Hidden Brain Shift Beginning Around Age 50 Discovered by Scientists

A study funded by the National Institutes of Health has uncovered a major shift in the immune environment of the hippocampus, the part of the brain that plays a central role in learning and memory. The findings suggest that this immune remodeling begins in midlife and may help explain how aging contributes to the long-lasting brain inflammation often seen in neurodegenerative diseases.

Researchers from the University of California, San Diego, the New York Genome Center, and the University of California, Irvine used advanced single-cell methods to study postmortem hippocampal tissue from 40 neurologically healthy adults between the ages of 20 and 95.

Their analysis found that microglia, the brain's main immune cells, gradually decline from about age 50 to age 75. At the same time, they appear to be replaced by cells with stronger inflammatory signals and other traits similar to immune cells that originate in peripheral blood. This discovery challenges a long-standing assumption that microglia remain in the brain and continually renew themselves throughout a person's life.

The team combined standard measurements of gene activity with newer methods that map the genome's 3D structure and its chemical modifications, known as the epigenome. These combined methods allowed the researchers to identify changes in immune cell identity and origin that would have remained hidden if they had examined gene activity alone.

The study also found signs of age-related decline in cells that help maintain the blood-brain barrier. Across many kinds of brain cells, aging was associated with widespread and coordinated changes in the genome's physical organization.

Future research will examine why resident microglia are lost with age and whether the newly identified immune cell transition directly contributes to Alzheimer's disease and other neurological conditions associated with aging. Understanding these cellular transitions may provide new opportunities to develop interventions that preserve brain function and reduce vulnerability to neurodegenerative disease.