Immature dendritic cells (DCs) can be instrumental in the induction of peripheral tolerance promoting the generation of regulatory T cells. Since NF-kB is central to DC maturation, rat bone marrow-derived DCs were transfected with an adenoviral vector encoding for a kinase-defective dominant negative form of IKK2, namely AdVdnIKK2, to block NF-kB activation and inhibit DC maturation. DnIKK2-DCs had an immature phenotype, as shown by the low expression of MHCII, CD86 and IL-12. Moreover, they had an impaired allostimulatory capacity and generated CD4+ T cells with regulatory function (CD4+ dnIKK2-Treg). I have investigated the potency, the phenotype, and the mechanism of action of CD4+dnIKK2 Treg. The results revealed that CD4+dnIKK2-Treg were CD4+CD25-/dim and expressed Foxp3, IL-10, TGF-P, IL-2, and inducible nitric oxide synthase (iNOS). CD4+ dnIKK2-Treg had an extremely potent donor-specific regulatory activity, in fact when cocultured with responder T cells in a primary MLR, suppressed T-cell proliferation to alloantigens until at up to 1:10¹⁵ ratio.