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Journal of Virology, October 2004, p. 10320-10327, Vol. 78, No. 19
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.19.10320-10327.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Unité d'Epidémiologie et Physiopathologie des Virus Oncogènes,1 URA CNRS 2581, Parasitology Department, Institut Pasteur,2 Service d'Anatomie et Cytologie Pathologiques, Hôpital Lariboisière,3 Service de Médecine Interne, Groupe Hospitalier Pitié-Salpêtrière,4 CNRS UPR 9051, Institut Universitaire d'Hématologie, Hôpital Saint Louis, Paris, France5
Received 6 January 2004/ Accepted 21 May 2004
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Besides or concomitantly with HAM/TSP development, HTLV-1 infection has been associated with inflammatory muscle diseases such as polymyositis (13, 18) and, more rarely, sporadic inclusion body myositis (sIBM) (4, 25). These disorders are believed to be mediated by the immune system (5, 26, 31). The muscle target antigens remain to be identified, although a viral etiology has been suspected in some cases (35). Similar to the lesions observed in the spinal cord of HAM/TSP patients, CD4+ and CD8+ T cells are found in inflammatory lesions; HTLV-1 is detected in infiltrating CD4+ lymphocytes but not in muscle fibers (4, 14, 25). Furthermore, anti-Tax CD8+ clonotypes can be generated from muscle cultures, at least in those from HTLV-1-infected polymyositis patients (28). These findings thus strongly suggest that an immune reaction between HTLV-1-infected CD4+ T cells and anti-HTLV-1 CD8+ T cells also accounts for HTLV-1-associated myositis. It is therefore tempting to speculate that the mechanisms involved in the pathogenesis of HTLV-1-associated myositis closely mirror those of HAM/TSP. As muscle biopsies are more readily available than spinal cord samples, the study of HTLV-1-associated myopathies might be of major help in generally understanding HTLV-1-associated inflammatory disorders.
It was previously reported that HTLV-1 Tax mRNA is produced in mononuclear cells infiltrating the muscle of an HTLV-1-infected patient with sIBM (25). The present study, of a new HTLV-1-infected Caribbean patient suffering from HAM/TSP and sIBM, provides definitive evidence for a chronic immune reaction involving Tax-positive mononuclear cells and cytolytic anti-Tax T lymphocytes within muscle inflammatory lesions.
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Muscle and blood cultures. Blood samples were collected at the time of the first biopsy and 2 months later to determine the global virological and immunological parameters of the patient (Table 1) and to purify the PBMCs.
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TABLE 1. Virological and immunological parameters in blood of patient CB
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Muscle-cell-infiltrating T lymphocytes were obtained from two independent biopsies of the left deltoid muscle carried out 5 months apart (November 2000 and April 2001). Fresh biopsies were washed in RPMI culture medium to remove contaminating blood and then cut in small pieces; the resulting aggregates were set in culture in a 24-well plate in 2 ml of medium. To permit muscle cell antigen-driven expansion, muscle cell cultures were performed without exogenous stimulation and with a low dose of IL-2 (20 U/ml). Massive expansion of mononuclear cells was apparent after 2 weeks. Then, these cells were sorted into CD4+ and CD8+ T lymphocyte populations by positive selection with magnetic beads (Miltenyi Biotec) and further cultivated in the same medium.
Determination of proviral loads and antibody titers. HTLV-1 proviral loads were quantified in PBMCs by a real-time TaqMan PCR assay as described previously (6) with the viral pol gene for HTLV-1 quantification and the endogenous albumin gene for normalization. The titers of anti-HTLV-1 antibodies in plasma were quantified by indirect immunofluorescence with the MT-2 cell lines as HTLV-1 antigen-producing cells and twofold dilutions of each plasma sample, as described previously (32).
Chromium release assay and ELISPOT. Cytotoxic activities of CD8+ T-cell lines were tested against autologous infected T cells (CB-CD4/HTLV) or against short-term culture of autologous PHA-stimulated CD8+ T cells (CB-CD4) pulsed or not pulsed with the HLA-A*02-restricted Tax 11-19 epitope (Neosystem, Strasbourg, France) by the chromium release cytotoxic assay as described previously (27).
The presence of circulating in vivo-primed CD8+ T cells directed to the immunodominant Tax 11-19 epitope was examined by the ELISPOT assay. Briefly, CD8+ T cells freshly sorted from PBMCs were plated at a density of 50,000/well in RPMI medium supplemented with 5% human serum and 5% fetal calf serum, with or without 10 µM purified Tax 11-19 peptide. Specific revelation was performed as described previously (27) after 24 h of stimulation.
TCR repertoire analysis and sequencing. Total mRNAs were prepared from CD8+ T cells from blood and muscle cell cultures, and immunoscopic analysis of T-cell receptor (TCR) diversity was performed following PCR amplification of each of the 24 Vß families, as previously described (32). Vß13.1-specific PCR products were further cloned with the TOPO TA cloning kit (Invitrogen, Cergy Pontoise, France) and sequenced by an automatic procedure.
In situ analyses. Tissue sections from an HLA-A*0201- and HTLV-1-negative patient also suffering from sIBM were used as controls. For electron microscopy, ultrathin sections of epon-embedded specimens were stained with uranyl acetate-lead citrate. In situ hybridization (ISH) was performed with a tax 33P antisense riboprobe or with a sense tax riboprobe (as a negative control), as described previously (25).
Immunohistochemistry was performed with frozen tissue sections fixed in acetone. The anti-CD8+ and anti-CD4+ (Leu-2a and NU-TH/1) Ki-M7 antibody to the macrophage surface marker CD68, anti-perforin monoclonal antibody (BD Biosciences-Ozyme, Saint Quentin Yvelines, France), and anti-human tumor necrosis factor alpha (anti-TNF-
) antibody (HyCult Biotechnology-TEBU, Le Perray en Yvelines, France) were used. The antigenic specificity of muscle CD8+ T cells was assessed with a phycoerythrin (PE)-conjugated HLA-A*02/Tax 11-19-tetramer (kindly provided by C. Bangham, Imperial College, London, United Kingdom) or a control PE-conjugated HLA-A*02/EBV tetramer (kindly provided by H. Teisserenc, Hôpital Saint Louis, Paris, France), followed by an anti-PE monoclonal antibody (Sigma). All simple labeling was revealed with alkaline phosphatase substrates (Dako, Glostrup, Denmark). Revelation with two secondary antibodies conjugated to alkaline phosphatase or horseradish peroxidase was used for double staining.
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Two biopsies from the left deltoid muscle were then obtained 5 months apart from patient CB. Electromyography and histological examination of the biopsies showed typical features of sIBM, including scattered vacuolated myofibers displaying abnormal subsarcolemmal filaments (Fig. 1A to C) and important interstitial mononuclear cells infiltrates containing macrophages as well as CD4+ (Fig. 1D) and CD8+ (Fig. 1E) T lymphocytes.
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FIG.1. Histological and virological features of muscle tissues and cultures. Electron micrographs of deltoid muscle sections from patient CB showing intracytoplasmic accumulation of abnormal filaments (magnification, x9,800) (A) and clusters of straight (B) and paired helical (C) filaments seen at higher magnification (x14,000 and x18,000). Immunochemical detection of CD4+ (D) and CD8+ (E) T cells on muscle sections counterstained with Harris hematoxylin. Detection of tax mRNA-positive cells among fresh PBMCs (F) and CD4+ T lymphocytes sorted from muscle culture (G) compared to PHA-stimulated CD4+ T cells from blood (H) (counterstained with Giemsa; exposure time, 8 days). In situ detection of tax mRNA with an antisense probe on muscle sections from the first (I and J) and the second (K) biopsies; arrows show focal positivity for HTLV-1 in mononuclear infiltrated cells and not in muscle fibers. Negative control analyses were performed with a tissue section from the second biopsy with a sense probe (L). Sections were counterstained with hematoxylin-eosin, and the exposure time was 21 days.
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ISH showed that the tax mRNA was produced among CD4+ T cells expanded from the second biopsy (Fig. 1G), consistent with previous observation of HTLV-1 expression in tissues of infected patients with inflammatory disorders (17, 25, 30). Although muscle cell cultures were carried out without mitogen, the number of tax mRNA-positive cells (Fig. 1G) was comparable to that found in a long-term culture of mitogen-stimulated CD4+ T cells from blood (Fig. 1H). This indicates that a large number of HTLV-1-infected cells were already present in the biopsy.
We next investigated whether the CD8+ T-cell populations purified from muscle cultures contain anti-HTLV-1 CTLs. Since no mitogen was added during culture, any specific cytotoxic activity would reflect the presence and efficient presentation of HTLV-1 antigens present in the biopsy. Indeed, significant CTL activity to peptide-pulsed target cells was observed with CD8+ T cells expanded from both biopsies (Fig. 2A). The cytolysis found in CD8+ T cells recovered from muscle cells without mitogenic stimulation represents 20 or 30% of that found for PHA-stimulated CD8+ cells from blood, showing that the muscle-driven stimulation led to the expansion of a relatively high amount of specific and functional cells. Consistently, a cytotoxic activity directed to autologous HTLV-1 chronically infected cells (CB-CD4/HTLV) was found for CD8+ T cells expanded from the second biopsy, even at a low effector:target ratio (Fig. 2B).
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FIG. 2. Cytotoxic activities of CD8+ T cells expanded from muscle cultures. (A) Chromium release assay to measure the cytotoxic activity directed to the immunodominant Tax 11-19 epitope of CD8+ T cells sorted from muscle cultures (performed in absence of mitogen), compared to PHA-stimulated CD8+ T cells from blood. For 4 days, PHA-stimulated CD4+ T lymphocytes from blood (in which HTLV-1 antigen production was still undetectable) incubated with or without the immunodominant Tax 11-19 epitope (10 µM) were used as targets cells (effector/target ratio, 40:1). (B) Chromium release assay to detect the ability of CD8+ T cells sorted after culture of biopsy 2 to kill chronically infected cells (CB-CD4/HTLV) at two effector/target ratios. Nonproducing autologous PHA-stimulated CD4+ cells were also used as a negative control. A percentage of cell lysis up to 10% was considered significant.
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FIG. 3. Molecular characterization of CD8+ T cells expanded from muscle cultures. Determination of amino acid composition of the TCR ß-chain CDR3 sequences of 9 (biopsy 1), 10 (biopsy 2), and 19 (blood) DNA products obtained from Vß13.1/Cß-specific PCRs. Amino acid sequences corresponding to the conserved P/GLA/RG and SPGTG motifs previously found in anti-Tax 11-19 CD8+ T clonotypes (2, 29) are in boldface. The number of clones containing the characteristic motifs among the total number of sequenced clones is indicated between brackets.
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We first investigated whether HTLV-1 expression occurs within muscle by performing ISH with an anti-sense probe specific for tax mRNA. Foci of tax mRNA-expressing cells were readily detected in the first (Fig. 1I and 1J) and second (Fig. 1K) biopsies, in infiltrated mononuclear cells but not in muscle fiber. In contrast, no signal was detected with a sense tax riboprobe, even in a zone of high inflammation (Fig. 1L). These data demonstrate that HTLV-1 expression occurs chronically in muscle cells. Rare foci of Tax expression were also seen among fresh PBMCs (Fig. 1F). This is consistent with previous findings that HTLV-1 expression in the blood of HAM/TSP patients is rare, approximately 1 positive cell for every 5,000 PBMCs (9).
MHC-1/peptide tetramers are now commonly used to visualize the population of CD8+ T cells that recognized a particular epitope. For example, in HLA-A2-positive individuals suffering from HAM/TSP, Tax 11-19/HLA-A*02 tetramer-reactive cells can represent up to 10% of circulating CD8+ T cells (1). In patient CB, around 5% of PBMCs were recognized by the Tax 11-19/HLA-A*02 tetramer (data not shown).
To determine the antigenic specificity of muscle CD8+ T cells, we decided to use the Tax 11-19 tetramer in situ. As shown in Fig. 4, reactivity with the Tax 11-19 tetramer was indeed found among the mononuclear cell infiltrates in both the first (Fig. 4A) and the second (Fig. 4B) biopsies. Only background was found in parallel experiments with muscle sections from an HLA-A*02-uninfected patient with sIBM (Fig. 4C) or a control HLA-A*02/EBV tetramer (Fig. 4D). Perforin-producing cells were also observed (Fig. 4E), and a double-staining experiment using both anti-perforin antibody and the anti-Tax 11-19 tetramer further revealed that such populations include anti-Tax 11-19 T cells (Fig. 4F). Finally, simultaneous detection with anti-CD68 and anti-TNF-
antibodies allowed us to demonstrate that macrophages producing TNF-
were present in inflammatory muscle (Fig. 4G). In addition, we detected the presence of apoptotic cells among mononuclear cell infiltrates (Fig. 4H).
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FIG. 4. Immunochemical analyses of inflammatory lesions within deltoid muscle. Serial 5-µm frozen sections were fixed in acetone, processed for antigen detection, and counterstained with Harris hematoxylin. Staining was performed with the specific PE-conjugated HLA-A*02 Tax 11-19 tetramer on sections from the first (A) and the second (B) biopsies or on a control section from an HLA-A*02 uninfected patient with sIBM (C). Staining with a control HLA-A*02 EBV tetramer was also carried out on the second biospy sample from patient CB (D). Sections from the first biopsy were stained with anti-perforin antibody (E), with both anti-perforin antibody and PE-conjugated HLA-A*02 Tax 11-19 (F) (arrowheads indicate double-stained cells), and with both anti-TNF- and CD68+ antibodies (G). Detection of apoptotic mononuclear cells by the terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling method (H) was also performed.
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Our findings first show that HTLV-1 expression exists within muscle lesions, as evidenced by the presence of tax mRNA-positive cells both in muscle cultures and in tissue sections. In tissue sections, only a small proportion of mononuclear cells were positive for tax mRNA, but HTLV-1 expression was maintained over time. This suggests that a low-level but persistent virus expression is sufficient to promote the inflammatory process. Consistent with previous data (14, 25), HTLV-1 expression was confined to mononuclear cell infiltrates, ruling out a direct pathogenic role of the virus on muscle fibers. As was previously shown (9), only rare foci of tax mRNA-positive cells were found among PBMCs. That HTLV-1-expressing cells in muscle were evidenced among foci containing a limited number of cells, whereas such cells were rare among the large population of PBMCs, might indicate that tax mRNA expression is higher in muscle than in blood.
A massive expansion of anti-HTLV-1 CTLs during muscle culture provided another strong argument for the chronic production of HTLV-1 antigens within tissue. Indeed, since no mitogenic stimulation was performed, any CD8+ T-cell expansion would reflect the production and subsequent presentation of local antigens. Specific amino acid motifs within the CDR3 region of Vß13.1 anti-Tax 11-19 clonotypes were previously identified (2, 29). Whether a specific CD8+ proliferation driven by HTLV-1 occurred during muscle culture was then examined by sequencing the Vß13.1 PCR products amplified from the two biopsies and PBMCs as a control. We found that 70 and 100% of DNA clones from the first and second biopsies, respectively, contained a motif found in anti-Tax 11-19 cells for only 31% of DNA clones amplified from PBMCs. Hence, not only cells bearing the molecular signatures of anti-Tax 11-19 clonotypes were present within lymphocytes recovered from muscle but these cells were highly enriched in muscle compared to blood. This is reminiscent of what was recently observed with cerebrospinal fluid samples from HAM/TSP patients, in which a 12-fold enrichment for a particular anti-Tax clone was documented (21). Chromium release assays further showed that CD8+ T cells from muscle were able to kill both Tax 11-19-presenting target cells and autologous infected cells. This demonstrates that CD8+ effectors were generated during muscle culture upon stimulation of CD8+ precursors by HTLV-1-producing cells. Importantly, we obtained direct evidence that such precursor-to effector-differentiation also occurred within tissue. Indeed, we were able to specifically detect anti-Tax 11-19 T cells within lesions by using the highly specific Tax 11-19 tetramer. Furthermore, we also evidenced that such anti-Tax 11-19 cells produce perforin, a hallmark of CD8+ effectors (11). This formally demonstrates that functional CTLs directed to Tax are indeed recruited within inflamed lesions.
The above findings revealed that both Tax-producing T cells and perforin-positive cells directed to a dominant epitope of Tax were present in muscle. Moreover, the recruitment of such populations was maintained over time, since both cell types were found in sections from two successive muscle biopsies. This provides the first direct demonstration that a chronic CTL-mediated immune reaction directed to HTLV-1 occurs locally in the inflammatory tissue of an HTLV-1-infected patient. That such an immune response takes place in the muscle cells of a patient suffering from sIBM strongly supports the notion that chronic HTLV-1 infection and immune recognition are sufficient to trigger the inflammatory process leading to sIBM development. This might also imply that other infectious agents can promote sIBM development, providing that they are chronically present in muscle and constitute potent targets for CD8+ T cells.
Activated CD8+ T cells were previously described in spinal cord lesions (19), but whether this population includes anti-HTLV-1 CTLs was not determined. Since perforin-positive CD8+ T lymphocytes directed to Tax are found within muscle, as we showed here with a patient suffering from both HAM/TSP and sIBM, it seems very likely that such effector cells are also recruited within spinal cord tissue. We propose, therefore, that the specific immune reaction we demonstrated with muscle represents a common pathogenic determinant for the triggering of HTLV-1-associated inflammatory disorders. A remaining issue is how this specific anti-HTLV-1 response is linked to local inflammation and tissue destruction. As documented here, activation of macrophages due to local apoptosis among infiltrated mononuclear cells or other stimulations, leading to secretion of TNF-
, could constitute a major event. Further studies of muscle, a relatively available tissue, could help to elucidate these complex mechanisms and contribute to the design of new approaches for the treatment of HTLV-1-associated inflammatory disorders.
This work was supported by the Association Française contre les Myopathies (AFM).
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