Microbial induction of CARD15 expression in intestinal epithelial cells via toll-like receptor 5 triggers an antibacterial response loop

With the discovery of CARD15 as susceptibility gene for Crohn's disease (CD) a first link to a potential defect in the innate immune system was made. In this work we aimed to analyze enterocyte NOD2/CARD15 expression and regulation in response to bacterial motifs and the consequences of the most common CD-specific CARD15 mutation on antibacterial responses of normal intestinal epithelial cells (IEC). Under normal conditions, IEC lines and ileal enterocytes did not express NOD2/CARD15 mRNA or protein, contrary to IEC derived from inflammatory CD sections. In vitro analyses revealed that the simple contact with non-pathogenic commensal E. Coli K12 was sufficient to induced NOD2/CARD15 mRNA and protein in human IEC (HIEC). We identified bacterial flagellin interacting with TLR5 as major motif in this regulation of NOD2/CARD15. E. Coli mutants not expressing flagellin (DeltaFliC) failed to induce CARD15. Similarly, in HIEC transfected with a plasmid encoding dominant negative TLR5, no CARD15 induction was observed after K12 contact. Isolated TLR2 or TLR4 stimulation had no or only a marginal effect on NOD2/CARD15 expression. NOD2/CARD15 negative HIEC were unresponsive to muramyl dipeptide (MDP), but once NOD2/CARD15 was induced, HIEC and Caco2 cells responded to intra or extracellular MDP presentation with the activation of the NFkB pathway. IEC transfected with the Crohn-specific CARD15 mutant (F3020insC, FS) failed to activate NFkB after MDP-challenge, in contrast to CARD15WT IEC. In response to MDP, IEC induced a massive antibacterial peptide (ABP) response, seen in the apical release of CCL20. This was completely abolished in IEC carrying CARD15FS. These data suggest a critical role of NOD2/CARD15 in the bacterial clearance of the intestinal epithelium while CD-specific mutated NOD2/CARD15 causes an impaired epithelial barrier.