Lewy body diseases (LBDs) evolve by spreading of intraneuronal protein aggregates comprised of α-synuclein, termed neuronal α-synucleinopathy (NAS). In the absence of reliable in vivo imaging to detect NAS, histopathological staging is critical to understand LBD pathogenesis but seldom accounts for microglial activity within well-established circuits. Here, we address this key knowledge gap by testing the hypothesis that microglial activity mirrors NAS accumulation within the well-defined hippocampal circuit.

NAS is associated with increased microglial activity but there is conflicting evidence to the role of microglia in LBDs. NAS in the hippocampus develops preferentially in certain subfields, and here, we used a focused digital histopathologic study to evaluate the association of microglial activity with NAS distribution using NAS positivity within the well-studied hippocampal circuit to model transsynaptic NAS spread.

We isolated autopsy-confirmed hippocampal NAS with minimal age-related co-pathologies (n=62). Consecutive hippocampal sections were immunostained for NAS and microglial markers, Iba1, HLA-DR, and CD68. We measured the percent area occupied (%AO) in hippocampal subfields (Subiculum, CA1-4, and Dentate Gyrus). We used linear-mixed effects models with subfield as a fixed factor to compare the normalized %AO across subfields while co-varying for demographics and determined the association of microglial activity with NAS within and between subfields using non-parametric correlations.

There was significantly higher NAS and microglial activity in the CA2 compared to most other hippocampal subregions (β=-0.99- -0.18, p<0.05). Interestingly, within-region correlations between NAS and CD68 and HLA-DR were observed only in the CA2 (ρ=0.33-0.26, p<0.05). In contrast, CA2 microglial activity with these markers correlated with NAS in distant subfields (CA4 and Dentate Gyrus) with retrograde connectivity to the CA2 (ρ=0.43-0.36, p<0.01).

Our data suggest that focal microglial activity in the CA2 is associated with NAS development specifically within the CA2 and with retrograde distribution of NAS to distant hippocampal subfields.