Influence of Microenvironment on Mesenchymal Stem Cell Therapeutic Potency: From Planar Culture to Microcarriers

Microcarrier-based stem cell bioprocessing: GMP-grade culture challenges and future trends for regenerative medicine

Recently, stem cell-based therapies have been proposed as an alternative for the treatment of many diseases. Stem cells (SCs) are well known for their capacity to preserve themselves, proliferate, and differentiate into multiple lineages. These characteristics allow stem cells to be a viable option for the treatment of diverse diseases. Traditional methodologies based on 2-dimensional culture techniques (T-flasks and Petri dishes) are simple and well standardized; however, they present disadvantages that limit the production of the cell yield required for regenerative medicine applications. Lately, microcarrier (MC)-based culture techniques have emerged as an attractive platform for expanding stem cells in suspension systems. Although the use of stem cell expansion on MCs has recently shown significant increase, their implementation for medical purposes is been hampered by bottlenecks in upstream and downstream processing. Therefore, there is an urgent need in the development of bioprocesses that simplify stem cell cultures under xeno-free conditions and detachment from MCs without diminishing their pluripotency and viability. A critical analysis of the factors that impact the up and downstream bioprocessing on MC-based stem cell cultures is presented in this review. This analysis aims to raise the awareness of the current drawbacks that limit MC-based stem cell bioprocessing in regenerative medicine and propose alternatives to overcome them.

Bioinspired in vitro microenvironments to control cell fate: focus on macromolecular crowding

The development of therapeutic regenerative medicine and accurate drug discovery cell-based products requires effective, with respect to obtaining sufficient numbers of viable, proliferative, and functional cell populations, cell expansion ex vivo. Unfortunately, traditional cell culture systems fail to recapitulate the multifaceted tissue milieu in vitro, resulting in cell phenotypic drift, loss of functionality, senescence, and apoptosis. Substrate-, environment-, and media-induced approaches are under intense investigation as a means to maintain cell phenotype and function while in culture. In this context, herein, the potential of macromolecular crowding, a biophysical phenomenon with considerable biological consequences, is discussed.

Multiomics analyses of cytokines, genes, miRNA, and regulatory networks in human mesenchymal stem cells expanded in stirred microcarrier-spinner cultures

Mesenchymal stem cells (MSCs) are of great clinical interest as a form of allogenic therapy due to their excellent regenerative and immunomodulatory effects for various therapeutic indications. Stirred suspension bioreactors using microcarriers (MC) have been used for large-scale production of MSCs compared to planar cultivation systems. Previously, we have demonstrated that expansion of MSCs in MC-spinner cultures improved chondrogenic, osteogenic, and cell migration potentials as compared to monolayer-static cultures. In this study, we sought to address this by analyzing global gene expression patterns, miRNA profiles and secretome under both monolayer-static and MC-spinner cultures in serum-free medium at different growth phases. The datasets revealed differential expression patterns that correlated with potentially improved MSC properties in cells from MC-spinner cultures compared to those of monolayer-static cultures. Transcriptome analysis identified a unique expression signature for cells from MC-spinner cultures, which correlated well with miRNA expression, and cytokine secretion involved in key MSC functions. Importantly, MC-spinner cultures and conditioned medium showed increased expression of factors that possibly enhance pathways of extracellular matrix dynamics, cellular metabolism, differentiation potential, immunoregulatory function, and wound healing. This systematic analysis provides insights for the efficient optimization of stem cell bioprocessing and infers that MC-based bioprocess manufacturing could improve post-expansion cellular properties for stem cell therapies.

Menaquinone-7 Supplementation Improves Osteogenesis in Pluripotent Stem Cell Derived Mesenchymal Stem Cells

Development of clinical stem cell interventions are hampered by immature cell progeny under current protocols. Human mesenchymal stem cells (hMSCs) are characterized by their ability to self-renew and differentiate into multiple lineages. Generating hMSCs from pluripotent stem cells (iPSCs) is an attractive avenue for cost-efficient and scalable production of cellular material. In this study we generate mature osteoblasts from iPSCs using a stable expandable MSC intermediate, refining established protocols. We investigated the timeframe and phenotype of cells under osteogenic conditions as well as the effect of menaquinone-7 (MK-7) on differentiation. From day 2 we noted a significant increase in RUNX2 expression under osteogenic conditions with MK-7, as well as decreases in ROS species production, increased cellular migration and changes to dynamics of collagen deposition when compared to differentiated cells that were not treated with MK-7. At day 21 OsteoMK-7 increased alkaline phosphatase activity and collagen deposition, as well as downregulated RUNX2 expression, suggesting to a mature cellular phenotype. Throughout we note no changes to expression of osteocalcin suggesting a non-canonical function of MK-7 in osteoblast differentiation. Together our data provide further mechanistic insight between basic and clinical studies on extrahepatic activity of MK-7. Our findings show that MK-7 promotes osteoblast maturation thereby increasing osteogenic differentiation.

Agitation in a Microcarrier-based Spinner Flask Bioreactor Modulates Homeostasis of Human Mesenchymal Stem Cells

Human mesenchymal stem cells (hMSCs) are well known in cell therapy due to their secretion of trophic factors, multipotent differentiation potential, and ability for self-renewal. As a result, the number of clinical trials has been steadily increasing over the last decade highlighting the need for in vitro systems capable of producing large quantities of cells to meet growing demands. However, hMSCs are highly sensitive to microenvironment conditions, including shear stress caused by dynamic bioreactor systems, and can lead to alteration of cellular homeostasis. In this study, hMSCs were expanded on microcarriers within a 125 mL spinner flask bioreactor system. Our results demonstrate a three-fold expansion over seven days. Furthermore, our results show that culturing hMSCs in the microcarrier-based suspension bioreactor (compared to static planar culture) results in smaller cell size and higher levels of reactive oxidative species (ROS) and ROS regulator Sirtuin-3, which have implications on the nicotinamide adenine dinucleotide metabolic pathway and metabolic homeostasis. In addition, hMSCs in the bioreactor showed the increased Prostaglandin E₂ secretion as well as reduced the Indoleamine-pyrrole 2,3-dioxygenase secretion upon stimulus with interferon gamma. The results of this study provide understanding of potential hMSC physiology alterations impacted by bioreactor microenvironment during scalable production of hMSCs for biomanufacturing and clinical trials.

Development of a Biodegradable Microcarrier for the Cultivation of Human Adipose Stem Cells (hASCs) with a Defined Xeno- and Serum-Free Medium

Stirred single-use bioreactors in combination with microcarriers (MCs) have established themselves as a technology that has the potential to meet the demands of current and future cell therapeutic markets. However, most of the published processes have been performed using fetal bovine serum (FBS) containing cell culture medium and non-biocompatible MCs. This approach has two significant drawbacks: firstly, the inevitable potential risks associated with the use of FBS for clinical applications; secondly, non-biocompatible MCs have to be removed from the cell suspension before implantation, requiring a step that causes loss of viable cells and adds further costs and complications. This study aimed to develop a new platform based on a chemically defined xeno- and serum-free cell culture medium and biodegradable MC that can support the growth of human adipose stem cells (hASCs) while still preserving their undifferentiated status. A specific combination of components and manufacturing parameters resulted in a MC prototype, called “BR44”, which delivered the desired functionality. MC BR44 allows the hASCs to stick to its surface and grow in a chemically defined xeno- and serum-free medium (UrSuppe). Although the cells’ expansion rate was not as high as with a commercial non-biodegradable standard MC, those cultured on BR44 maintained a better undifferentiated status in both static and dynamic conditions than those cultured on traditional 2D surfaces.

Xeno-Free Bioreactor Culture of Human Mesenchymal Stromal Cells on Chemically Defined Microcarriers

Human mesenchymal stromal cells (hMSC), also called mesenchymal stem cells, are adult cells that have demonstrated their potential in therapeutic applications, highlighted by their ability to differentiate down different lineages, modulate the immune system, and produce biologics. There is a pressing need for scalable culture systems for hMSC due to the large number of cells needed for clinical applications. Most current methods for expanding hMSC fail to provide a reproducible cell product in clinically required cell numbers without the use of serum-containing media or harsh enzymes. In this work, we apply a tailorable, thin, synthetic polymer coating-poly(poly(ethylene glycol) methyl ether methacrylate-ran-vinyl dimethyl azlactone-ran-glycidyl methacrylate) (P(PEGMEMA-r-VDM-r-GMA), PVG)-to the surface of commercially available polystyrene (PS) microcarriers to create chemically defined three-dimensional (3D) surfaces for large-scale cell expansion. These chemically defined microcarriers provide a reproducible surface that does not rely on the adsorption of xenogeneic serum proteins to mediate cell adhesion, enabling their use in xeno-free culture systems. Specifically, this work demonstrates the improved adhesion of hMSC to coated microcarriers over PS microcarriers in xeno-free media and describes their use in a readily scalable, bioreactor-based culture system. Additionally, these surfaces resist the adsorption of media-borne and cell-produced proteins, which result in integrin-mediated cell adhesion throughout the culture period. This feature allows the cells to be efficiently passaged from the microcarrier using a chemical chelating agent (ethylenediaminetetraacetic acid (EDTA)) in the absence of cleavage enzymes, an improvement over other microcarrier products in the field. Bioreactor culture of hMSC on these microcarriers enabled the production of hMSC over 4 days from a scalable, xeno-free environment.

Qualifying Osteogenic Potency Assay Metrics for Human Multipotent Stromal Cells: TGF-β2 a Telling Eligible Biomarker

Potency assays are critical for regenerative medicine, addressing the known challenge of functional heterogeneity among human multipotent stromal cells (hMSC). Necessary laboratory cell expansion allows analysis before implantation in the patient. Levels of induction of five signature gene biomarkers, ALPL, COL1A2, DCN, ELN and RUNX2, constituted a previously reported proof-of-principle osteogenic potency assay. We tested assay modification to enhance reproducibility using six consistent bone marrow derived hBM-MSC and explored applicability to three adipose tissue derived hAT-MSC. Using a potent proprietary osteogenic induction factor, the GUSB/YWAHZ reference gene pair provided real time PCR consistency. The novel assay conditions supported the concept that genes encoding extracellular matrix proteins one week after osteogenic induction were informative. Nonetheless, relatively low induction of COL1A2 and ELN encouraged search for additional biomarkers. TGFB2 mRNA induction, important for osteogenic commitment, was readily quantifiable in both hBM-MSC and hAT-MSC. Combined with DCN, TGFB2 mRNA induction data provided discriminatory power for resolving donor-specific heterogeneity. Histomorphometric decorin and TGF-β2 protein expression patterns in eight-week heterotopic bone implants also discriminated the two non-bone-forming hMSC. We highlight progress towards prompt osteogenic potency assays, needed by current clinical trials to accelerate improved intervention with enhanced stem cell therapy for serious bone fractures.

Cyclical aggregation extends in vitro expansion potential of human mesenchymal stem cells

Mesenchymal stem cell (MSC)-based therapy has shown great promises in various animal disease models. However, this therapeutic potency has not been well claimed when applied to human clinical trials. This is due to both the availability of MSCs at the time of administration and lack of viable expansion strategies. MSCs are very susceptible to in vitro culture environment and tend to adapt the microenvironment which could lead to cellular senescence and aging. Therefore, extended in vitro expansion induces loss of MSC functionality and its clinical relevance. To combat this effect, this work assessed a novel cyclical aggregation as a means of expanding MSCs to maintain stem cell functionality. The cyclical aggregation consists of an aggregation phase and an expansion phase by replating the dissociated MSC aggregates onto planar tissue culture surfaces. The results indicate that cyclical aggregation maintains proliferative capability, stem cell proteins, and clonogenicity, and prevents the acquisition of senescence. To determine why aggregation was responsible for this phenomenon, the integrated stress response pathway was probed with salubrial and GSK-2606414. Treatment with salubrial had no significant effect, while GSK-2606414 mitigated the effects of aggregation leading to in vitro aging. This method holds the potential to increase the clinical relevance of MSC therapeutic effects from small model systems (such as rats and mice) to humans, and may open the potential of patient-derived MSCs for treatment thereby removing the need for immunosuppression.

Designing topographically textured microparticles for induction and modulation of osteogenesis in mesenchymal stem cell engineering

Mesenchymal stem cells are the focus of intense research in bone development and regeneration. The potential of microparticles as modulating moieties of osteogenic response by utilizing their architectural features is demonstrated herein. Topographically textured microparticles of varying microscale features are produced by exploiting phase-separation of a readily soluble sacrificial component from polylactic acid. The influence of varying topographical features on primary human mesenchymal stem cell attachment, proliferation and markers of osteogenesis is investigated. In the absence of osteoinductive supplements, cells cultured on textured microparticles exhibit notably increased expression of osteogenic markers relative to conventional smooth microparticles. They also exhibit varying morphological, attachment and proliferation responses. Significantly altered gene expression and metabolic profiles are observed, with varying histological characteristics in vivo. This study highlights how tailoring topographical design offers cell-instructive 3D microenvironments which allow manipulation of stem cell fate by eliciting the desired downstream response without use of exogenous osteoinductive factors.

Bioreactor Parameters for Microcarrier-Based Human MSC Expansion under Xeno-Free Conditions in a Vertical-Wheel System

Human mesenchymal stem/stromal cells (hMSCs) have been investigated and proven to be a well-tolerated, safe therapy for a variety of indications, as shown by over 900 registered hMSC-based clinical trials. To meet the commercial demand for clinical manufacturing of hMSCs, production requires a scale that can achieve a lot size of ~100B cells, which requires innovative manufacturing technologies such as 3D bioreactors. A robust suspension bioreactor process that can be scaled-up to the relevant scale is therefore crucial. In this study, we developed a fed-batch, microcarrier-based bioreactor process, which enhances media productivity and drives a cost-effective and less labor-intensive hMSC expansion process. We determined parameter settings for various stages of the culture: inoculation, bioreactor culture, and harvest. Addition of a bioreactor feed, using a fed-batch approach, was necessary to replenish the mitogenic factors that were depleted from the media within the first 3 days of culture. Our study resulted in an optimized hMSC culture protocol that consistently achieved hMSC densities between 2 × 105-6 × 105 cells/mL within 5 days with no media exchange, maintaining the final cell population doubling level (PDL) at 16-20. Using multiple hMSC donors, we showed that this process was robust and yielded hMSCs that maintained expansion, phenotypic characteristic, and functional properties. The developed process in a vertical-wheel suspension bioreactor can be scaled to the levels needed to meet commercial demand of hMSCs.