A dural lymphatic vascular system that drains brain interstitial fluid and macromolecules
“Any neuroscience textbook that has ever been written will say that the central nervous system is devoid of a lymphatic system and that is one of the reasons the brain is immune privileged,” Louveau said. “When we started our project, our question was if there are so many immune cells surrounding the brain, how do they traffic there? By addressing this question we found vessels that weren’t supposed to exist. They were very well hidden and we think that is why it took so long to discover them.”
Let's learn the history:
The theory of immune privilege was invented to explain why foreign tissue grafts placed on brain tissue didn't cause an immediate immune reaction, as similar grafts did in other parts of the body, like the skin. It was believed that antigens in the brain were concealed from the immune system by the blood brain barrier. That's why it was assumed that when immune cells showed up in the brain--if they weren't there fighting an infection or as an inflammatory reaction after a stroke---there could only be one explanation--it was some sort of "auto-immune"and destructive inflammatory reaction.
This theory of immune privilege was developed in the 1940s and became the foundation of transplantation immunology---or the reason why we need to block the immune system when patients receive a donated organ or a skin graft. The body's immune response needs to be turned off, so that it will not reject the foreign tissue. This process was discovered by Sir Peter Medawar, who wanted to understand how to help skin grafts survive.
Not coincidently, the EAE model for MS was developed around the same time as the theory of immune privilege, as an example of how t-cells could break through the blood brain barrier and attack the brain in an auto-immune reaction. It has been a prevailing theory in MS treatment development, which has also never been proven. Another 70 year old theory which has lead to disastrous assumptions about the brain.
The presence of mild scant lymphocytic infiltrates in the demyelinating lesions has been generally interpreted as the evidence of an inflammatory autoimmune process. Because specific T-cell mediated autoimmunity can be reproduced in animals after myelin protein sensitisation (Experimental Allergic Encephalo- myelitis (EAE)) it has been assumed (but never proven) that a similar T-cell driven immune mechanism is responsible for demyelination in MS.
The acceptance of EAE as a model for MS is an unfortunate error that has its basis on faith rather than science. Whilst EAE is a good example of an experimental organ-specific autoimmune disorder in animals, it cannot be accepted as a model for MS for a wide variety of reasons. This is particularly important in relation to the development of MS pharmacotherapy. We have analysed the literature on immune-modifying therapy in MS and it is clear that none of these agents can qualify as a candidate therapy under scrutiny.
The brain's relationship to the peripheral immune system is obviously much more communicative and complex than previously imagined. The discovery of lymphatic vessels by neuroimmunologists means we need a do-over in MS research, as this science writer comments:
For decades, several axioms have prevailed with respect to the relationships between the CNS and circulating immune cells. Specifically, immune cell entry was largely considered to be pathological or to mark the beginning of pathology within the brain. Moreover, local inflammation associated with neurodegenerative diseases such Alzheimer's disease or amyotrophic lateral sclerosis, were considered similar in their etiology to inflammatory diseases, such as remitting relapsing-multiple sclerosis. The ensuing confusion reflected a lack of awareness that the etiology of the disease as well as the origin of the immune cells determines the nature of the inflammatory response, and that inflammation resolution is an active cellular process. The last two decades have seen a revolution in these prevailing dogmas, with a significant contribution made by the authors. Microglia and infiltrating monocyte-derived macrophages are now known to be functionally distinct and of separate origin. Innate and adaptive immune cells are now known to have protective/healing properties in the CNS, as long as their activity is regulated, and their recruitment is well controlled; their role is appreciated in maintenance of brain plasticity in health, aging, and chronic neurodevelopmental and neurodegenerative diseases.
Israel Steiner, a neurologist at the Hadassah University Hospital in Jerusalem, agrees that EAE has blocked "effective progress" for decades. He thinks alternative theories should be put to the test. "I definitely believe it's high time to reconsider the entire field. It has not led us into understanding the disease or to a better therapy for patients," he says.