CD9 marks myeloid/MegE-biased human hematopoiesis

Rare hematopoietic stem cells make up an infrequent but critical population in the bone marrow (BM), maintaining and replenishing the entire hematopoietic system. Importantly, despite sharing the unique stem cell properties of multilineage differentiation and self-renewal, individual HSCs are functionally heterogeneous, and this heterogeneity increases during aging. While HSCs in young mice are qualitatively more similar, ageing is marked by an increased size of the HSC pool and substantial functional variation of individual HSCs. CD9 is a cell surface marker that is highly expressed in HSCs in mice, while CD9 expression within the human HSC population has been reported to be low during neonatal hematopoiesis. Here, we have investigated CD9 expression levels in the human HSPC population over time and identified that early in life; CD9 is infrequent in HSCs, but marks progenitor populations with low engraftment potential and high proliferation capacity. However, during situations of myeloid/Megakaryocyte-erythoid (MegE) biased hematopoiesis, such as during ageing or in leukemia, there is a substantial increase of CD9 expression in HSPCs. Thus, CD9 represents an HSC marker for myeloid/MegE-biased hematopoiesis. ### Competing Interest Statement The authors have declared no competing interest.