Association Between Capsule Diameter, Adequacy of Encapsulation, and Survival of Microencapsulated Rat Islet Allografts1

Abstract

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.

Long-term biocompatibility, chemistry, and function of microencapsulated pancreatic islets

“…Also, an inadequate mechanical integrity of the capsules [9,12] can cause immunological reactions in the recipient. Finally, we found that grafts can fail as the consequence of incomplete encapsulation of islets [13,14].…”

“…Purification of alginate was performed as described in detail by our group elsewhere [8]. All identified factors influencing biological responses were tested in the present study [5,[8][9][10][11][12][13][14]. The composition of the alginate was studied after purification by nuclear magnetic resonance analysis [8].…”

“…This ratio is required to prevent protrusion of islets [21]. The adequacy of encapsulation was controlled before grafting by applying our lectin-binding assay [13,14] for identification of protruding and incompletely encapsulated islets. The percentage of inadequate capsules was always less than 3% of the total amount of capsules.…”

Considerations for successful transplantation of encapsulated pancreatic islets

“…Inadequately encapsulated islets are identified by positive fluorescence (Fig. 3), and the overall adequacy of an islet encapsulation procedure can be quantified by expressing the number of inadequate capsules as the percentage of the total number of capsules containing islets [58,95].…”

Improved survival of microencapsulated islets during in vitro culture and enhanced metabolic function following transplantation

“…The literature reveals, however, that success has so far been limited and non-reproducible. It is thought that the inability of encapsulated islet grafts to reproducibly achieve and maintain long-term euglycaemia is related not only to insufficient immunoprotection, but also to a variety of technical and metabolic issues, including: the encapsulation process, composition/purity of the microcapsule [12], capsule size and surface structure [13,14], poor biocompatibility [15,16], and endotoxin levels of the alginate [17,18]. Furthermore, while the addition of poly-L-lysine has been used to reduce capsule porosity [19], its addition has resulted in an increased fibrotic response [15].…”