ScienceDaily (Oct. 23, 2009) — In a surprising reversal of long-standing 
scientific belief, researchers at the Mayo Clinic campus in Florida have 
discovered that inflammation in the brain is not the trigger that leads to 
buildup of amyloid deposits and development of Alzheimer's disease. 

In fact, inflammation helps clear the brain of these noxious amyloid plaques 
early in the disease development, as seen from studies in mice that are 
predisposed to the disorder, say the researchers in the online issue of the 
FASEB Journal.
"This is the opposite of what most people who study Alzheimer's disease, 
including our research group, believed," says the study's lead investigator 
Pritam Das, Ph.D., an assistant professor in the Department of Neuroscience. 
"And it also suggests that we can take advantage of the brain's own immune 
cells by directing them to remove amyloid plaques from the brain, thus 
protecting the brain against their harmful effects."
The study tested the widely held belief that inflammation in the brain 
increases the production and buildup of a toxic protein known as amyloid beta 
(Aβ). Clumps of this protein in the brain are the hallmark pathological 
feature of Alzheimer's disease.
"The belief was that when the brain's immune cells, microglia, are activated 
following the initial buildup of amyloid plaques, the inflammation that ensues 
stimulates the brain cell's machinery to produce more Aβ, which then leads to 
more inflammation," Dr. Das says. "This chronic activation of immune cells 
results in a self-reinforcing feedback loop that promotes more and more Aβ 
deposition and inflammation, eventually leading to malfunction and death of 
brain neurons."
Although this notion, which came mostly from studies in laboratory cells, was 
accepted throughout the scientific community, the Mayo Clinic researchers 
developed a way to test it in a living organism -- and they expected to see 
the same result.
"We had initiated these studies using our new in vivo model to confirm whether 
inducing inflammation in the brain would in fact exacerbate the disease," Dr. 
Das says.
The researchers used a technique known as "Somatic Brain Transgenesis" to 
increase expression of Interleukin-6 (IL-6), a cytokine that stimulates an 
inflammatory immune response in the brains of young mice predisposed to 
developing age-progressive amyloid plaques. This powerful technology allows 
researchers to express any gene of interest in specific parts of the body by 
tagging the gene onto Adeno-associated viruses, which are inert. In this way, 
they can study the function of any protein in the brain, and also test its 
potential therapeutic use.
They found that IL-6 triggered inflammation throughout the brain, and they 
expected to see a big buildup of plaque as well as damage to brain neurons. 
"Instead, to our surprise, we found that the inflammation prevented plaques 
from forming and cleared whatever plaque that was already there," Dr. Das 
says.
Given this unexpected result, they performed additional experiments using 
different strategies. "First, we expressed IL-6 in the brains of newly born 
mice that are yet to develop any amyloid plaques and, secondly, we expressed 
IL-6 in the brains of mice with pre-existing plaque pathology," he says. "In 
both these cases, we got similar results -- the presence of IL-6 leads to the 
clearance of amyloid plaques from the brain."
The researchers then performed experiments to determine how the amyloid 
plaques were removed from the brain. Their analysis revealed that the 
inflammation induced by IL-6 in the brain directed the microglia cells to 
remove the amyloid plaques from the brain. Microglial cells do this by 
phagocytosis. "They gobble up the plaque, which they 'see' as a foreign 
invader, and break it apart," Dr. Das says. Researchers also found that 
activated microglia cells were closely attached to the plaques and expressed 
proteins that help in removing the amyloid plaques from the brain. Dr. Das 
hypothesizes that inflammation helps clear plaque early in the development of 
Alzheimer's disease, but that at some point, continued production of the 
amyloid clumps in the brain overwhelms the ability of microglial cells to do 
their job. At that point, inflammation, chronically activated by presence of 
the amyloid plaque, can produce its own unhealthy effects on brain function.
"Indeed, it may be feasible to transiently and selectively manipulate the 
microglia cells to alter amyloid plaques in a manner that is both effective and 
tolerable," he says. "However, given that chronic inflammation over years of 
insult may be detrimental, any intervention based on activation of the brain's 
immune system must clearly strike a balance between the neuroprotective and 
neurotoxic effects," cautions Dr. Das. "We need to study this phenomenon more 
thoroughly, but if we are right, it could have implications not only for 
Alzheimer's disease but also other neurodegenerative disorders characterized 
by protein buildup in the brain, such as Parkinson's disease."
The study was funded by grants from the American Health Assistance Foundation 
(AHAF), Mayo Clinic and the National Institutes of Health (NIH). 

Adapted from materials provided by Mayo Clinic.

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