Stanford scientists have discovered that immune cells from the bloodstream actively infiltrate the human brain as people age. This process begins in middle age, challenging previous assumptions that the brain remains isolated from the body's immune system.
The study, published in the journal Nature, revealed that cells originating from bone marrow can transform into microglia—specialized immune cells of the brain. Previously, it was believed that microglia renewed only within the brain, without external replenishment.
“We typically think of the brain as a closed system,” states Julia Belk, the study’s lead author and a postdoctoral scholar at Stanford University School of Medicine. “It turns out that quite a few immune cells enter the brain during the aging process.”
The team, led by Siddhartha Jaiswal and Howard Chang, utilized brain and blood tissue samples from the Stanford Rapid Autopsy Program and the University of Washington's Alzheimer’s Disease Sequencing Project. By comparing somatic mutations in DNA, researchers traced the cells' origins and confirmed their 'blood-derived' nature.
This process is not observed in mice and primates, suggesting it is a unique feature of human aging. The discovery raises the question: how precisely does the history of hematopoietic stem cells influence the risk of neurodegenerative diseases?
Scientists see promising avenues for new therapeutic approaches stemming from this finding. Strategies can now be developed where peripheral immune cells specifically target amyloid and tau plaques. The Knight Initiative for Brain Resilience at the Wu Tsai Neurosciences Institute provided support for this research.
The results indicate that bone marrow and hematopoietic health may directly impact brain function in old age. This opens up a new direction in the study of aging and neurodegenerative diseases.
