Platelet and fibrin deposition at the damaged vessel wall: cooperative substrates for neutrophil adhesion under flow conditions.

At sites of vessel wall damage, the primary hemostatic reaction involves platelet and fibrin deposition. At these sites, circulating leukocytes marginate and become activated. Adhered platelets can support leukocyte localization; however, the role of fibrin in this respect is not known. We studied the adhesion of human neutrophils (polymorphonuclear leukocytes [PMNs]) to endothelial extracellular matrix (ECM)-bound fibrin and platelets under flow conditions. ECM alone did not show PMN adhesion. ECM-coated cover slips were perfused with plasma to form a surface-bound fibrin network, and/or with whole blood to allow platelet adhesion. Unstimulated PMNs adhered to fibrin at moderate shear stress (20 to 200 mPa). ECM-bound platelets induced rolling adhesion and allowed more PMNs to adhere at higher shear (320 mPa). ECM coated with both platelets and fibrin induced more static and shear-resistant PMN adhesion. PMN adhesion to fibrin alone but not to platelet/fibrin surfaces was inhibited by soluble fibrinogen. Adhesion to fibrin alone was inhibited by CD11b and CD18 blocking antibodies. Furthermore, fibrin formed under flow conditions showed up to threefold higher PMN adhesion compared with fibrin formed under static conditions, due to structural differences. These results indicate that circulating PMNs adhere to fibrin in an integrin-dependent manner at moderate shear stresses. However, at higher shear rates (> 200 mPa), additional mechanisms (ie, activated platelets) are necessary for an interaction of PMNs with a fibrin network.