Microglia in Alzheimer brain: a neuropathological perspective

Microglia have long been noted to be present and activated in Alzheimer brain. Demonstrations that these microglia are associated with the specific lesions of Alzheimer disease-Aβ plaques and neurofibrillary tangles-and that these microglia overexpress the potent proinflammatory cytokine interleukin-1 led to the recognition of a potential pathogenic role for these cells in initiation and progression of disease. Activated, cytokine-overexpressing microglia are near-universal components of Aβ plaques at early (diffuse) and mid (neuritic) stages of progression in Alzheimer brain, and only decline in end-stage, dense core plaques. They correlate with plaque distribution across cerebral cortical cytoarchitectonic layers and across brain regions. They also show close associations with tangle-bearing neurons in Alzheimer brain. Microglial activation is a consistent feature in conditions that confer increased risk for Alzheimer disease or that are associated with accelerated appearance of Alzheimer-type neuropathological changes. These include normal ageing, head injury, diabetes, heart disease, and chronic intractable epilepsy. The neuropathological demonstration of microglial activation in Alzheimer brain and in Alzheimer-related conditions opened the field of basic and applied investigations centered on the idea of a pathogenically important neuroinflammatory process in Alzheimer disease.