Regulatory CD4+CD25+ T cells (Treg) derived from the thymus and/or peripheral tissues have been demonstrated to broadly control T cell reactivity (14). Importantly, Tregs have been shown to control immune responsiveness to alloantigens and significantly contribute to operational tolerance in transplantation models (15, 16). However, there have been limited efforts to harness the therapeutic potential of directly isolated CD4+CD25+ Treg cells for controlling graft rejection and inducing transplantation tolerance, such as for kidney transplants. In order for CD4+CD25+ Treg cells to be used as a clinical treatment, the following cell properties could be necessary: ex vivo generation of sufficient numbers of cells, migration in vivo to sites of antigenic reactivity, ability to suppress rejection in an alloantigen-specific manner, and survival/expansion after infusion for a critical, but currently unknown, period of time. Our published work and that of other investigators has demonstrated 1) the feasibility of expanding Treg ex vivo, 2) the ability of these cells to downregulate allogeneic immune responses in vitro, and 3) the efficacy of Treg for prevention of allograft rejection in animal models (15,16). We have developed strategies for the ex vivo expansion of naturally occurring human Tregs (nTregs) that allow for the practical employment of this cellular therapy in the clinic. Our central hypothesis is that sufficient human nTreg can be expanded ex vivo and used to both prevent renal transplant rejection and facilitate the reduction and subsequent withdrawal of drug-based immunosuppression. This study will allow for us to define the safety of Treg adoptive cellular transfer (TRACT) in living donor renal transplant recipients that draws upon our extensive preclinical experience with expanded Tregs, as well as our recognized clinical expertise with designing immunosuppressive regimens compatible with this type of therapeutic cell transfer.