Inhibition of leukocyte chemiluminescence by platelets: role of platelet-bound fibrinogen

Tethering of PMNL by platelets via CD62P has been shown to cause PMNL activ ation. Go-incubation of purified PMNL with platelets that were activated wi th thrombin and then fixed and washed, resulted in the formation of platele t-PMNL conjugates as well as in a generation of reactive oxygen species tha t were measured as luminol-enhanced chemiluminescence. When platelets were thrombin activated in the presence of RGDS to prevent binding of fibrinogen to membrane receptors, they had a reduced capacity to adhere to PMNL, but ROS generation was enhanced. In samples of citrated whole blood RGDS as wel l as the more specific platelet fibrinogen receptor antagonist GR144053F or a dissociation of the platelet glycoprotein IIb/IIIa complex markedly enha nced ROS generation that was induced by stirring the samples for 10 min at 1000 rpm, by 175%, 95% and 138%, respectively. Removal of platelets from th e whole blood samples also resulted in an enhancement of stirring-induced R OS generation, which was inversely correlated to the platelet count. These data provide some evidence that platelets are capable of inhibiting ROS gen eration in PMNL by a mechanism that involves platelet-bound fibrinogen and probably depends on fibrinogen-mediated platelet-PMNL contact.