Organ Transplants from Animals : Part 2

by Food and Drug Administration (FDA)

(Page 2 of 3)

Pig organs have been transplanted to humans several times in the last few years. In 1992, two women received pig liver transplants as "bridges" to hold them over until human transplants were found. In one patient, the liver was kept outside the body in a plastic bag and hooked up to her main liver arteries. She survived long enough to receive a human liver. In the other patient, the pig liver was implanted alongside the old diseased liver, to spare the patient the rigors of removing it. Although that patient died before a human transplant could be found, there was some evidence that the pig liver had functioned for her.

By genetically altering pig livers, some scientists believe they can make a pig liver bridge more successful. In July 1995, FDA permitted the Duke University Medical Center to test genetically altered pig livers in a small number of patients with end-stage liver disease. The pig livers contained three human genes that will produce human proteins to counter the rejection process.

Safe or Disastrous?

Xenotransplantation could be very good news for patients with end-stage organ diseases. There would be no more anxious months of waiting for an organ donor. Disease-free pigs would provide most of the organs. Raised in sterile environments, they would be genetically altered with human DNA so that the chance of rejection is greatly reduced.

Transplant surgery would be scheduled at the patient's convenience, as opposed to emergency surgery performed whenever a human donor is found. Patients wouldn't have to wait until their diseases were at a critical stage, so they would be stronger for recovery.

Today, however, xenotransplantation is still experimental, and there are serious risks to the procedures.

Although many researchers believe it is slight, one legitimate concern is that animal diseases will be transmitted into the human population. Baboons and swine both carry myriad transmittable agents that we know about — and perhaps many more we cannot yet detect. These bacteria, viruses and fungi may be fairly harmless in their natural host, a baboon or pig, yet extremely toxic — even deadly — in humans.

The two types of animal viruses that are especially troublesome are herpes viruses and retroviruses. Both types have already been proven to be rather harmless in monkeys, but fatal to humans. HIV, for example, is a retrovirus that many researchers believe was transmitted to humans from monkeys. The problem occurs in reverse as well. Measles, for example, a serious but manageable disease in humans, can destroy a whole colony of monkeys quickly.

By regulating xenotransplants, FDA will provide a framework for collecting safety data and tracking patients' health. The process should involve open and public discussion by scientists about their experiments, allowing their peers to evaluate and critique them, and their patients to understand the risks and make informed decisions.

"Will [xenotransplants] cause an outbreak of a new infectious disease? We don't know," says Phil Nogouchi, M.D., a pathologist and director of FDA's division of cellular and gene therapies. "But we want all these procedures discussed in public. We need to make people aware of the hazards."

Nogouchi emphasizes the importance of monitoring and tracking all recipients of xenotransplants so that if any new diseases do develop, they will be detected quickly and the threat to public health will be minimized.

"We cannot say that's not a possibility," says Nogouchi. "But we do feel the potential benefits are great and that efforts can be made to make everyone responsible. There are ways to deal with problems should they arise."

At press time, FDA, the national Centers for Disease Control and Prevention, and the National Institutes of Health were working on recommendations for researchers doing xenotransplant experiments.

Although the new recommendations will be for researchers, patients will likely also recognize their importance.

"Our biggest allies are the patients," says Nogouchi. "They should be asking, 'Where'd you get that pig?'" Xenotransplants cannot be "fresh off the farm." They should be bred and raised in a biomedical animal facility under strict conditions.

Battling Rejection

The other formidable obstacle to xenotransplants is that posed by the human body's own immune system. Even before a person is born, his or her immune system learns to detect and resist foreign substances in the body called antigens. These could be from anything that's not supposed to be there: viruses, bacteria, bacterial toxins, any animal organs, or even artificial parts.

Antigens trigger the body's white blood cells, called lymphocytes, to produce antibodies. Different lymphocytes recognize and produce antibodies against particular antigens. B cell lymphocytes produce antibodies in the blood that remove antigens by causing them to clump or by making them more susceptible to other immune cells. T cell lymphocytes activate other cells that cause direct destruction of antigens or assist the B cells.

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.