IL-17-producing CD4+ T cells have been identified in synovial tissues from patients with RA, including their inducing cytokines IL-6 IL-1β, IL-21, transforming growth factor (TGF)-β, and IL-23 [13,14,15,16,17], and have been demonstrated to be increased/maintained in the peripheral blood of RA patients [18,19,20,21], whereas IL-17 was shown to induce bone resorption and contribute to neutrophil recruitment, and particularly into the synovial fluid, a hallmark of RA [22,23]. Here, IL21 is linked to rheumatoid arthritis.