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Abstract

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Objective

Although interleukin-17 (IL-17)–producing γ/δ T cells were reported to play pathogenic roles in collagen-induced arthritis (CIA), their characteristics remain unknown. The aim of this study was to clarify whether γ/δ T cells or CD4+ T cells are the predominant IL-17–producing cells, and to determine what stimulates γ/δ T cells to secret IL-17 in mice with CIA. The involvement of IL-17–producing γ/δ T cells in SKG mice with autoimmune arthritis and patients with rheumatoid arthritis (RA) was also investigated.

Methods

IL-17–producing cells in the affected joints of mice with CIA were counted by intracellular cytokine staining during 6 distinct disease phases, and these cells were stimulated with various combinations of cytokines or specific antigens to determine the signaling requirements. Similar studies were performed using SKG mice with arthritis and patients with RA.

Results

Gamma/delta T cells were the predominant population in IL-17–producing cells in the swollen joints of mice with CIA, and the absolute numbers of these cells increased in parallel with disease activity. IL-17–producing γ/δ T cells expressed CC chemokine receptor 6, were maintained by IL-23 but not by type II collagen in vitro, and were induced antigen independently in vivo. Furthermore, IL-17 production by γ/δ T cells was induced by IL-1β plus IL-23 independently of T cell receptor. In contrast to what was observed in mice with CIA, IL-17–producing γ/δ T cells were nearly absent in the affected joints of SKG mice and patients with RA, and Th1 cells were predominant in the joints of patients with RA.

Conclusion

Gamma/delta T cells were antigen independently stimulated by inflammation at affected joints and produced enhanced amounts of IL-17 to exacerbate arthritis in mice with CIA but not in SKG mice with arthritis or patients with RA.

Rheumatoid arthritis (RA) is a chronic autoimmune disease that results in the destruction of cartilage and bone in joints. Collagen-induced arthritis (CIA) is a well-established murine model of this disease and shares many features with RA (1, 2). Specifically, susceptibility to both CIA and RA is associated with the specific class II major histocompatibility complex allele (3, 4). In addition, autoantibodies to type II collagen have been detected in the synovial fluid of patients with RA, and these autoantibodies have an aggravating effect on CIA in mice (5–7). In addition, pathogenic contributions of CD4+ T helper cells have been reported in both CIA and RA (8, 9).

Interleukin-17 (IL-17) is a cytokine secreted by T cells, natural killer (NK) cells, and neutrophils (10), and it induces IL-6, IL-8, chemokine, and metalloproteinase production by target cells (11). Central pathogenic roles of IL-17 in CIA have been reported recently. For example, systemic or local IL-17 gene transfer aggravated CIA, whereas administration of an IL-17–blocking antibody ameliorated CIA even after the onset of arthritis (12, 13), and IL-17–deficient mice also showed reduced severity of CIA (14). Furthermore, IL-23–deficient mice, which show an impaired Th17 response, do not exhibit CIA, because IL-23 is an essential factor for the maintenance of Th17 cells (15).

Although Roark et al recently reported the infiltration of IL-17–producing γ/δ T cells together with IL-17–producing CD4+ T (Th17) cells in inflamed joints of mice with CIA (16), the precise predominance, distribution, kinetics, cytokine-production requirements, and characteristics of these cells, especially in the context of IL-17–producing γ/δ T versus Th17 cells, remain unclear. Elucidation of these factors will be critical in terms of understanding the pathogenesis of CIA, finding novel therapeutic targets associated with IL-17, and determining the optimal timing and site for therapeutic intervention in CIA.

In the current study, we performed spatiotemporal analysis of IL-17–producing cells in CIA and demonstrated that γ/δ T cells are the predominant source of IL-17 in swollen joints of mice with CIA. IL-17–producing γ/δ T cells were maintained by IL-23 but not by type II collagen in vitro. Furthermore, IL-17 production by γ/δ T cells was efficiently stimulated by inflammatory cytokines independently of T cell receptor (TCR). Contrary to the results observed in mice with CIA, IL-17–producing γ/δ T cells could not be detected in the affected joints of patients with RA.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Animals.

DBA1/J mice and BALB/c mice were purchased from Charles River (Wilmington, MA). Eight-week-old male mice were used for induction of CIA, and 2-week-old mice were used to analyze thymocytes. The procedures for the induction of arthritis in SKG mice were described previously (17). Mice were maintained in our animal facility under specific pathogen–free conditions, and all animal procedures were approved by the Ethics Committee of Kyoto University.

Induction of CIA.

Immunization-grade bovine type II collagen was purchased from Chondrex (Redmond, WA) and reconstituted at 2 mg/ml in 0.05M acetic acid and then emulsified with an equal volume of Freund's complete adjuvant (CFA) containing 4 mg/ml of heat-killed Mycobacterium tuberculosis (Arthrogen-CIA; Chondrex). In order to examine the immune process at the immunized site, CIA was initiated by subcutaneous injection with 100 μl of emulsified type II collagen into the left footpad rather than the tail base. This altered method of immunization did not result in skewed disease kinetics, severity, or cytokine profiles of cells in swollen joints (data not shown). A booster immunization was not given. Each joint was designated as follows: immunized joint = left hind paw that received immunization; swollen joint = a fore paw in which arthritis developed; nonswollen joint = right hind paw that was not immunized and in which arthritis did not develop macroscopically (Figure 1A). SKG and BALB/c mice were also immunized with CFA plus type II collagen into the left hind paw to analyze locally infiltrated cells 10 days later. In some experiments, control mice were treated with type II collagen emulsified in Freund's incomplete adjuvant (IFA; Difco, Detroit, MI) or 0.05M acetic acid emulsified in IFA or phosphate buffered saline (PBS) alone.

Preparation of mononuclear cells from joints.

To prepare cells from the joints, the previously described technique (3) was used. Although a previous report confirmed that contamination of bone marrow cells had not occurred using this procedure (18), we compared the absolute counts of γ/δ T cells and CD19+ cells collected by this procedure and collected from the remaining tissues of the normal joints of naive DBA1/J mice. Cells in the remaining tissues were collected by mincing the remaining tissues, including bone marrow. The cells were stained with biotinylated anti-CD19 monoclonal antibody (mAb) (1D3; BD Biosciences, San Jose, CA) or anti-γ/δ TCR mAb (UC7-13D5), detected with streptavidin–allophycocyanin, and analyzed using fluorescence-activated cell sorting. Human synovial tissue or synovial fluid was obtained from patients with RA who were undergoing joint replacement surgery or subcutaneous puncture of the knee joints. Synovial tissue was dissected into small pieces with scissors, and lymphocytes were collected by density-gradient centrifugation with Ficoll-Paque PLUS (GE Healthcare, Amersham, UK). All human procedures were approved by the Ethics Committee of Kyoto University and were performed after receiving informed consent.

Intracellular cytokine staining.

Cell stimulation with phorbol myristate acetate (PMA) and ionomycin and intracellular cytokine staining were performed as described previously (16). When IL-17 production requirements were analyzed, 2 × 104 cells/well in a 96-well plate were stimulated with 10 μg/ml of plate-bound anti-γ/δ TCR (UC7-13D5), 2 μg/ml of soluble anti-CD28 (37. 51; BD Biosciences), 5 ng/ml of recombinant mouse IL-23 (1887-ML-010; R&D Systems, Minneapolis, MN), 50 ng/ml of recombinant mouse IL-1β (094-04681; Wako, Osaka, Japan), or 50 ng/ml of recombinant human transforming growth factor β1 (TGFβ1) (240-B; R&D Systems) for 24 hours, in the presence of 15 μM monensin for the last 4 hours. Other stimulants were not included in the analysis of IL-17 production requirements.

To analyze surface antigens, the following antibodies were used: fluorescein isothiocyanate (FITC)–labeled anti-CD8 (53-6.7), FITC-conjugated anti-CD3e (145-2C11), FITC-conjugated anti-mouse CC chemokine receptor 6 (CCR6) (140706; R&D Systems), peridinin chlorophyll protein complex–labeled anti-CD4 (L3T4; BD Biosciences), biotinylated anti-γ/δ TCR (UC7-13D5), and biotinylated anti-CD49b (DX5) mAb detected using streptavidin–allophycocyanin or streptavidin–Cy-Chrome (BD Biosciences). Cytokines were detected using FITC- or allophycocyanin-labeled anti–interferon-γ (IFNγ) (XMG1.2), phycoerythrin (PE)–labeled anti–IL-17 mAb (TC11-18H10; BD Biosciences), or an isotype control. When human synoviocytes were analyzed, the cells were stained using FITC-conjugated anti-human IL-17A (eBio64DEC17), allophycocyanin-conjugated anti-human IFNγ (4S. B3), Cy-Chrome–conjugated anti-human CD4 (PM-30158X; BD Biosciences), and PE-conjugated anti-human γ/δ TCR mAb (B1.1). Unless specified otherwise, all antibodies were purchased from eBioscience (San Diego, CA).

Flow cytometry analysis.

The absolute numbers of cytokine-producing cells were analyzed using a FACSCalibur flow cytometer (BD Biosciences). Lymphocytes were gated based on their forward and side scatter. The cytokine-positive subsets were determined by a comparison with isotype control staining. By applying cells from a whole joint, the absolute numbers of cytokine-positive cells in each joint were counted, and the data were analyzed using CellQuest software (BD Biosciences).

Sorting of γ/δ T cells.

To analyze the IL-17 production requirements, cells were collected from peripheral lymph nodes of naive DBA1/J mice or from the draining lymph nodes (DLNs) of the swollen joints of mice with CIA. Cells were stained with FITC-conjugated anti-mouse γ/δ TCR mAb (UC7-13D5) and anti-FITC microbeads, and then γ/δ T cells were prepared by positive selection using an MS column (Miltenyi Biotec, Bergisch Gladbach, Germany).

Cell culture in the presence of IL-23 or type II collagen.

Cells were prepared from the DLNs of swollen joints of mice with CIA. Then, 5 × 105 cells/well were cultured in 200 μl of RPMI 1640 complete medium in the presence or absence of 1 ng/ml of IL-23. For type II collagen, cells were cultured in the presence or absence of 15 μg/ml of type II collagen. After 7 days, the cells were stimulated with PMA and ionomycin for 4 hours. IL-17–producing cells were detected by intracellular cytokine staining. The ratios of the numbers of IL-17–producing cells in the presence of IL-23 or type II collagen to those in medium alone were calculated.

Analysis of the γ/δ TCR repertoire of IL-17–producing γ/δ T cells (CCR6+ γ/δ T cells).

Cells from the DLNs of swollen joints were stained with FITC-conjugated anti-mouse CCR6 mAb (140706; R&D Systems) and anti-FITC microbeads, and then CCR6+ cells were prepared by positive selection using an MS column (Miltenyi Biotec). The purity of CCR6+ cells among γ/δ T cells was >99%. RNA isolation, complementary DNA synthesis, and TCR repertoire analysis with polymerase chain reaction (PCR) were performed as described previously (19, 20) with the same PCR primer sets.

Adoptive transfer experiments with CCR6+ γ/δ T cells.

Cells from the DLNs of swollen joints of mice with CIA were prepared. To enrich CCR6+ γ/δ T cells, single-cell suspensions were depleted of CD4+, CD8a+, CD45R+, CD49b+, CD11b+, and Ter-119+ cells by negative selection with a biotin antibody cocktail and antibiotin microbeads of a CD4+ T Cell Isolation Kit, CD4+ microbeads, and an LS column (Miltenyi Biotec). The remaining γ/δ TCR–positive–enriched cells were stained with FITC-conjugated anti-mouse CCR6 mAb (140706; R&D Systems) and anti-FITC microbeads, and the CCR6+ γ/δ T cells were prepared by positive selection using an MS column (Miltenyi Biotec). Control naive CD4+ T cells were purified using the CD4+CD62L+ T Cell Isolation Kit II (Miltenyi Biotec) in accordance with the manufacturer's instructions. Using a Hamilton microsyringe (Osaka Chemical, Osaka, Japan), 60,000 CCR6+ γ/δ T cells or naive CD4+ T cells in 10 μl of PBS, or PBS alone, were injected around each wrist or ankle of naive mice or mice immunized with type II collagen plus CFA 2 weeks previously (n = 79). Arthritis in each joint was examined every 3–4 days according to the scoring system described previously (21).

Patients with RA.

Eleven female patients ages 37– 81 years (mean ± SD 59 ± 12 years) with a diagnosis of RA based on the 1987 criteria of the American College of Rheumatology (ACR; formerly, the American Rheumatism Association) (22) were included. The duration of RA ranged from 4 years to 33 years (mean ± SD 15 ± 9 years). Eight patients were receiving disease-modifying antirheumatic drugs (6 methotrexate, 2 bucillamine, and 2 sulfasalazopyridine) either as monotherapy or in combination, 9 patients were receiving prednisolone (2–10 mg/day), and 1 patient was being treated with an anti–tumor necrosis factor α biologic (etanercept). The 28-joint Disease Activity Score (23) ranged from 2.22 to 6.49 (mean ± SD 4.3 ± 1.6). The C-reactive protein level and the erythrocyte sedimentation rate ranged from 0 to 5.9 mg/dl (mean ± SD 1.9 ± 2.1) and from 9 to 83 mm/hour (mean ± SD 41.9 ± 22.5), respectively. According to the Steinbrocker criteria (24), 27% of the patients had stage III disease, and 73% had stage IV disease. According to the ACR 1991 revised criteria for the classification of global functional status in RA, 50% of the patients had stage II disease, and 50% had stage III disease (25).

Statistical analysis.

All statistical analyses were performed using the Mann-Whitney U test with Microsoft Excel software (Microsoft, Redmond, WA) and Statcel2 add-in software (Hisae Yanai, Department of Mathematics, Faculty of Science, Saitama University, Japan). P values less than 0.05 were considered significant.

RESULTS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

Predominance of IL-17–producing γ/δ T cells in swollen joints of mice with CIA.

In the present study, it was first confirmed that cells from the joints were not contaminated by bone marrow cells. The number of CD19+ cells in the joints was negligible (Figure 1B). Next, IL-17–producing γ/δ T cells in the swollen joints of mice were analyzed at the peak of CIA. Interestingly, the percentage of IL-17–producing γ/δ T cells was 4.4-fold higher than that of Th17 cells (Figure 1C). Almost all of the IL-17–producing cells in swollen joints were either γ/δ T cells or CD4+ T cells, and neither CD8+ cells nor DX5+ NK cells produced IL-17 (Figures 1C and D).

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Figure 1. Predominance of interleukin-17 (IL-17)–producing γ/δ T cells in the swollen joints of mice with collagen-induced arthritis (CIA). A, Schematic of the analyzed joints and draining lymph nodes (DLNs) in mice with CIA. B, Comparison of the absolute counts of γ/δ T cells and CD19+ cells in the joints and remaining tissues of naive mice, as detected by fluorescence-activated cell sorting analysis. Values are the mean and SEM results from 3 different mice. C and D, Analysis of IL-17–producing γ/δ T cells in the swollen joints of mice. Cells were obtained from the swollen joints of mice with CIA at the peak of arthritis and stained with antibodies against CD3, CD4, CD8, DX5, and γ/δ T cell receptor (TCR). IL-17–producing cells were detected by intracellular cytokine staining. Live lymphocytes were gated based on their forward and side scatter. The percentage of cells in each region or quadrant is noted. One of 5 experiments with similar results is shown. In C, IL-17–producing cells were gated and plotted by their expression of CD4 and γ/δ TCR. Non-swo = nonswollen.

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Distribution and kinetics of IL-17–producing γ/δ and CD4+ T cells in CIA.

To analyze the distribution and kinetics of IL-17–producing γ/δ T cells and Th17 cells in mice with CIA, intracellular cytokine staining was performed using cells obtained from the joints of mice with CIA during 6 distinct phases, as follows: before immunization (naive mice), 1 day after immunization (day 1), before onset of arthritis (day 10), onset of arthritis (day 32), peak of arthritis (day 42), and ankylosing phase of arthritis (day 70). At each phase, cells were collected from the swollen joint, an immunized joint, a nonswollen joint, the DLNs of each joint, and the spleen (Figure 1A).

In swollen joints, the absolute numbers of IL-17–producing γ/δ T cells were higher than the absolute numbers of Th17 cells, with the maximal counts obtained at the peak of arthritis (Figures 2A and B). Surprisingly, neither IFNγ-producing CD4+ (Th1) cells nor IFNγ-producing γ/δ T cells were detected in the swollen joints at any of the time points analyzed. In contrast, Th1 cells were detected in the DLNs of swollen joints (Figure 2A). In immunized joints, IL-17–producing γ/δ T cells and Th17 cells were already observed on day 1, reached the first peak on day 10 after immunization, and then reached their highest counts at the peak of arthritis. The absolute numbers of IL-17–producing γ/δ T cells were consistently higher than the numbers of Th17 cells at most time points analyzed. In contrast to what was observed in swollen joints, Th1 cells were detected in immunized joints after immunization (Figures 2A and B). In both swollen and immunized joints, the percentages of IL-17–producing γ/δ T cells among IL-17–producing cells were higher than those in DLNs of swollen and immunized joints (Figure 2A). In nonswollen joints, both IL-17–producing T cells and IFNγ-producing T cells were rarely observed. In addition, IFNγ-producing γ/δ T cells were a minor population at the sites of CIA (Figure 2A).

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Figure 2. Distribution and kinetics of IL-17–producing γ/δ and CD4+ T cells in CIA. A, Cells were obtained from the joints, their DLNs, and the spleens of mice with CIA at the peak of arthritis. Viable lymphocytes were gated based on their forward and side scatter. Using intracellular cytokine staining, IL-17–producing cells and interferon-γ (IFNγ)–producing cells were detected (top row). IL-17–producing cells (second row) or IFNγ-producing cells (bottom row) were gated and plotted by their expression of γ/δ TCR and CD4. In the panels showing analysis of joints, the absolute number of IL-17–producing cells and the percentage of CD4+ cells and γ/δ TCR+ cells among them are indicated. In the panels showing analysis of DLNs and spleen, the percentage of cells in each quadrant is noted. One of 3 experiments with similar results is shown. B, Cells were recovered from the swollen joints, immunized joints, and nonswollen joints of mice with CIA during the 6 distinct phases of arthritis described in Materials and Methods. IL-17–producing cells and IFNγ+ cells were detected by intracellular cytokine staining, and their absolute numbers were calculated using fluorescence-activated cell sorting analysis. Values for each phase represent the mean from at least 3 different mice. In B, only 3 phases after the onset of arthritis are applicable for the DLNs of swollen joints. See Figure 1 for other definitions.

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Efficient stimulation of IL-17 production from γ/δ T cells by IL-1β and IL-23.

A recent study showed that a subset of γ/δ T cells already differentiate to acquire an IL-17–producing function in the thymus (26). In other studies, specific expression of CCR6 on Th17 has been suggested (27–30). Therefore, the expression of CCR6 on IL-17–producing γ/δ T cells in the thymus of naive DBA1/J mice was evaluated. IL-17–producing, but not IFNγ-producing, γ/δ T cells preferentially expressed CCR6 (Figure 3A). A small number of γ/δ T cells are present in the normal joints of mice (18). To elucidate whether de novo CCR6+ IL-17–producing γ/δ T cells are present in the normal joints of naive DBA1/J mice, cells were collected from the normal joints of naive mice, and intracellular cytokine staining was performed. By analyzing cells from 2 normal paws and ankles at a time, CCR6+ IL-17–producing γ/δ T cells could be detected (Figure 3B). In addition, in mice with CIA, 92% of CCR6+ γ/δ T cells produced IL-17 (Figure 3C).

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Figure 3. Efficient stimulation of IL-17 production from γ/δ T cells by IL-1β and IL-23. A, Thymocytes from naive mice were stimulated with phorbol myristate acetate and ionomycin for 4 hours. TCR+ cells were gated, and CCR6+ cells among IL-17–producing or interferon-γ (IFNγ)–producing γ/δ T cells were detected. B, Cells were collected from the paws and ankles of naive mice and stained for γ/δ TCR and CCR6. Gamma/delta TCR+ cells were gated, and CCR6+ IL-17–producing cells were detected by intracellular cytokine staining. In A and B, the percentages of cells in each quadrant are shown. C, Cells were collected from the DLNs of swollen joints, and IL-17–producing cells were detected by intracellular cytokine staining. CCR6+ cells were gated, and IL-17–producing cells were analyzed. The percentage of IL-17–producing cells among CCR+ γ/δ T cells is shown. D, Gamma/delta T cells were sorted from the peripheral lymph nodes of naive DBA1/J mice (upper panel) or from the DLNs of swollen joints of mice with CIA at the peak of arthritis (lower panel) and stimulated with cytokines, activating anti-γ/δ TCR antibodies, and anti-CD28 antibodies for 24 hours. The percentages of IL-17–producing cells among γ/δ T cells were determined by intracellular cytokine staining. Bars show the mean and SEM results from 3 different mice. TGFβ = transforming growth factor β (see Figure 1 for other definitions).

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Next, the IL-17 production requirements for γ/δ T cells were analyzed. Gamma/delta T cells from naive DBA1/J mice were analyzed by stimulation with cytokines in the presence or absence of anti-γ/δ TCR–activating mAb (Figure 3D). IL-17–producing γ/δ T cells were detected with anti-γ/δ TCR mAb, IL-23, and IL-1β alone. In addition, additive stimulatory effects were observed when anti-γ/δ TCR mAb was combined with IL-23, IL-1β, or anti-CD28. Surprisingly, IL-23 plus IL-1β induced IL-17 production quite efficiently. These observations indicated that TCR signaling was not necessary to stimulate IL-17 production by γ/δ T cells. Furthermore, a combination of IL-23 and IL-1β was a much more potent stimulator than was TCR signaling. Similar results were obtained with γ/δ T cells sorted from DLNs of swollen joints at the peak of CIA (Figure 3D, lower panel).

Type II collagen–independent induction and maintenance of IL-17–producing γ/δ T cells.

Because IL-23 plays important roles in the maintenance of Th17 cells (31–36), we next addressed the maintaining effect of IL-23 or type II collagen on IL-17–producing γ/δ T cells. To this end, cells from the DLNs of swollen joints were cultured with IL-23, type II collagen, or medium alone (Figure 4A). Both IL-17–producing γ/δ T cells and Th17 cells were maintained in the presence of IL-23. In contrast, IL-17–producing γ/δ T cells were not type II collagen dependently maintained, whereas Th17 cells showed type II collagen dependency. To further investigate the factors that enhanced the accumulation of IL-17–producing γ/δ T cells in inflamed joints, the numbers of IL-17–producing γ/δ T cells in the differently immunized joints of mice were counted on day 10. Mice were immunized with PBS, IFA plus solution (0.05 mM acetic acid), IFA plus type II collagen, or CFA plus type II collagen (Figure 4B). The numbers of IL-17–producing γ/δ T cells were not significantly different between mice immunized with IFA plus solution, IFA plus type II collagen, or CFA plus type II collagen. In contrast, the numbers of IL-17–producing γ/δ T cells were significantly smaller in mice immunized with PBS compared with the 3 other treatments. The numbers of Th17 cells were significantly higher in mice immunized with IFA plus type II collagen than those in mice treated with IFA plus solution. These data indicate that IL-17–producing γ/δ T cells do not specifically respond to type II collagen and may only respond to adjuvant (IFA plus solution) or adjuvant-induced IL-23.

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Figure 4. Exacerbation of arthritis by IL-17–producing γ/δ T cells. A, Cells were prepared from the DLNs of swollen joints and cultured for 7 days in the presence of IL-23, type II collagen (CII), or medium alone. IL-17–producing cells were detected by fluorescence-activated cell sorting (FACS) analysis. The ratio of the numbers of IL-17–producing cells in the presence of IL-23 or type II collagen to those in medium alone was calculated. Bars show the mean and SEM results from at least 3 different experiments. B, Various combinations of substances were administered into the footpads of DBA1/J mice. Ten days later, the absolute numbers of IL-17–producing cells were counted using FACS analysis. Bars show the mean and SEM results from at least 3 different mice. C, The use of γ/δ TCR by CCR6+ γ/δ T cells was analyzed by reverse transcription–polymerase chain reaction. D, CCR6+ γ/δ T cells from the DLNs of swollen joints were enriched. CCR6+ γ/δ T cells or phosphate buffered saline (PBS) alone was injected into nonimmunized wrists or ankles of mice that had been immunized with type II collagen plus Freund's complete adjuvant (CFA) 2 weeks previously. For naive mice, CCR6+ γ/δ T cells or PBS alone was injected. Values are the mean ± SEM arthritis scores in affected joints. ∗ = P < 0.05 versus PBS. NS = not significant; IFA = Freund's incomplete adjuvant; M = marker (see Figure 1 for other definitions).

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Next, the γ/δ TCR repertoire was analyzed (Figure 4C). The Vγ repertoire of IL-17–producing γ/δ T cells was composed of Vγ1, Vγ2, Vγ4, and Vγ6 rather than a single Vγ chain in CIA. In addition, the Vδ repertoire of IL-17–producing γ/δ T cells was composed of Vδ1 and Vδ5.

Exacerbation of arthritis by IL-17–producing γ/δ T cells.

Next, the pathogenic roles of IL-17–producing γ/δ T cells in CIA were analyzed. When transferred to the joints of naive mice, CCR6+ γ/δ T cells did not induce arthritis. However, when transferred to the joints of mice immunized with type II collagen plus CFA, CCR6+ γ/δ T cells significantly worsened the arthritis score of joints with arthritis compared with the scores of joints treated with PBS (Figure 4D). The arthritis-exacerbating effect of CCR6+ γ/δ T cells from swollen joints was equivalent to that of CCR6+ γ/δ T cells from the DLNs of swollen joints (data not shown).

Absence of IL-17–producing γ/δ T cells in swollen joints of SKG mice or affected joints of patients with RA.

To elucidate the pathologic differences from other arthritis models, the same analysis was performed using SKG mice (21). SKG mice carry a point mutation of the gene encoding ZAP-70, and homozygous mice show IL-17–dependent arthritis resembling RA. Although the present study could detect only a few IL-17–producing γ/δ T cells in the DLNs of swollen joints, surprisingly, almost all of the IL-17–producing cells were Th17 cells, and the number of IL-17–producing γ/δ T cells was negligible in the swollen joints of SKG mice (Figure 5A).

SKG is a BALB/c background strain, and autoimmune arthritis in SKG mice is induced using zymosan as an adjuvant (17, 21). To exclude the possibility that IL-17–producing γ/δ T cells are absent in the joints of SKG mice with arthritis because of the differences in strain and adjuvant compared with CIA, the absolute numbers of cell subsets from the joints of SKG or BALB/c mice immunized with CFA plus type II collagen were counted. Even with this protocol, IL-17–producing γ/δ T cells were not detected in SKG mice, whereas IL-17–producing γ/δ T cells were more abundant than Th17 cells in BALB/c mice (Figure 5B).

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Figure 5. Absence of IL-17–producing γ/δ T cells in the swollen joints of SKG mice. A, Cells were collected from the ankles with maximum arthritis (and their DLNs) of SKG mice that had been treated with zymosan 7 weeks previously. Lymphocytes were gated based on their forward and side scatter. IL-17–producing cells and interferon-γ (IFNγ)–producing cells were detected by intracellular cytokine staining (left column). IL-17–producing IFNγ-negative cells (middle column) or IFNγ-producing IL-17–negative cells (right column) were gated and their expression of γ/δ TCR and CD4 was plotted. In the panels showing analysis of joints, the absolute numbers and percentages of CD4+ cells and γ/δ TCR+ cells are indicated. In the panels showing analysis of DLNs, the percentage of cells in each quadrant is noted. One experiment representative of 3 that were performed is shown. B, SKG or BALB/c mice were immunized with Freund's complete adjuvant plus type II collagen, and cells from the immunized joints were collected 10 days later. The absolute numbers of cells were counted using fluorescence-activated cell sorting analysis. Bars show the mean and SEM results for 3 different mice. See Figure 1 for other definitions.

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Finally, cells in RA synovial tissue or fluid were analyzed to determine the presence of IL-17–producing γ/δ T cells and Th17 cells at the effector sites of arthritis. In contrast to what was observed in CIA, IL-17–producing γ/δ T cells could not be detected in the synovial tissue of affected joints, whereas IFNγ-producing γ/δ T cells were present in synovial tissue (Figure 6). Among the CD4+ T cells in synovial tissue, IL-17–producing cells were present. However, the proportions of Th1 cells among CD4+ T cells were much larger than those of Th17 cells in affected joints. Similar results were obtained in cells from synovial fluid.

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Figure 6. Absence of interleukin-17 (IL-17)–producing γ/δ T cells in the affected joints of patients with rheumatoid arthritis (RA). Cells in RA synovial tissue (n = 4) or synovial fluid (n = 7) were stained with antibodies against CD4 and γ/δ T cell receptor (TCR). IL-17–producing and interferon-γ (IFNγ)–producing cells were analyzed. The percentages of cells among total γ/δ T cells plus CD4+ T cells were determined. Bars show the mean and SEM.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

The present study first focused on IL-17–producing T cells in the swollen joints of mice with CIA. It was observed that γ/δ T cells were the predominant source of IL-17 and were more abundant than Th17 cells. DX5+ NK cells did not secrete IL-17 in swollen joints. A direct comparison of the absolute numbers of IL-17–producing γ/δ T cells with the absolute numbers of Th17 cells simultaneously in each joint of mice with CIA was performed for the first time. Although it is known that γ/δ T cells are not necessary for the induction of CIA, because γ/δ TCR–deficient mice can mount CIA (37), the present findings in the kinetics study and adoptive transfer experiments, together with previous reports (16, 18, 38), suggest that not only Th17 cells but also IL-17–producing γ/δ T cells contribute to the exacerbation of CIA. In contrast, α/β τ cells, especially Th17 cells, are essential for the induction of CIA, because α/β TCR–deficient mice cannot mount CIA (37). In addition, IL-17–producing invariant NK T cells in CIA have been reported recently (37), but these cells were not analyzed in the current study.

The origin and functions of IL-17–producing γ/δ T cells in physiologic and pathologic conditions have been elucidated recently. It was reported that a subset of γ/δ T cells acquired an IL-17–producing function in the thymus (26) and produced cytokines immediately in response to initial stimulation. In various murine infectious disease models, these γ/δ T cells predominantly produce IL-17 and eradicate pathogens (40–43). However, the precise requirements of IL-17 production by γ/δ T cells especially in CIA are unknown, although IL-23 was known as a sufficient stimulant of IL-17 production by γ/δ T cells in naive mice (42). Here, it was demonstrated that the combination of IL-23 and IL-1β synergistically stimulated IL-17 production, but stimulation via γ/δ TCR had a limited effect. Given the enhanced expression of IL-1β and IL-23 in the inflamed joints of mice with CIA (44, 45), these findings suggest that IL-17 production by γ/δ T cells in CIA might mainly be an inflammatory cytokine–driven process rather than a TCR signal–driven process.

The present study showed that IL-17–producing γ/δ T cells were CCR6 positive, and CCR6 was already expressed on IL-17–producing γ/δ T cells in the thymus of naive mice. CCL20, the only chemokine known to interact with CCR6, is physiologically expressed at epithelial surfaces (46) and fibroblast-like synoviocytes (29) and is up-regulated in inflammatory conditions (30, 46). These findings suggest that CCR6 might have some roles in determining the physiologic distribution of IL-17–producing γ/δ T cells. In fact, it was found that a small number of CCR6+ IL-17–producing γ/δ T cells were present in the joints of naive mice.

Next, we focused on the differences between IL-17–producing γ/δ T cells and Th17 cells. IL-17–producing γ/δ T cells were maintained by IL-23 but not by a specific antigen (type II collagen, in this case). In contrast, Th17 cells responded to type II collagen and IL-23. Furthermore, IL-17–producing γ/δ T cells were induced equivalently in response to stimulation by IFA plus solution in the absence of type II collagen. Together with results from the previous study demonstrating that IL-17–producing γ/δ T cells are induced equally by CFA plus type II collagen and CFA (16), the present data suggest that IL-17–producing γ/δ T cells do not recognize the specific antigen (type II collagen) but rather proliferate in response to IL-23, which may be produced locally by synovial cells (44). The ligands of γ/δ T cells are largely unknown, and further analysis of possible antigens of IL-17–producing γ/δ T cells in CIA could be difficult (47). However, the present study confirmed the diverse usage of γ/δ TCR in IL-17–producing γ/δ T cells in CIA (Figure 4C), which supported the present conclusion that IL-17–producing γ/δ T cells are antigen independently induced by inflammatory cytokines.

In summary, it is speculated that the sequence of pathology of CIA is as follows. First, type II collagen–specific Th17 cells are induced by type II collagen plus CFA, which then infiltrate into the joints and cause primary inflammation. Although antigen-independent IL-17–producing γ/δ T cells could be induced simultaneously by CFA, they are not essential for the induction of arthritis. Next, primary inflammation induces local production of IL-23 from synoviocytes and increases the expression of IL-1β in joint cartilage and pannus (45). Locally produced IL-23 induces the proliferation of resident IL-17–producing γ/δ T cells. These γ/δ T cells, stimulated by IL-1β and IL-23, produce enhanced amounts of IL-17 and exacerbate the arthritis of CIA. Another, but not mutually exclusive, possibility is that primary inflammation enhances CCL20 expression in vascular endothelial cells and fibroblast-like synoviocytes (30) in inflamed joints and recruits CCR6+ IL-17–producing cells. In the ankylosing phase, the burned-out tissue does not produce inflammatory cytokines, and the activities and the number of IL-17–producing γ/δ T cells decrease to the basal level.

Finally, the cytokine profiles of T cells in the inflamed joints of SKG mice and patients with RA were compared with those in mice with CIA. In contrast to what was observed in mice with CIA, IL-17–producing γ/δ T cells were not detected in the swollen joints of SKG mice. A lack of IL-17–producing γ/δ T cells in SKG mice was not caused by the differences in strain or adjuvant. It was also observed that IL-17–producing γ/δ T cells are hardly induced in immunized joints, their DLNs, non-DLNs, and spleens of SKG mice (data not shown) 10 days after immunization with CFA plus type II collagen. Given that TCR signals in SKG mice are attenuated because of a point mutation in ZAP-70 (21), and differentiation of γ/δ T cells needs a strong signal via the TCR (48, 49), there may be some defects in γ/δ T cell differentiation in SKG mice. This speculation was supported by data showing impaired development of specific subsets of γ/δ T cells in ZAP-70–knockout mice (50). Furthermore, IL-17 production from γ/δ T cells in the synovial tissue of patients with RA has not yet been detected. In contrast to IL-17–producing γ/δ T cells, IFNγ-producing γ/δ T cells were present. In addition, among CD4+ T cells, Th1 cells were predominant; this finding was consistent with a previous report (51).

These results suggest that IFNγ-producing cells, but not IL-17–producing cells including γ/δ T cells, play predominant pathogenic roles in RA. These distinct pathogenic cell populations may result from differences between CIA and RA such as species and age-related susceptibility. Alternatively, IL-17–producing γ/δ T cells may play an important role in RA as well but are suppressed by the effects of medical treatment. It should be noted that in the present study, we could access joint materials only from patients with progressed stages of RA. Therefore, further studies with patients with recent-onset RA who have not received medical treatment are necessary to determine whether IL-17–producing γ/δ T cells are present.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. Usui had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design. Y. Ito, Usui.

Acquisition of data. Y. Ito, Usui, Kobayashi, Iguchi-Hashimoto, H. Ito, Yoshitomi, Nakamura, Shimizu, Kawabata, Yukawa, Hashimoto, N. Sakaguchi, S. Sakaguchi, Yoshifuji, Nojima, Ohmura, Fujii, Mimori.

Analysis and interpretation of data. Y. Ito, Usui.

Acknowledgements

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES

We thank all personnel at Dr. Mimori's laboratory for helpful discussions.

REFERENCES

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. Acknowledgements
  8. REFERENCES