CD4 and rheumatoid arthritis: Thus, FoxP3 expression and CXCL13 production are regulated by discrete mechanisms, and CXCL13‐producing CD4+ T cells may preferentially differentiate in an IL‐2‐limited environment, such as the RA synovium, where the levels of T cell derived cytokines are relatively low compared with those of proinflammatory cytokines [23, 24].