Immunogenicity and protection efficacy of enhanced fitness recombinant Typhi monovalent and bivalent vaccine strains against acute toxoplasmosis.

The proficiency of Typhi to induce cell-mediated immunity has allowed its exploitation as a live vector against the obligate intracellular protozoan . vaccine research is of great medical value due to the lack of a suitable toxoplasmosis vaccine. In the present work, we integrated antigen into a growth-dependent chromosome locus of . Typhi CVD910 strain to form recombinant . Typhi monovalent CVD910-SAG1 expressed SAG1 antigen and monovalent CVD910-GRA2 expressed GRA2 antigen. Furthermore, a low-copy stabilized recombinant plasmid encoding SAG1 antigen was transformed into CVD910-GRA2 to form bivalent CVD910-GS strain. An osmolarity-regulated promoter was also incorporated to control the gene transcription, whereas clyA export protein was included to translocate the antigen out of the cytoplasm. Both CVD910-GRA2 and CVD910-GS displayed healthy growth fitness and readily expressed the encoded antigens. When administered , CVD910-GS successfully induced both humoral and cellular immunity in the immunized BALB/c mice, and extended mice survival against virulent . In particular, the mice immunized with bivalent CVD910-GS presented the highest titers of IgG, percentages of CD4 T, CD8 T, B cells and memory T cells, and total IgG memory B cells as compared to the CVD910-GRA2 and control strains. The CVD910-GS group also generated mixed Th1/Th2 cytokine profile with secretions of IFN-ɣ, IL-2 and IL-10. This study demonstrated the importance of enhancing live vector fitness to sustain heterologous antigen expression for eliciting robust immune responses and providing effective protection against pathogen.