SPECIFIC ACCUMULATION OF CIRCULATING MONOCYTES AND POLYMORPHONUCLEAR LEUKOCYTES ON PLATELET THROMBI IN A VASCULAR INJURY MODEL

The adhesion of leukocytes to platelets deposited at the site of vascu lar injury may represent an important mechanism by which leukocytes co ntribute to hemostasis and thrombosis. In this study, we examined whet her, in comparison with their distribution in circulating blood, certa in leukocyte types are enriched at sites of platelet deposition. We us ed an experimental vascular injury model, in which human fibrillar col lagen was exposed to anticoagulated human whole blood flowing through parallel-plate chambers (venous shear rate, 65/s). The platelet-adhere nt leukocytes were detached by EDTA treatment and analyzed by flow cyt ometry using cell-type-specific antibodies. The predominant leukocytes found in platelet thrombi were polymorphonuclear leukocytes, accounti ng for 76% of bound leukocytes (62% in circulating blood), whereas T a nd B lymphocytes did not significantly accumulate on thrombi, comprisi ng a fraction of less than 5% (32% in circulating blood). Monocytes co nstituted 16% of platelet thrombus-bound leukocytes, which represents an almost fourfold enrichment as compared with their proportion in cir culating blood. Almost identical results were obtained when we analyze d leukocytes adhering to platelet monolayers, which were formed by blo cking glycoprotein IIb-IIIa, thus preventing platelet aggregation on t op of the collagen-adherent platelets. Furthermore, leukocyte adhesion to platelet monolayers was completely inhibited by an anti-P-selectin antibody (50% inhibitory concentration, 0.3 mu g/mL), whereas it reac hed a plateau at about 70% inhibition on platelet thrombi. This differ ence could be explained by a possible function of glycoprotein IIb-III a in leukocyte immobilization to thrombi or by the high local concentr ation of P-selectin in the growing thrombi. The results suggest that, because of their known abilities to promote coagulation and thrombolys is, the monocytes and polymorphonuclear leukocytes accumulating on for ming platelet thrombi could play an important role in modulating throm botic and hemostatic processes. (C) 1997 by The American Society of He matology.