The ontogeny and heterogeneity of Fc-gamma receptor bearing cells in the human placenta and fetal membranes.

The human fetus is donated passive immunity, via the placenta, in the form of maternal IgG class antibodies. This provides protection against pathogens while the fetus is immunologically immature. Immunocytochemistry has been performed on placental tissues to localise endogenous IgG and three subtypes of IgG receptor designated FcgammaRI (CD64), FcgammaRII (CDw32) and FcgammaRIII (CD16). Ultrastructural immunocytochemistry has been used to investigate the subcellular distribution of IgG. FcgammaRI is expressed by mesenchymal cells of term and first trimester placentae. FcgammaRII is expressed by endothelial cells and FcgammaRIII is expressed by syncytiotrophoblast. Endogenous IgG is restricted to the syncytiotrophoblast in first trimester villi indicating that transport is inhibited in early stages of gestation. At term IgG is associated with extracellular matrix, serum, endothelial cells, mesenchymal cells and syncytiotrophoblast suggesting that it is efficiently transported across the trophoblast. Ultrastructural observations of IgG distribution indicate that it is taken up by receptor-mediated endocytosis into the syncytiotrophoblast and reaches an endosome-like compartment. In term amniochorion FcgammaRI, FcgammaRII and FcgammaRIII are expressed by leucocytes in the maternal decidua and heterogeneous populations of mesenchymal cells in the fetal connective tissues suggesting that they possess an immunoregulatory role. Endogenous IgG is restricted to the extracellular matrix and cells possessing Fcgamma receptors. Cytotrophoblast and amniotic epithelial cells, which do not possess Fcgamma receptors, contain no endogenous antibodies. The hypothesis is proposed that cytotrophoblast cells are a barrier to IgG transmission across the placenta. This accounts for the paradox that there are low levels of transport in the first trimester when the syncytiotrophoblast expresses Fcgamma receptors. Cytotrophoblast cells form an almost continuous layer underlying the syncytiotrophoblast in the first trimester which becomes attenuated as the placenta matures. Furthermore, IgG may diffuse non-specifically between cytotrophoblast cells of the amniochorion and thus contribute to the acquisition of passive immunity. These data suggest that IgG is transmitted across the vascular system of the placenta and is not acquired via amniotic fluid.