CD4 and rheumatoid arthritis: In human studies, IL-17 is spontaneously produced by RA synovial membrane cultures [8], high levels have been observed in the synovial fluid of patients with RA [8], [9], IL-17 producing CD4+ T cells have been detected in RA synovial membranes [10]–[12] and neutralization of IL-17 seems to be effective in RA clinical trials [13].