Difference between bacterial and food antigens in mucosal immunogenicity

The mucosa-associated lymphoid tissue may deviate from its systemic counterpart in being able to discriminate between microbial and nonmicrobial antigens. To study this, the systemic and mucosal antibody responses to bacterial and food antigens were followed in parallel in female rats during two pregnancies and lactation periods. Germfree rats were monocolonized with an Escherichia coli O6K13H1 strain, and their diet was switched to pellets containing large amounts of ovalbumin and beta-lactoglobulin. Antibodies against O6 lipopolysaccharide already appeared in serum and bile 1 week after colonization, and those against type 1 fimbriae appeared a few weeks later. Serum immunoglobulin G antibodies against the E. coli enzyme beta-galactosidase were found in moderate titers in all rats after 16 weeks of exposure. In contrast, few rats had detectable antibody levels against the dietary proteins ovalbumin and beta-lactoglobulin in serum or bile even after 16 weeks of exposure. In the milk, antibodies against E. coli beta-galactosidase and type 1 fimbriae reached the highest titers, while moderate titers were found against the food antigens and against O6 lipopolysaccharide. The difference in immune reactivity against bacterial versus dietary antigens was not likely due to insufficient amounts of the latter reaching lymphoid tissue, since (i) uptake studies indicated that ovalbumin was more efficiently taken up than endotoxin and (ii) the same difference in antigenicity between ovalbumin and E. coli was seen after immunization directly into Peyer's patches. We therefore suggest that a prerequisite for a strong mucosal antibody response is that the antigen be encountered by the gut-associated lymphoid tissue within microorganisms capable of stimulating antigen presentation.