Survival and function of syngeneic rat islet grafts placed within the thymus versus under the kidney capsule

Considerations for an Alternative Site of Islet Cell Transplantation

Islet cell transplantation has been limited most by poor graft survival. Optimizing the site of transplantation could improve clinical outcomes by minimizing required donor cells, increasing graft integration, and simplifying the transplantation and monitoring process. In this article, we review the history and significant human and animal data for clinically relevant sites, including the liver, spleen, and kidney subcapsule, and identify promising new sites for further research. While the liver was the first studied site and has been used the most in clinical practice, the majority of transplanted islets become necrotic. We review the potential causes for graft death, including the instant blood-mediated inflammatory reaction, exposure to immunosuppressive agents, and low oxygen tension. Significant research exists on alternative sites for islet cell transplantation, suggesting a promising future for patients undergoing pancreatectomy.

Accessory cells for β-cell transplantation

Despite recent advances, insulin therapy remains a treatment, not a cure, for diabetes mellitus with persistent risk of glycaemic alterations and life-threatening complications. Restoration of the endogenous β-cell mass through regeneration or transplantation offers an attractive alternative. Unfortunately, signals that drive β-cell regeneration remain enigmatic and β-cell replacement therapy still faces major hurdles that prevent its widespread application. Co-transplantation of accessory non-islet cells with islet cells has been shown to improve the outcome of experimental islet transplantation. This review will highlight current travails in β-cell therapy and focuses on the potential benefits of accessory cells for islet transplantation in diabetes.

Bioengineered sites for islet cell transplantation

Although islet transplantation has demonstrated its potential use in treating type 1 diabetes, this remains limited by the need for daily immunosuppression. Islet encapsulation was then proposed with a view to avoiding any immunosuppressive regimen and related side effects. In order to obtain a standard clinical procedure in terms of safety and reproducibility, two important factors have to be taken into account: the encapsulation design (which determines the graft volume) and the implantation site. Indeed, the implantation site should meet certain requirements: (1) its space must be large enough for the volume of transplanted tissues; (2) there must be proximity to abundant vascularization with a good oxygen supply; (3) there must be real-time access to physiologically representative blood glucose levels; (4) there must be easy access for implantation and the reversibility of the procedure (for safety); and finally, (5) the site should have minimal early inflammatory reaction and promote long-term survival. The aim of this article is to review possible preclinical/clinical implantation sites (in comparison with free islets) for encapsulated islet transplantation as a function of the encapsulation design: macro/microcapsules and conformal coating.

Revascularization of transplanted pancreatic islets and role of the transplantation site

Since the initial reporting of the successful reversal of hyperglycemia through the transplantation of pancreatic islets, significant research efforts have been conducted in elucidating the process of revascularization and the influence of engraftment site on graft function and survival. During the isolation process the intrinsic islet vascular networks are destroyed, leading to impaired revascularization after transplant. As a result, in some cases a significant quantity of the beta cell mass transplanted dies acutely following the infusion into the portal vein, the most clinically used site of engraftment. Subsequently, despite the majority of patients achieving insulin independence after transplant, a proportion of them recommence small, supplemental exogenous insulin over time. Herein, this review considers the process of islet revascularization after transplant, its limiting factors, and potential strategies to improve this critical step. Furthermore, we provide a characterization of alternative transplant sites, analyzing the historical evolution and their role towards advancing transplant outcomes in both the experimental and clinical settings.

Alternative transplantation sites for pancreatic islet grafts

The liver is the current site of choice for pancreatic islet transplantation, even though it is far from being an ideal site because of immunologic, anatomic, and physiologic factors leading to a significant early graft loss. A huge amount of alternative sites have been used for islet transplantation in experimental animal models to provide improved engraftment and long-term survival minimizing surgical complications. The pancreas, gastric submucosa, genitourinary tract, muscle, omentum, bone marrow, kidney capsule, peritoneum, anterior eye chamber, testis, and thymus have been explored. Site-specific differences exist in term of islet engraftment, but few alternative sites have potential clinical translation and generally the evidence of a post-transplant islet function better than that reached after intraportal infusion is still lacking. This review discusses site-specific benefits and drawbacks taking into account immunologic, metabolic, and technical aspects to identify the ideal microenvironment for islet function and survival.

Resolving the conundrum of islet transplantation by linking metabolic dysregulation, inflammation, and immune regulation

Although type 1 diabetes cannot be prevented or reversed, replacement of insulin production by transplantation of the pancreas or pancreatic islets represents a definitive solution. At present, transplantation can restore euglycemia, but this restoration is short-lived, requires islets from multiple donors, and necessitates lifelong immunosuppression. An emerging paradigm in transplantation and autoimmunity indicates that systemic inflammation contributes to tissue injury while disrupting immune tolerance. We identify multiple barriers to successful islet transplantation, each of which either contributes to the inflammatory state or is augmented by it. To optimize islet transplantation for diabetes reversal, we suggest that targeting these interacting barriers and the accompanying inflammation may represent an improved approach to achieve successful clinical islet transplantation by enhancing islet survival, regeneration or neogenesis potential, and tolerance induction. Overall, we consider the proinflammatory effects of important technical, immunological, and metabolic barriers including: 1) islet isolation and transplantation, including selection of implantation site; 2) recurrent autoimmunity, alloimmune rejection, and unique features of the autoimmune-prone immune system; and 3) the deranged metabolism of the islet transplant recipient. Consideration of these themes reveals that each is interrelated to and exacerbated by the other and that this connection is mediated by a systemic inflammatory state. This inflammatory state may form the central barrier to successful islet transplantation. Overall, there remains substantial promise in islet transplantation with several avenues of ongoing promising research. This review focuses on interactions between the technical, immunological, and metabolic barriers that must be overcome to optimize the success of this important therapeutic approach.