Abstract
Tularemia is a severe infectious zoonotic disease caused by Francisella tularensis. Although F. tularensis is considered to be a potential biological weapon due to its high infectivity and mortality rate, no vaccine has been currently licensed. Recently, we reported that F. tularensis SCHU P9 derived ΔpdpC strain lacking the pathogenicity determinant protein C gene conferred stable and good protection in a mouse lethal model. In this study, the protective effect of ΔpdpC was evaluated using a monkey lethal model. Two cynomolgus macaques (Macaca fascicularis) intratracheally challenged with the virulent strain SCHU P9 were euthanized on 7 and 11 days post-challenge after the development of severe clinical signs. The bacterial replication in alveolar macrophages and type II epithelial cells in the lungs would cause severe pneumonia accompanied by necrosis. Conversely, two animals subcutaneously immunized with ΔpdpC survived 3 weeks after SCHU P9 challenge. Though one of the two animals developed mild symptoms of tularemia, bacterial replication was limited in the respiratory organs, which may be due to a high level of humoral and cellular immune responses against F. tularensis. These results suggest that the ΔpdpC mutant would be a safe and promising candidate as a live attenuated tularemia vaccine.
Highlights
Tularemia is a severe and sometimes fatal zoonotic disease caused by the Gram-negative intracellular bacterium Francisella tularensis
The cynomolgus macaque is a better model of inhalation tularemia than the rhesus macaque[33], as the pathology of cynomolgus macaques infected with aerosolized F. tularensis is similar to those observed in human inhalation tularemia[20]
The results showed that bacteria invaded macrophages (Fig. 5A) and other cells (Fig. 5B), which were shown to be angiotensin-converting enzyme 2 (ACE2) positive alveolar epithelial type II (AT-II) cells (Fig. 5C)
Summary
Tularemia is a severe and sometimes fatal zoonotic disease caused by the Gram-negative intracellular bacterium Francisella tularensis. Rat, rabbit, guinea pig, and macaque models have been applied in various tularemia vaccine studies[18]. Among these animal models to evaluate the efficacy of a vaccine, macaques develop clinical signs of pulmonary infection, quite similar to those in humans due to their close genetic backgrounds[19,20,21]. A macaque model is well-suited to study the pathogenesis of tularemia and develop countermeasures in humans[3,10]. The cynomolgus macaque is a better model of inhalation tularemia than the rhesus macaque[33], as the pathology of cynomolgus macaques infected with aerosolized F. tularensis is similar to those observed in human inhalation tularemia[20]. The 50% lethal dose (LD50) of the virulent strain SCHU S4 in cynomolgus macaque is approximately 20 CFU via aerosol infection[10,33]
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