Use of incompatible plasmids to control expression of antigen by Salmonella typhimurium and analysis of immunogenicity in mice

Abstract

Salmonella spp. have been investigated as live vaccine vectors because they are heat stable and can elicit humoral, cellular, and secretory immune responses. However, the expression of some foreign antigens is toxic to bacterial vectors. We therefore studied an approach for the controlled expression of antigen in Salmonella typhimurium wherein the antigen is not expressed in vitro but is expressed in vivo. A model antigen, β-galactosidase, was expressed from the trc promoter on one plasmid, while repression was achieved by Lacl expressed in trans from a second plasmid. The second repressor plasmid was incompatible with the expression plasmid encoding β-galactosidase. Loss by segregation of the repressor plasmid in vitro correlated with increased expression of β-galactosidase. Oral inoculation of mice with salmonellae containing both plasmids induced serum IgG but not nasal, salivary, or biliary IgA antibody to β-galactosidase. Serum IgG as well as biliary IgA anti- S. typhimurium antibody, but not salivary or nasal IgA, were also detected. This salmonella vector system for the controlled expression of recombinant antigens may be of value for inducing systemic but not mucosal immunity to antigens that are toxic to bacterial vectors.