Key Points

  • 5-Azacytidine depletes hematopoietic stem cells and enables donor engraftment in immunocompetent mice

  • Anti-CD117 mAb in combination with 5-Azacytidine significantly enhances hematopoietic stem cell depletion and donor engraftment in immunocompetent mice

Depletion of hematopoietic stem cells (HSC) is used therapeutically in many malignant and non-malignant blood disorders in the setting of a hematopoietic cell transplantation (HCT) to eradicate diseased HSC allowing donor HSC to engraft. Current treatments to achieve HSC elimination rely on modalities that cause DNA strand breakage (i.e., alkylators, radiation) resulting in multiple short-term and long-term toxicities, and sometimes even death. These risks have severely limited HCT utilization to patients with few to no co-morbidities, and excluded many others with diseases curable by HCT. 5-Azacytidine (AZA) is a widely used hypomethylating agent that is thought to preferentially target leukemic cells in myeloid malignancies. Here, we reveal a previously unknown effect of AZA on HSC. We show that AZA induces early HSC proliferation in vivo and exerts a direct cytotoxic effect on proliferating HSC in vitro. When used to pretreat recipient mice for transplant, AZA permitted low level donor HSC engraftment. Moreover, by combining AZA with a monoclonal antibody (mAb), targeting CD117 (c-Kit), a molecule expressed on HSC, more robust HSC-depletion and substantially higher levels of multilineage donor cell engraftment was achieved in immunocompetent mice. The enhanced effectiveness of this combined regimen correlated with increased apoptotic cell death in HSPC. Together, these findings highlight a previously unknown therapeutic mechanism for AZA which may broaden its utilization in clinical practice. Moreover, the synergy we show between AZA and anti-CD117 mAb is a novel strategy to eradicate abnormal HSC which can be rapidly tested in the clinical setting.

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