Abstract
FcRn, a non-classical MHCI molecule, transports IgG from mother to young and regulates the rate of IgG degradation throughout life. Brambell proposed a mechanism that unified these two functions, saying that IgG was pinocytosed nonspecifically by the cell into an FcRn-expressing endosome, where, at low pH, it bound to FcRn and was exocytosed. This theory was immediately challenged by claims that FcRn specificity for ligand could be conferred at the cell surface in neonatal jejunum. Assessing Brambell's hypothesis we found abundant nonspecifically endocytosed IgG present in the cytoplasm of FcRn−/− enterocytes. Further, IgG was present in the intercellular clefts and the cores of FcRn+/+ but not FcRn−/− jejunum. FcRn specificity for ligand could be determined within the cell.
Highlights
The father of maternal-fetal IgG transport, Rogers Brambell, hypothesized that a single Fc receptor was responsible remarkably for two widely disparate critical bodily functions; for the transport of IgG across fetal or neonatal tissue barriers and for the regulation of the rate of IgG degradation throughout the life span of the individual [1,2]
The receptor served as an effective ‘transporter’ by moving IgG across the cell, and it regulated the rate of IgG degradation by ‘protecting’ IgG from the lysosomal degradation pathway
We found that all r values were near zero indicating that most of the variability was within the animals and that variation among the animals was minimal
Summary
The father of maternal-fetal IgG transport, Rogers Brambell, hypothesized that a single Fc receptor was responsible remarkably for two widely disparate critical bodily functions; for the transport of IgG across fetal or neonatal tissue barriers and for the regulation of the rate of IgG degradation throughout the life span of the individual [1,2] These two functions took place in entirely different organs during entirely different developmental periods; transport in the transient placenta or yolk sac (YS) or neonatal gut, regulation of degradation at unspecified sites in the long-lived body. His model, accommodating all observations of the time, predicted that the single receptor worked at both sites in the same fashion. It was found to bind and protect albumin from degradation in a like manner, explaining many old observations about albumin turnover [8,9,10]
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