S100A8/A9 as a biomarker for synovial inflammation and joint damage in patients with rheumatoid arthritis

S100A8 and S100A9 are major leukocyte proteins, known as damage-associated molecular patterns, found at high concentrations in the synovial fluid of patients with rheumatoid arthritis (RA). A heterodimeric complex of S100A8/A9 is secreted by activated leukocytes and binds to Toll-like receptor 4, which mediates downstream signaling and promotes inflammation and autoimmunity. Serum and synovial fluid levels of S100A8/A9 are markedly higher in patients with RA than in patients with osteoarthritis or miscellaneous inflammatory arthritis. Serum levels of S100A8/A9 are significantly correlated with clinical and laboratory markers of inflammation, such as C-reactive protein, erythrocyte sedimentation rate, rheumatoid factor, and the Disease Activity Score for 28 joints. Significant correlations have also been found between S100A8/A9 and radiographic and clinical assessments of joint damage, such as hand radiographs and the Rheumatoid Arthritis Articular Damage score. In addition, among known inflammatory markers, S100A8/A9 has the strongest correlation with total sum scores of ultrasonography assessment. Furthermore, baseline levels of S100A8/A9 are independently associated with progression of joint destruction in longitudinal studies and are responsive to change during conventional and biologic treatments. These findings suggest S100A8/A9 to be a valuable diagnostic and prognostic biomarker for RA.

MRP8 promotes Th17 differentiation via upregulation of IL-6 production by fibroblast-like synoviocytes in rheumatoid arthritis

Myeloid-related protein (MRP)8/MRP14 is an endogenous Toll-like receptor 4 (TLR4) ligand and is abundant in synovial fluid (SF) of rheumatoid arthritis (RA) patients. Belonging to damage-associated molecular patterns, it amplifies proinflammatory mediators and facilitates a wide range of inflammatory and autoimmune diseases. Interleukin (IL)-17-producing T-helper (Th)17 cells have a crucial role in RA pathogenesis, and IL-6 is the key factor promoting Th17 differentiation. We investigated whether the level of MRP8/MRP14 is positively associated with IL-6 and IL-17 levels in RA SF and found that MRP8/MRP14 level had a significant correlation with IL-6 and IL-17 levels in RA SF. We also observed that MRP8-induced IL-17 production by peripheral blood mononuclear cells but MRP14 did not. Upon stimulation with MRP8, IL-6 production was enhanced by RA fibroblast-like synoviocytes (FLS) and was further elevated by coculturing RA FLS with activated CD4+ T cells. Moreover, we demonstrated that MRP8-activated IL-6 production by RA FLS promoted differentiation of Th17 cells using the coculture system consisting of CD4+ T cells and RA FLS. In addition, IL-6 blockade attenuated Th17 polarization of CD4+ T cells in the cocultures. Inhibitor studies revealed that MRP8 increased IL-6 production in RA FLS via TLR4/phosphoinositide 3-kinase/nuclear factor-kappaB and mitogen-activated protein kinase signaling pathways. Our results show that MRP8 has a crucial role in stimulating IL-6 expression by RA FLS, and subsequently promotes Th17 differentiation in RA, suggesting that neutralizing MRP8 level in RA synovium may be an effective therapeutic strategy in RA treatment.

Reciprocal Regulation of TH17 and Regulatory T Cells by Methotrexate and Its Therapeutic Effects in Collagen-induced Arthritis (CIA)

OBJECTIVE: Methotrexate is the first-line drug in treatment of rheumatoid arthritis (RA) exhibiting higher efficacy and better tolerability than most other DMARDs. To have a better understanding of the anti-arthritic mechanism of methotrexate, we investigated the effect of methotrexate on suppressing the autoimmune inflammatory and destructive arthritis in collagen-induced arthritis (CIA) mice. METHODS: The effects of methotrexate on joint inflammation were assessed by clinical scoring and histologic analysis. Levels of cytokines and autoreactive antibodies were analyzed by immunohistochemistry and ELISA. The population of TH17 and Foxp3+ regulatory T (Treg) cells and phosphorylation of their critical transcription activators, STAT3 and STAT5, were examined by fluorescence microscopy and flow cytometry, respectively. RESULTS: Treatment with methotrexate significantly alleviated joint inflammation and cartilage destruction in CIA. Serum levels of total immunoglobulins G, G1, G2a specific to type II collagen were also reduced considerably in methotrexate-treated mice. The drug inhibited the expression of proinflammatory cytokines such as IL-1beta, TNF-alpha, IL-6 and IL-17 in arthritic joints ex vivo as well as by splenocytes in vitro. Moreover, methotrexate treatment resulted in reciprocal modulation of TH17 cells and Foxp3+ regulatory T (Treg) cells in spleen tissues, in which TH17 cells were decreased and Treg cells in number were increased. Subsequent analysis of CD4+T cells showed that phosphorylation of STAT3 was decreased whereas phosphorylation of STAT5 was increased in methotrexate-treated mice. CONCLUSION: Methotrexate treatment effectively suppressed autoimmune arthritis and restored homeostasis of the immune system by reciprocal regulation of TH17 and Treg cells in a mouse model of collagen-induced arthritis.