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BackgroundThe alarmin cytokines IL-25 and IL-33 are key promoters of type 2 inflammation. Basophils respond to alarmin cytokines, however the relationship of these cytokines with basophil activation and recruitment in human studies of allergic asthma has not been well characterized. This study investigated the effect of IL-25 and IL-33 on basophils in a model of allergic asthma.Methods10 mild allergic asthmatics underwent allergen and diluent inhalation challenges. Bone marrow aspirates were collected at pre-challenge and 24 h (h) post challenge. Peripheral blood and sputum samples were collected at pre-challenge, 7 h, and 24 h post-challenge to measure basophil expression of IL-17RB, ST2, and intracellular IL-25. Freshly isolated peripheral blood basophils from allergic donors were incubated overnight with IL-25 and IL-33, or sputum supernatant collected post-allergen to assess pro-inflammatory effects of mediators released in the airways.ResultsThere were increased percentage of basophils expressing IL-17RB, ST2, and intracellular IL-25 collected from bone marrow, peripheral blood, and sputum after allergen inhalation challenge. In vitro stimulation with IL-25 and IL-33 increased the percentage of basophils expressing intracellular type 2 cytokines and surface activation markers, and primed eotaxin-induced migratory potential of basophils, which was mediated directly through IL-17RB and ST2, respectively. Stimulation of basophils with sputum supernatants collected post-allergen challenge up-regulated the percentage of basophils expressing markers of activation and intracellular type 2 cytokines, which was reversed following blockade of the common β chain (βc).ConclusionsOur findings indicate that the alarmin cytokines IL-33 and IL-25 increase basophil activation and migratory potential, and may pose as a novel therapeutic targets for the treatment of allergic asthma.Electronic supplementary materialThe online version of this article (doi:10.1186/s12931-016-0321-z) contains supplementary material, which is available to authorized users.

Activated bronchial epithelial cells (BEC) release various alarmins, including thymic stromal lymphopoietin (TSLP), that drive type 2 inflammation. We hypothesize that BEC-derived factors promote in situ eosinophil differentiation and maturation, a process that is driven by an IL-5-rich microenvironment in asthmatic airways. To assess the eosinophilopoietic potential of epithelial-derived factors, eosinophil/basophil colony forming units (Eo/B-CFU) were enumerated in 14-day methylcellulose cultures of blood-derived nonadherent mononuclear cells incubated with BEC supernatants (BECSN) from healthy nonatopic controls (n = 8), mild atopic asthmatics (n = 9), and severe asthmatics (n = 5). Receptor-blocking antibodies were used to evaluate the contribution of alarmins. Modulation of the mRNA expression of transcription factors that are crucial for eosinophil differentiation was evaluated. BECSN stimulated the clonogenic expansion of eosinophil progenitors in vitro. In the presence of IL-5, Eo/B-CFU numbers were significantly greater in cocultures of BESCN from severe asthmatics compared with other groups. This was attenuated in the presence of a TSLP (but not an IL-33) receptor-blocking antibody. Recombinant human TSLP (optimal at 100 pg/ml) stimulated Eo/B-CFU growth, which was significantly enhanced in the presence of IL-5 (1 ng/ml). Overnight culture of CD34 cells with IL-5 and TSLP synergistically increased GATA-binding factor 2 and CCAAT/enhancer-binding protein α mRNA expression. The eosinophilopoietic potential of factors derived from BEC is increased in severe asthma. Our data suggest that TSLP is a key alarmin that is produced by BECs and promotes in situ eosinophilopoiesis in a type 2-rich microenvironment.

Background: Interleukin (IL)-25 plays a pivotal role in type 2 immune responses. In a baseline cross-sectional study, we previously showed that IL-25 plasma levels and IL-25 receptor (IL-25R: IL-17RA, IL-17RB, and IL-17RA/RB) expression on mature blood eosinophils are increased in atopic asthmatics compared to normal nonatopic controls. This study investigated allergen-induced changes in IL-25 and IL-25R expression in eosinophils from asthmatics. Methods: Dual responder atopic asthmatics (n = 14) were enrolled in this randomized diluent-controlled crossover allergen challenge study. Blood was collected before and 24 h after the challenge. The surface expression of IL-25R was evaluated by flow cytometry on eosinophils and Th2 memory cells. In addition, plasma levels of IL-25 were measured by ELISA, and functional responses to IL-25 including type 2 cytokine expression, degranulation, and the migrational responsiveness of eosinophils were evaluated in vitro. Results: Following the allergen but not the diluent inhalation challenge, significant increases in the expression of IL-17RB and IL-17RA/B were found on eosinophils but not on Th2 memory cells. IL-25 plasma levels and the number of eosinophils but not of Th2 memory cells expressing intracellular IL-25 increased significantly in response to the allergen but not the diluent challenge. Stimulation with physiologically relevant concentrations of IL-25 in vitro caused (i) degranulation of eosinophils (measured by eosinophil peroxidase release), (ii) enhanced intracellular expression of IL-5 and IL-13, and (iii) priming of eosinophil migration to eotaxin. IL-25 stimulated intracellular cytokine expression, and the migration of eosinophils was blocked in the presence of a neutralizing IL-25 antibody. Conclusions: Our findings suggest that the IL-25/IL-25R axis may play an important role in promoting the recruitment and proinflammatory function of eosinophils in allergic asthma.

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