Xenotransplantation: Animal organs to save human lives
The human immune system is the body's main defense against infectious organisms. In this capacity it works like a well drilled army. Penetrate one line of defense and a flanking maneuver from a second line quickly moves in.
Although the immune system is essential for life, when a person needs an organ transplant, the immune system suddenly becomes a deadly force, attacking and destroying the implant. Immune system suppressing drugs such as cyclosporin have made human organ transplantation possible by restricting attack on the transplanted organ while sparing the immune system's most life-saving benefits.
But the challenges facing researchers trying to adapt animal organs for transplantation into humans, a process called xenotransplantation, are much more steep. Duke University scientists are systematically evading the immune system's defenses to make xenotransplantation a practical option for the almost 4,000 people who die each year waiting for human organs.
The prospect of using animal organs to save people is a goal Duke immunologist Jeffrey Platt believes is worth pursuing. He is looking to pigs, an animal that supplies humans with food and leather, for that ready organ supply. Surgeons already use pig heart valves to replace human heart valves that wear out.
"Pigs are good potential donor animals because their organs are about the same size as human organs and work like human organs," Platt said. "In addition, much is already known about raising pigs, and there is a ready supply of the animals."
The idea of using animal organs as substitutes for failing human organs has been attempted in the past. The results were dismal. Within minutes, an animal organ is recognized and targeted by protein sentries that circulate in the blood. These sentries flag the animal tissue and set in motion a cascade of events, involving the complement system, that cut off the blood supply to the organ and basically starve it to death.
When early xenotransplant pioneers witnessed implanted animal organs failing and turning black, they all but abandoned the idea of using animal organs. It was not until the last decade that transplantation of animal organs into humans became a realistic goal. The advances that made it possible involve a new understanding of the complex human immune response.
Platt, professor of experimental surgery at Duke University Medical Center, has been at the center of these discoveries. He has devoted his career to understanding how the immune system recognizes implanted tissue as foreign and destroys it.
The immune system has several effective lines of defense to protect the body from infection by foreign organisms such as bacteria and parasites. These defenses also attack transplants. Platt and his colleagues are meticulously dissecting each defense mechanism on the molecular level to understand the players and to design ways around some of those defenses.
"These defense mechanisms are a double-edged sword," Platt said. "On the one hand they protect us from infection, but on the other hand they can sometimes attack the body's own tissues, as happens in autoimmune diseases such as arthritis. In xenotransplantation, we are walking a fine line between asking the immune system to accept an organ from an animal, but still protect us from other threats, such as infectious disease."
Platt and his colleagues are working to dissect which parts of the immune response are attacking and destroying the animal organ transplant, and finding ways to interfere selectively with these pathways. One approach is to make a pig organ look to the human body more like one of its own.
"Animal donors provide a unique opportunity for us as researchers," said Platt. "In human-to-human organ transplantation, we have to treat the organ recipient with powerful immune suppressing drugs to suppress the rejection process. But if we could engineer the donor animal organ to match a human organ as closely as possible, we could possibly reduce or even eliminate the need for immune suppressing drugs."
To do that, Platt and his colleagues are using three main tactics:
Introduce into the pig human genes that stop hyperacute rejection, the first-line immune response that attacks the pig organ in the first few minutes to hours that it is implanted. This is the initial hurdle to cross-species transplantation. There is now good evidence that it has been achieved. Remove pig genes that flag the organ as foreign and alert the immune system to its presence. Identify factors that lead to acute vascular rejection, the second-line immune response, which destroys transplanted organs in the weeks-to-months time frame. This is the second major hurdle in cross-species transplantation.
Stopping Hyperacute Rejection
The first steps to stop hyperacute rejection are already complete. In 1995, Duke researchers, in collaboration with scientists at Nextran, a biotechnology company based in Princeton, NJ, succeeded in introducing two human genes into pig hearts and testing them in baboons. These genes encode two key proteins called delay accelerating factor (DAF) and CD59, which can stop the complement cascade before it does any damage. These proteins play a crucial role in a person's normal immune system. They guard blood components such as red blood cells and blood vessels from attack by the complement system. Platt explained that the body's own blood components and vessels can be the target of a complement system that can be set off like a hair trigger leading to autoimmune disease. Proteins such as DAF and CD59 work by snuffing out complement cascade before it can act. Platt reasoned that if the researchers could put human DAF and CD59 in pig cells, they might be able to stop the complement cascade from attacking the pig organs as well.
In May 1995, Platt's theory was proven correct when Duke surgeons transplanted pig hearts into baboons. The hearts made the human DAF and CD59 proteins. The researchers reported in the May 1995 issue of the journal Nature Medicine that they had succeeded in keeping a pig heart containing the human proteins DAF and CD59 alive and functioning for 30 hours. Subsequent studies showed that transgenic organ xenografts could function for days without further treatments. An unmodified pig heart would be destroyed within minutes. The researchers are now working to identify other proteins to help pig organs escape destruction by the complement system.
Making Pig Organs 'Invisible'
Platt and his colleagues have taken a step in that direction by focusing on what the complement systems actually "sees" when it identifies a pig organ as foreign. The Duke researchers have studied a sugar molecule that acts as a flag for the human immune system. This complex sugar molecule is made by pig cells, and in all mammals except some monkeys, apes and humans. The sugar is made inside the cell and transported to the cell surface where it acts as a flag for antibodies circulating in human blood. The antibodies recognize the sugar and tag it as foreign. Once this "molecular scarlet letter" marks the pig organ, the complement system comes to destroy it. Platt and his colleagues reasoned that if the flag were missing, perhaps the human antibodies would not recognize the pig organ as foreign. So they designed an experiment to stop it from being made in mice, a common experimental animal, and in the pigs the researchers eventually hope to use in humans. Preliminary results in the mice that don't make the sugar show that without this flag, the human immune system has a harder time identifying the animal organ as foreign. This research was published in the July 9, 1996 issue of the Proceedings of the National Academy of Sciences.
Halting Acute Vascular Rejection
The third major research focus in Platt's laboratory is understanding what happens to an organ when it is attacked by another immune system weapon called acute vascular rejection, which can destroy transplanted organs in the weeks-to-months time frame. Platt first identified this type of rejection and showed its similarity to types of rejection sometimes seen in human to human transplants. This research is important for understanding hurdles still to be overcome in cross-species transplantation and also to reducing rejection by the body of human organ transplants.
"Acute vascular rejection is a major cause of rejection in all transplanted organs," said Platt. "A better understanding of acute vascular rejection will certainly help bring xenotransplantation closer to reality, but in the near term it may also help reduce disability and death of human transplant patients."
Platt's research team is in the process of identifying the components of the blood clotting system that collect in foreign organs and cause deadly clots. Researchers already know that the cells lining the blood vessels of the transplanted organ change in acute vascular rejection. These cells, called endothelial cells, become "activated." They change shape and begin churning out coagulation factors and other substances that lead to blood clots. Platt and his team are investigating how this process happens with the hope of identifying strategies to halt it.
"We believe this three-pronged approach is bringing us close to the point where animal organs may begin to be routinely be transplanted in to humans within two years," Platt said.