Rheumatoid arthritis sufferers have a built-in protection against Alzheimer's that could soon become a promising new treatment for the brain disease.
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, from the Health Byrd Alzheimer's Institute at Florida State University, 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.
Lead researcher Professor Huntington Potter said: "Our study, along with the drug's track record for safety, suggests Leukine should be tested in humans as a potential treatment for Alzheimer's disease."
Prof Potter's team took a different tack by investigating immune system mechanisms in RA. "Our findings provide a compelling explanation for why rheumatoid arthritis is a negative risk factor for Alzheimer's disease," said the professor, whose research is published online in the Journal of Alzheimer's Disease.
