Loss of ATF3 exacerbates pulmonary fibrosis via enhanced neutrophil recruitment and profibrotic macrophage polarization.

Idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disease characterized by irreversible scarring, excessive extracellular matrix (ECM) deposition, and progressive loss of pulmonary function. Although innate immune cells, including neutrophils and macrophages, contribute to disease progression, upstream transcriptional mechanisms regulating their profibrotiac activity remain unclear. Activating transcription factor 3 (ATF3) is a stress-inducible transcriptional repressor that modulates inflammation; however, its role in immune cell-driven fibrosis remains uncharacterized. We investigated ATF3 function in pulmonary fibrosis using wild-type and ATF3 knockout (ATF3-/-) mice subjected to bleomycin-induced lung injury. We assessed fibrosis severity, lung function, immune cell infiltration, cytokine profiles in bronchoalveolar lavage fluid (BALF), and global transcriptomic changes. ATF3 deficiency markedly exacerbated bleomycin-induced fibrosis, as evidenced by greater weight loss, increased collagen deposition, and elevated expression of Col1a1, Col3a1, and fibronectin. Lung function decline was more pronounced in ATF3-/- mice. Flow cytometry revealed increased neutrophil infiltration (CD45+CD11b+Ly6G+) and M2 macrophage polarization (CD45+CD11b+F4/80+CD206+CD163+) in ATF3-/- lungs, alongside higher BALF levels of neutrophil- and macrophage-associated cytokines (CXCL1, CCL2, TGF-β, and IL-1β). Transcriptomics revealed the up-regulation of inflammation- and fibrosis-associated genes, with the enrichment of cytokine signaling pathway and neutrophil extracellular trap formation. Bone marrow-derived macrophages from ATF3-/- mice displayed increased M2 marker and profibrotic mediator expression upon TGF-β stimulation. ATF3 acts as a transcriptional checkpoint limiting immune-mediated fibrotic remodeling by restraining neutrophil recruitment and M2c macrophage polarization. ATF3 loss triggers a profibrotic gene program, amplifying inflammation and ECM deposition. Thus, ATF3 may represent a promising therapeutic target for IPF and related fibrotic lung diseases.