Seeding of Pancreatic Islets into Prevascularized Tissue Engineering Chambers

Innovations in bio-engineering and cell-based approaches to address immunological challenges in islet transplantation

Human allogeneic pancreatic islet transplantation is a life-changing treatment for patients with severe Type 1 Diabetes (T1D) who suffer from hypoglycemia unawareness and high risk of severe hypoglycemia. However, intensive immunosuppression is required to prevent immune rejection of the graft, that may in turn lead to undesirable side effects such as toxicity to the islet cells, kidney toxicity, occurrence of opportunistic infections, and malignancies. The shortage of cadaveric human islet donors further limits islet transplantation as a treatment option for widespread adoption. Alternatively, porcine islets have been considered as another source of insulin-secreting cells for transplantation in T1D patients, though xeno-transplants raise concerns over the risk of endogenous retrovirus transmission and immunological incompatibility. As a result, technological advancements have been made to protect transplanted islets from immune rejection and inflammation, ideally in the absence of chronic immunosuppression, to improve the outcomes and accessibility of allogeneic islet cell replacement therapies. These include the use of microencapsulation or macroencapsulation devices designed to provide an immunoprotective environment using a cell-impermeable layer, preventing immune cell attack of the transplanted cells. Other up and coming advancements are based on the use of stem cells as the starting source material for generating islet cells 'on-demand'. These starting stem cell sources include human induced pluripotent stem cells (hiPSCs) that have been genetically engineered to avoid the host immune response, curated HLA-selected donor hiPSCs that can be matched with recipients within a given population, and multipotent stem cells with natural immune privilege properties. These strategies are developed to provide an immune-evasive cell resource for allogeneic cell therapy. This review will summarize the immunological challenges facing islet transplantation and highlight recent bio-engineering and cell-based approaches aimed at avoiding immune rejection, to improve the accessibility of islet cell therapy and enhance treatment outcomes. Better understanding of the different approaches and their limitations can guide future research endeavors towards developing more comprehensive and targeted strategies for creating a more tolerogenic microenvironment, and improve the effectiveness and sustainability of islet transplantation to benefit more patients.

Accelerated absorption of regular insulin administered via a vascularizing permeable microchamber implanted subcutaneously in diabetic Rattus norvegicus

In Type 1 diabetes patients, even ultra-rapid acting insulins injected subcutaneously reach peak concentrations in 45 minutes or longer. The lag time between dosing and peak concentration, as well as intra- and inter-subject variability, render prandial glucose control and dose consistency difficult. We postulated that insulin absorption from subcutaneously implantable vascularizing microchambers would be significantly faster than conventional subcutaneous injection. Male athymic nude R. norvegicus rendered diabetic with streptozotocin were implanted with vascularizing microchambers (single chamber; 1.5 cm2 surface area per side; nominal volume, 22.5 μl). Plasma insulin was assayed after a single dose (1.5 U/kg) of diluted insulin human (Humulin®R U-100), injected subcutaneously or via microchamber. Microchambers were also implanted in additional animals and retrieved at intervals for histologic assessment of vascularity. Following conventional subcutaneous injection, the mean peak insulin concentration was 22.7 (SD 14.2) minutes. By contrast, when identical doses of insulin were injected via subcutaneous microchamber 28 days after implantation, the mean peak insulin time was shortened to 7.50 (SD 4.52) minutes. Peak insulin concentrations were similar by either route; however, inter-subject variability was reduced when insulin was administered via microchamber. Histologic examination of tissue surrounding microchambers showed mature vascularization on days 21 and 40 post-implantation. Implantable vascularizing microchambers of similar design may prove clinically useful for insulin dosing, either intermittently by needle, or continuously by pump including in "closed loop" systems, such as the artificial pancreas.

Bioengineering the Vascularized Endocrine Pancreas: A Fine-Tuned Interplay Between Vascularization, Extracellular-Matrix-Based Scaffold Architecture, and Insulin-Producing Cells

Intrahepatic islet transplantation is a promising β-cell replacement strategy for the treatment of type 1 diabetes. Instant blood-mediated inflammatory reactions, acute inflammatory storm, and graft revascularization delay limit islet engraftment in the peri-transplant phase, hampering the success rate of the procedure. Growing evidence has demonstrated that islet engraftment efficiency may take advantage of several bioengineering approaches aimed to recreate both vascular and endocrine compartments either ex vivo or in vivo. To this end, endocrine pancreas bioengineering is an emerging field in β-cell replacement, which might provide endocrine cells with all the building blocks (vascularization, ECM composition, or micro/macro-architecture) useful for their successful engraftment and function in vivo. Studies on reshaping either the endocrine cellular composition or the islet microenvironment have been largely performed, focusing on a single building block element, without, however, grasping that their synergistic effect is indispensable for correct endocrine function. Herein, the review focuses on the minimum building blocks that an ideal vascularized endocrine scaffold should have to resemble the endocrine niche architecture, composition, and function to foster functional connections between the vascular and endocrine compartments. Additionally, this review highlights the possibility of designing bioengineered scaffolds integrating alternative endocrine sources to overcome donor organ shortages and the possibility of combining novel immune-preserving strategies for long-term graft function.

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.

Bioengineering a pre-vascularized pouch for subsequent islet transplantation using VEGF-loaded polylactide capsules

The effectiveness of cell transplantation can be improved by optimization of the transplantation site. For some types of cells that form highly oxygen-demanding tissue, e.g., pancreatic islets, a successful engraftment depends on immediate and sufficient blood supply. This critical point can be avoided when cells are transplanted into a bioengineered pre-vascularized cavity which can be formed using a polymer scaffold. In our study, we tested surface-modified poly(lactide-co-caprolactone) (PLCL) capsular scaffolds containing the pro-angiogenic factor VEGF. After each modification step (i.e., amination and heparinization), the surface properties and morphology of scaffolds were characterized by ATR-FTIR and XPS spectroscopy, and by SEM and AFM. All modifications preserved the gross capsule morphology and maintained the open pore structure. Optimized aminolysis conditions decreased the Mw of PLCL only up to 10% while generating a sufficient number of NH2 groups required for the covalent immobilization of heparin. The heparin layer served as a VEGF reservoir with an in vitro VEGF release for at least four weeks. In vivo studies revealed that to obtain highly vascularized PLCL capsules (a) the optimal VEGF dose for the capsule was 50 μg and (b) the implantation time was four weeks when implanted into the greater omentum of Lewis rats; dense fibrous tissue accompanied by vessels completely infiltrated the scaffold and created sparse granulation tissue within the internal cavity of the capsule. The prepared pre-vascularized pouch enabled the islet graft survival and functioning for at least 50 days after islet transplantation. The proposed construct can be used to create a reliable pre-vascularized pouch for cell transplantation.

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.

Bio-engineering a tissue flap utilizing a porous scaffold incorporating a human induced pluripotent stem cell-derived endothelial cell capillary network connected to a vascular pedicle

Tissue flaps are used to cover large/poorly healing wounds, but involve complex surgery and donor site morbidity. In this study a tissue flap is assembled using the mammalian body as a bioreactor to functionally connect an artery and vein to a human capillary network assembled from induced pluripotent stem cell-derived endothelial cells (hiPSC ECs). In vitro: Porous NovoSorb™ scaffolds (3 mm × 1.35 mm) were seeded with 200,000 hiPSC ECs ± 100,000 human vascular smooth muscle cells (hvSMC), and cultured for 1-3 days, with capillaries formed by 24 h which were CD31⁺, VE-Cadherin⁺, EphB4⁺, VEGFR2⁺ and Ki67⁺, whilst hvSMCs (calponin⁺) attached abluminally. In vivo: In SCID mice, bi-lateral epigastric vascular pedicles were isolated in a silicone chamber for a 3 week 'delay period' for pedicle capillary sprouting, then reopened, and two hiPSC EC ± hvSMCs seeded scaffolds transplanted over the pedicle. The chamber was either resealed (Group 1), or removed and surrounding tissue secured around the pedicle + scaffolds (Group 2), for 1 or 2 weeks. Human capillaries survived in vivo and were CD31⁺, VE-Cadherin⁺ and VEGFR2⁺. Human vSMCs remained attached, and host mesenchymal cells also attached abluminally. Systemically injected FITC-dextran present in human capillary lumens indicated inosculation to host capillaries. Human iPSC EC capillary morphometric parameters at one week in vivo were equal to or higher than the same parameters measured in human abdominal skin. This 'proof of concept' study has demonstrated that bio-engineering an autologous human tissue flap based on hiPSC EC could minimize the use of donor flaps and has potential applications for complex wound coverage. STATEMENT OF SIGNIFICANCE: Tissue flaps, used for surgical reconstruction of wounds, require complex surgery, often associated with morbidity. Bio-engineering a simpler alternative, we assembled a human induced pluripotent stem cell derived endothelial cell (hiPSC ECs) capillary network in a porous scaffold in vitro, which when transplanted over a mouse vascular pedicle in vivo formed a functional tissue flap with mouse blood flow in the human capillaries. Therefore it is feasible to form an autologous tissue flap derived from a hiPSC EC capillary network assembled in vitro, and functionally connect to a vascular pedicle in vivo that could be utilized in complex wound repair for chronic or acute wounds.

Angiogenesis and tissue formation driven by an arteriovenous loop in the mouse

The rapid vascularisation of biomaterials and artificial tissues is a key determinant for their in vivo viability and ultimately for their integration in a host; therefore promoting angiogenesis and maintaining the newly formed vascular beds has become a major goal of tissue engineering. The arteriovenous loop (AVL) has been an extensively studied platform which integrates microsurgery with cells scaffolds and growth factors to form neotissues. Most AVL studies to date are limited to larger animal models, which are surgically easier to perform, but have inherent limits for the understanding and interrogation of the underlying in vivo mechanisms due the paucity of transgenic models. Here, we demonstrate for the first time in a mouse model the utility of the AVL in the de novo production of vascularized tissue. We also present the combined use of the model with 3D printed chambers, which allow us to dictate size and shape of the tissues formed. This novel platform will allow for an understanding of the fundamental mechanisms involved in tissue generation de novo.

Oxygenation strategies for encapsulated islet and beta cell transplants

Human allogeneic islet transplantation (ITx) is emerging as a promising treatment option for qualified patients with type 1 diabetes. However, widespread clinical application of allogeneic ITx is hindered by two critical barriers: the need for systemic immunosuppression and the limited supply of human islet tissue. Biocompatible, retrievable immunoisolation devices containing glucose-responsive insulin-secreting tissue may address both critical barriers by enabling the more effective and efficient use of allogeneic islets without immunosuppression in the near-term, and ultimately the use of a cell source with a virtually unlimited supply, such as human stem cell-derived β-cells or xenogeneic (porcine) islets with minimal or no immunosuppression. However, even though encapsulation methods have been developed and immunoprotection has been successfully tested in small and large animal models and to a limited extent in proof-of-concept clinical studies, the effective use of encapsulation approaches to convincingly and consistently treat diabetes in humans has yet to be demonstrated. There is increasing consensus that inadequate oxygen supply is a major factor limiting their clinical translation and routine implementation. Poor oxygenation negatively affects cell viability and β-cell function, and the problem is exacerbated with the high-density seeding required for reasonably-sized clinical encapsulation devices. Approaches for enhanced oxygen delivery to encapsulated tissues in implantable devices are therefore being actively developed and tested. This review summarizes fundamental aspects of islet microarchitecture and β-cell physiology as well as encapsulation approaches highlighting the need for adequate oxygenation; it also evaluates existing and emerging approaches for enhanced oxygen delivery to encapsulation devices, particularly with the advent of β-cell sources from stem cells that may enable the large-scale application of this approach.

The emerging field of pancreatic tissue engineering: A systematic review and evidence map of scaffold materials and scaffolding techniques for insulin-secreting cells

A bioartificial endocrine pancreas is proposed as a future alternative to current treatment options. Patients with insulin-secretion deficiency might benefit. This is the first systematic review that provides an overview of scaffold materials and techniques for insulin-secreting cells or cells to be differentiated into insulin-secreting cells. An electronic literature survey was conducted in PubMed/MEDLINE and Web of Science, limited to the past 10 years. A total of 197 articles investigating 60 different materials met the inclusion criteria. The extracted data on materials, cell types, study design, and transplantation sites were plotted into two evidence gap maps. Integral parts of the tissue engineering network such as fabrication technique, extracellular matrix, vascularization, immunoprotection, suitable transplantation sites, and the use of stem cells are highlighted. This systematic review provides an evidence-based structure for future studies. Accumulating evidence shows that scaffold-based tissue engineering can enhance the viability and function or differentiation of insulin-secreting cells both in vitro and in vivo.

3D biofabrication of vascular networks for tissue regeneration: A report on recent advances

Rapid progress in tissue engineering research in past decades has opened up vast possibilities to tackle the challenges of generating tissues or organs that mimic native structures. The success of tissue engineered constructs largely depends on the incorporation of a stable vascular network that eventually anastomoses with the host vasculature to support the various biological functions of embedded cells. In recent years, significant progress has been achieved with respect to extrusion, laser, micro-molding, and electrospinning-based techniques that allow the fabrication of any geometry in a layer-by-layer fashion. Moreover, decellularized matrix, self-assembled structures, and cell sheets have been explored to replace the biopolymers needed for scaffold fabrication. While the techniques have evolved to create specific tissues or organs with outstanding geometric precision, formation of interconnected, functional, and perfused vascular networks remains a challenge. This article briefly reviews recent progress in 3D fabrication approaches used to fabricate vascular networks with incorporated cells, angiogenic factors, proteins, and/or peptides. The influence of the fabricated network on blood vessel formation, and the various features, merits, and shortcomings of the various fabrication techniques are discussed and summarized.

Vascularization of PLGA-based bio-artificial beds by hypoxia-preconditioned mesenchymal stem cells for subcutaneous xenogeneic islet transplantation

BACKGROUND

Subcutaneous tissue is an attractive extra-hepatic heterotopic site for islet transplantation; however, poor oxygen tension and blood supply during early engraftment of implanted islets have limited the use of this site in clinical applications.

METHODS

This study investigated the vascularization potential of hypoxia-preconditioned mesenchymal stem cells (3% O₂ ; hypo-MSCs) in PLGA-based bio-artificial beds for subsequent subcutaneous islet transplantation. Sheet-typed polymeric PLGA scaffolds coated with hypo-MSCs or normo-MSCs (MSCs cultured under normoxia conditions, 21% O₂ ) were implanted subcutaneously in mice.

RESULTS

Compared to normo-MSCs, hypo-MSCs significantly enhanced vasculogenesis, both on the interior and exterior surfaces of the implanted PLGA devices, which peaked 4 weeks after implantation. Further, infusion of porcine islets inside the prevascularized PLGA bed restored normal glycemic control in 6 of 6 STZ-induced diabetic mice. The mass of the marginal islet was approximately 2000 IEQs, which is comparable to that required for the renal subcapsular space, a highly vascularized site.

CONCLUSIONS

Therefore, PLGA-based bio-artificial devices prevascularized with hypo-MSCs could be a useful modality for successful subcutaneous islet transplantation, which is of high clinical relevance.

Local, Controlled Release In Vivo of Vascular Endothelial Growth Factor Within a Subcutaneous Scaffolded Islet Implant Reduces Early Islet Necrosis and Improves Performance of the Graft

Islet transplantation remains the only alternative to daily insulin therapy for control of type 1 diabetes (T1D) in humans. To avoid the drawbacks of intrahepatic islet transplantation, we are developing a scaffolded islet implant to transplant islets into nonhepatic sites. The implant test bed, sized for mice, consists of a limited (2-mm) thickness, large-pore polymeric sponge scaffold perforated with peripheral cavities that contain islets suspended in a collagen hydrogel. A central cavity in the scaffold holds a 2-mm diameter alginate sphere for controlled release of the angiogenic cytokine vascular endothelial growth factor ( VEGF). Host microvessels readily penetrate the scaffold and collagen gel to vascularize the islets. Here, we evaluate the performance of the implant in a subcutaneous (SC) graft site. Implants incorporating 500 syngeneic islets reversed streptozotocin-induced diabetes in mice approximately 30 d after SC placement. Controlled release of a modest quantity (20 ng) of VEGF within the implant significantly reduced the time to normoglycemia compared to control implants lacking VEGF. Investigation of underlying causes for this effect revealed that inclusion of 20 ng of VEGF in the implants significantly reduced central necrosis of islets 24 h after grafting and increased implant vascularization (measured 12 d after grafting). Collectively, our results demonstrate (1) that the scaffolded islet implant design can reverse diabetes in SC sites in the absence of prevascularization of the graft site and (2) that relatively low quantities of VEGF, delivered by controlled release within the implant, can be a useful approach to limit islet stress after grafting.

An engineered macroencapsulation membrane releasing FTY720 to precondition pancreatic islet transplantation

Macroencapsulation is a powerful approach to increase the efficiency of extrahepatic pancreatic islet transplant. FTY720, a small molecule that activates signaling through sphingosine-1-phosphate receptors, is immunomodulatory and pro-angiogenic upon sustained delivery from biomaterials. While FTY720 (fingolimod, Gilenya) has been explored for organ transplantation, in the present work the effect of locally released FTY720 from novel nanofiber-based macroencapsulation membranes is explored for islet transplantation. We screened islet viability during culture with FTY720 and various biodegradable polymers. Islet viability is significantly reduced by the addition of high doses (≥500 ng/mL) of soluble FTY720. Among the polymers screened, islets have the highest viability when cultured with poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Therefore, PHBV was blended with polycaprolactone (PCL) for mechanical stability and electrospun into nanofibers. Islets had no detectable function ex vivo following 5 days or 12 h of subcutaneous implantation within our engineered device. Subsequently, we explored a preconditioning scheme in which islets are transplanted 2 weeks after FTY720-loaded nanofibers are implanted. This allows FTY720 to orchestrate a local regenerative milieu while preventing premature transplantation into avascular sites that contain high concentrations of FTY720. These results provide a foundation and motivation for further investigation into the use of FTY720 in preconditioning sites for efficacious islet transplantation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 555-568, 2018.

Tissue Engineering by Intrinsic Vascularization in an In Vivo Tissue Engineering Chamber

In reconstructive surgery, there is a clinical need for an alternative to the current methods of autologous reconstruction which are complex, costly and trade one defect for another. Tissue engineering holds the promise to address this increasing demand. However, most tissue engineering strategies fail to generate stable and functional tissue substitutes because of poor vascularization. This paper focuses on an in vivo tissue engineering chamber model of intrinsic vascularization where a perfused artery and a vein either as an arteriovenous loop or a flow-through pedicle configuration is directed inside a protected hollow chamber. In this chamber-based system angiogenic sprouting occurs from the arteriovenous vessels and this system attracts ischemic and inflammatory driven endogenous cell migration which gradually fills the chamber space with fibro-vascular tissue. Exogenous cell/matrix implantation at the time of chamber construction enhances cell survival and determines specificity of the engineered tissues which develop. Our studies have shown that this chamber model can successfully generate different tissues such as fat, cardiac muscle, liver and others. However, modifications and refinements are required to ensure target tissue formation is consistent and reproducible. This article describes a standardized protocol for the fabrication of two different vascularized tissue engineering chamber models in vivo.

Repurposed biological scaffolds: kidney to pancreas

Advances in organ regeneration have been facilitated by gentle decellularization protocols that maintain distinct tissue compartments, and thereby allow seeding of blood vessels with endothelial lineages separate from populations of the parenchyma with tissue-specific cells. We hypothesized that a reconstituted vasculature could serve as a novel platform for perfusing cells derived from a different organ: thus discordance of origin between the vascular and functional cells, leading to a hybrid repurposed organ. The need for a highly vascular bed is highlighted by tissue engineering approaches that involve transplantation of just cells, as attempted for insulin production to treat human diabetes. Those pancreatic islet cells present unique challenges since large numbers are needed to allow the cell-to-cell signaling required for viability and proper function; however, increasing their number is limited by inadequate perfusion and hypoxia. As proof of principle of the repurposed organ methodology we harnessed the vasculature of a kidney scaffold while seeding the collecting system with insulin-producing cells. Pig kidneys were decellularized by sequential detergent, enzymatic and rinsing steps. Maintenance of distinct vascular and collecting system compartments was demonstrated by both fluorescent 10 micron polystyrene microspheres and cell distributions in tissue sections. Sterilized acellular scaffolds underwent seeding separately via the artery (fibroblasts or endothelioma cells) and retrograde (murine βTC-tet cells) up the ureter. After three-day bioreactor incubation, histology confirmed separation of cells in the vasculature from those in the collecting system. βTC-tet clusters survived in tubules, glomerular Bowman's space, demonstrated insulin immunolabeling, and thereby supported the feasibility of kidney-to-pancreas repurposing.

Fibrin gels engineered with pro-angiogenic growth factors promote engraftment of pancreatic islets in extrahepatic sites in mice

With a view toward reduction of graft loss, we explored pancreatic islet transplantation within fibrin matrices rendered pro-angiogenic by incorporation of minimal doses of vascular endothelial growth factor-A165 and platelet-derived growth factor-BB presented complexed to a fibrin-bound integrin-binding fibronectin domain. Engineered matrices allowed for extended release of pro-angiogenic factors and for their synergistic signaling with extracellular matrix-binding domains in the post-transplant period. Aprotinin addition delayed matrix degradation and prolonged pro-angiogenic factor availability within the graft. Both subcutaneous (SC) and epididymal fat pad (EFP) sites were evaluated. We show that in the SC site, diabetes reversal in mice transplanted with 1,000 IEQ of syngeneic islets was not observed for islets transplanted alone, while engineered matrices resulted in a diabetes median reversal time (MDRT) of 38 days. In the EFP site, the MDRT with 250 IEQ of syngeneic islets within the engineered matrices was 24 days versus 86 days for islets transplanted alone. Improved function of engineered grafts was associated with enhanced and earlier (by day 7) angiogenesis. Our findings show that by engineering the transplant site to promote prompt re-vascularization, engraftment and long-term function of islet grafts can be improved in relevant extrahepatic sites.

Vascularisation to improve translational potential of tissue engineering systems for cardiac repair

Cardiac tissue engineering is developing as an alternative approach to heart transplantation for treating heart failure. Shortage of organ donors and complications arising after orthotopic transplant remain major challenges to the modern field of heart transplantation. Engineering functional myocardium de novo requires an abundant source of cardiomyocytes, a biocompatible scaffold material and a functional vasculature to sustain the high metabolism of the construct. Progress has been made on several fronts, with cardiac cell biology, stem cells and biomaterials research particularly promising for cardiac tissue engineering, however currently employed strategies for vascularisation have lagged behind and limit the volume of tissue formed. Over ten years we have developed an in vivo tissue engineering model to construct vascularised tissue from various cell and tissue sources, including cardiac tissue. In this article we review the progress made with this approach and others, together with their potential to support a volume of engineered tissue for cardiac tissue engineering where contractile mass impacts directly on functional outcomes in translation to the clinic. It is clear that a scaled-up cardiac tissue engineering solution required for clinical treatment of heart failure will include a robust vascular supply for successful translation. This article is part of a directed issue entitled: Regenerative Medicine: the challenge of translation.

Intraportal islet oxygenation

Islet transplantation (IT) is a promising therapy for the treatment of diabetes. The large number of islets required to achieve insulin independence limit its cost-effectiveness and the number of patients who can be treated. It is believed that >50% of islets are lost in the immediate post-IT period. Poor oxygenation in the early post-IT period is recognized as a possible reason for islet loss and dysfunction but has not been extensively studied. Several key variables affect oxygenation in this setting, including (1) local oxygen partial pressure (pO(2)), (2) islet oxygen consumption, (3) islet size (diameter, D), and (4) presence or absence of thrombosis on the islet surface. We discuss implications of oxygen-limiting conditions on intraportal islet viability and function. Of the 4 key variables, the islet size appears to be the most important determinant of the anoxic and nonfunctional islet volume fractions. Similarly, the effect of thrombus formation on the islet surface may be substantial. At the University of Minnesota, average size distribution data from clinical alloislet preparations (n = 10) indicate that >150-µm D islets account for only ~30% of the total islet number, but >85% of the total islet volume. This suggests that improved oxygen supply to the islets may have a profound impact on islet survivability and function since most of the β-cell volume is within large islets which are most susceptible to oxygen-limiting conditions. The assumption that the liver is a suitable islet transplant site from the standpoint of oxygenation should be reconsidered.

Macrophages play a key role in angiogenesis and adipogenesis in a mouse tissue engineering model

We have previously described a mouse adipose tissue engineering model using a silicon chamber enclosing the superficial epigastric pedicle in a Matrigel based environment. We have shown that when Zymosan, a sterile inflammatory agent, is added to the chamber, angiogenesis and adipogenesis are significantly improved. As Zymosan interacts with toll-like receptors on macrophages, the role of macrophages in new tissue development in the tissue engineering chamber was assessed. Morphological and histological results showed that macrophages were presenting in high numbers at 2 weeks but had decreased significantly by 4 and 6 weeks in the chamber. Numerous immature new blood vessels had formed by 2 weeks, becoming more mature at 4 and 6 weeks. Immature adipocytes were visualized at 4 weeks and mature cells, at 6 weeks. To investigate the functional role of macrophages in the tissue engineering process, we knocked out the local macrophage population by inserting Clodronate liposomes in this chamber. This study shows for the first time that when macrophages are depleted, there is minimal new vascular and adipose tissue development. We propose a new theory for tissue engineering in which macrophages play a central role in both neovascularisation and adipogenesis.

Engineering a local microenvironment for pancreatic islet replacement

Intraportal islet transplantation has emerged as a promising treatment for type 1 diabetes mellitus (T1DM). Nevertheless, long-term efficacy has been limited to a marginal number of patients. Outcomes have been restricted, in part, by challenges associated with the transplant site, poor vascularization, and disruption of the native islet architecture during the isolation process. Engineering a biomaterial platform that recapitulates critical components of the pancreatic environment can serve to address these hurdles. This review highlights the challenges and opportunities in engineering 3D niches for islets, specifically: the importance of site selection; the application of scaffold functionalization to present bioactive motifs; and the development of technologies for enhancing implant nutritional profiles. The potential of these novel approaches to improve islet engraftment and duration of function is discussed.

Improved function and growth of pancreatic cells in a three-dimensional bioreactor environment

Methods of three-dimensional (3D) cell culture have made significant progress in recent years due to a better understanding of cell to cell interactions and the cell's interface with their surrounding environment. We hypothesized that a microgravity 3D culture system would improve upon the growth and function of a pancreatic progenitor cell population. We developed a rotating wall vessel bioreactor and established a culture system using a pancreatic cell line. Cells in the bioreactors showed robust proliferation, enhanced transcriptional signaling, and improved translation of pancreatic genes compared with two-dimensional static culture. Cells also gained the ability to respond to glucose stimulation, which was not observed in the control cultures. These findings suggest that a 3D microgravity bioreactor environment mimics the niche of the pancreas yielding a cell source with potential for cell-based therapy in the treatment of diabetes.

In situ application of hydrogel-type fibrin-islet composite optimized for rapid glycemic control by subcutaneous xenogeneic porcine islet transplantation

Maximum engraftment of transplanted islets is essential for the clinical application of a subcutaneous site. Significant barriers to the current approaches are associated with their low effectiveness, complexity and unproven biosafety. Here, we evaluated and optimized a fibrin-islet composite for effective glycemic control in a subcutaneous site whose environment is highly hypoxic due to low vascularization potential. In the setting of xenogeneic porcine islet transplantation into the subcutaneous space of a diabetic mouse, the in vivo islet functions were greatly affected by the concentrations of fibrinogen and thrombin. The optimized hydrogel-type fibrin remarkably reduced the marginal islet mass to approximately one tenth that of islets without fibrin. This marginal islet mass was comparable to that in the setting of the subcapsular space of the kidney, which is a highly vascularized organ. Highly vascularized structures were generated inside and on the outer surface of the grafts. A hydrogel-type fibrin-islet composite established early diabetic control within an average of 3.4days after the transplantation. In the mechanistic studies, fibrin promoted local angiogenesis, enhanced islet viability and prevented fragmentation of islets into single cells. In conclusion, in situ application of hydrogel-type fibrin-islet composite may be a promising modality in the clinical success of subcutaneous islet transplantation.

The use of biomaterials in islet transplantation

Pancreatic islet transplantation is a therapeutic option to replace destroyed β cells in autoimmune diabetes. Islets are transplanted into the liver via the portal vein; however, inflammation, the required immunosuppression, and lack of vasculature decrease early islet viability and function. Therefore, the use of accessory therapy and biomaterials to protect islets and improve islet function has definite therapeutic potential. Here we review the application of niche accessory cells and factors, as well as the use of biomaterials as carriers or capsules, for pancreatic islet transplantation.

PLGA/hydrogel biopapers as a stackable substrate for printing HUVEC networks via BioLP

Two major challenges in tissue engineering are mimicking the native cell-cell arrangements of tissues and maintaining viability of three-dimension (3D) tissues thicker than 300 µm. Cell printing and prevascularization of engineered tissues are promising approaches to meet these challenges. However, the printing technologies used in biofabrication must balance the competing parameters of resolution, speed, and volume, which limit the resolution of thicker 3D structures. We suggest that high-resolution conformal printing techniques can be used to print 2D patterns of vascular cells onto biopaper substrates which can then be stacked to form a thicker tissue construct. Towards this end we created 1 cm × 1 cm × 300 µm biopapers to be used as the transferable, stackable substrate for cell printing. 3.6% w/v poly-lactide-co-glycolide was dissolved in chloroform and poured into molds filled with NaCl crystals. The salt was removed with DI water and the scaffolds were dried and loaded with a Collagen Type I or Matrigel. SEM of the biopapers showed extensive porosity and gel loading throughout. Biological laser printing (BioLP) was used to deposit human umbilical vein endothelial cells (HUVEC) in a simple intersecting pattern to the surface of the biopapers. The cells differentiated and stretched to form networks preserving the printed pattern. In a separate experiment to demonstrate "stackability," individual biopapers were randomly seeded with HUVECs and cultured for 1 day. The mechanically stable and viable biopapers were then stacked and cultured for 4 days. Three-dimensional confocal microscopy showed cell infiltration and survival in the compound multilayer constructs. These results demonstrate the feasibility of stackable "biopapers" as a scaffold to build 3D vascularized tissues with a 2D cell-printing technique.

Immunoisolating semi-permeable membranes for cell encapsulation: focus on hydrogels

Cell-based medicine has recently emerged as a promising cure for patients suffering from various diseases and disorders that cannot be cured/treated using technologies currently available. Encapsulation within semi-permeable membranes offers transplanted cell protection from the surrounding host environment to achieve successful therapeutic function following in vivo implantation. Apart from the immunoisolation requirements, the encapsulating material must allow for cell survival and differentiation while maintaining its physico-mechanical properties throughout the required implantation period. Here we review the progress made in the development of cell encapsulation technologies from the mass transport side, highlighting the essential requirements of materials comprising immunoisolating membranes. The review will focus on hydrogels, the most common polymers used in cell encapsulation, and discuss the advantages of these materials and the challenges faced in the modification of their immunoisolating and permeability characteristics in order to optimize their function.

Creation of a prevascularized site for cell transplantation in rats

INTRODUCTION

Transplanted cells, especially islet cells, are likely to become apoptotic due to local hypoxia leading to graft dysfunction. Isolated pancreatic islet cells depend on the diffusion of oxygen from the surrounding tissue; therefore, access to sufficient oxygen supply is beneficial, particularly when microcapsules are used for immunoisolation in xenotransplantation. The aim of this study was to create a prevascularized site for cell transplantation in rats and test its effectiveness with microencapsulated HEK293 cells.

METHODS

The combination of implantation of a foam dressing, vacuum-assisted wound closure (foam+VAC) and hyperbaric oxygenation (HBO) was used in 40 Sprague-Dawley rats. Blood flow and vascular endothelial growth factor (VEGF) levels were determined. Sodium cellulose sulphate (SCS)-microencapsulated HEK293 cells were xenotransplanted into the foam dressing in rats pre-treated with HBO, and angiogenesis and apoptosis were assessed.

RESULTS

Vessel ingrowth and VEGF levels increased depending on the duration of HBO treatment. The area containing the foam was perfused significantly better in the experimental groups when compared to controls. Only a small amount of apoptosis occurs in SCS-microencapsulated HEK293 cells after xenotransplantation.

CONCLUSION

As ischemia-damaged cells are likely to undergo cell death or loose functionality due to hypoxia, therefore leading to graft dysfunction, the combination foam+VAC and HBO might be a promising method to create a prevascularized site to achieve better results in xenogeneic cell transplantation.

Pancreatic Islet Culture and Preservation Strategies: Advances, Challenges, and Future Outlook

Postisolation islet survival is a critical step for achieving successful and efficient islet transplantation. This involves the optimization of islet culture in order to prolong survival and functionality in vitro. Many studies have focused on different strategies to culture pancreatic islets in vitro through manipulation of culture media, surface modified substrates, and the use of various techniques such as encapsulation, embedding, scaffold, and bioreactor culture strategies. This review aims to present and discuss the different methodologies employed to optimize pancreatic islet culture in vitro as well as address their respective advantages and drawbacks.

Engineering more than a cell: vascularization strategies in tissue engineering

Host integration and performance of engineered tissues have been severely limited by the lack of robust strategies to generate patent vascularization and tissue perfusion. This review highlights a selection of exciting developments in vascularization approaches for tissue engineering research. Current strategies for vascularization in tissue engineering are related to growth factor signaling and delivery, cell transplantation, bioactive smart matrix materials, and directed fabrication. Application of these techniques to in vivo models has resulted in a number of robust host vascular responses, especially with synergistic and engineered bioactive systems. The future outlook of the field includes refinement and development of new technologies for vascularization and combining these techniques with functional repair models for metabolically active tissues and relevant disease states.