Induction of Long-Term Immune Responses to Protect Mice against Pneumonic Plague: Development and Testing of Novel Live-Attenuated Mutants of Yersinia pestis CO92

Currently, there is no FDA-approved vaccine against the causative agent of the pneumonic plague, Yersinia pestis. Since both humoral and cell-mediated immunity are essential in providing protection against the plague, we developed three novel live-attenuated vaccine strains. These mutants were either deleted for genes encoding Braun lipoprotein (Lpp), an acetyltransferase (MsbB), the attachment invasion locus (Ail), and/or the plasminogen-activator protease (Pla) creating the Δlpp ΔmsbB Δail and Δlpp ΔmsbB Δpla triple mutants of Y. pestis CO92. Another stain containing a modified version of the ail gene with diminished virulence (Δlpp ΔmsbB::ailL2) was also developed. All three live-attenuated mutants were highly attenuated (100% survival) in mice after intranasal, intramuscular, and/or subcutaneous administration suggesting these vaccine candidates are safe with no untoward clinical symptoms or histopathological lesions in various organs. These live-attenuated mutants were able to stimulate both long-term humoral- and cell-mediated immune responses, which protected mice against exposure to highly lethal pneumonic challenge with wild-type Y. pestis CO92 strain. In summary, the ∆lpp ∆msbB ∆ail, ∆lpp ∆msbB::ailL2, and ∆lpp ∆msbB ∆pla live-attenuated mutants are viable vaccine candidates; however, future studies involving immunocompromised mice as well as evolutionary higher animal models of pneumonic plague are needed to further determine the efficacy and safety of these potential live-attenuated plague vaccines.