Toll‐like receptors‐2, ‐3 and ‐4 expression patterns on human colon and their regulation by mucosal‐associated bacteria

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Abstract
The colonic epithelium provides an interface between the host and micro-organisms colonising the gastrointestinal tract. Molecular recognition of bacteria is facilitated through Toll-like receptors (TLR). The colonic epithelium expresses relatively high levels of mRNA for TLR3 and less for TLR2 and -4. Little is known of the expression patterns and mode of induction of expression for these pattern recognition receptors in human colon. The aim of this study was to investigate their localization in the gut and induction of expression in epithelial cell lines by mucosal bacteria. TLR2 and -4 were expressed only in crypt epithelial cells, expression was lost as the cells matured and moved towards the gut lumen. In contrast, TLR3 was only produced in mature epithelial cells. HT29 and CACO-2 had different levels of expression for TLR1–4. Co-culture of HT29 cells with different mucosal isolates showed that they were highly responsive to bacterial challenge, with up-regulation of mRNA for TLR1–4. In contrast, CACO-2 cells were refractive to bacterial challenge, showing little difference in mRNA levels. TLR3 was induced in HT29 only by Gram-positive commensals with up-regulation of both mRNA and protein and an enhancement of the antiviral immune response. This pattern of expression allows induction of responsiveness to bacteria only by the crypt epithelium so that tolerance to commensal organisms can be maintained. In contrast, mature columnar epithelium is able to respond to viral pathogens, which are not part of the normal gut commensal microbiota.