Unrevealing Hematopoietic Reconstruction Discrepancies after Allo-UCBT and Allo-PBSCT at Single Cell Resolution

Umbilical cord blood (UCB) and G-CSF mobilizing peripheral blood stem cell (G-PBSC) are the main sources of hematopoietic stem cells for transplantation on clinic. Clinical data have shown that hematopoietic reconstruction in allogeneic cord blood transplantation (allo-UCBT) is significantly delayed compared with allogeneic peripheral blood stem cell transplantation (allo-PBSCT), suggesting that the cellular composition and reconstruction dynamics of UCB are significantly different from that of G-PBSC. Following on the heels of our recently study that found HSC/MPPs show differentiation bias to megakaryocytic-erythroid lineage and strong proliferation tendency, and defined a neutrophil progenitor population named S100A^hi Neu2 which is inversely associated with the incidence of GVHD after allo-PBSCT (Yingying Huo et al, Science Immunology 2023), we next aimed to analyze the differences of hematopoietic reconstitution between allo-UCBT and allo-PBSCT and the causes of delayed hematopoietic reconstitution after allo-UCBT.

In this study, we collected peripheral blood (PB) and bone marrow (BM) samples from 8 patients with bone marrow failure syndrome (BMFs) who received allo-UCBT (n=4) or allo-PBSCT (n=4) at different time points (d14, d21, d30, d60, d90, d180, d365) after transplantation. At the same time, UCB, G-PBSC and BM of normal children were collected as controls. Lin^-CD34⁺ cells from BM, G-PBSC and UCB were separated by FACS for STRT-seq, the total nucleated cells (TNCs) of all samples were isolated for 10X transcriptome sequencing. We used 276628 hematopoietic cells to map the single cell transcriptome profiles of hematopoietic reconstruction by different transplantation methods and annotated 44 cell types. Compared with the G-PBSC, the immunogenicity of HSC/MPPs in Lin^-CD34⁺ cells derived from UCB decreased, the proportion increased, and the differentiation bias of HSC/MPPs to monocytic and erythroid lineage was stronger, but the differentiation ability to myeloid and megakaryocytic lineage was decreased, the proportion of EBM, MEP and CMP decreased. For TNCs, the proportion of T cells and NK cells increased, and NK_CD56dim_CD16 had a stronger immune function. Compared with allo-PBSCT, the differentiation of HSC/MPPs and CMP into neutrophils in the early stage after allo-UCBT was weakened. The proportion of MEP cells decreased, and the differentiation ability of HSC/MPPs to MEP, HSC/MPPs and MEP to Mk was weakened.

In summary, we found that the delayed reconstruction of neutrophils and platelets in patients with allo-UCBT are related to the nature of the graft, suggesting the importance of reasonable intervention of HSC derived from UCB to accelerate platelet implantation.