Endothelial Angptl2 orchestrates influenza-associated pulmonary fibrosis via EndMT and EDN1-mediated fibroblast activation

Pulmonary fibrosis (PF) is a severe consequence of viral respiratory infections, including influenza. Endothelial dysfunction is a hallmark of PF progression; however, the molecular circuitry bridging viral insults and fibrotic remodeling remains largely undefined. Angiopoietin-like protein 2 (Angptl2), an endothelial-associated transcription factor and multifunctional mediator of inflammation and tissue remodeling, may be involved in this process. This study systematically examines Angptl2’s role in the pathogenesis of influenza-associated PF. Transcriptome sequencing was performed on bronchoalveolar lavage samples from patients with influenza-associated PF and non-fibrotic controls. In vivo, a murine model of PF induced by H1N1 virus and Angptl2 knockout mice were established, followed by single-cell RNA sequencing (scRNA-seq). In vitro, Angptl2 knockdown and overexpression were performed in endothelial cells, and endothelial-fibroblast interactions were studied using transwell co-culture. Angptl2 expression was elevated in PF patients. In the murine model of influenza-induced PF, both transcriptome and scRNA-seq confirmed that elevated Angptl2 expression in fibrotic lung tissues, particularly in endothelial cells, correlating with PF development. Angptl2 deficiency significantly attenuated fibrosis, accompanied by expansion of a protective G-protein signaling 2-positive (Rgs2 + ) endothelial subpopulation. Mechanistic studies suggested that Angptl2 contributes to endothelial-to-mesenchymal transition (EndMT) by binding to the α5β1 integrin receptor, activating MAPK and PI3K-AKT signaling pathways, and driving extracellular matrix (ECM) deposition. Additionally, during EndMT, endothelial cells secrete endothelin-1 (EDN1), enhancing fibroblast activation via NF-κB signaling and further contributing to ECM accumulation and PF progression. Our findings demonstrate that Angptl2 is a pivotal mediator of influenza-associated PF by orchestrating endothelial dysfunction and fibroblast activation through coordinated cellular communication. Angptl2 drives EndMT and promotes endothelial-mediated fibroblast activation via the EDN1 signaling. In Angptl2-deficient mice, where PF is markedly attenuated, the expansion of Rgs2⁺ endothelial cells indicates a potential protective mechanism that could inform future therapeutic approaches.