Transcriptomic characterisation of haematopoietic stem and progenitor cells from adult human homeostatic (non-mobilised) and mobilised peripheral blood

Haematopoietic stem and progenitor cells (HSPCs), the precursors of all blood cells, reside predominantly in the bone marrow. Yet, a small proportion (<1%) of phenotypic HSPCs circulates through peripheral blood (PB) at any given time. In contrast, mobilising agents, such as G-CSF can increase HPSC content in PB and are widely used to source HSPCs for clinical transplantation. To date, the detailed cellular and molecular composition of non-mobilised and mobilised PB (mPB) HSPCs in adult humans remains very poor. Here, we characterized the single-cell transcriptomes of >51,000 adult human HSPCs within non-mobilised PB from six healthy donors as well as of >25,000 HSPCs isolated from four mPB samples. We combined this analysis with single-cell functional analysis using most immature haematopoietic stem cells and multipotent progenitors (HSC/MPPs). We find that long-term functional HSC/MPPs are very rare in non-mobilised PB, and that a large fraction of circulating HSPCs is biased towards the erythroid lineage. In particular, we detect the enrichment of a subset of exclusively erythroid/megakaryocyte-primed quiescent HSC-like cells within the phenotypic PB HSC/MPP compartment. In contrast, the features of mPB HSPCs are in between those of BM and non-mobilised PB HSPCs. Similar to BM, mPB contains more myeloid progenitors than non-mobilised PB. However, the HSC/MPPs of both PB sources share a common transcriptional signature and increased abundance of lineage-primed subsets that distinguishes them from BM HSC/MPPs.