Bronchiolitis Obliterans Syndrome (BOS) is a fibrotic disease heavily responsible for high mortality rates after lung transplantation. Myofibroblasts are primary effectors of this fibrotic process, but their origin is still under debate. The purpose of this work was to identify the precursors of mesenchymal cells responsible for post-transplant airway fibro-obliteration.Lineage-tracing tools were used to track or deplete potential sources of myofibroblasts in the heterotopic tracheal transplantation model. Allografts were analysed by histology, confocal microscopy, flow cytometry or single-cell transcriptomic analysis. BOS explants were evaluated by histology and confocal microscopy.Myofibroblasts in the allografts were recipient-derived. Still, when recipient mice were treated with tacrolimus, we observed rare epithelial-to-mesenchymal transition phenomena and an overall increase in donor-derived myofibroblasts (p=0.0467), but the proportion of these cells remained low (7%). Hematopoietic cells, and specifically the mononuclear phagocyte system, gave rise to the majority of myofibroblasts found in occluded airways. Ablation of Cx3cR1+ cells decreased fibro-obliteration (p=0.0151) and myofibroblasts accumulation (p=0.0020). Single-cell RNA-sequencing unveiled similarities between myeloid-derived cells from allografts and both murine and human samples of lung fibrosis. Finally, myofibroblasts expressing the macrophage marker CD68 were increased in BOS explants when compared to controls (14.4% versus 8.5% p=0.0249).Recipient-derived myeloid progenitors represent a clinically-relevant source of mesenchymal cells infiltrating the airways after allogeneic transplantation. Therefore, therapies targeting the mononuclear phagocyte system could improve long-term outcomes after lung transplantation.

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