Encapsulation: Protection for Islet Cells

by Thania Benios

For people with type 1 diabetes (T1D) awaiting an islet transplant, a single phone call could change their life. The waiting list for these transplants stretches on, and far too often only serendipity propels a patient to the front of the line. These transplanted islets hold the promise of supplying a patient with the critical hormone insulin, without which the body cannot regulate blood sugar and maintain optimal health.

When islet transplantations were performed at the National Institutes of Health in Bethesda, MD, as part of the Edmonton protocol, they were heralded as a new frontier for treating severe cases of T1D. The initial reports of these transplants were promising, showing that 80 percent of all patients who received new islets did not require insulin injections for at least a year. But those numbers didn’t last, and the burden for patients having to take anti-rejection drugs for life also weighed heavily on physicians, since the drugs were spawning problems worse than those that the transplant was solving.

For people who have severe symptoms of diabetes, the idea of taking immune-suppressing drugs is an acceptable risk. But the risks may not outweigh the benefits in otherwise healthy children and adults with diabetes. To make islet cell transplantation available for a wider number of people with the disease, it became increasingly clear that transplantation couldn’t come at the cost of suppressing the entire immune system, says Julia Greenstein, Ph.D., assistant vice president of cure therapies at JDRF. “So we said that if we can’t suppress the immune system, we have to find a way to hide the transplanted cells from it.

That’s the principle behind islet encapsulation: before sending islets into a battlefield that’s highly stacked against them, dress them in a coat of armor, a protective barrier that hides them from the immune system and shields them from attack. All the while, this armor would support the longevity of the islets by allowing nutrients to seep in and insulin to seep out.

Now transplanting encapsulated islet cells is at the forefront of JDRF’s priorities, with a research portfolio that has been steadily growing since 2006. By embracing this field, JDRF is also taking on encapsulation’s battered history and checkered past, as well as the extraordinary cultural, technological, and biological hurdles that must be overcome to position this field for success and build on its most recent advances. JDRF believes that the fundamental key to this success will be to bring the right scientists with the right expertise to the right projects, and has met this challenge on many fronts.

On March 22-23 of this year, hosted its second Encapsulation Workshop in seven years, bringing together more than 50 industry and academic experts on beta cell encapsulation, with backgrounds in fields as diverse as medicine, immunology, chemistry and engineering. The purpose was to hear the most recent progress and to identify the barriers that need to be overcome. The recommendations from the workshop will guide JDRF’s next wave of research efforts to accelerate and champion the field.

Building a capsule

Ever since encapsulated islets were shown to correct diabetes in rats in the 1980s, researchers in the field have sought to extend that success to humans. The approach seemed simple enough: encase islets with a material that hides them from the immune system and keeps them alive. But the field was plagued with several limitations, including impure materials or sub-par ones that were not developed with islets in mind. Working with these materials yielded widely different results, and researchers, unable to verify their own results or those of their colleagues, became frustrated. Industry support dwindled, and the field never delivered on its promises.

Many of these materials were developed for encapsulating drugs, work that enjoyed tremendous success throughout the 1990s, especially for the delivery of cancer drugs to patients, explains Ron Gill, Ph.D., an immunologist at the University of Colorado who attended the workshop. “But encapsulating islets is a different story because you have to throw this material around a living, functioning, metabolically active tissue and ask it to survive.” In other words, encapsulating islets is not like encapsulating a medicine—it’s a lot more complicated because islets are living cells.

In 2007, JDRF launched a three-year, $4.3 million initiative that sought to develop and identify the best materials specifically for encapsulating islets. The initiative, led by Robert Langer, Sc.D., a chemical engineer at MIT, is unlike any encapsulation program to date due to its specific focus on islets, as well as its approach. In the past, researchers, tweaked existing materials developed for other uses in a way they thought would make them compatible with islets. Langer’s approach, however, eliminates this human bias.

Rather than subtly tweak materials used for other purposes, inching along one modification at a time, Langer and his team take a shotgun approach. They put a piece of alginate – a tiny coat of islet armor – in hundreds of thousands of individual wells that look like miniature egg cartons. The alginate in each well is modified in a different chemical reaction, yielding thousands of unique coats of armor, each of which is slightly different from the next. The goal is to identify the strongest coat of armor that does not alarm the immune system. In essence, it is a process of evolution whereby the worst performers are discarded and the best survive.

Regardless of the material the Langer group is using (alginate is only one of many), even the best-performing armor must pull off a daunting feat, explains Dan Anderson, Ph.D., a biomaterials engineer who works with Langer. Not only does the material need to be compatible with the recipient’s immune system, but also with the islets it protects. “And remember, these islets are not the recipient’s,” he says. “They are coming from either another human or pig donor, so we are asking a material to be compatible with two vastly different biological systems.”

A capsule must also keep islets safe and healthy by protecting them from the immune system while allowing insulin and glucose to pass through its pores. However, the same capsule that shields the islets from an attack can also starve them of their much-needed oxygen supply. To resolve these conflicting needs of islet protection and viability, scientists must finesse the design of the capsule—pore size, thickness, etc.—to accommodate both.

“It’s a really tough problem to solve because these pores in the capsules can’t be too big or too small,” says Albert Hwa, Ph.D., a scientific program manager at JDRF. “It’s just like Goldilocks. They have to be just right.”

Sustaining islets

Building a capsule is one thing, but providing them with a nurturing environment in the body is another, says Cherie Stabler, Ph.D., a biomaterials engineer at the University of Miami.

Healthy islet cells, she says, are constantly surveying the amount of glucose in the blood and calculating how much insulin to release to counteract it. That’s why in a normal pancreas they are surrounded by an intricate network of blood vessels, which are constantly providing them with a fresh supply of oxygen and nutrients.

Working with a cross-disciplinary team of immunologists and clinicians, Stabler is trying to create such an environment in which transplanted islets can flourish. ”Even if we build the perfect capsule, the islets may not be happy in it once we transplant it into the body,” says Stabler. “So that’s why we engineer the sites—to make sure that when we do transplant our encapsulated islets, they will have lots of oxygen and blood vessels all around them.”

The sites that Stabler and her colleagues are engineering are molecular scaffolds—like the steel frame of a rising building. Before transplanting them into the body, they sprinkle compounds on the scaffold that generate oxygen and promote the growth of blood vessels. Some compounds also suppress the immune system, but only in the area surrounding the scaffold, not throughout the entire body. The team calls this system the “sprinkler system.” Just like a sprinkler promotes the growth and long-term health of your backyard flora, the seeded scaffold creates an environment in which islets can thrive.

Stabler points out that these scaffolds, which are 90 percent porous and 10 percent material, do not physically protect the islets from the immune system. To shield them from an immune attack, Stabler and her interdisciplinary team of immunologists and clinicians use a technique called nano-encapsulation. With this technique, they wrap individual islets with a microscopically thin coat of armor, around 40,000 times smaller than the width of an average human hair. Because this layer is so thin, as though it is shrink-wrapped around the islet, oxygen and nutrients can easily reach the islet. At the same time, the islet can release insulin into the bloodstream through that same, ultra-thin layer.

While Stabler creates a favorable environment for the islets outside of the capsule, Adam Mendelsohn, Ph.D., from the University of California San Francisco, creates a favorable environment inside of it. Also, rather than shrink-wrap individual islets, Mendelsohn places several islets in a capsule, like peas in a balloon, a technique known as macro-encapsulation. It is well known, he says, that the number of islets that contact each other inside the capsule impact each islet’s ability to produce and secrete insulin. Mendelsohn and his colleagues have shown that the larger the cluster the better—but up to a point. If clusters get too large, some of the islets are buried in the middle, where they have very little access to nutrients and oxygen. “Very few people are looking at cluster size and how that might affect the transplant efficacy and viability,” he says. “We are developing methods to determine just that.”

Identifying the barriers

With better materials, including the advent of exceptionally pure alginate, and biocompatible scaffolds that keep islets healthy, researchers have been able to encapsulate islets and keep them alive for up to six months.

However, this is a far cry from the goal of keeping islets alive and correcting diabetes for a lifetime. Despite major steps forward in capsule design, transplanted islets are still failing and researchers are desperately trying to understand why. “We have identified certain gaps in knowledge that prevent us from knowing the answer,” says Hwa. “These are the priorities that we need to address.”

At the Encapsulation Workshop, some 50 researchers—experts in immunology, beta cell biology, chemistry, biomaterials, and islet transplantation—discussed the critical gaps in knowledge about how the immune system works and how these gaps make it difficult to design a material that protects the islets from it.

One gap in knowledge is understanding exactly how the immune system identifies and attacks islets, which can happen either from direct contact between immune cells and beta cells—a so-called “kiss of death”—or by signals sent over longer distances that can alert the immune system that there are islets in the neighborhood. “The immune system is capable of mounting an attack on islets in two ways,” says Gill. “But the problem in building a suit of armor for them is that we don’t know which immune response we are protecting them from.”

Further complicating matters is the question of whether the immune system is the only culprit in the islet’s demise. “These islets may be dying because they are asked to correct diabetes in a large body,” says Hwa. “If they work too hard, the stress may exhaust them.” When islets are stressed, he explains, they release alarm signals that can trigger any number of events in the body that ultimately end in an islet’s death.

At this point, it is not clear what percentage of the islets dies because of stress and what percentage dies because of the immune attack. To find out, researchers must either develop imaging technologies such as MRI or identify biological markers—molecular clues that indicate what going on inside the body. These tools would allow researchers to untangle the factors—healthy tissue, good blood supply, a functioning immune system—that are most important for keeping islets healthy. The same techniques might also help researchers gauge whether transplanted islets are flourishing in their new home.

Forging ahead

Despite the many difficult challenges ahead, JDRF believes that the potential benefit of eliminating the need for immunosuppression justifies the investment in encapsulation research. In order to maximize its chances for success, JDRF will be significantly expanding funding opportunities for encapsulation research in FY12, while emphasizing and promoting cross-disciplinary interaction.

In addition to co-sponsoring the encapsulation workshops in 2004 and hosting it in 2011, and funding the Langer program in 2007, JDRF also released a Request for Applications (RFA) in 2009. The RFA, which formally invites researchers to submit grants, specifically required a biologist and an engineer to co-apply. The idea is to reward and foster interdisciplinary collaboration, with the hope that multiple fields inform and learn from each other. Based on the March 2011 workshop recommendations, a similar RFA was released in November 2011 to expand JDRF effort in encapsulation. This new RFA will support a number of projects aimed to improve our understanding of the limitations of the current technologies and to test novel and innovative approaches to protect islet cells. Most importantly, the funded projects will be required to participate in data and tool sharing in a consortium fashion, which is critically needed to move this field forward.

JDRF also recently renewed the program led by Langer, and by adding two more members to the team underscored its fundamental strategy of rewarding cross-disciplinary collaboration, making sure that the right experts are involved in these projects. “We feel that we really rounded out the program by bringing together key players in the field,” says Hwa. “By bringing together the right people with the right combination of skills, we have a real shot of making an impact in this field.”

Islets are clusters of cells in the pancreas that include beta cells, which produce insulin. In type 1 diabetes, beta cells are destroyed and no insulin can be produced. An islet transplant replaces these insulin-producing cells with functioning ones from a donor source.

The Edmonton protocol was a new way to perform islet transplantation. It is still performed today with some modifications that have improved its success, but it continues to require immune-suppressing drugs, which have side effects that severely limit its applicability.

Encapsulation provides a physical barrier around islets that hides them from the immune system, circumventing the need for immune-suppressing drugs.