Francisella tularensis Catalase Restricts Immune Function by Impairing TRPM2 Channel Activity.

Abstract As an innate defense mechanism, macrophages produce reactive oxygen species (ROS) which weaken pathogens and serve as secondary messengers involved in immune function. The gram-negative bacterium F. tularensis utilizes its antioxidant armature to limit the host immune response but the mechanism behind this suppression is not defined. Here we establish that F. tularensis limits Ca2+ entry in macrophages thereby limiting actin reorganization and IL-6 production in a redox-dependent fashion. Wild-type (LVS) or catalase deficient F. tularensis (ΔkatG) show distinct profiles in their H2O2 scavenging rates, 1 pM/sec and 0.015 pM/sec, respectively. Murine alveolar macrophages infected with ΔkatG display abnormally high basal intracellular Ca2+ concentration that did not increase further in response to H2O2. Additionally, ΔkatG-infected macrophages displayed limited Ca2+ influx in response to ionomycin, as a result of the ionophores H2O2-sensitivity. Exogenously added H2O2 or H2O2 generated by ΔkatG likely oxidizes ionomycin and alters its ability to transport Ca2+. Basal increases in cytosolic Ca2+ as well as insensitivity to H2O2-mediated Ca2+ entry in ΔkatG-infected cells are reversed by the Ca2+ channel inhibitors, 2-Aminoethyl diphenylborinate (2APB) and SKF-96365. 2APB but not SKF abrogated ΔkatG-dependent increases in macrophage actin remodeling and IL-6 secretion, suggesting a role for H2O2-mediated Ca2+ entry through the transient receptor potential melastatin 2 (TRPM2) channel in macrophages. Indeed, increases in basal Ca2+, actin polymerization and IL-6 production are reversed in TRPM2-null macrophages infected with ΔkatG. Together our findings provide compelling evidence that F. tularensis catalase restricts ROS to temper macrophage TRPM2-mediated Ca2+ signaling and limit host immune function.