TLRs in the Gut I. The role of TLRs/Nods in intestinal development and homeostasis

The innate immune system includes microbial pattern recognition receptors that detect bacteria and viral products at the cell surface, in vesicles, and within the cytoplasm. Transmembrane signaling occurs through Toll-like receptors (TLRs). Cytoplasmic receptors are generally members of the nucleotide-binding domain (NOD)-leucine-rich repeat (LRR) family (CATERPILLER family). They influence the effects of other family members and of TLRs. Most NOD-LRR members enhance signal transduction, but Monarch-1 counterbalances TLR activity. NOD-LRR family members also act within the adaptive immune system. The class II transactivator regulates major histocompatibility complex class II expression. In the intestine, it is developmentally regulated, and its expression depends on weaning and, independently, on age.