Important Link between the Brain and Immune System Found

When the ancient Egyptians prepared a mummy they would scoop out the brain through the nostrils and throw it away. While other organs were preserved and entombed, the brain was considered separately from the rest of the body, and unnecessary for life or afterlife. Eventually, of course, healers and scientists realised that the three pounds of entangled neurons beneath our crania serve some rather critical functions. Yet even now the brain is often viewed as somewhat divorced from the rest of the body; a neurobiological Oz crewing our bodies and minds from behind the scenes with unique biology and unique pathologies. Perhaps the most commonly cited division between body and brain concerns the immune system. When exposed to foreign bacteria, viruses, tumours, and transplant tissue, the body stirs up a torrent of immune activity: white blood cells devour invading pathogens and burst compromised cells; antibodies tag outsiders for destruction. Except, that is, in the brain. Thought to be too vulnerable to host an onslaught of angry defensive cells, the brain was assumed to be protected from this immune cascade. However research published recently 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. As early as 1921 scientists recognised that the brain is different, immunologically speaking. 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 defences. 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, lead author and University of Virginia neuroscience professor Dr. Jonathan Kipnis and his group 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. Kipnis and colleagues had previously shown that a type of white blood cell called T-cells in the meninges are associated with significant influences on cognition and hence were curious about the role of meningeal immunity on brain function. 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 to the peripheral immune system. “We stumbled upon these vessels completely by serendipity,” Kipnis commented.

Scientific American, 21 July 2015 ; ;