Resistance to Yersinia pestis infection decreases with age in B10.T(6R) mice

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.

Difference in Strain Pathogenicity of Septicemic Yersinia pestis Infection in a TLR2-/- Mouse Model

Yersinia pestis is the causative agent of bubonic, pneumonic, and septicemic plague. We demonstrate that Toll-like receptor 2-deficient (TLR2^-/-) mice are resistant to septicemic infection by the KIM5 strain of Y. pestis but not to infection by the CO92 Δpgm strain. This resistance is dependent on TLR2, the route of infection, and the isoform of YopJ. Elevated bacterial burdens were found in the spleens of CO92 Δpgm-infected animals by 24 h postinfection and in the livers by 4 days. The YopJ isoform present contributed directly to cytotoxicity and inflammatory cytokine production of bone marrow-derived macrophages from TLR2^-/- mice. Immune cell trafficking is altered in CO92 Δpgm infections, with an increased neutrophil infiltration to the spleen 5 days postinfection. Immune cell infiltration to the liver was greater and earlier in KIM5-infected TLR2^-/- mice. The functionality of the immune cells was assessed by the ability to develop reactive oxygen and nitrogen species. Our data suggest an inhibition of granulocytes in forming these species in CO92 Δpgm-infected TLR2^-/- mice. These findings suggest that resistance to KIM5 in TLR2^-/- mice is dependent on early immune cell trafficking and functionality.

The Impact of Age and Sex on Mouse Models of Melioidosis

Mouse models have been used to generate critical data for many infectious diseases. In the case of Burkholderia pseudomallei, mouse models have been invaluable for bacterial pathogenesis studies as well as for testing novel medical countermeasures including both vaccines and therapeutics. Mouse models of melioidosis have also provided a possible way forward to better understand the chronicity associated with this infection, as it appears that BALB/c mice develop an acute infection with B. pseudomallei, whereas the C57BL/6 model is potentially more suggestive of a chronic infection. Several unanswered questions, however, persist around this model. In particular, little attention has been paid to the effect of age or sex on the disease outcome in these animal models. In this report, we determined the LD₅₀ of the B. pseudomallei K96243 strain in both female and male BALB/c and C57BL/6 mice in three distinct age groups. Our data demonstrated a modest increase in susceptibility associated with sex in this model, and we documented important histopathological differences associated with the reproductive systems of each sex. There was a statistically significant inverse correlation between age and susceptibility. The older mice, in most cases, were more susceptible to the infection. Additionally, our retrospective analyses suggested that the impact of animal supplier on disease outcome in mice may be minimal. These observations were consistent regardless of whether the mice were injected with bacteria intraperitoneally or if they were exposed to aerosolized bacteria. All of these factors should be considered when designing experiments using mouse models of melioidosis.

Resistance of Mice of the 129 Background to Yersinia pestis Maps to Multiple Loci on Chromosome 1

Yersinia pestis is a Gram-negative bacterium that is the causative agent of bubonic and pneumonic plague. It is commonly acquired by mammals such as rodents and humans via the bite of an infected flea. We previously reported that multiple substrains of the 129 mouse background are resistant to pigmentation locus-negative (pgm(-)) Yersinia pestis and that this phenotype maps to a 30-centimorgan (cM) region located on chromosome 1. In this study, we have further delineated this plague resistance locus to a region of less than 20 cM through the creation and phenotyping of recombinant offspring arising from novel crossovers in this region. Furthermore, our experiments have revealed that there are at least two alleles in this initial locus, both of which are required for resistance on a susceptible C57BL/6 background. These two alleles work in trans since resistance is restored in offspring possessing one allele contributed by each parent. Our studies also indicated that the Slc11a1 gene (formerly known as Nramp1) located within the chromosome1 locus is not responsible for conferring resistance to 129 mice.