Regulatory T (Treg) cells have been shown to be involved in downregulating immune responses in autoimmunity, transplant rejection, and graft-versus-host disease. Moreover, the important role of Treg cells in tumor progression has also been extensively investigated and Treg-mediated immune suppression has emerged as a crucial mechanism of tumor evasion. Recently, a novel subset of TCRαβ+ CD4−CD8− (double-negative, DN) T cells has been characterized to specifically suppress immune responses in mice. Here we demonstrate that human DN T cells belong to the family of inducible Treg cells with potent suppressor activity towards CD4+ and CD8+ T-cell responses. Resting DN T cells failed to suppress responder cells, whereas antigen presenting cell (APC)-stimulated DN T-cells and freshly isolated CD4+CD25+ Treg cells revealed a strong suppressive activity. Of importance, when more potent stimulators such as allogeneic dendritic cells were used for activation of responder T cells, CD4+CD25+ Treg cells were unable to mediate any suppressor function, while APC-primed DN T cells were still able to suppress. The suppressive activity of DN T cells is neither mediated indirectly by modulation of APCs nor by competition for T-cell growth factors. Furthermore, DN T-cell mediated suppression towards responder T cells requires cell-cell contact and is TCR dependent. Based on the importance of Treg cells in tumor immunity, we determined the frequency of circulating DN T cells in patients with malignant diseases. Of interest, the DN T-cell pool revealed an increased frequency in cancer patients compared to healthy controls, indicating that these suppressor cells might limit tumor-specific T-cell responses and thereby impair tumor immunosurveillance. Taken together, our results demonstrate that human DN T cells are a new subset of inducible Treg cells exerting a very potent suppressive activity towards cellular immune responses. Further understanding of the mechanisms involved in human DN T-cell suppression may have important implications for novel immunotherapies.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.