Objective To investigate the changes of T helper 9( Th9) cells,interleukin-9( IL-9),and related transcription factors in previously untreated patients with chronic hepatitis C,as well as their association with clinical indices. Methods A total of 29 previously untreated patients with chronic hepatitis C who attended Hainan Provincial People's Hospital from December 2018 to July 2019 were enrolled,and 15 healthy individuals were enrolled as healthy controls. The patients with chronic hepatitis C received sofosbuvir/velpatasvir antiviral therapy for 12 weeks,and then plasma and peripheral mononuclear cells( PBMCs) were isolated. Flow cytometry was used to measure the percentage of CD3+CD4+IL-9+Th9 cells in PBMCs; ELISA was used to measure the plasma level of IL-9; quantitative real-time PCR was used to measure the relative mRNA expression of IL-9 and the transcription factors PU. 1 and Foxo1 in PBMCs. The t-test or the paired t-test was used for comparison between two groups,and a Pearson correlation analysis was used to investigate correlation. Results Compared with the healthy controls,the previously untreated chronic hepatitis C patients had significantly lower percentage of peripheral Th9 cells( 0. 92% ± 0. 14% vs 1. 14% ± 0. 21%,t = 4. 31,P < 0. 001) and plasma IL-9 level( 248. 2 ± 66. 97 pg/ml vs 309. 02 ± 88. 48 pg/ml,t = 2. 63,P = 0. 012). The previously untreated chronic hepatitis C patients had significantly lower relative mRNA expression of IL-9 and PU. 1 than the healthy controls( t = 20. 67 and 23. 21,both P < 0. 001),while there was no significant difference in the relative mRNA expression of Foxo1 between the previously untreated chronic hepatitis C patients and the healthy controls( P > 0. 05). In the previously untreated chronic hepatitis C patients,the percentage of peripheral Th9 cells,IL-9 level,and mRNA expression of IL-9 and PU. 1 were negatively correlated with HCV RNA( r =-0. 46,-0. 38,-0. 52,and-0. 41,all P < 0. 05),but they were not correlated with the level of alanine aminotransferase( all P > 0. 05). Sofosbuvir/velpatasvir antiviral therapy achieved virologic response in 29 chronic hepatitis Cpatients,and the percentage of peripheral Th9 cells and the mRNA expression of PU. 1 after antiviral therapy were significantly higher than those at baseline( t = 2. 20 and 6. 52,both P < 0. 05),while there were no significant changes in the plasma level of IL-9 and the relative mRNA expression of IL-9 from baseline to after treatment( both P > 0. 05). Conclusion Chronic hepatitis C virus infection may suppress the activation of Th9 cells,suggesting that Th9 cells might be involved in the chronicity of hepatitis C virus infection.