Synergistic effects of certain combinations of inhibitory receptors on NK-cell responsiveness. (A) Relative frequency of CD107a⁺ NK cells expressing the combinations of Ly49A, Ly49C, and NKG2A in D^b single mice. Ly49C and Ly49I were gated away. (B) A similar analysis in D^d single mice, including Ly49A, Ly49G2, and NKG2A, excluding Ly49C and Ly49I. (C) A similar analysis in K^bD^b mice, including Ly49C, Ly49I, and NKG2A, excluding Ly49A and Ly49G2. (D) Effect on CD107a-expression of adding the 2 excluded receptors in each mouse. Data are expressed as mean percentages of CD107a⁺ NK cells in a given subset relative to the receptor-negative NK cells from the same mouse strain and experiment. Mean fold change with SEM from at least 8 experiments. *P < .05; **P < .01; ***P < .001.