Von Willebrand factor can support platelet aggregation via interaction with activated GPIIb-IIIa and GPIb

In this study we investigated mechanisms of platelet interaction with von Willebrand factor (vWF) induced by activating anti-glycoprotein (GP)IIb-IIIa antibody CRC54 directed against LIBS (ligand-induced binding site epitope) in GPIIIa. It was demonstrated that aggregation of washed platelets (measured in Born aggregometer) could be stimulated by CRC54 not only in the presence of fibrinogen but vWF as well. The level of aggregation induced in the presence of saturating concentrations of vWF (∼80 μg ml) was even higher than that in the presence of 1 mg ml of fibrinogen. Aggregation supported by vWF unlike fibrinogen supported aggregation was almost completely inhibited not only by GPIIb-IIIa antagonists (F(ab )2 fragment of blocking anti-GPIIb-IIIa antibody CRC64 and peptidomimetic aggrastat) but also by anti-GPIb blocking antibody AK2. Aggregation response induced by CRC54 in the presence of vWF was much lower when normal platelets were substituted with GP-Ib-deficient platelets and this residual aggregation was not affected by anti-GPIb antibody AK2 but still inhibited by anti-GPIIb-IIIa blocking antibody fragment. CRC54-induced aggregation supported by vWF (as well as by fibrinogen) was only partially inhibited by prostaglandin E1, indicating that at least its initiation does not require activation of platelets. CRC54, both in the presence of vWF and fibrinogen, failed to stimulate serotonin secretion at physiological Ca2+ concentration of 1 mM, although substantial release reaction was detected when Ca2+ concentration was decreased to 0.1 mM. CRC54 could also stimulate platelet interaction with immobilized vWF and fibrinogen. However, unlike platelet aggregation in suspension mediated by flow phase vWF, platelet adhesion to adsorbed vWF (in a same way as to fibrinogen) was inhibited only by GPIIb-IIIa but not GPIb antagonists. The data obtained indicated that vWF support platelet aggregation induced by activating anti-GPIIb-IIIa via interaction with two receptors - activated GPIIb-IIIa and GPIb.