In cancer patients undergoing immune therapy with lipopolysaccharides/interferon-gamma activated interleukin (IL)-12 secreting dendritic cells (DCs) we observed enhanced proliferative capacity of pyrophosphate-responsive peripheral blood (PB) gammadelta T-cells. This was not noted before as in other clinical trials DCs were used that were not enabled for IL-12 secretion and mice do not have a corresponding subset of PB gammadelta T-cells. In vitro examination of IL-12 DC/PB gammadelta T-cell interactions revealed a potential of PB gammadelta T-cells to negatively regulate the proliferative capacity of CD4 and CD8 T-cells. We further demonstrate that IL-12 is critical in the activation of PB gammadelta T-cells. In contrast, the regulatory activity of PB gammadelta T-cells in immune responses against strong recall antigens or alloantigens did not require activated DCs, but depended on pyrophosphate activation of PB gammadelta T-cells. Depletion of PB gammadelta T-cells abrogated the regulatory activity in IL-12 DC/peripheral blood mononuclear cell cocultures; adding back graded numbers of PB gammadelta T-cells restored it. Our observations revealed a potential PB gammadelta T cell-mediated negative regulatory feedback mechanism triggered by IL-12 DCs, which may critically impact on the design of DC cancer vaccines.