Platelet FcgammaRIIA binds and internalizes IgG-containing complexes

The physiologic role of platelet FcgammaRIIA, the only Fc receptor for IgG on human platelets, is largely unknown. FcgammaRIIA 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 FcgammaRIIA on platelets and macrophages at levels comparable to that seen in humans. Using the transgenic mouse model, we observed that FcgammaRIIA mediates the clearance of IgG-coated cells. With the hypothesis that FcgammaRIIA on platelets may serve to remove IgG complexes from the circulation, we studied the capacity of human platelet FcgammaRIIA to bind and internalize such complexes.

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 FcgammaRIIA transgenic mice to study endocytosis of IgG complexes by platelet FcgammaRIIA.

Wild-type mouse platelets do not express Fcgamma receptors. While platelets from wild-type mice did not bind or endocytose IgG complexes, the presence of transgenic FcgammaRIIA on mouse platelets allowed the platelets to bind and endocytose IgG complexes.

Our data indicate that platelet FcgammaRIIA binds and internalizes IgG complexes and suggest that human platelets may function to clear soluble IgG complexes from the circulation.