PD-1 on NK cells impairs degranulation and IFN-γ production against PD-L1–expressing targets and is restored by PD-1 blockade. (A-D) NK cells were purified from peripheral blood and co-incubated with K562 (A-B) or THP-1 (C-D) cells with or without PD-1 blocking antibodies or an irrelevant anti-CD19 scFv for 4 hours. (E-F) The ability of PD-1 blockade to enhance ADCC was also tested with an anti-CD33 (aCD33) antibody against THP-1. (G-H) PD-1 blockade was also evaluated on NK cells preactivated with 10 ng/mL IL-15 against THP-1 targets. Function was assessed via flow cytometry staining for CD107a and IFN-γ (n = 5). Error bars indicate the mean ± SEM. *P < .05; **P < .01; ***P < .001. Unstim, unstimulated.