“[The principle, in building a sewer system, was] of diverting the cause of the mischief to a locality where it can do no mischief. “ Joseph Bazelgette 1819 – 1891
For years, we thought the brain was a fortress, safely locked behind the blood brain barrier’s fortifications. We knew it was patrolled by its own immune cells, isolated from the rest of the body’s immune system. We knew it was fed nutrients passed across the blood brain barrier from the bloodstream. But what we didn’t know was how the brain disposed of its garbage, the waste generated by cells. The lymphatic system is the body’s sewer system, carrying unwanted cellular debris and excess fluids away from cells. But, as far as we knew, the brain wasn’t tapped into the body’s lymphatic plumbing. So how was it taking out the trash?
The first breakthrough was the discovery of the “glymphatic system.” In a 2013 paper, researchers proposed a method by which the cerebral spinal fluid flushes out the brain’s interstitial fluid (the fluid outside of and between cells) by flowing along arteries into the brain’s tissue, picking up waste for disposal, and exiting along veins. (Learn how sleep is important for this cleansing flow of the brain.)
Next up: a 2015 Nature paper unveiled the presence of lymphatic vessels in membranes that cover the brain and the CNS. These vessels drain into the deep cervical lymph nodes in the neck, perhaps explaining how they hid without detection for so many years.
“We think these vessels may play a role in pathogenesis of neurological conditions that have an immune component” –Jonathan Kipnis
The discovery of lymphatic vessels in the brain immediately captured the attention of MS researchers. Why? Because while the lymphatic system is the body’s sewer system, it is also the immune system’s highway, circulating immune cells throughout the body. Therefore, these lymphatic vessels in the brain are the subject of intense speculation over whether they’re involved with the development and/or progression of MS.
For example, here’s one potential model: perhaps these vessels allow bits of myelin to travel from the brain into the lymphatic system. Once there, perhaps the immune system’s T cells could learn to recognize the myelin as their target for attack. Perhaps these myelin-primed T cells could then migrate to the CNS and promote the destruction of myelin. Perhaps.
We’re eagerly awaiting more research on this topic to be published, and hopefully we won’t have too long to wait: Dr. Jonathan Kipnis, author on the 2015 paper above, just presented a talk titled “Meningeal lymphatics in MS” at the 2017 ECTRIMS meeting!
Cover image credit: Sears, Roebuck & Co., 1922