The novel cytokine interleukin-36α is expressed in psoriatic and rheumatoid arthritis synovium

Background Interleukin (IL)-36 is a recently described member of the IL-1 cytokine family with pro-inflammatory and clearly pathogenic properties in psoriasis. Objective To determine the IL-36 expression in psoriatic arthritis (PsA) compared to rheumatoid arthritis (RA) and osteoarthritis (OA). Methods Synovial tissues obtained from arthritis patients were stained for IL-36, IL-36 receptor (IL-36R) and IL-36R antagonist (IL-36Ra) by immunohistochemistry and immunofluorescence. Lysates were examined for IL-36 by western blot analysis. Synovial fibroblasts (FLS) cultured in the presence of IL-36 were assayed for cytokine expression by quantitative real time PCR and multiplex assay. IL-36-induced signal transduction in FLS was analysed by immunoblotting. Results Expression of IL-36R and its ligands IL-36 and IL-36Ra was detected in the synovial lining layer and cellular infiltrates of patients with inflammatory arthritis. IL-36 was expressed significantly higher in PsA and RA than in OA synovium. CD138-positive plasma cells were identified as the main cellular source of IL-36. No differences were observed for the expression of IL-36R and IL-36Ra between PsA, RA and OA. Functionally, IL-36 induced the expression of IL-6 and IL-8 in FLS through p38/NFkB activation. Conclusions IL-36 is up-regulated in PsA and RA synovium, expressed by tissue plasma cells and leads to IL-6 and IL-8 production by synovial fibroblasts. Hence, IL-36 links plasma cells to inflammatory cytokine production by FLS and may represent a key link between autoimmunity and the induction of synovitis.