Along with their immunogenic role, dendritic cells (DCs) are also critical in maintaining tolerance to self-antigens by inducing regulatory T cells (Tregs) via the expression of the immunomodulatory enzyme indoleamine 2,3-dioxygenase 1 (IDO1). In turn, Tregs modulate the maturation and/or function of DCs. In immune thrombocytopenia (ITP), the interaction between DCs and Tregs has never been investigated although decreased number/function of Tregs as well as altered DCs have been described. Here, we ask whether, in ITP: (1) IDO1 expression/activity is decreased in mature DCs; (2) IDO1-mediated Treg generation is impaired; and (3) DC maturation is abnormally modulated by Tregs. We found that in ITP, DCs show reduced capability of upregulating the expression/activity of IDO1. This finding results in the reduced ability of mature DCs of converting T cells into Tregs. In turn, Tregs are characterized by decreased interleukin-10 production and show lower ability of inhibiting DC maturation. In conclusion, these data point out the role of IDO1 in the impaired regulatory T cell development of ITP patients and suggest that the cross-talk between Tregs and DCs is hampered and plays a pathogenetic role.