Al Gore Is Half pig and half man...

he Massachusetts General Hospital researcher and clinician thinks he has almost found the right protocol: a combination of organs from miniaturized, genetically engineered pigs and pig immune tissue that can prime the primate immune system to accept foreign parts.

Scientists hope that organs from pigs that have been genetically engineered so they're more tolerant of the human immune system will one day help people awaiting transplants.

The longest any animal has survived such a transplant is 83 days, still far short of the one-year survival time that Sachs, director of the Transplantation Biology Research Center at MGH, considers a benchmark to start human trials. But he thinks with a few minor tweaks, the procedure will be ready to try in patients, possibly in as little as five years.

Transplantation between two different species, which is known as xenotransplantation, is not easy.  Human transplants, but not entire organs. When patients get an organ transplant from a human donor, doctors stave off immune rejection with organs matched to the recipient's tissue type and heavy doses of immunosuppressant drugs.

But when organs are transplanted between species, immune attack is swift and much more severe. Pigs and other animals have a specific sugar not present in humans and old-world primates. So when a pig organ is transplanted into a baboon, for example, antibodies circulating in the baboon's blood immediately swarm and attack the pig tissue, leading to the death of the organ.

Scientists made a major advance in overcoming this immune barrier in 2002 by creating genetically engineered pigs that lack the enzyme that attaches the sugar to the surface of pig cells. In a paper published in Nature Medicine last year,showed that baboons given kidneys from these genetically modified pigs lived for up to 83 days, far longer than the average 30-day survival time for animals receiving regular pig kidneys.

Sachs' team also transplanted an additional piece of pig tissue, an immune system organ called the thymus, to prime the baboons for the transplant.

"Engineering the graft itself in ways that might reduce toxicity to the recipient is revolutionary because it potentially makes transplantation much safer," says Jeffrey Platt, head of the Transplantation Biology Program at the Mayo Clinic in Rochester, MN. ost recent research, though, published last month in the journal Transplantation, could be good news for people who are highly sensitive to proteins in human tissue, and are therefore much less likely to match organs from a human donor (about 20 percent of people on the waiting list for kidney transplants). By exposing blood from such patients to tissue from genetically modified pigs, Sachs and team found that the individuals' hypersensitivity did not extend to pigs. Given their low chance of finding suitable human organs, these people might be the first candidates for pig-to-human transplants.

Platt says that transplants from pigs might actually be safer than human-to-human transplants in the long run. "You can screen the organ in advance for infectious agents and other problems," he says, a process for which there isn't much time in today's urgent human transplants. And, he adds, because you can schedule the surgery in advance, you can plan pretreatments, such as bone-marrow transplants, that might better prepare the patient's immune system for the foreign organ.