vitro but may be insufficient for full transformation of these cells in vivo. Our in vivo system using retrovirally tax-transduced rat T cells could facilitate investigation of the additional genetic events that cooperatively transform T cells transduced with tax gene.

Human T lymphotropic virus type I in arthropathy and autoimmune disorders

The progressive nature of the disease and the persistent inflammation affecting various organs are common features of idiopathic autoimmune disorders of unknown etiology. Therefore, the HTLV-I-associated disorders described in the present review are outstandingly important models for our understanding of the pathologic mechanisms of organ-specific immune disorders. HTLV-I arthropathy is characterized by chronic inflammatory and proliferative synovitis with lymphoid follicles and pannus formation in the affected joints, indistinguishable from the findings in idiopathic RA. The presence of the tax gene in HTLV-I-negative SS patients suggests that it is responsible for the exocrine gland abnormality, characterized by extensive lymphoproliferative epithelial lesions. Furthermore, the pulmonary lesions of HTLV-I bronchopneumonopathy are similar to those of idiopathic interstitial pneumonitis. Based on these observations, the clinical findings associated with the immunologic abnormalities in HTLV-I-infected patients provide us with valuable information for understanding the pathogenetic mechanisms of chronic inflammatory conditions associated with immune regulatory disorders. Although the clinical and pathologic features of the 2 common HTLV-I-associated disorders, ATL and HAM/TSP, have been well characterized and are clearly distinguishable from those of the idiopathic forms of these disorders, other HTLV-I-related autoimmune diseases, e.g., arthropathy, SS, or bronchopneumonopathy, are clinically indistinguishable from the idiopathic forms of the diseases. Such similarity may serve as a clue to the pathogenetic mechanisms of idiopathic autoimmune disorders.

Negative regulation of gene expression from the HTLV type II long terminal repeat by Rex: functional and structural dissociation from positive posttranscriptional regulation

Regulation of human T cell leukemia virus type II (HTLV-II) gene expression by Rex is mediated by cis-acting elements in the 5' viral long terminal repeat (LTR). Rex acts posttranscriptionally to enhance cytoplasmic accumulation of incompletely spliced viral mRNAs encoding structural proteins. We report a distinct negative regulatory function mediated by Rex affecting expression from the viral 5' LTR. Using both LTR-driven CAT reporters and a full-length HTLV-II proviral construct, we demonstrate that Rex decreases total cellular levels of LTR-containing mRNA in a dose-dependent manner. Negative regulation is an independent function as demonstrated by structural and functional dissociation from Rex positive posttranscriptional regulation. This negative regulatory action was dependent on nuclear localization sequences, but did not require the previously defined Rex-responsive element (RxRE). Negative regulation was observed in T cell lines but not in B cell lines, suggesting the involvement of cell type-specific factors distinct from those involved in posttranscriptional regulation. An internal deletion mutant of Rex removing aa 38-80 retained the ability to repress, but did not posttranscriptionally increase expression, while negative regulation requires a previously uncharacterized carboxy-terminal region (aa 154-170). These findings suggest that Rex may serve two simultaneous functions: to decrease overall levels of transcribed viral mRNA, and to facilitate nuclear to cytoplasmic export of mRNAs encoding structural proteins. The negative regulatory function of Rex may play a role in viral latency.

Presence of antibodies to p21X and/or p27rex proteins in sera from human T-cell leukemia virus type I-infected individuals

The human T-cell leukemia virus type I (HTLV-I) pX gene encodes three nonstructural proteins, p40tax, p27rex and p21X. So far, natural antibodies to p27rex and/or p21X have not been found in sera from HTLV-I-infected individuals, although antibodies to p40tax have been found. Recently, the viral transcripts specific for these proteins were detected in fresh peripheral blood mononuclear cells from HTLV-I-infected individuals by the polymerase chain reaction coupled to reverse transcription, showing the in vivo expression of these proteins. We detected antibodies to p21X and p27rex by an enzyme-linked immunosorbent assay (ELISA) system using a recombinantly produced p21X protein as a common antigen, because p21X is identical to the C-terminal portion of p27rex. The sensitivity of the ELISA was determined to be approximately 100 times greater than that of Western blotting. From the analyzed sera of 31 ATL patients, 30 asymptomatic carriers, 18 HAM patients and 100 healthy donors, three specimens from one ATL patient and two carriers were found to be positive for anti-p21X/p27rex antibodies. The specificity of the ELISA reaction was confirmed by the competitive ELISA test with the highly purified recombinant p21X protein. As of result, we first determined the presence of anti-p21X/p27rex antibodies in a small percentage (3.8%) of the sera from HTLV-I-infected individuals. Even sera from the ATL patients, whose fresh PBMCs contained the transcripts for these proteins, were not found to contain these antibodies, suggesting that the immune response to these proteins is low in HTLV-I-infected humans.

Interleukin (IL) 15/IL-T production by the adult T-cell leukemia cell line HuT-102 is associated with a human T-cell lymphotrophic virus type I region /IL-15 fusion message that lacks many upstream AUGs that normally attenuates IL-15 mRNA translation

We reported previously that the human T-cell lymphotrophic virus type I (HTLV-I)-associated adult T-cell leukemia line HuT-102 produces a cytokine designated interleukin (IL) T that requires interleukin (IL) 2 receptor beta-subunit expression for its action. Using anti-cytokine antibodies, we demonstrated that IL-T is identical to the simultaneously described IL-15. When compared to activated monocytes, IL-15 mRNA expression was 6- to 10-fold greater in HuT-102 cells. The predominant IL-15 message from HuT-102 is a chimeric mRNA joining a segment of the R region of the long terminal repeat of HTLV-I and the 5'-untranslated region (UTR) of IL-15. Normally, by alternative splicing, this 118-nucleotide R element represents the most 5' region of several HTLV-I transcripts including tax, rex, and env. The introduction of the R element eliminated over 200 nucleotides of the IL-15 5'-UTR, including 8 of 10 upstream AUGs that are present in normal IL-15 messages. On analysis of the 5'-UTR of normal IL-15, we demonstrated that the presence of these 10 upstream AUGs interferes with IL-15 mRNA translation. Thus, IL-15 synthesis by the adult T-cell leukemia line HuT- 102 involves an increase in IL-15 mRNA transcription and translation secondary to the production of an HTLV-I R element fusion message that lacks many upstream AUGs.

Differential Tax expression in HTLV type I-infected asymptomatic carriers

tax gene expression in a family cluster of three HTLV-I-infected asymptomatic individuals was investigated. Two carriers had normal tax mRNA, Tax-specific humoral antibody, and cell-mediated immune (CMI) response. In one carrier who had only weak Tax-specific humoral and no Tax-specific CMI response, an abnormal Tax-related mRNA product was detected. This product was sequenced and found to consist of two exons derived from the LTR gag and pX regions. The abnormal mRNA has an ORF predicting a 17-kDa protein, the translation of which is initiated in the first exon. The presence of this protein, of antibody to it, and of its function remain to be elucidated.

Induction of ICAM-1 and LFA-3 by Tax1 of human T-cell leukemia virus type 1 and mechanism of down-regulation of ICAM-1 or LFA-1 in adult-T-cell-leukemia cell lines

The present study was undertaken to determine the role of HTLV-I TaxI in the up-regulation of ICAM-I and LFA-3 in human T cells transformed with HTLV-I and the mechanism of down-regulation of ICAM-I and LFA-I in ATL-derived cell lines. Induction of TaxI in a human T-cell line Jurkat carrying the TaxI gene under the metallothionein promoter led to increases in mRNA and surface expression of ICAM-I. The response of LFA-3 to TaxI induction was, on the other hand, relatively slow and weak, and might be indirect. Transactivation of the ICAM-I promoter by TaxI was further shown by co-transfection of a CAT reporter construct with the ICAM-I promoter and a plasmid expressing TaxI. The mechanism of down-regulation of ICAM-I or LFA-I in 4 ATL cell lines was next examined. ICAM-I mRNA was quite low in MT-I, but no genomic changes were found. The CAT reporter with the ICAM-I promoter was inactive in MT-I. Finally, combined treatment of MT-I with 5-azacytidine and IFN-gamma induced re-expression of ICAM-I. Collectively, (a) transcriptional factor(s) necessary for expression of ICAM-I gene may be repressed in MT-I through DNA methylation. Three other ATL cell lines (TL-OmI, H582, HuT102) were found to have little mRNA for the LFA-I beta chain (CD18). H582 and HuT102 were also negative for the LFA-I alpha chain (CDIIa) mRNA. No genomic changes were found, and a CAT reporter gene with the CD18 promoter was inactive in the 3 of them, again suggesting lack of (a) transcriptional factor(s) necessary for CD18 expression.

Immunopathological mechanisms of human T cell lymphotropic virus type 1 (HTLV-I) uveitis. Detection of HTLV-I-infected T cells in the eye and their constitutive cytokine production

The immunopathology of human T cell lymphotropic virus type 1 (HTLV-I) uveitis was addressed by using T cell clones (TCC) established from the intraocular fluid of patients with HTLV-I uveitis. Proviral DNA of HTLV-I was identified in 55 out of 94 (59%) or 13 out of 36 (36%) TCC from the ocular fluid or the peripheral blood of these patients, respectively. Most of HTLV-I-infected TCC had a CD3+ CD4+ CD8- phenotype. HTLV-I infection on TCC was confirmed by analysis of the viral mRNA, nucleotide sequence, virus-associated proteins, and virus particles. HTLV-I-infected TCC, but not HTLV-I negative TCC, constitutively produced high amounts of IL-6 (1,336 +/- 1,050 pg/ml) and TNF-alpha (289 +/- 237 pg/ml) in the absence of any stimuli. HTLV-I-infected TCC from the ocular lesion also constitutively produced high amounts of IL-1 alpha (12,699 pg/ml), IL-2 (61 pg/ml), IL-3 (428 pg/ml), IL-8 (1,268 pg/ml), IL-10 (28 pg/ml), IFN-gamma (5,095 pg/ml), and GM-CSF (2,886 pg/ml). Hydrocortisone, a drug effective in vivo for the treatment of HTLV-I uveitis, severely depressed cytokine production in vitro in most cases. In summary, the results demonstrated direct evidence of HTLV-I infection of the eye and suggest that cytokines produced by HTLV-I-infected T cells are responsible for the intraocular inflammation in patients with HTLV-I uveitis.