The role of human T-lymphotropic virus type 1 (HTLV-1)-infected dendritic cells in the development of HTLV-1-associated myelopathy/tropical spastic paraparesis

Use of human antigen presenting cell gene array profiling to examine the effect of human T-cell leukemia virus type 1 Tax on primary human dendritic cells

Human T-cell leukemia virus type 1 (HTLV-1) is etiologically linked to adult T-cell leukemia and a progressive demyelinating disorder termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). One of the most striking features of the immune response in HAM/TSP centers on the expansion of HTLV-1-specific CD8(+) cytotoxic T lymphocyte (CTL) compartment in the peripheral blood and cerebrospinal fluid. More than 90% of the HTLV-1-specific CTLs are directed against the viral Tax (11-19) peptide implying that Tax is available for immune recognition by antigen presenting cells, such as dendritic cells (DCs). DCs obtained from HAM/TSP patients have been shown to be infected with HTLV-1 and exhibit rapid maturation. Therefore, we hypothesized that presentation of Tax peptides by activated DCs to naIve CD8(+) T cells may play an important role in the induction of a Tax-specific CTL response and neurologic dysfunction. In this study, a pathway-specific antigen presenting cell gene array was used to study transcriptional changes induced by exposure of monocyte-derived DCs to extracellular HTLV-1 Tax protein. Approximately 100 genes were differentially expressed including genes encoding toll-like receptors, cell surface receptors, proteins involved in antigen uptake and presentation and adhesion molecules. The differential regulation of chemokines and cytokines characteristic of functional DC activation was also observed by the gene array analyses. Furthermore, the expression pattern of signal transduction genes was also significantly altered. These results have suggested that Tax-mediated DC gene regulation might play a critical role in cellular activation and the mechanisms resulting in HTLV-1-induced disease.

Regulation of human T-cell leukemia virus type 1 gene expression by Sp1 and Sp3 interaction with TRE-1 repeat III

Transcription factors of the Sp family are known to play key roles in the regulation of both constitutive as well as cell type- and differentiation stage-specific gene expression. Binding sites for factors of the Sp family (Sp1 and Sp3) have previously been identified within the U3 region of the human T-cell leukemia virus type 1 (HTLV-1) long terminal repeat (LTR). Although previous studies have demonstrated that Sp1 and Sp3 can interact with the Tax-responsive element 1 (TRE-1) repeat III, the sequences required for Sp1/Sp3 binding have not been mapped in detail. Herein, we demonstrate that the GC-rich regions flanking the viral cAMP-responsive element (CRE) within TRE-1 repeat III exhibit substantial affinity for both Sp1 and Sp3. We demonstrate that purified Sp1 competes with purified CREB for binding to TRE-1 repeat III due to the physical proximity of the Sp1/Sp3 and ATF/CREB binding sites, while purified Sp1 forms a multiprotein complex with purified CREB in the presence of Tax as demonstrated by electrophoretic mobility shift (EMS) analyses. Sp1 and Sp3 binding to the U3 region of the HTLV-1 LTR in the presence of Tax in vivo was confirmed by chromatin immunoprecipitation using HTLV-1-infected T cells (SLB-1 and C8166). Overexpression of Sp1 was modestly enhanced, while overexpression of Sp3 inhibited basal and Tax-mediated transactivation of the HTLV-1 LTR in U-937 cells (which express relatively low levels of endogenous Sp1 and Sp3). Furthermore, the modest upregulation of LTR activation caused by overexpression of Sp1 could be blocked by site-directed mutagenesis of the GC-rich Sp1/Sp3 binding sites within TRE-1 repeat III. These results suggest that both Sp1 and Sp3 transcription factor binding to TRE-1 repeat III participate in regulation of HTLV-1 viral gene expression.

DC-SIGN facilitates fusion of dendritic cells with human T-cell leukemia virus type 1-infected cells

Interactions between the oncogenic retrovirus human T-cell leukemia virus type 1 (HTLV-1) and dendritic cells (DCs) are poorly characterized. We show here that monocyte-derived DCs form syncytia and are infected upon coculture with HTLV-1-infected lymphocytes. We examined the role of DC-specific ICAM-3-grabbing nonintegrin (DC-SIGN), a C-type lectin expressed in DCs, in HTLV-1-induced syncytium formation. DC-SIGN is known to bind with high affinity to various viral envelope glycoproteins, including human immunodeficiency virus (HIV) and hepatitis C virus, as well as to the cellular receptors ICAM-2 and ICAM-3. After cocultivating DCs and HTLV-1-infected cells, we found that anti-DC-SIGN monoclonal antibodies (MAbs) were able to decrease the number and size of HTLV-1-induced syncytia. Moreover, expression of the lectin in epithelial-cell lines dramatically enhanced the ability to fuse with HTLV-1-positive cells. Interestingly, in contrast to the envelope (Env) glycoproteins of HIV and other viruses, that of HTLV-1 does not bind directly to DC-SIGN. The facilitating role of the lectin in HTLV-1 syncytium formation is mediated by its interaction with ICAM-2 and ICAM-3, as demonstrated by use of MAbs directed against these adhesion molecules. Altogether, our results indicate that DC-SIGN facilitates HTLV-1 infection and fusion of DCs through an ICAM-dependent mechanism.

Identification of an Immunomodulating Agent from Mycobacterium leprae

A search for an immunomodulating agent from mycobacteria was carried out using Mycobacterium leprae. The antigenicity of each fraction of the bacterial membrane, which contains the most antigenic components of M. leprae, was assessed by using sera from paucibacillary leprosy. N-terminal sequencing of the serum-reactive protein and functional assessment of the membrane fractions using monocyte-derived dendritic cells (DCs) identified major membrane protein II (MMP-II) as one of the efficient T-cell-activating candidates. Purified MMP-II stimulated DCs from healthy individuals to produce interleukin-12 p70 and up-regulated the surface expression of major histocompatibility complex class I and II, CD86, and CD83 molecules. Also, there was an increase in the percentage of CD83(+) cells in the DC population. Furthermore, MMP-II-pulsed DCs expressed their derivatives on their surfaces. Using Toll-like receptor 2 (TLR-2)-dependent receptor constructs, we found that TLR-2 signaling was involved in DC maturation induced by MMP-II. Taken together, MMP-II can be recognized as an immunomodulating protein in terms of activation of antigen-presenting cells and innate immunity.

Immunostimulatory activity of major membrane protein-II from Mycobacterium leprae

We examined the antigenicity of an immunomodulatory protein, major membrane protein (MMP)-II, from Mycobacterium leprae, since host defense against M. leprae largely depends on adaptive immunity. Both unprimed and memory T cells from healthy individuals were stimulated by autologous MMP-II-pulsed monocyte-derived dendritic cells (DCs) to produce IFN-gamma. The DC-mediated IFN-gamma production was dependent on the expression of MHC, CD86, and MMP-II antigens. Memory T cells from paucibacillary (PB) leprosy more extensively responded to MMP-II-pulsed DCs than T cells from healthy individuals, while comparable IFN-gamma was produced by unprimed T cells. Memory T cells from multibacillary leprosy, which are normally believed to be anergic, were activated similarly to those from healthy individuals by MMP-II-pulsed DCs. These results suggest that memory T cells from PB leprosy are primed with MMP-II prior to the manifestation of the disease, and MMP-II is highly antigenic in terms of activation of adaptive immunity.

Virus infection of dendritic cells: portal for host invasion and host defense

Dendritic cells (DCs) act as a portal for virus invasion and as the most potent antigen-presenting cells in antiviral host defense. Human immunodeficiency virus (HIV)-1 has served as the paradigm for virus interaction with DCs. HIV-1 infection of DCs via its primary CD4 receptor and secondary chemokine receptors leads to full virus replication (cis infection), whereas binding to C-type lectin receptors results both in cis replication, as well as transfer and replication of virus in CD4(pos) T cells (trans infection). DCs respond to this invasion by processing viral proteins through MHC class I and II pathways and undergoing a maturation that enhances their presentation of antigen to T cells for induction of adaptive antiviral immunity. HIV-1 and other viruses have evolved mechanisms to subvert this immune function. Engineering of DCs with various forms of viral immunogens and co-treatment with cytokines and chemokines is being used as an immunotherapy for HIV-1 and other viral infections.

Upregulation of T-cell-stimulating activity of mycobacteria-infected macrophages

Macrophages are one of the most abundant host cells to come in contact with mycobacteria. However, the infected macrophages less efficiently stimulate autologous T cells in vitro. We investigated the effect of the induction of phenotypic change of macrophages on the host cell activities by using Mycobacterium leprae as a pathogen. The treatment of macrophages with interferon-gamma (IFN-gamma), GM-CSF and interleukin-4 deprived macrophages of CD14 antigen expression but instead provided them with CD1a, CD83 and enhanced CD86 antigen expression. These phenotypic features resembled those of monocyte-derived dendritic cells (DC). These macrophage-derived DC-like cells (MACDC) stimulated autologous CD4+ and CD8+ T cells when infected with M. leprae. Further enhancement of the antigen-presenting function and CD1a expression of macrophages was observed when treated with IFN-gamma. The M. leprae-infected and -treated macrophages expressed bacterial cell membrane-derived antigens on the surface and were efficiently cytolysed by the cell membrane antigen-specific CD8+ cytotoxic T lymphocytes (CTL). These results suggest that the induction of phenotypic changes in macrophages can lead to the upregulation of host defence activity against M. leprae.

Depletion and impaired interferon-α -producing capacity of blood plasmacytoid dendritic cells in human T-cell leukaemia virus type I-infected individuals

Dendritic cells (DCs) play an important role in innate and adaptive immunity. There are two major populations of blood DCs, myeloid DCs (myDCs) and plasmacytoid DCs (pcDCs). pcDCs are particularly important in antiviral as well as in general host defence, as they are the principal producers of type I interferons (IFNs). In this study, we analysed myDCs and pcDCs in healthy controls, human T-cell leukaemia virus type I (HTLV-I)-infected asymptomatic carriers (ACs), and patients with adult T-cell leukaemia (ATL). ATL patients had significantly decreased number of pcDCs and myDCs compared with controls. IFN-alpha production by peripheral blood mononuclear cells (PBMCs) was markedly reduced in ATL patients. Purified pcDCs from ACs were found to have impaired IFN-alpha-producing capacity, suggesting a functional defect in pcDCs in HTLV-I-infected individuals. Interestingly, pcDCs were shown to be susceptible to HTLV-I infection. Thus, impaired IFN-alpha production by pcDCs may contribute to the immunodeficiency observed in ATL. Furthermore, IFN-alpha-producing capacity was inversely correlated with HTLV-I proviral load in PBMCs from ACs, suggesting a role for pcDCs in maintaining the carrier state. Taken together, we hypothesize that the depletion and impaired IFN-alpha-producing capacity of blood DCs may contribute to the immunodeficiency in ATL and/or the development of ATL.

Maturation of monocyte-derived dendritic cells by Hochu-ekki-to, a traditional Japanese herbal medicine

To investigate the immunological effect of the traditional Japanese herbal medicine (kampo), Hochu-ekki-to (HOT), on dendritic cells (DC), we examined in vitro if HOT would stimulate the maturation process of human monocyte-derived DC as do TNF-alpha and LPS. Monocytes from a healthy volunteer were cultured in the presence of IL-4 and GM-CSF, and the generated immature DC were stimulated with HOT, TNF-alpha, or LPS (HOT-DC, TNF-DC, and LPS-DC, respectively) for 2 days. Flow cytometric analysis showed that HOT stimulated DC to express the surface maturation markers CD80, CD83, and CD86 dose-dependently and that the up-regulation level was identical to TNF-alpha and LPS. The antigen-uptake capacity of HOT-DC was determined by FITC-labeled albumin uptake. HOT-DC lost albumin uptake capacity comparable to LPS-DC, indicating DC maturity. IL-12 (p70) production by HOT-DC and TNF-DC was not increased in comparison with LPS-DC. The antigen-presenting capacity of HOT-DC as analyzed by allogeneic T cell proliferation was significantly increased in comparison with immature DC and was identical to LPS-DC. These results demonstrate that HOT stimulates DC maturation as well as the other known maturation factors, despite low IL-12 production, and suggests the possibility that DC maturation by HOT can play an important role in the improvement of the immunoregulatory function in patients with impaired host defense.

Mediators of central nervous system damage during the progression of human T-cell leukemia type I-associated myelopathy/tropical spastic paraparesis

Human T-cell leukemia virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) represents one of the most devastating diseases associated with HTLV-I infection. Despite the delineation of clinical features associated with this neurologic disease, more progress needs to be made with respect to understanding the molecular mechanisms relating to the genesis of HAM/TSP. Several factors have been hypothesized to contribute to whether an HTLV-I-infected individual remains asymptomatic, develops adult T-cell leukemia (ATL), or progresses to HAM/TSP. Among the most intriguing of these factors is the immune response mounted by the host against HTLV-I. Several cell populations are crucial with respect to generating an efficient immune response against the virus. This includes CD4(+) T cells, CD8(+) T cells, dendritic cells (DCs), monocytes/macrophages, and HTLV-I-infected cells that interact with immune cells to stimulate their effector functions. Although all of these cell types likely play important roles in the etiology of HAM/TSP, this review focuses specifically on the potential function of the CD8(+) T-cell population during the progression of HTLV-I-induced neurologic disease. The immune response in HAM/TSP patients may transition from a beneficial response aimed at controlling the viral infection, to a detrimental response that ultimately participates in mediating the pathology observed in HAM/TSP. In this respect, the generation of a hyperactive CD8(+) cytotoxic T lymphocyte (CTL) response primarily targeting the HTLV-I Tax protein likely plays a key role in the genesis of pathologic abnormalities associated with HAM/TSP. The efficiency and activity of Tax-specific CD8(+) CTLs may be regulated at a number of levels, and deregulation of Tax-specific CTL activation may contribute to HAM/TSP. This review focuses on potential mechanisms of central nervous system (CNS) damage associated with the genesis of HAM/TSP following HTLV-I infection, focusing on the role of the Tax-specific CTL compartment.

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.

Assessment of cell mediated immunogenicity of Mycobacterium leprae-derived antigens

The antigenicity of Mycobacterium leprae (M. leprae)-derived cell membrane fraction was examined using human dendritic cells (DCs). Immature DCs internalized and processed the cell membrane components, and expressed M. leprae-derived antigens (Ags) on their surface. The expression of MHC class II, CD86, and CD83 Ags on DCs and CD40 ligand (L)-associated IL-12 p70 production from DCs were up-regulated by the membrane Ags. Moreover these stimulated DCs induced significantly higher level of interferon-gamma (IFN-gamma) production by autologous CD4(+) and CD8(+) T cells than those pulsed with equivalent doses of live M. leprae or its cytosol fraction. Both subsets of T cells from tuberculoid leprosy patients also produced several fold more IFN-gamma than those from normal individuals. Furthermore, the intracellular perforin production in CD8(+) T cells was up-regulated in an Ag-dose dependent manner. These results suggest that M. leprae membrane Ags might be useful as the vaccinating agents against leprosy.

Fusion of mature dendritic cells and human T-lymphotropic virus type I infected T cells: its efficiency as an antigen-presenting cell

Monocyte-derived dendritic cells (DCs) from adult T cell leukemia are impaired in taking up exogenous antigens. To overcome this impairment, we fused unpulsed DCs to human T-lymphotropic virus type I (HTLV-I)-infected CD4(+) T cells (fusion DC-T cells). The efficiency of fusion was 50% and the fusion cells expressed higher HLA-ABC and CD86 Ags than HTLV-I-infected DCs. The fusion DC-T cells stimulated autologous CD4(+) and CD8(+) T cells, but DCs fused to itself or PHA-blasts did not stimulate any subsets. The functionally highest fusion DC-T cells was obtained when a DC and HTLV-I-infected T cells were fused at a ratio of 3:1. Expression of HTLV-Igag Ag on CD4(+) T cells was up-regulated when infected in the presence of 8-azaguanine, and these fusion DC-T cells were quite efficient in induction of higher CD8(+) T cell response. The results suggest that fusion DC-T cells produce functionally competent Ag-presenting cells and may be a likely candidate for immunotherapeutic use.

Mycobacterium leprae infection in monocyte-derived dendritic cells and its influence on antigen-presenting function

Host defense against Mycobacterium leprae infection is chiefly mediated by gamma interferon (IFN-gamma)-secreting cytotoxic T cells. Since which antigen-presenting cell populations act to stimulate these T cells is not fully understood, we addressed the role of monocyte-derived dendritic cells (DCs). The DCs phagocytosed M. leprae and expressed bacterially derived antigens (Ags), such as phenolic glycolipid 1 (PGL-1), in the cytoplasm, as well as on the cell surface. The expression of HLA-ABC and -DR Ags on DCs was down-regulated by M. leprae infection, and that of CD86 was up-regulated, but not as fully as by Mycobacterium bovis BCG infection. Induction of CD83 expression required a large number of M. leprae cells. When a multiplicity of infection of >40 was used, the DCs induced a significant proliferative and IFN-gamma-producing response in autologous T cells. However, these responses were significantly lower than those induced by BCG- or Mycobacterium avium-infected DCs. A CD40-mediated signaling in M. leprae-infected DCs up-regulated the expression of HLA Ags, CD86, and CD83 but did not enhance T-cell-stimulating ability. Therefore, M. leprae-infected DCs are less efficient at inducing T-cell responses. However, when the surface PGL-1 on M. leprae-infected DCs was masked by a monoclonal antibody, the DCs induced enhanced responses in both CD4(+)- and CD8(+)-T-cell subsets. M. leprae is a unique pathogen which remains resistant to DC-mediated T-cell immunity, at least in the early stages of infection.

Dendritic cell susceptibility to hepatitis C virus genotype 1 infection

In vitro infection of human monocyte-derived dendritic cells was carried out to study their susceptibility to hepatitis C virus (HCV) infection. Immature dendritic cells and mature dendritic cells were incubated overnight at 37 degrees C with HCV-positive (genotype 1) serum samples; the presence of the viral genome associated with the production of its replicative intermediate was used as evidence of infection. In immature dendritic cells, HCV RNA was detectable from days 1-10 post-infection (p.i.), and de novo synthesis of negative-strand HCV RNA could be demonstrated by a strand-specific rTth reverse transcription-polymerase chain reaction at day 2. In mature dendritic cells, the positive-strand form was detectable from days 1-5 p.i., while the negative-strand HCV RNA appeared at days 1 and 2 p.i. Quasispecies present in the inoculum and 6 days p.i. were analyzed by sequencing hypervariable region 1 of the E2 protein. Only two of seven HVR variants present in the inoculum were found in HCV-infected immature dendritic cells. Another two HVR variants not found in the inoculum were recovered from infected immature dendritic cells, suggesting serum minor variants selection or virus evolution during in vitro replication. Analysis by single-strand conformation polymorphism assay of 5' untranslated region of HCV sequences showed that the patterns obtained from the inoculum and infected immature dendritic cells and mature dendritic cells differed slightly. These findings indicate that both immature dendritic cells and mature dendritic cells are susceptible to HCV genotype 1 infection, supporting at least HCV RNA replication. This model should be a valuable tool for the study of modulation of dendritic cell functions in HCV infection.

Association of CD40 ligand expression on HTLV-I-infected T cells and maturation of dendritic cells

Human T lymphotropic virus type I (HTLV-I) induces HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and adult T-cell leukemia (ATL). The development of HAM/TSP is associated with rapid maturation of dendritic cells (DCs), while ATL is accomplished with their maturation defect. The DC maturation is induced by cell-to-cell contact with CD4+ T cells expressing CD40 ligand (L). We determined the influence of CD40L expressed on various HTLV-I-infected T cells on the DC maturation. Around 60% of CD4+ T cells infected with HTLV-I for 1 week, expressed CD40L molecules involved in DC maturation. DCs matured by the CD40L+ T cells activated autologous CD4+ and CD8+ T cells. HTLV-I-immortalized T-cell lines established from healthy donors consistently expressed CD40L molecules for 3 months, however, some lines lost the expression soon thereafter. Interleukin (IL)-2-independent and transformed lines lacked that expression. Furthermore, T cells obtained from HAM/TSP patients expressed CD40L molecules for at least 3 weeks, whereas T cells from ATL patients did not express that. The CD40L T cells did not induce DC maturation, and required exogenous CD40L molecules for maturation. The CD40L+ T-cell-induced maturation was blocked by anti-CD40L antibody. Therefore, the lack of CD40L expression on HTLV-I-infected T cells may be associated with the development of ATL.

HTLV-1 and its neurological complications

BACKGROUND

Since its discovery in 1980, human T-cell lymphotropic virus type-1 (HTLV-1) has been associated with a number of neurological diseases. The distribution of HTLV-1-associated neurological disease is worldwide. In endemic areas, up to 30% of the population may be infected with HTLV-1; however, only a small percentage of infected persons develops neurological disease.

REVIEW SUMMARY

In 1986, HTLV-1 infection was reported in patients of chronic progressive myelopathy of uncertain etiology, and the disease entity was called HTLV-1-associated myelopathy/tropical spastic paraparesis. Recently, HTLV-1 infection has been associated with polymyositis and uveitis. Interestingly, a single patient may display more than one syndrome. Although other neurological syndromes occur in HTLV-1-infected individuals, there is not enough epidemiologic data that show a strong association. Treatment of HTLV-1-associated neurological disease is challenging, and well-controlled studies are lacking.

CONCLUSION

As neurologists and other scientists begin to understand the pathophysiology of HTLV-1 infection, improved therapies should be developed. Randomized trials with longer follow-up are required to understand the effect of treatment on disability and quality of life.

Fratricide among CD8(+) T lymphocytes naturally infected with human T cell lymphotropic virus type I

Infection and gene expression by the human T lymphotropic virus type I (HTLV-I) in vivo have been thought to be confined to CD4(+) T lymphocytes. We show here that, in natural HTLV-I infection, a significant proportion of CD8(+) T lymphocytes are infected by HTLV-I. Interestingly, HTLV-I-specific but not Epstein-Barr virus-specific CD8(+) T lymphocytes were shown to be infected. Furthermore, HTLV-I protein expression in naturally infected CD8(+) T lymphocytes renders them susceptible to fratricide mediated by autologous HTLV-I-specific CD8(+) T lymphocytes. Fratricide among virus-specific CTLs could impair the immune control of HTLV-I and possibly other lymphotropic viruses.

A human B-lineage dendritic cell line, HBM-noda and its potential role in human T-cell leukemia/lymphoma virus type I infection

An Epstein-Barr virus-transformed B-cell line, HBM-Noda (Noda), that has a dendritic morphology as well as several characteristic features of dendritic cells (DC) has been established. We therefore refer to Noda as B-lineage DC. Although human T-cell leukemia/lymphoma virus type I (HTLV-I) exhibit substantial cellular tropism, the roles of DC in HTLV-I infection remain unknown. To further clarify the characteristics of Noda cells, we performed infection experiments using a concentrated HTLV-I fraction from the adult T-cell leukemia cell line, HPB-ATL-2. Noda, as well as other cell lines examined, were sensitive to HTLV-I infection as detected by proviral DNA using polymerase chain reaction, but most infected Noda cells underwent necrosis within 7 days. The most striking feature of Noda cells was the abundant expression of viral antigen (p19) on the cell surface following infection (approximately day 4), probably due to strong viral adsorption. In cocultivation experiments using Noda cells at day 1 of post-infection and peripheral blood activated T cells, we detected a few (1.3%) viral antigen expressing T cells after 5 days of coculture by flow cytometry. These results suggest that B-lineage DC such as Noda cells play a role in the establishment of HTLV-I infection at an early phase.

Monocyte-Driven Activation-Induced Apoptotic Cell Death of Human T-Lymphotropic Virus Type I-Infected T Cells

We attempted apoptotic cell death induction of T cells infected with human T lymphotropic virus type I (HTLV-I) which induces HTLV-I-associated myelopathy/tropical spastic paraparesis and adult T cell leukemia. T cells acutely infected and expressing HTLV-Igag Ags were killed by cross-linking their TCR with anti-CD3 mAb. Cells in apoptotic process were found by staining with annexin V. The apoptosis was not affected by costimulation through CD28 molecules and was resistant to ligation of Fas molecules. Whereas the virus-infected T cells expressed higher levels of HLA-DR, CD25, CD80, and CD86 Ags than apoptosis-resistant PHA-blasts, the T cell apoptosis was enhanced by addition of exogenous IL-2. Furthermore, in this apoptosis, monocytes played an important role because T cells infected in the absence of monocytes were resistant to the death signals. The apoptosis-sensitive T cells responded to TCR signaling more strongly by proliferating than those apoptosis-resistant cells. Monocytes weakly affected the expression levels of viral Ags on T cells. However, HTLV-I-infected monocytes primed T cells to die by subsequent TCR signaling. T cells primed with the monocytes, subsequently infected in the absence of monocytes, were killed by TCR signaling. These observations suggest that primed and infected T cells could be killed by activation-induced cell death.