Helios, a member of the Ikaros transcription factor family, is preferentially expressed at the mRNA level by regulatory T cells (Treg cells). We evaluated Helios protein expression using a newly generated mAb and demonstrated that it is expressed in all thymocytes at the double negative 2 stage of thymic development. Although Helios was expressed by 100% of CD4⁺CD8⁻Foxp3⁺ thymocytes, its expression in peripheral lymphoid tissues was restricted to a subpopulation (∼70%) of Foxp3⁺ T cells in mice and humans. Neither mouse nor human naive T cells induced to express Foxp3 in vitro by TCR stimulation in the presence of TGF-β expressed Helios. Ag-specific Foxp3⁺ T cells induced in vivo by Ag feeding also failed to express Helios. Collectively, these results demonstrate that Helios is potentially a specific marker of thymic-derived Treg cells and raises the possibility that a significant percentage of Foxp3⁺ Treg cells are generated extrathymically.

Thymic-derived natural T-regulatory cells (nTregs) are important for the induction of self-tolerance and the control of autoimmunity. Murine CD4+CD25−Foxp3− cells can be induced to express Foxp3 after T-cell receptor (TCR) activation in the presence of transforming growth factor β (TGFβ) and are phenotypically similar to nTregs. Some studies have suggested that TCR stimulation of human CD4+CD25− cells results in the induction of transient expression of FOXP3, but that the induced cells lack a regulatory phenotype. We demonstrate here that TCR stimulation alone was insufficient to induce FOXP3 expression in the absence of TGFβ, whereas high levels of FOXP3 expression could be induced in the presence of TGFβ. Although FOXP3 expression was stable, the TGFβ-induced FOXP3+ T cells were neither anergic nor suppressive and produced high levels of effector cytokines. These results suggest that even high levels of FOXP3 expression are insufficient to define a human CD4+ T cell as a T-regulatory cell.