Bcl-2 and Mcl-1 inhibition for induction of hematopoietic chimerism and renal allograft tolerance without myelosuppression in nonhuman primates

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SUMMARY Organ transplantation has become the standard of care for many end-state diseases, but currently requires life-long administration of potent immunosuppressive drugs. This results in increased morbidity and mortality from infection, malignancy and other metabolic disorders. Establishing a reliable method to achieve allograft survival without ongoing immunosuppression (I.S.) remains an important goal. We previously reported achievement of long-term I.S.-free renal allograft survival in humans after induction of only transient hematopoietic chimerism through donor bone marrow (BM) transplantation. To expand the application of our approach, it is imperative to improve the levels and consistency of hematopoietic chimerism without increasing myelosuppression associated with the current conditioning regimen. We have identified a novel strategy in nonhuman primates that addresses this obstacle by enhancing intrinsic apoptosis of selective hematopoietic cells using a B cell lymphoma-2 inhibitor (Bcl- 2i). This approach significantly improves chimerism levels and duration and achieves I.S.-free renal allograft survival without neutropenia and thrombocytopenia. These studies did reveal that costimulatory blockade (CB) remains essential for tolerance induction with Bcl-2i. Therefore, we will first define a protocol using only FDA approved (or in the process of being FDA approved) CB, including 1) anti-CD2 mAb, 2) Fc-modified anti-CD154 mAb, and 3) Belatacept. More recently, with the support of an exploratory R21 grant, we have found that induction of hematopoietic chimerism appears to be possible even without any chemo/radiation therapy, if hematopoietic stem cells are adequately depleted from BM niches with a Bcl-2i in combination with another proapoptotic agent that inhibits Myeloid cell leukemia 1 (Mcl-1). In our proposal, we will therefore further pursue the ultimate goal to induce hematopoietic chimerism without any radiation or chemotherapeutic drugs. Also of major importance for more widespread clinical applicability, we will extend the most successful Bcl-2i based protocol to our novel “delayed tolerance” approach. This will, for the first time, make tolerance induction strategies available to recipients of deceased donor allografts using cryopreserved BM, as well as to ongoing living donor transplant recipients whose kidney donor is available to provide hematopoietic stem cells. Finally, we will elucidate the mechanistic pathways involved in successful I.S.-free renal allograft survival by transient hematopoietic chimerism and proapoptotic agents, utilizing extensive in vitro and in vivo experiments with novel immunological approaches.