Construction of atetR-IntegratedSalmonella entericaSerovar Typhi CVD908 Strain That Tightly Controls Expression of the Major Merozoite Surface Protein ofPlasmodium falciparumfor Applications in Human Vaccine Production

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Abstract
AttenuatedSalmonellastrains are an attractive live vector for delivery of a foreign antigen to the human immune system. However, the problem with this vector lies with plasmid segregation and the low level of expression of the foreign gene in vivo when constitutive expression is employed, leading to a diminished immune response. We have established inducible expressions of foreign genes in theSalmonella entericaserovar Typhi CVD908 vaccine strain using the tetracycline response regulatory promoter. To set up this system, a tetracycline repressor (tetR) was integrated into a defined ΔaroClocus of the chromosome via suicide plasmid pJG12/tetR-neo. To remove theneogene conferring kanamycin resistance from the locus, acreexpression vector under the control of the tetracycline response promoter was transformed into the clone; expression of the Cre recombinase excised theneogene and generated the end strain CVD908-tetR. Expression of the luciferase reporter gene in this strain is dependent on the presence of tetracycline in the medium and can be regulated up to 4,773-fold. Moreover, the tightly controlled expression of major merozoite surface protein 1 (MSP1) and parts ofPlasmodium falciparumwas achieved, and the product yield was increased when the inducible expression system was employed. Inoculation of bacteria harboring plasmid pZE11/MSP142in mice produced the protein in liver and spleen controlled by the inducer. The persistence of the plasmid-carrying bacteria in mice was determined. Peak colonization of both liver and spleen was detected on the third day postinoculation and was followed by a decline in growth curves. After 14 days postinfection, the majority of the bacteria (>90%) recovered from the liver and spleen of the mice retained the plasmid when expression was induced; this clearly indicated that stability of the expression vector in vivo was improved by inducible expression. Establishment of the regulatory system in the vaccine strain may broaden the range of its use by enhancing plasmid stability and expression levels in vivo. Moreover, the availability of the vaccine strain inducibly expressing the entire MSP1 provides possibilities for examining its immunogenicity, particularly the cellular response in animal models.