Microglial activation plays a crucial role in neuronal death during Alzheimer's disease (AD). In healthy brains, homeostatic microglia maintain physiological immune responses and neuronal health. In AD, microglia respond to the accumulation of amyloid-beta (Aβ) and tau pathology by increasing their phagocytic activity and cytokine production. The initial neuroprotective microglial response becomes dysregulated and neurotoxic significantly. While the mechanisms behind this damage are known, the key factors that influence the microglial response in AD remain unclear. We aim to investigate how tau, Aβ, and other damaging stimuli affect the microglial response and subsequent neurotoxicity.

We treated a mouse immortalized microglial (IMG) cell line with different tau isoforms ON3R and ON4R tau to mimic early increases in tau levels. IMG cells were also treated with etoposide to simulate the damage resulting from the failure in DNA repair mechanisms. In addition, we used a combination of lipopolysaccharide and interferon-gamma, pro-inflammatory, and interleukin-4, anti-inflammatory, microglial activation controls. All treatments were performed at 37°C for 24 hours. We then utilized flow cytometry to assess changes in the expression of markers associated with pro-inflammatory activation (CD11c, CD45). We also evaluated morphological changes through surface area quantification.

Our results indicate that tau-driven activation leads to decreased CD45 and CD11c expression. Conversely, etoposide activation increased the CD45 expression. These findings were corroborated through surface area quantification, which showed increases in the area of cells treated with etoposide, a change not observed in tau-treated cells.

Our findings suggest that the microglial activation phenotype promoted by tau does not exhibit changes typically associated with pro-inflammatory activation; instead, it appears to align more closely with an anti-inflammatory profile. Conversely, the microglial response to etoposide resembles the pro-inflammatory profile. This indicates that different AD-associated stimuli activate microglia in distinct ways, potentially resulting in differences in neurotoxicity.