JAK3 and rheumatoid arthritis: Tofacitinib, with specificity for JAK1 and JAK3, reduces IL-23 triggered IL-17A secretion from T cells of rheumatoid arthritis (RA) and psoriatric arthritis (PSA) patients in vitro13,14, and was the first JAK inhibitor to be licensed for treatment of RA in the United States15,16.