Porker potential: but the effects of fully deleting a gene that blocks xenotransplants are not known. Credit: PPL

The safe transplantation of pig organs into human patients remains several steps from realization, experts say. Two announcements that research teams have cloned pigs lacking a gene involved in graft rejection still leave several obstacles to xenotransplantation intact, according to most specialists in the field.

On 2 January, PPL Therapeutics, the Scottish company that cloned Dolly the sheep, announced the birth on Christmas Day of five cloned knockout female piglets. Each had an inactivated gene for α-1,3-galactosyl transferase, an enzyme that adds the sugar α-1,3-galactosyl, or alpha-gal, to the surface of pig cells. The immune systems of humans and Old World primates, who lost this enzyme in evolution, recognize the sugar as foreign and kill transplanted pig organs in minutes.

The timing of PPL's statement was widely interpreted as an attempt to steal the thunder from a similar finding that was peer-reviewed and published by Science on 3 January (L. Lai et al. 10.1126/science.1068228) by groups at the University of Missouri at Columbia and Massachusetts-based Immerge BioTherapeutics. Their four cloned piglets, all female, were born in September and October.

Alan Colman is confident of the pigs' prospects. Credit: PPL

“The promise of xenotransplantation is now a reality,” Alan Colman, research director of PPL Therapeutics, claimed in his company's statement. But others find this contention premature. They point out that the elimination of the alpha-gal is incomplete in the new work — and that even if it were completely removed, it would by no means assure that xenotransplantation could succeed.

Each research team only knocked out one of the two copies of the gene — all of the piglets still make alpha-gal with the other copy. The companies intend to breed from their piglets and planned male litters to get offspring that have both genes knocked out and produce no alpha-gal. But the success of this step, which will take around 18 months, is not guaranteed. Jeffrey Platt, director of transplantation biology at the Mayo Clinic in Rochester, Minnesota, points out that although mice with double knockouts for this gene have been produced, this modification could prove lethal to pigs.

Nor is it clear that organs from pigs that lack alpha-gal altogether will be accepted by the human body. When pig organs are transplanted into primates, a violent immune response is unleashed. This 'hyperacute rejection' destroys the blood vessels in the organ, cutting off oxygen, and turning it black within minutes.

Alpha-gal is known to play a major role in this acute response. But other foreign sugars will remain and the acute rejection may not be abrogated completely, says Robin Weiss, a virologist at University College London. “We won't know until we try,” admits Ron James, PPL's managing director.

Should scientists succeed in overcoming hyperacute rejection, they will still face other rejection mechanisms including the 'delayed xenograft rejection' that occurs within days, when antibodies, macrophages and natural killer cells invade the organ. Data are insufficient to predict whether the absence of alpha-gal will alleviate these, says Fritz Bach, a xenotransplantation expert at the Harvard Medical School.

Alpha-gal also plays no role in the remaining major immunological barrier, T-cell-mediated chronic rejection, which can come into play months or years after the transplant. In human-to-human transplants this is controlled by lifelong immunosuppression, but this would probably be insufficient for xenotransplants which have a lot more potential targets for the immune system to attack. One promising approach, tricking the recipient into recognizing donor tissues as 'self', is being pursued by David Sachs, director of the Transplantation Biology Research Center at Massachusetts General Hospital.

Sachs is collaborating with Immerge BioTherapeutics on a system where cells from the thymus of the donor pig are first engrafted into the recipient while their immune system is temporarily disabled. As the system recovers, the recipient develops tolerance to the donor organs and tissues.

Even if all these hurdles are cleared, there is still the risk that animal viruses might jump to humans and cause man-made pandemics (see Nature 391, 320–324; 1998).

Despite these problems James predicts that clinical trials of pig organs may be just four years away. Julia Greenstein, chief executive of Immerge BioTherapeutics, is more circumspect. “I'm saying that in three years time we want to have completed the preclinical trial in primates that would justify human trials,” she says.

Financial analysts are also cautious — despite the jump in PPL's share price just after its statement was released. “There are huge difficulties and it will be a long way getting there,” says Kevin Scotcher, a pharmaceuticals analyst at S G Cowen, the US-based investment bank. But Scotcher adds that a global market for transplant organs that could soon be worth $6 billion annually will ensure intense investor interest in research related to xenotransplantation.