cMPL-Based Purification and Depletion of Human Hematopoietic Stem Cells: Implications for Pretransplant Conditioning

• High surface expression of cMPL is a hallmark feature of LT-HSCs within the adult human CD34+ HSPC subset.

• Anti-cMPL immunotoxin selectively depletes cMPLhigh LT-HSCs and facilitates donor HSPC engraftment as a conditioning strategy.

The thrombopoietin (TPO):cMPL signaling axis is a critical regulator of early hematopoiesis. However, the utility of cMPL as a standalone marker for identifying long-term repopulating hematopoietic stem cells (LT-HSCs) within the adult human CD34+ cell hematopoietic stem and progenitor cell (HSPC) population has not been validated. In this study, we established high cMPL surface expression as a defining feature of human LT-HSCs. Targeting the cMPL receptor facilitated the separation of human LT-HSCs from mature progenitors, a delineation not achievable with cKIT. Leveraging this finding, we explored the therapeutic potential of cMPL as a novel target for pre-transplant conditioning regimens. We developed a cMPL-targeting immunotoxin, demonstrating its ability to preferentially deplete host cMPLhigh LT-HSCs in murine xenograft models. Evaluation in rhesus macaques confirmed these findings and highlighted a favorable safety profile with rapid systemic clearance within 24 hours of administration. Proof-of-concept experiments validated the immunotoxin as a novel conditioning agent, enabling donor HSPC engraftment without the use of chemotherapy or irradiation. These findings advance our understanding of the molecular determinants of human hematopoiesis and underscore the potential of cMPL-targeting preparative regimens to improve therapeutic transplantation outcomes.