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Indoleamine 2, 3- dioxygenase (IDO) plays an important role in immunosuppressive pathway, as inhibits responses of T cells and promotes immune tolerance. Host response to Helicobacter pylori (H. pylori) is involved in the infection persistence and it is also associated with different clinical outcomes. The aim of this study was to investigate the role of IDO in H. pylori -infected patients with gastritis diseases and peptic ulcer diseases (PUD) through the assessment of the relationship among IDO protein expression and the numbers of T helper (Th)-1, Th17, Th22, and T regulator (Treg) cells. Antrum biopsy was obtained from H. pylori -negative patients (n = 48) and H. pylori -positive subjects (55 patients with gastritis and 47 patients with PUD), for performing H. pylori status and histopathological assessments. IDO protein expression was evaluated by Western blotting. IDO protein expression was significantly higher in gastric biopsies from H. pylori -positive subjects compared to the H. pylori -negative subjects, and also in H. pylori -positive subjects with gastritis disease compared to H. pylori -positive subjects with PUD. Moreover, in H. pylori -positive subjects, a positive correlation was observed between IDO protein expression and the frequency of Treg cells. In addition, a negative correlation was observed between IDO protein expression and the number of Th1, Th17, and Th22. Increased IDO protein expression is able to change the number of Th1, Th17, Th22, and Treg cells and these changes are possibly associated with an increase in the risk of PUD development in H. pylori -infected patients. [ABSTRACT FROM AUTHOR]