INCREASED EXPRESSION OF IL-15 IN THE SYNOVIUM OF PATIENTS WITH RHEUMATOID ARTHRITIS COMPARED WITH PATIENTS WITH YERSINIA-INDUCED ARTHRITIS AND OSTEOARTHRITIS

Recently, a new player in the cytokine network has been described that is produced by monocytes and can be detected in the rheumatoid synovium: interleukin‐15 (IL‐15). Since this cytokine may play a role in the accumulation and activation of T‐cells, B‐cells, and natural killer (NK) cells characteristic of synovial tissue (ST) from patients with rheumatoid arthritis (RA), the expression of IL‐15 was studied in ST from RA patients in comparison with ST from patients with reactive arthritis (ReA) and osteoarthritis (OA) and the phenotype of IL‐15‐positive cells was determined. IL‐15 expression was investigated by immunohistochemical analysis of ST from ten patients with RA, ten patients with Yersinia enterocolitica‐induced ReA, and nine patients with OA. The immunohistological findings were quantified and the results obtained in the different patient groups were compared. To determine the phenotype of IL‐15‐expressing cells, double‐labelling immunofluorescence was performed. The expression of IL‐15 was significantly higher in ST from patients with RA than in ST from patients with ReA or OA. In double‐label experiments, co‐expression was observed with markers for macrophages, T‐cells, and NK cells. The composition of the cellular infiltrate in the synovium of patients with RA might be partly explained by the specific increase in expression of IL‐15 in rheumatoid ST. It can be speculated that IL‐15 production by inflammatory cells other than macrophages may occur in the rheumatoid synovium. © 1997 by John Wiley & Sons, Ltd.