Models Derived fromIn VitroAnalyses of Spleen, Liver, and Lung Leukocyte Functions Predict Vaccine Efficacy against the Francisella tularensis Live Vaccine Strain (LVS)
Authors: De Pascalis Roberto , Chou Alicia Y. , Ryden Patrik , Kennett Nikki J. , Sjöstedt Anders , Elkins Karen L. 
Published: April 2014

Currently, there are no licensed vaccines and no correlates of protection againstFrancisella tularensis, which causes tularemia. We recently demonstrated that measuringin vitrocontrol of intramacrophage bacterial growth by murineF. tularensis-immune splenocytes, as well as transcriptional analyses, discriminatedFrancisellavaccines of different efficacies. Further, we identified potential correlates of protection against systemic challenge. Here, we extended this approach by studying leukocytes derived from lungs and livers of mice immunized by parenteral and respiratory routes withF. tularensisvaccines. Liver and lung leukocytes derived from intradermally and intranasally vaccinated mice controlledin vitro FrancisellaLive Vaccine Strain (LVS) intramacrophage replication in patterns similar to those of splenocytes. Gene expression analyses of potential correlates also revealed similar patterns in liver cells and splenocytes. In some cases (e.g., tumor necrosis factor alpha [TNF-α], interleukin 22 [IL-22], and granulocyte-macrophage colony-stimulating factor [GM-CSF]), liver cells exhibited even higher relative gene expression, whereas fewer genes exhibited differential expression in lung cells. In contrast with their strong ability to control LVS replication, splenocytes from intranasally vaccinated mice expressed few genes with a hierarchy of expression similar to that of splenocytes from intradermally vaccinated mice. Thus, the relative levels of gene expression vary between cell types from different organs and by vaccination route. Most importantly, because studies comparing cell sources and routes of vaccination supported the predictive validity of this coculture and gene quantification approach, we combinedin vitroLVS replication with gene expression data to develop analytical models that discriminated between vaccine groups and successfully predicted the degree of vaccine efficacy. Thus, this strategy remains a promising means of identifying and quantifying correlative T cell responses.

IMPORTANCEIdentifying and quantifying correlates of protection is especially challenging for intracellular bacteria, includingFrancisella tularensis.F. tularensisis classified as a category A bioterrorism agent, and no vaccines have been licensed in the United States, but tularemia is a rare disease. Therefore, clinical trials to test promising vaccines are impractical. In this report, we further evaluated a novel approach to developing correlates by assessing T cell immune responses in lungs and livers of differentially vaccinated mice; these nonprofessional immune tissues are colonized byFrancisella. The relative degree of vaccine efficacy against systemic challenge was reflected by the ability of immune T cells, particularly liver T cells, to control the intramacrophage replication of bacteriain vitroand by relative gene expression of several immunological mediators. We therefore developed analytical models that combined bacterial replication data and gene expression data. Several resulting models provided excellent discrimination between vaccines of different efficacies.