The brain is protected by a very selective shield called the blood-brain barrier, which only allows certain things to pass through. This barrier also protects the brain from the immune system, and employs immune cells of its own called microglia. Scientists have been debating whether or not white blood cells have a role in the brain. New work reported in Cell has now suggested that a subset of immune cells can be found in the brains of mice and humans, and that these cells indicate the importance of white blood cells to the development of mouse brains.
"A misconception about white blood cells comes from their name," explained study author Dr. Oliver Burton of the Babraham Institute. "These immune cells are not just present in the blood. They are constantly circulating around our body and enter all of our organs, including, as it turns out, the brain. We are only just starting to discover what white blood cells do when they leave the blood. This research indicates that they act as a go-between, transferring information from the rest of the body to the brain environment."
Research has suggested that white blood cells are involved in some brain disorders including stroke, Alzheimer's, and Parkinson's.
In this work, the scientists zeroed in on a population of T helper cells (a type of white blood cell) that resides in mouse and human brain tissue. The researchers were able to study how T cells circulating in the blood moved into the brain and began to behave like T cells that reside there.
They found that if the brain lacks T helper cells, the microglia end up suspended between an underdeveloped juvenile and mature adult state. There were also behavioral differences when the mice lost these cells. The question remains as to whether this T cell population plays a similar role in humans.
"In mice, the wave of entry of immune cells at birth triggers a switch in brain development," said Liston. "Humans have a much longer gestation than mice though, and we don't know about the timing of immune cell entry into the brain. Does this occur before birth? Is it delayed until after birth? Did a change in timing of entry contribute to the evolution of enhanced cognitive capacity in humans?"
This research shows that we have a lot more to learn about how the brain interacts with the immune system. "It has been really exciting to work on this project. We are learning so much about how our immune system can alter our brain, and how our brain modifies our immune system. The two are far more interconnected than we previously thought," said Dr. Emanuela Pasciuto of VIB-KU Leuven.
The researchers are also aware of the importance of considering the microbiome when studying the brain and how it develops and functions."There are now multiple links between the bacteria in our gut and different neurological conditions, but without any convincing explanations for what connects them," Liston noted. "We show that white blood cells are modified by gut bacteria, and then take that information with them into the brain. This could be the route by which our gut microbiome influences the brain."