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-{beta} (A{beta}) plaque burden--one of the hallmarks of AD--and microglial states in young 5XFAD mouse models of amyloid pathology. We hypothesized that an elevated A{beta} burden will directly correlate with an increase in microglial cell number and phagocytic activity, shaping 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 A{beta} 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 A{beta} plaques and microglial clustering in 4-month-old female 5XFAD compared to male mice. Conversely by 6 months, higher total plaque load in males and no sex difference in the number of plaque-associated microglial (PAM) cluster was observed. Our spatiotemporal characterization of microglial CD68, Dectin-1 (Clec7a), and CD11c (Itgax) expression revealed transient sex differences in the upregulation of only CD68 among these phagocytic markers in cortical microglia. In 4-months-old males, greater phagolysosomal activity may benefit A{beta} clearance and delay plaque formation. In females, lower microglial phagolysosomal activity and increased microgliosis may have led to the increase in plaque deposition and compaction from 2-4 months. Our results suggest that during early amyloidosis, sex differences in CD68-associated phagolysosomal 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.