PD-L1 KO mice and NK cell depletion show impaired antitumor activity in a PD-L1 knock-out YAC-1 tumor model treated with or without anti-PD-L1 mAb. (A and B) Representative flow cytometry plots and summary data (n = 5) of (A) PD-L1 expression of spleen NK cells and (B) CD107a expression of spleen and lung NK cells of BALB/c mice after being challenged with PD-L1 KO YAC-1 cells. (C and D) Representative flow cytometry plots and summary data (n = 5) of NK cell CD107a expression in the (C) spleen and (D) lung of WT and PD-L1−/− BALB/c mice challenged with PD-L1 KO YAC-1 cells treated without or with anti-PD-L1 mAb. (E) The number of PD-L1 KO YAC-1 cells in spleens of WT, NK cell-depleted or PD-L1−/− BALB/c mice (n = 5) treated with anti-PD-L1 mAb or IgG control. Paired t test (B and C) was used for two-group comparisons and one-way ANOVA with repeated measures for 3 or more groups (A and E). P values were adjusted by the Holm-Sidak method. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001.