Human CD25+CD4+ T regulatory cells suppress naive and memory T cell proliferation and can be expanded in vitro without loss of function

Active suppression by T regulatory (Tr) cells plays an important role in the downregulation of T cell responses to foreign and self-antigens. Mouse CD4⁺ Tr cells that express CD25 possess remarkable suppressive activity in vitro and in autoimmune disease models in vivo. Thus far, the existence of a similar subset of CD25⁺CD4⁺ Tr cells in humans has not been reported. Here we show that human CD25⁺CD4⁺ Tr cells isolated from peripheral blood failed to proliferate and displayed reduced expression of CD40 ligand (CD40L), in response to T cell receptor-mediated polyclonal activation, but strongly upregulated cytotoxic T lymphocyte-associated antigen (CTLA)-4. Human CD25⁺CD4⁺ Tr cells also did not proliferate in response to allogeneic antigen-presenting cells, but they produced interleukin (IL)10, transforming growth factor (TGF)-β, low levels of interferon (IFN)-γ, and no IL-4 or IL-2. Importantly, CD25⁺CD4⁺ Tr cells strongly inhibited the proliferative responses of both naive and memory CD4⁺ T cells to alloantigens, but neither IL-10, TGF-β, nor CTLA-4 seemed to be directly required for their suppressive effects. CD25⁺CD4⁺ Tr cells could be expanded in vitro in the presence of IL-2 and allogeneic feeder cells and maintained their suppressive capacities. These findings that CD25⁺CD4⁺ Tr cells with immunosuppressive effects can be isolated from peripheral blood and expanded in vitro without loss of function represent a major advance towards the therapeutic use of these cells in T cell-mediated diseases.