Reprogramming of airway mononuclear phagocytes in response to type 2 inflammation

Abstract

There is a global epidemic of asthma which affects all age groups and leads to significant public health challenges. Allergy-associated asthma is the most common phenotype; thus, it is important to understand the mechanistic pathways that drive allergic asthma pathogenesis. In this study, we assessed the reprogramming of airway mononuclear phagocytes (aMPs) in response to type 2 inflammation in allergic asthmatics. aMPs comprising macrophages, monocytes, and dendritic cells (DCs) are abundant immune cells located in the lung, and are found to be associated with asthma pathogenesis. We utilized bronchoscopic segmental allergen challenge (SAC), a human in vivo asthma exacerbation model, to compare airway immune responses to aeroallergen between allergic asthmatics (AA) and allergic non-asthmatics (AC). Only airway conventional dendritic cells (cDCs) isolated from AA bronchoalveolar lavage (BAL) fluid responded to IL-13 stimulation ex vivo by producing the T helper 2 (Th2)-specific chemokines CCL17 and CCL22. The amount of IL-13 receptor alpha (IL-13Rα1), as assessed by mean fluorescence intensity, was higher on type 2 dendritic cells (DC2s) isolated from BAL fluid of AA compared to AC. In addition, the number of CCL17+ DC2s in the BAL after allergen challenge was higher from AA compared to AC. This corresponded with a positive correlation between DC2s and eosinophils, and significant increases in the eosinophil cell count and higher levels of type 2 cytokines and chemokines, such as IL-4, IL-5, IL-13, CCL17, and CCL22 in AA after SAC, demonstrating a greater type 2 inflammatory response to allergen. We also found that these changes were localized to the airway and were not seen in circulating DCs. Finally, we established a novel 22-marker flow cytometry panel to comprehensively explore airway immune cell populations and heterogeneity in AA post-SAC. Overall, these data demonstrate a more robust allergen-induced type 2 inflammatory response in AA compared to AC, and suggests that DC2s, and in particular their response to IL-13, may play a previously under-explored role in allergic asthma pathogenesis. The exploratory panel further examines DC2 subsets that express IL-13Rα1, the IL-4 receptor alpha (IL-4Rα), and the high-affinity IgE receptor (FCεR1), as well as other airway mononuclear phagocytes, including alveolar macrophages (AMs), alternatively activated macrophages (AAMs), CCR7+ migratory DCs, CCR2high immature monocytes, and Th2 cells both in the airway and blood. Overall, this study contributes novel insights into the mechanisms of asthma pathogenesis and lays out a direction for future studies.