CTLs cause target cell death of Bcl-2–overexpressing cells. (A) CTLs induce caspase activation in target cells overexpressing Bcl-2. Jneo cells (top row) and JBcl-2 cells (bottom row) were preloaded with Cell Trace Far red (FL-4) prior to incubation with CTLs (E/T ratio = 2:1) or grB/AD (20 nM grB). Cells were stained with the caspase indicator zVAD-FITC, and FL-4⁺ target cells were analyzed for caspase activation in the FL-1 (FITC) channel. This is a representative example of 1 of 4 independent experiments done in triplicate. (B) CTLs induce caspase activation in target cells overexpressing Bcl-2. Jneo cells (□) and JBcl-2 cells (■) were preloaded with CTFR (FL-4) and incubated for 2 hours with CTLs at E/T ratios of 0.25:1, 0.5:1, 1:1, and 2:1 (CTLs), or 2.5 nM, 5 nM, 10 nM, and 20 nM grB with AD, 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 4 independent experiments done in triplicate, with the corresponding standard deviation. (C) CTLs induce tritium release in target cells overexpressing Bcl-2. Jneo cells (□) and JBcl-2 cells (■) were preloaded with ³H-thymidine and incubated for 2 hours with CTLs at E/T ratios of 0.6:1, 2:1, and 6:1 (CTLs), or 10 nM, 30 nM, and 90 nM grB (grB). The numbers indicate the average of 3 independent experiments done in triplicate, with the corresponding standard deviation.