Objective: subclinical gut inflammation has been demonstrated in ankylosing spondylitis(AS) patients. Aim of this study was to determine the frequency of regulatoryCD4+CD25high T cells (Treg), and to evaluate Treg-related cytokines (IL-2, TGF-β, IL-10), andtranscription factors (FOXP3 and STAT5) in the ileum of AS patients.Methods: Quantitative gene expression analysis, by rt-PCR, of Treg-related cytokines (IL-2,TGF-β, IL-10) and transcription factor (STAT-5 and FOXP3) was performed on ilealbiopsies of 18 AS and 15 active Crohn’s disease (CD) patients, and 15 healthy subjects(HS). Tissue and circulating Treg cells were also analyzed by flow cytometry.Results: A significant up-regulation of IL-2, TGF-β, FOXP3, STAT-5 and IL-10 transcriptsin the terminal ileum of AS patients displaying chronic ileal inflammation was observed.Flow cytometric analysis of Treg cells showed significant peripheral expansion in bothchronic inflamed AS and CD patients (1.08%±0.4% and 1.05±0.3% respectively),compared to HS (0.25±0.12; p<0.05). Interestingly a five-fold increase in the proportion ofTreg cells was observed in the gut of AS patients (5±3%; p<0.001) compared to HS(1.2%±0.4%), with 70 to 80% of these cells also producing IL-10. In vitro studies showedthat block of IL-10 was sufficient to induce Th17 polarization on isolated lamina propriamononuclear cells (LPMC) from AS patients.Conclusion: Here we provide the first evidence that an active Treg cell response, mainlydominated by IL-10 production, occurs in the gut of AS patients, probably responsible forthe absence of a clear TH17 polarization observed in the ileum of AS patients.