Adaptation to ex vivo culture reduces human hematopoietic stem cell activity independently of cell cycle.

Loss of long-term hematopoietic stem cell (LT-HSC) function ex vivo hampers the success of clinical protocols reliant on culture. However, the kinetics and mechanisms by which this occurs remain incompletely characterized. Here, through time-resolved scRNA-Seq, matched in vivo functional analysis and the use of a reversible in vitro system of early G1 arrest, we define the sequence of transcriptional and functional events occurring during the first ex vivo division of human LT-HSCs. We demonstrate that the sharpest loss of LT-HSC repopulation capacity happens early on, between 6 and 24 hours of culture, before LT-HSCs commit to cell cycle progression. During this time window, LT-HSCs adapt to the culture environment, limiting global variability in gene expression and transiently upregulating gene networks involved in signaling and stress responses. From 24 hours, LT-HSC progression past early G1 contributes to the establishment of differentiation programmes in culture. However, contrary to current assumptions, we demonstrate that loss of HSC function ex vivo is independent of cell cycle progression. Finally, we show that targeting LT-HSC adaptation to culture by inhibiting early activation of JAK/STAT signaling improves HSC long-term repopulating function ex vivo. Collectively, our study demonstrates that controlling early LT-HSC adaptation to ex vivo culture, for example via JAK inhibition, is of critical importance to improve HSC gene therapy and expansion protocols.

Unique molecular and functional features of extramedullary hematopoietic stem and progenitor cell reservoirs in humans.

Rare hematopoietic stem and progenitor cell (HSPC) pools outside the bone marrow (BM) contribute to blood production in stress and disease but remain ill-defined. Although nonmobilized peripheral blood (PB) is routinely sampled for clinical management, the diagnosis and monitoring potential of PB HSPCs remain untapped, as no healthy PB HSPC baseline has been reported. Here we comprehensively delineate human extramedullary HSPC compartments comparing spleen, PB, and mobilized PB to BM using single-cell RNA-sequencing and/or functional assays. We uncovered HSPC features shared by extramedullary tissues and others unique to PB. First, in contrast to actively dividing BM HSPCs, we found no evidence of substantial ongoing hematopoiesis in extramedullary tissues at steady state but report increased splenic HSPC proliferative output during stress erythropoiesis. Second, extramedullary hematopoietic stem cells/multipotent progenitors (HSCs/MPPs) from spleen, PB, and mobilized PB share a common transcriptional signature and increased abundance of lineage-primed subsets compared with BM. Third, healthy PB HSPCs display a unique bias toward erythroid-megakaryocytic differentiation. At the HSC/MPP level, this is functionally imparted by a subset of phenotypic CD71+ HSCs/MPPs, exclusively producing erythrocytes and megakaryocytes, highly abundant in PB but rare in other adult tissues. Finally, the unique erythroid-megakaryocytic-skewing of PB is perturbed with age in essential thrombocythemia and ß-thalassemia. Collectively, we identify extramedullary lineage-primed HSPC reservoirs that are nonproliferative in situ and report involvement of splenic HSPCs during demand-adapted hematopoiesis. Our data also establish aberrant composition and function of circulating HSPCs as potential clinical indicators of BM dysfunction.

Latest culture techniques: cracking the secrets of bone marrow to mass-produce erythrocytes and platelets ex vivo.

Since the dawn of medicine, scientists have carefully observed, modeled and interpreted the human body to improve healthcare. At the beginning there were drawings and paintings, now there is three-dimensional modeling. Moving from two-dimensional cultures and towards complex and relevant biomaterials, tissue-engineering approaches have been developed in order to create three-dimensional functional mimics of native organs. The bone marrow represents a challenging organ to reproduce because of its structure and composition that confer it unique biochemical and mechanical features to control hematopoiesis. Reproducing the human bone marrow niche is instrumental to answer the growing demand for human erythrocytes and platelets for fundamental studies and clinical applications in transfusion medicine. In this review, we discuss the latest culture techniques and technological approaches to obtain functional platelets and erythrocytes ex vivo. This is a rapidly evolving field that will define the future of targeted therapies for thrombocytopenia and anemia, but also a long-term promise for new approaches to the understanding and cure of hematologic diseases.