Intestinal epithelial toll-like receptors: to protect. And serve?

The innate immune system uses a series of pattern recognition receptors to detect the presence of pathogens thus allowing for rapid host defense responses to invading microbes. A key component of such receptors are the "toll-like receptors" (TLRs), which recognize a panel of microbial molecules that tend to be somewhat invariant, at least in select regions, thus permitting a relatively small number of receptors to recognize a large number of different microbes. Accordingly, this panel of TLRs bears little ability to distinguish between commensal and pathogenic microbes as such organisms generally bear far more structural similarities than differences between them. For the professional phagocytic cells classically considered to be the primary mediators of innate immunity such distinction between commensal and pathogenic microbes is not particularly important since any microbe that breaches the outer host defensive barriers to reach these phagocytes, whether doing so by a pathogen-specific or opportunistic mechanism, is likely potentially hazardous to its host. However, epithelial cells that line mucosal surfaces, thus being on the front line of host defense, also play an active role in innate immunity particularly by secreting chemokines and other immune mediators in response to pathogenic microbes. Epithelial cells have been reported to express several TLRs suggesting these receptors play a role in intestinal epithelial innate immune signaling pathways. However, since some mucosal surfaces such as the intestinal epithelium are normally densely colonized by a wide variety of microbes, the ability to distinguish the occasional pathogen from the sea of commensals presents an important challenge. This minireview considers the current findings regarding TLR expression in the intestinal epithelium and the role these receptors might serve in host defense.