Single Cell Transcriptomics of Fibrotic Lungs Unveils Aging-associated Alterations in Endothelial and Epithelial Cell Regeneration.

Lung regeneration deteriorates with aging leading to increased susceptibility to pathologic conditions, including fibrosis. Here, we investigated bleomycin-induced lung injury responses in young and aged mice at single-cell resolution to gain insights into the cellular and molecular contributions of aging to fibrosis. Analysis of 52,542 cells in young (8 weeks) and aged (72 weeks) mice identified 15 cellular clusters, many of which exhibited distinct injury responses that associated with age. We identified alveolar fibroblasts as a major source of collagen expression following bleomycin challenge, with those from aged lungs exhibiting a more persistent activation compared to young ones. We also observed age-associated transcriptional abnormalities affecting lung progenitor cells, including ATII pneumocytes and general capillary (gCap) endothelial cells (ECs). Transcriptional analysis combined with lineage tracing identified a sub-population of gCap ECs marked by the expression of Tropomyosin Receptor Kinase B (TrkB) that appeared in bleomycin-injured lungs and accumulated with aging. This newly emerged TrkB EC population expressed common gCap EC markers but also exhibited a distinct gene expression signature associated with aberrant YAP/TAZ signaling, mitochondrial dysfunction, and hypoxia. Finally, we defined ACKR1 venous ECs that exclusively emerged in injured lungs of aged animals and were closely associated with areas of collagen deposition and inflammation. Immunostaining and FACS analysis of human IPF lungs demonstrated that ACKR1 venous ECs were dominant cells within the fibrotic regions and accumulated in areas of myofibroblast aggregation. Together, these data provide high-resolution insights into the impact of aging on lung cell adaptability to injury responses.