Astrocytes inhibit microglial surface expression of dendritic cell‐related co‐stimulatory molecules through a contact‐mediated process

Murine microglia cultured in isolation were treated sequentially with granulocyte/monocyte colony‐stimulating factor (GM‐CSF) (5 days) and lipopolysaccharide (LPS) (2 days) to elicit a mature dendritic cell‐like (DC‐like) phenotype. Examined by flow cytometry microglia thus isolated show high surface expression of CD11c together with the co‐stimulatory molecules CD40, CD80, and CD86 that are necessary for T‐cell activation. In contrast, microglia co‐cultured with astrocytes fail to achieve a mature DC‐like phenotype. Contact with the astrocytic environment is necessary for the inhibition. Failure was not because of a more rapid degradation of protein. Bone marrow‐derived cells, like microglia, were prevented by astrocytes from attaining a mature DC phenotype. Although GM‐CSF pre‐treatment substantially increases mRNA of co‐stimulatory molecules and major histocompatibility complex (MHC) Class II in isolated microglia, co‐cultured microglia await treatment with LPS to up‐regulate them. In contrast, western blot and immunocytochemical analysis revealed that it is not a failure of transcription or translation, nor is it a more rapid degradation of mRNA that is responsible for the low surface expression; rather microglia co‐cultured with astrocytes produce mRNA and protein but do not traffic the protein onto the cell surface.