Impact of <scp>CD34</scp> ⁺ Bone Marrow Cell Heterogeneity and <scp>HMOX1</scp> Promoter Polymorphism on Neutrophil Engraftment in Pediatric Patients Undergoing Hematopoietic Stem Cell Transplantation

Hematopoietic stem cell transplantation (HSCT) is a therapeutic option for various pediatric diseases. In this study, we analyzed 52 pediatric patients undergoing allogeneic HSCT to determine how the content of myeloid and lymphoid CD34⁺ progenitor cells, as well as HMOX1 promoter polymorphisms, affects neutrophil (NE) and platelet engraftment (PE). We found that NE was significantly accelerated in patients who received a higher dose of CD34⁺ myeloid cells (> 4 × 10⁶/kg), while lymphoid progenitors had no impact on engraftment. PE was not affected by any CD34⁺ cell subtype. Moreover, in a subset of 28 patients, the presence of short HMOX1 promoter alleles in recipients, but not in donors, was associated with delayed NE, particularly during early post-transplantation. These results highlight the importance of graft composition-specifically myeloid CD34⁺ cell dose-and recipient HMOX1 genotype in determining the kinetics of neutrophil recovery. This may offer a new perspective for graft optimization and potential use of HO-1-targeted therapies to enhance engraftment.