TLR4 signalling in the intestine in health and disease

The colonic epithelium is lined along its apical membrane with approximately 10(14) bacteria/g of tissue. Commensal bacteria outnumber mammalian cells in the gut severalfold. The reason for this degree of commensalism probably resides in the recent recognition of the microbiome as an important source of metabolic energy in the setting of poorly digestible nutrients. As in many themes in biology, the host may have sacrificed short-term benefit, i.e. nutritional advantages, for long-term consequences, such as chronic inflammation or colon cancer. In the present review, we examine the role of TLR (Toll-like receptor) signalling in the healthy host and the diseased host. We pay particular attention to the role of TLR signalling in idiopathic IBD (inflammatory bowel disease) and colitis-associated carcinogenesis. In general, TLR signalling in health contributes to homoeostatic functions. These include induction of antimicrobial peptides, proliferation and wound healing in the intestine. The pathogenesis of IBD, ulcerative colitis and Crohn's disease may be due to increased TLR or decreased TLR signalling respectively. Finally, we discuss the possible role of TLR signalling in colitis-associated neoplasia.