Allergen-induced airway inflammation is characterized by Th2-mediated eosinophilic inflammation in the lungs. While the molecular mechanisms leading to this abnormal Th2 response remain unclear. Recent studies have demonstrated that MicroRNAs (miRNAs) modulate allergic airway inflammation. In this study, the role of miRNAs in allergic asthma pathogenesis was examined. Differentially expressed miRNAs were identified via miRNA microarray, with miR-155 being among the most highly expressed in asthma mice lungs. Examination of miR-155 overexpression resulted in enhanced inflammation and mucus hypersecretion in the lungs of allergen-challenged mice compared with control animals. Furthermore, CTLA-4, an important negative regulator of T-cell activation, was identified as a direct miR-155 target. Moreover, miR-155 overexpression in CD4(+) T cells resulted in decreased CTLA-4 levels and a subsequent increased proliferative response. Collectively, these findings suggest that miR-155 might contribute to allergic asthma by increasing the proliferative response of Th cells via CTLA-4 down regulation and thus may be a potential therapeutic target for allergic asthma. (C) 2017 Elsevier Inc. All rights reserved.