Evidence for the chronic in vivo production of human T cell leukemia virus type I Rof and Tof proteins from cytotoxic T lymphocytes directed against viral peptides

YAP suppresses human T-cell leukemia virus type 1 transcription

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that causes adult T-cell leukemia/lymphoma (ATL). HTLV-1 encodes Tax protein that activates transcription from viral long terminal repeats (LTR). Multiple cofactors are involved in the regulation of HTLV-1 transcription via association with Tax. Yes-associated protein (YAP), which is the key effector of Hippo pathway, is elevated and activated in ATL cells. In this study, we reported that YAP protein suppressed Tax activation of HTLV-1 5' LTR but not 3' LTR. The activation of the 5' LTR by Tax was potentiated when YAP was depleted. Moreover, overexpression of YAP repressed HTLV-1 plus-strand viral gene expression and virion production, whereas compromising YAP by RNA inference augmented the expression of HTLV-1 protein. As mechanisms of YAP-mediated viral transcription inhibition, we found that YAP interacted with Tax, and prevented the association between Tax and p300. It finally led to the inhibition of recruitment of Tax to the Tax-responsive element in the 5' LTR of HTLV-1. Taken together, our results demonstrate the negative regulatory function of YAP in Tax activation of HTLV-1 transcription. It may achieve sufficient transcriptional repression to maintain persistent infection and long-term latency of HTLV-1 in the host cells.

Hijacking Host Immunity by the Human T-Cell Leukemia Virus Type-1: Implications for Therapeutic and Preventive Vaccines

Human T-cell Leukemia virus type-1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and other inflammatory diseases. High viral DNA burden (VL) in peripheral blood mononuclear cells is a documented risk factor for ATLL and HAM/TSP, and patients with HAM/TSP have a higher VL in cerebrospinal fluid than in peripheral blood. VL alone is not sufficient to differentiate symptomatic patients from healthy carriers, suggesting the importance of other factors, including host immune response. HTLV-1 infection is life-long; CD4+-infected cells are not eradicated by the immune response because HTLV-1 inhibits the function of dendritic cells, monocytes, Natural Killer cells, and adaptive cytotoxic CD8+ responses. Although the majority of infected CD4+ T-cells adopt a resting phenotype, antigen stimulation may result in bursts of viral expression. The antigen-dependent "on-off" viral expression creates "conditional latency" that when combined with ineffective host responses precludes virus eradication. Epidemiological and clinical data suggest that the continuous attempt of the host immunity to eliminate infected cells results in chronic immune activation that can be further exacerbated by co-morbidities, resulting in the development of severe disease. We review cell and animal model studies that uncovered mechanisms used by HTLV-1 to usurp and/or counteract host immunity.

Short-term cultured autologous peripheral blood mononuclear cells as a potential immunogen to activate Tax-specific CTL response in adult T-cell leukemia patients

Activation of CD8⁺ Tax‐specific CTL is a new therapeutic concept for adult T‐cell leukemia (ATL) caused by HTLV‐1. A recent clinical study of the dendritic cell vaccine pulsed with Tax peptides corresponding to CTL epitopes showed promising outcomes in ATL patients possessing limited human leukocyte antigen (HLA) alleles. In this study, we aimed to develop another immunotherapy to activate Tax‐specific CTL without HLA limitation by using patients’ own HTLV‐1‐infected cells as a vaccine. To examine the potential of HTLV‐1‐infected T‐cells to activate CTL via antigen presenting cells, we established a unique co–culture system. We demonstrated that mitomycin C‐treated HLA‐A2‐negative HTLV‐1‐infected T‐cell lines or short‐term cultured peripheral blood mononuclear cells (PBMC) derived from ATL patients induced cross–presentation of Tax antigen in co–cultured HLA‐A2‐positive antigen presenting cells, resulting in activation of HLA‐A2‐restricted CD8⁺ Tax‐specific CTL. This effect was not inhibited by a reverse transcriptase inhibitor. IL‐12 production and CD86 expression were also induced in antigen presenting cells co–cultured with HTLV‐1‐infected cells at various levels, which were improved by pre–treatment of the infected cells with histone deacetylase inhibitors. Furthermore, monocyte‐derived dendritic cells induced from PBMC of a chronic ATL patient produced IL‐12 and expressed enhanced levels of CD86 when co–cultured with autologous lymphocytes that had been isolated from the same PBMC and cultured for several days. These findings suggest that short‐term cultured autologous PBMC from ATL patients could potentially serve as a vaccine to evoke Tax‐specific CTL responses.

Human T-cell lymphotropic virus HBZ and tax mRNA expression are associated with specific clinicopathological features in adult T-cell leukemia/lymphoma

Adult T-cell leukemia/lymphoma (ATLL) is caused by human T-cell leukemia virus type 1 (HTLV-1). HTLV-1-associated mRNA, including HBZ and tax, is deeply involved in the pathogenesis of ATLL. Using 88 ATLL tissue samples, we performed in situ mRNA analysis of HBZ and tax, and investigated its association with clinicopathological characteristics of ATLL. The median value of HBZ signals (/1000 ATLL cells) was 795.2 (range: 0.4-4013.1) and of tax signals (/1000 ATLL cells) was 5.1 (range: 0.1-891.2). The low-expression HBZ group displayed significant increase in the number of skin lesion (P = 0.0283). The high-expression tax group displayed significant increase in the number of PD-1-positive tumor-infiltrating lymphocytes (P < 0.0001). In addition, we identified patients with very high-expression of tax signals (400 or more signals/1000 ATLL cells). These patients displayed significant reductions in the expression of HLA class I (P = 0.0385) and β2M (P = 0.0124). Moreover, these patients displayed significantly poor overall survival (median survival time [MST] 7.7 months, 95% confidence interval [CI] [4.7-NA]), compared with the survival in patients with less than 400 tax signals (MST 22.6 months, 95% CI [13.7-41.7]) (P = 0.0499). These results suggest that Tax-mediated treatment of ATLL should be performed carefully in the high-expression tax group. More detailed studies could elucidate the clinicopathological significance of HBZ and tax mRNA expressions in ATLL.

p30 protein: a critical regulator of HTLV-1 viral latency and host immunity

The extraordinarily high prevalence of HTLV-1 subtype C (HTLV-1C) in some isolated indigenous communities in Oceania and the severity of the health conditions associated with the virus impress the great need for basic and translational research to prevent and treat HTLV-1 infection. The genome of the virus’s most common subtype, HTLV-1A, encodes structural, enzymatic, and regulatory proteins that contribute to viral persistence and pathogenesis. Among these is the p30 protein encoded by the doubly spliced Tax-orf II mRNA, a nuclear/nucleolar protein with both transcriptional and post-transcriptional activity. The p30 protein inhibits the productive replication cycle via nuclear retention of the mRNA that encodes for both the viral transcriptional trans-activator Tax, and the Rex proteins that regulate the transport of incompletely spliced viral mRNA to the cytoplasm. In myeloid cells, p30 inhibits the PU-1 transcription factor that regulates interferon expression and is a critical mediator of innate and adaptive immunity. Furthermore, p30 alters gene expression, cell cycle progression, and DNA damage responses in T-cells, raising the hypothesis that p30 may directly contribute to T cell transformation. By fine-tuning viral expression while also inhibiting host innate responses, p30 is likely essential for viral infection and persistence. This concept is supported by the finding that macaques, a natural host for the closely genetically related simian T-cell leukemia virus 1 (STLV-1), exposed to an HTLV-1 knockout for p30 expression by a single point mutation do not became infected unless reversion and selection of the wild type HTLV-1 genotype occurs. All together, these data suggest that inhibition of p30 may help to curb and eventually eradicate viral infection by exposing infected cells to an effective host immune response.

Silencers of HTLV-1 and HTLV-2: the pX-encoded latency-maintenance factors

Of the members of the primate T cell lymphotropic virus (PTLV) family, only the human T-cell leukemia virus type-1 (HTLV-1) causes disease in humans—as the etiological agent of adult T-cell leukemia/lymphoma (ATLL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other auto-inflammatory disorders. Despite having significant genomic organizational and structural similarities, the closely related human T-cell lymphotropic virus type-2 (HTLV-2) is considered apathogenic and has been linked with benign lymphoproliferation and mild neurological symptoms in certain infected patients. The silencing of proviral gene expression and maintenance of latency are central for the establishment of persistent infections in vivo. The conserved pX sequences of HTLV-1 and HTLV-2 encode several ancillary factors which have been shown to negatively regulate proviral gene expression, while simultaneously activating host cellular proliferative and pro-survival pathways. In particular, the ORF-II proteins, HTLV-1 p30^II and HTLV-2 p28^II, suppress Tax-dependent transactivation from the viral promoter—whereas p30^II also inhibits PU.1-mediated inflammatory-signaling, differentially augments the expression of p53-regulated metabolic/pro-survival genes, and induces lymphoproliferation which could promote mitotic proviral replication. The ubiquitinated form of the HTLV-1 p13^II protein localizes to nuclear speckles and interferes with recruitment of the p300 coactivator by the viral transactivator Tax. Further, the antisense-encoded HTLV-1 HBZ and HTLV-2 APH-2 proteins and mRNAs negatively regulate Tax-dependent proviral gene expression and activate inflammatory signaling associated with enhanced T-cell lymphoproliferation. This review will summarize our current understanding of the pX latency-maintenance factors of HTLV-1 and HTLV-2 and discuss how these products may contribute to the differences in pathogenicity between the human PTLVs.

Impact of host immunity on HTLV-1 pathogenesis: potential of Tax-targeted immunotherapy against ATL

Human T-cell leukemia virus type-1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATL), HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), and other inflammatory diseases. There is no disease-specific difference in viral strains, and it is unclear how HTLV-1 causes such different diseases manifesting as lymphoproliferation or inflammation. Although some progress has been made in therapies for these diseases, the prognosis for ATL is still dismal and HAM/TSP remains an intractable disease. So far, two regulatory proteins of HTLV-1, Tax and HBZ, have been well studied and shown to have pleiotropic functions implicated in viral pathogenesis. Tax in particular can strongly activate NFκB, which is constitutively activated in HTLV-1-infected cells and considered to contribute to both oncogenesis and inflammation. However, the expression level of Tax is very low in vivo, leading to confusion in understanding its role in viral pathogenesis. A series of studies using IL-2-dependent HTLV-1-infected cells indicated that IL-10, an anti-inflammatory/immune suppressive cytokine, could induce a proliferative phenotype in HTLV-1-infected cells. In addition, type I interferon (IFN) suppresses HTLV-1 expression in a reversible manner. These findings suggest involvement of host innate immunity in the switch between lymphoproliferative and inflammatory diseases as well as the regulation of HTLV-1 expression. Innate immune responses also affect another important host determinant, Tax-specific cytotoxic T lymphocytes (CTLs), which are impaired in ATL patients, while activated in HAM/TSP patients. Activation of Tax-specific CTLs in ATL patients after hematopoietic stem cell transplantation indicates Tax expression and its fluctuation in vivo. A recently developed anti-ATL therapeutic vaccine, consisting of Tax peptide-pulsed dendritic cells, induced Tax-specific CTL responses in ATL patients and exhibited favorable clinical outcomes, unless Tax-defective ATL clones emerged. These findings support the significance of Tax in HTLV-1 pathogenesis, at least in part, and encourage Tax-targeted immunotherapy in ATL. Host innate and acquired immune responses induce host microenvironments that modify HTLV-1-encoded pathogenesis and establish a complicated network for development of diseases in HTLV-1 infection. Both host and viral factors should be taken into consideration in development of therapeutic and prophylactic strategies in HTLV-1 infection.

The human T-cell leukemia virus type-1 tax oncoprotein dissociates NF-κB p65RelA-Stathmin complexes and causes catastrophic mitotic spindle damage and genomic instability

Genomic instability is a hallmark of many cancers; however, the molecular etiology of chromosomal dysregulation is not well understood. The human T-cell leukemia virus type-1 (HTLV-1) oncoprotein Tax activates NF-κB-signaling and induces DNA-damage and aberrant chromosomal segregation through diverse mechanisms which contribute to viral carcinogenesis. Intriguingly, Stathmin/oncoprotein-18 (Op-18) depolymerizes tubulin and interacts with the p65^RelA subunit and functions as a cofactor for NF-κB-dependent transactivation. We thus hypothesized that the dissociation of p65^RelA-Stathmin/Op-18 complexes by Tax could lead to the catastrophic destabilization of microtubule (MT) spindle fibers during mitosis and provide a novel mechanistic link between NF-κB-signaling and genomic instability. Here we report that the inhibition of Stathmin expression by the retroviral latency protein, p30^II, or knockdown with siRNA-stathmin, dampens Tax-mediated NF-κB transactivation and counters Tax-induced genomic instability and cytotoxicity. The Tax-G148V mutant, defective for NF-κB activation, exhibited reduced p65^RelA-Stathmin binding and diminished genomic instability and cytotoxicity. Dominant-negative inhibitors of NF-κB also prevented Tax-induced multinucleation and apoptosis. Moreover, cell clones containing the infectious HTLV-1 ACH. p30^II mutant provirus, impaired for p30^II production, exhibited increased multinucleation and the accumulation of cytoplasmic tubulin aggregates following nocodozole-treatment. These findings allude to a mechanism whereby NF-κB-signaling regulates tubulin dynamics and mitotic instability through the modulation of p65^RelA-Stathmin/Op-18 interactions, and support the notion that p30^II enhances the survival of Tax-expressing HTLV-1-transformed cells.

The TP53-Induced Glycolysis and Apoptosis Regulator mediates cooperation between HTLV-1 p30II and the retroviral oncoproteins Tax and HBZ and is highly expressed in an in vivo xenograft model of HTLV-1-induced lymphoma

The human T-cell leukemia virus type-1 (HTLV-1) is an oncoretrovirus that infects and transforms CD4+ T-cells and causes adult T-cell leukemia/lymphoma (ATLL) -an aggressive lymphoproliferative disease that is highly refractive to most anticancer therapies. The HTLV-1 proviral genome encodes several regulatory products within a conserved 3' nucleotide sequence, known as pX; however, it remains unclear how these factors might cooperate or dynamically interact in virus-infected cells. Here we demonstrate that the HTLV-1 latency-maintenance factor p30^II induces the TP53-induced glycolysis and apoptosis regulator (TIGAR) and counters the oxidative stress, mitochondrial damage, and cytotoxicity caused by the viral oncoproteins Tax and HBZ. The p30^II protein cooperates with Tax and HBZ and enhances their oncogenic potential in colony transformation/foci-formation assays. Further, we have shown that TIGAR is highly expressed in HTLV-1-induced tumors associated with oncogene dysregulation and increased angiogenesis in an in vivo xenograft model of HTLV-1-induced T-cell lymphoma. These findings provide the first evidence that p30^II likely collaborates as an ancillary factor for the major oncoproteins Tax and HBZ during retroviral carcinogenesis.

Human T Cell Leukemia Virus Type 1: Persistence and Pathogenesis

Human T cell leukemia virus type 1 (HTLV-1), also known as human T lymphotropic virus type 1, was the first exogenous human retrovirus discovered. Unlike the distantly related lentivirus HIV-1, HTLV-1 causes disease in only 5-10% of infected people, depending on their ethnic origin. But whereas HIV-1 infection and the consequent diseases can be efficiently contained in most cases by antiretroviral drug treatment, there is no satisfactory treatment for the malignant or inflammatory diseases caused by HTLV-1. The purpose of the present article is to review recent advances in the understanding of the mechanisms by which the virus persists in vivo and causes disabling or fatal diseases.

Atomic Coordination Reflects Peptide Immunogenicity

We demonstrated that the immunological identity of variant peptides may be accurately predicted on the basis of atomic coordination of both unprotonated and protonated tertiary structures, provided that the structure of the native peptide (index) is known. The metric which was discovered to account for this discrimination is the coordination difference between the variant and the index; we also showed that increasing coordination difference in respect to the index was correlated to a correspondingly weakening immunological outcome of the variant. Additionally, we established that this metric quickly seizes to operate beyond the peptide scale, e.g., within a coordination shell inclusive of atoms up to a distance of 7 Å away from the peptide or over the entire pMHC-TCR complex. Analysis of molecular orbital interactions for a range of formal charges further revealed that the N-terminus of the agonists was always able to sustain a stable ammonium (NH3+) group which was consistently absent in antagonists. We deem that the presence of NH3+ constitutes a secondary observable with a biological consequence, signifying a change in T cell activation. While our analysis of protonated structures relied on the quantum chemical relaxation of the H species, the results were consistent across a wide range of peptide charge and spin polarization conditions.

The coordination of unprotonated peptide tertiary structure as a metric of pMHC-TCR functional avidity

The coordination difference between the unprotonated tertiary structures of a native (Tax) peptide and a number of its variants – all peptides presented by HLA-A201 and bound to the human A6 T cell receptor-was discovered to constitute a metric of pMHC–TCR functional avidity. Moreover, increasing coordination deviations from the index were found to flag correspondingly weakening immunological outcomes of the variant peptides. The prognostic utility of the coordination difference of unprotonated tertiary structure was established to operate strictly on the peptide scale, seizing to be of relevance either to the immediate peptide environment (i.e. within the realm of peptide short range order, within 7 Å of any peptide atom) or over the entirety of the pMHC–TCR complex. Additionally, the imprint of peptide immunological identity was expressed both by the total coordination as well as by its C–C partial.

Quantum chemical calculations predict biological function: the case of T cell receptor interaction with a peptide/MHC class I

A combination of atomic correlation statistics and quantum chemical calculations are shown to predict biological function. In the present study, various antigenic peptide-Major Histocompatibility Complex (pMHC) ligands with near-identical stereochemistries, in complexation with the same T cell receptor (TCR), were found to consistently induce distinctly different quantum chemical behavior, directly dependent on the peptide's electron spin density and intrinsically expressed by the protonation state of the peptide's N-terminus. Furthermore, the cumulative coordination difference of any variant in respect to the native peptide was found to accurately reflect peptide biological function and immerges as the physical observable which is directly related to the immunological end-effect of pMHC-TCR interaction.

Discovery and characterization of auxiliary proteins encoded by type 3 simian T-cell lymphotropic viruses

Human T-cell lymphotropic virus type 1 (HTLV-1) and HTLV-2 encode auxiliary proteins that play important roles in viral replication, viral latency, and immune escape. The presence of auxiliary protein-encoding open reading frames (ORFs) in HTLV-3, the latest HTLV to be discovered, is unknown. Simian T-cell lymphotropic virus type 3 (STLV-3) is almost identical to HTLV-3. Given the lack of HTLV-3-infected cell lines, we took advantage of STLV-3-infected cells and of an STLV-3 molecular clone to search for the presence of auxiliary transcripts. Using reverse transcriptase PCR (RT-PCR), we first uncovered the presence of three unknown viral mRNAs encoding putative proteins of 5, 8, and 9 kDa and confirmed the presence of the previously reported RorfII transcript. The existence of these viral mRNAs was confirmed by using splice site-specific RT-PCR with ex vivo samples. We showed that p5 is distributed throughout the cell and does not colocalize with a specific organelle. The p9 localization is similar to that of HTLV-1 p12 and induced a strong decrease in the calreticulin signal, similarly to HTLV-1 p12. Although p8, RorfII, and Rex-3 share an N-terminal sequence that is predicted to contain a nucleolar localization signal (NoLS), only p8 is found in the nucleolus. The p8 location in the nucleolus is linked to a bipartite NoLS. p8 and, to a lesser extent, p9 repressed viral expression but did not alter Rex-3-dependent mRNA export. Using a transformation assay, we finally showed that none of the STLV-3 auxiliary proteins had the ability to induce colony formation, while both Tax-3 and antisense protein of HTLV-3 (APH-3) promoted cellular transformation. Altogether, these results complete the characterization of the newly described primate T-lymphotropic virus type 3 (PTLV-3).

IMPORTANCE

Together with their simian counterparts, HTLVs form the primate T-lymphotropic viruses. HTLVs arose from interspecies transmission between nonhuman primates and humans. HTLV-1 and HTLV-2 encode auxiliary proteins that play important roles in viral replication, viral latency, and immune escape. The presence of ORFs encoding auxiliary proteins in HTLV-3 or STLV-3 genomes was unknown. Using in silico analyses, ex vivo samples, or in vitro experiments, we have uncovered the presence of 3 previously unknown viral mRNAs encoding putative proteins and confirmed the presence of a previously reported viral transcript. We characterized the intracellular localization of the four proteins. We showed that two of these proteins repress viral expression but that none of them have the ability to induce colony formation. However, both Tax and the antisense protein APH-3 promote cell transformation. Our results allowed us to characterize 4 new retroviral proteins for the first time.

The need to accessorize: molecular roles of HTLV-1 p30 and HTLV-2 p28 accessory proteins in the viral life cycle

Extensive studies of human T-cell leukemia virus (HTLV)-1 and HTLV-2 over the last three decades have provided detailed knowledge on viral transformation, host–viral interactions and pathogenesis. HTLV-1 is the etiological agent of adult T cell leukemia and multiple neurodegenerative and inflammatory diseases while HTLV-2 disease association remains elusive, with few infected individuals displaying neurodegenerative diseases similar to HTLV-1. The HTLV group of oncoretroviruses has a genome that encodes structural and enzymatic proteins Gag, Pro, and Env, regulatory proteins Tax and Rex, and several accessory proteins from the pX region. Of these proteins, HTLV-1 p30 and HTLV-2 p28 are encoded by the open reading frame II of the pX region. Like most other accessory proteins, p30 and p28 are dispensable for in vitro viral replication and transformation but are required for efficient viral replication and persistence in vivo. Both p30 and p28 regulate viral gene expression at the post-transcriptional level whereas p30 can also function at the transcriptional level. Recently, several reports have implicated p30 and p28 in multiple cellular processes, which provide novel insight into HTLV spread and survival and ultimately pathogenesis. In this review we summarize and compare what is known about p30 and p28, highlighting their roles in viral replication and viral pathogenesis.

Human T-lymphotropic virus type 1 non-structural proteins: Requirements for latent infection

It has been more than 30 years since the discovery of human T-lymphotropic virus type 1 (HTLV-1), the first human retrovirus identified. Human T-lymphotropic virus type 1 infects 15-20 million people worldwide causing two major diseases: adult T-cell leukemia/lymphoma and HTLV-1-associated myelopathy/tropical spastic paraparesis. Human T-lymphotropic virus type 1 establishes several decades of latent infection, during which viral-host interaction determines disease segregation. This review highlights non-structural proteins that are encoded on the viral genome and manage latent infection. Latent infection is a key in HTLV pathology, so that effective inhibition of these proteins might lead to successful disease management.

Overview on HTLV-1 p12, p8, p30, p13: accomplices in persistent infection and viral pathogenesis

The human T-lymphotropic virus type-1 (HTLV-1) is etiologically linked to adult T cell leukemia/lymphoma and tropical spastic paraparesis/HTLV-1-associated myelopathy. While the role of Tax and Rex in viral replication and pathogenesis has been extensively studied, recent evidence suggests that additional viral proteins are essential for the virus life cycle in vivo. In this review, we will summarize possible molecular mechanisms evoked in the literature to explain how p12, p8, p30, and p13 facilitate persistent viral infection of the host. We will explore several stratagems used by HTLV-1 accessory genes to escape immune surveillance, to establish latency, and to deregulate cell cycle and apoptosis to participate in virus-mediated cellular transformation.

The roles of acquired and innate immunity in human T-cell leukemia virus type 1-mediated diseases

Human T-cell leukemia virus type 1 (HTLV-1) causes adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis in small subsets of HTLV-1 carriers. HTLV-1-specific T-cell responses play critical roles in anti-viral and anti-tumor host defense during HTLV-1 infections. Some HTLV-1 carriers exhibit selective loss or anergy of HTLV-1-specific T-cells at an asymptomatic stage. This is also observed in ATL patients and may therefore be an underlying risk factor of ATL in combination with elevated proviral loads. HTLV-1-specific T-cells often recognize the viral oncoprotein Tax, indicating expression of Tax protein in vivo, although levels of HTLV-1 gene expression are known to be very low. A type-I interferon (IFN) response can be induced by HTLV-1-infected cells and suppresses HTLV-1 expression in vitro, suggesting a role of type-I IFN response in viral suppression and pathogenesis in vivo. Both acquired and innate immune responses control the status of HTLV-1-infected cells and could be the important determinants in the development of HTLV-1-mediated malignant and inflammatory diseases.

Orf-I and orf-II-encoded proteins in HTLV-1 infection and persistence

The 3′ end of the human T-cell leukemia/lymphoma virus type-1 (HTLV-1) genome contains four overlapping open reading frames (ORF) that encode regulatory proteins. Here, we review current knowledge of HTLV-1 orf-I and orf-II protein products. Singly spliced mRNA from orf-I encodes p12, which can be proteolytically cleaved to generate p8, while differential splicing of mRNA from orf-II results in production of p13 and p30. These proteins have been demonstrated to modulate transcription, apoptosis, host cell activation and proliferation, virus infectivity and transmission, and host immune responses. Though these proteins are not essential for virus replication in vitro, p8, p12, p13, and p30 have an important role in the establishment and maintenance of HTLV-1 infection in vivo.

Human T-lymphotropic virus type 1 p30 interacts with REGgamma and modulates ATM (ataxia telangiectasia mutated) to promote cell survival

Human T-lymphotropic virus type 1 (HTLV-1) is a causative agent of adult T cell leukemia/lymphoma and a variety of inflammatory disorders. HTLV-1 encodes a nuclear localizing protein, p30, that selectively alters viral and cellular gene expression, activates G(2)-M cell cycle checkpoints, and is essential for viral spread. Here, we used immunoprecipitation and affinity pulldown of ectopically expressed p30 coupled with mass spectrometry to identify cellular binding partners of p30. Our data indicate that p30 specifically binds to cellular ATM (ataxia telangiectasia mutated) and REGγ (a nuclear 20 S proteasome activator). Under conditions of genotoxic stress, p30 expression was associated with reduced levels of ATM and increased cell survival. Knockdown or overexpression of REGγ paralleled p30 expression, suggesting an unexpected enhancement of p30 expression in the presence of REGγ. Finally, size exclusion chromatography revealed the presence of p30 in a high molecular mass complex along with ATM and REGγ. On the basis of our findings, we propose that HTLV-1 p30 interacts with ATM and REGγ to increase viral spread by facilitating cell survival.

Hijacking the T-cell communication network by the human T-cell leukemia/lymphoma virus type 1 (HTLV-1) p12 and p8 proteins

The non-structural proteins encoded by the orf-I, II, III, and IV genes of the human T-cell leukemia/lymphoma virus type 1 (HTLV-1) genome, are critical for the modulation of cellular gene expression and T-cell proliferation, the escape from cytotoxic T-cells and natural killer cells, and virus expression. In here, we review the main functions of the HTLV-1 orf-I products. The 12kDa product from orf-I (p12) is proteolytically cleaved within the endoplasmic reticulum (ER) to generate the 8kDa protein (p8). At the steady state, both proteins are expressed at similar levels in transfected T-cells. The p12 protein remains in the ER and cis-Golgi, whereas the p8 protein traffics to the cell surface and is recruited to the immunological synapse. The p12 and the p8 proteins have seemingly opposite effects on T-cells; the ER resident p12, modulates T-cell activation and proliferation, whereas p8 induces T-cell anergy. The p8 protein also increases the formation of cellular conduits, is transferred to neighboring T-cells, and increases virus transmission. The requirement for HTLV-1 infectivity of orf-I is demonstrated by the loss of virus infectivity in macaques exposed to an engineered virus, whereby expression of orf-I was ablated. Altogether the current knowledge demonstrates that the concerted activity of p8 and p12 is essential for the persistence of virus infected cells in the host.

Neuroimmunity of HTLV-I Infection

Human T-lymphotrophic virus type I (HTLV-I) is an oncogenic retrovirus and its infection is associated with a variety of human diseases including HTLV-I-associated myelopathy/tropic spastic paraparesis (HAM/TSP). Large numbers of epidemiological, virological, immunological, and clinical studies on HTLV-I- and HTLV-I-associated diseases have been published, although the pathogenesis of HAM/TSP remains to be fully understood. In the last several years, researchers have shown that several key factors are important in HTLV-I-associated neurologic disease including high HTLV-I proviral load and a strong immune response to HTLV-I. Here, we review pathophysiological findings on HAM/TSP and focus on viral-host immune responses to the virus in HTLV-I infected individuals. In particular, the role of HTLV-I-specific CD8+ T cell response is highlighted.

Novel role for interleukin-2 receptor-Jak signaling in retrovirus transmission

Human T-lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma, and it encodes a number of nonstructural proteins that are involved in virus replication and immune evasion. The viral protein p12 previously has been characterized to interfere with major histocompatibility complex class, ICAM-1, and ICAM-2 expression, and it activates STAT5. Using a previously established T-cell line immortalized with an HTLV-1 molecular clone deleted for p12, we assessed the role of p12 in regulating cellular growth and virus transmission. These cells were complemented for p12 expression by the transduction of a lentivirus vector expressing p12. We report that p12 conferred a selective growth advantage in vitro and increased the colony formation of human T cells in soft-agar assays. Consistently with previous studies, p12- and p12+ cell lines produced similar amounts of virus particles released into the supernatant of cultured cells, although we found that p12 expression greatly enhanced virus transmission. Moreover, we found that interleukin-2 (IL-2) stimulation also increased HTLV-1 transmission whether p12 was expressed or not, and inversely, that the inhibition of Jak signaling significantly reduced HTLV-1 transmission. Intriguingly, IL-2/Jak signaling was not associated with changes in viral gene expression, viral RNA encapsidation, the maturation of the virus particle, cell-cell adherence, or Gag polarization and virological synapse formation. We do demonstrate, however, that IL-2 stimulation and p12 expression significantly increased the rate of syncytium formation, revealing a novel role for IL-2 signaling and Jak activation in HTLV-1 virus transmission.

Presentation of human T cell leukemia virus type 1 (HTLV-1) Tax protein by dendritic cells: the underlying mechanism of HTLV-1-associated neuroinflammatory disease

HTLV-1 is the etiologic agent of a debilitating neurologic disorder, HAM/TSP. This disease features a robust immune response including the oligoclonal expansion of CD8+ CTLs specific for the viral oncoprotein Tax. The key pathogenic process resulting in the proliferation of CTLs and the presentation of Tax peptide remains uncharacterized. We have investigated the role of APCs, particularly DCs, in priming of the anti-Tax CTL response under in vitro and in vivo conditions. We investigated two routes (direct vs. indirect) of Tax presentation using live virus, infected primary CD4+/CD25+ T cells, and the CD4+ T cell line (C8166, a HTLV-1-mutated line that only expresses Tax). Our results indicated that DCs are capable of priming a pronounced Tax-specific CTL response in cell cultures consisting of naïve PBLs as well as in HLA-A*0201 transgenic mice (line HHD II). DCs were able to direct the presentation of Tax successfully through infected T cells, live virus, and cell-free Tax. These observations were comparable with those made with a known stimulant of DC maturation, a combination of CD40L and IFN-gamma. Our studies clearly establish a role for this important immune cell component in HTLV-1 immuno/neuropathogenesis and suggest that modulation of DC functions could be an important tool for therapeutic interventions.

The avidity and lytic efficiency of the CTL response to HTLV-1

In human T-lymphotropic virus type 1 (HTLV-1) infection, a high frequency of HTLV-1-specific CTLs can co-exist stably with a high proviral load and the proviral load is strongly correlated with the risk of HTLV-1-associated inflammatory diseases. These observations led to the hypothesis that HTLV-1 specific CTLs are ineffective in controlling HTLV-1 replication but contribute to the pathogenesis of the inflammatory diseases. But evidence from host and viral immunogenetics and gene expression microarrays suggests that a strong CTL response is associated with a low proviral load and a low risk of HAM/TSP. Here, we quantified the frequency, lytic activity and functional avidity of HTLV-1-specific CD8(+) cells in fresh, unstimulated PBMCs from individuals with natural HTLV-1 infection. The lytic efficiency of the CD8(+) T cell response-the fraction of autologous HTLV-1-expressing cells eliminated per CD8(+) cell per day-was inversely correlated with both the proviral load and the rate of spontaneous proviral expression. The functional avidity of HTLV-1-specific CD8(+) cells was strongly correlated with their lytic efficiency. We conclude that efficient control of HTLV-1 in vivo depends on the CTL lytic efficiency, which depends in turn on CTL avidity of Ag recognition. CTL quality determines the position of virus-host equilibrium in persistent HTLV-1 infection.

HTLV gene regulation: because size matters, transcription is not enough

Despite being discovered in animals in the early 20th century, the scientific interest in retroviruses was boosted with the discovery of human retroviruses (human T-leukemia/lymphoma virus [HTLV] and HIV), which are responsible for significant morbidity and mortality. HTLV was identified more than 25 years ago as the etiological agent of adult T-cell leukemia/lymphoma. It was then shown to be a complex retrovirus, given that it not only encodes the characteristic retroviral Gag, Pol and Env proteins, but also regulatory and accessory proteins. Since the first studies documenting the role of these proteins in viral expression, the picture has become increasingly more complex. Indeed, owing to the limited size of its genome that contains overlapping open-reading frames, HTLV has evolved unique ways to regulate its expression. Retroviral expression was originally thought to be mainly controlled through the regulation of transcription from the 5 long-terminal repeats, but we now know that the 3 long-terminal repeats also serve as promoters. Regulation of splicing and mRNA export, and post-translational modifications of viral protein also play a major role. This review discusses the latest insights gained into the field of HTLV gene expression.

HTLV-1 Yin and Yang: Rex and p30 master regulators of viral mRNA trafficking

Human retroviruses are associated with a variety of malignancies including Kaposi's sarcoma and Epstein-Barr virus-associated lymphoma in HIV infection, T-cell leukemia/lymphoma and a neurologic disorder in human T-cell lymphotropic virus type 1 (HTLV-1) infection. Both HIV and human T-cell lymphotropic virus type 1 have evolved a complex genetic organization for optimal use of their limited genome and production of all necessary structural and regulatory proteins. Use of alternative splicing is essential for balanced expression of multiple viral regulators from one genomic polycistronic RNA. In addition, nuclear export of incompletely spliced RNA is required for production of structural and enzymatic proteins and virus particles. Decisions controlling these events are largely guarded by viral proteins. In human T-cell lymphotropic virus type 1, Rex and p30 are both nuclear/nucleolar RNA binding regulatory proteins. Rex interacts with a Rex-responsive element to stimulate nuclear export of incompletely spliced RNA and increase production of virus particles. In contrast, human T-cell lymphotropic virus type 1 p30 is involved in the nuclear retention of the tax/rex mRNA leading to inhibition of virus expression and establishment of viral latency. How these two proteins, with apparently opposite functions, orchestrate virus replication and ensure vigilant control of viral gene expression is discussed.

In vivo genetic mutations define predominant functions of the human T-cell leukemia/lymphoma virus p12I protein

The human T-cell leukemia/lymphoma virus type 1 (HTLV-1) ORF-I encodes a 99-amino acid hydrophobic membrane protein, p12(I), that affects receptors in different cellular compartments. We report here that proteolytic cleavage dictates different cellular localization and functions of p12(I). The removal of a noncanonical endoplasmic reticulum (ER) retention/retrieval signal within the amino terminus of p12(I) is necessary for trafficking to the Golgi apparatus and generation of a completely cleaved 8-kDa protein. The 8-kDa protein in turn traffics to the cell surface, is recruited to the immunologic synapse following T-cell receptor (TCR) ligation, and down-regulates TCR proximal signaling. The uncleaved 12-kDa form of p12(I) resides in the ER and interacts with the beta and gamma(c) chains of the interleukin-2 receptor (IL-2R), the heavy chain of the major histocompatibility complex (MHC) class I, as well as calreticulin and calnexin. Genetic analysis of ORF-I from ex vivo samples of HTLV-1-infected patients reveals predominant amino acid substitutions within ORF-I that affect proteolytic cleavage, suggesting that ER-associated functions of p12(I) may contribute to the survival and proliferation of the infected T cells in the host.

Immunologic control of human T-cell leukemia virus type I and adult T-cell leukemia

Host T-cell responses to human T-cell leukemia virus type I (HTLV-I) control the expansion of HTLV-I-infected cells and are determinants of the equilibrium proviral load in vivo. Insufficient T-cell responses are regarded as an immunologic risk factor for adult T-cell leukemia (ATL) because they allow increased proviral loads, which represent an epidemiologic risk factor for ATL. ATL cells from approximately half of ATL cases retain the ability to express HTLV-I Tax, a major target antigen of HTLV-I-specific cytotoxic T-lymphocytes (CTL), whereas Tax-specific CTL in ATL patients are inactive. Tax-specific CTL responses are strongly activated after hematopoietic stem cell transplantation in some ATL patients in long-term remission, indicating that HTLV-I Tax is expressed in vivo rather than being silent, and that the donor-derived T-cell system can recognize it. These findings strongly suggest that reactivation of Tax-specific CTL by vaccines may be promising for prophylaxis of ATL in the high-risk group of HTLV-I carriers and for therapy of ATL in patients whose tumor cells are capable of expressing Tax.

Human T-lymphotropic virus type-1 p30 alters cell cycle G2 regulation of T lymphocytes to enhance cell survival

Background

Human T-lymphotropic virus type-1 (HTLV-1) causes adult T-cell leukemia/lymphoma and is linked to a number of lymphocyte-mediated disorders. HTLV-1 contains both regulatory and accessory genes in four pX open reading frames. pX ORF-II encodes two proteins, p13 and p30, whose roles are still being defined in the virus life cycle and in HTLV-1 virus-host cell interactions. Proviral clones of HTLV-1 with pX ORF-II mutations diminish the ability of the virus to maintain viral loads in vivo. p30 expressed exogenously differentially modulates CREB and Tax-responsive element-mediated transcription through its interaction with CREB-binding protein/p300 and while acting as a repressor of many genes including Tax, in part by blocking tax/rex RNA nuclear export, selectively enhances key gene pathways involved in T-cell signaling/activation.

Results

Herein, we analyzed the role of p30 in cell cycle regulation. Jurkat T-cells transduced with a p30 expressing lentivirus vector accumulated in the G2-M phase of cell cycle. We then analyzed key proteins involved in G2-M checkpoint activation. p30 expression in Jurkat T-cells resulted in an increase in phosphorylation at serine 216 of nuclear cell division cycle 25C (Cdc25C), had enhanced checkpoint kinase 1 (Chk1) serine 345 phosphorylation, reduced expression of polo-like kinase 1 (PLK1), diminished phosphorylation of PLK1 at tyrosine 210 and reduced phosphorylation of Cdc25C at serine 198. Finally, primary human lymphocyte derived cell lines immortalized by a HTLV-1 proviral clone defective in p30 expression were more susceptible to camptothecin induced apoptosis. Collectively these data are consistent with a cell survival role of p30 against genotoxic insults to HTLV-1 infected lymphocytes.

Conclusion

Collectively, our data are the first to indicate that HTLV-1 p30 expression results in activation of the G2-M cell cycle checkpoint, events that would promote early viral spread and T-cell survival.

In vivo T lymphocyte dynamics in humans and the impact of human T-lymphotropic virus 1 infection

Human T-lymphotropic virus type 1 (HTLV-1) is a persistent CD4+ T-lymphotropic retrovirus. Most HTLV-1-infected individuals remain asymptomatic, but a proportion develop adult T cell leukemia or inflammatory disease. It is not fully understood how HTLV-1 persists despite a strong immune response or what determines the risk of HTLV-1-associated diseases. Until recently, it has been difficult to quantify lymphocyte kinetics in humans in vivo. Here, we used deuterated glucose labeling to quantify in vivo lymphocyte dynamics in HTLV-1-infected individuals. We then used these results to address four questions. (i) What is the impact of HTLV-1 infection on lymphocyte dynamics? (ii) How does HTLV-1 persist? (iii) What is the extent of HTLV-1 expression in vivo? (iv) What features of lymphocyte kinetics are associated with HTLV-1-associated myelopathy/tropical spastic paraparesis? We found that CD4+CD45RO+ and CD8+CD45RO+ T lymphocyte proliferation was elevated in HTLV-1-infected subjects compared with controls, with an extra 10(12) lymphocytes produced per year in an HTLV-1-infected subject. The in vivo proliferation rate of CD4+CD45RO+ cells also correlated with ex vivo viral expression. Finally, the inflammatory disease HTLV-1-associated myelopathy/tropical spastic paraparesis was associated with significantly increased CD4+CD45RO+ cell proliferation. We suggest that there is persistent viral gene expression in vivo, which is necessary for the maintenance of the proviral load and determines HTLV-1-associated myelopathy/tropical spastic paraparesis risk.

Preferential selection of human T-cell leukemia virus type 1 provirus lacking the 5' long terminal repeat during oncogenesis

In adult T-cell leukemia (ATL) cells, a defective human T-cell leukemia virus type 1 (HTLV-1) provirus lacking the 5' long terminal repeat (LTR), designated type 2 defective provirus, is frequently observed. To investigate the mechanism underlying the generation of the defective provirus, we sequenced HTLV-1 provirus integration sites from cases of ATL. In HTLV-1 proviruses retaining both LTRs, 6-bp repeat sequences were adjacent to the 5' and 3' LTRs. In 8 of 12 cases with type 2 defective provirus, 6-bp repeats were identified at both ends. In five of these cases, a short repeat was bound to CA dinucleotides of the pol and env genes at the 5' end, suggesting that these type 2 defective proviruses were formed before integration. In four cases lacking the 6-bp repeat, short (6- to 26-bp) deletions in the host genome were identified, indicating that these defective proviruses were generated after integration. Quantification indicated frequencies of type 2 defective provirus of less than 3.9% for two carriers, which are much lower than those seen for ATL cases (27.8%). In type 2 defective proviruses, the second exons of the tax, rex, and p30 genes were frequently deleted, leaving Tax unable to activate NF-kappaB and CREB pathways. The HTLV-1 bZIP factor gene, located on the minus strand, is expressed in ATL cells with this defective provirus, and its coding sequences are intact, suggesting its significance in oncogenesis.

Detection and quantitation of HTLV-1 and HTLV-2 mRNA species by real-time RT-PCR

HTLV-1 and HTLV-2 are highly related delta-retroviruses that infect and transform T-lymphocytes, but have distinct pathogenic properties. HTLV replication and survival requires the expression of multiple gene products from an unspliced and a series of highly related alternatively spliced mRNA species. To date, the comparative levels of all known HTLV-1 and HTLV-2 viral mRNAs in different transformed cell lines and at different stages of virus infection have not been assessed. In this study, we compiled a series of oligonucleotide primer pairs and probes to quantify both HTLV-1 and HTLV-2 mRNA species using real-time RT-PCR. The optimized reaction for detection of each mRNA had amplification efficiency greater than 90% with a linear range spanning 25-2.5 x 10(7) copies. The R(2)'s of all standard curves were greater than 0.97. Quantitation of HTLV mRNAs between different cell lines showed variability (gag/pol>or=tax/rex>env>or=accessory proteins), but the overall levels of each mRNA relative to each other within a cell line were similar. These results provide a method to quantify all specific mRNAs from both HTLV-1 and HTLV-2, which can be used to evaluate further viral gene expression and correlate transcript levels to key stages of the virus life cycle and ultimately, pathogenesis.

In vitro peptide immunization of target tax protein human T-cell leukemia virus type 1-specific CD4+ helper T lymphocytes

PURPOSE

Adult T-cell leukemia/lymphoma induced by human T-cell leukemia virus type 1 (HTLV-1) is usually a fatal lymphoproliferative malignant disease. HTLV-1 Tax protein plays a critical role in HTLV-1-associated leukemogenesis and is an attractive target for vaccine development. Although HTLV-1 Tax is the most dominant antigen for HTLV-1-specific CD8(+) CTLs in HTLV-1-infected individuals, few epitopes recognized by CD4(+) helper T lymphocytes in HTLV-1 Tax protein have been described. The aim of the present study was to study T-helper-cell responses to HTLV-1 Tax and to identify naturally processed MHC class II-restricted epitopes that could be used for vaccine development.

EXPERIMENTAL DESIGN

An MHC class II binding peptide algorithm was used to predict potential T-helper cell epitope peptides from HTLV-1 Tax. We assessed the ability of the corresponding peptides to elicit helper T-cell responses by in vitro vaccination of purified CD4(+) T lymphocytes.

RESULTS

Peptides Tax(191-205) and Tax(305-319) were effective in inducing T-helper-cell responses. Although Tax(191-205) was restricted by the HLA-DR1 and DR9 alleles, responses to Tax(305-319) were restricted by either DR15 or DQ9. Both these epitopes were found to be naturally processed by HTLV-1(+) T-cell lymphoma cells and by autologous antigen-presenting cells that were pulsed with HTLV-1 Tax(+) tumor lysates. Notably, the two newly identified helper T-cell epitopes are found to lie proximal to known CTL epitopes, which will facilitate the development of prophylactic peptide-based vaccine capable of inducing simultaneous CTL and T-helper responses.

CONCLUSION

Our data suggest that HTLV-1 Tax protein could serve as tumor-associated antigen for CD4(+) helper T cells and that the present epitopes might be used for T-cell-based immunotherapy against tumors expressing HTLV-1.

Histone acetyltransferase (HAT) activity of p300 modulates human T lymphotropic virus type 1 p30II-mediated repression of LTR transcriptional activity

Human T-lymphotropic virus type-1 (HTLV-1) is a deltaretrovirus that causes adult T cell leukemia/lymphoma, and is implicated in a variety of lymphocyte-mediated inflammatory disorders. HTLV-1 provirus has regulatory and accessory genes in four pX open reading frames. HTLV-1 pX ORF-II encodes two proteins, p13II and p30II, which are incompletely defined in virus replication or pathogenesis. We have demonstrated that pX ORF-II mutations block virus replication in vivo and that ORF-II encoded p30II, a nuclear-localizing protein that binds with CREB-binding protein (CBP)/p300, represses CREB and Tax responsive element (TRE)-mediated transcription. Herein, we have identified p30II motifs important for p300 binding and in regulating TRE-mediated transcription in the absence and presence of HTLV-1 provirus. Within amino acids 100-179 of p30II, a region important for repression of LTR-mediated transcription, we identified a single lysine residue at amino acid 106 (K3) that significantly modulates the ability of p30II to repress TRE-mediated transcription. Exogenous p300, in a dose-responsive manner, reverses p30II-dependent repression of TRE-mediated transcription, in the absence or presence of the provirus, In contrast to wild type p300, p300 HAT mutants (defective in histone acetyltransferase activity) only partially rescued p30(II)-mediated LTR repression. Deacetylation by histone deacetylase-1 (HDAC-1) enhanced p30II-mediated LTR repression, while inhibition of deacetylation by trichostatin A decreases p30(II)-mediated LTR repression. Collectively, our data indicate that HTLV-1 p30II modulates viral gene expression in a cooperative manner with p300-mediated acetylation.

Enhancement of LFA-1-mediated T cell adhesion by human T lymphotropic virus type 1 p12I1

Cell-to-cell transmission of retroviruses, such as human T lymphotropic virus type 1 (HTLV-1), is well documented, but the roles of viral regulatory or other nonstructural proteins in the modulation of T cell adhesion are incompletely understood. In this study we tested the role of the HTLV-1 accessory protein, p12(I), on LFA-1-mediated cell adhesion. p12(I) is critical for early HTLV-1 infection by causing the release of calcium from the endoplasmic reticulum to activate NFAT-mediated transcription. We tested the role of this novel viral protein in mediating LFA-1-dependent cell adhesion. Our data indicated that T cells expressing a mutant HTLV-1 provirus that does not produce p12(I) mRNA (ACH.p12(I)) exhibited reduced LFA-1-mediated adhesion compared with wild-type HTLV-1-expressing cells (ACH). Furthermore, the expression of p12(I) in Jurkat T cells using lentiviral vectors enhanced LFA-1-mediated cell adhesion, which was inhibited by the calcium chelator BAPTA-AM, the calcium channel blocker SK&F 96365, and calpeptin, an inhibitor of the calcium-dependent protease calpain. Similar to the intracellular calcium mobilizer, thapsigargin, the expression of p12(I) in Jurkat T cells induced cell surface clustering of LFA-1 without changing the level of integrin expression. Our data are the first to indicate that HTLV-1 p12(I), in addition to enhancing T cell activation, promotes cell-to-cell spread by inducing LFA-1 clustering on T cells via calcium-dependent signaling.

Human T-lymphotropic virus type 1 mitochondrion-localizing protein p13(II) is required for viral infectivity in vivo

Human T-lymphotropic virus type 1 (HTLV-1), the etiological agent of adult T-cell leukemia, encodes unique regulatory and accessory proteins in the pX region of the provirus, including the open reading frame II product p13(II). p13(II) localizes to mitochondria, binds farnesyl pyrophosphate synthetase, an enzyme involved in posttranslational farnesylation of Ras, and alters Ras-dependent cell signaling and control of apoptosis. The role of p13(II) in virus infection in vivo remains undetermined. Herein, we analyzed the functional significance of p13(II) in HTLV-1 infection. We compared the infectivity of a human B-cell line that harbors an infectious molecular clone of HTLV-1 with a selective mutation that prevents the translation of p13(II) (729.ACH.p13) to the infectivity of a wild-type HTLV-1-expressing cell line (729.ACH). 729.ACH and 729.ACH.p13 producer lines had comparable infectivities for cultured rabbit peripheral blood mononuclear cells (PBMC), and the fidelity of the start codon mutation in ACH.p13 was maintained after PBMC passage. In contrast, zero of six rabbits inoculated with 729.ACH.p13 cells failed to establish viral infection, whereas six of six rabbits inoculated with wild-type HTLV-1-expressing cells (729.ACH) were infected as measured by antibody responses, proviral load, and HTLV-1 p19 matrix antigen production from ex vivo-cultured PBMC. Our data are the first to indicate that the HTLV-1 mitochondrion-localizing protein p13(II) has an essential biological role during the early phase of virus infection in vivo.

Human T-cell leukemia/lymphoma virus type 1 nonstructural genes and their functions

The human T-cell leukemia/lymphoma virus (HTLV) genome, in addition to the structural Gag and Env proteins and retroviral enzymes, carries a region at its 3' end originally designated pX. To date, we know that this region encodes two essential transcriptional and post-transcriptional positive regulators of viral expression, the Tax and Rex proteins, respectively (reviewed elsewhere in this issue). Here, we will review current knowledge of the functions of three additional proteins encoded in the pX region, p12I, p13II, and p30II.

Cellular immune response to HTLV-1

There is strong evidence at the individual level and the population level that an efficient cytotoxic T lymphocyte (CTL) response to HTLV-1 limits the proviral load and the risk of associated inflammatory diseases such as HAM/TSP. This evidence comes from host population genetics, viral genetics, DNA expression microarrays and assays of lymphocyte function. However, until now there has been no satisfactory and rigorous means to define or to measure the efficiency of an antiviral CTL response. Recently, methods have been developed to quantify lymphocyte turnover rates in vivo and the efficiency of anti-HTLV-1 CTLs ex vivo. Data from these new techniques appear to substantiate the conclusion that variation between individual hosts in the rate at which a single CTL kills HTLV-1-infected lymphocytes is an important determinant, perhaps the decisive determinant, of the proviral load and the risk of HAM/TSP. With these experimental data, it is becoming possible to refine, parameterize and test mathematical models of the immune control of HTLV-1, which are a necessary part of an understanding of this complex dynamic system.

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

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

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

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

Human T-lymphotropic virus type 1 mitochondrion-localizing protein p13II sensitizes Jurkat T cells to Ras-mediated apoptosis

Human T-lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia. In addition to typical retroviral structural and enzymatic gene products, HTLV-1 encodes unique regulatory and accessory proteins, including a singly spliced pX open reading frame II (ORF II) product, p13(II). We have demonstrated that proviral clones of HTLV-1 which are mutated in pX ORF II fail to obtain typical proviral loads and antibody responses in a rabbit animal model. p13(II) localizes to mitochondria and reduces cell growth and tumorigenicity in mice, but its function in human lymphocytes remains undetermined. For this study, we analyzed the functional properties of Jurkat T cells expressing p13(II), using both transient and stable expression vectors. Our data indicate that p13(II)-expressing Jurkat T cells are sensitive to caspase-dependent, ceramide- and FasL-induced apoptosis. p13(II)-expressing Jurkat T cells also exhibited reduced proliferation when cultured at a high density. Furthermore, preincubation of the p13(II)-expressing cells with a farnesyl transferase inhibitor, which blocks the posttranslational modification of Ras, markedly reduced FasL-induced apoptosis, indicating the participation of the Ras pathway in p13(II)'s influence on lymphocyte survival. Our data are the first to demonstrate that p13(II) alters Ras-mediated apoptosis in T lymphocytes, and they reveal a potential mechanism by which HTLV-1 alters lymphocyte proliferation.

The human T-cell leukemia virus type 1 p13II protein: effects on mitochondrial function and cell growth

p13(II) of human T-cell leukemia virus type 1 (HTLV-1) is an 87-amino-acid protein that is targeted to the inner mitochondrial membrane. p13(II) alters mitochondrial membrane permeability, producing a rapid, membrane potential-dependent influx of K(+). These changes result in increased mitochondrial matrix volume and fragmentation and may lead to depolarization and alterations in mitochondrial Ca(2+) uptake/retention capacity. At the cellular level, p13(II) has been found to interfere with cell proliferation and transformation and to promote apoptosis induced by ceramide and Fas ligand. Assays carried out in T cells (the major targets of HTLV-1 infection in vivo) demonstrate that p13(II)-mediated sensitization to Fas ligand-induced apoptosis can be blocked by an inhibitor of Ras farnesylation, thus implicating Ras signaling as a downstream target of p13(II) function.

Tumor immunity against adult T-cell leukemia

Human T-cell leukemia virus type-I (HTLV-I) causes adult T-cell leukemia (ATL) in a small population of infected individuals after a long incubation period. Although the process of clonal evolution of ATL cells may involve multiple steps, ATL cells from half of the ATL cases still retain the ability to express HTLV-I Tax, a key molecule of HTLV-I leukemogenesis. A recent finding of reactivation of Tax-specific cytotoxic T lymphocytes (CTL) in ATL patients after hematopoietic stem cell transplantation suggests the presence of Tax expression in vivo and potential contribution of the CTL to antitumor immunity. This is consistent with the results of a series of animal experiments indicating that Tax-specific CTL limit the growth of HTLV-I-infected cells in vivo, although the animal model mimics only an early phase of HTLV-I infection and leukemogenesis. Establishment of an insufficient HTLV-I-specific T-cell response and an increased viral load in orally HTLV-I-infected rats suggests that host HTLV-I-specific T-cell response at a primary HTLV-I infection can be a critical determinant of persistent HTLV-I levels thereafter. These findings indicate that Tax-targeted vaccines may be effective for prophylaxis of ATL in a high-risk group, and also for therapy of ATL in at least half the cases.

Human Dlg protein binds to the envelope glycoproteins of human T-cell leukemia virus type 1 and regulates envelope mediated cell-cell fusion in T lymphocytes

Human homologue of the Drosophila Dlg tumor suppressor (hDlg) is a widely expressed scaffold protein implicated in the organization of multi-protein complexes at cell adhesion sites such as the neuronal synapse. hDlg contains three PDZ domains that mediate its binding to the consensus motifs present at the C-termini of various cell surface proteins, thus inducing their clustering and/or stabilization at the plasma membrane. Using a yeast two-hybrid screen, we identified hDlg as a cellular binding partner of a viral membrane integral protein, the envelope glycoprotein (Env) of human T-cell leukemia virus type 1 (HTLV-1). HTLV-1 is a human retrovirus that infects CD4+ T lymphocytes and is preferentially transmitted via direct contacts between infected and target cells, through a structure referred to as the virological synapse. Here, we demonstrate that hDlg interacts with a classical PDZ domain-binding motif present at the C-terminus of the cytoplasmic domain of HTLV-1 Env and conserved in the related HTLV-2 virus. We further document that, in HTLV-1 infected primary T cells, hDlg and Env are concentrated in restricted areas of the plasma membrane, enriched in molecules involved in T-cell contacts. The presence of Gag proteins responsible for viral assembly and budding in these areas indicated that they constitute platforms for viral assembly and transmission. Finally, a mutant virus unable to bind hDlg exhibited a decreased ability to trigger Env mediated cell fusion between T lymphocytes. We thus propose that hDlg stabilizes HTLV-1 envelope glycoproteins at the virological synapse formed between infected and target cells, hence assisting the cell-to-cell transmission of the virus.

HTLV-1 p30II: selective repressor of gene expression

Human T-lymphotropic virus type-1 (HTLV-1) is a complex retrovirus that causes adult T-cell leukemia/lymphoma (ATL) and is implicated in a variety of lymphocyte-mediated disorders. HTLV-1 pX ORF II encodes two proteins, p13^II and p30^II whose roles are beginning to be defined in the virus life cycle. Previous studies indicate the importance of these viral proteins in the ability of the virus to maintain viral loads and persist in an animal model of HTLV-1 infection. Intriguing new studies indicate that p30^II is a multifunctional regulator that differentially modulates CREB and Tax-responsive element-mediated transcription through its interaction with CREB-binding protein (CBP)/p300 and specifically binds and represses tax/rex mRNA nuclear export. A new study characterized the role of p30^II in regulation of cellular gene expression using comprehensive human gene arrays. Interestingly, p30^II is an overall repressor of cellular gene expression, while selectively favoring the expression of regulatory gene pathways important to T lymphocytes. These new findings suggest that HTLV-1, which is associated with lymphoproliferative diseases, uses p30^II to selectively repress cellular and viral gene expression to favor the survival of cellular targets ultimately resulting in leukemogenesis.

Human T lymphotropic virus type-1 p30II alters cellular gene expression to selectively enhance signaling pathways that activate T lymphocytes

Background

Human T-lymphotropic virus type-1 (HTLV-1) is a deltaretrovirus that causes adult T-cell leukemia/lymphoma and is implicated in a variety of lymphocyte-mediated disorders. HTLV-1 contains both regulatory and accessory genes in four pX open reading frames. pX ORF-II encodes two proteins, p13^II and p30^II, which are incompletely defined in the virus life cycle or HTLV-1 pathogenesis. Proviral clones of the virus with pX ORF-II mutations diminish the ability of the virus to maintain viral loads in vivo. Exogenous expression of p30^II differentially modulates CREB and Tax-responsive element-mediated transcription through its interaction with CREB-binding protein/p300 and represses tax/rex RNA nuclear export.

Results

Herein, we further characterized the role of p30^II in regulation of cellular gene expression, using stable p30^II expression system employing lentiviral vectors to test cellular gene expression with Affymetrix U133A arrays, representing ~33,000 human genes. Reporter assays in Jurkat T cells and RT-PCR in Jurkat and primary CD4+ T-lymphocytes were used to confirm selected gene expression patterns. Our data reveals alterations of interrelated pathways of cell proliferation, T-cell signaling, apoptosis and cell cycle in p30^II expressing Jurkat T cells. In all categories, p30^II appeared to be an overall repressor of cellular gene expression, while selectively increasing the expression of certain key regulatory genes.

Conclusions

We are the first to demonstrate that p30^II, while repressing the expression of many genes, selectively activates key gene pathways involved in T-cell signaling/activation. Collectively, our data suggests that this complex retrovirus, associated with lymphoproliferative diseases, relies upon accessory gene products to modify cellular environment to promote clonal expansion of the virus genome and thus maintain proviral loads in vivo.