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

Retroviral b-Zip protein (HBZ) contributes to the release of soluble and exosomal immune checkpoint molecules in the context of neuroinflammation

HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a chronic, progressive, neuroinflammatory demyelinating condition of the spinal cord. We have previously shown that aberrant expression and activity of immune checkpoint (ICP) molecules such as PD-1 and PD-L1/PD-L2, negatively associates with the cytolytic potential of T cells in individuals with HAM/TSP. Interestingly, ICPs can exist in a soluble cell-free form and can be carried on extracellular vesicles (EVs) and exosomes (small EVs, <300nm) while maintaining their immunomodulatory activity. Therefore, we investigated the role of soluble and exosomal ICPs in HTLV-1 associated neuroinflammation. For the very first time, we demonstrate a unique elevated presence of several stimulatory (CD27, CD28, 4-1BB) and inhibitory (BTLA, CTLA-4, LAG-3, PD-1, PD-L2) ICP receptors in HAM/TSP sera, and in purified exosomes from a HAM/TSP-derived HTLV-1-producing (OSP2) cells. These ICPs were found to be co-localized with the endosomal sorting complex required for transport (ESCRT) pathway proteins and exhibited functional binding with their respective ligands. Viral proteins and cytokines (primarily IFNγ) were found to be present in purified exosomes. IFNγ exposure enhanced the release of ICP molecules while antiretroviral drugs (Azidothymidine and Lopinavir) significantly inhibited this process. HTLV-1 b-Zip protein (HBZ) has been linked to factors that enhance EV release and concurrent knockdown here led to the reduced expression of ESCRT associated genes (eg. Hrs, Vsp4, Alix, Tsg101) as well as abrogated the release of ICP molecules, suggesting HBZ involvement in this process. Moreso, exosomes from OSP2 cells adversely affected CD8 T-cell functions by dimishing levels of cytokines and cytotoxic factors. Collectively, these findings highlight exosome-mediated immunmodulation of T-cell functions with HBZ and ESCRT pathways as an underlying mechanism in the context of HTLV-1-induced neuroinflammation.

Vaccination with short-term-cultured autologous PBMCs efficiently activated STLV-1-specific CTLs in naturally STLV-1-infected Japanese monkeys with impaired CTL responses

A small proportion of human T-cell leukemia virus type-1 (HTLV-1)-infected individuals develop adult T-cell leukemia/lymphoma, a chemotherapy-resistant lymphoproliferative disease with a poor prognosis. HTLV-1-specific cytotoxic T lymphocytes (CTLs), potential anti-tumor/virus effectors, are impaired in adult T-cell leukemia/lymphoma patients. Here, using Japanese monkeys naturally infected with simian T-cell leukemia/T-lymphotropic virus type-1 (STLV-1) as a model, we demonstrate that short-term-cultured autologous peripheral blood mononuclear cells (PBMCs) can serve as a therapeutic vaccine to activate such CTLs. In a screening test, STLV-1-specific CTL activity was detectable in 8/10 naturally STLV-1-infected monkeys. We conducted a vaccine study in the remaining two monkeys with impaired CTL responses. The short-term-cultured PBMCs of these monkeys spontaneously expressed viral antigens, in a similar way to PBMCs from human HTLV-1 carriers. The first monkey was subcutaneously inoculated with three-day-cultured and mitomycin C (MMC)-treated autologous PBMCs, and then boosted with MMC-treated autologous STLV-1-infected cell line cells. The second monkey was inoculated with autologous PBMC-vaccine alone twice. In addition, a third monkey that originally showed a weak STLV-1-specific CTL response was inoculated with similar autologous PBMC-vaccines. In all three vaccinated monkeys, marked activation of STLV-1-specific CTLs and a mild reduction in the STLV-1 proviral load were observed. Follow-up analyses on the two monkeys vaccinated with PBMCs alone indicated that STLV-1-specific CTL responses peaked at 3-4 months after vaccination, and then diminished but remained detectable for more than one year. The significant reduction in the proviral load and the control of viral expression were associated with CTL activation but also diminished 6 and 12 months after vaccination, respectively, suggesting the requirement for a booster. The vaccine-induced CTLs in these monkeys recognized epitopes in the STLV-1 Tax and/or Envelope proteins, and efficiently killed autologous STLV-1-infected cells in vitro. These findings indicated that the autologous PBMC-based vaccine could induce functional STLV-1-specific CTLs in vivo.

Novel perspectives on antisense transcription in HIV-1, HTLV-1, and HTLV-2

The genome of retroviruses contains two promoter elements (called long terminal repeat or LTR) at the 5' and 3' end of their genome. Although the expression of retroviral genes generally depends on the promoter located in the 5' LTR, the 3' LTR also has promoter activity responsible for producing antisense transcripts. These natural antisense transcripts (NATs) are a class of RNA molecules transcribed from the opposite strand of a protein-coding gene. NATs have been identified in many prokaryotic and eukaryotic systems, as well as in human retroviruses such as human immunodeficiency virus type 1 (HIV-1) and HTLV-1/2 (human T-cell leukemia virus type 1/2). The antisense transcripts of HIV-1, HTLV-1, and HTLV-2 have been briefly characterized over the past several years. However, a complete appreciation of the role these transcripts play in the virus lifecycle and the cellular factors which regulate their transcription is still lacking. This review provides an overview of antisense transcription in human retroviruses with a specific focus on the MEF-2 family of transcription factors, the function(s) of the antisense protein products, and the application of antisense transcription models in therapeutics against HIV-1 and HTLV-1 in the context of co-infection.

Flow cytometric methodology for the detection of de novo human T-cell leukemia virus -1 infection in vitro: A tool to study novel infection inhibitors

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MT-2 cells express more Tax, gp46 and p19 than HUT102′s.

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HUT78 cells express higher levels of the HTLV-1 permissive receptors neuropilin and GLUT-1 than CEM or JURKAT.

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Irradiation does not eliminate all MT-2 donor cells in HTLV-1 co-culture protocols.

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Flow cytometry and Lt-4 anti-tax antibody can detect de novo HTLV-1 infection at early time points.

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Cytochalasin B and sodium valproate inhibit HTLV-1 infection at early time points.

Methodology to detect and study de novo human T-cell leukemia virus (HTLV)-1 infection is required to further our knowledge of the viruses’ mechanisms of infection and to study potential therapeutic interventions. Whilst methodology currently exists, utilisation of an anti-Tax antibody to detect de novo Tax expression in permissive cells labelled with cell tracker allowing for the detection by flow cytometry of new infection after co-culture with donor cell lines productively infected with HTLV-1 is an alternative strategy. Using this methodology, we have been able to detect de novo infection of the T cell line HUT78 following co-culture with the productively infected HTLV-1 donor cell line MT-2 and to confirm that infection can be effectively blocked with well characterised infection inhibitors. This methodology will benefit experimental studies examining HTLV infection in vitro and may aid identification of therapeutic agents that block this process.

MEF-2 isoforms' (A-D) roles in development and tumorigenesis

Myocyte enhancer factor (MEF)-2 plays a critical role in proliferation, differentiation, and development of various cell types in a tissue specific manner. Four isoforms of MEF-2 (A-D) differentially participate in controlling the cell fate during the developmental phases of cardiac, muscle, vascular, immune and skeletal systems. Through their associations with various cellular factors MEF-2 isoforms can trigger alterations in complex protein networks and modulate various stages of cellular differentiation, proliferation, survival and apoptosis. The role of the MEF-2 family of transcription factors in the development has been investigated in various cell types, and the evolving alterations in this family of transcription factors have resulted in a diverse and wide spectrum of disease phenotypes, ranging from cancer to infection. This review provides a comprehensive account on MEF-2 isoforms (A-D) from their respective localization, signaling, role in development and tumorigenesis as well as their association with histone deacetylases (HDACs), which can be exploited for therapeutic intervention.

Human T cell leukemia virus type 1 and Zika virus: tale of two reemerging viruses with neuropathological sequelae of public health concern

Human T cell leukemia virus type 1 (HTLV-1) and Zika virus (ZIKV) have been considered neglected viruses of low public health concern until recently when incidences of HTLV-1 and ZIKV were observed to be linked to serious immune-related disease and neurological complications. This review will discuss the epidemiology, genomic evolution, virus-host interactions, virulence factors, neuropathological sequelae, and current perspectives of these reemerging viruses. There are no FDA-approved therapeutics or vaccines against these viruses, and as such, it is important for clinical trials to focus on developing vaccines that can induce cell-mediated immune response to confer long-term protective immunity. Furthermore, attention should be paid to reducing the transmission of these viruses through unprotected sex, infected blood during sharing of contaminated needles, donated blood and organs, and vertical transmission from mother to baby via breastfeeding. There is an urgent need to re-evaluate repurposing current antiviral therapies as well as developing novel antiviral agents with enhanced efficacy due to the high morbidity rate associated with these two reemerging chronic viral diseases.

Impairment of the humoral and CD4(+) T cell responses in HTLV-1-infected individuals immunized with tetanus toxoid

T cells from HTLV-1-infected individuals have a decreased ability to proliferate after stimulation with recall antigens. This abnormality may be due to the production of regulatory cytokine or a dysfunctional antigen presentation. The aims of this study were to evaluate the antibody production and cytokine expression by lymphocytes before and after immunization with tetanus toxoid (TT) and to evaluate the immune response of monocytes after stimulation with TT and frequency of dendritic cells (DC) subsets. HTLV-1 carriers (HC) and uninfected controls (UC) with negative serology for TT were immunized with TT, and the antibody titers were determined by ELISA as well as the cell activation markers expression by monocytes. The frequencies of DC subsets were determined by flow cytometry. Following immunization, the IgG anti-TT titers and the frequency of CD4(+) T cells expressing IFN-γ, TNF-α and IL-10 in response to TT were lower in the HC than in the UC. Additionally, monocytes from HC did not exhibit increased HLA-DR expression after stimulation with TT, and presented low numbers of DC subsets, therefore, it's necessary to perform functional studies with antigen-presenting cells. Collectively, our finding suggests that HC present an impairment of the humoral and CD4(+) T cell immune responses after vaccination.

Myocyte enhancer factor (MEF)-2 plays essential roles in T-cell transformation associated with HTLV-1 infection by stabilizing complex between Tax and CREB

Background

The exact molecular mechanisms regarding HTLV-1 Tax-mediated viral gene expression and CD4 T-cell transformation have yet to be fully delineated. Herein, utilizing virus-infected primary CD4⁺ T cells and the virus-producing cell line, MT-2, we describe the involvement and regulation of Myocyte enhancer factor-2 (specifically MEF-2A) during the course of HTLV-1 infection and associated disease syndrome.

Results

Inhibition of MEF-2 expression by shRNA and its activity by HDAC9 led to reduced viral replication and T-cell transformation in correlation with a heightened expression of MEF-2 in ATL patients. Mechanistically, MEF-2 was recruited to the viral promoter (LTR, long terminal repeat) in the context of chromatin, and constituted Tax/CREB transcriptional complex via direct binding to the HTLV-1 LTR. Furthermore, an increase in MEF-2 expression was observed upon infection in an extent similar to CREB (known Tax-interacting transcription factor), and HATs (p300, CBP, and p/CAF). Confocal imaging confirmed MEF-2 co-localization with Tax and these proteins were also shown to interact by co-immunoprecipitation. MEF-2 stabilization of Tax/CREB complex was confirmed by a novel promoter-binding assay that highlighted the involvement of NFAT (nuclear factor of activated T cells) in this process via Tax-mediated activation of calcineurin (a calcium-dependent serine-threonine phosphatase). MEF-2-integrated signaling pathways (PI3K/Akt, NF-κB, MAPK, JAK/STAT, and TGF-β) were also activated during HTLV-1 infection of primary CD4⁺ T cells, possibly regulating MEF-2 activity.

Conclusions

We demonstrate the involvement of MEF-2 in Tax-mediated LTR activation, viral replication, and T-cell transformation in correlation with its heightened expression in ATL patients through direct binding to DNA within the HTLV-1 LTR.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0140-1) contains supplementary material, which is available to authorized users.

Human T-lymphotropic virus type 1-infected cells secrete exosomes that contain Tax protein

Human T-lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. The HTLV-1 transactivator protein Tax controls many critical cellular pathways, including host cell DNA damage response mechanisms, cell cycle progression, and apoptosis. Extracellular vesicles called exosomes play critical roles during pathogenic viral infections as delivery vehicles for host and viral components, including proteins, mRNA, and microRNA. We hypothesized that exosomes derived from HTLV-1-infected cells contain unique host and viral proteins that may contribute to HTLV-1-induced pathogenesis. We found exosomes derived from infected cells to contain Tax protein and proinflammatory mediators as well as viral mRNA transcripts, including Tax, HBZ, and Env. Furthermore, we observed that exosomes released from HTLV-1-infected Tax-expressing cells contributed to enhanced survival of exosome-recipient cells when treated with Fas antibody. This survival was cFLIP-dependent, with Tax showing induction of NF-κB in exosome-recipient cells. Finally, IL-2-dependent CTLL-2 cells that received Tax-containing exosomes were protected from apoptosis through activation of AKT. Similar experiments with primary cultures showed protection and survival of peripheral blood mononuclear cells even in the absence of phytohemagglutinin/IL-2. Surviving cells contained more phosphorylated Rb, consistent with the role of Tax in regulation of the cell cycle. Collectively, these results suggest that exosomes may play an important role in extracellular delivery of functional HTLV-1 proteins and mRNA to recipient cells.

Understanding MHC class I presentation of viral antigens by human dendritic cells as a basis for rational design of therapeutic vaccines

Effective viral clearance requires the induction of virus-specific CD8⁺ cytotoxic T lymphocytes (CTL). Since dendritic cells (DC) have a central role in initiating and shaping virus-specific CTL responses, it is important to understand how DC initiate virus-specific CTL responses. Some viruses can directly infect DC, which theoretically allow direct presentation of viral antigens to CTL, but many viruses target other cells than DC and thus the host depends on the cross-presentation of viral antigens by DC to activate virus-specific CTL.

Research in mouse models has highly enhanced our understanding of the mechanisms underlying cross-presentation and the dendritic cells (DC) subsets involved, however, these results cannot be readily translated toward the role of human DC in MHC class I-antigen presentation of human viruses. Here, we summarize the insights gained in the past 20 years on MHC class I presentation of viral antigen by human DC and add to the current debate on the capacities of different human DC subsets herein. Furthermore, possible sources of viral antigens and essential DC characteristics for effective induction of virus-specific CTL are evaluated.

We conclude that cross-presentation is not only an efficient mechanism exploited by DC to initiate immunity to viruses that do not infect DC but also to viruses that do infect DC, because cross-presentation has many conceptual advantages and bypasses direct immune modulatory effects of the virus on its infected target cells.

Since knowledge on the mechanism of viral antigen presentation and the preferred DC subsets is crucial for rational vaccine design, the obtained insights are very instrumental for the development of effective anti-viral immunotherapy.

In vivo immunogenicity of Tax(11-19) epitope in HLA-A2/DTR transgenic mice: implication for dendritic cell-based anti-HTLV-1 vaccine

Viral oncoprotein Tax plays key roles in transformation of human T-cell leukemia virus (HTLV-1)-infected T cells leading to adult T-cell leukemia (ATL), and is the key antigen recognized during HTLV-associated myelopathy (HAM). In HLA-A2+ asymptomatic carriers as well as ATL and HAM patients, Tax(11-19) epitope exhibits immunodominance. Here, we evaluate CD8 T-cell immune response against this epitope in the presence and absence of dendritic cells (DCs) given the recent encouraging observations made with Phase 1 DC-based vaccine trial for ATL. To facilitate these studies, we first generated an HLA-A2/DTR hybrid mouse strain carrying the HLA-A2.1 and CD11c-DTR genes. We then studied CD8 T-cell immune response against Tax(11-19) epitope delivered in the absence or presence of Freund's adjuvant and/or DCs. Overall results demonstrate that naturally presented Tax epitope could initiate an antigen-specific CD8T cell response in vivo but failed to do so upon DC depletion. Presence of adjuvant potentiated Tax(11-19)-specific response. Elevated serum IL-6 levels coincided with depletion of DCs whereas decreased TGF-β was associated with adjuvant use. Thus, Tax(11-19) epitope is a potential candidate for the DC-based anti-HTLV-1 vaccine and the newly hybrid mouse strain could be used for investigating DC involvement in human class-I-restricted immune responses.

An altered maturation and adhesion phenotype of dendritic cells in diseased individuals compared to asymptomatic carriers of human T cell leukemia virus type 1

The immunopathogenic mechanisms underlying human T cell leukemia virus type 1 (HTLV-1)-mediated diseases such as adult T cell leukemia (ATL) and HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP) are not clearly understood. As critical effectors of antiviral immune response, dendritic cells (DCs) are implicated to play an important role in determining the outcome of HTLV-1 infection. However, a complete understanding of their role in any disease pathogenesis requires extensive assessment of the phenotypic and functional state of DCs. To enable this, we developed a polychromatic antibody cocktail comprising key phenotypic and functional markers of DCs and applied it in a patient cohort from the HTLV-1 endemic region, Jamaica, consisted of seronegative controls, asymptomatic carriers (ACs), ATL, and HAM/TSP patients. This ex vivo analyses included two major subsets of blood DCs, myeloid and plasmacytoid (mDCs and pDCs, respectively). The comparative analyses of results demonstrated a decreased pDC frequency in both ATL and HAM/TSP patients as compared to ACs and seronegative controls. Similarly, CD86 expression on both mDCs and pDCs was significantly higher in HAM/TSP (but not ATL) patients compared to ACs. Interestingly, HLA-DR expression was significantly lower on pDCs of patients as compared to carriers; however, for mDCs, only the HAM/TSP group had significantly lower expression of HLA-DR. Unlike HAM/TSP individuals, ATL individuals had higher HLA-ABC expression on mDCs compared to ACs. Finally, both mDCs and pDCs of HAM/TSP patients had significantly higher expression of the programmed death ligand 1 (PD-L1) compared to ACs. Overall, this study suggests that DCs exhibit a differential phenotypic and functional profile between patients (ATL and HAM/TSP) and carriers of HTLV-1 and could provide an important tool for understanding HTLV-1 immunopathogenesis during infection and disease.

Neuroimmunological aspects of human T cell leukemia virus type 1-associated myelopathy/tropical spastic paraparesis

Human T cell leukemia virus type 1 (HTLV-1) is a human retrovirus etiologically associated with adult T cell leukemia/lymphoma and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Only approximately 0.25-4 % of infected individuals develop HAM/TSP; the majority of infected individuals remain lifelong asymptomatic carriers. Recent data suggest that immunological aspects of host-virus interactions might play an important role in the development and pathogenesis of HAM/TSP. This review outlines and discusses the current understanding, ongoing developments, and future perspectives of HAM/TSP research.

Dendritic Cells in HIV-1 and HCV Infection: Can They Help Win the Battle?

Persistent infections with human immunodeficiency virus type 1 (HIV-1) and hepatitis C virus (HCV) are a major cause of morbidity and mortality worldwide. As sentinels of our immune system, dendritic cells (DCs) play a central role in initiating and regulating a potent antiviral immune response. Recent advances in our understanding of the role of DCs during HIV-1 and HCV infection have provided crucial insights into the mechanisms employed by these viruses to impair DC functions in order to evade an effective immune response against them. Modulation of the immunological synapse between DC and T-cell, as well as dysregulation of the crosstalk between DCs and natural killer (NK) cells, are emerging as two crucial mechanisms. This review focuses on understanding the interaction of HIV-1 and HCV with DCs not only to understand the immunopathogenesis of chronic HIV-1 and HCV infection, but also to explore the possibilities of DC-based immunotherapeutic approaches against them. Host genetic makeup is known to play major roles in infection outcome and rate of disease progression, as well as response to anti-viral therapy in both HIV-1 and HCV-infected individuals. Therefore, we highlight the genetic variations that can potentially affect DC functions, especially in the setting of chronic viral infection. Altogether, we address if DCs’ potential as critical effectors of antiviral immune response could indeed be utilized to combat chronic infection with HIV-1 and HCV.

Dendritic cell CNS recruitment correlates with disease severity in EAE via CCL2 chemotaxis at the blood-brain barrier through paracellular transmigration and ERK activation

BACKGROUND

Transmigration of circulating dendritic cells (DCs) into the central nervous system (CNS) across the blood-brain barrier (BBB) has not thus far been investigated. An increase in immune cell infiltration across the BBB, uncontrolled activation and antigen presentation are influenced by chemokines. Chemokine ligand 2 (CCL2) is a potent chemoattractant known to be secreted by the BBB but has not been implicated in the recruitment of DCs specifically at the BBB.

METHODS

Experimental autoimmune encephalomyelitis (EAE) was induced in C57BL/6 mice by injection of MOG35-55 peptide and pertussis toxin intraperitoneally. Animals with increasing degree of EAE score were sacrificed and subjected to near-infrared and fluorescence imaging analysis to detect and localize the accumulation of CD11c+-labeled DCs with respect to CCL2 expression. To further characterize the direct effect of CCL2 in DC trafficking at the BBB, we utilized an in vitro BBB model consisting of human brain microvascular endothelial cells to compare migratory patterns of monocyte-derived dendritic cells, CD4+ and CD8+ T cells. Further, this model was used to image transmigration using fluorescence microcopy and to assess specific molecular signaling pathways involved in transmigration.

RESULTS

Near-infrared imaging of DC transmigration correlated with the severity of inflammation during EAE. Ex vivo histology confirmed the presence of CCL2 in EAE lesions, with DCs emerging from perivascular spaces. DCs exhibited more efficient transmigration than T cells in BBB model studies. These observations correlated with transwell imaging, which indicated a paracellular versus transcellular pattern of migration by DCs and T cells. Moreover, at the molecular level, CCL2 seems to facilitate DC transmigration in an ERK1/2-dependent manner.

CONCLUSION

CNS recruitment of DCs correlates with disease severity in EAE via CCL2 chemotaxis and paracellular transmigration across the BBB, which is facilitated by ERK activation. Overall, these comprehensive studies provide a state-of-the-art view of DCs within the CNS, elucidate their path across the BBB, and highlight potential mechanisms involved in CCL2-mediated DC trafficking.

Immunopathogenesis of human T-cell leukemia virus type-1-associated myelopathy/tropical spastic paraparesis: recent perspectives

Human T-cell leukemia virus type-1 (HTLV-1) is a replication-competent human retrovirus associated with two distinct types of disease only in a minority of infected individuals: the malignancy known as adult T-cell leukemia (ATL) and a chronic inflammatory central nervous system disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HAM/TSP is a chronic progressive myelopathy characterized by spastic paraparesis, sphincter dysfunction, and mild sensory disturbance in the lower extremities. Although the factors that cause these different manifestations of HTLV-1 infection are not fully understood, accumulating evidence from host population genetics, viral genetics, DNA expression microarrays, and assays of lymphocyte function suggests that complex virus-host interactions and the host immune response play an important role in the pathogenesis of HAM/TSP. Especially, the efficiency of an individual's cytotoxic T-cell (CTL) response to HTLV-1 limits the HTLV-1 proviral load and the risk of HAM/TSP. This paper focuses on the recent advances in HAM/TSP research with the aim to identify the precise mechanisms of disease, in order to develop effective treatment and prevention.

The tug-of-war between dendritic cells and human chronic viruses

The human immune system is under constant challenge from many viruses, some of which the body is successfully able to clear. Other viruses have evolved to escape the host immune responses and thus persist, leading to the development of chronic diseases. Dendritic cells are professional antigen-presenting cells that play a major role in both innate and adaptive immunity against different pathogens. This review focuses on the interaction of different chronic viruses with dendritic cells and the viruses' ability to exploit this critical cell type to their advantage so as to establish persistence within the host.

Monocytes from HTLV-1–infected patients are unable to fully mature into dendritic cells

Human T-cell lymphotropic virus type 1 (HTLV-1) is a causative agent of adult T-cell leukemia and HTLV-1–associated myelopathy/tropical spastic paraparesis. HTLV-1–associated myelopathy/tropical spastic paraparesis is a chronic inflammatory disease characterized by loss of motor movement in response to spinal marrow cell destruction by T lymphocytes. To perform their cellular function, T cells need to be activated by antigen-presenting cells, such as dendritic cells (DCs). The aim of this work was to analyze DC differentiation and activation from monocytes of HTLV-1–infected individuals. We demonstrated that monocytes from HTLV-1–infected patients who had been stimulated to differentiate had an impaired loss of CD14 expression, expressed low levels of CD1a, and maintained secretion of tumor necrosis factor-α compared with monocytes from noninfected donors. We further evaluated DC activation by tumor necrosis factor-α. We observed that in response to activation, DCs that were derived from noninfected donors had an increase in the percentage of CD83+, CD86+, and human leukocyte antigen-DR+ cells, whereas in DCs derived from HTLV-1–infected patients, the percentage of CD83+, CD86+, and human leukocyte antigen-DR+ cells remained similar to that of nonactivated cells. Moreover, these cells had an impaired capacity to stimulate allogeneic T lymphocytes. We demonstrated that DC maturation was altered in HTLV-1–infected patients, which could contribute to the development of HTLV-1–associated diseases.

Murine FLT3 ligand-derived dendritic cell-mediated early immune responses are critical to controlling cell-free human T cell leukemia virus type 1 infection

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. We observed previously that depletion of dendritic cells (DCs) in CD11c-diphtheria toxin receptor transgenic mice followed by infection with cell-free virus led to greater proviral and Tax mRNA loads and diminished cellular immune response compared with mice infected with cell-associated virus. To understand the significance of these in vivo results and explore the host-pathogen interaction between DCs and cell-free HTLV-1, we used FLT3 ligand-cultured mouse bone marrow-derived DCs (FL-DCs) and chimeric HTLV-1. Phenotypically, the FL-DCs upregulated expression of surface markers (CD80, CD86, and MHC class II) on infection; however, the level of MHC class I remained unchanged. We performed kinetic studies to understand viral entry, proviral integration, and expression of the viral protein Tax. Multiplex cytokine profiling revealed production of an array of proinflammatory cytokines and type 1 IFN (IFN-α) by FL-DCs treated with virus. Virus-matured FL-DCs stimulated proliferation of autologous CD3(+) T cells as shown by intracellular nuclear Ki67 staining and produced IFN-γ when cultured with infected FL-DCs. Gene expression studies using type 1 IFN-specific and DC-specific arrays revealed upregulation of IFN-stimulated genes, most cytokines, and transcription factors, but a distinct downregulation of many chemokines. Overall, these results highlight the critical early responses generated by FL-DCs on challenge with cell-free chimeric HTLV-1.

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.

Immunogenetics and the Pathological Mechanisms of Human T-Cell Leukemia VirusType 1- (HTLV-1-)Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP)

Human T-cell leukemia virus type 1 (HTLV-1) is a replication-competent human retrovirus associated with two distinct types of disease only in a minority of infected individuals: the malignancy known as adult T-cell leukemia (ATL) and a chronic inflammatory central nervous system disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Although the factors that cause these different manifestations of HTLV-1 infection are not fully understood, accumulating evidence suggests that complex virus-host interactions play an important role in determining the risk of HAM/TSP. This review focuses on the role of the immune response in controlling or limiting viral persistence in HAM/TSP patients, and the reason why some HTLV-1-infected people develop HAM/TSP whereas the majority remains asymptomatic carriers of the virus.