Cross-linking PECAM-1 induces conformational changes in α_IIbβ₃ and exposure of P-selectin on the platelet surface. Washed platelets (5 × 10⁶/mL) were incubated with the indicated agonists as described in the Materials and Methods. All antibodies were used at a final concentration of 10 μg/mL and were used in the form of intact IgG (not shown), F(ab′)₂ fragments, or as monovalent Fab fragments. After the addition of FITC-conjugated D3 (A) or FITC-conjugated S12 (B) for 30 minutes at room temperature, platelets were washed and transferred to 450 μL of RCD, pH 7.4, and analyzed by flow cytometry. Results are expressed as the mean ± SD of fluorescence intensity for three independent experiments. The observation that bivalent PECAM-1.2 or 4G6 F(ab′)₂ fragments, but not their monovalent Fab counterparts, induced conformational changes in the α_IIbβ₃ integrin complex and exposure of P-selectin on the platelet surface suggests that receptor dimerization is required for outside/in signal transduction mediated by PECAM-1.