Francisella tularensis Catalase Restricts Immune Function by Impairing TRPM2 Channel Activity

As an innate defense mechanism, macrophages produce reactive oxygen species that weaken pathogens and serve as secondary messengers involved in immune function. The Gram-negative bacterium Francisella 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 (live vaccine strain) or catalase-deficient F. tularensis (katG) show distinct profiles in their H2O2 scavenging rates, 1 and 0.015 pm/s, respectively. Murine alveolar macrophages infected with katG display abnormally high basal intracellular Ca2+ concentration that did not increase further in response to H2O2. Additionally, katG-infected macrophages displayed limited Ca2+ influx in response to ionomycin, as a result of ionophore H2O2 sensitivity. Exogenously added H2O2 or H2O2 generated by katG likely oxidizes ionomycin and alters its ability to transport Ca2+. Basal increases in cytosolic Ca2+ and insensitivity to H2O2-mediated Ca2+ entry in katG-infected cells are reversed by the Ca2+ channel inhibitors 2-aminoethyl diphenylborinate and SKF-96365. 2-Aminoethyl diphenylborinate but not SKF-96365 abrogated katG-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 katG. Together, our findings provide compelling evidence that F. tularensis catalase restricts reactive oxygen species to temper macrophage TRPM2-mediated Ca2+ signaling and limit host immune function.