The Fischer 344 Rat Reflects Human Susceptibility to Francisella Pulmonary Challenge and Provides a New Platform for Virulence and Protection Studies

BACKGROUND The pathogenesis of Francisella tularensis, the causative agent of tularemia, has been primarily characterized in mice. However, the high degree of sensitivity of mice to bacterial challenge, especially with the human virulent strains of F. tularensis, limits this animal model for screening of defined attenuated vaccine candidates for protection studies. METHODS AND FINDINGS We analyzed the susceptibility of the Fischer 344 rat to pulmonary (intratracheal) challenge with three different subspecies (subsp) of F. tularensis that reflect different levels of virulence in humans, and characterized the bacterial replication profile in rat bone marrow-derived macrophages (BMDM). In contrast to the mouse, Fischer 344 rats exhibit a broader range of sensitivity to pulmonary challenge with the human virulent subsp. tularensis and holarctica. Unlike mice, Fischer rats exhibited a high degree of resistance to pulmonary challenge with LVS (an attenuated derivative of subsp. holarctica) and subsp. novicida. Within BMDM, subsp. tularensis and LVS showed minimal replication, subsp. novicida showed marginal replication, and subsp. holartica replicated robustly. The limited intramacrophage replication of subsp. tularensis and novicida strains was correlated with the induction of nitric oxide production. Importantly, Fischer 344 rats that survived pulmonary infection with subsp. novicida were markedly protected against subsequent pulmonary challenge with subsp. tularensis, suggesting that subsp. novicida may be a useful platform for the development of vaccines against subsp. tularensis. CONCLUSIONS The Fischer 344 rat exhibits similar sensitivity to F. tularensis strains as that reported for humans, and thus the Fischer 344 ray may serve as a better animal model for tularemia vaccine development.