Storage and microencapsulation of islets for transplantation

Microencapsulation is an effective means of immunoisolation for pancreatic islet transplants. However. the process of isolating, purifying, encapsulat ing, and transplanting islets in a single day is labor intensive and diffic ult for routine use. There is an apparent need for reliable methods of isle t storage, and cryopreservation has emerged as an attractive system of isle t banking. While studies have shown that cryopreserved islets are viable wh en tested unencapsulated after thawing, it is not clear if the combination of freezing and encapsulation would affect islet function. The purpose of t he present study was to determine the in vitro function of cryopreserved is lets following thawing and microencapsulation. Islets were isolated from th e pancreata of Sprague-Dawley rats and cryopreserved under liquid nitrogen for either 1 week or 1 month, following an overnight culture at 37 degrees C. Upon thawing, the islets were tested either unencapsulated or after enca psulation in polylysine-alginate membrane. In all experiments islets were p reperifused for 1 h at 37 degrees C with a modified Krebs-Ringer bicarbonat e buffer containing 3.3 mM (60 mg/dl) glucose and maintained at pH 7.3 by c ontinuous gassing with 95% air/5% CO2. Following basal effluent sample coll ection on ice, the glucose concentration was raised to 16.7 mM (300 mg/dl). It was found that, within 10 min of high glucose stimulation, an average o f twofold increase in insulin secretion (p < 0.01) was obtained in islets w ithin or without microcapsules. We conclude that islets cryopreserved for m onth prior to thawing and microencapsulation retained functional viability as determined in in vitro experiments.