Investigating Ectopic Sites for Islet Transplantation

• Author / Creator

  Salama, Bassem

• Diabetes is a chronic metabolic condition characterized by hyperglycemia resulting from impaired glucose metabolism. Type 1 diabetes mellitus (T1DM) is a result of progressive autoimmune destruction of insulin-producing pancreatic beta (β)-cells. By far, parenteral insulin replacement is considered to be the most reliable replacement therapy to control hyperglycemia, and to delay the inevitable complications associated with diabetes. Nonetheless, insulin hardly imitates the physiological secretion pattern of endogenous insulin, and fails to control glycemic excursions in subset of diabetic patients suffering from what is known as “brittle diabetes”
  Pancreatic islets transplantation represents a conceivable alternative to exogenous insulin administration. Virtually, it provides a real-time response to fluctuating blood glucose levels, replacing the lost pancreatic control on glucose metabolism. In 2000, seven patients attained long-term insulin independence after being transplanted with fresh islet allografts in the liver, using non-steroidal immunosuppressive therapy. Since then, this transplant protocol – known as the Edmonton Protocol – has been utilized by many clinical islet transplantation centers worldwide. Yet, many obstacles (such as the restricted availability of human cadaveric islets, and hostile transplantation microenvironment) have impeded its widespread clinical application.
  Porcine islet xenografts offer a logical alternative tissue source for islet transplantation. Till recent time, many diabetic patients were relying on porcine insulin as a life-saving replacement therapy to control hyperglycemia. Neonatal porcine islets (NPI) possess numerous merits as an alternate clinical β-cell source, compared to their adult or juvenile counterparts. They can be isolated and cultured easily in a large-scale setting, and they are capable of controlling hyperglycemia after proliferating within the recipient, despite being immature at the time of transplantation.
  Finding a more suitable transplant site is as important as finding a virtually unlimited donor tissue source. The hepatic microenvironment contributes greatly to the acute post-transplantation graft loss. Instant blood-mediated inflammatory response leads to loss of more than half of the initial transplanted islet mass. Other sites have been investigated in order to prevent graft attrition and loss of insulin independence. An optimal ectopic site should be easily accessible with minor perioperative morbidity, can accommodate large volume of islets or engineered insulin secreting cells, and allows graft retrieval for cellular monitoring. It also should be well vascularized to enhance graft survival, glucose sensing, and insulin secretion. Most importantly, the microenvironment in that proposed site should enhance graft maturation and differentiation, with minimal host’s reaction towards transplanted tissue.
  This doctoral thesis provides the reader with results from multiple studies aimed to investigate two proposed ectopic transplant sites. Also, it underscores the impact of the recipient’s sex on the behaviour of islet graft post-transplantation. I hypothesize that augmented transplantation of islets subcutaneously or in the intra-abdominal fat folds can control hyperglycemia and reverse diabetes mellitus in diabetic animal models. Also, recipient’s sex would have a profound influence on graft function due to difference in circulating gonadal hormones.
  
  This thesis is organized in a paper-based format, based on original experimental manuscripts published or under review. Initially, the reader is provided with a thorough literature review including historical background, clinical applications, current limitations, and recent attempts to circumvent those obstacles.
  Next, we explore an attractive and clinically translatable extra-hepatic site for islet transplantation; we investigate the subcutaneous space and its potential to accommodate islet xenografts. Due to its poor vascularity, it is expected that islets will not survive without amending its local microenvironment. Hence, we used fibrin as a biocompatible scaffold for cell delivery and local vascular enrichment. We also demonstrated that intra-peritoneal fat folds (such as epididymal fat pads in mice) could support islet grafts. We used alginate (proven immune barrier) as another form of scaffold to deliver human islets in the fat pad. Shifting focus to the global recipient’s influence on the graft, we demonstrated the effect of host’s sex hormones on graft function and response to glucose-stimulated insulin secretion.
  Taken together, this thesis offers clear results pertaining the capability of two different ectopic transplantation sites in supporting islet grafts, aiming to improve post-transplantation outcomes.

• Subjects / Keywords

  • Subcutaneous
  • Xenotransplantation
  • Neonatal Porcine Islets
  • Ectopic sites
  • Fibrin

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-md1t-t023

• License

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