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DIABETES TECHNOLOGY & THERAPEUTICS Volume 7, Number 6, 2005 © Mary Ann Liebert, Inc.
Review The Bioartificial Pancreas: Progress and Challenges SEDA KIZILEL, Ph.D.,1 MARC GARFINKEL, M.D.,1 and EMMANUEL OPARA, Ph.D.2
ABSTRACT Diabetes remains a devastating disease, with tremendous cost in terms of human suffering and healthcare expenditures. A bioartificial pancreas has the potential as a promising approach to preventing or reversing complications associated with this disease. Bioartificial pancreatic constructs are based on encapsulation of islet cells with a semipermeable membrane so that cells can be protected from the host’s immune system. Encapsulation of islet cells eliminates the requirement of immunosuppressive drugs, and offers a possible solution to the shortage of donors as it may allow the use of animal islets or insulin-producing cells engineered from stem cells. During the past 2 decades, several major approaches for immunoprotection of islets have been studied. The microencapsulation approach is quite promising because of its improved diffusion capacity, and technical ease of transplantation. It has the potential for providing an effective long-term treatment or cure of Type 1 diabetes.
INTRODUCTION
D
represents a major public health problem, as it is the most frequent endocrine disease in industrialized countries.1,2 According to the latest estimates, the number of people with this disease worldwide is 177 million, and this figure will double by the year 2025.3 The complications of diabetes are associated with multiple medical problems related to ophthalmic, renal, neurological, cerebrovascular, cardiovascular, and peripheral vascular disease.4–9 The economic burden of diabetes is related to its management, to the treatment of its secondary complications, and to resultant productivity loss.10 While one estimate shows that the cost of treating diabetes in the United States in 1997 was $44 billion,3 another study IABETES MELLITUS
1Section
has shown that it cost about $100 billion to treat diabetes and its complications in this country in 1992.10 Type 1 diabetes, also previously known as insulin-dependent diabetes mellitus, occurs when pancreatic islet cells are unable to produce insulin. Consequently, the blood glucose concentration becomes high while tissues are starving for metabolic fuel. In Type 2 diabetes, previously referred to as non–insulin-dependent diabetes, the body continues to make at least some insulin, but is unable to respond properly to the action of insulin produced by the pancreas. The Diabetes Control and Complications Trial11 showed that an improved metabolic control was achieved using intensive insulin treatment in Type 1 diabetes patients. However, the aggressive management of dia-
of Transplantation, Department of Surgery, The University of Chicago, Chicago, Illinois. Institute of Biomedical Science and Engineering, Illinois Institute of Technology, Chicago, Illinois.
2Pritzker
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betes with exogenous insulin only delayed the progression and development of complications of this disease. Also, intensive insulin treatment could only be applied to fewer than 10% of patients due an increased risk of severe episodes of hypoglycemia. Furthermore, it appears that C-peptide, a cleavage product from the processing of pro-insulin to insulin, prevents diabetes and hyperglycemia-induced vascular and neural dysfunction in animal and clinical models of diabetes and should therefore be used in combination with insulin to maintain vascular and neural functions as well as blood glucose levels.12–15 Advances in transplantation and in immunosuppression have made pancreas transplantation another treatment option for Type 1 diabetes. The main objective for pancreas transplantation is to achieve normal level of glucose in the blood and to free the patient from exogenous insulin requirements based on multiple finger stick glucose measurements.16 Even though successful pancreas transplantation provides normal glucose homeostasis, the requirement of lifelong immunosuppression makes it unclear whether transplantation is advantageous over continued insulin treatment.17 Also, because of the need for immunosuppression, most pancreas transplantations have been done simultaneously with a kidney transplant in patients who have advanced nephropathy.18 In contrast to pancreas transplantation, islet transplantation requires no major surgery. Also, islet transplantation offers an alternative to exogenous insulin treatment, as it can result in better glycemic control19 and potentially avoids surgical complications associated with whole-organ pancreas transplantation. It is known that islet cell transplantation can prevent, and, in some cases, reverse existing complications of diabetes probably because of the role played by C-peptide.20 However, major challenges remain to be addressed before islet transplantation can be used routinely and applied more widely to patients with diabetes. One major barrier is the shortage of human pancreas as a source of islet cells. Also, lifelong use of immunosuppressive drugs to overcome the rejection of transplanted tissue poses a significant risk to patients, as the use of glucocorticoids and cyclosporine have dose-dependent
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deleterious effects on glucose homeostasis and -cell function, and result in increased incidences of infection and cancer.20–24 The introduction of the steroid-free immunosuppressive drug regimen has resulted in long-term graft survival of islets with concomitant control of blood glucose levels.25 However, this immunosuppressive drug regimen is also not without risks to graft recipients.26 The routine application of islet transplantation would be tremendously enhanced by an unlimited supply of donor tissue, a standard implantation procedure, and the ability to maintain the transplanted tissue without the requirement of immunosuppressive drugs.27 To achieve these goals, immunoisolation (encapsulation) of islet cells has been proposed to protect cells from attack by the host immune system. Using immunoisolation of the islets, chronic administration of immunosuppressive drugs can be theoretically eliminated or minimized, as transplanted cells are separated from the host immune system by a biocompatible and semipermeable membrane.28,29 Moreover, immunoisolation of pancreatic islet cells for transplantation into patients with diabetes theoretically permits grafting of xenogenic islets, thus expanding the potential supply of donor islet tissue.25 Hence, immunoisolation could open up the possibility of using islets harvested from animals, or insulin-producing cells engineered from stem cells without the requirement of immunosuppression of transplant recipients. Porcine and bovine insulin amino acid sequences have considerable sequence homology with human insulin,30 and therefore pigs and cows are considered attractive as options for xenogeneic donors. One important consideration in using microencapsulated islets is defining the optimal site for transplantation. To date, most research has considered peritoneal cavity as the optimal site for implantation of encapsulated islets, as it is possible to transplant microcapsules into the peritoneal cavity using a simple procedure that could be suitable for routine outpatient use.29,31–34 Also, this site allows for implantation of high numbers of islets, which can be retrieved by peritoneal lavage35 if necessary.36 However, this site limits graft viability because of low blood supply, oxygen tension, and con-
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centration of essential minerals.34 In addition, the release of insulin from the peritoneal cavity to the blood stream is delayed compared with hormones released to the portal vein.37 Also, peritoneal macrophages have a high toxicity against encapsulated islets.38 Several researchers have also explored the portal vein of the liver as an alternative site for transplantation of immunoisolated islets.39 Based on its double vascularization (hepatic artery and portal vein), the intraportal location has a high oxygen and nutrient supply, which could be beneficial for the performance and the longevity of microencapsulated cells.39 Also, the liver is considered to be an immunologically privileged site.40 But, even more clinically relevant, is the use of percutaneous transhepatic catheterization, which provides relatively simple, inexpensive, and non-surgical access to the liver.41 In addition, The International Islet Registry reports more C-peptide-positive cases (more than 0.5 ng/mL) 1 year after transplantation into the portal vein than from any other site.42 This article reviews the recent progress and challenges remaining for the successful development of a bioartificial pancreas, and discusses the requirements to bring this technology closer to clinical application.
HISTORICAL DEVELOPMENT The pancreatic origin of diabetes was discovered in 1889 by Mering and Minkowski, when surgical removal of the pancreas caused dogs to develop diabetes. In 1913, Murlin and Kramer prepared extracts of bovine pancreas to lower the blood glucose in a dog with diabetes. However, the benefit of the extract was explained by the presence of lactate. Finally, in 1922, Banting and Best developed a method for preparing a pancreatic extract, and a few years later, injection of the active factor in this extract, insulin, became the main therapy for Type 1 diabetes. As of today, the treatment options for Type 1 diabetes are exogenous insulin with external glucose monitoring, whole-organ pancreas transplantation, islet cell transplantation, the artificial pancreas, and the bioartificial pancreas.
Among these treatments, the bioartificial pancreas avoids the use of immunosuppressive drugs while providing moment-to-moment glucose homeostasis consistent with a wellfunctioning native pancreas. It is important in this context to distinguish between the terms “artificial pancreas” (sometimes called “mechanical artificial pancreas”) and “bioartificial pancreas.” The mechanical artificial pancreas consists of a glucose sensor, an insulin pump, and a computer, which determines the rate of insulin delivery. The sensor may be implanted in the vena cava, and may require periodic replacement. The development of a glucose biosensor has been challenged by the difficulty of creating a sensitive, stable, and accurate sensor, despite the capability of a mechanical artificial pancreas to regulate glucose levels. An error in sensing, computation, or delivery could result in insulin overdosing, which could be potentially risky with life-threatening hypoglycemia,43–46 an unacceptable potential side effect of an artificial pancreas. It is also very difficult to produce a mechanical artificial pancreas that responds quickly enough to changes in blood sugar (islets respond in less than 10 min).47 In contrast, a bioartificial pancreas is a device that substitutes for the endocrine portion of the pancreas. Devices in this category contain synthetic materials and functional islets encapsulated within a semipermeable membrane to protect cells from the host immune response. The semipermeable membrane permits exchange of nutrients, glucose, and insulin with the host, but excludes the diffusion of immunoglobulins, complement, and white blood cells. The device may be implanted into a vascularized site48,49 or into the peritoneal cavity.36 The idea of using ultrathin polymer membrane microcapsules was proposed by Chang50 in 1964 for the immunoprotection of transplanted cells. Nearly 2 decades later, the concept of bioencapsulation was successfully shown to maintain glucose homeostasis in rats with diabetes using encapsulated allogeneic islets.32 When alginate microcapsules containing islets were implanted in rats, normal blood glucose levels were achieved for 13 weeks. The microcapsule used in that study consisted of an inner alginate core surrounded by a polylysine
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membrane, which was then surrounded by an outer polyethyleneimine coating. In later studies, the outer layer was replaced by the more biocompatible alginate coating because of an inflammatory reaction induced by polyethyleneimine.51 Since then, a variety of studies have been performed to understand the requirements for successful transplantation of encapsulated islet cells, but there is still a need for convincing evidence of success of encapsulated islet transplantation in either non-human primates or humans.49
IMPEDIMENTS TO THE PROGRESS OF CELL ENCAPSULATION TECHNOLOGY Fibrotic overgrowth of the implanted microcapsules is one of the major obstacles to progress in cell encapsulation technology. Invariably, the materials used for encapsulation are not completely inert, and can induce foreign body and inflammatory reactions. As a result of this fibrous tissue overgrowth, the diffusion of nutrients, oxygen, hormones, and waste products through the capsule is diminished, and encapsulated islets are destroyed because of hypoxia, starvation, and the secretion of nitric oxide by the stimulated macrophages.52 This fibrotic reaction to implanted microcapsules had previously been associated with commercially available alginate with high mannuronic acid content, which activates macrophages in vivo, resulting in fibroblast proliferation and eventual overgrowth.53 However, later studies did not support that notion.33 It has also been shown that polylysine may cause a fibrotic response through the induction of cytokines.54 Another important component of capsular design is permselectivity. Most capsule designs are based on the presumption that the effectiveness of immunoisolation of a polymer capsule is closely related to the pore size of the capsule membrane.55–57 However, pore sizes of polymer membranes are not homogeneous by nature, and consist of a broad spectrum of sizes.58 Total immunoprotection of encapsulated cells can only be provided if the polymer membrane does not have any pores larger than the antibody complement component.59 This fact has been largely ignored by researchers
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and has resulted in inconsistent findings in the field.60–66 Mechanism of immunologic attack of allografts and xenografts With regard to specific immunologic attack of encapsulated cell implants, there are differences in the immune protection requirement of allografts versus xenografts. An allograft is a graft between genetically different individuals within the same species, while a xenograft is a graft between individuals from different species. Allograft rejection occurs as result of activation of cellular immunity by interactions of host T cells with a graft, while humoral immunity, including antibodies and complement proteins, is responsible for the rejection of xenografts.67,68 In the direct pathway of antigen presentation, the T cell receptor recognizes T cells presented by donor-type antigen-presenting cells along with Class I or II major histocompatibility complex (MHC) molecules. The idea of immunoisolation using a polymer membrane is to separate allogeneic or xenogeneic tissue from the immune system of the recipient. Therefore, microencapsulation of islets within effective permselective membranes would prevent contact with immunoglobulin, complement components, and inflammatory cells. However, there is also a potential immune response towards antigens shed by encapsulated allogeneic or xenogeneic cells. Such antigens could be therapeutic agent themselves, cell surface molecules, or cell components (phospholipids, DNA), including those released upon cell death. Shedding of antigens from encapsulated cells would initiate a molecular tissue response around the implant, which could affect the viability and function of the encapsulated cells. As a result of this immunological reaction, these shedded antigens may be internalized, processed, and presented in association with host Class II MHC molecules (macrophages and dendritic cells) to host CD4 helper T cells (Fig. 1). The recognition of antigens through this indirect pathway may lead to the activation of T helper cells, which then secrete cytokines and regulate cellmediated immune response and inflammation.69,70 Small molecules, such as reactive oxy-
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FIG. 1. Schematic representation of antigen recognition via (A) the direct pathway and (B) the indirect pathway. APC, antigen-presenting cells; TCR, T cell receptor.
gen species and cytokines, may also pass through the polymer membrane and damage the transplanted tissue. Owing to the complexity of the immune response mechanism, it is important to understand the mechanism of immune protection, and considerable efforts have been made to investigate this issue. Chen et al.71 developed cell lines that have resistance to cytokines and oxygen radical-induced damage. Recently, macrophage depletion has also been used as a tool to protect xenografts from immune destruction. It has been found that depletion of peritoneal macrophages with clodronate liposomes improved the survival of macroencapsulated porcine neonatal pancreatic cell clusters.35 Safley et al.72 evaluated the cellular immune response in non-obese diabetic (NOD) mice after two different pathways of T cell co-stimulation were blocked. Alginate–poly-L-lysine-encapsulated porcine islet xenografts were transplanted intraperitoneally in NOD mice treated with CTLA4-immunoglobulin to block CD28/ B7 and with anti-CD154 monoclonal antibody to inhibit CD40/CD40–ligand interactions. It was concluded that blocking two different pathways of T cell co-stimulation inhibited T cell-dependent inflammatory responses, and
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significantly prolonged the survival of encapsulated islet xenografts. Another approach to improving the survival of microencapsulated islets can be to use local immunosuppression. Studies have shown that preimplantation of an immunoisolating device improved the survival of an encapsulated islet graft and reduced fibroblast outgrowth.73 Functional capsules may also be designed to release the immunosuppressive agent in a controlled manner. Local immunosuppression induced using this approach may prevent the occurrence of an undesirable reaction around the implant. Inflammatory reaction in response to the implantation of cell-free capsules involves a sequence of events similar to a foreign body reaction, starting with an acute inflammatory response and leading to a chronic inflammatory response or granulation tissue development and fibrous capsule development. The intensity and duration of each of these responses are dependent upon several factors, such as the extent of injury created in the implantation, biomaterial chemical composition, surface free energy, surface charge, porosity, surface roughness, and implant size and shape.74,75 The biocompatibility of the material is determined by the extent and deviation from the optimal wound healing conditions.76 Sources of donor islets Despite all of the advances in islet transplantation and in encapsulation technology, some challenges must be still overcome before the bioartificial pancreas can be broadly applied. The most significant of these challenges is the shortage of donor islets. This shortage is the motivation for researchers to find alternative sources of insulin-producing cells. Promising approaches for resolving this problem are differentiation of stem cells into cells with -celllike characteristics77–80 and genetic engineering of adult cells for secretion of recombinant insulin.81–86 Based on similarities between mechanisms that control the development of both the adult pancreas and the central nervous system, methods promoting neural differentiation of embryonic stem cells have been adapted by researchers to derive insulin-producing cells.87–91 Hori et al.92 investigated the possibility of directing neural progenitors to develop into glu-
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cose-responsive insulin-producing cells using inductive signals involved in normal pathways of islet development. Compared with other methods for insulin-producing cell development from human stem cells,91,93 the method developed by this group produced insulin at the highest levels yet achieved from an expandable, human stem cell-derived tissue. However, the method developed in the study still needs to be improved, perhaps through the addition of glucagon-like peptide-1, transforming growth factor- ligands, or other factors to potentiate -cell maturation, growth, and insulin secretion.94–96 Gene therapy has also been suggested as a treatment of insulin-dependent diabetes mellitus. This strategy can be applied by preventing the autoimmune destruction of -cells,97,98 by regenerating -cells,99,100 or by engineering insulin-secreting surrogate -cells.99,101–103 Sapir et al.104 recently suggested the potential use of adult human liver as an alternate tissue for autologous -cell-replacement therapy. Using pancreatic and duodenal homeobox gene 1 (PDX-1) and soluble factors, they managed to induce a comprehensive developmental shift of adult human liver cells into functional insulinproducing cells. According to the findings of that study, PDX-1-treated human liver cells expressed insulin, stored it in defined granules, and secreted the hormone in a glucose-regulated manner. When these cells were implanted under the renal capsule of immunodeficient mice with diabetes, the cells ameliorated hyperglycemia for prolonged periods of time. The technique offers both the potential of a cell-replacement therapy for patients with diabetes and allows the patient to be the donor of his or her own insulin-producing tissue. Intestinal K cells have also been engineered to express and secrete insulin in a glucose dependent manner, generating functional cells.105,106 With the development of robust and safe gene therapy, this strategy may lead to effective therapy in the future. Preservation of encapsulated cell systems The last challenge for a bioartificial pancreas to become a routine clinical application is the long-term cryopreservation of encapsulated cells. Validated technologies are required for
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long-term preservation of encapsulated cell systems to maintain a product inventory, and in order to meet end-user demands. One of the current strategies for overcoming the problem of cryopreservation of tissue is the reduction of cryoprotectant [dimethyl sulfoxide (DMSO)] concentration or complete replacement by equally powerful substances. Recent work in Dr. Opara’s laboratory has demonstrated that cryopreservation of islet cells cultured overnight in the presence of 10% polyvinylpyrrolidone yielded higher intact islet recovery compared with islets frozen in the presence of DMSO and glycerol.107 The lower islet cell integrity and function were explained on the basis of hypothesis that low-molecular-weight compounds, such as DMSO and glycerol, permeate the cell and interact hydrophobically with intracellular proteins, which results in perturbed cytoskeletal architecture of the frozen cells and diminished islet cell integrity and function. The other promising technical improvement of the cryopreservation technology is the reduction of total sample volume. Miniaturization of the cryosubstrates from 1 mL to microliters reduces temperature gradients in cryosubstrates, which makes it possible to achieve higher freezing rates and more homogeneous freezing of the sample.108 Islets cryopreserved with this strategy were highly functional even with the lower DMSO concentrations.109 Misler et al.110 studied stimulus–secretion coupling in whole islets as well as single cells from carefully selected cryopreserved and thawed human islets of Langerhans. They found that without using any of the recent advances in cryopreservation technique, cryopreserved and thawed human islets, which were selected based on their smooth surface and diameter, responded to glucose in a calcium- and metabolicdependent fashion. In another study by von Mach et al.,111 viability of pancreatic islets after cryopreservation was correlated negatively with their size, and suppression of insulin release was not observed for islets 300 m.
DIFFERENT FORMS OF BIOARTIFICIAL PANCREAS Various configurations have been studied for the purpose of immunoisolation of islets. These
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include biohybrid vascular devices, extravascular chambers, and encapsulation.112,113 Encapsulation is the technique of islet immunoisolation using biopolymeric spheres of different sizes. Currently, results in rodents and dogs with diabetes indicate that spherical micro- and macrocapsules appear to have the highest therapeutic potential.60,65,114 A microcapsule typically contains only a few hundred cells or a single islet, and to provide a therapeutic dosage, tens of hundreds of thousands of cells are required. In contrast, macrocapsules employ much larger depots to contain the full therapeutic dosage in one or a few implants. Macrocapsules are shaped as cylinders or planes, and typically one dimension is in the range of 2–6 mm (Fig. 2a).115,116 One advantage of the implantation of macrocapsules is the ease of retrieval in case of complications. However, despite reports of reduced risks in the implantation of macrocapsules, it has been shown that macroencapsulated islets have significantly impaired insulin secretion because of necrosis in the center of aggregated islet clumps as a result of diffusion limitations of nutrients and oxygen.116,117 Encouraging results have been obtained by immobilization of the islets in a matrix before final macroencapsulation.115,118 Limited diffusion of nutrients, as well as slow exchange of glucose and insulin, occurs as a result of relative large surface-to-volume ratio of these devices. To ensure sufficient nutrient and oxygen diffusion to the cells, islet cell density within the macrocapsules is kept to around 5–10% of the volume fraction. This approach, however, requires the implantation of large devices in order to provide adequate masses of insulin-producing cells, and large grafts are not practical and cannot be transplanted in conventional sites.119 In contrast, microencapsulation offers an optimal volume-to-surface ratio for fast exchange of hormones and nutrients without a large device. The technique involves enclosure of one or two islets within small spheres, which have diameters of less than 1 mm (Fig. 2b). Also, because of reduced volume of the islet transplants, and in contrast to macroencapsulated islets, microcapsules can theoretically be transplanted in delicate sites, such as the portal vein. The trans-
KIZILEL ET AL. (a) Macroencapsulation
(b) Microencapsulation
FIG. 2. Immunoisolated islets within (a) membrane diffusion chambers (macrocapsules) and (b) microcapsules.
plantation of microcapsules into the peritoneal cavity, an outpatient-based procedure,33,120–122 makes it particularly attractive to patients and doctors. Interestingly, intraperitoneally implanted microencapsulated islets have shown promising results in large animal122–124 models as well as in limited clinical trials.33
TECHNIQUES IN MICROENCAPSULATION Materials used for microencapsulation Various materials have been used as biopolymeric coats in islet microencapsulation. These have included alginate, agarose, tissue-engineered chondrocytes, polyacrylates,125,126 and poly(ethylene glycol) (PEG). Alginate–polylysine-based microencapsulation of islets, first described by Lim and Sun,32 has been found not to interfere with cellular function, and these microcapsules have been shown to be stable for years in small and large animals as well as in human beings.127–132 Alginate is a linear polysaccharide extracted from seaweed.133 It is composed of three types of 1 4-linked polyuronic acid blocks containing poly-L-guluronic acid segments (G blocks), poly-D-manuronic acid segments (M blocks), and segments of alternating L-guluronic and D-mannuronic acid residues, and it has gel-forming properties in the presence of most polyvalent cations.133,134 Impurities, such as monomers, catalysts, and initiators present in synthetic polymers, may contribute to the failure of the encapsulated islet im-
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plants.135 Other impurities, such as pyrogens and mitogens, can be found in polymers derived from natural substances, such as seaweed.135 Up to 90% of the impurities can be removed by electrophoresis or by the Klock extraction purification procedure.130,135 Conflicting reports exist on whether alginate with high M blocks or high G blocks results in increased fibrosis.53 It has been shown, however, that the inflammatory response to alginate becomes less with purification, irrespective of the composition used.36,136 The immunoisolating permselectivity of alginate microcapsules can be achieved by soaking the initial alginate beads in 0.05–0.1% polylysine dissolved in normal saline for 6–20 min. This occurs as a result of the binding of negatively charged carboxyl groups on the alginate to positively charged -amino groups on the amino acid polymer. The droplets are again rinsed with normal saline, followed by the addition of an outer alginate layer by soaking in a lower concentration of 0.06–0.25% sodium alginate for 4 min. It is extremely important that the outer coating of alginate on top of the polylysine membrane is complete. In the case of incomplete cover of the polylysine coating and when an impure alginate is used for the outer coating, microcapsules may become completely covered by cell overgrowths within 1 week of transplantation because of the inflammatory reactions attributable to polylysine and the impurities in the alginate.137 To dissolve the intracapsular sodium alginate by chelation of the cross-linking calcium, the resulting microcapsules are treated with 55 mM sodium citrate for 7 min. The hollow microcapsules containing a few islets are then washed with saline (Fig. 3). It appears that liquefaction of the inner alginate core of microcapsules is required for optimal function of the enclosed islets.138 One successful transplantation of encapsulated islets performed with this technique was reported by a team of investigators at Duke University. The encapsulated islets were made with an outer alginate coating and inner polyL-ornithine layer for permselectivity. To liquefy the inner alginate core of each bead a salt treatment was used. After transplantation, encapsulated islet cells were shown to keep a baboon with diabetes from requiring exogenous insulin for more than 9 months.122
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FIG. 3. Microencapsulation of islets with alginate– polylysine.
For a capsule to be stable in vivo it has to be water insoluble, as capsule dissolution will elicit a continuing inflammatory response and will ultimately expose the transplanted cells to the host. Both alginate and polylysine are water-soluble polymers and are both expected to elicit an inflammatory or foreign body response. However, findings indicate that the polyelectrolyte complex formed during encapsulation is biocompatible and less soluble in water than the individual components.62,139,140 Differences in biocompatibility were also noted between empty and cell-containing capsules in NOD mice,141,142 which could be due to the difficulty of producing reproducible cell-containing capsules. For better biocompatibility, one group has suggested the use of high guluronic acid-containing alginates,53 while another group modified alginate–polylysine capsules with PEG.132 An alternative approach has been chosen to obtain stronger and more biocompatible microcapsules using synthetic polyacrylates such as the water-insoluble synthetic copolymer hydroxyethyl methacrylate-methyl methacrylate.143 This research group has selected the acrylate monomers because of their diversity and has shown that mammalian cells
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may be microencapsulated in uncharged and polyelectrolyte polymer, in polyelectrolyte complexes, and inside a cohesive precipitate from a destabilized emulsion, without loss of viability. Polymerization of acrylamide monomer on islet cells encapsulated in agarose microspheres has also been reported.144 However, when polymerization is in direct contact with tissue, this can generate excessive local heating and cytotoxicity.145 Poly(styrene sulfonic acid), one of the most potent complement-interacting polymers, was selected for the purpose of consuming cytolytic complement activity and mixed with agarose for the preparation of microbeads to protect xenogeneic islets in mice with diabetes from the humoral immune reaction.63,146 Antibodies and complement proteins play a major role in the rejection of xenografts. Binding of antibodies to the antigens on the xenogeneic cell surface cannot alone damage the islet cell. For destruction of the xenogeneic cells to occur, complement activation by antigen–antibody complexes present on the cell surface is required.147 Various polymers bearing hydroxyl groups such as regenerated cellulose, cellulose acetate, and poly(ethylene-covinyl alcohol) activate the complement system through the alternative pathway,148 whereas other polymers bearing sulfonic acid or sulfate groups strongly interact with complement proteins and decrease the cytolytic complement activity.149 As previously noted, a fibrotic reaction after implantation would limit the diffusion of essential nutrients or metabolites, leading to reduced viability or impaired functional activity of the encapsulated cells. Several studies have reported the usefulness of coating membrane surfaces with poly(ethylene oxide) (PEO) in order to reduce protein adsorption and associated fibrotic reaction.150 This was achieved by exposing the poly(acrylonitrile-vinyl chloride) membrane surface to amine-terminated PEO after acid hydrolysis of the nitrile group to a carboxylic acid. Membrane surfaces were also exposed to PEO-succinimide after base reduction to an amine.151 The in vivo response proved that a smaller number of macrophages and foreign body giant cells were present on the PEO-grafted membrane surface without
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any change in transport properties. The other application to modify the membrane surface was achieved by synthesizing a graft copolymer having poly-L-lysine as the backbone and the monomethoxy PEG as pendant chains. Microcapsules with sodium alginate were formed using this polycationic copolymer, which demonstrated reduced protein adsorption, complement binding, and cell adhesion in vitro compared with materials with unmodified polyL-lysine.132 That artificial materials used for immunoisolation purposes are not completely inert and can induce foreign body and inflammatory reactions have led researchers to study tissue-engineered capsules of rat chondrocytes. Pollok et al.126 proposed the encapsulation of islets with a layer of chondrocytes and their matrix to prevent immunorecognition and destruction of transplanted allogeneic or xenogeneic islets. This investigation demonstrated the membrane’s ability as an immunoisolation barrier by utilizing the immunoprivileged properties of the chondrocyte matrix. This approach might offer a therapeutic advantage for patients with diabetes, using the patient’s own chondrocytes from a cartilage biopsy specimen as the encapsulation material. PEG is another polymer that has been utilized for the purpose of encapsulating islets. In order to obtain stable and biocompatible gels without any cytotoxicity generation, Pathak et al.152 and Cruise et al.153 reported rapid photopolymerization of water-soluble PEG-based macromers in direct contact with islet cells. PEG is biocompatible, nontoxic, non-immunogenic, and hydrophilic and can be chemically cross-linked into hydrogels for a variety of applications.154,155 Diffusion profiles of proteins through PEG diacrylate hydrogels showed that gels formed with the proper formulation were capable of being immunoprotective. The semipermeable properties of cross-linked PEG hydrogels made by interfacial photopolymerization of PEG diacrylate were also studied by Cruise et al.156 by forming hydrogels upon poly(vinylidiene fluoride) microporous filters. This approach allowed them to study the effect of molecular weight and concentration of the PEG diacrylate on the diffusivity of biological molecules.
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Techniques used for droplet generation The other major consideration in microencapsulation technology is how to get the cells into the individual polymer capsules. Techniques to form such a physical barrier include air-jet syringe pump droplet generator, interfacial photopolymerization, and selective withdrawal. One of the main devices used for microencapsulation of islets with alginate–polylysine is the air-jet syringe pump droplet generator.157 The device consists of an air jacket surrounding an alginate nozzle through which alginate solution is injected.158,159 Islets are suspended in a solution of 1.4–3% sodium alginate, and spherical droplets of this suspension are formed by an air-jet syringe pump generator. As alginate droplets are forced out of the end of the needle by the syringe pump, the droplets are pulled off by the shear forces set up by the flowing air stream. The size of the spherical droplets is controlled by adjusting the flow rate of the air jacket. The spherical droplets are collected in a funnel containing HEPES acid buffer supplemented with 1.1% CaCl2 solution, which transformed alginate droplets into rigid beads by gelation.110 Excess fluid is drained with the aid of a filtering device consisting of a nylon mesh and a stopcock at the exit end of the funnel, and the droplets are washed in normal saline. As noted earlier, an air jet-syringe pump extrusion method generates gel droplets containing entrapped islets from the suspension of the islets in aqueous sodium alginate solution. However, there are various constraints concerning the use of this procedure to produce microcapsule diameters of less than 700 m. To form perfectly spherical capsules, the viscosity of the gel-forming liquid must be greater than 30 cp. Also, to prevent the blockage of the needle by the islets, the minimum internal diameter of the needle must be greater than 300 m. Finally, the volumetric airflow rate must remain below 2,000 mL/min in order to produce capsules of uniform diameter. This procedure was improved by employing an electrostatic droplet generator to form uniform, smooth, and perfectly spherical microcapsules having a diameter about 150–500 m.160 A droplet,
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which is charged with high static voltage, is suspended from a needle and attracted to a collecting vessel with opposing polarity. Once the voltage threshold potential is passed, the droplet moves from the needle to the collecting vessel. Droplets with predetermined sizes may be repeatedly generated as the voltage pulse height, pulse frequency and length, and extrusion rate of the droplets are adjustable. As shown in Figure 4, droplets are formed as the plunger is driven by the syringe pump and expelled towards a collecting vessel containing a hardening solution, which may be aqueous calcium chloride in the case of an aqueous droplet forming liquid containing sodium alginate. Negative polarity was attached to the needle, while positive polarity was attached to the metal ring. As the voltage applied during droplet formation is a static voltage, the viability of the encapsulated islets is not compromised. Biomolecules such as proteins and cells can also be deposited as patterned droplets onto surfaces for the development of cell-based biosensing devices for applications such as high-throughput drug screening.161 There are numerous techniques for microfabrication of patterned polymer surfaces for protein delivery. Lithographic techniques and microcontact printing have been most widely used to generate patterned droplets of cells.162–168 PEG hy-
FIG. 4. Schematic representation of an electrostatic droplet generator.
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drogel is the most commonly used material for cell patterning purposes. In the case of surfaceinitiated photopolymerization of PEG, the technique involves immobilization of initiator onto a substrate surface using a stamp such as poly(dimethyl siloxane), followed by polymerization of precursor solution.169 One recent study focused on the cellular delivery based on micro- and nanotechnology.170 To provide an immunoisolating microenvironment for islet cells, nanoporous biocapsules were bulk and surface micromachined to present uniform and well-controlled pore sizes. The designs of the membranes with defined nanopores were fabricated using a thermally grown silicon oxide sandwiched between two structural layers of silicon. Another technique used to generate droplets of encapsulated islet cells was proposed by Cruise et al.153 using interfacial photopolymerization. The technique involves physical adsorption of the initiator eosin Y on the islet cell surface, which then allows polymerization to occur on the islet–prepolymer interface. The interfacial photoinitiation process employed with this technique results in conformal coating of cross-linked PEG-based hydrogel on the islet cell surface. The process of selective withdrawal is another novel method reported recently for the purpose of coating cell clusters, such as islet cells.171 Briefly, the process involves the insertion of a tube into a container such that its tip is suspended at a specific height above an interface separating two immiscible fluids (Fig. 5). In the case of a low fluid withdrawal rate, only the upper fluid (oil) is withdrawn through the tube. Increasing the fluid flow rate or de-
To pump
Oil
Water
FIG. 5.
Diagram of the selective withdrawal apparatus.
creasing the height of the tip above the interface results in a transition where the lower liquid (water) is incorporated in a thin spout along with the oil. The technique was demonstrated through encapsulation of a poppy seed in a poly(styrenesulfonic acid). The particles were coated with the prepolymer solution, which has styrene sulfonic acid sodium salt and triethylene glycol diacrylate, and mixed with the initiator eosin Y and co-initiator triethanolamine used for the purpose of initiating polymerization.171 After the selective withdrawal step, the coated particles were collected and photopolymerized with a halogen lamp. The technique could also be used to encapsulate cells through surface initiated photopolymerization of PEG. The photoinitiator eosin Y could be immobilized on the islet cell surface through covalent attachment,169 and after the selective withdrawal process, cells could be collected and illuminated for 2–3 min with an argon ion laser in order to cross-link the coat around the cells.
CONCLUSIONS The successful development of a bioartificial pancreas involves several considerations. Two major obstacles in islet transplantation are the limited supply of islet cells and the use of immunosuppressive drugs to prevent transplant rejection. It is hoped that the use of encapsulated islets as a form of bioartificial pancreas would overcome these obstacles. Three potential sources of islet cell tissue, including human or allogeneic cells, porcine or xenogenic cells, and engineered cells, are currently under investigation. Among these sources, human islet cells would be the least immunogenic; however, there is a shortage of pancreata retrieved from human cadaveric donors, and these cells have a limited secretory capacity and life span. The use of porcine islet cells could be a better option, because these are readily available, and there is an unlimited supply of donors. However, one major concern about the use of islets harvested from pigs has been the possibility of transmittance of porcine endogenous retroviruses (PERVs) to humans during transplantation. This concept arose from studies on the
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infection of humans and immunodeficient mice after pig cell xenotransplantation.172,173 There are studies, however, showing no possibility of transmission of PERVs from pigs to human recipients of islet xenografts.120,174,175 Indeed, one recent study showed that the porcine genome harbors a limited number of infectious PERV sequences, which suggests that many breeds of pig fail to produce PERVs capable of infecting human cells even in laboratory testing.176 This raises the possibility of using special breeds of pig for an unlimited supply of islet cells for encapsulation and use as a bioartificial pancreas in clinical xenotransplantation. The other attractive approach to generate unlimited supply of islets is the use of embryonic stem cells, which have the ability to differentiate into a variety of cell lineages in vitro. Recent studies in small animals showing successful transplantation of pancreatic stem cells suggest that this approach could provide an unlimited source of allogeneic insulin-producing tissue in the future.177–179 The maintenance of cell viability is another important concern in the development of a successful bioartificial pancreas. When cells are encapsulated and implanted in an environment without a natural circulation, lack of an adequate supply of oxygen and nutrients results in cell necrosis, making implantation unsuccessful. To improve long-term islet survival and function, vascularization of transplanted islets has been proposed.180–182 An ideal architecture for implantation would be to promote vascularization around encapsulated cells using a novel approach. Recently, a technique to form covalently bonded multilayers of PEG hydrogels has been developed. This approach may be used to encapsulate cells with bilayers such that outer layer would promote vascularization around the inner layer.183,184 There are other areas of urgent need for the successful development of a bioartificial pancreas through islet cell microencapsulation. All current techniques for cell microencapsulation are slow in producing desirable microcapsules, requiring days to generate enough encapsulated islets for one transplantation.185 An ideal procedure for routine use of encapsulated islets would require the development of massively parallel concepts in microencapsulation, which
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would generate sufficient quantities of microcapsules for multiple transplantations in a matter of minutes. In the absence of a technique based on such a principle, there is a need for an adequate procedure for long-term storage of encapsulated cells to enhance their use for transplantation. It is also highly desirable to develop rapid in vitro techniques for determination of the functional viability of encapsulated islets prior to transplantation. In summary, a potential cure for Type 1 diabetes could be the use of bioartificial pancreatic constructs based on insulin-secreting cells (allogeneic or xenogeneic islet cells) that are immunoisolated with the microencapsulation technique. Additionally, enhanced survival of the graft might be supported with a novel approach to induce neovascularization, which could make this technology a clinical reality in the near future. This will in turn serve as an important progress in cell therapy for the treatment of diabetes as well as other diseases, such as cancer, hemophilia, liver failure, and Parkinson’s disease.
REFERENCES 1. Kleinman JC, Donahue RP, Harris MI, Finucane FF, Madans JH, Brock DB: Mortality among diabetics in a national sample. Am J Epidemiol 1988;128:389–401. 2. Drury TF: Disability among adult diabetics. In: National Diabetes Group, eds. Diabetes in America: Diabetes Data Compiled. Publication 85-1468. Bethesda, MD: National Diabetes Data Group, 1985. 3. World Health Organization: Definition, Diagnosis and Classification of Diabetes Mellitus and Its Complications. Report of a WHO Consultation. Geneva: World Health Organization, 1999. 4. Nathan DM: Long term complications of diabetes mellitus. N Engl J Med 1993;328:1676–1685. 5. Barrett-Connor E, Orchard T: Insulin-dependent diabetes mellitus and ischemic heart disease. Diabetes Care 1985;8:65–70. 6. Barret-Conor E, Khaw KT: Diabetes mellitus and independent risk factor for stroke. Am J Epidemiol 1988;128:116–123. 7. Bild DE, Selby JV, Sinnock P, Browner WS, Braveman P, Showstack JA: Lower extremity amputation in people with diabetes: epidemiology and prevention. Diabetes Care 1989;12:24–29. 8. Geiss LS, Herman WH, Teutsch SM: Diabetes and renal mortality in the United States. Am J Public Health 1985;75:1325–1326.
6019_13_p968-985
12/21/05
3:09 PM
Page 980
980 9. Moss SE, Klein R, Klein BE: The incidence of vision loss in a diabetic population. Ophthalmology 1988; 95:1340–1348. 10. Rubin RJ, Altman WM, Mendelson DN: Health care expenditures for people with diabetes mellitus. J Clin Endocrinol Metab 1994;78:809A–809F. 11. Diabetes Control and Complications Trial Research Group: The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. N Engl J Med 1993;329:977–986. 12. Ido Y, Vindignia A, Chang K, Stramm L, Chance R, Heath WF, DiMarchi RD, Di Cera E, Williamson JR: Prevention of vascular and neural dysfunction in diabetic rats by C-peptide. Science 1997;277:563–566. 13. Johansson BL, Borg K, Fernqvist-Forbes E, Kernell A, Odergren T, Wahren J: Beneficial effects of C-peptide on incipient nephropathy and neuropathy in patients with Type 1 diabetes mellitus. Diabet Med 2000;17:181–189. 14. Hansen A, Johansson BL, Wahren J, Bibra HV: Cpeptide exerts beneficial effects on myocardial blood flow and function in patients with Type 1 diabetes. Diabetes 2002;51:3077–3082. 15. Ekberg K, Brismar T, Johansson BL, Jonsson B, Lindström P, Wahren J: Amelioration of sensory nerve dysfunction by C-peptide in patients with Type 1 diabetes. Diabetes 2003;52:536–541. 16. Sutherland DE, Gruessner RW, Gruessner AC: Pancreas transplantation for treatment of diabetes mellitus. World J Surg 2001;25:487–496. 17. Venstrom JM, McBride MA, Rother KI, Hirshberg B, Orchard TJ, Harlan DM: Survival after pancreas transplantation in patients with diabetes and preserved kidney function. JAMA 2003;290:2817–2823. 18. Sutherland DE, Stratta RJ, Gruessner AC: Pancreas transplant outcome by recipient category: single pancreas versus combined kidney-pancreas. Curr Opin Organ Transplant 1998;3:231–241. 19. de Groot M, Schuurs TA, van Schilfgaarde R: Causes of limited survival of microencapsulated pancreatic islet grafts. J Surg Res 2004;121:141–150. 20. Kendall WFJ, Opara EC: Immunoisolation techniques for islet cell transplantation. Expert Opin Biol Ther 2002;2:503–511. 21. Alejandro R, Feldman EC, Bloom AD, Kenyon NS: Effects of cyclosporin on insulin and C-peptide secretion in healthy beagles. Diabetes 1989;38:698–703. 22. Gunnarsson R, Klintmalm G, Lundgren G, Wilczek H, Ostman J, Groth CG: Deterioration in glucose metabolism in pancreatic transplant recipients given cyclosporine. Lancet 1983;2:571–572. 23. Schlumpf R, Largiader F, Uhlschmid GK, Baumgartner D: Is cyclosporine toxic for transplanted pancreatic islets? Transplant Proc 1986;28:1169–1170. 24. Van Schilfgaarde R, van der Burg MPM, van Suylichem HG, Goosen HG, Frolich M: Does cyclosporine influence beta cell function? Transplant Proc 1986;28:1175–1176.
KIZILEL ET AL. 25. Shapiro AM, Lakey JR, Ryan EA, Korbutt GS, Toth E, Warnock GL, Kneteman NM, Rajotte RV: Islet transplantation in seven patients with Type I diabetes mellitus using a glucocorticoid-free immunosuppressive regimen. N Engl J Med 2000;343:230–238. 26. Ryan EA, Lakey JRT, Paty BW, Imes S, Korbutt GS, Kneteman, NM, Bigam D, Rajotte RV, Shapiro AM: Successful islet transplantation: continued insulin reverse provides long-term glycemic control. Diabetes 2002;51:2148–2157. 27. Brunicardi FC, Mullen Y: Issues in clinical islet transplantation. Pancreas 1994;9:281–290. 28. De Vos P, Van Schilfgaarde R: Biocompatibility issues. In: Chick WL, ed. Cell Encapsulation Technology and Therapeutics. Boston: Birkhauser, 1999: 63–79. 29. De Vos P, Van Straaten JF, Nieuwenhuizen AG, de Groot M, Ploeg RJ, De Haan BJ, Van Schlifgaarde R: Why do microencapsulated islet grafts fail in the presence of fibrotic overgrowth? Diabetes 1999;48: 1381–1388. 30. Ortiz C, Zhang D, Xie Y, Davisson VJ, Ben-Amotz D: Identification of insulin variants using Raman spectroscopy. Anal Biochem 2004;332:245–252. 31. Lanza RP, Ecker DM, Kuhtreiber WM, Marsh JP, Ringeling J, Chick WL: Transplantation of islets using microencapsulation: studies in diabetic rodents and dogs. J Mol Med 1999;77:206–210. 32. Lim F, Sun AM: Microencapsulated islets as bioartificial endocrine pancreas. Science 1980;210:908–910. 33. Soon-Shiong P, Heintz RE, Merideth N, Yao QX, Yao ZW, Zheng TL, Murphy M, Moloney MK, Schmehl M, Harris M, Mendez R, Mendez R, Sandford PA: Insulin independence in a type I diabetic patient after encapsulated islet transplantation. Lancet 1994;343:950–951. 34. Zekorn TD, Horcher A, Siebers U, Federlin K, Bretzel RG: Synergistic effect of microencapsulation and immunoalteration on islet allograft survival in bioartificial pancreas. J Mol Med 1999;77:193–199. 35. Omer A, Keegan M, Czismadia E, De Vos P, Van Rooijen N, Bonner-Weir S, Weir GC: Macrophage depletion improves survival of porcine neonatal pancreatic cell clusters contained in alginate macrocapsules transplanted into rats. Xenotransplantation 2003;10:240–251. 36. de Vos P, Hamel AF, Tatarkiewicz K: Considerations for successful transplantation of encapsulated pancreatic islets. Diabetologia 2002;45:159–173. 37. De Vos P, Vegter D, De Haan BJ, Strubbe JH, Bruggink JE, Van Schilfgaarde R: Kinetics of intraperitoneally infused insulin in rats. Functional implications for the bioartificial pancreas. Diabetes 1996; 45:1102–1107. 38. Kessler L, Jesser C, Lombard Y, Karsten V, Belcourt A, Pinget M, Poindron P: Cytotoxicity of peritoneal murine macrophages against encapsulated pancreatic rat islets: in vivo and in vitro studies. J Leuk Biol 1996;60:729–736.
6019_13_p968-985
12/21/05
3:09 PM
Page 981
THE BIOARTIFICIAL PANCREAS 39. Toso C, Oberholzer J, Ceausoglu I, Ris F, Rochat B, Rehor A, Bucher P, Wandrey C, Schuldt U, Belenger J, Bosco D, Morel P, Hunkeler D: Intra-portal injection of 400 microcapsules in a large animal model. Transplant Int 2003;16:405–410. 40. Qian JH, Hashimoto T, Fujiwara H, Hamaoka T: Studies on the induction of tolerance of alloantigens. I. The abrogation of potentials for delayed-type-hypersensitivity responses to alloantigens by portal venous inoculations with allogeneic cells. J Immunol 1985;134:3656–3661. 41. Alejandro R, Mintz DH, Noel J, Latif Z, Koh N, Russell E, Miller J: Islet cell transplantation in Type I diabetes mellitus. Transplant Proc 1987;19:2359– 2361. 42. Brendel MD, Hering B, Schultz AO, Bretzel RG: International Islet Transplant Registry. Newsletter 8. Giessen, Germany: Justus-Liebig-University of Giessen, 1999. 43. Pfeiffer EF: Artificial pancreas, glucose sensors and the impact upon diabetology. Int J Artif Organs 1993;16:636–644. 44. Pfeiffer EF, Kerner W, Beischer W: Substitution of islet cell function by mechanical device: extracorporeal artificial pancreas. Adv Exp Med Biol 1979; 119:501–508. 45. Kruse-Jarres JD, Braun G, Naegele R, Bresch M, Lehmann U: Blood glucose monitoring and computer regulation by means of an artificial endocrine pancreas. Horm Metab Res Suppl 1979;8:42–45. 46. Kerner W, Beischer W, Herfarth C, Pfeiffer EF: Application of an artificial endocrine pancreas in the management of the diabetic surgical patient. Horm Metab Res Suppl 1979;8:159–161. 47. Hunkeler D: Allo transplants xeno: as bioartificial organs move to the clinic. Ann N Y Acad Sci 2001;944: 1–6. 48. Encapsulation & immunoprotective strategies of islet cells. Diabetes Technol Ther 2002;4:361. 49. Prokop A: Bioartificial pancreas: materials, devices, function and limitations. Diabetes Technol Ther 2001;3:431–449. 50. Chang TMS: Semipermeable microcapsules. Science 1964;146:524–525. 51. Sun AM, O’Shea GM, Goosen MF: Injectable microencapsulated islet cells as bioartificial pancreas. Appl Biochem Biotechnol 1984;10:87–99. 52. Wiegand F, Kroncke KD, Kolb-Bachofen V: Macrophage generated nitric-oxide as cytptoxic factor in destruction of alginate-encapsulated islets. Transplant Proc 1993;56:1206–1212. 53. Soon-Shiong P, Otterlie M, Skjak-Braek G, Smidsrod O, Heintz R, Lanza RP, Espevik T: An immunologic basis for the fibrotic reaction to implanted microcapsules. Transplant Proc 1991;23:758–759. 54. Strand BL, Ryan TL, In’t Veld P, Kulseng B, Rokstad AM, Skjak-Brek G, Espevik T: Poly-L-lysine induces fibrosis on alginate microcapsules via the induction of cytokines. Cell Transplant 2001;10:263–275.
981 55. De Vos P, Wolters GHJ, Fritschy WM, Van Schilfgaarde R: Obstacles in the application of microencapsulation in islet transplantation. Int J Artif Organs 1993;16:205–212. 56. Goosen MFA: In: Lanza RR, Chick WL, eds. Immunoisolation of Pancreatic Islets. Austin, TX: R.G. Landes Co., 1994:21–44. 57. Soon-Shiong P, Feldman E, Nelson R, Heintz R, Yao Q, Yao Z, Zheng T, Merideth N, Skjak-Braek G, Espevik T, Smidsrod O, Sandford P: Long-term reversal of diabetes by the injection of immunoprotected islets. Proc Natl Acad Sci U S A 1993;90:5843–5847. 58. Lanza R, Langer R, Chick WL: Principles of Tissue Engineering, 2nd ed. Orlando, FL: Academic Press, 2000. 59. Colton CK, Avgoustiniatos ES: Bioengineering in development of the hybrid artificial pancreas. J Biomech Eng 1991;113:152–170. 60. Brissova M, Lacik I, Powers AC, Anilkumar AV, Wang TG: Control and measurement of permeability for design of microcapsule cell delivery system. J Biomed Mater Res 1998;39:61–70. 61. Cole DR, Waterfall M, McIntyre M, Baird JD: Microencapsulated islet grafts in the BB/E rat: a possible role for cytokines in graft failure. Diabetologia 1992;35:231–237. 62. Fan MY, Lum Z, Levesque L, Tai IT, Sun AM: Reversal of diabetes in BB rats by transplantation of encapsulated rat islets. Diabetes 1990;39:519–522. 63. Iwata H, Takagi T, Kobayashi K, Oka T, Tsuki T, Ito F: Strategy for developing microbeads applicable to islet xenotransplantation into a spontaneous diabetic NOD mice. J Biomed Mater Res 1994;28:1201–1207. 64. Lanza RR, Chick WL, eds.: Immunoisolation of Pancreatic Islets. Austin, TX: R.G. Landes Co., 1994. 65. Lacik I, Brissova M, Anilkumar AV, Powers AC, Wang TG: New capsule with tailored properties for the encapsulation of living cells. J Biomed Mater Res 1998;39:52–60. 66. Lum ZP, Tai P, Krestow I, Tai T, Sun AM: Xenografts of microencapsulated rat islets into diabetic mice. Transplant Proc 1992;53:1180–1183. 67. Iwata H, Murakami Y, Ikada Y: Control of complement activities for immunoisolation. Ann N Y Acad Sci 1999;875:7–23. 68. Paul LC: Mechanism of humoral xenograft rejection. In: Cooper DKC, Kemp E, Reemtsma K, White D, eds. Xenotransplantation. Berlin: Springer-Verlag, 1991:47–67. 69. Auchincloss HJ, Moses R, Conti D, Sundt T, Smith C, Sachs DH, Winn HJ: Rejection of transgenic skin expressing a xeno-class I antigen is CD4-dependent and CD8-independent. Transplant Proc 1990;22: 1059–1060. 70. Sayegh MH, Watschinger B, Carpenter CB: Mechanisms of T cell recognition of alloantigen. The role of peptides. Transplantation 1994;57:1295–1302. 71. Chen G, Hohmeier HE, Gasa R, Tran VV, Newgard CB: Selection of insulinoma cell lines with resistance
6019_13_p968-985
12/21/05
3:09 PM
Page 982
982
72.
73.
74. 75.
76.
77. 78.
79.
80.
81.
82.
83.
84.
85.
86.
87.
KIZILEL ET AL. to interleukin-1beta- and gamma-interferon-induced cytotoxicity. Diabetes 2000;49:562–570. Safley SA, Kapp LM, Tucker-Burden C, Hering B, Kapp JA, Weber CJ: Inhibition of cellular immune responses to encapsulated porcine islet xenografts by simultaneous blockade of two different costimulatory pathways. Transplantation 2005;79:409–418. Rafael E, Wu GS, Hultenby K, Tibell A, Wernerson A: Improved survival of macroencapsulated islets of Langerhans by preimplantation of the immunoisolating device: a morphometric study. Cell Transplant 2003;12:407–412. Anderson JM: Inflammatory response to implants. ASAIO Trans 1988;34:101–107. Anderson JM: Mechanisms of inflammation and infection with implanted devices. Cardiovasc Pathol 1993;2(Suppl):33S–41S. Anderson JM: Biocompatibility of tissue engineered implants. In: CW Patrick Jr, AG Mikos, LV McIntire, eds. Frontiers in Tissue Engineering. Oxford: Elsevier Science, 1998:152–165. Bonner-Weir S, Sharma A: Pancreatic stem cells. J Pathol 2002;197:519–526. Lumelsky N, Blondel O, Laeng P, Velasco I, Ravin R, McKay R: Differentiation of embryonic stem cells to insulin-secreting structures similar to pancreatic islets. Science 2001;292:1389–1394. Peck AB, Cornelius JG, Schatz D, Ramiya VK: Generation of islets of Langerhans from adult pancreatic stem cells. J Hepatobil Pancreat Surg 2002;9:704–709. Ramiya VK, Maraist M, Arfors KE, Schatz DA, Peck AB, Cornelius JG: Reversal of insulin-dependent diabetes using islets generated in vitro from pancreatic stem cells. Nat Med 2000;6:278–282. Chen R, Meseck ML, Woo SL: Auto-regulated hepatic insulin gene expression in type I diabetic rats. Mol Ther 2001;3:584–590. Lee HC, Kim SJ, Kim KS, Shin HC, Yoon JW: Remission in models of Type I diabetes by gene therapy using a single-chain insulin analogue. Nature 2000;408:483–488. Tang SC, Sambanis A: Development of genetically engineered human intestinal cells for regulated insulin secretion using rAAV-mediated gene transfer. Biochem Biophys Res Commun 2003;303:645–652. Tang SC, Sambanis A: Preproinsulin mRNA engineering and its application to the regulation of insulin secretion from human hepatomas. FEBS Lett 2003;537:193–197. Thule PM, Liu JM: Regulated hepatic insulin gene therapy of STZ-diabetic rats. Gene Ther 2000; 7:1744–1752. Thule PM, Liu J, Philips LS: Glucose regulated production of human insulin in rat hepatocytes. Gene Ther 2000;7:205–214. Lumelsky N, Blondel O, Laeng P, Velasco I, Ravin R, McKay R: Differentiation of embryonic stem cells to insulin secreting structures similar to pancreaticislets. Science 2001;292:1389–1394.
88. Hori Y, Rulifson IC, Tsai BC, Heit JJ, Cahoy JD, Kim SK: Growth inhibitors promote differentiation of insulin-producing tissue from embryonic stem cells. Proc Natl Acad Sci U S A 2002;99:16105–16110. 89. Blyszczuk P, Czyz J, Kania G, Wagner M, Roll U, StOnge L, Wobus AM: Expression of Pax4 in embryonic stem cells promotes differentiation of nestinpositive progenitor and insulin-producing cells. Proc Natl Acad Sci U S A 2003;100:998–1003. 90. Moritoh Y, Yamato E, Yasui Y, Miyazaki S, Miyazaki J: Analysis of insulin-producing cells during in vitro differentiation from feeder-free embryonic stem cells. Diabetes 2003;52:1163–1168. 91. Segev H, Fishman B, Ziskind A, Shulman M, Itskovitz-Eldor J: Differentiation of human embryonic stem cells into insulin-producing clusters. Stem Cells 2004;22:265–274. 92. Hori Y, Gu X, Xie X, Kim SK: Differentiation of insulin-producing cells from human neural progenitor cells. PLoS Med 2005;2:e103. 93. Assady S, Maor G, Amit M, Itskovitz-Eldor J, Skorecki KL, Tzukerman M: Insulin production by human embryonic stem cells. Diabetes 2001;50:1691– 1697. 94. Xu G, Stoffers DA, Habener JF, Bonner-Weir S: Exendin-4 stimulates both beta-cell replication and neogenesis, resulting in increased beta-cell mass and improved glucose tolerance in diabetic rats. Diabetes 1999;48:2270–2276. 95. Gromada J, Brock B, Schmitz O, Rorsman P: Glucagon-like peptide-1: regulation of insulin secretion and therapeutic potential. Basic Clin Pharmacol Toxicol 2004;95:252–262. 96. Harmon EB, Apelqvist AA, Smart NG, Gu X, Osborne DH, Kim SK: GDF11 modulates NGN3 islet progenitor cell number and promotes beta-cell differentiation in pancreas development. Development 2004;131:6163–6174. 97. Bach JF: Immunotherapy of insulin-dependent diabetes mellitus. Curr Opin Immunol 2001;13:601–605. 98. Giannoukakis N, Rudert WA, Robbins PD, Trucco M: Targeting autoimmune diabetes with gene therapy. Diabetes 1999;48:2107–2121. 99. Docherty K: Gene therapy for diabetes mellitus. Clin Sci (Lond) 1997;92:321–330. 100. Soria B, Skoudy A, Martin F: From stem cells to beta cells: new strategies in cell therapy of diabetes mellitus. Diabetologia 2001;44:407–415. 101. Selden RF, Skoskiewicz MJ, Russell PS, Goodman HM: Regulation of insulin-gene expression. Implications for gene therapy. N Engl J Med 1987; 317:1067–1076. 102. Newgard CB: Cellular engineering and gene therapy strategies for insulin replacement in diabetes. Diabetes 1994;43:341–350. 103. Halban PA, Kahn SE, Lernmark A, Rhodes CJ: Gene and cell-replacement therapy in the treatment of type 1 diabetes: how high must the standards be set? Diabetes 2001;50:2181–2191.
6019_13_p968-985
12/21/05
3:09 PM
Page 983
THE BIOARTIFICIAL PANCREAS 104. Sapir T, Shternhall K, Meivar-Levy I, Blumenfeld T, Cohen H, Skutelsky E, Eventov-Friedman S, Barshack I, Goldberg I, Pri-Chen S, Ben-Dor L, PolakCharcon S, Karasik A, Shimon I, Mor E, Ferber S: Cell-replacement therapy for diabetes: generating functional insulin-producing tissue from adult human liver cells. Proc Natl Acad Sci U S A 2005;102:7964–7969. 105. Fujita Y, Cheung AT, Kieffer TJ: Harnessing the gut to treat diabetes. Pediatr Diabetes 2004;5:57–69. 106. Cheung AT, Dayanandan B, Lewis JT, Korbutt GS, Rajotte RV, Bryer-Ash M, Boylan MO, Wolfe MM, Kieffer TJ: Glucose-dependent insulin release from genetically engineered K cells. Science 2000;290: 1959–1962. 107. El-Shewy HM, Kendall WFJ, Darrabie M, Collins BH, Opara EC: Polyvinyl pyrrolidone: a novel cryoprotectant in islet cell cryopreservation. Cell Transplant 2004;13:237–243. 108. Zimmermann H, Katsen AD, Ihmig FR, Durst CHP, Shirley SG, Fuhr GR: First steps of an interdisciplinary approach towards miniaturised cryopreservation for cellular nanobiotechnology. IEE Proc Nanobiotechnol 2004;151:134–138. 109. Zimmermann H, Zimmermann D, Reuss R, Feilen PJ, Manz B, Katsen A, Weber M, Ihmig FR, Ehrhart F, Gessner P, Behringer M, Steinbach A, Wegner LH, Sukhorukov VL, Vasquez JA, Schneider S, Weber MM, Volke F, Wolf R, Zimmermann U: Towards a medically approved technology for alginate-based microcapsules allowing long-term immunoisolated transplantation. J Mater Sci Mater Med 2005;16: 491–501. 110. Misler S, Dickey A, Barnett DW: Maintenance of stimulus-secretion coupling and single beta-cell function in cryopreserved-thawed human islets of Langerhans. Pflugers Arch 2005;450:395–404. 111. von Mach MA, Schlosser J, Weiland M, Feilen PJ, Ringel M, Hengstler JG, Weilemann LS, Beyer J, Kann P, Schneider S: Size of pancreatic islets of Langerhans: a key parameter for viability after cryopreservation. Acta Diabetol 2003;40:123–129. 112. Lanza RP, Chick WL: Transplantation of encapsulated cells and tissues. Surgery 1997;121:1–9. 113. Lanza RP, Chick WL: Immunoisolation strategies for the transplantation of pancreatic islets. Ann N Y Acad Sci 1997;831:323–331. 114. Wang TG: Progress in bioartificial pancreas. J Regen Med 2000;1:93–98. 115. Jain K, Yang H, Cai BR, Haque B, Hurvitz AI, Diehl C, Miyata T, Smith BH, Stenzel K, Suthanthiran M: Retrievable, replaceable, macroencapsulated pancreatic islet xenografts. Long-term engraftment without immunosuppression. Transplantation 1995;59:319–324. 116. Trivedi N, Keegan M, Steil GM, Hollister-Lock J, Hasenkamp WM, Colton CK, Bonner-Weir S, Weir GC: Islets in alginate macrobeads reverse diabetes despite minimal acute insulin secretory responses. Transplantation 2001;71:203–211.
983 117. Lacy PE, Hegre OD: Maintenance of normoglycemia in diabetic mice by subcutaneous xenografts of encapsulated islets. Science 1991;254:1782–1784. 118. Scharp DW, Swanson CJ, Olack BJ, Latta PP, Hegre OD, Doherty EJ, Gentile FT, Flavin KS, Ansara MF, Lacy P: Protection of encapsulated human islets implanted without immunosuppression in patients with type I or type II diabetes and in nondiabetic control subjects. Diabetes 1994;43:1167–1170. 119. van Suylichem PT, Strubbe JH, Houwing H, Wolters GH, van Schilfgaarde R: Insulin secretion by rat islet isografts of a defined endocrine volume after transplantation to three different sites. Diabetologia 1992;35:917–923. 120. Elliott RB, Escobar L, Garkavenko O, Croxson MC, Schroeder BA, McGregor M, Ferguson G, Beckman N, Ferguson S: No evidence of infection with porcine endogenous retrovirus in recipients of encapsulated porcine islet xenografts. Cell Transplant 2000;9: 895–901. 121. Sun YL, Ma X, Zhou D, Vacek I, Sun AM: Normalization of diabetes in spontaneously diabetic cynomologus monkeys by xenografts of microencapsulated porcine islets without immunosuppression. J Clin Invest 1996;98:1417–1422. 122. Kendall WF, Collins BH, Hobbs HA, Bollinger RR, Opara EC: Long-term normoglycemia induced by microencapsulated porcine islet xenotransplantation in a diabetic baboon [abstract]. Acta Chir Austriaca 2001;33(Suppl 174):56. 123. Soon-Shiong P, Feldman E, Nelson R, Komtebedde J, Smidsrod O, Skjak-Braek G, Espevik T, Heintz R, Lee M: Successful reversal of spontaneous diabetes in dogs by intraperitoneal microencapsulated islets. Transplantation 1992;54:769–774. 124. Sun Y, Ma X, Zhou D, Vacek I, Sun AM: Normalization of diabetes in spontaneously diabetic cynomologous monkeys by xenografts of microencapsulated porcine islets without immunosuppression. J Clin Invest 1996;98:1417–1422. 125. Uludag H, De Vos P, Tresco PA: Technology of mammalian cell encapsulation. Adv Drug Deliv Rev 2000;42:29–64. 126. Pollok JM, Kolin PA, Lorenzen M, Torok E, Kaufmann PM, Kluth D, Bohuslavizki KH, Gundlach M, Rogiers X: Islets of Langerhans encapsulated with a tissue engineered membrane of rat chondrocytes maintain insulin secretion and glucose insulin feedback for at least 30 days in culture. Transplant Proc 2001;33:1713–1714. 127. Sandler S, Andersson A, Eizirik DL, Hellerstrom C, Espevik T, Kulseng B, Thu B, Pipeleers DG, SkjakBraek G: Assessment of insulin secretion in vitro from microencapsulated fetal porcine islet like cell clusters and rat, mouse, and human pancreatic islets. Transplantation 1997;63:1712–1718. 128. Halle JP, Leblond FA, Pariseau JF, Jutras P, Brabant MJ, Lepage Y: Studies on small (300 microns) microcapsules: II—Parameters governing the produc-
6019_13_p968-985
12/21/05
3:09 PM
Page 984
984
129.
130.
131.
132.
133. 134.
135.
136.
137.
138.
139.
140.
141.
142.
143.
144.
KIZILEL ET AL. tion of alginate beads by high voltage electrostatic pulses. Cell Transplant 1994;3:365–372. Duvivier-Kali VF, Omer A, Parent RJ, O’Neil JJ, Weir GC: Complete protection of islets against allorejection and autoimmunity by a simple barium alginate membrane. Diabetes 2001;50:1698–1705. Klock G, Frank H, Houben R, Zekorn T, Horcher A, Siebers U, Wöhrle M, Federlin K, Zimmermann U: Production of purified alginates suitable for use in immunoisolated transplantation. Appl Microbiol Biotechnol 1994;40:638–643. Kulseng B, Thu T, Espevik T, Skjk-Brk G: Alginate poly-lysine microcapsules as immune barrier: permeability of cytokines and immunoglobins over the capsule membrane. Cell Transplant 1997;6:387–394. Sawhney AS, Hubbell JA: Poly(ethylene oxide)graft-poly(L-lysine) copolymers to enhance the biocompatibility of poly(L-lysine)-alginate microcapsule membranes. Biomaterials 1992;13:863–870. Lim F: Microencapsulation of living mammalian cells. Adv Biotechnol Processes 1988;7:185–197. Smidsrod O, Skjak-Braek G: Alginate as immobilization matrix for cells. Trends Biotechnol 1990;8: 71–78. Prokop A, Wang TG: Purification of polymers used for fabrication of an immunoisolation barrier. Ann N Y Acad Sci 1997;831:223–231. Zekorn T, Klock G, Horcher A, Siebers U, Wohrle M, Kowalski M, Arnold M, Federlin K, Bretzel R, Zimmermann U: Lymphoid activation by different crude alginates and the effect of purification. Transplant Proc 1992;24:2952–2953. Ricker A, Stockberger S: Hyperimmune response to microencapsulated xenogeneic tissue in NOD mice [abstract]. Diabetes 1986;35:61A. Garfinkel MR, Harland RC, Opara EC: Optimization of the microencapsulated islet for transplantation. J Surg Res 1998;76:7–10. Ar’Rajab A, Bengmark S, Ahren B: Insulin secretion in streptozotocin-diabetic rats transplanted with immunoisolated islets. Transplantation 1991;51:571– 574. Calafiore R, Calcinaro F, Basta G, Pietropoalo M, Falorni A, Piermattei M, Brunetti P: A method for the massive separation of highly purified, adult porcine islets of Langerhans. Metabolism 1990;39: 175–181. Carani BS, Edwin G, eds.: Annals of the New York Academy of Sciences, Vol. 1037: Immunology of Diabetes III. New York: New York Academy of Sciences, 2005. Mazaheri R, Atkison P, Stiller C, Dupre J, Vose J, O’Shea G: Transplantation of encapsulated allogeneic islets into diabetic BB/W rats. Effects of immunosuppression. Transplantation 1991;51:750–754. Sefton MV, Stevenson WTK: Microencapsulation of live animal cells using polyacrylates. Adv Polym Sci 1993;107:145–198. Dupuy B, Gin H, Baquey C, Ducassou D: In situ polymerization of a microencapsulating medium
145.
146.
147.
148.
149.
150.
151.
152.
153.
154.
155.
156.
157.
158.
159.
round living cells. J Biomed Mater Res 1988;22: 1061–1070. Gin H, Dupuy B, Baquey C, Ducassou D, Aubertin J: Agarose encapsulation of islets of Langerhans: reduced toxicity in vitro. J Microencapsul 1987;4: 239–242. Takagi T, Iwata H, Tashiro H, Tsuji T, Ito F: Development of a novel microbead applicable to xenogeneic islet transplantation. J Control Rel 1994;31: 283–291. Iwata H, Murakami Y, Ikada Y: Control of complement activities for immunoisolation. Ann N Y Acad Sci 1999;875:7–23. Deppsich R, Goehl H, Ritz E, Haensch GM: Complement activation on artificial surfaces in biomedical therapies. In: Rother K, Till GO, Hänsch GM, eds. The Complement System, 2nd ed. Berlin: SpringerVerlag, 1998, pp. 487–504. Pangburn MK: Alternative pathway: activation and regulation. In: Rother K, Till GO, Hänsch GM, eds. The Complement System, 2nd ed. Berlin: SpringerVerlag, 1998:93–115. Chaikof EL: Engineering and material considerations in islet cell transplantation. Annu Rev Biomed Eng 1999;1:103–127. Shoichet MS, Winn SR, Gentile FT, Athavale S, Harris JM: Poly(ethylene oxide) grafted thermoplastic membranes for use as cellular hybrid bioartificial organs in the central nervous system. Biotechnol Bioengin 1994;43:563–572. Pathak CP, Sawhney AS, Hubbell JA: Rapid photopolymerization of immunoprotective gels in contact with cells and tissue. J Am Chem Soc 1992; 114:8311–8312. Cruise GM, Hegre OD, Scharp DS, Hubbell JA: A sensitivity study of the key parameters in the interfacial photopolymerization of poly(ethylene glycol) diacrylate upon porcine islets. Biotechnol Bioengin 1998;57:655–665. Harris JM: Poly(ethylene Glycol) Chemistry: Biotechnical and Biomedical Applications. New York: Plenum Press, 1992. Sawhney AS, Pathak CP, Hubbell JA: Bioerodible hydrogels based on photopolymerized poly(ethylene glycol)-co-poly(-hydroxy acid) diacrylate macromers. Macromolecules 1993;26:581–587. Cruise GM, Scharp DS, Hubbell JA: Characterization of permeability and network structure of interfacially photopolymerized poly(ethylene glycol) diacrylate hydrogels. Biomaterials 1998;19:1287– 1294. Wolters GH, Fritschy WM, Gerrits D, Van Schilfgaarde R: A versatile alginate droplet generator applicable for microencapsulation of pancreatic islets. J Appl Biomater 1992;3:281–286. Opara EC: The therapeutic potential of islet cell transplants in the treatment of diabetes. Expert Opin Invest Drugs 1998;7:1–11. Hobbs H, Kendall W, Darrabie M, Collins B, Bridges S, Opara EC: Substitution of polyornithine for
6019_13_p968-985
12/21/05
3:09 PM
Page 985
THE BIOARTIFICIAL PANCREAS
160.
161.
162.
163.
164.
165.
166. 167.
168.
169.
170.
171.
172.
173.
174.
175.
polylysine in alginate microcapsules [abstract]. Diabetes 2000;49:A111. Hommel M, Sun AM, Goosen MFA, inventors; Connaught Laboratories Limited, assignee: Droplet generation. US Patent 4,956,128. September 11, 1990. Sundberg C: High-throughput and ultra-highthroughput screening: solution- and cell-based approaches. Curr Opin Biotechnol 2000;11:47–53. Kane RS, Takayama S, Ostuni E, Ingber DE, Whitesides GM: Patterning proteins and cells using soft lithography. Biomaterials 1999;20:2363–2376. Singhvi R, Kumer A, Lopez GP, Stephanopoulos GN, Wang DIC, Whitesides GM, Ingber DE: Engineering cell shape and function. Science 1994;264:696–698. Amirpour ML, Ghosh P, Lackowski WM, Crooks RM, Pishko MV: Mammalian cell cultures on micropatterned surfaces of weak-acid, polyelectrolyte hyperbranched thin films on gold. Anal Chem 2001;73:1560–1566. Chen CS, Mrksich M, Huang S, Whitesides GM, Ingber DE: Micropatterned surfaces for control of cell shape, position, and function. Biotechnol Prog 1998;14:356–363. Ito Y: Surface micropatterning to regulate cell functions. Biomaterials 1999;20:2333–2342. Koh W, Revzin A, Simonian A, Reeves T, Pishko M: Control of mammalian cell and bacteria adhesion on substrates micropatterned with poly(ethylene glycol) hydrogels. Biomed Microdev 2003;5:11–19. Matsuda T, Sugawara T: Development of surface photochemical modification method for micropatterning of cultured cells. J Biomed Mater Res 1995;29:749–756. Kizilel S, Perez-Luna VH, Teymour F: Photopolymerization of poly(ethylene glycol) diacrylate on eosin-functionalized surfaces. Langmuir 2004;20: 8652–8658. Desai TA, West T, Cohen M, Boiarski T, Rampersaud A: Nanoporous microsystems for islet cell replacement. Adv Drug Deliv Rev 2004;56:1661–1673. Cohen I, Li H, Hougland JL, Mrksich M, Nagel SR: Using selective withdrawal to coat microparticles. Science 2001;292:265–267. Patience C, Takeuchi Y, Weiss RA: Infection of human cells by an endogeneous retrovirus of pigs. Nat Med 1997;3:282–286. van der Laan LJW, Lockey C, Griffeth BC, Frasier FS, Wilson CA, Onions DE, Hering BJ, Long Z, Otto E, Torbett BE, Salomon DR: Infection by porcine endogeneous retrovirus after islet xenotransplantation in SCID mice. Nature 2000;407:90–94. Heneine W, Tibell A, Switzer WM, Sandstrom P, Rosales GV, Mathews A, Korsgren O, Chapman LE, Folks TM, Groth CG: No evidence of infection with porcine endogeneous retrovirus in recipients of porcine islet-cell xenografts. Lancet 1998;352:695– 699. Paradis K, Langford G, Long Z, Heneine W, Sandstrom P, Switzer WM, Chapman LE, Lockey C, Onions D, Otto E: Search for cross species transmis-
985
176.
177.
178.
179.
180.
181.
182.
183.
184.
185.
sion of porcine endogeneous retrovirus in patients treated with living pig tissue. The XEN 111 Study Group. Science 1999;285:1236–1241. Niebert M, Rogel-Gaillard C, Chardon P, Tonjes RR: Characterization of chromosomally assigned replication-component gamma porcine endogenous retroviruses derived from a large white pig and expression in human cells. J Virol 2002;76:2714–2720. Soria B, Roche E, Berna G, Leon-Quinto T, Reig JA, Martin F: Insulin secreting cells derived from embryonic stem cells normalize glycemia in streptozotocin induced diabetic mice. Diabetes 2000;49:157–162. Soria B, Andreu E, Berna G, Fuentes E, Gil A, LeonQuinto T, Martin F, Montanya E, Nadal A, Reig JA, Ripoll C, Roche E, Sanchez-Andres JV, Segura J: Engineering pancreatic islets. Pflugers Arch 2000; 440:1–18. Ramiya VK, Maraist M, Arfors KE, Schatz DA, Peck AB, Cornelius JG: Reversal of insulin dependent diabetes using islets generated in vitro from pancreatic stem cells. Nat Med 2000;6:278–282. Zhang N, Richter A, Suriawinata J, Harbaran S, Altomonte J, Cong L, Zhang H, Song K, Meseck M, Bromberg J, Dong H: Elevated vascular endothelial growth factor production in islets improves islet graft vascularization. Diabetes 2004;53:963–970. Sigrist S, Mechine-Neuville A, Mandes K, Calenda V, Braun S, Legeay G, Bellocq JP, Pinget M, Kessler L: Influence of VEGF on the viability of encapsulated pancreatic rat islets after transplantation in diabetic mice. Cell Transplant 2003;12:627–635. Narang AS, Cheng K, Henry J, Zhang C, Sabek O, Fraga D, Kotb M, Gaber AO, Mahato RI: Vascular endothelial growth factor gene delivery for revascularization in transplanted human islets. Pharm Res 2004;21:15–25. Kizilel S: Theoretical and experimental investigation for interfacial photopolymerization of poly(ethylene glycol) diacrylate [Ph.D. thesis]. Chicago: Biomedical Engineering, Illinois Institute of Technology, 2004. Kizilel S, Sawardecker E, Teymour F, Perez-Luna VH: Sequential formation of covalently bonded hydrogel multilayers through surface initiated photopolymerization. Biomaterials 2005 Sept 10 [Epub ahead of print]. De Vos P, De Haan BJ, Van Schilfgaarde R: Upscaling the production of microencapsulated pancreatic islets. Biomaterials 1997;18:1085–1090.
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