PE-9 CTLs cultured with lysates of LSA or EL-4 cells exhibit apoptosis. PE-9 cells were cultured in the presence of cell lysates as described in Fig 6. They were stained with TdT + FITC-dUTP and analyzed using a flow cytometer. (A) Irradiated thymocytes as a positive control for apoptosis. PE-9 cells cultured with lysates from FasL-deficient P815 (B) or FasL⁺ LSA (C) or EL-4 (D) cells are depicted. Bold histograms represent negative controls consisting of cells cultured in the presence of medium; broken histograms show cells irradiated (A) or cultured with cell lysates (B, C, and D).