By David N. Leff
CAMBRIDGE, Mass. - No cruise ship takes tourists to the Islets of Langerhans.
The discoverer of this ubiquitous archipelago, Paul Langerhans (1847-1888), was no 19th century sea captain, spotting the small islands on the horizon through his telescope. Rather, the German anatomist, through his microscope, discerned those specks of tissue lurking in the pancreas.
The pancreatic gland is an organ the size and shape of a banana, but with the consistency of wet toilet paper. Nestled between the upper intestine and spleen, a pancreas contains about 1 million Islets of Langerhans.
Their main assignment in the body is to produce insulin, which metabolizes dietary sugars, or glucose. When your annual physical's glucose metabolism test shows too high a level of blood sugar, the doctor may suspect diabetes mellitus. This widespread disease comes in two versions, Type 1 and Type 2. Both reflect inadequate insulin, but with a difference. In Type 1, also known as juvenile-onset diabetes because it strikes the very young, the immune system starts eyeing those islets as foreign invaders, and chews them up. This means a life sentence of daily injections of supplemental insulin to replace the loss from the destroyed islets. Type 2 diabetes is gentler, often responding to diet and exercise.
Ever since the first successful organ transplant a generation ago, surgeons have been trying to transfer healthy islets into Type 1 patients, to replace those cells that fell victim to the autoimmune wipeout. Those efforts haven't succeeded, but not for lack of trying.
"Islet transplantation at the present time is miserably unsuccessful." That was the message from transplant surgeon Hugh Auchincloss Tuesday to a session here on future diabetes therapies, at this year's science symposium of the Massachusetts Biotechnology Council.
Auchincloss, who directs the Juvenile Diabetes Center for Islet Transplantation at Harvard Medical School, set the stage for two reports on ongoing islet transplant research, each approaching that grail from opposite directions.
"You will hear of two interesting new approaches to protecting islets from the immune system," he told his audience. "Immunologist Lisa Burkly, at [Cambridge-based] Biogen Inc., will talk about biologic mechanisms to achieve tolerance, and avoid long-term immunosuppression. Then, chemical engineer Clark Colton, at MIT [Massachusetts Institute of Technology, also in Cambridge] will describe physical barrier devices to separate islets from immunologic attack.
"When you develop Type 1 diabetes," Auchincloss explained, "your islets become infiltrated and eventually destroyed by lymphocytes. We know that this is an autoimmune disease, mediated by T cells. The net result of Type 1 autoimmune diabetes is the destruction of the beta cells that make insulin.
"We also know that it is in fact possible to cure this juvenile-onset disease by islet replacement therapy. I'll now introduce one person to you who illustrates that point."
At that point, Auchincloss put a slide on the screen depicting a pleasant, smiling woman. "She is Jill Aszling," he said, "a patient of mine who works at MIT. Jill grew up with juvenile diabetes, which she had for roughly 20 years. Then, 11 years ago, in a seven-hour operation, she had a combined kidney and pancreas transplant. The diabetes had destroyed her kidney, but the pancreas was transplanted as well to cure her of diabetes. Jill in fact has been able to say for these past 11 years that she used to have diabetes. But she has not taken a single unit of insulin during any of that time.
"Jill is not alone in that situation," Auchincloss continued. "Many hundreds of patients in the U.S. have been cured of diabetes by this transplant approach to replace the missing islet cells. But there are problems with Jill's solution. For one thing, in place of her insulin, Jill is now taking long-term immunosuppression. I would suggest to you that as bad as diabetes is, and as much of a pain in the neck taking insulin is, the same number of years of immunosuppression are actually worse than insulin therapy. It would not be appropriate to turn to a 15-year-old child and say, 'Take away the insulin that you now need, but we're going to give you prednisone, cyclosporine and a number of other drugs for the remainder of your life.'
"There are roughly 300 to 400 people," Auchincloss pointed out, "who have had autoimmune diabetes, and who received islet transplants from a different individual. They then got standard immunosuppression, and the number of such people who were free of insulin therapy for a period of longer than one year is five to six to seven individuals - out of 300 to 400. That's in the range of 1 percent of patients successfully treated with islet transplantation."
To solve this problem Auchincloss proposed four elements:
"One is simply accomplishing successful islet replacement as a form of transplantation surgery. Second, it is essential that tolerance be induced, i.e. that there not be a need for long-term immuno suppression. Third, we need to prevent the recurrence of autoimmunity, which seems to be one of the critical features for the success of islet transplantation. And finally," he pointed out, "if all of those problems were solved, we'd need a larger source of islets for human transplantation, This is going to mean either turning to animal donors - xenotransplantation - or learning to expand the number of human islets that are available - a problem that has not been solved yet."
Auchincloss then introduced Biogen's Burkly, "who will discuss using biologics to overcome the immune system's autoimmune reaction to transplanted foreign islets."
Biogen's Antibody Keeps Diabetic Monkeys Off Insulin
Biogen's strategy, Burkly recounted, focuses on a proprietary antibody to the CD40 molecule, expressed on the surface of antibody-producing B cells, and its ligand, CD40L, which perches on activated T cells. "This CD40 molecular pathway," she told the diabetes session, "plays an essential role in regulating humoral as well as cell-mediated immunity. And the potential for blockade of this pathway is a new approach to enable clinical islet allotransplantation."
Burkly described preclinical primate studies using the humanized antibody specific to the CD40 ligand, which Biogen is developing.
"That ligand," she explained, "is a member of the continuously growing tumor necrosis factor superfamily of molecules. Besides TNF itself, it includes lymphotoxin alpha and beta, the fas ligand, etc. These play key roles in promoting inflammation and regulating cell survival. CD40 is especially interesting as regards immune regulation. This makes it a specific and attractive target for immune modulation.
"We tested this potential in primates," Burkly recounted, "using our anti-CD40L monoclonal antibodies in rhesus monkeys transplanted with allogeneic islets from genetically mismatched donors."
She and her co-workers rendered the recipient monkeys diabetic by completely ablating their pancreas glands. From other animals they isolated the Islets of Langerhans, and transplanted them into pancreatectomized hosts. Then they monitored the animals' blood glucose levels as a measure of islet cell function.
"We are testing here," Burkly pointed out, "the ability of an agent to inhibit or prevent rejection of genetically mismatched islets. These animals do not have underlying autoimmune disease."
They introduced the antibody treatment concurrently with the islet transplant, of 10,000 to 20,000 islets per kilogram of body weight, "and chose to continue the antibody therapy in a maintenance mode on a monthly basis for at least one year. A control animal," she went on, "received an alloislet transplant without any treatment; instead, it got intensive insulin therapy.
"By contrast, four animals are our longest-term recipients at this point. They showed stable glucose metabolic control for well over a year post-transplant. That is typical of a human patient with Type 1 diabetes, being maintained on an intensive insulin regimen. In ongoing studies," Burkly continued, "eight of nine monkeys that received islet allografts are insulin-independent for days ranging from 43 to well over 500."
Human trials, she announced, are in the planning stage at the universities of Miami and Minnesota.
Implantable Insulin-Secreting, Antigen-Proof Barriers
MIT's Colton then described the "two main approaches that are being used in the area of capsulation or immune barrier: A microcapsule runs anywhere from 500 to 1,000 microns. Islets have diameters in different animals ranging from 100 to 200 microns."
The immune barrier he presented "involves a sandwich-like structure called a planar fusion chamber. It consists of two membrane laminates. A spacer between them defines an inner chamber where insulin-secreting tissue is placed. The cellular and humoral immune components are prevented from entering the tissue compartment by a semipermeable membrane, but nutrients and oxygen are able to diffuse into tissue. Insulin and wastes are secreted out.
"One of the biggest problems," Colton observed, "is that islets are normally very heavily vascularized, and are normally exposed to arterial oxygen. In an immunobarrier situation, the islets are removed from their blood supply, and must obtain oxygen from a source outside of the membrane."
He and his associates have developed a technique to generate oxygen electrochemically at a site adjacent to the implanted device. "These are very recent results," Colton told the session. "The whole idea is that if we have a tissue here, at one face we're going to interpose a flux of oxygen, generated by electric splitting of water. The oxygen diffuses from that face, and as we increase the flux, the insulin-secreting tissue increases."
The MIT group also is testing islets encapsulated in alginate gels, 1 centimeter in diameter by 1 millimeter in thickness. "The issue of those experiments," he explained, "is to try to get pancreatic cellular clusters from neonatal piglets. They mature, differentiate and eventually become insulin-secreting porcine beta cells."
Colton then switched to immune protection. "Antigens small enough to pass through the membrane micropores stimulate antigen-presenting cells, and initiate a process that's very similar to what happens with normal organ transplantation, in which both cellular and humoral responses are activated. The cells that can concentrate outside the barrier in an inflammatory response, as a result of the activation of the immune system by the shed antigens, can secrete cytokines, free radicals, nitrous oxide."
"Even though you can keep out large molecules, such as antibodies," Colton continued, "you can't necessarily prevent the occurrence of an immune response outside the device. These are some of the issues that we are beginning to tackle in the immune area."