Limited efficacy and unacceptable toxicity of cyclophosphamide for the induction of mixed chimerism and renal allograft tolerance in cynomolgus monkeys

Although numerous protocols for the induction of allograft tolerance have been successfully applied in rodent studies, only a limited number of these have been translated to nonhuman primates. We have previously reported a nonyeloablative treatment regimen for the induction of mixed chimerism and renal allograft tolerance in nonhuman primates (1–3). The regimen included total body irradiation (TBI), local thymic irradiation (TI), antithymocyte globulin (ATG), either splenectomy or a short course of CD154 blockade (4) and a 1-month postoperative course of cyclosporine. With this protocol, the recipients of MHC mismatched kidney allografts developed transient multilineage mixed chimerism and approximately 50% to 60% of them acquired renal allograft tolerance. More recently, we extended this approach to HLA matched (5–7) or mismatched (8) human kidney transplant recipients. In the clinical trials, we have substituted cyclophosphamide (CP), in place of TBI, because of concern for secondary malignancy after TBI. In our HLA-mismatched kidney transplant recipients, all developed transient mixed chimerism, and four of the five have been off all immnosuppression with a follow-up of 2 to 5 years (8). Encouraged by these clinical studies and prompted by the desire to develop a more similar preclinical model, we evaluated CP in place of TBI in our monkey conditioning regimen. As reported here, we found that cynomolgus monkeys were significantly more resistant to the modulatory effects of CP, requiring larger and more toxic doses to induce chimerism than what was observed in humans. The TBI-based regimen therefore has continued to be essential for the induction of multilineage chimerism and renal allograft tolerance in our nonhuman primate studies.