BATF regulates collagen-induced arthritis by regulating T helper cell differentiation
We recently demonstrated that BATF, a member of the activator protein-1 (AP-1) family, regulates osteoarthritic cartilage destruction. Here, we explored the roles and regulatory mechanisms of BATF in collagen-induced arthritis (CIA) in mice.
CIA and K/BxN serum transfer were used to generate inflammatory arthritis models in wild-type (WT) and Batf−/− mice. RA manifestations were determined by examining CIA incidence, clinical score, synovitis, synovial hyperplasia, angiogenesis in inflamed synovium, pannus formation, bone erosion, and cartilage destruction. Immune features in RA were analyzed by examining immune cell populations and cytokine production.
BATF was upregulated in the synovial tissues of joints in which inflammatory arthritis had been caused by CIA or K/BxN serum transfer. The increases in CIA incidence, clinical score, and autoantibody production in CIA-induced WT mice were completely abrogated in the corresponding Batf−/− DBA/1 J mice. Genetic ablation of Batf also inhibited CIA-induced synovitis, synovial hyperplasia, angiogenesis in synovial tissues, pannus formation, bone erosion, and cartilage destruction. Batf knockout inhibited the differentiation of T helper (Th)17 cells and the conversion of CD4+Foxp3+ cells to CD4+IL-17+ cells. However, BATF did not modulate the functions of fibroblast-like synoviocytes (FLS), including the expressions of chemokines, matrix-degrading enzymes, vascular endothelial growth factor, and receptor activator of NF-κB ligand (RANKL).
Our findings indicate that BATF crucially mediates CIA by regulating Th cell differentiation without directly affecting the functions of FLS.
KeywordsBATF (basic leucine zipper transcription factor ATF-like) Th cells Collagen-induced arthritis Fibroblast-like synoviocytes (FLS) Cartilage Bone
Basic leucine zipper transcription factor, ATF-like
Enzyme-linked immunosorbent assay
Hematoxylin and eosin
Receptor activator of NF-κB ligand
T cell receptor
Transforming growth factor
Tumor necrosis factor
Tartase-resistant acid phosphatase
Vascular endothelial growth factor
BATF (basic leucine zipper transcription factor, ATF-like) is a member of the activator protein-1 (AP-1) family whose members regulate various biological functions [1, 2, 3]. BATF, which lacks a transactivation domain, heterodimerizes with JUN to bind the AP-1 site for transcriptional regulation [3, 4]. We recently demonstrated that BATF regulates osteoarthritis (OA) in mice by modulating anabolic and catabolic gene expression in chondrocytes . We found that overexpression of BATF upregulated matrix-degrading enzymes and downregulated cartilage matrix molecules; we also found that BATF expression in mouse joint tissues promoted OA cartilage destruction and that, conversely, knockout of Batf in mice (Batf−/−) suppressed experimental OA .
OA and rheumatoid arthritis (RA), which are the most common types of joint arthritis, share certain phenotypic features, such as cartilage destruction . However, these diseases clearly differ in their etiologies, pathogenic mechanisms, and the cell types associated with each pathogenesis. OA is a degenerative joint disease that begins with the destruction of surface articular cartilage [6, 7]. Mechanical stresses (i.e., joint instability) and factors that predispose toward OA (i.e., aging) are important causes of OA pathogenesis [6, 7]. In contrast, RA is an inflammatory autoimmune disease that mainly targets the synovium, resulting in destruction of the joint architecture. Various cell types of joint tissues are associated with RA pathogenesis, including T cells, B cells, macrophages, synoviocytes, chondrocytes, and osteoclasts [8, 9, 10]. T cell-mediated autoimmune responses play a critical role in the RA pathogenesis, in which interleukin (IL)-17-producing T helper (Th) cells act as crucial effectors [8, 11, 12]. RA is characterized by synovitis with infiltration of immune cells, synovial hyperplasia that arises via proliferation of synovial cells, such as macrophage-like synoviocytes (MLS) and fibroblast-like synoviocytes (FLS), and angiogenesis in the hyperplastic synovium [8, 9, 10]. Synovial cells express numerous cytokines that have been implicated in many of the immune processes involved in RA . RA manifestations also include erosion of the bone and cartilage which is caused by the formation of the pannus, which is an aggressive front of hyperplastic synovium. The pannus invades and destroys mineralized cartilage and bone through the action of osteoclasts [8, 9, 10].
BATF is known to regulate OA cartilage destruction, and we previously showed that BATF overexpression in joint tissues causes synovial inflammation  suggesting that BATF could contribute to inflammatory arthritis. This notion is supported by reports that proinflammatory cytokines, such as IL-1β and IL-6, increase BATF expression in naive CD4+ T cells [14, 15], BATF directly regulates IL-17 expression, and Batf-deficient mice show resistance to experimental autoimmune encephalomyelitis . BATF also controls the development of follicular Th cells (Tfh) and class-switch recombination in B cells . Additionally, inhibition of the transcriptional activity of c-Fos/AP-1 suppresses arthritic joint destruction in a mouse RA model . However, while these previous reports suggest that BATF may be involved in RA pathogenesis, the role of BATF and its regulatory mechanisms are not yet well understood.
In this study, we examined whether BATF is required for collagen-induced arthritis (CIA), which is a commonly used experimental model of inflammatory arthritis caused by a T cell-dependent, antibody-mediated autoimmune response directed against cartilage type II collagen . Here, we show that genetic ablation of Batf in mice suppresses the manifestations of CIA, including synovitis, synovial hyperplasia, angiogenesis in the inflamed synovium, and cartilage/bone erosion in the joint tissues. We also reveal that BATF regulates CIA by regulating Th cell differentiation without directly affecting the functions of FLS.
Mice and experimental RA
Male wild-type (WT) and Batf−/− DBA/1 J mice were used to generate the CIA models. C57BL/6-background Batf−/− mice  were backcrossed with DBA/1 J mice to generate Batf−/− DBA/1 J mice. All mice were used in accordance with protocols approved by the Animal Care and Ethics Committees of the Gwangju Institute of Science and Technology. CIA was induced by a standard protocol [19, 20]. Mice were intradermally injected with incomplete Freund’s adjuvant alone (nonimmunized; NI) or Freund’s adjuvant containing 100 μg collagen type II (CIA). A booster injection was given 21 days later. The incidence and severity of arthritis were evaluated on the indicated days after the first immunization. Severity was evaluated using a clinical score (grade 0−4) of paw swelling [19, 20]. Joint tissues were fixed, decalcified with 0.5 M EDTA, embedded in paraffin, and sectioned at 5-μm thickness. Synovitis was evaluated by hematoxylin and eosin (H&E) staining, and synovial inflammation (grade 0−4) was scored as previously described [19, 20]. The pannus was visualized by H&E staining and quantified by scoring (grade 0−4) [19, 20]. Cartilage destruction was examined by safranin-O staining and scored using the OARSI (Osteoarthritis Research Society International) grading system [4, 20]. Inflammatory arthritis was also induced by K/BxN serum transfer  in WT and Batf−/− C57BL/6 mice. Arthritic transgenic mice (K/BxN) and nontransgenic littermates (BxN) were generated by crossing KRN T cell receptor (TCR)-transgenic (K/B) mice with nonobese diabetic (NOD) mice. K/BxN and control sera were collected from K/BxN and BxN mice, respectively, and administered intraperitoneally to recipient mice on days 0 and 2. Mice were sacrificed on day 14 after serum transfer.
Immunohistochemistry, immunofluorescence microscopy, and tartrate-resistant acid phosphatase (TRAP) staining
Antigens were retrieved by incubating joint sections at 60 °C overnight with sodium citrate buffer (10 mM sodium citrate, 0.05% Tween 20, pH 6.0). The sections were blocked with 2% bovine serum albumin in phosphate-buffered saline (PBS), and then incubated with primary antibodies, including rabbit anti-BATF (Brookwood Biomedical), rabbit anti-RANKL (receptor activator of NF-κB ligand) (Abcam), goat anti-IL-6 (R&D Systems), rabbit anti-TNF-α (tumor necrosis factor alpha) (Novus Biologicals), and rabbit anti-Ki67 (Abcam). The Dako REAL Envision Detection system was used for chromogenic color development. BATF-expressing cells in synovial tissues were identified by double immunofluorescence labeling of vimentin for FLS, CD11b for macrophages, CD4 for T cells, and B220 for B cells. The following primary antibodies were used: mouse anti-CD4, mouse anti-B220, rat anti-CD11b (Abcam), rabbit anti-BATF (ThermoFisher Scientific), and mouse anti-vimentin (BD Pharmingen). Blood vessels in synovial tissues were detected with mouse anti-CD31 (Dianova). TRAP activity was determined in joint sections as previously described [20, 22], and the numbers of TRAP-positive osteoclasts were counted in regions containing pannus-cartilage and pannus-bone interfaces.
FLS culture and proliferation assays
FLS were isolated from WT or Batf knockout (KO) mice and cultured as described by Zhao et al. . FLS of passages 4–8 were used for further analysis. Pure FLS (> 90% CD90+/< 1% CD14+) were identified by flow cytometry using antibodies against CD90 and CD14 (Abcam). FLS proliferation in culture was quantified by measuring bromodeoxyuridine (BrdU) incorporation . Briefly, FLS cultured in a 96-well plate were treated with or without tumor necrosis factor (TNF)-α (100 ng/ml), and BrdU labeling was detected using the cell proliferation enzyme-linked immunosorbent assay (ELISA) BrdU kit (Roche). Proliferating cells in synovial sections were identified by detecting Ki67 using an antibody obtained from Abcam. The empty adenovirus (Ad-C) and BATF-expressing adenovirus (Ad-Batf) were as previously described . FLS were infected with Ad-Batf and Ad-C at the indicated multiplicities of infection (MOI) for 2 h, washed, and maintained for 48 h before analysis.
ELISA of autoantibody production
Collagen type II-specific antibodies were measured by ELISA . Sera from NI and CIA mice were loaded to type II collagen-coated 96-well plates, incubated overnight at 4 °C, washed, and incubated for 1 h with alkaline phosphatase-labeled monoclonal antibodies against mouse IgG1, IgG2a, or IgG2b (Immunology Consultants Lab). p-Nitrophenyl phosphate was used as a substrate for chromogenic reactions, and the resulting color reaction was quantified using an ELISA plate reader.
RT-PCR, qRT-PCR, and Western blotting
Total RNA was isolated from cultured FLS using the TRI reagent. The isolated RNA was reverse-transcribed, and the resulting cDNA was used for RT-PCR. The PCR primers and experimental conditions were as previously described for BATF, matrix metalloproteinase (MMP)3, and MMP13 , as well as CCL2, CCL5, CXCL1, CXCL5, CXCL10, GAPDH, RANKL, and vascular endothelial growth factor (VEGF) . For Western blotting, FLS cells were incubated on ice for 30 min with radioimmune precipitation assay buffer (10 mM sodium phosphate, pH 7.2, 150 mM NaCl, 1% SDS, 1% deoxycholate, 1% Nonidet P-40). Whole-cell lysates were fractionated by polyacrylamide gel electrophoresis and immunoblotted using rabbit anti-BATF (Brookwood Biomedical) and goat anti-Lamin B (Santa Cruz).
Flow cytometric analysis
Thymocytes, splenocytes, and lymphocytes were isolated from 6- to 8-week-old WT and Batf−/− mice as previously described . For detection of cell surface antigens, cells (1 × 106) were labeled with fluorochrome-conjugated primary antibodies. For detection of intracellular antigens, surface-stained cells were fixed and permeabilized with a permeabilization buffer (eBioscience) or Foxp3/Transcription Factor Staining Buffer (eBioscience). The following antibodies were purchased from eBioscience for cell staining: Alexa Fluor 488®-conjugated anti-mouse CD4 and anti-mouse/rat Foxp3; FITC-conjugated anti-mouse TCRβ and anti-mouse CD8; PerCP Cy5.5-conjugated anti-mouse CD25, anti-mouse CD62L, and anti-mouse interferon (IFN)-γ; APC-conjugated anti-mouse CD4, anti-mouse B220, and anti-mouse IL-17A; PE-conjugated anti-human/mouse CD44, anti-mouse IL-4, anti-mouse CD25, and anti-mouse/rat Foxp3; and eFluor 450®-conjugated anti-mouse CD4, and PE-Cyanine7-conjugated anti-mouse CD4.
In-vitro differentiation of Th cells
CD4+ T cell were isolated from the spleens of WT and Batf−/− mice using an EasySep™ Mouse CD4+ T cell Isolation Kit (Stem Cell). The cells (2.5 × 105) were cultured for 120 h under Th cell-differentiating conditions . Briefly, for in-vitro differentiation of CD4+ T cells into Th1 cells, isolated cells were cultured with anti-mouse CD3 (5 μg/ml), anti-mouse CD28 (5 μg/ml), IL-12 (20 ng/ml), IL-2 (20 ng/ml), and anti-IL-4 (10 μg/ml). For in-vitro differentiation of CD4+ T cells into Th2 cells, isolated cells were cultured with anti-mouse CD3 (5 μg/ml), anti-mouse CD28 (5 μg/ml), IL-4 (25 ng/ml), IL-2 (20 ng/ml), anti-IFN-γ (10 μg/ml), and anti-IL-12R (10 μg/ml). For in-vitro differentiation of CD4+ T cells into Th17 cells, isolated cells were cultured with anti-mouse CD3 (5 μg/ml), anti-mouse CD28 (5 μg/ml), IL-6 (100 ng/ml), mouse-transforming growth factor (TGF)-β (5 ng/ml), anti-IFN-γ (10 μg/ml), and anti-IL-4 (10 μg/ml). For in-vitro differentiation of CD4+ T cells into Treg cells, isolated cells were cultured with anti-mouse CD3 (5 μg/ml), anti-mouse CD28 (5 μg/ml), mouse-TGF-β (5 ng/ml), and IL-2 (20 ng/ml). After the Th cell differentiation, cytokine production was analyzed by flow cytometry analysis after activation with phorbol-12-myristate 13-acetate (PMA; 50 ng/ml), ionomycin (500 ng/ml), and Brefeldin A solution (eBioscience) for an additional 4 h. To test the plasticity of Treg cells to Th17 cells in vitro, isolated cells were cultured with anti-mouse CD3 (5 μg/ml), anti-mouse CD28 (5 μg/ml), mouse-TGF-β (5 ng/ml), and IL-2 (20 ng/ml) for 2 days. Then, the cells were washed and further incubated with anti-mouse CD3 (5 μg/ml), anti-mouse CD28 (5 μg/ml), IL-6 (100 ng/ml), mouse-TGF-β (5 ng/ml), anti-IFN-γ (10 μg/ml), and anti-IL-4 (10 μg/ml) for 3 days.
For measurement of secreted cytokine levels, lymphocytes were isolated from the lymph nodes of CIA mice. Isolated lymphocytes (1 × 106) were cultured for 4 h with PMA (50 ng/ml) and ionomycin (500 ng/ml) . The LEGEND MAX mouse IL-6 ELISA kit and mouse IL-13 Platinum ELISA kit (Invitrogen) were used to detect IL-6 and IL-10, respectively. The BD Cytometric Bead Array solution (BD Biosciences) and a FACS Canto II flow cytometer (BD Biosciences) were used to measure secreted cytokines such as IL-2, IL-4, IL-10, IL-17A, IFN-γ, and TNF-α.
The nonparametric Mann-Whitney U test was used for the analysis of data based on an ordinal grading system, such as the synovitis, pannus, and OARSI grades. For results obtained from ELISA and analyses of joint thickness, TRAP-positive cells, and BrdU incorporation, the data were first tested for conformation to a normal distribution using the Shapiro-Wilk test. The data were analyzed by the Student’s t test (pair-wise comparisons) or analysis of variance (ANOVA) with post-hoc tests (multi-comparisons) as appropriate. Significance was accepted at the 0.05 level of probability (P < 0.05).
BATF is upregulated in synovial tissues of arthritic joints
Genetic ablation of Batf inhibits CIA
Genetic ablation of Batf inhibits CIA-induced synovitis, synovial hyperplasia, and angiogenesis in synovial tissues
Inflammatory synovial tissues express VEGF, which stimulates angiogenesis to maintain the chronic inflammatory status . Consistent with the above results, neovascularization (as determined by CD31 staining) was observed in CIA-induced WT mice but not in Batf−/− mice (Fig. 3e). Previous studies showed that FLS contribute to CIA by producing angiogenic factors associated with blood vessel formation [10, 20, 27, 28], and our group reported that hypoxia-inducible factor (HIF)-2α, which causes RA pathogenesis by modulating FLS functions, stimulates VEGF expression in FLS . Here, we found that overexpression of BATF in FLS did not cause detectable expression of VEGF (Fig. 3f). Additionally, although BATF was upregulated in vimentin-expressing FLS of CIA synovium (Fig. 1c), treatment of FLS with IL-6 or TNF-α, cytokines that critically regulate RA pathogenesis , did not induce BATF expression (Fig. 3f). Furthermore, TNF-α- or IL-6-induced upregulation of matrix-degrading enzymes (MMP3 and MMP13), chemokines (CCL2 and CCL5), and chemokine receptors (CXCL1, CXCL5, and CXCL10) in FLS were not affected by Batf KO (Fig. 3g).
Genetic ablation of Batf inhibits CIA-induced bone and cartilage erosion
CIA in WT mice caused cartilage destruction, as determined by safranin-O staining and OARSI grading (Fig. 4d). Similar to the case of bone erosion, cartilage destruction was completely abrogated in Batf−/− littermates (Fig. 4d). Cartilage destruction during CIA can be caused by pannus, which invades and destroys mineralized cartilage, or matrix-degrading enzymes produced by synoviocytes and chondrocytes [31, 32]. Although the matrix-degrading enzymes in inflammatory joint tissues are primarily produced by synoviocytes, such as FLS [31, 32], BATF overexpression in FLS did not cause any upregulation of the tested matrix-degrading enzymes (MMP3 and MMP13) or chemokines and chemokine receptors (CCL2, CCL5, CXCL1, CXCL5, and CXCL10) (Fig. 3f). Notably, however, we previously showed that BATF overexpression causes chondrocytes to produce MMP3 and MMP13 . These results collectively suggest that matrix-degrading enzymes produced by chondrocytes, not FLS, may contribute to cartilage destruction during CIA.
Genetic ablation of Batf does not affect thymic T cell development
Batf KO modulates Th cell differentiation
Genetic ablation of Batf does not affect inflammatory arthritis caused by K/BxN serum transfer
We herein show that BATF is required for CIA since Batf−/− mice exhibit complete suppression of the manifestations of CIA. We further demonstrate that BATF regulates Th cell differentiation during CIA. The pathogenesis of RA is associated with enrichment of Th17 cells in the inflamed synovium and enhancement of IL-17 production [8, 9, 36, 37]. A recent report indicated that the conversion of Treg cells to Th17 cells contributes to bone destruction in CIA mice , suggesting that an imbalance between these cell populations contributes to CIA. BATF is known to regulate Th17 cell differentiation by modulating the expression of the transcription factor RORγt , and to control class-switch recombination in T and B cells . Thus, BATF was previously known to function at multiple hierarchical levels in two cell types to globally regulate switched-antibody responses. Here, we demonstrated that BATF deficiency decreases Th17 cell differentiation both in vivo and in vitro. It was previously reported that a portion of Foxp3-positive cells also expresses IL-17, and that CD4+Foxp3+IL-17+ cells can be found in CIA mice . However, we found here that Batf deficiency completely abolished CD4+Foxp3+IL-17+ cell generation under CIA-inducing conditions. An additional interesting finding of our current study is that BATF regulates Th2 cell differentiation; we demonstrated that IL-4-producing CD4+ T cells were increased in CIA-induced Batf−/− mice compared with WT control mice. This is consistent with a previous report that IL-4 negatively regulates the induction and progression of CIA . In addition, BATF is not detected in the infiltrated B cells of CIA synovial tissue. Although this does not rule out expression and the possible role of BATF in B cells of peripheral lymphatic tissues, our results indicate that upregulation of BATF in the infiltrated B cells is not essential for pathogenesis of CIA. Furthermore, we found that T cell-independent inflammatory arthritis caused by K/BxN serum transfer was not affected by Batf−/− mice. Based on these findings, we propose that BATF deficiency alters Th cell differentiation, enabling Batf−/− mice to resist CIA. Thus, BATF inhibition could be a useful strategy for the treatment of RA.
We also report that FLS are not directly associated with the BATF-mediated regulation of CIA, although BATF appears to regulate the proliferation of FLS in response to TNF-α. Accumulating evidence indicates that FLS are key players in RA pathogenesis [10, 28]. Cytokines and chemokines produced by FLS attract T cells to RA synovium, and the interaction of FLS with T cells results in the activation of both cell types. FLS also produce matrix-degrading enzymes involved in cartilage destruction, angiogenic factors associated with neovascularization, and RANKL [10, 28]. The latter factor regulates osteoclastogenesis, which requires physical contact of precursor cells with RANKL-expressing FLS or T cells [29, 30, 39]. We showed previously that HIF-2α modulates various functions of FLS, including proliferation, the expressions of RANKL and various catabolic factors, and osteoclastogenic potential . Here, we found that, unlike HIF-2α, TNF-α and IL-6 do not cause BATF expression in FLS, and BATF overexpression in FLS does not modulate the expressions of various matrix-degrading enzymes or chemokines. These results collectively support our notion that the ability of BATF to regulate CIA is not due to a direct BATF-mediated modulation of FLS functions.
It has proven difficult to elucidate the role of chondrocytes in cartilage destruction during RA pathogenesis . Such destruction occurs primarily at the interface of the pannus and calcified cartilage [5, 40]. There is evidence that proteoglycans are lost from the superficial zone, where cartilage contacts with synovial fluid, but not with the pannus . Because RA synovium produces various matrix-degrading enzymes [5, 8, 9], cartilage destruction at the superficial zone may be due to synovial cell functions. However, proteoglycans can also be lost from the middle and deep zones of the cartilage , suggesting that a chondrocyte may help degrade its own matrix by releasing matrix-degrading enzymes. Indeed, we recently demonstrated that BATF upregulates MMP3 and MMP13 in chondrocytes, leading to cartilage destruction during OA pathogenesis . Therefore, our current and previous  results suggest that the BATF-mediated regulation of MMP3 and MMP13 expression in chondrocytes is associated with cartilage destruction during CIA.
The results of the present study collectively indicate that BATF regulates CIA in mice, and our previous work showed that BATF functions as a catabolic regulator of OA cartilage destruction by upregulating catabolic enzymes (e.g., MMP3 and MMP13) in chondrocytes . Thus, despite their different etiologies and pathogeneses, both RA and OA are regulated by BATF. However, different mechanisms are involved; BATF regulates OA pathogenesis by upregulating matrix-degrading enzymes in chondrocytes, whereas it appears to regulate RA pathogenesis by regulating Th cell differentiation. Thus, BATF could be a useful target for the regulation of RA.
In summary, we demonstrated here that BATF regulates CIA, including synovitis, synovial hyperplasia, angiogenesis in the inflamed synovium, cartilage destruction, and bone erosion in the joint tissues. We also reveal that BATF regulates CIA by regulating Th cell differentiation without directly affecting the functions of FLS.
This work was supported by grants from the National Research Foundation of Korea (2016R1A3B1906090, 2016R1A5A1007318, and 2016R1A2B4008819), the Korea Healthcare Technology R&D project through the Korea Health Industry Development Institute (HI16C0287 and HI14C3484), and the GIST Research Institute (2017).
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SGP and JSC contributed equally as corresponding authors by conceiving and designing the experiments. SHP, JR, and SKK performed the experiments. JAK, JSK, YS, and WSC analyzed the data. SHP, JR, SGP, and JSC wrote the manuscript. All authors read and approved the final manuscript.
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