Dysregulation between pro- and anti-inflammatory cytokines activity in rheumatoid arthritis (RA) contributes to immune dysregulation, chronic inflammation and subsequent joint destruction. Interleukin-37 (IL-37) has been described as an anti-inflammatory cytokine in several autoimmune diseases. The main aim of this work was to determine the levels of IL-37 in serum and synovial fluid (SF) of RA patients and to compare them with the levels in patients with osteoarthritis (OA) and further explore the association of IL-37 with disease activity and other clinical parameters. Subsequent goal was to study its anti-inflammatory function on RA synovial fibroblasts and describe other cells types of synovial tissue contributing to its production. IL-37 levels were detected using enzyme-linked immunosorbent assay (ELISA). Synovial fibroblasts were stimulated by lipopolysaccharide (LPS) and recombinant IL-37 (rIL-37). The levels of studied genes were detected by PCR. Synovial tissues and immune cells were visualized by immunohistochemical and by immunofluorescence staining. We found increased levels of IL-37 in SF of patients with RA in comparison to OA patients. There was a significant correlation between serum and SF levels of IL-37. RA as well as OA patients showed increased levels of IL-37 in serum than in...

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