Obstacles in the Application of Microencapsulation in Islet Transplantation

Several factors stand in the way of successful clinical transplantation of alginate-polylysine-alginate microencapsulated pancreatic islets. These obstacles can be classified into three categories. The first regards the technical aspects of the production process. Limiting factors are the insufficient ability to produce small capsules with an adequate production rate, and insufficient insight into the factors determining the optimal chemical and mechanical properties of the capsules. The second category regards the functional aspects of the microencapsulated islets, such as the limitations of the transplantation site and the absence of a physiologic insulin response of the encapsulated islets to elevated blood glucose levels. The third category regards the fact that survival times of encapsulated islet grafts are still limited to several weeks or months, which is mainly explained by a pericapsular fibrotic overgrowth reaction as a consequence of the bioincom-patibility of the capsule membrane. This study describes these obstacles, and thereby summarizes the requirements needed for successful clinical application of encapsulated islet transplantation.

Response of Encapsulated Rat Pancreatic Islets to Hypoxia

Hypoxia contributes to encapsulated pancreatic islet graft failure. To gain insight into the mechanisms that lead to hypoxia-induced graft failure, encapsulated islet function, vitality, and cell replication were assessed after 2 and 5 days of hypoxic (1% O₂) and normoxic (20% O₂) culture. The mRNA expression levels of Bcl-2, Bax, inducible nitric oxide synthase (iNOS), and monocyte chemoattractant protein 1 (MCP-1) were assessed, as well as the amount of nitrite and MCP-1 in the culture medium. Hypoxia was associated with loss of encapsulated islet function and vitality, but not with an increase in islet cell replication. Loss of vitality was due to necrosis, and only modestly due to apoptosis. Hypoxia was not associated with changes in the Bcl-2/Bax mRNA ratio, but it did increase the expression of iNOS and MCP-1 mRNA. The increased mRNA levels were, however, not associated with elevated concentrations of nitrite nor with elevated levels of MCP-1 protein. The increased iNOS mRNA levels imply a role for NO in the completion of cell death by hypoxia. The increased MCP-1 mRNA levels suggest that encapsulated islets in vivo contribute to their own graft failure by attracting cytokine-producing macrophages. The discrepancy between iNOS mRNA and nitrite is explained by the longer half-life of NO during hypoxia. MCP-1 protein levels are underestimated as a consequence of the lower number of vital cells in combination with a higher proteolytic activity due to necrosis. Thus, strategies to eliminate hypoxia may not only improve islet function and vitality, but may also reduce the attraction of macrophages by encapsulated islets.

C-peptide responses after meal challenge in mice transplanted with microencapsulated rat islets

AIMS/HYPOTHESIS

This study aimed to assess a response of microencapsulated rat islets to a meal challenge after being transplanted intraperitoneally into diabetic mice.

METHODS

Microencapsulated rat islets or control naked syngeneic mouse islets were transplanted intraperitoneally into mice with streptozotocin-induced diabetes. Meal challenges were done 3, 6 and 9 weeks after transplantation. Glucose-induced insulin secretion from microencapsulated islets before and after transplantation was assessed in vitro.

RESULTS

Within the first week, all animals transplanted with either microencapsulated rat islets or with syngeneic murine islets became normoglycaemic (< 11 mmol/l). At 4 and 6 weeks, body weight was less than normal in the non-diabetic control mice. Mice with the encapsulated rat islets had lower fasting glucose concentrations and more rapid glucose clearance after a meal challenge than the control mice. The group of mice with transplanted syngeneic islets had similar glucose profiles to control mice, except for slightly accelerated glucose clearance. The C peptide responses of mice with either microencapsulated or naked islets were clearly lower than the controls. An increase of C peptide appeared as early as 20 min in the plasma of the group with encapsulated islets, but this was considerably slower than in the other two groups. Microencapsulated rat islets retrieved 9 weeks after transplantation did not lose their ability to respond to glucose, but their output was less than half of the pretransplant control islets.

CONCLUSION/INTERPRETATION

The delivery of C peptide and presumably the accompanying insulin are delayed by restrictions of the capsules and the peritoneal location. However, this delay in reaching peripheral target organs does not prevent microencapsulated grafts from efficiently clearing glucose after a meal.

Why do microencapsulated islet grafts fail in the absence of fibrotic overgrowth?

The survival of microencapsulated islet grafts is limited, even if capsular overgrowth is restricted to a small percentage of the capsules. In search of processes other than overgrowth contributing to graft failure, we have studied the islets in non-overgrown capsules at several time points after allotransplantation in the rat. All recipients of islet allografts became normoglycemic. Grafts were retrieved at 4 and 8 weeks after implantation and at 15.3 +/- 2.3 weeks postimplant, 2 weeks after the mean time period at which graft failure occurred. Overgrowth of capsules was complete within 4 weeks postimplant, and it was usually restricted to <10% of the capsules. During the first 4 weeks of implantation, 40% of the initial number of islets was lost. Thereafter, we observed a decrease in function rather than in numbers of islets, as illustrated by a decline in the ex vivo glucose-induced insulin response. At 4 and 8 weeks postimplant, beta-cell replication was 10-fold higher in encapsulated islets than in islets in the normal pancreas, but these high replication rates were insufficient to prevent a progressive increase in the percentage of nonviable tissue in the islets. Necrosis and not apoptosis proved to be the major cause of cell death in the islets. The necrosis mainly occurred in the center of the islets, which indicates insufficient nutrition as a major causative factor. Our study demonstrates that not only capsular overgrowth but also an imbalance between beta-cell birth and beta-cell death contributes to the failure of encapsulated islet grafts. Our observations indicate that we should focus on finding or creating a transplantation site that, more than the unmodified peritoneal cavity, permits for close contact between the blood and the encapsulated islet tissue.

Insulin levels after portal and systemic insulin infusion differ in a dose-dependent fashion

The role of the liver in the regulation of systemic insulin levels is not well understood. The reported extraction rates vary between 0 to 85%, and extraction of a constant fraction of 50% of the portally delivered insulin is generally assumed. In the present study, we have investigated the role of the liver in the regulation of systemic insulin levels in the normal rat. Insulin was infused into the portal vein of conscious and freely moving rats in doses of 20, 40, 80 pmol/min during 15 min to mimic the gradual release of insulin by the native endocrine rat pancreas. The profiles of plasma insulin and glucose levels in the systemic circulation were compared to those obtained after direct infusion into the systemic circulation. The effect of intraportal and direct systemic infusion on plasma insulin and blood glucose levels were virtually similar where 20 pmol/min was applied. But, these effects were different if the dose was 40 pmol/min, and this difference increased when the dose was increased to 80 pmol/min, since hypoglycemia was less severe and normoglycemia was restored more rapidly with portal than with systemic infusion. Thus, our results show that the fraction of intraportally infused insulin reaching the systemic circulation decreases with higher doses of insulin. This suggests that the liver contains adaptable mechanisms to reduce the systemic insulin levels.

Evaluation of two outcome prediction models on an independent database

OBJECTIVE

To evaluate the performance of the New Simplified Acute Physiology Score (SAPS II) and the admission Mortality Probability Model (MPM0) in a large independent database, using formal statistical assessment.

DESIGN

Analysis of the database of a multicenter, multinational, prospective cohort study, EURICUS-I.

SETTING

Eighty nine intensive care units (ICUs) from 13 European areas.

PATIENTS

Data of 16,060 patients consecutively admitted to the participating ICUs were collected during a period of 4 months. Following the original SAPS II and MPM0 criteria, the analysis excluded: patients <18 ys of age; readmissions; patients admitted with acute myocardial infarction; burns; and patients in the postoperative period after coronary artery bypass surgery. All patients with a length of stay <8 hrs were excluded from the study to keep comparability between both systems. A total of 10,027 patients were analyzed.

INTERVENTIONS

Collection of the first 24 hrs' admission data necessary for the calculation of SAPS II and MPM0 and basic demographic statistics. Vital status at discharge from the hospital was registered.

MEASUREMENTS AND MAIN RESULTS

Despite having a good discriminative capability, as measured by the area under the receiver operating characteristic (ROC) curves (SAPS II: ROC = 0.822 +/- 0.005 SEM; MPM0: ROC = 0.785 +/- 0.006 SEM), both models presented poor calibration, with significant differences between observed and predicted mortality (Hosmer-Lemeshow goodness-of-fit tests H and C, p < .0001). Both SAPS II (predicted risk >40%) and MPM0 (predicted risk >30%) overestimated the risk of death. The evaluation of the uniformity of fit of SAPS II and MPM0 demonstrated large variations across the various subgroups of patients.

CONCLUSIONS

The original SAPS II and MPM0 models did not accurately predict mortality on an independent large international multicenter ICU patient database. Results of studies utilizing general outcome prediction models without previous validation in the target population should be interpreted with prudence.

Upscaling the production of microencapsulated pancreatic islets

Presently used single-needle air-driven droplet generators are incapable of producing sufficient numbers of islet-containing droplets in a sufficiently short time-period to allow for successfully grafting alginate-poly-L-lysine encapsulated islets in large animals or humans. We have designed an air-driven multineedle droplet generator, which increases the production rate by simultaneously producing multiple droplets. Although we have tested a four-needle device, the construction is such that the number of needles, and thereby the production rate, can be readily extended. The production rate can be further extended by increasing the number of islets per millilitre alginate in the reservoir. When tested with 500 and 800 microm capsules, an increase in the number of islets per millilitre alginate was found to be associated with an increase in the number of inadequately encapsulated islets in a diameter-dependent fashion. When small instead of large capsules are produced from a given volume of alginate, larger numbers of capsules are obtained, but also a larger portion of inadequate capsules. With 10,000 islets per millimetre alginate, these combined effects can be calculated to result in a two-fold increase in the production rate of adequate capsules when 500 microm instead of 800 microm capsules are produced. Hence, substantial upscaling of the production can be achieved by combining an increase in the number of needles with a decrease in the capsule diameter.

Efficacy of a prevascularized expanded polytetrafluoroethylene solid support system as a transplantation site for pancreatic islets

An intraperitoneally located and prevascularized expanded polytetrafluoroethylene solid support is potentially a suitable transplantation site for encapsulated pancreatic islets, because it allows for both the implantation of a large volume islet graft in the immediate vicinity of blood vessels, and its complete removal. The present study investigates the efficacy of such solid supports for the implantation of nonencapsulated islet isografts in streptozotocin diabetic rat recipients. These solid supports were always coated with acidic fibroblast growth factor, because we found that this growth factor enhances the neovascularization. The success rates of 5-microl (group A) and 10-microl (group B) islet isografts in solid supports were compared with the success rates of 5-microl (group C) and 10-microl (group D) islet isografts implanted in the unmodified peritoneal cavity. Four of seven rats in group A and all seven rats in group B became normoglycemic for at least 6 months. Only two of eight rats in group C and four of eleven rats in group D showed normoglycemia. The normoglycemia lasted for at least 6 months in zero of two animals in group C and in three of four animals in group D. Because of the low success rates in groups C and D, intravenous and oral glucose testing were restricted to the successful recipients in groups A and B. Glucose tolerance was found to be proportional to the grafted islet volume but, expectedly, in both groups the glucose tolerance and the insulin responses were somewhat lower than in controls. Thus, the prevascularized expanded polytetrafluoroethylene solid support, rather than the unmodified peritoneal cavity, is an efficacious transplantation site, potentially suitable for encapsulated islets.

Improved biocompatibility but limited graft survival after purification of alginate for microencapsulation of pancreatic islets

Graft failure of alginate-polylysine microencapsulated islets is often interpreted as the consequence of a non-specific foreign body reaction against the microcapsules, initiated by impurities present in crude alginate. The aim of the present study was to investigate if purification of the alginate improves the biocompatibility of alginate-polylysine microcapsules. Alginate was purified by filtration, extraction and precipitation. Microcapsules prepared from crude or purified alginate were implanted in the peritoneal cavity of normoglycaemic AO-rats and retrieved at 1, 2, 3, 6, 9, and 12 months after implantation. With crude alginate, all capsules were overgrown within 1 month after implantation. With purified alginate, however, the portion of capsules overgrown was usually less than 10%, even at 12 months after implantation. All recipients of islet allografts in capsules prepared of purified alginate became normoglycaemic within 5 days after implantation, but hyperglycaemia reoccurred after 6 to 20 weeks. With intravenous and oral glucose tolerance test, all recipients had impaired glucose tolerance and insulin responses were virtually absent. After graft failure, capsules were retrieved (80-100%) by peritoneal lavage. Histologically, the percentage of overgrown capsules was usually less than 10% and maximally 31%. This small portion cannot explain the occurrence of graft failure. The immunoprotective properties of the capsules were confirmed by similar if not identical survival times of encapsulated islet allo- and isografts. Our results show that purification of the alginate improves the biocompatibility of alginate-polylysine microcapsules. Nevertheless, graft survival was still limited, most probably as a consequence of a lack of blood supply to the encapsulated islets.

Effect of the alginate composition on the biocompatibility of alginate-polylysine microcapsules

Alginate-polylysine (PLL) capsules are commonly applied for immunoprotection of endocrine tissues. Alginate is composed of mannuronic acid (M) and guluronic acid (G). Different types of alginate have different ratios of G to M, but little is known of the influence of these differences on biocompatibility. Therefore, we have investigated in vivo the effect of the G-content of the alginate on the biocompatibility of the capsules. Capsules prepared of commercially available alginates with either a high or an intermediate G-content were implanted in the peritoneal cavity of rats and retrieved one month later for histological evaluation. The fibrotic reaction was more severe against high-G alginate capsules than to intermediate-G alginate capsules. The majority of the high-G capsules proved to be overgrown and adherent to the abdominal organs whereas with intermediate-G alginate most capsules were found freely floating in the peritoneal cavity and free of any adhesion of cells. This was not caused by the alginate as such but rather by inadequate binding of high-G alginate to PLL since in the absence of PLL, i.e. with beads instead of capsules, no fibrotic reaction was observed. As high-G alginates have beneficial effects for islet encapsulation, efforts should be made to apply polycations which more effectively interact with high-G alginate than PLL.

Association between capsule diameter, adequacy of encapsulation, and survival of microencapsulated rat islet allografts

As a consequence of its large volume, a microencapsulated islet graft can be implanted only into the peritoneal cavity. The graft volume can be reduced by using small capsules. However, reduction of the diameter of the capsules holds a certain risk, because with smaller capsules, more islets may be found to protrude from the capsules. We have developed a lectin binding assay which, after encapsulation, specifically labels islets or parts of islets that are insufficiently immunoprotected as a consequence of inadequate, and particularly incomplete, encapsulation. With this assay, we found that a reduction of the capsule diameter from 800 micrometers to 500 micrometers was associated with an increase in the percentage of inadequately encapsulated islets from 6.3+/-1.2% to 24.2+/-1.5%. The in vivo significance of this finding was investigated by performing allotransplantations with large diameter (700-800 micrometers) and small diameter (400-500 micrometers) capsules. With large-capsule islet grafts, all recipients (n=5) became normoglycemic for 7-16 weeks, whereas with small-capsule islet grafts, only one of seven recipients became normoglycemic. The in vivo significance of inadequate encapsulation was further substantiated by our finding that most large capsules were floating freely in the peritoneal cavity without any cell adhesion, whereas the vast majority of small capsules was found to be adherent to the surface of intra-abdominal organs and infiltrated by immune cell elements characteristic of both an allograft reaction and a foreign body reaction. We conclude that successful use of capsules with small diameters requires further study to determine which factors in the encapsulation procedure should be modified to reduce the number of inadequate small capsules.

Factors influencing the adequacy of microencapsulation of rat pancreatic islets

The observation that only a portion of all alginate-polylysine microcapsules are overgrown after implantation suggests that physical imperfections of individual capsules, rather than the chemical composition of the material applied, are responsible for inducing insufficient biocompatibility and thereby fibrotic overgrowth of those capsules. We recently developed a lectin binding assay that allows for quantifying the portion of inadequately encapsulated islets, and demonstrated that inadequately encapsulated islets induce a fibrotic response associated with graft failure. The present study investigates factors influencing the adequacy of encapsulation of pancreatic islets. We applied our lectin binding assay and found that the number of inadequate, and particularly incomplete, capsules is influenced by the following factors. (1) A capsule diameter of 800 micrometers is associated with a lower percentage of inadequate capsules than smaller (500 micrometers and 600 micrometers) or larger (1800 micrometers) capsules. (2) A high rather than low guluronic acid content of the alginate is associated with a lower percentage of inadequate capsules. This can be explained, at least in part, by smaller ranges of swelling and subsequent shrinkage during the encapsulation procedure. (3) An increase in viscosity caused by applying a higher alginate concentration compensates for a low guluronic acid content. This effect of increased viscosity cannot be explained by a reduced range of swelling and shrinkage during the encapsulation procedure. We conclude that alginates with a high guluronic acid content and a viscosity near the filtration limit are preferable in order to minimize the number of inadequate capsules.

Kinetics of intraperitoneally infused insulin in rats. Functional implications for the bioartificial pancreas

Intraperitoneal transplantation of encapsulated islets can restore normoglycemia in diabetic recipients but not normal glucose tolerance nor normal insulin responses to a physiological stimulus. This study investigates whether the intraperitoneal implantation site as such contributes to the interference with optimal transport kinetics between the islets and the bloodstream. Insulin was infused into the peritoneal cavity of conscious and freely moving rats in doses of 20, 40, and 80 pmol.l-1.min-1 during 15 min, to mimic the gradual release of insulin from an encapsulated, i.e., a nonvascularized, islet graft. With 20 pmol.l-1.min-1, we observed virtually no rise of insulin levels, and it took 30 min until glucose levels had dropped significantly. With 40 and 80 pmol.l-1.min-1 insulin infusions, there was a dose-dependent rise of insulin and decrease of glucose levels. When compared with intraportal infusions with the same insulin dosages, however, they were strongly delayed and reduced as well as prolonged. Similar results were obtained when inulin instead of insulin was intraperitoneally infused, with indicates that the transport of insulin from the peritoneal cavity to the bloodstream is mainly by passive diffusion. With a view on the clinical efficacy of the bioartificial pancreas, our findings indicate that we should focus on finding or creating a transplantation site that, more than the unmodified peritoneal cavity, permits close contact between the bloodstream and the encapsulated islet tissue.

Glycol methacrylate embedding of alginate-polylysine microencapsulated pancreatic islets

A method for processing and embedding alginate-polylysine microencapsulated pancreatic tissue in glycol methacrylate resin (GMA) is described. Fixation in 4% phosphate buffered formaldehyde, processing in ascending concentrations of glycol methacrylate monomer and embedding in Technovit 7100 results in well preserved morphological details of hydrogels, hydrogel-cell interfaces, and encapsulated pancreatic tissue. Routine staining with Loeffler's methylene blue, hematoxylin and eosin, and Romanovsky-Giemsa gave excellent images of the GMA embedded alginate polylysine membrane and tissues allowing cells on the outside of the capsule to be analyzed effectively as part of the foreign body reaction against the capsule membrane.

Distribution of collagens type I, type III and type V in the pancreas of rat, dog, pig and man

The presence of collagens type I, type III and type V was determined immunohistochemically in pancreatic tissue of rat, pig, dog and man. The reaction to anti-collagen type I is weak (pig, dog) or moderate (rat, man) in the peri-insular region and in the lobar, lobular and acinar septa, whereas the reaction to anti-collagen type III is well developed. In rat and dog, the latter reaction deposit on the lobar and acinar septa is prominent. These elements only show a moderate reaction intensity in pig and man. The peri-insular region displays a weak (rat, dog, man) or very weak (pig) reaction against collagen type III. Anti-collagen type V reacts moderately (rat, dog, man) or weakly (pig) in the lobar and lobular septa. The acinar septa show a moderate (rat, dog, man) or very weak (pig) reaction. This information regarding the types and distribution of the collagenous compounds in pancreatic extracellular matrix could lead to differentiated enzymatic pancreas dissociation and, ultimately, increased islet yield and improved reproducibility of pancreatic islet isolation procedures for transplantation purposes.

Obstacles in the application of microencapsulation in islet transplantation

Several factors stand in the way of successful clinical transplantation of alginate-polylysine-alginate microencapsulated pancreatic islets. These obstacles can be classified into three categories. The first regards the technical aspects of the production process. Limiting factors are the insufficient ability to produce small capsules with an adequate production rate, and insufficient insight into the factors determining the optimal chemical and mechanical properties of the capsules. The second category regards the functional aspects of the microencapsulated islets, such as the limitations of the transplantation site and the absence of a physiologic insulin response of the encapsulated islets to elevated blood glucose levels. The third category regards the fact that survival times of encapsulated islet grafts are still limited to several weeks or months, which is mainly explained by a pericapsular fibrotic overgrowth reaction as a consequence of the bioincompatibility of the capsule membrane. This study describes these obstacles, and thereby summarizes the requirements needed for successful clinical application of encapsulated islet transplantation.

Conservative surgical treatment for acute pancreatitis: the Lawson procedure

Twenty-seven patients with acute necrotizing pancreatitis were treated with the Lawson procedure, consisting of closed drainage of the lesser sac, cholecystostomy, gastrostomy and jejunostomy. Retrospectively, the effect of the treatment was judged in relation to mortality and morbidity. Thirteen patients (48%) died, all of them due to sepsis and multiple organ failure. This considerable mortality was undoubtedly also influenced by the serious condition of most patients as reflected by the number of positive Ranson criteria. Seven patients (26%) had to be reoperated on because of abscesses. The high incidence of abscesses and the sepsis in all deceased patients indicate an insufficient drainage of the lesser sac with this method. Of the three stomas only jejunostomy proved to be useful for quick resumption of enteral feeding.