Mechanisms regulating airway responses to the fungal allergen Alternaria alternata

Abstract

Human asthma is a heterogeneous disease characterized by the expression of both Th2 and Th17 cytokines. In vitro and in vivo studies have shown a reciprocal regulation between Th2 and Th17 pathways, suggesting a potential induction of neutrophil-promoting Th17 inflammation in the absence of a Th2 response. Alternaria alternata is a clinically relevant allergen that is associated with severe and fatal asthma exacerbations. Exposure to A. alternata is characterized by a predominant Th2 response, but can also induce the production of factors associated with Th17 responses (e.g., CXCL8) from epithelial cells. Using a mouse model, we found that wild-type mice develop an eosinophilic Th2 airway disease in response to A. alternata exposure, while IL-4-, IL-13-, and STAT6-deficient mice exhibit a primarily neutrophilic response. Neutrophilic asthma in STAT6-/- mice was accompanied by elevated lung levels of TNF-α, CXCL1, CXCL2, and CXCL5, and was steroid-resistant. Neutralization of Th17 signaling only partially reduced neutrophil numbers and total airway inflammation. Airway neutrophilia developed in RAG-deficient and CD4-depleted Balb/c mice, suggesting that the suppression of neutrophil responses is dependent on Th2 cytokine production by T cells and that airway neutrophilia is primarily an innate response to allergen. These results highlight the importance of combination therapies for treatment of asthma and establish a role for factors other than IL-17 as targets for neutrophilic asthma.