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

Significance Organ fibrosis is a major cause of global morbidity and mortality. It is driven by chronic inflammation and associated oxidative stress with depletion of cellular antioxidant defenses. We demonstrate a pathway in which the evolutionarily conserved receptor stabilin-1 on tissue-infiltrating macrophages provides a second-line defense to prevent tissue damage from oxidative stress. Stabilin-1+ monocytes take up malondialdehyde-LDL (MDA-LDL), a major product of oxidative lipid peroxidation, to form ceroid-laden macrophages. Through the uptake of MDA-LDL, stabilin-1 suppresses production of the profibrogenic chemokine CCL3 and prevents excessive collagen deposition in experimental models of liver fibrosis. We propose that macrophage stabilin-1 is a critical defense against oxidative tissue damage and thereby maintains tissue homeostasis. Macrophages are key regulators of fibrosis development and resolution. Elucidating the mechanisms by which they mediate this process is crucial for establishing their therapeutic potential. Here, we use experimental models of liver fibrosis to show that deficiency of the scavenger receptor, stabilin-1, exacerbates fibrosis and delays resolution during the recovery phase. We detected a subset of stabilin-1+ macrophages that were induced at sites of cellular injury close to the hepatic scar in mouse models of liver fibrosis and in human liver disease. Stabilin-1 deficiency abrogated malondialdehyde-LDL (MDA-LDL) uptake by hepatic macrophages and was associated with excess collagen III deposition. Mechanistically, the lack of stabilin-1 led to elevated intrahepatic levels of the profibrogenic chemokine CCL3 and an increase in GFAP+ fibrogenic cells. Stabilin-1−/− macrophages demonstrated a proinflammatory phenotype during liver injury and the normal induction of Ly6Clo monocytes during resolution was absent in stabilin-1 knockouts leading to persistence of fibrosis. Human stabilin-1+ monocytes efficiently internalized MDA-LDL and this suppressed their ability to secrete CCL3, suggesting that loss of stabilin-1 removes a brake to CCL3 secretion. Experiments with cell-lineage–specific knockouts revealed that stabilin-1 expression in myeloid cells is required for the induction of this subset of macrophages and that increased fibrosis occurs in their absence. This study demonstrates a previously unidentified regulatory pathway in fibrogenesis in which a macrophage scavenger receptor protects against organ fibrosis by removing fibrogenic products of lipid peroxidation. Thus, stabilin-1+ macrophages shape the tissue microenvironment during liver injury and healing.