Liver Macrophage Diversity in Health and Disease.

The liver fulfils a plethora of metabolic and immunological functions. Liver macrophages are a heterogeneous immune cell population with high plasticity and are important for maintaining normal liver function but are also critically involved in disease processes. In this chapter, we review the heterogeneity and multifaceted functions of hepatic macrophages in liver health and in disease conditions, including acute liver injury, chronic liver diseases, and hepatocellular carcinoma. Under homeostatic conditions, the tissue resident Kupffer cells are phagocytic cells that have important functions in immune surveillance, antigen presentation, and metabolic regulation while the roles of other populations such as capsular, peritoneal, or monocyte-derived macrophages in liver health are less clearly defined. Upon liver injury, Kupffer cell numbers are markedly reduced while monocyte-derived macrophages significantly expand and take critical roles in driving and resolving liver injury, including important pathogenic involvements in inflammation, fibrosis, and regeneration. They also create and maintain an immunosuppressive and immune-excluded microenvironment in hepatocellular carcinoma. Single-cell and spatial omics technologies are significantly expanding our understanding of the diversity and plasticity of macrophage populations under different conditions and enable the reliable identification of specific hepatic macrophage subsets. This knowledge can now be applied to dissect the exact contributions of distinct macrophage populations to disease processes and hopefully will pave the way for new therapeutic interventions.