Pathophysiological Actions of Anthrax Virulence Determinants

National Institute Of Allergy And Infectious Diseases

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Abstract

Anthrax toxin protective antigen protein (PA, or PA83, 83 kDa) binds to receptors on the surface of mammalian cells, is cleaved by the cell surface protease furin to 63 kDa size (PA63), and forms an oligomer allowing binding of two other toxin proteins, lethal factor (LF, 90 kDa) or edema factor (EF, 89 kDa). PA translocates LF and EF to the cytosol. EF is a potent calmodulin-dependent adenylyl cyclase that causes large and unregulated increases in intracellular cAMP concentrations. LF is a metalloprotease that cleaves several mitogen-activated protein kinase kinases (MEKs). Anthrax lethal toxin (LT, the combination of PA and LF) is the primary virulence factor of B. anthracis and immunization against either of its components provides full protection against challenge with anthrax spores. Injection of this toxin into sensitive mice leads to death in 2-4 days. Injection in Fischer rats was historically shown to lead to a rapid death in as little as 37 min. The organ or cellular target for the toxin and the molecular pathway(s) leading to death are unknown, although our previous studies implicate a vascular collapse resulting from targeting of the heart in mice. In work done during 2010, we focused on the mechanism of death in rats. We used a recombinant inbred rat panel of 19 strains generated by others from two progenitor strains that were LT-sensitive and LT-resistant. This allowed mapping of the gene controlling sensitivity to this toxin to the gene rNlrp1. The protein encoded by this gene is the sensor component of the Nlrp1 inflammasome, a multiprotein complex that activates caspase-1. This gene is the closest rat homolog of murine Nlrp1b, previously shown to control mouse macrophage sensitivity to the toxin. We found an exact correlation between macrophage and animal sensitivity to LT in inbred and recombinant rats, whereas the same correlation was not found in mice. The mechanism for LT-induced macrophage death through inflammasome activation, however, was found to be similar to cell death induced in mice. LT activated caspase-1 in sensitive rat macrophages, but not in resistant cells. Inflammasome activation in rat cells was dependent on proteasome activity, as the proteasome inhibitor lactacystin could protect against toxin-mediated macrophage death. Quinidine, caspase-1 inhibitors, cathepsin B inhibitors, and heat shock protected rat macrophages from LT toxicity. Further analysis of rNlrp1 from twelve LT-sensitive and LT-resistant rat strains allowed identification of five polymorphic alleles and assignment of specific polymorphisms in the first 100 amino acids of rNlrp1 as the determining factors for animal sensitivity. The mechanism through which these limited polymorphisms in an inflammasome component determine animal survival or death is currently under study.