Circulating Hematopoietic Stem/Progenitor Cells subsets contribute to human hematopoietic homeostasis.

In physiological conditions, few circulating hematopoietic stem/progenitor cells (cHSPC) are present in the peripheral blood but their contribution to hematopoietic homeostasis in humans remain unsolved. By integrating advanced immunophenotyping, cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq), functional single-cell assays and integration site (IS) clonal tracking, we unveiled the phenotypic composition, the transcriptional features and the biological role of human cHSPC subpopulations in relationship to their bone marrow (BM) counterpart. We found that cHSPC progressively reduced in cell count over aging and are enriched for primitive, lymphoid and erythroid subpopulations, showing pre-activated transcriptional and functional state. Moreover, cHSPC have low expression of multiple BM-retention molecules, but maintain their homing potential after xenotransplantation. By generating a comprehensive Human Organ-Resident HSPC (HuOR) dataset based on scRNAseq data, we detected organ-specific seeding properties of the distinct trafficking HSPC subpopulations. Of note, circulating multi-lymphoid progenitors (MLP) are primed for seeding the thymus and actively contribute to T-cell production at steady state in patients treated with HSPC-gene therapy (GT). Human clonal tracking data from GT patients also showed that cHSPC connect distant BM niches and participate to steady-state hematopoietic production, with primitive cHSPC having the highest re-circulation capability to travel in and out the BM. Finally, in case of hematopoietic impairment, cHSPC composition reflects the BM-HSPC content and might represent a biomarker of the BM state for clinical and research purposes. Overall, our comprehensive work unveiled fundamental insights into the in vivo dynamics of human HSPC trafficking and its role in sustaining hematopoietic homeostasis.