Integrin-α3 Is a Functional Marker of Ex Vivo Expanded Human Long-Term Hematopoietic Stem Cells

Hematopoietic stem cells in human fetal liver selectively express CD49f

Identification of phenotypes of human hematopoietic cells that display long-term mature cell outputs in vitro and repopulating capability in immunodeficient mice has been important to anticipating the therapeutic potential of fresh harvests of bone marrow or cord blood before or after their physical or genetic manipulation. However, characterizing their key properties and strategies for their isolation from multiple sources at increasing cell purities and elucidating the mechanisms that regulate their ability to sustain mature blood cell production continues to be of major interest. Previous studies have shown that fetal and adult human cells with long-term blood cell output potential are highly enriched in their respective glycosylphosphatidylinositol (GPI)-anchored surface protein GPI80+ and CD49f+ subsets of a developmentally preserved CD45+CD34+CD38-CD45RA-CD90+ population. The so-called "GPI80" hematopoietic cells found in first-trimester human fetal liver are of particular interest because of their very high regenerative capability compared with their adult or even neonatal (cord blood) "CD49f" counterparts. Here, it was hypothesized that high regenerative activity of the GPI80+ cells could be further enriched within a CD49f+ subset. We now demonstrated that coexpression of CD49f within the GPI80+ population identifies a subset with reduced short-term myeloid colony-forming activity in semisolid medium and greater progeny outputs in both 12-week growth factor-supplemented stromal cocultures and in transplanted immunodeficient mice. These findings demonstrated that CD49f is a pervasive marker of human hematopoietic stem cells (HSCs) throughout ontogeny and aging.

Engineered cytokine-expressing MSCs support ex vivo culture of human HSPCs and AML cells

CD34+ human hematopoietic stem and progenitor cells and primary patient-derived leukemia cells are important tools for basic and translational research. Their limited availability demands additional expansion ex vivo in many cases. The use of either cytokine cocktails or cocultures with mesenchymal stromal cells (MSCs) has advanced cell expansion but combinations of both have not been addressed extensively so far. Here, we presented a novel approach to generating human cytokine-expressing MSCs (ceMSCs) using genetic engineering. Coculture with ceMSCs and their culture supernatant led to an efficient expansion and maintenance of functional CD34+CD45RA-CD90+CD201+CD49c+ hematopoietic stem cells ex vivo. Similarly, ceMSCs and their culture supernatant support the growth of cytokine-dependent leukemic cell lines in vitro and improve the survival, maintenance, and expansion of patient-derived acute myeloid leukemia cells, a cell population very challenging to be cultured ex vivo. ceMSCs even surpass the support provided by wild-type MSCs or external cytokines alone. Therefore, ceMSCs offer a cost-effective, straightforward alternative to traditional cytokine supplementation, enhancing the feasibility of ex vivo studies on healthy and leukemic stem and progenitor cells, including therapeutic drug testing and mechanistic investigations.

A differentiated β-globin gene replacement strategy uses heterologous introns to restore physiological expression

β-Hemoglobinopathies are common monogenic disorders. In sickle cell disease (SCD), a single mutation in the β-globin (HBB) gene results in dysfunctional hemoglobin protein, while in β-thalassemia, over 300 mutations distributed across the gene reduce β-globin levels and cause severe anemia. Genetic engineering replacing the whole HBB gene through homology-directed repair (HDR) is an ideal strategy to restore a benign genotype and rescue HBB expression for most genotypes. However, this is technically challenging because (1) the insert must not be homologous to the endogenous gene and (2) synonymous codon-optimized, intron-less sequences may not reconstitute adequate β-globin levels. Here, we developed an HBB gene replacement strategy using CRISPR-Cas9 that successfully addresses these challenges. We determined that a DNA donor containing a diverged HBB coding sequence and heterologous introns to avoid sequence homology provides proper physiological expression. We identified a DNA donor that uses truncated γ-globin introns, results in 34% HDR, and rescues β-globin expression in in vitro models of SCD and β-thalassemia in hematopoietic stem and progenitor cells (HSPCs). Furthermore, while HDR allele frequency dropped in vivo, it was maintained at ∼15%, demonstrating editing of long-term repopulating HSPCs. In summary, our HBB gene replacement strategy offers a differentiated approach by restoring naturally regulated adult hemoglobin expression.

Multi-targeted, NOT gated CAR-T cells as a strategy to protect normal lineages for blood cancer therapy

Introduction

Despite advances in treatment of blood cancers, several-including acute myeloid leukemia (AML)-continue to be recalcitrant. Cell therapies based on chimeric antigen receptors (CARs) have emerged as promising approaches for blood cancers. However, current CAR-T treatments suffer from on-target, off-tumor toxicity, because most familiar blood cancer targets are also expressed in normal lineages. In addition, they face the common problem of relapse due to target-antigen loss. Cell therapeutics engineered to integrate more than one signal, often called logic-gated cells, can in principle achieve greater selectivity for tumors.

Methods

We applied such a technology, a NOT gated system called Tmod™ that is being developed to treat solid-tumor patients, to the problem of therapeutic selectivity for blood cancer cells.

Results

Here we show that Tmod cells can be designed to target 2-4 antigens to provide different practical and conceptual options for a blood cancer therapy: (i) mono- and bispecific activating receptors that target CD33, a well-known AML antigen expressed on the majority of AML tumors (as well as healthy myeloid cells) and CD43 (SPN), an antigen expressed on many hematopoietic cancers (and normal blood lineages); and (ii) mono- and bispecific inhibitory receptors that target CD16b (FCGR3B) and CLEC9A, antigens expressed on key normal blood cells but not on most blood cancers.

Discussion

These results further demonstrate the robust modularity of the Tmod system and generalize the Tmod approach beyond solid tumors.

Conversion of placental hemogenic endothelial cells to hematopoietic stem and progenitor cells

Hematopoietic stem and progenitor cells (HSPCs) are critical for the treatment of blood diseases in clinic. However, the limited source of HSPCs severely hinders their clinical application. In the embryo, hematopoietic stem cells (HSCs) arise from hemogenic endothelial (HE) cells lining the major arteries in vivo. In this work, by engineering vascular niche endothelial cells (VN-ECs), we generated functional HSPCs in vitro from ECs at various sites, including the aorta-gonad-mesonephros (AGM) region and the placenta. Firstly, we converted mouse embryonic HE cells from the AGM region (aHE) into induced HSPCs (iHSPCs), which have the abilities for multilineage differentiation and self-renewal. Mechanistically, we found that VN-ECs can promote the generation of iHSPCs via secretion of CX3CL1 and IL1A. Next, through VN-EC co-culture, we showed that placental HE (pHE) cells, a type of extra-embryonic HE cells, were successfully converted into iHSPCs (pHE-iHSPCs), which have multilineage differentiation capacity, but exhibit limited self-renewal ability. Furthermore, comparative transcriptome analysis of aHE-iHSPCs and pHE-iHSPCs showed that aHE-iHSPCs highly expressed HSC-specific and self-renewal-related genes. Moreover, experimental validation showed that retinoic acid (RA) treatment promoted the transformation of pHE cells into iHSPCs that have self-renewal ability. Collectively, our results suggested that pHE cells possess the potential to transform into self-renewing iHSPCs through RA treatment, which will facilitate the clinical application of placental endothelial cells in hematopoietic cell generation.

CEBPA repression by MECOM blocks differentiation to drive aggressive leukemias

Acute myeloid leukemias (AMLs) have an overall poor prognosis with many high-risk cases co-opting stem cell gene regulatory programs, yet the mechanisms through which this occurs remain poorly understood. Increased expression of the stem cell transcription factor, MECOM, underlies one key driver mechanism in largely incurable AMLs. How MECOM results in such aggressive AML phenotypes remains unknown. To address existing experimental limitations, we engineered and applied targeted protein degradation with functional genomic readouts to demonstrate that MECOM promotes malignant stem cell-like states by directly repressing pro-differentiation gene regulatory programs. Remarkably and unexpectedly, a single node in this network, a MECOM-bound cis-regulatory element located 42 kb downstream of the myeloid differentiation regulator CEBPA, is both necessary and sufficient for maintaining MECOM-driven leukemias. Importantly, targeted activation of this regulatory element promotes differentiation of these aggressive AMLs and reduces leukemia burden in vivo, suggesting a broadly applicable differentiation-based approach for improving therapy.

Endocytosis as a critical regulator of hematopoietic stem cell fate -implications for hematopoietic stem cell and gene therapy

Hematopoietic stem cells (HSCs) have emerged as one of the most therapeutically significant adult stem cells, paving way for a range of novel curative regimens over decades. HSCs are transplanted, either directly or post restorative genetic engineering in order to repopulate a healthy hematopoietic homeostasis in patients with disorders affecting the blood and immune cells. Despite being an extensively studied system, the maintenance and expansion of functional HSCs ex vivo remains a major bottleneck. The challenge primarily stems from difficulties in reproducing HSC self-renewal divisions and gradual depletion of stemness characters, in vitro. Refining the in vitro culture can be particularly beneficial in the case of cord blood HSCs (CB-HSCs), as inadequate numbers in a single umbilical cord limits its therapeutic potential. In recent years, molecular dissection of HSC stemness has significantly improved in vitro hematopoietic stem and progenitor cells (HSPCs) culture. Despite such significant progress, lacunae exist in fully understanding all the underlying mechanisms and their interplay active in bona fide HSCs, and how it transforms when cells proliferate in culture. A new groundbreaking study titled "MYCT1 controls environmental sensing in human haematopoietic stem cells", published in Nature in June 2024, sheds light on this complex field. Through a series of experiments, including knock-down, overexpression, single-cell RNA sequencing, and transplantation, the study identifies a previously unknown role of the MYC target 1 (MYCT1) protein in HSC maintenance. This protein acts as a crucial regulator of human HSCs, with high expression in primitive HSCs and subsequently downregulated during ex vivo culture. The study reveals that MYCT1 plays a vital role in moderating endocytosis and environmental sensing in HSCs, processes thereby essential for maintaining HSC stemness and function. This commentary will discuss the implications of the new findings for cord blood expansion in cell therapies and HSPC culture for gene therapy applications, providing valuable insights for the field of hematopoietic regenerative medicine.

Human post-implantation blastocyst-like characteristics of Muse cells isolated from human umbilical cord

Muse cells, identified as cells positive for the pluripotent surface marker SSEA-3, are pluripotent-like endogenous stem cells located in the bone marrow (BM), peripheral blood, and organ connective tissues. The detailed characteristics of SSEA-3(+) cells in extraembryonic tissue, however, are unknown. Here, we demonstrated that similar to human-adult tissue-Muse cells collected from the BM, adipose tissue, and dermis as SSEA-3(+), human-umbilical cord (UC)-SSEA-3(+) cells express pluripotency markers, differentiate into triploblastic-lineage cells at a single cell level, migrate to damaged tissue, and exhibit low telomerase activity and non-tumorigenicity. Notably, ~ 20% of human-UC-SSEA-3(+) cells were negative for X-inactive specific transcript (XIST), a naïve pluripotent stem cell characteristic, whereas all human adult tissue-Muse cells are XIST-positive. Single-cell RNA sequencing revealed that the gene expression profile of human-UC-SSEA-3(+) cells was more similar to that of human post-implantation blastocysts than human-adult tissue-Muse cells. The DNA methylation level showed the same trend, and notably, the methylation levels in genes particularly related to differentiation were lower in human-UC-SSEA-3(+) cells than in human-adult tissue-Muse cells. Furthermore, human-UC-SSEA-3(+) cells newly express markers specific to extraembryonic-, germline-, and hematopoietic-lineages after differentiation induction in vitro whereas human-adult tissue-Muse cells respond only partially to the induction. Among various stem/progenitor cells in living bodies, those that exhibit properties similar to post-implantation blastocysts in a naïve state have not yet been found in humans. Easily accessible human-UC-SSEA-3(+) cells may be a valuable tool for studying early-stage human development and human reproductive medicine.

Near-perfect precise on-target editing of human hematopoietic stem and progenitor cells

Precision gene editing in primary hematopoietic stem and progenitor cells (HSPCs) would facilitate both curative treatments for monogenic disorders as well as disease modelling. Precise efficiencies even with the CRISPR/Cas system, however, remain limited. Through an optimization of guide RNA delivery, donor design, and additives, we have now obtained mean precise editing efficiencies >90% on primary cord blood HSCPs with minimal toxicity and without observed off-target editing. The main protocol modifications needed to achieve such high efficiencies were the addition of the DNA-PK inhibitor AZD7648, and the inclusion of spacer-breaking silent mutations in the donor in addition to mutations disrupting the PAM sequence. Critically, editing was even across the progenitor hierarchy, did not substantially distort the hierarchy or affect lineage outputs in colony-forming cell assays or the frequency of high self-renewal potential long-term culture initiating cells. As modelling of many diseases requires heterozygosity, we also demonstrated that the overall editing and zygosity can be tuned by adding in defined mixtures of mutant and wild-type donors. With these optimizations, editing at near-perfect efficiency can now be accomplished directly in human HSPCs. This will open new avenues in both therapeutic strategies and disease modelling.

MYCT1 controls environmental sensing in human haematopoietic stem cells

The processes that govern human haematopoietic stem cell (HSC) self-renewal and engraftment are poorly understood and challenging to recapitulate in culture to reliably expand functional HSCs1-3. Here we identify MYC target 1 (MYCT1; also known as MTLC) as a crucial human HSC regulator that moderates endocytosis and environmental sensing in HSCs. MYCT1 is selectively expressed in undifferentiated human haematopoietic stem and progenitor cells (HSPCs) and endothelial cells but becomes markedly downregulated during HSC culture. Lentivirus-mediated knockdown of MYCT1 prevented human fetal liver and cord blood (CB) HSPC expansion and engraftment. By contrast, restoring MYCT1 expression improved the expansion and engraftment of cultured CB HSPCs. Single-cell RNA sequencing of human CB HSPCs in which MYCT1 was knocked down or overexpressed revealed that MYCT1 governs important regulatory programmes and cellular properties essential for HSC stemness, such as ETS factor expression and low mitochondrial activity. MYCT1 is localized in the endosomal membrane in HSPCs and interacts with vesicle trafficking regulators and signalling machinery. MYCT1 loss in HSPCs led to excessive endocytosis and hyperactive signalling responses, whereas restoring MYCT1 expression balanced culture-induced endocytosis and dysregulated signalling. Moreover, sorting cultured CB HSPCs on the basis of lowest endocytosis rate identified HSPCs with preserved MYCT1 expression and MYCT1-regulated HSC stemness programmes. Our work identifies MYCT1-moderated endocytosis and environmental sensing as essential regulatory mechanisms required to preserve human HSC stemness. Our data also pinpoint silencing of MYCT1 as a cell-culture-induced vulnerability that compromises human HSC expansion.

Iron/ROS/Itga3 mediated accelerated depletion of hippocampal neural stem cell pool contributes to cognitive impairment after hemorrhagic stroke

Hemorrhagic stroke, specifically intracerebral hemorrhage (ICH), has been implicated in the development of persistent cognitive impairment, significantly compromising the quality of life for affected individuals. Nevertheless, the precise underlying mechanism remains elusive. Here, we report for the first time that the accumulation of iron within the hippocampus, distal to the site of ICH in the striatum, is causally linked to the observed cognitive impairment with both clinical patient data and animal model. Both susceptibility-weighted imaging (SWI) and quantitative susceptibility mapping (QSM) demonstrated significant iron accumulation in the hippocampus of ICH patients, which is far from the actual hematoma. Logistical regression analysis and multiple linear regression analysis identified iron level as an independent risk factor with a negative correlation with post-ICH cognitive impairment. Using a mouse model of ICH, we demonstrated that iron accumulation triggers an excessive activation of neural stem cells (NSCs). This overactivation subsequently leads to the depletion of the NSC pool, diminished neurogenesis, and the onset of progressive cognitive dysfunction. Mechanistically, iron accumulation elevated the levels of reactive oxygen species (ROS), which downregulated the expression of Itga3. Notably, pharmacological chelation of iron accumulation or scavenger of aberrant ROS levels, as well as conditionally overexpressed Itga3 in NSCs, remarkably attenuated the exhaustion of NSC pool, abnormal neurogenesis and cognitive decline in the mouse model of ICH. Together, these results provide molecular insights into ICH-induced cognitive impairment, shedding light on the value of maintaining NSC pool in preventing cognitive dysfunction in patients with hemorrhagic stroke or related conditions.

Functional screening in human HSPCs identifies optimized protein-based enhancers of Homology Directed Repair

Homology Directed Repair (HDR) enables precise genome editing, but the implementation of HDR-based therapies is hindered by limited efficiency in comparison to methods that exploit alternative DNA repair routes, such as Non-Homologous End Joining (NHEJ). In this study, we develop a functional, pooled screening platform to identify protein-based reagents that improve HDR in human hematopoietic stem and progenitor cells (HSPCs). We leverage this screening platform to explore sequence diversity at the binding interface of the NHEJ inhibitor i53 and its target, 53BP1, identifying optimized variants that enable new intermolecular bonds and robustly increase HDR. We show that these variants specifically reduce insertion-deletion outcomes without increasing off-target editing, synergize with a DNAPK inhibitor molecule, and can be applied at manufacturing scale to increase the fraction of cells bearing repaired alleles. This screening platform can enable the discovery of future gene editing reagents that improve HDR outcomes.

Improving hematopoietic differentiation from human induced pluripotent stem cells by the modulation of Hippo signaling with a diarylheptanoid derivative

BACKGROUND

The diarylheptanoid ASPP 049 has improved the quality of adult hematopoietic stem cell (HSC) expansion ex vivo through long-term reconstitution in animal models. However, its effect on hematopoietic regeneration from human induced pluripotent stem cells (hiPSCs) is unknown.

METHOD

We utilized a defined cocktail of cytokines without serum or feeder followed by the supplementation of ASPP 049 to produce hematopoietic stem/progenitor cells (HSPCs). Flow cytometry and trypan blue exclusion analysis were used to identify nonadherent and adherent cells. Nonadherent cells were harvested to investigate the effect of ASPP 049 on multipotency using LTC-IC and CFU assays. Subsequently, the mechanism of action was explored through transcriptomic profiles, which were validated by qRT-PCR, immunoblotting, and immunofluorescence analysis.

RESULT

The supplementation of ASPP 049 increased the number of phenotypically defined primitive HSPCs (CD34+CD45+CD90+) two-fold relative to seeded hiPSC colonies, indicating enhanced HSC derivation from hiPSCs. Under ASPP 049-supplemented conditions, we observed elevated HSPC niches, including CD144+CD73- hemogenic- and CD144+CD73+ vascular-endothelial progenitors, during HSC differentiation. Moreover, harvested ASPP 049-treated cells exhibited improved self-renewal and a significantly larger proportion of different blood cell colonies with unbiased lineages, indicating enhanced HSC stemness properties. Transcriptomics and KEGG analysis of sorted CD34+CD45+ cells-related mRNA profiles revealed that the Hippo signaling pathway is the most significant in responding to WWTR1/TAZ, which correlates with the validation of the protein expression. Interestingly, ASPP 049-supplemented HSPCs upregulated 11 genes similarly to umbilical cord blood-derived HSPCs.

CONCLUSION

These findings suggest that ASPP 049 can improve HSC-generating protocols with proliferative potentials, self-renewal ability, unbiased differentiation, and a definable mechanism of action for the clinical perspective of hematopoietic regenerative medicine.

Ex vivo hematopoietic stem cell expansion technologies: recent progress, applications, and open questions

Hematopoietic stem cells (HSCs) are a rare but potent cell type that support life-long hematopoiesis and stably regenerate the entire blood and immune system following transplantation. HSC transplantation represents a mainstay treatment for various diseases of the blood and immune systems. The ex vivo expansion and manipulation of HSCs therefore represents an important approach to ask biological questions in experimental hematology and to help improve clinical HSC transplantation therapies. However, it has remained challenging to expand transplantable HSCs ex vivo. This review summarizes recent progress in ex vivo HSC expansion technologies and their applications to biological and clinical problems and discusses current questions in the field.

Inherited blood cancer predisposition through altered transcription elongation

Despite advances in defining diverse somatic mutations that cause myeloid malignancies, a significant heritable component for these cancers remains largely unexplained. Here, we perform rare variant association studies in a large population cohort to identify inherited predisposition genes for these blood cancers. CTR9, which encodes a key component of the PAF1 transcription elongation complex, is among the significant genes identified. The risk variants found in the cases cause loss of function and result in a ∼10-fold increased odds of acquiring a myeloid malignancy. Partial CTR9 loss of function expands human hematopoietic stem cells (HSCs) by increased super elongation complex-mediated transcriptional activity, which thereby increases the expression of key regulators of HSC self-renewal. By following up on insights from a human genetic study examining inherited predisposition to the myeloid malignancies, we define a previously unknown antagonistic interaction between the PAF1 and super elongation complexes. These insights could enable targeted approaches for blood cancer prevention.

Ciclopirox ethanolamine preserves the immature state of human HSCs by mediating intracellular iron content

Culture conditions in which hematopoietic stem cells (HSCs) can be expanded for clinical benefit are highly sought after. To elucidate regulatory mechanisms governing the maintenance and propagation of human HSCs ex vivo, we screened libraries of annotated small molecules in human cord blood cells using an optimized assay for detection of functional HSCs during culture. We found that the antifungal agent ciclopirox ethanolamine (CPX) selectively supported immature CD34+CD90+ cells during culture and enhanced their long-term in vivo repopulation capacity. Purified HSCs treated with CPX showed a reduced cell division rate and an enrichment of HSC-specific gene expression patterns. Mechanistically, we found that the HSC stimulating effect of CPX was directly mediated by chelation of the intracellular iron pool, which in turn affected iron-dependent proteins and enzymes mediating cellular metabolism and respiration. Our findings unveil a significant impact of iron homeostasis in regulation of human HSCs, with important implications for both basic HSC biology and clinical hematology.

Loss of α-1,2-mannosidase MAN1C1 promotes tumorigenesis of intrahepatic cholangiocarcinoma through enhancing CD133-FIP200 interaction

CD133 is widely used as a marker to isolate tumor-initiating cells in many types of cancers. The structure of N-glycan on CD133 is altered during the differentiation of tumor-initiating cells. However, the relationship between CD133 N-glycosylation and stem cell characteristics remains elusive. Here, we found that the level of α-1,2-mannosylated CD133 was associated with the level of stemness genes in intrahepatic cholangiocarcinoma (iCCA) tissues. α-1,2-mannosylated CD133+ cells possessed the characteristics of tumor-initiating cells. The loss of the Golgi α-mannosidase I coding gene MAN1C1 resulted in the formation of α-1,2-mannosylated CD133 in iCCA-initiating cells. Mechanistically, α-1,2-mannosylation promoted the cytoplasmic distribution of CD133 and enhanced the interaction between CD133 and the autophagy gene FIP200, subsequently promoting the tumorigenesis of α-1,2-mannosylated CD133+ cells. Analysis of iCCA samples showed that the level of cytoplasmic CD133 was associated with poor iCCA prognosis. Collectively, α-1,2-mannosylated CD133 is a functional marker of iCCA-initiating cells.

CEA cell adhesion molecule 5 enriches functional human hematopoietic stem cells capable of long-term multi-lineage engraftment

Hematopoietic stem cell (HSC) surface markers improve the understanding of cell identity and function. Here, we report that human HSCs can be distinguished by their expression of the CEA Cell Adhesion Molecule 5 (CEACAM5, CD66e), which serves as a marker and a regulator of HSC function. CD66e+ cells exhibited a 5.5-fold enrichment for functional long term HSCs compared to CD66e- cells. CD66e+CD34+CD90+CD45RA- cells displayed robust multi-lineage repopulation and serial reconstitution ability in immunodeficient mice compared to CD66e-CD34+CD90+CD45RA-cells. CD66e expression also identified almost all repopulating HSCs within the CD34+CD90+CD45RA- population. Together, these results indicated that CEACAM5 is a marker that enriches functional human hematopoietic stem cells capable of long-term multi-lineage engraftment.

Development of a gene edited next-generation hematopoietic cell transplant to enable acute myeloid leukemia treatment by solving off-tumor toxicity

Immunotherapy of acute myeloid leukemia (AML) has been challenging because the lack of tumor-specific antigens results in "on-target, off-tumor" toxicity. To unlock the full potential of AML therapies, we used CRISPR-Cas9 to genetically ablate the myeloid protein CD33 from healthy donor hematopoietic stem and progenitor cells (HSPCs), creating tremtelectogene empogeditemcel (trem-cel). Trem-cel is a HSPC transplant product designed to provide a reconstituted hematopoietic compartment that is resistant to anti-CD33 drug cytotoxicity. Here, we describe preclinical studies and process development of clinical-scale manufacturing of trem-cel. Preclinical data showed proof-of-concept with loss of CD33 surface protein and no impact on myeloid cell differentiation or function. At clinical scale, trem-cel could be manufactured reproducibly, routinely achieving >70% CD33 editing with no effect on cell viability, differentiation, and function. Trem-cel pharmacology studies using mouse xenograft models showed long-term engraftment, multilineage differentiation, and persistence of gene editing. Toxicology assessment revealed no adverse findings, and no significant or reproducible off-target editing events. Importantly, CD33-knockout myeloid cells were resistant to the CD33-targeted agent gemtuzumab ozogamicin in vitro and in vivo. These studies supported the initiation of the first-in-human, multicenter clinical trial evaluating the safety and efficacy of trem-cel in patients with AML (NCT04849910).

The Immunophenotypic Profile of Healthy Human Bone Marrow

Flow cytometry enables multiparametric characterization of hematopoietic cell immunophenotype. Deviations from normal immunophenotypic patterns comprise a cardinal feature of many hematopoietic neoplasms, underscoring the ongoing essentiality of flow cytometry as a diagnostic tool. However, understanding of aberrant hematopoiesis requires an equal understanding of normal hematopoiesis as a comparator. In this review, we outline key features of healthy adult hematopoiesis and lineage specification as illuminated by flow cytometry and provide diagrams illustrating what a diagnostician may observe in flow cytometric plots. These features provide a profile of baseline hematopoiesis, to which clinical samples with suspected neoplasia may be compared.

Human CD34+ hematopoietic stem cell hierarchy: how far are we with its delineation at the most primitive level?

The ability to isolate and characterize different hematopoietic stem cell (HSC) or progenitor cell populations opens avenues to understand how hematopoiesis is regulated during development, homeostasis, and regeneration as well as in age-related conditions such as clonal hematopoiesis and leukemogenesis. Significant progress has been made in the past few decades in determining the composition of the cell types that exist in this system, but the most significant advances have come from mouse studies. However, recent breakthroughs have made significant strides that have enhanced the resolution of the human primitive hematopoietic compartment. Therefore, we aim to review this subject not only from a historical perspective but also to discuss the progress made in the characterization of the human postnatal CD34+ HSC-enriched populations. This approach will enable us to shed light on the potential future translational applicability of human HSCs.

Controlling genetic heterogeneity in gene-edited hematopoietic stem cells by single-cell expansion

Gene editing using engineered nucleases frequently produces unintended genetic lesions in hematopoietic stem cells (HSCs). Gene-edited HSC cultures thus contain heterogeneous populations, the majority of which either do not carry the desired edit or harbor unwanted mutations. In consequence, transplanting edited HSCs carries the risks of suboptimal efficiency and of unwanted mutations in the graft. Here, we present an approach for expanding gene-edited HSCs at clonal density, allowing for genetic profiling of individual clones before transplantation. We achieved this by developing a defined, polymer-based expansion system and identifying long-term expanding clones within the CD201+CD150+CD48-c-Kit+Sca-1+Lin- population of precultured HSCs. Using the Prkdcscid immunodeficiency model, we demonstrate that we can expand and profile edited HSC clones to check for desired and unintended modifications, including large deletions. Transplantation of Prkdc-corrected HSCs rescued the immunodeficient phenotype. Our ex vivo manipulation platform establishes a paradigm to control genetic heterogeneity in HSC gene editing and therapy.

Biophysical cues of bone marrow-inspired scaffolds regulate hematopoiesis of hematopoietic stem and progenitor cells

Hematopoietic stem cells (HSCs) are adult multipotential stem cells with the capacity to differentiate into all blood cells and immune cells, which are essential for maintaining hematopoietic homeostasis throughout the lifespan and reconstituting damaged hematopoietic system after myeloablation. However, the clinical application of HSCs is hindered by the imbalance of its self-renewal and differentiation during in vitro culture. Considering the fact that HSC fate is uniquely determined by natural bone marrow microenvironment, various elaborate cues in this hematopoietic micro-niche provide an excellent reference for the regulation of HSCs. Inspired by the bone marrow extracellular matrix (ECM) network, we designed degradable scaffolds by orchestrating the physical parameters to investigate the decoupling effects of Young's modulus and pore size of three-dimensional (3D) matrix materials on the fate of hematopoietic stem and progenitor cells (HSPCs). We ascertained that the scaffold with larger pore size (80 μm) and higher Young's modulus (70 kPa) was more favorable for HSPCs proliferation and the maintenance of stemness related phenotypes. Through in vivo transplantation, we further validated that scaffolds with higher Young's modulus were more propitious in maintaining the hematopoietic function of HSPCs. We systematically screened an optimized scaffold for HSPC culture which could significantly improve the cell function and self-renewal ability compared with traditional two-dimensional (2D) culture. Together, these results indicate the important role of biophysical cues in regulating HSC fate and pave the way for the parameter design of 3D HSC culture system.

A genetic disorder reveals a hematopoietic stem cell regulatory network co-opted in leukemia

The molecular regulation of human hematopoietic stem cell (HSC) maintenance is therapeutically important, but limitations in experimental systems and interspecies variation have constrained our knowledge of this process. Here, we have studied a rare genetic disorder due to MECOM haploinsufficiency, characterized by an early-onset absence of HSCs in vivo. By generating a faithful model of this disorder in primary human HSCs and coupling functional studies with integrative single-cell genomic analyses, we uncover a key transcriptional network involving hundreds of genes that is required for HSC maintenance. Through our analyses, we nominate cooperating transcriptional regulators and identify how MECOM prevents the CTCF-dependent genome reorganization that occurs as HSCs differentiate. We show that this transcriptional network is co-opted in high-risk leukemias, thereby enabling these cancers to acquire stem cell properties. Collectively, we illuminate a regulatory network necessary for HSC self-renewal through the study of a rare experiment of nature.

De novo construction of T cell compartment in humanized mice engrafted with iPSC-derived thymus organoids

Hematopoietic humanized (hu) mice are powerful tools for modeling the action of human immune system and are widely used for preclinical studies and drug discovery. However, generating a functional human T cell compartment in hu mice remains challenging, primarily due to the species-related differences between human and mouse thymus. While engrafting human fetal thymic tissues can support robust T cell development in hu mice, tissue scarcity and ethical concerns limit their wide use. Here, we describe the tissue engineering of human thymus organoids from inducible pluripotent stem cells (iPSC-thymus) that can support the de novo generation of a diverse population of functional human T cells. T cells of iPSC-thymus-engrafted hu mice could mediate both cellular and humoral immune responses, including mounting robust proinflammatory responses on T cell receptor engagement, inhibiting allogeneic tumor graft growth and facilitating efficient Ig class switching. Our findings indicate that hu mice engrafted with iPSC-thymus can serve as a new animal model to study human T cell-mediated immunity and accelerate the translation of findings from animal studies into the clinic.

Clinical Progress and Preclinical Insights Into Umbilical Cord Blood Transplantation Improvement

The application of umbilical cord blood (UCB) as an important source of hematopoietic stem and progenitor cells (HSPCs) for hematopoietic reconstitution in the clinical context has steadily grown worldwide in the past 30 years. UCB has advantages that include rapid availability of donors, less strict HLA-matching demands, and low rates of graft-versus-host disease (GVHD) versus bone marrow (BM) and mobilized peripheral blood (PB). However, the limited number of HSPCs within a single UCB unit often leads to delayed hematopoietic engraftment, increased risk of transplant-related infection and mortality, and proneness to graft failure, thus hindering wide clinical application. Many strategies have been developed to improve UCB engraftment, most of which are based on 2 approaches: increasing the HSPC number ex vivo before transplantation and enhancing HSPC homing to the recipient BM niche after transplantation. Recently, several methods have shown promising progress in UCB engraftment improvement. Here, we review the current situations of UCB manipulation in preclinical and clinical settings and discuss challenges and future directions.

Update on preclinical and clinical efforts on ex-vivo expansion of hematopoietic stem and progenitor cells

PURPOSE OF REVIEW

Ex-vivo expansion of hematopoietic stem cells (HSCs) is one potential approach to enhance the clinical efficacy of hematopoietic cell transplantation-based therapy for malignant and nonmalignant blood diseases. Here, we discuss the major progress of preclinical and clinical studies on the ex-vivo expansion of human HSCs and progenitor cells (HPCs).

RECENT FINDINGS

Single-cell RNA sequencing identified ADGRG1 as a reliable marker of functional HSCs upon ex-vivo expansion-induced mitochondrial oxidative stress. Both SR1 and UM171 significantly promote ex-vivo expansion of human cord blood HSCs and HPCs, as determined in preclinical animal models. Encouraged by these findings from the bench, multiple phase I/II and phase II clinical trials have been conducted to evaluate the safety, feasibility and efficacy of SR1-expanded and UM171-expanded cord blood units in patients with hematological malignancy.

SUMMARY

Preliminary data from multiple phase I/II clinical trials regarding transplants of ex-vivo-expanded HSCs and HPCs have demonstrated that ex-vivo expansion may be used to overcome the limitation of the rarity of HSCs without compromising stemness.

Precise CRISPR-Cas-mediated gene repair with minimal off-target and unintended on-target mutations in human hematopoietic stem cells

While CRISPR-Cas9 is key for the development of gene therapy, its potential off-target mutations are still a major concern. Here, we establish a "spacer-nick" gene correction approach that combines the Cas9D10A nickase with a pair of PAM-out sgRNAs at a distance of 200 to 350 bp. In combination with adeno-associated virus (AAV) serotype 6 template delivery, our approach led to efficient HDR in human hematopoietic stem and progenitor cells (HSPCs including long-term HSCs) and T cells, with minimal NHEJ-mediated on-target mutations. Using spacer-nick, we developed an approach to repair disease-causing mutations occurring in the HBB, ELANE, IL7R, and PRF1 genes. We achieved gene correction efficiencies of 20 to 50% with minimal NHEJ-mediated on-target mutations. On the basis of in-depth off-target assessment, frequent unintended genetic alterations induced by classical CRISPR-Cas9 were significantly reduced or absent in the HSPCs treated with spacer-nick. Thus, the spacer-nick gene correction approach provides improved safety and suitability for gene therapy.

Single cell analyses identify a highly regenerative and homogenous human CD34+ hematopoietic stem cell population

The heterogeneous nature of human CD34+ hematopoietic stem cells (HSCs) has hampered our understanding of the cellular and molecular trajectories that HSCs navigate during lineage commitment. Using various platforms including single cell RNA-sequencing and extensive xenotransplantation, we have uncovered an uncharacterized human CD34+ HSC population. These CD34+EPCR+(CD38/CD45RA)- (simply as EPCR+) HSCs have a high repopulating and self-renewal abilities, reaching a stem cell frequency of ~1 in 3 cells, the highest described to date. Their unique transcriptomic wiring in which many gene modules associated with differentiated cell lineages confers their multilineage lineage output both in vivo and in vitro. At the single cell level, EPCR+ HSCs are the most transcriptomically and functionally homogenous human HSC population defined to date and can also be easily identified in post-natal tissues. Therefore, this EPCR+ population not only offers a high human HSC resolution but also a well-structured human hematopoietic hierarchical organization at the most primitive level.

Transient Inhibition of the JNK Pathway Promotes Human Hematopoietic Stem Cell Quiescence and Engraftment

The widespread clinical application of cord blood (CB) for hematopoietic stem cell (HSC) transplantation is limited mainly by the inadequate number of hematopoietic stem and progenitor cells (HSPCs) in single CB units, which results in unsuccessful or delayed engraftment in recipients. The identification of agents to promote CB HSPC engraftment has significant therapeutic value. Here, we found that transient inhibition of the JNK pathway increased the HSC frequency in CB CD34+ cells to 13.46-fold. Mechanistic studies showed that inhibition of the JNK pathway upregulated the expression of quiescence-associated and stemness genes in HSCs, preventing HSCs from entering the cell cycle, increasing glucose uptake and accumulating reactive oxygen species (ROS). Importantly, transient inhibition of the JNK pathway during CB CD34+ cell collection also enhanced long-term HSC (LT-HSC) recovery and engraftment efficiency. Collectively, these findings suggest that transient inhibition of the JNK pathway could promote a quiescent state in HSCs by preventing cell cycle entry and metabolic activation, thus enhancing the HSC number and engraftment potential. Together, these findings improve the understanding of the regulatory mechanisms governing HSC quiescence and stemness and have the potential to improve HSC collection and transplantation.

CEACAM1 is a novel culture-compatible surface marker of expanded long-term reconstituting hematopoietic stem cells

CEACAM1 is a novel cell surface marker of ex vivo expanded LT-HSCs.

Sorting umbilical cord blood cells based on CEACAM1 and known HSC marker expression allows purification of LT-HSCs with improved purity.

The extracellular matrix of hematopoietic stem cell niches

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Comprehensive overview of different classes of ECM molecules in the HSC niche.

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Overview of current knowledge on role of biophysics of the HSC niche.

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Description of approaches to create artificial stem cell niches for several application.

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Importance of considering ECM in drug development and testing.

Hematopoietic stem cells (HSCs) are the life-long source of all types of blood cells. Their function is controlled by their direct microenvironment, the HSC niche in the bone marrow. Although the importance of the extracellular matrix (ECM) in the niche by orchestrating niche architecture and cellular function is widely acknowledged, it is still underexplored. In this review, we provide a comprehensive overview of the ECM in HSC niches. For this purpose, we first briefly outline HSC niche biology and then review the role of the different classes of ECM molecules in the niche one by one and how they are perceived by cells. Matrix remodeling and the emerging importance of biophysics in HSC niche function are discussed. Finally, the application of the current knowledge of ECM in the niche in form of artificial HSC niches for HSC expansion or targeted differentiation as well as drug testing is reviewed.

Hematopoietic stem cell gene editing and expansion: State-of-the-art technologies and recent applications

Hematopoietic stem cell transplantation (HSCT) is a curative therapy for a range of hematological diseases, from leukemias to immunodeficiencies and anemias. The aim in using HSCT is to replace a patient's dysfunctional blood system with a functional one by transplanting healthy hematopoietic stem cells (HSCs). HSCs may be collected from a healthy donor (for allogeneic HSCT) or from the patient for genetic correction (for autologous HSCT gene therapies). Despite the curative potential of HSCT, several hurdles to its wider and safer use remain, including how to efficiently genetically correct HSCs and how to increase donor HSC numbers to improve the donor pool. In recent years, the development of state-of-the-art technologies, such as Cas9-AAV6 technologies and identification of the small molecule HSC agonist UM171, have accelerated progress in HSC gene editing and expansion. These translational research efforts were the focus of the Spring 2021 International Society for Experimental Hematology (ISEH) webinar. Here we present a summary and discussion of the implications of these new approaches to improve HSC-based therapy.

HLF expression defines the human hematopoietic stem cell state

Hematopoietic stem cells (HSCs) sustain blood cell homeostasis throughout life and can regenerate all blood lineages after transplantation. Despite this clear functional definition, highly enriched isolation of human HSCs can currently only be achieved through combinatorial assessment of multiple surface antigens. Although several transgenic HSC reporter mouse strains have been described, no analogous approach to prospectively isolate human HSCs has been reported. To identify genes with the most selective expression in human HSCs, we profiled population and single-cell transcriptomes of unexpanded and ex vivo cultured cord blood-derived hematopoietic stem and progenitor cells as well as peripheral blood, adult bone marrow, and fetal liver. On the basis of these analyses, we propose the master transcription factor HLF (hepatic leukemia factor) as one of the most specific HSC marker genes. To directly track its expression in human hematopoietic cells, we developed a genomic HLF reporter strategy, capable of selectively labeling the most immature blood cells on the basis of a single engineered parameter. Most importantly, HLF-expressing cells comprise all stem cell activity in culture and in vivo during serial transplantation. Taken together, these results experimentally establish HLF as a defining gene of the human HSC state and outline a new approach to continuously mark these cells with high fidelity.

The Laminin Receptors Basal Cell Adhesion Molecule/Lutheran and Integrin α7β1 on Human Hematopoietic Stem Cells

In the adult organism, hematopoietic stem and progenitor cells (HSPC) reside in the bone marrow (BM) in specialized hematopoietic stem cell niches of which the extracellular matrix (ECM) is an integral component. Laminins (LM) are a family of heterotrimeric ECM molecules of which mainly family members containing an α4 or α5 chain are expressed in cells from BM niches and involved in HSPC homing and proliferation. Various integrin and non-integrin laminin receptors have been identified and characterized. Among these, the integrins α6β1 and α3β1 were reported to be strongly expressed on human and mouse HSPC. In the present study, we focus on two further specific laminin receptors, namely integrin α7β1 and basal cell adhesion molecule/Lutheran (BCAM/Lu). Using RT-PCR analyses, immunofluorescence staining, immunoblotting and flow cytometry, we show that both are strongly expressed by human lineage-negative CD34 + HSPC. Treatment with function-blocking antibodies against BCAM/Lu neither inhibits the strong adhesive interaction of CD34 + HSPC with LM-511/LM-521 nor the LM-511/LM-521 mediated changes in CD34 + HSPC proliferation, but however, influences the cytokine-induced differentiation of HSPC in colony formation assays. In addition, stromal-derived factor (SDF) 1α-mediated transmigration of CD34 + HSPC through an endothelial cell layer was effectively diminished by BCAM/Lu antibodies, suggesting a direct involvement of BCAM/Lu in the migration process. This study indicates that both laminin receptors newly identified on human CD34 + HSPC should be taken into consideration in future studies.

Preferential expansion of human CD34+CD133+CD90+ hematopoietic stem cells enhances gene-modified cell frequency for gene therapy

CD34+CD133+CD90+ hematopoietic stem cells (HSCs) are responsible for long-term multi-lineage hematopoiesis and the high frequency of gene-modified HSCs is crucial for the success of hematopoietic stem and progenitor cell (HSPC) gene therapy. However, the ex vivo culture and gene manipulation steps of HSPC graft preparation significantly reduce the frequency of HSCs, thus necessitating large doses of HSPCs and reagents for the manipulation. Here, we identified a combination of small molecules, Resveratrol, UM729, and SR1 that preferentially expands CD34+CD133+CD90+ HSCs over other subpopulations of adult HSPCs in ex vivo culture. The preferential expansion enriches the HSCs in ex vivo culture, enhances the adhesion and results in a 6-fold increase in the long-term engraftment in NSG mice. Further, the culture enriched HSCs are more responsive to gene modification by lentiviral transduction and gene editing, increasing the frequency of gene-modified HSCs up to 10-fold in vivo. The yield of gene-modified HSCs obtained by the culture enrichment is similar to the sort-purification of HSCs and superior to Cyclosporin-H treatment. Our study addresses a critical challenge of low frequency of gene-modified HSCs in HSPC graft by developing and demonstrating a facile HSPC culture condition that increases the frequency of gene-modified cells in vivo. This strategy will improve the outcome of HSPC gene therapy and also simplify the gene manipulation process.

Asymmetric organelle inheritance predicts human blood stem cell fate

Understanding human hematopoietic stem cell fate control is important for their improved therapeutic manipulation. Asymmetric cell division, the asymmetric inheritance of factors during division instructing future daughter cell fates, was recently described in mouse blood stem cells. In human blood stem cells, the possible existence of asymmetric cell division remained unclear due to technical challenges in its direct observation. Here, we use long-term quantitative single-cell imaging to show that lysosomes and active mitochondria are asymmetrically inherited in human blood stem cells and that their inheritance is a coordinated, non-random process. Furthermore, multiple additional organelles, including autophagosomes, mitophagosomes, autolysosomes and recycling endosomes show preferential asymmetric co-segregation with lysosomes. Importantly, asymmetric lysosomal inheritance predicts future asymmetric daughter cell cycle length, differentiation and stem cell marker expression, while asymmetric inheritance of active mitochondria correlates with daughter metabolic activity. Hence, human hematopoietic stem cell fates are regulated by asymmetric cell division, with both mechanistic evolutionary conservation and differences to the mouse system.

Deciphering transcriptional and functional heterogeneity in hematopoiesis with single-cell genomics

PURPOSE OF REVIEW

Single-cell genomic approaches have uncovered cell fate biases and heterogeneity within hematopoietic subpopulations. However, standard single-cell transcriptomics suffers from high sampling noise, which particularly skews the distribution of lowly expressed genes, such as transcription factors (TFs). This might preclude the identification of rare transcripts that define cell identity and demarcate cell fate biases. Moreover, these studies need to go hand in hand with relevant functional assays to ensure that observed gene expression changes represent biologically meaningful alterations.

RECENT FINDINGS

Single-cell lineage tracing and functional validation studies have uncovered cell fate bias within transcriptionally distinct hematopoietic stem and progenitor subpopulations. Novel markers identified using these strategies have been proposed to prospectively isolate functionally distinct subpopulations, including long-term hematopoietic stem cells for ex vivo applications. Furthermore, the continuous nature of hematopoiesis has prompted the study of the relationship between stochastic transcriptional noise in hematopoietic TFs and cell fate determination.

SUMMARY

An understanding of the limitations of single-cell genomic approaches and follow-up functional assays is critical to discern the technical and biological contribution of noise in hematopoietic heterogeneity, to identify rare gene expression states, and to uncover functionally distinct subpopulations within hematopoiesis.

SUPPLEMENTARY VIDEO

http://links.lww.com/COH/A23.

UM171 Preserves Epigenetic Marks that Are Reduced in Ex Vivo Culture of Human HSCs via Potentiation of the CLR3-KBTBD4 Complex

Human hematopoietic stem cells (HSCs) exhibit attrition of their self-renewal capacity when cultured ex vivo, a process that is partially reversed upon treatment with epigenetic modifiers, most notably inhibitors of histone deacetylases (HDACs) or lysine-specific demethylase LSD1. A recent study showed that the human HSC self-renewal agonist UM171 modulates the CoREST complex, leading to LSD1 degradation, whose inhibition mimics the activity of UM171. The mechanism underlying the UM171-mediated loss of CoREST function remains undetermined. We now report that UM171 potentiates the activity of a CULLIN3-E3 ubiquitin ligase (CRL3) complex whose target specificity is dictated by the poorly characterized Kelch/BTB domain protein KBTBD4. CRL3^KBTBD4 targets components of the LSD1/RCOR1 corepressor complex for proteasomal degradation, hence re-establishing H3K4me2 and H3K27ac epigenetic marks, which are rapidly decreased upon ex vivo culture of human HSCs.

Endosialin defines human stem Leydig cells with regenerative potential

STUDY QUESTION

Is endosialin a specific marker of human stem Leydig cells (SLCs) with the ability to differentiate into testosterone-producing Leydig cells (LCs) in vitro and in vivo?

SUMMARY ANSWER

Endosialin is a specific marker of human SLCs which differentiate into testosterone-producing LCs in vitro and in vivo.

WHAT IS KNOWN ALREADY

Human SLCs have been identified and isolated using the marker platelet-derived growth factor receptor α (PDGFRα) or nerve growth factor receptor (NGFR). However, the specificity was not high; thus, LCs and germ cells could be mistakenly sorted as SLCs if PDGFRα or NGFR was used as a marker for human SLCs isolation.

STUDY DESIGN, SIZE, DURATION

Firstly, we re-evaluated the specificity of PDGFRα and NGFR for SLCs in adult human testes. Then we analysed the previously published single-cell sequencing data and found that endosialin may identify human SLCs. Subsequently, we sorted endosialin+ cells from four human donors and characterized their self-renewal and multipotent properties. To assess whether endosialin+ cells have the potential to differentiate into functional LCs in vitro, these cells were stimulated by differentiation-inducing medium. We next assessed the in vivo regenerative potential of human endosialin+ cells after xenotransplantation into the testes of immunodeficient mice.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Single-cell sequencing analysis, immunofluorescence and flow cytometry were used to characterize human testis tissues. In vitro colony formation, multipotent differentiation (adipogenic, osteogenic and chondrogenic) and Leydig cell-lineage induction were used to assess stem cell activity. Xenotransplantation into 3-week-old immunodeficient mice was used to determine in vivo regenerative potential. Endpoint measures included testosterone measurements, cell proliferation, immunofluorescence, flow cytometry and quantitative RT-PCR.

MAIN RESULTS AND THE ROLE OF CHANCE

The results indicate that endosialin is a specific marker of SLCs compared with PDGFRα and NGFR. Additionally, endosialin+ cells isolated from human testes show extensive proliferation and differentiation potential in vitro: their self-renewal ability was inferred by the formation of spherical clones derived from a single cell. Moreover, these cells could differentiate into functional LCs that secreted testosterone in response to LH in a concentration-dependent manner in vitro. These self-renewal and differentiation properties reinforce the proposal that human testicular endosialin+ cells are SLCs. Furthermore, transplanted human endosialin+ cells appear to colonize the murine host testes, localize to peritubular and perivascular regions, proliferate measurably and differentiate partially into testosterone-producing LCs in vivo.

LARGE SCALE DATA

NA.

LIMITATIONS, REASONS FOR CAUTION

Owing to the difficulty in collecting human testis tissue, the sample size was limited. The functions of endosialin on SLCs need to be elucidated in future studies.

WIDER IMPLICATIONS OF THE FINDINGS

A discriminatory marker, endosialin, for human SLCs purification is a prerequisite to advance research in SLCs and logically promote further clinical translation of SLCs-based therapies for male hypogonadism.

STUDY FUNDING/COMPETING INTEREST(S)

A.P.X. was supported by the National Key Research and Development Program of China (2017YFA0103802 and 2018YFA0107200). C.D. was supported by the National Natural Science Foundation of China (81971314) and the Natural Science Foundation of Guangdong Province, China (2018B030311039). The authors declare no conflict of interest.

Persistence of CRISPR/Cas9 gene edited hematopoietic stem cells following transplantation: A systematic review and meta-analysis of preclinical studies

Gene editing blood‐derived cells is an attractive approach to cure selected monogenic diseases but remains experimental. A systematic search of preclinical controlled studies is needed to determine the persistence of edited cells following reinfusion. All studies identified in our systematic search (to 20 October 2020) examining the use of CRISPR/Cas9 gene editing in blood‐derived cells for transplantation were included. Meta‐analysis was performed to determine the engraftment and persistence of gene edited cells. A total of 3538 preclinical studies were identified with 15 published articles meeting eligibility for meta‐analysis. These in vivo animal studies examined editing of hemoglobin to correct sickle cell disease (eight studies), inducing resistance to acquired immunodeficiency syndrome (two studies), and six other monogenic disorders (single studies). CRISPR‐Cas9 edited hematopoietic stem and progenitor cells demonstrated equivalent early engraftment compared to controls in meta‐analysis but persistence of gene‐edited cells was reduced at later time points and in secondary transplant recipients. Subgroup analysis in studies targeting the hemoglobin gene revealed a significant reduction in the persistence of gene‐edited cells whether homology‐directed repair or nonhomologous end‐joining were used. No adverse side effects were reported. Significant heterogeneity in study design and outcome reporting was observed and the potential for bias was identified in all studies. CRISPR‐Cas9 gene edited cells engraft similarly to unedited hematopoietic cells. Persistence of gene edited cells, however, remains a challenge and improved methods of targeting hematopoietic stem cells are needed. Reducing heterogeneity and potential risk of bias will hasten the development of informative clinical trials.

Cadherins, Selectins, and Integrins in CAM-DR in Leukemia

The interaction between leukemia cells and the bone microenvironment is known to provide drug resistance in leukemia cells. This phenomenon, called cell adhesion-mediated drug resistance (CAM-DR), has been demonstrated in many subsets of leukemia including B- and T-acute lymphoblastic leukemia (B- and T-ALL) and acute myeloid leukemia (AML). Cell adhesion molecules (CAMs) are surface molecules that allow cell-cell or cell-extracellular matrix (ECM) adhesion. CAMs not only recognize ligands for binding but also initiate the intracellular signaling pathways that are associated with cell proliferation, survival, and drug resistance upon binding to their ligands. Cadherins, selectins, and integrins are well-known cell adhesion molecules that allow binding to neighboring cells, ECM proteins, and soluble factors. The expression of cadherin, selectin, and integrin correlates with the increased drug resistance of leukemia cells. This paper will review the role of cadherins, selectins, and integrins in CAM-DR and the results of clinical trials targeting these molecules.

Inherited myeloproliferative neoplasm risk affects haematopoietic stem cells

Myeloproliferative neoplasms (MPNs) are blood cancers that are characterized by the excessive production of mature myeloid cells and arise from the acquisition of somatic driver mutations in haematopoietic stem cells (HSCs). Epidemiological studies indicate a substantial heritable component of MPNs that is among the highest known for cancers1. However, only a limited number of genetic risk loci have been identified, and the underlying biological mechanisms that lead to the acquisition of MPNs remain unclear. Here, by conducting a large-scale genome-wide association study (3,797 cases and 1,152,977 controls), we identify 17 MPN risk loci (P < 5.0 × 10-8), 7 of which have not been previously reported. We find that there is a shared genetic architecture between MPN risk and several haematopoietic traits from distinct lineages; that there is an enrichment for MPN risk variants within accessible chromatin of HSCs; and that increased MPN risk is associated with longer telomere length in leukocytes and other clonal haematopoietic states-collectively suggesting that MPN risk is associated with the function and self-renewal of HSCs. We use gene mapping to identify modulators of HSC biology linked to MPN risk, and show through targeted variant-to-function assays that CHEK2 and GFI1B have roles in altering the function of HSCs to confer disease risk. Overall, our results reveal a previously unappreciated mechanism for inherited MPN risk through the modulation of HSC function.

ARID3a expression in human hematopoietic stem cells is associated with distinct gene patterns in aged individuals

Background

Immunologic aging leads to immune dysfunction, significantly reducing the quality of life of the elderly. Aged-related defects in early hematopoiesis result in reduced lymphoid cell development, functionally defective mature immune cells, and poor protective responses to vaccines and pathogens. Despite considerable progress understanding the underlying causes of decreased immunity in the elderly, the mechanisms by which these occur are still poorly understood. The DNA-binding protein ARID3a is expressed in a subset of human hematopoietic progenitors. Inhibition of ARID3a in bulk human cord blood CD34⁺ hematopoietic progenitors led to developmental skewing toward myeloid lineage at the expense of lymphoid lineage cells in vitro. Effects of ARID3a expression in adult-derived hematopoietic stem cells (HSCs) have not been analyzed, nor has ARID3a expression been assessed in relationship to age. We hypothesized that decreases in ARID3a could explain some of the defects observed in aging.

Results

Our data reveal decreased frequencies of ARID3a-expressing peripheral blood HSCs from aged healthy individuals compared with young donor HSCs. Inhibition of ARID3a in young donor-derived HSCs limits B lineage potential, suggesting a role for ARID3a in B lymphopoiesis in bone marrow-derived HSCs. Increasing ARID3a levels of HSCs from aged donors in vitro alters B lineage development and maturation. Finally, single cell analyses of ARID3a-expressing HSCs from young versus aged donors identify a number of differentially expressed genes in aged ARID3A-expressing cells versus young ARID3A-expressing HSCs, as well as between ARID3A-expressing and non-expressing cells in both young and aged donor HSCs.

Conclusions

These data suggest that ARID3a-expressing HSCs from aged individuals differ at both molecular and functional levels compared to ARID3a-expressing HSCs from young individuals.

Controlled Cycling and Quiescence Enables Efficient HDR in Engraftment-Enriched Adult Hematopoietic Stem and Progenitor Cells

Genome editing often takes the form of either error-prone sequence disruption by non-homologous end joining (NHEJ) or sequence replacement by homology-directed repair (HDR). Although NHEJ is generally effective, HDR is often difficult in primary cells. Here, we use a combination of immunophenotyping, next-generation sequencing, and single-cell RNA sequencing to investigate and reprogram genome editing outcomes in subpopulations of adult hematopoietic stem and progenitor cells. We find that although quiescent stem-enriched cells mostly use NHEJ, non-quiescent cells with the same immunophenotype use both NHEJ and HDR. Inducing quiescence before editing results in a loss of HDR in all cell subtypes. We develop a strategy of controlled cycling and quiescence that yields a 6-fold increase in the HDR/NHEJ ratio in quiescent stem cells ex vivo and in vivo. Our results highlight the tension between editing and cellular physiology and suggest strategies to manipulate quiescent cells for research and therapeutic genome editing.

Mechanoregulation in hematopoiesis and hematologic disorders

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) are reliant on intrinsic and extrinsic factors for tight control of self-renewal, quiescence, differentiation, and homing. Given the intimate relationship between HSCs and their niche, increasing numbers of studies are examining how biophysical cues in the hematopoietic microenvironment impact HSC functions.

RECENT FINDINGS

Numerous mechanosensors are present on hematopoietic cells, including integrins, mechanosensitive ion channels, and primary cilia. Integrin-ligand adhesion, in particular, has been found to be critical for homing and anchoring of HSCs and progenitors in the bone marrow. Integrin-mediated interactions with ligands present on extracellular matrix and endothelial cells are key to establishing long-term engraftment and quiescence of HSCs. Importantly, disruption in the architecture and cellular composition of the bone marrow associated with conditioning regimens and primary myelofibrosis exposes HSCs to a profoundly distinct mechanical environment, with potential implications for progression of hematologic dysfunction and pathologies.

SUMMARY

Study of the mechanobiological signals that govern hematopoiesis represents an important future step toward understanding HSC biology in homeostasis, aging, and cancer.

A transcriptomic analysis of serial-cultured, tonsil-derived mesenchymal stem cells reveals decreased integrin α3 protein as a potential biomarker of senescent cells

BACKGROUND

Mesenchymal stem cells (MSCs) have been widely used for stem cell therapy, and serial passage of stem cells is often required to obtain sufficient cell numbers for practical applications in regenerative medicine. A long-term serial cell expansion can potentially induce replicative senescence, which leads to a progressive decline in stem cell function and stemness, losing multipotent characteristics. To improve the therapeutic efficiency of stem cell therapy, it would be important to identify specific biomarkers for senescent cells.

METHODS

Tonsil-derived mesenchymal stem cells (TMSCs) with 20-25 passages were designated as culture-aged TMSCs, and their mesodermal differentiation potentials as well as markers of senescence and stemness were compared with the control TMSCs passaged up to 8 times at the most (designated as young). A whole-genome analysis was used to identify novel regulatory factors that distinguish between the culture-aged and control TMSCs. The identified markers of replicative senescence were validated using Western blot analyses.

RESULTS

The culture-aged TMSCs showed longer doubling time compared to control TMSCs and had higher expression of senescence-associated (SA)-β-gal staining but lower expression of the stemness protein markers, including Nanog, Oct4, and Sox2 with decreased adipogenic, osteogenic, and chondrogenic differentiation potentials. Microarray analyses identified a total of 18,614 differentially expressed genes between the culture-aged and control TMSCs. The differentially expressed genes were classified into the Gene Ontology categories of cellular component (CC), functional component (FC), and biological process (BP) using KEGG (Kyoto encyclopedia of genes and genomes) pathway analysis. This analysis revealed that those genes associated with CC and BP showed the most significant difference between the culture-aged and control TMSCs. The genes related to extracellular matrix-receptor interactions were also shown to be significantly different (p < 0.001). We also found that culture-aged TMSCs had decreased expressions of integrin α3 (ITGA3) and phosphorylated AKT protein (p-AKT-Ser473) compared to the control TMSCs.

CONCLUSIONS

Our data suggest that activation of ECM-receptor signaling, specifically involved with integrin family-mediated activation of the intracellular cell survival-signaling molecule AKT, can regulate stem cell senescence in TMSCs. Among these identified factors, ITGA3 was found to be a representative biomarker of the senescent TMSCs. Exclusion of the TMSCs with the senescent TMSC markers in this study could potentially increase the therapeutic efficacy of TMSCs in clinical applications.

Feeder-free and serum-free in vitro assay for measuring the effect of drugs on acute and chronic myeloid leukemia stem/progenitor cells

Research on chronic and acute myeloid leukemia (CML/AML) is focused on the development of novel therapeutic strategies to eliminate leukemic stem/progenitor cells that are responsible for drug resistance and disease relapse. Methods to culture hematopoietic stem/progenitor cells (HSPCs) from blood or bone marrow samples are indispensable for investigating disease pathogenesis and delineating drug responses in individual patients. A key challenge in this area is that primary leukemic cells grow poorly in culture or rapidly differentiate and lose their hematopoietic potential. Access to patient samples can also be limiting or cell numbers too low to enable large-scale assays and/or to obtain reproducible quantitative data. Here we describe a feeder cell-free and serum-free liquid culture system for the expansion of CD34⁺ HSPCs from CML/AML samples and healthy control tissues. Following 7 or 14 days of culture, CD34⁺ cells are expanded 30- to 65-fold or 400- to 800-fold, yielding a purity of ∼80% and ∼60% CD34⁺ cells, respectively. This system was adapted to a 96-well format to measure the sensitivity of leukemic and normal HSPCs to cytotoxic drugs after only 7 days. The assay requires only 10³ cells per well to determine drug IC₅₀ values and can be performed with uncultured and culture-expanded cells. Importantly, resulting IC₅₀ values strongly correlate with those obtained in the classic colony-forming unit (CFU) assay. Compared with the CFU assay, this novel 96-well liquid-based assay designed specifically for leukemic and normal HSPCs is faster and simpler, with more flexible readout methods for selecting candidates for further drug development.

Haematopoietic stem cell self-renewal in vivo and ex vivo

The self-renewal capacity of multipotent haematopoietic stem cells (HSCs) supports blood system homeostasis throughout life and underlies the curative capacity of clinical HSC transplantation therapies. However, despite extensive characterization of the HSC state in the adult bone marrow and embryonic fetal liver, the mechanism of HSC self-renewal has remained elusive. This Review presents our current understanding of HSC self-renewal in vivo and ex vivo, and discusses important advances in ex vivo HSC expansion that are providing new biological insights and offering new therapeutic opportunities.