K24: Immunopathology of Indirect Airway Hyperresponsiveness in Asthma Project Summary Indirect or "endogenous" airway hyperresponsiveness (AHR) is a fundamental feature of asthma that is not fully understood. In contrast to other features of asthma such as airflow obstruction or the response to an exogenous bronchoconstrictor such as methacholine, endogenous AHR is specific for asthma and replicates many of the common triggers for asthma including the response to cold/dry air, hypertonic aerosols and allergens in individuals who are appropriately sensitized. We have focused on exercise-induced bronchoconstriction (EIB) as a prototypical feature of endogenous AHR because it is a common trigger for symptoms, has been associated with risk of asthma progression, does not require allergic sensitization and can be precisely measured in the laboratory. Recent studies from our lab have revealed a shift in the precise location of mast cells (MCs) in the airways from the submucosa to the epithelium and that MCs and eosinophils (Eos) interact with the airway epithelium in a manner that serves to propagate airway inflammation. We have also identified alterations in phospholipid metabolism and a specific enzyme called secreted phospholipase A2 group 10 (sPLA2-X) that is strongly associated with AHR and contributes to the dysregulated lipid mediator metabolism present in asthmatic airways. The overall goal of my research program is to understand the underlying alterations in the airways that lead to endogenous AHR in humans. We hypothesize that MCs and Eos act in concert with the epithelium to promote airway inflammation and that alterations in phospholipid metabolism play a key role through generation of mediators that serve to activate the sensory nerves. In the first aim, we determine differences in the number and proliferation potential of MC progenitors in the airways and utilize ex vivo models to examine how interactions among MCs, Eos and airway epithelial cells (AECs) serve to propagate airway inflammation. In the second aim, we examine the function of sPLA2-X in innate immune cells and explore the therapeutic potential of an extracellular inhibitor of this enzyme in a model of EIB. In the final aim, we use design-based stereology to examine the precise location of cells and structures in the airway wall and integrate this information with transcriptomic analyses of airway epithelial brushings to identify the underpinnings of AHR in asthma. These projects move the field forward through a better understanding of the basis for AHR in asthma and will support the career development of the next generation of patient-oriented researchers interested in the immunopathology of asthma.