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

Complete Rescue of HTLV-1p12KO Infectivity by Depletion of Monocytes Together with NK and CD8+ T Cells

The transient depletion of monocytes alone prior to exposure of macaques to HTLV-1 enhances both HTLV-1WT (wild type) and HTLV-1p12KO (Orf-1 knockout) infectivity, but seroconversion to either virus is not sustained over time, suggesting a progressive decrease in virus expression. These results raise the hypotheses that either HTLV-1 persistence depends on a monocyte reservoir or monocyte depletion provides a transient immune evasion benefit. To test these hypotheses, we simultaneously depleted NK cells, CD8+ T cells, and monocytes (triple depletion) prior to exposure to HTLV-1WT or HTLV-1p12KO. Remarkably, triple depletion resulted in exacerbation of infection by both viruses and complete rescue of HTLV-1p12KO infectivity. Following triple depletion, we observed rapid and sustained seroconversion, high titers of antibodies against HTLV-1 p24Gag, and frequent detection of viral DNA in the blood and tissues of all animals when compared with depletion of only CD8+ and NK cells, or monocytes alone. The infection of macaques with HTLV-1WT or HTLV-1p12KO was associated with higher plasma levels of IL-10 after 21 weeks, while IL-6, IFN-γ, IL-18, and IL-1β were only elevated in animals infected with HTLV-1WT. The repeat depletion of monocytes, NK, and CD8+ cells seven months following the first exposure to HTLV-1 did not further exacerbate viral replication. These results underscore the contribution of monocytes in orchestrating anti-viral immunity. Indeed, the absence of orf-1 expression was fully compensated by the simultaneous depletion of CD8+ T cells, NK cells, and monocytes, underlining the primary role of orf-1 in hijacking host immunity.

HTLV-1 p12 modulates the levels of prion protein (PrPC) in CD4+ T cells

Introduction

Infection with human T cell lymphotropic virus type 1 (HTLV-1) is endemic in Brazil and is linked with pro-inflammatory conditions including HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic neuroinflammatory incapacitating disease that culminates in loss of motor functions. The mechanisms underlying the onset and progression of HAM/TSP are incompletely understood. Previous studies have demonstrated that inflammation and infectious agents can affect the expression of cellular prion protein (PrPC) in immune cells.

Methods

Here, we investigated whether HTLV-1 infection affected PrPC content in cell lines and primary CD4+cells in vitro using flow cytometry and western blot assays.

Results

We found that HTLV-1 infection decreased the expression levels of PrPC and HTLV-1 Orf I encoded p12, an endoplasmic reticulum resident protein also known to affect post-transcriptionally cellular proteins such as MHC-class I and the IL-2 receptor. In addition, we observed a reduced percentage of CD4+ T cells from infected individuals expressing PrPC, which was reflected by IFN type II but not IL-17 expression.

Discussion

These results suggested that PrPC downregulation, linked to both HTLV-1 p12 and IFN-γ expression in CD4+ cells, may play a role in the neuropathogenesis of HTLV-1 infection.

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.

NK cells and monocytes modulate primary HTLV-1 infection

We investigated the impact of monocytes, NK cells, and CD8⁺ T-cells in primary HTLV-1 infection by depleting cell subsets and exposing macaques to either HTLV-1 wild type (HTLV-1_WT) or to the HTLV-1_p12KO mutant unable to infect replete animals due to a single point mutation in orf-I that inhibits its expression. The orf-I encoded p8/p12 proteins counteract cytotoxic NK and CD8⁺ T-cells and favor viral DNA persistence in monocytes. Double NK and CD8⁺ T-cells or CD8 depletion alone accelerated seroconversion in all animals exposed to HTLV-1_WT. In contrast, HTLV-1_p12KO infectivity was fully restored only when NK cells were also depleted, demonstrating a critical role of NK cells in primary infection. Monocyte/macrophage depletion resulted in accelerated seroconversion in all animals exposed to HTLV-1_WT, but antibody titers to the virus were low and not sustained. Seroconversion did not occur in most animals exposed to HTLV-1_p12KO.In vitro experiments in human primary monocytes or THP-1 cells comparing HTLV-1_WT and HTLV-1_p12KO demonstrated that orf-I expression is associated with inhibition of inflammasome activation in primary cells, with increased CD47 “don’t-eat-me” signal surface expression in virus infected cells and decreased monocyte engulfment of infected cells. Collectively, our data demonstrate a critical role for innate NK cells in primary infection and suggest a dual role of monocytes in primary infection. On one hand, orf-I expression increases the chances of viral transmission by sparing infected cells from efferocytosis, and on the other may protect the engulfed infected cells by modulating inflammasome activation. These data also suggest that, once infection is established, the stoichiometry of orf-I expression may contribute to the chronic inflammation observed in HTLV-1 infection by modulating monocyte efferocytosis.

The immune cells that inhibit or favor HTLV-1 infection are still unknown and their identification is critical for understanding viral pathogenesis and for the development of an effective HTLV-1 vaccine. Neutralizing antibodies are produced in natural HTLV-1 infection, but their impact is likely hampered by the virus’s ability to be transmitted from cell to cell via the virological synapse, cellular conduits, and biofilms. By depleting specific immune cell subsets in blood, we found that NK cells play a critical role in the containment of early HTLV-1 infection. Moreover, transient depletion of monocytes/macrophages results in early, but not sustained seroconversion, suggesting that early engagement of monocytes may be necessary for long-term productive infection. The engulfment of apoptotic T-cells infected by HTLV-1 may represent a viral strategy to persist in the host since the viral proteins encoded by orf-I and orf-II affect the function of receptors and proteins involved in efferocytosis. These results suggest that effective HTLV-1 vaccines must also elicit durable innate responses able to promptly clear virus invasion of monocytes through engulfment of infected T-cells to avoid the establishment of a vicious cycle that leads to chronic inflammation.

Epigenomic regulation of human T-cell leukemia virus by chromatin-insulator CTCF

Human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that causes an aggressive T-cell malignancy and a variety of inflammatory conditions. The integrated provirus includes a single binding site for the epigenomic insulator, CCCTC-binding protein (CTCF), but its function remains unclear. In the current study, a mutant virus was examined that eliminates the CTCF-binding site. The mutation did not disrupt the kinetics and levels of virus gene expression, or establishment of or reactivation from latency. However, the mutation disrupted the epigenetic barrier function, resulting in enhanced DNA CpG methylation downstream of the CTCF binding site on both strands of the integrated provirus and H3K4Me3, H3K36Me3, and H3K27Me3 chromatin modifications both up- and downstream of the site. A majority of clonal cell lines infected with wild type HTLV-1 exhibited increased plus strand gene expression with CTCF knockdown, while expression in mutant HTLV-1 clonal lines was unaffected. These findings indicate that CTCF binding regulates HTLV-1 gene expression, DNA and histone methylation in an integration site dependent fashion.

Human T-cell leukemia virus type 1 (HTLV-1) is a cause of leukemia and lymphoma as well as several inflammatory medical disorders. The virus integrates in the host cell DNA, and it has a single binding site for a protein designated CTCF. This protein is important in the regulation of many DNA viruses, as well as many properties of normal and malignant cells. In order to define the role of CTCF binding to HTLV, we analyzed a mutant virus lacking the binding site. We found that this mutation variably affected gene expression, DNA and histone modification, suggesting a key role in regulation of virus replication in infected cells.

Role of HTLV-1 orf-I encoded proteins in viral transmission and persistence

The human T cell leukemia virus type 1 (HTVL-1), first reported in 1980 by Robert Gallo's group, is the etiologic agent of both cancer and inflammatory diseases. Despite approximately 40 years of investigation, the prognosis for afflicted patients remains poor with no effective treatments. The virus persists in the infected host by evading the host immune response and inducing proliferation of infected CD4⁺ T-cells. Here, we will review the role that viral orf-I protein products play in altering intracellular signaling, protein expression and cell-cell communication in order to escape immune recognition and promote T-cell proliferation. We will also review studies of orf-I mutations found in infected patients and their potential impact on viral load, transmission and persistence. Finally, we will compare the orf-I gene in HTLV-1 subtypes as well as related STLV-1.

Non-Structural Proteins from Human T-cell Leukemia Virus Type 1 in Cellular Membranes-Mechanisms for Viral Survivability and Proliferation

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of illnesses, such as adult T-cell leukemia/lymphoma, myelopathy/tropical spastic paraparesis (a neurodegenerative disorder), and other diseases. Therefore, HTLV-1 infection is a serious public health concern. Currently, diseases caused by HTLV-1 cannot be prevented or cured. Hence, there is a pressing need to comprehensively understand the mechanisms of HTLV-1 infection and intervention in host cell physiology. HTLV-1-encoded non-structural proteins that reside and function in the cellular membranes are of particular interest, because they alter cellular components, signaling pathways, and transcriptional mechanisms. Summarized herein is the current knowledge about the functions of the membrane-associated p8^I, p12^I, and p13^II regulatory non-structural proteins. p12^I resides in endomembranes and interacts with host proteins on the pathways of signal transduction, thus preventing immune responses to the virus. p8^I is a proteolytic product of p12^I residing in the plasma membrane, where it contributes to T-cell deactivation and participates in cellular conduits, enhancing virus transmission. p13^II associates with the inner mitochondrial membrane, where it is proposed to function as a potassium channel. Potassium influx through p13^II in the matrix causes membrane depolarization and triggers processes that lead to either T-cell activation or cell death through apoptosis.

Masters of manipulation: Viral modulation of the immunological synapse

In order to thrive, viruses have evolved to manipulate host cell machinery for their own benefit. One major obstacle faced by pathogens is the immunological synapse. To enable efficient replication and latency in immune cells, viruses have developed a range of strategies to manipulate cellular processes involved in immunological synapse formation to evade immune detection and control T-cell activation. In vitro, viruses such as human immunodeficiency virus 1 and human T-lymphotropic virus type 1 utilise structures known as virological synapses to aid transmission of viral particles from cell to cell in a process termed trans-infection. The formation of the virological synapse provides a gateway for virus to be transferred between cells avoiding the extracellular space, preventing antibody neutralisation or recognition by complement. This review looks at how viruses are able to subvert intracellular signalling to modulate immune function to their advantage and explores the role synapse formation has in viral persistence and cell-to-cell transmission.

Mitochondrial Proteins Coded by Human Tumor Viruses

Viruses must exploit the cellular biosynthetic machinery and evade cellular defense systems to complete their life cycles. Due to their crucial roles in cellular bioenergetics, apoptosis, innate immunity and redox balance, mitochondria are important functional targets of many viruses, including tumor viruses. The present review describes the interactions between mitochondria and proteins coded by the human tumor viruses human T-cell leukemia virus type 1, Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, human hepatitis viruses B and C, and human papillomavirus, and highlights how these interactions contribute to viral replication, persistence and transformation.

Analyses of HTLV-1 sequences suggest interaction between ORF-I mutations and HAM/TSP outcome

The region known as pX in the 3' end of the human T-cell lymphotropic virus type 1 (HTLV-1) genome contains four overlapping open reading frames (ORF) that encode regulatory proteins. HTLV-1 ORF-I produces the protein p12 and its cleavage product p8. The functions of these proteins have been linked to immune evasion and viral infectivity and persistence. It is known that the HTLV-1 infection does not necessarily imply the development of pathological processes and here we evaluated whether natural mutations in HTLV-1 ORF-I can influence the proviral load and clinical manifestation of HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). For that, we performed molecular characterization, datamining and phylogenetic analysis with HTLV-1 ORF-I sequences from 156 patients with negative or positive diagnosis for HAM/TSP. Our analyses demonstrated that some mutations may be associated with the outcome of HAM/TSP (C39R, L40F, P45L, S69G and R88K) or with proviral load (P34L and F61L). We further examined the presence of mutations in motifs of HBZ and observed that P45L mutation is located within the HBZ nuclear localization signal and was found more frequently between patients with HAM/TSP and high proviral load. These results indicate that some natural mutations are located in functional domains of ORF-I and suggests a potential association between these mutations and the proviral loads and development of HAM/TSP. Therefore it is necessary to conduct functional studies aimed at evaluating the impact of these mutations on the virus persistence and immune evasion.

HTLV-1 ORF-I Encoded Proteins and the Regulation of Host Immune Response: Viral Induced Dysregulation of Intracellular Signaling

The human T-cell lymphotropic virus type 1 (HTLV-1) is a retrovirus associated with both proliferative and inflammatory disorders. This virus causes a persistent infection, mainly in CD4+ T lymphocyte. The ability to persist in the host is associated with the virus capacity to evade the immune response and to induce infected T-cell proliferation, once the HTLV-1 maintains the infection mainly by clonal expansion of infected cells. There are several evidences that ORF-I encoded proteins, such as p12 and p8, play an important role in this context. The present study will review the molecular mechanisms that HTLV-1 ORF-I encoded proteins have to induce dysregulation of intracellular signaling, in order to escape from immune response and to increase the infected T-cell proliferation rate. The work will also address the impact of ORF-I mutations on the human host and perspectives in this study field.

Wogonin inhibits osteoclast differentiation by inhibiting NFATc1 translocation into the nucleus

The aim of the present study was to identify a natural product with the ability to inhibit nuclear factor of activated T cells c1 (NFATc1) translocation from the cytoplasm to the nucleus by high-throughput screening, and to investigate the effect of the natural product upon osteoclast differentiation and its underlying mechanism. An NFATc1 antagonist redistribution assay was performed in U2OS-NFATc1 cells against a natural product library, and Wogonin was found to have the ability to inhibit the NFATc1 translocation from the cytoplasm to the nucleus. The effect of Wogonin on NFATc1 transcription activation was further determined by luciferase assay. An osteoclast differentiation assay was executed to evaluate the effect of Wogonin on osteoclast differentiation. The effect of Wogonin upon the vital genes in osteoclast differentiation was investigated using fluorescent quantitative polymerase chain reaction analysis. The natural product Wogonin significantly inhibited the translocation of NFATc1 from the cytoplasm to the nucleus and its transcriptional activation activity. Wogonin also significantly inhibited osteoclast differentiation and decreased the transcription of osteoclast-associated immunoglobulin-like receptor, tartrate-resistant acid phosphatase and calcitonin receptor. In conclusion, the natural product Wogonin inhibited osteoclast differentiation through the inhibition of NFATc1 translocation from the cytoplasm to the nucleus, and thus the downregulation of genes associated with osteoclast differentiation, which marked Wogonin as a potential treatment for osteoporosis.

Co-dependence of HTLV-1 p12 and p8 functions in virus persistence

HTLV-1 orf-I is linked to immune evasion, viral replication and persistence. Examining the orf-I sequence of 160 HTLV-1-infected individuals; we found polymorphism of orf-I that alters the relative amounts of p12 and its cleavage product p8. Three groups were identified on the basis of p12 and p8 expression: predominantly p12, predominantly p8 and balanced expression of p12 and p8. We found a significant association between balanced expression of p12 and p8 with high viral DNA loads, a correlate of disease development. To determine the individual roles of p12 and p8 in viral persistence, we constructed infectious molecular clones expressing p12 and p8 (D26), predominantly p12 (G29S) or predominantly p8 (N26). As we previously showed, cells expressing N26 had a higher level of virus transmission in vitro. However, when inoculated into Rhesus macaques, cells producing N26 virus caused only a partial seroconversion in 3 of 4 animals and only 1 of those animals was HTLV-1 DNA positive by PCR. None of the animals exposed to G29S virus seroconverted or had detectable viral DNA. In contrast, 3 of 4 animals exposed to D26 virus seroconverted and were HTLV-1 positive by PCR. In vitro studies in THP-1 cells suggested that expression of p8 was sufficient for productive infection of monocytes. Since orf-I plays a role in T-cell activation and recognition; we compared the CTL response elicited by CD4+ T-cells infected with the different HTLV-1 clones. Although supernatant p19 levels and viral DNA loads for all four infected lines were similar, a significant difference in Tax-specific HLA.A2-restricted killing was observed. Cells infected with Orf-I-knockout virus (12KO), G29S or N26 were killed by CTLs, whereas cells infected with D26 virus were resistant to CTL killing. These results indicate that efficient viral persistence and spread require the combined functions of p12 and p8.

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.

HTLV-1 and HTLV-2: highly similar viruses with distinct oncogenic properties

HTLV-1 and HTLV-2 share broad similarities in their overall genetic organization and expression pattern, but they differ substantially in their pathogenic properties. This review outlines distinctive features of HTLV-1 and HTLV-2 that might provide clues to explain their distinct clinical outcomes. Differences in the kinetics of viral mRNA expression, functional properties of the regulatory and accessory proteins, and interactions with cellular factors and signal transduction pathways are discussed.

Cell cycle regulation during viral infection

To replicate their genomes in cells and generate new progeny, viruses typically require factors provided by the cells that they have infected. Subversion of the cellular machinery that controls replication of the infected host cell is a common activity of many viruses. Viruses employ different strategies to deregulate cell cycle checkpoint controls and modulate cell proliferation pathways. A number of DNA and RNA viruses encode proteins that target critical cell cycle regulators to achieve cellular conditions that are beneficial for viral replication. Many DNA viruses induce quiescent cells to enter the cell cycle; this is thought to increase pools of deoxynucleotides and thus, facilitate viral replication. In contrast, some viruses can arrest cells in a particular phase of the cell cycle that is favorable for replication of the specific virus. Cell cycle arrest may inhibit early cell death of infected cells, allow the cells to evade immune defenses, or help promote virus assembly. Although beneficial for the viral life cycle, virus-mediated alterations in normal cell cycle control mechanisms could have detrimental effects on cellular physiology and may ultimately contribute to pathologies associated with the viral infection, including cell transformation and cancer progression and maintenance. In this chapter, we summarize various strategies employed by DNA and RNA viruses to modulate the replication cycle of the virus-infected cell. When known, we describe how these virus-associated effects influence replication of the virus and contribute to diseases associated with infection by that specific virus.

Palmitoylation and p8-mediated human T-cell leukemia virus type 1 transmission

The orf-I gene of human T-cell leukemia type 1 (HTLV-1) encodes p8 and p12 and has a conserved cysteine at position 39. p8 and p12 form disulfide-linked dimers, and only the monomeric forms of p8 and p12 are palmitoylated. Mutation of cysteine 39 to alanine (C39A) abrogated dimerization and palmitoylation of both proteins. However, the ability of p8 to localize to the cell surface and to increase cell adhesion and viral transmission was not affected by the C39A mutation.

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.

Mechanisms of human T-lymphotropic virus type 1 transmission and disease

Human T-lymphotrophic virus type-1 (HTLV-1) infects approximately 15-20 million people worldwide, with endemic areas in Japan, the Caribbean, and Africa. The virus is spread through contact with bodily fluids containing infected cells most often from mother to child through breast milk or via blood transfusion. After prolonged latency periods, approximately 3-5% of HTLV-1 infected individuals will develop either adult T-cell leukemia/lymphoma, or other lymphocyte-mediated disorders such as HTLV-1-associated myelopathy/tropical spastic paraparesis. The genome of this complex retrovirus contains typical gag, pol, and env genes, but also unique nonstructural proteins encoded from the pX region. These nonstructural genes encode the Tax and Rex regulatory proteins, as well as novel proteins essential for viral spread in vivo such as p30, p12, p13 and the antisense-encoded HTLV-1 basic leucine zipper factor (HBZ). While progress has been made in knowledge of viral determinants of cell transformation and host immune responses, host and viral determinants of HTLV-1 transmission and spread during the early phases of infection are unclear. Improvements in the molecular tools to test these viral determinants in cellular and animal models have provided new insights into the early events of HTLV-1 infection. This review will focus on studies that test HTLV-1 determinants in context to full-length infectious clones of the virus providing insights into the mechanisms of transmission and spread of HTLV-1.

Comparison of the Genetic Organization, Expression Strategies and Oncogenic Potential of HTLV-1 and HTLV-2

Human T cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) are genetically related complex retroviruses that are capable of immortalizing human T-cells in vitro and establish life-long persistent infections in vivo. In spite of these apparent similarities, HTLV-1 and HTLV-2 exhibit a significantly different pathogenic potential. HTLV-1 is recognized as the causative agent of adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). In contrast, HTLV-2 has not been causally linked to human malignancy, although it may increase the risk of developing inflammatory neuropathies and infectious diseases. The present paper is focused on the studies aimed at defining the viral genetic determinants of the pathobiology of HTLV-1 and HTLV-2 through a comparison of the expression strategies and functional properties of the different gene products of the two viruses.

Converging strategies in expression of human complex retroviruses

The discovery of human retroviruses in the early 1980s revealed the existence of viral-encoded non-structural genes that were not evident in previously described animal retroviruses. Based on the absence or presence of these additional genes retroviruses were classified as ‘simple’ and ‘complex’, respectively. Expression of most of these extra genes is achieved through the generation of alternatively spliced mRNAs. The present review summarizes the genetic organization and expression strategies of human complex retroviruses and highlights the converging mechanisms controlling their life cycles.

Molecular determinants of human T-lymphotropic virus type 1 transmission and spread

Human T-lymphotrophic virus type-1 (HTLV-1) infects approximately 15 to 20 million people worldwide, with endemic areas in Japan, the Caribbean, and Africa. The virus is spread through contact with bodily fluids containing infected cells, most often from mother to child through breast milk or via blood transfusion. After prolonged latency periods, approximately 3 to 5% of HTLV-1 infected individuals will develop either adult T-cell leukemia/lymphoma (ATL), or other lymphocyte-mediated disorders such as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The genome of this complex retrovirus contains typical gag, pol, and env genes, but also unique nonstructural proteins encoded from the pX region. These nonstructural genes encode the Tax and Rex regulatory proteins, as well as novel proteins essential for viral spread in vivo such as, p30, p12, p13 and the antisense encoded HBZ. While progress has been made in the understanding of viral determinants of cell transformation and host immune responses, host and viral determinants of HTLV-1 transmission and spread during the early phases of infection are unclear. Improvements in the molecular tools to test these viral determinants in cellular and animal models have provided new insights into the early events of HTLV-1 infection. This review will focus on studies that test HTLV-1 determinants in context to full length infectious clones of the virus providing insights into the mechanisms of transmission and spread of HTLV-1.

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.

How viruses hijack cell regulation

Viruses, as obligate intracellular parasites, are the pathogens that have the most intimate relationship with their host, and as such, their genomes have been shaped directly by interactions with the host proteome. Every step of the viral life cycle, from entry to budding, is orchestrated through interactions with cellular proteins. Accordingly, viruses will hijack and manipulate these proteins utilising any achievable mechanism. Yet, the extensive interactions of viral proteomes has yielded a conundrum: how do viruses commandeer so many diverse pathways and processes, given the obvious spatial constraints imposed by their compact genomes? One important approach is slowly being revealed, the extensive mimicry of host protein short linear motifs (SLiMs).

HTLV-I p30 inhibits multiple S phase entry checkpoints, decreases cyclin E-CDK2 interactions and delays cell cycle progression

BACKGROUND

Human T-cell leukemia virus type I (HTLV-I) has efficiently adapted to its host and establishes a persistent infection characterized by low levels of viral gene expression and slow proliferation of HTLV-I infected cells over decades. We have previously found that HTLV-I p30 is a negative regulator of virus expression.

RESULTS

In this study we show that p30 targets multiple cell cycle checkpoints resulting in a delayed entry into S phase. We found that p30 binds to cyclin E and CDK2 and prevents the formation of active cyclin E-CDK2 complexes. In turn, this decreases the phosphorylation levels of Rb and prevents the release of E2F and its transcriptional activation of genes required for G1/S transition. Our studies also show that HTLV-II p28 does not bind cyclin E and does not affect cell cycle progression.

CONCLUSIONS

In contrast to HTLV-I, the HTLV-II-related retrovirus is not oncogenic in humans. Here we report that the HTLV-I p30 delays cell cycle progression while its homologue, HTLV-II p28, does not, providing evidence for important differences between these two related retrovirus proteins.

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

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

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.

Requirement of the human T-cell leukemia virus p12 and p30 products for infectivity of human dendritic cells and macaques but not rabbits

The identification of the genes necessary for human T-cell leukemia virus (HTLV-1) persistence in humans may provide targets for therapeutic approaches. We demonstrate that ablation of the HTLV-1 genes encoding p12, p30, or the HBZ protein, does not affect viral infectivity in rabbits and in this species, only the absence of HBZ is associated with a consistent reduction in virus levels. We observed reversion of the HTLV-1 mutants to the HTLV-1 wild-type genotype in none of the inoculated rabbits. In contrast, in macaques, the absence of HBZ was associated with reversion of the mutant virus to the wild-type genotype in 3 of the 4 animals within weeks from infection. Similarly, reversion to the wild type was observed in 2 of the 4 macaque inoculated with the p30 mutant. The 4 macaques exposed to the p12 knock remained seronegative, and only 2 animals were positive at a single time point for viral DNA in tissues. Interestingly, we found that the p12 and the p30 mutants were also severely impaired in their ability to replicate in human dendritic cells. These data suggest that infection of dendritic cells may be required for the establishment and maintenance of HTLV-1 infection in primate species.

Role of post-translational modifications of HTLV-1 Tax in NF-κB activation

Human T-cell leukemia virus type 1 (HTLV-1), the first human retrovirus discovered, is the etiological agent of adult-T-cell leukemia/lymphoma. The HTLV-1 encoded Tax protein is a potent oncoprotein that deregulates gene expression by constitutively activating nuclear factor-κB (NF-κB). Tax activation of NF-κB is critical for the immortalization and survival of HTLV-1-infected T cells. In this review, we summarize the present knowledge on mechanisms underlying Tax-mediated NF-κB activation, with an emphasis on post-translational modifications of Tax.

Does the HBZ gene represent a new potential target for the treatment of adult T-cell leukemia?

Links between human T-cell leukemia virus type 1 and adult T-cell leukemia (ATL) were first suspected in 1980. Provirus integration has since been found in all ATL cells. Although the viral Tax protein is involved in the proliferation of the infected cells during the preleukemic stage, Tax expression is not systematically detected in primary leukemic cells. Recent studies found that the viral HBZ gene was always expressed in leukemic cells, suggesting its involvement in the progression of the infected cells toward malignancy. How could this new discovery be translated into possible new avenues for the prevention or treatment of ATL?

Progressive loss of CD3 expression after HTLV-I infection results from chromatin remodeling affecting all the CD3 genes and persists despite early viral genes silencing

Background

HTLV-I infected CD4⁺ T-cells lines usually progress towards a CD3^- or CD3^low phenotype. In this paper, we studied expression, kinetics, chromatin remodeling of the CD3 gene at different time-points post HTLV-I infection.

Results

The onset of this phenomenon coincided with a decrease of CD3γ followed by the subsequent progressive reduction in CD3δ, then CD3ε and CD3ζ mRNA. Transient transfection experiments showed that the CD3γ promoter was still active in CD3^- HTLV-I infected cells demonstrating that adequate amounts of the required transcription factors were available. We next looked at whether epigenetic mechanisms could be responsible for this progressive decrease in CD3 expression using DNase I hypersensitivity (DHS) experiments examining the CD3γ and CD3δ promoters and the CD3δ enhancer. In uninfected and cells immediately post-infection all three DHS sites were open, then the CD3γ promoter became non accessible, and this was followed by a sequential closure of all the DHS sites corresponding to all three transcriptional control regions. Furthermore, a continuous decrease of in vivo bound transcription initiation factors to the CD3γ promoter was observed after silencing of the viral genome. Coincidently, cells with a lower expression of CD3 grew more rapidly.

Conclusion

We conclude that HTLV-I infection initiates a process leading to a complete loss of CD3 membrane expression by an epigenetic mechanism which continues along time, despite an early silencing of the viral genome. Whether CD3 progressive loss is an epiphenomenon or a causal event in the process of eventual malignant transformation remains to be investigated.

Inhibition of T-cell receptor signal transduction and viral expression by the linker for activation of T cells-interacting p12(I) protein of human T-cell leukemia/lymphoma virus type 1

The p12(I) protein of human T-cell leukemia/lymphoma virus type 1 (HTLV-1) is a small oncoprotein that increases calcium release following protein kinase C activation by phorbol myristate acetate, and importantly, this effect is linker for activation of T cells (LAT) independent. Here, we demonstrate that p12(I) inhibits the phosphorylation of LAT, Vav, and phospholipase C-gamma 1 and decreases NFAT (nuclear factor of activated T cells) activation upon engagement of the T-cell receptor (TCR) with anti-CD3 antibody. Furthermore, we demonstrate that p12(I) localizes to membrane lipid rafts and, upon engagement of the TCR, relocalizes to the interface between T cells and antigen-presenting cells, defined as the immunological synapse. A p12(I) knockout molecular clone of HTLV-1 expresses more virus upon antigen stimulation than the isogenic wild type, suggesting that, by decreasing T-cell responsiveness, p12(I) curtails viral expression. Thus, p12(I) has contrasting effects on TCR signaling: it down-regulates TCR in a LAT-dependent manner on one hand, and on the other, it increases calcium release in a LAT-independent manner. The negative regulation of T-cell activation by p12(I) may have evolved to minimize immune recognition of infected CD4(+) T cells, to impair the function of infected cytotoxic CD8(+) T cells, and to favor viral persistence in the infected host.

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.

Cytobiological consequences of calcium-signaling alterations induced by human viral proteins

Since calcium-signaling regulates specific and fundamental cellular processes, it represents the ideal target of viral proteins, in order for the virus to control cellular functions and favour its persistence, multiplication and spread. A detailed analysis of reports focused on the impact of viral proteins on calcium-signaling has shown that virus-related elevations of cytosolic calcium levels allow increased viral protein expression (HIV-1, HSV-1/2), viral replication (HBx, enterovirus 2B, HTLV-1 p12(I), HHV-8, EBV), viral maturation (rotavirus), viral release (enterovirus 2B) and cell immortalization (EBV). Interestingly, virus-induced decreased cytosolic calcium levels have been found to be associated with inhibition of immune cells functions (HIV-1 Tat, HHV-8 K15, EBV LMP2A). Finally, several viral proteins are able to modulate intracellular calcium-signaling to control cell viability (HIV-1 Tat, HTLV-1 p13(II), HCV core, HBx, enterovirus 2B, HHV-8 K7). These data point out calcium-signaling as a key cellular target for viral infection and should stimulate further studies exploring new calcium-related therapeutic strategies.

Calcium-dependent enhancement of transcription of p300 by human T-lymphotropic type 1 p12I

Human T-lymphotropic virus type 1 (HTLV-1) p12I localizes to the endoplasmic reticulum and Golgi causing sustained release of calcium, T cell activation, and enhanced expression of several calcium-regulated genes. In recent microarray studies, p300 mRNA was increased in T cells expressing p12I. The co-activator p300 is a key regulator of cellular and viral transcription; however, factors that influence its transcriptional regulation are less well studied. We hypothesized that the transcription of p300 is calcium dependent and that sustained low magnitude increases in intracellular calcium may enhance the transcription of p300. Herein, we report enhanced expression of p300 in T cells by p12I in a calcium-dependent, but calcineurin-independent manner. Sustained low magnitude calcium release induced by ionomycin in T cells was sufficient to increased mRNA and protein levels of p300 resulting in enhanced transcription from a p300-dependent promoter. Promoter analysis of the p300 gene was used to predict calcium-responsive transcription factor binding sites. Using mutant forms of p12I, we demonstrate that ER localization of the viral protein is required to increase p300. In addition, p12I reversed the repression of HTLV-1 LTR-driven transcription by HTLV-1 p30II, a p300-binding protein. HTLV-1 p12I-mediated enhancement of p300 expression represents a novel mechanism of regulation of cellular gene expression by viral proteins. By targeting a ubiquitous second messenger such as calcium, HTLV-1 p12I may regulate the expression of the cellular transcriptional co-activator p300 to modulate viral gene expression and promote lymphocyte survival.

HTLV-1 p12I protein sequences from South America: truncated proteins and common genetic signatures

p12 pX ORF-I (p12I) of simian T cell lymphotropic virus 1 from Central and West Africa is a 91-amino acid (aa) protein, contrasting with the known p12 sequences from human T cell lymphotropic virus type 1, which are 99 aa in length. An in silico sequence analysis shows a premature termination codon in HTLV-1 p12I sequences from South American and Japanese strains. A translation analysis indicated the presence of 86-aa and 81-aa p12I proteins, shorter than the 99-aa protein found in the majority of HTLV-1 strains around the world. This report shows one more common feature between these two viruses. Additionally, the HTLV-1 p12 signature reported as a genetic marker of Brazilian sequences (p12I P63 and P91) is also highly prevalent in p12 Argentinean sequences. Because p12I may confer a proliferative advantage on HTLV-1-infected cells the existence of naturally truncated proteins might supply insights into its pathogenic mechanisms.

Targeting of Calcineurin to an NFAT-like Docking Site Is Required for the Calcium-dependent Activation of the Background K+ Channel, TRESK

The two-pore domain K(+) channel, TRESK (TWIK-related spinal cord K(+) channel) is activated in response to the calcium signal by the calcium/calmodulin-dependent protein phosphatase, calcineurin. In the present study we report that calcineurin also interacts with TRESK via an NFAT-like docking site, in addition to its enzymatic action. In its intracellular loop, mouse TRESK possesses the amino acid sequence, PQIVID, which is similar to the calcineurin binding consensus motif, PXIXIT (where X denotes any amino acids), necessary for NFAT (nuclear factor of activated T cells) activation and nuclear translocation. Mutations of the PQIVID sequence of TRESK to PQIVIA, PQIVAD, or PQAVAD increasingly deteriorated the calcium-dependent activation in the listed order and correspondingly reduced the benzocaine sensitivity (a property discriminating activated channels from resting ones), when it was measured after the calcium signal in Xenopus oocytes. Microinjection of VIVIT peptide, designed to inhibit the NFAT-calcineurin interaction specifically, also eliminated TRESK activation. The intracellular loop of TRESK, expressed as a GST fusion protein, bound constitutively active calcineurin in vitro. PQAVAD mutation as well as addition of VIVIT peptide to the reaction abrogated this calcineurin binding. Wild type calcineurin was recruited to GST-TRESK-loop in the presence of calcium and calmodulin. These results indicate that the PQIVID sequence is a docking site for calcineurin, and its occupancy is required for the calcium-dependent regulation of TRESK. Immunosuppressive compounds, developed to target the NFAT binding site of calcineurin, are also expected to interfere with TRESK regulation, in addition to their desired effect on NFAT.

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

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

Genetic markers on the HTLV-1 p12I protein sequences from Brazilian HAM/TSP patients and asymptomatic HTLV-1 carrier isolates

The human T cell leukemia/lymphotropic virus type-1 (HTLV-1) genome has approximately 9 kb and contains the pX region that codes for regulatory and accessory proteins. The pX ORF-I encodes for the p12 protein, a 99 aa peptide, which presents several functional putative domains, such as leucine zipper motifs, SH3- binding motifs, and a dileucine motif, p12I. Also, a rare p12IK88 allele was found mainly in HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients, suggesting it is a marker of pathogenesis, although recent studies showed p12IK in asymptomatic HTLV-1 carriers. To extend the observations on p12I motifs, we sequenced 26 p12I from HAM/TSP patients and asymptomatic HTLV-1 carriers. Amino acid analysis of 48 p12I motifs demonstrated the presence of several alleles, but the allelic variation, including p12IK, was not prevalent in HAM/TSP isolates. Nonetheless, some genetic markers were recognized in association with isolates from HTLV-1a subgroup B and Brazilian HTLV-1aA strains.

Human T lymphotropic virus type 1 accessory protein p12I modulates calcium-mediated cellular gene expression and enhances p300 expression in T lymphocytes

Human T-lymphotropic virus type 1 (HTLV-1) is the etiologic agent of adult T cell leukemia/lymphoma (ATLL), an aggressive CD4+ T lymphocyte malignancy. Activation of T lymphocytes is required for effective retroviral integration into the host cell genome and subsequent viral replication, but the molecular mechanisms involved in HTLV-1-mediated T cell activation remain unclear. HTLV-1 encodes various accessory proteins such as p12I, which has been demonstrated to be critical for HTLV-1 infectivity in vivo in rabbits and in vitro in quiescent primary human T lymphocytes. This hydrophobic protein localizes in the endoplasmic reticulum, increases intracellular calcium, and activates nuclear factor of activated T cell-mediated transcription. To further elucidate the role of p12I in regulation of cellular gene expression, we performed gene array analysis on stable p12I-expressing Jurkat T cells, using Affymetrix U133A arrays. Our data indicate that p12I altered the expression of genes associated with a network of interrelated pathways including T cell signaling, cell proliferation, and apoptosis. Expression of several calcium-regulated genes was found to be altered by p12I, consistent with known properties of the viral protein. Gene array findings were confirmed by semiquantitative RT-PCR in Jurkat T cells and primary CD4+ T lymphocytes. Furthermore, dose-dependent expression of p12I in Jurkat T cells resulted in significant increases in p300 and p300-dependent transcription. This is the first report of a viral protein influencing the transcription of p300, a rate-limiting coadapter critical in HTLV-1-mediated T cell activation. Collectively, our data strongly indicate that HTLV-1 p12I modulates cellular gene expression patterns to hasten the activation of T lymphocytes and thereby promote efficient viral infection.

Reduced proviral loads during primo-infection of sheep by Bovine Leukemia virus attenuated mutants

Background

The early stages consecutive to infection of sheep (e.g. primo-infection) by Bovine leukemia virus mutants are largely unknown. In order to better understand the mechanisms associated with this period, we aimed at analyzing simultaneously three parameters: B-lymphocytosis, cell proliferation and viral replication.

Results

Sheep were experimentally infected either with a wild type BLV provirus or with selected mutants among which: a virus harboring an optimalized LTR promoter with consensus cyclic AMP-responsive elements, two deletants of the R3 or the G4 accessory genes and a fusion-deficient transmembrane recombinant. Seroconversion, as revealed by the onset of an anti-viral antibody response, was detected at 3 to 11 weeks after inoculation. At seroconversion, all sheep exhibited a marked increase in the numbers of circulating B lymphocytes expressing the CD5 and CD11b cluster of differentiation markers and, interestingly, this phenomenon occurred independently of the type of virus. The net increase of the absolute number of B cells was at least partially due to accelerated proliferation as revealed, after intravenous injection of bromodeoxyuridine, by the higher proportion of circulating BrdU+ B lymphocytes. BLV proviral DNA was detected by polymerase chain reaction in the leucocytes of all sheep, as expected. However, at seroconversion, the proviral loads were lower in sheep infected by the attenuated proviruses despite similar levels of B cell lymphocytosis.

Conclusions

We conclude that the proviral loads are not directly linked to the extent of B cell proliferation observed during primo-infection of BLV-infected sheep. We propose a model of opportunistic replication of the virus supported by a general activation process of B lymphocytes.

Polycystin-1 activates the calcineurin/NFAT (nuclear factor of activated T-cells) signaling pathway

Regulation of intracellular Ca(2+) mobilization has been associated with the functions of polycystin-1 (PC1) and polycystin-2 (PC2), the protein products of the PKD1 and PKD2 genes. We have now demonstrated that PC1 can activate the calcineurin/NFAT (nuclear factor of activated T-cells) signaling pathway through Galpha(q) -mediated activation of phospholipase C (PLC). Transient transfection of HEK293T cells with an NFAT promoter-luciferase reporter demonstrated that membrane-targeted PC1 constructs containing the membrane proximal region of the C-terminal tail, which includes the heterotrimeric G protein binding and activation domain, can stimulate NFAT luciferase activity. Inhibition of glycogen synthase kinase-3beta by LiCl treatment further increased PC1-mediated NFAT activity. PC1-mediated activation of NFAT was completely inhibited by the calcineurin inhibitor, cyclosporin A. Cotransfection of a construct expressing the Galpha(q) subunit augmented PC1-mediated NFAT activity, whereas the inhibitors of PLC (U73122) and the inositol trisphosphate and ryanodine receptors (xestospongin and 2-aminophenylborate) and a nonspecific Ca(2+) channel blocker (gadolinium) diminished PC1-mediated NFAT activity. PC2 was not able to activate NFAT. An NFAT-green fluorescent protein nuclear localization assay demonstrated that PC1 constructs containing the C-tail only or the entire 11-transmembrane spanning region plus C-tail induced NFAT-green fluorescent protein nuclear translocation. NFAT expression was demonstrated in the M-1 mouse cortical collecting duct cell line and in embryonic and adult mouse kidneys by reverse transcriptase-PCR and immunolocalization. These data suggest a model in which PC1 signaling leads to a sustained elevation of intracellular Ca(2+) mediated by PC1 activation of Galpha(q) followed by PLC activation, release of Ca(2+) from intracellular stores, and activation of store-operated Ca(2+) entry, thus activating calcineurin and NFAT.

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.

Interleukin-13 overexpression by tax transactivation: a potential autocrine stimulus in human T-cell leukemia virus-infected lymphocytes

The human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein induces growth transformation and is critical for the pathogenesis of the HTLV-1-induced adult T-cell leukemia (ATL). It stimulates the cell cycle and transactivates cellular genes. Here we show that the expression of interleukin-13 (IL-13) is upregulated as a consequence of Tax in HTLV-1-transformed T cells and ATL-derived cultures. IL-13 exerts proliferative and antiapoptotic functions and is linked to leukemogenesis, since it stimulates Hodgkin lymphoma cells by an autocrine mechanism. Overexpression of IL-13 RNA and protein was confirmed in HTLV-1-positive and Tax-transformed cells. Induction of endogenous IL-13 levels in tax-transfected Jurkat cells and in conditional Tax-expressing transformed T lymphocytes suggested that Tax can replace signals required for IL-13 synthesis. For functional analysis, the IL-13 promoter and deletion variants were cloned into luciferase reporter plasmids. Experiments with transfected human T lymphocytes revealed a 16-fold stimulation of the IL-13 promoter by Tax. Experiments with Tax mutants indicated that none of the classical transactivation pathways (SRF, CREB, and NF-kappaB) is sufficient for the transactivation; at least two different Tax functions are required for full transactivation. The IL-13 promoter is stimulated via two elements; one is a NF-AT binding P element, and the other is a putative AP-1 site. The following observations suggest that IL-13 may stimulate HTLV-1-transformed cells by an autocrine mechanism: (i) the HTLV-1-transformed cells express the IL-13 receptor on their surface, and (ii) STAT6, a downstream effector of IL-13 signaling, is constitutively activated. Thus, in summary, Tax, by transactivating the promoter, induces IL-13 overexpression that possibly leads to an autocrine stimulation of HTLV-1-infected cells.

Human T-cell lymphotropic virus type 1 open reading frame II-encoded p30II is required for in vivo replication: evidence of in vivo reversion

Human T-cell lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma and exhibits high genetic stability in vivo. HTLV-1 contains four open reading frames (ORFs) in its pX region. ORF II encodes two proteins, p30(II) and p13(II), both of which are incompletely characterized. p30(II) localizes to the nucleus or nucleolus and has distant homology to the transcription factors Oct-1, Pit-1, and POU-M1. In vitro studies have demonstrated that at low concentrations, p30(II) differentially regulates cellular and viral promoters through an interaction with CREB binding protein/p300. To determine the in vivo significance of p30(II), we inoculated rabbits with cell lines expressing either a wild-type clone of HTLV-1 (ACH.1) or a clone containing a mutation in ORF II, which eliminated wild-type p30(II) expression (ACH.30.1). ACH.1-inoculated rabbits maintained higher HTLV-1-specific antibody titers than ACH.30.1-inoculated rabbits, and all ACH.1-inoculated rabbits were seropositive for HTLV-1, whereas only two of six ACH.30.1-inoculated rabbits were seropositive. Provirus could be consistently PCR amplified from peripheral blood mononuclear cell (PBMC) DNA in all ACH.1-inoculated rabbits but in only three of six ACH.30.1-inoculated rabbits. Quantitative competitive PCR indicated higher PBMC proviral loads in ACH.1-inoculated rabbits. Interestingly, sequencing of ORF II from PBMC of provirus-positive ACH.30.1-inoculated rabbits revealed a reversion to wild-type sequence with evidence of early coexistence of mutant and wild-type sequence. Our data provide evidence that HTLV-1 must maintain its key accessory genes to survive in vivo and that in vivo pressures select for maintenance of wild-type ORF II gene products during the early course of infection.

Calcineurin signaling in Saccharomyces cerevisiae: how yeast go crazy in response to stress

In the yeast Saccharomyces cerevisiae, Ca(2+) signaling mediated by the Ca(2+)/calmodulin dependent phosphatase, calcineurin, is required for survival during environmental stress. One role of the phosphatase under these conditions is to activate gene expression through its regulation of the Crz1p ("crazy") transcription factor. Calcineurin dephosphorylates Crz1p and causes its rapid translocation from the cytosol to the nucleus. Crz1p then activates the transcription of genes whose products promote cell survival. Recent studies concerning the regulation of Crz1p by calcineurin are discussed in this review and the mechanisms by which calcineurin controls gene expression in yeast and mammalian cells are compared.

Roles of viral and cellular proteins in the expression of alternatively spliced HTLV-1 pX mRNAs11The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government

The human T cell leukemia virus type 1 (HTLV-1) genome contains a cluster of at least five open reading frames (ORFs) near the 3' terminus within the pX region. The pX ORFs are encoded by mono- or bicistronic mRNAs that are generated by alternative splicing. The various pX mRNAs result from skipping of the internal exon (2-exon versus 3-exon isofoms) or from the utilization of alternative splice acceptor sites in the terminal exon. The Rex and Tax proteins, encoded by ORFs X-III and X-IV, have been studied intensively and are encoded by the most abundant of the alternative 3-exon mRNAs. The protein products of the other pX ORFs have not been detected in HTLV-1-infected cell lines and the levels of the corresponding mRNAs have not been accurately established. We have used real-time RT-PCR with splice-site specific primers to accurately measure the levels of individual pX mRNA species in chronically infected T cell lines. We have asked whether virus regulatory proteins or ectopic expression of cellular factors influence pX mRNA splicing in cells that were transfected with HTLV-1 provirus clones. In chronically infected cell lines, the pX-tax/rex mRNA was present at 500- to 2500-fold higher levels than the pX-tax-orfII mRNA and at approximately 1000-fold higher levels than pX-rex-orfI mRNA. Chronically infected cell lines that contain numerous defective proviruses expressed 2-exon forms of pX mRNAs at significantly higher levels compared to cell lines that contain a single full-length provirus. Cells transfected with provirus expression plasmids expressed similar relative amounts of 3-exon pX mRNAs but lower levels of 2-exon mRNA forms compared to cells containing a single, full-length provirus. The pX mRNA expression patterns were nearly identical in cells transfected with wild-type, Tax-minus, or Rex-minus proviruses. Cotransfection of cells with HTLV-1 provirus in combination with SF2/ASF expression plasmid resulted in a relative increase in pX-tax/rex mRNA compared to pX-tax-orfII and pX-rex-orfI mRNAs, but did not affect exon skipping. Ectopic expression of hnRNP A1 did not affect pX splice site utilization, but increased exon skipping, as the level of pX-p21rex mRNA was increased by almost 10-fold.