Construction and screening of attenuated ΔphoP/Q Salmonella typhimurium vectored plague vaccine candidates.

Preclinical studies evaluating plague vaccine candidates have demonstrated that the F1 and V protein antigens of Yersinia pestis confer protection against challenge from virulent strains. Live-attenuated ΔphoP/Q Salmonella typhimurium recombinants were constructed expressing either F1, V antigens, F1 and V antigens, or a F1-V fusion from Asd (+) balanced-lethal plasmids. To improve antigen delivery, genes encoding plague antigens were modified in order to localize antigens to specific bacterial cellular compartments which include cytoplasmic, outer membrane, or secreted. Candidate vaccine strains were evaluated for growth characteristics, full-length lipopolysaccharide (LPS), plasmid stability, and antigen expression in vitro. Plague vaccine candidate strains with favorable in vitro profiles were evaluated in murine or rabbit preclinical oral immunogenicity studies. Attenuated S. typhimurium strains expressing cytoplasmically localized F1-V and V antigen antigens were more immunogenic than strains that secreted or localized plague antigens to the outer membrane. In particular, S. typhimurium M020 and M023, which express Asd(+)-plasmid derived soluble F1-V and soluble V antigen, respectively, at high levels in the bacterial cell cytoplasm were found to induce the highest levels of plague-specific serum antibodies. To further evaluate balanced-lethal plasmid retention capacity, ΔphoP/Q S. typhimurium PurB(+) and GlnA(+) balanced-lethal plasmid systems harboring F1-V were compared with M020 in vitro and in BALB/c mice in a immunogenicity study. Although there was no detectable difference in plague antigen expression in vitro, S. typhimurium M020 was the most immunogenic plague antigen vector strain evaluated, inducing high-titer serum IgG antibodies specific against F1, V and F1-V.