Although commensal and pathogenic bacteria can be recognized by host pattern recognition receptors, intestinal epithelial cells target protective inflammatory responses towards pathogenic organisms through mechanisms that are incompletely understood. Additional mechanisms of pathogen sensing must exist that allow intestinal cells to target inflammatory defenses towards bona fide pathogens during an infection, and not harmless commensal bacteria. Pathogenic bacteria can express virulence determinants. Phenazine toxins are a family of redox active virulence determinants that are produced by a variety of human pathogens, including P. aeruginosa. P. aeruginosa can colonize the intestines of immunocompromised patients and cause fulminant septicemia and subsequent death. The mechanism by which intestinal epithelial cells detect P. aeruginosa, and whether this involves the surveillance of phenazine toxins, is not known. Nuclear hormone receptors (NHR) are transcription factors that program adaptive host responses following recognition of specific exogenous or endogenous ligands. In particular, HNF4? is an NHR expressed in the intestine. In the model organism C. elegans, the HNF4? homolog NHR-86 is required for the transcriptional activation of innate immune effector genes that protect against P. aeruginosa infection. The central hypothesis of this proposal is that intestinal epithelial cells detect infection through the surveillance of pathogen-derived phenazine toxins by NHR-86/ HNF4?, which directly activates protective anti-pathogen defenses in the intestinal epithelium. The following key preliminary findings support this central hypothesis: i) P. aeruginosa mutants that cannot make phenazine toxins do not activate C. elegans innate immune defenses; ii) synthetic phenazine toxins can activate immune genes; and iii) induction of immune genes by phenazine toxins is dependent on the expression of NHR-86/ HNF4?. In this proposal, Aim 1 will characterize the C. elegans immune response towards bacterial phenazine toxins, and Aim 2 will define the role of intestinal NHR-86/HNF4? in detecting P. aeruginosa infection in C. elegans. The research proposed here will define a new concept of immune activation in intestinal epithelial cells and will also attribute a novel role for NHRs in pathogen sensing in the intestine. Insights from these findings may identify unexplored approaches for the development of anti-inflammatory and anti-infective therapies.
The intestinal microbiome harbors a community of commensal and pathogenic bacteria, yet the mechanisms of pathogen detection by the intestinal epithelium remain poorly understood. Proper detection of pathogens promotes human intestinal health. This proposal will define a new mechanism of pathogen sensing by intestinal cells in a model organism.
