Chitinase 3-like-1 protects airway function despite promoting type 2 inflammation during fungal-associated allergic airway inflammation

Exposure to fungi can result in a wide range of comorbidities depending on the immune status of the host. Chronic exposure and reactivity to fungi such as Aspergillus fumigatus can result in conditions such as severe asthma with fungal sensitization (SAFS) or allergic bronchopulmonary aspergillosis (ABPA). However, the pathophysiology of SAFS and ABPA is not well understood. Here, we report that the chitinase-like protein YKL-40 is elevated in lung lavage fluid from humans with asthma that are sensitized to fungi. Initial studies demonstrated that mice deficient in the murine ortholog of YKL-40, breast regression protein-39 (BRP-39, chitinase-3-like 1, Chi3l1), were not more susceptible to acute infection with A. fumigatus. However, in an experimental model of fungal-associated allergic airway inflammation (fungal asthma), Chi3l1-/- mice had significantly increased airway hyperresponsiveness (AHR). Surprisingly, increased AHR in Chi3l1-/- mice occurred in the presence of significantly lower type 2 responses (decreased eosinophil numbers and decreased IL-4, IL-5, IL-33, CCL17, and CCL22 levels), although type 1 and type 17 responses were not different. Increased AHR was not associated with differences in periodic acid-Schiff staining of lung tissue, differences in the expression of Muc5ac and Clca3, nor differences in lung edema. Bone marrow chimera studies revealed that the presence of BRP-39 in either the hematopoietic or nonhematopoietic compartment was sufficient for controlling AHR during fungal asthma. Collectively, these results indicate that BRP-39 protects against AHR during fungal asthma despite contributing to type 2 inflammation, thus highlighting an unexpected protective role for BRP-39 in allergic fungal asthma.