The concept of immune regulation/suppression has been well-established and, besides thymus-derived CD4+CD25+ regulatory T (TR) cells, it became clear that a variety of additional peripherally induced TR cells play vital roles in protection from many harmful immune responses including intestinal inflammation. In the present study, we have analyzed in vivo-induced Ag-specific CD4+ TR cells with respect to their molecular and functional phenotype. By comparative genomics we could show that these Ag-specific TR cells induced by chronic Ag stimulation in vivo clearly differ in their genetic program from naturally occurring thymus-derived CD4+CD25+ TR cells. This distinct population of induced TR cells express neither CD25 nor the TR-associated transcription factor Foxp3. Strikingly, CD25 is not even up-regulated upon stimulation. Despite the lack in Foxp3 expression, these in vivo-induced CD25- TR cells are able to interfere with an Ag-specific CD8+ T cell-mediated intestinal inflammation without significant increase in CD25 and Foxp3 expression. Thus, our results demonstrate that in vivo-induced Ag-specific TR cells represent a distinct population of Foxp3-CD25- TR cells with regulatory capacity both in vitro and in vivo.