Platelet interactions with surface-adsorbed plasma proteins: exposure of CD62P induced by von Willebrand factor

The adhesion of isolated platelets to surface-adsorbed plasma proteins and subsequent activation of the cells was studied by immunofluorescence. The spreading of the cells, formation of F-actin and exposure of CD62P, a marker for alpha-granule release, were detected by specific antibodies and FITC-labelled phalloidine. The plasma proteins used were albumin, complement factors C1q and C3, fibrinogen, thrombin and von Willebrand factor. Normal plasma was used as a positive control. Of the pure plasma proteins adsorbed, von Willebrand factor was the only protein which induced a marked exposure (> 70%) of CD62P. Platelets adhering to fibrinogen and thrombin only showed a marked exposure (>70%) of CD62P when von Willebrand factor was added to the platelet suspension. This addition of von Willebrand factor inhibited the adhesion of platelets to surface-adsorbed C1q, C3 and albumin. The finding that pure von Willebrand factor was the only protein which induced a high exposure of CD62P was verified by the adhesion of platelets to surface-adsorbed factor-VIII deficient plasma. F-actin was seen around cell edges at all surfaces, although there was a difference in spreading. We conclude that CD62P exposure on the cell surface of adhering platelets is induced mainly by von Willebrand factor but could be influenced by other proteins, and that the spreading of platelets is not correlated to the exposure of CD62P. (C) 1998 Elsevier Science B.V.