Human T-cell leukemia virus type II nucleotide sequences between env and the last exon of tax/rex are not required for viral replication or cellular transformation

The PRMT5 inhibitor EPZ015666 is effective against HTLV-1-transformed T-cell lines in vitro and in vivo

Human T-cell leukemia virus type 1 (HTLV-1) is the infectious cause of adult T-cell leukemia/lymphoma (ATL), an extremely aggressive and fatal malignancy of CD4+ T-cells. Due to the chemotherapy-resistance of ATL and the absence of long-term therapy regimens currently available for ATL patients, there is an urgent need to characterize novel therapeutic targets against this disease. Protein arginine methyltransferase 5 (PRMT5) is a type II PRMT enzyme that is directly involved in the pathogenesis of multiple different lymphomas through the transcriptional regulation of relevant oncogenes. Recently, our group identified that PRMT5 is overexpressed in HTLV-1-transformed T-cell lines, during the HTLV-1-mediated T-cell immortalization process, and in ATL patient samples. The objective of this study was to determine the importance of PRMT5 on HTLV-1 infected cell viability, T-cell transformation, and ultimately disease induction. Inhibition of PRMT5 enzymatic activity with a commercially available small molecule inhibitor (EPZ015666) resulted in selective in vitro toxicity of actively proliferating and transformed T-cells. EPZ015666-treatment resulted in a dose-dependent increase in apoptosis in HTLV-1-transformed and ATL-derived cell lines compared to uninfected Jurkat T-cells. Using a co-culture model of infection and immortalization, we found that EPZ015666 is capable of blocking HTLV-1-mediated T-cell immortalization in vitro, indicating that PRMT5 enzymatic activity is essential for the HTLV-1 T-cell transformation process. Administration of EPZ015666 in both NSG xenograft and HTLV-1-infected humanized immune system (HIS) mice significantly improved survival outcomes. The cumulative findings of this study demonstrate that the epigenetic regulator PRMT5 is critical for the survival, transformation, and pathogenesis of HTLV-1, illustrating the value of this cellular enzyme as a potential therapeutic target for the treatment of ATL.

An acquired phosphatidylinositol 4-phosphate transport initiates T-cell deterioration and leukemogenesis

Lipid remodeling is crucial for malignant cell transformation and tumorigenesis, but the precise molecular processes involved and direct evidences for these in vivo remain elusive. Here, we report that oxysterol-binding protein (OSBP)-related protein 4 L (ORP4L) is expressed in adult T-cell leukemia (ATL) cells but not normal T-cells. In ORP4L knock-in T-cells, ORP4L dimerizes with OSBP to control the shuttling of OSBP between the Golgi apparatus and the plasma membrane (PM) as an exchanger of phosphatidylinositol 4-phosphate [PI(4)P]/cholesterol. The PI(4)P arriving at the PM via this transport machinery replenishes phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] and phosphatidylinositol (3,4,5) trisphosphate [PI(3,4,5)P₃] biosynthesis, thus contributing to PI3K/AKT hyperactivation and T-cell deterioration in vitro and in vivo. Disruption of ORP4L and OSBP dimerization disables PI(4)P transport and T-cell leukemogenesis. In summary, we identify a non-vesicular lipid transport machinery between Golgi and PM maintaining the oncogenic signaling competence initiating T-cell deterioration and leukemogenesis.

The oxysterol-binding protein-related protein 4 (ORP4L) is expressed in T-cell acute lymphoblastic leukemia and is required for leukemogenesis. Here the authors show that ORP4L orchestrates the transport of the phospholipid PI(4)P from Golgi to the plasma membrane, contributing to PI3K/AKT hyperactivation and T-cell leukemogenesis.

ORP4L is a prerequisite for the induction of T-cell leukemogenesis associated with human T-cell leukemia virus 1

ORP4L deletion blocks Tax-induced T-cell leukemia, whereas engineering ORP4L expression in T cells results in T-cell leukemia in mice.

Loss of miR-31 induced by Tax releases ORP4L expression, which initiates T-cell deterioration, but ORP4L inhibition eliminates ATL in PDX mice.

Human T-cell leukemia virus 1 (HTLV-1) causes adult T-cell leukemia (ATL), but the mechanism underlying its initiation remains elusive. In this study, ORP4L was expressed in ATL cells but not in normal T-cells. ORP4L ablation completely blocked T-cell leukemogenesis induced by the HTLV-1 oncoprotein Tax in mice, whereas engineering ORP4L expression in T-cells resulted in T-cell leukemia in mice, suggesting the oncogenic properties and prerequisite of ORP4L promote the initiation of T-cell leukemogenesis. For molecular insight, we found that loss of miR-31 caused by HTLV-1 induced ORP4L expression in T-cells. ORP4L interacts with PI3Kδ to promote PI(3,4,5)P₃ generation, contributing to AKT hyperactivation; NF-κB–dependent, p53 inactivation-induced pro-oncogene expression; and T-cell leukemogenesis. Consistently, ORP4L ablation eliminates human ATL cells in patient-derived xenograft ATL models. These results reveal a plausible mechanism of T-cell deterioration by HTLV-1 that can be therapeutically targeted.

Stability of the HTLV-1 Antisense-Derived Protein, HBZ, Is Regulated by the E3 Ubiquitin-Protein Ligase, UBR5

Human T-cell leukemia virus type 1 (HTLV-1) encodes a protein derived from the antisense strand of the proviral genome designated HBZ (HTLV-1 basic leucine zipper factor). HBZ is the only viral gene consistently expressed in infected patients and adult T-cell leukemia/lymphoma (ATL) tumor cell lines. It functions to antagonize many activities of the Tax viral transcriptional activator, suppresses apoptosis, and supports proliferation of ATL cells. Factors that regulate the stability of HBZ are thus important to the pathophysiology of ATL development. Using affinity-tagged protein and shotgun proteomics, we identified UBR5 as a novel HBZ-binding partner. UBR5 is an E3 ubiquitin-protein ligase that functions as a key regulator of the ubiquitin proteasome system in both cancer and developmental biology. Herein, we investigated the role of UBR5 in HTLV-1-mediated T-cell transformation and leukemia/lymphoma development. The UBR5/HBZ interaction was verified in vivo using over-expression constructs, as well as endogenously in T-cells. shRNA-mediated knockdown of UBR5 enhanced HBZ steady-state levels by stabilizing the HBZ protein. Interestingly, the related HTLV-2 antisense-derived protein, APH-2, also interacted with UBR5 in vivo. However, knockdown of UBR5 did not affect APH-2 protein stability. Co-immunoprecipitation assays identified ubiquitination of HBZ and knockdown of UBR5 resulted in a decrease in HBZ ubiquitination. MS/MS analysis identified seven ubiquitinated lysines in HBZ. Interestingly, UBR5 expression was upregulated in established T lymphocytic leukemia/lymphoma cell lines and the later stage of T-cell transformation in vitro. Finally, we demonstrated loss of UBR5 decreased cellular proliferation in transformed T-cell lines. Overall, our study provides evidence for UBR5 as a host cell E3 ubiquitin-protein ligase responsible for regulating HBZ protein stability. Additionally, our data suggests UBR5 plays an important role in maintaining the proliferative phenotype of transformed T-cell lines.

PRMT5 Is Upregulated in HTLV-1-Mediated T-Cell Transformation and Selective Inhibition Alters Viral Gene Expression and Infected Cell Survival

Human T-cell leukemia virus type-1 (HTLV-1) is a tumorigenic retrovirus responsible for development of adult T-cell leukemia/lymphoma (ATLL). This disease manifests after a long clinical latency period of up to 2-3 decades. Two viral gene products, Tax and HBZ, have transforming properties and play a role in the pathogenic process. Genetic and epigenetic cellular changes also occur in HTLV-1-infected cells, which contribute to transformation and disease development. However, the role of cellular factors in transformation is not completely understood. Herein, we examined the role of protein arginine methyltransferase 5 (PRMT5) on HTLV-1-mediated cellular transformation and viral gene expression. We found PRMT5 expression was upregulated during HTLV-1-mediated T-cell transformation, as well as in established lymphocytic leukemia/lymphoma cell lines and ATLL patient PBMCs. shRNA-mediated reduction in PRMT5 protein levels or its inhibition by a small molecule inhibitor (PRMT5i) in HTLV-1-infected lymphocytes resulted in increased viral gene expression and decreased cellular proliferation. PRMT5i also had selective toxicity in HTLV-1-transformed T-cells. Finally, we demonstrated that PRMT5 and the HTLV-1 p30 protein had an additive inhibitory effect on HTLV-1 gene expression. Our study provides evidence for PRMT5 as a host cell factor important in HTLV-1-mediated T-cell transformation, and a potential target for ATLL treatment.

Human T-cell leukemia virus type 2 antisense viral protein 2 is dispensable for in vitro immortalization but functions to repress early virus replication in vivo

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are closely related but pathogenically distinct human retroviruses. The antisense strand of the HTLV-1 genome encodes HTLV-1 basic leucine zipper (b-ZIP) protein (HBZ), a protein that inhibits Tax-mediated viral transcription, enhances T-cell proliferation, and promotes viral persistence. Recently, an HTLV-2 antisense viral protein (APH-2) was identified. Despite its lack of a typical b-ZIP domain, APH-2, like HBZ, interacts with cyclic AMP response element binding protein (CREB) and downregulates Tax-mediated viral transcription. Here, we provide evidence that the APH-2 C-terminal LXXLL motif is important for CREB binding and Tax repression. In order to investigate the functional role of APH-2 in the HTLV-2-mediated immortalization of primary T lymphocytes in vitro and in HTLV-2 infection in vivo, we generated APH-2 mutant viruses. In cell cultures, the immortalization capacities of APH-2 mutant viruses were indistinguishable from that of wild-type HTLV-2 (wtHTLV-2), indicating that, like HBZ, APH-2 is dispensable for viral infection and cellular transformation. In vivo, rabbits inoculated with either wtHTLV-2 or APH-2 mutant viruses established a persistent infection. However, the APH-2 knockout virus displayed an increased replication rate, as measured by an increased viral antibody response and a higher proviral load. In contrast to HTLV-1 HBZ, we show that APH-2 is dispensable for the establishment of an efficient infection and persistence in a rabbit animal model. Therefore, antisense proteins of HTLV-1 and HTLV-2 have evolved different functions in vivo, and further comparative studies will provide fundamental insights into the distinct pathobiologies of these two viruses.

Host-pathogen interactome mapping for HTLV-1 and -2 retroviruses

BACKGROUND

Human T-cell leukemia virus type 1 (HTLV-1) and type 2 both target T lymphocytes, yet induce radically different phenotypic outcomes. HTLV-1 is a causative agent of Adult T-cell leukemia (ATL), whereas HTLV-2, highly similar to HTLV-1, causes no known overt disease. HTLV gene products are engaged in a dynamic struggle of activating and antagonistic interactions with host cells. Investigations focused on one or a few genes have identified several human factors interacting with HTLV viral proteins. Most of the available interaction data concern the highly investigated HTLV-1 Tax protein. Identifying shared and distinct host-pathogen protein interaction profiles for these two viruses would enlighten how they exploit distinctive or common strategies to subvert cellular pathways toward disease progression.

RESULTS

We employ a scalable methodology for the systematic mapping and comparison of pathogen-host protein interactions that includes stringent yeast two-hybrid screening and systematic retest, as well as two independent validations through an additional protein interaction detection method and a functional transactivation assay. The final data set contained 166 interactions between 10 viral proteins and 122 human proteins. Among the 166 interactions identified, 87 and 79 involved HTLV-1 and HTLV-2 -encoded proteins, respectively. Targets for HTLV-1 and HTLV-2 proteins implicate a diverse set of cellular processes including the ubiquitin-proteasome system, the apoptosis, different cancer pathways and the Notch signaling pathway.

CONCLUSIONS

This study constitutes a first pass, with homogeneous data, at comparative analysis of host targets for HTLV-1 and -2 retroviruses, complements currently existing data for formulation of systems biology models of retroviral induced diseases and presents new insights on biological pathways involved in retroviral infection.

Bovine leukemia virus infection in a juvenile alpaca with multicentric lymphoma

A 13-month-old alpaca (Vicugna pacos) was presented for mandibular masses and weight loss. Histopathology of biopsy tissue was consistent with lymphoma. The alpaca was euthanized and necropsy revealed lymphoma masses in multiple organs. Immunohistochemistry for T- and B-cell typing was inconclusive. Serology and in-situ polymerase chain reaction hybridization were positive for bovine leukemia virus.

Human T-cell leukemia virus type 2 produces a spliced antisense transcript encoding a protein that lacks a classic bZIP domain but still inhibits Tax2-mediated transcription

Human T-cell leukemia virus type 1 (HTLV-1) and type 2 (HTLV-2) retroviruses infect T lymphocytes. The minus strand of the HTLV-1 genome encodes HBZ, a protein that could play a role in the development of leukemia in infected patients. Herein, we demonstrate that the complementary strand of the HTLV-2 genome also encodes a protein that we named APH-2 for "antisense protein of HTLV-2." APH-2 mRNA is spliced, polyadenylated, and initiates in the 3'-long terminal repeat at different positions. This transcript was detected in all HTLV-2-infected cell lines and short-term culture of lymphocytes obtained from HTLV-2 African patients tested and in 4 of 15 HTLV-2-infected blood donors. The APH-2 protein is 183 amino acids long, is localized in the cell nucleus, and is detected in vivo. Despite the lack of a consensus basic leucine zipper domain, APH-2 interacts with cyclic adenosine monophosphate-response element binding protein (CREB) and represses Tax2-mediated transcription in Tax2-expressing cells and in cells transfected with an HTLV-2 molecular clone. Altogether, our results demonstrate the existence of an antisense strand-encoded protein in HTLV-2, which could represent an important player in the development of disorders, such as lymphocytosis, which is frequently observed in HTLV-2 patients.

Human T-cell leukemia virus type 2 Rex carboxy terminus is an inhibitory/stability domain that regulates Rex functional activity and viral replication

Human T-cell leukemia virus (HTLV) regulatory protein, Rex, functions to increase the expression of the viral structural and enzymatic gene products. The phosphorylation of two serine residues (S151 and S153) at the C terminus is important for the function of HTLV-2 Rex (Rex-2). The Rex-2 phosphomimetic double mutant (S151D, S153D) is locked in a functionally active conformation. Since rex and tax genes overlap, Rex S151D and S153D mutants were found to alter the Tax oncoprotein coding sequence and transactivation activities. Therefore, additional Rex-2 mutants including P152D, A157D, S151Term, and S158Term were generated and characterized ("Term" indicates termination codon). All Rex-2 mutants and wild-type (wt) Rex-2 localized predominantly to the nucleus/nucleolus, but in contrast to the detection of phosphorylated and unphosphorylated forms of wt Rex-2 (p26 and p24), mutant proteins were detected as a single phosphoprotein species. We found that Rex P152D, A157D, and S158Term mutants are more functionally active than wt Rex-2 and that the Rex-2 C terminus and its specific phosphorylation state are required for stability and optimal expression. In the context of the provirus, the more active Rex mutants (A157D or S158Term) promoted increased viral protein production, increased viral infectious spread, and enhanced HTLV-2-mediated cellular proliferation. Moreover, these Rex mutant viruses replicated and persisted in inoculated rabbits despite higher antiviral antibody responses. Thus, we identified in Rex-2 a novel C-terminal inhibitory domain that regulates functional activity and is positively regulated through phosphorylation. The ability of this domain to modulate viral replication likely plays a key role in the infectious spread of the virus and in virus-induced cellular proliferation.

Human T-cell leukemia virus type 2 post-transcriptional control protein p28 is required for viral infectivity and persistence in vivo

Background

Human T-cell leukemia virus (HTLV) type 1 and type 2 are related but distinct pathogenic complex retroviruses. HTLV-1 is associated with adult T-cell leukemia and a variety of immune-mediated disorders including the chronic neurological disease termed HTLV-1-associated myelopathy/tropical spastic paraparesis. In contrast, HTLV-2 displays distinct biological differences and is much less pathogenic, with only a few reported cases of leukemia and neurological disease associated with infection. In addition to the structural and enzymatic proteins, HTLV encodes regulatory (Tax and Rex) and accessory proteins. Tax and Rex positively regulate virus production and are critical for efficient viral replication and pathogenesis. Using an over-expression system approach, we recently reported that the accessory gene product of the HTLV-1 and HTLV-2 open reading frame (ORF) II (p30 and p28, respectively) acts as a negative regulator of both Tax and Rex by binding to and retaining their mRNA in the nucleus, leading to reduced protein expression and virion production. Further characterization revealed that p28 was distinct from p30 in that it was devoid of major transcriptional modulating activity, suggesting potentially divergent functions that may be responsible for the distinct pathobiologies of HTLV-1 and HTLV-2.

Results

In this study, we investigated the functional significance of p28 in HTLV-2 infection, proliferation, and immortaliztion of primary T-cells in culture, and viral survival in an infectious rabbit animal model. An HTLV-2 p28 knockout virus (HTLV-2Δp28) was generated and evaluated. Infectivity and immortalization capacity of HTLV-2Δp28 in vitro was indistinguishable from wild type HTLV-2. In contrast, we showed that viral replication was severely attenuated in rabbits inoculated with HTLV-2Δp28 and the mutant virus failed to establish persistent infection.

Conclusion

We provide direct evidence that p28 is dispensable for viral replication and cellular immortalization of primary T-lymphocytes in cell culture. However, our data indicate that p28 function is critical for viral survival in vivo. Our results are consistent with the hypothesis that p28 repression of Tax and Rex-mediated viral gene expression may facilitate survival of these cells by down-modulating overall viral gene expression.

Cell-free HTLV-1 infects dendritic cells leading to transmission and transformation of CD4(+) T cells

Cell-free human T-lymphotropic virus type 1 (HTLV-1) virions are poorly infectious in vitro for their primary target cells, CD4(+) T cells. Here, we show that HTLV-1 can efficiently infect myeloid and plasmacytoid dendritic cells (DCs). Moreover, DCs exposed to HTLV-1, both before and after being productively infected, can rapidly, efficiently and reproducibly transfer virus to autologous primary CD4(+) T cells. This DC-mediated transfer of HTLV-1 involves heparan sulfate proteoglycans and neuropilin-1 and results in long-term productive infection and interleukin-2-independent transformation of the CD4(+) T cells. These studies, along with observations of HTLV-1-infected DCs in the peripheral blood of infected individuals, indicate that DCs have a central role in HTLV-1 transmission, dissemination and persistence in vivo. In addition to altering the current paradigm concerning how HTLV-1 transmission occurs, these studies suggest that impairment of DC function after HTLV-1 infection plays a part in pathogenesis.

Construction and characterization of a full-length infectious simian T-cell lymphotropic virus type 3 molecular clone

Together with their simian T-cell lymphotropic virus (STLV) equivalent, human T-cell lymphotropic virus type 1 (HTLV-1), HTLV-2, and HTLV-3 form the primate T-cell lymphotropic virus (PTLV) group. Over the years, understanding the biology and pathogenesis of HTLV-1 and HTLV-2 has been widely improved by the creation of molecular clones. In contrast, so far, PTLV-3 experimental studies have been restricted to the overexpression of the tax gene using reporter assays. We have therefore decided to construct an STLV-3 molecular clone. We generated a full-length STLV-3 proviral clone (8,891 bp) by PCR amplification of overlapping fragments. This STLV-3 molecular clone was then transfected into 293T cells. Reverse transcriptase PCR experiments followed by sequence analysis of the amplified products allowed us to establish that both gag and tax/rex mRNAs were transcribed. Western blotting further demonstrated the presence of the STLV-3 p24gag protein in the cell culture supernatant from transfected cells. Transient transfection of 293T cells and of 293T-long terminal repeat-green fluorescent protein cells with the STLV-3 clone promoted syncytium formation, a hallmark of PTLV Env expression, as well as the appearance of fluorescent cells, also demonstrating that the Tax3 protein was expressed. Virus particles were visible by electron microscopy. These particles are infectious, as demonstrated by our cell-free-infection experiments with purified virions. All together, our data demonstrate that the STLV-3 molecular clone is functional and infectious. This clone will give us a unique opportunity to study in vitro the different pX transcripts and the putative presence of antisense transcripts and to evaluate the PTLV-3 pathogenicity in vivo.

Human T-cell leukemia virus open reading frame II encodes a posttranscriptional repressor that is recruited at the level of transcription

Human T-cell leukemia virus (HTLV) infection is a chronic, lifelong infection that is associated with the development of leukemia and neurological disease after a long latency period, and the mechanism by which the virus is able to evade host immune surveillance is elusive. Besides the structural and enzymatic proteins, HTLV encodes regulatory (Tax and Rex) and accessory (open reading frame I [ORF I] and ORF II) proteins. Tax activates viral and cellular transcription and promotes T-cell growth and malignant transformation. Rex acts posttranscriptionally to facilitate cytoplasmic expression of incompletely spliced viral mRNAs. Recently, we reported that the accessory gene products of HTLV-1 and HTLV-2 ORF II (p30II and p28II, respectively) are able to restrict viral replication. These proteins act as negative regulators of both Tax and Rex by binding to and retaining their mRNA in the nucleus, leading to reduced protein expression and virion production. Here, we show that p28II is recruited to the viral promoter in a Tax-dependent manner. After recruitment to the promoter, p28II or p30II then travels with the transcription elongation machinery until its target mRNA is synthesized. Experiments artificially directing these proteins to the promoter indicate that p28II, unlike HTLV-1 p30II, displays no transcriptional activity. Furthermore, the tethering of p28II directly to tax/rex mRNA resulted in repression of Tax function, which could be attributed to the ability of p28II to block TAP/p15-mediated enhancement of Tax expression. p28II-mediated reduction of viral replication in infected cells may permit survival of the cells by allowing escape from immune recognition, which is consistent with the critical role of HTLV accessory proteins in viral persistence in vivo.

PDZ binding motif of HTLV-1 Tax promotes virus-mediated T-cell proliferation in vitro and persistence in vivo

HTLV-1 cellular transformation and disease induction is dependent on expression of the viral Tax oncoprotein. PDZ is a modular protein interaction domain used in organizing signaling complexes in eukaryotic cells through recognition of a specific binding motif in partner proteins. Tax-1, but not Tax-2, contains a PDZ-binding domain motif (PBM) that promotes the interaction with several cellular PDZ proteins. Herein, we investigate the contribution of the Tax-1 PBM in HTLV-induced proliferation and immortalization of primary T cells in vitro and viral survival in an infectious rabbit animal model. We generated several HTLV-1 and HTLV-2 Tax viral mutants, including HTLV-1deltaPBM, HTLV-2+C22(+PBM), and HTLV-2+ C18(deltaPBM). All Tax mutants maintained the ability to significantly activate the CREB/ATF or NFkappaB signaling pathways. Microtiter proliferation assays revealed that the Tax-1 PBM significantly increases both HTLV-1- and HTLV-2-induced primary T-cell proliferation. In addition, Tax-1 PBM was responsible for the micronuclei induction activity of Tax-1 relative to that of Tax-2. Viral infection and persistence were severely attenuated in rabbits inoculated with HTLV-1deltaPBM. Our results provide the first direct evidence suggesting that PBM-mediated associations between Tax-1 and cellular proteins play a key role in HTLV-induced cell proliferation and genetic instability in vitro and facilitate viral persistence in vivo.

Human T-cell leukemia virus type 1 expressing nonoverlapping tax and rex genes replicates and immortalizes primary human T lymphocytes but fails to replicate and persist in vivo

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus associated primarily with adult T-cell leukemia and neurological disease. HTLV-1 encodes the positive trans-regulatory proteins Tax and Rex, both of which are essential for viral replication. Tax activates transcription initiation from the viral long terminal repeat and modulates the transcription or activity of a number of cellular genes. Rex regulates gene expression posttranscriptionally by facilitating the cytoplasmic expression of incompletely spliced viral mRNAs. Tax and Rex mutants have been identified that have defective activities or impaired biochemical properties associated with their function. To ultimately determine the contribution of specific protein activities on viral replication and cellular transformation of primary T cells, mutants need to be characterized in the context of an infectious molecular clone. Since the tax and rex genes are in partially overlapping reading frames, mutation in one gene frequently disrupts the other, confounding interpretation of mutational analyses in the context of the virus. Here we generated and characterized a unique proviral clone (H1IT) in which the tax and rex genes were separated by expressing Tax from an internal ribosome entry site. We showed that H1IT expresses both functional Tax and Rex. In short- and long-term coculture assays, H1IT was competent to infect and immortalize primary human T cells similar to wild-type HTLV-1. In contrast, H1IT failed to efficiently replicate and persist in inoculated rabbits, thus emphasizing the importance of temporal and quantitative regulation of specific mRNA for viral survival in vivo.

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.

Enhancement of infectivity and persistence in vivo by HBZ, a natural antisense coded protein of HTLV-1

Natural antisense viral transcripts have been recognized in retroviruses, including human T-cell leukemia virus type 1 (HTLV-1), HIV-1, and feline immunodeficiency virus (FIV), and have been postulated to encode proteins important for the infection cycle and/or pathogenesis of the virus. The antisense strand of the HTLV-1 genome encodes HBZ, a novel nuclear basic region leucine zipper (b-ZIP) protein that in overexpression assays down-regulates Tax oncoprotein-induced viral transcription. Herein, we investigated the contribution of HBZ to HTLV-1-mediated immortalization of primary T lymphocytes in vitro and HTLV-1 infection in a rabbit animal model. HTLV-1 HBZ mutant viruses were generated and evaluated for viral gene expression, protein production, and immortalization capacity. Biologic properties of HBZ mutant viruses in vitro were indistinguishable from wild-type HTLV-1, providing the first direct evidence that HBZ is dispensable for viral replication and cellular immortalization. Rabbits inoculated with irradiated cells expressing HTLV-1 HBZ mutant viruses became persistently infected. However, these rabbits displayed a decreased antibody response to viral gene products and reduced proviral copies in peripheral blood mononuclear cells (PBMCs) as compared with wild-type HTLV-1-infected animals. Our findings indicated that HBZ was not required for in vitro cellular immortalization, but enhanced infectivity and persistence in inoculated rabbits. This study demonstrates that retroviruses use negative-strand-encoded proteins in the establishment of chronic viral 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.

Construction and characterization of deltaretrovirus indicator cell lines

The deltaretroviruses, which include bovine leukemia virus (BLV) and human T-cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2), replicate poorly in culture and the molecular details of their life cycles are limited. To facilitate the analysis of virus replication, mammalian cell lines were created with the long terminal repeats (LTRs) of each virus driving expression of the enhanced green fluorescent protein gene (egfp). The BLGFP, H1GFP and H2GFP cell lines detect virus infection by the expression of GFP via the transactivation of the LTR via the Tax protein of BLV, HTLV-1 or HTLV-2, respectively. GFP expression was measured by flow cytometry, yielding sensitive and rapid detection of virus infectivity. Interestingly, we observed that the Tax proteins of HTLV-1 and HTLV-2 could transactivate the BLV LTR at levels that were comparable to that of BLV Tax. In contrast, the BLV Tax showed low levels of transactivation in H1GFP and H2GFP cells. HTLV-1 and HTLV-2 Tax proteins efficiently transactivated both the HTLV-1 and HTLV-2 LTRs. Finally, spinoculation of BLV resulted in only a two-fold increase in viral titer.

Repression of human T-cell leukemia virus type 1 and type 2 replication by a viral mRNA-encoded posttranscriptional regulator

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are complex retroviruses that persist in the host, eventually causing leukemia and neurological disease in a small percentage of infected individuals. In addition to structural and enzymatic proteins, HTLV encodes regulatory (Tax and Rex) and accessory (open reading frame I and II) proteins. The viral Tax and Rex proteins positively regulate virus production. Tax activates viral and cellular transcription to promote T-cell growth and, ultimately, malignant transformation. Rex acts posttranscriptionally to facilitate cytoplasmic expression of viral mRNAs that encode the structural and enzymatic gene products, thus positively controlling virion expression. Here, we report that both HTLV-1 and HTLV-2 have evolved accessory genes to encode proteins that act as negative regulators of both Tax and Rex. HTLV-1 p30(II) and the related HTLV-2 p28(II) inhibit virion production by binding to and retaining tax/rex mRNA in the nucleus. Reduction of viral replication in a cell carrying the provirus may allow escape from immune recognition in an infected individual. These data are consistent with the critical role of these proteins in viral persistence and pathogenesis in animal models of HTLV-1 and HTLV-2 infection.

Role of accessory proteins of HTLV-1 in viral replication, T cell activation, and cellular gene expression

Human T-cell lymphotropic virus type 1 (HTLV-1), causes adult T cell leukemia/lymphoma (ATLL), and initiates a variety of immune mediated disorders. The viral genome encodes common structural and enzymatic proteins characteristic of all retroviruses and utilizes alternative splicing and alternate codon usage to make several regulatory and accessory proteins encoded in the pX region (pX ORF I to IV). Recent studies indicate that the accessory proteins p12I, p27I, p13II, and p30II, encoded by pX ORF I and II, contribute to viral replication and the ability of the virus to maintain typical in vivo expression levels. Proviral clones that are mutated in either pX ORF I or II, while fully competent in cell culture, are severely limited in their replicative capacity in a rabbit model. These HTLV-1 accessory proteins are critical for establishment of viral infectivity, enhance T-lymphocyte activation and potentially alter gene transcription and mitochondrial function. HTLV-1 pX ORF I expression is critical to the viral infectivity in resting primary lymphocytes suggesting a role for the calcineurin-binding protein p12I in lymphocyte activation. The endoplasmic reticulum and cis-Golgi localizing p12I activates NFAT, a key T cell transcription factor, through calcium-mediated signaling pathways and may lower the threshold of lymphocyte activation via the JAK/STAT pathway. In contrast p30II localizes to the nucleus and represses viral promoter activity, but may regulate cellular gene expression through p300/CBP or related co-activators of transcription. The mitochondrial localizing p13II induces morphologic changes in the organelle and may influence energy metabolism infected cells. Future studies of the molecular details HTLV-1 "accessory" proteins interactions will provide important new directions for investigations of HTLV-1 and related viruses associated with lymphoproliferative diseases. Thus, the accessory proteins of HTLV-1, once thought to be dispensable for viral replication, have proven to be directly involved in viral spread in vivo and represent potential targets for therapeutic intervention against HTLV-1 infection and disease.

HTLV-1 Rex is required for viral spread and persistence in vivo but is dispensable for cellular immortalization in vitro

Human T-cell leukemia virus type 1 (HTLV-1) is associated with leukemia/lymphoma and neurologic disorders. Although the viral transcriptional activator Tax is the critical viral oncoprotein, Rex, which regulates the expression of the viral structural and enzymatic genes, is essential for efficient viral replication. Herein, we investigate the contribution of Rex in HTLV-1 immortalization of primary T cells in vitro and viral survival in an infectious rabbit animal model. A Rex-deficient HTLV-1 (HTLVRex-) was constructed and characterized for viral gene expression, protein production, and immortalization capacity. Cells transiently transfected with the HTLVRex- proviral clone produced low detectable levels of p19 Gag. 729HTLVRex- stable transfectants produced functional Tax, but undetectable levels of Rex or p19 Gag. Coculture of irradiated 729HTLVRex- cells with peripheral blood mononuclear cells (PBMCs) resulted in sustained interleukin-2 (IL-2)-dependent growth of primary T lymphocytes. These cells carried the HTLVRex- genome and expressed tax/rex mRNA but produced no detectable Rex or p19 Gag. Rabbits inoculated with irradiated 729HTLVRex- cells or 729HTLVRex- cells transiently transfected with a Rex cDNA expression plasmid failed to become persistently infected or mount a detectable antibody response to the viral gene products. Together, our results provide the first direct evidence that Rex and its function to modulate viral gene expression and virion production is not required for in vitro immortalization by HTLV-1. However, Rex is critical for efficient infection of cells and persistence in vivo.

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.

Retroviral proteins that target the major histocompatibility complex class I

The human T-cell leukemia virus type 1 (HTLV-1) and human immunodeficiency virus type 1 (HIV-1) retroviruses are two evolutionary distinct human pathogens. HTLV-1 is the etiologic agent of two diverse diseases: adult T-cell leukemia/lymphoma, as well as the neurologic disorder tropical spastic paraparesis/HTLV-1-associated myelopathy. HTLV-1 is the only retrovirus known to be the etiologic agent of human cancer. HTLV-2, the other known oncovirus, is not apparently associated with human cancer. While HTLV-1 transforms T-cells in vitro, HIV kills CD4+ T-cells and is the etiological agent of human acquired immunodeficiency syndrome, characterized by a progressive loss of CD4+ cells, weakening of the immune system, and susceptibility to opportunistic infections and cancer. HTLV-1 and HIV-1 both cause lifelong infections, which suggests that they have evolved mechanism(s) to evade detection by the host's immune response; particularly to evade cytotoxic T-lymphocytes, which play a major role in cellular immunity against viruses and will be the focus of this review.

Critical role of human T-lymphotropic virus type 1 accessory proteins in viral replication and pathogenesis

Human T-cell lymphotropic virus type 1 (HTLV-1) infection is associated with a diverse range of lymphoproliferative and neurodegenerative diseases, yet pathogenic mechanisms induced by the virus remain obscure. This complex retrovirus contains typical structural and enzymatic genes but also unique regulatory and accessory genes in four open reading frames (ORFs) of the pX region of the viral genome (pX ORFs I to IV). The regulatory proteins encoded by pX ORFs III and IV, Tax and Rex, respectively, have been extensively characterized. In contrast the contribution of the four accessory proteins p12(I), p27(I), p13(II), and p30(II), encoded by pX ORFs I and II, to viral replication and pathogenesis remained unclear. Proviral clones that are mutated in either pX ORF I or II, while fully competent in cell culture, are severely limited in their replicative capacity in a rabbit model. Emerging evidence indicates that the HTLV-1 accessory proteins are critical for establishment of viral infectivity, enhance T-lymphocyte activation, and potentially alter gene transcription and mitochondrial function. HTLV-1 pX ORF I expression is critical to the viral infectivity in resting primary lymphocytes, suggesting a role for p12(I) in lymphocyte activation. The endoplasmic reticulum and cis-Golgi localizing p12(I), encoded from pX ORF I, activates NFAT, a key T-cell transcription factor, through calcium-mediated signaling pathways and may lower the threshold of lymphocyte activation via the JAK/STAT pathway. In contrast p30(II) localizes to the nucleus and represses viral promoter activity, but may regulate cellular gene expression through p300/CBP or related coactivators of transcription. p13(II) targets mitochondrial proteins, where it alters the organelle morphology and may influence energy metabolism. Collectively, studies of the molecular functions of the HTLV-1 accessory proteins provide insight into strategies used by retroviruses that are associated with lymphoproliferative diseases.

Oncoviral bovine leukemia virus G4 and human T-cell leukemia virus type 1 p13(II) accessory proteins interact with farnesyl pyrophosphate synthetase

G4 and p13(II) are accessory proteins encoded by the X region of bovine leukemia virus and human T-cell leukemia virus type 1 (HTLV-1), respectively. Disruption of the G4 and p13(II) open reading frames interferes with viral spread in animal model systems, indicating that the corresponding proteins play a key role in viral replication. In addition, G4 is oncogenic in primary cell cultures and is absolutely required for efficient onset of leukemogenesis in sheep. To gain insight into the function of these proteins, we utilized the yeast two-hybrid system to identify protein partners of G4. Results revealed that G4 interacts with farnesyl pyrophosphate synthetase (FPPS), a protein involved in the mevalonate/squalene pathway and in synthesis of FPP, a substrate required for prenylation of Ras. The specificity of the interaction was verified by glutathione S-transferase (GST) pull-down assays and by coimmunoprecipitation experiments. Furthermore, confocal microscopy showed that the subcellular localization of G4 was profoundly affected by FPPS. The G4 protein itself was not prenylated, at least in rabbit reticulocyte lysate-based assays. The domain of G4 required for binding to FPPS was restricted to an amphipathic alpha-helix rich in arginine residues. Subtle mutation of this alpha-helix abrogated G4 oncogenic potential in vitro, providing a biological relevance for FPPS-G4 complex formation in cells. Finally, HTLV-1 p13(II) was also found to specifically interact with FPPS (in yeast as well as in GST pull-down assays) and to colocalize with G4 in mitochondria, suggesting a functional analogy between these oncoviral accessory proteins. Identification of FPPS as a molecular partner for p13(II) and G4 accessory proteins opens new prospects for treatment of retrovirus-induced leukemia.

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

Human T cell leukemia virus type 1 (HTLV-1) and 2 (HTLV-2) are distinct oncogenic retroviruses that infect several cell types, but display their biologic/pathogenic activity only in T lymphocytes. HTLV-1 is associated with adult T cell leukemia, a malignancy of mature CD4(+) T cells, and a chronic neurological disorder termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-2 is less pathogenic and has been associated with a few cases of a variant of hairy cell leukemia and neurological disease. 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. The molecular mechanism that determines the cellular tropism of HTLV-1 and HTLV-2 has not been precisely determined. However, one study by our group has provided evidence that HTLV-1-enhanced viral transcription in CD4(+) T cells may be responsible for its tropism. In an effort to understand HTLV-2 tropism we tested the ability of HTLV-2 to infect, replicate in, and transform purified CD4(+) or CD8(+) T cells in cell culture. After cocultures of purified primary human CD4(+) and CD8(+) T cells with an HTLV-2-producer cell line we measured viral transcription by reverse transcription PCR analysis, virus production by p19(gag) ELISA, proviral integration by DNA slot-blot analysis, surface phenotype by FACS analysis, and cellular transformation. We also measured HTLV-2 long terminal repeat-directed transcription in the presence and absence of Tax in purified CD4(+) and CD8(+) T cells, using transient transfection assays. Our data indicate that CD4(+) and CD8(+) T cells are equally susceptible to HTLV-2 infection. We observed no significant difference in viral transcription based on mRNA and virus production in CD4(+) and CD8(+) T cell cocultures. Although LTR transcription was enhanced 12- to 16-fold in the presence of Tax, there was no significant difference in CD4(+) or CD8(+) T cells. Interestingly, we show that HTLV-2 preferentially transforms CD8(+) T cells in culture. Together, our data indicate that, unlike HTLV-1, HTLV-2 cell tropism is not due to inhibition of viral infection and inefficient gene expression in CD4(+) versus CD8(+) T cells, and likely involves unique interactions with viral and CD8(+) T cell-specific proteins.

The MHC class I heavy chain is a common target of the small proteins encoded by the 3' end of HTLV type 1 and HTLV type 2

Human T cell leukemia/lymphotropic virus types 1 and 2 are two immunologically and phylogenetically related retroviruses that differ in their pathogenicity in vivo. The overall genetic structure of HTLV-1 and -2 is similar. Each contains a unique region at the 3' end of the genome, designated the pX region. p12(I) is a membrane-associated protein encoded by the open reading frame I (ORF I) region of HTLV-1, which lies within the pX region. A corresponding protein, p10(I) is encoded by the ORF I region of HTLV-2 and an additional protein, p11(V), is encoded by ORF V, which overlaps the HTLV-2 ORF I region. As with HTLV-1, the small proteins encoded by the pX region of HTLV-2 appear to be dispensable for viral replication and cellular transformation in vitro. However, the small open reading frames of both viruses are important for viral replication in vivo, which suggests they may play an important role during the viral life cycle. This study was undertaken to investigate and compare the cellular targets of the p10(I), p11(V), and p12(I) putative proteins.

Human T-cell leukemia virus type 2 tax mutants that selectively abrogate NFkappaB or CREB/ATF activation fail to transform primary human T cells

Human T-cell leukemia virus (HTLV) Tax protein has been implicated in the HTLV oncogenic process, primarily due to its pleiotropic effects on cellular genes involved in growth regulation and cell cycle control. To date, several approaches attempting to correlate Tax activation of the CREB/activating transcription factor (ATF) or NFkappaB/Rel transcriptional activation pathway to cellular transformation have yielded conflicting results. In this study, we use a unique HTLV-2 provirus (HTLV(c-enh)) that replicates by a Tax-independent mechanism to directly assess the role of Tax transactivation in HTLV-mediated T-lymphocyte transformation. A panel of well-characterized tax-2 mutations is utilized to correlate the respective roles of the CREB/ATF or NFkappaB/Rel signaling pathway. Our results demonstrate that viruses expressing tax-2 mutations that selectively abrogate NFkappaB/Rel or CREB/ATF activation display distinct phenotypes but ultimately fail to transform primary human T lymphocytes. One conclusion consistent with our results is that the activation of NFkappaB/Rel provides a critical proliferative signal early in the cellular transformation process, whereas CREB/ATF activation is required to promote the fully transformed state. However, complete understanding will require correlation of Tax domains important in cellular transformation to those Tax domains important in the modulation of gene transcription, cell cycle control, induction of DNA damage, and other undefined activities.

Functional role of pX open reading frame II of human T-lymphotropic virus type 1 in maintenance of viral loads in vivo

Human T-lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma and is associated with a variety of immune-mediated disorders. The role of four open reading frames (ORFs), located between env and the 3' long terminal repeat of HTLV-1, in mediating disease is not entirely clear. By differential splicing, ORF II encodes two proteins, p13(II) and p30(II), both of which have not been functionally defined. p13(II) localizes to mitochondria and may alter the configuration of the tubular network of this cellular organelle. p30(II) localizes to the nucleolus and shares homology with the transcription factors Oct-1 and -2, Pit-1, and POU-M1. Both p13(II) and p30(II) are dispensable for infection and immortalization of primary human and rabbit lymphocytes in vitro. To test the role of ORF II gene products in vivo, we inoculated rabbits with lethally irradiated cell lines expressing the wild-type molecular clone of HTLV-1 (ACH.1) or a clone containing selected mutations in ORF II (ACH.30/13.1). ACH.1-inoculated animals maintained higher HTLV-1-specific antibody titers than animals inoculated with ACH.30/13.1. Viral p19 antigen was transiently detected in ex vivo cultures of peripheral blood mononuclear cells (PBMC) from only two ACH.30/13.1-inoculated rabbits, while PBMC cultures from all ACH.1-inoculated rabbits routinely produced p19 antigen. In only three of six animals exposed to the ACH. p30(II)/p13(II) clone could provirus be consistently PCR amplified from extracted PBMC DNA and quantitative competitive PCR showed the proviral loads in PBMC from ACH.p30(II)/p13(II)-infected rabbits to be dramatically lower than the proviral loads in rabbits exposed to ACH. Our data indicate selected mutations in pX ORF II diminish the ability of HTLV-1 to maintain high viral loads in vivo and suggest an important function for p13(II) and p30(II) in viral pathogenesis.

Proliferation response to interleukin-2 and Jak/Stat activation of T cells immortalized by human T-cell lymphotropic virus type 1 is independent of open reading frame I expression

Human T-cell lymphotropic virus type 1 (HTLV-1), a complex retrovirus, encodes a hydrophobic 12-kD protein from pX open reading frame (ORF) I that localizes to cellular endomembranes and contains four minimal SH3 binding motifs (PXXP). We have demonstrated the importance of ORF I expression in the establishment of infection and hypothesize that p12(I) has a role in T-cell activation. In this study, we tested interleukin-2 (IL-2) receptor expression, IL-2-mediated proliferation, and Jak/Stat activation in T-cell lines immortalized with either wild-type or ORF I mutant clones of HTLV-1. All cell lines exhibited typical patterns of T-cell markers and maintained mutation fidelity. No significant differences between cell lines were observed in IL-2 receptor chain (alpha, beta, or gamma(c)) expression, in IL-2-mediated proliferation, or in IL-2-induced phosphorylated forms of Stat3, Stat5, Jak1, or Jak3. The expression of ORF I is more likely to play a role in early HTLV-1 infection, such as in the activation of quiescent T cells in vivo.

Human T-cell leukemia virus type 2 Rex protein increases stability and promotes nuclear to cytoplasmic transport of gag/pol and env RNAs

The human T-cell leukemia virus (HTLV) Rex protein is essential for efficient expression of the viral structural and enzymatic gene products. In this study, we assessed the role of the HTLV-2 rex gene in viral RNA expression and Gag protein production. Following transfection of human JM4 T cells with wild-type and rex mutant full-length proviral constructs, PCR was used for semiquantitative analysis of specific viral RNA transcripts. In the presence of Rex, the total amount of steady-state viral RNA was increased fourfold. Rex significantly up-regulated the level of incompletely spliced RNAs by increasing RNA stability and was associated with a twofold down-regulation of the completely spliced tax/rex RNA. PCR analysis of subcellular RNA fractions, isolated from transfected cells, indicated that the level of gag/pol and env cytoplasmic RNAs were increased 7- to 9-fold in the presence of Rex, whereas Gag protein production was increased 130-fold. These data indicate that HTLV-2 Rex increases the stability and promotes nucleus-to-cytoplasm transport of the incompletely spliced viral RNAs, ultimately resulting in increased structural protein production. Moreover, this model system provides a sensitive approach to further characterize HTLV gene expression from full-length proviral clones following transfection of human T cells.

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.

Conservative mutations in the immunosuppressive region of the bovine leukemia virus transmembrane protein affect fusion but not infectivity in vivo

Many retroviruses, including bovine leukemia virus (BLV), contain a highly conserved region located about 40 amino acids downstream from the fusion peptide within the sequence of the external domain of the transmembrane (TM) protein. This region is notably thought to be involved in the presentation of the NH2-terminal peptide to allow cell fusion. By using hydrophobic cluster analysis and by analogy with the influenza A hemagglutinin structures, the core of the TM structure including this particular region was predicted to consist, in the BLV and other retroviral envelope proteins, of an alpha-helix followed by a loop region, both docked against a subsequent alpha-helix that forms a triple-stranded coiled coil. The loop region could undergo, as in hemagglutinin, a major refolding into an alpha-helix integrating the coiled coil structure and putting the fusion peptide to one tip of the molecule. Based on this model, we have identified amino acids that may be essential to the BLV TM structure, and a series of mutations were introduced in the BLV env gene of an infectious molecular clone. A first series of mutations was designed to disturb the coiled coil structure (substitutions with proline residues), whereas others would maintain the general TM structure. When expressed by Semliki Forest virus recombinants, all the mutated envelope proteins were stable and efficiently synthesized in baby hamster kidney cells. Both proline-substituted and conservative mutants were strongly affected in their capacity to fuse to CC81 indicator cells. In addition, it appeared that the integrity of the TM coiled coil structure is essential for envelope protein multimerization, as analyzed by metrizamide gradient centrifugation. Finally, to gain insight into the role of this coiled coil in the infectious potential of BLV in vivo, the mutated TM genes were introduced in an infectious and pathogenic molecular clone and injected into sheep. It appeared that only the conservative mutations (A60V and A64S) allowed maintenance of viral infectivity in vivo. Since these mutations destroyed the ability to induce syncytia, we conclude that efficient fusion capacity of the recombinant envelopes is not a prerequisite for the infectious potential of BLV in vivo. Viral propagation of these mutants was strongly affected in some of the infected sheep. However, the proviral loads within half of the infected animals (2 out of 2 for A60V and 1 out of 4 for A64S) were close to the wild-type levels. In these sheep, it thus appears that the A60V and A64S mutants propagate efficiently despite being unable to induce syncytia in cell culture.

Human T-cell leukemia virus type 1 pX-I and pX-II open reading frames are dispensable for the immortalization of primary lymphocytes

Human T-cell leukemia virus type 1 (HTLV-1) infects and transforms CD4+ T-lymphocytes both in vivo and in vitro. Although the Tax protein of HTLV-1 has been strongly implicated as a transforming agent, other virally encoded proteins may also play a role in the transformation process. In addition to the rex and tax genes, the pX region of the HTLV-1 genome contains two open reading frames (pX-I and pX-II) which encode the putative viral accessory proteins known as p12I, p30II, and p13II. Mutations in the ACH molecular clone of HTLV-1 that are predicted to abrogate the expression of p12I, p13II and p30II were constructed. These mutations had no effect on viral replication or the immortalization of primary lymphocytes. Although these proteins are dispensable for viral replication and immortalization in vitro, it remains possible that they alter infection in vivo.

Mutational analysis of human T-cell leukemia virus type 2 Tax

A mutational analysis of human T-cell leukemia virus type 2 (HTLV-2) Tax (Tax-2) was performed to identify regions within Tax-2 important for activation of promoters through the CREB/ATF or NF-kappaB/Rel signaling pathway. Tax-2 mutations within the putative zinc-binding region as well as mutations at the carboxy terminus disrupted CREB/ATF transactivation. A single mutation within the central proline-rich region of Tax-2 disrupted the transactivation of the NF-kappaB/Rel pathway. Surprisingly, this mutation, which is thought to be in a separate activation domain, was suppressed by mutations within or around the putative zinc-binding region, suggesting an interaction between these two regions. These analyses indicate that the functional regions or domains important for transactivation through the CREB/ATF or NF-kappaB/Rel signaling pathway are similar, but not identical, in Tax-1 and Tax-2. Identification of these distinct Tax-2 mutants should facilitate comparative biological studies of HTLV-1 and HTLV-2 and ultimately lead to the determination of the functional importance of Tax trans-acting capacities in T-lymphocyte transformation by HTLV.

X-I and X-II open reading frames of HTLV-I are not required for virus replication or for immortalization of primary T-cells in vitro

In contrast to other retroviruses of the oncovirinae subgroup, the primate and bovine leukemia viruses (HTLV, STLV, and BLV) encode genes in the X-region of the genome, between the env gene and the 3' long terminal repeat. In HTLV-I, two overlapping open reading frames (ORFs) in the distal half of the X-region encode tax and rex genes, while two ORFs (X-I and X-II) in the proximal half of this region potentially encode proteins designated p12(XI) (or rof) and p30(XII) (or tof). The biological functions and mechanisms of tax and rex have been studied extensively whereas the roles of the other ORFs have not yet been established. To identify possible functions for ORFs X-I and X-II, an infectious molecular clone of HTLV-I and a mutant provirus lacking these ORFs were compared with respect to virus replication, gene expression, and ability to immortalize primary T-cells. When transiently transfected into 293 cells, both intact and deleted proviruses directed the synthesis of virus mRNAs and proteins that were quantitatively and qualitatively identical. These viruses were also indistinguishable in their abilities to infect and replicate in DBS-FRhL cells, which are permissive for HTLV-I propagation. Immortalized T-cell lines were established after cell-free or coculture methods for infection of activated, human peripheral blood or cord blood lymphocytes with each of the cloned viruses. The growth kinetics, cytokine dependence, and cell surface markers of the infected T-cell cultures were similar for each provirus clone. Thus, ORFs X-I and X-II are not essential for virus infectivity, replication, gene expression, or T-cell immortalization in vitro.

Human T-cell leukemia viruses: epidemiology, biology, and pathogenesis

The human T-cell lymphotropic viruses type I and type II are closely related human retroviruses that have similar biological properties, genetic organization and tropism for T lymphocytes. Along with the simian T-cell lymphoma virus type I, they define the group of retroviruses known as the primate T-cell leukemia/lymphoma viruses. Initially identified in 1980, the human T-cell lymphotropic virus type I has been implicated as the etiologic agent of adult T-cell leukemia/lymphoma and of a degenerative neurologic disorder known as tropical spastic paraparesis or human T-cell lymphotropic virus type I-associated myelopathy. The intriguing link between human T-cell lymphotropic virus type, T-cell malignancy, and a totally unrelated and non-overlapping neurological disorder suggests divergent and unique pathogenetic mechanisms. This review will address the epidemiology, molecular biology, and pathogenesis of human T-cell leukemia viruses.

Inhibition of human T-cell leukemia virus type 2 Rex function by truncated forms of Rex encoded in alternatively spliced mRNAs

Three mRNA species encoding the x-III open reading frame are expressed in human T-cell leukemia virus type 2 (HTLV-2)-infected cells. An mRNA composed of exons 1, 2, and 3 produces the essential posttranscriptional regulator Rex; shorter 1-3 and 1-B mRNAs encode a family of x-III proteins of unknown function that represent truncated forms of Rex. This report presents an analysis of the functional interactions between Rex and the x-III proteins, results of which suggest a role for the x-III proteins as negative regulators of Rex function. Cotransfection assays demonstrated that the x-III proteins were able to inhibit the ability of Rex to activate the expression of a Rex-dependent mRNA. Analysis of intracellular compartmentalization in actinomycin D-treated cells showed that coexpression of the x-III proteins resulted in the sequestration of Rex into the nuclear compartment. Subcellular fractionation studies showed that Rex was preferentially localized in the cytoplasmic or nuclear fraction depending on its phosphorylation status and that coexpression of Rex with the x-III proteins changed the phosphorylation pattern of Rex and the intracellular distribution of the x-III proteins. In vitro protein binding assays demonstrated the formation of Rex-Rex homomultimeric complexes; however, mixed Rex/x-III multimers were not detected. These findings indicated a correlation between phosphorylation and intracellular trafficking of Rex and suggested that the mechanism underlying the inhibitory effects of the x-III proteins might result from an interference with these processes.

Both wild-type and strongly attenuated bovine leukemia viruses protect peripheral blood mononuclear cells from apoptosis

Bovine leukemia virus (BLV) and the human T-cell leukemia viruses belong to the same subfamily of oncoviruses. Although much attention has focused on the mechanisms of cell proliferation and transformation by these viruses, experiments on the apoptotic process have yielded conflicting data in in vitro cell culture. Experimental infection of sheep with BLV proviruses offers the opportunity to analyze apoptosis in vivo. Here, we show that BLV-infected peripheral mononuclear cells, cultivated ex vivo, are protected from spontaneous programmed cell death. Moreover, the virus is able to specifically interfere with the apoptotic program of infected B lymphocytes. Strongly attenuated mutant proviruses that harbor deletions in the G4 and/or R3 genes also decrease the global susceptibility to apoptosis at levels similar to those obtained with the wild-type virus. In addition, cell culture supernatants from wild-type and mutant viruses can prevent uninfected cells from undergoing programmed cell death. These observations demonstrate that the R3 and G4 genes are not required to maintain both direct and indirect protection against apoptosis. They also imply that the level of programmed cell death observed ex vivo is independent of the amounts of proviruses in the animals. The failure of these cells to undergo apoptosis might be related to the pathogenesis induced by BLV.

The tax gene of human T-cell leukemia virus type 2 is essential for transformation of human T lymphocytes

The mechanism of human T-cell leukemia virus (HTLV)-mediated transformation and induction of malignancy is unknown; however, several studies have implicated the viral gene product, Tax. Conclusive evidence for the role of Tax in the HTLV malignant process has been impeded by the inability to mutate tax in the context of an infectious virus and dissociate viral replication from cellular transformation. To circumvent this problem we constructed a mutant of HTLV type 2 (HTLV-2) that replicates by a Tax-independent mechanism. For these studies, the Tax response element in the viral long terminal repeat was replaced with the cytomegalovirus immediate-early promoter enhancer (C-enh). Transcription of the chimeric HTLV-2 (HTLVC-enh) was efficiently directed by this heterologous promoter. Also, the chimeric virus transformed primary human T lymphocytes with an efficiency similar to that of wild-type HTLV-2. A tax-knockout virus, termed HTLVC-enhDeltaTax, was constructed to directly assess the importance of Tax in cellular transformation. Transfection and infection studies indicated that HTLVC-enhDeltaTax was replication competent; however, HTLVC-enhDeltaTax failed to transform primary human T lymphocytes. We conclude that Tax is essential for HTLV-mediated transformation of human T lymphocytes. Furthermore, this chimeric HTLV, that replicates in the absence of Tax, should facilitate studies to determine the precise mechanism of T-lymphocyte transformation by HTLV.

Coding potential of the X region of human T-cell leukemia/lymphotropic virus type II

Human T-cell leukemia/lymphotropic viruses type I and type II (HTLV-I and HTLV-II) are complex human retroviruses showing a similar genetic organization but substantially different biologic and pathogenic properties. As in other complex retroviruses, the 3' portion of the HTLV genome contains the peculiar "X region" comprising several partially overlapping open reading frames (ORFs). To search for a possible basis for the pathogenic differences between the two viruses, a number of studies have been carried out to analyze the coding potential of the X region of HTLV-I and, more recently, of HTLV-II. This review focuses on the coding potential of the HTLV-II X region and presents a comparison with that of HTLV-I. Expression of different ORFs present in the X region may be accessed through two expression strategies: alternative splicing and translation of more than one protein from the same mRNA. Initial analyses of the X region proteins indicate that some differ significantly from their HTLV-I homologues, thus providing possible clues to the understanding of the complex life cycle and pathogenicity of the two viruses.