Toll-like receptor 2 drives liver senescence and fibrosis in aging through gut-derived microbial signaling

Background: While the role of endotoxins from Gram-negative bacteria has been studied extensively, the contribution of Gram-positive bacterial components—particularly those activating toll-like receptor 2 (TLR2), such as lipoteichoic acid (LTA)—to liver aging, inflammation, and fibrosis remains poorly understood. Here, we investigated the role of TLR2 and its ligand LTA in liver aging by using murine models, in vitro experiments, and human samples from young and elderly individuals. Methods: TLR2 ligands were evaluated in serum samples from young (aged 21–33 years) and elderly (aged 65–77 years) healthy individuals. Markers of liver damage, senescence, and inflammation were assessed in 4- and 20-month-old male C57BL/6 and TLR2 knockout (TLR2 ^−/−) mice. In addition, 17-month-old male C57BL/6 mice were treated either with the TLR2 inhibitor ortho-vanillin (60 mg/kg BW in drinking water) or plain water for 4 months and markers as determined above were assessed. The presence of markers of senescence was measured in J774A.1 cells and human peripheral blood mononuclear cells stimulated with LTA. Results: In humans and mice, aging was associated with significantly elevated circulating levels of TLR2 ligands. In aging mice, this was accompanied by increased hepatic Tlr2 mRNA expression. Strikingly, 20-month-old male TLR2 ^−/− mice exhibited reduced markers of senescence (e.g., plasma plasminogen activator inhibitor-1 and liver p16 expression), inflammation (e.g., hepatic neutrophil infiltration, Il1b mRNA expression), and fibrosis (e.g., α-smooth muscle actin expression, Sirius Red staining), compared with age-matched wild-type controls. Similarly, in aged male C57BL/6 J mice showing first signs of impaired intestinal barrier function i.e., rising peripheral TLR2 ligand levels in plasma, treatment with the TLR2 inhibitor ortho-vanillin for 4 months attenuated the progression of liver aging as indicated by attenuated senescence, liver inflammation, and fibrosis. In vitro, stimulation of J774A.1 macrophages and human peripheral blood mononuclear cells with LTA induced the expression of senescence-associated genes p16 and p21. Conclusions: Collectively, these findings suggest that increased translocation of TLR2 ligands and subsequent activation of TLR2-dependent pathways play a critical role in age-associated liver degeneration. Targeting TLR2 signaling may therefore represent a promising therapeutic approach to mitigate hepatic aging and associated pathologies.