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Enteropathogenic Escherichia coli (EPEC) effector‐mediated suppression of antimicrobial nitric oxide production in a small intestinal epithelial model system

In vivo studies with the mouse‐specific member of the attaching and effacing (A/E) family of pathogens raised the possibility that these non‐invasive enteric pathogens can specifically inhibit inducible nitric oxide synthase (iNOS) expression to prevent the production of antimicrobial nitric oxide (NO). In this study we use polarized Caco‐2 cells, a model of human small intestinal epithelia, to (i) demonstrate conclusively that an A/E member, human specific enteropathogenic Escherichia coli (EPEC), can inhibit cytokine‐induced iNOS expression, (ii) show that this activity is dependent on the delivery of effector molecules into host cells and (iii) investigate the mechanism of inhibition. Analysis of the level of iNOS‐related mRNA, protein and NO production demonstrated that EPEC can inhibit iNOS expression at the transcriptional, by direct and indirect mechanisms, and post‐transcriptional levels. This transcriptional block was linked to the failure of the iNOS‐related transcriptional factor NF‐κB, but not STAT1, to undergo phosphorylation‐associated activation. A selective pressure to prevent iNOS production was evidenced by the finding that iNOS activity had a potent antimicrobial effect on adherent but not non‐adherent bacteria. Moreover, given the central role NF‐κB plays in transcribing genes associated with early host immune responses, this inhibitory mechanism presumably represents an important role in pathogenesis. Our study also provides insights into the nature of NO production in response to bacterial infection as well as the role of the locus of enterocyte effacement (LEE)‐encoded effector molecules in inhibiting iNOS expression.