Primary liver tumors and liver metastasis currently represent the leading cause of cancer-related deaths. The liver intimately cross-talks with the gut and performs many essential functions related to digestion, metabolism of nutrients, and clearance of bacterial metabolites. Diseased livers are often associated with altered gut bacterial composition, or dysbiosis, and it has been suggested that gut bacterial products contribute to malignant transformation of hepatocytes. The liver is exposed to the gut microbiome through the portal vein and is an immunological organ that is heavily populated by immune cells. Emerging studies have shown that gut commensal bacteria are important regulators of antitumor immunity. Although it has been established that the gut microbiome influences the efficacy of cancer immunotherapy, the role of gut bacteria in antitumor surveillance in the liver is poorly understood.

The liver is exposed to gut bacterial metabolites and products by way of blood from the intestine, which comprises 70% of the whole liver blood supply. Changes in the gut microbiome may affect immune cell function in the liver, and commensal bacteria can mediate the metabolism of primary into secondary bile acids, which recirculate back into the liver through the enterohepatic circulation. Given that bile acids are known to be involved in liver cancer development, we focused on the role of bile acids in immunosurveillance of tumors growing in the liver. We altered gut bacteria and examined changes of hepatic immune cells and antitumor immunity directed against liver tumors. Uncovering how the gut microbiome uses bile acids to shape immunity to liver cancer may have future therapeutic applications.

Using one primary liver model and three liver metastasis models, we found that altering commensal gut bacteria induced a liver-selective antitumor effect. A selective increase of hepatic CXCR6⁺ natural killer T (NKT) cells was observed, independent of mouse strain, gender, or presence of liver tumors. The accumulated hepatic NKT cells showed an activated phenotype and produced more interferon-γ upon antigen stimulation. In vivo studies using both antibody-mediated cell depletion and NKT-deficient mice confirmed that NKT cells mediated the inhibition of tumor growth in the liver. Further investigation showed that NKT cell accumulation was regulated by the expression of CXCL16, the solo ligand for CXCR6, on liver sinusoidal endothelial cells, which form the lining of liver capillaries and the first barrier for the blood coming from the gut entering the liver. Primary bile acids increased CXCL16 expression, whereas secondary bile acids showed the opposite effect. Removing gram-positive bacteria by antibiotic treatment with vancomycin, which contains the bacteria mediating primary-to-secondary bile acid conversion, was sufficient to induce hepatic NKT cell accumulation and decrease liver tumor growth. Feeding secondary bile acids or colonization of bile acid–metabolizing bacteria, reversed both NKT cell accumulation and inhibition of liver tumor growth in mice with altered gut commensal bacteria. In nontumor liver tissue from human patients with primary liver cancer, primary bile acid chenodeoxycholic acid (CDCA) levels correlated with CXCL16 expression, whereas an inverse correlation was observed with secondary bile acid glycolithocholate (GLCA), suggesting that the finding may apply to humans.

We describe a mechanism by which the gut microbiome uses bile acids as messengers to control a chemokine-dependent accumulation of hepatic NKT cells and antitumor immunity in the liver …