Stanford researchers uncover blood-to-brain immune cell migration in human aging

Researchers at Stanford Medicine have identified a previously unrecognized phenomenon in which peripheral blood-derived immune cells migrate into the human brain during aging and differentiate into microglia — the brain's resident immune population — a finding that challenges a foundational assumption in neuroscience and opens a potential therapeutic route into the central nervous system, according to a study published July 30, 2026 in Nature.

The study, led by postdoctoral scholar Julia Belk and senior author Siddhartha Jaiswal, an associate professor of pathology at Stanford Medicine, used somatic mutation-based lineage tracing in matched human blood and postmortem brain tissue to confirm that circulating immune cells breach the blood-brain barrier and take up residence as functional microglia in the aging brain. Notably, the phenomenon was absent in both mice and non-human primates, making it a uniquely human feature of aging and one that prior animal model research would have entirely missed.

Tracking immune cells from blood to brain

Microglia have traditionally been understood as a self-renewing immune population established during embryonic development and maintained throughout life with little contribution from circulating blood cells. That model has shaped research into neuroinflammation and microglial dysfunction in conditions including Alzheimer's disease.

The Stanford team investigated that assumption using naturally occurring somatic mutations as cellular lineage markers. Mutations acquired by hematopoietic stem cells are inherited by their immune-cell descendants, allowing researchers to compare mutation patterns in blood and brain cells and reconstruct their ancestry.

By identifying shared mutation signatures in matched blood and brain samples, the researchers found evidence that some circulating immune cells had entered the brain and adopted characteristics associated with microglia. The process appeared to begin in middle age and continue during later life.

The study extends earlier work from Jaiswal's group that suggested blood-derived cells could contribute to the brain's immune-cell population.

Potential link between blood-cell clones and Alzheimer's

The findings may also help explain an unexpected association between clonal hematopoiesis of indeterminate potential (CHIP) and Alzheimer's disease.

CHIP occurs when hematopoietic stem cells carrying particular somatic mutations expand and generate a disproportionately large share of circulating blood cells. Previous epidemiological research has associated some forms of CHIP with a reduced risk of Alzheimer's disease, raising the question of how changes originating in the blood could influence neurodegeneration.

The new study provides a potential biological link. If immune cells derived from particular blood stem-cell clones can enter the brain and acquire microglial characteristics, the clonal composition of the blood could potentially alter the brain's immune environment.

"Our findings suggest that the life history of blood stem cells could influence the risk of brain diseases by altering the microglia," Jaiswal said.

The research used postmortem brain tissue and matched blood samples, including material from the Stanford Rapid Autopsy Center and the University of Washington Alzheimer's Disease Sequencing Project, alongside genetic and clonal hematopoiesis data from larger patient cohorts.

Human-specific biology complicates translation

The apparent absence of the process in mice and non-human primates has potentially important implications for neuroscience research. Animal models are widely used to investigate microglial biology, neuroinflammation, and potential therapies for neurodegenerative disease, but would not capture a human-specific pathway involving migration of peripheral immune cells into the aging brain.

That distinction could also complicate development of therapies designed to manipulate the newly identified pathway because there is currently no established animal model that reproduces it.

At the same time, the finding raises a potential therapeutic opportunity. If peripheral immune cells naturally migrate into the human brain, researchers could potentially engineer such cells outside the body to perform therapeutic functions before administering them back to patients.

One possibility proposed by the researchers would be to enhance the cells' ability to clear pathological proteins such as amyloid or tau, potentially exploiting their endogenous migration into the brain as a way to circumvent some of the challenges posed by the blood-brain barrier.

"Now that we know that these immune cells actually can get into the brain, we can think about all kinds of new engineering strategies to have those peripheral immune cells do useful things," Belk said.

Any therapeutic application remains highly speculative. The study was discovery-stage research using human tissue, and no therapeutic candidates or clinical development programs based on the mechanism have been disclosed. The absence of an animal model that reproduces the process also presents a significant challenge for conventional preclinical development.

Further research will be needed to determine what controls the migration of blood-derived immune cells into the brain, how closely their functions resemble those of embryonically derived microglia, and whether manipulating the process can meaningfully alter neurodegenerative disease.

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