Development of nonalcoholic steatohepatitis (NASH) involves the innate immune system and is mediated by Kupffer cells and hepatic stellate cells (HSCs). Toll-like receptor 9 (TLR9) is a pattern recognition receptor that recognizes bacteria-derived cytosine phosphate guanine (CpG)–containing DNA and activates innate immunity. We investigated the role of TLR9 signaling and the inflammatory cytokine interleukin-1β (IL-1β) in steatohepatitis, fibrosis, and insulin resistance.

Wild-type (WT), TLR9^−/−, IL-1 receptor (IL-1R)^−/−, and MyD88^−/− mice were fed a choline-deficient amino acid-defined (CDAA) diet for 22 weeks and then assessed for steatohepatitis, fibrosis, and insulin resistance. Lipid accumulation and cell death were assessed in isolated hepatocytes. Kupffer cells and HSCs were isolated to assess inflammatory and fibrogenic responses, respectively.

The CDAA diet induced NASH in WT mice, characterized by steatosis, inflammation, fibrosis, and insulin resistance. TLR9^−/− mice showed less steatohepatitis and liver fibrosis than WT mice. Among inflammatory cytokines, IL-1β production was suppressed in TLR9^−/− mice. Kupffer cells produced IL-1β in response to CpG oligodeoxynucleotide. IL-1β but not CpG-oligodeoxynucleotides, increased lipid accumulation in hepatocytes. Lipid accumulation in hepatocytes led to nuclear factor-κB inactivation, resulting in cell death in response to IL-1β. IL-1β induced fibrogenic responses in HSCs, including secretion of tissue inhibitor of metalloproteinase-1. IL-1R^−/− mice had reduced steatohepatitis and fibrosis, compared with WT mice. Mice deficient in MyD88, an adaptor molecule for TLR9 and IL-1R signaling, also had reduced steatohepatitis and fibrosis. TLR9^−/−, IL-1R^−/−, and MyD88^−/− mice had less insulin resistance than WT mice on the CDAA diet.

In a mouse model of NASH, TLR9 signaling induces production of IL-1β by Kupffer cells, leading to steatosis, inflammation, and fibrosis.