Prolonged reversal of diabetic state in NOD mice by xenografts of microencapsulated rat islets

Strategy for developing microbeads applicable to islet xenotransplantation into a spontaneous diabetic NOD mouse

A bioartificial pancreas (BAP) created through the encapsulation of islets of Langerhans (islets) in a semipermeable membrane has been proposed as a promising approach to treating insulin-dependent diabetes patients. A nonobese diabetic (NOD) mouse, which shares many features of human insulin-dependent diabetes mellitus, is an ideal model for evaluating the function of BAP. However, the functions of BAPs that have been developed have been limited in NOD mice. We propose novel microbeads that can realize long-term BAP function in NOD mice. The novel microbeads were composed of agarose and poly(styrene sulfonic acid) (PSSa) mixed gel. A polyion complex layer between PSSa and polycationic polybrene was formed on and just inside the microbead, and the microbead surfaces were further covered by polyanions to produce anionic surface charges. The islets in the novel microbeads were intraperitoneally implanted. Graft-functioning periods were dependent on both PSSa concentration and the kinds of polyanion. Islets in the microbeads composed of 5% agarose and 5% PSSa, which had an outermost surface covered by carboxymethyl cellulose, produced normoglycemic periods of more than 60 days in all five recipients. Control mice receiving either transplants of unenclosed islets or islets in agarose microbeads showed normoglycemic periods of less than 12 days. We believe that agarose/PSSa microbeads are promising for producing semipermeable membranes that enable xenotransplantation of islets in spontaneous diabetes mellitus.

Studies on small (< 300 microns) microcapsules: II--Parameters governing the production of alginate beads by high voltage electrostatic pulses

The size of microcapsules is a critical parameter in the immunoisolation of islets of Langerhans by microencapsulation. The use of smaller capsules decreases the total implant volume and improves insulin kinetics and oxygen supply. A high voltage electrostatic pulse system was used for the production of small (< 300 microns) alginate beads, the first step of the encapsulation technique. However, islets often protruded from capsules that were too small, further emphasizing the need for a method to control bead size. A study of 7 parameters [electrostatic pulse amplitude (A), duration (D) and wavelength (lambda), pump flow rate (P), needle gauge, alginate viscosity and distance between electrodes] showed that P (r = 0.981, p = 0.003) and lambda (r = 0.988, p = 0.0002) were the principal determinants of bead size. To detect potential interactions between parameters, 270 combinations of different levels of A, D, lambda, and P were studied. A multivariate regression analysis of these data confirmed that P and lambda are the prime determinants of bead size, and showed that a 2-parameter (P, lambda) model could be used to precisely predict bead size (R2 = 0.84), while keeping the application simple. The precision of the predictive model is only slightly improved by the use of additional parameters. The reliability of the data used to elaborate this model was demonstrated (p = 0.6226) by comparing them with a second data set obtained under the same conditions. A third set of experiments confirmed the applicability of the model. This work has major implications on the preclinical application of microencapsulation since it showed that it is possible to predetermine the bead size.

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.

Generation of alginate-poly-l-lysine-alginate (APA) biomicrocapsules: the relationship between the membrane strength and the reaction conditions

Alginate-poly-l-lysine-alginate (APA) microcapsules have proven effective in protecting enclosed live cells from immune rejection following transplantation into experimental animals, thereby eliminating the need for immunosuppressive therapy. However, in order for the capsules to remain intact for extended periods in vivo, the thickness of the membrane material must be optimized. In this study, the membrane thickness was examined as an indicator of membrane strength and measured under different reaction conditions. The thickness was found to increase 1) from 4.6 microns to 6.6 microns with an increase in the concentration of sodium alginate from 1.25 (w/v) to 2.0% (w.v); 2) from 4.2 microns to 6.2 microns with an increase in the concentration of the calcium solution from 20 mM to 100mM; 3) from 3.9 microns to 10.3 microns with an increase in the concentration of poly-l-lysine (PLL) from 0.02% (w/v) to 0.08% (w/v); and 4) from 2.3 microns to 7.4 microns with an increase in the reaction time with the PLL from two to seven minutes. On the other hand, membrane thickness decreased 1) from 9.8 microns to 8.6 microns with an increase of the pH in the PLL solution from 5.8 to 9.2; 2) from 13.2m to 5.8 microns with an increase in the molecular weight of PLL from 14,000 to 57,000; 3) from 8.4 microns to 6.0 microns with an increase in the treatment time with 0.9 (w/v) NaCl solution from zero to fifteen minutes and; 4) from 7.5 microns to 6.1 microns with an increase in the treatment time of the second sodium alginate coating from zero to ten minutes. Membrane thickness was inversely proportional to capsule volume expansion during membrane synthesis. By replacing calcium chloride by calcium lactate and eliminating the use of CHES in the construction of capsule membranes, we improved the strength and biocompatibility of our capsules, as evidenced by marked improvements in the survival rates of diabetic mice treated with islet transplants enclosed in the new capsules. These results indicate that it is possible to obtain optimal membrane thickness for a given purpose by creating specific reaction conditions under which membranes are synthesized.

Production of purified alginates suitable for use in immunoisolated transplantation

Alginate is used as a matrix for immunoisolation of cells and tissues in vivo. We have demonstrated previously that commercial alginates contain various fractions of mitogenic impurities and that they can be removed by free flow electrophoresis. The use of purified material is a necessity in order to reveal the parameters that control biocompatibility of the implanted material (such as stability, size, surface charge and curvature, etc.). In this study, we present a protocol for the chemical purification of alginates on a large-scale. Beads made from alginates purified by this multi-step chemical extraction procedure did not induce a significant foreign body reaction when implanted for 3 weeks either intraperitoneally or beneath the kidney capsule of Lewis or non-diabetic BB/Gi rats.

FT-IR of membranes made with alginate/polylysine complexes. Variations with the mannuronic or guluronic content of the polysaccharides

FT-IR spectra of polylysine/alginate membranes made with alginate containing various contents of mannuronic or guluronic acid residues have been recorded. The interpretation of the more important absorptions related to functional groups engaged in the complexes have been proposed and discussed using comparisons with spectra of cellulosic films and other published results. Mannuronnic rich alginate seemed to link stronger than guluronic rich alginate to the polylysine molecules which is illustrated by the continuum in absorption between 3000 cm-1 and 2000 cm-1. However, the analysis of the 2000-1000 cm-1 region prompted us to believe that the polymers were engaged in the same basic sort of molecular complexes. Therefore it is necessary that other parameters (either physical, as toughness, porosity, ...) other than variations in molecular structures are studied in order that the biological differences of the membranes may be explained.

Porcine pancreatic islets: isolation, microencapsulation, and xenotransplantation

To provide a plentiful source of pancreatic islets for future clinical transplants into diabetic patients, we have developed a simple and reliable method to isolate porcine islets of a high degree of purity. Porcine pancreata were perfused and digested with collagenase, and the islets were then purified on dextran density gradients. In order to avoid any damage to the islets, no mechanical devices nor any strenuous treatment was employed. As many as 5 x 10(5) islets were isolated from a single porcine pancreas. Islets were encapsulated in alginate-polylysine-alginate membranes with the aid of an electrostatic droplet generator. In vitro studies demonstrated that the isolated islets secreted insulin in response to glucose and 3-isobutyl-L-methylxanthine (IBMX) challenge for at least 4 weeks. Perifusion studies showed that the kinetics of insulin release from the encapsulated islets was similar to that exhibited by free islets. In in vivo studies, 18 diabetic BALB-c mice were transplanted with 1,500-2,500 encapsulated islets each. In 13 recipients, the diabetic condition was reversed for at least 85 days. When capsules were removed from 2 transplant recipients, their diabetic condition quickly recurred.

Islet microencapsulation: a review

The original report on the microencapsulation of islets of Langerhans used sodium alginate and poly-L-lysine (PLL) to form the capsules. Although several alternative materials have subsequently been used with vary-mg degrees of success, it is those studies using islets encapsulated in alginate-PLL-alginate which are reviewed in detail in this article. Since the first report of islet microencapsulation, many studies have demonstrated excellent in vitro viability of encapsulated islets. However, transplantation experiments into chemically induced diabetic recipients have yielded varied results, with some studies showing good long-term graft function whilst in others grafts failed due to pericapsular fibrosis. The use of naturally occurring animal models of type 1 (insulin-dependent) diabetes has demonstrated a decline in graft function, suggesting that this presents a more complex problem to be solved than that in chemically induced diabetic recipients. Fibrosis of capsules has been the major problem causing graft failure, and this has been demonstrated to be more severe in spontaneously diabetic models. However, recent advances in alginate purification and attempts to reduce the size of the encapsulated islets are major steps towards encapsulated islet transplants becoming a viable proposition for the treatment of type 1 diabetic patients.