Platelet FcγRIIA binds and internalizes IgG-containing complexes

Objective. The physiologic role of platelet Fc gamma RIIA, the only Fc receptor for IgG on human platelets, is largely unknown. Fc gamma RIIA is also expressed on phagocytes such as monocytes and neutrophils, where it mediates the binding and internalization of both soluble IgG-containing complexes and IgG-coated cells. We previously reported the creation and characterization of a transgenic mouse that expresses human Fc gamma RIIA on platelets and macrophages at levels comparable to that seen in humans. Using the transgenic mouse model, we observed that Fc gamma RIIA mediates the clearance of IgG-coated cells. With the hypothesis that Fc gamma RIIA on platelets may serve to remove IgG complexes from the circulation, we studied the capacity of human platelet Fc gamma RIIA to bind and internalize such complexes. Methods. We demonstrated by flow cytometry and electron microscopy that human platelets at 37 degrees C can bind and endocytose IgG complexes. We also utilized platelets from Fc gamma RIIA transgenic mice to study endocytosis of IgG complexes by platelet Fc gamma RIIA. Results. Wild-type mouse platelets do not express Fc gamma receptors. While platelets from wildtype mice did not bind or endocytose IgG complexes, the presence of transgenic Fc gamma RIIA on mouse platelets allowed the platelets to bind and endocytose IgG complexes. Conclusion. Our data indicate that platelet Fc gamma RIIA binds and internalizes IgG complexes and suggest that human platelets may function to clear soluble IgG complexes from the circulation. (c) 2006 International Society for Experimental Hematology. Published by Elsevier Inc.