Sweet success in freezing islets.

Sweet success in freezing islets

A surgical team recently removed most of a baby's pancreasbecause his pancreatic islets were producing too much insulin. While this procedure solved one problem, it may eventually make the child diabetic, because as he grows he will need more insulin-producing islets to control his blood sugar levels. So the surgeons have frozen the baby's islets in the hope of being able to transplant them back if the need arises in the future.

If the surgery is successful, it will be due largely to thecryopreservation studies of Ray Rajotte, Garth Warnock and Marilyne Coulombe at the University of Alberta in Edmonton. This group says it has now shown conclusively that animal islets can be successfully frozen, thawed and transplanted. The researchers report that they have reversed drug-induced diabetes in rats with healthy cryopreserved islets grafted in the animals' kidneys. They know that the transplanted islets were responsible because when they removed the kidneys, the animals became diabetic again. Testing the viability of islets in this way was not possible in past studies, Coulombe says, because cryopreserved islets were grafted into the liver, which could not be removed without killing the animal.

The ability to freeze and thaw organs and tissues withoutdamage has been a longtime goal of cryobiologists. Researchers have cryopreserved relatively simple tissues such as skin, but they are still far from storing organs at freezing temperatures. Islets, says Rajotte, "are the most complicated multicellular structures we've been able to freeze.'

Scientists have been interested in collecting and storingislets from donors mainly because they hope to use islet transplants as a safer and more effective treatment for some types of diabetes. The traditional approach--periodically giving large doses of insulin that are not tailored to the diabetic's actual blood sugar levels--often leads to later complications such as blindness.

According to Coulombe, there have been a few humantransplants with fresh islets, but these have met with only limited success, possibly because insufficient numbers of donor islets were collected. Cryopreservation would give surgeons time to collect many islets from different donors. But Rajotte says his group has also developed a technique for extracting high yields of pure islets in dogs, and this may help surgeons isolate sufficient quantities of islets in new human trials scheduled to begin shortly.

Another potential advantage of cryopreservation is thatcertain freeze-thaw conditions may preserve tissue while selectively killing off donor "passenger' leukocytes, the white blood cells that trigger an immunological rejection of the tissue. Rajotte's group has also demonstrated that cryopreserved rat islets grafted into mice generally withstand rejection longer than grafted fresh islets. However, the researchers have not yet proved that freezing and thawing per se are in fact solely responsible for the prolonged survival.

Working on the assumption that cryopreservation can affecttissue and leukocytes differently, Michael Taylor and his colleagues at the Medical Research Council in Cambridge, England, are looking for the best freezing regimes for reducing rejection of the transplanted islets. While Rajotte has shown that the survival of islets is optimal during a slow-freeze-quick-thaw procedure, Taylor suspects that this does not kill all leukocytes. Since there has been some suggestion that leukocytes do poorly when frozen quickly, he has set out to see if at least some islets can survive under rapid freezing. His preliminary conclusion is that they can. From his and other studies, says Taylor, islets appear to be able to withstand a much wider range of cryopreservation conditions than any other tissue system. "Why this should be,' he says, "is the question that is uppermost in cryobiologists' minds.'