Receptor-mediated transport of IgG across the intestinal epithelium of the neonatal rat.

The absorptive epithelium of the neonatal rat is developmentally specialized to transfer maternal immunoglobulin G (IgG) intact to the circulation while other milk protein are digested. The epithelial cells of the duodenum and proximal jejunum which are responsible for IgG transfer represent a particular striking experimental model for study of receptor-mediated intracellular transport. Receptors located on the luminal plasma membrane selectively bind the Fc region of IgG. The IgG enters the cell by constitutive endocytosis within coated vesicles and is then released at the basolateral plasma membrane. Morphological evidence supports a model in which IgG crosses the cell as a ligand-receptor complex that dissociates only on exposure to a pH 7.4 environment found at the basolateral cell surface. Although uptake of IgG at the luminal plasma membrane is highly selective, small but significant amounts of other proteins enter the cell apparently non-selectively. Nevertheless, these latter proteins are not transferred across the cell. Double-tracer experiments indicate that IgG and these other proteins enter the cell simultaneously within the same endocytic vesicles, but that non-membrane-bound proteins are removed from the IgG transport pathway by an as yet poorly defined mechanism and sequestered within small apical vacuoles and lysosomes.