It has been previously shown that immunoglobulins of the IgG class are absorbed and transported across the proximal small intestine of neonatal rats during the first 18 to 21 days of life. In the present work, we examined the binding of mouse immunoglobulins to the intestinal epithelium of neonatal rats. Radioiodinated mouse immunoglobulins of various classes and IgG subclasses, Fab and Fc fragments of IgG, and monomeric and heat-aggregated forms of IgG were instilled into loops of intestine prepared from rats of various ages. The specificity of the radiolabeled immunoglobulin binding was evaluated by competitive inhibition with unlabeled proteins. Monomeric IgG bound specifically to the apical regions of villi in the proximal jejunum. Specific binding progressively decreased from the 12th day of life and could not be demonstrated after about the 21st day of life. Mouse IgA and IgM did not bind specifically. Radioiodinated Fc fragments of IgG bound specifically, whereas radioiodinated Fab fragments did not. Treatment of 14-day-old rats with cortisone acetate caused binding of monomeric IgG to cease prematurely by day 16. Intestinal loops treated with trypsin no longer bound radioiodinated IgG.

Heat-aggregated IgG also bound specifically to jejunal epithelium. In contrast to the binding of monomeric immunoglobulin, this binding continued into adulthood and increased in response to cortisone treatment. Aggregated immunoglobulin did not inhibit the binding of monomeric immunoglobulin, but did inhibit the binding of aggregates.

These studies: 1) confirm the presence of receptors for the Fc region of monomeric IgG on absorptive epithelial cells of the rat jejunum; 2) demonstrate that the receptors disappear at the time when immunoglobulin transport is known to cease, and therefore appear to be critically involved in transport of monomeric IgG; 3) demonstrate that the receptors for monomeric IgG are sensitive to trypsin and disappear prematurely after cortisone treatment; 4) demonstrate specific binding of aggregated IgG, perhaps by receptor mechanisms different from those involved in the binding of monomeric IgG.

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This work was supported by the Medical Research Service of the Veterans Administration and by Grants AI-09758 and CA-17342 from the National Cancer Institute through the National Large Bowel Cancer Project.

This work was presented in part at the annual meeting of the American Gastroenterological Association, Miami Beach, Florida, May 25, 1976.

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