Activated microglial phenotypes in the hippocampal CA2 subfield are implicated in Lewy body disease progression

Histopathologic staging models of neuronal α-synuclein pathology (n-asyn) in Lewy body disease (LBD) seldom evaluate brain regions with direct synaptic connectivity to model the role of microglial processes. We address this gap by testing the hypothesis that, within the well-defined synaptic connectivity of the intrahippocampal circuit, n-asyn is associated with activated microglial phenotypes. We selected a cohort of autopsy-confirmed LBD patients and minimal age-related copathologies (n = 62) and a control cohort of cognitively healthy patients with isolated hippocampal tau accumulation (i.e., primary age-related tauopathy, PART; n = 12), to control for neurodegenerative pathology without amyloid plaques. We immunostained consecutive hippocampal sections for n-asyn and established markers of activated microglial phenotypes, Iba1, HLA-DR, and CD68. With validated digital histology methods, we measured percent area occupied (%AO) of each marker in 6 hippocampal subfields to compare and correlate microglial morphologic and proteomic activation phenotypes between cohorts and used linear mixed effects models to compare the %AO between subfields while covariying for demographics. We also constructed groups of n-asyn restricted to the cornu ammonis (CA) 2-3 subfields (Focal Subtype) or widespread n-asyn within additional subfields (Widespread Subtype) to model hypothesized n-asyn spread within the intrahippocampal circuit. LBD patients exhibited increased HLA-DR and CD68%AO in most hippocampal subfields compared with PART. In LBD patients, all microglial markers were the highest in the CA2. CA2 n-asyn correlated with HLA-DR and CD68 but not Iba1%AO. Patients classified as Widespread Subtype had worse cognitive impairment and increased CA2 HLA-DR and CD68%AO. CA2 HLA-DR and CD68%AO correlated with distal n-asyn in retrograde, but not anterograde connected subfields. Our data show that activated microglial phenotypes in the CA2 of LBD patients are associated with worse clinical outcomes and retrograde n-asyn transmission. These data suggest that measures of microglial states can refine LBD histopathological progression models.