Idiopathic pulmonary fibrosis (IPF) is a rare, chronic, and irreversible interstitial lung disease characterized by a progressive loss of lung function, limited therapeutic options and high mortality. The pathomechanism of IPF remains incompletely understood, but repetitive injuries to the alveolar epithelium and disturbed wound healing with delayed re-epithelialization seem to play a critical role in the development of this deleterious disease. These processes induce an enhanced proliferation of mesenchymal cells and their increased deposition of extracellular matrix proteins in the lung interstitium, finally leading to progressive fibrosis (Selman M; Ann Intern Med 2001). Taking into account the limited efficacy of current anti-fibrotic treatments, new therapeutic approaches should focus on the alveolar epithelium and its injury repair capacities. Recently we have demonstrated the presence of stem cell-like cells in the peripheral lung parenchyma of patients with IPF (Gazdhar A et al. PlosONE 2013; Hostettler KE et al. PlosOne 2017) . The secretome of these lung-resident stem cell-like cells exerted significant anti-fibrotic effects in vitro (Hostettler KE et al. PlosOne 2017) and in vivo (Khan P et al; manuscript submitted). Single cell RNA-sequencing characterizes the cells as cytokeratin 5-positive epithelial progenitor cells (KRT5+PC) with a subset of these progenitor cells co-expressing mesenchymal markers. Overall Objectives: The objectives of this project are to further investigate the characteristics, function and capacities of KRT5+PC in vitro and in vivo and thus study their function and their role in the pathogenesis of IPF. We hypothesize that strategies involving the use of KRT5+PC and/or their products for future treatment of IPF may be a promising approach. The results of this project will add knowledge towards strategies involving the use of lung-derived epithelial progenitor cells for future treatments in IPF patients. Specific Aims/Methods: This project is based on the in vitro culture of primary human peripheral lung-derived KRT5+PC. Cells will be obtained from patients with IPF and other non-IPF fibrotic lung diseases. (i) Lung tissue from IPF patients and from patients with non-IPF fibrotic lung diseases will be collected. Primary cultures of KRT5+PC will be established. KRT5+PC will be further characterized and their functional and differentiation properties will be studied in vitro by immunofluorescence, RT-PCR, and immunoblotting. The effect of a pro-fibrotic microenvironment on the KRT5+PC will be assessed. (ii) The effect of KRT5+PC in a mouse model for IPF will be studied (Prof. A. Prasse, Hannover, Germany). KRT5+PC will be injected into the mice and engraftment, differentiation state of cells and their effect on mouse lung architecture will be investigated. (iii) The gene expression profile of KRT5+PC obtained from patients with IPF as compared to KRT5+PC from patients with non-IPF fibrotic lung diseases will be studied and compared by RNA sequencing and single cell RNA-sequencing. Expected results and impact: This project will broaden our knowledge regarding the characteristics and function of KRT5+PC with regard to the pathomechanisms and treatment of IPF. Results from this project might enable a translational approach to develop epithelial-based therapeutic options for patients with IPF.