1. Figure 2 Pro-inflammatory and anti-inflammatory effects of the gut microbiota
Figure 2 | Pro-inflammatory and anti-inflammatory effects of the gut microbiota. Pathogenic microorganisms are sensed via Toll-like receptors (TLR) and NOD-like receptors (NLR) on innate immune cells (dendritic cells (DCs), macrophages), Paneth cells and epithelial cells. This process leads to differentiation of type 17 T helper (TH17) cells and type 3 innate lymphoid cells (ILC3s) under the influence of transforming growth factor (TGF)β, IL‑6 and IL‑1β, and activation of the IL‑23 inflammatory pathway. TLR and NLR signalling also leads to NF‑κB and inflammasome activation, secretion of pro-inflammatory cytokines, and type 1 T helper (TH1) cell activation. Ultimately, these inflammatory responses lead to epithelial damage, loss of mucus-secreting goblet cells, and bacterial translocation, which further stimulates the inflammatory response. Anti-inflammatory bacteria are also sensed via TLR and NLR; however, this process leads to regulatory T (Treg)-cell differentiation via TGFβ, retinoic acid (RA) and the aryl hydrocarbon receptor (AhR) signalling. Treg cells exert their immunoregulatory function via IL‑10 secretion. Moreover, NF‑κB and inflammasome activation lead to secretion of anti-apoptotic factors and antimicrobial peptides, alongside recruitment of stromal and myeloid cells necessary for epithelial repair. Finally, IgA (produced by B cells) prevents colitogenic bacteria from penetrating the mucus layer.
Ni, J. et al. (2017) Gut microbiota and IBD: causation or correlation?
Nat. Rev. Gastroenterol. Hepatol. doi: /nrgastro