Donor-derived unlicensed NK cells promote the hematopoietic engraftment and survival after allogeneic hematopoietic stem cell transplantation.

Successful hematopoietic engraftment is critical for the prognosis of allogeneic hematopoietic stem cell transplantation (HSCT), as delayed or failed engraftment remains a major cause of transplant failure. Natural killer (NK) cells, particularly recipient-derived unlicensed NK (U-NK) cells, have been reported to promote hematopoietic recovery under non-myeloablative conditioning (non-MAC). However, following myeloablative conditioning (MAC), recipient-derived NK cells are largely depleted, highlighting the need to investigate the role of donor-derived NK cells. In this study, we analyzed a clinical cohort of 976 patients who underwent haploidentical HSCT following MAC and validated our findings in both the original and propensity score-matched cohorts. Our results demonstrated that the presence of donor-derived U-NK cells was associated with faster hematopoietic recovery and improved outcomes, including reduced treatment-related mortality (TRM) and superior long-term survival. Notably, these effects were independent of recipient human leukocyte antigen (HLA) typing. Multivariate analyses further identified donor U-NK status as an independent protective factor for hematopoietic engraftment, TRM, and overall survival. Mechanistically, U-NK cells directly enhanced hematopoietic stem and progenitor cell colony formation in vitro, both independently and through stromal cell interactions. In vivo, infusion of U-NK cells markedly accelerated neutrophil and platelet engraftment. This was accompanied by increased granulocyte-monocyte progenitors, megakaryocyte-erythroid progenitors and multipotent progenitors early post-transplant, along with elevated plasma levels of FVII and serum amyloid A1 (SAA1). Collectively, donor-derived U-NK cells promote hematopoietic engraftment and improve transplant outcomes independently of recipient HLA typing, supporting their potential as a donor selection biomarker and providing a therapeutic strategy for addressing engraftment failure.