Stanford: Blood Immune Cells Become Brain's Microglia

A team at Stanford University School of Medicine has shown that immune cells from the bloodstream enter the human brain as early as middle age and turn into microglia, the specialized immune cells of brain tissue, once they arrive. The research was published in the journal Nature, and the university’s announcement came out on August 14. The lead author is Julia Belk, a postdoctoral researcher in pathology, and the senior authors are Siddhartha Jaiswal, associate professor of pathology, and Howard Chang, a cancer researcher.

The finding cuts against an assumption that had stood as a starting point in neurology for decades. The brain has its own population of immune cells and a blood-brain barrier that lets in very little from the rest of the body, so microglia were long thought to be present from birth and to renew themselves throughout life, with no outside supply.

“We usually think of the brain as a closed system. What we found is that a lot of immune cells are actually entering the human brain during aging.”

That is what Julia Belk said in Stanford’s announcement. Her team arrived at the question in stages. An earlier step had analyzed genetic data from thousands of people, including some followed for decades, and it showed that people who carry certain immune cell clones arising from mutated blood stem cells develop Alzheimer’s disease less often. That result raised the idea that these cells were somehow communicating with the brain.

How the Researchers Traced Where the Cells Came From

The method was cell-lineage tracing through somatic mutations. Mutations pile up randomly in blood stem cells as a person ages, and the immune cells that arise from those stem cells inherit the same pattern. The researchers compared the mutation record of immune cells from blood samples with the mutation record of cells from postmortem brain tissue of the same person.

“If we see the same mutations in the blood and in the brain’s microglia, we can be very confident that the immune cells in the brain are descendants of those immune cells from the blood.”

The samples came from two sources. One was Stanford’s rapid autopsy program, led by co-author Jody Hooper, and the other was the University of Washington’s Alzheimer’s disease sequencing project. Both programs collect blood and brain tissue from people who had Alzheimer’s disease and from people who did not. The mutation match held up, and the cells’ entry into the brain was already detectable in middle age.

One detail stood out to the team as unexpected. The same crossover was not seen in mice or in non-human primates.

“This is a uniquely human feature of aging that we had no idea about.”

What This Could Mean for Treatment

The authors conclude that a person’s blood stem cell history could influence their risk of brain disease by reshaping the microglia population. If a large share of microglia in older people really does originate in the blood, then anything that acts on blood and bone marrow has a potential path into brain tissue as well.

As a possible application, the researchers point to engineering peripheral immune cells to target amyloid and tau protein deposits in the brain, before those deposits build up. For now that is a proposed direction, not a tested therapy.

What Does This Finding Not Prove?

The study establishes where the cells came from, not what they do. The mutation match shows the origin of the microglia, but not whether the newly arrived cells help the brain, harm it, or have no effect either way. The link to Alzheimer’s disease still rests on the association found in earlier genetic analyses, not on proven causation.

The material itself sets limits too. The matched blood and brain samples come from postmortem tissue, which gives a snapshot rather than a record of the process over time. And the absence of the same pattern in mice and primates raises a further practical problem: the animal models most commonly used to test brain therapies do not show this mechanism, so results from them do not necessarily carry over to humans.

Stanford teams have published earlier findings on amyloid plaques, including one reporting a 35 percent reduction in plaques with estrogen therapy. This study is not about treatment but about where the cells come from, and it will take further research to show what these cells actually do once they reach the brain.