Research published this month reported a previously unknown line of communication between our brains and immune systems, adding to a fast-growing body of research suggesting that the brain and body are more connected than previously thought. The new work could have important implications for understanding and treating disorders of the brain. Outside tissue grafted into most parts of the body often results in immunologic attack; tissue grafted into the central nervous system on the other hand sparks a far less hostile response. Thanks in part to the blood-brain barrier — tightly packed cells lining the brain's vessels that let nutrients slip by, but, for the most part, keep out unwanted invaders like bacteria and viruses — the brain was long considered "immunologically privileged,” meaning it can tolerate the introduction of outside pathogens and tissues. The central nervous system was seen as existing separately from the peripheral immune system, left to wield its own less aggressive immune defenses. The brain’s privilege was also considered to be due to its lack of lymphatic drainage. The lymphatic system is our body's third and perhaps least considered set of vessels, the others being arteries and veins.
Lymphatic vessels return intracellular fluid to the bloodstream while lymph nodes – stationed periodically along the vessel network – serve has storehouses for immune cells. In most parts of the body, antigens – molecules on pathogens or foreign tissue that alert our immune system to potential threats – are presented to white blood cells in our the lymph nodes causing an immune response. But it was assumed that this doesn’t occur in the brain given its lack of a lymphatic network, which is why the new findings represent a dogmatic shift in understanding how the brain interacts with the immune system. Working primarily with mice, researchers at the University of Virginia identified a previously undetected network of lymphatic vessels in the meninges — the membranes that surround the brain and spinal cord — that shuttle fluid and immune cells from the cerebrospinal fluid to a group of lymph nodes in the neck, the deep cervical lymph nodes. By mounting whole mouse meninges and using neuroimaging the team noticed that T-cells were present in vessels separate from arteries and veins, confirming that the brain does in fact have a lymphatic system linking it directly the peripheral immune system.