Susceptibility of BL cell lines to Fas-mediated apoptosis can be regulated by caspase-8 and FLIP_L. (A) Percent apoptosis in Fas-sensitive BJAB and SKW6.4 cell lines cotransfected with pCEP4-FLIP_L (SmtA) plus the pCEP4-EGFP reporter construct. After incubation for 72 hours, the transfected cultures were either left untreated (▨) or treated with anti-Fas IgM (500 ng/mL, ▪) for an additional 8 hours. Percentage apoptosis was measured in 300 EGFP-positive cells using a combination of fluorescence and phase microscopy as described in Materials and Methods. (B) The Fas-resistant BL cell lines Mutu-BL III and Raji were cotransfected with pCEP4-CASP8(FL) and pCEP4-EGFP expression constructs. Transient transfection and evaluation of Fas-mediated apoptosis was performed as described in (A).