Microglial Activation in the Normal and Alzheimer's Disease Brain

Abstract

Microglia, the innate immune cells in the brain, become activated in response to damage and disease, changing cellular morphology and immunophenotypes. Microglial activation occurs in Alzheimer's disease (AD) and understanding the changes in activation may be key to understanding AD pathogenesis. The work presented in this thesis characterises microglial activation in immunohistochemically stained post-mortem human normal and AD middle temporal gyrus. Identifying microglia over other myeloid cell populations in the human brain has been challenging in the field. In this thesis, an unbiased classification method was established to distinguish between microglia and perivascular macrophages (PVMs). An anatomical classification method was subsequently used to characterise immunophenotype marker expression by microglia and PVMs to identify population differences in activation state and function. PVMs expressed higher levels of immunophenotype markers than microglia, suggesting that PVMs are more activated in the normal human brain. Immunophenotype markers have been used to indicate activation states in vitro, but it remained unclear whether immunophenotype markers could suggest activation in the human brain. To determine whether immunophenotype markers were more highly expressed on activated microglia in the human brain, an immunohistochemical analysis was carried out. Co-labelling of immunophenotype markers with a classical activation marker, human leukocyte antigen, DR isotype (HLA-DR), demonstrated that immunophenotype markers were more highly expressed on HLA-DRhigh 'activated' microglia. Immunophenotype markers were subsequently used to determine changes in microglial activation in AD. Fluorescent co-labelling of immunophenotype markers of interest (MOI) and the pan microglial marker, ionised calcium binding adaptor protein 1 (Iba1), was carried out, and a novel automated quantification method was developed to quantify the abundance of the MOI and Iba1 in each microglia in normal and AD middle temporal gyri. A higher proportion of an Iba1low MOIhigh population was observed in AD. The Iba1low population was best delineated by high abundance of the dysfunctional marker, L-Ferritin. The Iba1low population was therefore hypothesised to be dysfunctional, supporting the microglial dysfunction hypothesis of AD. The data presented in this thesis demonstrates that immunophenotype markers are a useful tool to characterise microglial activation and allowed for comparison between normal and AD brains. These data contribute to the current knowledge of microglial activation and function in the normal and AD human brain.