Aberrant and ectopic cell populations in restrictive allograft syndrome after lung transplantation

Rationale Restrictive allograft syndrome (RAS) is a major cause of mortality following lung transplantation due to progressive fibrosis of the lung allograft, with no therapeutic options. Knowledge of the cellular and molecular mechanisms driving fibrosis in RAS remains limited. Objective To characterise the cellular and molecular changes in human RAS lungs through single-nucleus transcriptomic profiling. Methods Single-nucleus RNA-sequencing (snRNA-seq) was performed in peripheral lung tissues from 15 RAS patients undergoing lung re-transplantation, and from nine healthy control lungs. Findings were validated and extended using histological techniques including immunofluorescence, RNA in situ hybridisation, Elastica van Gieson immunohistochemistry, quantitative histological analyses, and micro-computed tomography (CT) scans. Measurements and main results snRNA-seq analysis of RAS lungs revealed previously undescribed aberrant basaloid cells, ectopic COL15A1 + peribronchial vascular endothelial cells (pVECs), and CTHRC1 + fibrotic fibroblasts. Histological stains disclosed distinctive distribution patterns: aberrant basaloid cells, primarily localised at the fibrotic edge, together with juxtaposed CTHRC1 + fibrotic fibroblasts and ectopic COL15A1 + pVECs form the fibrotic niche of alveolar fibroelastosis (AFE). PRX + alveolar microvasculature is partially lost in AFE areas. Micro-CT scans revealed changes from pulmonary to systemic perfusion, facilitated by COL15A1 + pVECs. Lastly, our data reveal potential therapeutic targets in RAS, including integrin αvβ6, activator of transforming growth factor-β. Conclusion Considering the multifaceted differences of RAS and idiopathic pulmonary fibrosis, we revealed a surprising general principle of an entity-spanning composition of the fibrotic niche by aberrant basaloid cells localised at the fibrotic edge, ectopic COL15A1 + pVECs and CTHRC1 + fibrotic fibroblasts. This suggests a flexible, but cellular pathogenesis-guided, transferability of potential therapeutic approaches between progressive fibrotic lung diseases.