Stanford: Blood immune cells flood aging brain
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- Stanford researchers found that immune cells from the blood enter the human brain as early as middle age, where they transform into microglia, the brain's specialized immune cells.
- The discovery, published in Nature, overturns the long-standing assumption that microglia are a self-sustaining population established at birth and largely isolated from the body's wider immune system.
- Julia Belk, a postdoctoral scholar in pathology and first author, traced cell origins by matching DNA mutations shared between immune cells in paired blood and brain tissue samples, using mutations as biological 'ancestry markers.'
- Belk and senior authors Siddhartha Jaiswal and Howard Chang used samples from the Stanford Rapid Autopsy Center and the University of Washington's Alzheimer's Disease Sequencing Project, comparing immune cells from people with and without Alzheimer's.
- The blood-to-microglia transformation does not appear in mice or non-human primates, making it a uniquely human feature of brain aging that had previously gone undetected.
- The new study builds on earlier work by the same team showing that people carrying certain mutated immune cell clones — a condition known as clonal hematopoiesis of indeterminate potential — were much less likely to develop Alzheimer's.
- The researchers said the finding could enable new engineering strategies, such as modifying peripheral immune cells to target and break down amyloid and tau aggregates associated with neurodegenerative diseases, potentially delivered preventively.
Why it matters: Because peripheral immune cells can reach the brain and become microglia, anything that alters blood or bone marrow cells — aging, mutation, future gene therapies — could now influence brain disease risk, reframing Alzheimer's and neurodegeneration research around the body's broader immune history. The fact that this process appears uniquely human also means mouse models, long the backbone of microglia research, may be inadequate for testing brain immune therapies, reshaping how preclinical studies are designed.
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