CD11b ⁺ CD43 ^hi Ly6C ^lo splenocyte‐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(lo) MoMFs 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(hi)Ly6C(lo) splenic 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 (sM phi s) with macrophage features of higher expressions of CX(3)CR1, F4/80, MHC class II, and CD64 in the fibrotic hepatic milieu. Furthermore, sM-2s and sM phi s were demonstrated capable of activating hepatic stellate cells and thus exacerbating liver fibrosis. Conclusions CD11b(+)CD43(hi)Ly6C(lo) splenic 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.