Cathelicidin senses enteric pathogen Salmonella typhimurium/LPS for colonic chemokine generation: a new innate immune role for a host defense peptide
Authors: Holani Ravi , Lopes Fernando , Blyth Graham A. D. , Jijon Humberto , McKay Derek M. , Hollenberg Morley D. , Cobo Eduardo R. 
Published: August 2018

The mechanisms by which epithelia identify and respond to pathogens are manifold, nuanced and complex. Here, using human-colon derived HT29 epithelial cells, mouse and human primary colonoids, and cathelicidin null (Cramp) mice, we report a novel immunoregulatory role for the antimicrobial peptide, cathelicidin, that was found to recognize and synergise with Salmonella typhimurium or its derived virulence factor lipopolysaccharide (LPS) to promote epithelial synthesis of the chemokine IL-8/KC for neutrophil recruitment/activation during infectious colitis. Mechanistically, cathelicidin facilitated the internalization of LPS via GM1 lipid rafts and subsequent TLR4 activation to promote IL-8 production. Furthermore, IL-8 output required the integrated activity of two signal transduction pathways: NF-κB and MEK 1/2 kinase signaling was required for IL-8 mRNA synthesis, while Src-EGFR-p38MAPK (NF-κB independent) activity underlay IL-8 mRNA stabilization. This immunomodulatory function of cathelicidin was key in colon defense, since Cramp−/− mice infected with a natural murine Gram negative intestinal pathogen, Citrobacter rodentium, displayed diminished KC secretion, impaired mobilization and reduced clearance of the bacteria. Occurring at concentrations lower than those necessary for anti-microbial activity, cathelicidin’s capacity to sense pathogens/LPS and enhance neutrophil recruitment reveals a novel function for this peptide in directing innate immunity which may be of pivotal importance in the control of infections colitis.

Author summary

The gut lining has a well regulated immune system that tolerates resident bacteria and does not respond to them. However, when pathogenic bacteria enter, there needs to be a protective response. How the gut lining ‘switches’ from passive to protective is of interest. In our study, we determined host defense cathelicidin peptide (either naturally occurring or administered) “instructs” the colon lining to produce a compound (IL-8) that attracts white blood cells in response to a pathogen (Salmonella typhimurium) or lipopolysaccharide, a component of this pathogen’s cell wall. We discovered a novel mechanism by which cathelicidin facilitates uptake of lipopolysaccharide by the lining of the colon and how it activates receptors to increase synthesis and release of IL-8. In addition, we also detected a synergistic action between cathelicidin and intestinal pathogens in laboratory cultures of colon tissues from mice and humans, as well as in a mouse model of colitis with another pathogenic bacterium. Cathelicidin induced production of IL-8 which attracted and stimulated more white blood cells. Therefore, in addition to potential direct actions to supress harmful bacteria, cathelicidin also acts as a biological sensor in the gut lining, recognizing pathogens or factors they produce and increasing white cell responses.