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

CD4⁺CD25⁺FOXP3⁺ regulatory T-cells (T_regs) form an important arm of the immune system responsible for suppressing untoward immune responses. T_regs 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 T_regs, 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 T_reg 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 "T_regs" may paradoxically result in induction of effector T-cells, unless stability is confirmed.