Toll-like receptor 4 (TLR4) does not confer a resistance advantage on mice against low-dose aerosol infection with virulent type A Francisella tularensis.

Francisella tularensis, the causative agent of tularemia, is a gram-negative facultative intracellular bacterium. Toll-like receptor (TLR) 4 is considered to be critical for inducing host innate immunity against many gram-negative bacteria including many respiratory pathogens. To determine the role of TLR4 in host defense against airborne F. tularensis infection, TLR4-defective C3H/HeJ (TLR4(d)) or wild-type C3H/HeOuJ (WT) mice were challenged by low-dose aerosol with type A F. tularensis, and the course of the infection and host responses were compared at day 2 and 4 post-inoculation (dpi). At dpi 2, bacterial burdens in the lungs were similar between TLR4(d) and WT mice, but TLR4(d) mice surprisingly harbored approximately 10-fold fewer bacteria in their spleens and livers. However, the bacterial burdens at dpi 4, the mortality and median time to irreversible moribundity were indistinguishable between the two mouse strains. In addition, the inflammatory responses to the infection, as reflected by the cytokine levels and leukocyte influx in the bronchoalveolar lavage fluid and histopathological analysis, were similar between both mouse strains. Additionally, as with C3H mice, we found no difference in either the median time to death or the survival rate between TLR4-deleted C57BL/10ScNJ mice and WT C57BL/10 mice. Combined, these data suggest that TLR4 does not contribute to resistance of mice to airborne type A F. tularensis infection.