Leukocytes can enhance platelet-mediated aggregation and thromboxane release via interaction of P-selectin glycoprotein ligand 1 with P-selectin

Background: Platelet-leukocyte conjugates have been observed in patients with unstable coronary syndromes and after cardiopulmonary bypass. In vitro, the binding of platelet P-selectin to leukocyte P-selectin glycoprotein ligand-1 (PSGL1) mediates conjugate formation; however, the hemostatic implications of these cell-cell interactions are unknown. The aims of this study were to determine the ability of leukocytes to modulate platelet agonist-induced aggregation and secretion in the blood milieu, and to investigate the role of P-selectin and PSGL-1 in mediating these responses. Methods: Blood was drawn from healthy volunteers for in vitro analysis of platelet agonist-induced aggregation, secretion (adenosine triphosphate, β-thromboglobulin, and thromboxane), and platelet-leukocyte conjugate formation. Experiments were performed on live cells in whole blood or plasma to simulate physiologic conditions. Whole-blood impedance and optical aggregometry, flow cytometry, and enzyme-linked immunosorbent assays were performed in the presence and absence of blocking antibodies to P-selectin and PSGL1. The platelet-specific agonists, thrombin receptor activating peptide and adenosine diphosphate, were used to elicit platelet activation responses. Results: Inhibition of platelet-leukocyte adherence by P-selectin and PSGL1 antibodies decreased agonist-induced aggregation in whole blood. The presence of leukocytes in platelet-rich plasma increased aggregation, and this increase was attenuated by P-selectin blocking antibodies. Data from flow cytometry confirmed that platelet-leukocyte conjugate formation contributed to aggregation responses. Blocking antibodies reduced platelet agonist-induced thromboxane release but had no impact on adenosine triphosphate and β-thomboglobulin secretion. Conclusions: Leukocytes can enhance platelet agonist-induced aggregation and thromboxane release in whole blood and platelet-rich plasma under shear conditions in vitro. Interaction of platelet P-selectin with leukocyte PSGL1 contributes substantially to these effects.