The normal adult intestinal tract harbors large numbers of bacteria. The interaction between bacterial flora and the intestinal mucosal immune system causes continuous inflammation, which is well controlled in the healthy intestines. The recognition of commensal bacteria by the gut mucosal immune system is mediated by pattern recognition receptors. There are at least two different classes involved; one is the cell surface receptor class of Toll-like receptors (TLRs) and the other consists of cytoplasmic proteins composed of NBD and LRR domains (NRL family proteins). We have shown that one of these NLR proteins, Nod2, detects a bacterial cell wall component in vitro and in vivo and plays a significant role in the activation of host protection mechanisms in the intestine. Nod2 plays an important role in the activation of the adaptive immune system as an adjuvant receptor. Nod2 has been mapped to the IBD1 locus and loss of function mutations in Nod2 have been shown to be related to Crohn’s disease, an inflammatory disease of the intestine. Therefore, Nod2 is a critical innate immune molecule which regulates intestinal mucosal homeostasis.


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