Deletion of an immune evasion gene, steD, from a live Salmonella enterica serovar Typhimurium vaccine improves vaccine responses in aged mice

Introduction: Non-typhoidal Salmonella (NTS) generally causes self-limiting gastroenteritis. However, older adults (>= 65 years) can experience more severe outcomes from NTS infection. We have previously shown that a live attenuated S. Typhimurium vaccine, CVD 1926 (I77 Delta guaBA Delta clpP Delta pipA Delta htrA), was immunogenic in adult but not aged mice. Here we describe modification of CVD 1926 through deletion of steD, a Salmonella effector responsible for host immune escape, which we hypothesized would increase immunogenicity in aged mice. Methods: Mel Juso and/or mutuDC cells were infected with S. Typhimurium I77, CVD 1926, and their respective steD mutants, and the MHC-II levels were evaluated. Aged (18-month-old) C57BL/6 mice received two doses of PBS, CVD 1926, or CVD 1926 Delta steD perorally (10(9) CFU) and the number of FliC-specific CD4(+) T cells were determined. Lastly, aged C57BL/6 mice received three doses of PBS, CVD 1926, or CVD 1926 Delta steD perorally (10(9) CFU) and then were challenged perorally with wild-type S. Typhimurium SL1344 (10(8) CFU). These animals were also evaluated for antibody responses. Results: MHC-II induction was higher in cells treated with steD mutants, compared to their respective parental strains. Compared to PBS-vaccinated mice, CVD 1926 Delta steD elicited significantly more FliC-specific CD4(+) T cells in the Peyer's Patches. There were no significant differences in FliC-specific CD4(+) T cells in the Peyer's patches or spleen of CVD 1926- versus PBS-immunized mice. CVD 1926 and CVD 1926 Delta steD induced similar serum and fecal anti-core and O polysaccharide antibody titers after three doses. After two immunizations, the proportion of seroconverters for CVD 1926 Delta steD was 83% (10/12) compared to 42% (5/12) for CVD 1926. Compared to PBS-immunized mice, mice immunized with CVD 1926 Delta steD had significantly lower S. Typhimurium counts in the spleen, cecum, and small intestine upon challenge. In contrast, there were no differences in bacterial loads in the tissues of PBS-vaccinated and CVD 1926-immunized animals. Conclusion: These data suggest that the steD deletion enhanced the immunogenicity of our live attenuated S. Typhimurium vaccine. Deletion of immune evasion genes could be a potential strategy to improve the immunogenicity of live attenuated vaccines in older adults.