good or bad?
However, whether immune cells harm or help the brain remains an unanswered question. Wyss-Coray and his colleagues suggest that the immune system could damage neurons by releasing molecules that promote inflammation and trigger cell death. Other researchers believe that T cells and other immune cells may be protective instead. The Schwartz group reported that in mouse models of Alzheimer’s disease, an enhanced immune response clears amyloid plaques and improves cognitive performance.
It is now clear that the edges of the brain exhibit great immunological diversity: almost every type of immune cell in the body is also found in the vicinity of the brain. Neuroimmunologist Movahedi even calls the meninges – the fluid-filled membrane that envelops the brain – an “immunological wonderland”. In his work he focuses on macrophages in the border regions of the brain. “So much is happening out there,” he says.
Cerebrospinal fluid in mouse brain | CSF (red) seeps into brain tissue (blue) through tiny gaps in blood vessels, washing out waste products. This process is impaired in old age, which can contribute to the development of Alzheimer’s disease.
Some are actually found exclusively at the borders. Thus, in 2021, Jonathan Kipnis, a neuroimmunologist at Washington University in St. Louis, Missouri, and his colleagues reported that there is a local source of immune cells: the bone marrow of the skull. In further experiments, they investigated how the bone marrow mobilizes these cells. The researchers found that signals in the CSF are transmitted to the bone marrow of the skull in response to an injury to the central nervous system or to a pathogen. As a result, these cells are produced and released.
“The most exciting thing about neuroimmunology is that it is relevant to many diseases as well as to normal physiology.”(Beth Stevens, Neuroscientist)
However, it is still unclear what role these locally produced immune cells play. The group led by Kipnis suspects that they may have a “gentle task” than defense cells from other parts of the body. They might regulate the immune response rather than attacking themselves. If that distinction were correct, Kipnis said, it would have implications for treatment. In diseases like multiple sclerosis, preventing immune cells from other parts of the body from invading could improve symptoms. But with a brain tumor, he adds, “you want fighters.”
brain monitoring
Kipnis’ team has also discovered a network of channels stretching across the surface of the brain, teeming with immune cells. It forms the brain’s own lymphatic system. These vessels, located at the outermost part of the meninges, provide immune cells with a near-brain vantage point from which to monitor signs of infection or injury.
As evidence mounts that immune cells are involved in brain injury and disease, researchers have looked at their function in the healthy brain. “The most exciting thing about neuroimmunology is that it’s relevant to many diseases as well as normal physiology,” says Beth Stevens, a neuroscientist at Boston Children’s Hospital in Massachusetts.