IL-10 gene-deficient mice lack TGF-β/Smad signaling and fail to inhibit proinflammatory gene expression in intestinal epithelial cells after the colonization with colitogenic Enterococcus faecalis

Nonpathogcnic enteric bacterial species initiate and perpetuate experimental colitis in IL-10 gene-deficient mice (IL-10^-/-). Bacteria-specific effects on the epithelium are difficult to dissect due to the complex nature of the gut microflora. We showed that IL-10^-/- mice compared with wild-type mice fail to inhibit proinflammatory gene expression in native intestinal epithelial cells (IEC) after the colonization with colitogenic Gram-positive Enterococcus faecalis. Interestingly, proinflammatory gene expression was transient after 1 wk of E. faecalis monoassociation in IEC from wild-type mice, but persisted after 14 wk of bacterial colonization in IL-10^-/- mice. Accordingly, wild-type IEC expressed phosphorylated NF-κB subunit RelA (p65) and phosphorylated Smad2 only at day 7 after bacterial colonization, whereas E. faecalis-monoassociatcd IL-10^-/- mice triggered persistent RelA, but no Smad2 phosphorylation in IEC at days 3, 7, 14, and 28. Consistent with the induction of TLR2-mcdiated RelA phosphorylation and proinflammatory 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^-/- IEC. Of note, TGF-β1-activated Smad signaling was associated with the loss of TLR2 protein expression and the inhibition of NF-κB-dependent gene expression in IEC lines. In conclusion, E. faecalis-monoassaciated IL-10 ^-/-, but not wild-type mice lack protective TGF-β/Smad signaling and fail to inhibit TLR2-mediated proinflammatory gene expression in the intestinal epithelium, suggesting a critical role for IL-10 and TGF-β in maintaining normal epithelial cell homeostasis in the interplay with commensal enteric bacteria.