Flow sorted CD56dim NK cells were pre-activated with LD15, IL-12+IL-18, or IL-15+IL-18, washed, and re-plated in cytokine-free media for 2 days. Cells were then incubated for 1 hour with an isotype or anti-CD25 blocking antibody prior to activation with dilutions of IL-2. After 24 hours, cells were harvested and plated in a four-hour in vitro flow-based killing assay with CFSE-labeled K562 cells. Shown is mean ± SEM percent specific killing at a 4:1 effector:target ratio pre-incubated with (A) an isotype control or (B,C) an anti-CD25 blocking antibody. (D,E) Killing by pre-activated CD56dim NK cells is NK cell dose (effector:target ratio) dependent, shown at 10 pM IL-2, with anti-CD25 or isotype control pre-incubation. Data is summarized from 3–5 donors in 2 independent experiments.