Ki-M1P as a marker for microglia and brain macrophages in routinely processed human tissues

The monoclonal antibody Ki-M1P recognizes a formalin/paraffin-resistant differentiation epitope of monocytes and their macrophage derivatives [Radzun et al., Lab Invest 65:306, 1991]. To evaluate its usefulness for neuropathology, we examined a variety of routinely processed tissues using immunohistochemistry. In normal brains, positivity was restricted to ramified microglial cells. Intense labeling of macrophages, ramified and ameboid microglial cells, and rod cells was seen in brains with various degenerative and inflammatory disorders. Astrocytes were negative as determined by double-immunofluorescence labeling using Ki-M1P and anti-glial fibrillary acidic protein (GFAP). Histiocytic lesions (histiocytosis X, xanthogranulomas, granulomatous inflammation) were immunopositive. Among 107 tumors, reactivity of Ki-M1P was observed with some schwannoma and meningioma tumor cells. In addition to macrophages, most gliomas contained small, elongated Ki-M1P-positive cells, which were negative for GFAP. Positivity was also found in two glioblastoma cell lines. Immunoblotting performed on spleen, meningioma and glioblastoma specimens revealed one to three bands in the range of 110 to 130 kDa. We conclude that Ki-M1P can serve as a reliable marker for brain macrophages and microglial cells in routinely processed normal and non-neoplastic tissues, whereas due to the unexpected immunoreactivities results obtained with neoplastic tissues should be carefully interpreted.