In vitro cellular tropism of human T cell leukemia virus type 2

Human T-Cell Leukemia Virus Type 1 Envelope Protein: Post-Entry Roles in Viral Pathogenesis

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that is the causative infectious agent of adult T-cell leukemia/lymphoma (ATL), an aggressive and fatal CD4⁺ T-cell malignancy, and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic neurological disease. Disease progression in infected individuals is the result of HTLV-1-driven clonal expansion of CD4⁺ T-cells and is generally associated with the activities of the viral oncoproteins Tax and Hbz. A closely related virus, HTLV-2, exhibits similar genomic features and the capacity to transform T-cells, but is non-pathogenic. In vitro, HTLV-1 primarily immortalizes or transforms CD4⁺ T-cells, while HTLV-2 displays a transformation tropism for CD8⁺ T-cells. This distinct tropism is recapitulated in infected people. Through comparative studies, the genetic determinant for this divergent tropism of HTLV-1/2 has been mapped to the viral envelope (Env). In this review, we explore the emerging roles for Env beyond initial viral entry and examine current perspectives on its contributions to HTLV-1-mediated disease development.

Use of Precision-Cut Lung Slices as an Ex Vivo Tool for Evaluating Viruses and Viral Vectors for Gene and Oncolytic Therapy

Organotypic slice cultures recapitulate many features of an intact organ, including cellular architecture, microenvironment, and polarity, making them an ideal tool for the ex vivo study of viruses and viral vectors. Here, we describe a procedure for generating precision-cut ovine and murine tissue slices from agarose-perfused normal and murine melanoma tumor-bearing lungs. Furthermore, we demonstrate that these precision-cut lung slices can be maintained up to 1 month and can be used for a range of applications, which include characterizing the tissue tropism of viruses that cannot be propagated in cell monolayers, evaluating the transducing properties of gene therapy vectors, and, finally, investigating the tumor specificity of oncolytic viruses. Our results suggest that ex vivo lung slices are an ideal platform for studying the tissue specificity and cancer cell selectivity of gene therapy vectors and oncolytic viruses prior to in vivo studies, providing justification for pre-clinical work.

Stability of HTLV-2 antisense protein is controlled by PML nuclear bodies in a SUMO-dependent manner

Since the identification of the antisense protein of HTLV-2 (APH-2) and the demonstration that APH-2 mRNA is expressed in vivo in most HTLV-2 carriers, much effort has been dedicated to the elucidation of similarities and/or differences between APH-2 and HBZ, the antisense protein of HTLV-1. Similar to HBZ, APH-2 negatively regulates HTLV-2 transcription. However, it does not promote cell proliferation. In contrast to HBZ, APH-2 half-life is very short. Here, we show that APH-2 is addressed to PML nuclear bodies in T-cells, as well as in different cell types. Covalent SUMOylation of APH-2 is readily detected, indicating that APH-2 might be addressed to the PML nuclear bodies in a SUMO-dependent manner. We further show that silencing of PML increases expression of APH-2, while expression of HBZ is unaffected. On the other hand, SUMO-1 overexpression leads to a specific loss of APH-2 expression that is restored upon proteasome inhibition. Furthermore, the carboxy-terminal LAGLL motif of APH-2 is responsible for both the targeting of the protein to PML nuclear bodies and its short half-life. Taken together, these observations indicate that natural APH-2 targeting to PML nuclear bodies induces proteasomal degradation of the viral protein in a SUMO-dependent manner. Hence, this study deciphers the molecular and cellular bases of APH-2 short half-life in comparison to HBZ and highlights key differences in the post-translational mechanisms that control the expression of both proteins.

A case of peripheral T-cell lymphoma, not otherwise specified in a HCV and HTLV-II-positive patient, diagnosed by abdominal fluid cytology

BACKGROUND

Peripheral T-cell lymphoma, not otherwise specified (PTCL, NOS) is a rare neoplasm that typically presents as generalized lymphadenopathy. PTCL, NOS presenting as malignant ascites is rare.

METHODS

A 61-year-old African-American man with past medical history of HCV, cryoglobulinemia, and cryptococcal pneumonia was admitted for dyspnea on exertion over a period of 1 month and new onset of abdominal distension.

RESULTS

Ascites, splenomegaly, hepatomegaly and extensive lymphadenopathy were found by imaging. Paracentesis obtained 1.3 liter of abdominal fluid, the cytologic evaluation showed a monomorphic population of intermediate-sized lymphoid cells with irregular to convoluted nuclear contours. Fluid sent for flow cytometry showed an abnormal T-lymphocyte population expressing CD4, weak surface CD3 and absence of CD7. PCR studies of ascitic fluid detected a clonal T-lymphocyte population with T-cell receptor gamma gene rearrangement. Serologic testing for human T lymphotropic virus (HTLV) was positive for HTLV-II. Subsequent bone marrow biopsy revealed lymphomatous involvement. CD30 and ALK-1 immunostaining were negative. This case was classified as PTCL, NOS.

CONCLUSIONS

PTCL, NOS can have unusual clinical presentation such as ascites and pleural effusion, and may also occur as a complication of immunodeficiency state. Further studies are needed to determine if HCV or HTLV-II viral infection is associated with PTCL.

Clonality of HTLV-2 in natural infection

Human T-lymphotropic virus type 1 (HTLV-1) and type 2 (HTLV-2) both cause lifelong persistent infections, but differ in their clinical outcomes. HTLV-1 infection causes a chronic or acute T-lymphocytic malignancy in up to 5% of infected individuals whereas HTLV-2 has not been unequivocally linked to a T-cell malignancy. Virus-driven clonal proliferation of infected cells both in vitro and in vivo has been demonstrated in HTLV-1 infection. However, T-cell clonality in HTLV-2 infection has not been rigorously characterized. In this study we used a high-throughput approach in conjunction with flow cytometric sorting to identify and quantify HTLV-2-infected T-cell clones in 28 individuals with natural infection. We show that while genome-wide integration site preferences in vivo were similar to those found in HTLV-1 infection, expansion of HTLV-2-infected clones did not demonstrate the same significant association with the genomic environment of the integrated provirus. The proviral load in HTLV-2 is almost confined to CD8⁺ T-cells and is composed of a small number of often highly expanded clones. The HTLV-2 load correlated significantly with the degree of dispersion of the clone frequency distribution, which was highly stable over ∼8 years. These results suggest that there are significant differences in the selection forces that control the clonal expansion of virus-infected cells in HTLV-1 and HTLV-2 infection. In addition, our data demonstrate that strong virus-driven proliferation per se does not predispose to malignant transformation in oncoretroviral infections.

The two human retroviruses HTLV-1 and HTLV-2 are similar in their structure, replication cycle and the manner through which they spread between and within individuals. They differ in their preferred host T-cell type and in their possible clinical outcomes. HTLV-2 has not been linked with a specific disease, whereas HTLV-1 infection can cause leukemia and profound neuropathology. It is well established that HTLV-1-infected cells undergo clonal expansion in infected individuals, but little is known about clonality in HTLV-2 infection. In this work, we demonstrate that the extent of HTLV-2-infected cell expansion significantly exceeds that of HTLV-1-infected cells in healthy carriers, approximating instead to that observed in patients with HTLV-1-associated leukemia. Furthermore, we show that HTLV-2 characteristically resides in a small number of expanded clones that persist over time, and that the degree of oligoclonality significantly correlates with viral burden in HTLV-2-infected individuals. These results highlight the distinction between in vivo clonal proliferation and malignant transformation, and suggest that the infected cell type may be a more important determinant of clinical outcome in retroviral infections.

Human T lymphotropic virus type 1 SU residue 195 plays a role in determining the preferential CD4+ T cell immortalization/transformation tropism

Human T lymphotropic virus type 1 (HTLV-1) mainly causes adult T cell leukemia and predominantly immortalizes/transforms CD4(+) T cells in culture. HTLV-2 is aleukemic and predominantly immortalizes/transforms CD8(+) T cells in culture. We have shown previously that the viral envelope is the genetic determinant of the differential T cell tropism in culture. The surface component (SU) of the HTLV-1 envelope is responsible for binding to the cellular receptors for entry. Here, we dissect the HTLV-1 SU further to identify key domains that are involved in determining the immortalization tropism. We generated HTLV-1 envelope recombinant virus containing the HTLV-2 SU domain. HTLV-1/SU2 was capable of infecting and immortalizing freshly isolated peripheral blood mononuclear cells in culture. HTLV-1/SU2 shifted the CD4(+) T cell immortalization tropism of wild-type HTLV-1 (wtHTLV-1) to a CD8(+) T cell preference. Furthermore, a single amino acid substitution, N195D, in HTLV-1 SU (Ach.195) resulted in a shift to a CD8(+) T cell immortalization tropism preference. Longitudinal phenotyping analyses of the in vitro transformation process revealed that CD4(+) T cells emerged as the predominant population by week 5 in wtHTLV-1 cultures, while CD8(+) T cells emerged as the predominant population by weeks 4 and 7 in wtHTLV-2 and Ach.195 cultures, respectively. Our results indicate that SU domain independently influences the preferential T cell immortalization tropism irrespective of the envelope counterpart transmembrane (TM) domain. We further showed that asparagine at position 195 in HTLV-1 SU is involved in determining this CD4(+) T cell immortalization tropism. The slower emergence of the CD8(+) T cell predominance in Ach.195-infected cultures suggests that other residues/domains contribute to this tropism preference.

HTLV-2 Tax immortalizes human CD4+ memory T lymphocytes by oncogenic activation and dysregulation of autophagy

Human T cell leukemia virus type 1 and type 2 (HTLV-1 and -2) are two closely related retroviruses with the former causing adult T cell leukemia. HTLV-2 infection is prevalent among intravenous drug users, and the viral genome encodes the viral transactivator Tax, which is highly homologous to the transforming protein Tax from HTLV-1. However, the link between HTLV-2 infection and leukemia has not been established. In the present study, we evaluated the activity of HTLV-2 Tax in promoting aberrant proliferation of human CD4 T lymphocytes. Tax2 efficiently immortalized CD4(+) memory T lymphocytes with a CD3/TCRαβ/CD4/CD25/CD45RO/CD69 immunophenotype, promoted constitutive activation of PI3K/Akt, IκB kinase/NF-κB, mitogen-activated protein kinase, and STAT3, and it also increased the level of Mcl-1. Disruption of these oncogenic pathways led to growth retardation and apoptotic cell death of the Tax2-established T cell lines. We further found that Tax2 induced autophagy by interacting with the autophagy molecule complex containing Beclin1 and PI3K class III to form the LC3(+) autophagosome. Tax2-mediated autophagy promoted survival and proliferation of the immortalized T cells. The present study demonstrated the oncogenic properties of Tax2 in human T cells and also implicated Tax2 in serving as a molecular tool to generate distinct T cell subtype lines.

Comparative host protein interactions with HTLV-1 p30 and HTLV-2 p28: insights into difference in pathobiology of human retroviruses

Background

Human T lymphotropic virus type-1 (HTLV-1) and type 2 (HTLV-2) are closely related human retroviruses, but have unique disease associations. HTLV-1 is the causative agent of an aggressive T-cell leukemia known as adult T-cell leukemia (ATL), HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other inflammatory diseases. HTLV-2 infection has not been clearly associated with any disease condition. Although both viruses can transform T cells in vitro, the HTLV-1 provirus is mainly detected in CD4+ T cells whereas HTLV-2 is mainly detected in CD8+ T cells of infected individuals. HTLV-1 and HTLV-2 encode accessory proteins p30 and p28, respectively, which share partial amino acid homology and are required for viral persistence in vivo. The goal of this study was to identify host proteins interacting with p30 and p28 in order to understand their role in pathogenesis.

Results

Affinity-tag purification coupled with mass spectrometric (MS) analyses revealed 42 and 22 potential interacting cellular partners of p30 and p28, respectively. Of these, only three cellular proteins, protein arginine methyltransferase 5 (PRMT5), hnRNP K and 60 S ribosomal protein L8 were detected in both p30 and p28 fractions. To validate the proteomic results, four interacting proteins were selected for further analyses using immunoblot assays. In full agreement with the MS analysis two cellular proteins REGγ and NEAF-interacting protein 30 (NIP30) selectively interacted with p30 and not with p28; heterogeneous nuclear ribonucleoprotein H1 (hnRNP H1) bound to p28 and not to p30; and PRMT5 interacted with both p30 and p28. Further studies demonstrated that reduced levels of PRMT5 resulted in decreased HTLV-2 viral gene expression whereas the viral gene expression of HTLV-1 was unchanged.

Conclusion

The comparisons of p30 and p28 host protein interaction proteome showed striking differences with some degree of overlap. PRMT5, one of the host proteins that interacted with both p30 and p28 differentially affected HTLV-1 and HTLV-2 viral gene expression suggesting that PRMT5 is involved at different stages of HTLV-1 and HTLV-2 biology. These findings suggest that distinct host protein interaction profiles of p30 and p28 could, in part, be responsible for differences in HTLV-1 and HTLV-2 pathobiology. This study provides new avenues of investigation into mechanisms of viral infection, tropism and persistence.

Distinct transformation tropism exhibited by human T lymphotropic virus type 1 (HTLV-1) and HTLV-2 is the result of postinfection T cell clonal expansion

Human T lymphotropic virus type 1 (HTLV-1) and HTLV-2 are related but pathogenically distinct viruses. HTLV-1 mainly causes adult T cell leukemia, while HTLV-2 is not associated with leukemia. In vitro, HTLV-1 and HTLV-2 predominantly transform CD4(+) and CD8(+) T cells, respectively: the genetic determinant maps to the viral envelope. Herein, we investigate whether this transformation tropism occurs during initial infection or subsequently during the cellular transformation process. Since most individuals are chronically infected at the time of detection, we utilized an established rabbit model to longitudinally measure the early HTLV-1 and HTLV-2 infection and replication kinetics in purified CD4(+) and CD8(+) T cells. HTLV-1 and HTLV-2 were detected in both CD4(+) and CD8(+) T cells within 1 week postinoculation. In HTLV-1-infected rabbit CD4(+) T cells, proviral burden and tax/rex mRNA expression peaked early, and expression levels were directly proportional to each other. The late expression of the antisense transcript (Hbz or Aph-2) correlated directly with a late proviral burden peak in HTLV-1- or HTLV-2-infected rabbit CD8(+) T cells, respectively. This study provides the first in vivo evidence that these viruses do not exhibit cellular preference during initial infection. We further evaluated the transformation tropism of HTLV-1 and HTLV-2 over a 9-week period using in vitro cell growth/immortalization assays. At the early weeks, both HTLV-1 and HTLV-2 showed proportionate growth of CD4(+) and CD8(+) T cells. However, beyond week 5, the predominance of one particular T cell type emerged, supporting the conclusion that transformation tropism is a postinfection event due to selective clonal expansion over time.

Human T Lymphotropic Virus Type 1 (HTLV-1): Molecular Biology and Oncogenesis

Human T lymphotropic viruses (HTLVs) are complex deltaretroviruses that do not contain a proto-oncogene in their genome, yet are capable of transforming primary T lymphocytes both in vitro and in vivo. There are four known strains of HTLV including HTLV type 1 (HTLV-1), HTLV-2, HTLV-3 and HTLV-4. HTLV-1 is primarily associated with adult T cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-2 is rarely pathogenic and is sporadically associated with neurological disorders. There have been no diseases associated with HTLV-3 or HTLV-4 to date. Due to the difference in the disease manifestation between HTLV-1 and HTLV-2, a clear understanding of their individual pathobiologies and the role of various viral proteins in transformation should provide insights into better prognosis and prevention strategies. In this review, we aim to summarize the data accumulated so far in the transformation and pathogenesis of HTLV-1, focusing on the viral Tax and HBZ and citing appropriate comparisons to HTLV-2.

Word add-in for ontology recognition: semantic enrichment of scientific literature

BACKGROUND

In the current era of scientific research, efficient communication of information is paramount. As such, the nature of scholarly and scientific communication is changing; cyberinfrastructure is now absolutely necessary and new media are allowing information and knowledge to be more interactive and immediate. One approach to making knowledge more accessible is the addition of machine-readable semantic data to scholarly articles.

RESULTS

The Word add-in presented here will assist authors in this effort by automatically recognizing and highlighting words or phrases that are likely information-rich, allowing authors to associate semantic data with those words or phrases, and to embed that data in the document as XML. The add-in and source code are publicly available at http://www.codeplex.com/UCSDBioLit.

CONCLUSIONS

The Word add-in for ontology term recognition makes it possible for an author to add semantic data to a document as it is being written and it encodes these data using XML tags that are effectively a standard in life sciences literature. Allowing authors to mark-up their own work will help increase the amount and quality of machine-readable literature metadata.

Cell polarity proteins: common targets for tumorigenic human viruses

Loss of polarity and disruption of cell junctions are common features of epithelial-derived cancer cells, and mounting evidence indicates that such defects have a direct function in the pathology of cancer. Supporting this idea, results with several different human tumor viruses indicate that their oncogenic potential depends in part on a common ability to inactivate key cell polarity proteins. For example, adenovirus (Ad) type 9 is unique among human Ads by causing exclusively estrogen-dependent mammary tumors in experimental animals and in having E4 region-encoded open reading frame 1 (E4-ORF1) as its primary oncogenic determinant. The 125-residue E4-ORF1 protein consists of two separate protein-interaction elements, one of which defines a PDZ domain-binding motif (PBM) required for E4-ORF1 to induce both cellular transformation in vitro and tumorigenesis in vivo. Most notably, the E4-ORF1 PBM mediates interactions with a selected group of cellular PDZ proteins, three of which include the cell polarity proteins Dlg1, PATJ and ZO-2. Data further indicate that these interactions promote disruption of cell junctions and a loss of cell polarity. In addition, one or more of the E4-ORF1-interacting cell polarity proteins, as well as the cell polarity protein Scribble, are common targets for the high-risk human papillomavirus (HPV) E6 or human T-cell leukemia virus type 1 (HTLV-1) Tax oncoproteins. Underscoring the significance of these observations, in humans, high-risk HPV and HTLV-1 are causative agents for cervical cancer and adult T-cell leukemia, respectively. Consequently, human tumor viruses should serve as powerful tools for deciphering mechanisms whereby disruption of cell junctions and loss of cell polarity contribute to the development of many human cancers. This review article discusses evidence supporting this hypothesis, with an emphasis on the human Ad E4-ORF1 oncoprotein.

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 use different receptor complexes to enter T cells

Studies using adherent cell lines have shown that glucose transporter-1 (GLUT-1) can function as a receptor for human T-cell leukemia virus type 1 (HTLV). In primary CD4(+) T cells, heparan sulfate proteoglycans (HSPGs) are required for efficient entry of HTLV-1. Here, the roles of HSPGs and GLUT-1 in HTLV-1 and HTLV-2 Env-mediated binding and entry into primary T cells were studied. Examination of the cell surface of activated primary T cells revealed that CD4(+) T cells, the primary target of HTLV-1, expressed significantly higher levels of HSPGs than CD8(+) T cells. Conversely, CD8(+) T cells, the primary target of HTLV-2, expressed GLUT-1 at dramatically higher levels than CD4(+) T cells. Under these conditions, the HTLV-2 surface glycoprotein (SU) binding and viral entry were markedly higher on CD8(+) T cells while HTLV-1 SU binding and viral entry were higher on CD4(+) T cells. Binding studies with HTLV-1/HTLV-2 SU recombinants showed that preferential binding to CD4(+) T cells expressing high levels of HSPGs mapped to the C-terminal portion of SU. Transfection studies revealed that overexpression of GLUT-1 in CD4(+) T cells increased HTLV-2 entry, while expression of HSPGs on CD8(+) T cells increased entry of HTLV-1. These studies demonstrate that HTLV-1 and HTLV-2 differ in their T-cell entry requirements and suggest that the differences in the in vitro cellular tropism for transformation and in vivo pathobiology of these viruses reflect different interactions between their Env proteins and molecules on CD4(+) and CD8(+) T cells involved in entry.

Inhibition of human T cell leukemia virus type 2 replication by the suppressive action of class II transactivator and nuclear factor Y

The master regulator of MHC-II gene transcription, class II transactivator (CIITA), acts as a potent inhibitor of human T cell leukemia virus type 2 (HTLV-2) replication by blocking the activity of the viral Tax-2 transactivator. Here, we show that this inhibitory effect takes place at the nuclear level and maps to the N-terminal 1-321 region of CIITA, where we identified a minimal domain, from positions 64-144, that is strictly required to suppress Tax-2 function. Furthermore, we show that Tax-2 specifically cooperates with cAMP response element binding protein-binding protein (CBP) and p300, but not with p300/CBP-associated factor, to enhance transcription from the viral promoter. This finding represents a unique difference with respect to Tax-1, which uses all three coactivators to transactivate the human T cell leukemia virus type 1 LTR. Direct sequestering of CBP or p300 is not the primary mechanism by which CIITA causes suppression of Tax-2. Interestingly, we found that the transcription factor nuclear factor Y, which interacts with CIITA to increase transcription of MHC-II genes, exerts a negative regulatory action on the Tax-2-mediated HTLV-2 LTR transactivation. Thus, CIITA may inhibit Tax-2 function, at least in part, through nuclear factor Y. These findings demonstrate the dual defensive role of CIITA against pathogens: it increases the antigen-presenting function for viral determinants and suppresses HTLV-2 replication in infected cells.

Envelope is a major viral determinant of the distinct in vitro cellular transformation tropism of human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are related deltaretroviruses but are distinct in their disease-inducing capacity. These viruses can infect a variety of cell types, but only T lymphocytes become transformed, which is defined in vitro as showing indefinite interleukin-2-independent growth. Studies have indicated that HTLV-1 has a preferential tropism for CD4+ T cells in vivo and is associated with the development of leukemia and neurological disease. Conversely, the in vivo T-cell tropism of HTLV-2 is less clear, although it appears that CD8+ T cells preferentially harbor the provirus, with only a few cases of disease association. The difference in T-cell transformation tropism has been confirmed in vitro as shown by the preferential transformation of CD4+ T cells by HTLV-1 versus the transformation of CD8+ T cells by HTLV-2. Our previous studies showed that Tax and overlapping Rex do not confer the distinct T-cell transformation tropisms between HTLV-1 and HTLV-2. Therefore, for this study HTLV-1 and HTLV-2 recombinants were generated to assess the contribution of LTR and env sequences in T-cell transformation tropism. Both sets of proviral recombinants expressed p19 Gag following transfection into cells. Furthermore, recombinant viruses were replication competent and had the capacity to transform T lymphocytes. Our data showed that exchange of the env gene resulted in altered T-cell transformation tropism compared to wild-type virus, while exchange of long terminal repeat sequences had no significant effect. HTLV-2/Env1 preferentially transformed CD4+ T cells similarly to wild-type HTLV-1 (wtHTLV-1), whereas HTLV-1/Env2 had a transformation tropism similar to that of wtHTLV-2 (CD8+ T cells). These results indicate that env is a major viral determinant for HTLV T-cell transformation tropism in vitro and provides strong evidence implicating its contribution to the distinct pathogenesis resulting from HTLV-1 versus HTLV-2 infections.

Comparative biology of human T-cell lymphotropic virus type 1 (HTLV-1) and HTLV-2

HTLV-1 and HTLV-2 are highly related complex retroviruses that have been studied intensely for nearly three decades because of their association with neoplasia, neuropathology, and/or their capacity to transform primary human T lymphocytes. The study of HTLV also represents an attractive model that has allowed investigators to dissect the mechanism of various cellular processes, several of which may be critical steps in HTLV-mediated pathogenesis. Both HTLV-1 and HTLV-2 can efficiently immortalize and transform T lymphocytes in cell culture and persist in infected individuals or experimental animals. However, the clinical manifestations of these two viruses differ significantly. HTLV-1 is associated with adult T-cell leukemia (ATL) and a variety of immune-mediated disorders including the chronic neurological disease termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). In contrast, HTLV-2 is much less pathogenic with reports of only a few cases of variant hairy cell leukemia and neurological disease associated with infection. The limited number of individuals shown to harbor HTLV-2 in association with specific diseases has, to date, precluded convincing epidemiological demonstration of a definitive etiologic role of HTLV-2 in human disease. Therefore, it has become clear that comparative studies designed to elucidate the mechanisms by which HTLV-1 and HTLV-2 determine distinct outcomes are likely to provide fundamental insights into the initiation of multistep leukemogenesis.

Deregulation of cell-signaling pathways in HTLV-1 infection

Human T-lymphotropic virus type 1 (HTLV-1) infection is associated with the clonal expansion and transformation of mature T lymphocytes. While the mechanisms involved are incompletely understood the viral regulatory protein Tax plays a central role in these processes. Recent studies employing genomic and proteomic approaches have demonstrated the marked complexity of gene deregulation associated with Tax expression and confirmed the remarkable pleiotropism of this protein as evidenced by the numerous Tax-cellular protein interactions in infected cells. In this review, we summarize the role of Tax in the deregulation of selected cellular-signaling pathways. Specifically, this has focused on the influence and interaction of Tax with the AP-1 and NF-AT transcription factors, PDZ domain-containing proteins, Rho-GTPases, and the Janus kinase/signal transducer and activator of transcription and transforming growth factor-beta-signaling pathways. In addition to identifying the deregulation of events within these pathways, attempts have been made to highlight differences between HTLV-1 and -2, which may relate to differences in their pathogenic properties.

The human T-cell leukemia virus Rex protein

A critical step in the life cycle of complex retroviruses, including HTLV-1 and HTLV-2 is the ability of these viruses to adopt a mechanism by which the genome-length unspliced mRNA as well as the partially spliced mRNAs are exported from the nucleus instead of being subjected to splicing or degradation. In HTLV, this is accomplished through the expression of the viral Rex, which recognizes a specific response element on the incompletely spliced mRNAs, stabilizes them, inhibits their splicing, and utilizes the CRM1-dependent cellular pathway for transporting them from the nucleus to the cytoplasm. Rex itself is regulated by phosphorylation, which implies that proper activation of the protein in response to certain cellular cues is an important tool for the virus to ensure that specific viral gene expression is allowed only when the host cell can provide the best conditions for virion production. Having such a critical role in HTLV life cycle, Rex is indispensable for efficient viral replication, infection and spread. Indeed, Rex is considered to regulate the switch between the latent and productive phases of the HTLV life cycle. Without a functional Rex, the virus would still produce regulatory and some accessory gene products; however, structural and enzymatic post-transcriptional gene expression would be severely repressed, essentially leading to non-productive viral replication. More detailed understanding of the exact molecular mechanism of action of Rex will thus allow for better design of therapeutic drugs against Rex function and ultimately HTLV replication. Herein we summarize the progress made towards understanding Rex function and its role in the HTLV life cycle.

HTLV-1 and -2 envelope SU subdomains and critical determinants in receptor binding

Background

Human T-cell leukemia virus (HTLV) -1 and -2 are deltaretroviruses that infect a wide range of cells. Glut1, the major vertebrate glucose transporter, has been shown to be the HTLV Env receptor. While it is well established that the extracellular surface component (SU) of the HTLV envelope glycoprotein (Env) harbors all of the determinants of interaction with the receptor, identification of SU subdomains that are necessary and sufficient for interaction with the receptor, as well as critical amino acids therein, remain to be precisely defined. Although highly divergent in the rest of their genomes, HTLV and murine leukemia virus (MLV) Env appear to be related and based on homologous motifs between the HTLV and MLV SU, we derived chimeric HTLV/MLV Env and soluble HTLV-1 and -2 truncated amino terminal SU subdomains.

Results

Using these SU constructs, we found that the 183 and 178 amino terminal residues of the HTLV-1 and -2 Env, respectively, were sufficient to efficiently bind target cells of different species. Binding resulted from bona fide interaction with the HTLV receptor as isolated SU subdomains specifically interfered with HTLV Env-mediated binding, cell fusion, and cell-free as well as cell-to-cell infection. Therefore, the HTLV receptor-binding domain (RBD) lies in the amino terminus of the SU, immediately upstream of a central immunodominant proline rich region (Env residues 180 to 205), that we show to be dispensible for receptor-binding and interference. Moreover, we identified a highly conserved tyrosine residue at position 114 of HTLV-1 Env, Tyr₁₁₄, as critical for receptor-binding and subsequent interference to cell-to-cell fusion and infection. Finally, we observed that residues in the vicinity of Tyr₁₁₄ have lesser impact on receptor binding and had various efficiency in interference to post-binding events.

Conclusions

The first 160 residues of the HTLV-1 and -2 mature cleaved SU fold as autonomous domains that contain all the determinants required for binding the HTLV receptor.

A 10-amino acid domain within human T-cell leukemia virus type 1 and type 2 tax protein sequences is responsible for their divergent subcellular distribution

Human T-cell leukemia virus type 1 and type 2 (HTLV-1/2) are related retroviruses that infect T-lymphocytes. Whereas HTLV-1 infection can cause leukemia, HTLV-2 has not been demonstrated to be the agent of a hematological malignant disease. Nevertheless, the virally encoded Tax-1 and Tax-2 transactivators display a high percentage of similarity. Tax-1 is a shuttling protein that contains a noncanonical nuclear localization signal as well as a nuclear export signal. The presence of the nuclear localization signal and the nuclear export signal domains in the Tax-2 sequence has not been determined. The distribution of Tax-2 in infected cells is not known but has been assumed to be similar to that of Tax-1. By using a Tax-2-specific antibody, we report here that Tax-2 is located predominantly in the cytoplasm of the HTLV-2 immortalized or transformed infected T-cells. These results were confirmed after transient transfection of untagged Tax-1 and Tax-2 constructs, histidine tag Tax1/Tax2, GFP-Tax, and Tax-GFP fusion constructs in several cell lines. We show that this unanticipated localization is not due to a default in the Tax-2 nuclear localization signal functions nor to major differences in Tax-2 versus Tax-1 binding to the IKKgamma/NEMO protein. In addition, we demonstrate that inhibiting the proteasome results in a relocalization of Tax-1 in the cytoplasm, similar to that of Tax-2. By using a series of Tax-1/Tax-2 chimeras, we determined that the minimal domain that is necessary for Tax-2 peculiar distribution encompasses amino acids 90-100. Finally, we show a high correlation between intracellular localization of Tax and their NF-kappaB or CREB transactivating ability.

Utilization of the CBP but not the p300 co-activator by human T-lymphotropic virus type-2 Tax for p53 inhibition

We previously reported, both in transfected cells and in human T-cell leukemia virus type-2 subtype B infected cells, that the viral transactivator Tax-2B protein could inhibit p53 functions. We have now investigated the mechanism through which Tax-2B represses p53 using GFPTax-2B fusion proteins. We present evidence that Tax-2B inhibition of p53 function is not linked to CREB/ATF activation, but is uniquely correlated with the interaction of CREB binding protein (CBP), but not p300, with the C-terminus of Tax-2B. Wild type, but not a Tax-2B-M47 mutant, inhibits p53 function in adherent cells. We demonstrate that both Tax-2B and Tax-2B-M47 can bind p300, while Tax-2B-M47 is impaired for CBP binding. Importantly, transfection of increasing amounts of CBP but not p300 or p300/CBP-associated factor (P/CAF) could rescue p53 transcriptional activity in the presence of Tax-2B in nonlymphocytic cells. In lymphoid cells, Tax-2B mediated inhibition of p53 is correlated with the NF-kappaB pathway activation and could be prevented by the overexpression of an IkappaBalpha mutant. Given the similarities between the functional domains of CBP and p300, these results are intriguing and suggest that Tax-2B must bind the CR2 domain of CBP, but not that of p300 in order to repress p53.

Tax and overlapping rex sequences do not confer the distinct transformation tropisms of human T-cell leukemia virus types 1 and 2

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are distinct oncogenic retroviruses that infect several cell types but display their biological and pathogenic activity only in T cells. Previous studies have indicated that in vivo HTLV-1 has a preferential tropism for CD4+ T cells, whereas HTLV-2 in vivo tropism is less clear but appears to favor CD8+ T cells. Both CD4+ and CD8+ T cells are susceptible to HTLV-1 and HTLV-2 infection in vitro, and HTLV-1 has a preferential immortalization and transformation tropism of CD4+ T cells, whereas HTLV-2 immortalizes and transforms primarily CD8+ T cells. The molecular mechanism that determines this tropism of HTLV-1 and HTLV-2 has not been determined. HTLV-1 and HTLV-2 carry the tax and rex transregulatory genes in separate but partially overlapping reading frames. Since Tax has been shown to be critical for cellular transformation in vitro and interacts with numerous cellular processes, we hypothesized that the viral determinant of transformation tropism is encoded by tax. Using molecular clones of HTLV-1 (Ach) and HTLV-2 (pH6neo), we constructed recombinants in which tax and overlapping rex genes of the two viruses were exchanged. p19 Gag expression from proviral clones transfected into 293T cells indicated that both recombinants contained functional Tax and Rex but with significantly altered activity compared to the wild-type clones. Stable transfectants expressing recombinant viruses were established, irradiated, and cocultured with peripheral blood mononuclear cells. Both recombinants were competent to transform T lymphocytes with an efficiency similar to that of the parental viruses. Flow cytometry analysis indicated that HTLV-1 and HTLV-1/TR2 had a preferential tropism for CD4+ T cells and that HTLV-2 and HTLV-2/TR1 had a preferential tropism for CD8(+) T cells. Our results indicate that tax/rex in different genetic backgrounds display altered functional activity but ultimately do not contribute to the different in vitro transformation tropisms. This first study with recombinants between HTLV-1 and HTLV-2 is the initial step in elucidating the different pathobiologies of HTLV-1 and HTLV-2.

CD8(+) T cells are an in vivo reservoir for human T-cell lymphotropic virus type I

It is thought that human T-cell lymphotropic virus type I (HTLV-I) preferentially infects CD4(+) T cells in vivo. However, observations of high HTLV-I proviral load in patients with HTLV-I-associated myelopathy/tropical spastic paraparesis suggest that HTLV-I may infect other cell types in addition to CD4(+) T cells. To identify in vivo T-cell tropisms of HTLV-I, real-time quantitative polymerase chain reaction (PCR) and intracellular protein staining were used. A high amount of HTLV-I proviral DNA was detected from purified CD8(+) T cells by quantitative PCR (between 1.64 and 62.83 copies of HTLV-I provirus per 100 isolated CD8(+) T cells). CD8(+) T cells expressed HTLV-I-related antigens (HTLV-I Tax and p19 protein) after a short time in cultivation. These results demonstrate that CD8(+) T cells are also infected with HTLV-I and express HTLV-I antigens at levels that are comparable to HTLV-I-infected CD4(+) cells. Therefore, CD8(+) cells are an additional viral reservoir in vivo for HTLV-I and may contribute to the pathogenesis of HTLV-I-mediated disorders.