Lymphatic System in the Brain and MS

June 25, 2015

A recent report has generated excitement in the scientific community. This report announces the discovery of a lymphatic system in the brain.

First, here is a bit of background on how the immune system works. When damage occurs to some area of the body, protein fragments are released. These fragments get picked up by macrophages at the site of injury. The macrophages travel through the lymphatic system to lymph nodes that drain that region. The lymphatic system is a system of vessels, sort of like blood vessels, but they don't transport blood. Rather, they bring fluid from between the cells (interstitial fluid) and white blood cells/macrophages from the tissues to lymph nodes, and then from the lymph nodes back into the blood circulation. The lymphatic system has no pump (it isn't pumped by the heart) but relies on low pressure flow and the action of muscles to slowly squeeze the fluid along.

The protein fragments are picked up by macrophages are presented to lymphocytes as these lymphocytes traffic through the lymph nodes. If the protein fragment matches to the receptor on the surface of the lymphocyte, then that lymphocyte becomes activated and exits the lymph node to help protect us from bacteria or viruses that have that protein fragments.

One of the mysteries of medicine is how this system might work in the brain. It was thought that lymph vessels did not exist in the central nervous system, so how would macrophages get protein fragments from the brain to their local lymph node? How would lymphocytes trafficking through the brain get back into the circulation? This paper tells us how.

In fact, the brain does have a lymphatic system. It was not previously recognized because the vessels are small and tucked up next to some of the major veins that drain the brain, making them hard to see. Nevertheless, we now know of their existence and now understand how macrophages, antigens and lymphocytes from the brain are able to participate in immune activities the same as they can from other tissues.

It is uncertain whether this will have any impact on the way we treat MS. All of our MS disease-modifying therapies are already directed at immune activity and I would expect that new therapies in the near future would similarly be directed towards the immune system. However, it is nice to understand the pathway by which these immune cells operate.