Bulk Cryopreservation of Isolated Islets of Langerhans

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Abstract
Current methods to isolate human islets of Langerhans are limited and multiple donors are required for successful reversal of longstanding Type 1 diabetes mellitus. Cryopreservation of isolated islets is an effective method of storing and pooling islets. Current cryopreservation protocols are cumbersome due to current practices of placing small aliquots of islets per individual freezer tube. In the present study, we examined the application of a blood freezer bag for the cryopreservation of isolated islets by slow cooling and rapid thawing. Freezing and thawing profiles generated using thermocouples placed inside a 500 mL Cryocyte (Baxter) blood freezer bag showed that a longer equilibration period at −7.4°C was necessary to consistently achieve nucleation and cooling profiles similar to those observed in glass tubes. When known numbers of rat islets were placed in the freezer bag and the cryoprotectant dimethyl sulfoxide (DMSO) was added in a stepwise fashion and removed using a sucrose dilution, the islet recovery compared with glass tubes was 92 ± 4.8 vs. 90 ± 2.3% (n = 4, p = ns, Mann-Whitney U-test). When purified canine islets were cryopreserved in a single freezer bag or in multiple glass tubes, the recovery was similar (78.8 ± 12.5% recovery for freezer bag vs. 82.3 ± 5.3% for glass tubes; n = 6, p = ns). In vitro function was equivalent for both groups. The stimulation index of insulin release during glucose perifusion (stimulated over basal insulin secretion) for canine islets cryopreserved in a freezer bag vs. glass tubes was 3.2 ± 1.0 and 2.3 ± 1.3, respectively (n = 6, p = ns). These values were significantly lower than the nonfrozen control islets (6.9 ± 2.4, p < 0.05). When 2000 canine islets cryopreserved in either a freezer bag, or glass tubes were transplanted into diabetic nude mice, the animals became and remained normoglycemic posttransplant. We conclude that the survival of freshly isolated canine islets cryopreserved in a single freezer bag is equivalent to the glass tube method. Bulk cryopreservation of islets in a single freezer bag will facilitate effective low temperature tissue banking to support ongoing clinical trials of islet transplantation.