Artificial Niches: Biomimetic Materials for Hematopoietic Stem Cell Culture

A Multifunctional Nanostructured Hydrogel as a Platform for Deciphering Niche Interactions of Hematopoietic Stem and Progenitor Cells

For over half a century, hematopoietic stem cells (HSCs) have been used for transplantation therapy to treat severe hematologic diseases. Successful outcomes depend on collecting sufficient donor HSCs as well as ensuring efficient engraftment. These processes are influenced by dynamic interactions of HSCs with the bone marrow niche, which can be revealed by artificial niche models. Here, a multifunctional nanostructured hydrogel is presented as a 2D platform to investigate how the interdependencies of cytokine binding and nanopatterned adhesive ligands influence the behavior of human hematopoietic stem and progenitor cells (HSPCs). The results indicate that the degree of HSPC polarization and motility, observed when cultured on gels presenting the chemokine SDF-1α and a nanoscale-defined density of a cellular (IDSP) or extracellular matrix (LDV) α4β1 integrin binding motif, are differently influenced on hydrogels functionalized with the different ligand types. Further, SDF-1α promotes cell polarization but not motility. Strikingly, the degree of differentiation correlates negatively with the nanoparticle spacing, which determines ligand density, but only for the cellular-derived IDSP motif. This mechanism potentially offers a means of predictably regulating early HSC fate decisions. Consequently, the innovative multifunctional hydrogel holds promise for deciphering dynamic HSPC-niche interactions and refining transplantation therapy protocols.

Hydrogel-Based Pre-Clinical Evaluation of Repurposed FDA-Approved Drugs for AML

In vivo models of acute myeloid leukemia (AML) are low throughput, and standard liquid culture models fail to recapitulate the mechanical and biochemical properties of the extracellular matrix-rich protective bone marrow niche that contributes to drug resistance. Candidate drug discovery in AML requires advanced synthetic platforms to improve our understanding of the impact of mechanical cues on drug sensitivity in AML. By use of a synthetic, self-assembling peptide hydrogel (SAPH) of modifiable stiffness and composition, a 3D model of the bone marrow niche to screen repurposed FDA-approved drugs has been developed and utilized. AML cell proliferation was dependent on SAPH stiffness, which was optimized to facilitate colony growth. Three candidate FDA-approved drugs were initially screened against the THP-1 cell line and mAF9 primary cells in liquid culture, and EC50 values were used to inform drug sensitivity assays in the peptide hydrogel models. Salinomycin demonstrated efficacy in both an 'early-stage' model in which treatment was added shortly after initiation of AML cell encapsulation, and an 'established' model in which time-encapsulated cells had started to form colonies. Sensitivity to Vidofludimus treatment was not observed in the hydrogel models, and Atorvastatin demonstrated increased sensitivity in the 'established' compared to the 'early-stage' model. AML patient samples were equally sensitive to Salinomycin in the 3D hydrogels and partially sensitive to Atorvastatin. Together, this confirms that AML cell sensitivity is drug- and context-specific and that advanced synthetic platforms for higher throughput are valuable tools for pre-clinical evaluation of candidate anti-AML drugs.

Functionalized 3D scaffolds for engineering the hematopoietic niche

Hematopoietic stem cells (HSCs) reside in a subzone of the bone marrow (BM) defined as the hematopoietic niche where, via the interplay of differentiation and self-renewal, they can give rise to immune and blood cells. Artificial hematopoietic niches were firstly developed in 2D in vitro cultures but the limited expansion potential and stemness maintenance induced the optimization of these systems to avoid the total loss of the natural tissue complexity. The next steps were adopted by engineering different materials such as hydrogels, fibrous structures with natural or synthetic polymers, ceramics, etc. to produce a 3D substrate better resembling that of BM. Cytokines, soluble factors, adhesion molecules, extracellular matrix (ECM) components, and the secretome of other niche-resident cells play a fundamental role in controlling and regulating HSC commitment. To provide biochemical cues, co-cultures, and feeder-layers, as well as natural or synthetic molecules were utilized. This review gathers key elements employed for the functionalization of a 3D scaffold that demonstrated to promote HSC growth and differentiation ranging from 1) biophysical cues, i.e., material, topography, stiffness, oxygen tension, and fluid shear stress to 2) biochemical hints favored by the presence of ECM elements, feeder cell layers, and redox scavengers. Particular focus is given to the 3D systems to recreate megakaryocyte products, to be applied for blood cell production, whereas HSC clinical application in such 3D constructs was limited so far to BM diseases testing.

Engineered Tissue Models to Replicate Dynamic Interactions within the Hematopoietic Stem Cell Niche

Hematopoietic stem cells are the progenitors of the blood and immune system and represent the most widely used regenerative therapy. However, their rarity and limited donor base necessitate the design of ex vivo systems that support HSC expansion without the loss of long-term stem cell activity. This review describes recent advances in biomaterials systems to replicate features of the hematopoietic niche. Inspired by the native bone marrow, these instructive biomaterials provide stimuli and cues from cocultured niche-associated cells to support HSC encapsulation and expansion. Engineered systems increasingly enable study of the dynamic nature of the matrix and biomolecular environment as well as the role of cell-cell signaling (e.g., autocrine feedback vs paracrine signaling between dissimilar cells). The inherent coupling of material properties, biotransport of cell-secreted factors, and cell-mediated remodeling motivate dynamic biomaterial systems as well as characterization and modeling tools capable of evaluating a temporally evolving tissue microenvironment. Recent advances in HSC identification and tracking, model-based experimental design, and single-cell culture platforms facilitate the study of the effect of constellations of matrix, cell, and soluble factor signals on HSC fate. While inspired by the HSC niche, these tools are amenable to the broader stem cell engineering community.

Isolation, Maintenance and Expansion of Adult Hematopoietic Stem/Progenitor Cells and Leukemic Stem Cells

Hematopoietic stem cells (HSCs) are rare, self-renewing cells that perch on top of the hematopoietic tree. The HSCs ensure the constant supply of mature blood cells in a tightly regulated process producing peripheral blood cells. Intense efforts are ongoing to optimize HSC engraftment as therapeutic strategy to treat patients suffering from hematopoietic diseases. Preclinical research paves the way by developing methods to maintain, manipulate and expand HSCs ex vivo to understand their regulation and molecular make-up. The generation of a sufficient number of transplantable HSCs is the Holy Grail for clinical therapy. Leukemia stem cells (LSCs) are characterized by their acquired stem cell characteristics and are responsible for disease initiation, progression, and relapse. We summarize efforts, that have been undertaken to increase the number of long-term (LT)-HSCs and to prevent differentiation towards committed progenitors in ex vivo culture. We provide an overview and compare methods currently available to isolate, maintain and enrich HSC subsets, progenitors and LSCs and discuss their individual advantages and drawbacks.

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.

Alginate-Chitosan Microencapsulated Cells for Improving CD34+ Progenitor Maintenance and Expansion

Protocols for isolation, characterization, and transplantation of hematopoietic stem cells (HSCs) have been well established. However, difficulty in finding human leucocyte antigens (HLA)-matched donors and scarcity of HSCs are still the major obstacles of allogeneic transplantation. In this study, we developed a double-layered microcapsule to deliver paracrine factors from non-matched or low-matched HSCs to other cells. The umbilical cord blood-derived hematopoietic progenitor cells, identified as CD34+ cells, were entrapped in alginate polymer and further protected by chitosan coating. The microcapsules showed no toxicity for surrounding CD34+ cells. When CD34+ cells-loaded microcapsules were co-cultured with bare CD34+ cells that have been collected from unrelated donors, the microcapsules affected surrounding cells and increased the percentage of CD34+ cell population. This study is the first to report the potency of alginate-chitosan microcapsules containing non-HLA-matched cells for improving proliferation and progenitor maintenance of CD34+ cells.

Decellularized adipose tissue scaffolds guide hematopoietic differentiation and stimulate vascular regeneration in a hindlimb ischemia model

Cellular therapies to stimulate therapeutic angiogenesis in individuals with critical limb ischemia (CLI) remain under intense investigation. In this context, the efficacy of cell therapy is dependent on the survival, biodistribution, and pro-angiogenic paracrine signaling of the cells transplanted. Hematopoietic progenitor cells (HPC) purified from human umbilical cord blood using high aldehyde dehydrogenase-activity (ALDH^hi cells) and expanded ex vivo, represent a heterogeneous mixture of progenitor cells previously shown to support limb revascularization in mouse models of CLI. The objectives of this study were to investigate the utility of bioscaffolds derived from human decellularized adipose tissue (DAT) to guide the differentiation of seeded HPC in vitro and harness the pro-angiogenic capacity of HPC at the site of ischemia after implantation in vivo. Probing whether the DAT scaffolds altered HPC differentiation, label-free quantitative mass spectrometry and flow cytometric phenotype analyses indicated that culturing the HPC on the DAT scaffolds supported their differentiation towards the pro-angiogenic monocyte/macrophage lineage at the expense of megakaryopoiesis. Moreover, implantation of HPC in DAT scaffolds within a unilateral hindlimb ischemia model in NOD/SCID mice increased cell retention at the site of ischemia relative to intramuscular injection, and accelerated the recovery of limb perfusion, improved functional limb use and augmented CD31⁺ capillary density when compared to DAT implantation alone or saline-injected controls. Collectively, these data indicate that cell-instructive DAT scaffolds can direct therapeutic HPC differentiation towards the monocyte/macrophage lineage and represent a promising delivery platform for improving the efficacy of cell therapies for CLI.

Kinetics of leukemic cells in 3D culture with stromal cells and with arginine deprivation stress

Previously, we established a three-dimensional (3D) bone marrow culture system that maintains normal hematopoiesis, including prolongation of hematopoietic stem cell proliferation and differentiation. To analyze the role of bone marrow stromal cells that compose the microenvironment, the growth of a leukemic cell line (K562) in the 3D condition and with arginine deprivation stress was compared with two-dimensional stromal cell monolayers (2D) and suspension cultures without stromal cells (stroma (-)). Arginine is essential for the proliferation and differentiation of erythrocytes. The proliferation and differentiation of K562 cells cultured in the 3D system were stabilized compared with cells in 2D or stroma (-). Furthermore, the number of K562 cells in the G₀/G₁ phase in 3D was increased significantly compared with cells grown in 2D or stroma (-). Interestingly, the mRNA expression of various hematopoietic growth factors of stromal cells in 3D was not different from 2D, even though supportive activity on K562 cell growth was observed in the arginine deprivation condition. Thus, the hematopoietic microenvironment involves multi-dimensional and complex systems including biochemical and physiochemical factors that regulate quiescence, proliferation, activation, and differentiation of normal hematopoietic cells and cloned leukemic cells. Our 3D culture system may be a valuable new tool for investigating leukemic cell-stromal cell interactions in vitro.

Thermoresponsive Inverted Colloidal Crystal Hydrogel Scaffolds for Lymphoid Tissue Engineering

Inverted colloidal crystal (ICC) hydrogel scaffolds represent unique opportunities in modeling lymphoid tissues and expanding hematopoietic-lymphoid cells. Fully interconnected spherical pore arrays direct the formation of stromal networks and facilitate interactions between stroma and hematopoietic-lymphoid cells. However, due to the intricate architecture of these materials, release of expanded cells is restricted and requires mechanical disruption or chemical dissolution of the hydrogel scaffold. One potent biomaterials strategy to release pore-entrapped hematopoietic-lymphoid cells without breaking the scaffolds apart is to transiently increase the dimensions of these materials using stimuli-responsive polymers. Having this mindset, thermoresponsive ICC scaffolds that undergo rapid (<1 min) and substantial (>300%) diameter change over a physiological temperature range (4-37 °C) by using poly(N-isopropylacrylamide) (PNIPAM) with nanogel crosslinkers is developed. For a proof-of-concept study, the stromal niche by creating osteospheroids, aggregates of osteoblasts, and bone chips is first replicated, and subsequently Nalm-6 model hematopoietic-lymphoid cells are introduced. A sixfold increase in cell count is harvested when ICC hydrogel scaffolds are expanded without termination of the established 3D stromal cell culture. It is envisioned that thermoresponsive ICC hydrogel scaffolds will enable for scalable and sustainable ex vivo expansion of hematopoietic-lymphoid cells.

Decellularized Wharton jelly matrix: a biomimetic scaffold for ex vivo hematopoietic stem cell culture

Hematopoietic stem progenitor cells (HSPCs) reside in the bone marrow (BM) hematopoietic "niche," a special 3-dimensional (3D) microenvironment that regulates HSPC self-renewal and multipotency. In this study, we evaluated a novel 3D in vitro culture system that uses components of the BM hematopoietic niche to expand umbilical cord blood (UCB) CD34+ cells. We developed this model using decellularized Wharton jelly matrix (DWJM) as an extracellular matrix (ECM) scaffold and human BM mesenchymal stromal cells (MSCs) as supporting niche cells. To assess the efficacy of this model in expanding CD34+ cells, we analyzed UCB CD34+ cells, following culture in DWJM, for proliferation, viability, self-renewal, multilineage differentiation, and transmigration capability. We found that DWJM significantly expanded UCB HSPC subset. It promoted UCB CD34+ cell quiescence, while maintaining their viability, differentiation potential with megakaryocytic differentiation bias, and clonogenic capacity. DWJM induced an increase in the frequency of c-kit+ cells, a population with enhanced self-renewal ability, and in CXCR4 expression in CD34+ cells, which enhanced their transmigration capability. The presence of BM MSCs in DWJM, however, impaired UCB CD34+ cell transmigration and suppressed CXCR4 expression. Transcriptome analysis indicated that DWJM upregulates a set of genes that are specifically involved in megakaryocytic differentiation, cell mobility, and BM homing. Collectively, our results indicate that the DWJM-based 3D culture system is a novel in vitro model that supports the proliferation of UCB CD34+ cells with enhanced transmigration potential, while maintaining their differentiation potential. Our findings shed light on the interplay between DWJM and BM MSCs in supporting the ex vivo culture of human UCB CD34+ cells for use in clinical transplantation.

Phage-Based Artificial Niche: The Recent Progress and Future Opportunities in Stem Cell Therapy

Self-renewal and differentiation of stem cells can be the best option for treating intractable diseases in regenerative medicine, and they occur when these cells reside in a special microenvironment, called the "stem cell niche." Thus, the niche is crucial for the effective performance of the stem cells in both in vivo and in vitro since the niche provides its functional cues by interacting with stem cells chemically, physically, or topologically. This review provides a perspective on the different types of artificial niches including engineered phage and how they could be used to recapitulate or manipulate stem cell niches. Phage-based artificial niche engineering as a promising therapeutic strategy for repair and regeneration of tissues is also discussed.

3D models of the bone marrow in health and disease: yesterday, today and tomorrow

The complex interaction between hematopoietic stem cells (HSCs) and their microenvironment in the human bone marrow ensures a life-long blood production by balancing stem cell maintenance and differentiation. This so-called HSC niche can be disturbed by malignant diseases. Investigating their consequences on hematopoiesis requires deep understanding of how the niches function in health and disease. To facilitate this, biomimetic models of the bone marrow are needed to analyse HSC maintenance and hematopoiesis under steady-state and diseased conditions. Here, 3D bone marrow models, their fabrication methods (including 3D bioprinting) and implementations recapturing bone marrow functions in health and diseases, are presented.

Tissue engineered models of healthy and malignant human bone marrow

Tissue engineering is becoming increasingly successful in providing in vitro models of human tissues that can be used for ex vivo recapitulation of functional tissues as well as predictive testing of drug efficacy and safety. From simple tissue models to microphysiological platforms comprising multiple tissue types connected by vascular perfusion, these "tissues on a chip" are emerging as a fast track application for tissue engineering, with great potential for modeling diseases and supporting the development of new drugs and therapeutic targets. We focus here on tissue engineering of the hematopoietic stem and progenitor cell compartment and the malignancies that can develop in the human bone marrow. Our overall goal is to demonstrate the utility and interconnectedness of improvements in bioengineering methods developed in one area of bone marrow studies for the remaining, seemingly disparate, bone marrow fields.

Stem cell niche: Dynamic neighbor of stem cells

Stem cell niche is a specialized and dynamic microenvironment around the stem cells which plays a critical role in maintaining the stemness properties of stem cells. Over the years, advancement in the research activity has revealed the various important aspects of stem cell niche including cell-cell interaction, cell-extracellular matrix interaction, a large number of soluble signaling factors and various biochemical and biophysical cues (such as oxygen tension, flow, and shear and pore size). Stem cells have the potential to be a powerful tool in regenerative medicine due to their self-renewal property and immense differentiation potential. Recent progresses in in vitro culture conditions of embryonic stem cells, adult stem cells and induced pluripotent stem cells have enabled the researchers to investigate and understand the role of the microenvironment in stem cell properties. The engineered artificial stem cell niche has led to a better execution of stem cells in regenerative medicine. Here we elucidate the key components of stem cell niche and their role in niche engineering and stem cell therapeutics.

Magnetic Macroporous Hydrogels as a Novel Approach for Perfused Stem Cell Culture in 3D Scaffolds via Contactless Motion Control

There is an urgent need for 3D cell culture systems that avoid the oversimplifications and artifacts of conventional culture in 2D. However, 3D culture within the cavities of porous biomaterials or large 3D structures harboring high cell numbers is limited by the needs to nurture cells and to remove growth-limiting metabolites. To overcome the diffusion-limited transport of such soluble factors in 3D culture, mixing can be improved by pumping, stirring or shaking, but this in turn can lead to other problems. Using pumps typically requires custom-made accessories that are not compatible with conventional cell culture disposables, thus interfering with cell production processes. Stirring or shaking allows little control over movement of scaffolds in media. To overcome these limitations, magnetic, macroporous hydrogels that can be moved or positioned within media in conventional cell culture tubes in a contactless manner are presented. The cytocompatibility of the developed biomaterial and the applied magnetic fields are verified for human hematopoietic stem and progenitor cells (HSPCs). The potential of this technique for perfusing 3D cultures is demonstrated in a proof-of-principle study that shows that controlled contactless movement of cell-laden magnetic hydrogels in culture media can mimic the natural influence of differently perfused environments on HSPCs.

Lectin-mediated reversible immobilization of human cells into a glycosylated macroporous protein hydrogel as a cell culture matrix

3D cell culture is a helpful approach to study cell-cell interaction in a native-like environment, but is often limited due the challenge of retrieving cells from the material. In this study, we present the use of recombinant lectin B, a sugar-binding protein with four binding cavities, to enable reversible cell integration into a macroporous protein hydrogel matrix. By functionalizing hydrogel precursors with saccharose, lectin B can both bind to sugar moieties on the cellular surface as well as to the modified hydrogel network. Confocal microscopy and flow cytometry analysis revealed cells to be integrated into the network and to adhere and proliferate. Furthermore, the specificity and reversibility was investigated by using a recombinantly produced yellow fluorescent - lectin B fusion protein and a variety of sugars with diverging affinities for lectin B at different concentrations and elution times. Cells could be eluted within minutes by addition of L-fucose to the cell-loaded hydrogels to make cells available for further analysis.

3D models of the hematopoietic stem cell niche under steady-state and active conditions

Hematopoietic stem cells (HSCs) in the bone marrow are able to differentiate into all types of blood cells and supply the organism each day with billions of fresh cells. They are applied to cure hematological diseases such as leukemia. The clinical need for HSCs is high and there is a demand for being able to control and multiply HSCs in vitro. The hematopoietic system is highly proliferative and thus sensitive to anti-proliferative drugs such as chemotherapeutics. For many of these drugs suppression of the hematopoietic system is the dose-limiting toxicity. Therefore, biomimetic 3D models of the HSC niche that allow to control HSC behavior in vitro and to test drugs in a human setting are relevant for the clinics and pharmacology. Here, we describe a perfused 3D bone marrow analog that allows mimicking the HSC niche under steady-state and activated conditions that favor either HSC maintenance or differentiation, respectively, and allows for drug testing.

Biomimetic Macroporous PCL Scaffolds for Ex Vivo Expansion of Cord Blood-Derived CD34+ Cells with Feeder Cells Support

Ex vivo expansion of hematopoietic stem cells (HSCs) with most current methods can hardly satisfy clinical application requirement. While in vivo, HSCs efficiently self-renew in niche where they interact with 3D extracellular matrix and stromal cells. Therefore, co-cultures of CD34⁺ cells and mesenchyme stem cells derived from human amniotic membrane (hAMSCs) on the basis of biomimetic macroporous three-dimensional (3D) poly(ε-caprolactone) (PCL) scaffolds are developed, where scaffolds and hAMSCs are applied to mimic structural and cellular microenvironment of HSCs. The influence of scaffolds, feeder cells, and contact manners on expansion and stemness maintenance of CD34⁺ cells is investigated in this protocol. Biomimetic scaffolds-dependent co-cultures of CD34⁺ cells and hAMSCs can effectively promote the expansion of CD34⁺ cells; meanwhile, indirect contact is superior to direct contact. The combination of biomimetic scaffolds and hAMSCs represents a new strategy for achieving clinical-scale ex vivo expansion of CD34⁺ cells.

Poly-l-Lactic Acid Nanofiber-Polyamidoamine Hydrogel Composites: Preparation, Properties, and Preliminary Evaluation as Scaffolds for Human Pluripotent Stem Cell Culturing

Electrospun poly-l-lactic acid (PLLA) nanofiber mats carrying surface amine groups, previously introduced by nitrogen atmospheric pressure nonequilibrium plasma, are embedded into aqueous solutions of oligomeric acrylamide-end capped AGMA1, a biocompatible polyamidoamine with arg-gly-asp (RGD)-reminiscent repeating units. The resultant mixture is finally cured giving PLLA-AGMA1 hydrogel composites that absorb large amounts of water and, in the swollen state, are translucent, soft, and pliable, yet as strong as the parent PLLA mat. They do not split apart from each other when swollen in water and remain highly flexible and resistant, since the hydrogel portion is covalently grafted onto the PLLA nanofibers via the addition reaction of the surface amine groups to a part of the terminal acrylic double bonds of AGMA1 oligomers. Preliminary tested as scaffolds, the composites prove capable of maintaining short-term undifferentiated cultures of human pluripotent stem cells in feeder-free conditions.

Challenges and Opportunities to Harnessing the (Hematopoietic) Stem Cell Niche

In our body, stem cells reside in a microenvironment termed the niche. While the exact composition and therefore the level of complexity of a stem cell niche can vary significantly tissue-to-tissue, the stem cell niche microenvironment is dynamic, typically containing spatial and temporal variations in both cellular, extracellular matrix, and biomolecular components. This complex flow of secreted or bound biomolecules, cytokines, extracellular matrix components, and cellular constituents all contribute to the regulation of stem cell fate specification events, making engineering approaches at the nano- and micro-scale of particular interest for creating an artificial niche environment in vitro. Recent advances in fabrication approaches have enabled biomedical researchers to capture and recreate the complexity of stem cell niche microenvironments in vitro. Such engineered platforms show promise as a means to enhance our understanding of the mechanisms underlying niche-mediated stem cell regulation as well as offer opportunities to precisely control stem cell expansion and differentiation events for clinical applications. While these principles generally apply to all adult stem cells and niches, in this review, we focus on recent developments in engineering synthetic niche microenvironments for one of the best-characterized stem cell populations, hematopoietic stem cells (HSC). Specifically, we highlight recent advances in platforms designed to facilitate the extrinsic control of HSC fate decisions.

Bone-marrow mimicking biomaterial niches for studying hematopoietic stem and progenitor cells

This review discusses the considerations and approaches that have been employed for designing biomaterial based cultures for replicating the hematopoietic stem and progenitor cell niche.

Encapsulated feeder cells within alginate beads for ex vivo expansion of cord blood-derived CD34+ cells

A co-culture system based on encapsulated feeder cells within alginate beads was developed through optimizing the detailed aspects of the cell culture system to expand CD34-positive (CD34⁺) cells ex vivo.

Identifying States along the Hematopoietic Stem Cell Differentiation Hierarchy with Single Cell Specificity via Raman Spectroscopy

A major challenge for expanding specific types of hematopoietic cells ex vivo for the treatment of blood cell pathologies is identifying the combinations of cellular and matrix cues that direct hematopoietic stem cells (HSC) to self-renew or differentiate into cell populations ex vivo. Microscale screening platforms enable minimizing the number of rare HSCs required to screen the effects of numerous cues on HSC fate decisions. These platforms create a strong demand for label-free methods that accurately identify the fate decisions of individual hematopoietic cells at specific locations on the platform. We demonstrate the capacity to identify discrete cells along the HSC differentiation hierarchy via multivariate analysis of Raman spectra. Notably, cell state identification is accurate for individual cells and independent of the biophysical properties of the functionalized polyacrylamide gels upon which these cells are cultured. We report partial least-squares discriminant analysis (PLS-DA) models of single cell Raman spectra enable identifying four dissimilar hematopoietic cell populations across the HSC lineage specification. Successful discrimination was obtained for a population enriched for long-term repopulating HSCs (LT-HSCs) versus their more differentiated progeny, including closely related short-term repopulating HSCs (ST-HSCs) and fully differentiated lymphoid (B cells) and myeloid (granulocytes) cells. The lineage-specific differentiation states of cells from these four subpopulations were accurately identified independent of the stiffness of the underlying biomaterial substrate, indicating subtle spectral variations that discriminated these populations were not masked by features from the culture substrate. This approach enables identifying the lineage-specific differentiation stages of hematopoietic cells on biomaterial substrates of differing composition and may facilitate correlating hematopoietic cell fate decisions with the extrinsic cues that elicited them.

Engineering the hematopoietic stem cell niche: Frontiers in biomaterial science

Hematopoietic stem cells (HSCs) play a crucial role in the generation of the body's blood and immune cells. This process takes place primarily in the bone marrow in specialized 'niche' microenvironments, which provide signals responsible for maintaining a balance between HSC quiescence, self-renewal, and lineage specification required for life-long hematopoiesis. While our understanding of these signaling mechanisms continues to improve, our ability to engineer them in vitro for the expansion of clinically relevant HSC populations is still lacking. In this review, we focus on development of biomaterials-based culture platforms for in vitro study of interactions between HSCs and their local microenvironment. The tools and techniques used for both examining HSC-niche interactions as well as applying these findings towards controlled HSC expansion or directed differentiation in 2D and 3D platforms are discussed. These novel techniques hold the potential to push the existing boundaries of HSC cultures towards high-throughput, real-time, and single-cell level biomimetic approaches that enable a more nuanced understanding of HSC regulation and function. Their application in conjunction with innovative biomaterial platforms can pave the way for engineering artificial bone marrow niches for clinical applications as well as elucidating the pathology of blood-related cancers and disorders.

Towards unravelling the kinetics of an acute myeloid leukaemia model system under oxidative and starvation stress: a comparison between two- and three-dimensional cultures

A great challenge when conducting ex vivo studies of leukaemia is the construction of an appropriate experimental platform that would recapitulate the bone marrow (BM) environment. Such a 3D scaffold system has been previously developed in our group [1]. Additionally to the BM architectural characteristics, parameters such as oxygen and glucose concentration are crucial as their value could differ between patients as well as within the same patient at different stages of treatment, consequently affecting the resistance of leukaemia to chemotherapy. The effect of oxidative and glucose stress-at levels close to human physiologic ones-on the proliferation and metabolic evolution of an AML model system (K-562 cell line) in conventional 2D cultures as well as in 3D scaffolds were studied. We observed that the K-562 cell line can proliferate and remain alive for 2 weeks in medium with glucose close to physiological levels both in 20 and 5% O2. We report interesting differences on the cellular response to the environmental, i.e., oxidative and/or nutritional stress stimuli in 2D and 3D. Higher adaptation to oxidative stress under non-starving conditions is observed in the 3D system. The glucose level in the medium has more impact on the cellular proliferation in the 3D compared to the 2D system. These differences can be of significant importance both when applying chemotherapy in vitro and also when constructing mathematical tools for optimisation of disease treatment.

Space constraints govern fate of hematopoietic stem and progenitor cells in vitro

Deciphering exogenous cues that determine stem cell fate decisions is a persisting challenge of cell biology and bioengineering. In an effort to unravel the role of spatial constraints in the cell-instructive characteristics of bone marrow microenvironments, murine hematopoietic stem and progenitor cells (HSPC) were exposed to fibronectin-coated microcavities in vitro. Microcavity sizes were chosen to allow for the inclusion of either individual or multiple cells. Repopulation experiments using lethally irradiated mice showed that the maintenance of functional HSPC in culture critically depends on cavity dimensions. Short-term repopulating hematopoietic stem cells (ST-HSC) were found to be best supported within single-cell sized compartments while long-term repopulating HSC (LT-HSC) were maintained within both cavity sizes. In sum, the reported data reveal spatial restriction to be a simple but powerful means for directing HSPC fate ex vivo.

Cardiac stem cell niches

The critical role that stem cell niches have in cardiac homeostasis and myocardial repair following injury is the focus of this review. Cardiac niches represent specialized microdomains where the quiescent and activated state of resident stem cells is regulated. Alterations in niche function with aging and cardiac diseases result in abnormal sites of cardiomyogenesis and inadequate myocyte formation. The relevance of Notch1 signaling, gap-junction formation, HIF-1α and metabolic state in the regulation of stem cell growth and differentiation within the cardiac niches are discussed.

Biophysical regulation of hematopoietic stem cells

Blood is renewed throughout the entire life. The stem cells of the blood, called hematopoietic stem cells (HSCs), are responsible for maintaining a supply of all types of fresh blood cells. In contrast to other stem cells, the clinical application of these cells is well established and HSC transplantation is an established life-saving therapy for patients suffering from haematological disorders. Despite their efficient functionality throughout life in vivo, controlling HSC behaviour in vitro (including their proliferation and differentiation) is still a major task that has not been resolved with standard cell culture systems. Targeted HSC multiplication in vitro could be beneficial for many patients, because HSC supply is limited. The biology of these cells and their natural microenvironment - their niche - remain a matter of ongoing research. In recent years, evidence has come to light that HSCs are susceptible to physical stimuli. This makes the regulation of HSCs by engineering physical parameters a promising approach for the targeted manipulation of these cells for clinical applications. Nevertheless, the biophysical regulation of these cells is still poorly understood. This review sheds light on the role of biophysical parameters in HSC biology and outlines which knowledge on biophysical regulation identified in other cell types could be applied to HSCs.

Covalent immobilization of stem cell factor and stromal derived factor 1α for in vitro culture of hematopoietic progenitor cells

Hematopoietic stem cells (HSCs) are currently utilized in the treatment of blood diseases, but widespread application of HSC therapeutics has been hindered by the limited availability of HSCs. With a better understanding of the HSC microenvironment and the ability to precisely recapitulate its components, we may be able to gain control of HSC behavior. In this work we developed a novel, biomimetic PEG hydrogel material as a substrate for this purpose and tested its potential with an anchorage-independent hematopoietic cell line, 32D clone 3 cells. We immobilized a fibronectin-derived adhesive peptide sequence, RGDS; a cytokine critical in HSC self-renewal, stem cell factor (SCF); and a chemokine important in HSC homing and lodging, stromal derived factor 1α (SDF1α), onto the surfaces of poly(ethylene glycol) (PEG) hydrogels. To evaluate the system's capabilities, we observed the effects of the biomolecules on 32D cell adhesion and morphology. We demonstrated that the incorporation of RGDS onto the surfaces promotes 32D cell adhesion in a dose-dependent fashion. We also observed an additive response in adhesion on surfaces with RGDS in combination with either SCF or SDF1α. In addition, the average cell area increased and circularity decreased on gel surfaces containing immobilized SCF or SDF1α, indicating enhanced cell spreading. By recapitulating aspects of the HSC microenvironment using a PEG hydrogel scaffold, we have shown the ability to control the adhesion and spreading of the 32D cells and demonstrated the potential of the system for the culture of primary hematopoietic cell populations.

Regulation of hematopoietic stem cell behavior by the nanostructured presentation of extracellular matrix components

Hematopoietic stem cells (HSCs) are maintained in stem cell niches, which regulate stem cell fate. Extracellular matrix (ECM) molecules, which are an essential part of these niches, can actively modulate cell functions. However, only little is known on the impact of ECM ligands on HSCs in a biomimetic environment defined on the nanometer-scale level. Here, we show that human hematopoietic stem and progenitor cell (HSPC) adhesion depends on the type of ligand, i.e., the type of ECM molecule, and the lateral, nanometer-scaled distance between the ligands (while the ligand type influenced the dependency on the latter). For small fibronectin (FN)-derived peptide ligands such as RGD and LDV the critical adhesive interligand distance for HSPCs was below 45 nm. FN-derived (FN type III 7-10) and osteopontin-derived protein domains also supported cell adhesion at greater distances. We found that the expression of the ECM protein thrombospondin-2 (THBS2) in HSPCs depends on the presence of the ligand type and its nanostructured presentation. Functionally, THBS2 proved to mediate adhesion of HSPCs. In conclusion, the present study shows that HSPCs are sensitive to the nanostructure of their microenvironment and that they are able to actively modulate their environment by secreting ECM factors.