Antioxidant defenses of Francisella tularensis perturb Aim2 inflammasome activation

ABSTRACT 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-1β and IL-18, key pro-inflammatory 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 live vaccine strain (LVS) mutant lacking OxyR (Δ oxyR ), a transcriptional regulator that controls the expression of major antioxidant enzymes. Our results show that macrophages infected with the Δ oxyR mutant exhibit significantly higher levels of Aim2-dependent caspase-1 and IL-1β 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 Δ oxyR -infected macrophages than in controls. These changes were absent in Δ oxyR -infected NADPH oxidase-deficient macrophages, which are unable to generate reactive oxygen species. Collectively, these findings demonstrate that the macrophage redox environment plays a key role in activating the Aim2 inflammasome. This work advances understanding of how F. tularensis -encoded factors subvert host innate immune defenses.