Differential Ly-6C expression identifies the recruited macrophage phenotype, which orchestrates the regression of murine liver fibrosis

Abstract

Although macrophages are widely recognized to have a profibrotic role in inflammation, we have used a highly tractable CCl 4 -induced model of reversible hepatic fibrosis to identify and characterize the macrophage phenotype responsible for tissue remodeling: the hitherto elusive restorative macrophage. This CD11B hi F4/80 int Ly-6C lo macrophage subset was most abundant in livers during maximal fibrosis resolution and represented the principle matrix metalloproteinase (MMP) -expressing subset. Depletion of this population in CD11B promoter-diphtheria toxin receptor (CD11B-DTR) transgenic mice caused a failure of scar remodeling. Adoptive transfer and in situ labeling experiments showed that these restorative macrophages derive from recruited Ly-6C hi monocytes, a common origin with profibrotic Ly-6C hi macrophages, indicative of a phenotypic switch in vivo conferring proresolution properties. Microarray profiling of the Ly-6C lo subset, compared with Ly-6C hi macrophages, showed a phenotype outside the M1/M2 classification, with increased expression of MMPs, growth factors, and phagocytosis-related genes, including Mmp9, Mmp12, insulin-like growth factor 1 (Igf1), and Glycoprotein (transmembrane) nmb (Gpnmb). Confocal microscopy confirmed the postphagocytic nature of restorative macrophages. Furthermore, the restorative macrophage phenotype was recapitulated in vitro by the phagocytosis of cellular debris with associated activation of the ERK signaling cascade. Critically, induced phagocytic behavior in vivo, through administration of liposomes, increased restorative macrophage number and accelerated fibrosis resolution, offering a therapeutic strategy to this orphan pathological process.

Stabilin-1 expression defines a subset of macrophages that mediate tissue homeostasis and prevent fibrosis in chronic liver injury

“…S8 B-E). This is in keeping with previous studies where defining macrophages through their polarization to classical M1 or M2 macrophages is unreliable in liver injury, as M1/M2 markers can be expressed simultaneously by liver macrophages (25).…”

Neutrophils alleviate fibrosis in the CCl4‐induced mouse chronic liver injury model

“…FACS analysis showed that the Ly6G + CD11b + cells were markedly decreased at 72 hours after the final CCl 4 administration when the scar resolution was most active (Fig. 7B) 6. Consistently, immunostaining of liver sections demonstrated a significant decrease of Ly6G + cells in liver at 96 hours after the final CCl 4 administration (Fig.…”

Identifying Molecular Factors That Contribute to Resolution of Liver Fibrosis

“…4,7,8 Work by a number of groups, including the study by Yang et al in this issue of Gastroenterology, has demonstrated that macrophages are crucial to the resolution of fibrosis. 4,7,[9][10][11] Indeed, the removal of the macrophage population at the onset of spontaneous fibrosis resolution in rodent models of liver injury prevents remodeling of fibrosis. Additionally, deletion of the macrophage population is associated with a critical drop in liver levels of key enzymes such as MMP13 and MMP12-identifying the macrophage as a crucial source of these enzymes in fibrosis resolution.…”

“…Intriguingly, in the carbon tetrachloride-induced model of liver injury, the macrophages crucial for resolution are the same population that is recruited during fibrogenesis, and that contribute to fibrosis. 9,10 Associated with the onset of fibrosis resolution, this same macrophage population undergoes a phenotypic switch in situ, expressing markers that define a distinct phenotype and up-regulate the expression of matrix-degrading enzymes (and survival and proliferative signals for hepatocytes and hepatic progenitor cells) after ingestion of debris. 6,10 Against this background, the work by Yang et al 4 provides another crucial insight to the molecular regulators of fibrosis resolution.…”

“…9,10 Associated with the onset of fibrosis resolution, this same macrophage population undergoes a phenotypic switch in situ, expressing markers that define a distinct phenotype and up-regulate the expression of matrix-degrading enzymes (and survival and proliferative signals for hepatocytes and hepatic progenitor cells) after ingestion of debris. 6,10 Against this background, the work by Yang et al 4 provides another crucial insight to the molecular regulators of fibrosis resolution. As identified by the authors, vascular endothelial growth factor (VEGF) has previously been found to play a role in fibrogenesis via a proinflammatory effect acting primarily on endothelial cells.…”