Mucosal immune responses following oral immunisation of pigs with fimbrial antigens of enterotoxigenic E. coli

The intestinal mucosal immune system can discriminate actively between harmful pathogens and harmless food antigens resulting in different immune responses namely IgA production and oral tolerance, respectively. Whereas particulate antigen (microorganisms) can induce an IgA response, soluble antigen often leads to tolerance. Recently, it has been demonstrated that F4 fimbrial antigens of enterotoxigenic E. coli (F4 ETEC) can be used to immunise piglets. Oral administration of soluble F4 to F4R+ piglets (pigs with a receptor for these fimbriae (F4R+) on their small intestinal villous enterocytes) results in an intestinal mucosal immune response that completely protects the piglets against a challenge infection. In F4R- pigs, such an intestinal mucosal immune response does not occur. However, a priming of the systemic immune system can be seen similar to the priming in pigs fed with the same dose of a food antigen, suggesting that F4 in F4R- pigs behaves as a food antigen. These results indicate that a receptor-mediated mechanism is involved in the induction of a protective intestinal mucosal immune response using soluble antigen. However, oral administration of soluble F18 fimbriae of verotoxigenic E. coli to F18R+ piglets could not induce a similar protective immune response. So the type of the receptor and/or the nature of the antigen seem to be important to obtain an intestinal IgA response.