Toll-like receptors and immune cell crosstalk in the intestinal epithelium

Abstract

The intestinal epithelium consists of a barrier one cell thick found along the length of the gastrointestinal tract composed of many cell subtypes such as absorptive enterocytes and secretory Paneth cells, Goblet cells and enteroendocrine cells. Primarily known as a cell layer used to absorb nutrients from the products of digestion and as a protective barrier from infection, this has changed in recent years with numerous discoveries indicating its importance in priming and tolerising immune cells. Toll-like receptors are a family of pathogen recognition receptors that are widely expressed in human cells including the intestinal epithelium and are known primarily as initiators of inflammatory responses. However, recent evidence suggest that they may have a variety of roles and are involved in cross-talk with a variety of cell types. This review discusses TLR signalling pathways in the context of the intestinal epithelial microenvironment, namely innate and adaptive immune cells as well as microorganisms that resident in the lumen of the gut. TLR signalling is not only involved in defence against such microorganisms but also in communicating with the underlying immune cells. This review describes the many mechanisms by which such communication is executed. It also highlights potential sources of variation in such signalling in the general population in particular the effects of genetic variation, diversity of the microbiota, concomitant disease, diet and age.