A new immunoperoxidase marker for microglia in paraffin section.

We tested antisera against monocytic and lymphocytic lineages on 27 formalin-fixed, paraffin-embedded brains, obtained at autopsy, using the avidin-biotin-complex immunoperoxidase method. Patients ranged from one day to 69 years with 12 cases under the age of two. LN-1-positive microglia were present in 22 brains. LN-1 did not stain any other glial or neuronal cells. Five negative brains included two irradiated gliomas, two cases of multiple sclerosis and one normal case. Immunostaining was confined to cells with bipolar processes and rod-shaped nuclei recapitulating the characteristic features of microglia in silver-impregnated sections. LN-1-positive microglia were most prominent in the grey matter and in the grey-white junction with fewer positive cells seen in the white matter. Double immunostaining with LN-1 and glial fibrillary acidic protein (GFAP) clearly distinguished LN-1-positive microglia and GFAP-positive astrocytes. The expression of LN-1, a B-lymphocyte antigen, by microglia contradicts the macrophage derivation theory and supports data indicating a functional role of microglia in immune processes. LN-1, while not specific for microglia, should be considered a useful marker, more reliable than silver impregnation, for detecting microglia in paraffin section.