Interleukin‐22 promotes osteoclastogenesis in rheumatoid arthritis through induction of RANKL in human synovial fibroblasts

Abstract Objective To examine the regulatory role of interleukin‐22 (IL‐22) in the expression of RANKL and induction of osteoclastogenesis in rheumatoid arthritis (RA). Methods Concentrations of IL‐22 and RANKL in the serum and synovial fluid of RA patients were measured using enzyme‐linked immunosorbent assay. RA synovial fibroblasts were treated with recombinant human IL‐22 (rhIL‐22), and the expression of RANKL messenger RNA (mRNA) and protein was measured using real‐time polymerase chain reaction, Western blotting, and intracellular immunostaining. Human monocytes were cocultured with IL‐22–prestimulated RA synovial fibroblasts and macrophage colony‐stimulating factor, and osteoclastogenesis was assessed by counting the multinucleated cells (those staining positive for tartrate‐resistant acid phosphatase). Results The IL‐22 concentration in the synovial fluid was higher in RA patients than in patients with osteoarthritis (OA). The serum IL‐22 concentration was also higher in RA patients than in OA patients and healthy volunteers, and this correlated with serum titers of rheumatoid factor and anti–cyclic citrullinated peptide antibodies. In RA synovial fibroblasts treated with rhIL‐22, the expression of RANKL mRNA and protein was increased in a dose‐dependent manner. IL‐22–induced RANKL expression was down‐regulated significantly by the inhibition of p38 MAPK/NF‐κB or JAK‐2/STAT‐3 signaling. In human monocytes cocultured with IL‐22–prestimulated RA synovial fibroblasts in the absence of exogenous RANKL, the monocytes differentiated into osteoclasts, but this osteoclastogenesis decreased after p38 MAPK/NF‐κB or JAK‐2/STAT‐3 signaling was inhibited. Conclusion These results show that IL‐22 up‐regulates RANKL expression in RA synovial fibroblasts and induces osteoclastogenesis. These effects are mediated by the p38 MAPK/NF‐κB and JAK‐2/STAT‐3 signaling pathways.