MHC class II‐positive microglia in human brain: Association with alzheimer lesions

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Abstract
Cells of the mononuclear phagocytic system (MPS) present foreign antigen on their cell surfaces bound to major histocompatibility complex (MHC) class II molecules. Previous studies of normal human brain samples reported MHC class II expression primarily by perivascular MPS cells and white matter microglial cells. Marked increases in MHC class II‐expressing microglia have been shown in many neuropathologic disorders, including Alzheimer's disease (AD). A close morphologic association between these cells and Alzheimer senile plaque β‐amyloid has been demonstrated. The present study used a mixed aldehyde fixative to enhance the localization of MHC class II‐expressing MPS cells in non‐AD and AD brain. Two antibodies against MHC class II (HLA‐DR; LN3), as well as the lectin Ricinus communis agglutinin (which recognizes both ramified and activated microglia) were used for light and electron microscopic analyses. We now report that MHC class II‐expressing ramified microglia are distributed in a uniform reticular array throughout the grey, as well as the white matter in non‐AD cases. In AD cases, immunolabelled cells had the morphology of activated microglia, with darkly stained plump somata and short, thick processes. Microglia clustered around senile plaque amyloid and neurofibrillary tangles (NFT), rather than forming the uniform array characteristic of control tissue. Finally, we report that perivascular MPS cells are found in a morphologic relationship with vascular amyloid identical to that seen between microglial cells and senile plaque β‐amyloid. These data suggest that MHC class II‐expressing cells may be involved in the degradation of NFT‐laden neurons and the posttranslational modification of extracellular‐NFT epitopes. In addition, both parenchymal and perivascular MPS cells are ideally situated to uptake and process the β‐amyloid protein precursor and deposit β‐amyloid on senile plaques, NFT, and the cerebrovasculature.