PD-1 is upregulated on NK cells in transplant recipients by day 28 posttransplant compared with donor NK cells and impairs NK cell function against PD-L1–expressing tumor targets. (A) NK cells from 10 sibling-matched donors and recipients 28 days posttransplant were stained for PD-1 expression. (B-C) PBMCs from the same 10 donors and recipients were analyzed by CyTOF up to day 100 posttransplant and visualized by bulk lymphocyte subsets (B) or focused specifically on NK cells (C) (n = 10). Horizontal bars indicate the mean ± SEM. *P < .05; **P < .01. MFI, mean fluorescence intensity.