PROJECT SUMMARY/ABSTRACT Lower respiratory infections were the fourth leading cause of death worldwide for nearly twenty years, and in 2019 with the onset of the COVID-19 pandemic, respiratory infections quickly rose to the leading cause of death in many countries killing over 5 million people worldwide thus far. Our lungs are equipped with stem cells to help us recover from lung related injury caused by inhaling toxins and pathogens. However, like in many COVID-19 cases, the stem cells cannot always handle the injury burden. Alveolar type II (AT2) cells are facultative stem cells of the lung that secrete surfactant and aid with gas exchange. During injury, AT2 cells withdraw from homeostatic quiescence to repair damage through proliferation and differentiation into alveolar type I (AT1) cells, then revert to quiescence when the epithelium is restored. The mechanism behind AT2 cell reversible activation is unclear. To better understand this mechanism, we modeled respiratory injury in mice through a murine parainfluenza virus known as Sendai virus. Lineage tracing of AT2 cells and proliferation revealed that AT2 cells not only activate to perform in situ wound repair, but also cluster and proliferate at regions away from damage, specific to the edges of airways and vessels, as well as the most distal border of the tissue, representing de novo growth. ATAC-sequencing was performed on AT2 cells from infected mice at the peak of AT2 cell proliferation and in the recovery phase after returning to quiescence. AT2 cells gained accessibility of AP-1 motifs during injury repair and subsequently lost accessibility for AP-1 motifs at recovery. The hypothesis of this proposal is that AP-1 transcriptionally regulates AT2 cell activation to induce both in situ and de novo repair during lung injury response. We will first identify the spatiotemporal activation of AT2 cells during infection through lineage tracing AT2 cell proliferation and differentiation for detection with light sheet microscopy to generate 3D images for deciphering if the proposed in situ and de novo regions of activation are truly distinct. We will also perturb an airway stem cell population known to contribute to in situ wound repair to study region specific changes in AT2 cell activation when airway assistance is disrupted (Aim1). Second, we will investigate the regulatory mechanism in AT2 cell activation by looking at the kinetics of epigenetic change over time through injury response with bulk ATAC and single-cell mutiome-sequencing. We will also examine the role of AP-1 through AT2 cell specific conditional knockout of FOSB to examine its impact on AT2 cell activation and repair (Aim2). This study will help characterize the novel concept of in situ and de novo AT2 cell activation as well as the undiscovered epigenetic and transcriptional regulation of lung regeneration.