Progress on the research and development of plague vaccines with a call to action

There is a compelling demand for approved plague vaccines due to the endemicity of Yersinia pestis and its potential for pandemic spread. Whilst substantial progress has been made, we recommend that the global funding and health security systems should work urgently to translate some of the efficacious vaccines reviewed herein to expedite clinical development and to prevent future disastrous plague outbreaks, particularly caused by antimicrobial resistant Y. pestis strains.Content includes material subject to Crown Copyright © 2024.This is an open access article under the Open Government License ( http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/ ).

Plague vaccines: new developments in an ongoing search

As the reality of pandemic threats challenges humanity, exemplified during the ongoing SARS-CoV-2 infections, the development of vaccines targeting these etiological agents of disease has become increasingly critical. Of paramount concern are novel and reemerging pathogens that could trigger such events, including the plague bacterium Yersinia pestis. Y. pestis is responsible for more human deaths than any other known pathogen and exists globally in endemic regions of the world, including the four corners region and Northern California in the USA. Recent cases have been scattered throughout the world, including China and the USA, with serious outbreaks in Madagascar during 2008, 2013-2014, and, most recently, 2017-2018. This review will focus on recent advances in plague vaccine development, a seemingly necessary endeavor, as there is no Food and Drug Administration-licensed vaccine available for human distribution in western nations, and that antibiotic-resistant strains are recovered clinically or intentionally developed. Progress and recent development involving subunit, live-attenuated, and nucleic acid-based plague vaccine candidates will be discussed in this review. KEY POINTS: • Plague vaccine development remains elusive yet critical. • DNA, animal, and live-attenuated vaccine candidates gain traction.

Construction of a Live-Attenuated Vaccine Strain of Yersinia pestis EV76-B-SHUΔpla and Evaluation of Its Protection Efficacy in a Mouse Model by Aerosolized Intratracheal Inoculation

Plague, which is caused by Yersinia pestis, is one of the most dangerous infectious diseases. No FDA-approved vaccine against plague is available for human use at present. To improve the immune safety of Y. pestis EV76 based live attenuated vaccine and to explore the feasibility of aerosolized intratracheal inoculation (i.t.) route for vaccine delivery, a plasminogen activator protease (pla) gene deletion mutant of the attenuated Y. pestis strain EV76-B-SHU was constructed, and its residual virulence and protective efficacy were evaluated in a mouse model via aerosolized intratracheal inoculation (i.t.) or via subcutaneous injection (s.c.). The residual virulence of EV76-B-SHUΔpla was significantly reduced compared to that of the parental strain EV76-B-SHU following i.t. and s.c. infection. The EV76-B-SHUΔpla induced higher levels of mucosal antibody sIgA in the bronchoalveolar lavage fluid of mice immunized by i.t. but not by s.c.. Moreover, after lethal challenge with Y. pestis biovar Microtus strain 201 (avirulent in humans), the protective efficacy and bacterial clearance ability of the EV76-B-SHUΔpla-i.t. group were comparable to those of the EV76-B-SHUΔpla-s.c. and EV76-B-SHU immunized groups. Thus, the EV76-B-SHUΔpla represents an excellent live-attenuated vaccine candidate against pneumonic plague and aerosolized i.t. represents a promising immunization route in mouse model.

Yersinia pestis biovar Microtus strain 201, an avirulent strain to humans, provides protection against bubonic plague in rhesus macaques

Yersinia pestis biovar Microtus is considered to be a virulent to larger mammals, including guinea pigs, rabbits and humans. It may be used as live attenuated plague vaccine candidates in terms of its low virulence. However, the Microtus strain's protection against plague has yet to be demonstrated in larger mammals. In this study, we evaluated the protective efficacy of the Microtus strain 201 as a live attenuated plague vaccine candidate. Our results show that this strain is highly attenuated by subcutaneous route, elicits an F1-specific antibody titer similar to the EV and provides a protective efficacy similar to the EV against bubonic plague in Chinese-origin rhesus macaques. The Microtus strain 201 could induce elevated secretion of both Th1-associated cytokines (IFN-γ, IL-2 and TNF-α) and Th2-associated cytokines (IL-4, IL-5, and IL-6), as well as chemokines MCP-1 and IL-8. However, the protected animals developed skin ulcer at challenge site with different severity in most of the immunized and some of the EV-immunized monkeys. Generally, the Microtus strain 201 represented a good plague vaccine candidate based on its ability to generate strong humoral and cell-mediated immune responses as well as its good protection against high dose of subcutaneous virulent Y. pestis challenge.

Modulation of host immune defenses by Aeromonas and Yersinia species: convergence on toxins secreted by various secretion systems

Like other pathogenic bacteria, Yersinia and Aeromonas species have been continuously co-evolving with their respective hosts. Although the former is a bonafide human pathogen, the latter has gained notararity as an emerging disease-causing agent. In response to immune cell challenges, bacterial pathogens have developed diverse mechanism(s) enabling their survival, and, at times, dominance over various host immune defense systems. The bacterial type three secretion system (T3SS) is evolutionarily derived from flagellar subunits and serves as a vehicle by which microbes can directly inject/translocate anti-host factors/effector proteins into targeted host immune cells. A large number of Gram-negative bacterial pathogens possess a T3SS empowering them to disrupt host cell signaling, actin cytoskeleton re-arrangements, and even to induce host-cell apoptotic and pyroptotic pathways. All pathogenic yersiniae and most Aeromonas species possess a T3SS, but they also possess T2- and T6-secreted toxins/effector proteins. This review will focus on the mechanisms by which the T3SS effectors Yersinia outer membrane protein J (YopJ) and an Aeromonas hydrophila AexU protein, isolated from the diarrheal isolate SSU, mollify host immune system defenses. Additionally, the mechanisms that are associated with host cell apoptosis/pyroptosis by Aeromonas T2SS secreted Act, a cytotoxic enterotoxin, and Hemolysin co-regulated protein (Hcp), an A. hydrophila T6SS effector, will also be discussed.

Physiological levels of glucose induce membrane vesicle secretion and affect the lipid and protein composition of Yersinia pestis cell surfaces

Yersinia pestis grown with physiologic glucose increased cell autoaggregation and deposition of extracellular material, including membrane vesicles. Membranes were characterized, and glucose had significant effects on protein, lipid, and carbohydrate profiles. These effects were independent of temperature and the biofilm-related locus pgm and were not observed in Yersinia pseudotuberculosis.