Although transplantation tolerance to organ allografts has been achieved using a wide variety of immunologic interventions in laboratory animals, few tolerance induction protocols with complete immunosuppressive drug withdrawal have been tested in humans. In preclinical rodent models our group showed that long-term tolerance to vascularized organ allografts and donor specific unresponsiveness was achieved when hosts developed persistent mixed hematopoietic chimerism following conditioning with nonmyeloablative fractionated total lymphoid irradiation (TLI) combined with depletive anti-T cell antibodies (ATG) and infusions of donor bone marrow cells. We now report the translation of the preclinical model to human transplantation and have studied 8 patients enrolled in a tolerance induction protocol with a goal of complete immunosuppressive drug withdrawal after combined HLA- matched kidney and hematopoietic cell transplantation. Donors received a 5 day course of G-CSF 4-6 weeks before kidney transplant surgery, and purified CD34+ hematopoietic progenitor cells from the apheresis products were mixed with 1x10^6 donor CD3+ T cells/kg recipient weight (n = 7) or in the case of the patient with life-threatening lupus 10 x10^6 donor CD3+ T cells/kg before cryopreservation. After kidney transplantation surgery, recipients were conditioned with 10 equal doses of TLI administered once daily to a total dose of 800 (n=3) or 1200 (n=5) cGy combined with 5 intravenous infusions of rabbit anti-thymocyte globulin (ATG) at 1.5mg/kg/ dose, total dose of 7.5 mg/kg. Immediately after conditioning (day 10 after kidney transplantation), recipients were given an intravenous injection of the thawed donor cells in the outpatient clinic. Mycophenolate mofetil was given until day 30, and a tapering course of cyclosporine until day 180. Follow up for the first enrolled patient is 1230 days and the last enrolled patient is 105 days. The mean duration of hospitalization for the transplant was 5 days. The mean nadir WBC was 1100 cells/mm3 and no patient developed significant thrombocytopenia. Of the 8 patients transplanted, five have developed persistent chimerism; four of the five have persistent mixed chimerism and the one patient with lupus has complete chimerism. Among the patients with persistent chimerism two have had complete immunosuppressive drug withdrawal and three are in the midst of taper; none of these five patients have had clinical rejection episodes or have histologic evidence of rejection on protocol kidney biopsies. The three patients with transient hematopoietic cell chimerism are on maintenance low doses of immune suppression medications. All patients have excellent graft function with serum creatinine levels between 0.8 to 1.3 mg/dL. None of the patients developed acute or chronic GVHD. Two patients developed 1–2 dermatomal zoster and one patient had CMV reactivation in the blood; all were treated to resolution without hospitalization. Analysis of the gene array profiles of PBMC from the two patients off immune suppressive drugs showed the development of a “ tolerance” pattern after transplantation, and donor specific unresponsivesness in the MLR after drug withdrawal. In conclusion, using a CD34 selected G-CSF mobilized graft with a defined T cell dose and 1200 cGy of TLI we can establish persistent mixed hematopoietic cell chimerism for tolerance induction in patients undergoing kidney transplantation. Tolerance does not develop as soon as chimerism is established, and drug withdrawal without allograft rejection was feasible several months after persistent chimerism. The results suggest that tolerance induction in this protocol is an active and dynamic process.

Disclosures: Lowsky:Genzyme: Consultancy, Honoraria.

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