Cooper, C.

Abstract Rationale: Application of single cell RNAseq (scRNAseq) in idiopathic pulmonary fibrosis (IPF) and multiple murine models has revealed two unique disease associated fibroblast subsets, the Cthrc1+ fibrotic population, and a population expressing inflammatory markers whose function is unclear. We, and others, have demonstrated that inflammatory fibroblasts are derived from homeostatic fibroblasts and can differentiate to Cthrc1+ fibroblasts. There is also data in mice suggesting that TGFβ inhibition prevents the emergence of Cthrc1+ fibrotic fibroblasts without impacting the inflammatory fibroblast population. Using our inducible genetic murine model of PF, reductionist organoid systems, and scRNAseq from IPF we present evidence and mechanism for inflammatory fibroblasts as a potential regenerative population in the alveolar niche. Methods: Tamoxifen administration to SftpcI73T conditional allelic knock-in mice, induces expression of mutant protein and promotes an inflammatory cascade followed by fibrogenesis. ScRNA-seq and population RNA-seq of bleomycin and SftpcI73T murine models were compared with publicly available data sets from IPF to identify population specific surface marker expression and potential epithelial ligand-receptor pairs. Flow cytometry approaches developed to isolate fibroblast populations were applied for in-vitro organoid modeling and validation of signaling pathways. In-vivo murine intervention in TGFβ signaling was further applied to confirm functional biology of inflammatory fibroblasts. Results: Informatic analysis of human and mouse inflammatory fibroblasts identifies expression of multiple BMP antagonists that have previously been associated with epithelial proliferation. Furthermore, human inflammatory fibroblasts also express key epithelial growth factors including FGF2 and FGF7. Murine organoid modeling confirms that inflammatory fibroblasts support alveolar organoid formation through the inhibition of BMP signaling. As was previously demonstrated in bleomycin, the inhibition of TGFβ signaling in SftpcI73T does not alter the inflammatory fibroblast population, rather this population is sustained, and alveolar epithelial proliferation increases. Additionally, we did not find any evidence for increase inflammation in the mouse lung due to inflammatory fibroblasts preservation. Finally, human data suggests that two populations of inflammatory fibroblasts are present in the lung with only one subset found to express the BMP antagonists and growth factors required to promote alveolar epithelial proliferation. Conclusions: Our data identifies that inflammatory fibroblasts in the alveolar niche inhibit BMP signaling and allow for epithelial expansion. We also determine differences between murine and human inflammatory fibroblasts suggesting that human inflammatory fibroblasts have a greater potential to drive epithelial expansion. These data shed light on the functional role of inflammatory fibroblasts, further refining our understanding of fibroblast heterogeneity in IPF.