CD4 and rheumatoid arthritis: More than two decades ago, CD4+ Tregs have been identified as critical immunosuppressive cells [40] which play an important role in maintaining immune homeostasis [41] and preventing autoimmunity [42], including in autoimmune disorders like rheumatoid arthritis (RA) [43], systemic lupus erythematosus (SLE) [44] and juvenile idiopathic arthritis [45].