Development of a Bioartificial Vascular Pancreas
Abstract
Transplantation of pancreatic islets has been shown to be effective, in some patients, for the long-term treatment of type 1 diabetes. However, transplantation of islets into either the portal vein or the subcutaneous space can be limited by insufficient oxygen transfer, leading to islet loss. Furthermore, oxygen diffusion limitations can be magnified when islet numbers are increased dramatically, as in translating from rodent studies to human-scale treatments. To address these limitations, an islet transplantation approach using an acellular vascular graft as a vascular scaffold has been developed, termed the BioVascular Pancreas (BVP). To create the BVP, islets are seeded as an outer coating on the surface of an acellular vascular graft, using fibrin as a hydrogel carrier. The BVP can then be anastomosed as an arterial (or arteriovenous) graft, which allows fully oxygenated arterial blood with a pO2 of roughly 100 mmHg to flow through the graft lumen and thereby supply oxygen to the islets. In silico simulations and in vitro bioreactor experiments show that the BVP design provides adequate survivability for islets and helps avoid islet hypoxia. When implanted as end-to-end abdominal aorta grafts in nude rats, BVPs were able to restore near-normoglycemia durably for 90 days and developed robust microvascular infiltration from the host. Furthermore, pilot implantations in pigs were performed, which demonstrated the scalability of the technology. Given the potential benefits provided by the BVP, this tissue design may eventually serve as a solution for transplantation of pancreatic islets to treat or cure type 1 diabetes.
Keywords
Tissue engineering, Islet transplantation, Type 1 diabetes, Vascular graft, Cell-based therapeutics
Introduction
Type 1 diabetes results from the autoimmune destruction of pancreatic islets and is a growing and chronic health problem throughout the world.1,2 Monitoring of blood sugar levels, and recurrent intervention with exogenous insulin, allow many patients to lead relatively normal lives. However, diabetes still causes numerous short-term complications such as hyper- and hypo-glycemic episodes, and long-term complications such as cardiovascular disease and diabetic nephropathy.3,4 Transplantation of pancreatic islet cells can restore endocrine control of blood sugar levels and provides patients with improved glycemic control to avoid the debilitating side effects of diabetes.5 Unfortunately, current strategies for clinical islet transplantation suffer from issues such as islet inflammation from direct blood contact, hypoxia, and inadequate nutrient transfer, leading to islet cell death due to limitations in nutrient transfer/oxygenation, and transplant failure.
Native islets require high levels of oxygen to survive: islets utilize 5%–20% of the oxygen provided to the pancreas, despite making up only 1%–2% of pancreatic mass.
Given the high metabolic demands of islets, it is evident that islet transplant strategies require attention to oxygenation to achieve sufficient islet survival. Many islet therapies such as implantation of islets into the intra-abdominal cavity or into the sub-cutaneous space, transplant islets into environments where the local pO2 is typically only ~40 mmHg. This can lead to islet death during the early post-transplant period before revascularization of transplanted islets by the host. Islets transplanted into the portal vein of the liver using the Edmonton Protocol, where the pO2 of the portal blood is often near 40 mmHg, face local hypoxia as well as acute thrombosis around the islets. Up to 50% of transplanted islets are immediately lost upon transplantation due to the instant blood mediated inflammatory reaction caused by a combination of acute thrombosis and leukocyte infiltration. The islets then face hypoxia, due to low oxygen levels in portal vein blood. This hypoxic environment can lead to eventual graft failure, if capillary ingrowth does not occur quickly enough to support islet survival. As a result, multiple injections of islets are required, and the success rate of the Edmonton Protocol after 5 years is only 25%–50%.
To protect transplanted islets from immune recognition, microencapsulation is commonly used,27,28 wherein isletembedded microcapsules are implanted either subcutaneously or intraperitoneally.29–32 While each individual microcapsule is able to support enough diffusion for islet survival, combining tens to hundreds of thousands of microcapsules in the subcutaneous or intraperitoneal space can lead to clumping and local depletion of oxygen and nutrients.33 Low oxygen levels in islet encapsulation techniques suggest that focusing on improving islet oxygen delivery and vascular integration may be a road toward better islet transplantation results.
Higher local oxygen levels increase the likely hood that islets will survive the early, avascular post-transplant period long enough to allow for revascularization. To facilitate oxygen delivery, some approaches utilize implantable devices with external oxygen ports or oxygen generating materials. However, these techniques have not yet demonstrated sustained islet viability in islet transplant recipients. For vascular integration, directly connecting an islet transplant device with the arterial system of the host, to create a vascularized islet delivery device would allow islets to be in close proximity to constantly replenishing oxygen in the arterial bloodstream. Initial attempts to create such a vascular delivery device begun to take shape in the 1990s with work by Monaco and Sullivan. However, these perfused, vascular artificial pancreata were not successful in vivo, in part because of high rates of thrombosis of the synthetic materials used for the vascular conduit. Progressions in vascular engineering, specifically in decellularized vessel engineering, have yielded vascular grafts with improved biocompatibility that allow us to re-examine this methodology for islet transplantation.
Vascular grafts made from acellular biological extracellular matrices do not utilize oxygen from the bloodstream and have a lower rate of thrombosis than grafts made from artificial materials,39–47 and oxygen can diffuse from the bloodstream through the wall of the acellular matrices, thereby providing a means to oxygenate cells or tissues that are applied on the outer surface of the vascular conduits. An acellular vascular graft that is 40 cm in length and 6 mm in diameter – similar to that used as an arteriovenous graft in the arm for dialysis access – could accommodate roughly 800,000 islets on its outer surface, which is approximately the normal total islet complement of an adult human.48,49 Therefore, clinically-relevant dimensions of acellular vessels could accommodate islet numbers that may be suitable for therapy in type 1 diabetes.
It is this general approach for islet delivery that we sought to test in this report. We evaluated the delivery of pancreatic islets using an acellular vessel as a scaffold, to deliver islets in close proximity to, but not within, the arterial blood circulation. We hypothesized that seeding islets on the outer surface of an acellular vessel would allow for improved islet survival and functionality compared to transplantation of islets without connection to the arterial circulation. We term this approach the “Biovascular Pancreas,” or BVP (Figure 1). To determine the ultimate utility of the BVP concept and to optimize parameters of its design, we performed in silico modeling (computersimulations) using finite element analysis. These results were validated by in vitro bioreactor studies designed to mimic oxygen levels in implanted BVP constructs. Finally, to demonstrate the therapeutic effect of the BVP, we performed in vivo implantations into nude rats and piloted implantation studies into adult swine.
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