CD107a-deficient NK cells are more susceptible to apoptosis after target cell encounter. (A) Purified splenic IL-2–expanded NK cells from WT (top) or CD107a^−/− (bottom) mice were incubated for 4 hours in the absence or presence of RMA/S cells. NK cells were identified by gating on CD3^–NK1.1⁺ cells, and Annexin-V–positive cells were quantified. (B) Statistical summary of the data. Percentage of specific increase in Annexin-V–positive cells was calculated as described in “Material and methods”. NK cells conjugated to target cells were ignored to avoid false-positive events through apoptotic targets. Statistical analysis was performed using the 2-tailed Mann-Whitney U test. ns, not significant.