CD33 epitope editing unlocks UM171-expanded cord blood grafts for AML immunotherapy

Immunotherapies in acute myeloid leukemia (AML) are limited by shared antigen expression between leukemic and healthy hematopoietic cells, resulting in on-target toxicity. To overcome this, we developed a clinically scalable approach to engineer cord blood (CB) hematopoietic stem and progenitor cell (HSPC) grafts resistant to an established CD33-directed therapy. Combining UM171-mediated expansion with adenine base editing (ABE), we precisely disrupted the P67 epitope of CD33, centered on phenylalanine 21, which is essential for gemtuzumab ozogamicin (GO) binding. Edited variants retained CD33 expression and sialic acid-dependent ligand binding, distinguishing this strategy from complete gene ablation. In UM171 cultures, P67 editing was efficient across multiple donors, enriched in primitive subsets, and minimally impacted expansion. In xenografts, edited grafts preserved multilineage engraftment, T cell output, and myeloid persistence. Upon GO challenge, edited graft-derived myeloid cells were protected while CD33⁺ AML cells were eliminated, confirming on-tumor activity with reduced myelotoxicity. Ultra-deep exome sequencing and GUIDE sequencing revealed minimal off-target activity. Together, these findings establish CD33 P67 epitope-directed base editing as a precise, scalable strategy to generate immunotherapy-compatible, UM171-expanded CB grafts that safely enable GO integration into post-transplant care.