Inhibition of aggregated IgG FcγRIIA-mediated platelet aggregation by anti-PECAM-1 intact and Fab fragments. (A) Addition of aggregated IgG to washed platelets results in a lag phase of 2 minutes followed by platelet aggregation. Pretreatment of platelets with anti-CD151 mAb 11B1 (10 μg/mL) did not block aggregated IgG-mediated platelet aggregation. Pretreatment of platelets with either anti-FcγRIIA mAb IV.3 (10 μg/mL) or anti-PECAM-1.1 mAb (10 μg/mL) completely blocked aggregated IgG-mediated platelet aggregation. (B) Platelet aggregation of washed platelets induced by subthreshold concentration of aggregated IgG. Pretreatment of washed platelets with biotinylated anti-CD151 mAb 11B1 Fab (10 μg/mL) for 5 minutes prior to Neutravidin cross-linking did not block aggregated IgG-mediated platelet aggregation. Pretreatment of washed platelets with either FcγRIIA mAb IV.3 Fab (10 μg/mL) or biotinylated anti-PECAM-1 mAb 2BD4 Fab (10 μg/mL) for 5 minutes prior to 10 μg/mL Neutravidin cross-linking completely blocked aggregated IgG-mediated platelet aggregation.