CTL target cell apoptosis is dependent on grB, and the Bcl-2 block is still reversed in the absence of grA. CTLs were derived from splenocytes of control WT mice (WT), grA KO mice (grA KO), and grB-cluster KO mice (grB KO). (A) Control mouse L cells (Lneo; □) and Bcl-2–overexpressing L cells (LBcl-2; ■) were preloaded with CTFR (FL-4) and incubated for 2 hours with CTLs at E/T ratios of 0.4:1, 2:1, and 10:1. Cells were then loaded with the caspase indicator zVAD-FITC (FL-1), and caspase activation of target cells (FL-4⁺ cells) were assessed by flow cytometry. Results of each treatment are graphed as the percentage of specific caspase-positive cells relative to the percentage of specific caspase-positive target cells treated with WT CTLs at an E/T ratio of 10:1. (B) Lneo cells (□) and LBcl-2 cells (■) were preloaded with ³H-thymidine and incubated for 2 hours with CTLs at E/T ratios of 0.4:1, 2:1, and 10:1. The percentage of specific tritium release was determined as outlined in “Apoptosis detection systems.” The numbers indicate the average of 3 independent experiments done in triplicate, with the corresponding standard deviation.