Interplay Between the IL-33/ST2 Axis and Bone Marrow ILC2s in Protease Allergen-Induced IL-5-Dependent Eosinophilia

Background:Eosinophils develop from CD34(+)progenitor cells in the bone marrow under the influence of interleukin (IL)-5. Several cell types produce IL-5, including type 2 innate lymphoid cells (ILC2s). The alarmin cytokine IL-33 is known to activate ILC2s in mucosal tissues, but little is known about IL-33-responsive ILC2s in the bone marrow in allergen-induced airway inflammation. Methods:Wild type (WT) and Rag1 deficient (Rag1(-/-)) mice, which lack mature T and B cells, received intranasal doses of papain to induce acute allergic inflammation. In some experiments, mice were pre-treated with anti-IL-5 prior to the papain challenge. Furthermore, recombinant IL-33 was administered to WT mice,Rag1(-/-)mice, lymphocyte deficient mice (Rag2(-/-)Il2rg(-/-)) and toex vivowhole bone marrow cultures. Bone marrow eosinophils and ILC2s were analyzed by flow cytometry. Eosinophil count was assessed by differential cell count and secreted IL-5 from bone marrow cells by ELISA. Results:Intranasal administration of papain or IL-33 increased the number of mature eosinophils in the bone marrow despite the absence of adaptive immune cells inRag1(-/-)mice. In parallel, an increased number of eosinophils was observed in the airways together with elevated levels of Eotaxin-2/CCL24. Bone marrow ILC2s were increased after papain or IL-33 administration, whereas ILC2s was found to be increased at baseline inRag1(-/-)mice compared to WT mice. An upregulation of the IL-33 receptor (ST2) expression on bone marrow ILC2s was observed after papain challenge in bothRag1(-/-)and WT mice which correlated to increased number of bone marrow eosinophilia. Furthermore, an increased number of ST2(+)mature eosinophils in the bone marrow was observed after papain challenge, which was further dependent on IL-5. In addition, bone marrow-derived ILC2s from both mouse strains produced large amounts of IL-5ex vivoafter IL-33 stimulation of whole bone marrow cultures. In contrast, IL-33-induced bone marrow and airway eosinophilia were abolished in the absence of ILC2s inRag2(-/-)Il2rg(-/-)mice and no production of IL-5 was detected in IL-33-stimulated bone marrow cultures. Conclusion:These findings establish bone marrow ILC2s and the IL-33/ST2 axis as promising targets for modulation of uncontrolled IL-5-dependent eosinophilic diseases including asthma.