We show that IFN-a promotes the maturation of tolerogenic IL-10 DC. However, addition of IFN-a to IL-10 DC did not induce full maturation as triggered by supplementation of a maturation cocktail. Similar observations have been made by Santini et al., demonstrating that IFN-a-induced generation of DC resulted in an increased, but incomplete expression of the maturation parameter CD83. IFN-a is known to Bortezomib clinical trial enhance the expression of the IL-12 receptor b1 and b2 chains on human T cells enhancing Th1/Tc1 immune responses, but, more dominantly, it negatively regulates IL-12 p40 and p70 production by APC. Our study confirmed these results by demonstrating that incubation of mDC as well as of IL-10 DC with IFN-a strongly inhibited IL12p40 secretion. As previously reported, human monocyte-derived DC generated by the protocol used in this study did not produce significant amounts of IL-12p70. Here, we demonstrated that IFN-a treatment did not increase levels of IL-12p70 secretion. However, we found that IFN-a stimulation of IL-10 DC was followed by an enhanced CD4+ and CD8+ T cell proliferation and increased IFN-c levels, indicating an amplified Th1/Tc1 cell response. If this effect is partially due to loss of the immunosuppressive function of the IL-12 p40 subunit or of the IL-23 heterodimer needs further evaluation. In accordance to our data, IFN-a-treated human DC have been shown to act as effective APC in driving the development of Th1/Tc1 immune responses in vitro an in vivo and, more recently, to expand both Th1 and Th17 populations. In T cells, contradictory effects of IFN-a on proliferation, function and cell death were observed. IFN-a is the cytokine with the longest record of use in clinical oncology. Clinical treatment of melanoma patients with high dose IFN-a is well established and direct antitumor effects as well as modulation of the immune system are supposed to contribute to the beneficial effect of the therapy. In melanoma patients, a striking correlation between the clinical response and the development of autoimmune reactions has been demonstrated. Reports of a prospective study of high dose IFN-a regime linked the appearance of clinical and laboratory evidence for autoimmunity with improved outcomes as demonstrated for relapse-free and overall survival, assuming that IFN-a inhibits tolerance mechanisms. However, the impact of IFN-a on tolerance mechanisms had not been addressed in detail. To date, the effects of IFN-a on human tolerogenic DC are unknown, but there is evidence that the ability of DC to attract Tregs.