β-Glucan exacerbates allergic asthma independent of fungal sensitization and promotes steroid-resistant T H 2/T H 17 responses.

Background: Allergic sensitization to fungi has been associated with asthma severity. As a result, it has been largely assumed that the contribution of fungi to allergic disease is mediated through their potent antigenicity.
Objective: We sought to determine the mechanism by which fungi affect asthma development and severity.
Methods: We integrated epidemiologic and experimental asthma models to explore the effect of fungal exposure on asthma development and severity.
Results: We report that fungal exposure enhances allergen-driven T _H 2 responses, promoting severe allergic asthma. This effect is independent of fungal sensitization and can be reconstituted with β-glucan and abrogated by neutralization of IL-17A. Furthermore, this severe asthma is resistant to steroids and characterized by mixed T _H 2 and T _H 17 responses, including IL-13 ⁺ IL-17 ⁺ CD4 ⁺ double-producing effector T cells. Steroid resistance is dependent on fungus-induced T _H 17 responses because steroid sensitivity was restored in IL-17rc ^-/- mice. Similarly, in children with asthma, fungal exposure was associated with increased serum IL-17A levels and asthma severity.
Conclusion: Our data demonstrate that fungi are potent immunomodulators and have powerful effects on asthma independent of their potential to act as antigens. Furthermore, our results provide a strong rationale for combination treatment strategies targeting IL-17A for this subgroup of fungus-exposed patients with difficult-to-treat asthma.
Competing Interests: The authors have declared that no conflict of interest exists. AL Budelsky is a paid employee and stockholder in Amgen, Inc.
(Copyright © 2016 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)