Early-life wheezing associated respiratory tract infection by rhinovirus (RV) is considered a risk factor for asthma development. We have shown that RV infection of 6 day-old BALB/c mice, but not mature mice, induces an asthma-like phenotype which is associated with an increase in the population of type 2 innate lymphoid cells (ILC2s) and dependent on IL-13 and IL-25. We hypothesize that ILC2s are required and sufficient for development of the asthma-like phenotype in immature mice. Mice were infected with RV1B on day 6 of life and treated with vehicle or a chemical inhibitor of retinoic acid receptor-related orphan receptor (ROR)-α, SR3335 (15 mg/kg/day intraperitoneally for 7 days). We also infected Rorasg/sg mice without functional ILC2s. ILC2s were identified as negative for lineage markers and positive for CD25/IL-2Rα and CD127/IL-7Rα. Effects of SR3335 on proliferation and function of cultured ILC2s were determined. Finally, sorted ILC2s were transferred into naïve mice and lungs harvested 14 days later for assessment of gene expression and histology. SR3335 decreased the number of RV-induced lung lineage-negative, CD25+, CD127+ ILC2s in immature mice. SR3335 also attenuated lung mRNA expression of IL-13, Muc5ac and Gob5 as well as mucous metaplasia. We also found reduced expansion of ILC2s in RV-infected Rorasg/sg mice. SR3335 also blocked IL-25 and IL-33-induced ILC2 proliferation and IL-13 production ex vivo. Finally, adoptive transfer of ILC2s led to development of asthma like phenotype in immature and adult mice. RORα-dependent ILC2s are required and sufficient for type 2 cytokine expression and mucous metaplasia in immature mice.-