Molecular biology of HTLV-I: recent progress

Transcriptional activation of immediate-early gene ETR101 by human T-cell leukaemia virus type I Tax

Human T-cell leukaemia virus type I (HTLV-I) Tax regulates viral and cellular gene expression through interactions with multiple cellular transcription pathways. This study describes the finding of immediate-early gene ETR101 expression in HTLV-I-infected cells and its regulation by Tax. ETR101 was persistently expressed in HTLV-I-infected cells but not in HTLV-I uninfected cells. Expression of ETR101 was dependent upon Tax expression in the inducible Tax-expressing cell line JPX-9 and also in Jurkat cells transiently transfected with Tax-expressing vectors. Tax transactivated the ETR101 gene promoter in a transient transfection assay. A series of deletion and mutation analyses of the ETR101 gene promoter indicated that a 35 bp region immediately upstream of the TATA-box sequence, which contains a consensus cAMP response element (CRE) and a G+C-rich sequence, is the critical responsive element for Tax activation. Site-directed mutagenesis analysis of the 35 bp region suggested that both the consensus CRE motif and its upstream G+C-rich sequence were critical for Tax transactivation. Electrophoretic mobility shift analysis (EMSA) using the 35 bp sequence as probe showed the formation of a specific protein-DNA complex in HTLV-I-infected cell lines. EMSA with specific antibodies confirmed that the CREB transcription factor was responsible for formation of this specific protein-DNA complex. These results suggested that Tax directly transactivated ETR101 gene expression, mainly through a CRE sequence via the CREB transcription pathway.

Reduced cell turnover in bovine leukemia virus-infected, persistently lymphocytotic cattle

Although nucleotide analogs like bromodeoxyuridine have been extensively used to estimate cell proliferation in vivo, precise dynamic parameters are scarce essentially because of the lack of adequate mathematical models. Besides recent developments on T cell dynamics, the turnover rates of B lymphocytes are largely unknown particularly in the context of a virally induced pathological disorder. Here, we aim to resolve this issue by determining the rates of cell proliferation and death during the chronic stage of the bovine leukemia virus (BLV) infection, called bovine persistent lymphocytosis (PL). Our methodology is based on direct intravenous injection of bromodeoxyuridine in association with subsequent flow cytometry. By this in vivo approach, we show that the death rate of PL B lymphocytes is significantly reduced (average death rate, 0.057 day(-1) versus 0.156 day(-1) in the asymptomatic controls). Concomitantly, proliferation of the PL cells is also significantly restricted compared to the controls (average proliferation rate, 0.0046 day(-1) versus 0.0085 day(-1)). We conclude that bovine PL is characterized by a decreased cell turnover resulting both from a reduction of cell death and an overall impairment of proliferation. The cell dynamic parameters differ from those measured in sheep, an experimental model for BLV infection. Finally, cells expressing p24 major capsid protein ex vivo were not BrdU positive, suggesting an immune selection against proliferating virus-positive lymphocytes. Based on a comparative leukemia approach, these observations might help to understand cell dynamics during other lymphoproliferative disease such as chronic lymphocytic leukemia or human T-cell lymphotropic virus-induced adult T-cell leukemia in humans.

Modulation of infection and type 1 cytokine expression parameters by morphine during in vitro coinfection with human T-cell leukemia virus type I and HIV-1

Infection of injection drug users (IDUs) with the human T-cell leukemia viruses (HTLVs) or HIV is considerably higher than in the non-IDU population. Also, coinfection with HIV-1 and HTLV type I (HTLV-I) occurs more frequently. There is little or no information on the effects of opiates (i.e., morphine) on HTLV infection alone or on coinfection of HTLV-I-infected cells with HIV-1. Therefore, in this report, we analyzed the in vitro effects of morphine on HIV or HTLV infection alone as well as on dual infection with HTLV-I and HIV-1. Morphine decreased the in vitro levels of interferon-gamma (IFN gamma) and IL-2 during single infections, and this effect was reversed by the addition of the opioid antagonist naloxone. In contrast, treatment with morphine resulted in a 31% and 36% increase in IFN gamma and IL-2 levels, respectively, during dual infection. In addition, naloxone had an apparent additive effect on the morphine-associated enhancement of IFN gamma and IL-2 expression in the dual-infection model. Despite the high levels of IFN gamma expression, the viability of the coinfected cells in the presence of morphine was maintained. Importantly, morphine treatment was associated with augmented viral reverse transcription activity in dually infected cultures, apparently to the benefit of HTLV-I. If a similar putative morphine-induced advantage for HTLV-I production also occurs during in vivo coinfection, opiates such as morphine could contribute to the observed increased rate of HIV-1/HTLV-I infection in the IDU population in a more direct fashion than was previously believed.

Mutant tax protein from bovine leukemia virus with enhanced ability to activate the expression of c-fos

Bovine leukemia virus (BLV) is the etiologic agent of enzootic bovine leukosis. We previously identified several mutants of the BLV Tax protein with an ability to transactivate transcription via the BLV enhancer that is significantly greater than that of the wild-type Tax protein. Moreover, the mutant proteins also activated other viral enhancers, such as the enhancer of human T-cell leukemia virus type 1, which cannot be activated by wild-type BLV Tax. In this study, we demonstrated that the mutant proteins but not wild-type protein activate the upstream sequence of the human c-fos gene, which contains two major cis-acting elements, the CArG box and cyclic AMP-responsive element (CRE) motif. The mutant protein also strongly increased levels of endogenous c-fos mRNA in both human and bovine cell lines. On the other hand, the wild-type Tax protein has no activity to activate the expression of human c-fos, indicating that wild-type BLV Tax might discriminate between human and bovine c-fos promoter sequences. Deletion and point-mutational analysis of the cis-acting elements revealed that both the CArG box and the CRE motif were indispensable for the activation of c-fos by the mutant BLV Tax protein. Our results suggest that the mutant BLV Tax proteins might not only have the ability to enhance the production of virus particles but might also have increased ability to induce leukemia.

Human T-cell lymphotropic-I-associated leukemia/lymphoma

Human T-cell lymphotropic virus-I (HTLV-I)-related adult T-cell leukemia/lymphoma (ATL) is a model disease for proof of viral oncogenesis. HTLV-I infection is endemic in southern Japan and the Caribbean basin, and occurs sporadically in Africa, Central and South America, the Middle East, and the southeastern United States. ATL occurs in only 2% to 4% of HTLV-I-infected people [1-3]. When it does occur, it is usually aggressive and difficult to treat; most people survive for less than 1 year [1-3]. Combination chemotherapy with cytotoxic agents has yielded complete response rates of 20% to 45%, but responses usually last only a few months [3]. Recently, novel treatments, such as monoclonal antibodies directed at the interleukin-2 receptor and the combination of interferon alfa and zidovudine, have been shown to be active in the treatment of patients with ATL. A small percentage of patients achieve long-lasting remissions [2,3].

The region between amino acids 245 and 265 of the bovine leukemia virus (BLV) tax protein restricts transactivation not only via the BLV enhancer but also via other retrovirus enhancers

Bovine leukemia virus (BLV) is associated with enzootic bovine leukosis and is closely related to human T-cell leukemia virus type 1 (HTLV-1). The Tax protein of BLV acts through the 5' long terminal repeat (LTR) of BLV and activates the transcription of BLV. In this study, we amplified tax genes from BLV-infected cattle using PCR. We cloned the genes and monitored the transcriptional activities of the products. Seven independent mutant Tax proteins, with at least one amino acid substitution between residues 240 and 265, exhibited a markedly stronger ability to stimulate the viral LTR-directed transcription than the wild-type Tax protein. Analysis of chimeric Tax proteins derived from wild-type and mutant Tax proteins clearly demonstrated that a single substitution between residue 240 and 265 might be critical for the higher activities of the Tax mutant proteins. Furthermore, it appeared that transient expression of a Tax mutant protein was better able to increase the production of viral proteins and particles from a defective recombinant proviral clone of BLV than was wild-type Tax. Analysis of mutations within the U3 region of the LTR revealed that a cyclic AMP-responsive element in Tax-responsive element 2 might be sufficient for the enhanced activation mediated by the mutant proteins. In addition to the LTR of BLV, other viral enhancers, such as the enhancers of HTLV-1 and of mouse mammary tumor virus, which cannot be activated by wild-type BLV Tax protein, were activated by a Tax mutant protein. Our observations suggest that the transactivation activity and target sequence specificity of BLV Tax might be limited or negatively regulated by the region of the protein between amino acids 240 and 265.

Discordance between bovine leukemia virus tax immortalization in vitro and oncogenicity in vivo

Bovine leukemia virus (BLV) Tax protein, a transcriptional activator of viral expression, is essential for viral replication in vivo. Tax is believed to be involved in leukemogenesis because of its second function, immortalization of primary cells in vitro. These activities of Tax can be dissociated on the basis of point mutations within specific regions of the protein. For example, mutation of the phosphorylation sites at serines 106 and 293 abrogates immortalization potential in vitro but maintains transcriptional activity. This type of mutant is thus particularly useful for unraveling the role of Tax immortalization activity during leukemogenesis independently of viral replication. In this report, we describe the biological properties of BLV recombinant proviruses mutated in the Tax phosphorylation sites (BLVTax106+293). Titration of the proviral loads by semiquantitative PCR revealed that the BLV mutants propagated at wild-type levels in vivo. Furthermore, two animals (sheep 480 and 296) infected with BLVTax106+293 developed leukemia or lymphosarcoma after 16 and 36 months, respectively. These periods of time are within the normal range of latencies preceding the onset of pathogenesis induced by wild-type viruses. The phenotype of the mutant-infected cells was characteristic of a B lymphocyte (immunoglobulin M positive) expressing CD11b and CD5 (except at the final stage for the latter marker), a pattern that is typical of wild-type virus-infected target cells. Interestingly, the transformed B lymphocytes from sheep 480 also coexpressed the CD8 marker, a phenotype rarely observed in tumor biopsies from chronic lymphocytic leukemia patients. Finally, direct sequencing of the tax gene demonstrated that the leukemic cells did not harbor revertant proviruses. We conclude that viruses expressing a Tax mutant unable to transform primary cells in culture are still pathogenic in the sheep animal model. Our data thus provide a clear example of the discordant conclusions that can be drawn from in vitro immortalization assays and in vivo experiments. These observations could be of interest for other systems, such as the related human T-cell leukemia virus type 1, which currently lack animal models allowing the study of the leukemogenic process.

Smoldering HTLV-1-induced T-cell lymphoma localized within the skin; a radiation-resistant tumor

BACKGROUND

Human T-cell lymphotrophic virus type 1 (HTLV-1)-induced lymphoproliferative disease occurs in approximately 3-5% of people in endemic areas who have been HTLV-1 positive for decades. Lymphoproliferative disease may present as four subtypes, including an acute adult T-cell leukemia/lymphoma (ATLL), an aggressive HTLV-1 lymphoma, chronic ATLL, and smoldering ATLL.

MATERIALS AND METHODS

A 72-year-old HTLV-1+ Haitian woman presented with a 2-year history of a cutaneous eruption localized to the right arm. The eruption had evolved into multinodular lesions over the past 6-7 months. Peripheral blood and cutaneous biopsy specimens were evaluated. Immunohistochemical studies for lymphoid markers were performed on the cutaneous biopsy material, and polymerase chain reaction (PCR) and Southern blot assay were evaluated for the presence and integration of HTLV-1 within the genome.

RESULTS

The biopsy specimen showed a pleomorphic T-cell infiltrate with epidermotrophism, and an immunohistochemical phenotype showing CD3+, CD4+, CD8-, CD25, CD30-, HLDA-DR+ cells. PCR and Southern blot assay evaluation showed a single clonal integration of HTLV-1 provirus within a monoclonal tumor cell population. The patient had no abnormal lymphoid forms on peripheral smear at presentation, and no evidence of other organ involvement.

CONCLUSIONS

Smoldering HTLV-1-induced lymphoma is uncommon even in endemic areas. In previously reported cases, the smoldering variant was accompanied by abnormal forms in the peripheral blood and/or by other signs of systemic disease. This case illustrates that smoldering disease may be localized to the skin with no detected morphologic abnormalities on peripheral smear.

The human T cell leukemia/lymphotropic virus type 1 Tax protein represses MyoD-dependent transcription by inhibiting MyoD-binding to the KIX domain of p300. A potential mechanism for Tax-mediated repression of the transcriptional activity of basic helix-loop-helix factors

The human T cell leukemia/lymphotropic virus type 1 (HTLV-1) Tax protein strongly activates viral and cellular gene transcription. It mainly functions by interacting with cellular transcription factors and the KIX domain of the p300/CBP coactivators. Tax can also repress the transcription of cellular genes through the basic helix-loop-helix (bHLH) protein family. To investigate the molecular mechanisms of this Tax-mediated inhibition, we analyzed its effect on the transcriptional activity of the myogenic MyoD protein, which was used as a paradigm of bHLH factors. In this study, we show that overexpression of the p300 coactivator in transient transfection assays was sufficient to rescue MyoD repression by Tax. Furthermore, an N-terminal domain of p300 (amino acids 379-654) containing the region of KIX serving as the Tax binding site was found, when overexpressed, to potentiate Tax-mediated transactivation of HTLV-1 proviral as well as MyoD-dependent transcription, and to antagonize the inhibition by Tax of the transcriptional activity of MyoD. These results revealing the presence of an N-terminal MyoD binding site were confirmed by in vitro protein-protein interaction assays that demonstrate that MyoD binds to the KIX domain of p300 and that Tax competes with MyoD binding in a nonreciprocal manner. These observations provide evidence that Tax binding to the KIX domain of p300 prevents bHLH proteins from contacting this N-terminal domain of the coactivator, thus resulting in their transcriptional repression. As bHLH proteins are implicated in many developmental fate decisions, especially during thymopoiesis, Tax-mediated inhibition of their transcriptional activity may contribute to the induction of HTLV-1-linked leukemogenesis.

Reovirus-induced apoptosis requires activation of transcription factor NF-kappaB

Reovirus infection induces apoptosis in cultured cells and in vivo. To identify host cell factors that mediate this response, we investigated whether reovirus infection alters the activation state of the transcription factor nuclear factor kappa B (NF-kappaB). As determined in electrophoretic mobility shift assays, reovirus infection of HeLa cells leads to nuclear translocation of NF-kappaB complexes containing Rel family members p50 and p65. Reovirus-induced activation of NF-kappaB DNA-binding activity correlated with the onset of NF-kappaB-directed transcription in reporter gene assays. Three independent lines of evidence indicate that this functional form of NF-kappaB is required for reovirus-induced apoptosis. First, treatment of reovirus-infected HeLa cells with a proteasome inhibitor prevents NF-kappaB activation following infection and substantially diminishes reovirus-induced apoptosis. Second, transient expression of a dominant-negative form of IkappaB that constitutively represses NF-kappaB activation significantly reduces levels of apoptosis triggered by reovirus infection. Third, mutant cell lines deficient for either the p50 or p65 subunits of NF-kappaB are resistant to reovirus-induced apoptosis compared with cells expressing an intact NF-kappaB signaling pathway. These findings indicate that NF-kappaB plays a significant role in the mechanism by which reovirus induces apoptosis in susceptible host cells.

Human T-cell leukemia virus type 1 Tax shuttles between functionally discrete subcellular targets

Human T-cell leukemia virus type 1 (HTLV-1) Tax is a nuclear protein with striking pleiotropic functionality. We recently demonstrated that Tax localizes to a multicomponent nuclear speckled structure (Tax speckled structure [TSS]). Here, we examine these structures further and identify a partial overlap of TSS with transcription hot spots. We used a strategy of directed expression via fusion proteins to determine if these transcription sites are the subtargets within TSS required for Tax function. When fused to human immunodeficiency virus type 1 (HIV-1) Tat, the resulting Tat-Tax fusion protein displayed neither a Tat-like nor a Tax-like pattern but rather was targeted specifically to the transcription subsites. The Tat-Tax fusion was able to activate both the HIV-1 long terminal repeat (LTR) and the HTVL-1 LTR at the same level as the individual component; thus, targeting proteins to transcription hot spots was compatible with both Tax and Tat transcription function. In contrast, the fusion with HIV-1 Rev, Rev-Tax, resulted in a pattern of expression that was largely Rev-like (nucleolar and cytoplasmic). The reduced localization of Rev-Tax to transcription sites was reflected in a 10-fold drop in activation of the HTLV-1 LTR. However, there was no loss in the ability of Tax to activate via NF-kappaB. Thus, NF-kappaB-dependent Tax function does not require targeting of Tax to these transcription sites and suggests that activation via NF-kappaB is a cytoplasmic function. Selective mutation of the nuclear localization signal site in the Rev portion resulted in retargeting of Rev-Tax to TSS and subsequent restoration of transcription function, demonstrating that inappropriate localization preceded loss of function. Mutation of the nuclear export signal site in the Rev portion had no effect on transcription, although the relative amount of Rev-Tax in the cytoplasm was reduced. Finally, in explaining how Tax can occupy multiple subcellular sites, we show that Tax shuttles from the nucleus to the cytoplasm in a heterokaryon fusion assay. Thus, pleiotropic functionality by Tax is regulatable via shuttling between discrete subcellular compartments.

Titration of cellular export factors, but not heteromultimerization, is the molecular mechanism of trans-dominant HTLV-1 rex mutants

The HTLV-1 Rex protein is an essential shuttle protein required for nuclear export of unspliced and incompletely-spliced viral RNAs. Several trans-dominant (TD) mutant Rex proteins have been reported, however, the mechanism of trans-dominance is not known. We compared TD Rex mutants and found that a natural occurring Rex mutant, Rexp21, lacking the RNA binding domain, was highly TD and inhibited also HIV-1 Rev function. Using fusions to the green fluorescent protein (GFP) we observed that Rexp21-GFP displayed a cytoplasmic localization but was actively shuttling between the nucleus and the cytoplasm in live human cells. The presence of Rexp21-GFP inhibited the nuclear export of Rex and HIV-1 Rev as assayed by cotransfection and microinjection experiments. However, Rex-GFP or Rexp21-GFP did not form heteromultimers with nuclear Rex mutants in vivo. In contrast, shuttling was essential for trans-dominance. Thus, we propose that TD Rex mutants do not function by retaining WT Rex in the nucleus by protein-protein interactions, as demonstrated for Rev, but to titrate factors essential for Rex/Rev export. Our findings demonstrate differences between the regulatory proteins Rex and Rev and implicate a novel strategy to generate highly TD Rex mutants also applicable to other proteins.

Gene regulation mediated by interaction between HTLV-1 promoter elements and transcription factors Tax and CREB

In this work we examine the role of three genetic control components in the regulation of HTLV-1 transcription: cyclic AMP-responsive element (CRE)-binding protein (CREB), the HTLV-1 trans-activator Tax, and the three Tax-responsive elements (TREs). We demonstrate that the in vivo efficiency of the HTLV-1 promoter basal expression in cell culture depends on the spacing between the three TRE elements, located at the HTLV-1 LTR (long terminal repeat), whereas the level of transcription activation mediated by Tax is affected by the number of TREs. In the presence of only one TRE, the enhancement of expression by Tax is affected by the distance between the single TRE and the transcription start site. Following CREB binding to the LTR, additional DNase I hypersensitive sites are generated in the region between the two distal TREs (I and II), while in the presence of Tax, such sites are generated also in the region between TREs II and III. Neither cooperative binding of CREB to the TREs nor preferential binding of CREB to a particular TRE was observed. Tax binding to the CREB/TRE complex does not change the DNase I protection pattern. Taken together, these results suggest that the basal CREB-mediated transcription is determined by the number and the position of the viral TREs relative to each other. Tax protein stabilizes the protein/DNA complex and suppresses the spacing limitations, probably by bridging between the CREB/TRE complexes and the basal initiation transcription complex.

In vivo rescue of a silent tax-deficient bovine leukemia virus from a tumor-derived ovine B-cell line by recombination with a retrovirally transduced wild-type tax gene

The lack of bovine leukemia virus (BLV) expression is a consistent finding in freshly isolated ovine tumor cells and in the B-cell lines derived from these tumors. In order to gain further insight into the mechanisms of BLV silencing in these tumors, we have used the YR2 B-cell line, which was derived from the leukemic cells of a BLV-infected sheep. This cell line contains a single, monoclonally integrated, silent provirus, which cannot be reactivated either by stimulation in vitro or by in vivo injection of the tumor cells or cloned proviral DNA in sheep. Sequence analysis of the tax gene from the YR2 cell line identified two G-to-A transitions (G7924 to A7924 and G8149 to A8149) that result in E-to-K amino acid changes at positions 228 and 303 in the Tax protein. Following retroviral vector-mediated transfer of a wild-type tax gene into YR2 cells, we showed that BLV mRNA, viral proteins, and virions were produced, demonstrating that the cellular factors required for virus expression were present in the original YR2 cell line. Injection of this transduced YR2 cell line in sheep led to the rescue of replication-competent BLV proviruses. The integrated competent proviruses exhibited unique chimeric tax genes, which arose from homologous recombination between the transduced wild-type tax and the YR2-derived tax sequences. Furthermore, in one of these functional recombinant proviruses, only the A8149-to-G8149 reversion was present, providing clear evidence that the defect underlying the silent phenotype in YR2 cells results from a single C-terminal E303-to-K303 amino acid substitution in the BLV Tax protein. Our observations suggest that a single strategically located mutation in tax provides a mechanism for BLV inactivation in B-cell tumors.

Selective Ablation of Human T-Cell Lymphotropic Virus Type 1 p12I Reduces Viral Infectivity In Vivo

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia and HTLV-1-associated myelopathy. Novel, yet conserved RNA transcripts encoded from open reading frames (ORFs) I and II of the viral pX region are expressed both in vitro and in infected individuals. The ORF I mRNA encodes the protein p12I, which has been shown to localize to cellular endomembranes, cooperate with bovine papillomavirus E5 in transformation, as well as bind to the IL-2 receptor β and γ chains and the H+ vacuolar ATPase. It is unknown what role p12I plays in the viral life cycle. Using an infectious molecular clone of HTLV-1 (ACH) and a derivative clone, ACH.p12I, which fails to produce the p12Imessage, we investigated the importance of p12I in infected primary cells and in a rabbit model of the infection. ACH.p12I was infectious in vitro as shown by viral passage in culture and no qualitative or quantitative differences were noted between ACH and ACH.p12I in posttransfection viral antigen production. However, in contrast to ACH, ACH.p12I failed to establish persistent infection in vivo as indicated by reduced anti-HTLV-1 antibody responses, failure to demonstrate viral p19 antigen production in peripheral blood mononuclear cell (PBMC) cultures, and only transient detection of provirus by polymerase chain reaction in PBMC from ACH.p12I-inoculated rabbits. These results are the first to show the essential role of HTLV-1 p12I in the establishment of persistent viral infection in vivo and suggest potential new targets in antiviral strategies to prevent HTLV-1 infection.

Selective Ablation of Human T-Cell Lymphotropic Virus Type 1 p12I Reduces Viral Infectivity In Vivo

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia and HTLV-1-associated myelopathy. Novel, yet conserved RNA transcripts encoded from open reading frames (ORFs) I and II of the viral pX region are expressed both in vitro and in infected individuals. The ORF I mRNA encodes the protein p12I, which has been shown to localize to cellular endomembranes, cooperate with bovine papillomavirus E5 in transformation, as well as bind to the IL-2 receptor β and γ chains and the H+ vacuolar ATPase. It is unknown what role p12I plays in the viral life cycle. Using an infectious molecular clone of HTLV-1 (ACH) and a derivative clone, ACH.p12I, which fails to produce the p12Imessage, we investigated the importance of p12I in infected primary cells and in a rabbit model of the infection. ACH.p12I was infectious in vitro as shown by viral passage in culture and no qualitative or quantitative differences were noted between ACH and ACH.p12I in posttransfection viral antigen production. However, in contrast to ACH, ACH.p12I failed to establish persistent infection in vivo as indicated by reduced anti-HTLV-1 antibody responses, failure to demonstrate viral p19 antigen production in peripheral blood mononuclear cell (PBMC) cultures, and only transient detection of provirus by polymerase chain reaction in PBMC from ACH.p12I-inoculated rabbits. These results are the first to show the essential role of HTLV-1 p12I in the establishment of persistent viral infection in vivo and suggest potential new targets in antiviral strategies to prevent HTLV-1 infection.

In vitro and in vivo oncogenic potential of bovine leukemia virus G4 protein

In addition to the genes involved in the structure of the viral particle, the bovine leukemia virus (BLV) genome contains a region called X which contains at least four genes. Among them, the tax and rex genes, respectively, are involved in transcriptional and posttranscriptional regulation of viral transcription. Two other genes, R3 and G4, were identified after cloning of the corresponding mRNAs from BLV-infected lymphocytes. Although the function of the two latter genes is still unknown, they appear to have important roles, since deletion of them restricts viral propagation in vivo. In order to assess the oncogenic potential of the R3 and G4 proteins, we first analyzed their ability to immortalize and/or transform primary rat embryo fibroblasts (Refs). In this assay, the G4 but not the R3 protein cooperated with the Ha-ras oncogene to induce tumors in nude mice. It thus appears that G4 exhibited oncogenic potential in vitro. To extend these observations in vivo, the pathology induced by recombinant viruses with mutations in G4 and in R3 and G4 was next evaluated with the sheep animal model. Viral propagation, as measured by semiquantitative PCR, appeared to be reduced when the R3 and G4 genes were deleted. These observations confirm and extend our previous data underlining the biological function of these genes. In addition, we present the results of a clinical survey that involves 39 sheep infected with six different BLV recombinants. Over a period of 40 months, 83% of the sheep infected with a wild-type virus developed leukemias and/or lymphosarcomas. In contrast, none out of 13 sheep infected with viruses with mutations in G4 or in R3 and G4 developed disease. We conclude that in addition to its oncogenic potential in vitro, G4 is required for pathogenesis in vivo. These observations should help us gain insight into the process of leukemogenesis induced by the related human T-cell leukemia virus type 1.

HTLV-1 and Adult T-Cell Leukemia/Lymphoma: A Review

Infection with the human T-lymphotropic virus type 1 (HTLV-1) has been shown to be fundamental to the etiology of Adult T-cell Leukemia/Lymphoma (ATL). The disease is endemic in specific geographic areas but is increasingly reported from non-endemic regions. With increasing number of patients with this entity, the diversity in the clinical features has become apparent. In the past treatment strategies using combination chemotherapy have been unsatisfactory, but more recent trials using adenosine analouges, interferons, and combination of interferons and AZT have shown promise. With increased understanding of the etiology and molecular basis of the disease more effective therapies can be anticipated.

Human TAF(II)28 interacts with the human T cell leukemia virus type I Tax transactivator and promotes its transcriptional activity

The Tax protein encoded by human T cell leukemia virus type I transactivates the viral promoter by forming a complex with several cellular factors bound to three repeats of a specific upstream regulatory sequence. We have shown that transactivation by Tax was correlated with its ability to interact with the C-terminal moiety of the TATA box-binding protein (TBP). In the present study, the ability of Tax to interact with several human TBP-associated factors (TAF(II)s) was analyzed. We show that Tax interacts selectively with hTAF(II)28 in transfected HeLa cells. A direct interaction between Tax and hTAF(II)28 was also observed in vitro with purified proteins. In transient expression studies we show that overexpression of hTAF(II)28 significantly increased transactivation by Tax, both in the absence and in the presence of overexpressed TBP. The ability of hTAF(II)28 to potentiate transactivation correlated with the ability of Tax to interact with hTAF(II)28 and also with the ability of hTAF(II)28 to interact with TBP. Coexpression of TBP and hTAF(II)28 resulted in an additive increase in transactivation by Tax. From these observations we propose that transcriptional activation by Tax involves multiple interactions with TFIID via its TBP and hTAF(II)28 subunits.