INDUCTION OF ORAL TOLERANCE WITH EFFECTS ON NUMBERS OF IGE-CARRYING MAST-CELLS AND ON BYSTANDER SUPPRESSION IN YOUNG-RATS

The presence of IgE(+) mast cells in the small intestine, bystander su ppression of DTH and antibody responses to human serum albumin (HSA) w ere studied in young rats, made tolerant to ovalbumin (OA) by feeding an OA-containing diet for 1-4 weeks starting from weaning, and in sens itized control rats. One week after finishing the OA diet, both groups of rats were immunized with a mixture of OA and HSA in Freund's compl ete adjuvant (FCA) at one site on the back. The animals were then colo nized far 5 days with a genetically manipulated Escherichia coli produ cing OA. Immunohistochemical staining of the small intestine of the ra ts fed the OA diet for 4 weeks showed significantly fewer IgE(+) mast cells in the lamina propria, a lower level of MHC class antigen was fo und in the epithelial cells and in the lamina propria, and the villus crypt depth was also significantly less in tolerant compared with sens itized rats (P = 0.003, 0.007, 0.003, respectively). Sensitized rats s howed a mild diarrhoea during the colonization in contrast to tolerant rats. All rats fed OA showed a significantly reduced IgE anti-OA anti body and DTH response to OA before colonization compared with the sens itized rats. Bystander suppression of IgG and IgE anti-HSA antibody re sponses was also seen, but only in the rats fed OA for either 1 or 4 w eeks. Rats fed the OA-containing diet for 1, 3, or 4 weeks showed byst ander suppression of the DTH response to HSA. After colonization with E. coli producing OA, rats tolerant to OA after either 1 or 4 weeks on an OA diet maintained tolerance to OA and bystander suppression HSA. These results suggest that oral tolerance to OA down-regulates signs o f local inflammatory response by IgE, IgG antibody and T cell response s to OA, but also provides bystander suppression to an unrelated antig en, HSA.