Effect of granulocytes on platelet aggregation

Introduction: Platelet-leukocyte conjugates have been observed in patients with MI, stroke, and sepsis. Platelets adhere to leukocytes in a process that is known to involve P-selectin but the significance of this in-teraction in the hemostatic process is unclear. To determine the contribution of platelet-leukocyte interactions to aggregation induced by platelet agonists, we performed standard impedance and optical aggregometry assays in the presence and absence of antibodies to P-selectin and P-selectin glycoprotein ligand 1 (PSGL). Methods: Blood was drawn from healthy volunteers. Whole blood aggregation in response to thrombin receptor activating peptide (TRAP) was determined by impedance aggregometry. Optical aggregometry was performed in platelet rich plasma (PRP)in the presence and absence of isolated granulocytes. Aggregation was performed in the presence and absence of antibodies to P-selectin, PSGL, or an IgG control. Data were analyzed by one-way ANOVA and Neuman-Keuls post-hoc analyses. Results: Whole blood aggregation was inhibited by P-Selectin antibody (^Δp<0.01 vs. No Ab or IgG). Aggregation in PRP increased in the presence of granulocytes, but this increase was blocked by P-Selectin antibody (*p<0.01 vs HBSS control, ⁺p<0.05 vs No Ab and IgG). Similar results were seen in the presence of PSGL antibody. Conclusions: The presence of granulocytes in whole blood and plasma contributes substantially to aggregation induced by platelet activators. However, the in vivo thrombotic potential of this interaction remains unclear.