Phosphotyrosine antibodies specifically label ameboid microglia in vitro and ramified microglia in vivo.

Using an affinity-purified, polyclonal antibody to phosphotyrosine (Wang: Molecular and Cellular Biology 5:3640-3643, 1985) we have previously demonstrated that phosphotyrosine immunoreactivity is restricted to a population of multipolar GFAP-negative neuroglia in adult rat brain (Tillotson and Wood: Journal of Comparative Neurology 282:133-141, 1989) and retina (Tillotson and Wood: Journal of Cell Biology 107:724a, 1988). In this study, we show that the phosphotyrosine-immunoreactive cells are microglia. This conclusion is supported by numerous morphological and ultrastructural similarities between the phosphotyrosine-immunoreactive cells and microglia. Furthermore, phosphotyrosine co-localizes with the microglial-specific B4 isolectin of Bandeiraea simplifolia-1 lectin. Phosphotyrosine antibodies also stain ameboid microglia in primary cultures of neonatal rat brain. In addition, after 7 days in vitro, microglia are the only phosphotyrosine-immunoreactive element in the cultures. This temporal pattern of staining in vitro mimics the developmental progression of phosphotyrosine immunoreactivity in situ, in which a variety of structures stain during postnatal neural development (Tillotson and Wood: Journal of Comparative Neurology 282:133-141, 1989), but only microglia stain in mature brain. The significance of phosphotyrosine-containing proteins potentially expressed in a microglial-specific manner is discussed.