Phenotypic differences between human monocytes/macrophages and microglial cells studied in situ and in vitro

This report describes a phenotypic differentiation pattern conceived to distinguish invading monocytes from resident microglia in frozen and formalin-fixed human CNS. Phagocytic cells in normal and diseased CNS (multiple sclerosis and encephalitis) were studied immunohistochemically with a panel of antibodies, and phenotypic characteristics were compared with cultured monocytes/macrophages and microglia. Monocytes/macrophages were positive for the markers non-specific esterase, myeloperoxidase, L1, lysozyme, RFD7, and CD14, whereas microglia were negative for the same markers. Both populations of cells were positive for CD11c and CD68. Our results indicate that invading monocytes/macrophages mainly have a perivascular location in active multiple sclerosis lesions, whereas invading monocytes/macrophages also infiltrate the parenchyma in acute inflammatory CNS diseases such as in encephalitis.