Parenteral vaccination of mice and piglets with F4+ Escherichia coli suppresses the enteric anti-F4 response upon oral infection.

We studied with a mouse model and in piglets the requirements to prime for a secondary, mucosal B-cell response against Escherichia coli F4 fimbriae, an important virulence factor of enterotoxigenic E. coli, the agent associated with postweaning diarrhoea in piglets. The major observation obtained with the mouse model was verified for piglets. Mice and piglets were primed orally or parenterally with purified F4ac antigen or whole bacterial cells carrying the F4ac antigen and were later orally infected with live F4ac+ E. coli bacteria. Cell suspensions of murine spleen or porcine serum were used to study the systemic B-cell response. Cell suspensions were also made of murine and porcine enteric lamina propria and were used to study the mucosal B-cell response. Enzyme-linked immunospot assays and enzyme-linked immunosorbent assays specific to E. coli F4ac antigen were used to quantify either the antibody-secreting cells or antibody titres in serum. Results showed that in mice only primary oral immunization with live bacteria induced an enteric immune response against the E. coli F4ac+ fimbriae. Oral immunization with killed bacteria induced hardly any mucosal immune response. Parenteral immunization induced a state of suppression that was reflected by the lack of an enteric immune response upon a subsequent oral infection with live bacteria. A comparable induction of suppression was observed in piglets using the same protocol. We conclude that parenteral vaccination of piglets with the E. coli F4ac antigen is ineffective to induce protective immunity at the mucosal level against postweaning diarrhoea and is possibly detrimental.