HUMAN NEUTROPHIL CATHEPSIN-G IS A POTENT PLATELET ACTIVATOR

Purpose: Neutrophil activation has been implicated in the pathophysiol ogic condition of ischemia-reperfusion injury, the formation of arteri al aneurysms, the progression of myocardial ischemia, and the initiati on of deep venous thrombosis. Activated neutrophils release cathepsin G, a serine protease, from their granules, which may cause platelet ac tivation that leads to intravascular thrombosis, tissue infarction, an d systemic release of the thrombogenic products of platelet granules. This study used flow cytometry to quantify the extent of cathepsin G-i nduced platelet activation and degranulation through changes in the ex pression of platelet surface glycoproteins. Methods: Increasing concen trations of human neutrophil-derived cathepsin G were incubated with w ashed platelets or whole blood from healthy human donors. The platelet surface expression of glycoproteins, including P-selectin, a platelet membrane glycoprotein only expressed after platelet alpha granule rel ease, were determined by quantifying the platelet binding of a panel o f fluorescently labeled monoclonal antibodies. Results were compared w ith the effect of a maximal dose of thrombin, the most potent known pl atelet activator. Results: In a washed platelet system, cathepsin G in creased platelet surface expression of P-selectin (an activation-depen dent neutrophil binding site), the glycoprotein IIb/IIIa complex (fibr inogen receptor), and glycoprotein IV (thrombospondin receptor), and d ecreased surface expression of glycoprotein Ib (von Willebrand factor receptor) to an extent comparable to maximal thrombin. However, these effects were not observed in a whole blood system. Further experiments revealed that preexposure to plasma completely inhibited cathepsin G- induced washed platelet activation and degranulation. Prostacyclin tre atment of washed platelets markedly inhibited cathepsin G-induced plat elet activation. Conclusions: Cathepsin G is a very potent platelet ag onist and degranulator, comparable to maximal thrombin, which alters p latelet surface glycoprotein expression for enhanced neutrophil bindin g and effective platelet aggregation. This study helps to elucidate a possible pathway through which neutrophils may directly activate plate lets, leading to intravascular thrombosis, irreversible ischemia, and tissue death in cardiovascular disease states. Patients with diseased endothelium that is deficient in prostacyclin production may be partic ularly prone to the detrimental effects of neutrophil-derived cathepsi n G platelet activation.