A JUXTACRINE MECHANISM FOR NEUTROPHIL ADHESION ON PLATELETS INVOLVES PLATELET-ACTIVATING-FACTOR AND A SELECTIN-DEPENDENT ACTIVATION PROCESS

The aim of this study was to identify the molecular mechanisms involve d in neutrophil adhesion to immobilized platelets with particular focu s on the possible existence of a juxtacrine system for neutrophil-plat elet interactions. Platelets were immobilized onto collagen (type I)-c oated coverslips that were placed in a flow chamber and neutrophils we re perfused across these confluent monolayers at a shear stress of 1 t o 4 dynes/cm(2). Neutrophils rolled, and a significant proportion (25% to 50%) adhered to platelet monolayers. P-selectin was expressed in v ery large quantities on the surface of platelets and mediated all of t he rolling, whereas the beta(2)-integrin mediated firm adhesion. An ac tivation mechanism for adhesion was necessary inasmuch as fixed neutro phils continued to roll on immobilized platelets, but did not adhere. Platelets adherent to collagen produced significant levels of platelet -activating factor (PAF). Accordingly, the firm adhesion of neutrophil s to platelets was significantly inhibited by a PAF receptor antagonis t (WEB 2086). Treatment of only the platelets with acetylhydrolase, wh ich converts membrane-associated PAF to lyso PAF, prevented 60% of the adhesion. These data suggest that PAF, on the surface of platelets, m ediated a significant portion of the adhesive interaction. Addition of some selectin-binding carbohydrates (fucoidan or soluble SLEx analogs but not dextran sulfate) to the platelets caused rolling neutrophils to immediately adhere, an event that was not observed on histamine or thrombin-treated endothelium or P-selectin transfectants. These data s upport the view that a juxtacrine activation process exists on immobil ized platelets for neutrophils. This process can be greatly enhanced o n platelets and may involve a signaling mechanism through P-selectin. (C) 1998 by The American Society of Hematology.