IL17A and rheumatoid arthritis: RA patients display elevated peripheral Th17 cells, secreting pro-inflammatory cytokines (Interleukin-17 (IL-17), Interleukin-21, Interleukin-22 (IL-22), granulocyte-macrophage colony-stimulating factor, TNF-α) that induce synovial fibroblast receptor activator of nuclear factor kappa-B ligand (RANKL) expression.