IL17A and rheumatoid arthritis: In contrast, IL-17, the main cytokine secreted by Th17 cells, not only directly upregulates the expression of RANKL in synovial fibroblasts of patients with RA but also promotes the differentiation of Th17 cells by upregulating the expression of prostaglandin E2 (PGE2) in OBs, thus inducing OC production and joint destruction (20, 83).