Scientists Discover A New Link Between The Brain And The Immune System
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Friday, July 10, 2015 06:10 PM

Researchers have announced they had discovered a physical connection between the immune system and the brain’s blood supply.  Careful studies have shown that the brain does interact with the peripheral immune system, albeit in unique ways. Immune cells do, somehow, circulate through the brain, and antigens—which would normally stoke an immune response—do drain from the brain into the lymph nodes

The finding gives researchers a novel approach to understanding diseases and strengthens the bridge between neuroscience and immunology.  Previous speculations about the association of immune system and disease processes become fair game with this newfound appreciation of the brain’s connection to the immune system.  Scientists predict that in the next decade we will be on our way toward achieving an understanding of the neuro-immune system that matches the depth and breadth of our knowledge of the peripheral immune system;  we’ll be seeing new ways of using peripheral pathways to hack into brain function and behavior, to treat symptoms of neurological disease.

Neuroscience and immunology now have even greater reason to collaborate. These findings are going to crack open the chasm of the field of neuroimmunology in a wonderful way, Immunologists and neuroscientists have been politely ignoring each other for far too long.

For decades, researchers had assumed that the lymphatic system stopped short of the brain. This recent discovery turns that assumption on its head.  The immune system, which tracks and addresses threats to the body by way of the bloodstream, is directly exposed to neither the inside of the gut nor the brain. The gut microbiome is separated from the bloodstream by the lining of the intestines, and from the brain by the aptly named blood-brain barrier (BBB). Indeed, the brain was long considered to be “immune privileged,” or exempt from normal immune surveillance, both good and bad—a necessity given that, for instance, the brain can’t tolerate swelling from inside the skull.

But in the last twenty years, the notion that the brain is immune privileged has been slowly dismantled. Careful studies have shown that the brain does interact with the peripheral immune system, albeit in unique ways. Immune cells do, somehow, circulate through the brain, and antigens—which would normally stoke an immune response—do drain from the brain into the lymph nodes. Moreover, neurological diseases like multiple sclerosis and Alzheimer’s have long been linked to changes in immune system function, and autoimmune diseases of the gut, like Crohn’s disease, correlate with psychiatric illness.

Scientists at the University of Virginia stumbled across such a bridge: a network of lymphatic vessels that appears to directly link the brain with the immune system.  Scientists were studying the circulation of immune cells in the meninges, the blood-vessel-rich tissue that lies between the skull and the brain. They were looking at T-cells, specifically, a class of immune cell that detect trouble in the body and communicate it to the rest of the immune system. Other labs had noticed that T-cells injected into the brain eventually found their way to the cervical lymph nodes. What wasn’t clear was how they got there. 

One scientists came up with a nifty method for visualizing the meninges of a mouse brain without destroying it. “Shockingly, there were lymphatic vessels that closely followed blood vessels down into the sinuses of the brain—anatomists had apparently missed this direct conduit for decades

It’s a big discovery.  There may be more intimate interactions between the peripheral immune system and brain than we’ve ever realized.  For example in recent years the study of the importance of both the immune system and particular gut microbes in mouse models of autism. Scientists have now shown that a particular type of gut microbe is responsible for spurring cells in the gut to produce serotonin—nearly all the serotonin found in the body.  While little of this peripheral serotonin crosses the blood-brain barrier, “there is an increasing appreciation that serotonin mediates pro-inflammatory responses.” Her recent study showed that without this microbe-induced serotonin production, a host of problems with gastrointestinal function emerge. Her lab is intrigued by the potential impact of this serotonin on other elements of physiology, such as immunity. Moreover, the recent discovery from the Kipnis lab “raises the question of whether microbial influences on the neuroimmune system, or neuro-immune interactions, can lead to changes in brain function and/or behavior.”