Apoptosis of NK cells following costimulation with IL-15 and IL-12. (A) Purified NK cells cultured for 3 days in medium containing 15 ng/mL IL-15 alone have only 1% of nuclei within the hypodiploid DNA fraction. (B) A parallel culture of NK cells from the same individual in the presence of IL-15 and 3 ng/mL IL-12 shows 39% of nuclei in the hypodiploid DNA region that characterizes apoptosis. (C) Agarose gel electrophoresis of total DNA from CD3⁻CD56⁺ NK cells that were cultured for 3 days in IL-15 alone (lane 1) or IL-15 and IL-12 (lane 2). Genomic DNA was isolated from an equal number of wells initially containing 1.4 × 10⁶ sorted NK cells. MW indicates a molecular weight standard. The DNA laddering effect characteristic of apoptosis is seen in lane 2. (D) Cytospin preparation of CD3⁻CD56⁺ NK cells cultured 3 days in IL-15 alone or (E) IL-15 and IL-12, with nuclear fragmentation and membrane blebbing characteristic of apoptosis. (Wright Giemsa stain, original magnification × 330.) These results are representative of four independent experiments. Similar results were obtained when IL-2 was substituted for IL-15 (data not shown).