Detection of human T-lymphotropic virus type I (HTLV-I) tax RNA in the central nervous system of HTLV-I-associated myelopathy/tropical spastic paraparesis patients by in situ hybridization

STLV-1 Commonly Targets Neurons in the Brain of Asymptomatic Non-Human Primates

The human T-cell leukemia virus (HTLV)-1 is responsible for an aggressive neurodegenerative disease (HAM/TSP) and multiple neurological alterations. The capacity of HTLV-1 to infect central nervous system (CNS) resident cells, together with the neuroimmune-driven response, has not been well-established. Here, we combined the use of human induced pluripotent stem cells (hiPSC) and of naturally STLV-1-infected nonhuman primates (NHP) as models with which to investigate HTLV-1 neurotropism. Hence, neuronal cells obtained after hiPSC differentiation in neural polycultures were the main cell population infected by HTLV-1. Further, we report the infection of neurons with STLV-1 in spinal cord regions as well as in brain cortical and cerebellar sections of postmortem NHP. Additionally, reactive microglial cells were found in infected areas, suggesting an immune antiviral response. These results emphasize the need to develop new efficient models by which to understand HTLV-1 neuroinfection and suggest an alternative mechanism that leads to HAM/TSP.

CRISPR Genome Editing Applied to the Pathogenic Retrovirus HTLV-1

CRISPR editing of retroviral proviruses has been limited to HIV-1. We propose human T-cell leukemia virus type 1 (HTLV-1) as an excellent model to advance CRISPR/Cas9 genome editing technologies against actively expressing and latent retroviral proviruses. HTLV-1 is a tumorigenic human retrovirus responsible for the development of both leukemia/lymphoma (ATL) and a neurological disease (HAM/TSP). The virus immortalizes and persists in CD4⁺ T lymphocytes that survive for the lifetime of the host. The most important drivers of HTLV-1-mediated transformation and proliferation are the tax and hbz viral genes. Tax, transcribed from the plus-sense or genome strand, is essential for de novo infection and cellular immortalization. Hbz, transcribed from the minus-strand, supports proliferation and survival of infected cells in both its protein and mRNA forms. Abrogating the function or expression of tax and/or hbz by genome editing and mutagenic double-strand break repair may disable HTLV-1-infected cell growth/survival and prevent immune modulatory effects and ultimately HTLV-1-associated disease. In addition, the HTLV-1 viral genome is highly conserved with remarkable sequence homogeneity, both within the same host and even among different HTLV isolates. This offers more focused guide RNA targeting. In addition, there are several well-established animal models for studying HTLV-1 infection in vivo as well as cell immortalization in vitro. Therefore, studies with HTLV-1 may provide a better basis to assess and advance in vivo genome editing against retroviral infections.

Immunopathogenesis and Cellular Interactions in Human T-Cell Leukemia Virus Type 1 Associated Myelopathy/Tropical Spastic Paraparesis

HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) is a neuropathological disorder in 1–3% of individuals infected with Human T-lymphotropic virus 1 (HTLV-1). This condition is characterized by progressive spastic lower limb weakness and paralysis, lower back pain, bladder incontinence, and mild sensory disturbances resembling spinal forms of multiple sclerosis. This disease also causes chronic disability and is therefore associated with high health burden in areas where HTLV-1 infection is endemic. Despite various efforts in understanding the virus and discovery of novel diagnostic markers, and cellular and viral interactions, HAM/TSP management is still unsatisfactory and mainly focused on symptomatic alleviation, and it hasn’t been explained why only a minority of the virus carriers develop HAM/TSP. This comprehensive review focuses on host and viral factors in association with immunopathology of the disease in hope of providing new insights for drug therapies or other forms of intervention.

Regulation of Expression and Latency in BLV and HTLV

Human T-lymphotrophic virus type 1 (HTLV-1) and Bovine leukemia virus (BLV) belong to the Deltaretrovirus genus. HTLV-1 is the etiologic agent of the highly aggressive and currently incurable cancer adult T-cell leukemia (ATL) and a neurological disease HTLV-1-associated myelopathy (HAM)/tropical spastic paraparesis (TSP). BLV causes neoplastic proliferation of B cells in cattle: enzootic bovine leucosis (EBL). Despite the severity of these conditions, infection by HTLV-1 and BLV appear in most cases clinically asymptomatic. These viruses can undergo latency in their hosts. The silencing of proviral gene expression and maintenance of latency are central for the establishment of persistent infection, as well as for pathogenesis in vivo. In this review, we will present the mechanisms that control proviral activation and retroviral latency in deltaretroviruses, in comparison with other exogenous retroviruses. The 5′ long terminal repeats (5′-LTRs) play a main role in controlling viral gene expression. While the regulation of transcription initiation is a major mechanism of silencing, we discuss topics that include (i) the epigenetic control of the provirus, (ii) the cis-elements present in the LTR, (iii) enhancers with cell-type specific regulatory functions, (iv) the role of virally-encoded transactivator proteins, (v) the role of repressors in transcription and silencing, (vi) the effect of hormonal signaling, (vii) implications of LTR variability on transcription and latency, and (viii) the regulatory role of non-coding RNAs. Finally, we discuss how a better understanding of these mechanisms may allow for the development of more effective treatments against Deltaretroviruses.

HTLV-1 infection of myeloid cells: from transmission to immune alterations

Human T cell leukemia virus type 1 (HTLV-1), the etiological agent of adult T-cell leukemia/lymphoma (ATLL) and the demyelinating neuroinflammatory disease known as HTLV-1-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP), was the first human retrovirus to be discovered. T-cells, which represent the main reservoir for HTLV-1, have been the main focus of studies aimed at understanding viral transmission and disease progression. However, other cell types such as myeloid cells are also target of HTLV-1 infection and display functional alterations as a consequence. In this work, we review the current investigations that shed light on infection, transmission and functional alterations subsequent to HTLV-1 infection of the different myeloid cells types, and we highlight the lack of knowledge in this regard.

Immunovirological markers in HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP)

Human T cell lymphotropic virus 1 (HTLV-1) is a human retrovirus and infects approximately 10–20 million people worldwide. While the majority of infected people are asymptomatic carriers of HTLV-1, only 4% of infected people develop HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HAM/TSP is a chronic, progressive, neurological disease which usually progresses slowly without remission, and is characterized by perivascular inflammatory infiltrates in chronic inflammatory lesions of the central nervous system (CNS), primarily affecting the spinal cord. A high HTLV-1 proviral load, high levels of antibodies against HTLV-1 antigens, and elevated concentration of proteins are detected in cerebrospinal fluid (CSF) of HAM/TSP patients. These chronically activated immune responses against HTLV-1 and infiltration of inflammatory cells including HTLV-1 infected cells into the CNS contribute to clinical disability and underlie the pathogenesis of HAM/TSP. Since the disease development of HAM/TSP mainly occurs in adults, with a mean age at onset of 40–50 years, it is important for HTLV-1-infected carriers and HAM/TSP patients to be monitored throughout the disease process. Recent advances in technologies and findings provide new insights to virological and immunological aspects in both the CNS as well as in peripheral blood. In this review, we focus on understanding the inflammatory milieu in the CNS and discuss the immunopathogenic process in HTLV-1-associated neurologic diseases.

Cytokine Networks Dysregulation during HTLV-1 Infection and Associated Diseases

Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of a neural chronic inflammation, called HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and of a malignant lymphoproliferation, called the adult T-cell leukemia/lymphoma (ATLL). The mechanisms through which the HTLV-1 induces these diseases are still unclear, but they might rely on immune alterations. HAM/TSP is associated with an impaired production of pro-inflammatory cytokines and chemokines, such as IFN-γ, TNF-α, CXCL9, or CXCL10. ATLL is associated with high levels of IL-10 and TGF-β. These immunosuppressive cytokines could promote a protumoral micro-environment. Moreover, HTLV-1 infection impairs the IFN-I production and signaling, and favors the IL-2, IL-4, and IL-6 expression. This contributes both to immune escape and to infected cells proliferation. Here, we review the landscape of cytokine dysregulations induced by HTLV-1 infection and the role of these cytokines in the HTLV-1-associated diseases progression.

HTLV-1 infection-induced motor dysfunction, memory impairment, depression, and brain tissues oxidative damage in female BALB/c mice

AIMS

The HTLV-1 infection is associated with a neuro-inflammatory disease. In the present study, the behavioral consequences and brain oxidative damages were evaluated in HTLV-1-infected BALB/c mice.

MATERIAL AND METHODS

20 female BALB/c mice were divided into two groups comprising control and HTLV-1-infected. The HTLV-1-infected group was inoculated with a 10⁶ MT-2 HTLV-1-infected cell line. Two months later, the behavioral tests were conducted. Finally, oxidative stress was assessed in the cortex and hippocampus tissues.

KEY FINDINGS

In the HTLV-1-infected group, running time and latency to fall, travel distance and time spent in the peripheral zone, total crossing number and total traveled distance in open field test, the latency of entrance into the dark compartment in the passive avoidance test, the new object exploration percentage, and discrimination ratio were significantly lower than in the control group. The immobility time, time spent in the dark compartment in passive avoidance test, and total exploration time significantly increased in the HTLV-1-infected group compared to the control group. In the cortical tissue of the HTLV-1 group, the malondialdehyde levels were elevated while the total thiol levels decreased in comparison to the control group. The activity of superoxide dismutase in the cortical and hippocampal tissues, and catalase activity in cortical tissue significantly decreased in the HTLV-1 group in comparison to the control group.

SIGNIFICANCE

The HTLV-1 infection seems to induce depression-like behavior, motor dysfunction, disruption in working and fear memory and also oxidative stress in the cortex and hippocampus.

Role of HTLV-1 Tax and HBZ in the Pathogenesis of HAM/TSP

Human T cell lymphotropic virus type 1 (HTLV-1) infection can lead to development of adult T cell leukemia/lymphoma (ATL) or HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) in a subset of infected subjects. Understanding the interaction between host and HTLV-1 and the molecular mechanisms associated with disease pathogenesis is critical for development efficient therapies. Two HTLV-1 genes, tax and HTLV-1 basic leucine zipper factor (HBZ), have been demonstrated to play important roles in HTLV-1 infectivity and the growth and survival of leukemic cells. Increased HTLV-1 Tax expression induces the expression of various cellular genes such as IL-2 and IL-15, which directly contributes to lymphocyte activation and immunopathogenesis in HAM/TSP patients. However, little is known about the molecular and cellular mechanism of HBZ in development of HAM/TSP. It has been reported that HBZ mRNA expression was detected in HAM/TSP patients higher than in asymptomatic carriers and correlated with proviral load and disease severity. Unlike HTLV-1 tax, HBZ escapes efficient anti-viral immune responses and therefore these reactivities are difficult to detect. Thus, it is important to focus on understanding the function and the role of HTLV-1 tax and HBZ in disease development of HAM/TSP and discuss the potential use of these HTLV-1 viral gene products as biomarkers and therapeutic targets for HAM/TSP.

Synaptic Plasticity and Neurological Disorders in Neurotropic Viral Infections

Based on the type of cells or tissues they tend to harbor or attack, many of the viruses are characterized. But, in case of neurotropic viruses, it is not possible to classify them based on their tropism because many of them are not primarily neurotropic. While rabies and poliovirus are considered as strictly neurotropic, other neurotropic viruses involve nervous tissue only secondarily. Since the AIDS pandemic, the interest in neurotropic viral infections has become essential for all clinical neurologists. Although these neurotropic viruses are able to be harbored in or infect the nervous system, not all the neurotropic viruses have been reported to cause disrupted synaptic plasticity and impaired cognitive functions. In this review, we have discussed the neurotropic viruses, which play a major role in altered synaptic plasticity and neurological disorders.

HAM/TSP-derived HTLV-1-infected T cell lines promote morphological and functional changes in human astrocytes cell lines: possible role in the enhanced T cells recruitment into Central Nervous System

BACKGROUND

The mechanisms through which HTLV-1 leads to and maintains damage in the central nervous system of patients undergoing HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) are still poorly understood. In recent years, increasing evidence indicates that, not only lymphocytes but also glial cells, in particular astrocytes, play a role in the pathophysiology of HAM/TSP. In this study we used a model of co-culture between human HTLV-1-infected (CIB and C91PL) and non-infected (CEM) T lymphocyte cell lines and astrocyte (U251 and U87) cell lines to mimic the in vivo T cell-astrocyte interactions.

RESULTS

We first observed that CIB and C91PL adhere strongly to cultured astrocytes cell lines, and that co-cultures of HTLV-1 infected and astrocyte cell lines cells resulted in rapid syncytium formation, accompanied by severe morphological alterations and increased apoptotic cell death of astrocyte cells. Additionally, cultures of astrocyte cell lines in presence of supernatants harvested from HTLV-1-infected T cell cultures resulted in significant increase in the mRNA of CCL2, CXCL1, CXCL2, CXCL3, CXCL10, IL-13, IL-8, NFKB1, TLR4, TNF, MMP8 and VCAM1, as compared with the values obtained when we applied supernatants of non-infected T- cell lines. Lastly, soluble factors secreted by cultured astrocytic cell lines primed through 1-h interaction with infected T cell lines, further enhanced migratory responses, as compared to the effect seen when supernatants from astrocytic cell lines were primed with non-infected T cell lines.

CONCLUSION

Collectively, our results show that HTLV-1 infected T lymphocyte cell lines interact strongly with astrocyte cell lines, leading to astrocyte damage and increased secretion of attracting cytokines, which in turn may participate in the further attraction of HTLV-1-infected T cells into central nervous system (CNS), thus amplifying and prolonging the immune damage of CNS.

Human T-lymphotropic virus type 1-associated myelopathy/tropical spastic paraparesis: Clinical presentation and pathophysiology

Human T-cell lymphotropic virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a slowly progressive neurodegenerative disorder in which lesions of the central nervous system cause progressive weakness, stiffness, and a lower limb spastic paraparesis. In some cases, polymyositis, inclusion body myositis, or amyotrophic lateral sclerosis-like syndromes are associated with HTLV-1. TSP was first described in Jamaica in 1888 and known as Jamaican peripheral neuritis before TSP was related to HTLV-1 virus, the first retrovirus being identified, and the disease is since named HAM/TSP. There is no established treatment program for HAM/TSP. Prevention is difficult in low-income patients (i.e., HTLV-1 infected breast feeding mothers in rural areas, sex workers). Thus, there is a need for new therapeutic avenues. Therapeutic approaches must be based on a better understanding, not only of clinical and clinicopathological data, but also of the pathophysiology of the affection. Consequently, a better understanding of existing or newly developed animal models of HAM/TSP is a prerequisite step in the development of new treatments.

[Intercellular transmission of HTLV-1: not all mechanisms have been revealed]

HTLV-1 is the etiological agent of Adult T cell Leukemia/Lymphoma (ATLL) and of HTLV-1-Associated Myelopathy/tropical spastic paraparesis (HAM/TSP). It is mainly detected in CD4+ lymphocytes in vivo, but proviral genomes have also been detected although less frequently, in CD8+ T lymphocytes, B lymphocytes, monocytes, macrophages, dendritic cells and other non-lymphoid cells. Virus spread is highly dependent on cell-cell contact. This mode of transmission is correlated with an increased ability of infected cells to migrate, a property linked to cytoskeleton reorganization induced by the viral Tax protein. Cell-to-cell transmission relies on at least three non-exclusive molecular pathways. First, a specialized area, the "virological synapse'' (VS) promotes direct transmission of budding HTLV-1 particles into a synaptic cleft formed between infected and uninfected cells. Second, HTLV-1 particles accumulate at the plasma membrane of infected cells in a biofilm-like extracellular viral assembly that resembles a bacterial biofilm. Viral biofilm is rapidly transmitted to uninfected cells when infected cells contact target cells. Finally, membrane extensions called inter-cellular conduits facilitate HTLV-1 proteins transfer from infected to uninfected target cells, and may stabilize cell-cell contacts. The aim of this review is to summarize the molecular mechanisms of these HTLV-1 transmission pathways.

HTLV-1-associated myelopathy/tropical spastic paraparesis

Human T-lymphotropic virus 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a progressive disease of the CNS that causes weakness or paralysis of the legs, lower back pain and urinary symptoms. HAM/TSP was first described in Jamaica in the nineteenth century, but the aetiology of the condition, infection with the retrovirus HTLV-1, was only identified in the 1980s. HAM/TSP causes chronic disability and, accordingly, imposes a substantial health burden in areas where HTLV-1 infection is endemic. Since the discovery of the cause of HAM/TSP, considerable advances have been made in the understanding of the virology, immunology, cell biology and pathology of HTLV-1 infection and its associated diseases. However, progress has been limited by the lack of accurate animal models of the disease. Moreover, the treatment of HAM/TSP remains highly unsatisfactory: antiretroviral drugs have little impact on the infection and, although potential disease-modifying therapies are widely used, their value is unproved. At present, clinical management is focused on symptomatic treatment and counselling. Here, we summarize current knowledge on the epidemiology, pathogenesis and treatment of HAM/TSP and identify areas in which further research is needed. For an illustrated summary of this Primer, visit: http://go.nature.com/tjZCFM.

Adult T-cell leukemia: future prophylaxis and immunotherapy

A small population of human T-cell leukemia virus Type I (HTLV-I) carriers develop adult T-cell leukemia after a long incubation period. The results of a series of experiments using animal models suggest that insufficiency of HTLV-I-specific T-cell response induced by vertical HTLV-I infection allows enlargement of the HTLV-I-infected cell reservoir in vivo, a crucial risk factor of adult T-cell leukemia. In this review it is proposed that prophylactic Tax-targeted vaccines for the high-risk group of adult T-cell leukemia, which is characterized by low HTLV-I-specific T-cell response and high proviral load, can reduce the risk. Immunological studies on adult T-cell leukemia patients after hematopoietic stem cell transplantation also suggest that Tax-targeted immunotherapy may be effective against full-blown disease, although its indication may be limited.

Human T-cell leukemia virus type 1 Tax-deregulated autophagy pathway and c-FLIP expression contribute to resistance against death receptor-mediated apoptosis

The human T-cell leukemia virus type 1 (HTLV-1) Tax protein is considered to play a central role in the process that leads to adult T-cell leukemia/lymphoma (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-1 Tax-expressing cells show resistance to apoptosis induced by Fas ligand (FasL) and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL). The regulation of Tax on the autophagy pathway in HeLa cells and peripheral T cells was recently reported, but the function and underlying molecular mechanism of the Tax-regulated autophagy are not yet well defined. Here, we report that HTLV-1 Tax deregulates the autophagy pathway, which plays a protective role during the death receptor (DR)-mediated apoptosis of human U251 astroglioma cells. The cellular FLICE-inhibitory protein (c-FLIP), which is upregulated by Tax, also contributes to the resistance against DR-mediated apoptosis. Both Tax-induced autophagy and Tax-induced c-FLIP expression require Tax-induced activation of IκB kinases (IKK). Furthermore, Tax-induced c-FLIP expression is regulated through the Tax-IKK-NF-κB signaling pathway, whereas Tax-triggered autophagy depends on the activation of IKK but not the activation of NF-κB. In addition, DR-mediated apoptosis is correlated with the degradation of Tax, which can be facilitated by the inhibitors of autophagy.

IMPORTANCE

Our study reveals that Tax-deregulated autophagy is a protective mechanism for DR-mediated apoptosis. The molecular mechanism of Tax-induced autophagy is also illuminated, which is different from Tax-increased c-FLIP. Tax can be degraded via manipulation of autophagy and TRAIL-induced apoptosis. These results outline a complex regulatory network between and among apoptosis, autophagy, and Tax and also present evidence that autophagy represents a new possible target for therapeutic intervention for the HTVL-1 related diseases.

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.

Cellular Factors Involved in HTLV-1 Entry and Pathogenicit

Human T cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia (ATL) and HTLV-1 – associated myelopathy and tropical spastic paraparesis (HAM/TSP). HTLV-1 has a preferential tropism for CD4 T cells in healthy carriers and ATL patients, while both CD4 and CD8 T cells serve as viral reservoirs in HAM/TSP patients. HTLV-1 has also been detected other cell types, including monocytes, endothelial cells, and dendritic cells. In contrast to the limited cell tropism of HTLV-1 in vivo, the HTLV receptor appears to be expressed in almost all human or animal cell lines. It remains to be examined whether this cell tropism is determined by host factors or by HTLV-1 heterogeneity. Unlike most retroviruses, cell-free virions of HTLV-1 are very poorly infectious. The lack of completely HTLV-1-resistant cells and the low infectivity of HTLV-1 have hampered research on the HTLV entry receptor. Entry of HTLV-1 into target cells is thought to involve interactions between the env (Env) glycoproteins, a surface glycoprotein (surface unit), and a transmembrane glycoprotein. Recent studies have shown that glucose transporter GLUT1, heparan sulfate proteoglycans (HSPGs), and neuropilin-1 (NRP-1) are the three proteins important for the entry of HTLV-1. Studies using adherent cell lines have shown that GLUT1 can function as a receptor for HTLV. HSPGs are required for efficient entry of HTLV-1 into primary CD4 T cells. NRP-1 is expressed in most established cell lines. Further studies have shown that these three molecules work together to promote HTLV-1 binding to cells and fusion of viral and cell membranes. The virus could first contact with HSPGs and then form complexes with NRP-1, followed by association with GLUT1. It remains to be determined whether these three molecules can explain HTLV-1 cell tropism. It also remains to be more definitively proven that these molecules are sufficient to permit HTLV-1 entry into completely HTLV-1-resistant cells.

HTLV-1 Tax mediated downregulation of miRNAs associated with chromatin remodeling factors in T cells with stably integrated viral promoter

RNA interference (RNAi) is a natural cellular mechanism to silence gene expression and is predominantly mediated by microRNAs (miRNAs) that target messenger RNA. Viruses can manipulate the cellular processes necessary for their replication by targeting the host RNAi machinery. This study explores the effect of human T-cell leukemia virus type 1 (HTLV-1) transactivating protein Tax on the RNAi pathway in the context of a chromosomally integrated viral long terminal repeat (LTR) using a CD4⁺ T-cell line, Jurkat. Transcription factor profiling of the HTLV-1 LTR stably integrated T-cell clone transfected with Tax demonstrates increased activation of substrates and factors associated with chromatin remodeling complexes. Using a miRNA microarray and bioinformatics experimental approach, Tax was also shown to downregulate the expression of miRNAs associated with the translational regulation of factors required for chromatin remodeling. These observations were validated with selected miRNAs and an HTLV-1 infected T cells line, MT-2. miR-149 and miR-873 were found to be capable of directly targeting p300 and p/CAF, chromatin remodeling factors known to play critical role in HTLV-1 pathogenesis. Overall, these results are first in line establishing HTLV-1/Tax-miRNA-chromatin concept and open new avenues toward understanding retroviral latency and/or replication in a given cell type.

Blood-brain barrier and retroviral infections

Homeostasis in the central nervous system (CNS) is maintained by active interfaces between the bloodstream and the brain parenchyma. The blood-brain barrier (BBB) constitutes a selective filter for exchange of water, solutes, nutrients, and controls toxic compounds or pathogens entry. Some parasites, bacteria, and viruses have however developed various CNS invasion strategies, and can bypass the brain barriers. Concerning viruses, these strategies include transport along neural pathways, transcytosis, infection of the brain endothelial cells, breaching of the BBB, and passage of infected-leukocytes. Moreover, neurotropic viruses can alter BBB functions, thus compromising CNS homeostasis. Retroviruses have been associated to human neurological diseases: HIV (human immunodeficiency virus 1) can induce HIV-associated dementia, and HTLV-1 (human T lymphotropic virus 1) is the etiological factor of tropical spastic paraparesis/HTLV-1 associated myelopathy (TSP/HAM). The present review focuses on how the different retroviruses interact with this structure, bypass it and alter its functions.

Cotranscriptional Chromatin Remodeling by Small RNA Species: An HTLV-1 Perspective

Cell type specificity of human T cell leukemia virus 1 has been proposed as a possible reason for differential viral outcome in primary target cells versus secondary. Through chromatin remodeling, the HTLV-1 transactivator protein Tax interacts with cellular factors at the chromosomally integrated viral promoter to activate downstream genes and control viral transcription. RNA interference is the host innate defense mechanism mediated by short RNA species (siRNA or miRNA) that regulate gene expression. There exists a close collaborative functioning of cellular transcription factors with miRNA in order to regulate the expression of a number of eukaryotic genes including those involved in suppression of cell growth, induction of apoptosis, as well as repressing viral replication and propagation. In addition, it has been suggested that retroviral latency is influenced by chromatin alterations brought about by miRNA. Since Tax requires the assembly of transcriptional cofactors to carry out viral gene expression, there might be a close association between miRNA influencing chromatin alterations and Tax-mediated LTR activation. Herein we explore the possible interplay between HTLV-1 infection and miRNA pathways resulting in chromatin reorganization as one of the mechanisms determining HTLV-1 cell specificity and viral fate in different cell types.

Inhibition of immune activation by a novel nuclear factor-kappa B inhibitor in HTLV-I-associated neurologic disease

The human T-lymphotropic virus type I (HTLV-I) causes a chronic inflammatory disorder of the central nervous system termed HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-I encodes a protein known to activate several host-signaling pathways involved in inflammation, such as the nuclear factor-κB (NF-κB). The contribution of the NF-κB pathway to the pathogenesis of HAM/TSP, however, has not been fully defined. We show evidence of canonical NF-κB activation in short-term cultures of peripheral blood mononuclear cells (PBMCs) from subjects with HAM/TSP. NF-κB activation was closely linked to HTLV-I viral protein expression. The NF-κB activation in HAM/TSP PBMCs was reversed by a novel small-molecule inhibitor that demonstrates potent and selective NF-κB antagonist activity. Inhibition of NF-κB activation led to a reduction in the expression of lymphocyte activation markers and resulted in reduced cytokine signaling in HAM/TSP PBMCs. Furthermore, NF-κB inhibition led to a reduction in spontaneous lymphoproliferation, a key ex vivo correlate of the immune activation associated with HAM/TSP. These results indicate that NF-κB activation plays a critical upstream role in the immune activation of HAM/TSP, and identify the NF-κB pathway as a potential target for immunomodulation in HAM/TSP.

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.

Depletion of dendritic cells enhances susceptibility to cell-free infection of human T cell leukemia virus type 1 in CD11c-diphtheria toxin receptor transgenic mice

Human T cell leukemia virus type 1 (HTLV-1) is associated with two immunologically distinct diseases: HTLV-1-associated myelopathy/tropical spastic paraparesis and adult T cell leukemia. The genesis of these diseases is believed to be associated with the route (mucosa versus blood) and mode (cell-free versus cell-associated) of primary infection as well as the modulation of dendritic cell (DC) functions. To explore the role of DCs during early HTLV-1 infection in vivo, we used a chimeric HTLV-1 with a replaced envelope gene from Moloney murine leukemia virus to allow HTLV-1 to fuse with murine cells, which are generally not susceptible to infection with human retroviruses. We also used a CD11c-diphtheria toxin receptor transgenic mouse model system that permits conditional transient depletion of CD11c(+) DCs. We infected these transgenic mice with HTLV-1 using both cell-free and cell-associated infection routes in the absence and presence of DCs. The ablation of DCs led to an enhanced susceptibility to infection with cell-free but not cell-associated HTLV-1 in both CD4 and non-CD4 fractions, as measured by the proviral load. Infection with cell-free virus in the absence of DCs was also found to have increased levels of Tax mRNA in the non-CD4 fraction. Moreover, depletion of DCs significantly dampened the cellular immune response (IFN-gamma(+)CD8(+) T cells) against both cell-free and cell-associated virus. These results uniquely differentiate the involvement of DCs in early cell-free versus late cell-associated infection of HTLV-1 and highlight a significant aspect of viral immunopathogenesis related to the progression of adult T cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis after the initial infection.

Role of resident CNS cell populations in HTLV-1-associated neuroinflammatory disease

Human T cell leukemia virus type 1 (HTLV-1), the first human retrovirus discovered, is the etiologic agent for a number of disorders; the two most common pathologies include adult T cell leukemia (ATL) and a progressive demyelinating neuroinflammatory disease, HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The neurologic dysfunction associated with HAM/TSP is a result of viral intrusion into the central nervous system (CNS) and the generation of a hyperstimulated host response within the peripheral and central nervous system that includes expanded populations of CD4+ and CD8+ T cells and proinflammatory cytokines/chemokines in the cerebrospinal fluid (CSF). This robust, yet detrimental immune response likely contributes to the death of myelin producing oligodendrocytes and degeneration of neuronal axons. The mechanisms of neurological degeneration in HAM/TSP have yet to be fully delineated in vivo and may involve the immunogenic properties of the HTLV-1 transactivator protein Tax. This comprehensive review characterizes the available knowledge to date concerning the effects of HTLV-1 on CNS resident cell populations with emphasis on both viral and host factors contributing to the genesis of HAM/TSP.

Treatment of HTLV-I-associated myelopathy/tropical spastic paraparesis: toward rational targeted therapy

The treatment of HAM/TSP is a challenge. No agent has shown to significantly modify the long-term disability associated with HAM/TSP. Advances in our understanding of the pathogenesis of HAM/TSP have led to the identification of several biomarkers and therapeutic targets. Clinical trials in HAM/TSP continue to be opportunities for further qualification and refinement of biomarkers and therapeutic targets. The validation of HAM/TSP relevant biomarkers and the identification of new targets remain key challenges in the development of effective targeted therapy in HAM/TSP.

Viruses and rickettsiae

In this review I shall try to provide a brief, up-to-date, account of the neuropathology of those viral and rickettsial diseases that are particularly prevalent in tropical regions. These diseases are not, however, exclusive to the tropics. Some, such as AIDS, are common in temperate regions as well, though others are closer to being exclusively tropical, such as some of the arthropod-borne (ARBO) virus encephalides. The latter are dependent for their dissemination on an existence during part of their infectious cycle in insects which are, in turn, climatically and seasonally sensitive. This necessarily limits their geographical distribution. Factors that influence some of the other diseases are less closely dependent on climate and geography and reflect more the social or cultural conditions under which people live. Thus, diseases that depend for their spread on forms of human behavior such as promiscuity or drug abuse (AIDS), or poor hygiene and living conditions (polio, rickettsial diseases) or on contact with domestic and other animals (rabies) may occur in a more widespread distribution, for the tropics are not the only places that afford opportunities for these diseases to flourish. I shall select for discussion aspects of the pathology of these diseases that are currently undergoing investigation but will aim to present these against the backdrop of more established aspects of their pathology. Recent reviews of the pathology of viral encephalitis can be found in Hamilton and Wiley (33) and Esiri and Kennedy (20) and of HIV-1 infection in Price & Sidtis (78) and Scaravilli (85).

Dysregulation of TGF-beta signaling and regulatory and effector T-cell function in virus-induced neuroinflammatory disease

We previously demonstrated that CD4(+)CD25(+) T regulatory cells (Tregs), important for the maintenance of immune tolerance and prevention of autoimmune disease, from patients with human T lymphotropic virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) exhibit reduced Foxp3 expression and Treg suppressor function compared with healthy donors. Since TGF-beta signaling has been previously reported to be critical for both Foxp3 expression and Treg function, we examined whether this signaling pathway was dysregulated in patients with HAM/TSP. Levels of TGF-beta receptor II (TGF-betaRII) as well as Smad7 (a TGF-beta-inducible gene) were significantly reduced in CD4(+) T cells in patients with HAM/TSP compared with healthy donors, and the expression of TGF-betaRII inversely correlated with the HTLV-I tax proviral load. Importantly, both CD4(+)CD25(+) and CD4(+)CD25(-) T cells from HAM/TSP patients exhibited reduced TGF-betaRII expression compared with healthy donors, which was associated with functional deficits in vitro, including a block in TGF-beta-inducible Foxp3 expression that inversely correlated with the HTLV-I tax proviral load, loss of Treg suppressor function, and escape of effector T cells from Treg-mediated control. This evidence suggests that a virus-induced breakdown of immune tolerance affecting both regulatory and effector T cells contributes to the pathogenesis of HAM/TSP.

Proinflammatory cytokine gene induction by human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 Tax in primary human glial cells

Infection with human T-cell leukemia virus type 1 (HTLV-1) can result in the development of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic inflammatory disease of the central nervous system (CNS). HTLV-2 is highly related to HTLV-1 at the genetic level and shares a high degree of sequence homology, but infection with HTLV-2 is relatively nonpathogenic compared to HTLV-1. Although the pathogenesis of HAM/TSP remains to be fully elucidated, previous evidence suggests that elevated levels of the proinflammatory cytokines in the CNS are associated with neuropathogenesis. We demonstrate that HTLV-1 infection in astrogliomas results in a robust induction of interleukin-1beta (IL-1beta), IL-1alpha, tumor necrosis factor alpha (TNF-alpha), TNF-beta, and IL-6 expression. HTLV encodes for a viral transcriptional transactivator protein named Tax that also induces the transcription of cellular genes. To investigate and compare the effects of Tax1 and Tax2 expression on the dysregulation of proinflammatory cytokines, lentivirus vectors were used to transduce primary human astrocytomas and oligodendrogliomas. The expression of Tax1 in primary human astrocytomas and oligodendrogliomas resulted in significantly higher levels of proinflammatory cytokine gene expression compared to Tax2. Notably, Tax1 expression uniquely sensitized primary human astrocytomas to apoptosis. A Tax2/Tax1 chimera encoding the C-terminal 53 amino acids of the Tax1 fused to the Tax2 gene (Tax(221)) demonstrated a phenotype that resembled Tax1, with respect to proinflammatory cytokine gene expression and sensitization to apoptosis. The patterns of differential cytokine induction and sensitization to apoptosis displayed by Tax1 and Tax2 may reflect differences relating to the heightened neuropathogenicity associated with HTLV-1 infection and the development of HAM/TSP.

Immunostimulation by induced expression of NKG2D and its MIC ligands in HTLV-1-associated neurologic disease

The NKG2D receptor costimulates effector/memory CD8 T cells and is normally absent on CD4 T cells but can be induced by T cell antigen receptor complex stimulation and interleukin-15 (IL-15). Among its ligands are the human major histocompatibility complex class I-related MICA and MICB, which have a restricted tissue distribution but are frequently associated with malignancies and some microbial infections. Moreover, aberrant expression of MIC may promote autoimmune disease progression. Human T cell lymphotropic virus type I (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a chronic inflammatory disease of the central nervous system that resembles multiple sclerosis. Disease progression involves production of IL-15 and its receptor through transactivation by the viral Tax regulator protein, an activated immune response state, and local cytokine production and T cell fratricide by Tax-specific cytotoxic T lymphocytes (CTL). This study shows that as with CD8 T cells, substantial proportions of HAM/TSP patient CD4 T cells are positive for NKG2D and that large numbers of T cells from both subsets express MIC, which can be transactivated by Tax independent of nuclear factor kappaB. Engagement of MIC by NKG2D promotes spontaneous HAM/TSP T cell proliferation and, apparently, CTL activities against HTLV-1-infected T cells. These results reveal a viral strategy that may exploit immune stimulatory mechanisms to negotiate a balance between promotion and limitation of infected host T cell expansions.

CCAAT/enhancer-binding proteins modulate human T cell leukemia virus type 1 long terminal repeat activation

CCAAT/enhancer-binding protein (C/EBP) basic region/leucine zipper (bZIP) transcription factors have been shown to form heterodimers with cAMP-responsive element binding protein 2 (CREB-2), a transcription factor involved in regulating basal and Tax-mediated transactivation of the human T cell leukemia virus type 1 (HTLV-1) long terminal repeat (LTR). In cells of the monocyte-macrophage lineage (proposed to play a role in HTLV-1 pathogenesis as an accessory target cell), several members of the C/EBP family are expressed at high levels and may have functional impact on both basal and Tax-mediated transactivation of the HTLV-1 LTR. Basal activation of the HTLV-1 LTR was enhanced by overexpression of C/EBPbeta, C/EBPdelta, or C/EBPepsilon, whereas transactivation of the LTR by Tax was inhibited by overexpression of C/EBPalpha and C/EBPbeta. Inhibition of Tax-mediated transactivation of the HTLV-1 LTR was co-activator-independent, did not require C/EBP binding to the Tax-responsive elements, and may involve heterodimerization with CREB factors.

The human T-cell leukemia virus type 1 (HTLV-1): New insights into the clinical aspects and molecular pathogenesis of adult t-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-associated myelopathy (TSP/HAM)

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus to be identified in the early 1980s. The isolation and identification of a related virus, HTLV-2, and the distantly related human immunodeficiency virus (HIV) immediately followed. Of the three retroviruses, two are associated definitively with specific diseases, HIV, with acquired immune deficiency syndrome (AIDS) and HTLV-1, with adult T-cell leukemia/lymphoma (ATLL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). While an estimated 10-20 million people worldwide are infected with HTLV-I, infection is endemic in the Caribbean, parts of Africa, southwestern Japan, and Italy. Approximately 4% of HTLV-I infected individuals develop ATLL, a disease with a poor prognosis. The clinical manifestations of infection and the current biology of HTLV viruses with emphasis on HTLV-1 are discussed in detail. The implications for improvements in diagnosis, treatment, intervention, and vaccination are included, as well as a discussion of the emergence of HTLV-1 and -2 as copathogens among HIV-1-infected individuals.

HTLV-1 Tax nucleocytoplasmic shuttling, interaction with the secretory pathway, extracellular signaling, and implications for neurologic disease

The human T cell leukemia virus type 1 (HTLV-1) oncoprotein Tax interacts with numerous cellular pathways promoting both the survival and pathogenesis of the virus in the human population. Tax has been studied extensively with respect to its role in transcriptional transactivation and its involvement in the up-regulation of a number of cellular genes during the process of oncogenic transformation. These processes are dependent on Tax localization to the nucleus where it interacts with a number of cellular transcription factors during its course of nuclear action. However, there is mounting evidence suggesting that Tax may shuttle between the nucleus and cytoplasm, localize to several cytoplasmic organelles with subsequent secretion from both Tax-transfected cells as well as HTLV-1-infected cells. In addition, the presence of cell-free Tax in cerebral spinal fluid (CSF) was recently demonstrated to occur during all stages of HAM/TSP. This has brought about an increased interest in the cytoplasmic localization of Tax and the implications this localization may have with respect to the progression of HTLV-1-associated disease processes. This review addresses the functional implications relevant to the localization and accumulation of Tax in the cytoplasm including the Tax amino acid signals and cellular protein interactions that may regulate this process. Specifically, we have discussed three important processes associated with the cytoplasmic localization of Tax. First, the process of Tax shuttling between the nucleus and cytoplasm will be described and how this process may be involved in regulating different transcriptional activation pathways. Second, cytoplasmic localization of Tax will be discussed with relevance to Tax secretion and the interaction of Tax with proteins in the cellular secretory pathway. Finally, the secretion of Tax and the effects of extracellular Tax on HTLV-1 pathogenesis will be addressed.

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.

HTLV-1 tropism and envelope receptor

The identification of CD4 as the primary receptor for HIV followed shortly after the discovery of the virus, but the HTLV receptor remained long elusive, until its recent identification as the GLUT1 glucose transporter. In the present review, we describe the status of the literature that surrounded this discovery as well as the in vitro and in vivo observations that led to the identification of GLUT1. Also, we will explore a few tracks to conciliate the in vitro and in vivo data on HTLV-1 tropism within the context of the HTLV literature that has accumulated over the past 25 years. A close examination of these data leads us to conclude that the preferential detection of HTLV-1 in CD4+ T lymphocyte subsets in vivo, even in the absence of leukemia, is not likely to be directly related to envelope receptor interactions, but rather to an array of postentry selection bottlenecks in vivo. Furthermore, we propose that infection of other hematopoietic and nonhematopoietic cells is likely to take place during the lifetime of an individual, with a burst early during the infection.

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.

Tropical Spastic Paraparesis/Human T Leukemia Virus Type 1--Associated Myelopathy in HIV Type 1--Coinfected Patients

BACKGROUND

Tropical spastic paraparesis/human T leukemia virus type 1 (HTLV-1)-associated myelopathy (TSP/HAM) is rarely reported in the United States. The causative agents of TSP/HAM are HTLV-1 and, possibly, its cosmopolitan variant, human T leukemia virus type 2 (HTLV-2). Among HTLV-1- or HTLV-2-monoinfected individuals, the estimated lifetime risk for development of TSP/HAM is <2%. However, it has been suggested that HIV/HTLV coinfection may increase the risk for development of TSP/HAM.

METHODS

A total of 2239 human immunodeficiency virus (HIV)-infected patients were tested for HTLV-1 and HTLV-2 infection at the New Orleans Outpatient Clinic (Louisiana) during the period 1991-1998. HTLV-1-infected patients with suspected myelopathy were referred for additional evaluation.

RESULTS

Four cases of TSP/HAM (9.7%) were identified among 41 individuals with Western blot-confirmed HTLV-1 infection. The diagnosis was confirmed with use of molecular diagnostic assays and viral isolation. No TSP/HAM cases were identified among 65 patients with HIV-HTLV-2 coinfection. An additional patient with HIV-HTLV-1 coinfection also received a diagnosis of TSP/HAM at the New Orleans Veteran's Affairs HIV Outpatient Clinic (Louisiana). All patients had normal CD4+ T cell counts at the time of diagnosis.

CONCLUSIONS

Given the high rates of HIV-HTLV coinfection in the United States, a heightened suspicion for TSP/HAM should be considered in HIV-infected patients who present with normal CD4+ T cell counts and myelopathy in the absence of other acquired immunodeficiency syndrome-defining conditions.

Global epidemiology of HTLV-I infection and associated diseases

Epidemiologic aspects of human T-lymphotropic virus type I (HTLV-I) infection have been thoroughly studied over the course of approximately 25 years since its first description. The geographic distribution of the virus has been defined, with Japan, Africa, Caribbean islands and South America emerging as the areas of highest prevalence. The reasons for HTLV-I clustering, such as the high ubiquity in southwestern Japan but low prevalence in neighboring regions of Korea, China and eastern Russia are still unknown. The major modes of transmission are well understood, although better quantitative data on the incidence of transmission, and on promoting/inhibiting factors, are needed. Epidemiologic proof has been obtained for HTLV-I's causative role in major disease associations: adult T-cell leukemia (ATL), HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), HTLV-associated uveitis and infective dermatitis. However, more and better studies are needed for other apparent disease outcomes such as rheumatologic, psychiatric and infectious diseases. Since curative treatment of ATL and HAM/TSP is lacking and a vaccine is unavailable, the social and financial cost for the individual, his/her family and the health system is immense. For this reason, public health interventions aimed at counseling and educating high-risk individuals and populations are of paramount importance.

Interferon-beta1a therapy in human T-lymphotropic virus type I-associated neurologic disease

Human T-lymphotropic virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is an immune-mediated inflammatory disorder of the central nervous system. Immune activation in the host, which results from high levels of persistent antigenic stimulation and from transactivation of host immunoregulatory genes by HTLV-I, appears important in the pathogenesis of HAM/TSP. In a single-center, open-label trial, 12 patients with HAM/TSP were treated with doses of interferon-beta1a of up to 60mug twice weekly, based on its antiviral and immunomodulatory effects. Primary end points were immunological and virological measures that are potential biomarkers for HAM/TSP. Interferon-beta1a therapy reduced the HTLV-I tax messenger RNA load and the frequency of potentially pathogenic HTLV-I-specific CD8(+) cells. The HTLV-I proviral DNA load remained unchanged. Spontaneous lymphoproliferation, a marker of T-cell activation in HAM/TSP, also was reduced. Some measures of motor function were improved, and no significant clinical progression occurred during therapy. These results indicate that interferon-beta1a may beneficially affect the immune mechanisms central to the pathogenesis of HAM/TSP.

Autoimmunity and molecular mimicry in tropical spastic paraparesis/human T-lymphotropic virus-associated myelopathy

Viruses share antigenic sites with normal host cell components, a phenomenon known as molecular mimicry. It has long been suggested that viral infections might trigger an autoimmune response by several mechanisms including molecular mimicry. More than 600 antiviral monoclonal antibodies generated against 11 different viruses have been reported to react with 3.5% of cells specific for uninfected mouse organs. The main pathological feature of tropical spastic paraparesis/human T-lymphotropic virus type I (HTLV-I)-associated myelopathy (TSP/HAM) is a chronic inflammation of the spinal cord characterized by perivascular cuffing of mononuclear cells accompanied by parenchymal lymphocytic infiltration. We detected the presence of autoantibodies against a 98- to 100-kDa protein of in vitro cultured human astrocytes and a 33- to 35-kDa protein from normal human brain in the serum of HTLV-I-seropositive individuals. The two cell proteins exhibited molecular mimicry with HTLV-I gag and tax proteins in TSP/HAM patients, respectively. Furthermore, the location of 33- to 35-kDa protein cross-reaction correlated with the anatomical spinal cord areas (in the rat model) in which axonal damage has been reported in several cases of TSP/HAM patients. Our experimental evidence strongly suggests that the demyelinating process occurring in TSP/HAM may be mediated by molecular mimicry between domains of some viral proteins and normal cellular targets of the spinal cord sections involved in the neurodegeneration.

Direct evidence for a chronic CD8+-T-cell-mediated immune reaction to tax within the muscle of a human T-cell leukemia/lymphoma virus type 1-infected patient with sporadic inclusion body myositis

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) infection can lead to the development of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), concomitantly with or without other inflammatory disorders such as myositis. These pathologies are considered immune-mediated diseases, and it is assumed that migration within tissues of both HTLV-1-infected CD4(+) T cells and anti-HTLV-1 cytotoxic T cells represents a pivotal event. However, although HTLV-1-infected T cells were found in inflamed lesions, the antigenic specificity of coinfiltrated CD8(+) T cells remains to be determined. In this study, we performed both ex vivo and in situ analyses using muscle biopsies obtained from an HTLV-1-infected patient with HAM/TSP and sporadic inclusion body myositis. We found that both HTLV-1-infected CD4(+) T cells and CD8(+) T cells directed to the dominant Tax antigen can be amplified from muscle cell cultures. Moreover, we were able to detect in two successive muscle biopsies both tax mRNA-positive mononuclear cells and T cells recognized by the Tax11-19/HLA-A*02 tetramer and positive for perforin. These findings provide the first direct demonstration that anti-Tax cytotoxic T cells are chronically recruited within inflamed tissues of an HTLV-1 infected patient, which validates the cytotoxic immune reaction model for the pathogenesis of HTLV-1-associated inflammatory disease.

Immunological risks of adult T-cell leukemia at primary HTLV-I infection

A small percentage of human T-cell leukemia virus type-I (HTLV-I)-infected individuals develop adult T-cell leukemia (ATL). In animal experiments, inoculation of HTLV-I via the oral route, which is the main route of mother-to-child viral transmission in humans as a result of breastfeeding, induced host HTLV-I-specific T-cell unresponsiveness and resulted in increased viral load. This strongly suggested that the known epidemiological risk factors for ATL (i.e. vertical HTLV-I infection and elevated viral load) are linked by an insufficient HTLV-I-specific T-cell response. Recent findings on the anti-tumor effects of Tax-targeted vaccination in rats and the reactivation of Tax-specific T cells in ATL patients as a result of hematopoietic stem cell transplantation imply promising immunological approaches for the prophylaxis and therapy of ATL.

Human T-lymphotropic virus type 1 (HTLV-1): persistence and immune control

The human retrovirus human T-lymphotropic virus type 1 (HTLV-1) is associated with two distinct types of disease: the malignancy known as adult T-cell leukemia and a range of chronic inflammatory conditions including the central nervous system disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Until recently, it was believed that HTLV-1 was largely latent in vivo. However, evidence from a number of types of experiments shows that HTLV-1 persistently expresses its genes, and that the "set point" of an individual's proviral load of HTLV-1 is mainly determined by the efficiency of that individual's cellular immune response to the virus. These conclusions have two main consequences. First, HTLV-1 may be vulnerable to antiretroviral drug therapy or immunotherapy. Second, HTLV-1 infection has become a useful system to analyze the determinants of the efficiency of the antiviral immune response.

Provirus expansion and deregulation of apoptosis-related genes in the spinal cord of a rat model for human T-lymphocyte virus type I-associated myeloneuropathy

Apoptosis of the spinal oligodendrocytes is the main factor linked to the pathogenesis of human T-lymphocyte virus type I (HTLV-I)-induced myeloneuropathy in rats (HAM rat). To clarify apoptosis-related mechanisms, expression of apoptosis-related genes in the spinal cord of these rats was chronologically examined by means of a semiquantitative reverse transcriptase-polymerase chain reaction. Provirus expansion and increment of HTLV-I pX mRNA were evident at 7 months after the induced infection. Tumor necrosis factor-alpha increased gradually soon after pX expression. The expression of a major apoptosis-resistant gene, bcl-2, was markedly suppressed at a period of the provirus expansion and bax was also down-regulated. p53 was consistently expressed at high levels. These findings were never observed in spinal cords of HAM-resistant strains with HTLV-I infection even throughout their entire life. Collective evidence suggests that the local provirus expansion and deregulation of apoptosis-related genes, especially down-regulation of bcl-2, may lead to apoptosis of oligodendrocytes, thus being a major pathogenetic pathway in the HTLV-I-induced myeloneuropathy.

The immune control and cell-to-cell spread of human T-lymphotropic virus type 1

Human T-lymphotropic virus type 1 (HTLV-1) varies little in sequence compared with human immunodeficiency virus type 1 (HIV) and it is difficult to detect HTLV-1 mRNA, proteins or virions in fresh blood. But the strong and chronically activated T cell response to the virus indicates that HTLV-1 proteins are expressed persistently. It now appears that the efficiency of an individual's cytotoxic T cell (CTL) response to HTLV-1 is the chief single determinant of that person's provirus load, which can differ between HTLV-1-infected people by more than 10 000-fold. Progress is now being made towards defining this CTL ‘efficiency’ in terms of host genetics, T cell function, T cell gene expression and mathematical dynamics. Lymphocytes that are naturally infected with HTLV-1 do not produce enveloped extracellular virions in short-term culture and this has reinforced the erroneous conclusion that the virus is latent. But recent evidence shows that HTLV-1 can spread directly between lymphocytes across a specialized, virus-induced cell–cell contact – a ‘viral synapse’. Instead of making extracellular virions, HTLV-1 uses the mobility of the host cell to spread within and between hosts. In this review the evidence is summarized on the persistent gene expression of HTLV-1 in vivo, the role of the immune system in protection and pathogenesis in HTLV-1 infection, and the mechanism of cell-to-cell spread of HTLV-1.

Provirus expansion and deregulation of apoptosis-related genes in the spinal cord of a rat model for human T-lymphocyte virus type I-associated myeloneuropathy

Apoptosis of the spinal oligodendrocytes is the main factor linked to the pathogenesis of human T-lymphocyte virus type I (HTLV-I)-induced myeloneuropathy in rats (HAM rat). To clarify apoptosis-related mechanisms, expression of apoptosis-related genes in the spinal cord of these rats was chronologically examined by means of a semiquantitative reverse transcriptase-polymerase chain reaction. Provirus expansion and increment of HTLV-I pX mRNA were evident at 7 months after the induced infection. Tumor necrosis factor-alpha increased gradually soon after pX expression. The expression of a major apoptosis-resistant gene, bcl-2, was markedly suppressed at a period of the provirus expansion and bax was also down-regulated. p53 was consistently expressed at high levels. These findings were never observed in spinal cords of HAM-resistant strains with HTLV-I infection even throughout their entire life. Collective evidence suggests that the local provirus expansion and deregulation of apoptosis-related genes, especially down-regulation of bcl-2, may lead to apoptosis of oligodendrocytes, thus being a major pathogenetic pathway in the HTLV-I-induced myeloneuropathy.

Human T-cell lymphotropic virus type I and neurological diseases

Human T-cell lymphotropic virus type I (HTLV-I) infection is associated with a variety of human diseases. In particular, there are two major diseases caused by HTLV-I infection. One is an aggressive neoplastic disease called adult T-cell leukemia (ATL), and another is a chronic progressive inflammatory neurological disease called HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). It is still unknown why one virus causes these different diseases. With regard to HAM/TSP, virus-host immunological interactions are an considered to be important cause of this disease. Coexisting high HTLV-I proviral load and HTLV-I-specific T cells (CD4+ T cells and CD8+ T cells) is an important feature of HAM/TSP. Histopathological studies indicate the existence of an inflammatory reaction and HTLV-I-infected cells in the affected lesions of HAM/TSP. Therefore, the immune response to HTLV-I probably contributes to the inflammatory process of the central nervous system lesions in HAM/TSP patients.