NK cells costimulated with immobilized IgG and IL-12 secrete high levels of IFN-γ. (a) Human NK cells were cultured on wells precoated with huIgG. rhuIL-12 was added at a concentration of 10 ng/ml. Control wells consisted of NK cells cultured with medium alone (Medium), immobilized huIgG alone (IgG), or rhuIL-12 alone (IL-12). Culture supernatants were harvested after 72 hours and analyzed for IFN-γ content by ELISA. (b) Human NK cells were cultured on immobilized huIgG in varying concentrations of rhuIL-12 (0.01–20 ng/ml), and supernatants were harvested after 72 hours. (c) In time-course experiments, NK cells were cultured with 10 ng/ml IL-12 on immobilized huIgG. Supernatants were harvested at varying times (6–72 hours). *P < 0.001 versus medium, IgG, and IL-12.