Abstract
Helicobacter pylori infection of the stomach causes peptic ulcer disease and gastric cancer. Despite eliciting a vigorous immune response, the bacterium persists for the life of the host. An important antimicrobial mechanism is the production of NO derived from inducible NO synthase (iNOS). We have reported that macrophages can kill H. pylori in vitro by an NO-dependent mechanism, but supraphysiologic levels of the iNOS substrate l-arginine are required. Because H. pylori induces arginase activity in macrophages, we determined if this restricts NO generation by reducing l-arginine availability. Inhibition of arginase with S-(2-boronoethyl)-l-cysteine (BEC) significantly enhanced NO generation in H. pylori-stimulated RAW 264.7 macrophages by enhancing iNOS protein translation but not iNOS mRNA levels. This effect resulted in increased killing of H. pylori that was attenuated with an NO scavenger. In contrast, inhibition of arginase in macrophages activated by the colitis-inducing bacterium Citrobacter rodentium increased NO without affecting iNOS levels. H. pylori upregulated levels of arginase II (Arg2) mRNA and protein, which localized to mitochondria, whereas arginase I was not induced. Increased iNOS protein and NO levels were also demonstrated by small interfering RNA knockdown of Arg2 and in peritoneal macrophages from C57BL/6 Arg2(-/-) mice. In H. pylori-infected mice, treatment with BEC or deletion of Arg2 increased iNOS protein levels and NO generation in gastric macrophages, but treatment of Arg2(-/-) mice with BEC had no additional effect. These studies implicate Arg2 in the immune evasion of H. pylori by causing intracellular depletion of l-arginine and thus reduction of NO-dependent bactericidal activity.