Drugs that are used to suppress antiallograft immunity in kidney transplant recipients (KTR) are nephrotoxic and increases the incidence of malignancy and cardiovascular pathology.1 Investigations are underway to use immune regulatory cellular therapy to induce immunological unresponsiveness to donor alloantigens,2 which may permit a reduction of immunosuppression in the longer term. Infusion of Treg cells is 1 proposed cellular therapy undergoing investigation in KTRs.3 Regulatory T cells infused into transplant patients must be able to survive, retain suppressive function, and potentially continue to expand to replace Treg cells that die as part of natural cell turnover in the presence of immunosuppressants, including calcineurin inhibitors (CNIs), such as tacrolimus (TAC). Regulatory T cells require low doses of IL-2 to expand and become functionally active.4,5 CNIs interfere with IL-2 signaling and have been implicated to reduce Treg numbers posttransplant.6,7 The ONE study clinical trial is examining Treg cell adoptive transfer therapy in combination with withdrawal of steroids and mycophenolate mofetil immunosuppression in KTRs; however, the withdrawal of TAC therapy is not currently planned and therefore the identification of Treg populations that remain suppressive in the presence of TAC may be critical to implementing Treg cell therapy successfully in an allograft transplant setting.