KIR⁺ NK cells produce higher levels of IFNγ after target cell exposure than do KIR⁻ NK cells. (A) CD56⁺CD3⁻–gated NK cells from donors and recipient post-HCT samples at the indicated time points were further gated as KIR⁺ (black bars) or KIR⁻ (white bars) based on staining with a cocktail of antibodies containing CD158a/h, CD158b/j, and CD158e. Expression of CD107a and production of IFNγ were measured after incubation with K562 cells for 4 hours. IFNγ production also was measured after overnight stimulation with IL-12 and IL-18. Bars represent the mean ± SEM. KIR⁺ NK cells were compared with KIR⁻ NK cells at each time point using the Wilcoxon signed rank test; statistical significance is indicated as *P < .05, **P < .01, and ***P < .0001. (B) CD56⁺CD3⁻–gated NK cells from donors and recipient post-HCT samples from the indicated time points were further gated as CD56^bright or CD56^dim. Production of IFNγ was measured after overnight stimulation with IL-12 and IL-18. Bars represent the mean ± SEM. CD56^bright NK cells were compared with CD56^dim NK cells at each time point using the Wilcoxon signed rank test; statistical significance is indicated as *P < .05, **P < .01, and ***P < .0001.