Expression of ligands for NK-cell–activating receptors in leukemic cells. (A) Flow cytometric analysis for the expression of ligands in KG-1 and KG-1(R) cells, and HL-60 and HL-60(R) cells. One representative of at least 3 separate experiments is shown. (B) Flow cytometric analysis of CD107a expression NK cells cocultured for 4 hours with candidate tumor cells at E:T ratio of 2:1 (representative blot of n = 3). (C) Sensitive and resistant leukemia cells incubated with anti-NKG2D mAb or isotype as negative control. The lytic activity of the NKL cells toward these target cells was assessed in a standard 4-hour chromium release assay at different indicated E:T ratios.