CYTOTROPHOBLAST CELLS - A BARRIER TO MATERNOFETAL TRANSMISSION OF PASSIVE-IMMUNITY

The human fetus receives passive immunity via the chorioallantoic plac enta 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 confoc al laser scanning fluorescence microscopy and immunoelectron microscop y of frozen sections of chorionic villi from early and late gestation. With confocal microscopy we also investigated the distribution of a r eceptor for IgG (Fc gamma RIII; CD16) that is typically expressed on t he surface of human leukocytes. Endogenous IgG was present in the sync ytiotrophoblast that surrounds chorionic villi but underlying cytotrop hoblast cells were devoid of endogenous antibody. Fc gamma RIII immuno reactivity was confined to the syncytiotrophoblast and was also absent from cytotrophoblast cells. We propose that cytotrophoblast cells rep resent a barrier to the transmission of maternally derived IgG across the human placenta. This accounts for the paradox that there are low l evels 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 placen ta matures, thus removing the cellular impediment to IgG transmission.