Vaccines against enterotoxigenic bacterial pathogens based on hybrid Salmonella that express heterologous antigens

In this report, we examine two aspects in the development of a vaccine against enterotoxigenic bacterial pathogens based on hybrid Salmonella that express heterologous antigens. First, we describe the construction of a non-toxic fusion peptide for immunization against Escherichia coli that produce heat-labile (LT) and heat-stable (ST) enterotoxins. For that construction, the 5' terminus of the gene coding for ST was fused to the 3' terminus of the gene coding for the binding subunit of LT(LT-B). The ST gene was constructed synthetically with appropriate restriction sites to permit in-frame, downstream insertion. Maximum expression of ST antigenicity was obtained when a seven-amino-acid proline-containing linker was included between the LT-B and ST moieties. The purified LT-B/ST fusion peptide consisted of a single polypeptide chain with an apparent molecular weight of 18,000. The LT-B/ST fusion peptide was non-toxic and immunologic determinants of both LT and ST were recognized by antibodies directed against the native toxins. Animals immunized with either crude or purified preparations containing the hybrid molecule produced antibodies that were able to recognize native toxin in vitro. Significantly, these antibodies were able to neutralize the biological activity of native ST. The second aspect reported here examines a mechanism for stabilizing expression of heterologous antigens in attenuated Salmonella mutants by integration of the heterologous gene (LT-B) into the chromosome of the carrier. A comparative in vitro study of the levels of expression of LT-B between the cointegrate strain and an isogenic strain carrying the LT-B gene on a multicopy plasmid demonstrated that the initial levels of expression of both strains is similar, that the plasmid-carrying strain loses the ability to express the heterologous antigen very quickly and that the cointegrate continues to maintain and express the antigen without the requirement for a stabilizing antibiotic.