NLRC4 expression in intestinal epithelial cells mediates protection against an enteric pathogen

Abstract

The inflammasomes play an important role in connecting the detection of endogenous and microbial danger signals to caspase-1 activation and induction of protective immune responses. NLRC4 is a cytosolic NOD-like receptor (NLR) that can trigger inflammasome formation in response to bacterial flagellin, an immunodominant antigen in the intestine. To characterize the role of NLRC4 in bacterially-triggered intestinal inflammation, we used the murine pathogen Citrobacter rodentium, an extracellular, attaching/effacing bacterium similar to EHEC and EPEC. Following infection with C. rodentium, we found that Nlrc4−/− mice developed more severe weight loss, increased bacterial colonization levels and exacerbated intestinal inflammation compared to WT counterparts. Nlrc4−/− mice mounted robust adaptive immune responses, but were unable to control early colonization by C. rodentium, suggesting that a defect in innate immunity was responsible. Experiments using bone marrow chimeras revealed that the protective effects of NLRC4 were dependent on its expression in non-hematopoietic cells and quantitative PCR analyses revealed that NLRC4 was highly expressed in epithelial crypts but not in intestinal stroma. Thus, early NLRC4 sensing in intestinal epithelial cells regulates colonization by an extracellular bacterial pathogen and limits subsequent intestinal damage.

NOD-Like Receptors: Guardians of Intestinal Mucosal Barriers

“…Bacterial stimulation of intestinal mononuclear phagocytes (iMP) revealed that the commensals Bacterioides fragilis, Enterococcus fecalis, and Lactobacillus plantarum do not induce NLRC4-dependent release of IL-1␤, whereas Salmonella and Pseudomonas are potent stimulators of this cytokine (27). Indeed, NLRC4 seems to be a specialized guardian of the intestinal mucosal barrier, since its expression is highly elevated in resting iMPs and epithelial crypts (27,83). Therefore, it is not surprising that NLRC4 was found to be protective against intestinal pathogens such as C. rodentium or S. Tm, as well as against experimental colitis models such as DSS and AOM/DSS-induced tumorigenesis (12,14,26,43,83).…”

“…Indeed, NLRC4 seems to be a specialized guardian of the intestinal mucosal barrier, since its expression is highly elevated in resting iMPs and epithelial crypts (27,83). Therefore, it is not surprising that NLRC4 was found to be protective against intestinal pathogens such as C. rodentium or S. Tm, as well as against experimental colitis models such as DSS and AOM/DSS-induced tumorigenesis (12,14,26,43,83). Most recently, NLRC4 was found to restrict S. Tm infections through its activation of pyroptosis and expulsion of infected intestinal epithelial cells in to the lumen (98).…”

Inflammasomes

“…Similar to S. typhimurium, C. rodentium infection results in NLRP3-and NLRC4-dependent inflammatory responses that limit bacterial burden and tissue pathology, which was proposed to be IL-18 dependent (Kayagaki et al 2011b;Gurung et al 2012b;Liu et al 2012a;Alipour et al 2013). Unlike for S. typhimurium, NLRP3 and NLRC4 were shown to sense C. rodentium in nonhematopoietic cells, most likely in intestinal epithelial cells Nordlander et al 2013;Song-Zhao et al 2013). This matches the localization of C. rodentium, which does not invade host cells and instead remains firmly attached to intestinal epithelial cells.…”

Activation of NLRC4 downregulates TLR5-mediated antibody immune responses against flagellin

“…The current observations whereby flagellin lacking NLRC4-mediated activity could induce higher antibody responses against flagellin than could WT FliC was consistent with previous observations using A/J MyD88 2/232 and NLRC4 KO mice. 50 We also observed that FliC-L3A (deficient in NLRC4-mediated activity) induced significantly higher antibody titers than did FliC after intranasal immunization (data not shown). In summary, we could conclude that abrogating the NLRC4-mediated activity of flagellin upregulated the TLR5-mediated adaptive immune responses against flagellin.…”