The Development and Function of IL-9-Secreting T Helper Cells During Chronic and Allergen Recall-Induced Allergic Airway Disease

Abstract

Asthma is a chronic inflammatory lung disease with intermittent flares predominately mediated through memory T cells. The majority of the T cells in tissues such as the lung are tissue-resident memory (Trm) cells, defined as cells that maintain long-lasting presence in the tissue and have rapid functional recall following challenge. Allergen-specific CD4 T helper cells that secrete the cytokine IL-9 have been shown to be a necessary component of asthma pathogenesis. However, the precise characterization and function of IL-9-secreting CD4+ cells (Th9 cells) are unknown. Here we demonstrate that IL-9 production is progressively lost in Th9 cells over several rounds of culture and that environmental cues dictate the instability or effector function of the Th9 phenotype. We show Th9 cells are long-lived tissue-resident cells with the capacity to rapidly respond to secondary allergen challenge causing allergic airway disease (AAD). We found in a memory model of Aspergillus fumigatus challenge, Th9 cells maintain tissue residency throughout a 12-week period of antigen-free rest. Additionally, we demonstrated increased frequency of IL-9-producing cells and quantity of IL-9 upon rechallenge, characteristic of a secondary response. Antibody blockade of IL-9 immediately prior to the recall challenge significantly reduced overall allergic lung inflammation, suggesting that IL-9 plays an obligate role in the allergic memory response following pulmonary allergen challenge. The protection afforded by IL-9 antibody blockade was not seen in a chronic model asthma-like disease demonstrating IL-9 has a specific role in allergic memory responses. Interestingly, IL-9-secreting cells have a polyfunctional multi-cytokine phenotype demonstrating a highly pathogenic state that we reproduced in culture. These observations suggest that IL-9 from Trm cell populations and Th9 cells play a novel role in allergen recall responses and are potential therapeutic targets for patients suffering from chronic intermittent asthma.