Dendritic cells partially abrogate the regulatory activity of CD4+CD25+ T cells present in the human peripheral blood

The factors that influence the functionality of human CD4+CD25+ regulatory T cells are not well understood. We sought to characterize the effects of dendritic cells (DCs) on the in vitro regulatory activity of CD4+CD25+ T cells obtained from peripheral blood of healthy human donors. Flow cytometry showed that a higher proportion of CD4+CD25+(High) T cells expressed surface glucocorticoid-induced tumor necrosis factor receptor family-related protein (GITR) and CTL-associated antigen 4 than CD4+CD25− or CD4+CD25+(Med–low) T cells. Intracellular Foxp3 was equivalently expressed on CD4+CD25+(All), CD4+CD25+(High), CD4+CD25+(Med–low) and CD4+CD25− T cell populations, irrespective of GITR and CTL-associated antigen 4 expression. CD4+CD25+ T cells were isolated and then cultured in vitro with CD4+CD25− responder T cells and stimulated with anti-CD3 antibodies, and immature dendritic cells (iDCs), mature dendritic cells (mDCs), PBMCs or PBMCs plus anti-CD28 antibodies to provide co-stimulation. In addition, secretion of the Th1 cytokine IFN-γ, IL-2 and the immunoregulatory cytokines, IL-10 and transforming growth factor (TGF)-β, were also assessed in these cultures. We found that iDCs and mDCs were capable of reversing the suppression of proliferation mediated by CD4+CD25+ regulatory T cells. However, the reversal of suppression by DCs was not dependent upon the increase of IFN-γ and IL-2 production or inhibition of IL-10 and/or TGF-β production. Therefore, DCs are able to reverse the suppressive effect of regulatory T cells independent of cytokine production. These results suggest for the first time that human DCs possess unique abilities which allow them to influence the functions of regulatory T cells in order to provide fine-tuning in the regulation of T cell responses.