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Published online: 20
January 2005; | doi:10.1038/news050117-11
Prions discovered in unexpected organsRoxanne Khamsi
Immune system helps BSE
proteins spread through the body.
One assumption lies at the root of
efforts to keep the meat we eat safe from mad cow disease: that
tissues beyond an animal's brain, spinal cord and immune system
are free of the prions that cause the disease.
A disturbing study now shows that
assumption to be false. Researchers have found that if an
animal falls ill with another infection, its immune response
can carry large numbers of prions to organs throughout its
body.
"The rules no longer apply," warns pathologist
Adriano Aguzzi at Zurich University Hospital, Switzerland, who
led the research.
Prion problems
Mad cow
disease, more correctly known as bovine spongiform
encephalopathy (BSE), is believed to be caused by rogue
proteins called prions. When these prions enter the human food
chain, they can cause the equivalent disease in humans, called
new-variant Creutzfeldt-Jakob disease (vCJD).
Patients develop rapidly increasing dementia, and die
soon after the onset of symptoms. Since vCJD first appeared in
Britain in the mid-1990s, only a handful of people have
succumbed, but uncertainty about the incubation time of the
disease leaves open the possibility that the number of people
infected is actually much larger.
The outbreak has
caused massive public concern over the effects of beef
consumption. To prevent the spread of disease, several
countries affected by BSE including Britain, Canada and the
United States have implemented regulations that exclude the
brain, spinal cord and immune-system tissues, such as the
spleen, from the food chain.
It was thought that
other body parts were safe to eat. "This is why you can still
eat products from susceptible animals in the countries with
BSE," says Aguzzi.
Inflamed infection
Aguzzi's research has begun to chip away at that
belief. In 2003, he and his colleagues announced that they had
detected small amounts of prions in the muscle tissue of
people who had died from vCJD, suggesting that animal meat
might harbour trace amounts of these disease proteins
too.
But nothing prepared Aguzzi and his team for what
they found in their most recent study. They took mice with the
equivalent of BSE and induced an 'inflammatory response' in
them. This is the type of immune response that the body mounts
in the face of a wide range of injuries and illnesses,
including cuts, the common cold and type I diabetes.
There
was an explosion of prions in the animals' pancreases, kidneys
and livers, the researchers report in this week's issue of
Science1.
The amount of prions in these organs was just as high as is
generally found in diseased spleens.
"If the animal has
an additional infection in the body, the prions are no longer
confined to the areas where they normally are," explains
Surachai Supattapone, an expert in infectious diseases at the
Dartmouth Medical School in Hanover, New Hampshire.
The
researchers believe that the cells involved in the
inflammatory response somehow help the prions to replicate,
and to spread to the parts of the body being targeted by the
immune reaction.
References
- Heikenwalder, M. et al. Science Published
online: doi:10.1126/science.1106460 (2005).
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