Figure 2.

Association of biliary pathway and host immunity. (1) Primary bile acids (cholic acid and chenodeoxycholic acid) are mainly synthesized and conjugated with glycine and taurine respectively in the liver and are then released into the gastrointestinal tract. (2) In the intestinal lumen, gut microbiota metabolize these primary bile acids to secondary bile acids (deoxycholic and lithocholic acid, respectively). Secondary bile acids can regulate immune response in gut by decreasing proinflammatory cytokines. (3) Subsequently, enterohepatic circulation reabsorbs 95% of bile acids in the terminal ileum, and they are deconjugated in the liver through the portal vein. Nonreabsorbed secondary bile acids (5%) in the terminal ileum will be then excreted in the feces. (4) In the liver, deconjugated secondary bile acids become primary bile acids that could inhibit hepatic tumor growth by the induction of CXCL16-activated NKT cells. (5) However, it is uncertain whether these primary bile acids are able to inhibit tumor growth in the bile duct. CXCL16, C-X-C Motif Chemokine Ligand 16; FXR, farnesoid X receptors; G + PBAR1, G protein-coupled bile acid receptor 1; NKT cells, natural killer T cells.