Host stress and immune responses during aerosol challenge of Brown Norway rats with Yersinia pestis

Sex differences in immune protection in mice conferred by heterologous vaccines for pneumonic plague

Background

Yersinia pestis is the etiological agent of plague, which can manifest as bubonic, septicemic, and/or pneumonic disease. Plague is a severe and rapidly progressing illness that can only be successfully treated with antibiotics initiated early after infection. There are no FDA-approved vaccines for plague, and some vaccine candidates may be less effective against pneumonic plague than bubonic plague. Y. pestis is not known to impact males and females differently in mechanisms of pathogenesis or severity of infection. However, one previous study reported sex-biased vaccine effectiveness after intranasal Y. pestis challenge. As part of developing a safe and effective vaccine, it is essential that potential sex differences are characterized.

Methods

In this study we evaluated novel vaccines in male and female BALB/c mice using a heterologous prime-boost approach and monitored survival, bacterial load in organs, and immunological correlates. Our vaccine strategy consisted of two subcutaneous immunizations, followed by challenge with aerosolized virulent nonencapsulated Y. pestis. Mice were immunized with a combination of live Y. pestis pgm- pPst-Δcaf1, live Y. pestis pgm- pPst-Δcaf1/ΔyopD, or recombinant F1-V (rF1-V) combined with adjuvants.

Results

The most effective vaccine regimen was initial priming with rF1-V, followed by boost with either of the live attenuated strains. However, this and other strategies were more protective in female mice. Males had higher bacterial burden and differing patterns of cytokine expression and serum antibody titers. Male mice did not demonstrate synergy between vaccination and antibiotic treatment as repeatedly observed in female mice.

Conclusions

This study provides new knowledge about heterologous vaccine strategies, sex differences in plague-vaccine efficacy, and the immunological factors that differ between male and female mice.

Protection Induced by Oral Vaccination with a Recombinant Yersinia pseudotuberculosis Delivering Yersinia pestis LcrV and F1 Antigens in Mice and Rats against Pneumonic Plague

A newly attenuated Yersinia pseudotuberculosis strain (designated Yptb1) with triple mutation Δasd ΔyopK ΔyopJ and chromosomal insertion of the Y. pestis caf1R-caf1M-caf1A-caf1 operon was constructed as a live vaccine platform. Yptb1 tailored with an Asd⁺ plasmid (pYA5199) (designated Yptb1[pYA5199]) simultaneously delivers Y. pestis LcrV and F1. The attenuated Yptb1(pYA5199) localized in the Peyer's patches, lung, spleen, and liver for a few weeks after oral immunization without causing any disease symptoms in immunized rodents. An oral prime-boost Yptb1(pYA5199) immunization stimulated potent antibody responses to LcrV, F1, and Y. pestis whole-cell lysate (YPL) in Swiss Webster mice and Brown Norway rats. The prime-boost Yptb1(pYA5199) immunization induced higher antigen-specific humoral and cellular immune responses in mice than a single immunization did, and it provided complete short-term and long-term protection against a high dose of intranasal Y. pestis challenge in mice. Moreover, the prime-boost immunization afforded substantial protection for Brown Norway rats against an aerosolized Y. pestis challenge. Our study highlights that Yptb1(pYA5199) has high potential as an oral vaccine candidate against pneumonic plague.

Aerosolization and recovery of viable murine norovirus in an experimental setup

Noroviruses are the major cause for viral acute gastroenteritis in the world. Despite the existing infection prevention strategies in hospitals, the disease continues to spread and causes extensive and numerous outbreaks. Hence, there is a need to investigate the possibility of airborne transmission of norovirus. In this study, we developed an experimental setup for studies on the infectivity of aerosolized murine norovirus (MNV), a model for the human norovirus. Two aerosol generation principles were evaluated: bubble bursting, a common natural aerosolization mechanism, and nebulization, a common aerosolization technique in laboratory studies. The aerosolization setup was characterized by physical and viral dilution factors, generated aerosol particle size distributions, and the viral infectivity after aerosolization. We found a lower physical dilution factor when using the nebulization generator than with the bubble bursting generator. The viral dilution factor of the system was higher than the physical dilution; however, when comparing the physical and viral dilution factors, bubble bursting generation was more efficient. The infectivity per virus was similar using either generation principle, suggesting that the generation itself had a minor impact on MNV infectivity and that instead, the effect of drying in air could be a major reason for infectivity losses.

Standardized Method for Aerosol Challenge of Rodents with Yersinia pestis for Modeling Primary Pneumonic Plague

Primary pneumonic plague occurs when Yersinia pestis is inhaled into the lower respiratory tract where it invades the alveoli and grows. Rapid bacterial growth eventually elicits a neutrophilic inflammatory response that is ineffective and damaging, leading to accelerated progression of disease. In the laboratory, modeling of primary pneumonic plague can be accomplished by instillation of bacterial culture in the nares of anesthetized mice and rats. Although primary pneumonic plague can develop from this method, variability in dosing and side effects of anesthesia can complicate data interpretation. In contrast, aerosol challenge models allow for well-controlled studies of pneumonic plague with minimal experimental bias and unwanted side effects. For these reasons, antibiotic testing and the licensing of new treatments depend on efficacy data generated from aerosol delivery of Y. pestis in order to more accurately model transmission and the early stages of human pneumonic plague. In order to meet this need, we have extensively characterized pneumonic plague in mice and rats challenged by nose-only exposure to Yersinia pestis. With this approach, simultaneous challenge of large cohorts of animals, gently restrained and not anesthetized, assures safe, well-controlled, unbiased, and uniform infection. In this chapter, we present a standardized method for reproducible aerosol delivery of wild-type Y. pestis to rodents for experimental models of primary pneumonic plague.

Remote monitoring of the progression of primary pneumonic plague in Brown Norway rats in high-capacity, high-containment housing

Development of new vaccines, diagnostics, and therapeutics for biodefense or other relatively rare infectious diseases is hindered by the lack of naturally occurring human disease on which to conduct clinical trials of efficacy. To overcome this experimental gap, the U.S. Food and Drug Administration established the Animal Rule, in which efficacy testing in two well-characterized animal models that closely resemble human disease may be accepted in lieu of large-scale clinical trials for diseases with limited natural human incidence. In this report, we evaluated the Brown Norway rat as a model for pneumonic plague and describe the natural history of clinical disease following inhalation exposure to Yersinia pestis. In high-capacity, high-containment housing, we monitored temperature, activity, heart rate, and rhythm by capturing electronic impulses transmitted from abdominal telemeter implants. Using this system, we show that reduced activity and development of fever are sensitive indications of disease progression. Furthermore, we identified heart arrhythmias as contributing factors to the rapid progression to lethality following the fever response. Together, these data validate the Brown Norway rat as an experimental model for human pneumonic plague and provide new insight that may ultimately lead to novel approaches in postexposure treatment of this devastating infection.