We conducted peroxidase-labeled antibody immunocytochemical studies on the gastrointestinal tracts of neonatal rats to determine the disposition of IgA ingested in maternal milk.
From the onset of suckling to sometime between the 15th and 21st days of life, IgA and the secretory component (SC) were localized on the luminal surfaces of villi in the proximal one-third of the small intestine. At the ultrastructural level, these components of secretory IgA were associated with the plasma membranes of the upper one-third to one-half of microvilli. The IgA could not be removed by vigorous washing or trypsin treatment; it was not internalized or transported through the enterocytes to intercellular spaces; it could not be attributed to endogenous sources within the neonates. By contrast with IgA, ingested IgG was bound to the lower portions of microvilli and intervening apical plasma membranes of jejunal enterocytes, and was internalized and transported through the cells in membranous vesicles.
In the distal small intestine, neither IgA nor IgG was bound to the epithelial surface, but both immunoglobulins were found in cytoplasmic vacuoles that did not fuse with lateral cell membranes or discharge their contents into intercellular spaces.
We conclude that villus enterocytes of neonatal rat jejunum have specific binding sites for maternal secretory IgA that are distinct from the sites that bind IgG. IgG binding is involved in transepithelial translocation of immunoglobulin into the circulation, but bound IgA remains on the epithelial surfaces. In the distal small intestine, IgA and IgG are taken nonspecifically into villus epithelial cells but are not transported through the cells intact.