Immunocytochemical and labelled tracer approaches to uptake and intracellular routing of immunoglobulin-G (IgG) in the human placenta.

Isolated lobules of freshly delivered human term placenta were (a) subjected to an indirect immunoelectron ultracryo method in which the immunoreactivity of endogenous Immunoglobulin-G (IgG) to rabbit anti-human IgG antibody was localized with protein-A-colloidal gold and (b) extracorporeally perfused and human IgG molecules complexed to horseradish peroxidase (HRP) added to the maternal perfusate and the uptake of IgG-HRP over different perfusion durations visualized ultrastructurally by using diaminobenzidine cytochemistry. Immunoreactivity to anti-human IgG antibody was localized all along the apical plasmalemma, in apical coated and uncoated vesicles, in apical and juxtanuclear multivesicular bodies, and in basal vesicles of the syncytiotrophoblast layer of the placenta. The stroma separating the syncytiotrophoblast from the foetal endothelium as well as vesicles within the endothelium were immunoreactive. No immunoreactivity was localized in paracellular clefts of endothelia. A similar distribution of exogenous IgG-HRP was observed for the perfused placentae. When bovine IgG-HRP or HRP alone were used as control tracers no uptake was seen for the former whilst the latter was observed only in early endosomal vesicles of the syncytiotrophoblast. The pattern of localization visualized in both studies is consistent with receptor-mediated uptake of IgG by the syncytiotrophoblast and a vesicular transport of IgG across the foetal endothelium.