Study Reveals Aging Human Brains Replace Resident Immune Cells with Blood-Derived Cells Starting at Age 50

A study published in Nature reveals that aging human brains do not rely solely on self-renewing resident microglia; instead, starting around age 50, a massive influx of peripheral blood-derived immune cells enters the brain to replace them. Using DNA somatic mutation lineage tracing, researchers from Stanford and other institutions proved these blood-derived cells transform into functional microglia. This uniquely human phenomenon, absent in mice, opens new therapeutic avenues for engineering peripheral cells to target neurodegenerative diseases like Alzheimer's.

Blood-derived immune cell brain migration

• ▪A study published in Nature shows that aging human brains experience a large influx of peripheral blood-derived immune cells that enter the brain and replace resident microglia.
• ▪The discovery upends a long-held neuroimmunology dogma that the brain's immune system is a closed system with microglia self-renewing exclusively inside the brain.
• ▪In the human hippocampus, resident microglia steadily decline between the ages of 50 and 75 and are replaced by cells carrying stronger inflammatory signals.
• ▪The study also found that aging is associated with a decline in cells maintaining the blood-brain barrier and synchronized structural disruptions in the genome.

DNA mutation lineage tracing methodology

• ▪The study analyzed postmortem hippocampal tissue and blood samples from neurologically healthy adults aged 20 to 95 to track immune cell changes.
• ▪Researchers traced the lineage of brain immune cells by comparing DNA from blood and postmortem brain samples to identify shared somatic mutations.
• ▪The research team paired standard gene activity measurements with tools mapping the genome's 3D structure and chemical modifications, known as the epigenome, to identify cell origins.

Human-specific aging phenomenon

• ▪Comparative analyses showed that standard laboratory animal models do not exhibit this influx of peripheral immune cells into the brain during aging.
• ▪The migration and transformation of peripheral blood cells into microglial phenotypes during aging is a uniquely human phenomenon not observed in mice or nonhuman primates.

Clonal hematopoiesis protective effects

• ▪Prior genetic analysis of thousands of people showed that individuals with specific mutant blood stem cell clones, known as clonal hematopoiesis of indeterminate potential, have a reduced risk of developing Alzheimer's disease.
• ▪The protective association of clonal hematopoiesis suggested to researchers that mutant peripheral blood cells actively migrate into the brain and alter disease trajectories.

Engineered cell therapeutic delivery

• ▪The discovery that peripheral immune cells can enter the brain opens new avenues for engineering these cells to deliver therapies or clear toxic amyloid and tau aggregates.
• ▪Researchers suggest that understanding these cellular transitions could lead to interventions that preserve brain function and reduce vulnerability to neurodegenerative diseases.