Published protocols for expanding CD4+ regulatory T cells ex vivo rely on repetitive stimulation

via the TCR in combination with cytokine exposure.26–28 Within the CD8+ regulatory T-cell subset, adaptive CD8+ regulatory T cells are by far the most dominant group. These cells can be induced by stimulation through the T-cell receptor under certain conditions resulting in a variety of different phenotypes. Recently, it was demonstrated that MLN0128 CD8+ CD25+ Foxp3+ regulatory T cells can be generated by the treatment with anti-CD3 antibody.29,30 In addition, another population of human CD8+ CD25+ Foxp3+ regulatory T cells has been described by Siegmund et al.31 Here, TGF-β and CD3/CD28 antibodies were required to expand these cells. For the CD4+ T-cell subset it was shown that TGF-β-induced conversion of CD4+ T cells into the Foxp3+ phenotype by gut-associated DCs is augmented by the key metabolite of Vitamin A, RA, in vitro.32,33 Ideally, if unwanted uncontrolled immunosuppression is to be avoided, regulatory T cells should be manipulated to express homing molecules that direct them to the tissue of interest. Most interesting in this

context is the observation that the RA is synthesized in abundance by gut and gut-associated DCs21,32,33 and induces the specific gut-homing molecules CCR9 and α4β7 integrin on T cells.21 Therefore RA seems to play a predominant role in the homeostasis and homing of lymphoid populations IWR-1 molecular weight of the gut-associated lymphoid Vasopressin Receptor tissue (GALT). The important role of RA in controlling Foxp3 expression in combination with TGF-β suggests that the

GALT has evolved a specific system for maintaining a balanced symbiosis between the gut flora and the immune system.18,32–34 Intriguingly, in the current study we could demonstrate that the potential of TGF-β and RA to convert naive CD4+ T cells into Foxp3+ T cells is also true for both murine and human CD8+ T cells. Our work has shown that treating naive CD8+ T cells with TGF-β and RA induces murine and human CD8+ Foxp3+ T cells with suppressive activity. Although these CD8+ Foxp3+ T cells possess proliferative capability they exhibit a phenotype that is strikingly similar to that of naturally occurring CD4+ Foxp3+ regulatory T cells and TGF-β/RA-induced CD4+ regulatory T cells. Most notably, they specifically express higher levels of CD25, Gpr83 and CTLA-4 than do CD8+ Foxp3− T cells activated in vitro. In vitro and in vivo experimental systems investigating polyclonal populations of CD8+ regulatory T cells have assumed the existence of separate subsets of CD8+ regulatory T cells on the basis of several apparently distinct mechanisms of immune regulation.