Fas susceptibility and EBV status of cell lines used in this study. Fas-mediated apoptosis was assayed with MTT. Triplicate cultures of 100 μL (3 × 10⁴ cells) were incubated with anti-Fas IgM (250 ng/mL) for 24 hours at 37°C. MTT was then added for an additional 2 hours followed by solubilization of the formazan crystals. Absorbance was read at 570 nmol/L and percentage apoptosis was calculated as previously described.40 Results are from 3 separate experiments and are expressed as mean percentage apoptosis ± standard deviation. Cell-surface expression of the Fas receptor was determined by flow cytometry and the results indicate Fas-positive (▪) and Fas-negative (□) cell lines. The EBV status (negative or positive) of each cell line was obtained from the literature and later confirmed in appropriate cell lines by LMP1 immunoblot analysis.61 62