Successful Engraftment of Hematopoietic Stem Cells into Immunocompetent Recipients Using Only Anti-CD117 Antibody and CD47 Blockade As Conditioning

Bone marrow transplantation (BMT) is an efficacious therapy for many otherwise incurable hematologic malignancies and disorders that affect production of hematopoietic cells, including life-threatening anemias, and immunodeficiency syndromes. In a successful transplantation, clearance of bone-marrow niches must be achieved for donor hematopoietic stem cell (HSC) to engraft. Current methods to clear niche space rely on radiation and/or chemotherapy, which can impart toxic adverse effects that greatly limit the potential clinical utility of BMT. Thus, there is a major clinical need for safer conditioning regimens. To this end, a monoclonal antibody against CD117 (ACK2) has been shown to transiently deplete HSCs in immunocompromised mice and enable donor cell engraftment. However, this regimen has proved ineffective in immunocompetent animals. We hypothesized that HSC-depletion by this antibody may occur, in part, through Fc-mediated effector functions, such as antibody-dependent cellular phagocytosis (ADCP). We thus sought to enhance the ADCP activity of anti-CD117 antibodies by blocking the inhibitory CD47-SIRPα “don’t eat me” pathway. Consistent with previous reports, we found treatment of wild-type mice (C57BL/6) with ACK2 alone resulted in only modest and transient depletion of hematopoietic stem and progenitor cells (HSPCs). Strikingly, the addition of the CD47-antagonist CV1 (Weiskopf & Ring, 2013) in combination with ACK2 resulted in rapid and prolonged depletion of bone marrow HSPCs as well as clearance of the bone marrow niche. This depletion was accompanied by profound reductions in hematocrit and blood leukocyte counts not observed in mice treated with ACK2 alone. In support of the role of Fc receptors in the treatment regimen, depletion of HSCs was not observed in mice lacking functional Fc receptors, nor in animals treated with high-doses F(ab) fragments of ACK2. Finally, high levels of granulocyte chimerism (~70%) were achieved post-BMT in mice receiving the combination of ACK2 and CV1 as compared to no chimerism in animals treated with ACK2 alone (p-value <0.0001). Our results show that targeted biologic agents have the potential to replace the toxic preconditioning therapies that are currently clinically utilized. This can lead to safer preparative regimens, which will allow BMT to treat a broader patient population and larger spectrum of hematologic disorders.