Macrophages are critical inducers of bleomycin-induced fibrosis in a systemic scleroderma model

Macrophages are critical regulators of inflammation with an essential role in maintaining and re-establishing homeostasis after inflammatory insults. However, excessive macrophage activation could promote fibrosis, highlighting their potential as a therapeutic target in chronic inflammatory diseases. Two preclinical models of systemic sclerosis, bleomycin-induced systemic scleroderma and sclerodermatous graft-versus-host disease, were used to analyze the role of macrophages in the establishment of fibrosis. In both models, macrophage numbers increase in the skin and lungs, in association with elevated collagen content and correlating with fibrosis development. These macrophages had a Ly6clowCD206+MerTk+ phenotype, suggesting a profibrotic role during disease progression. Using macrophage depletion and differentiation blocking approaches, this work shows that reduced macrophage accumulation effectively prevented bleomycin-induced fibrosis development. Direct profibrotic activity of bleomycin-exposed macrophages was revealed by s.c. macrophage injections in naïve mice, which was sufficient to induce systemic fibrosis. Finally, bleomycin-treated primary mouse and human macrophages display reduced clearance of apoptotic cells and secrete factors that promote fibroblast activation and collagen production. Metabolic and mitochondrial dysfunction, changes in receptor shedding, and cytoskeletal reorganization in bleomycin-treated macrophages further contribute to their impaired efferocytosis and enhanced profibrotic activity. Collectively, this work identifies macrophages as critical promoters of tissue fibrosis and suggests that inhibition of macrophage activation represents a new potent therapeutic avenue in efforts to reverse fibrosis associated with chronic inflammation.