SUMMARY Recurrent alveolar injury and dysregulated signaling in alveolar cell niche have been implicated in the pathogenesis of pulmonary fibrosis. While genome-wide association studies further supported this model, the precise cellular triggers and the molecular mechanisms that drive alveolar fibroblasts replication and their conversion into excessive extracellular matrix producing myofibroblasts remain elusive. Currently we lack a comprehensive understanding of the cell intrinsic and extrinsic homeostatic mechanisms that normally control fibroblasts replication and prevent their conversion into pathological myofibroblasts. Our recent finding that either conditional ablation of alveolar epithelial type-1 (AT1) cells, or loss of Pdgfra specifically in PDGFRA-expressing fibroblasts (alveolar fibroblasts) in vivo, results in spontaneous conversion of alveolar fibroblasts into myofibroblasts and fibrosis. Single cell transcriptome guided ligand-receptor pair predictions coupled with alveolar fibroblast cultures in serum-free conditions suggest that AT1 cell-derived PDGFA is required in proper amounts to both maintain alveolar fibroblasts identity (i.e., prevent their conversion to myofibroblasts) and control replication. Exploring downstream mechanisms, we have uncovered an increase in the expression of the transcription factor RUNX1 in both replicating alveolar fibroblasts as well as TGFβ- induced myofibrogenesis both in vivo and ex vivo. Significantly, our preliminary data also indicate that genetic loss of Runx1 abrogates PDGFA or TGFβ-induced alveolar fibroblasts proliferation or conversion into myofibroblasts, respectively. These preliminary data lead us to hypothesize that AT1 cells maintain alveolar fibroblasts identity via PDGFA – PDGFRA signaling axis and that a fine balance in the amount of PDGFA is essential for fibroblast quiescence and replication at homeostasis and injury repair. We also hypothesize that RUNX1 is an essential regulator of both PDGFRA and TGFβ signaling to regulate replication and myofibrogenic programs in alveolar fibroblasts during regeneration and fibrosis. The major objectives of this proposal are to define the communication between AT1 cells and alveolar fibroblasts and to study transcriptional control of myofibrogenesis. In Aim1, we will test the hypothesis that PDGFA-PDGFRA signaling between AT1 cells and alveolar fibroblasts is essential for alveolar homeostasis. In Aim2, we will determine context specific functions of RUNX1 in alveolar fibroblasts replication and myofibrogenesis. We will use novel in vivo genetic mouse models and pharmacological loss-of-function models, human lung fibroblasts and precision cut lung slice cultures, and molecular assays to study these specific aims. Our prior expertise in lung regeneration and transcriptional control of cell fates will aid us in studying the proposed aims. The outcomes from the proposed studies will have broader impact on lung regenerative medicine and will for...