Behaviour of two IgG subclasses in transport of immunoglobulin across the human placenta.

The human IgG subclasses are a family of highly related yet distinct molecules. Each of these four subclasses performs a discrete function within the human immune system. Previous studies have shown that one of these molecules, hIgG2, may be discriminated against in transport across the human placenta. We have aimed to elucidate the mechanism of this discrimination in order to gain a more comprehensive understanding of the process of transport of immunoglobulin across the human placenta. We have used a combination of immunocyctochemical localisation and biochemical analysis to detail the behaviour of hIgG2. Confocal laser scanning microscopy was used to compare the localisation of hIgG1 (chosen as representative of the efficiently transported subclasses) and hIgG2 in term and first trimester chorionic villi. Complementary evidence was provided from immunoblot analysis of isolated placental coated vesicles. The data presented here suggest that the hIgG2 is transported into the syncytiotrophoblast and appears to accumulate in the stroma of the villi. This leads us to the hypothesis that the fetal capillary endothelium is the cellular impediment to the transport of hIgG2 into the fetal circulation.