Chemically defined cytokine-free expansion of human haematopoietic stem cells

Synergistic effect and molecular mechanism of nicotinamide and UM171 in ex vivo expansion of long-term hematopoietic stem cells

Introduction

Several approaches to expand human hematopoietic stem cells (HSCs) have been reported, but the ability of these methods to expand long-term hematopoietic stem cells (LT-HSCs) remains to be improved, which limits the application of HSCs-based therapies.

Methods

CD34+ cells were purified from umbilical cord blood using MacsCD34 beads, and then cultured for 12 d in a serum-free medium. Flow cytometry was used to detect phenotype, cell cycle distribution, and apoptosis of the cultured cells. Colony-forming cell (CFC) assays can evaluate multi-lineage differentiation potential of HSCs. Real-time polymerase chain reaction was employed to detect the expression of genes related to self-renewal programs and antioxidant activity. DCFH-DA probes were used to evaluate intracellular production of reactive oxygen species (ROS). Determination of the effect of different culture conditions on the balance of cytokine by cytometric bead array.

Results

Here, we show a combination, Nicotinamide (NAM) combined with pyrimidoindole derivative UM171, can massively expand LT-HSCs ex vivo, and the expanded cells maintained the capability of self-renewal and multilineage differentiation. Additionally, our data indicated that UM171 promoted self-renewal of HSCs by inducing HSCs entry into the cell cycle and activating Notch and Wnt pathways, but the infinite occurrence of this process may lead to mitochondrial metabolism disorder and differentiation of HSCs. NAM kept HSCs in their primitive and dormant states by reducing intracellular ROS levels and upregulating the expression of stemness related genes, so we believed that NAM can act as a brake to control the above process.

Conclusions

The discovery of the synergistic effect of NAM and UM171 for expanding LT-HSCs provides a new strategy in solving the clinical issue of limited numbers of HSCs.

Improvement of multilineage hematopoiesis in hematopoietic stem cell-transferred c-kit mutant NOG-EXL humanized mice

Human hematopoietic stem cell (HSC)-transferred humanized mice are valuable models for exploring human hematology and immunology. However, sufficient recapitulation of human hematopoiesis in mice requires large quantities of enriched human CD34+ HSCs and total-body irradiation for adequate engraftment. Recently, we generated a NOG mouse strain with a point mutation in the c-kit tyrosine kinase domain (W41 mutant; NOGW mice). In this study, we examined the ability of NOGW mice to reconstitute human hematopoietic cells. Irradiated NOGW mice exhibited high engraftment levels of human CD45+ cells in the peripheral blood, even when only 5,000-10,000 CD34+ HSCs were transferred. Efficient engraftment of human CD45+ cells was also observed in non-irradiated NOGW mice transferred with 20,000-40,000 HSCs. The bone marrow (BM) of NOGW mice exhibited significantly more engrafted human HSCs or progenitor cells (CD34+CD38- or CD34+CD38+ cells) than the BM of NOG mice. Furthermore, we generated a human cytokine (interleukin-3 and granulocyte-macrophage colony-stimulating factor) transgenic NOG-W41 (NOGW-EXL) mouse to achieve multilineage reconstitution with sufficient engraftment of human hematopoietic cells. Non-irradiated NOGW-EXL mice showed significantly higher engraftment levels of human CD45+ and myeloid lineage cells, particularly granulocytes and platelets/megakaryocytes, than non-irradiated NOGW or irradiated NOG-EXL mice after human CD34+ cell transplantation. Serial BM transplantation experiments revealed that NOGW mice exhibited the highest potential for long-term HSC compared with other strains. Consequently, c-kit mutant NOGW-EXL humanized mice represent an advanced model for HSC-transferred humanized mice and hold promise for widespread applications owing to their high versatility.

Second bone marrow transplantation into regenerating hematopoiesis enhances reconstitution of immune system

In bone marrow transplantation (BMT), hematopoiesis-reconstituting cells are introduced following myeloablative treatment, which eradicates existing hematopoietic cells and disrupts stroma within the hematopoietic tissue. Both hematopoietic cells and stroma then undergo regeneration. Our study compares the outcomes of a second BMT administered to mice shortly after myeloablative treatment and the first BMT, with those of a second BMT administered to mice experiencing robust hematopoietic regeneration after the initial transplant. We evaluated the efficacy of the second BMT in terms of engraftment efficiency, types of generated blood cells, and longevity of function. Our findings show that regenerating hematopoiesis readily accommodates newly transplanted stem cells, including those endowed with a robust capacity for generating B and T cells. Importantly, our investigation uncovered a window for preferential engraftment of transplanted stem cells coinciding with the resumption of blood cell production. Repeated BMT could intensify hematopoiesis reconstitution and enable therapeutic administration of genetically modified autologous stem cells.

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.

Advances in ex vivo expansion of hematopoietic stem and progenitor cells for clinical applications

As caretakers of the hematopoietic system, hematopoietic stem cells assure a lifelong supply of differentiated populations that are responsible for critical bodily functions, including oxygen transport, immunological protection and coagulation. Due to the far-reaching influence of the hematopoietic system, hematological disorders typically have a significant impact on the lives of individuals, even becoming fatal. Hematopoietic cell transplantation was the first effective therapeutic avenue to treat such hematological diseases. Since then, key use and manipulation of hematopoietic stem cells for treatments has been aspired to fully take advantage of such an important cell population. Limited knowledge on hematopoietic stem cell behavior has motivated in-depth research into their biology. Efforts were able to uncover their native environment and characteristics during development and adult stages. Several signaling pathways at a cellular level have been mapped, providing insight into their machinery. Important dynamics of hematopoietic stem cell maintenance were begun to be understood with improved comprehension of their metabolism and progressive aging. These advances have provided a solid platform for the development of innovative strategies for the manipulation of hematopoietic stem cells. Specifically, expansion of the hematopoietic stem cell pool has triggered immense interest, gaining momentum. A wide range of approaches have sprouted, leading to a variety of expansion systems, from simpler small molecule-based strategies to complex biomimetic scaffolds. The recent approval of Omisirge, the first expanded hematopoietic stem and progenitor cell product, whose expansion platform is one of the earliest, is predictive of further successes that might arise soon. In order to guarantee the quality of these ex vivo manipulated cells, robust assays that measure cell function or potency need to be developed. Whether targeting hematopoietic engraftment, immunological differentiation potential or malignancy clearance, hematopoietic stem cells and their derivatives need efficient scaling of their therapeutic potency. In this review, we comprehensively view hematopoietic stem cells as therapeutic assets, going from fundamental to translational.

Complexities of modeling the bone marrow microenvironment to facilitate hematopoietic research

Hematopoiesis occurs in the bone marrow (BM), within a specialized microenvironment referred to as the stem cell niche, where the hematopoietic stem cells (HSCs) reside and are regulated for quiescence, self-renewal and differentiation through intrinsic and extrinsic mechanisms. The BM contains at least two distinctive HSC-supportive niches: an endosteal osteoblastic niche that supports quiescence and self-renewal and a more vascular/perisinusoidal niche that promotes proliferation and differentiation. Both associate with supporting mesenchymal stromal cells. Within the more hypoxic osteoblastic niche, HSCs specifically interact with the osteoblasts that line the endosteal surface, which secrete several important HSC quiescence and maintenance regulatory factors. In vivo imaging indicates that the HSCs and progenitors located further away, in the vicinity of sinusoidal endothelial cells, are more proliferative. Here, HSCs interact with endothelial cells via specific cell adhesion molecules. Endothelial cells also secrete several factors important for HSC homeostasis and proliferation. In addition, HSCs and mesenchymal stromal cells are embedded within the extracellular matrix (ECM), an important network of proteins such as collagen, elastin, laminin, proteoglycans, vitronectin, and fibronectin. The ECM provides mechanical characteristics such as stiffness and elasticity important for cell behavior regulation. ECM proteins are also able to bind, sequester, display, and distribute growth factors across the BM, thus directly affecting stem cell fate and regulation of hematopoiesis. These important physical and chemical features of the BM require careful consideration when creating three-dimensional models of the BM.

Label-free metabolic optical biomarkers track stem cell fate transition in real time

Tracking stem cell fate transition is crucial for understanding their development and optimizing biomanufacturing. Destructive single-cell methods provide a pseudotemporal landscape of stem cell differentiation but cannot monitor stem cell fate in real time. We established a metabolic optical metric using label-free fluorescence lifetime imaging microscopy (FLIM), feature extraction and machine learning-assisted analysis, for real-time cell fate tracking. From a library of 205 metabolic optical biomarker (MOB) features, we identified 56 associated with hematopoietic stem cell (HSC) differentiation. These features collectively describe HSC fate transition and detect its bifurcate lineage choice. We further derived a MOB score measuring the "metabolic stemness" of single cells and distinguishing their division patterns. This score reveals a distinct role of asymmetric division in rescuing stem cells with compromised metabolic stemness and a unique mechanism of PI3K inhibition in promoting ex vivo HSC maintenance. MOB profiling is a powerful tool for tracking stem cell fate transition and improving their biomanufacturing from a single-cell perspective.

Biomaterial-assisted therapeutic cell production and modification in vivo

Hematopoietic stem cell transplantation remains the preferred treatment for a variety of hematopoietic function disorders. To address the issue of limited numbers of hematopoietic stem/progenitor cells (HSPCs), significant progress has been made in the technology for ex vivo expansion of HSPCs. In addition, biomaterial-assisted in vivo production technology for therapeutic cells, including HSPCs, is gradually gaining attention. With the aid of specifically functional biomaterials, researchers can construct bone-like tissues exhibiting typical bone marrow-like structures (termed in vivo osteo-organoids in this article) for the production of therapeutic cells. These in vivo osteo-organoids mimic the native bone marrow niche and provide a microenvironment conducive to the expansion and differentiation of HSPCs. In this perspective article, we systematically summarize the history of in vivo osteo-organoids as a model for studying hematopoiesis and cancer metastasis and propose the challenges faced by the in vivo osteo-organoid production platform for therapeutic cells in terms of clinical translation. Ultimately, we hope to achieve functional customization of in vivo osteo-organoid-derived cells through continuously developed material design methods, so as to meet the treatment needs of different types of diseases and bring hope for life to more people.

Hematopoietic stem cell collection for sickle cell disease gene therapy

PURPOSE OF REVIEW

Gene therapy for sickle cell disease (SCD) is advancing rapidly, with two transformative products recently approved by the US Food and Drug Administration and numerous others under study. All current gene therapy protocols require ex vivo modification of autologous hematopoietic stem cells (HSCs). However, several SCD-related problems impair HSC collection, including a stressed and damaged bone marrow, potential cytotoxicity by the major therapeutic drug hydroxyurea, and inability to use granulocyte colony stimulating factor, which can precipitate severe vaso-occlusive events.

RECENT FINDINGS

Peripheral blood mobilization of HSCs using the CXCR4 antagonist plerixafor followed by apheresis collection was recently shown to be safe and effective for most SCD patients and is the current strategy for mobilizing HSCs. However, exceptionally large numbers of HSCs are required to manufacture an adequate cellular product, responses to plerixafor are variable, and most patients require multiple mobilization cycles, increasing the risk for adverse events. For some, gene therapy is prohibited by the failure to obtain adequate numbers of HSCs.

SUMMARY

Here we review the current knowledge on HSC collection from individuals with SCD and potential improvements that may enhance the safety, efficacy, and availability of gene therapy for this disorder.

The potency of hematopoietic stem cell reprogramming for changing immune tone

Innate immune memory endows innate immune cells with antigen independent heightened responsiveness to subsequent challenges. The durability of this response can be mediated by inflammation induced epigenetic and metabolic reprogramming in hematopoietic stem and progenitor cells (HSPCs) that are maintained through differentiation to mature immune progeny. Understanding the mechanisms and extent of trained immunity induction by pathogens and vaccines, such as BCG, in HSPC remains a critical area of exploration with important implications for health and disease. Here we review these concepts and present new analysis to highlight how inflammatory reprogramming of HSPC can potently alter immune tone, including to enhance specific anti-tumor responses. New findings in the field pave the way for novel HSPC targeting therapeutic strategies in cancer and other contexts of immune modulation. Future studies are expected to unravel diverse and extensive effects of infections, vaccines, microbiota, and sterile inflammation on hematopoietic progenitor cells and begin to illuminate the broad spectrum of immunologic tuning that can be established through altering HSPC phenotypes. The purpose of this review is to draw attention to emerging and speculative topics in this field where we posit that focused study of HSPC in the framework of trained immunity holds significant promise.

Generating human bone marrow organoids for disease modeling and drug discovery

The bone marrow supports and regulates hematopoiesis, responding to physiological requirements for blood cell production over ontogeny and during pathological challenges. Interactions between hematopoietic cells and niche components are challenging to study mechanistically in the human context, but are important to delineate in order to explore the pathobiology of blood and bone marrow disorders. Organoids are proving transformative in many research settings, but an accurate human bone marrow model incorporating multiple hematopoietic and stromal elements has been lacking. This protocol describes a method to generate three-dimensional, multilineage bone marrow organoids from human induced pluripotent stem cells (hiPSCs), detailing the steps for the directed differentiation of hiPSCs using a series of cytokine cocktails and hydrogel embedding. Over 18 days of differentiation, hiPSCs yield the key lineages that are present in central myelopoietic bone marrow, organized in a well-vascularized architecture that resembles native hematopoietic tissues. This presents a robust, in vitro system that can model healthy and perturbed hematopoiesis in a scalable three-dimensional microenvironment. Bone marrow organoids also support the growth of immortalized cell lines and primary cells from healthy donors and patients with myeloid and lymphoid cancers, including cell types that are poorly viable in standard culture systems. Moreover, we discuss assays for the characterization of organoids, including interrogation of pathogenic remodeling using recombinant TGF-ß treatment, and methods for organoid engraftment with exogenous cells. This protocol can be readily adapted to specific experimental requirements, can be easily implemented by users with tissue culture experience and does not require access to specialist equipment.

Purging myeloma cell contaminants and simultaneous expansion of peripheral blood-mobilized stem cells

Human hematopoietic stem cells (HSCs) are widely used as a cellular source for hematopoietic stem cell transplantation (HSCT) in the clinical treatment of hematological malignancies. After transplantation therapy, delays in hematopoietic recovery due to insufficient donor-derived HSCs can lead to increased risks of life-threatening infections and bleeding. Our previous studies developed an efficient ex vivo expansion culture medium (3a medium) for umbilical cord blood-derived HSCs (CBSCs), offering a potential solution to this problem. Nevertheless, the broader applicability of our culture method to alternative cell sources and, of greater significance, its efficacy in eliminating potentially disease-associated contaminated tumor cells, especially in autologous transplantation, raise critical clinical questions. In this study, we modified the 3a medium by incorporating UM729 to replace UM171, adding FMS-like tyrosine kinase 3 (Flt3) ligand, and adjusting the concentrations of butyzamide, 740Y-P, polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer (PCL-PVAc-PEG, Soluplus) to create the modified-3a medium. This sophistication allowed the efficient expansion of not only CBSCs but also peripheral blood-mobilized HSCs (PBSCs). Additionally, we successfully removed contaminated myeloma cells by adding bortezomib and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) at appropriate concentrations, although we maintained HSCs through the addition of lenalidomide. Our research findings present the potential for widespread clinical application of the modified-3a medium and suggest a safe ex vivo culture technique for expanding human HSCs within peripheral blood-derived donor grafts used for autologous HSCT.

Ex vivo expansion of human hematopoietic stem cells and clinical applications

Hematopoietic stem cells (HSCs) are a rare population of cells found in the bone marrow that play a critical role in lifelong hematopoiesis and the reconstitution of the hematopoietic system after hematopoietic stem cell transplantation. Hematopoietic stem cell transplantation remains the only curative treatment for patients with refractory hematologic disorders, and umbilical cord blood (CB) serves as an alternative stem cell source due to its several advantageous characteristics, including human leukocyte antigen flexibility and reduced donor burden. However, CB also has the disadvantage of containing a small number of cells, resulting in limited donor selection and a longer time for engraftment. Therefore, the development of techniques to expand HSCs ex vivo, particularly umbilical CB, is a goal in hematology. While various combinations of cytokines were once the mainstream approach, these protocols had limited expansion rates and did not lead to clinical application. However, in recent years, the development of a technique in which small molecules are added to cytokines has enabled the stable, long-term ex vivo expansion of human HSCs. Clinical trials of expanded umbilical CB using these techniques have been undertaken and have confirmed their efficacy and safety. In addition, we have successfully developed a recombinant-cytokine-free and albumin-free culture system for the long-term expansion of human HSCs. This approach could offer the potential for more selective expansion of human HSCs compared to previous protocols. This review discusses ex vivo culture protocols for expanding human HSCs and presents the results of clinical trials using these techniques, along with future perspectives.

cMPL-Based Purification and Depletion of Human Hematopoietic Stem Cells: Implications for Pre-Transplant Conditioning

The transplantation of gene-modified autologous hematopoietic stem and progenitor cells (HSPCs) offers a promising therapeutic approach for hematological and immunological disorders. However, this strategy is often limited by the toxicities associated with traditional conditioning regimens. Antibody-based conditioning strategies targeting cKIT and CD45 antigens have shown potential in mitigating these toxicities, but their long-term safety and efficacy in clinical settings require further validation. In this study, we investigate the thrombopoietin (TPO) receptor, cMPL, as a novel target for conditioning protocols. We demonstrate that high surface expression of cMPL is a hallmark feature of long-term repopulating hematopoietic stem cells (LT-HSCs) within the adult human CD34+ HSPC subset. Targeting the cMPL receptor facilitates the separation of human LT-HSCs from mature progenitors, a delineation not achievable with cKIT. Leveraging this finding, we developed a cMPL-targeting immunotoxin, demonstrating its ability to selectively deplete host cMPLhigh LT-HSCs with a favorable safety profile and rapid clearance within 24 hours post-infusion in rhesus macaques. These findings present significant potential to advance our understanding of human hematopoiesis and enhance the therapeutic outcomes of ex vivo autologous HSPC gene therapies.

Effects of different concentrations of nicotinamide on hematopoietic stem cells cultured in vitro

BACKGROUND

In vitro expansion to increase numbers of hematopoietic stem cells (HSCs) in cord blood could improve clinical efficacy of this vital resource. Nicotinamide (NAM) can promote HSC expansion ex vivo, but its effect on hematopoietic stem and progenitor cells (HSPCs, CD34+CD38) and functional subtypes of HSCs - short-term repopulating HSCs (ST-HSCs, CD34+CD38CD45RACD49f+) and long-term repopulating HSCs (LT-HSCs, CD34+CD38CD45RACD49f+CD90+) is not yet known. As a sirtuin 1 (SIRT1) inhibitor, NAM participates in regulating cell adhesion, polarity, migration, proliferation, and differentiation. However, SIRT1 exhibits dual effects by promoting or inhibiting differentiation in different tissues or cells. We propose that the concentration of NAM may influence proliferation, differentiation, and SIRT1 signaling of HSCs.

AIM

To evaluate the effects and underlying mechanisms of action of different concentrations of NAM on HSC proliferation and differentiation.

METHODS

CD34+ cells were purified from umbilical cord blood using MacsCD34 beads, and cultured for 10-12 d in a serum-free medium supplemented with cytokines, with different concentrations of NAM added according to experimental requirements. Flow cytometry was used to detect phenotype, cell cycle distribution, and apoptosis of the cultured cells. Real-time polymerase chain reaction was used to detect the transcription levels of target genes encoding stemness-related factors, chemokines, components of hypoxia pathways, and antioxidant enzymes. Dichloro-dihydro-fluorescein diacetate probes were used to evaluate intracellular production of reactive oxygen species (ROS). Determination of the effect of different culture conditions on the balance of cytokine by cytometric bead array.

RESULTS

Compared with the control group, the proportion and expansion folds of HSPCs (CD34+CD38) incubated with 5 mmol/L or 10 mmol/L NAM were significantly increased (all P < 0.05). The ST-HSCs ratio and fold expansion of the 5 mmol/L NAM group were significantly higher than those of the control and 10 mmol/L NAM groups (all P < 0.001), whereas the LT-HSCs ratio and fold expansion of the 10 mmol/L NAM group were significantly higher than those of the other two groups (all P < 0.05). When the NAM concentration was > 10 mmol/L, cell viability significantly decreased. In addition, compared with the 5 mmol/L NAM group, the proportion of apoptotic cells in the 10 mmol/L NAM group increased and the proportion of cells in S and G2 phase decreased. Compared with the 5 mmol/L NAM group, the HSCs incubated with 10 mmol/L NAM exhibited significantly inhibited SIRT1 expression, increased intracellular ROS content, and downregulated expression of genes encoding antioxidant enzymes (superoxide dismutase 1, peroxiredoxin 1).

CONCLUSION

Low concentrations (5 mmol/L) of NAM can better regulate the balance between proliferation and differentiation, thereby promoting expansion of HSCs. These findings allow adjustment of NAM concentrations according to expansion needs.

The interactions between ineffective erythropoiesis and ferroptosis in β-thalassemia

In Guangxi, Hainan, and Fujian Province in southern China, β-thalassemia is a frequent monogenic hereditary disorder that is primarily defined by hemolytic anemia brought on by inefficient erythropoiesis. It has been found that ineffective erythropoiesis in β-thalassemia is closely associated with a high accumulation of Reactive oxygen species, a product of oxidative stress, in erythroid cells. During recent years, ferroptosis is an iron-dependent lipid peroxidation that involves abnormalities in lipid and iron metabolism as well as reactive oxygen species homeostasis. It is a recently identified kind of programmed cell death. β-thalassemia patients experience increased iron release from reticuloendothelial cells and intestinal absorption of iron, ultimately resulting in iron overload. Additionally, the secretion of Hepcidin is inhibited in these patients. What counts is both ineffective erythropoiesis and ferroptosis in β-thalassemia are intricately linked to the iron metabolism and Reactive oxygen species homeostasis. Consequently, to shed further light on the pathophysiology of β-thalassemia and propose fresh ideas for its therapy, this paper reviews ferroptosis, ineffective erythropoiesis, and the way they interact.

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.

In situ correction of various β-thalassemia mutations in human hematopoietic stem cells

β-thalassemia (β-thal) is the most common monogenic disorder caused by various mutations in the human hemoglobin β (HBB) gene and affecting millions of people worldwide. Electroporation of Cas9 and single-guide RNA (sgRNA)-ribonucleoprotein (RNP) complex-mediated gene targeting in patient-derived hematopoietic stem cells (HSCs), followed by autologous transplantation, holds the promise to cure patients lacking a compatible bone marrow donor. In this study, a universal gene correction method was devised to achieve in situ correction of most types of HBB mutations by using validated CRISPR/sgRNA-RNP complexes and recombinant adeno-associated viral 6 (rAAV6) donor-mediated homology-directed repair (HDR) in HSCs. The gene-edited HSCs exhibited multi-lineage formation abilities, and the expression of β-globin transcripts was restored in differentiated erythroid cells. The method was applied to efficiently correct different mutations in β-thal patient-derived HSCs, and the edited HSCs retained the ability to engraft into the bone marrow of immunodeficient NOD-scid-IL2Rg-/- (NSI) mice. This study provides an efficient and safe approach for targeting HSCs by HDR at the HBB locus, which provides a potential therapeutic approach for treating other types of monogenic diseases in patient-specific HSCs.

Nardilysin determines hematopoietic stem cell fitness by regulating protein synthesis

Nardilysin (NRDC) is a multifunctional protein required for maintaining homeostasis in various cellular and tissue contexts. However, its role in hematopoietic stem cells (HSCs) remains unclear. Here, through the conditional deletion of NRDC in hematopoietic cells, we demonstrate that NRDC is required for HSCs expansion in vitro and the reconstitution of hematopoiesis in vivo after transplantation. We found NRDC-deficient HSCs lose their self-renewal ability and display a preferential bias to myeloid differentiation in response to replication stress. Transcriptome data analysis revealed the upregulation of heat shock response-related genes in NRDC-deficient HSCs. Additionally, we observed increased protein synthesis in cultured NRDC-deficient HSCs. Thus, loss of NRDC may cause the inability to control protein synthesis in response to replication induced protein stress, leading to the impaired HSC self-renewal ability. This highlights a novel model of action of NRDC specifically in HSCs.

Understanding genetic heterogeneity in gene-edited hematopoietic stem cell products

CRISPR/Cas gene editing has transformed genetic research and is poised to drive the next generation of gene therapies targeting hematopoietic stem cells (HSCs). However, the installation of the "desired" edit is most often only achieved in a minor subset of alleles. The array of cellular pathways triggered by gene editing tools produces a broad spectrum of "undesired" editing outcomes, including short insertions and deletions (indels) and chromosome rearrangements, leading to considerable genetic heterogeneity in gene-edited HSC populations. This heterogeneity may undermine the effect of the genetic intervention since only a subset of cells will carry the intended modification. Also, undesired mutations represent a potential safety concern as gene editing advances toward broader clinical use. Here, we will review the different sources of "undesired" edits and will discuss strategies for their mitigation and control.

A defined serum-free culture system for human long-term haematopoietic stem cells

Long-term repopulating haematopoietic stem cells (LT-HSCs) have the ability to reconstitute the entire haematopoietic system following transplantation permanently. Despite great achievements in HSC transplantation, the limited transplantable HSC number, especially LT-HSCs, remains critical for successful transplantation and broader applications. In this study, we established a defined serum-free culture system for in vitro expansion of LT-HSCs. This culture system (E1) expanded LT-HSCs from umbilical cord blood, human mobilization peripheral blood and bone marrow. These E1-expanded HSCs reconstituted the haematopoietic and immune systems in primary and secondary transplanted mice in a short time. Better haematopoietic reconstitution was observed in secondary xenografted mice. Moreover, we obtained the comprehensive expression profile and cellular components of LT-HSCs from umbilical cord blood. Our study provides a valuable tool for LT-HSC research and may improve clinical applications of HSCs.

Haematopoietic development and HSC formation in vitro: promise and limitations of gastruloid models

Haematopoietic stem cells (HSCs) are the most extensively studied adult stem cells. Yet, six decades after their first description, reproducible and translatable generation of HSC in vitro remains an unmet challenge. HSC production in vitro is confounded by the multi-stage nature of blood production during development. Specification of HSC is a late event in embryonic blood production and depends on physical and chemical cues which remain incompletely characterised. The precise molecular composition of the HSC themselves is incompletely understood, limiting approaches to track their origin in situ in the appropriate cellular, chemical and mechanical context. Embryonic material at the point of HSC emergence is limiting, highlighting the need for an in vitro model of embryonic haematopoietic development in which current knowledge gaps can be addressed and exploited to enable HSC production. Gastruloids are pluripotent stem cell-derived 3-dimensional (3D) cellular aggregates which recapitulate developmental events in gastrulation and early organogenesis with spatial and temporal precision. Gastruloids self-organise multi-tissue structures upon minimal and controlled external cues, and are amenable to live imaging, screening, scaling and physicochemical manipulation to understand and translate tissue formation. In this review, we consider the haematopoietic potential of gastruloids and review early strategies to enhance blood progenitor and HSC production. We highlight possible strategies to achieve HSC production from gastruloids, and discuss the potential of gastruloid systems in illuminating current knowledge gaps in HSC specification.

Umbilical cord blood and cord tissue banking as somatic stem cell resources to support medical cell modalities

Human umbilical cord blood (CB) and umbilical cord tissue (UC) are attractive sources of somatic stem cells for gene and cell therapies. CB and UC can be obtained noninvasively from donors. CB, a known source of hematopoietic stem cells for transplantation, has attracted attention as a new source of immune cells, including universal chimeric antigen receptor-T cell therapy (CAR-T) and, more recently, universal CAR-natural killer cells. UC-derived mesenchymal stromal cells (UC-MSCs) have a higher proliferation potency than those derived from adult tissues and can be used anon-HLA restrictively. UC-MSCs meet the MSC criteria outlined by the International Society of Gene and Cellular Therapy. UC-MSCs are negative for HLA-DR, CD80, and CD86 and have an immunosuppressive ability that mitigates the proliferation of activated lymphocytes through secreting indoleamine 2,3-dioxygenase 1 and prostaglandin E2, and the expression of PD-L2 and PD-L1. We established the off-the-shelf cord blood/cord bank IMSUT CORD to support novel cell therapy modalities, including the CB-derived immune cells, MSCs, MSCs-derived extracellular vesicles, biological carriers loaded with chemotherapy drugs, prodrug, oncolytic viruses, nanoparticles, human artificial chromosome, combinational products with a scaffold, bio3D printing, and so on.

Expansion of human hematopoietic stem cells by inhibiting translation

Hematopoietic stem cell (HSC) transplantation using umbilical cord blood (UCB) is a potentially life-saving treatment for leukemia and bone marrow failure but is limited by the low number of HSCs in UCB. The loss of HSCs after ex vivo manipulation is also a major obstacle to gene editing for inherited blood disorders. HSCs require a low rate of translation to maintain their capacity for self-renewal, but hematopoietic cytokines used to expand HSCs stimulate protein synthesis and impair long-term self-renewal. We previously described cytokine-free conditions that maintain but do not expand human and mouse HSCs ex vivo. Here we performed a high throughput screen and identified translation inhibitors that allow ex vivo expansion of human HSCs while minimizing cytokine exposure. Transplantation assays show a ~5-fold expansion of long-term HSCs from UCB after one week of culture in low cytokine conditions. Single cell transcriptomic analysis demonstrates maintenance of HSCs expressing mediators of the unfolded protein stress response, further supporting the importance of regulated proteostasis in HSC maintenance and expansion. This expansion method maintains and expands human HSCs after CRISPR/Cas9 editing of the BCL11A+58 enhancer, overcoming a major obstacle to ex vivo gene correction for human hemoglobinopathies.

Stem cell mutations, associated cancer risk, and consequences for regenerative medicine

Mutation accumulation in stem cells has been associated with cancer risk. However, the presence of numerous mutant clones in healthy tissues has raised the question of what limits cancer initiation. Here, we review recent developments in characterizing mutation accumulation in healthy tissues and compare mutation rates in stem cells during development and adult life with corresponding cancer risk. A certain level of mutagenesis within the stem cell pool might be beneficial to limit the size of malignant clones through competition. This knowledge impacts our understanding of carcinogenesis with potential consequences for the use of stem cells in regenerative medicine.

VEGF signaling governs the initiation of biliary-mediated liver regeneration through the PI3K-mTORC1 axis

Biliary epithelial cells (BECs) are a potential source to repair the damaged liver when hepatocyte proliferation is compromised. Promotion of BEC-to-hepatocyte transdifferentiation could be beneficial to the clinical therapeutics of patients with end-stage liver diseases. However, mechanisms underlying the initiation of BEC transdifferentiation remain largely unknown. Here, we show that upon extreme hepatocyte injury, vegfaa and vegfr2/kdrl are notably induced in hepatic stellate cells and BECs, respectively. Pharmacological and genetic inactivation of vascular endothelial growth factor (VEGF) signaling would disrupt BEC dedifferentiation and proliferation, thus restraining hepatocyte regeneration. Mechanically, VEGF signaling regulates the activation of the PI3K-mammalian target of rapamycin complex 1 (mTORC1) axis, which is essential for BEC-to-hepatocyte transdifferentiation. In mice, VEGF signaling exerts conserved roles in oval cell activation and BEC-to-hepatocyte differentiation. Taken together, this study shows VEGF signaling as an initiator of biliary-mediated liver regeneration through activating the PI3K-mTORC1 axis. Modulation of VEGF signaling in BECs could be a therapeutic approach for patients with end-stage liver diseases.

Structure of the thrombopoietin-MPL receptor complex is a blueprint for biasing hematopoiesis

Thrombopoietin (THPO or TPO) is an essential cytokine for hematopoietic stem cell (HSC) maintenance and megakaryocyte differentiation. Here, we report the 3.4 Å resolution cryoelectron microscopy structure of the extracellular TPO-TPO receptor (TpoR or MPL) signaling complex, revealing the basis for homodimeric MPL activation and providing a structural rationalization for genetic loss-of-function thrombocytopenia mutations. The structure guided the engineering of TPO variants (TPOmod) with a spectrum of signaling activities, from neutral antagonists to partial- and super-agonists. Partial agonist TPOmod decoupled JAK/STAT from ERK/AKT/CREB activation, driving a bias for megakaryopoiesis and platelet production without causing significant HSC expansion in mice and showing superior maintenance of human HSCs in vitro. These data demonstrate the functional uncoupling of the two primary roles of TPO, highlighting the potential utility of TPOmod in hematology research and clinical HSC transplantation.

Mobilization-based engraftment of haematopoietic stem cells: a new perspective for chemotherapy-free gene therapy and transplantation

INTRODUCTION

In haematopoietic stem cell transplantation (HSCT), haematopoietic stem cells (HSCs) from a healthy donor replace the patient's ones. Ex vivo HSC gene therapy (HSC-GT) is a form of HSCT in which HSCs, usually from an autologous source, are genetically modified before infusion, to generate a progeny of gene-modified cells. In HSCT and HSC-GT, chemotherapy is administered before infusion to free space in the bone marrow (BM) niche, which is required for the engraftment of infused cells. Here, we review alternative chemotherapy-free approaches to niche voidance that could replace conventional regimens and alleviate the morbidity of the procedure.

SOURCES OF DATA

Literature was reviewed from PubMed-listed peer-reviewed articles. No new data are presented in this article.

AREAS OF AGREEMENT

Chemotherapy exerts short and long-term toxicity to haematopoietic and non-haematopoietic organs. Whenever chemotherapy is solely used to allow engraftment of donor HSCs, rather than eliminating malignant cells, as in the case of HSC-GT for inborn genetic diseases, non-genotoxic approaches sparing off-target tissues are highly desirable.

AREAS OF CONTROVERSY

In principle, HSCs can be temporarily moved from the BM niches using mobilizing drugs or selectively cleared with targeted antibodies or immunotoxins to make space for the infused cells. However, translation of these principles into clinically relevant settings is only at the beginning, and whether therapeutically meaningful levels of chimerism can be safely established with these approaches remains to be determined.

GROWING POINTS

In pre-clinical models, mobilization of HSCs from the niche can be tailored to accommodate the exchange and engraftment of infused cells. Infused cells can be further endowed with a transient engraftment advantage.

AREAS TIMELY FOR DEVELOPING RESEARCH

Inter-individual efficiency and kinetics of HSC mobilization need to be carefully assessed. Investigations in large animal models of emerging non-genotoxic approaches will further strengthen the rationale and encourage application to the treatment of selected diseases.

Chemical reprogramming for cell fate manipulation: Methods, applications, and perspectives

Chemical reprogramming offers an unprecedented opportunity to control somatic cell fate and generate desired cell types including pluripotent stem cells for applications in biomedicine in a precise, flexible, and controllable manner. Recent success in the chemical reprogramming of human somatic cells by activating a regeneration-like program provides an alternative way of producing stem cells for clinical translation. Likewise, chemical manipulation enables the capture of multiple (stem) cell states, ranging from totipotency to the stabilization of somatic fates in vitro. Here, we review progress in using chemical approaches for cell fate manipulation in addition to future opportunities in this promising field.

Expanding hematopoietic stem cell ex vivo: recent advances and technical considerations

Hematopoietic stem cells (HSCs) are the most primitive cell type in the hematopoietic hierarchy, which are responsible for sustaining the lifelong production of mature blood and immune cells. Due to their superior long-term regenerative capacity, HSC therapies such as stem cell transplantation have been used in a broad range of hematologic disorders. However, the rarity of this population in vivo considerably limits its clinical applications and large-scale analyses such as screening and safety studies. Therefore, ex vivo culture methods that allow long-term expansion and maintenance of functional HSCs are instrumental in overcoming the difficulties in studying HSC biology and improving HSC therapies. In this perspective, we discuss recent advances and technical considerations for three ex vivo HSC expansion methods including 1) polyvinyl alcohol-based HSC expansion, 2) mesenchymal stromal cell-HSC co-culture, and 3) two-/three-dimensional hydrogel HSC culture. This review summarizes the presentations and discussions from the 2022 International Society for Experimental Hematology (ISEH) Annual Meeting New Investigator Technology Session.

The genesis of human hematopoietic stem cells

Developmental hematopoiesis consists of multiple, partially overlapping hematopoietic waves that generate the differentiated blood cells required for embryonic development while establishing a pool of undifferentiated hematopoietic stem cells (HSCs) for postnatal life. This multilayered design in which active hematopoiesis migrates through diverse extra and intraembryonic tissues has made it difficult to define a roadmap for generating HSCs vs non-self-renewing progenitors, especially in humans. Recent single-cell studies have helped in identifying the rare human HSCs at stages when functional assays are unsuitable for distinguishing them from progenitors. This approach has made it possible to track the origin of human HSCs to the unique type of arterial endothelium in the aorta-gonad-mesonephros region and document novel benchmarks for HSC migration and maturation in the conceptus. These studies have delivered new insights into the intricate process of HSC generation and provided tools to inform the in vitro efforts to replicate the physiological developmental journey from pluripotent stem cells via distinct mesodermal and endothelial intermediates to 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.

Unraveling Heterogeneity in the Aging Hematopoietic Stem Cell Compartment: An Insight From Single-cell Approaches

Specific cell types and, therefore, organs respond differently during aging. This is also true for the hematopoietic system, where it has been demonstrated that hematopoietic stem cells alter a variety of features, such as their metabolism, and accumulate DNA damage, which can lead to clonal outgrowth over time. In addition, profound changes in the bone marrow microenvironment upon aging lead to senescence in certain cell types such as mesenchymal stem cells and result in increased inflammation. This heterogeneity makes it difficult to pinpoint the molecular drivers of organismal aging gained from bulk approaches, such as RNA sequencing. A better understanding of the heterogeneity underlying the aging process in the hematopoietic compartment is, therefore, needed. With the advances of single-cell technologies in recent years, it is now possible to address fundamental questions of aging. In this review, we discuss how single-cell approaches can and indeed are already being used to understand changes observed during aging in the hematopoietic compartment. We will touch on established and novel methods for flow cytometric detection, single-cell culture approaches, and single-cell omics.

Genetic engineering meets hematopoietic stem cell biology for next-generation gene therapy

The growing clinical success of hematopoietic stem/progenitor cell (HSPC) gene therapy (GT) relies on the development of viral vectors as portable "Trojan horses" for safe and efficient gene transfer. The recent advent of novel technologies enabling site-specific gene editing is broadening the scope and means of GT, paving the way to more precise genetic engineering and expanding the spectrum of diseases amenable to HSPC-GT. Here, we provide an overview of state-of-the-art and prospective developments of the HSPC-GT field, highlighting how advances in biological characterization and manipulation of HSPCs will enable the design of the next generation of these transforming therapeutics.

Long-term expansion of human hematopoietic stem cells

Hematopoietic stem cells (HSCs) are critical for the treatment of a variety of hematological diseases. However, the low number of HSCs lead to the clinical application difficult. To gain more functional human HSCs ex vivo, Sakurai et al. established a recombinant-cytokine-free and albumin-free culture system, i.e. PCL-PVAc-PEG-based culture, in combination with 740Y-P, butyzamide and UM171, to improve the long-term expansion of human cord blood HSCs.

Human hematopoietic stem cells expand beyond cytokines

The paucity of hematopoietic stem cells (HSCs) presents a challenge for both transplantation and the study of HSCs.¹ Sakurai et al.² now present a cytokine-free culture system for robust ex vivo expansion of functional human HSCs that may lead to exciting clinical outcomes.

In Vitro Human Haematopoietic Stem Cell Expansion and Differentiation

The haematopoietic system plays an essential role in our health and survival. It is comprised of a range of mature blood and immune cell types, including oxygen-carrying erythrocytes, platelet-producing megakaryocytes and infection-fighting myeloid and lymphoid cells. Self-renewing multipotent haematopoietic stem cells (HSCs) and a range of intermediate haematopoietic progenitor cell types differentiate into these mature cell types to continuously support haematopoietic system homeostasis throughout life. This process of haematopoiesis is tightly regulated in vivo and primarily takes place in the bone marrow. Over the years, a range of in vitro culture systems have been developed, either to expand haematopoietic stem and progenitor cells or to differentiate them into the various haematopoietic lineages, based on the use of recombinant cytokines, co-culture systems and/or small molecules. These approaches provide important tractable models to study human haematopoiesis in vitro. Additionally, haematopoietic cell culture systems are being developed and clinical tested as a source of cell products for transplantation and transfusion medicine. This review discusses the in vitro culture protocols for human HSC expansion and differentiation, and summarises the key factors involved in these biological processes.