Pro-fibrolytic Myeloid TLR4 Signaling for Resolution of Murine Liver Fibrosis: Roles of Gut Microbiome-Bile Acid Axis

Background & Aims:
Roles of TLR4 signaling and gut microbiome amid resolution phase of fibrosis remodeling are unelucidated.
Methods:
We investigated the effects of inhibition of intracellular TLR4 signaling with TAK-242 during resolution of murine liver fibrosis (CCl₄ and MCD diet). Single-cell transcriptome, bone marrow transplantation, gut sterilization and transplant of the fecal material to germ-free mice were used for mechanistic evaluation.
Results:
Contrary to previously known pro-fibrogenic roles of TLR4, significantly retarded hepatic fibrosis resolution was observed as TLR4-signaling was pharmacologically inhibited in vivo in murine models. Single-cell transcriptome analysis of hepatic CD11b⁺ cells, main producers of matrix metalloproteinases, revealed a prominent cluster of Tlr4-expressing Ly6c2-low myeloid cells with a restorative phenotype. Gut sterilization and fecal material transplant demonstrated its microbiome-dependent nature. Metagenomic predictive functional profiling demonstrated enrichment of multiple metabolic pathways linking to the upregulation of bile salt hydrolase-possessing family Erysipelotrichaceae during resolution. Moreover, farnesoid X receptor (FXR)-stimulating secondary bile acids including 7-oxo-lithocholic acids and inhibitory tauro-β-muricholic acids phenotypically correlated with resolution. TLR4- and FXR-dependent upregulation of MMP12 was confirmed in post-phagocytic bone marrow-derived macrophages in vitro.
Conclusions:
Myeloid TLR4 signaling, along with the gut microbiota and the FXR activation via agonizing bile acids, promoted murine liver fibrosis resolution. These finding may provide possible “pro-biotic/post-biotic” targets for anti-fibrotic therapy in the future.