74 Toll-Like Receptor (TLR) 2 in Kupffer Cells Plays a Crucial Role in the Development of Steatohepatitis in Mice

Abstract

Background: Gut-derived bacterial products are candidates for a promoting factor to progress non-alcoholic steatohepatitis (NASH). Toll-like receptors are pattern recognition receptors that sense microbial products such as LPS. We have previously shown that TLR9, a receptor for bacterial DNA, plays an important role in the development of NASH. However, little information on TLR2, a receptor for Gram-positive bacterial products such as peptidoglycan, is available in that process. In addition, the role of TLR2 in Kupffer cells, hepatic macrophages, remain unknown. Materials and Methods: WT and TLR2 KO were fed choline-deficient (CDAA) or Choline-supplemented control (CSAA) diet for 22 weeks. Steatosis, inflammation and fibrosis were assessed by histological examination, quantitative real-time PCR and ELISA. To examine the role of Kupffer cells in the development of steatohepatitis, WT mice were injected clodronate to deplete Kupffer cells, followed by CDAA diet feeding for 2 weeks. To further explore whether Kupffer cells mediate TLR2-dependent inflammation and fibrosis, we generated TLR2-chimeric mice using a combination of Kupffer cell depletion, irradiation and bone-marrow transplantation, followed by CDAA diet feeding for 22 weeks. Results: After 22 weeks of CDAA diet feeding, WT mice showed typical steatohepatitis characterized by severe steatosis, inflammatory cell infiltration and hepatocyte ballooning as well as perisinusoidal fibrosis. In contrast, TLR2 KOmice exhibited less inflammatory cell infiltration and hepatocyte ballooning. NAFLD activity score in TLR2KO mice was significantly lower than that in WT mice. In addition, TLR2KO mice showed a significant reduction in fibrosis compared with WT mice. In Kupffer cells-depleted WT mice, diet-induced steatohepatitis and fibrosis were significantly suppressed compared with WT mice without Kupffer cell depletion. In chimeric mice, full reconstitution of Kupffer cells was achieved, but hepatic stellate cells were not replaced by bone marrow-derived cells. WT mice containing Tlr2deficient bone marrow showed suppressed steatohepatitis and fibrosis, whereas Tlr2-deficient mice containing wild-type bone marrow showed advanced steatohepatitis and fibrosis. Conclusion: TLR2 expression in Kupffer cells plays a crucial role in CDAA diet-induced steatohepatitis and fibrosis in mice.