CTLA4 and systemic lupus erythematosus: IL‐21 could suppress FOXP3+ Tregs differentiation and suppressive activity by inhibiting CTLA‐4 and GATA‐3 expression, whereas IL‐2 and TGF‐β promoted the expression of CTLA‐4+ Tregs and GATA‐3+ Tregs in SLE.