Rationale: Group 2 innate lymphoid cells (ILC2), a major source of type 2 cytokines, initiate eosinophilic inflammatory responses in murine models of asthma.

Objectives: To investigate the role of ILC2 in allergen-induced airway eosinophilic responses in subjects with atopy and asthma.

Methods: Using a diluent-controlled allergen challenge crossover study, where all subjects (n = 10) developed allergen-induced early and late responses, airway eosinophilia, and increased methacholine airway responsiveness, bone marrow, blood, and sputum samples were collected before and after inhalation challenge.

Measurements and Main Results: ILC2 (lin⁻FcεRI⁻CD45⁺CD127⁺ST2⁺) and CD4⁺T lymphocytes were enumerated by flow cytometry, as well as intracellular IL-5 and IL-13 expression. Steroid sensitivity of ILC2 and CD4⁺ T cells was investigated in vitro. A significant increase in total, IL-5⁺, IL-13⁺, and CRTH2⁺ ILC2 was found in sputum, 24 hours after allergen, coincident with a significant decrease in blood ILC2. Total, IL-5⁺, and IL-13⁺, but not CRTH2⁺, CD4⁺ T cells significantly increased at 24 and 48 hours after allergen in sputum. In blood and bone marrow, only CD4⁺ cells demonstrated increased activation after allergen. Airway eosinophilia correlated with IL-5⁺ ILC2 at all time points and allergen-induced changes in IL-5⁺ CD4⁺ cells at 48 hours after allergen. Dexamethasone significantly attenuated IL-2– and IL-33–stimulated IL-5 and IL-13 production by both cell types.

Conclusions: Innate and adaptive immune cells are increased in the airways associated with allergic asthmatic responses. Total and type 2 cytokine–positive ILC2 are increased only within the airways, whereas CD4⁺ T lymphocytes demonstrated local and systemic increases. Steroid sensitivity of both cells may explain effectiveness of this therapy in those with mild asthma.