Date of Award

5-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Pharmaceutical Sciences

Research Advisor

Ram I. Mahato, Ph.D.

Committee

Ivan C. Gerling, Ph.D. Yi Lu, Ph.D. Bernd Meibohm, Ph.D. Duane D. Miller, Ph.D.

Abstract

Since its first introduction in 1990s, the Edmonton Protocol for human islet transplantation has helped more than 500 patients with type 1 diabetes worldwide and met great success from bench to bedside. The Edmonton Protocol involves isolating islets from a cadaveric donor pancreas by enzymatic digestion and infusing into the patient's portal vein. Each recipient usually receives islets from two or three donors for an optimal outcome and is dosed with two immunosuppressive drugs, sirolimus and tacrolimus and a monoclonal antibody drug daclizumab to prevent the graft rejection. However, despite the administration of immunosuppressive drugs and the recent improvement in islet isolation, preparation and transplantation, insulin independence is rarely sustained for long term after islet transplantation. It was reported that by a year after transplant, 50-68% of patients do not need to receive additional insulin, but by five years after the procedure, few than 10% of total patients are free of daily insulin injection.

The wide application of human islet transplantation is hindered by two major barriers, the immune rejection from the organ recipients and the PNF of islet grafts. Immune rejection describes a process where transplanted islets are attached, recognized and attacked by the host immune system while the PNF is characterized as the loss of islet viability and function caused by non-immune reasons, such as the disruption of islet microvasculature during islet isolation and purification process, hypoxia in the core of islet grafts and production of inflammatory cytokines at the transplantation sites.

Several strategies have been proposed to address the immune rejection and the PNF. Gene therapy, which relies on “vectors” to deliver therapeutic genes into human islets, was successful in immunodeficient animal models because the potent expressions of therapeutic proteins usually lead to an advantage in the post-transplantation survival of human islets. However, gene therapy provided limited success to overcome the immune rejection. Also gene therapy faced inherited problems such as the low transfection efficiency of non-viral vectors and the increasing safety concerns of the viral vectors.

Cell therapy especially stem cell therapy, on the other hand, has recently met great success as a novel regenerative medicine to support solid organ transplantation including human islet transplantation. Among all types of stem cells, mesenchymal stem cells (MSCs) are given special interest for their self-renewal potential, multi-lineage capacities, paracrine effects (trophic mediator) and immune modulatory effects, making it a great candidate for improving human islet transplantation. MSCs are mostly found in bone marrow, adipose and umbilical cord blood, are one of the most extensively studies adult stem cells used in treating degenerative diseases as well as solid organ transplantation. Unlike embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs), adult stem cells show restricted proliferation and lineage differentiation, and consequently have less risk of inducing tumor. MSC-based therapy has been used to improve human islet transplantation from three major aspects. First, MSC was used as an immune modulator to prevent the immune rejection of human islets. Second, MSC was used as a trophic mediator to improve the revascularization of human islets. Third, MSC was induced in vitro to form insulin producing cell as an alternative sources of human islets. The third aspect seems to be quite promising theoretically, but no group has successfully generated enough “artificial” islets for human transplantation. Our focus is mainly on the immunomodulatory effects and the trophic effects of MSCs to improve the human islet transplantation.

We also introduce humanized mouse model into the studies of islet transplantation. Humanized mouse model contains mature human immunity and is a good model to determine the immune rejection to human islet after transplantation. Our study is innovative because this is the first time humanized animal is used test the efficacy of MSC based regenerative medicine on solid organ transplantation. We plan to answer the following questions in this study. 1) Can humanized mouse model pose an immune rejection to human islets after transplantation? 2) What is the duration and limitation of the adoptively transferred human immunity? 3) What is the efficacy and mechanism of stem cell therapy in improving human islet transplantation in humanized mouse model?

DOI

10.21007/etd.cghs.2013.0358

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