Inhibition of aggregated IgG binding to human platelets by anti-PECAM-1 antibodies. (A) Binding of aggregated human IgG to washed platelets was measured by flow cytometry. Platelets were (1) untreated or (2) incubated with aggregated human IgG. Platelets were also incubated with aggregated human IgG after pretreatment with 10 μg/mL Fab fragments of (3) IV.3 mAb; (4) PECAM-1.1 mAb; (5) PECAM-1.3 mAb; (6) B2B1 mAb; (7) 2BD4 mAb; or negative control (8) CD151 mAb. * denotes statistical significance (P < .05) (n = 3). (B) Binding of biotinylated IV.3 to washed platelets in the presence of anti-PECAM-1 Fab fragments was measured by flow cytometry. Platelets were (1) untreated or (2) incubated with biotinylated IV.3 Fab (10 μg/mL). Platelets were also incubated with biotinylated IV.3 Fab (10 μg/mL) after pretreatment with increasing doses of anti-PECAM-1 2BD4 Fab fragments (3) 5 μg/mL, (4) 10 μg/mL, (5) 20 μg/mL, (6) 40 μg/mL, and IV.3 binding detected with streptavidin-PE and measured with flow cytometry. Results are representative of 3 experiments. Error bars represent mean ± SEM.