ONTOGENY AND DISTRIBUTION OF FC-GAMMA RECEPTORS IN THE HUMAN PLACENTA- TRANSPORT OR IMMUNE SURVEILLANCE

The human fetus acquires maternal IgG via the chorioallantoic placenta . Utilising antibodies against 3 characterised subtypes of IgG Fc rece ptor (Fc gamma R) expressed by human leucocytes, we show by confocal i mmunofluorescence microscopy that these molecules are also expressed b y cells of the placenta. Fc gamma RI (CD64) is expressed by undifferen tiated mesenchymal or fibroblast cells of Ist trimester. and term chor ionic villi. Punctate immunoreactivity for Fc gamma RII (CDw32) is fou nd on capillary endothelial cells of term and Ist trimester villi. Fc gamma RIII (CD16) expression is observed in the trophoblast surroundin g chorionic villi that forms the functional 'barrier' between mother a nd fetus. In 1st trimester villi this receptor is associated with a po pulation of marginated vesicular inclusions of the syncytiotrophoblast . In term villi the receptor is concentrated in the apex of the syncyt iotrophoblast, suggesting a possible role in the maternofetal transmis sion of passive immunity. All 3 subtypes of receptor are expressed by Hofbauer cells. We have been unable to demonstrate these receptors in cytotrophoblast cells. Results obtained using immunofluorescence and i mmunoelectron microscopic detection of endogenous IgG are consistent w ith the hypothesis that IgG is internalised into clathrin-coated pits and vesicles. Endogenous IgG was not demonstrable in cytotrophoblast c ells. The significance of our inability to demonstrate IgG or specific receptor molecules for IgG in cytotrophoblast cells, and possible rol es of Fc gamma receptor-bearing cells of the placenta are discussed.