MPR: From Pigs to People

From Pigs to People
By Brent Wolfe
May 11, 1999

Each day, 12 people die in the United States waiting for an organ transplant. Solving the organ shortage will require more than just additional donors. Demand will still far outstrip the supply. So researchers at the Mayo Clinic and elsewhere are developing a new source of organs - pigs.

Organs transplanted from animals - known as xenotransplants - are a challenge for medical researchers. They present several problems including rejection by the human host and the possibility that viruses will jump from pigs to people.

57-YEAR-OLD JOHN MCGARRY IS WAITING for a new liver and a new heart at the Mayo Clinic. A rare condition makes his liver produce too much protein. The protein attacked his heart, which is now so weak he's been hospitalized for 10 weeks. Any day, he could get the word that a donated heart and liver are available but, in the meantime, he sits propped up in bed in a purple robe with small, gold Irish good-luck angels pinned to the lapel. He spends a lot of time looking out the window.

McGarry: I count all the cars in the gas station. And I count all the bricks in the hotel across the street. I've counted them about 40 times.
MPR:How many bricks are there in the hotel across the street? McGarry: 3,014 I think. No. 3,214.

McGarry says he's confident a donor will become available. He'll make it through the surgery and then return to his home in Chicago in a few months. One of McGarry's doctors, Christopher MacGregor, is Mayo's director of cardio-thorasic transplantation. MacGregor has spent years watching patients wait for organs, wanting another option for them. He says Mayo is recruiting xenotransplant researchers hoping to reduce the waiting time for patients like John McGarry.

MacGregor: Not just one investigator, but a critical mass of investigators. And it's this commitment to a critical mass of scientists over a long period of time that, hopefully, will produce positive results.

Dr. Jeffrey Platt leads the team and he's setting up a lab in one of Mayo's research buildings in downtown Rochester.

Platt: This is the main part of the laboratory. Its quite a large room and its designed as an open room.

Platt says scientists are trying to build artificial organs and "grow"" organs using genetic engineering. But he believes xenotransplants, or organs from animals, are more promising because researchers don't have to start from scratch. Doctors have used baboon organs in some cases, but Platt says pigs are the donors of the future because there aren't enough primates available.

Platt: Another reason for using pigs is that they're the same size as humans and their tissues and organs would work the same in a human, we think, as they would in a pig and vice versa.

Doctors already use pig valves to replace leaky heart valves in cardiac patients but since that tissue is sterilized and essentially dead, it doesn't present the potential for viral transmission. Mayo researchers aren't the only ones studying xenotransplants. Scientists at the University of Minnesota have transplanted several pig hearts into baboons.

One of the largest barriers facing xenotransplantation is human rejection of the pig organ. The human immune system immediately recognizes the new organ as a foreign object and attacks it until it's destroyed. So researchers are looking at how to suppress the immune system and how to genetically engineer pig cells with human genes so they don't look so foreign to the human immune system.

People with suppressed immune systems become more vulnerable to infections by human viruses. These patients - or their new pig organ - might also be susceptible to viruses that affect pigs. Harvard Medical School Professor Fritz Bach says xenotransplantation could become a bridge for pig viruses to move to people. Another reason for using pigs is that they're the same size as humans and their tissues and organs would work the same in a human, we think, as they would in a pig and vice versa.

Bach: The real danger is that that virus would mutate, would change genetically in such a way that it could then spread from the recipient of the porcine organ - presumably through close contacts of the patient into the general population - and cause a pandemic, perhaps not unlike AIDS.

Scientists believe the virus that causes AIDS moved from chimpanzees or monkeys to humans and mutated into a form that spread from person to person. Researchers will try to stem the spread of pig-borne viruses by keeping the pigs in sterilized environments, not an ordinary feedlot. Mayo and Baxter-Nextran, a Chicago-based drug company, are building a special facility in northwest Rochester where the pigs will breathe filtered air, eat sterilized food and be housed separately.

However, Bach says that won't eliminate at least one virus known as the Porcine Androgonous Retrovirus. Like the human immunodeficiency virus that causes AIDS, it moves into the genetic material of cells. Bach says there's no evidence so far of the retrovirus passing to human cells in the laboratory, but he says attempting a pig to human transplant would be the wrong way to find out if it could happen.

Bach: We don't know the extent of risk. Many people say it's very small. I don't know what that means. And to be honest, I think if they were asked they wouldn't know either in quantitative terms. But we have to remember that if it were to occur, the results are catastrophic. If we say that, if it were to occur, that is the release of a pandemic, we may not recognize that for 10 years that it's happened, and then it'll burst on the scene.

Bach worries that someone might move to clinical trials of xenotransplants too quickly in the pursuit of profits or prestige. He's calling for a moratorium on clinical trials until there's been adequate public discussion on the risk of an epidemic - like AIDS - spreading from pigs to people.

Mayo viral researcher David Persing believes the risk of a viral transmission to humans is small. Even so, he's leading a study of 300 workers at Hormel's Austin pork-processing plant to look for any evidence of a pig virus infecting people who regularly come in contact with pig flesh and blood.

Persing: I think that since humans and pigs have co-mingled for so many thousands of years, I think that we would have seen something already. We would have known about something happening within individuals that were studying. More likely, I think, if transmission of viruses occurs, they're likely to be, essentially, harmless.

Persing and other supporters of xenotransplant research say there are adequate safeguards in place to prevent a viral jump including oversight by the Food and Drug Administration. They say many more studies will be conducted before anyone can move to clinical trials and when those trials do begin, the human recipients are likely to be quarantined while scientists study them for any signs of viral transmission.

In the meantime, thousands of people like John McGarry are still waiting for new organs.

McGarry: No time to get bored around here. There's always something going on around here. I go for a walk. If I get bored, I play my music.

McGarry plays traditional Irish music on a small bedside tape player to help pass the time. A nurse comes in with another dose of pills and asks if she can get him anything else. Just a heart and a liver he answers with a smile.

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