Endothelial ER protein TXNDC5 promotes lung fibrosis by augmenting endothelial-to-mesenchymal transition

<bold>Background:</bold> Pulmonary fibrosis (PF) is a severe lung disease with limited treatment options. Endothelial-to-mesenchymal transition (EndoMT) contributes critically to the development of PF. This study aims to investigate the role of endothelial thioredoxin domain-containing 5 (TXNDC5), a novel EndoMT driver, in the pathogenesis of PF. <bold>Methods:</bold> A triple transgenic mouse line (<italic>Cdh5-Cre^ERT2;ROSA26-tdTomato;Col1a1-GFP^Tg</italic>, abbreviated as EndoMT^3Tg) was utilized to lineage-trace endothelial cells (ECs) undergoing EndoMT in bleomycin (BLM)-induced PF. The extent and functional consequences of PF were evaluated in EndoMT3Tg with and without EC-specific Txndc5 deletion. <bold>Results:</bold> Single-cell RNASeq re-analyses revealed increased TXNDC5 expression in ECs from human fibrotic lungs. Using EndoMT^3Tg mice, we showed that 48% of myofibroblasts in BLM-induced PF originated from ECs. Deletion of Txndc5 in ECs markedly reduced EndoMT, lung permeability, and fibrosis, resulting in preserved lung function compared to controls. BLM-induced loss of EC markers, upregulation of fibrogenic proteins and increase in EndoMT transcription factors were reversed in mice with EC-specific Txndc5 deletion. Inducing EC Txndc5 deletion in mice with pre-established PF arrested the progression of fibrosis and improved lung function. <bold>Conclusion:</bold> This study unveils a significant contribution of EndoMT to the development of PF, driven, at least in part, by the upregulation of ER protein TXNDC5 in pulmonary ECs. Targeted deletion of Txndc5 in pulmonary ECs reduced the extent of EndoMT and lung fibrosis, thereby preserving lung function. These findings underscore the therapeutic potential of inhibiting endothelial TXNDC5 in PF.