Abstract
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 Ca(2+) 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 (ΔkatG) show distinct profiles in their H2O2 scavenging rates, 1 and 0.015 pm/s, respectively. Murine alveolar macrophages infected with ΔkatG display abnormally high basal intracellular Ca(2+) concentration that did not increase further in response to H2O2. Additionally, ΔkatG-infected macrophages displayed limited Ca(2+) influx in response to ionomycin, as a result of ionophore H2O2 sensitivity. Exogenously added H2O2 or H2O2 generated by ΔkatG likely oxidizes ionomycin and alters its ability to transport Ca(2+). Basal increases in cytosolic Ca(2+) and insensitivity to H2O2-mediated Ca(2+) entry in ΔkatG-infected cells are reversed by the Ca(2+) channel inhibitors 2-aminoethyl diphenylborinate and SKF-96365. 2-Aminoethyl diphenylborinate but not SKF-96365 abrogated ΔkatG-dependent increases in macrophage actin remodeling and IL-6 secretion, suggesting a role for H2O2-mediated Ca(2+) entry through the transient receptor potential melastatin 2 (TRPM2) channel in macrophages. Indeed, increases in basal Ca(2+), 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 reactive oxygen species to temper macrophage TRPM2-mediated Ca(2+) signaling and limit host immune function.