ABSTRACT

Background: Older age is the main risk factor for chronic lung diseases including idiopathic pulmonary fibrosis (IPF). Halting or reversing progression of IPF remains an unmet clinical need due to limited knowledge of underlying mechanisms. The lung circulatory system, composed of blood (pulmonary and bronchial) and lymphatic vessels networks, has been implicated in IPF pathophysiology in elderly people, based solely on reports of altered density and increased permeability of vessels.

Aim: We aimed to define heterogeneity and IPF-associated changes of lung endothelial cells (EC or endothelium) by comparing gene expression in tissues from elderly people - transplant donors and recipients with IPF.

Methods: Single-cell RNA sequencing (scRNAseq) datasets of “ageing lung” tissues were selected only from those publicly available sources that contain age-matching samples for both groups (49- 77 years old donors and IPF patients; nine pairs in total), integrated and compared. Findings were validated by immunohistochemistry using EC-specific markers.

Results: The generation of integrated single-cell maps of ageing lung tissues revealed 17 subpopulations of endothelium (12 for blood and 5 for lymphatic vessels, including 9 novel), with distinct transcriptional profiles. In IPF lung, the heterogeneity of ageing lung endothelium was significantly altered - both in terms of cell numbers (linked to disease- related changes in tissue composition) and differentially expressed genes (associated with fibrosis, inflammation, differentiation and vasodilation) in individual pulmonary, bronchial and lymphatic EC subpopulations.

Conclusions: These findings reveal underappreciated extent of heterogeneity and IPF-associated changes of ageing lung endothelium. Our data suggest direct involvement of specific subpopulations of ageing lung endothelium in IPF pathophysiology, uncovering cellular and molecular targets which may have potential diagnostic, prognostic and therapeutic relevance. This study creates a conceptual framework for appreciating the disease-specific heterogeneity of ageing lung endothelium as a hallmark of IPF.