Background: Cellular senescence (CS) is marked by cell cycle arrest and secretion of inflammatory factors. While CS has been strongly implicated in the pathogenesis of pulmonary fibrosis, its role in pulmonary arterial hypertension (PAH) remains unclear, with conflicting results in the few studies performed to date.
Objective: We hypothesized that senescent endothelial cells (SECs) play a dual role in PAH, at first facilitating vascular repair but later promoting inflammation and arterial remodeling. We investigated the dual role of SECs by evaluating of timing of senolytic therapy on PAH severity in the rat SU5416 (SU), chronic hypoxia model.
Methods: CS was assessed by single-cell RNA-Seq (scRNAseq), flowcytometry and immunofluorescent staining (IF) at different time points after SU. The senolytic agent, ABT-263 (targeting anti-apoptotic BCL-2 proteins), was administered for 1 week beginning at Day 0, 14 or 35 after SU.
Results: scRNAseq revealed emergence of a PAH-specific population of ?activated? arterial endothelial cells, with a dysregulated growth profile, localized to regions of arterial remodeling, a subset of which uniquely expressed senescence-related genes. Flow cytometry and IF staining confirmed increased SECs in PAH lung tissues. ABT-263 improved hemodynamics when administered 14 days (or later) after SU; however, early treatment significantly worsened PAH, associated with a paradoxical amplification of CS at day 35.  
Conclusion: These findings suggest SECs may facilitate vascular repair in early PAH but promote disease progression in later stages, making them a potential therapeutic target.