IL15RA and rheumatoid arthritis: However, the role of IL-15R-independent membrane-bound IL-15 in RA has not been completely studied until now, and no data has been reported so far concerning the existence of natural sIL-15Rα in synovial fluid of RA patients, two elements necessary to induce proinflammatory cytokines through reverse signaling that could contribute to pathogenesis of RA.