Brain's Link To Immune System Might Help Explain Alzheimer's | KUOW News and Information

Brain's Link To Immune System Might Help Explain Alzheimer's

Oct 3, 2017
Originally published on October 6, 2017 2:39 pm

Fresh evidence that the body's immune system interacts directly with the brain could lead to a new understanding of diseases from multiple sclerosis to Alzheimer's.

A study of human and monkey brains found lymphatic vessels — a key part of the body's immune system — in a membrane that surrounds the brain and nervous system, a team reported Tuesday in the online journal eLife.

Lymphatic vessels are a part of the lymphatic system, which extends throughout the body much like our network of veins and arteries. Instead of carrying blood, though, these vessels carry a clear fluid called lymph, which contains both immune cells and waste products.

The new finding bolsters recent evidence in rodents that the brain interacts with the body's lymphatic system to help fend off diseases and remove waste. Until a few years ago, scientists believed that the brain's immune and waste removal systems operated independently.

The discovery of lymphatic vessels near the surface of the brain could lead to a better understanding of multiple sclerosis, which seems to be triggered by a glitch in the immune system, says Dr. Daniel Reich, an author of the study and a senior investigator at the National Institute of Neurological Disorders and Stroke.

"How the immune system interacts with the brain is fundamental to how multiple sclerosis develops and how we treat multiple sclerosis," Reich says.

Current treatments for multiple sclerosis often involve drugs that suppress the immune system.

The research also has implications for diseases like Parkinson's and Alzheimer's.

These diseases occur as certain toxic waste products accumulate in the brain. And lymphatic vessels appear to be part of the system that usually removes these waste products.

"The discovery of a lymphatic system in the brain raises the possibility that a disorder of the lymphatic system is somehow involved in the causation of Alzheimer's disease," says Dr. Michael Weiner, a professor of radiology at the University of California San Francisco, who was not connected with the study.

That sort of thinking is a radical change from just a few years ago, Reich says.

For centuries, most scientists believed that the body's lymphatic system didn't connect to the brain, Reich says. "The brain is thought to be what is called immune-privileged," he says. "It has a different immune system from the rest of the body."

So Reich was intrigued when he heard a talk in 2015 by Jonathan Kipnis, who directs the neuroscience department at the University of Virginia.

"He showed very clearly in this talk that there are lymph vessels in the head, which I had learned in medical school didn't exist," Reich says.

But the evidence was in mice. So Reich and a team of scientists used MRI to study the brains of several human volunteers.

The scientists injected a special dye into the bloodstream, then watched to see where it went. They focused on the dura mater, the outermost membrane that protects the brain and nervous system.

As expected, the team saw some of the dye leak out of blood vessels in the dura mater. But then they could see that the leaked dye was being collected by different vessels – which is exactly what happens in the lymphatic system.

"That gave us some evidence that there are vessels here that are behaving different from blood vessels," Reich says. "But we weren't sure that they were lymphatic vessels."

To be certain, Reich's team spent years perfecting a technique to reveal the lymphatic vessels in the dura mater of brains taken from human cadavers. This allowed the scientists to confirm the presence of these vessels near the surface of the brain. And it strongly suggested that the lymphatic system interacts directly with the brain.

The results extend the findings of a landmark study published in 2013. It found that the brain appears to flush out waste products during sleep.

But it wasn't clear how these waste products were draining out of the head. Now it appears that at least some of the waste might be exiting through the lymphatic system.

Copyright 2017 NPR. To see more, visit http://www.npr.org/.

KELLY MCEVERS, HOST:

Scientists have found new evidence that the body's immune system extends to the brain. The finding challenges longstanding beliefs about how the brain protects itself from diseases. And as NPR's Jon Hamilton reports, it may change our understanding of disorders ranging from multiple sclerosis to Alzheimer's.

JON HAMILTON, BYLINE: In the body, cells that fight disease often travel through a network called the lymphatic system. Its vessels extend all over the place, sort of like blood vessels, except these vessels carry a clear fluid called lymph. But for centuries, scientists thought the brain simply wasn't connected to this system. Daniel Reich is a neurologist and radiologist at the National Institutes of Health.

DANIEL REICH: The brain is thought to be what is called immune privileged. It has a different immune system from the rest of the body.

HAMILTON: Except no one could figure out exactly how the brain's immune system worked. So Reich was intrigued when he heard a talk by a scientist from the University of Virginia a couple of years ago.

REICH: And he showed very clearly in this talk that there are real lymph vessels in the head, which I had learned in medical school didn't exist.

HAMILTON: Reich says that suggested the lymphatic system was interacting directly with the brain. But the evidence was in mice.

REICH: I immediately thought, well, he's showing this in mice. Do they exist in people, as well?

HAMILTON: To find out, Reich and a team of scientists used MRI to study the brains of several people. The team injected a special dye into the bloodstream, then watched to see where it went. They focused on a membrane that surrounds the brain and nervous system. And as expected, they saw some of the dye leak out of the blood vessels in this membrane. But then, Reich says, the leaking dye was collected by different vessels. That's how the lymph system works. It collects and carries away things that don't belong there.

REICH: That gave us some evidence that there are vessels here that are behaving different from blood vessels. But we weren't sure that they were lymphatic vessels.

HAMILTON: To be certain, Reich's team spent several years perfecting a technique that revealed the lymphatic vessels in brains taken from human cadavers. This allowed them to confirm the presence of these vessels near the surface of the brain. And it strongly suggested that the lymphatic system is connected to the brain. Reich says the finding could help explain multiple sclerosis, a disease in the brain and nervous system that seems to be triggered by the immune system.

REICH: How the immune system interacts with the brain is fundamental to how multiple sclerosis develops and how we treat multiple sclerosis.

HAMILTON: The new research published in the online journal eLife also has implications for diseases like Parkinson's and Alzheimer's. These diseases occur as toxic waste products accumulate in the brain. Michael Weiner is a professor of radiology at the University of California, San Francisco.

MICHAEL WEINER: The discovery of a lymphatic system in the brain raises the possibility that a disorder of the lymphatic system is somehow involved in the causation of Alzheimer's disease.

HAMILTON: Weiner says the discovery also could explain why people with sleep disorders seem to be more likely to develop Alzheimer's. A few years ago, researchers found evidence that the brain flushes out waste products during sleep. But it wasn't clear how those waste products drain out of the head. Weiner says the new research suggests the answer may be the lymphatic system.

WEINER: So it may be that the reason why sleep and Alzheimer's disease are related is because of the effects of sleep on the lymphatic system.

HAMILTON: A system once thought to have nothing to do with the brain. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.