Increased killing of targets cells by IL-15–stimulated NK cells. Purified CD56⁺ NK cells (98%) were cultured in the absence (○) or presence of IL-15 ([□] 10 or [•] 50 ng/mL) for 3 days. NK cells were washed with PBS and mixed, in triplicate, with ⁵¹Cr-labeled target cells at effector:target ratios varying between 0.1:1 and 10:1. Cells were incubated in V-bottomed wells for 16 hours at 37°C, after which the supernatants (0.1 mL) were analyzed for radioactivity using a γ-counter. Results are expressed as the mean of triplicate cultures ± SD. (A) K562 target cells; (B) EBV-immortalized lymphoblastoid cell line; (C) HSB-2 target cells; (D) HHV-6–infected HSB-2 target cells.