CTLs cause target cell death of Bid- and Bax/Bak-null target cells. (A) CTLs induce caspase activation in target cells lacking expression of Bid and Bax/Bak. Wild-type (WT) cells (□) and Bax/Bak double-KO cells (Bax Bak DKO; ▩) and Bid KO cells (■) 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, then loaded with the caspase indicator zVAD-FITC. FL-4⁺ target cells were analyzed for caspase activation by flow cytometry. The numbers indicate the average of 3 independent experiments done in triplicate, with the corresponding standard deviation. (B) CTLs induce tritium release in target cells lacking expression of Bid and Bax/Bak. WT cells (□) and Bax Bak DKO (▩) and Bid KO 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 numbers indicate the average of 3 independent experiments done in triplicate, with the corresponding standard deviation.