Thrombospondin inhibits adhesion of platelets to glass and protein-covered substrata

Glass and protein-covered surfaces when treated with the platelet- secreted glycoprotein thrombospondin lose their capacity to bind unstimulated platelets. In comparison to the number that bind to fibronectin-covered glass surfaces, less than 3% bind to thrombospondin- covered glass surfaces. When the fibronectin-covered surface is incubated with thrombospondin, it loses 87% of its binding capacity for platelets. The inhibitory effect of thrombospondin on platelet binding increases with increasing amounts of the adsorbed protein and reaches maximal values at 65% saturation of the adsorption of thrombospondin to the surface. Platelet spreading on the surface is also completely inhibited by thrombospondin. These data suggest that thrombospondin is nonthrombogenic and can modulate platelet adhesion to the subendothelium.