Ability of Different Glycoprotein IIb-IIIa Ligands to Support Platelet Aggregation Induced by Activating Antibody CRC54

The ability of different ligands of glycoprotein (GP) IIb-IIIa (alphaIIb/beta3-integrin) to support platelet aggregation stimulated by activating anti-GP IIb-IIIa monoclonal antibody (monoAB) CRC54 has been investigated. Antibody CRC54 stimulated aggregation of washed platelets not only in the presence of fibrinogen, the main GP IIb-IIIa ligand, but also in the presence of von Willebrand factor (vWF). Unlike these ligands, fibronectin failed to support CRC54-induced aggregation. Fibrinogen and vWF dependent platelet aggregation was completely suppressed by GP IIb-IIIa antagonists--preparations Monafram (F(ab')2 fragments of monoAB that blocked GP IIb-IIIa receptor activity) and aggrastat (RGD-like peptidomimetic). However, aggregation stimulated in the presence of vWF was also completely inhibited by monoAB AK2 directed against GP Ib and capable of blocking its binding with vWF. CRC54-induced aggregation of platelets from patient with GP Ib deficiency in the presence of vWF was significantly lower than aggregation of platelets from normal donors and was not inhibited by anti-GP Ib antibody but still blocked by GP IIb-IIIa antagonist Monafram. Monafram also suppressed CRC54-stimulated platelet adhesion to plastic-adsorbed fibrinogen, vWF, and fibronectin. Unlike CRC54-induced platelet aggregation supported by fluid phase vWF, CRC54-induced adhesion to adsorbed vWF was not affected by anti-GP Ib antibody. Aggregation induced by CRC54 in the presence of fibrinogen and vWF was only partially suppressed by prostaglandin E1, an inhibitor of platelet activation, and was associated with serotonin release from platelet granules only when Ca2+ concentration was decreased from 1 mM (physiological level) to 0.1 mM. The data indicate that vWF supports CRC54-induced platelet aggregation via interaction with two receptors--GP IIb-IIIa and GP Ib. Aggregation induced by CRC54 in the presence of vWF or fibrinogen is only partially dependent on platelet activation and is accompanied with granule secretion only at low Ca2+ concentrations.