CXCL9 Contributes to Antimicrobial Protection of the Gut during Citrobacter rodentium Infection Independent of Chemokine-Receptor Signaling

Chemokines have been shown to be effective bactericidal molecules against a variety of bacteria and fungi in vitro. These direct antimicrobial effects are independent of their chemotactic activities involving immunological receptors. However, the direct biological role that these proteins may play in host defense, particularly against intestinal pathogens, is poorly understood. Here, we show that CXCL9, an ELR- chemokine, exhibits direct antimicrobial activity against Citrobacter rodentium, an attaching/effacing pathogen that infects the gut mucosa. Inhibition of this antimicrobial activity in vivo using anti-CXCL9 antibodies increases host susceptibility to C. rodentium infection with pronounced bacterial penetration into crypts, increased bacterial load, and worsened tissue pathology. Using Rag1(-/-) mice and CXCR3(-/-) mice, we demonstrate that the role for CXCL9 in protecting the gut mucosa is independent of an adaptive response or its immunological receptor, CXCR3. Finally, we provide evidence that phagocytes function in tandem with NK cells for robust CXCL9 responses to C. rodentium. These findings identify a novel role for the immune cell-derived CXCL9 chemokine in directing a protective antimicrobial response in the intestinal mucosa.