PD-1 blockade of reconstituting NK cells from sibling transplant recipients results in enhanced NK function against PD-L1–positive tumor targets. PBMCs from sibling (A-B) or UCB (C-D) recipients 28 days posttransplant were cocultured with K562 cells with or without PD-1 blockade for 4 hours and stained for CD107a and IFN-γ (n = 10). Horizontal bars indicate the mean ± SEM. **P < .01; ***P < .001.