Abstract： Objective To investigate the expression pattern of histone deacetylase 9 (HDAC9) in peripheral blood of asthmatics and its effect on immune cells (Th2,Th17,Tregs ) involved in the pathogenesis of asthma.Methods Forty-seven asthmatics from Ruijin Hospital were recruited and assigned to intermittent,mild and moderate-severe groups.Lung function test and Asthma Control Questionnaire were performed to evaluate asthma control and severity.Twenty healthy donors were enrolled as controls.GATA3,IL-4,and HDAC9 mRNA expression levels were measured by SYRB Green Real-time PCR.The cytokine IL-17-mainly produced by Th17 cells and TGF-β-mainly produced by Treg cells,were measured by ELISA.Results The GATA3 and IL-4 mRNA expression levels ( 28.12 ± 7.57 and 743.6 ± 312.8 ) were upregulated in asthmatics as compared to the healthy controls [ 0.56 ± 0.22,0.7 ± 0.8 ( U =16.00,37.00,P＜0.01 )].The HDAC9 mRNA expression levels of intermittent,mild and moderate-severe groups were 3.20 ± 0.50,89.6 ± 18.0,323.0 ± 65.3,respectively,which were associated with the severity of disease (H =11.32,P ＜0.05).The level of IL-17 in asthmatic group was(83 ±55)ng/L,which was up-regulated as compared to the healthy control [ (34 ± 22 ) ng/L ( U =153.50,P ＜ 0.01 ) ].The level of TGF-β was decreased in the asthmatic groups as compared to the healthy control,but the difference did not reach significance.HDAC9 mRNA expression level was positively correlated with GATA3 mRNA expression level (r =0.482,P ＜0.05),and also with IL-4 mRNA expression (r =0.432,P＜0.05) and IL-17 (r=0.538,P ＜ 0.05 ),but negatively correlated with TGF-β ( r =- 0.417,P ＜ 0.05 ).In patients with moderate-severe asthma,HDAC9 mRNA expression level was negatively correlated with FEV1％ (r =-0.657,P ＜ 0.05 ).Conclusion HDAC9 mRNA expression was up-regulated in peripheral blood of asthmatics,which was not only associate with the Th2 master transcriptional factors GATA3,cytokine IL-4 mRNA,Thl7 and Treg cell-related cytokines,but also with FEV1％ in moderate-severe asthma.