Interferon-gamma (IFN-gamma)-producing CD3(+)CD4(+)CD25(+)Foxp3(+) peripheral blood lymphocytes (PBL) are more frequently detectable in patients with good than in patients with impaired long-term kidney graft function, suggesting an immunoregulatory role of this induced T regulatory (iTreg) subtype. Herein, the in vitro function of separated CD3(+)CD4(+)CD25(+)Foxp3(+)IFN-gamma(+) PBL that were induced by phorbol 12-myristate 13-acetate (PMA)/ionomycin or alloantigenic stimulation was investigated using cell coculture techniques and flow cytometry. CD4(+)CD25(+)Foxp3(+) PBL with intracellular IFN-gamma production increased to 26% in cell cultures stimulated with PMA/ionomycin for 6 hours. Recombinant IFN-gamma augmented and anti-IFN-gamma monoclonal antibody blocked induction of CD4(+)CD25(+)Foxp3(+)IFN-gamma C FBI., suggesting their IFN-gamma-dependent induction. In addition, CD4(+)CD25(+)Foxp3(+)IFN-gamma(+) PBL produced immunosuppressive interleukin (IL)-10, transforming growth factor-beta, and IL-4 intracellularly and expressed both IFN-gamma and IFN-gamma receptors (CD119) on the cell surface, allowing separation of CD4(+)CD25(+)IFN-gamma(+) PBL with 98% purity. Addition of enriched CD4(+)CD25(+)IFN-gamma(+) PBL to autologous PMA/ionomycin stimulated PBL decreased blast formation (p < 0.05), indicating suppression of cell proliferation by CD4(+)CD25(+)IFN-gamma(+) PBL CD4(+)CD25(+)IFN-gamma(+) PBL separated from primary mixed leukocyte cultures (MLC) and added to autologous or third-party secondary MLC suppressed allogeneic T-cell activation nonspecifically (p < 0.05). We conclude that CD4(+)CD25(+)Foxp3(+)IFN-gamma(+) PBL are induced by IFN-gamma, making them sensors for IFN-gamma and initial immune responses. Circulating CD4(+)CD25(+)Foxp3(+)IFN-gamma(+) PBL could suppress allogeneic T-cell responses in patients and may be involved in inhibition of the posttransplant alloresponse. (C) 2011 American Society for Histocompatibility and Immunogenetics. Published by Elsevier Inc. All rights reserved.