Identification and characterization of infiltrating macrophages in acetaminophen-induced liver injury

TLDR

Macrophages in acetaminophen‑induced liver injury have been reported to have both pro‑toxicant and hepatoprotective roles, a controversy likely driven by their heterogeneity and the difficulty of distinguishing subpopulations. This study aimed to separate distinct macrophage populations and define their specific functions in liver inflammation and repair. Using flow cytometry with fluorescence‑labeled antibodies against surface markers, the authors identified a novel APAP‑induced macrophage (IM) population distinct from resident Kupffer cells. The IMs, derived from infiltrating monocytes, displayed an alternatively activated phenotype, phagocytosed apoptotic cells, induced neutrophil apoptosis, and their absence in CCR2‑/‑ mice delayed hepatic damage resolution, indicating a role in tissue repair through anti‑inflammatory and angiogenic actions.

Abstract

Abstract The role of macrophages in the pathogenesis of acetaminophen (APAP)-induced liver injury remains controversial, as it has been demonstrated that these cells display pro-toxicant and hepato-protective functions. This controversy may stem from the heterogeneity and/or plasticity of macrophages and the difficulty in distinguishing and differentially studying subpopulations of macrophages in the liver. In the present study, using flow cytometric analysis and fluorescence-labeled antibodies against specific cell surface macrophage markers, we were able to, for the first time, identify an APAP-induced macrophage (IM) population distinct from resident Kupffer cells. The data demonstrated that the IMs were derived from circulating monocytes that infiltrated the liver following APAP-induced liver injury. The IMs exhibited a phenotype consistent with that of alternatively activated macrophages and demonstrated the ability to phagocytize apoptotic cells and induce apoptosis of neutrophils. Furthermore, in the absence of the IMs, the resolution of hepatic damage following APAP-induced hepatotoxicity was delayed in CCR2−/− mice compared with wild-type mice. These findings likely contribute to the role of the IMs in the processes of tissue repair, including counteracting inflammation and promoting angiogenesis. The present study also demonstrated the ability of separating populations of macrophages and delineating distinct functions of each group in future studies of inflammatory disease in the liver and other tissues.