Single-cell RNA seq identifies Plg-RKT-PLG as signals inducing phenotypic transformation of scar-associated macrophage in liver fibrosis

Hepatic macrophages play a central role in liver fibrosis. Scar-associated macrophages (SAMs), a recently identified subgroup of macrophages, play an important role in this process. However, the mechanism by which SAMs transform during liver fibrosis is still unclear. In this study, we aimed to characterize SAMs and elucidate the underlying mechanism of SAM transformation. Bile duct ligation (BDL) and carbon tetrachloride (CCl4) were used to induce mouse liver fibrosis. Non-parenchymal cells were isolated from normal/fibrotic livers and were analyzed using single cell RNA sequencing (scRNA-seq) or mass cytometry (CyTOF). The glucan-encapsulated siRNA particles (siRNA-GeRPs) was employed to perform macrophage selective gene knockdown. The results of scRNA-seq and CyTOF revealed that SAMs, which derived from bone marrow-derived macrophages (BMMs), accumulated in mouse fibrotic livers. Further analysis showed that SAMs highly expressed genes related to fibrosis, indicating the pro-fibrotic functions of SAMs. Moreover, plasminogen receptor Plg-RKT was highly expressed by SAMs, suggesting the role of Plg-RKT and plasminogen (PLG) in SAM transformation. In vitro, PLG-treated BMMs transformed into SAMs and expressed SAM functional genes. Knockdown of Plg-RKT blocked the effects of PLG. In vivo, selective knockdown of Plg-RKT in intrahepatic macrophages of BDL- and CCl4-treated mice reduced the number of SAMs and alleviated BDL- and CCl4-induced liver fibrosis, suggesting that Plg-RKT-PLG played an important role in liver fibrosis by mediating SAM transformation. Our findings reveal that SAMs are crucial participants in liver fibrosis. Inhibition of SAM transformation by blocking Plg-RKT might be a potential therapeutic target for liver fibrosis.