Rheumatoid arthritis sufferers have a built-in protection against Alzheimer's that could soon become a promising new treatment for the brain disease, it was revealed today.
The protein GM-CSF plays a role in the defective immune system response that leads to the arthritic disorder.
But it also marshals the immune system to remove harmful deposits in the brain that are linked to Alzheimer's.
Tests on mice showed that the protein could reverse Alzheimer's symptoms in just 20 days.
Scientists, who say they are "amazed" at the finding, believe the discovery could quickly lead to a practical new treatment for Alzheimer's, the most common form of dementia.
A laboratory-made version of GM-CSF, called Leukine, has been used for years to boost the immune systems of certain cancer patients.
Because its safety profile is already well known, it should not take long to convert into an Alzheimer's therapy.
The US scientists are now planning a pilot trial later this year that will put the theory to the test.
Patients with mild or moderate levels of Alzheimer's will be given the protein to see if their symptoms improve.
It was already known that people with rheumatoid arthritis (RA), an auto-immune disease that attacks the joints, had a reduced risk of Alzheimer's.
Until recently, most experts assumed this was due to the anti-inflammatory drugs commonly given to RA sufferers. But recent clinical trials showed the drugs do not help Alzheimer's patients, putting the theory in doubt.
Prof Potter's team took a different tack by investigating immune system mechanisms in RA. Three RA proteins were studied in mice bred to develop the rodent equivalent of human Alzheimer's.
One, GM-CSF, proved most likely to be protective against the dementia condition.
The researchers then injected the protein into "Alzheimer's" mice, and normal old-age mice.
Behavioural tests confirmed that the Alzheimer's mice were suffering signs of memory loss by the age of 12 months.
After 20 days, the treated Alzheimer's mice were performing substantially better in tests measuring their working memory and learning ability than those not given the protein.