Study of Pig
Virus Eases One Worry
By LAURAN NEERGAARD, AP
02:05 PM ET 08/19/99
WASHINGTON (AP) - The most rigorous study ever
on the safety of transplanting animal parts into humans
found no evidence that people caught a worrisome pig
virus. The reassuring finding could spur experiments using
pigs and other animals as organ donors. At
issue is ``xenotransplantation,'' transplanting organs or
cells from one species into another. Doctors hope this
still highly experimental field could one day save
thousands of lives by easing a worldwide shortage of
donated organs.
But some scientists fear animal-to-human
transplants also could cause new epidemics. The AIDS virus
is thought to have jumped from monkeys to people long ago,
and just last year pigs in Malaysia were sickened by a
deadly new virus that they spread to people. So
scientists are breeding pigs - the most likely species for
xenotransplants - specially tested against infections. But
concern rose when researchers recently discovered the
genes of all pigs harbor a previously unknown virus that
test-tube experiments showed could infect human cells.
The virus did not harm the pigs, but nobody knew
if it would hurt humans.
The new study, published in Friday's edition of
the journal Science, suggests this so-called ``porcine
endogenous retrovirus,'' also known as PERV, is not a big
threat.
Scientists at the British biotechnology company
Imutran Ltd. tracked down 160 patients from eight
countries who were treated experimentally with living pig
tissue. Using the most rigorous viral testing available -
and supported by testing at the U.S. Centers for Disease
Control and Prevention - Imutran scientists found no
evidence PERV had infected the 160 patients. Among
the virus-free patients were 36 people deemed at high risk
because they had very weak immune systems.
``It's really good news for all those people
waiting for this potentially lifesaving technology,'' said
lead researcher Dr. Khazal Paradis of Imutran, which is
developing ways to transplant entire animal organs.
The study is not proof that xenotransplantation
is safe, cautioned CDC's Dr. Louisa Chapman.
``It's a substantial advance that ... gives us
more confidence'' about allowing certain experiments to
proceed, she said. But all xenotransplants must continue
to be closely watched for viruses, she added.
The patients studied included diabetics who had
received transplants of pig pancreas cells, burn victims
who had temporary pigskin grafts, or liver- or
kidney-failure patients who had their blood filtered
through pig livers or kidneys outside the body while
awaiting a human organ transplant.
The study's biggest surprise: Pig cells remained
alive, with no detectable ill effects, inside a small
group of Russian patients eight years after they had an
unusual treatment.
``We did not expect the pig cells to survive for
that long,'' Paradis said.
The Russians had undergone therapy apparently
used nowhere else. The blood of 100 child burn victims was
filtered through a fresh pig spleen for one hour to help
them fight severe infection. Blood normally runs through
the spleen, part of the immune system, and Russian
scientists have contended this therapy works. Regardless,
Paradis found living pig cells in 23 of the 100 Russians.
In most other experiments, the human immune
system quickly has killed animal cells - another roadblock
to attempting transplants of entire animal organs.
The downside when animal cells survive is that
``any virus may not be seen by the immune system either,''
said Dr. Jonathan Allan, a virus expert at the Southwest
Foundation for Biomedical Research in San Antonio, who has
been outspoken about the potential risk of
xenotransplantation.
``This study is not a green light'' to rush
ahead, warned Allan, who wants xenotransplant experiments
limited to small numbers of closely monitored patients.
For now, that is the U.S. Food and Drug
Administration's approach. It is allowing a few small
pig-to-people experiments, including implants of fetal pig
cells into Parkinson's disease patients' brains, while
scientists hunt potential infections.
___
Attempts at transplanting animal organs into people date
to 1905, but they have failed to keep patients alive for
more than a few days. The most famous was the 1984
transplant of a baboon heart into tiny Baby Fae, who died
20 days later. The problem is people's immune
systems quickly recognize the animal organs as foreign and
launch a vicious attack. Scientists are
working on ways to prevent such attacks and make animal
organ transplants feasible. Research ranges from better
immuno-suppressing drugs to work by Imutran Ltd., which is
genetically engineering pigs to harbor a human gene that
scientists hope will make people's bodies more tolerant of
the pigs' organs. But while they can't yet
transplant entire organs, scientists are experimenting
with transplants of living animal cells. A U.S.
corporation, for instance, is implanting fetal pig cells
into the brains of Parkinson's disease patients, and
reports some early signals that the treatment is
promising.
Also, scientists are trying to keep some liver
transplant patients alive by filtering their blood through
pig livers outside the body, a temporary ``bridge'' until
they got a donated human liver for transplant.
Pigs are the most likely species for
animal-to-human transplants because their major organs are
similar in size and structure to people's.
LISTEN TO NPR's STORY
IN AUDIO
Transplants
Biomedical scientists have spent years trying to
genetically engineer pigs whose cells, skin and even
organs could be transplanted to humans. One major
roadblock has been the fear that pig viruses could end up
in humans. NPR's David Baron reports on new research
suggesting that the most common pig virus does NOT pose a
threat, which should hasten the day when organs can be
transplanted.
