How aging changes brain immune cells in female and male mice

Can ordinary aging make brain immune cells look more like cells seen in disease? A study published on August 16, 2023, examined that question in the hippocampus of young and old female and male mice. The researchers found that sex differences in microglial gene activity were limited in young animals but became broader with age. In old females, several of the additional features were associated with disease-related microglial states and cellular senescence, a condition in which cells stop dividing and alter the signals they release.

The finding offers a baseline for thinking about Alzheimer’s disease, not an explanation of why human women and men experience the disease differently. These mice were not being followed through Alzheimer’s pathology. The study instead asked what aging alone does to hippocampal microglia, the brain’s resident immune cells.

The age comparison came before any disease model

The experiment used four groups: young females, young males, old females and old males. The principal sequencing groups included mice aged 5–6 months and 22–25 months. These were different animals at different ages, so the study compares age groups rather than tracking the same microglial cells through time.

The researchers used two complementary ways to examine RNA. In one, they enriched CD11b-positive microglial material from acutely isolated hippocampal tissue and measured the broader transcriptome, meaning the collection of RNA present in the enriched preparation. In the other, they used Cx3cr1-NuTRAP mice to isolate RNA associated with tagged polysomes, the structures where RNA is used to make proteins. This translating RNA provided a second view of the genes that were being actively used by the cells.

The sequencing groups contained roughly 3–5 animals per sex and age for the CD11b-based method and 5–7 for the ribosome-associated method. The authors also used flow cytometry and other validation approaches in separate cohorts. One flow-cytometry comparison used mice aged 2–4 months and 23 months, with 6–7 animals per sex and age. The preparation methods were designed to reduce activation caused by handling tissue outside the brain, although the enriched material could include contributions from other brain-resident macrophages as well as microglia.

In young hippocampi, the researchers found few sex-linked differences beyond genes located on the sex chromosomes. By old age, the differences extended to many autosomal genes. This shift matters because it suggests that the female–male contrast was not simply a fixed genetic difference present from early life. It became more visible as the brain environment aged.

Old female microglia showed a different mix of cell states

The gene-expression results were followed by measurements of marker combinations on individual cells. These markers do not describe every function of a cell, but they can help estimate how much of a tissue sample falls into broad phenotypic categories.

Old females had a higher proportion of cells carrying the combination of markers used in the study to identify a disease-associated microglial state. Old males, in contrast, had a higher proportion of cells carrying a homeostatic combination associated with a more typical maintenance state. The disease-associated comparison used high CD11c and CLEC7A expression, while the homeostatic comparison used P2RY12 together with the absence of CLEC7A.

The marker results reinforce the RNA findings without turning them into a claim that every old female microglial cell had the same behavior. Microglia are heterogeneous, and a bulk RNA average or a marker-defined gate can conceal several subpopulations. The authors therefore identified single-cell analysis as an important way to test whether the sex difference reflects a change in the abundance of particular subtypes, a change in their activity, or both.

There was also an important exception to any simple ranking of female mice as more inflammatory in every respect. Interferon-response signals increased with age in both sexes. The broader transcriptomic results suggested a stronger increase in some interferon-related pathways in old males, and a flow-cytometry measure of CD317 showed a male-leaning difference among old animals in post hoc analysis, even though the overall sex effect for that measure was not statistically significant. Aging therefore did not push every immune program in the same direction.

Senescence was supported by a pattern of measurements

Cellular senescence is not just another word for activation. The paper describes it as a state involving cell-cycle arrest, resistance to programmed cell death and a changed secretory program known as the senescence-associated secretory phenotype, or SASP. A senescent cell may remain present while releasing signals that influence its surroundings.

Several results supported the authors’ interpretation that old female microglia carried a stronger senescence-related program. SASP-associated genes were expressed at higher levels in old females than in the other groups. A gene-set analysis also found enrichment for senescent markers when old females were compared with old males. Csf1, one of the genes showing female-biased expression in the aging comparison, is part of the SASP described in the study.

The authors added a network-based analysis of 19 genes that were more highly expressed in old female microglia than in old male microglia. Among the highly connected neighboring genes was Cdkn2a, which encodes p16INK4A, a commonly used senescence marker. That network result is a computational relationship, not a direct demonstration that every cell expressing the female-biased genes was senescent.

The distinction is important because the study did not establish all of the defining features of senescence in the same cells. The authors called for additional confirmation using approaches such as senescence-associated beta-galactosidase staining and single-cell methods. The evidence supports a stronger senescence-associated expression pattern in aged female hippocampal microglia; it does not convert that pattern into a complete functional diagnosis of each cell.

The signatures do not settle what helps or harms the brain

The study’s pathway analysis highlighted inflammatory mediators including IFNG, TNF and IL1B, along with Alzheimer’s disease risk-related genes such as TREM2 and APP. These were inferred upstream regulators from expression patterns. The experiment did not perturb those molecules to show that any one of them caused the age-by-sex difference.

That limitation also applies to the disease-associated microglial markers. Microglial activity can involve surveillance, debris clearance, movement, survival and communication with nearby cells. A state that resembles one seen in disease may contain both potentially harmful and potentially useful functions, depending on the surrounding signals and the task the cell is performing. Broadly suppressing every cell with a disease-associated signature could therefore remove useful immune activity along with damaging inflammation.

The results also leave open where the sex divergence begins. It could involve cell-intrinsic factors such as sex-chromosome regulation or hormone-receptor activity. It could also arise from differences in the aging environment around microglia, including the material and signals those cells encounter. The authors specifically point to sex hormones and chromosome-linked regulation as possibilities for future study, not as established explanations.

The Alzheimer’s connection remains a hypothesis built from this aging baseline. The female-biased microglial features appeared in mice without the disease pathology used in Alzheimer’s models. They may help explain why the same later pathological challenge could be handled differently, but this experiment did not test that challenge. Nor does it show that the observed mouse pattern causes the sex disparity in human Alzheimer’s disease.

How sex differences could shape Alzheimer’s treatments

The practical possibility raised by this work is more selective treatment of brain inflammation. If future experiments show that particular age-by-sex microglial programs contribute causally to harmful responses around Alzheimer’s pathology, therapies might be designed for the immune environment in which that pathology develops rather than assuming that female and male brains age immunologically in the same way.

For people at risk of Alzheimer’s, the potential benefit would be the ability to reduce damaging inflammatory activity while preserving surveillance, debris handling and other useful microglial functions. Reaching that point would require causal tests of the candidate regulators, confirmation that the mouse signatures matter in humans, and interventions that act safely. The 2023 study provides a detailed starting comparison for that work, not evidence that such a treatment already works.

Key Sources

• Primary source: Microglial senescence contributes to female-biased neuroinflammation in the aging mouse hippocampus: implications for Alzheimer's disease.
  DOI or Journal Page · Peer-Reviewed Paper · 2023-08-16