Project summary Idiopathic pulmonary fibrosis (IPF) is the single most common and lethal type of lung fibrosis of unknown cause, which starts with the build-up of scar tissue from the edges of the lung, and advances toward the inside center over time. This intriguing edge-to-center fibrotic progression suggest the presence of unique biological or physical cues in the lung periphery that initiate the disease, the study of which is still in its infancy. Given the fact that few treatment options are available once interstitial fibrosis develops, early diagnosis and treatment are thus critically important, which is hindered by our poor knowledge on the etiology of fibrotic progression. By using cutting-edge technologies including lineage tracing, mouse modeling and multimodal single cell sequencing, Dr. Xu found that elevated mechanical tension drives the differentiation of a novel population of fibroblasts in the lung periphery, leading to the subpleural fibrosis. These fibroblasts are characterized by expression of Wilms' tumor 1 (WT1), signatures of Epithelial-Mesenchymal Transition (EMT), and secretion of multiple kinds of chemokines and cytokines. Fibrosis progression is achieved through additional lung injury, suggesting that a synergistic effort between these WT1+ subpleural fibroblasts and injured epithelial cells may drive the progression of pulmonary fibrosis. These preliminary data raised the Central Hypothesis that WT1+ subpleural fibroblasts function as a stromal niche that drives the progression of pulmonary fibrosis, which will be rigorously tested here in three Specific Aims: Aim 1. To determine the cellular origins and molecular drivers of WT1+ SPFBs [K99] To address the progenitors of WT1+ SPFBs, and transcription factors that drive their induction. Aim 2. To determine if WT1+ SPFBs act as a stromal niche to promote fibrosis progression [K99/R00] To address the biological roles of WT1+ SPFBs in promoting fibrosis progression. Aim 3. To determine if the recruitment and function of myeloid cells in the subpleural region promote pulmonary fibrosis progression [R00] To address the contribution of subpleural myeloid cells, which are hypothesized to be recruited by WT1+ SPFBs, to fibrosis progression. Dr. Xu is well on track towards his career goal as an independent investigator, evidenced by his multidiscipline training record and academic productivity. To accomplish this, he will receive support from his outstanding and complementary mentor committee, including Dr. Xin Sun (primary mentor), Dr. Zea Borok (co- mentor, lung fibrosis), Dr. Laura Alexander (co-mentor, immunology) and Dr. Kyle Gaulton (co-mentor, epigenomics/single cell technology).