Construction and oral immunogenicity of a Salmonella typhimurium strain expressing a streptococcal adhesin linked to the A2/B subunits of cholera toxin

A major adhesin from the oral pathogen Streptococcus mutans has been shown to be mucosally immunogenic upon genetic fusion with the cholera toxin A2/B subunits. To take advantage of the ability of Salmonella typhimurium to deliver cloned antigens to the mucosal inductive sites that would obviate the need for antigen purification, we expressed this chimeric construct in an attenuated S. typhimurium strain under the control of bacteriophage T7 transcription. Residual expression of the temperature-regulated T7 RNA polymerase at 30°C allowed production of the chimeric protein at 2-3% of the total soluble protein, but it was increased five to six times following induction at 37°C. Oral administration of a single dose of 10⁹ recombinant Salmonella to mice resulted in serum IgG and salivary IgA antibody responses to Salmonella, cholera toxin, and the streptococcal adhesin, which were generally enhanced after a booster immunization.