Distinct Roles of Reactive Nitrogen and Oxygen Species To Control Infection with the Facultative Intracellular BacteriumFrancisella tularensis

Abstract

Reactive nitrogen species (RNS) and reactive oxygen species (ROS) are important mediators of the bactericidal host response. We investigated the contribution of these two mediators to the control of infection with the facultative intracellular bacteriumFrancisella tularensis. When intradermally infected with the live vaccine strainF. tularensisLVS, mice deficient in production of RNS (iNOS^−/−mice) or in production of ROS by the phagocyte oxidase (p47^phox−/−mice) showed compromised resistance to infection. The 50% lethal dose (LD₅₀) for iNOS^−/−mice was 500,000 CFU for wild-type mice. The iNOS^−/−mice survived for 26.4 ± 1.8 days, and the p47^phox−/−mice survived for 10.1 ± 1.3 days. During the course of infection, the serum levels of gamma interferon (IFN-γ) and interleukin-6 were higher in iNOS^−/−and p47^phox−/−mice than in wild-type mice. Histological examination of livers of iNOS^−/−mice revealed severe liver pathology. Splenocytes obtained 5 weeks after primary infection from antibiotic-treated iNOS^−/−mice showed an in vitro recall response that was similar in magnitude and greater secretion of IFN-γ compared to cells obtained from wild-type mice. In summary, mice lacking expression of RNS or ROS showed extreme susceptibility to infection withF. tularensisLVS. The roles of RNS and ROS seemed to be distinct since mice deficient in production of ROS showed dissemination of infection and died during the early phase of infection, whereas RNS deficiency led to severe liver pathology and a contracted course of infection.