IL-20 is a novel cytokine that is involved in skin inflammation, in part by serving as an autocrine factor for keratinocytes. The aim of this study was to investigate the expression of IL-20 in the inflamed synovium of patients with psoriatic arthritis (PsA) and rheumatoid arthritis (RA) to provide more insight into the possible role of this cytokine in inflammatory joint disease. To further assess the regulation of IL-20 expression, in vitro experiments were conducted to determine the effects of IL-1β and TNF-α on the expression of IL-20 by RA fibroblast-like synoviocytes (FLS).

Synovial biopsy specimens of patients with PsA (n = 11) and RA (n = 10) were obtained from actively inflamed joints by needle arthroscopy. Immunohistologic analysis was performed using a monoclonal antibody (mAb) specific for IL-20. In addition, we measured IL-20 expression in RA FLS in the presence or absence of 250 pg/ml IL-1β and 100 ng/ml TNF-α by immunocytochemistry.

IL-20 was expressed in the intimal lining layer, in the synovial sublining and on endothelium in both PsA and RA patients. Digital microscopic analysis of synovial tissue revealed clear IL-20 expression with comparable overall scores in both patient groups (RA 149775 ± 56466, PsA 123862 ± 50630 [mean integrated optical density ± SEM]; NS). The in vitro experiments revealed marked induction of IL-20 after stimulation with IL-1β and TNF-α in RA FLS.

IL-20 expression is induced in FLS by proinflammatory cytokines, including TNF-α and IL-1β. In addition, IL-20 is markedly expressed in psoriatic skin lesions, but also in the synovium of patients with PsA and RA, suggesting a role in various forms of inflammatory joint disease.