Project Summary LTE4 is the stable cysteinyl leukotriene (cysLT), detected in the biologic fluids of patients with asthma triggered by allergen challenge, aspirin, and respiratory viruses. LTE4 elicits cutaneous edema in normal controls and elicits basophil and eosinophil recruitment to the lung in patients with asthma. Our group recently identified CysLT3R (also called Oxgr1 or GPR99) as the high affinity receptor for LTE4 and demonstrated that it mediates LTE4-elicited cutaneous edema in mice lacking the conventional cysLT receptors, CysLT1R and CysLT2R. However, the mechanism(s) by which LTE4 induces lung pathobiology remains poorly understood and the role of CysLT3R has not been elucidated. We have found that CysLT3R is expressed on both murine and human respiratory epithelial cells (EpCs) and controls both their activation and development. As CysLT3R is resistant to currently available cysLT receptor inhibitors, the findings from this proposal will determine whether CysLT3R is a logical therapeutic target to reduce mucus production, airflow obstruction, or type 2 inflammation in asthma. Using single cell RNA-Seq and a novel CysLT3R floxed strain that we have developed, in addition to several lineage reporter and null strains, Aim 1 will define the cellular mechanisms by which CysLT3R regulates murine lung EpC activation and development. In Aim 2, we will characterize candidate effector pathways in CysLT3R-expanded airway EpCs that we have defined in an RNA-seq dataset. This aim proposes to expand our RNA-seq data on a purified rare EpC subset, and use a cell-specific knockout to characterize an EpC effector protein of interest. In Aim 3 we will take advantage of a growing single cell RNA-seq data base we are developing from patients with nasal polyps. We will exploit the tremendous transcriptional distinction we see across different disease states (aspirin-exacerbated respiratory disease, aspirin-tolerant patients with nasal polyposis, and control samples) to look for unique EpC effector programs and define those which may be regulated by CysLT3R. Finally, we will use an in vitro system in normal human bronchial epithelial cell culture to determine the extent to which CysLT3R regulates human airway EpC differentiation and validate the pathways through which this occurs.