Lung injury-induced endothelial cell states persist in aging-associated non-resolving fibrosis

The capacity of lung tissue to heal without scarring deteriorates with aging. The pulmonary vasculature has emerged as a central regulator of lung repair, and aging-associated endothelial cell dysfunction has been associated with defective lung response to injury and sustained fibrosis. To shed light into aging-associated cellular and transcriptional responses of lung endothelial cells during lung fibrogenesis, we carried out a single cell RNA-seq analysis of bleomycin-treated young (2 months) and aged (72 months) lungs during the early phase of fibrosis resolution (30 days post bleomycin). This analysis revealed that lung injury was associated with the appearance of distinct populations of endothelial cells that exhibit “activated” transcriptional states, originating from virtually all vascular beds. Activated endothelial cells deriving from venous and capillary beds were the most represented in aged compared to young lungs. Longitudinal single-cell analysis together with lineage tracing revealed that activated endothelial cells appeared transiently during the peak of collagen production in young lungs and disappeared during the resolution phase. Conversely, activated endothelial cell states persisted in aged lungs around fibrotic areas, indicating a failure of the aged vascular tissue to return to quiescence. We provide evidence that an axis between YAP and TrkB signaling acts as a node that directs injury-induced capillary endothelial cell activation in vivo and lung capillary morphogenesis ex vivo. Together, these data provide insights into the aging-related dynamics of lung endothelial cells in response to injury.