Identification of a broadly fibrogenic macrophage subset induced by type 3 inflammation

Macrophages are central orchestrators of the tissue response to injury, with distinct macrophage activation states playing key roles in fibrosis progression and resolution. Identifying key macrophage populations found in human fibrotic tissues could lead to new treatments for fibrosis. Here, we used human liver and lung single-cell RNA sequencing datasets to identify a subset of CD9(+)TREM2(+) macrophages that express SPP1, GPNMB, FABP5, and CD63. In both human and murine hepatic and pulmonary fibrosis, these macrophages were enriched at the outside edges of scarring and adjacent to activated mesenchymal cells. Neutrophils expressing MMP9, which participates in the activation of TGF-beta 1, and the type 3 cytokines GM-CSF and IL-17A coclustered with these macrophages. In vitro, GM-CSF, IL-17A, and TGF-beta 1 drive the differentiation of human monocytes into macrophages expressing scar-associated markers. Such differentiated cells could degrade collagen IV but not collagen I and promote TGF- beta 1-induced collagen I deposition by activated mesenchymal cells. In murine models blocking GM-CSF, IL-17A or TGF-beta 1 reduced scar-associated macrophage expansion and hepatic or pulmonary fibrosis. Our work identifies a highly specific macrophage population to which we assign a profibrotic role across species and tissues. It further provides a strategy for unbiased discovery, triage, and preclinical validation of therapeutic targets based on this fibrogenic macrophage population.

Automated summary

Macrophages are key regulators of the tissue response to injury and play important roles in fibrosis progression and resolution. A specific subset of CD9(+)TREM2(+) macrophages expressing SPP1, GPNMB, FABP5, and CD63 were identified in human fibrotic tissues. These macrophages were enriched at the edges of scarring and adjacent to activated mesenchymal cells, and their expansion was associated with the presence of neutrophils expressing MMP9, GM-CSF, and IL-17A. In vitro studies revealed that GM-CSF, IL-17A, and TGF-beta 1 promoted the differentiation of monocytes into scar-associated macrophages. In mouse models, blocking GM-CSF, IL-17A, or TGF-beta 1 reduced scar-associated macrophage expansion and hepatic or pulmonary fibrosis. This study provides valuable insights into the role of a specific macrophage population in fibrosis and suggests potential therapeutic targets.