TNFSF11 and rheumatoid arthritis: IL-22 has also been reported in relation to RA as follows: (1) IL-22 levels are elevated in peripheral blood and correlate with disease activity (21); (2) IL-22 facilitates RANKL expression in synovial fibroblasts and, thereby, promotes osteoclast differentiation; and (3) IL-22 facilitates the proliferation of synovial fibroblasts in vitro (25).