Microglial reactivity in the hippocampal CA2 is associated with advanced neuronal α-synucleinopathy

Lewy body diseases are thought to evolve by the spread of intraneuronal α-synuclein pathology. However, staging models of α-synuclein pathology in the human brain seldom evaluate regions with direct synaptic connectivity to model microglial processes in disease. Here, we address this gap by testing the hypothesis that, within the well-defined synaptic connectivity of the intrahippocampal circuit, neuronal α-synuclein pathology is associated with microglial reactivity. We selected a cohort of autopsy-confirmed Lewy body disease patients with hippocampal neuronal α-synuclein pathology and minimal age-related co-pathologies (<i>n</i>=62) and as a control for other intraneuronal hippocampal pathology, we used a cohort of cognitively healthy patients with focal hippocampal tau accumulation with minimal amyloid plaques, termed primary age-related tauopathy (<i>n</i>=12). We immunostained consecutive hippocampal sections for α-synuclein pathology and established markers of microglial reactivity, Iba1, HLA-DR, and CD68. With validated digital histology methods, we measured percent area occupied in 6 hippocampal subfields to compare the percent area occupied within subfields between patient cohorts, correlate the percent area occupied within and between subfields, and use linear-mixed effects models to compare the percent area occupied across subfields while co-varying for demographics. We also constructed two groups of either low-level hippocampal α-synuclein pathology restricted to the cornu ammonis 2-3 subfields (Focal Subtype) or widespread α-synuclein pathology within additional subfields (Widespread Subtype) to model hypothesized α-synuclein pathological spread within the intrahippocampal circuit. Lewy body disease patients exhibited increased HLA-DR and CD68 in most hippocampal subfields compared with primary age-related tauopathy. In Lewy body disease patients, staining for all microglial markers was highest in the cornu ammonis field 2. Neuronal α-synuclein pathology in the cornu ammonis field 2 consistently correlated with only HLA-DR and CD68 but not Iba1. Patients classified as the Widespread Subtype had worse cognitive impairment and increased HLA-DR and CD68 within the cornu ammonis field 2. Furthermore, HLA-DR and CD68 in the cornu ammonis field 2 correlated with distal neuronal α-synuclein pathology only in subfields with retrograde connectivity. We found increased hippocampal microglial abnormalities in Lewy body disease patients compared with cognitively normal controls with primary age-related tauopathy, suggesting a relatively specific microglial/inflammatory response to neuronal α-synuclein pathology. Our data is consistent with predominantly retrograde transmission of neuronal α-synuclein pathology across hippocampal subfields, where the focal microglial response in the cornu ammonis field 2 may influence clinical outcomes and α-synuclein pathological spread. These data suggest that microglial processes can help refine Lewy body disease histopathological staging.