Airway mucus is essential for healthy lung defense1, but excessive mucus in asthma obstructs airflow, leading to severe and potentially fatal outcomes2–5. Current asthma therapies reduce allergic inflammation and relax airway smooth muscle, but treatments are often inadequate due to their minimal effects on mucus obstruction6,7. The lack of efficacious mucus-targeted treatments stems from a poor understanding of healthy mucus function and pathological mucus dysfunction at a molecular level. The

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... chief macromolecules in mucus, polymeric mucins, are massive glycoproteins whose sizes and biophysical properties are dictated in part by covalent disulfide bonds that link mucin molecules into assemblies of 10 or more subunits8. Once secreted, mucin glycopolymers can aggregate to form plugs that block airflow. Here we show that reducing mucin disulfide bonds depolymerizes mucus in human asthma and reverses pathological effects of mucus hypersecretion in a mouse allergic asthma model. In mice challenged with a fungal allergen, inhaled mucolytic treatment acutely loosened mucus mesh, enhanced mucociliary clearance (MCC), and abolished airway hyperreactivity (AHR) to the bronchoprovocative agent methacholine. AHR reversal was directly related to reduced mucus plugging. Furthermore, protection in mucolytic treated mice was identical to prevention observed in mice lacking Muc5ac, the polymeric mucin required for allergic AHR in murine models9. These findings establish grounds for developing novel fast-acting agents to treat mucus hypersecretion in asthma10,11. Efficacious mucolytic therapies could be used to directly improve airflow, help resolve inflammation, and enhance the effects of inhaled treatments for asthma and other respiratory conditions11,12.