Salmonella Vaccine Vectors Displaying Delayed Antigen Synthesis In Vivo To Enhance Immunogenicity

We have developed a regulated delayed antigen synthesis (RDAS) system for use in recombinant attenuated Salmonella vaccine (RASV) strains to enhance immune responses by reducing the adverse effects of high-level antigen synthesis. This system includes a chromosomal repressor gene, lacI, expressed from the arabinoseregulated araC P-BAD promoter. LacI serves to regulate expression from a plasmid promoter, P-trc, that directs antigen synthesis. In the presence of arabinose LacI is produced, which binds to P-trc, blocking antigen synthesis. In vivo, an arabinose-poor environment, the concentration of LacI decreases with each cell division, allowing increased antigen synthesis. To optimize the system and for comparison, we altered the lacI ribosomebinding site, start codon, and/or codon content to construct RDAS strains chi 9095, chi 9959, and chi 9241, synthesizing from low to high levels of LacI, respectively, and non-RDAS strain chi 9555 as a control. We evaluated this system with two test antigens, the green fluorescent protein for initial in vitro assessment and the Streptococcus pneumoniae PspA protein for validation of our system in mice. All RASV strains expressing PspA generated high antilipopolysaccharide antibody titers, indicating that expression of lacI did not interfere with the capacity to induce an immune response. Strain chi 9241 induced significantly higher anti-PspA IgG and IgA antibody titers than strain chi 9555, which expressed PspA constitutively. Anti-PspA antibody titers were inversely correlated to the level of LacI synthesis. Strain chi 9241 also induced significantly greater protective efficacy against challenge with virulent S. pneumoniae. These results suggest that regulated delayed antigen synthesis is useful for improving immunogenicity of RASV strains.