Platelet microparticle formation and thrombin generation under high shear are effectively suppressed by a monoclonal antibody against GPIba.

We studied the inhibition of platelet microparticle (MP) formation and thrombin generation under high shear forces. We hypothesized that an inhibitor of the GPIb a -von Willebrand factor (vWF) interaction would be more effective in suppressing MP formation and thrombin generation than GPIIb/IIIa inhibitors. Platelet-rich plasma (PRP) anticoagulated with PPACK (D-Phe-Pro-Arg chloromethyl ketone) was exposed in a cone-and-plate viscometer (shear: 5,000 s(-1) for 5 min) in the presence of antagonists to GPIb a (the monoclonal antibody [Mab] Ib-23) or to GPIIb/IIIa (abciximab, tirofiban, eptifibatide) at their IC90 determined in platelet aggregometry with ristocetin or ADP, respectively. We used double labeling (CD41-PE and annexin-V-FITC) for flow cytometric detection of MP and their aminophospholipid exposure. Thrombin generation was measured using PRP prepared from ACD anticoagulated blood. About 40% of the thrombin generation was found to be mediated by the MP fraction of the PRP. Blockade of GPIb a with Mab Ib-23 reduced MP formation and thrombin generation by 50%, and was more effective than any GPIIb/IIIa antagonist. The combination of Mab Ib-23 with one of the GPIIb/IIIa inhibitors further reduced the MP formation to ~ 30%. The antibody also partially inhibited thrombin induced platelet aggregation. Epitope mapping suggested that Mab Ib-23 binds between the amino acids 201 and 268 of GPIb a , explaining the interference with vWF and thrombin interaction. In contrast to the commonly used GPIIb/IIIa antagonists, the blockade of GPIb a with Mab Ib-23 effectively reduces the prothrombotic MP generation and thrombin formation at shear rates typically found in arterial stenoses.