Dissecting the Mechanisms Shaping Liver Macrophage Heterogeneity and Function in Metabolic Liver Disease

Abstract

Macrophages play an integral role in host defense, tissue homeostasis, and disease progression. Altered macrophage polarization, characterized by changes in its transcriptional and functional states, has been causally linked to metabolic disease. Metabolic dysfunction-associated steatohepatitis (MASH) represents a severe stage of metabolic liver disease characterized by hepatocyte injury, inflammation, and liver fibrosis. Single-cell transcriptome analysis on non-parenchymal cells (NPC) isolated from healthy and MASH livers revealed unprecedented insights into the nature of intercellular crosstalk and reprogramming of diverse liver cell types during MASH pathogenesis. We observed a marked change of liver macrophage composition upon diet-induced MASH, characterized by a depletion of resident macrophages, and expansion of infiltrated macrophages. TREM2+ macrophages represent a unique population of monocyte-derived macrophages induced in both mouse and human MASH liver. These findings illustrate a new dimension of macrophage biology in MASH progression and raise several important questions regarding the mechanisms and pathophysiological role of liver macrophages during MASH progression and the development of MASH-associated liver cancer.

Macrophages play an important role in tissue homeostasis and disease pathogenesis; however, the nature of macrophage heterogeneity, disease-associated reprogramming, and contribution to MASH remains incompletely understood. In my thesis, we performed bulk and single-cell RNA sequencing analysis to delineate the landscape of macrophage transcriptomes in healthy and MASH liver. Our analysis uncovered cell type-specific transcriptomic signatures of liver cells upon diet-induced MASH. Gene ontology analysis indicated strongly activated inflammatory pathways in MASH. We identified brain abundant membrane attached signal protein 1 (Basp1) as a myeloid-enriched gene that is markedly induced in mouse and human MASH liver. Myeloid-specific inactivation of BASP1 attenuates the severity of diet-induced MASH pathologies as shown by reduced hepatocyte injury and liver fibrosis in mice. Mechanistically, cultured macrophages lacking BASP1 exhibited a diminished response to pro-inflammatory stimuli, impaired NLRP3 inflammasome activation, and reduced cytokine secretion. These findings uncover BASP1 as a critical regulator of myeloid inflammatory signaling that underlies MASH pathogenesis.

Cellular heterogeneity of macrophages in the liver is a hallmark of MASH pathogenesis. We identified TGF-beta signaling as a crucial regulator of disease-associated macrophages in the MASH liver. Myeloid-specific inactivation of Tgfbr1 in mice exacerbated diet-induced MASH. Mechanistically, ablation of TGF-beta signaling in myeloid cells altered liver macrophage composition characterized by a reduction of TREM2+ macrophages and a corresponding expansion of FCRL5+ macrophages in the liver. Additionally, macrophages lacking Tgfbr1 exhibited gene signatures associated with inflammasome activation, cytokine signaling, cellular senescence, and immunosuppression. These changes in macrophage composition and function promoted effector T cell exhaustion and the development of MASH-associated hepatocellular carcinoma in Tgfbr1-deficient mice. These studies uncover myeloid TGF-beta signaling as a driver that governs liver macrophage heterogeneity and polarization within the liver microenvironment during the development of MASH and MASH-associated liver cancer. Together, my thesis work has revealed disease-associated liver macrophage reprogramming, the molecular nature of inflammatory responses, and intrahepatic signaling pathways that shape macrophage heterogeneity in metabolic liver disease.