Alterations of autophagic and innate immune responses by the Crohn's disease-associated mutation.

Crohn's disease (CD) is driven by the loss of tolerance to intestinal microbiota and excessive production of pro-inflammatory cytokines. These pro-inflammatory cytokines are produced by macrophages and dendritic cells (DCs) upon sensing the intestinal microbiota by the pattern recognition receptors (PRRs). Impaired activation of PRR-mediated signaling pathways is associated with chronic gastrointestinal inflammation, as shown by the fact that loss-of-function mutations in the nucleotide-binding oligomerization domain 2 gene increase the risk of CD development. Autophagy is an intracellular degradation process, during which cytoplasmic nutrients and intracellular pathogens are digested. Given that impaired reaction to intestinal microbiota alters signaling pathways mediated by PRRs, it is likely that dysfunction of the autophagic machinery is involved in the development of CD. Indeed, the loss-of-function mutation T300A in the autophagy related 16 like 1 (ATG16L1) protein, a critical regulator of autophagy, increases susceptibility to CD. Recent studies have provided evidence that ATG16L1 is involved not only in autophagy, but also in PRR-mediated signaling pathways. ATG16L1 negatively regulates pro-inflammatory cytokine responses of macrophages and DCs after these cells sense the intestinal microbiota by PRRs. Here, we discuss the molecular mechanisms underlying the development of CD in the T300A mutation by focusing on PRR-mediated signaling pathways.