Loss of Toll-like receptor 9 protects from hepatocellular carcinoma in murine models of chronic liver disease

Toll-like receptor 9 (Tlr9) is a pathogen recognition receptor detecting unmethylated DNA derivatives of pathogens and damaged host cells. It is therefore an important modulator of innate immunity. Here we investigated the role of Tlr9 in fibrogenesis and progression of hepatocellular carcinoma in chronic liver disease. We treated mice with a constitutive deletion of Tlr9 (Tlr9−/−) with DEN/CCl4 for 24 weeks. As a second model, we used hepatocyte-specific Nemo knockout (NemoΔhepa) mice and generated double knockout (NemoΔhepaTlr9−/−) animals. We show that Tlr9 is in the liver primarily expressed in Kupffer cells, suggesting a key role of Tlr9 in intercellular communication during hepatic injury. Tlr9 deletion resulted in reduced liver fibrosis as well as tumor burden. We observed down-regulation of hepatic stellate cell activation and consequently decreased collagen production in both models. Tlr9 deletion was associated with decreased apoptosis and compensatory proliferation of hepatocytes, modulating the initiation and progression of hepatocarcinogenesis. These findings were accompanied by a decrease in interferon-β and an increase in chemokines having an anti-tumoral effect. Our data define Tlr9 as an important receptor involved in fibrogenesis, but also in the initiation and progression of hepatocellular carcinoma during chronic liver diseases. •Tlr9 is primarily expressed in Kupffer cells, suggesting a key role in intracellular communication during liver injury.•Tlr9 deletion results in reduced liver fibrosis and tumor burden.•Absence of Tlr9 is associated with decreased apoptosis and compensatory proliferation of hepatocytes.•The tumor microenvironment is altered in Tlr9−/− mice, via upregulation of antitumoral chemokines and dowregulation of Ifnb1.