IFN-induced sensitization to Fas-mediated apoptosis is independent of Apo2L/TRAIL. U-266-1970 cells were preincubated with IL-6 (20 U/mL) alone or in combination with IFN-γ (1000 U/mL) or IFN-α (1000 U/mL) for 96 hours, followed by a 24-hour incubation with agonistic anti-Fas antibody CH-11, recombinant soluble Apo2L/TRAIL (TRAIL), or isotype-specific control IgM. In addition, blocking agent rhTRAIL-R1:Fc (Fc-DR4) was continually added during the incubation time in the experiments shown in the right panels. The percentage of annexin V⁺/PI^- apoptotic cells was evaluated by flow cytometry as described in “Patients, materials, and methods.” Four independent experiments were performed. The percentage number in the lower right quadrants of each graph represents the proportion of annexin V⁺/PI^- cells, that is, early apoptotic cells from one representative experiment.