Identification of a new class of inducible receptors on platelets. Thrombospondin interacts with platelets via a GPIIb-IIIa-independent mechanism.

Thrombospondin with fibrinogen, fibronectin, and von Willebrand factor binds to platelets stimulated with agonists and support platelet adhesive functions. The receptors for the latter three proteins are associated with membrane glycoprotein GPIIb-IIIa. Thrombasthenic platelets deficient in GPIIb-IIIa have been utilized to examine the role of this membrane protein in the interactions of thrombospondin with platelets. Radioiodinated thrombospondin bound to thrombin-stimulated platelets from normal and thrombasthenic donors with a similar affinity and capacity. As monitored with a monoclonal antibody to thrombospondin, the divalent ion-dependent and -independent pathways for the expression of the endogenous pool of thrombospondin on the surface of thrombin-stimulated platelets from normal and thrombasthenic donors were also qualitatively and quantitatively similar. GPIIb-IIIa or ligands associated with GPIIb-IIIa thus are not essential for the binding of thrombospondin to platelets. Therefore, thrombospondin interacts with unique receptors on platelets.