CONSTRUCTION AND EVALUATION OF AN EXPRESSION VECTOR ALLOWING THE STABLE EXPRESSION OF FOREIGN ANTIGENS IN A SALMONELLA-TYPHIMURIUM VACCINE STRAIN

Salmonella strains have great potential as live carriers of heterologo us antigens to induce immunity against a variety of infectious disease s. However, the amount of heterologous antigen required to induce an a dequate immune response may be toxic for the bacterium and result in c ell death, overattenuation or loss of expression of the heterologous a ntigen. To solve this problem an expression vector was developed with a strong promoter located on a DNA fragment which is inverted at rando m. Antigen is only expressed in one particular orientation of the prom oter. Thus a bacterial population harbouring the plasmid will consist of a subpopulation which does not produce heterologous antigen, and is therefore not affected in growth, persistence and dissemination withi n the host. Further, this non-producing population will continuously s egregate antigen-producing bacteria. To evaluate the system, CtxB was used as a model antigen. Analysis of the plasmid DNA isolated from Sal monella revealed a selection against the promoter orientation that dir ects transcription of the ctxB gene. In spite of this, the vector was stably maintained in vivo and induced CtxB-specific IgA and IgG in mic e. These results indicate that this kind of expression vector may offe r a solution to the problem of unstable expression of foreign antigens in live bacterial vaccine strains.