CTLs from mice immunized with GM-CSF/IL-12–expressing vaccines lyse MPC11 cells and their MDR variants via granzyme B/perforin- but not Fas-dependent mechanism. Mice were immunized with GM-CSF/IL-12–expressing MPC11 cells (A) or MPC11Dox10-1 cells (B) as described. Splenocytes isolated at day 21 after the initial injection of the vaccine were stimulated with irradiated MPC11 (A) or irradiated MPC11Dox10-1 (B) cells for 5 days, and CTL activity in the absence (▪) or presence (□) of 250 nmol/L CMA was determined. (C) Cells were incubated with recombinant FasL for 17 hours. The cytotoxicity was determined in a colorimetric test (see Materials and Methods). The experiments were repeated twice with similar results. The S49.1 cell line was included as a positive control for FasL-mediated apoptosis.