A LYSOSOMAL MARKER FOR ACTIVATED MICROGLIAL CELLS INVOLVED IN ALZHEIMER CLASSIC SENILE PLAQUES

One of the major histopathological lesions in brains of patients with dementia of the Alzheimer type (DAT) is the senile plaque. Although pr evious studies have shown that senile plaques are often accompanied by microglial cells, the role of these cells in DAT pathology is still u nclear: In an immunohistochemical and immuno-electron microscopical an alysis of DAT and control brain tissues we addressed this issue using two monoclonal antibodies (mAbs KP1 and 25F9) directed against lysosom al antigens in monocytes and macrophages. Whereas KP1 stained lysosome s in both resting and activated microglial cells, 25F9-staining was pr edominantly found in lysosomes of activated microglial cells in classi c senile plaques. The number and size of 25F9-positive lysosomes in ac tivated microglial cells was increased compared to 25F9-staining in un affected areas in DAT and control sections. We conclude that mAb 25F9 is a unique and useful lysosomal marker, with a higher specificity tha n other known markers, for activated microglial cells associated with classic, but not with diffuse, senile plaques.