Allergic asthma is a chronic inflammatory disease of the lung and the airway,which is characterized by aberrant type 2 immune responses to otherwise unharmful aeroallergens.While the central role of Th2 cells and type 2 cytokines in the pathogenesis of allergic asthma is well documented,the regulation and plasticity of Th2 cells remain incompletely understood.By using an animal model of allergic asthma in IL-4-reporter mice,we found that Th2 cells in the lung expressed higher levels of Rora than those in the lymph nodes,and that treatment with an RORα agonist SRI 078 resulted in diminished Th2 cell responses in vivo.To determine the T cell-intrinsic role of RORα in allergic asthma in vivo,we established T cell-specific RORα-deficient(Cd4creRoraf/f)mice.Upon intranasal allergen challenges,Cd4creRoraf/f mice exhibited a significantly increased Th2 cells in the lungs and the airway and showed an enhanced eosinophilic inflammation compared to littermate control mice.Studies with Foxp3YFP-crereRoraf/f mice and CD8+T cell depletion showed that the increased Th2 cell responses in the Cd4creRoraf/f mice were independent of Treg cells and CD8+T cells.Our findings demonstrate a critical regulatory role of RORα in Th2 cells,which suggest that RORα agonists could be effective for the treatment of allergic diseases.