Mouse Susceptibility to Anthrax Lethal Toxin Is Influenced by Genetic Factors in Addition to Those Controlling Macrophage Sensitivity

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Abstract
Bacillus anthracis lethal toxin (LT) produces symptoms of anthrax in mice and induces rapid lysis of macrophages (Mφ) derived from certain inbred strains. We used nine inbred strains and two inducible nitric oxide synthase (iNOS) knockout C57BL/6J strains polymorphic for the LT Mφ sensitivity Kif1C locus to analyze the role of Mφ sensitivity (to lysis) in LT-mediated cytokine responses and lethality. LT-mediated induction of cytokines KC, MCP-1/JE, MIP-2, eotaxin, and interleukin-1β occurred only in mice having LT-sensitive Mφ. However, while iNOS knockout C57BL/6J mice having LT-sensitive Mφ were much more susceptible to LT than the knockout mice with LT-resistant Mφ, a comparison of susceptibilities to LT in the larger set of inbred mouse strains showed a lack of correlation between Mφ sensitivity and animal susceptibility to toxin. For example, C3H/HeJ mice, harboring LT-sensitive Mφ and having the associated LT-mediated cytokine response, were more resistant than mice with LT-resistant Mφ and no cytokine burst. Toll-like receptor 4 (Tlr4)-deficient, lipopolysaccharide-nonresponsive mice were not more resistant to LT. We also found that CAST/Ei mice are uniquely sensitive to LT and may provide an economical bioassay for toxin-directed therapeutics. The data indicate that while the cytokine response to LT in mice requires Mφ lysis and while Mφ sensitivity in the C57BL/6J background is sufficient for BALB/cJ-like mortality of that strain, the contribution of Mφ sensitivity and cytokine response to animal susceptibility to LT differs among other inbred strains. Thus, LT-mediated lethality in mice is influenced by genetic factors in addition to those controlling Mφ lysis and cytokine response and is independent of Tlr4 function.