Systemic lupus erythematosus (SLE; lupus) is a prototypical autoimmune disease characterized by circulating autoantibodies to nuclear antigens and immune complex deposition, resulting in damage to target organs. To investigate the effects of tacrolimus (TAC) on effector T cells and B cells, we examined its involvement in the development of effector T cells, germinal center (GC) B cells, and plasma cells in an in vitro system using wild-type (WT) and lupus-prone mice. The population of T helper (Th) 1, Th2, and Th17 cells interleukin (IL)-17-producing T (Th17) cells and the production of interferon-γ and interleukin-17A IL-17A were suppressed by TAC. TAC also reduced the population of regulatory T (Treg) cells; however, a combination treatment with the signal transducer and activator of transcription 3 (STAT3) inhibitor STA-21 promoted the population of Treg cells. TAC also suppressed the populations of GC B cells and plasma cells synergistically with STA-21. These findings suggest that the application of TAC with a STAT3 signal inhibitor may provide benefits in SLE treatment.