Effect of Anti-Thrombospondin Antibodies on the Hemagglutination Activities of the Endogenous Platelet Lectin and Thrombospondin

The proposal that thrombospondin is the endogenous platelet lectin was evaluated using antisera and monoclonal antibodies to thrombospondin. The platelet-bound hemagglutinin activity of human platelets stimulated with A23187 was inhibited by rabbit anti-thrombospondin sera and by a monoclonal anti-thrombospondin IgG. A second monoclonal IgG did not inhibit platelet-bound agglutinin activity. Preparations of purified platelet thrombospondin differed in their hemagglutination activities. The hemagglutination activity of an active preparation of thrombospondin was inhibited by the monoclonal antibody that inhibited platelet-bound lectin activity. The hemagglutination activity of an almost inactive preparation of thrombospondin was enhanced by the anti-thrombospondin monoclonal antibody that did not block platelet-bound lectin activity. The results demonstrate that expression of the platelet-bound form of the endogenous lectin is thrombospondin-dependent and suggest that thrombospondin must become part of a larger complex, either by binding to the platelet surface or by becoming aggregated in solution, before hemagglutination activity can be expressed.