MEK inhibitors effectively suppress human alloreactivity alone and synergistically with calcineurin inhibitors. (A) CFSE-labeled PBMC were cultured with allogeneic DCs with DMSO, tacrolimus, U0126, or selumetinib for 7 days, with division profiles measured by CFSE dye dilution. (B) Aggregate results (n = 6) depicting the percentage of CFSE^low CD4⁺ and CD8⁺ T cells in the presence of DMSO, Tacrolimus (Tacro), U0126, and selumetinib (Sel). Results shown depict means + SD. *P < .05; **P < .01. (C) Naive (CD45RA⁺CD27⁺) CD4⁺ and CD8⁺ T cells were sorted, CFSE-labeled and cultured with allogeneic DCs in the presence of DMSO, tacrolimus, or selumetinib (Sel). In addition to CFSE dilution, functional differentiation was assessed by examining CD27 and CD45RA expression. Data representative of 5 consistent and independent experiments is shown. (D-E) CFSE-labeled PBMCs were cultured with DCs along DMSO (control), tacrolimus, and selumetinib (alone and in combination with suboptimal tacrolimus levels). (E) Aggregate results (n = 4) depicting the percentage of CFSE^low CD4⁺ and CD8⁺ T cells demonstrate synergistic inhibition by tacrolimus (Tacro) and selumetinib (Sel). Results depict means + SD. *P < .05.