The increasing number of bubonic/pneumonic plague cases globally (2010-2018), including the U.S., with a ~18% case fatality rate may reflect climate changes and a rodent carrier range shift. The 2017-18 plague outbreak in Madagascar with ~2400 cases (>75% pneumonic) and ~9% causalities has led WHO (April 2018) to intensify the need for developing new generation subunit and live-attenuated plague vaccines. This need is exemplified by deadly plague cases in China (2019) and Congo (2020 with a 35% fatality rate). Y. pestis? (Yp) ability to persist in dead hosts to resurge after years of silence, existence of antibiotic-resistant strains that occur naturally or have been intentionally developed, and no FDA-approved plague vaccine, is fearsome. Two- component subunit vaccines composed of capsular antigen F1 and a T3SS component and effector LcrV (low calcium response V antigen), which only generate a humoral immune response, provide variable protection in African green monkeys (AGM) and generate poor T cell responses in humans. Such vaccines will not be effective against Yp strains lacking F1 or possessing LcrV variants. Since the cellular immunity is also critical for protection, we focused first on identifying new virulence genes of Yp and then to delete them in combination to develop novel live-attenuated vaccine (LAV) strains. Two such LAVs were 100% attenuated in inducing bubonic/pneumonic plague in mice/rats and generated long-term humoral and cellular immune responses to provide 100% protection to rodents against developing plague. No clinical symptoms of the disease or histopathological lesions were noted either during immunization or when the vaccinated animals were subsequently exposed to Yp CO92 in a more stringent pneumonic plague model. Therefore, further immunological characterization of these mutants and their testing in higher animals, such as cynomolgus macaques (CM) and AGM, will provide a rationale for future clinical studies. There is a precedent for using a LAV against plague (EV76 strain) in humans. However, this vaccine is reactogenic, represents a spontaneous mutant, and causes disease in patients with over iron load.
In Aim 1, we will demonstrate efficacy and immune responses of two vaccine candidates generated from Yp CO92 (biovar Orientalis) against other Yp biovars (Antiqua and Medievalis), the F1-minus mutant of CO92, and Yp CO92 with LcrV variants, in bubonic and pneumonic mouse models. Our data with the mutants indicated a role of IL-17 (a Th17 cytokine), Th1-IFN-?, and antibodies, in protection.
In Aim 2, we will study the mechanistic basis of this protection (one chosen mutant) by using ROR?t-/- mice, which lack Th17 cells, as well as IFN-? and IgA k/o mice, to discern their links to neutrophil recruitment and mucosal immunity, to combat Yp infection in bubonic/pneumonic plague models.
In Aim 3, CM and AGM will be used with one mutant to demonstrate its short- and long-term efficacy in causing bubonic and/or pneumonic plague as well as reactogenicity. The correlates of protective immunity will then be established. These innovative mechanistic/translational approaches will result in effective new generation plague vaccines.
An increasing number of global deadly plague cases, including within the U.S., highlights the importance of studying new virulence genes of this pathogen and to develop safe plague vaccines, especially because antibiotic-resistant Yersinia pestis strains exist in nature or have been developed for bioweaponization. Based on the identification of new virulence genes, we have developed two excellent vaccine candidates which trigger robust protective humoral and cellular immune responses in rodents. The immunological basis of protection as well as long-term efficacy of the vaccine(s) will now be tested in nonhuman primates before launching final preclinical toxicology studies.
