Engagement of GITR on NK cells augments IL-2-, IFN-α-, and NKG2D-dependent NK cytotoxicity and IFN-γ production. (A) Cytotoxic activity of NK cells against L cells (squares) or GITRL-L cells (circles) were evaluated at various E/T ratios in the presence (closed symbols) or absence (open symbols) of suboptimal doses of IL-2 (20 U/mL), IFN-α (100 U/mL), and anti-NKG2D (0.1 μg/mL). Data are represented as the mean percentage of specific lysis ± SD and are representative of 3 experiments. (B) NK cells were cultured with irradiated L cells or GITRL-L cells in the presence or absence of suboptimal doses of IL-2 (20 U/mL), IFN-α (100 U/mL), and anti-NKG2D (0.1 μg/mL or 5 μg/mL) for 48 hours. IFN-γ in the supernatant of cultures was determined by ELISA. The data shown are representative of 5 experiments.