Vibrio cholerae evades neutrophil extracellular traps by the activity of two extracellular nucleases

The Gram negative bacterium Vibrio cholerae is the causative agent of the secretory diarrheal disease cholera, which has traditionally been classified as a noninflammatory disease. However, several recent reports suggest that a V. cholerae infection induces an inflammatory response in the gastrointestinal tract indicated by recruitment of innate immune cells and increase of inflammatory cytokines. In this study, we describe a colonization defect of a double extracellular nuclease V. cholerae mutant in immunocompetent mice, which is not evident in neutropenic mice. Intrigued by this observation, we investigated the impact of neutrophils, as a central part of the innate immune system, on the pathogen V. cholerae in more detail. Our results demonstrate that V. cholerae induces formation of neutrophil extracellular traps (NETs) upon contact with neutrophils, while V. cholerae in return induces the two extracellular nucleases upon presence of NETs. We show that the V. cholerae wild type rapidly degrades the DNA component of the NETs by the combined activity of the two extracellular nucleases Dns and Xds. In contrast, NETs exhibit prolonged stability in presence of the double nuclease mutant. Finally, we demonstrate that Dns and Xds mediate evasion of V. cholerae from NETs and lower the susceptibility for extracellular killing in the presence of NETs. This report provides a first comprehensive characterization of the interplay between neutrophils and V. cholerae along with new evidence that the innate immune response impacts the colonization of V. cholerae in vivo. A limitation of this study is an inability for technical and physiological reasons to visualize intact NETs in the intestinal lumen of infected mice, but we can hypothesize that extracellular nuclease production by V. cholerae may enhance survival fitness of the pathogen through NET degradation.
Author Summary Although several reports describe an inflammatory component of the diarrheal disease cholera, the innate immune response to V. cholerae and its impact on the pathogenesis of the disease is poorly understood. In the present study we can link the presence of host neutrophils with a colonization defect of a V. cholerae mutant deleted for both extracellular nucleases, Dns and Xds. Neutrophils can be seen as a first line of defense of the innate immunity and can effectively entrap and kill pathogens in neutrophil extracellular traps (NETs). Herein we show for the first time that V. cholerae induces NET formation, but effectively uses its two extracellular nucleases to degrade NETs and evade from this innate immunity weapons. Interestingly, we recently characterized the two extracellular nucleases as modulators of extracellular DNA during biofilm formation, which is rather associated with environmental lifestyle of this facultative human pathogen in aquatic ecosystems. Thus, V. cholerae seems to utilize the activity of the extracellular nucleases under both stages of its lifecycle, inside the host as a defense mechanism against NETs and during biofilm formation in the environment.