Roles of the Host Oxidative Immune Response and Bacterial Antioxidant Rubrerythrin during Porphyromonas gingivalis Infection

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Abstract
The efficient clearance of microbes by neutrophils requires the concerted action of reactive oxygen species and microbicidal components within leukocyte secretory granules. Rubrerythrin (Rbr) is a nonheme iron protein that protects many air-sensitive bacteria against oxidative stress. Using oxidative burst-knockout (NADPH oxidase–null) mice and an rbr gene knockout bacterial strain, we investigated the interplay between the phagocytic oxidative burst of the host and the oxidative stress response of the anaerobic periodontal pathogen Porphyromonas gingivalis. Rbr ensured the proliferation of P. gingivalis in mice that possessed a fully functional oxidative burst response, but not in NADPH oxidase–null mice. Furthermore, the in vivo protection afforded by Rbr was not associated with the oxidative burst responses of isolated neutrophils in vitro. Although the phagocyte-derived oxidative burst response was largely ineffective against P. gingivalis infection, the corresponding oxidative response to the Rbr-positive microbe contributed to host-induced pathology via potent mobilization and systemic activation of neutrophils. It appeared that Rbr also provided protection against reactive nitrogen species, thereby ensuring the survival of P. gingivalis in the infected host. The presence of the rbr gene in P. gingivalis also led to greater oral bone loss upon infection. Collectively, these results indicate that the host oxidative burst paradoxically enhances the survival of P. gingivalis by exacerbating local and systemic inflammation, thereby contributing to the morbidity and mortality associated with infection. The physiological role of neutrophils is to seek out and destroy invading microbes. Professional phagocytes engulf (phagocytose) these organisms and kill them using bactericidal peptides, enzymes, toxic reactive oxygen species, and reactive nitrogen species produced by neutrophils and macrophages. Unfortunately, the reactive oxygen species unleashed in an oxidative burst response can cause considerable collateral damage and are directly responsible for infection-associated tissues injuries, especially if the invaders are protected against killing by neutrophils. The authors investigated the pathogenesis of Porphyromonas gingivalis, an anaerobic bacterium that is responsible for human periodontal disease and is protected against oxidative stress by the cytoplasmic protein rubrerythrin. We show that P. gingivalis is not only resistant to reactive oxygen species, but that in mice, rubrerythrin shields the bacterium against reactive nitrogen species. These features allow P. gingivalis to proliferate in animals that possess a fully functional oxidative burst response. Furthermore, we demonstrate that the neutrophil oxidative burst response, rather than eliminating the bacteria, exacerbates disease by damaging host tissues and facilitating growth and systemic dissemination of the pathogen. Collectively, this study provides important information on how oxygen-dependent killing mechanisms operate during anaerobic infection and on the role of rubrerythrin in protecting against a pathogenic anaerobic organism, while emphasizing the importance of limiting host-mediated tissue injury in inflammatory diseases caused by bacteria.