Fig. 1.

The intestinal microbiota is involved in regulating intestinal homeostasis and the immune system. Gut microbiota components or metabolites regulate microbial-host interactions and immune tolerance. TLR4 in epithelial and immune cells recognizes LPS and activates NF-κB and NLRP3 inflammasome via MyD88 to produce pro-inflammatory cytokines, ultimately leading to pyroptosis. TLR5 expressed by the basolateral epithelium of the intestine detects bacterial translocation and causes chronic inflammation by recognizing the FLG of Gram-negative bacteria. TRL2 recognizes LTA in the cell wall of Gram-positive bacteria and activates mast cells via PGE2, causing edema and increased vascular permeability. PGN activates defensin synthesis in Paneth cells and increases transcription of pro-inflammatory genes by promoting NF-κB and MAPK activation via NOD family proteins. Microbial metabolites modulate immune responses through different pathways and coordinate host gut health and immune function.