The etiologic agent of plague, Yersinia pestis, is a globally distributed pathogen which poses both a natural and adversarial threat. Due largely to the rapid course and high mortality of pneumonic plague, vaccines are greatly needed. Two-component protein vaccines have been unreliable and potentially vulnerable to vaccine resistance. We evaluated the safety and efficacy of eight live Y. pestis strains derived from virulent strains CO92 or KIM6+ and mutated in one or more virulence-associated gene(s) or cured of plasmid pPst. Stringent, single-dose vaccination allowed down-selection of the two safest and most protective vaccine candidates, CO92 mutants pgm- pPst- and ΔyscN. Both completely protected BALB/c mice against subcutaneous and aerosol challenge with Y. pestis. Strain CD-1 outbred mice were more resistant to bubonic (but not pneumonic) plague than BALB/c mice, but the vaccines elicited partial protection of CD-1 mice against aerosol challenge, while providing full protection against subcutaneous challenge. A ΔyscN mutant of the nonencapsulated C12 strain was expected to display antigens previously concealed by the capsule. C12 ΔyscN elicited negligible titers to F1 but comparable antibody levels to whole killed bacteria, as did CO92 ΔyscN. Although one dose of C12 ΔyscN was not protective, vaccination with two doses of either CO92 ΔyscN, or a combination of the ΔyscN mutants of C12 and CO92, protected optimally against lethal bubonic or pneumonic plague. Protection against encapsulated Y. pestis required inclusion of F1 in the vaccine and was associated with high anti-F1 titers.

Melioidosis is an emerging disease that is caused by the facultative intracellular pathogen Burkholderia pseudomallei. It is intrinsically resistant to many antibiotics and host risk factors play a major role in susceptibility to infection. Currently, there is no human or animal vaccine against melioidosis. In this study, multiple B. pseudomallei MSHR668 deletion mutants were evaluated as live attenuated vaccines in the sensitive BALB/c mouse model of melioidosis. The most efficacious vaccines after an intraperitoneal challenge with 50-fold over the 50% median lethal dose (MLD₅₀) with B. pseudomallei K96243 were 668 ΔhisF and 668 ΔilvI. Both vaccines completely protected mice in the acute phase of infection and showed significant protection (50% survivors) during the chronic phase of infection. The spleens of the survivors that were examined were sterile. Splenocytes from mice vaccinated with 668 ΔhisF and 668 ΔilvI expressed higher amounts of IFN-γ after stimulation with B. pseudomallei antigens than splenocytes from mice vaccinated with less protective candidates. Finally, we demonstrate that 668 ΔhisF is nonlethal in immunocompromised NOD/SCID mice. Our results show that 668 ΔhisF and 668 ΔilvI provide protective cell-mediated immune responses in the acute phase of infection and promote long term survival in the sensitive BALB/c mouse model of melioidosis.

The administration of antipyretic analgesics prior to, in conjunction with, or due to sequelae associated with vaccination is a common yet somewhat controversial practice. In the context of human vaccination, it is unclear if even short-term analgesic regimens can significantly alter the resulting immune response, as literature exists to support several scenarios including substantial immune interference. In this report, we used a live attenuated Yersinia pestis vaccine to examine the impact of analgesic administration on the immune response elicited by a single dose of a live bacterial vaccine in mice. Mice were assessed by evaluating natural and provoked behavior, as well as food and water consumption. The resulting immune responses were assessed by determining antibody titers against multiple antigens and assaying cellular responses in stimulated splenocytes collected from vaccinated animals. We observed no substantial benefit to the mice associated with the analgesic administration. Splenocytes from both C57BL/6 and BALB/c vaccinated mice receiving acetaminophen have a significantly reduced interferon-gamma (IFN-γ) recall response. Additionally, there is a significantly lower immunoglobulin (Ig)G2a/IgG1 ratio in vaccinated BALB/c mice treated with either acetaminophen or meloxicam and a significantly lower IgG2c/IgG1 ratio in vaccinated C57BL/6 mice treated with acetaminophen. Taken together, our data indicate that the use of analgesics, while possibly ethically warranted, may hinder the accurate characterization and evaluation of novel vaccine strategies with little to no appreciable benefits to the vaccinated mice.