Objective: CD4(+)FoxP3(+)CD25(+) regulatory T-cells (T-regs) are important for preventing tissue destruction. Here, we investigate the role of T-regs for protection against experimental arthritis by IFN-alpha. Methods: Arthritis was triggered by intra-articular injection of methylated bovine serum albumin (mBSA) in wild-type mice, Foxp3DTReGFP(+/-) mice [allowing selective depletion of T-regs by diphtheria toxin (DT)] and CD4-Cre(+/-) IFNA1R flox/flox mice (devoid of IFNAR signaling in T-cells) earlier immunized with mBSA, with or without treatment with IFN-alpha or the indoleamine 2,3-dioxygenase (IDO)-metabolite kynurenine. T-regs were depleted in DT-treated Foxp3DTReGFP(+/-) mice and enumerated by FoxP3 staining. Suppressive capacity of FACS-sorted CD25(+high)CD4(+) T-regs was tested in vivo by adoptive transfer and ex vivo in cocultures with antigen-stimulated CFSE-stained T-responder (CD25-CD4(+)) cells. IDO was inhibited by 1-methyl tryptophan. Results: Both control mice and mice devoid of IFNAR-signaling in T helper cells were protected from arthritis by IFN-alpha. Depletion of T-regs in the arthritis phase, but not at immunization, abolished the protective effect of IFN-alpha and kynurenine against arthritis. IFN-alpha increased the number of T-regs in ex vivo cultures upon antigen recall stimulation but not in naive cells. IFN-alpha also increased the suppressive capacity of T-regs against mBSA-induced T-responder cell proliferation ex vivo and against arthritis when adoptively transferred. The increased suppressive activity against proliferation conferred by IFN-alpha was clearly reduced by in vivo inhibition of IDO at immunization, which also abolished the protective effect of IFN-alpha against arthritis. Conclusion: By activating IDO during antigen sensitization, IFN-alpha activates T-regs, which prevent arthritis triggered by antigen rechallenge. This is one way by which IFN-alpha suppresses inflammation.