Transient regulatory T-cells: A state attained by all activated human T-cells

CD4+CD25+FOXP3+ regulatory T-cells (Tregs) form an important arm of the immune system responsible for suppressing untoward immune responses. Tregs can be thymically derived or peripherally induced, even from CD4+CD25−FOXP3− T-cells. FOXP3 expression and in vitro suppressive activity are considered unique hallmarks of this dedicated and stable lineage of regulatory cells. Here we show that virtually all human CD4+CD25−FOXP3− T-cells and CD8+CD25−FOXP3− T-cells attain a transient FOXP3+CD25+ state during activation. In this state of activation, these cells possess the classic phenotype of Tregs, in that they express similar markers and inhibit in vitro proliferation of autologous CD4+CD25− T-cells. This state is characterized by suppressed IFN-γ production and robust TNF-α and IL-10 production. Interestingly, the great majority of the activated cells eventually downregulate FOXP3 expression, with a concomitant drop in suppressive ability. Our results show that, in humans, FOXP3 expression and Treg functionality are not exclusive features of a stable or unique lineage of T-cells but may also be a transient state attained by almost all T-cells. These results warrant caution in interpreting human studies using FOXP3 and suppressive activity as readouts and suggest that attempts to induce “Tregs” may paradoxically result in induction of effector T-cells, unless stability is confirmed.