FIGURE 6.

Mechanisms underlying the regulation of TLR4 gene expression in IECs. Mechanisms underlying the regulation of TLR4 gene expression in IECs are schematically drawn. TLR4 gene expression is repressed due to block of transcription by epigenetic mechanisms in differentiated villus IECs to prevent triggering an excessive inflammatory reaction against commensal bacteria ([A]). However, when invading microbes reach the crypt, undifferentiated IECs in this region expressing TLR4 at a higher level may respond to them. TLR4 expression is beyond control of epigenetic repression and is increased by stimulation with commensal bacteria in undifferentiated crypt IECs ([B]). Commensal bacteria contribute to epigenetic repression of TLR4 gene expression in IECs of the large intestine.