Differential Effects of IL-17A and TNF-α on Osteoblastic Differentiation of Isolated Synoviocytes and on Bone Explants from Arthritis Patients.

OBJECTIVE: TNF-α and IL-17A act on fibroblast-like synoviocytes (FLS) and contribute to cytokine production, inflammation, and tissue destruction in rheumatoid arthritis (RA). The aim of this study was to compare their effects on osteogenic differentiation of isolated FLS and on whole bone explants from RA and osteoarthritis (OA) patients.

METHODS: Fibroblast-like synoviocytes and bone explants were cultured in the presence or absence of TNF-α and/or IL-17A. Mineralization of extracellular matrix of FLS was measured by alizarin red and alkaline phosphatase activity (ALP). mRNA expression was analyzed by qRT-PCR for Wnt5a, BMP2, and RUNX2, key genes associated with osteogenesis. IL-6 and IL-8 levels were measured by enzyme-linked immunosorbent assays. Bone explant structure was quantified by histomorphometry.

RESULTS: In isolated OA and RA FLS, the combination of TNF-α and IL-17A induced matrix mineralization, increased ALP activity and expression of the osteogenesis-associated genes Wnt5a, BMP2, and Runx2, indicating an osteogenic differentiation. Wnt5a levels increased with TNF-α alone and in combination with IL-17A. BMP2 expression decreased with IL-17A and TNF-α after 12 h with OA FLS and 24 h with RA FLS. Runx2 expression decreased only with combination of TNF-α and IL-17A in OA FLS and with cytokines alone and combined in RA FLS. IL-6 and IL-8 production increased with IL-17A and/or TNF-α in both FLS and bone samples, especially from RA. Treatment of bone explants with cytokine combination increased ALP in OA but not RA samples. A decrease in bone volume was seen with cytokine combination, especially with RA explants.

CONCLUSION: Differences were observed for the effects of IL-17A and TNF-α on osteogenic differentiation. In isolated FLS, increased osteoblastogenesis was observed, contrasting with the inhibitory effect in whole bone, specifically in RA. The net effect of IL-17A and TNF-α appears to depend on the disease state and the presence of other cells.