Antioxidant defenses of Francisella tularensis perturb Aim2 Inflammasome Activation

Francisella tularensis is a Gram-negative bacterium that causes tularemia, a fatal zoonotic disease. F. tularensis has been used in the bioweapon programs of several countries. Its potential use as a bioterrorism agent led the CDC to classify F. tularensis as a Tier 1 Select Agent. The cytosolic sensor absent in melanoma 2 (Aim2) detects double-stranded DNA in the cytosol of infected cells and subsequently assembles a multiprotein complex known as the inflammasome. Inflammasome activation drives the secretion of IL-1{beta} and IL-18, key proinflammatory cytokines required for controlling F. tularensis infection. Prior studies have shown that F. tularensis actively suppresses Aim2 inflammasome activation; however, the underlying mechanism remains unknown. We hypothesized that F. tularensis suppresses Aim2-mediated responses by modulating the intracellular redox environment. We utilized an F. tularensis mutant lacking OxyR ({Delta}oxyR), a transcriptional regulator that controls the expression of major antioxidant enzymes. Our results show that macrophages infected with the {Delta}oxyR mutant exhibit significantly higher levels of Aim2-dependent Caspase-1 and IL-1{beta} than those infected with wild-type bacteria. The expression of interferon regulatory factor 1 and the guanylate-binding proteins GBP2 and GBP5, upstream signaling components of the Aim2 inflammasome, is markedly higher in {Delta}oxyR-infected macrophages than in controls. These changes were absent in {Delta}oxyR-infected NADPH oxidase-deficient macrophages, which are unable to generate reactive oxygen species. Collectively, these findings demonstrate that macrophage redox environment plays a key role in activating signaling components required for Aim2 inflammasome activation. This work advances our understanding of how F. tularensis-encoded factors subvert host innate immune defenses.