Ectopic expression does not render MFF cells resistant. (A) Target cells were labeled with ⁵¹Cr-chromate for 1 hour, loaded with relevant E1B peptide, and incubated with the E1B-specific CTL clone at different effector-target ratios. After 5 hours, the released label was determined, which served as a measure of the percentage of CTL-induced lysis when compared with the control. (B) Ectopic expression of SPI-CI and SPI-6 in MFF cells. Transfected MFF cells were analyzed for SPI-CI (top panel) and SPI-6 (bottom panel) expression. Transfected SPI-6 and SPI-CI are VSV-tagged and therefore migrate slightly more slowly than their endogenous counterparts. (C) Target cells were incubated at 37°C with different concentrations of purified LAK granules. After 1 hour, cells were directly lysed in Laemmli buffer and analyzed for PARP cleavage. (D) Target cells were incubated at 37°C with different concentrations of purified LAK granules. After 1 hour, cells were harvested and membrane integrity was measured using propidium iodide. (E) Target cells were labeled with ⁵¹Cr-chromate for 1 hour, loaded with relevant E1A peptide, and incubated with the E1A-specific CTL clone at different effector-target ratios. After 5 hours, the released label was determined, which served as a measure for CTL-induced lysis.