Lineage Tracing of Cells Expressing the Mesenchymal Profibrotic Transcription Factor Prrx1 in the Normal and Fibrotic Lung

Rationale: Idiopathic pulmonary fibrosis is a rare pulmonary disease, resulting from alterations of alveolar epithelial cells leading to the recruitment and activation of mesenchymal cells. Prrx1 transcription factor (Paired related Homeobox protein 1) acts in controlling human pulmonary fibroblasts phenotype in vitro and in mouse pulmonary fibrogenesis in vivo. Recently, lineage tracing showed that a Prrx1 positive cell population was amplified and associated with profibrotic properties after a skin injury (Currie et al. 2019). We aim to characterize this Prrx1 positive cell population in the fibrotic lung. Methods: The Prrx1:CreERT2-IRES-GFP ;Rosa26iTomato transgenic mouse line contains the tamoxifen inducible Cre recombinase under the control of a 2.4 kb Prrx1 minimal promoter sequence (Prrx1-enh), that leads to expression of tdTomato protein. Prrx1-enh cells are labelled and tracked in normal or fibrotic lung (bleomycin model). Two protocols were used: 1) cell labelling at basal state to study their fate during fibrosis development and 2) cell labelling during fibrosis development. The tdTomato positive cells were characterized using immunohistochemistry and double immunofluorescence. Results: Lineage tracing experiments showed that tdTomatoPOS cells were very rare in the fibrotic lung using the first protocol, while numerous tdTomatoPOS cells were detected with the second protocol. Double immunofluorescence studies identified these cells mostly as mesenchymal cells in a fibrotic lung. Conclusion: We identified a mesenchymal subpopulation of Prrx1-expressing cells. This subpopulation is rare in the normal lung and accumulates only during fibrosis development.