Impact of MHC class I blockade on NK cells targeting KIR ligand–matched compared with KIR ligand–mismatched tumor targets. (A) NK cell lines 2, 6, and 7 were isolated by limiting dilution from the PBMCs of a healthy donor, SKEM (C-G1 homozygous: HLA-identical sibling to RCC patient SJ). (B) All 3 lines lysed a minority of C-G1 homozygous KIR-matched VOLLE EBV-LCLs. Preincubation of VOLLE EBV-LCLs with W6/32 significantly enhanced the cytotoxic effects of all 3 lines, consistent with MHC class I molecules inhibiting NK cell function. (C) NK lines 2, 6, and 7 were significantly more cytotoxic to KIR ligand–mismatched R-MEL cells (C-G2 homozygous) compared with KIR-matched (C-G1 homozygous) SJ cells, in which virtually no cytotoxicity was observed. Preincubation of SJ RCC cells with W6/32 significantly increased the cytotoxic effects of all 3 lines against this KIR-matched RCC target. Mean values +SD of 3 samples are shown.