Stable episomal expression system under control of a stress inducible promoter enhances the immunogenicity of Bacillus subtilis as a vector for antigen delivery

ABSTRACT

Bacillus subtilis has been successfully engineered to express heterologous antigens genetically fused to surface-exposed spore coat proteins asa vaccine vehicle endowed with remarkable heat resistance and probiotic effects for both humans and animals. Nonetheless, the immunogenicityof passenger antigens expressed by B. subtilis spores is low particularly following oral delivery. In this work, we describe a new episomalexpression system promoting enhanced immunogenicity of heterologous antigens carried by B. subtilis strains, either in the form of sporesor vegetative cells, following oral or parenteral delivery to mice. Based on a bi-directional replicating multicopy plasmid, the gene encodingthe B subunit of the heat-labile toxin (LTB), produced by enterotoxigenic Escherichia coli (ETEC) strains, was cloned under the controlof the B. subtilis glucose starvation inducible (gsiB) gene promoter, active in vegetative cells submitted to heat and other stress conditions.The recombinant plasmid proved to be structurally and segregationally stable in both cells and spores under in vitro and in vivo conditions.Moreover, BALB/c mice orally immunized with B. subtilis cells or spores elicited enhanced anti-LTB systemic (serum IgG) and secreted(fecal IgA) antibody responses, thus, suggesting that antigen expression occurred during in vivo transit. These results indicate that the newepisomal expression system may improve the performance of B. subtilis as a live orally-delivered vaccine carrier.