Perforin-deficient NK cells activate caspase biosensors by Fas and TRAIL death receptor ligation. HVS-CL cells from a PRF^−/− patient were preincubated with blocking antibodies or with isotype control antibodies, and then incubated with tumor target cells in a 4-hour luciferase assay at 1:1. Fold signal activation over target cells alone was calculated. (A,C) Representative time course of signal activation in target cells expressing GLS.IETD caspase 8 biosensor. Each datum point represents the mean of 4 replicate wells. (B,D) The fold change in signal from 3 independent reads at 240 minutes was normalized to the isotype control. Data are represented as mean ± SEM from 3 experiments. (A-B) K562 target cells. (C-D) Jurkat target cells. **P < .01; ***P < .001; ****P < .0001; ns, not significant by one-way ANOVA.