(A) Sublethal radiation enhances Fas-mediated apoptosis of LCLs generated from human EBV⁺ B-cell tumors of hu-PBL-SCID mice. Irradiated (□) or nonirradiated (⋄) LCLs were incubated for 24 hours without or with an increasing concentration of immobilized anti-Fas MoAb, and then assessed for induction of apoptosis using nuclear staining with PI.30 Each point represents the mean ± SEM of three different experiments on three different LCLs. (B) Exogenous IL-2 upregulates Fas expression on fresh human EBV⁺ B-cell tumors from hu-PBL-SCID mice and (C) potentiates Fas-mediated apoptosis. FACS profile of Fas expression on EBV⁺ tumor cells stained with anti-Fas MoAb after a 96-hour incubation in the absence ( — ) or presence (– – –) of IL-2 (10 ng/mL). Nonreactive isotype control MoAb profile is represented by the solid peak. X̄ = mean fluorescence intensity. Apoptosis was assessed following a 24-hour incubation with 10 μg/ml of immobilized anti-Fas MoAb, and then quantitated by nuclear staining with PI as described in Materials and Methods.