Activation rather than Foxp3 expression determines that TGF-β-induced regulatory T cells out-compete naïve T cells in dendritic cell clustering.

Regulatory T (Treg) cells are critically important for the maintenance of immunological tolerance. Both centrally arising natural nTreg cells and those emerging in the periphery in response to TGF-β, iTreg cells, play a role in the control of unwanted immune responses. Treg cells adopt multiple mechanisms to inhibit effector T cells, yet it is unclear whether these mechanisms are shared by nTreg cells and iTreg cells alike. Here, we show that iTreg cells, like nTreg cells, are able to out-compete naive T cells in clustering around dendritic cells (DCs). However, using both a tamoxifen-responsive inducible Foxp3 retroviral construct and TGF-β-induced iTreg cells from hCD2-Foxp3 knock in reporter mice, we show that it is prior antigen-induced activation rather than Foxp3 expression per se that determines the ability of iTreg cells to competitively cluster around DCs. We found no difference in the capacity of iTreg cells to displace naive T cells around DCs to that of Tr1, Th1, Th2, or Th9 cells. An important difference was, however, that clustering of iTreg cells around DCs, just as for naive T cells, did not effectively activate DCs.