Primary liver tumors and liver metastasis currently represent the leading cause of cancer-related death. Commensal bacteria are important regulators of antitumor immunity, and although the liver is exposed to gut bacteria, their role in antitumor surveillance of liver tumors is poorly understood. We found that altering commensal gut bacteria in mice induced a liver-selective antitumor effect, with an increase of hepatic CXCR6 natural killer T (NKT) cells and heightened interferon-γ production upon antigen stimulation. In vivo functional studies showed that NKT cells mediated liver-selective tumor inhibition. NKT cell accumulation was regulated by CXCL16 expression of liver sinusoidal endothelial cells, which was controlled by gut microbiome-mediated primary-to-secondary bile acid conversion. Our study suggests a link between gut bacteria-controlled bile acid metabolism and liver antitumor immunosurveillance.

Background: Cytokine-induced killer (CIK) cell-based immunotherapy is effective as adjuvant therapy in early stage hepatocellular carcinoma (HCC) but lacks efficacy in advanced HCC. We investigated immune suppressor mechanisms focuing on CIKs and myeloid-derived suppressor cells (MDSCs).Methods: MDSCs were quantified by flow cytometry and quantitative real-time PCR. Cytokines were detected by cytokine array. An LDH cytotoxicity assay was performed in the presence or absence of MDSCs to study CIK function against HCC cells in vitro. An FDA approved PDE5 inhibitor, tadalafil, was used to target MDSCs in vitro and in vivo. Two different murine HCC cell lines were tested in subcutaneous and orthotopic tumor models in C57BL/6 and BALB/c mice. The anti-tumor effects of human CIKs and MDSCs were also tested in vitro.Results: Adoptive cell transfer of CIKs into tumor bearing mice induced inflammatory mediators (e.g., CX3CL1, IL-13) in the tumor microenvironment and an increase of tumor infiltrating MDSCs leading to impaired anti-tumor activity in two different HCC tumor models. MDSCs efficiently suppressed the cytotoxic activity of CIKs in vitro. In contrast, treatment with a PDE5 inhibitor reversed the MDSC suppressor function via ARG1 and iNOS blockade and systemic treatment with a PDE5 inhibitor prevented MDSC accumulation in the tumor microenvironment