Porcine pancreatic islets: isolation, microencapsulation, and xenotransplantation

Improving cell encapsulation through size control

Capsules based on the polyelectrolyte complexation between the polyanions sodium alginate and sodium cellulose sulphate with the polycation poly(methylene-co-guanidine) hydrochloride in the presence of calcium chloride have previously shown important advantages for cell encapsulation. However, in vivo long-term applications require capsule features that are well suited for the functionality of encapsulated cells. These should be targeted to the site of implantation with an appropriate size, a relative stability, and suitable diffusion properties. This study shows the effect of capsule size reduction, from 1 mm to 400 microm, on capsule quality control, mechanical stability, diffusion properties, and in vitro activities of the encapsulated cells. Following a controlled preparation, it was determined that the capsule mechanical stability was largely dependent on the volume ratio of the capsule over the membrane. The molecule diffusion time was related to the surface/volume ratio of the capsule even for the capsules exhibiting an identical cut-off towards the proteins and the dextran molecules. Finally, the in vitro cellular activities, for both primary cultures of rat islets and murine hepatocytes, were improved for cells encapsulated into the 400 microm capsules compared with those in the 1 mm capsules. All of these findings suggest that the smaller capsules present better properties for future clinical applications, at the same time widening the choice of implantation site, and strengthen the notion that slight changes in the capsular morphological parameters can largely influence the graft function in vivo.

Alginate macroencapsulation of pig islets allows correction of streptozotocin-induced diabetes in primates up to 6 months without immunosuppression

BACKGROUND

This study assessed the capacity of alginate-encapsulated islets to reverse diabetes in a pig-to-primate model.

METHODS

Adult pig islets were encapsulated in microcapsules implanted under the kidney capsule (n=4) or in a subcutaneous macrodevice (n=5) in diabetic primates. Fasting blood glucose (FBG), insulin, porcine C-peptide, glycosylated hemoglobin (HbA1C), and cellular and humoral responses were followed.

RESULTS

Nonencapsulated pig islets were rejected within 7 days. A transient decrease of FBG was observed only during the 2 weeks after microencapsulated pig islet implantation under the kidney capsule. After subcutaneous transplantation of a macrodevice, diabetes was corrected up to a maximum of 6 months in five animals: FBG less than 107 mg/dL and HbA1C at 8% ± 1.4%. Two of the five animals received a new macrodevice between 25 and 35 weeks after the first graft dysfunction (HbA1C ≥ 13), and diabetes was controlled for an additional 18 weeks in these animals. Although a strong humoral response was elicited after transplantation of encapsulated islets, a total impermeability of alginate 3% wt/vol to IgG was demonstrated before and up to 20 weeks after transplantation of the subcutaneous macrodevice.

CONCLUSIONS

Pig islets encapsulated in a subcutaneous macrodevice can control diabetes up to 6 months without immunosuppression.

Intraperitoneal alginate-encapsulated neonatal porcine islets in a placebo-controlled study with 16 diabetic cynomolgus primates

BACKGROUND

A nonhuman primate model of diabetes is valuable for assessing porcine pancreatic islet transplants that might have clinical benefits in humans.

METHODS

Neonatal porcine islets, microencapsulated in alginate-polyornithine-alginate, were injected intraperitoneally (10,000 IEQs/kg islets) into eight adult male cynomolgus monkeys rendered diabetic with streptozotocin. Eight diabetic controls were given an equivalent dose of empty placebo capsules. All subjects received a repeat transplant 3 months after the first.

RESULTS

The transplant was well tolerated and no adverse or hypoglycemic events occurred. There were two deaths from nontransplant treatment or diabetic complications unrelated to the transplants. After transplantation, the average insulin dose was reduced in the islet-treated group and increased in the control group. At 12 weeks after the first transplant there was a mean 36% (95% CI: 6% to 65%, P = .02) drop in daily insulin dose compared with the control group. After 24 weeks the difference increased to a mean of 43% (95% CI: 12% to 75%, P = .01) without significant differences in blood glucose values between the two groups. Individual responses after islet transplant varied and one monkey was weaned off insulin by 36 weeks. At terminal autopsy, organs appeared normal and there was no visible peritoneal reaction. No animal had polymerase chain reaction (PCR)-amplified signals of porcine endogenous retrovirus or exogenous virus infections in blood or tissues.

CONCLUSION

Repeated intraperitoneal transplantation of microencapsulated neonatal porcine islets is a safe procedure in diabetic primates. It was shown to result in a significant reduction in insulin dose requirement in the majority of animals studied, whereas insulin requirement increased in controls.

Survival of microencapsulated adult pig islets in mice in spite of an antibody response

The aim of this study was to assess the capacity of simple alginate capsules to protect adult pig islets in a model of xenotransplantation. Adult pig islets were microencapsulated in alginate, with either single alginate coats (SAC) or double alginate coats (DAC), and transplanted into the streptozotocin-induced diabetic B6AF1 mice. Normalization of glucose levels was associated with an improvement of the glucose clearance during intravenous glucose tolerance tests. After explantation, all mice became hyperglycemic, demonstrating the efficacy of the encapsulated pig islets. Explanted capsules were mainly free of fibrotic reaction and encapsulated islets were still functional, responding to glucose stimulation with a 10-fold increase in insulin secretion. However, a significant decrease in the insulin content and insulin responses to glucose was observed for encapsulated islets explanted from hyperglycemic mice. An immune response of both IgG and IgM subtypes was detectable after transplantation. Interestingly, there were more newly formed antibodies in the serum of mice transplanted with SAC capsules than in the serum of mice transplanted with DAC capsules. In conclusion, alginate capsules can prolong the survival of adult pig islets transplanted into diabetic mice for up to 190 days, even in the presence of an antibody response.

Pancreatic Islet Cell Transplantation

Pancreatic islet cell transplantation as a treatment for diabetes has hitherto been confined to small patient cohorts with limited success. This article summarizes the results of islet cell transplantation before and after the advent of the new 'Edmonton protocol' of immunosuppression and management of the donor pancreas. Adopting this regimen has achieved unprecedented success and renewed interest in this potential cure for diabetes. Central to recent improvements in the technique has been the transplantation of an adequate islet mass. Improved methods to procure, isolate, and purify islets for clinical use are now being adopted as a new 'gold standard'. The use of new immunosuppressive drugs has further improved clinical results. Corticosteroid sparing-based regimens, and agents such as humanized monoclonal antibodies, are likely to form the mainstay of immunosuppressive protocols with the aim of achieving donor-specific tolerance. Alternative sources of islet cells are also required to expand the technique in an era of reduced numbers of donor pancreata. Manipulation of stem cells and xenotransplantation may yet yield sufficient islets to overcome the problem of donor shortage. Islet cell transplantation now forms the basis of a prospective multicenter trial under the aegis of the Immune Tolerance Network. The results of this are awaited, but it appears that islet cell transplantation may yet emerge as an effective treatment option for some members of the diabetic population.

Complete protection of islets against allorejection and autoimmunity by a simple barium-alginate membrane

We describe a new technique for microencapsulation with high-mannuronic acid (high-M) alginate crosslinked with BaCl(2) without a traditional permselective component, which allows the production of biocompatible capsules that allow prolonged survival of syngeneic and allogeneic transplanted islets in diabetic BALB/c and NOD mice for >350 days. The normalization of the glycemia in the transplanted mice was associated with normal glucose profiles in response to intravenous glucose tolerance tests. After explantation of the capsules, all mice became hyperglycemic, demonstrating the efficacy of the encapsulated islets. The retrieved capsules were free of cellular overgrowth and islets responded to glucose stimulation with a 5- to 10-fold increase of insulin secretion. Transfer of splenocytes isolated from transplanted NOD mice to NOD/SCID mice adoptively transferred diabetes, indicating that NOD recipients maintained islet-specific autoimmunity. In conclusion, we have developed a simple technique for microencapsulation that prolongs islet survival without immunosuppression, providing complete protection against allorejection and the recurrence of autoimmune diabetes.

Animal models in xenotransplantation

The severe shortage of donor organs has provided a strong impetus to push the investigation into the use of animal organs for humans. Xenotransplantation will not only benefit patients, but also represents a unique and potentially profitable business opportunity. However, there are many barriers to successful clinical xenotransplantation, including immunological barriers, physiological incompatibility, zoonosis and ethical concerns. This overview will focus on currently available animal models used in attempts to break through the immunological barriers to xenotransplantation. There are many advantages to using small animal, namely rodent, models in xenotransplantation research. For example, the use of the mouse model allows the use of knockout mice and careful dissection of rejection mechanisms at the molecular level. The following models can be used to study hyperacute rejection (HAR): guinea-pig-to-rat, mouse-to-rabbit, guinea-pig-to-mouse, rat-to-presensitised mouse and rat-to-alpha-Gal knockout mouse. The hamster-to-rat, mouse-to-rat and rat-to-mouse models are commonly used to study acute vascular rejection. Large animal models are complex and expensive, but they are more relevant to clinical xenotransplantation. Based on experiments using transgenic pig-to-primate models, HAR can be overcome. However, acute vascular rejection remains a major barrier at the present time. A pig cartilage-to-monkey model has been developed to study chronic rejection. Other novel models such as pig venous segment-to-monkey model and rat-to-primate model may represent viable options to study immunological barriers following xenotransplantation. Like many other medical breakthroughs, animal research will continue to make enormous contributions towards the eventual success of xenotransplantation.

Technology of mammalian cell encapsulation

Entrapment of mammalian cells in physical membranes has been practiced since the early 1950s when it was originally introduced as a basic research tool. The method has since been developed based on the promise of its therapeutic usefulness in tissue transplantation. Encapsulation physically isolates a cell mass from an outside environment and aims to maintain normal cellular physiology within a desired permeability barrier. Numerous encapsulation techniques have been developed over the years. These techniques are generally classified as microencapsulation (involving small spherical vehicles and conformally coated tissues) and macroencapsulation (involving larger flat-sheet and hollow-fiber membranes). This review is intended to summarize techniques of cell encapsulation as well as methods for evaluating the performance of encapsulated cells. The techniques reviewed include microencapsulation with polyelectrolyte complexation emphasizing alginate-polylysine capsules, thermoreversible gelation with agarose as a prototype system, interfacial precipitation and interfacial polymerization, as well as the technology of flat sheet and hollow fiber-based macroencapsulation. Four aspects of encapsulated cells that are critical for the success of the technology, namely the capsule permeability, mechanical properties, immune protection and biocompatibility, have been singled out and methods to evaluate these properties were summarized. Finally, speculations regarding future directions of cell encapsulation research and device development are included from the authors' perspective.

Xenotransplantation

BACKGROUND

Over the past 10 years xenotransplantation has generated much interest in the hope that it will enable us to overcome the current lack of human organ donors. This review examines the evolution and current therapeutic strategies that have been developed to overcome the predominant problem of graft rejection.

METHODS

A literature review was undertaken using a Medline search from January 1966 to August 1999.

RESULTS AND CONCLUSION

Despite the considerable advances that have been made in molecular biological techniques, xenograft rejection cannot be prevented without significant immunosuppression and toxic side-effects. The problem of delayed rejection, in particular, will probably be very difficult to overcome, although some of the difficulties associated with hyperacute rejection have been resolved. The potential risk of porcine endogenous retrovirus transmission has generated much debate recently, but it is likely that some of the important issues relating to xenotransplantation will never be resolved until carefully regulated clinical trials are allowed to begin.

Control and measurement of permeability for design of microcapsule cell delivery system

Transplantation of immunoisolated islets of Langerhans has been proposed as a promising approach to treating insulin-dependent diabetes mellitus. Recently, a cell delivery system based on a multicomponent microcapsule has been designed for the immunoisolation of insulin-secreting pancreatic islets. The capsule, formed by polyelectrolyte complexation of sodium alginate and cellulose sulfate with poly(methylene-co-guanidine), markedly has improved mechanical strength compared with the widely used alginate/poly(L-lysine) capsules. It also provides a flexibility for readily adjusting membrane thickness and capsule size, and, more important, the membrane permeability can be altered over a wide range of molecular sizes. To rigorously test the capsule diffusion properties, we have improved capsule permeability measurement by using two complementary methods: (1) size exclusion chromatography with dextran standards; and (2) newly developed methodology for assessing permeability to a series of biologically relevant proteins. Viability and function of rat pancreatic islets enclosed in the capsules with different permeability were tested in vitro. The insulin secretion of encapsulated islets was well preserved even though slightly delayed in comparison with a control group of free islets. We believe that the unique features of this encapsulation system together with the precise characterization of its physical parameters will enable us to find the optimal range of capsule permeability for in vitro and in vivo survival and function of encapsulated pancreatic islets.

New capsule with tailored properties for the encapsulation of living cells

A new capsule for the encapsulation and transplantation of pancreatic islets has been developed. Five active ingredients are involved in the capsule formation process: high viscosity sodium alginate (SA-HV), cellulose sulfate (CS), poly(methylene-co-guanidine) hydrochloride (PMCG), calcium chloride, and sodium chloride. Complexation reaction exhibits several unique features: (1) solution of SA-HV with CS represents a physical mixture of two entangled polyanions that provide both pH-sensitive (carboxylic) and permanently charged (sulfate) groups; (2) presence of CaCl2 in the cation solution ensures formation of the gelled bead after the drop of polyanion solution is immersed in the cation solution; (3) character of the polycation (PMCG), i.e., low molecular weight and unusually high charge density, combines both high mobility and reactivity; (4) presence of PMCG in cation solution, together with CaCl2, gives rise to the competitive binding of these two cations based on their diffusion and affinity towards the anion groups; and (5) NaCl provides the anti-gelling sodium ions that significantly affect the reaction of CaCl2 with the polyanion matrix, thus altering the final properties of the capsule surface, shape, and permeability. The capsule size, mechanical strength, membrane thickness, and permeability can be precisely adjusted and quantified. Detailed information on the permeability aspects is given in another paper by Brissová et al. [J. Biomed. Mater. Sci., 39, 61 (1998)]. The new features concerning capsule processing and testing are presented. We believe that the capsule characteristics can be optimized in the next step to meet the biological criteria. The initial transplantation results suggest that this capsule is biocompatible and noncytotoxic and is a promising candidate for the immunoisolation of cells such as pancreatic islets.

Cryopreservation of microencapsulated porcine pancreatic islets: in vitro and in vivo studies

BACKGROUND

If the transplantation of immunoisolated porcine islets into human diabetics is to become reality, the development of a long-term storage method represents an important prerequisite. However, information on cryogenic storage of porcine islets is scanty and fragmentary.

METHODS

Porcine pancreatic islets microencapsulated in alginate-polylysine-alginate membranes were cryopreserved and assessed both in vitro by static glucose challenge and in vivo in a transplantation study. Two separate methods of islet cryopreservation were compared: method A, using the Bio Cool III freezing machine, and method B, which uses the Nalgene isopropyl alcohol insulated cooler.

RESULTS

Method A was found to have better preserved the ability of the microencapsulated cryopreserved islets to respond to high-glucose static challenge (7 out of 10 lots) compared with method B (1 out of 10 lots). Upon exposure to high glucose, the islet batches that did retain the ability to respond to glucose were shown to have secreted an average of 1220+/-73 pM/24 hr/islet of insulin as compared with 1528+/-118 pM/24 hr/islet for fresh islets. The presence of isobutyl methylxanthine further potentiated insulin secretion to 1805+/-81 pM/24 hr/islet and to 2410+/-104 pM/24 hr/islet for cryopreserved and free islets, respectively. Intraperitoneal transplantation of 2000 cryopreserved microencapsulated porcine islets into streptozotocin-diabetic mice resulted in the reversal of hyperglycemia in 6 out of 10 recipients for the duration of the 90-day study.

CONCLUSIONS

The effective protection of the delicate porcine endocrine tissue during the cryopreservation process and the subsequent long-term storage were demonstrated with considerable success in this study.

An encapsulation system for the immunoisolation of pancreatic islets

Over a thousand combinations of polyanions and polycations were tested to search for new polymer candidates that would be suitable for encapsulation of living cells. The combination of sodium alginate, cellulose sulfate, poly (methylene-co-guanidine) hydrochloride, calcium chloride, and sodium chloride was most promising. In parallel, a novel multiloop chamber reactor was developed to control the time of complex formation and to negate gravitational effects such as pancreatic islet sedimentation and droplet deformation during the encapsulation process. Encapsulated rat islets demonstrated glucose-stimulated insulin secretion in vitro, and reversed diabetes in mice. This new capsule formulation and encapsulation system allows independent adjustments of capsule size, wall thickness, mechanical strength, and permeability, which may offer distinct advantages for immunoisolating cells.

Normalization of diabetes in spontaneously diabetic cynomologus monkeys by xenografts of microencapsulated porcine islets without immunosuppression

Porcine pancreatic islets were microencapsulated in alginate-polylysine-alginate capsules and transplanted intraperitoneally into nine spontaneously diabetic monkeys. After one, two, or three transplants of 3-7 x 10(4) islets per recipient, seven of the monkeys became insulin independent for periods ranging from 120 to 804 d with fasting blood glucose levels in the normoglycemic range. Glucose clearance rates in the transplant recipients were significantly higher than before the graft administration and the insulin secretion during glucose tolerance tests was significantly higher compared with pretransplant tests. Porcine C-peptide was detected in all transplant recipients throughout their period of normoglycemia while none was found before the graft administration. Hemoglobin A1C levels dropped significantly within 2 mo after transplantation. While ketones were detected in the urine of all recipients before the graft administration, all experimental animals became ketone free 2 wk after transplantation. Capsules recovered from two recipients 3 mo after the restoration of normoglycemia were found physically intact with enclosed islets clearly visible. The capsules were free of cellular overgrowth. Examination of internal organs of two of the animals involved in our transplantation studies for the duration of 2 yr revealed no untoward effect of the extended presence of the microcapsules.

Normalization of diabetes by xenotransplantation of cryopreserved microencapsulated pancreatic islets. Application of a new strategy in islet banking

To develop a requisite islet bank for the clinical implementation of an injectable bioartificial endocrine pancreas, microencapsulated islets were cryopreserved and assessed both in vitro by static glucose challenge and in a transplantation study. The insulin response of cryopreserved encapsulated rat islets was comparable with fresh islets. Transplantation of 800-900 banked rat islets resulted in the normalization of the metabolic blood glucose perturbation, body weight, and general health characteristics in 8 out of 8 diabetic mice for the study duration of 90 days. Whereas free islets are easily fragmented and lost during the freezing process, the capsule protects the fragile islets from freezing damage, increasing the retrieval rate from 79.5 +/- 9.8% to 97.2 +/- 1.3.

The rat epididymal fat pad as an implantation site for the study of microcapsule biocompatibility: validation of the method

The study of microcapsule biocompatibility is hindered by their uneven distribution and low recovery when implanted into the peritoneum. We evaluated the use of the rat epididymal fat pad as a microcapsule implantation site for biocompatibility studies. The recovery rate of microcapsules containing 85Sr-labeled microspheres was 99.6 +/- 0.75%. Microcapsules made from the same batch of nonpurified alginate, were injected into both fat pads of male Lewis rats (n = 18) and retrieved 14 days later. A semiquantitative fibrosis score scaled from 0 to 3.0 showed that the pericapsular reaction was uniform throughout a fat pad, and that the results of the two fat pads were equivalent because the null hypothesis of inequivalence was rejected (P < .001). Thus, this method can be used to compare the biocompatibility of microcapsule of differing compositions.

Xenotransplantation of microencapsulated canine islets into diabetic rats

Islets of Langerhans were isolated in high yields from canine pancreata. In the procedure, the pancreata were perfused and digested with collagenase, and the islets were then purified on histopaque density gradients. As many as 60,000 islets were isolated from a single pancreas. Islets were encapsulated in alginate-polylysine-alginate membranes with the aid of an air-jet droplet generator. In vitro studies demonstrated that the isolated and encapsulated islets secreted insulin in response to glucose and IBMX challenge for at least 9 weeks. In in vivo studies 6 diabetic Wistar rats were transplanted with 5,000 to 8,000 encapsulated islets each. The diabetic condition was reversed in all recipients for up to 112 days. In control animals, which received free, unencapsulated islets, the xenografts remained functional for fewer than 21 days. Microcapsules retrieved from normoglycemic transplant recipients 1 and 2 months posttransplantation were shown to contain viable islet tissue, and no cellular overgrowth was observed on capsular surfaces. The results of the study indicate a considerable clinical potential of microencapsulated canine islet xenografts.