Brain immune cells from a sibling can live in your brain
What if some of the cells in your brain carried a sibling’s genome? In one group of primates, that is a biological reality, and it reveals something unexpected about how brain immune cells work.
Twin births in marmosets are unusual. In the womb, twins share a connected blood supply, allowing stem cells from one fetus to migrate into the other. The result is lifelong chimerism: animals that carry cells with two distinct genomes. The researchers analyzed how far that chimerism extends into the brain, using cell-type analysis of RNA sequencing data from multiple brain regions and organs across several marmosets. The study was published in eLife.
The findings were strikingly specific. Chimerism in the brain was found exclusively in microglia, the brain’s immune cells, and in brain macrophages. In some animals, 20 to 52 percent of microglia in a given brain region were cells derived from a sibling. Neurons, support cells (glia), and other brain cell types showed no chimerism. All sibling-derived cells traced back to blood cell lineages.
Microglia follow their environment, not their genes
Notably, the gene activity of microglia was more strongly shaped by the local brain environment than by the cells’ genetic origin. Cells carrying a sibling’s genome that settled in a given brain region began behaving like the local microglia. That suggests the regional brain environment exerts a dominant influence on how microglia function, regardless of which genome they carry.
This creates a unique research model. Normally it is nearly impossible to study the effects of gene variants in microglia independently of other cell types. In marmosets, natural chimerism allows exactly that: comparing how microglia with two different genomes behave within the same brain environment.
Relevance for brain health in aging
Microglia play a central role in neuroinflammation, clearing cellular debris, and the risk for neurodegenerative diseases. Changes in their behavior and composition are associated with aging and conditions such as Alzheimer’s. The marmoset model may help answer how genetics versus environment drives microglial function as the brain ages. Direct implications for humans have not yet been demonstrated, but the model opens new avenues for investigation.
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