A New Look at Crohn's Disease: Breakdown of the Mucosal Antibacterial Defense

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Abstract
Crohn's disease (CD), a chronic inflammatory disease of the intestinal mucosa is usually located in the small intestine (ileum) and or in the colon. Ileal CD has been linked to a mutation in the NOD2 gene, a bacterial recognition protein. A disturbed antimicrobial defense as provided by an arsenal of different epithelial defensins seems to be a critical factor in disease pathogenesis. Defensins are antimicrobial peptides and in the ileum are mainly expressed in Paneth cells (PCs), epithelial cells that also express NOD2. In the colon, defensins are expressed by enterocytes or metaplastic PCs. Ileal CD patients are characterized by a reduced antibacterial activity and a specific reduction of ileal PC defensins. In ileal Crohn's patients displaying a NOD2 mutation, this decrease is even more pronounced. In contrast, CD of the colon is characterized by an impaired induction of beta defensins in enterocytes. In conclusion, the regional localizations of CD, either ileal or colonic disease, can be linked to different defects in defensin expression. In line with these new findings, we predict that future therapeutic strategies aimed at restoring the host-microbe balance at the intestinal mucosa may prove superior to those that broadly suppress inflammation and adaptive immunity.