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Article first published online: 26 MAR 2009
DOI: 10.1111/j.1600-6143.2009.02630_1.x
© 2009 The Authors Journal compilation © 2009 The American Society of Transplantation and the American Society of Transplant Surgeons
Additional Information
PONDROM, S. (2009), News and issues that affect organ and tissue transplantation. American Journal of Transplantation, 9: 653–654. doi: 10.1111/j.1600-6143.2009.02630_1.x
How close are we to developing an alternative to human islet cell transplantation?
Three years ago, the field of xenotransplantation was revitalized after groups in Minneapolis and Atlanta/Edmonton reported in Nature Medicine that pig islet transplantation had successfully corrected diabetes in monkeys.1,2 Although it appeared that human clinical trials were just around the corner, a cloud still hung over the celebration. The researchers had used an experimental anti-CD154 costimulation pathway blocker that caused elevated incidents of thromboembolism in humans.
For those researchers and others, it was time to look at alternatives.
The question was, which immunosuppressant could substitute for the anti-CD154 antibody that both the Atlanta/Edmonton and Minneapolis groups had used in the trials reported in Nature Medicine, says Bernhard Hering, MD, scientific director of the Schulze Diabetes Institute, University of Minnesota, Minneapolis and lead author of the Minneapolis study. “Since 2006, in the pig-to-monkey islet model, we have studied the critical immune recognition and rejection pathways and currently approved immunosuppressants that can substitute for anti-CD154 antibody when combined with baseline immunosuppression with CD28 antagonist and rapamycin,” he says. He also says that the approach has led to significant progress in developing a safe, efficacious and clinically applicable immunosuppressive protocol for taking a first-generation pig islet cell therapy product to the clinic.
CD154 is just one way of doing this, according to David Sutherland, MD, PhD, director of the Schulze Diabetes Institute. “There are other immunosuppressant approaches, such as targeting lymphocyte functionassociated antigen-1 (LFA-1) and CD40,” he says.
New generation strategies are likely going to involve privileged microenvironments for islet implantation, such as the bioengineered omentum instead of the portal vein, according to Drs. Hering and Sutherland. For example, researchers from Miami and France recently reported in the American Journal of Transplantation the successful implantation of non-human primate islets in an omental pouch on a biodegradable scaffold.3 The Minnesota group is also refining pre-transplant islet immunomodulation and investigating the possibility of local immunosuppression.
Christian Larsen, MD, PhD, chief of the Division of Transplantation at Emory University School of Medicine in Atlanta says, “Our priorities are to develop cocktails that avoid CD154. This might include targeting the other side of the pathway, CD40. We're also looking at CD28 pathway blockers such as belatacept, and antibodies against LFA-1.”
Additionally, there may be approaches that would still allow targeting of CD154. “If you make the therapy duration short enough, perhaps just a few doses, you could anti-coagulate around the time of the procedure,” says Dr. Larsen.
David Cooper, MD, PhD, professor of surgery at the Starzl Transplantation Institute, University of Pittsburgh, has a different slant on the issue. “I wouldn't concentrate on the immunosuppression,” he says. “It shouldn't be a problem, even with currently available drugs.”
Dr. Cooper believes Drs. Hering and Larsen are at a disadvantage by using wild-type pigs and relying on immunosuppressants. He contends that with a genetically engineered pig, “We have achieved pig islet graft survival for greater than one year in one case, and more than three months consistently.”
In addition to Gal knockout pigs, he says animals that express a complement-regulatory protein appear to offer some protection from the instant blood-mediated inflammatory reaction. Another option under investigation is the insertion of anticoagulant genes, such as tissue factor pathway inhibitor and/or CD39 into the islets. “The more you can do to the pig to protect it from the human immune response, the less you have to do for the patient,” he says.
Although researchers contacted for this article see benefit to genetically altered pigs, most say they aren't required for success. “We've achieved long-term islet survival, and so has Bernhard Hering, using wild-type pigs,” says Allan Kirk, MD, PhD, professor of surgery in the Department of Transplantation at Emory University. “For example, in islets, the Gal antigen doesn't seem to be as relevant as it is in vascularized transplantation.”
Meanwhile, numerous groups in the U.S. and abroad are looking at pig islet encapsulation to protect islets from the immune response. Although long-term survival of the islets has been a problem with capsules in the past, Julia L. Greenstein, PhD, scientific program director of beta cell replacement at the Juvenile Diabetes Research Foundation, says that over the past few years there has been a higher quality of clinical alginate available for the capsule material. “We're starting to see some intriguing results in rodent models and a little bit in primates,” she says.
One of the promising reports in non-human primates came from Pierre Gianello, MD, PhD, and Denis Dufrane, MD, PhD, of Université Catholique de Louvain, Belgium.4 Pig islets encapsulated in alginate supported liquid membrane cross-linked with calcium were transplanted as subcutaneous monolayer cellular devices (to increase oxygen diffusion) in five monkeys, with diabetes corrected for up to six months. “In addition, we have shown in a pig-to-primate model that subcutaneous is an interesting way to treat diabetes,” says Dr. Gianello.
The Belgium team is now working to extend the period of diabetes correction and soon will test the monolayer cellular device in human subjects with allo-islets.
Among the commercial entities investigating encapsulated islets are Living Cell Technologies (LCT) based in Auckland, New Zealand and MicroIslet and Novocell in San Diego, Calif. LCT's medical director, Bob Elliott, MD, says the company has tested neonatal porcine islets protected with an alginate/polyornithine/alginate capsule in non-human primates and six human patients in Russia, with additional human trials to be conducted in New Zealand. At MicroIslet, a calcium alginate capsule was tested in non-human primates, and the company plans to test these encapsulated islets in a phase I/IIa clinical trial in eight patients. Novocell has conducted phase I/II human clinical trials for its polyethylene glycol encapsulation of human islets.
But no one has a perfect capsule yet, says Ray Rajotte, PhD, professor with the Department of Surgery, University of Alberta. “The problem with encapsulation is that you're putting in a lot of material and you're asking the islets to survive by nutrient diffusion, not by revascularization,” he says. “I'm not sure how long islets will survive.”
Another area of investigation with pig islets is the use of immuneprivileged Sertoli cells, the elongated cells of the seminiferous tubules of the testis.
Dr. Rajotte, who did some of the original work with Sertoli cells, determined in 2006 that the “magic bullet” secreted by the cells is serpina3n, which his team continues to investigate.5 His colleague, Gina Rayat, PhD, assistant professor, Department of Surgery, University of Alberta, recently reported on successful and unsuccessful islet/Sertoli cell transplants.6 And David White, PhD, professor with the Transplantation and Regenerative Medicine Group at Robarts Research Institute in London, Ontario, has transplanted adult porcine Sertoli cells mixed with pig islets into mice, which then produced insulin and survived for six months.
Be it one method or another, an alternative to human islet cell transplantation is needed. “All of us in the field have said we need to develop an alternative source,” says Dr. Rajotte, adding that this might be xenotransplantation in the near term and possibly stem cells in the long term. Dr. Larsen adds, “To me, the priority is to get a clinically relevant immunosuppressive platform and then start to plug some of these other ideas into it.”
