Construction of Bifidobacterium infantis as a live oral vaccine that expresses antigens of the major fimbrial subunit (CfaB) and the B subunit of heat-labile enterotoxin (LTB) from enterotoxigenic Escherichia coli

We sought to develop Bifidobacterium infantis (BI) as a vehicle for the expression of heterologous antigens. Two proteins of enterotoxigenic Escherichia coli (ETEC) were expressed in BI: CfaB, a major fimbrial subunit protein, and LTB, the B subunit of heat-labile enterotoxin. The expression of CfaB and LTB in BI was verified by electrophoretic analysis. Sprague-Dawley rats were then subjected to intragastric immunization with BI-CfaB and BI-LTB systems both separately and together. ELISA was used to characterize the serum and mucosal immune responses against ETEC antigens. The immunized rats were intraperitoneally challenged with wild-type ETEC H10407 to study the immune response in vivo. The serum titres of IgG and faecal IgA antibodies in the BI-CfaB plus BI-LTB mixed vaccination group were significantly greater than those in the other two groups, which were immunized with a single vaccine (P<0.05). However, no significant difference was seen between the two groups that received a single immunization. These results suggest that expressing CfaB and LTB in BI provides a probiotic system with immunogenic properties. Furthermore, the expression of LTB in BI preserved its mucosal adjuvant effect. So this study confirms that BI can be used as a novel oral vaccine expression system for a heterologous antigen and BI-LTB can provide mucosal adjuvant properties.