Vasculogenic bio-synthetic hydrogel for enhancement of pancreatic islet engraftment and function in type 1 diabetes

VEGF-delivering PEG hydrogels promote vascularization in the porcine subcutaneous space

For cell therapies, the subcutaneous space is an attractive transplant site due to its large surface area and accessibility for implantation, monitoring, biopsy, and retrieval. However, its poor vascularization has catalyzed research to induce blood vessel formation within the site to enhance cell revascularization and survival. Most studies focus on the subcutaneous space of rodents, which does not recapitulate important anatomical features and vascularization responses of humans. Herein, we evaluate biomaterial-driven vascularization in the porcine subcutaneous space. Additionally, we report the first use of cost-effective fluorescent microspheres to quantify perfusion in the porcine subcutaneous space. We investigate the vascularization-inducing efficacy of vascular endothelial growth factor (VEGF)-delivering synthetic hydrogels based on 4-arm poly(ethylene) glycol macromers with terminal maleimides (PEG-4MAL). We compare three groups: a non-degradable hydrogel with a VEGF-releasing PEG-4MAL gel coating (Core+VEGF gel); an uncoated, non-degradable hydrogel (Core-only); and naïve tissue. After 2 weeks, Core+VEGF gel has significantly higher tissue perfusion, blood vessel area, blood vessel density, and number of vessels compared to both Core-only and naïve tissue. Furthermore, healthy vital signs during surgery and post-procedure metrics demonstrate the safety of hydrogel delivery. We demonstrate that VEGF-delivering synthetic hydrogels induce robust vascularization and perfusion in the porcine subcutaneous space.

Engineered 3D Hydrogel Matrices to Modulate Trophoblast Stem Cell-Derived Placental Organoid Phenotype

Placental organoid models are a promising platform to study human placental development and function. Organoid systems typically use naturally derived hydrogel extracellular matrices (ECM), resulting in batch-to-batch variability that limits experimental reproducibility. As an alternative, synthetic ECM-mimicking hydrogel matrices offer greater consistency and control over environmental cues. Here, we generated trophoblast stem cell-derived placental organoids using poly(ethylene glycol) (PEG) hydrogels with tunable degradability and placenta-derived ECM cues to evaluate trophoblast differentiation relative to Matrigel and two-dimensional (2D) culture controls. Our data demonstrate that PEG hydrogels support trophoblast viability and metabolic function comparable to gold standard Matrigel. Additionally, phenotypic characterization via proteomic analysis revealed that PEG and Matrigel matrices drive syncytiotrophoblast and extravillous trophoblast-dominant placental organoid phenotypes, respectively. Further, three-dimensional (3D) environments promoted greater integrin expression and ECM production than 2D culture. This study demonstrates that engineered 3D culture environments can be used to reliably generate placental organoids and guide trophoblast differentiation.

Encapsulation and immune protection for type 1 diabetes cell therapy

Type 1 Diabetes (T1D) involves the autoimmune destruction of insulin-producing β-cells in the pancreas. Exogenous insulin injections are the current therapy but are user-dependent and cannot fully recapitulate physiological insulin secretion dynamics. Since the emergence of allogeneic cell therapy for T1D, the Edmonton Protocol has been the most promising immunosuppression protocol for cadaveric islet transplantation, but the lack of donor islets, poor cell engraftment, and required chronic immunosuppression have limited its application as a therapy for T1D. Encapsulation in biomaterials on the nano-, micro-, and macro-scale offers the potential to integrate islets with the host and protect them from immune responses. This method can be applied to different cell types, including cadaveric, porcine, and stem cell-derived islets, mitigating the issue of a lack of donor cells. This review covers progress in the efforts to integrate insulin-producing cells from multiple sources to T1D patients as a form of cell therapy.

Recent Advances in Alginate-Based Hydrogels for Cell Transplantation Applications

With its exceptional biocompatibility, alginate emerged as a highly promising biomaterial for a large range of applications in regenerative medicine. Whether in the form of microparticles, injectable hydrogels, rigid scaffolds, or bioinks, alginate provides a versatile platform for encapsulating cells and fostering an optimal environment to enhance cell viability. This review aims to highlight recent studies utilizing alginate in diverse formulations for cell transplantation, offering insights into its efficacy in treating various diseases and injuries within the field of regenerative medicine.

Microfluidic vascular formation model for assessing angiogenic capacities of single islets

Pancreatic islet transplantation presents a promising therapy for individuals suffering from type 1 diabetes. To maintain the function of transplanted islets in vivo, it is imperative to induce angiogenesis. However, the mechanisms underlying angiogenesis triggered by islets remain unclear. In this study, we introduced a microphysiological system to study the angiogenic capacity and dynamics of individual islets. The system, which features an open-top structure, uniquely facilitates the inoculation of islets and the longitudinal observation of vascular formation in in vivo like microenvironment with islet-endothelial cell communication. By leveraging our system, we discovered notable islet-islet heterogeneity in the angiogenic capacity. Transcriptomic analysis of the vascularized islets revealed that islets with high angiogenic capacity exhibited upregulation of genes related to insulin secretion and downregulation of genes related to angiogenesis and fibroblasts. In conclusion, our microfluidic approach is effective in characterizing the vascular formation of individual islets and holds great promise for elucidating the angiogenic mechanisms that enhance islet transplantation therapy.

Controlling placental spheroid growth and phenotype using engineered synthetic hydrogel matrices

The human placenta is a complex organ comprised of multiple trophoblast subtypes, and inadequate models to study the human placenta in vitro limit the current understanding of human placental behavior and development. Common in vitro placental models rely on two-dimensional culture of cell lines and primary cells, which do not replicate the native tissue microenvironment, or poorly defined three-dimensional hydrogel matrices such as Matrigel™ that provide limited environmental control and suffer from high batch-to-batch variability. Here, we employ a highly defined, synthetic poly(ethylene glycol)-based hydrogel system with tunable degradability and presentation of extracellular matrix-derived adhesive ligands native to the placenta microenvironment to generate placental spheroids. We evaluate the capacity of a hydrogel library to support the viability, function, and phenotypic protein expression of three human trophoblast cell lines modeling varied trophoblast phenotypes and find that degradable synthetic hydrogels support the greatest degree of placental spheroid viability, proliferation, and function relative to standard Matrigel controls. Finally, we show that trophoblast culture conditions modulate cell functional phenotype as measured by proteomics analysis and functional secretion assays. Engineering precise control of placental spheroid development in vitro may provide an important new tool for the study of early placental behavior and development.

Hydrogel degradation promotes angiogenic and regenerative potential of cell spheroids for wound healing

Chronic nonhealing wounds are debilitating and diminish one's quality of life, necessitating the development of improved strategies for effective treatment. Biomaterial- and cell-based therapies offer an alternative treatment compared to conventional wound care for regenerating damaged tissues. Cell-based approaches frequently utilize endothelial cells (ECs) to promote vascularization and mesenchymal stromal cells (MSCs) for their potent secretome that promotes host cell recruitment. Spheroids have improved therapeutic potential over monodisperse cells, while degradable scaffolds can influence cellular processes conducive to long-term tissue regeneration. However, the role of biomaterial degradation on the therapeutic potential of heterotypic EC-MSC spheroids for wound healing is largely unknown. We formed poly(ethylene) glycol (PEG) hydrogels with varying ratios of matrix metalloproteinase (MMP)-degradable and non-degradable crosslinkers to develop three distinct constructs - fully degradable, partially degradable, and non-degradable - and interrogate the influence of degradation rate on engineered cell carriers for wound healing. We found that the vulnerability to degradation was critical for cellular proliferation, while inhibition of degradation impaired spheroid metabolic activity. Higher concentrations of degradable crosslinker promoted robust cell spreading, outgrowth, and secretion of proangiogenic cytokines (i.e., VEGF, HGF) that are critical in wound healing. The degree of degradation dictated the unique secretory profile of spheroids. When applied to a clinically relevant full-thickness ex vivo skin model, degradable spheroid-loaded hydrogels restored stratification of the epidermal layer, confirming the efficacy of scaffolds to promote wound healing. These results highlight the importance of matrix remodeling and its essential role in the therapeutic potential of heterotypic spheroids.

Hydrolytic hydrogels tune mesenchymal stem cell persistence and immunomodulation for enhanced diabetic cutaneous wound healing

Diabetes is associated with an altered global inflammatory state with impaired wound healing. Mesenchymal stem/stromal cells (MSC) are being explored for treatment of diabetic cutaneous wounds due to their regenerative properties. These cells are commonly delivered by injection, but the need to prolong the retention of MSC at sites of injury has spurred the development of biomaterial-based MSC delivery vehicles. However, controlling biomaterial degradation rates in vivo remains a therapeutic-limiting challenge. Here, we utilize hydrolytically degradable ester linkages to engineer synthetic hydrogels with tunable in vivo degradation kinetics for temporally controlled delivery of MSC. In vivo hydrogel degradation rate can be controlled by altering the ratio of ester to amide linkages in the hydrogel macromers. These hydrolytic hydrogels degrade at rates that enable unencumbered cutaneous wound healing, while enhancing the local persistence MSC compared to widely used protease-degradable hydrogels. Furthermore, hydrogel-based delivery of MSC modulates local immune responses and enhances cutaneous wound repair in diabetic mice. This study introduces a simple strategy for engineering tunable degradation modalities into synthetic biomaterials, overcoming a key barrier to their use as cell delivery vehicles.

Staphylococcal protein A-modified hydrogel facilitates in situ immunomodulation by capturing anti-HMGB1 for islet grafts

Islet transplantation is regarded as the most promising therapy for type 1 diabetes. However, both hypoxia and immune attack impair the grafted islets after transplantation, eventually failing the islet graft. Although many studies showed that biomaterials with nanoscale pores, like hydrogels, could protect islets from immune cells, the pores on biomaterials inhibited vascular endothelial cells (VECs) to creep in, which resulted in poor revascularization. Thus, a hydrogel device that can facilitate in situ immune modulations without the cost of poor revascularization should be put forward. Accordingly, we designed a spA-modified hydrogel capturing anti-HMGB1 mAB (mAB-spA Gel): the Staphylococcus aureus protein A (spA) was conjugated on the network of hydrogel to capture anti-HMGB1mAB which can inactivate immune cells, while the pore sizes of the hydrogel were more than 100μm which allows vascular endothelial cells (VECs) to creep in. In this study, we screened the optimal spA concentration in mAB-spA Gel according to the physical properties and antibody binding capability, then demonstrated that it could facilitate in situ immunomodulation without decreasing the vessel reconstruction in vitro. Further, we transplanted islet graft in vivo and showed that the survival of islets was elongated. In conclusion, mAB-spA Gel provided an alternative islet encapsulation strategy for type 1 diabetes. STATEMENT OF SIGNIFICANCE: Although various studies have shown that the backbone of the hydrogels can isolate islets grafts from immune cells and the survival of the islets can be prolonged by this way, it is also reported that when the pore size of the backbone is too small the revascularization will be adversely affected. According to this point, it is hard to adjust hydrogel's pore size to protect the islets from the immune attack while allowing endothelial vascular cells to creep in. To solve this dilemma, we designed an immunomodulatory hydrogel inhibiting the activation of T cells by immunosuppressive IgGs instead of the backbone network, so the hydrogel can prolong the survival of islets without the sacrifice of revascularization.

Mimicking Molecular Pathways in the Design of Smart Hydrogels for the Design of Vascularized Engineered Tissues

Biomaterials are pivotal in supporting and guiding vascularization for therapeutic applications. To design effective, bioactive biomaterials, understanding the cellular and molecular processes involved in angiogenesis and vasculogenesis is crucial. Biomaterial platforms can replicate the interactions between cells, the ECM, and the signaling molecules that trigger blood vessel formation. Hydrogels, with their soft and hydrated properties resembling natural tissues, are widely utilized; particularly synthetic hydrogels, known for their bio-inertness and precise control over cell-material interactions, are utilized. Naturally derived and synthetic hydrogel bases are tailored with specific mechanical properties, controlled for biodegradation, and enhanced for cell adhesion, appropriate biochemical signaling, and architectural features that facilitate the assembly and tubulogenesis of vascular cells. This comprehensive review showcases the latest advancements in hydrogel materials and innovative design modifications aimed at effectively guiding and supporting vascularization processes. Furthermore, by leveraging this knowledge, researchers can advance biomaterial design, which will enable precise support and guidance of vascularization processes and ultimately enhance tissue functionality and therapeutic outcomes.

Tuning the Microenvironment to Create Functionally Distinct Mesenchymal Stromal Cell Spheroids

Mesenchymal stromal cells (MSCs) are under investigation for wound healing and tissue regeneration due to their potent secretome. Compared to monodisperse cells, MSC spheroids exhibit increased cell survival and enhanced secretion of endogenous factors such as vascular endothelial growth factor (VEGF) and prostaglandin E2 (PGE2), two key factors in wound repair. We previously upregulated the proangiogenic potential of homotypic MSC spheroids by manipulating microenvironmental culture conditions. However, this approach depends on the responsiveness of host endothelial cells (ECs)-a limitation when attempting to restore large tissue deficits and for patients with chronic wounds in which ECs are dysfunctional and unresponsive. To address this challenge, we used a Design of Experiments (DOE) approach to engineer functionally distinct MSC spheroids that maximize VEGF production (VEGFMAX) or PGE2 production (PGE2,MAX) while incorporating ECs that could serve as the basic building blocks for vessel formation. VEGFMAX produced 22.7-fold more VEGF with enhanced endothelial cell migration compared to PGE2,MAX, while PGE2,MAX produced 16.7-fold more PGE2 with accelerated keratinocyte migration compared to VEGFMAX. When encapsulated together in engineered protease-degradable hydrogels as a model of cell delivery, VEGFMAX and PGE2,MAX spheroids exhibited robust spreading into the biomaterial and enhanced metabolic activity. The distinct bioactivities of these MSC spheroids demonstrate the highly tunable nature of spheroids and provide a new approach to leverage the therapeutic potential of cell-based therapies.

Recent advances in organoid engineering: A comprehensive review

Organoid, a 3D structure derived from various cell sources including progenitor and differentiated cells that self-organize through cell-cell and cell-matrix interactions to recapitulate the tissue/organ-specific architecture and function in vitro. The advancement of stem cell culture and the development of hydrogel-based extracellular matrices (ECM) have made it possible to derive self-assembled 3D tissue constructs like organoids. The ability to mimic the actual physiological conditions is the main advantage of organoids, reducing the excessive use of animal models and variability between animal models and humans. However, the complex microenvironment and complex cellular structure of organoids cannot be easily developed only using traditional cell biology. Therefore, several bioengineering approaches, including microfluidics, bioreactors, 3D bioprinting, and organoids-on-a-chip techniques, are extensively used to generate more physiologically relevant organoids. In this review, apart from organoid formation and self-assembly basics, the available bioengineering technologies are extensively discussed as solutions for traditional cell biology-oriented problems in organoid cultures. Also, the natural and synthetic hydrogel systems used in organoid cultures are discussed when necessary to highlight the significance of the stem cell microenvironment. The selected organoid models and their therapeutic applications in drug discovery and disease modeling are also presented.

Type 1 diabetes and engineering enhanced islet transplantation

The development of new therapeutic approaches to treat type 1 diabetes mellitus (T1D) relies on the precise understanding and deciphering of insulin-secreting β-cell biology, as well as the mechanisms responsible for their autoimmune destruction. β-cell or islet transplantation is viewed as a potential long-term therapy for the millions of patients with diabetes. To advance the field of insulin-secreting cell transplantation, two main research areas are currently investigated by the scientific community: (1) the identification of the developmental pathways that drive the differentiation of stem cells into insulin-producing cells, providing an inexhaustible source of cells; and (2) transplantation strategies and engineered transplants to provide protection and enhance the functionality of transplanted cells. In this review, we discuss the biology of pancreatic β-cells, pathology of T1D and current state of β-cell differentiation. We give a comprehensive view and discuss the different possibilities to engineer enhanced insulin-secreting cell/islet transplantation from a translational perspective.

IL-10-Functionalized Hydrogels Support Immunosuppressive Dendritic Cell Phenotype and Function

Biomaterial systems such as hydrogels enable localized delivery and postinjection modulation of cellular therapies in a wide array of contexts. Biomaterials as adjuvants have been an active area of investigation, but the study of functionalized biomaterials supporting immunosuppressive cell therapies for tolerogenic applications is still nascent. Here, we developed a 4-arm poly(ethylene-glycol)-maleimide (PEG-4MAL) hydrogel functionalized with interleukin-10 (IL-10) to improve the local delivery and efficacy of a cell therapy against autoimmune disease. The biophysical and biochemical properties of PEG-4MAL hydrogels were optimized to support dendritic cell (DC) viability and an immature phenotype. IL-10-functionalized PEG-4MAL (PEG-IL10) hydrogels exhibited controlled IL-10 release, extended the duration of DC support, and protected DCs from inflammatory assault. After incorporation in PEG-IL10 hydrogels, these DCs induced CD25+FoxP3+ regulatory T cells (Tregs) during in vitro coculture. These studies serve as a proof-of-concept for improving the efficacy of immunosuppressive cell therapies through biomaterial delivery. The flexible nature of this system enables its widespread application across a breadth of other tolerogenic applications for future investigation.

Adhesive peptide and polymer density modulate 3D cell traction forces within synthetic hydrogels

Cell-extracellular matrix forces provide pivotal signals regulating diverse physiological and pathological processes. Although mechanobiology has been widely studied in two-dimensional configurations, limited research has been conducted in three-dimensional (3D) systems due to the complex nature of mechanics and cellular behaviors. In this study, we established a platform integrating a well-defined synthetic hydrogel system (PEG-4MAL) with 3D traction force microscopy (TFM) methodologies to evaluate deformation and force responses within synthetic microenvironments, providing insights that are not tractable using biological matrices because of the interdependence of biochemical and biophysical properties and complex mechanics. We dissected the contributions of adhesive peptide density and polymer density, which determines hydrogel stiffness, to 3D force generation for fibroblasts. A critical threshold of adhesive peptide density at a constant matrix elasticity is required for cells to generate 3D forces. Furthermore, matrix displacements and strains decreased with matrix stiffness whereas stresses, and tractions increased with matrix stiffness until reaching constant values at higher stiffness values. Finally, Rho-kinase-dependent contractility and vinculin expression are required to generate significant 3D forces in both collagen and synthetic hydrogels. This research establishes a tunable platform for the study of mechanobiology and provides new insights into how cells sense and transmit forces in 3D.

A Novel ex vivo Method for Investigating Vascularization of Transplanted Islets

Revascularization of transplanted pancreatic islets is critical for survival and treatment of type 1 diabetes. Questions concerning how islets influence local microvascular networks and how networks form connections with islets remain understudied and motivate the need for new models that mimic the complexity of real tissue. Recently, our laboratory established the rat mesentery culture model as a tool to investigate cell dynamics involved in microvascular growth. An advantage is the ability to observe blood vessels, lymphatics, and immune cells. The objective of this study was to establish the rat mesentery tissue culture model as a useful tool to investigate islet tissue integration. DiI-labeled islets were seeded onto adult rat mesentery tissues and cultured for up to 3 days. Live lectin labeling enabled time-lapse observation of vessel growth. During culture, DiI-positive islets remained intact. Radial lectin-positive capillary sprouts with DiI labeling were observed to form from islets and connect to host networks. Lectin-positive vessels from host networks were also seen growing toward islets. PECAM and NG2 labeling confirmed that vessels sprouting from islets contained endothelial cells and pericytes. Our results introduce the rat mesentery culture model as a platform for investigating dynamics associated with the initial revascularization of transplanted islets.

Synthetic Matrix Scaffolds Engineer the In Vivo Tumor Immune Microenvironment for Immunotherapy Screening

Immunotherapy has emerged as one of the most powerful anti-cancer therapies but is stymied by the limits of existing preclinical models with respect to disease latency and reproducibility. Additionally, the influence of differing immune microenvironments within tumors observed clinically and associated with immunotherapeutic resistance cannot be tuned to facilitate drug testing workflows without changing model system or laborious genetic approaches. To address this testing platform gap in the immune oncology drug development pipeline, the authors deploy engineered biomaterials as scaffolds to increase tumor formation rate, decrease disease latency, and diminish variability of immune infiltrates into tumors formed from murine mammary carcinoma cell lines implanted into syngeneic mice. By altering synthetic gel formulations that reshape infiltrating immune cells within the tumor, responsiveness of the same tumor model to varying classes of cancer immunotherapies, including in situ vaccination with a molecular adjuvant and immune checkpoint blockade, diverge. These results demonstrate the significant role the local immune microenvironment plays in immunotherapeutic response. These engineered tumor immune microenvironments therefore improve upon the limitations of current breast tumor models used for immune oncology drug screening to enable immunotherapeutic testing relevant to the variability in tumor immune microenvironments underlying immunotherapeutic resistance seen in human patients.

Analyzing immune response to engineered hydrogels by hierarchical clustering of inflammatory cell subsets

Understanding the immune response to hydrogel implantation is critical for the design of immunomodulatory biomaterials. To study the progression of inflammation around poly(ethylene glycol) hydrogels presenting Arg-Gly-Asp (RGD) peptides and vascular endothelial growth factor, we used temporal analysis of high-dimensional flow cytometry data paired with intravital imaging, immunohistochemistry, and multiplexed proteomic profiling. RGD-presenting hydrogels created a reparative microenvironment promoting CD206+ cellular infiltration and revascularization in wounded dorsal skin tissue. Unbiased clustering algorithms (SPADE) revealed significant phenotypic transition shifts as a function of the cell-adhesion hydrogel properties. SPADE identified an intermediate macrophage subset functionally regulating in vivo cytokine secretion that was preferentially recruited for RGD-presenting hydrogels, whereas dendritic cell subsets were preferentially recruited to RDG-presenting hydrogels. Last, RGD-presenting hydrogels controlled macrophage functional cytokine secretion to direct polarization and vascularization. Our studies show that unbiased clustering of single-cell data provides unbiased insights into the underlying immune response to engineered materials.

Star polymer networks: a toolbox for cross-linked polymers with controlled structure

The synthesis of precisely controlled polymer networks has been a long-cherished dream of polymer scientists. Traditional random cross-linking strategies often lead to uncontrolled networks with various kinds of defects. Recent...

Targeting the extracellular matrix for immunomodulation: applications in drug delivery and cell therapies

Host immune cells interact bi-directionally with their extracellular matrix (ECM) to receive and deposit molecular signals, which orchestrate cellular activation, proliferation, differentiation, and function to maintain healthy tissue homeostasis. In response to pathogens or damage, immune cells infiltrate diseased sites and synthesize critical ECM molecules such as glycoproteins, proteoglycans, and glycosaminoglycans to promote healing. When the immune system misidentifies pathogens or fails to survey damaged cells effectively, maladies such as chronic inflammation, autoimmune diseases, and cancer can develop. In these conditions, it is essential to restore balance to the body through modulation of the immune system and the ECM. This review details the components of dysregulated ECM implicated in pathogenic environments and therapeutic approaches to restore tissue homeostasis. We evaluate emerging strategies to overcome inflamed, immune inhibitory, and otherwise diseased microenvironments, including mechanical stimulation, targeted proteases, adoptive cell therapy, mechanomedicine, and biomaterial-based cell therapeutics. We highlight various strategies that have produced efficacious responses in both pre-clinical and human trials and identify additional opportunities to develop next-generation interventions. Significantly, we identify a need for therapies to address dense or fibrotic tissue for the treatment of organ tissue damage and various cancer subtypes. Finally, we conclude that therapeutic techniques that disrupt, evade, or specifically target the pathogenic microenvironment have a high potential for improving therapeutic outcomes and should be considered a priority for immediate exploration. A schematic showing the various methods of extracellular matrix disruption/targeting in both fibrotic and cancerous environments. a Biomaterial-based cell therapy can be used to deliver anti-inflammatory cytokines, chemotherapeutics, or other factors for localized, slow release of therapeutics. b Mechanotherapeutics can be used to inhibit the deposition of molecules such as collagen that affect stiffness. c Ablation of the ECM and target tissue can be accomplished via mechanical degradation such as focused ultrasound. d Proteases can be used to improve the distribution of therapies such as oncolytic virus. e Localization of therapeutics such as checkpoint inhibitors can be improved with the targeting of specific ECM components, reducing off-target effects and toxicity.

In Vitro Disease Models of the Endocrine Pancreas

The ethical constraints and shortcomings of animal models, combined with the demand to study disease pathogenesis under controlled conditions, are giving rise to a new field at the interface of tissue engineering and pathophysiology, which focuses on the development of in vitro models of disease. In vitro models are defined as synthetic experimental systems that contain living human cells and mimic tissue- and organ-level physiology in vitro by taking advantage of recent advances in tissue engineering and microfabrication. This review provides an overview of in vitro models and focuses specifically on in vitro disease models of the endocrine pancreas and diabetes. First, we briefly review the anatomy, physiology, and pathophysiology of the human pancreas, with an emphasis on islets of Langerhans and beta cell dysfunction. We then discuss different types of in vitro models and fundamental elements that should be considered when developing an in vitro disease model. Finally, we review the current state and breakthroughs in the field of pancreatic in vitro models and conclude with some challenges that need to be addressed in the future development of in vitro models.

Designing biomaterials for the modulation of allogeneic and autoimmune responses to cellular implants in Type 1 Diabetes

The effective suppression of adaptive immune responses is essential for the success of allogeneic cell therapies. In islet transplantation for Type 1 Diabetes, pre-existing autoimmunity provides an additional hurdle, as memory autoimmune T cells mediate both an autoantigen-specific attack on the donor beta cells and an alloantigen-specific attack on the donor graft cells. Immunosuppressive agents used for islet transplantation are generally successful in suppressing alloimmune responses, but dramatically hinder the widespread adoption of this therapeutic approach and fail to control memory T cell populations, which leaves the graft vulnerable to destruction. In this review, we highlight the capacity of biomaterials to provide local and nuanced instruction to suppress or alter immune pathways activated in response to an allogeneic islet transplant. Biomaterial immunoisolation is a common approach employed to block direct antigen recognition and downstream cell-mediated graft destruction; however, immunoisolation alone still permits shed donor antigens to escape into the host environment, resulting in indirect antigen recognition, immune cell activation, and the creation of a toxic graft site. Designing materials to decrease antigen escape, improve cell viability, and increase material compatibility are all approaches that can decrease the local release of antigen and danger signals into the implant microenvironment. Implant materials can be further enhanced through the local delivery of anti-inflammatory, suppressive, chemotactic, and/or tolerogenic agents, which serve to control both the innate and adaptive immune responses to the implant with a benefit of reduced systemic effects. Lessons learned from understanding how to manipulate allogeneic and autogenic immune responses to pancreatic islets can also be applied to other cell therapies to improve their efficacy and duration. STATEMENT OF SIGNIFICANCE: This review explores key immunologic concepts and critical pathways mediating graft rejection in Type 1 Diabetes, which can instruct the future purposeful design of immunomodulatory biomaterials for cell therapy. A summary of immunological pathways initiated following cellular implantation, as well as current systemic immunomodulatory agents used, is provided. We then outline the potential of biomaterials to modulate these responses. The capacity of polymeric encapsulation to block some powerful rejection pathways is covered. We also highlight the role of cellular health and biocompatibility in mitigating immune responses. Finally, we review the use of bioactive materials to proactively modulate local immune responses, focusing on key concepts of anti-inflammatory, suppressive, and tolerogenic agents.

An Overview of Engineered Hydrogel-Based Biomaterials for Improved β-Cell Survival and Insulin Secretion

Islet transplantation provides a promising strategy in treating type 1 diabetes as an autoimmune disease, in which damaged β-cells are replaced with new islets in a minimally invasive procedure. Although islet transplantation avoids the complications associated with whole pancreas transplantations, its clinical applications maintain significant drawbacks, including long-term immunosuppression, a lack of compatible donors, and blood-mediated inflammatory responses. Biomaterial-assisted islet transplantation is an emerging technology that embeds desired cells into biomaterials, which are then directly transplanted into the patient, overcoming the aforementioned challenges. Among various biomaterials, hydrogels are the preferred biomaterial of choice in these transplants due to their ECM-like structure and tunable properties. This review aims to present a comprehensive overview of hydrogel-based biomaterials that are engineered for encapsulation of insulin-secreting cells, focusing on new hydrogel design and modification strategies to improve β-cell viability, decrease inflammatory responses, and enhance insulin secretion. We will discuss the current status of clinical studies using therapeutic bioengineering hydrogels in insulin release and prospective approaches.

Prevascularized Micro-/Nano-Sized Spheroid/Bead Aggregates for Vascular Tissue Engineering

Efficient strategies to promote microvascularization in vascular tissue engineering, a central priority in regenerative medicine, are still scarce; nano- and micro-sized aggregates and spheres or beads harboring primitive microvascular beds are promising methods in vascular tissue engineering. Capillaries are the smallest type and in numerous blood vessels, which are distributed densely in cardiovascular system. To mimic this microvascular network, specific cell components and proangiogenic factors are required. Herein, advanced biofabrication methods in microvascular engineering, including extrusion-based and droplet-based bioprinting, Kenzan, and biogripper approaches, are deliberated with emphasis on the newest works in prevascular nano- and micro-sized aggregates and microspheres/microbeads.

Localized hydrogel delivery of dendritic cells for attenuation of multiple sclerosis in a murine model

In multiple sclerosis (MS), abnormally activated immune cells responsive to myelin proteins result in widespread damage throughout the central nervous system (CNS) and ultimately irreversible disability. Immunomodulation by delivering dendritic cells (DCs) utilizes a potent and rapid MS disease progression driver therapeutically. Here, we investigated delivering DCs for disease severity attenuation using an experimental autoimmune encephalomyelitis preclinical MS model. DCs treated with interleukin-10 (IL-10) (DC10s) were transplanted using in situ gelling poly(ethylene glycol)-based hydrogel for target site localization. DC delivery increased hydrogel longevity and altered the injection site recruited, endogenous immune cell profile within 2 days postinjection. Furthermore, hydrogel-mediated DC transplantation efficacy depended on the injection-site. DCs delivered to the neck local to MS-associated CNS-draining cervical lymph nodes attenuated paralysis, compared to untreated controls, while delivery to the flank did not alter paralysis severity. This study demonstrates that local delivery of DC10s modulates immune cell recruitment and attenuates disease progression in a preclinical model of MS.

A Hydrogel Strategy to Augment Tissue Adenosine to Improve Hindlimb Perfusion

[Figure: see text].

Islet organoid as a promising model for diabetes

Studies on diabetes have long been hampered by a lack of authentic disease models that, ideally, should be unlimited and able to recapitulate the abnormalities involved in the development, structure, and function of human pancreatic islets under pathological conditions. Stem cell-based islet organoids faithfully recapitulate islet development in vitro and provide large amounts of three-dimensional functional islet biomimetic materials with a morphological structure and cellular composition similar to those of native islets. Thus, islet organoids hold great promise for modeling islet development and function, deciphering the mechanisms underlying the onset of diabetes, providing an in vitro human organ model for infection of viruses such as SARS-CoV-2, and contributing to drug screening and autologous islet transplantation. However, the currently established islet organoids are generally immature compared with native islets, and further efforts should be made to improve the heterogeneity and functionality of islet organoids, making it an authentic and informative disease model for diabetes. Here, we review the advances and challenges in the generation of islet organoids, focusing on human pluripotent stem cell-derived islet organoids, and the potential applications of islet organoids as disease models and regenerative therapies for diabetes.

Guest-host interlinked PEG-MAL granular hydrogels as an engineered cellular microenvironment

We report the development of a polyethylene glycol (PEG) hydrogel scaffold that provides the advantages of conventional bulk PEG hydrogels for engineering cellular microenvironments and allows for rapid cell migration. PEG microgels were used to assemble a densely packed granular system with an intrinsic interstitium-like negative space. In this material, guest-host molecular interactions provide reversible non-covalent linkages between discrete PEG microgel particles to form a cohesive bulk material. In guest-host chemistry, different guest molecules reversibly and non-covalently interact with their cyclic host molecules. Two species of PEG microgels were made, each with one functional group at the end of the four arm PEG-MAL functionalized using thiol click chemistry. The first was functionalized with the host molecule β-cyclodextrin, a cyclic oligosaccharide of repeating d-glucose units, and the other functionalized with the guest molecule adamantane. These two species provide a reversible guest-host interaction between microgel particles when mixed, generating an interlinked network with a percolated interstitium. We showed that this granular configuration, unlike conventional bulk PEG hydrogels, enabled the rapid migration of THP-1 monocyte cells. The guest-host microgels also exhibited shear-thinning behavior, providing a unique advantage over current bulk PEG hydrogels.

A novel prevascularized tissue-engineered chamber as a site for allogeneic and xenogeneic islet transplantation to establish a bioartificial pancreas

Although sites for clinical or experimental islet transplantation are well established, pancreatic islet survival and function in these locations remain unsatisfactory. A possible factor that might account for this outcome is local hypoxia caused by the limited blood supply. Here, we modified a prevascularized tissue-engineered chamber (TEC) that facilitated the viability and function of the seeded islets in vivo by providing a microvascular network prior to transplantation. TECs were created, filled with Growth Factor-Matrigel^™ (Matrigel^™) and then implanted into the groins of mice with streptozotocin-induced diabetes. The degree of microvascularization in each TECs was analyzed by histology, real-time PCR, and Western blotting. Three hundred syngeneic islets were seeded into each chamber on days 0, 14, and 28 post-chamber implantation, and 300, 200, or 100 syngeneic islets were seeded into additional chambers on day 28 post-implantation, respectively. Furthermore, allogeneic or xenogeneic islet transplantation is a potential solution for organ shortage. The feasibility of TECs as transplantation sites for islet allografts or xenografts and treatment with anti-CD45RB and/or anti-CD40L (MR-1) was therefore explored. A highly developed microvascularized network was established in each TEC on day 28 post-implantation. Normalization of blood glucose levels in diabetic mice was negatively correlated with the duration of prevascularization and the number of seeded syngeneic islets. Combined treatment with anti-CD45RB and MR-1 resulted in long-term survival of the grafts following allotransplantation (5/5, 100%) and xenotransplantation (16/20, 80%). Flow cytometry demonstrated that the frequency of CD4⁺Foxp3-Treg and CD4⁺IL-4⁺-Th2 cells increased significantly after tolerogenic xenograft transplantation, while the number of CD4+IFN-γ-Th1 cells decreased. These findings demonstrate that highly developed microvascularized constructs can facilitate the survival of transplanted islets in a TECs, implying its potential application as artificial pancreas in the future.

From Mesenchymal Stromal/Stem Cells to Insulin-Producing Cells: Progress and Challenges

Mesenchymal stromal cells (MSCs) are an attractive option for cell therapy for type 1 diabetes mellitus (DM). These cells can be obtained from many sources, but bone marrow and adipose tissue are the most studied. MSCs have distinct advantages since they are nonteratogenic, nonimmunogenic and have immunomodulatory functions. Insulin-producing cells (IPCs) can be generated from MSCs by gene transfection, gene editing or directed differentiation. For directed differentiation, MSCs are usually cultured in a glucose-rich medium with various growth and activation factors. The resulting IPCs can control chemically-induced diabetes in immune-deficient mice. These findings are comparable to those obtained from pluripotent cells. PD-L1 and PD-L2 expression by MSCs is upregulated under inflammatory conditions. Immunomodulation occurs due to the interaction between these ligands and PD-1 receptors on T lymphocytes. If this function is maintained after differentiation, life-long immunosuppression or encapsulation could be avoided. In the clinical setting, two sites can be used for transplantation of IPCs: the subcutaneous tissue and the omentum. A 2-stage procedure is required for the former and a laparoscopic procedure for the latter. For either site, cells should be transplanted within a scaffold, preferably one from fibrin. Several questions remain unanswered. Will the transplanted cells be affected by the antibodies involved in the pathogenesis of type 1 DM? What is the functional longevity of these cells following their transplantation? These issues have to be addressed before clinical translation is attempted. Graphical Abstract Bone marrow MSCs are isolated from the long bone of SD rats. Then they are expanded and through directed differentiation insulin-producing cells are formed. The differentiated cells are loaded onto a collagen scaffold. If one-stage transplantation is planned, a drug delivery system must be incorporated to ensure immediate oxygenation, promote vascularization and provide some growth factors. Some mechanisms involved in the immunomodulatory function of MSCs. These are implemented either by cell to cell contact or by the release of soluble factors. Collectively, these pathways results in an increase in T-regulatory cells.

Organoid Polymer Functionality and Mode of Klebsiella Pneumoniae Membrane Antigen Presentation Regulates Ex Vivo Germinal Center Epigenetics in Young and Aged B Cells

Antibiotic-resistant bacteria are a major global health threat that continues to rise due to a lack of effective vaccines. Of concern are Klebsiella pneumoniae that fail to induce in vivo germinal center B cell responses, which facilitate antibody production to fight infection. Immunotherapies using antibodies targeting antibiotic-resistant bacteria are emerging as promising alternatives, however, they cannot be efficiently derived ex vivo, necessitating the need for immune technologies to develop therapeutics. Here, PEG-based immune organoids were developed to elucidate the effects of polymer end-point chemistry, integrin ligands, and mode of K. pneumoniae antigen presentation on germinal center-like B cell phenotype and epigenetics, to better define the lymph node microenvironment factors regulating ex vivo germinal center dynamics. Notably, PEG vinyl sulfone or acrylate failed to sustain primary immune cells, but functionalization with maleimide (PEG-4MAL) led to B cell expansion and germinal center-like induction. RNA sequencing analysis of lymph node stromal and germinal center B cells showed niche associated heterogeneity of integrin-related genes. Incorporation of niche-mimicking peptides revealed that collagen-1 promoted germinal center-like dynamics and epigenetics. PEG-4MAL organoids elucidated the impact of K. pneumoniae outer membrane-embedded protein antigen versus soluble antigen presentation on germinal centers and preserved the response across young and aged mice.

Polymer microcapsules and microbeads as cell carriers for in vivo biomedical applications

Polymer microcarriers are being extensively explored as cell delivery vehicles in cell-based therapies and hybrid tissue and organ engineering. Spherical microcarriers are of particular interest due to easy fabrication and injectability. They include microbeads, composed of a porous matrix, and microcapsules, where matrix core is additionally covered with a semipermeable membrane. Microcarriers provide cell containment at implantation site and protect the cells from host immunoresponse, degradation and shear stress. Immobilized cells may be genetically altered to release a specific therapeutic product directly at the target site, eliminating side effects of systemic therapies. Cell microcarriers need to fulfil a number of extremely high standards regarding their biocompatibility, cytocompatibility, immunoisolating capacity, transport, mechanical and chemical properties. To obtain cell microcarriers of specified parameters, a wide variety of polymers, both natural and synthetic, and immobilization methods can be applied. Yet so far, only a few approaches based on cell-laden microcarriers have reached clinical trials. The main issue that still impedes progress of these systems towards clinical application is limited cell survival in vivo. Herein, we review polymer biomaterials and methods used for fabrication of cell microcarriers for in vivo biomedical applications. We describe their key limitations and modifications aiming at improvement of microcarrier in vivo performance. We also present the main applications of polymer cell microcarriers in regenerative medicine, pancreatic islet and hepatocyte transplantation and in the treatment of cancer. Lastly, we outline the main challenges in cell microimmobilization for biomedical purposes, the strategies to overcome these issues and potential future improvements in this area.

Mesentery - a 'New' organ

The mesentery is the organ in which all abdominal digestive organs develop, and which maintains these in systemic continuity in adulthood. Interest in the mesentery was rekindled by advancements of Heald and Hohenberger in colorectal surgery. Conventional descriptions hold there are multiple mesenteries centrally connected to the posterior midline. Recent advances first demonstrated that, distal to the duodenojejunal flexure, the mesentery is a continuous collection of tissues. This observation explained how the small and large intestines are centrally connected, and the anatomy of the associated peritoneal landscape. In turn it prompted recategorisation of the mesentery as an organ. Subsequent work demonstrated the mesentery remains continuous throughout development, and that abdominal digestive organs (i.e. liver, spleen, intestine and pancreas) develop either on, or in it. This relationship is retained into adulthood when abdominal digestive organs are directly connected to the mesentery (i.e. they are 'mesenteric' in embryological origin and anatomical position). Recognition of mesenteric continuity identified the mesenteric model of abdominal anatomy according to which all abdominal abdomino-pelvic organs are organised into either a mesenteric or a non-mesenteric domain. This model explains the positional anatomy of all abdominal digestive organs, and associated vasculature. Moreover, it explains the peritoneal landscape and enables differentiation of peritoneum from the mesentery. Increased scientific focus on the mesentery has identified multiple vital or specialised functions. These vary across time and in anatomical location. The following review demonstrates how recent advances related to the mesentery are re-orientating the study of human biology in general and, by extension, clinical practice.

Multifunctional Islet Transplantation Hydrogel Encapsulating A20 High-Expressing Islets

Islet transplantation is regarded as the most promising treatment for type 1 diabetes (T1D). However, the function of grafted islet could be damaged on account of transplant rejection and/or hypoxia several years later after transplantation. We proposed a hypothetical functionalized hydrogel model, which encapsulates sufficient A20 high-expressing islets and supporting cells, and performs as a drug release system releasing immunosuppressants and growth factors, to improve the outcome of pancreatic islet transplantation. Once injected in vivo, the hydrogel can gel and offer a robust mechanical structure for the A20 high-expressing islets and supporting cells. The natural biomaterials (eg, heparin) added into the hydrogel provide adhesive sites for islets to promote islets’ survival. Furthermore, the hydrogel encapsulates various supporting cells, which can facilitate the vascularization and/or prevent the immune system attacking the islet graft. Based on the previous studies that generally applied one or two combined strategies to protect the function of islet graft, we designed this hypothetical multifunctional encapsulation hydrogel model with various functions. We hypothesized that the islet graft could survive and maintain its function for a longer time in vivo compared with naked islets. This hypothetical model has a limitation in terms of clinical application. Future development work will focus on verifying the function and safety of this hypothetical islet transplantation hydrogel model in vitro and in vivo.

Angiogenic biomaterials to promote therapeutic regeneration and investigate disease progression

The vasculature is a key component of the tissue microenvironment. Traditionally known for its role in providing nutrients and oxygen to surrounding cells, the vasculature is now also acknowledged to provide signaling cues that influence biological outcomes in regeneration and disease. These cues come from the cells that comprise vasculature, as well as the dynamic biophysical and biochemical properties of the surrounding extracellular matrix that accompany vascular development and remodeling. In this review, we illustrate the larger role of the vasculature in the context of regenerative biology and cancer progression. We describe cellular, biophysical, biochemical, and metabolic components of vascularized microenvironments. Moreover, we provide an overview of multidimensional angiogenic biomaterials that have been developed to promote therapeutic vascularization and regeneration, as well as to mimic elements of vascularized microenvironments as a means to uncover mechanisms by which vasculature influences cancer progression and therapy.

Enhanced stem cell retention and antioxidative protection with injectable, ROS-degradable PEG hydrogels

Poly(ethylene glycol) (PEG) hydrogels crosslinked with enzyme-cleavable peptides are promising biodegradable vehicles for therapeutic cell delivery. However, peptide synthesis at the level required for bulk biomaterial manufacturing is costly, and fabrication of hydrogels from scalable, low-cost synthetic precursors while supporting cell-specific degradation remains a challenge. Reactive oxygen species (ROS) are cell-generated signaling molecules that can also be used as a trigger to mediate specific in vivo degradation of biomaterials. Here, PEG-based hydrogels crosslinked with ROS-degradable poly(thioketal) (PTK) polymers were successfully synthesized via thiol-maleimide chemistry and employed as a cell-degradable, antioxidative stem cell delivery platform. PTK hydrogels were mechanically robust and underwent ROS-mediated, dose-dependent degradation in vitro, while promoting robust cellular infiltration, tissue regeneration, and bioresorption in vivo. Moreover, these ROS-sensitive materials successfully encapsulated mesenchymal stem cells (MSCs) and maintained over 40% more viable cells than gold-standard hydrogels crosslinked with enzymatically-degradable peptides. The higher cellular survival in PTK-based gels was associated with the antioxidative function of the ROS-sensitive crosslinker, which scavenged free radicals and protected encapsulated MSCs from cytotoxic doses of ROS. Improved MSC viability was also observed in vivo as MSCs delivered within injectable PTK hydrogels maintained significantly more viability over 11 days compared against cells delivered within gels crosslinked with either a PEG-only control polymer or a gold-standard enzymatically-degradable peptide. Together, this study establishes a new paradigm for scalable creation and application of cell-degradable hydrogels, particularly for cell delivery applications.

White Adipose Tissue as a Site for Islet Transplantation

Although islet transplantation is recognized as a useful cellular replacement therapy for severe diabetes, surgeons face difficulties in islet engraftment. The transplant site is a pivotal factor that influences the engraftment. Although the liver is the current representative site for clinical islet transplantation, it is not the best site because of limitations in immunity, inflammation, and hypoxia. White adipose tissue, including omentum, is recognized as a useful candidate site for islet transplantation. Its effectiveness has been evaluated in not only various basic and translational studies using small and large animals but also in some recent clinical trials. In this review, we attempt to shed light on the characteristics and usefulness of white adipose tissue as a transplant site for islets.

White Adipose Tissue as a Site for Islet Transplantation

Although islet transplantation is recognized as a useful cellular replacement therapy for severe diabetes, surgeons face difficulties in islet engraftment. The transplant site is a pivotal factor that influences the engraftment. Although the liver is the current representative site for clinical islet transplantation, it is not the best site because of limitations in immunity, inflammation, and hypoxia. White adipose tissue, including omentum, is recognized as a useful candidate site for islet transplantation. Its effectiveness has been evaluated in not only various basic and translational studies using small and large animals but also in some recent clinical trials. In this review, we attempt to shed light on the characteristics and usefulness of white adipose tissue as a transplant site for islets.

Control of adhesive ligand density for modulation of nucleus pulposus cell phenotype

Cells of the nucleus pulposus have been observed to undergo a shift from their notochordal-like juvenile phenotype to a more fibroblast-like state with age and maturation. It has been demonstrated that culture of degenerative adult human nucleus pulposus cells upon soft (<1 kPa) full length laminin-containing hydrogel substrates promotes increased levels of a panel of markers associated with the juvenile nucleus pulposus cell phenotype. In the current work, we observed an ability to use soft polymeric substrates functionalized with short laminin-mimetic peptide sequences to recapitulate the behaviors elicited by soft, full-length laminin containing materials. Furthermore, our work suggests an ability to mimic features of soft systems through control of peptide density upon stiffer substrates. Specifically, results suggest that stiffer polymer-peptide hydrogel substrates can be used to promote the expression of a more juvenile-like phenotype for cells of the nucleus pulposus by reducing adhesive ligand presentation. Here we show how polymer stiffness combined with adhesive ligand presentation can be controlled to be supportive of nucleus pulposus cell phenotype and biosynthesis.

Small Molecule-Induced Pancreatic β-Like Cell Development: Mechanistic Approaches and Available Strategies

Diabetes is a metabolic disease which affects not only glucose metabolism but also lipid and protein metabolism. It encompasses two major types: type 1 and 2 diabetes. Despite the different etiologies of type 1 and 2 diabetes mellitus (T1DM and T2DM, respectively), the defining features of the two forms are insulin deficiency and resistance, respectively. Stem cell therapy is an efficient method for the treatment of diabetes, which can be achieved by differentiating pancreatic β-like cells. The consistent generation of glucose-responsive insulin releasing cells remains challenging. In this review article, we present basic concepts of pancreatic organogenesis, which intermittently provides a basis for engineering differentiation procedures, mainly based on the use of small molecules. Small molecules are more auspicious than any other growth factors, as they have unique, valuable properties like cell-permeability, as well as a nonimmunogenic nature; furthermore, they offer immense benefits in terms of generating efficient functional beta-like cells. We also summarize advances in the generation of stem cell-derived pancreatic cell lineages, especially endocrine β-like cells or islet organoids. The successful induction of stem cells depends on the quantity and quality of available stem cells and the efficient use of small molecules.

Integrin-specific hydrogels modulate transplanted human bone marrow-derived mesenchymal stem cell survival, engraftment, and reparative activities

Stem cell therapies are limited by poor cell survival and engraftment. A hurdle to the use of materials for cell delivery is the lack of understanding of material properties that govern transplanted stem cell functionality. Here, we show that synthetic hydrogels presenting integrin-specific peptides enhance the survival, persistence, and osteo-reparative functions of human bone marrow-derived mesenchymal stem cells (hMSCs) transplanted in murine bone defects. Integrin-specific hydrogels regulate hMSC adhesion, paracrine signaling, and osteoblastic differentiation in vitro. Hydrogels presenting GFOGER, a peptide targeting α2β1 integrin, prolong hMSC survival and engraftment in a segmental bone defect and result in improved bone repair compared to other peptides. Integrin-specific hydrogels have diverse pleiotropic effects on hMSC reparative activities, modulating in vitro cytokine secretion and in vivo gene expression for effectors associated with inflammation, vascularization, and bone formation. These results demonstrate that integrin-specific hydrogels improve tissue healing by directing hMSC survival, engraftment, and reparative activities.

Bioengineering strategies for bone and cartilage tissue regeneration using growth factors and stem cells

Bone and cartilage tissue engineering is an integrative approach that is inspired by the phenomena associated with wound healing. In this respect, growth factors have emerged as important moieties for the control and regulation of this process. Growth factors act as mediators and control the important physiological functions of bone regeneration. Herein, we discuss the importance of growth factors in bone and cartilage tissue engineering, their loading and delivery strategies, release kinetics, and their integration with biomaterials and stem cells to heal bone fractures. We also highlighted the role of growth factors in the determination of the bone tissue microenvironment based on the reciprocal signaling with cells and biomaterial scaffolds on which future bone and cartilage tissue engineering technologies and medical devices will be based upon.

Biofabrication of microcapsules encapsulating beta-TC-6 cells via scalable device and in-vivo evaluation in type 1 diabetic mice

Type 1 diabetes mellitus (T1DM) is a disease characterized by lack of pancreatic islet function. Whole tissue transplantation appears to be a viable alternative in the management of T1DM. This study aims at the fabrication and evaluation of alginate-chitosan microcapsules encapsulating insulin-secreting beta-TC-6 cells using an automated spraying nozzle. Uniform spherical microcapsules (250-350 µm) encapsulated with beta-TC-6 cells were fabricated in large quantities in a short span of time. Microencapsulated beta-TC-6 cells were transplanted intraperitoneally into streptozotocin (STZ) induced diabetic mice and monitored for immune tolerance and decrease in blood glucose levels. Mice that received microencapsulated beta-TC-6 cells maintained normoglycemia for 35 ± 5 days before rejection. However, the group that received naked beta-TC-6 cells rejected the cells within 1-2 days, unable to control elevated blood glucose levels. They also exhibited high immune reactions, as evidenced by elevated levels of CD8⁺ and CD62L T cells. CD4⁺ T cells that mediated a Th2 response (humoral response) were predominant in microencapsulated beta-TC-6 cells and cells only group as evidenced by elevated levels of CD45R. Our findings from the in-vivo study demonstrated that transplantation of microencapsulated beta-TC-6 cells can be a viable alternative in the management of T1DM with acceptable immune response.

Brief exposure to hyperglycemia activates dendritic cells in vitro and in vivo

Dendritic cells are key players in regulating immunity. These cells both activate and inhibit the immune response depending on their cellular environment. Their response to hyperglycemia, a condition common amongst diabetics wherein glucose is abnormally elevated, remains to be elucidated. In this study, the phenotype and immune response of dendritic cells exposed to hyperglycemia were characterized in vitro and in vivo using the streptozotocin-induced diabetes model. Dendritic cells were shown to be sensitive to hyperglycemia both during and after differentiation from bone marrow precursor cells. Dendritic cell behavior under hyperglycemic conditions was found to vary by phenotype, among which, tolerogenic dendritic cells were particularly sensitive. Expression of the costimulatory molecule CD86 was found to reliably increase when dendritic cells were exposed to hyperglycemia. Additionally, hydrogel-based delivery of the anti-inflammatory molecule interleukin-10 was shown to partially inhibit these effects in vivo.

Hydrogel-Mediated Sustained Systemic Delivery of Mesenchymal Stem Cell-Derived Extracellular Vesicles Improves Hepatic Regeneration in Chronic Liver Failure

Extracellular vesicles derived from mesenchymal stem cells (MSC-EVs) have been widely reported as promising cell-free products that show therapeutic effects of the parental cells but not their limitations. Due to the intrinsic liver tropism of MSC-EVs, they have been widely used as therapeutics or drug carriers for treatment of liver diseases. However, rapid clearance from the target site may attenuate the efficiency of systemically administered MSC-EVs. Herein, sustained release into the peritoneum has been proposed as a new strategy to prolong the bioavailability of the MSC-EVs in the target liver. During intraperitoneal injection, clickable polyethylene glycol (PEG) macromeres were mixed with MSC-EVs to form EV-encapsulated PEG hydrogels via a fast, biocompatible click reaction. Upon biodegradation, the EV-laden hydrogels were swollen gradually to release EVs in a sustained manner over 1 month. In vivo tracking of the labeled EVs revealed that the accumulation of EVs in the liver was extended by hydrogel-mediated delivery for 1 month. Four weeks after injection in a rat model of chronic liver fibrosis, the physical and histopathological investigations of the harvested liver showed superior antifibrosis, anti-apoptosis, and regenerative effects of the EVs when delivered by the sustained systemic release (Gel-EV) to the conventional bolus injection (Free-EV). Specifically, the Gel-EV system improved the antifibrosis, anti-inflammation, anti-apoptosis, and regenerative effects of the EVs to nearly 40, 50, 40, and 50% compared to Free-EV, respectively, as was specified by quantification of the fibrotic area, α-SMA density, and caspase-3 density in the harvested tissues and ALT enzyme in serum. This study may potentiate the use of MSC-EVs as cell-free therapeutics for chronic liver failure. The sustained systemic delivery strategy may open a new paradigm to extend the effects of disease-targeting EVs over time.

Engineering transferrable microvascular meshes for subcutaneous islet transplantation

The success of engineered cell or tissue implants is dependent on vascular regeneration to meet adequate metabolic requirements. However, development of a broadly applicable strategy for stable and functional vascularization has remained challenging. We report here highly organized and resilient microvascular meshes fabricated through a controllable anchored self-assembly method. The microvascular meshes are scalable to centimeters, almost free of defects and transferrable to diverse substrates, ready for transplantation. They promote formation of functional blood vessels, with a density as high as ~220 vessels mm-2, in the poorly vascularized subcutaneous space of SCID-Beige mice. We further demonstrate the feasibility of fabricating microvascular meshes from human induced pluripotent stem cell-derived endothelial cells, opening a way to engineer patient-specific microvasculature. As a proof-of-concept for type 1 diabetes treatment, we combine microvascular meshes and subcutaneously transplanted rat islets and achieve correction of chemically induced diabetes in SCID-Beige mice for 3 months.

Engineering the vasculature for islet transplantation

The microvasculature in the pancreatic islet is highly specialized for glucose sensing and insulin secretion. Although pancreatic islet transplantation is a potentially life-changing treatment for patients with insulin-dependent diabetes, a lack of blood perfusion reduces viability and function of newly transplanted tissues. Functional vasculature around an implant is not only necessary for the supply of oxygen and nutrients but also required for rapid insulin release kinetics and removal of metabolic waste. Inadequate vascularization is particularly a challenge in islet encapsulation. Selectively permeable membranes increase the barrier to diffusion and often elicit a foreign body reaction including a fibrotic capsule that is not well vascularized. Therefore, approaches that aid in the rapid formation of a mature and robust vasculature in close proximity to the transplanted cells are crucial for successful islet transplantation or other cellular therapies. In this paper, we review various strategies to engineer vasculature for islet transplantation. We consider properties of materials (both synthetic and naturally derived), prevascularization, local release of proangiogenic factors, and co-transplantation of vascular cells that have all been harnessed to increase vasculature. We then discuss the various other challenges in engineering mature, long-term functional and clinically viable vasculature as well as some emerging technologies developed to address them. The benefits of physiological glucose control for patients and the healthcare system demand vigorous pursuit of solutions to cell transplant challenges. STATEMENT OF SIGNIFICANCE: Insulin-dependent diabetes affects more than 1.25 million people in the United States alone. Pancreatic islets secrete insulin and other endocrine hormones that control glucose to normal levels. During preparation for transplantation, the specialized islet blood vessel supply is lost. Furthermore, in the case of cell encapsulation, cells are protected within a device, further limiting delivery of nutrients and absorption of hormones. To overcome these issues, this review considers methods to rapidly vascularize sites and implants through material properties, pre-vascularization, delivery of growth factors, or co-transplantation of vessel supporting cells. Other challenges and emerging technologies are also discussed. Proper vascular growth is a significant component of successful islet transplantation, a treatment that can provide life-changing benefits to patients.

IFN-γ-tethered hydrogels enhance mesenchymal stem cell-based immunomodulation and promote tissue repair

Because of their immunomodulatory activities, human mesenchymal stem cells (hMSCs) are being explored to treat a variety of chronic conditions such as inflammatory bowel disorders and graft-vs-host disease. Treating hMSCs with IFN-γ prior to administration augments these immunomodulatory properties; however, this ex vivo treatment limits the broad applicability of this therapy due to technical and regulatory issues. In this study, we engineered an injectable synthetic hydrogel with tethered recombinant IFN-γ that activates encapsulated hMSCs to increase their immunomodulatory functions and avoids the need for ex vivo manipulation. Tethering IFN-γ to the hydrogel increases retention of IFN-γ within the biomaterial while preserving its biological activity. hMSCs encapsulated within hydrogels with tethered IFN-γ exhibited significant differences in cytokine secretion and showed a potent ability to halt activated T-cell proliferation and monocyte-derived dendritic cell differentiation compared to hMSCs that were pre-treated with IFN-γ and untreated hMSCs. Importantly, hMSCs encapsulated within hydrogels with tethered IFN-γ accelerated healing of colonic mucosal wounds in both immunocompromised and immunocompetent mice. This novel approach for licensing hMSCs with IFN-γ may enhance the clinical translation and efficacy of hMSC-based therapies.