Novel Conditioning Regimens for Blood Stem Cell Transplantation
Authors: Shizuru Judith A. 
Published: April 2021

Hematopoietic stem cell (HSC) therapies rely on chemotherapy and/or radiation to overcome host barriers to permit cell engraftment and provide life-long donor derived hematopoiesis. In conventional allogeneic transplantation these modalities function to both suppress immune rejection and create sufficient niche space to allow HSC to engraft. For gene-corrected or gene-modified cells that are derived from autologous patient sources, access to the HSC niche constitutes the primary barrier. Non-genotoxic approaches are being developed with the goal to target molecules expressed on hematopoietic stem and progenitors. We have studied the use of monoclonal antibodies (mAbs) that target the molecule CD117, also known as cKit, to deplete endogenous stem cells and safely allow engraftment of purified HSC. CD117 is a cell surface molecule expressed at high levels on HSC and early myeloid progenitors as well as other non-hematopoietic cell types. It is a receptor tyrosine kinase that upon binding to its ligand, stem cell factor (SCF), propagates intracellular signaling pathways that regulate activities including cell survival, proliferation and differentiation. Studies in immune deficient mice showed that a single dose of anti-CD117 mAb transiently depletes HSC, creating a therapeutic window for donor HSC to engraft1. We have developed for use in clinical transplantation an anti-human CD117 (anti-hCD117) mAb. This antibody significantly inhibits in vitro proliferation of human HSC and effectively depletes human HSC in mice xenografted with human cord blood cells2, and is effective in depleting HSC and progenitors from the bone marrow of non-human primates (NHP). Although CD117 is expressed on non-hematopoietic cells including mast cells, melanocytes, and cells in the gastrointestinal tract, antibody administration in animals and in healthy human volunteers, show little to no off-target toxicity. We are currently testing in a Phase I clinical trial the anti-hCD117 mAb as the sole conditioning agent for the transplantation of children with severe combined immunodeficiency (SCID).
We have observed that the effect of the mouse anti-CD117 mAb is more potent in immune deficient mice compared to immune sufficient wildtype mice as evidenced by markedly inferior HSC engraftment results in wildtype recipients. Thus, we are exploring next generation approaches to achieve deeper and more extended depletion of HSC and progenitors in immune competent animals. In vivo HSC clearance by the mouse anti-CD117 mAb is dependent on Fc-mediated antibody effector functions. We therefore tested combination therapy with anti-CD117 mAb plus biologic agents that inhibit CD47, a myeloid-specific immune checkpoint, with the goal to promote phagocytosis of target cells that bind the anti-CD117 mAb. The combined treatment led to elimination of &gt;99% of host HSCs and robust multilineage blood reconstitution following HSC transplantation3. These studies show that a conditioning approach using anti-CD117 antibodies is feasible, and has the potential to replace genotoxic agents with targeted and safer biologic agents.
1. Czechowicz A, Kraft D, Weissman IL, et al, Efficient transplantation via antibody-based clearance of hematopoietic stem cell niches. Science. 2007; 318: 1296.
2. Logan AC, Czechowicz A, Kelley BV, et al. Anti-CD117 (c-Kit) monoclonal antibodies deplete human hematopoietic stem cells and facilitate their replacement in humanized NOD/SCID/IL2Rg-/- mice: A non-toxic conditioning regimen for allotransplantation. Blood . 2012; 120: 4099.
3. Chhabra A, Ring AM, Weiskopf K, et al.Hematopoietic stem cell transplantation in immunocompetent hosts without radiation or chemotherapy. Sci Transl Med . 2016; 8: 351RA105.

Disclosures
No relevant conflicts of interest to declare.