In vitro-staining specificity of the antibody 5-D-4 for microglia but not for monocytes and macrophages indicates that microglia are a unique subgroup of the myelomonocytic lineage.

We have previously shown that (i) the ramified phenotype and (ii) the microglia-specific pattern of membrane currents are induced not only in microglia, but also in monocytes and macrophages if they are cultured in the presence of astrocytes. These findings indicated that microglia are not a separate type of cell of the myelomonocytic lineage, but are induced to take on their unique characteristics by astrocytes. Recently, it was discovered that the antibody 5-D-4 selectively stains ramified microglia in situ. We therefore studied the influence of astrocytes and other epithelial cells on the expression of the keratan sulfate epitope recognized by 5-D-4 in microglia and other myelomonocytic cells. Our findings show that this antigen is exclusively expressed in microglia only if they are induced to ramify by coculture with either astrocytes or epithelial cells. By contrast monocytes and macrophages, even if induced to take on the ramified phenotype do not stain positive with 5-D-4. These findings indicate (i) that 5-D-4 is a specific marker for ramified microglia in vitro, and (ii) that microglia are a separate class of myelomonocytic cells, distinct from monocytes and macrophages.