Rheumatology Advance Access originally published online on April 25, 2006
Rheumatology 2006 45(12):1477-1484; doi:10.1093/rheumatology/kel119
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© 2006 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Decreased CD161+CD8+ T cells in the peripheral blood of patients suffering from rheumatic diseases
Department of Rheumatology and Internal Medicine, Juntendo University School of Medicine, Tokyo, Japan
Correspondence to: A. Mitsuo, Department of Rheumatology and Internal Medicine, Juntendo University School of Medicine, 2-1-1 Hongo, Bunkyo-Ku, Tokyo 113-8421, Japan. E-mail: iwai-kkr{at}umin.ac.jp
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Abstract |
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Objectives. Although it has been reported that the numbers of both CD4–CD8– and CD4+ natural killer T (NKT) cells are selectively decreased in the peripheral blood of patients with rheumatic diseases, there have been no reports concerning a novel subpopulation of CD8+ NKT cells. To examine whether CD161+CD8+ T cells, which are closely related to CD8+ NKT cells, are also decreased in patients with rheumatic diseases, we have investigated the expression of CD161, together with that of CD28, CD25 and CD62L, on T cells in the peripheral blood of these patients.
Methods. The rheumatic diseases evaluated in this study were systemic lupus erythematosus (SLE) (n= 54), mixed connective-tissue disease (MCTD) (n= 15), systemic sclerosis (SSc) (n= 14), polymyositis/dermatomyositis (PM/DM) (n= 13) and rheumatoid arthritis (RA) (n= 24). Healthy donors were examined as controls (n= 18). The expression of CD161, CD28, CD25 and CD62L on T cells was analysed by flow cytometry.
Results. Both the frequency of CD161 expression on CD8+ cells and the absolute number of CD161+CD8+ cells were significantly decreased in patients with SLE, MCTD, SSc and PM/DM. Only the absolute number of CD161+CD8+ T cells was significantly decreased in RA. CD161 expression on CD28–CD8+ T cells was significantly decreased in SLE, MCTD and SSc. The absolute number of CD161+CD8+CD62L– T cells was significantly decreased in SLE, MCTD and SSc.
Conclusions. Both the frequency and the absolute number of CD161+CD8+ T cells were decreased in the peripheral blood of patients suffering from SLE, MCTD, SSc and PM/DM. This result suggests that there is also an abnormality of NKT cells in the CD8+ population.
KEY WORDS: CD161+CD8+ T cells, CD8+ NKT cells, Rheumatic diseases
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Introduction |
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Autoimmune diseases are characterized by the reaction of lymphocytes against auto-antigens and by the production of auto-antibodies. Although the etiologies of autoimmune diseases are not clearly defined, most models for the pathogenesis of these disorders include the activation of both T and B cells including self-reactive clones. Immunological tolerance in healthy individuals regulates this activation of lymphocytes and prevents the onset of autoimmune diseases. Various mechanisms are involved in this tolerance, with an important role for regulatory T cells that suppress self-antigen-reactive cells.
Natural killer T (NKT) cells have been identified as a subpopulation of CD4–CD8– (double negative, DN) or CD4+ T cells, and express both specific 
/ß T cell receptors (TCRs) and NK markers in humans [1] and mice [2]. In humans, NKT cells have the invariant TCR V24JQ gene, which is recognized by CD1d [3]. The majority of human NKT cells express TCR V24J18Vß11 [4] and the CD161 (NKR-P1A) molecule on their surface as an NK marker. Human NKT cells are believed to have regulatory effects on immune tolerance or autoimmunity [1], and in fact the number of both CD4–CD8– and CD4+ NKT cells is selectively decreased in the peripheral blood of patients with systemic lupus erythematosus (SLE), systemic sclerosis (SSc), rheumatoid arthritis (RA) and Sjögren's syndrome (SS) [4].
CD161 is also expressed on CD8+ T cells [5, 6]. The TCRs of CD161+CD8+ T cells have V24J36, V24J45 or V24J37 [7], which are different from those of CD4–CD8– or CD4+ NKT cells. A recent study has further demonstrated a novel subpopulation of human V24+ CD8+ NKT cells, a majority of which express the CD161 molecule and recognize the CD1d molecules [8]. Moreover, it has been demonstrated that CD8+ NKT cells have a regulatory function of inhibiting the proliferation of antigen-specific activated T cells in humans [9]. Thus, these CD161+CD8+ T cells may be a different population or a subpopulation of the conventional CD4–CD8– or CD4+ NKT cells, and have a regulatory function. However, there have been no reports concerning CD161+CD8+ T cells or CD8+ NKT cells in rheumatic diseases, and the function of the CD161 molecule is still unclear.
In the present study, we investigated the expression of CD161 on peripheral blood T cells from patients with various rheumatic diseases to examine whether CD161+CD8+ T cells are also decreased. This is the first report to document a decrease of CD161+CD8+ T cells in the peripheral blood of patients suffering from SLE, mixed connective-tissue disease (MCTD), SSc and polymyositis/dermatomyositis (PM/DM). We also investigated CD161 expression on CD28–CD8+ T cells and CD4+CD25+ T cells, and the expression of the CD62L molecule on CD161+CD8+ T cells to help clarify how the CD161 molecule or CD161+CD8+ T cells might function in the peripheral blood of these patients.
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Patients and methods |
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Patients
The study population comprised 120 patients (18 men and 102 women), and each patient was followed at the Department of Rheumatology and Internal Medicine, Juntendo University School of Medicine, from October 1999 to February 2004. The demographic features of the patients and healthy donors are shown in Table 1. SLE, SSc and RA were diagnosed according to the American College of Rheumatology criteria [10–12]. MCTD was diagnosed according to Kasukawa's criteria [13], and PM/DM was diagnosed according to Bohan's criteria [14]. The PM/DM group did not include patients with amyopathic DM. The control group that included the healthy donors did not match the age of some of the patient groups, because of the predominant age at onset of each disease. The treatments that the patients received are shown in Table 2. Eight patients with SLE, four with MCTD, seven with SSc, one with PM/DM and two with RA did not receive any therapy, which means that these patients were not treated with steroids, immunosuppressants or plasmapheresis. For example, among the eight patients with SLE, two were treated with anti-allergic drugs, one with an antihistaminic, one with diclofenac sodium, one with an oral cephem, and three were fully drug-free before analysis. After analysis, all of these eight SLE patients received prednisolone as soon as possible. Among them, two patients received not only prednisolone but also 500 mg/month cyclophosphamide pulse therapy, and another patient received not only prednisolone but also methylprednisolone pulse therapy. Another two patients with SLE were treated by plasmapheresis once a week and peripheral blood samples from these patients were obtained just before the procedure. All the patients and healthy donors were fully informed and gave their consent to participate.
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Clinical assessment
Disease activity of SLE patients was assessed in terms of the SLE Disease Activity Index (SLEDAI) score [15], the level of CH50 (normal range 25–54 units), and anti-DNA antibodies (normal range <6 IU/ml). Other disease activities were assessed by the level of C-reactive protein (CRP, normal range <0.3 mg/dl) in SSc, PM/DM and RA patients and by the level of creatinine kinase (CK) (normal range 9–93 IU/l) in PM/DM patients.
Monoclonal antibodies (mAbs)
The following monoclonal antibodies (mAbs) were used in this study: fluorescein isothiocyanate (FITC)-conjugated anti-CD4 (RPA-T4), anti-CD161 (DX12), phycoerythrin (PE)-conjugated anti-CD8 (RPA-T8), anti-CD25 (M-A 251), anti-CD28 (CD28.2), anti-CD62L (Dreg 56), antigen-presenting cell (APC)-conjugated anti-CD3 (HIT 3a), biotin-conjugated anti-CD8 and anti-CD4, and purified anti-CD161 (DX12, mouse IgG1) (a blocking antibody) and isotype-matched control mAb (mouse IgG1). All mAbs were obtained from PharMingen (San Diego, CA, USA). Alexa 488-conjugated streptavidin was obtained from Molecular Probes (Eugene, OR, USA).
Cell preparation
Peripheral venous blood of patients and healthy donors was diluted 1:2 with phosphate-buffered saline (PBS) (Dulbecco, Nissui, Japan), and the peripheral blood mononuclear cells (PBMCs) were isolated by density-gradient centrifugation with lymphocyte separation (Lymphoprep, Nycomed Pharma AS, Oslo, Norway). After two washings with PBS, the PBMC count was determined with a blood cell counter, and the cell suspension was adjusted to a concentration of 1x 106 cells/ml in PBS.
Staining and flow cytometric analysis
The PBMCs (1x 106 cells) were incubated with FITC-conjugated anti-CD161 mAb, PE-conjugated anti-CD25, anti-CD28 or anti-CD62L mAb, APC-conjugated anti-CD3 mAb, and biotin-conjugated anti-CD8 or anti-CD4 mAb on ice for 30 min. After two washings with PBS, Alexa 488-conjugated streptavidin was added and incubation was continued for a further 30 min. After two additional washes, the samples were fixed with 0.5% paraformaldehyde in PBS. Flow cytometric analysis was performed with an FACScaliber (Becton Dickinson, San Jose, CA, USA) and data were processed with the CellQuest program (Becton Dickinson).
Effect of anti-CD161 mAb cocultured with pokeweed mitogen (PWM) and quantification of IgG by ELISA
The PBMCs from healthy donors (1x 105 cells/well) were cultured with 0.1 µg of PWM (Sigma, St Louis, MO) in 96-well round-bottom plates in 0.2 ml of culture medium for 7 days at 37°C in a humidified atmosphere with 5% CO2. Various amounts of purified anti-CD161 mAb (a blocking antibody) (0, 0.5 and 10 µg/ml) or isotype-matched control mAb (mouse IgG1) (0, 0.5 and 10 µg/ml) were added at the beginning of the experiments. The culture supernatants were harvested and added to goat anti-human IgG (Bethyl, Mongomery TX)-coated 96-well flat-bottom ELISA plates. After discarding the supernatants and washing with Tween-20 PBS, the bound human IgG was detected with horseradish peroxidase (HRP)-labelled goat anti-human IgG at a dilution of 1:100 000 followed by the addition of tetramethylbenzidine (Bio-Rad Laboratories, Hercules, CA), and the amount of IgG present was assessed by spectrophotometric analysis at 450 nm.
Statistical analysis
Data are expressed as median and mean± S.D. Data were analysed using a statistical software package (StatView 5.0: SAS Institute Inc., USA), the Mann–Whitney U-test and Spearman's rank correlation. Differences at P< 0.05 were considered to be statistically significant.
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Representative patterns of CD161 expression by flow cytometric analysis
Figure 1 demonstrates the representative patterns of CD161 expression on T cells from one healthy donor and patients with SLE and RA by flow cytometric analysis. Since CD161 was expressed mainly on NK cells, PBMCs were gated with APC-conjugated anti-CD3 mAb, and then three-colour analysis was performed. The population of CD161+CD8+ cells was decreased in patients with both SLE and RA in comparison with the healthy donors. The numbers indicate the percentage of each population.
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Decreased frequency and absolute number of CD161+CD8+ T cells in patients with SLE, MCTD, SSc and PM/DM
We then investigated the expression of CD161 on T cells from patients with various rheumatic diseases and healthy donors. Both the frequency and the absolute number of CD161+CD8+ T cells were significantly decreased in patients with SLE (both P< 0.001), MCTD (P< 0.01 and P< 0.001), SSc (P< 0.05 and P< 0.001) and PM/DM (P< 0.05 and P< 0.001), when compared with the healthy donors (Fig. 2). In patients with RA, the absolute number of CD161+CD8+ T cells was significantly decreased (P< 0.001), although the frequency of CD161+CD8+ T cells was not (Fig. 2). Furthermore, this reduction was correlated with the decrease of CD8+ T cells in patients with RA (data not shown). On the other hand, the absolute number of CD161+CD4+ T cells was significantly decreased only in SLE patients when compared with the healthy donors (P< 0.001), although the frequency of CD161+CD4+ T cells was not decreased significantly (data not shown).
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Lack of significant correlation between the frequency or absolute number of CD161+ T cells and age or sex
Next, since an influence of age was expected, we investigated the correlation between the frequency or absolute number of CD161+ T cells and age in each patient group and in the healthy donor group. However, no significant correlations could be demonstrated (data not shown). In addition, to exclude any influence of sex, we compared the frequency or absolute number of CD161+ T cells between males and females in each patient group and in the healthy donor group, but no significant differences were evident.
No influence of treatment on the frequency or absolute number of CD161+ T cells
To exclude the influence of treatment, we compared the frequency and absolute number of CD161+ T cells between the patients given treatment and those who did not receive any steroid, immunosuppressant or plasmapheresis in each rheumatic disease group, but no significant differences were evident. In addition, we investigated the correlation between the frequency or absolute number of CD161+ T cells and the dose of steroid in each patient group, but again no significant correlation could be demonstrated (data not shown). Moreover, we found no influence of any kind of immunosuppressant. On the other hand, although we compared the frequency or absolute number of these cells between the patients with SLE who received plasmapheresis and those who did not, no differences were evident. We also compared the absolute number of these cells between the inflow and outflow circuits during plasmapheresis in SLE patients, but found no differences. Finally, we determined whether there was any correlation between the frequency or absolute number of these cells and treatment in the eight patients with SLE who did not receive any steroid, immunosuppressant or plasmapheresis, and found no significant differences between the pre- and post-treatment results (data not shown).
Lack of significant correlation between CD161+ T cells and clinical parameters
Next, we investigated the correlation between the frequency or absolute number of CD161+ T cells and the clinical parameters CH50, anti-DNA antibody and SLEDAI in SLE patients, but no significant correlations could be demonstrated (data not shown). We also investigated the correlation between the levels of CRP in RA patients, or the levels of CRP or CK in PM/DM patients, or CRP in SSc patients, and the frequency or absolute number of CD161+ T cells in these patients, but again were unable to demonstrate any correlation (data not shown).
Decreased frequency of CD161 expression on CD28–CD8+ T cells in patients with SLE, MCTD and SSc
A recent study [16] has revealed that human suppressor T cells are derived from an oligoclonal population of CD8+ CD28– T cells. Therefore, we examined the relationship between CD161 expression and these cells. First, we examined both the frequency and the absolute number of CD28–CD8+ T cells in the patient groups, but there were no significant differences in comparison with the healthy donors (data not shown). Then, we analysed the frequency of CD161 expression on CD28–CD8+ T cells. The frequency of CD161 expression on CD28–CD8+ T cells was significantly decreased in patients with SLE, MCTD and SSc when compared with the healthy donors (Fig. 3). Thus, the population of CD28–CD8+ T cells contained a decreased proportion of CD161+CD8+ T cells. Moreover, we compared the frequency of CD161 expression on CD28–CD8+ T cells between patients with and without steroid, immunosuppressant or plasmapheresis treatment in each group of rheumatic diseases, but there were no significant differences. Thus, no other influence of treatments was evident.
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Lack of difference in the frequency of CD161 expression on CD4+CD25+ T cells between each of the patient groups and the healthy donors
It has been reported that a proportion of CD4+CD25+ T cells expressing Fox p3 exert regulatory effects [17, 18]. To examine the function of the CD161 molecule, we investigated its relationship with CD4+CD25+ T cells. Neither the frequency nor the absolute number of CD4+CD25+ T cells differed significantly between each of the patient groups and the healthy donors (data not shown). Next, we examined CD161 expression on CD4+CD25+ T cells in healthy donors, and found that the frequency of CD161 expression was significantly higher than that on CD4+CD25– T cells (data not shown). Finally, we analysed the frequency of CD161 expression on CD4+CD25+ T cells in each of the patient groups, but found no significant differences from the healthy donors (data not shown).
Lack of enhanced production of IgG upon treatment with purified anti-CD161 mAb
To investigate whether the CD161 molecule has an inhibitory function, we examined the production of IgG by PBMCs from healthy donors by treatment with purified anti-CD161 mAb (a blocking antibody) (0, 0.5 and 10 µg/ml) and co-culture with PWM. However, no enhanced production of IgG was observed upon treatment with purified anti-CD161 mAb when compared with isotype-matched control mAb (data not shown).
Lack of significant difference in the frequency of CD62L expression on CD8+ T cells between patients with rheumatic diseases and healthy donors
CD62L (L-selectin) molecules are constitutively expressed by most leucocytes (lymphocytes, neutrophils, monocytes and NK cells), and cooperate with other selectins and integrins in supporting leucocyte ‘rolling’ along the inflamed vascular endothelium prior to adhesion and transmigration. This molecule is cleaved from the cell surface after activation of the cells [19], and its expression is related to cell migration into inflammatory tissue. Therefore, to examine the possibility that CD161+CD8+ T cells may migrate into inflammatory tissue and become decreased in peripheral venous blood, we investigated CD62L expression on CD8+ T cells. However, there was no significant difference in the frequency of CD62L expression on CD8+ T cells between patients with rheumatic diseases and healthy donors (data not shown). In addition, age, sex or treatment had no influence on CD62L expression.
Increased frequency of CD62L expression on CD161+CD8+ T cells in patients with SLE and SSc
We investigated the frequency of CD62L expression on CD161+CD8+ T cells and the absolute numbers of both CD161+CD8+CD62L+ and CD161+CD8+CD62L– T cells. The mean± S.D. and median (range) of the frequency of CD62L expression on CD161+CD8+ T cells were 55.1± 20.4 and 49.2 (15.1–92.4) in patients with SLE, 51.1± 23.9 and 51.3 (22.1–91.5) in patients with MCTD, 54.0± 22.1 and 50.1 (35.3–80.6) in patients with SSc, 52.1± 25.4 and 56.2 (18.3–92.5) in patients with PM/DM, 52.7± 24.2 and 58.0 (11.9–81.1) in patients with RA, and 34.4± 13.5 and 35.0 (13.8–60.0) in the healthy donors, respectively. The mean± S.D. and median (range) of the frequency of CD62L expression on CD161– CD8+ T cells were 73.8± 18.8 and 77.0 (35.6–98.8) in patients with SLE, 72.8± 21.3 and 72.2 (40.6–94.8) in patients with MCTD, 82.9± 3.7 and 82.1 (79.7–87.7) in patients with SSc, 74.8± 19.8 and 72.5 (39.1–97.9) in patients with PM/DM, 67.9± 25.1 and 74.7 (19.5–93.0) in patients with RA, and 83.0± 9.9 and 84.9 (61.9–99.8) in the healthy donors, respectively. As shown in Fig. 4A, the frequency of CD62L expression on CD161+CD8+ T cells was significantly decreased in SLE (P< 0.01), MCTD (P< 0.05), SSc (P< 0.05), and the healthy donors (P< 0.001) when compared with that on CD161–CD8+ T cells. However, the frequency of CD62L expression on CD161+CD8+ T cells was significantly increased in SLE (P< 0.01) and SSc (P< 0.01), when compared with the healthy donors.
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Decreases in the absolute numbers of CD161+CD8+CD62L– T cells in patients with SLE, MCTD and SSc, and in absolute numbers of CD161+CD8+CD62L+ T cells in patients with SLE
The absolute numbers of CD161+CD8+CD62L– T cells were significantly decreased in patients with SLE, MCTD and SSc (Fig. 4B), and the absolute numbers of CD161+CD8+CD62L+ T cells were significantly decreased only in patients with SLE (Fig. 4C). Although we compared the absolute numbers of CD161+CD8+CD62L– T cells between the patients with and without treatment for each group of rheumatic diseases, no significant differences were found. Moreover, we compared the frequency of CD161 expression on these cells between patients given low-dose steroids and those given high-dose steroids, but there were no evident differences.
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Discussion |
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The present study is the first to demonstrate a decrease of CD161+CD8+ T cells in the peripheral blood of patients suffering from SLE, MCTD, SSc and PM/DM. This interesting finding is of considerable significance, since there has been little information concerning CD8+ NKT cells in autoimmune diseases. With regard to human NKT cells, it has been reported that both CD4–CD8– and CD4+ NKT cells are generally decreased in patients with SLE, SSc, RA and SS [4], suggesting that these cells play a role in the pathogenesis. Therefore, the results of the present study suggest that CD161+CD8+ T cells, which are closely related to CD8+ NKT cells, also contribute specifically to the pathogenesis of rheumatic diseases, as well as CD4–CD8– and CD4+ NKT cells.
On the other hand, the common contribution of CD4–CD8– and CD4+ NKT cells to pathogenesis is their regulatory function. Indeed, a previous study using a mouse model has suggested that these NKT cells may act as regulatory T cells, and that a reduction in their number is associated with the occurrence of autoimmune diseases [4]. A recent investigation has also suggested that CD4+ NKT cells are regulators of not only tumour immunity, but also of autoimmune diseases in humans [20]. Furthermore, a recent study has shown that CD8+ NKT cells may exert a regulatory role in humans through lysis of APCs or activated T cells [9]. Thus, it is possible that CD161+CD8+ T cells, like CD8+ NKT cells, may prevent autoimmunity. Indeed, in the present study, the decrease of CD161+CD8+ T cells observed in SLE, MCTD and SSc included a high proportion of CD28–CD8+ T cells. Therefore, suppressor T cells may also have been depleted. The observations suggest that the decrease of of CD161+CD8+ T cells may also play a role in the pathogenesis of rheumatic diseases though the disruption of the regulatory mechanism associated with the decrease of other populations of NKT cells. However, since the decrease in CD161+CD8+ T cells was not related to disease activity or to other populations of NKT cells, it may play only a partial role in the development of rheumatic diseases.
Another point of interest is whether the CD161 molecule has a regulatory function. We have demonstrated that the CD161 molecule is expressed more frequently on CD4+CD25+ T cells than on CD4+CD25– T cells, suggesting that it may play a regulatory role. However, CD4+CD25+ T cells do not represent a homogeneous population of regulatory cells, because CD25 is also expressed diffusely on activated effector T cells. Therefore, it may be necessary to investigate the frequency of CD161 expression on Foxp3+CD4+CD25+ T cells. In addition, anti-CD161 blocking antibody did not enhance the production of IgG upon stimulation with PWM. Therefore, clearer evidence for a regulatory function of CD161 will be needed. Since anti-CD161 antibody possessing an enhancing function is not commercially available, we are unable to investigate other effects such as the decrease of antibody generation. If anti-CD161 antibody with an enhancing function becomes available in the future, we would certainly be interested in investigating other effects.
On the other hand, there was no reduction in the frequency of CD161 expression on CD8+ T cells in RA patients, suggesting that the pathogenesis of RA may differ from that of other rheumatic diseases. Our previous study has shown that the expression of NK-B1 on CD8+ T cells is decreased only in RA patients, suggesting a specific mechanism for the activation of CD8+ T cells in this disease [21]. Therefore, the absence of any reduction in the frequency of CD161 expression on CD8+ T cells in RA may reflect the fact that the characteristics of the CD8+ T cells involved differ according to the type of disease.
Although we also examined the expression of CD161 on CD4+ T cells, the absolute number of CD161+CD4+ T cells was significantly decreased only in SLE patients. It is likely that this decrease is a consequence of the decrease in CD4+ T cells commonly observed in SLE patients. In addition, there was also no difference in the expression of CD161 on CD4+CD25+ T cells. Therefore, our results suggest that there may be no essential difference in CD161+CD4+ T cells, including CD4+ NKT cells, between patients with rheumatic diseases and healthy donors. However, this result differs from that of another study [4], and the discrepancy may be related to the fine difference in cell population between CD4+CD161+ T cells and CD4+ NKT cells. Thus, our present study demonstrated abnormal CD161 expression only in CD8+ T cells, and not in CD4+ T cells.
Finally, we consider the reason for the decrease of CD161+CD8+ T cells. Our observations on the regulation of CD62L provide an important clue. In brief, the main population of CD161+CD8+ T cells was CD62L-negative in healthy donors, and this was the main cell component that was decreased in patients with rheumatic diseases. The former observation is in agreement with a report that CD161+CD8+ T cells are present in extra-thymic or extra-lymphoid tissues [22], since CD62L-negative cells are localized mainly in tissue and peripheral blood. The latter observation provides some clue as to the cause of the decrease in these cells. First, it is possible that CD161+CD8+CD62L– T cells migrated into the inflammatory tissue. However, the observation that the decrease in CD161+CD8+ T cells was not related to disease activity, and a report that CD4+CD62L+ T cells migrate into inflammatory tissue [23], argue against this possibility. In addition, it is difficult to collect many samples of inflamed tissue from humans, even though direct observation of inflamed tissue for infiltration of CD161+CD8+ T cells would be desirable in order to investigate the tissue-homing properties of these cells. Second, it is possible that the production of CD161+CD8+ T cells in the bone marrow or extra-thymic/lymphoid tissues may be decreased. Most studies [24, 25] have found that CD62L-negative cells are derived from inflammatory tissue, although one study has demonstrated that naive T cells lack CD62L expression in human neonates [26]. Therefore, since it is believed that CD161+CD8+CD62L– T cells develop mainly in extra-thymic/lymphoid tissues, production of this T cell population at these sites may be suppressed by some mechanism, although this is difficult to prove. Third, it is possible that CD161+CD8+ T cells are decreased by apoptosis. However, since no significant correlation could be demonstrated between the absolute number of CD161+CD8+ T cells and that of apoptotic CD8+ T cells expressing Annexin V+/propidium iodide – (data not shown), we think this possibility is unlikely. Moreover, inadequate antigen presentation, dysfunction of CD8+ NKT cells, and abnormal antigen presentation [4] may be partly involved in the decrease of CD161+CD8+ T cells, as is the case for CD4–CD8– and CD4+ NKT cells. Moreover, a possible relationship between CD8+ NKT cells and infection, for example HIV or previous vaccination, cannot be ignored. However, a previous report has denied this possibility [27], and none of the patients included in the present study were injected with vaccines in a month prior to our analysis. A number of aspects concerning the role and origin of CD161+CD8+ T cells remain unclarified, and further studies will therefore be required.
In summary, we have demonstrated that both the frequency and the absolute number of CD161+CD8+ T cells are decreased in the peripheral blood of patients suffering from SLE, MCTD, SSc and PM/DM. We are convinced that our present findings provide a new clue to the etiology of these disorders.
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Funding to pay the Open Access publication charges for this article was provided by Department of Rheumatology and Internal Medicine, Juntendo University School of Medicine, Tokyo, Japan.
The authors have declared no conflicts of interest.
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