MEK inhibitors spare polyfunctional CMV- and EBV-specific T cells at doses inhibiting alloreactivity. (A-D) PBMCs from a healthy CMV-seropositive donor were stimulated with CMV pp65 peptide mixture along with DMSO (control), tacrolimus (Tacro), U0126, or selumetinib (Sel). (A) Tacrolimus-associated loss of polyfunctional CMV-specific CD4⁺ T cells. (B) Relative sparing of polyfunctional CMV-specific CD4⁺ T cells despite MEK inhibition. (C) Tacrolimus leads to loss of polyfunctional CMV-specific CD8⁺ T cells. (D) Relative sparing of polyfunctional CMV-specific CD8⁺ T cells despite MEK inhibition. (E) Aggregate results (n = 5 donors) depicting the frequencies of polyfunctional (TNFα⁺IFNγ⁺) CMV-specific cells are shown as mean + SD. Y-axis depicts the percentage of TNFα⁺IFNγ⁺ T cells, relative to control. (F) Results (n = 5) depicting the MFI of IFNγ-producing cells are shown as mean + SD. Y-axis depicts the percentage of IFNγ⁺ T cells, relative to control.*P < .01; **P < .05. (G-H) Healthy donor PBMCs were stimulated with an EBV-derived EBNA1 peptide mixture along with DMSO, tacrolimus, U0126, or selumetinib (Sel) and EBV-specific CD8⁺ T cells responses were analyzed. (H) Aggregate results (n = 3 donors) depicting the relative sparing of polyfunctional EBV-specific CD8⁺ T cells by MEK inhibitors. Differences are shown as means + SD. *P < .05.