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a Else-Kroener-Fresenius Centre for Experimental Nutritional Medicine, Technical University of Munich, 85350 Freising-Weihenstephan, Germany b Department of Medicine, University of North Carolina, Chapel Hill, North Carolina, USA
Key Words: chronic intestinal inflammation • intestinal epithelial cells • Enterococus faecalis • nuclear factor (NF)-  B • toll-like receptor (TLR) 2 • TGF- /Smad signal transduction
Address for correspondence: Dr. Dirk Haller, Else-Kroener-Fresenius Centre for Experimental Nutritional Medicine, Technical University of Munich, 85350 Freising-Weihenstephan, Germany. Voice: +49-8161-71-2026; fax: +49-8161-71-2097. e-mail: haller{at}wzw.tum.de
Nonpathogenic enteric bacterial species initiate and perpetuate experimental colitis in interleukin-10 geneeficient mice (IL-10-/-). Bacteria-specific effects on the epithelium are difficult to distinguish because of the complex nature of the gut microflora. We showed that IL-10-/- mice compared to wild-type mice fail to inhibit pro-inflammatory gene expression in native intestinal epithelial cells after the colonization with colitogenic Gram-positive Enterococcus faecalis. Of interest, pro-inflammatory gene expression was transient after 1 week of E. faecalis monoassociation in IECs from wild-type mice but persisted after 14 weeks of bacterial colonization in IL-10-/- mice. Accordingly, wild-type IECs expressed phosphorylated NF-kappaB subunit RelA (p65) and phosphorylated Smad2 only at day 7 after bacterial colonization, whereas E. faecalis-monoassociated IL-10-/- mice triggered persistent RelA but no Smad2 phosphorylation in IECs at days 3, 7, 14, and 28. Consistent with the induction of TLR2-mediated RelA phosphorylation and pro-inflammatory gene expression in E. faecalis-stimulated cell lines, TLR2 protein expression was absent after day 7 from E. faecalis-monoassociated wild-type mice but persisted in IL-10-/- IECs. Of note, TGF-beta-activated Smad signaling was associated with the loss of TLR2 protein expression and the inhibition of NF-kappa Bependent gene expression in E. faecalis-stimulated IEC lines. In conclusion, E. faecalis-monoassociated IL-10-/- but not wild-type mice lack protective TGF-beta/Smad signaling and fail to inhibit TLR2-mediated pro-inflammatory gene expression in the intestinal epithelium, suggesting a critical role for IL-10 and TGF-beta in maintaining normal epithelial cell homeostasis in the interplay with commensal enteric bacteria.
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