A new study shows that APOE4 restructures brain-related immune and lymphatic pathways in strikingly different ways in women and men, leading to opposite cognitive effects when innate immunity is suppressed.
Study: Sex-specific APOE4-dependent innate immunity modulates meningeal lymphatics, brain lipids, neuroinflammation, and cognition. Image credit: ahmetmapush / Shutterstock
In a recent study published in the journal neuronResearchers have shown that apolipoprotein E ε4 (APOE4) expression triggers sexually dimorphic inflammatory, lymphatic, lipid, neuroinflammatory, and cognitive responses.
APOE is the major lipid carrier in the brain. In humans, the APOE gene has three alleles: APOE2, APOE3 (E3), and APOE4 (E4), with E3/E3 being the most common genotype in the population. In Caucasians, expression of one or two E4 alleles increases AD risk by 3- to 4-fold or 9- to 15-fold, respectively.
APOE4 and sex can independently regulate glial, immune, and vascular responses in the brain. Recent rodent studies have shown that E4-expressing peripheral and neuronal cells exhibit sexual dimorphism. However, it is unclear why women are at higher risk for Alzheimer’s disease or how E4 shifts the balance toward dementia.
Changes in meningeal immune cells in APOE4 mice
In this study, researchers showed that E4 expression triggers sexually dimorphic inflammatory and lymphatic responses. First, we collected meningeal dura samples from middle-aged mice (12–13 months old) expressing two alleles of E4 or E3 instead of murine Apoe. Innate immune cell frequency was assessed by flow cytometry.
Although the frequency and number of innate immune cells in E4/E4 and E3/E3 mice were similar in both sexes, females had increased macrophages expressing major histocompatibility complex (MHC) class II and cluster of differentiation 206 (CD206), regardless of genotype. The researchers next investigated the effects of E4 or E3 expression on lymphatic vessel structure.
Furthermore, exposure to the CSF1R inhibitor PLX5622 (PLX) for 4 weeks decreased the frequency of macrophages. Notably, CD206+MHC class II+ macrophages were consistently and selectively depleted in both male and female mice, regardless of APOE genotype. PLX exposure also decreased APOE expression in macrophages, blood endothelial cells (BECs), mast cells, and fibroblasts, regardless of gender or genotype.
Macrophages showed the highest differentially expressed genes (DEGs) across group comparisons. E4/E4 PLX female mice had particularly elevated DEGs in macrophages, B cells, and BECs compared to female E4/E4 controls. In E4/E4 PLX male mice, BECs and B cells showed the highest DEGs compared to male E4/E4 controls. They also had significantly higher DEGs in natural killer cells, T cells, dendritic cells, fibroblasts, and innate lymphocytes than male E4/E4 controls.
Effect of APOE4 on lymphatic vessels and CSF drainage
Analysis of the meningeal dura mater of young (2-4 months old) or middle-aged mice showed a sex- and age-dependent effect of E4 expression on the total length of vessels expressing lymphatic endothelial hyaluronan receptor 1 (LYVE-1). Specifically, longer LYVE-1 + vessels were observed in 12- to 13-month-old male E4/E4 mice, but not in females, compared to age-matched E3/E3 males. There were no group differences in lymphoid morphology at younger ages.
Furthermore, middle-aged male E4/E4 mice had reduced cerebrospinal fluid (CSF) outflow to deep cervical lymph nodes (LNs) compared to age-matched E4/E4 females. This finding suggests that a wide range of meningeal lymphatic vessels in men is not necessarily beneficial, as it is associated with impaired drainage. Single-cell RNA-seq analysis reveals that two E4 alleles cause distinct and significant changes in gene expression in meningeal dural immune cells of middle-aged mice.
Neuroinflammation, brain lipids, and cognitive performance
Next, the research team investigated the effects of E4 expression on inflammatory chemokines and cytokines in the forebrain of middle-aged mice. E4/E4 females had higher levels of inflammatory cytokines and chemokines than E3/E3 females. In contrast, E3/E3 and E4/E4 males have comparable inflammatory profiles, consistent with the authors’ interpretation that at this stage males may exhibit relative resilience to APOE4-related neuroinflammatory changes. PLX exposure was associated with increased interleukin (IL)-22 and decreased levels of IL-1α, IL-16, and CC motif chemokine ligand 12 (CCL12), regardless of APOE genotype and sex.
Additionally, E4/E4 PLX females had lower levels of programmed cell death ligand 2, while E4/E4 PLX males had higher levels of CCL4, CSF1, fibroblast growth factor 21 (FGF21), CCL2, and hepatocyte growth factor (HGF) than sex-matched E4/E4 controls. This study also identified sex-specific effects of APOE4 on brain lipid profiles. The researchers then examined the cognitive function of middle-aged E4/E4 and E3/E3 mice using a fear conditioning test and an open field test. In open field tests, total distance traveled was similar between female groups.
However, the distance traveled through the center of the arena was significantly shorter between the female E4/E4 control and PLX groups. The male groups did not differ in the open field test. In fear conditioning tests, E4/E4 females showed less time to freeze during cue trials, indicating poorer cognitive function than E3/E3 females. In particular, PLX exposure decreased performance in the context test in E3/E3 females, but improved performance in the cued test in E4/E4 females, suggesting that suppressing the innate immune response may reduce cognitive vulnerability in female E4/E4 mice.
In contrast, E4/E4 males, who had similar fear freezing times as E3/E3 males, showed shorter freezing times on both trials after PLX exposure, indicating worsened cognitive function. Thus, suppression of innate immunity has mixed effects, appearing to be beneficial in E4/E4 females but detrimental in E4/E4 males. Finally, the researchers integrated six publicly available single-nuclear RNA-seq datasets of human brain cells from Alzheimer’s and non-Alzheimer’s patients. They noted that each brain leukocyte population responded differently to E4 expression in men and women.
Effects of sex-specific APOE4 on Alzheimer’s disease treatment
Taken together, our results highlight that the meningeal dura may represent a neuroimmune niche associated with age-related neurodegenerative diseases, the risk of which varies by gender, age, and E4 status. Elucidating the roles and mechanisms underlying immune responses in men and women is important for developing customized immunotherapeutic approaches against APOE4-induced cognitive decline. Overall, this finding suggests an early presence of APOE4-related neuroinflammatory and cognitive vulnerability in women, whereas men may show relative resilience at this stage despite changes in lymphatic structure and drainage.
