Non-genotoxic transplantation and in vivo selection through epitope editing

The short-term and long-term effects of genotoxic pre-transplant conditioning remain barriers to the broader application of haematopoietic stem/progenitor cell (HSPC) transplantation and gene therapies^1-4. Although monoclonal antibodies targeting KIT have been proposed as alternatives to chemotherapy or radiotherapy^5-7, their pharmacokinetics hinder clinical applications owing to the risk of depleting transplanted HSPCs. Here, to address this issue, we identified amino acid changes in the extracellular domain of KIT that disrupt the binding of two therapeutic monoclonal antibodies^8,9, which impair stem cell factor (SCF)-mediated signalling without affecting KIT expression or functionality. We exploited adenine base editing¹⁰ or prime editing¹¹ to efficiently introduce these mutations in HSPCs and combined them with the disruption of the BCL11A erythroid enhancer to promote expression of fetal haemoglobin (HbF)^12,13, a therapeutic approach for several haemoglobinopathies. This strategy enables in vivo co-selection of gene-engineered cells to reach the threshold required to provide therapeutic benefit in patients affected by sickle cell disease and β-thalassaemia. We show progressive enrichment of KIT plus BCL11A multiplex-edited haematopoiesis under selective pressure with KIT monoclonal antibody, in vitro and in vivo. We report that extended treatment with anti-KIT regimens leads to superior in vivo enrichment while avoiding clonal selection, as assessed by a lentiviral barcoded library. Finally, by overcoming the limitations of monoclonal antibody pharmacokinetics, epitope editing enables novel haematopoietic replacement regimens that are not limited by on-target graft elimination, allowing prolonged immune-based conditioning that maximizes haematopoietic niche clearance without chemo-radiotherapy or monoclonal antibody wash-out.