IgG in murine intestinal secretions. Aging effect and possible physiological role.

To assess the role of, and age-associated alterations in, intraluminal IgG in the intestine, total IgG and subclass distribution in small intestinal perfusates and pilocarpine-stimulated intestinal secretions were measured under non-fasting conditions in three groups of BALB/c mice: 2-3, 12-14, and over 24 months old. Total intraluminal IgG and its subclasses (i.e. IgG1, IgG2a, and IgG2b) increased with age. However, the binding capacities of natural intraluminal IgG antibody (Ab) for outer membrane antigen (Ag) from normal habitant bacteria (Escherichia coli and Enterobacter agglomerans) were markedly reduced in the aged lavage samples as compared with such binding in samples from the other two age groups. The striking similarity to serum in the magnitude of the increase of intraluminal IgG and also in the distribution patterns of its subclasses in the small intestinal perfusates from all age groups suggests that the serum is a major source of the IgG. The intraluminal IgG may play some role in humoral immune protection under physiologic conditions of the gut lumen.