Cytotoxic activity of human peripheral blood basophils. (A) Nonstimulated basophils, IL-3–stimulated (50 ng/mL, 40 hours) basophils, and NK-92 cells were analyzed in a direct killing assay against [⁵¹Cr]-labeled B-LCL 721.221 cells. Effector cells were incubated for 10 hours with target cells at E/T (effector-target) ratios of 15:1, 30:1, and 60:1. (B) IL-3–stimulated basophils and NK-92 cells were incubated with B-LCL 721.221 target cells in the absence or presence of Z-VAD-fmk (50 μM, pan-caspase inhibitor) or Z-IETD-fmk (50 μM, GzmB inhibitor) for 10 hours at an E/T ratio of 60:1. Cytotoxicity was estimated as described in “Materials and methods.” The data shown (mean of duplicates) are representative of 3 independent experiments. The enhancement of the cytotoxic activity of basophils by IL-3 and its inhibition by the pan-caspase and GzmB inhibitors were statistically significant (P > .01 for all conditions as determined by the paired Student t test).