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Background: Neuroinflammation is a key pathological feature of Alzheimer's disease (AD), with microglia being the main inflammatory cell type in the CNS. In order to examine these cells in vivo, PET ligands targeting the microglial translocator protein (TSPO) have been developed. However, it remains unclear the specific target of these ligands, whether this is all microglia or a subset of cells. We aim to elucidate this by identifying the microglial phenotype most associated with TSPO in humans. Method: Human post‐mortem sections of temporal lobe (TL) and cerebellum (Cb) from cases classified by Braak stage (0‐II, III‐IV, V‐VI; each n = 10) were immunostained for pan‐Aß, pTau, and microglial markers TSPO, Iba1 and HLA‐DR with protein load quantified. Double immunofluorescent staining was performed with TSPO and microglial markers HLA‐DR and CD68 with positive cells counted. Result: TL showed an increase in Aβ (P<0.0001), pTau (P<0.0001) and TSPO (P<0.0001) protein load, but not Iba1 or HLA‐DR, with progressing Braak stage. Aβ (P = 0.0008) and Iba1 (P = 0.012) load increased in the Cb, but not pTau, TSPO or HLA‐DR. Of note, TSPO load in TL was associated with pTau. In both areas, no difference was found in number of HLA+ or CD68+ cells, though TL TSPO+ cell number increased significantly (P = 0.0347). There was no difference in the number of HLA‐DR+/TSPO+ or CD68+/TSPO+ cells with progressing Braak stage. However, significantly more CD68+/TSPO+ than HLA‐DR+/TSPO+ cells were detected in both the TL (P<0.0001) and Cb (P = 0.009) regions. Conclusion: The results suggest that TSPO expression is related to late stage disease (pTau) and microglial phagocytosis (CD68), a function we have previously identified associated with the presence of dementia, cognitive decline and tau pathology. Further additional analysis with other microglial markers is currently ongoing. [ABSTRACT FROM AUTHOR]

Background: Microglia are the resident immune cells in the brain and contribute to the neuropathology of Alzheimer's disease (AD) by mechanisms not completely known. Iba1 is a microglial marker associated with homeostatic and motility functions. Whereas TSPO (PBR) is a marker of the mitochondrial membrane in microglia, macrophages and endothelial cells and is proposed to be associated with the activated subtype of microglia. Method: To assess both Iba1 and PBR, we retrieved human temporal cortex and cerebellum samples from 57 cases at different stages of the disease, based on the Braak staging (Group 0‐II n=19, Group III‐IV n=20, Group V‐VI n=18), using immunohistochemical analysis. Additionally, a V‐Plex Mesoscale Discovery (MSD) analysis was performed to assess the inflammatory environment. Result: There was a significant increase in PBR percentage load (P<0.001) across the Braak stages in the temporal cortex (average load 0.93%) but not in the cerebellum (average load 0.76%). Conversely, there was a significant increase in cerebellar Iba1 load (average load 0.96%, P=0.012) but not in the temporal cortex (average load 1.12%). Furthermore, it was observed that Iba1 forms clusters, putatively around amyloid plaques, whereas this phenomenon was not seen with the PBR marker. The only significant inflammatory molecule was the cytokine IL15 in the temporal cortex (P=0.024), which increased across all Braak stages. However, when the MSD data was correlated with the Iba1 and PBR markers it showed that GM‐CSF was significantly correlated with PBR in the temporal cortex (P<0.0099) and IL‐16 was significantly correlated with Iba1 in the cerebellum (P<0.0012). Conclusion: This emphasises the outcome that, in AD, PBR may contribute to inflammation in the temporal cortex and Iba1 may be correlated to inflammation in the cerebellum. The findings indicate that TSPO relates to microglial activation in the brain which is Braak‐stage specific and consequently supports the use of TSPO ligands for positron emission tomography (PET) imaging of Alzheimer's disease‐relevant microglial activity in vivo. Moreover, the absence of an increase in PBR load in the cerebellum could qualify this structure as a reference region for PET quantification. [ABSTRACT FROM AUTHOR]