Studies with a murine monoclonal antibody that abolishes ristocetin-induced binding of von Willebrand factor to platelets: Additional evidence in support of GPIb as a platelet receptor for von Willebrand factor

A murine monoclonal antibody directed at or near a platelet membrane receptor for the von Willebrand factor was produced by the hybridoma technique. Purified F(ab')₂ fragments and/or intact antibody completely blocked the agglutination of platelets induced by both ristocetin and bovine von Willebrand factor and the binding of von Willebrand factor antigen to platelets. The antibody also decreased platelet retention, prevented the reduction in platelet electrophoretic mobility caused by bovine von Willebrand factor, and decreased the serum prothrombin time. Radiolabeled F(ab')₂ fragments bound to ~2.5 x 10⁴ sites on normal platelets with high affinity (K(D) ~1.5 x 10^-8 M); there was no binding to platelets from 2 patients with the Bernard-Soulier syndrome. Immunoprecipitation and affinity chromatography studies indicated that the antibody binds to glycoprotein Ib at a site contained on the externally oriented portion of the GPIba chain (glycocalicin). An unidentified mol wt ~20,000 molecule labeled by periodate/NaB³H₄ coprecipitated and copurified with GPIb.