Role of Osteopontin in Synovial Th17 Differentiation in Rheumatoid Arthritis

Objective. Osteopontin (OPN) that is aberrantly produced in rheumatoid synovium is thought to play an important role in rheumatoid arthritis (RA). This study was undertaken to investigate the role of OPN in the differentiation and accumulation of Th17 cells in rheumatoid synovium. Methods. Peripheral blood mononuclear cells and purified CD4+ T cells derived from patients with RA or healthy controls were used to test the effect of OPN in vitro. Cytokine expression was determined by enzyme-linked immunosorbent assay and quantitative polymerase chain reaction. Intracellular staining and flow cytometry were used to detect the percentages of Th17 cells and OPN receptors. Signaling and molecular events were analyzed by immunoblotting and chromatin immunoprecipitation. Results. The levels of OPN correlated significantly with interleukin-17 (IL-17) production and the frequency of Th17 cells in the synovial fluid (SF) of RA patients. Endogenous OPN produced in RA SF was responsible for markedly increased production of IL-17 in T cells, which was blocked by OPN antibody. The effect of OPN in Th17 differentiation was mediated through a mechanism independent of the IL-6/STAT-3 pathway or other cytokines and specifically involved the OPN receptors CD44 and CD29 and the transcription factor retinoic acid-related orphan receptor (ROR). Furthermore, OPN was found to induce H3 acetylation of the IL17A gene promoter, mainly through the CD44 binding domain in CD4+ T cells, allowing the interaction of the IL17A gene locus with ROR. Conclusion. This study reveals new evidence of the critical role of OPN in Th17 differentiation in rheumatoid synovitis.