Spatiotemporal Characterization of Amyloidosis-Associated Microglial States Reveals Sex Difference in Early Plaque Formation

Twice as many women develop Alzheimer’s disease (AD) compared to men. Several key aspects, such as genetic risk factors, hormonal vulnerability, social responsibilities, and differences in longevity, contribute to the strong female bias in AD. To assess whether sex differences can be detected during the onset of AD, we examined the amyloid-β (Aβ) plaque burden––one of the hallmarks of AD––and microglial states in young 5XFAD mouse models of amyloid pathology. We hypothesized that an increase in microglial cell number and phagocytic activity will directly correlate with an elevated Aβ burden and shape the appearance of compact dense-core plaques in the cortex from 2 to 6 months of age. As expected, no change in microglial density and phenotype was found in Aβ plaque-free hypothalamus of 5XFAD male and female mice when compared to age-matched wildtype controls. By quantifying the number and coverage of diffuse and dense-core plaques in the cortex, we discovered a pronounced increase in Aβ plaques and microglial clustering in 4-month-old female 5XFAD compared to male mice. By 6 months, no sex difference in plaque load and microglial density was observed. Our spatiotemporal characterization of microglial Clec7a /Dectin-1 and CD68 expression revealed sex differences in the upregulation of these phagocytic markers in plaque-proximal microglia. In 2-months-old males, greater phagolysosomal activity around diffuse plaques may benefit Aβ clearance. However, in females, the lower initial microglial reactivity and subsequent rise in Dectin-1-driven phagocytic activity may have led to the increase in dense-core plaques at 4 months. Our results suggest that during early amyloidosis, sex differences in CD68-associated lysosomal activity and microglia-driven plaque compaction may cause disproportionate AD risk and severity that is compounded by other exacerbating factors during aging. Taken together, sex-specific targeting of microglial proliferation and phagocytic activity may be a promising intervention in presymptomatic patients with known AD risks.