1. Close contact between platelets and neutrophils modulates their cellular interactions in thrombotic and inflammatory states, with stimulation of P-selectin expression on platelets by agonists such as thrombin and neutrophil-derived cathepsin G being critical in mediating platelet—neutrophil adhesion. This study compared the effects of thrombin and cathepsin G on platelet P-selectin expression and on P-selectin-mediated platelet—neutrophil adhesion.

2. Washed platelets and platelet—neutrophil mixed cell suspensions (platelet/neutrophil ratio, 10:1) were incubated with either the supernatant of activated neutrophils, purified cathepsin G or thrombin. Platelet P-selectin expression and platelet adhesion to neutrophils was quantified by flow fluorocytometric analysis.

3. The supernatant from activated neutrophils stimulated platelet P-selectin expression comparable to that produced by purified cathepsin G or thrombin. P-selectin expression induced by both activated neutrophil supernatant and purified cathepsin G was completely inhibited by α1-antichymotrypsin, a specific inhibitor of cathepsin G. Unlike thrombin, which induced maximum platelet P-selectin expression by 10 min, sustained to 120 min, cathepsin G induced an initial large increase in platelet P-selectin expression, followed by a progressive reduction over 30–60 min to baseline levels.

4. Co-incubation of neutrophils with thrombin-stimulated platelets resulted in a significant increase in P-selectin-mediated platelet—neutrophil adhesion, which was completely inhibited by preincubation of neutrophils with anti-sialyl Lewisx monoclonal antibody. Thrombin produced maximum platelet—neutrophil adhesion by 10 min which remained stable over 120 min. In contrast, cathepsin G-stimulated platelets did not adhere to neutrophils over 120 min of co-incubation. Addition of cathepsin G to thrombin-stimulated platelets caused a progressive reduction over 30–60 min to baseline levels of platelet—neutrophil adhesion.

5. Neutrophil-derived cathepsin G is a potent platelet activator, but unlike thrombin it causes a time-dependent loss of platelet P-selectin expression and inhibits P-selectin-mediated platelet—neutrophil adhesion. Therefore, cathepsin G may modulate thrombin-mediated platelet—neutrophil adhesive interactions in inflammation and thrombosis.

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