PECAM-1 suppresses mitochondrial, but not Fas-induced, apoptosis. (A) Stable PECAM-1^- Jurkat cell lines that had been transfected with empty pcDNA3 vector, wild-type PECAM-1, or ITIM-less PECAM-1 were resuspended in 200 μL DPBS containing 25 μg/mL anti-Fas antibody (mouse IgM, clone CH11, Upstate Biotechnologies) or normal mouse serum. After incubation for 1 hour at 4°C, cells were washed in ice-cold PBS containing sodium azide and incubated with fluorescein isothiocyanate (FITC)—conjugated goat-mouse IgM. After incubation for 30 minutes at 4°C, cells were washed, fixed in 2% paraformaldyhede, and subjected to flow cytometric analysis. All 3 cell lines express similar levels of Fas on their cell surface. (B) Jurkat cells were induced to undergo apoptosis by incubation with 10 ng/mL anti-Fas antibody or by exposure to 500 J/m² UV-C irradiation, as described in “Materials and methods.” The percent of apoptotic cells was determined by microscopic examination of nuclear morphology of triplicate blinded samples taken at the indicated time points. PECAM-1 suppresses UV-induced, endogenous Bax-mediated apoptosis but has no significant effect on Fas-induced programmed cell death.