Characterization of chelator-mediated recovery of pancreatic islets from barium-stabilized alginate microcapsules

INTRODUCTION

Islet recovery from within alginate-based microcapsules is necessary for certain analytical assays like flow cytometry; however, this technology has not been widely characterized. In this study, we explore the ability of EDTA, EGTA, and sodium citrate to induce reverse alginate polymerization via chelation and assess the toxicity of each chelator on pancreatic islets.

METHODS

EDTA, EGTA, and sodium citrate were used to dissolve single-layered Ba²⁺ alginate encapsulated islets and the rate of capsule breakdown calculated from analysis of imaging data. The effect of chelator exposure on islet viability and recovery was assessed using flow cytometry, while glucose-stimulated insulin release (GSIR) assay was used to measure effects on islet function.

RESULTS

EGTA demonstrated the most rapid microcapsule dissolving rate followed by EDTA and sodium citrate. Islet recovery was significantly better when encapsulated islets were treated with EDTA than EGTA and Na⁺ citrate. A decrease in viability and increase in apoptotic cells were observed when encapsulated islets were treated with Na⁺ citrate compared to islets treated with EDTA and EGTA. Islets treated with EDTA and EGTA demonstrated comparable stimulation index values to non-treated control. Conversely, islets treated with Na⁺ citrate exhibited significantly decreased SI values compared to control. All chelator groups showed significantly lower insulin secretion than non-treated islets.

CONCLUSION

Islet recovery from alginate microcapsule is possible using common chelators like Na⁺ citrate, EDTA, and EGTA. Chelation of encapsulated islets using EDTA demonstrated the most efficient dissolving capabilities with the least toxicity toward islet recovery and health.

Use of small animal models for screening immunoisolation approaches to cellular transplantation

It has been recognized for many years that immunoisolation strategies form an attractive approach to preventing the rejection of cellular allografts and xenografts. Although immunoisolation has proven dramatically successful in some cases, the results have tended to be somewhat variable. Although many advances have been made in the development of biocompatible materials for separating host immune cells from the transplanted tissues, much of the experimentation in this area has been outcome driven. That is, the nature of host reactivity and/or biomaterial design resulting in the failure of some immunoisolation strategies has mostly been undefined. A first premise of this discussion is that immunoisolation is primarily cell isolation and not antigen isolation, per se. That is, although varied membrane barriers are designed to prevent cell-cell contact between host and donor cells, soluble antigens derived from the transplant are likely to gain access to the host immune system. A key question centers on the degree and consequence of this type of antigen presentation in the host to the immunoisolated transplant. To address this and related concerns, this overview presents a simple paradigm for using defined rodent (mouse) models for systematically screening the efficacy of immunoisolated cellular transplants. The proposition is made that understanding the basis of graft failure will aid in the design of future immunoisolation technologies.

Immunocompatibility and biocompatibility of cell delivery systems

Immunoisolation therapy overcomes important disadvantages of implanting free cells. By mechanically blocking immune attacks, synthetic membranes around grafted cells should obviate the need for immunosuppression. The membrane used for encapsulation must be biocompatible and immunocompatible to the recipient and also to the encapsulated graft. The ability of the host to accept the implanted graft depends not only on the material used for encapsulation, but also on the defense reaction of the recipient, which is very individual. Such a reaction usually starts as absorption of cell-adhesive proteins, immunoglobulins, complement components, growth factors and some other proteins on the surface of the device. The absorption of proteins is difficult to avoid, but the amount and specificity of absorbed proteins can be controlled to some extent by selection and modification of the device material. If the adsorption of proteins to the surface of the implanted material is reduced, the overgrowth of the device with fibroblast-like and macrophage-like cells is also reduced. Cell adhesion at the surface of the implanted device is, in addition to the selected polymeric material, greatly influenced by the device content. Xenografts trigger a more vigorous inflammatory reaction than allografts, most probably due to the release of antigenic products from encapsulated deteriorated and dying cells which diffuse through the membrane and activate adhering immune cells. There is an evident effect of autoimmune status on the fate of the encapsulated graft. While encapsulated xenogeneic islets readily reverse streptozotocin-induced diabetes in mice, the same xenografts are short-functioning in NOD autoimmune diabetes-prone mice. Autoantibodies, to which most devices are impermeable, are not involved. Among the cytotoxic factors which are responsible for the limited survival of the encapsulated graft the most important are cytokines and perhaps some other low-molecular-weight factors released by activated macrophages at the surface of the encapsulating membrane.

Cytokine-induced functional suppression of microencapsulated rat pancreatic islets in vitro

BACKGROUND

It is likely that inflammatory cytokines are released near microencapsulated islets in vivo.

METHODS

Rates of insulin release or glucose oxidation were measured after culture of microencapsulated rat islets with interleukin (IL)-1beta and tumor necrosis factor-(TNF-alpha). Their ability to recover from IL-1beta-induced suppression was also investigated.

RESULTS

Microencapsulated islets were suppressed after exposure to IL-1beta, which was potentiated by TNF-alpha. After exposure to lower IL-1beta concentrations, microencapsulated islets had similar oxidation rates as corresponding controls. At higher concentrations, microencapsulated islets were more suppressed than nonencapsulated islets. Microencapsulated and control islets were able to recover from suppression after exposure to 2.5 U/ml of IL-1beta.

CONCLUSIONS

Microencapsulation using the present alginate/poly-L-lysine/alginate capsules does not protect islets against the detrimental effects of IL-1beta and TNF-alpha. Indeed, microencapsulated islets seem to be more susceptible to suppression at higher concentrations of IL-1beta. However, after exposure to a lower concentration of IL-1beta, microencapsulated islets can recover.

Studies on small (<350 microm) alginate-poly-L-lysine microcapsules. V. Determination of carbohydrate and protein permeation through microcapsules by reverse-size exclusion chromatography

Membrane molecular weight (MW) cut-off is a critical factor for immunoprotection of transplanted microencapsulated cells as well as for graft survival. Our goal was to study dextran and protein permeation through small (<350 microm in diameter) alginate-poly-L-lysine microcapsules made with an electrostatic system. Microcapsules were packed into a column, and gel-sieving chromatography was performed with proteins and dextrans of known MW. The objectives of this study were (1) to validate this approach for the assessment of the MW cut-off of <350 microm-in-diameter microcapsules and (2) to evaluate the effect on MW cut-off of changes in experimental conditions. Elution profiles of proteins suggest that the MW cut-off of our small microcapsules lies between 14,500 and 44,000 Da whereas dextrans > or =19,000 Da were excluded. The increase in poly-L-lysine (PLL) concentration from 0.02 to 0.08% reduced the MW cut-off. Increasing the PLL MW from 11.6 to 69.6 kDa induced no change in the MW cut-off. The results also show that the method can be used to discriminate between adsorption and absorption and that insulin diffuses freely across the microcapsule membrane. This method will be useful in establishing the ideal MW cut-off, in optimizing microcapsule characteristics, and in performing routine quality controls.