A new study has found that large numbers of immune cells from the blood can enter the brain as people grow older. Some of these cells can then transform into microglia, specialised immune cells that help protect and maintain the brain.
The discovery challenges a long held belief that the brain’s immune system remains largely separate from the rest of the body throughout a person’s life.
Researchers say the finding could change how scientists understand brain ageing and may eventually open new possibilities for treating neurological diseases. The study was published in the journal Nature.
For many years, scientists believed that microglia were established early in life and remained largely self sustaining. Unlike many immune cells elsewhere in the body, they were thought to renew themselves without receiving new cells from the bone marrow.
However, Stanford researchers found evidence that this may not be the case in ageing humans.
To investigate, the researchers examined human blood and brain tissue collected from people with and without Alzheimer’s disease. They compared the genetic characteristics of immune cells found in the blood with those found in the brain.
The researchers used mutations in the DNA of the cells as markers to trace where the cells came from.
When the same mutations were found in immune cells in both the blood and brain, the scientists were able to determine that the cells shared the same origin.
Their analysis showed that immune cells originating outside the brain can enter the human brain, with this process beginning as early as middle age.
Once inside the brain, some of these cells can take on the characteristics and functions of microglia.
The researchers said the finding appears to be different from what is seen in some other animals, including mice and non human primates.
The discovery could also have important implications for the study of diseases such as Alzheimer’s.
Scientists believe it may eventually be possible to use blood based immune cells as part of new treatments for brain diseases.
One possible approach could involve modifying immune cells so that they can enter the brain and help remove harmful substances associated with neurodegenerative diseases, including amyloid and tau proteins.
Researchers also believe the health and history of blood stem cells could influence brain health as people age.
Changes in these cells may affect the microglia that develop in the brain and, in turn, potentially influence the risk of neurological diseases.
The Stanford team says the findings provide a new understanding of how the immune system and the ageing brain interact and could lead to further research into ways of protecting brain health later in life.
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