Material Viscoelastic Properties Modulate the Mesenchymal Stem Cell Secretome for Applications in Hematopoietic Recovery

Manufacturing mesenchymal stromal cells in a microcarrier-microbioreactor platform can enhance cell yield and quality attributes: case study for acute respiratory distress syndrome

Mesenchymal stem and stromal cells (MSCs) hold potential to treat a broad range of clinical indications, but clinical translation has been limited to date due in part to challenges with batch-to-batch reproducibility of potential critical quality attributes (pCQAs) that can predict potency/efficacy. Here, we designed and implemented a microcarrier-microbioreactor approach to cell therapy manufacturing, specific to anchorage-dependent cells such as MSCs. We sought to assess whether increased control of the biochemical and biophysical environment had the potential to create product with consistent presentation and elevated expression of pCQAs relative to established manufacturing approaches in tissue culture polystyrene (TCPS) flasks. First, we evaluated total cell yield harvested from dissolvable, gelatin microcarriers within a microbioreactor cassette (Mobius Breez) or a flask control with matched initial cell seeding density and culture duration. Next, we identified 24 genes implicated in a therapeutic role for a specific motivating indication, acute respiratory distress syndrome (ARDS); expression of these genes served as our pCQAs for initial in vitro evaluation of product potency. We evaluated mRNA expression for three distinct donors to assess inter-donor repeatability, as well as for one donor in three distinct batches to assess within-donor, inter-batch variability. Finally, we assessed gene expression at the protein level for a subset of the panel to confirm successful translation. Our results indicated that MSCs expanded with this microcarrier-microbioreactor approach exhibited reasonable donor-to-donor repeatability and reliable batch-to-batch reproducibility of pCQAs. Interestingly, the baseline conditions of this microcarrier-microbioreactor approach also significantly improved expression of several key pCQAs at the gene and protein expression levels and reduced total media consumption relative to TCPS culture. This proof-of-concept study illustrates key benefits of this approach to therapeutic cell process development for MSCs and other anchorage-dependent cells that are candidates for cell therapies.

Cause and consequence of heterogeneity in human mesenchymal stem cells: Challenges in clinical application

Human mesenchymal stem cells (hMSCs) are mesoderm-derived adult stem cells with self-proliferation capacity, pluripotent differentiation potency, and excellent histocompatibility. These advantages make hMSCs a promising tool in clinical application. However, the majority of clinical trials using hMSC therapy for diverse human diseases do not achieve expectations, despite the prospective pre-clinical outcomes in animal models. This is partly attributable to the intrinsic heterogeneity of hMSCs. In this review, the cause of heterogeneity in hMSCs is systematically discussed at multiple levels, including isolation methods, cultural conditions, donor-to-donor variation, tissue sources, intra-tissue subpopulations, etc. Additionally, the effect of hMSCs heterogeneity on the contrary role in tumor progression and immunomodulation is also discussed. The attempts to understand the cellular heterogeneity of hMSCs and its consequences are important in supporting and improving therapeutic strategies for hMSCs.

Mechanical Unloading Promotes Osteoclastic Differentiation and Bone Resorption by Modulating the MSC Secretome to Favor Inflammation

Aging, space flight, and prolonged bed rest have all been linked to bone loss, and no effective treatments are clinically available at present. Here, with the rodent hindlimb unloading (HU) model, we report that the bone marrow (BM) microenvironment was significantly altered, with an increased number of myeloid cells and elevated inflammatory cytokines. In such inflammatory BM, the osteoclast-mediated bone resorption was greatly enhanced, leading to a shifted bone remodeling balance that ultimately ends up with disuse-induced osteoporosis. Using Piezo1 conditional knockout (KO) mice (Piezo1fl/fl;LepRCre), we proved that lack of mechanical stimuli on LepR+ mesenchymal stem cells (MSCs) is the main reason for the pathological BM inflammation. Mechanically, the secretome of MSCs was regulated by mechanical stimuli. Inadequate mechanical load leads to increased production of inflammatory cytokines, such as interleukin (IL)-1α, IL-6, macrophage colony-stimulating factor 1 (M-CSF-1), and so on, which promotes monocyte proliferation and osteoclastic differentiation. Interestingly, transplantation of 10% cyclic mechanical stretch (CMS)-treated MSCs into HU animals significantly alleviated the BM microenvironment and rebalanced bone remodeling. In summary, our research revealed a new mechanism underlying mechanical unloading-induced bone loss and suggested a novel stem cell-based therapy to potentially prevent disuse-induced osteoporosis.

A New Bioactive Fibrin Formulation Provided Superior Cartilage Regeneration in a Caprine Model

The effective and long-term treatment of cartilage defects is an unmet need among patients worldwide. In the past, several synthetic and natural biomaterials have been designed to support functional articular cartilage formation. However, they have mostly failed to enhance the terminal stage of chondrogenic differentiation, leading to scar tissue formation after the operation. Growth factors substantially regulate cartilage regeneration by acting on receptors to trigger intracellular signaling and cell recruitment for tissue regeneration. In this study, we investigated the effect of recombinant insulin-like growth factor 1 (rIGF-1), loaded in fibrin microbeads (FibIGF1), on cartilage regeneration. rIGF-1-loaded fibrin microbeads were injected into full-thickness cartilage defects in the knees of goats. The stability, integration, and quality of tissue repair were evaluated at 1 and 6 months by gross morphology, histology, and collagen type II staining. The in vivo results showed that compared to plain fibrin samples, particularly at 6 months, FibIGF1 improved the functional cartilage formation, confirmed through gross morphology, histology, and collagen type II immunostaining. FibIGF1 could be a promising candidate for cartilage repair in the clinic.

Acoustofluidic Interfaces for the Mechanobiological Secretome of MSCs

While mesenchymal stem cells (MSCs) have gained enormous attention due to their unique properties of self-renewal, colony formation, and differentiation potential, the MSC secretome has become attractive due to its roles in immunomodulation, anti-inflammatory activity, angiogenesis, and anti-apoptosis. However, the precise stimulation and efficient production of the MSC secretome for therapeutic applications are challenging problems to solve. Here, we report on Acoustofluidic Interfaces for the Mechanobiological Secretome of MSCs: AIMS. We create an acoustofluidic mechanobiological environment to form reproducible three-dimensional MSC aggregates, which produce the MSC secretome with high efficiency. We confirm the increased MSC secretome is due to improved cell-cell interactions using AIMS: the key mediator N-cadherin was up-regulated while functional blocking of N-cadherin resulted in no enhancement of the secretome. After being primed by IFN-γ, the secretome profile of the MSC aggregates contains more anti-inflammatory cytokines and can be used to inhibit the pro-inflammatory response of M1 phenotype macrophages, suppress T cell activation, and support B cell functions. As such, the MSC secretome can be modified for personalized secretome-based therapies. AIMS acts as a powerful tool for improving the MSC secretome and precisely tuning the secretory profile to develop new treatments in translational medicine.

Poly(ethylene glycol)-Based Hydrogel Microcarriers Alter Secretory Activity of Genetically Modified Mesenchymal Stromal Cells

In order to scale up culture therapeutic cells, such as mesenchymal stromal cells (MSCs), culture in suspension bioreactors using microcarriers (μCs) is preferred. However, the impact of microcarrier type on the resulting MSC secretory activity has not been investigated. In this study, two poly(ethylene glycol) hydrogel formulations with different swelling ratios (named "stiffer" and "softer") were fabricated as μC substrates to culture MSCs and MSCs genetically modified to express the interleukin-1 receptor antagonist (IL-1Ra-MSCs). Changes in cell number, secretory and angiogenic activity, and changes in MAPK signaling were evaluated when cultured on hydrogel μCs, as well as on tissue culture plastic-based Synthemax μCs. We demonstrated that culture on stiffer μCs increased secretion of IL-1Ra compared to culture on Synthemax μCs by IL-1Ra-MSCs by 1.2- to 1.6-fold, as well as their in vitro angiogenic activity, compared to culture on Synthemax μCs, while culture on both stiffer and softer μCs altered the secretion of several other factors compared to culture on Synthemax μCs. Changes in angiogenic activity corresponded with increased gene expression and secretion of hepatocyte growth factor by MSCs cultured on softer μCs by 2.5- to 6-fold compared to MSCs cultured on Synthemax μCs. Quantification of phosphoprotein signaling with the MAPK pathway revealed broad reduction of pathway activation by IL-1Ra-MSCs cultured on both stiffer and softer μCs compared to Synthemax, where phosphorylated c-Jun, ATF2, and MEK1 were reduced specifically on softer μCs. Overall, this study showed that μC surfaces can influence the secretory activity of genetically modified MSCs and identified associated changes in MAPK pathway signaling, which is a known central regulator of cytokine secretion.

Delivery of Stem Cell Secretome for Therapeutic Applications

Intensive studies on stem cell therapy reveal that benefits of stem cells attribute to the paracrine effects. Hence, direct delivery of stem cell secretome to the injured site shows the comparative therapeutic efficacy of living cells while avoiding the potential limitations. However, conventional systemic administration of stem cell secretome often leads to rapid clearance in vivo. Therefore, a variety of different biomaterials are developed for sustained and controllable delivery of stem cell secretome to improve therapeutic efficiency. In this review, we first introduce current approaches for the preparation and characterization of stem cell secretome as well as strategies to improve their therapeutic efficacy and production. The up-to-date delivery platforms are also summarized, including nanoparticles, injectable hydrogels, microneedles, and scaffold patches. Meanwhile, we discuss the underlying therapeutic mechanism of stem cell secretome for the treatment of various diseases. In the end, future opportunities and challenges are proposed.

Matrix biophysical cues direct mesenchymal stromal cell functions in immunity

Hydrogels have been used to design synthetic matrices that capture salient features of matrix microenvironments to study and control cellular functions. Recent advances in understanding of both extracellular matrix biology and biomaterial design have shown that biophysical cues are powerful mediators of cell biology, especially that of mesenchymal stromal cells (MSCs). MSCs have been tested in many clinical trials because of their ability to modulate immune cells in different pathological conditions. While roles of biophysical cues in MSC biology have been studied in the context of multilineage differentiation, their significance in regulating immunomodulatory functions of MSCs is just beginning to be elucidated. This review first describes design principles behind how biophysical cues in native microenvironments influence the ability of MSCs to regulate immune cell production and functions. We will then discuss how biophysical cues can be leveraged to optimize cell isolation, priming, and delivery, which can help improve the success of MSC therapy for immunomodulation. Finally, a perspective is presented on how implementing biophysical cues in MSC potency assay can be important in predicting clinical outcomes. STATEMENT OF SIGNIFICANCE: Stromal cells of mesenchymal origin are known to direct immune cell functions in vivo by secreting paracrine mediators. This property has been leveraged in developing mesenchymal stromal cell (MSC)-based therapeutics by adoptive transfer to treat immunological rejection and tissue injuries, which have been tested in over one thousand clinical trials to date, but with mixed success. Advances in biomaterial design have enabled precise control of biophysical cues based on how stromal cells interact with the extracellular matrix in microenvironments in situ. Investigators have begun to use this approach to understand how different matrix biophysical parameters, such as fiber orientation, porosity, dimensionality, and viscoelasticity impact stromal cell-mediated immunomodulation. The insights gained from this effort can potentially be used to precisely define the microenvironmental cues for isolation, priming, and delivery of MSCs, which can be tailored based on different disease indications for optimal therapeutic outcomes.

Culture Substrates for Improved Manufacture of Mesenchymal Stromal Cell Therapies

Recent developments in mesenchymal stromal cell (MSC) therapies have increased the demand for tools to improve their manufacture, including the selection of optimal culture substrate materials. While many clinical manufacturers use planar tissue culture plastic (TCP) surfaces for MSC production, others have begun exploring the use of alternative culture substrates that present a variety of spatial, mechanical, and biochemical cues that influence cell expansion and resulting cell quality. In this review, the effects of culture and material properties distinct from traditional planar TCP surfaces on MSC proliferation, surface marker expression, and commonly used indications for therapeutic potency are examined. The different properties summarized include the use of alternative culture formats such as cellular aggregates or 3D scaffolds, as well as the effects of culture substrate stiffness and presentation of specific adhesive ligands and topographical cues. Specific substrate properties can be related to greater cell expansion and improvement in specific therapeutic functionalities, demonstrating the utility of culture materials in further improving the clinical-scale manufacture of highly secretory MSC products.

Engineering the MSC Secretome: A Hydrogel Focused Approach

The therapeutic benefits of exogenously delivered mesenchymal stromal/stem cells (MSCs) have been largely attributed to their secretory properties. However, clinical translation of MSC-based therapies is hindered due to loss of MSC regenerative properties during large-scale expansion and low survival/retention post-delivery. These limitations might be overcome by designing hydrogel culture platforms to modulate the MSC microenvironment. Hydrogel systems could be engineered to i) promote MSC proliferation and maintain regenerative properties (i.e., stemness and secretion) during ex vivo expansion, ii) improve MSC survival, retention, and engraftment in vivo, and/or iii) direct the MSC secretory profile using tailored biochemical and biophysical cues. Herein, it is reviewed how hydrogel material properties (i.e., matrix modulus, viscoelasticity, dimensionality, cell adhesion, and porosity) influence MSC secretion, mediated through cell-matrix and cell-cell interactions. In addition, it is highlighted how biochemical cues (i.e., small molecules, peptides, and proteins) can improve and direct the MSC secretory profile. Last, the authors' perspective is provided on future work toward the understanding of how microenvironmental cues influence the MSC secretome, and designing the next generation of biomaterials, with optimized biophysical and biochemical cues, to direct the MSC secretory profile for improved clinical translation outcomes.

The Effect of Physical Cues of Biomaterial Scaffolds on Stem Cell Behavior

Stem cells have been sought as a promising cell source in the tissue engineering field due to their proliferative capacity as well as differentiation potential. Biomaterials have been utilized to facilitate the delivery of stem cells in order to improve their engraftment and long-term viability upon implantation. Biomaterials also have been developed as scaffolds to promote stem cell induced tissue regeneration. This review focuses on the latter where the biomaterial scaffold is designed to provide physical cues to stem cells in order to promote their behavior for tissue formation. Recent work that explores the effect of scaffold physical properties, topography, mechanical properties and electrical properties, is discussed. Although still being elucidated, the biological mechanisms, including cell shape, focal adhesion distribution, and nuclear shape, are presented. This review also discusses emerging areas and challenges in clinical translation.

Harnessing mesenchymal stem cell secretome: Effect of extracellular matrices on proangiogenic signaling

The low engraftment and retention rate of mesenchymal stem cells (MSCs) at the target site indicates that the potential benefits of MSC-based therapies can be attributed to their paracrine signaling. In this study, the extracellular matrices (ECMs) deposited by bone marrow-derived human MSCs in the presence and absence of ascorbic acid was characterized. MSCs were seeded on top of decellularized ECM (dECM) and the concentrations of proangiogenic and antiangiogenic molecules released in culture (conditioned) media was compared. Effects of ECM derived from MSCs with different passage numbers on MSC secretome was also investigated. Our study revealed that the expression of proangiogenesis-related factors were upregulated when MSCs were harvested on dECMs, irrespective of media supplementation, as compared with those cultured on tissue culture plates. In addition, dECM generated in the presence of ascorbic acid promoted the expression of proangiogenic molecules as compared with dECM-derived in absence of media supplementation. Further, it was observed that the effectiveness of dECM to stimulate proangiogenic signaling of MSCs was reduced as cell passage number was increased from P3 to P5. The proliferation as well as capillary morphogenesis of human umbilical vein endothelial cells (HUVECs) in the presence of conditioned media were enhanced compared with the normal HUVECs culture media. These data indicate that the secretory signatures of MSCs and consequently, the therapeutic efficacy of MSCs can be regulated by presentation of dECM composition and variation of its composition.

Soft extracellular matrix enhances inflammatory activation of mesenchymal stromal cells to induce monocyte production and trafficking

Mesenchymal stromal cells (MSCs) modulate immune cells to ameliorate multiple inflammatory pathologies. Biophysical signals that regulate this process are poorly defined. By engineering hydrogels with tunable biophysical parameters relevant to bone marrow where MSCs naturally reside, we show that soft extracellular matrix maximizes the ability of MSCs to produce paracrine factors that have been implicated in monocyte production and chemotaxis upon inflammatory stimulation by tumor necrosis factor-α (TNFα). Soft matrix increases clustering of TNF receptors, thereby enhancing NF-κB activation and downstream gene expression. Actin polymerization and lipid rafts, but not myosin-II contractility, regulate mechanosensitive activation of MSCs by TNFα. We functionally demonstrate that human MSCs primed with TNFα in soft matrix enhance production of human monocytes in marrow of xenografted mice and increase trafficking of monocytes via CCL2. The results suggest the importance of biophysical signaling in tuning inflammatory activation of stromal cells to control the innate immune system.

Improving hematopoietic recovery through modeling and modulation of the mesenchymal stromal cell secretome

BACKGROUND

Efficient and sustained hematopoietic recovery after hematopoietic stem cell or bone marrow transplantation is supported by paracrine signaling from specific subpopulations of mesenchymal stromal cells (MSCs). Here, we considered whether in vitro mechanopriming of human MSCs could be administered to predictively and significantly improve in vivo hematopoietic recovery after irradiation injury.

METHODS

First, we implemented regression modeling to identify eight MSC-secreted proteins that correlated strongly with improved rescue from radiation damage, including hematopoietic recovery, in a murine model of hematopoietic failure. Using these partial least squares regression (PLSR) model parameters, we then predicted recovery potential of MSC populations that were culture expanded on substrata of varying mechanical stiffness. Lastly, we experimentally validated these predictions using an in vitro co-culture model of hematopoiesis and using new in vivo experiments for the same irradiation injury model used to generate survival predictions.

RESULTS

MSCs grown on the least stiff (elastic moduli ~ 1 kPa) of these polydimethylsiloxane (PDMS) substrata secreted high concentrations of key proteins identified in regression modeling, at concentrations comparable to those secreted by minor subpopulations of MSCs shown previously to be effective in supporting such radiation rescue. We confirmed that these MSCs expanded on PDMS could promote hematopoiesis in an in vitro co-culture model with hematopoietic stem and progenitor cells (HSPCs). Further, MSCs cultured on PDMS of highest stiffness (elastic moduli ~ 100 kPa) promoted expression of CD123+ HSPCs, indicative of myeloid differentiation. Systemic administration of mechanoprimed MSCs resulted in improved mouse survival and weight recovery after bone marrow ablation, as compared with both standard MSC expansion on stiffer materials and with biophysically sorted MSC subpopulations. Additionally, we observed recovery of white blood cells, platelets, and red blood cells, indicative of complete recovery of all hematopoietic lineages.

CONCLUSIONS

These results demonstrate that computational techniques to identify MSC biomarkers can be leveraged to predict and engineer therapeutically effective MSC phenotypes defined by mechanoprimed secreted factors, for translational applications including hematopoietic recovery.