The carbohydrate epitope 3-fucosyl-N-acetyllactosamine (CD15) is involved in cell-to-cell recognition processes in various tissues. In the CNS of the adult rat, immunoreactivity for CD15 reveals a region-specific distribution pattern by light microscopy. In the present study we investigated the ultrastructural localization of CD15 in the rat brain using preembedding immunocytochemical methods. In addition we studied CD15 expression in cultured astrocytes from optic nerves of 11-day-old rats. In optic nerve sections, immunostaining was found on the surface of astrocytes at various contact sites, i.e. astrocyte-astrocyte, astrocyte-oligodendrocyte, astrocyte-axon myelin, and astrocyte-blood vessel contacts. Oligodendrocyte-oligodendrocyte contacts, however, were always negative. In the telencephalic cortex, CD15 immunoreactivity was found in glial cell processes around synapses and in the cerebellar cortex in Bergmann glial cells. In astrocytes grown in serum-containing medium, CD15 was expressed on the surface of fibroblast-like glial fibrillary acidic protein-positive astrocytes, which were identified as type 1 astrocytes as well as on process-bearing A2B5-positive cells, representing type 2 astrocytes. The present data support the assumption that in the adult rodent brain, CD15 is exclusively expressed by astrocytes. The in vivo distribution of this carbohydrate molecule on distinct astroglial contact sites supports the notion that CD15 could act in cell-to-cell recognition processes.
