A Whole New Meaning to ‘Thinking Yourself Well’: the new brain immunity discoveryPosted: 08 18, 2015
Comprised of the brain and spinal cord, the central nervous system (CNS) stands apart from other organ systems. While all other organs share a common, loosely filtered blood supply, the brain is highly selective in what it allows to cross the blood-brain barrier and enter into this space. Furthermore, cells in many other organs can replicate and divide following injury, but most (although not all) neurons in the brain cannot divide, and thus following birth we are continuously losing but not replacing neurons. It has been long thought that the CNS is also unique because it lacks connection to the lymphatic vasculature. Recent findings, however, demonstrate that this is untrue. A study published in Nature by Antoine Louveau as first author and Jonathan Kipnis as senior author at University of Virginia School of Medicine demonstrates that the CNS does in fact have a lymphatic drainage system.
The lymphatic system is part of the circulatory system and vital in immune function. The lymphatic vessels (analogous to veins of the circulatory system) carry a clear fluid called lymph, which transports white-blood cells and other immune response molecules, and helps with fluid balance in the body. The previously-believed lack of a lymphatic system in the CNS did not mean the brain lacks immune capability. Rather, two systems of circulating cerebrospinal fluid (CSF) were thought to be responsible for maintaining immune function throughout the brain. As the CSF bathes and circulates around brain tissue, waste products enter the CSF and are filtered out and removed. The CSF eventually drains into the bloodstream.
Why has the lymphatic system in the CNS remained undiscovered until now? Simply, it was difficult for researchers to find! The discovering authors conducted precise dissection and whole-mount preparation of mouse brain meninges, the fine tissue covering the brain. They found lymphatic vessels embedded within the outermost soft tissue protective layer around the brain (called dura, located just inside the skull). Fluid and immune cells move from the brain parenchyma (the ‘meat’ of the brain, made of neurons and glial cells), into the CSF, and then into the lymphatic vessels.
So why are neuroscientists so excited about finding a new set of fluid-carrying tubes connected to the brain? Part of this answer is because of new questions that can be asked. A vast array of neurological and neurodegenerative diseases are believed to stem from dysregulation of the immune system within the brain. These include multiple sclerosis, an autoimmune disorder, in which the body’s natural defenses attack the protective coating around nerves; this disrupts communication within the brain and between the brain and the body. Another example is Alzheimer’s disease, which is thought to be caused by deficits in clearing amyloid-β from cerebrospinal fluid that then forms amyloid plaques and triggers inflammatory processes and neuronal dysfunction. Future research can now investigate the function of CNS lymphatic vessels related to physiological and pathological clearing of this peptide. Of course, further investigations are needed to determine whether the brain-immune system link found in mice translates to humans.
Peer edited by Lydia Arbar and Rachel Dee
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This article was co-published on the SWAC Blog, The Pipettepen.