PI3Kδ is dispensable for the initial formation of aggregates with leukemic target cells, but is indispensable for the maintenance of aggregates and for degranulation. (A) Initial aggregate formation (top panels) and maintainance of aggregates (bottom panels) of PI3Kδ^+/− and PI3Kδ^−/− NK cells with wt v-abl target cells was analyzed by light microscopy (magnification: top panel, ×100; bottom panel, ×430; one example for each genotype is depicted, obtained with a Zeiss Axiovert 135, 10× Ph1 [NA 0.25], camera: Cool Snap HQ [Ottobrunn, Germany], software: Metamorph 4.0 [Visitron, Puchheim, Germany]), (B) quantified by FACS, and observed over time. Error bars represent SEM (*P < .05; **P < .01). (C) A selective inhibitor of PI3Kδ was used to describe the requirement for PI3Kδ for killing, aggregate formation, and degranulation of NK cells when incubated with leukemic target cells. As normalized to PI3Kδ^+/− NK cells, PI3Kδ inhibition severely impaired killing (61.3% of PI3Kδ^+/− NK cells) and degranulation (37.7% of PI3Kδ^+/− NK cells), whereas initial aggregate formation was not affected (115% of PI3Kδ^+/− NK cells).