De novo, Foxp3+iT reg cells were found in the periphery (and, importantly, not the thymus;Fig. rejecting it, but are prevented from doing so by therapeutically induced Foxp3+T reg cells. Finally, Foxp3+cells have been confirmed as the critical missing link through which infectious tolerance operates in vivo. Peripherally induced Foxp3+cells sustain tolerance by converting naive T cells into the next generation of Foxp3+cells. Empowering Foxp3+regulatory T cells in vivo offers a tractable route to avoid and correct SLx-2119 (KD025) tissue immunopathology. An enduring goal of therapeutic immunosuppression is to exploit tolerance mechanisms so that short-term therapy can provide long-lived benefit (Kendal and Waldmann, 2010). The SLx-2119 (KD025) discovery that short pulses of blocking antibodies to co-receptors and co-stimulatory molecules could initiate the self-sustaining process of infectious transplantation tolerance, and that this was dependent on CD4+regulatory T cells, provided the impetus to harness regulatory T cells as a therapeutic strategy (Qin et al., 1993). Such approaches would benefit from the knowledge of exactly which regulatory cells are responsible, and where and when they act. Until now, much has been assumed in the absence of definitive proof. For example, despite the identification of many CD4+regulatory T cell subsets, including Foxp3+T cells (Bennett et al., 2001;Fontenot et al., 2003;Hori et al., 2003), IL-10secreting Tr1 cells (Levings et al., 2002), TGF-secreting Th3 cells (Weiner, 2001), and most recently Foxp3negiT(R)35 cells (Collison et al., 2010), Foxp3+T reg cells have been championed over the rest as essential for graft tolerance by largely descriptive data. This circumstantial evidence typically falls into three categories: (1) the enhancement of graft survival by adoptive transfer of Foxp3+-enriched but impure (Graca et al., 2002b) CD25+CD4+populations (Feng et al., 2008;Xia et al., 2009); (2) the peripheral induction of Foxp3+T reg cells as a result of tolerogenic protocols (Cobbold et al., 2004;Battaglia et al., 2006;Turnquist et al., 2007); and (3) the detection of small numbers of Foxp3+T reg cells in tolerated grafts (Lee et al., 2005;Fan et al., 2010;Semiletova et al., 2010). However, simply demonstrating their presence, regardless of how well it is performed, fails to confirm a genuine operational role for Foxp3+T reg cells. Achieving that conclusive evidence has been hampered by the lack of a natural cell surface marker by SLx-2119 (KD025) which Foxp3+T reg cells can be specifically manipulated in vivo. In addition, although diphtheria toxin ablation of Foxp3+cells in DEREG models (depletion of regulatory T cells) has dramatically demonstrated the importance of Foxp3+T reg cells in self-tolerance, the resultant systemic immunopathology is so severe that it limits the investigation of long-term induced tolerance to foreign antigen (Kim et al., 2007). Few mice survive beyond 3 wk, and any destruction of SLx-2119 (KD025) foreign graft tissue within that time period would have to be interpreted in the context of global tissue disruption (whether foreign or host) by an unregulated immune system (Kim et al., 2007;Lahl et al., 2007). Using a different approach, in this paper we provide the first definitive evidence of an essential role for Foxp3+T reg cells in therapeutic transplant tolerance in otherwise healthy mice. We use a transgenic reporter mouse containing a sequence coding for a GPI-linked human CD2_CD52 fusion protein in the 3UTR of the X-linked foxp3 gene (B6.Foxp3hCD2) such that all Foxp3-expressing cells coexpress the fusion protein on their cell surface (Komatsu et al., 2009). Using ablative antihuman CD2 antibodies, we show that Foxp3+T reg cells are necessary Rabbit Polyclonal to APOL2 for therapeutic tolerance induced by co-receptor/co-stimulatory blockade across full MHC, multiple minors, and single minor histocompatibility barriers. T cells capable of rejecting the tolerated graft remain quiescent in a tolerant host as a result of constant suppression by Foxp3+T reg cells. A key site of this suppression turns out to be the graft tissue itself, which contains T cells otherwise capable of rejection. Finally, the ability of antigen-specific induced Foxp3+(i)T reg cells to recruit new Foxp3+T reg cells.