Engagement of CTLA-4 accompanying reduced FasL surface expression.

(A) Anti–CTLA-4 reduced expression of FasL. Cells transfected with CTLA-4 WT were stimulated with the indicated antibodies bound to plates: anti–CTLA-4 (10 μg/mL), anti-TNP (10 μg/mL) control, and anti-CD3 (0.5 μg/mL). The cells were harvested after 6 hours, stained with biotinylated anti-FasL antibody followed by fluorescein isothiocyanate-labeled avidin. Subsequently, the cells were gated and analyzed by flow cytometry. A histogram of FasL staining on FL-1 is displayed. (B) Anti-FasL blocks anti-CD3–induced apoptosis in DC27.10 transfectants. CTLA-4 WT transfectants were stimulated with the indicated antibodies bound to plates: anti–CTLA-4 (10 μg/mL), anti-TNP (10 μg/mL) control, and anti-CD3 (0.5 μg/mL) with or without soluble blocking anti-FasL antibody (10 μg/mL). The cells were harvested 24 hours later and stained with PI. Apoptosis was measured as described in “Study design.” ■ indicates control; ▨, αCD3; ▪, αCD3 + αFasL. (C) Model of CTLA-4 blockage of IL-2 production and AICD. The coreceptor acts to dampen TCR/CD3 signaling at an early stage in T-cell activation prior to the bifurcation of signals leading to IL-2 production and FasL expression and AICD.