Sensor molecules in intestinal innate immunity against bacterial infections

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Abstract
Purpose of review Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD)-containing proteins are innate immune sensors for microbial signature molecules. This review highlights new insights into the functions of these sensors in intestinal physiology. Recent findings TLRs are membrane bound and survey the extracellular space for microbe-derived molecules, while NOD-containing proteins are cytoplasmic and detect microbial molecules in the cytoplasm. Most microbial sensors recognize components of the bacterial cell wall and its appendages. For example, TLR4 detects lipopolysaccharide in the Gram-negative bacterial cell wall. TLR5 recognizes flagellin, a component of bacterial flagella required for motility. NOD1 recognizes diaminopimelic acid-containing dipeptide or tripeptide motifs in the Gram-positive bacterial cell wall, while NOD2 detects muramyl dipeptide, a ubiquitous cell wall peptidoglycan motif. These sensors are important for host defense against gastrointestinal pathogens. Thus, TLR4 is required for Salmonella eradication, NOD1 contributes to controlling Helicobacter pylori infection, and NOD2 is involved in mucosal defense against Listeria monocytogenes. These sensors also regulate mucosal inflammation independent of pathogen infections. Summary Toll-like receptors and nucleotide-binding oligomerization domain-containing proteins not only play critical roles in host defense against known gastrointestinal bacterial pathogens, but also contribute to mucosal homeostasis in the apparent absence of such pathogens.