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Characterization of innate immune responses to enteric bacterial pathogens in intestinal epithelial cells Khan, Mohammed Aatif

Abstract

Enteropathogenic Escherichia coli (EPEC) belongs to the attaching and effacing (A/E) family of bacterial pathogens, which infect intestinal enterocytes with type three secretion system (T3SS), leading to diarrheal disease. While A/E bacteria also express flagellin (FliC) which induces secretion of pro-inflammatory interleukin (IL)-8 from intestinal epithelial cells (IECs), it is unclear if flagellin is the sole trigger of epithelial responses. IECs express innate toll-like receptors (TLRs) to recognize flagellin and trigger inflammatory responses. Activation of TLRs is strictly controlled in IECs producing a state of hyporesponsiveness which prevents unwanted inflammation. The Single IgG IL-1 related receptor (Sigirr) is described as a negative regulator of IL-1β and TLR4 responses. While IECs express Sigirr, it is unknown if Sigirr inhibits innate responses to bacterial flagellin. The aims of this study were to characterize the role of flagellin in innate responses to EPEC infection and determine how Sigirr regulates these responses. Following infection of Caco-2 IECs by wild type and △fliC EPEC, activation of several proinflammatory genes including IL-8, MCP-1 and MIP3α occurred in a FliC-dependent manner. At later time points, a subset of these pro-inflammatory genes (IL-8, MlP3α) was also induced in cells infected with △fliC EPEC. The mouse adapted A/E pathogen Citrobacter Rodentium, triggered a similar innate response through a TLR5-independent but partially NF-kB-dependent mechanism. Moreover, the EPEC F1iC-independent responses increased in the absence of T3SS, suggesting that translocated bacterial effectors attenuated these response. While exploring regulatory mechanisms, we found that exposure of non-transformed TECs to bacterial flagellin transiently downregulated Sigirr expression correlating with IL-8 response. Transient silencing of the Sigirr gene augmented IL-S responses to flagellin, whereas stable over-expression of Sigirr diminished the NF-kB mediated IL-8 response to TLR ligands and inflammatory cytokines. The expression of Sigirr increased as IECs differentiated in tissue culture. Similarly, Sigirr expression was prominent in differentiated cells on the apex while diminished at the base of intestinal crypts in human colonic tissues. Thus, we demonstrate that A/E pathogens trigger pro-inflammatory responses through both FliC-dependent and -independent pathways that are regulated by Sigirr in differentiated human IEC, with clinical implications for infectious and inflammatory bowel diseases.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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