Cytotrophoblast cells: a barrier to maternofetal transmission of passive immunity.

The human fetus receives passive immunity via the chorioallantoic placenta in the form of maternal immunoglobulin G (IgG) class antibodies. This provides protection against pathogens at a time when the fetus is immunologically naive. We localized endogenous human IgG using confocal laser scanning fluorescence microscopy and immunoelectron microscopy of frozen sections of chorionic villi from early and late gestation. With confocal microscopy we also investigated the distribution of a receptor for IgG (Fc gamma RIII; CD16) that is typically expressed on the surface of human leukocytes. Endogenous IgG was present in the syncytiotrophoblast that surrounds chorionic villi but underlying cytotrophoblast cells were devoid of endogenous antibody. Fc gamma RIII immunoreactivity was confined to the syncytiotrophoblast and was also absent from cytotrophoblast cells. We propose that cytotrophoblast cells present a barrier to the transmission of maternally derived IgG across the human placenta. This accounts for the paradox that there are low levels of transport in the first trimester when the syncytiotrophoblast is known to express receptors for IgG. Cytotrophoblast cells form an almost complete epithelial layer underlying the syncytiotrophoblast at this stage of gestation, but this becomes discontinuous as the placenta matures, thus removing the cellular impediment to IgG transmission.