CD11b⁺CD43^hiLy6C^losplenocyte-derived macrophages exacerbate liver fibrosis via spleen-liver axis

Background and Aims: Monocyte-derived macrophages (MoMFs), a dominant population of hepatic macrophages under inflammation, play a crucial role in liver fibrosis progression. The spleen serves as an extra monocyte reservoir in inflammatory conditions; however, the precise mechanisms of involvement of the spleen in the pathogenesis of liver fibrosis remain unclear. Approach and Results: By splenectomy and splenocyte transfusion, it was observed that splenic CD11b⁺cells accumulated intrahepatically as Ly6C^loMoMFs to exacerbate CCl4-induced liver fibrosis. The splenocyte migration into the fibrotic liver was further directly visualized by spleen-specific photoconversion with KikGR mice and confirmed by CD45.1⁺/CD45.2⁺spleen transplantation. Spleen-derived CD11b⁺cells purified from fibrotic livers were then annotated by single-cell RNA sequencing, and a subtype of CD11b⁺CD43^hiLy6C^losplenic monocytes (sM-1s) was identified, which was markedly expanded in both spleens and livers of mice with liver fibrosis. sM-1s exhibited mature feature with high expressions of F4/80, produced much ROS, and manifested preferential migration into livers. Once recruited, sM-1s underwent sequential transformation to sM-2s (highly expressed Mif, Msr1, Clec4d, and Cstb) and then to spleen-derived macrophages (sMϕs) with macrophage features of higher expressions of CX3CR1, F4/80, MHC class II, and CD64 in the fibrotic hepatic milieu. Furthermore, sM-2s and sMϕs were demonstrated capable of activating hepatic stellate cells and thus exacerbating liver fibrosis. Conclusions: CD11b⁺CD43^hiLy6C^losplenic monocytes migrate into the liver and shift to macrophages, which account for the exacerbation of liver fibrosis. These findings reveal precise mechanisms of spleen-liver axis in hepatic pathogenesis and shed light on the potential of sM-1 as candidate target for controlling liver diseases.