Which NK cell subset helps promote engraftment after hematopoietic stem cell transplantation?

Successful hematopoietic engraftment is a critical determinant of outcomes after allogeneic hematopoietic stem cell transplantation (HSCT). Natural killer (NK) cells are among the earliest immune cell populations to reconstitute after transplantation and contribute to anti-infective and anti-tumor immunity. NK-cell functional maturation is shaped by education, a process mediated by interactions between inhibitory receptors and self-major histocompatibility complex class I (MHC-I) molecules. Unlicensed NK (U-NK) cells, which lack receptors for self-MHC-I, are generally hypo-responsive. Although previous studies suggest that recipient-derived U-NK cells may influence post-transplant engraftment, recipient immune cells are largely eliminated after myeloablative conditioning and replaced by donor-derived cells. Thus, the role of donor-derived U-NK cells in hematopoietic recovery and engraftment remains an important question. A research team from Peking University People’s Hospital has now reported that donor-derived U-NK cells may accelerate hematopoietic recovery and improve clinical outcomes in patients undergoing myeloablative conditioning.

U-NK cells associated with faster recovery and improved outcomes

The study analyzed the clinical outcomes of 976 patients who underwent allogeneic hematopoietic stem cell transplantation (allo-HSCT) after myeloablative conditioning (Figure 2). Based on donor KIR ligand profiles at the HLA-A, -B, and -C loci, patients were categorized into the U-NK⁺ group (n = 693), defined as donors lacking one or more KIR ligands and harboring at least one U-NK cell subset, and the U-NK⁻ group (n = 283), defined as donors retaining all KIR ligands. Compared with those in the donor U-NK⁻ group, patients receiving grafts from U-NK⁺ donors experienced faster platelet engraftment, lower treatment-related mortality, and improved long-term survival. Further analysis of a 1:2 propensity score-matched (PSM) cohort showed that, after baseline characteristics were balanced, the beneficial effect of U-NK cells on hematopoietic reconstitution was even more pronounced, with both neutrophil and platelet engraftment occurring more rapidly, further supporting this association.

Notably, the clinical benefits associated with donor-derived U-NK cells were not influenced by recipient HLA type. Multivariable analyses further demonstrated that donor U-NK status was an independent protective factor for platelet engraftment, treatment-related mortality, and overall survival. These findings suggest that donor U-NK status may serve as an ancillary biomarker for donor selection.

U-NK cells promote the differentiation of HPSCs toward granulocytic and megakaryocytic lineages directly or in cooperation with stromal cells

To further investigate the potential mechanisms underlying these findings, the researchers performed in vitro co-culture experiments. Co-culture of U-NK cells with hematopoietic stem and progenitor cells (HSPCs) for 5 days significantly enhanced the colony-forming capacity of HSPCs. Transwell separation partially attenuated the pro-hematopoietic activity of U-NK cells, but did not completely abolish it, suggesting that both contact-dependent interactions and soluble factors may be involved. Furthermore, when endothelial cells were added to the co-culture system, just 1 day of co-culture with U-NK cells significantly enhanced the in vitro colony-forming capacity of HSPCs, indicating that stromal cells can markedly accelerate U-NK cell–mediated hematopoietic promotion.

The researchers further investigated the differentiation profile of HSPCs and found that colonies formed by HSPCs co-cultured with U-NK cells were predominantly granulocyte–macrophage colony-forming units (CFU-GM). In addition, the proportion of Lin⁻CD34⁺CD41⁺ megakaryocytic progenitor cells was significantly increased after 5 days of co-culture with U-NK cells. These findings suggest that U-NK cells promote the differentiation of HSPCs toward the granulocytic and megakaryocytic lineages in vitro, either through direct interactions or in cooperation with stromal cells.

Infusion of U-NK cells promotes hematopoietic engraftment in vivo

Building on the findings described above, the researchers established a murine transplantation model involving myeloablative conditioning followed by NK cell infusion, using CD45.1 mice as donors and CD45.2 mice as recipients. Compared with the control group, recipients receiving U-NK cells exhibited faster recovery of white blood cells, neutrophils, and platelets. Flow cytometric analysis demonstrated that the recovered white blood cells were entirely donor-derived.

Further analysis revealed that co-infusion of U-NK cells increased the numbers of granulocyte–monocyte progenitors, megakaryocyte–erythroid progenitors, and multipotent progenitors during the early post-transplant period, providing insight into the potential mechanisms by which U-NK cells promote hematopoietic engraftment. Serum proteomic analysis showed elevated plasma levels of coagulation factor VII and serum amyloid A1 (SAA1) in recipients receiving U-NK cell infusion, providing additional clues to the molecular mechanisms underlying U-NK cell–mediated hematopoietic recovery.

The findings indicate that donor-derived U-NK cells may promote hematopoietic engraftment and improve outcomes after allogeneic HSCT, independently of recipient HLA typing. In the future, assessment of donor U-NK status could help guide donor selection and support the development of therapeutic strategies for poor or failed hematopoietic engraftment.

The authors are affiliated with Peking University People’s Hospital, the Peking University Institute of Hematology, the National Clinical Research Center for Hematologic Diseases, and the Beijing Key Laboratory of Cell and Gene Therapy for Hematologic Malignancies. This study was supported by the National Key Research and Development Program of China, the Noncommunicable Chronic Diseases-National Science and Technology Major Project, the National Natural Science Foundation of China, and the Beijing Outstanding Young Scientists Project.The authors also express their gratitude to all colleagues in the Department of Hematology at Peking University People’s Hospital and to the patients who participated in the study.

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