FEARS about the wisdom of giving people pigs鈥� organs were reawakened last
week. A study in the US shows that transplanting pig tissue into mice can lead
to their becoming infected by viruses lurking in the pigs鈥� DNA, while another in
France suggests that controlling these viruses will be even harder than anyone
thought.
It鈥檚 been known since 1997 that porcine endogenous retroviruses, or PERVs,
can infect human cells growing in culture. Now David Onions of Glasgow
University and his colleagues in the US have shown that when pig pancreatic
islet cells鈥攖he cells that make insulin鈥攁re transplanted into mice
with deficient immune systems, PERVs jump the species barrier and infect mouse
tissues. Their results will appear in Nature.
鈥淭he act of transplanting tissue may actually provoke transcription [of viral
genes],鈥� says Onions, although he stresses that no signs of infection have yet
been found in human patients given live pig tissues. 鈥淭hat鈥檚 comforting,鈥� he
says.
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Alan Colman of PPL Therapeutics in Edinburgh, a company working on
xenotransplantation, points out that these mice were very susceptible to
infection because their immune systems had been completely wiped out, whereas
those of human patients would only be slightly suppressed. 鈥淚f you did to humans
what you do to these mice, they would not be alive.鈥�
The results aren鈥檛 surprising, says Robin Weiss of University College London,
who first showed that PERVs can infect human cells. Patients who desperately
need organs would probably be prepared to go ahead despite the risk of
infection, he says.
鈥淭he question is whether the infection will spread to the population at
large,鈥� he says. 鈥淎re we setting off a new epidemic? No one has any idea. It鈥檚
very unlikely. But so was HIV.鈥�
Some researchers have suggested that you could control PERVs by preventing
their expression in transplant patients. But such hopes are 鈥渋llusory鈥�, say
Andr茅 Jestin and his colleagues at the French Agency for Food Safety in
Ploufragan.
They found that the complete genomes of no fewer than 11 types of PERVs are
expressed in pig organs, including the heart, liver, pancreas and kidneys. Only
a few of these genomes could code for a functional virus, however.
The real surprise was how easy it was to find the PERVs, Jestin says. 鈥淥ther
groups have struggled to detect transcripts.鈥� A closer look at pigs might reveal
as yet undetected PERVs, his team will report in the Journal of
Virology.
Most researchers now agree the way forward is to eradicate viral genes from
donor pigs, using a combination of 鈥渒nock out鈥� technology and conventional
breeding. It remains to be seen whether this is feasible. Onions says he is
cautiously optimistic.