Platelet FcγRIIA binds and internalizes IgG-containing complexes

Objective

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

Methods

We demonstrated by flow cytometry and electron microscopy that human platelets at 37°C can bind and endocytose IgG complexes. We also utilized platelets from FcγRIIA transgenic mice to study endocytosis of IgG complexes by platelet FcγRIIA.

Results

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

Conclusion

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