Persistence of asthma requires multiple feedback circuits involving type 2 innate lymphoid cells and IL-33

Background: Asthma in a mouse model spontaneously resolves after cessation of allergen exposure. We developed a mouse model in which asthma features persisted for 6 months after cessation of allergen exposure. Objective: We sought to elucidate factors contributing to the persistence of asthma. Methods: We used a combination of immunologic, genetic, microarray, and pharmacologic approaches to dissect the mechanism of asthma persistence. Results: Elimination of T cells though antibody-mediated depletion or lethal irradiation and transplantation of recombination-activating gene (Rag1) 2/2 bone marrow in mice with chronic asthma resulted in resolution of airway inflammation but not airway hyperreactivity or remodeling. Elimination of T cells and type 2 innate lymphoid cells (ILC2s) through lethal irradiation and transplantation of Rag2(-/-) gamma c(-/-) bone marrow or blockade of IL-33 resulted in resolution of airway inflammation and hyperreactivity. Persistence of asthma required multiple interconnected feedback and feed-forward circuits between ILC2s and epithelial cells. Epithelial IL-33 induced ILC2s, a rich source of IL-13. The latter directly induced epithelial IL-33, establishing a positive feedback circuit. IL-33 autoinduced, generating another feedback circuit. IL-13 upregulated IL-33 receptors and facilitated IL-33 autoinduction, thus establishing a feed-forward circuit. Elimination of any component of these circuits resulted in resolution of chronic asthma. In agreement with the foregoing, IL-33 and ILC2 levels were increased in the airways of asthmatic patients. IL-33 levels correlated with disease severity. Conclusions: We present a critical network of feedback and feed-forward interactions between epithelial cells and ILC2s involved in maintaining chronic asthma. Although T cells contributed to the severity of chronic asthma, they were redundant in maintaining airway hyperreactivity and remodeling.