Partly because of sheer luck, researchers at the University of Pittsburgh and scientists from a private company have overcome a critical barrier to using pigs as a possible source of organs for transplantation into humans.
Pigs genetically engineered by the team were used as a source of tissue that was successfully transplanted into a mouse without inciting immediate, devastating rejection.
The feat makes plausible the idea that pig organs might someday be transplanted into humans, ultimately reducing the demand for scarce human donor organs.
The findings were published yesterday at www.scienceexpress.org, an online, early version of the journal Science.
The use of pig organs for transplant has not been feasible until now because the human immune system mounts a rapid, unstoppable antibody response, called hyperacute rejection, that destroys the animal cells within hours.
That's because the surfaces of non-human animal cells carry sugar molecules called galactose, which is foreign to humans. Standard anti-rejection drugs are ineffective against the antibodies that attack the sugars.
But hyperacute rejection might be prevented if scientists could breed pigs that didn't have an enzyme necessary to make the sugar. To do that, they used genetic engineering methods to "knock out" the two copies of the gene in each cell that make the galactose enzyme.
Scientists at the Virginia branch of Scotland-based PPL Therapeutics inserted that lab-modified genetic material into a developing pig embryo, using the same techniques that created Dolly, the cloned sheep. There was a chance that the pigs could not survive without the galactose enzyme.
In July, a litter of four piglets that had cell surfaces bare of galactose were born. But curiously, when scientists looked more closely at the litter's genes, they found that the piglets carried one seemingly normal copy of the galactose gene, which should have been sufficient to make enough of the sugar to trigger hyperacute rejection.
"What the heck is happening here?" wondered Dr. Massimo Trucco, of Pitt and Children's Hospital, who conducted the research along with transplant pioneer Dr. Thomas Starzl and others.
Extensive study in his lab revealed that the galactose gene actually contained a mutation that led to the production of a non-functional enzyme. The chances of that occurring are about one in a trillion.
"So that was a pure stroke of luck," Trucco said. "But luck is useful."
Study author and PPL's vice president of research David Ayares said that if not for a special technique used to select for galactose-negative cells, the scientists may not have even realized what they had. Conventional methods would have made it appear that their gene knockout efforts had failed.
The next step was to prove that the cells could be transplanted without being quickly rejected.
One of the pigs died from causes unrelated to the cloning or the genetic manipulation. The researchers made the most of the opportunity and isolated its pancreatic islet cells, which produce insulin. They injected the cells into experimental mice engineered to mount a furious antibody response to galactose, as humans would do.
Nothing happened.
"You don't have any reaction," Trucco said. "It's working beautifully because really these pigs are negative for galactose."
Once the initial risk of hyperacute rejection has passed, conventional drugs can be used to forestall rejection over the long term, he said.
"Now we really have the genetics that we were trying to get to and ... the field has been talking about for the last decade or more," Ayares said.
Trucco hopes to one day use such islet cells to treat people with Type I or juvenile diabetes, perhaps by creating a packet of cells encased in a biocompatible material that can be implanted in the belly.
"Instead of having to prick their fingers [to check blood sugar] and inject insulin twice a day, if they come once or twice a year to the hospital, we remove the little thing and we put in a new one," Trucco said.
But before that can happen, much more work needs to be done. There is a risk that pig DNA might harbor viruses that could cause human illness after transplant. The researchers will be talking with federal health officials about how to ensure safety. Transplants of pig tissue to monkeys will have to be done to further test the techniques.
"We're going to be initiating those studies in the early part of next year, around March or April," Ayares said, noting that islet cell transplants and kidney transplantation will likely be tried initially.
The other vital step is to breed the special pigs so enough are available by the time human clinical trials are possible.
"We obviously want this pig as soon as possible," Trucco said. "We can do so many things that actually we are fighting to decide which one will be the first."
Anita Srikameswaran can be reached at anitas@post-gazette.com or 412-263-3858.
