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

Abstract Macrophages are central orchestrators of the tissue response to injury, with distinct macrophage activation states playing key roles in the progression and resolution of fibrosis. Identifying the unique fibrogenic macrophages that are found in human fibrotic tissues could lead to new and more effective treatments for fibrosis. Here we used human liver and lung single cell RNA sequencing datasets to identify a unique subset of CD9 + TREM2 + macrophages expressing SPP1, GPNMB, FABP5, and CD63 with strong pro-fibrotic activity. This population was validated across orthogonal techniques, species and tissues. These macrophages were enriched at the outside edges of scarring adjacent to activated mesenchymal cells, and in the fibrotic niche across species and organs. Neutrophils producing the type 3 cytokines GM-CSF and IL-17A, and expressing MMP9, which participates in the activation of TGF-β1, clustered with these scar-associated macrophages. Using in vitro primary human cell assays, we determined that GM-CSF, IL-17A and TGF-β1 drive the differentiation of these scar-associated macrophages, and that co-culture of monocyte-derived macrophages with hepatic stellate cells and TGF-β1 augmented type 1 collagen deposition. In vivo blockade of GM-CSF, IL-17A or TGF-β1 with small or large molecules reduced scar-associated macrophage expansion and fibrosis in multiple models of hepatic and pulmonary fibrosis. Our work demonstrates that a specific scar-associated macrophage population is linked with fibrosis across species and tissues. It further provides a strategy for unbiased discovery, triage and preclinical validation of therapeutic targets within this fibrogenic macrophage population.