Mucosal immunization with a bacterial protein antigen genetically coupled to cholera toxin A2/B subunits

The generation of secretory IgA Abs for specific immune protection of mucosal surfaces depends on stimulation of the mucosal immune system, but this is not effectively achieved by parenteral or even oral administration of most soluble Ags. To harness the exceptional mucosal immunogenicity of cholera toxin (CT), which is largely attributed to the cell-binding property of its B subunit, for the generation of other oral vaccines, we have genetically replaced the toxic A1 subunit of CT with a 42-kDa segment of a streptococcal protein adhesin. This construct was expressed in Escherichia coli as a chimeric protein that retained the GM1 ganglioside-binding activity of CT subunit B and the antigenicity of the streptococcal adhesin, as shown by GM1-ELISA developed with Abs to the steptococcal segment. The protein composition of chromatographically purified chimeric protein was verified by SDS-PAGE and Western blotting with Abs to both antigenic components of the construct. Peroral administration of this chimeric immunogen in mice elicited high levels of mucosal IgA and serum IgG Abs to the streptococcal adhesin, which persisted for at least 6 mo. This strategy allows the development of similar constructs from other candidate Ags for oral immunization against a variety of mucosally acquired infections.