Irradiated mice lose the capacity to 'process' fed antigen for systemic tolerance of delayed-type hypersensitivity.

'Intestinal antigen processing' is a function of the gastro-intestinal tract whereby shortly after an animal has been fed an immunogenic protein antigen, such as ovalbumin (OVA), a tolerogenic form of the protein is generated and can be detected in the circulation. The effect of damage to the intestinal epithelium on the processing of OVA has been examined in lethally irradiated mice. Irradiated animals were fed 25 mg OVA and their serum collected 1 h later. When this serum was transferred intraperitoneally into naive recipient mice, this did not induce the typical suppression of systemic delayed-type hypersensitivity. Results were similar when the serum donors were at 2 days after irradiation, with crypt hypoplasia, and at 5 days after irradiation when there was reactive crypt hyperplasia. However reconstitution of donors with normal spleen cells immediately after irradiation restored their capacity to generate a tolerogenic form of the antigen. Immunoreactive OVA was detected by ELISA in both tolerizing and non-tolerizing sera, and the immunological properties of these sera were not related to serum levels of OVA after feeding. Thus subtle immunochemical alterations in the nature of a protein antigen are likely to be more important than the quantity of absorbed antigen, in influencing systemic cell-mediated immune responses after feeding. The lack of generation of a tolerogenic form of the protein in irradiated mice, unrelated to the pattern of epithelial cell kinetics, and the restoration of this function by normal spleen cells, suggests that lymphoid cells may be involved in the phenomenon of antigen processing.