Allergen specific immunotherapy (SIT) is the only specific treatment of allergic diseases at present. How SIT impacts pulmonary innate immunity against bacteria currently remains unclear. In this study, dust mite extracts (HDM)-sensitized mice were immunized with a subcutaneous injection of HDM. These mice were then challenged with an intranasal administration of HDM. After the last challenge, mice were infected with an intranasal instillation withWe measured the score of tissue inflammation, the expression of cathelicidin-related antimicrobial peptide (CRAMP) and 25-Hydroxyvitamin D-1Alpha-hydroxylase (CYP27B1) in lung. We analyzed the effect of TGF-β1 on CRAMP and CYP27B1 in airway cells (16HBE), and investigate the role of TGF-β1-induced CYP27B1 in defense against bacteria in16HBE cell. We found that SIT attenuates HDM-induced airway inflammation and airway responsiveness (AHR), which is involved in the increased levels of HDM-specific IgG2a, IL-10, TGF-β1, IFN-γ, CRAMP and CYP27B1. SIT ameliorates pulmonary infectious inflammation associated with an improving defense of HDM-challenged mice againstMeanwhile, TGF-β1 significantly increased the expression of CYP27B1 in a dose-dependent manner. TGF-β1 did not increase the levels of CRAMP in airway epithelial cells. Furthermore, 25-dihydroxyvitamin D3 (25VD) is required for TGF-β1-induced CRAMP in airway epithelial cells. CRAMP was significantly increased in TGF-β1/25VD-treated 16HBE cells. These findings illustrated that TGF-β1 is a major player against bacterial infections in SIT models via induction of CYP27B1 rather than CRAMP. Collectively, these findings highlight a role for SIT enhancing host defense against bacteria depending on TGF-β1-induced CYP27B1in asthma.