Expression of Fc gamma binding sites on human extraplacental membranes.

Binding sites for the Fc portion of immunoglobulin G (IgG) molecules (Fc gamma BS) were identified on amnionic epithelial cells and chorionic trophoblast cells in human extraplacental membranes using direct immunofluorescence with labeled human IgG. The binding sites are similar to FC gamma BS on placental trophoblast in their specificity and high degree of relative binding affinity for IgG monomers. Neither immunoglobulin A (IgA) nor IgM bound to a significant degree to the membranes. Binding was completely inhibited with unlabeled IgG but not F(ab')2 fragments of IgG. Studies on isolated amnion cells demonstrated that the binding sites are expressed on the cell membrane. At the level of sensitivity of the immunohistologic methods used, aggregated IgG or antigen-antibody complexes failed to bind amnionic epithelial cells or chorionic trophoblasts. This is in contrast to Fc gamma BS on macrophages which, being present in the same tissue, failed to exhibit significant binding of deaggregated IgG but bound complexes avidly.