PLATELET-DERIVED MICROPARTICLE FORMATION INVOLVES GLYCOPROTEIN-IIB-IIIA - INHIBITION BY RGDS AND A GLANZMANN THROMBASTHENIA DEFECT

While the physiologic role of platelet microparticles may include a st able, physical dispersion of concentrated surface procoagulant activit y the mechanism(s) of platelet vesiculation remains unknown. We demons trate using flow cytometric methods a central role for the beta3 integ rin glycoprotein (GP) IIb-IIIa complex and its ligand tetrapeptide Arg -Gly-Asp-Ser (RGDS) binding site in platelet vesiculation. Time- and c alcium-dependent vesiculation of platelets in response to ADP, collage n, thrombin, phorbol myristate acetate, and the thrombin peptide SFLLR N were dramatically inhibited, in a concentration-dependent manner, by monoclonal antibodies to GPIIb-IIIa (A2A9, 7E3, PAC1) and RGDS. Compl ete inhibition with A2A9 and RGDS occurred at 7.5 mug/ml and 75 muM, r espectively, while control antibodies and a mock peptide had no effect . Platelet vesiculation requires intact GPIIb-IIIa and is fully suppor ted by the intracellular pool of GPIIb-IIIa alone since de-complexing of this heterodimer by calcium chelation completely abolished micropar ticle formation in response to collagen (no alpha-granule release) but not to thrombin or SFLLRN. A central role for GPIIb-IIIa is supported by the near total inability of Glanzmann's thrombasthenic (type I) pl atelets to vesiculate in response to thrombin, ADP, collagen, and phor bol 12-myristate 13-acetate. This extends the biologic roles of GPIIb- IIIa to include platelet vesiculation and suggests that one or all of its binding ligands play a role.