Endothelial cell–astrocyte interactions and TGFβ are required for induction of blood–neural barrier properties

We sought to establish a blood–neural barrier (BNB) model of astrocyte contact with endothelial cells (EC) to test the hypothesis that transforming growth factor β (TGFβ) promotes an EC barrier-phenotype. Astrocyte–EC contact induced BNB properties in EC. Transendothelial resistance was augmented by direct contact between astrocytes–EC, but not by astrocyte-conditioned medium or astrocyte–EC coculture conditioned medium. Coculture of EC and astrocytes led to significant increase in endothelial occludin levels and junctional localization. EC γ-glutamyl-transferase (GGT) activity was increased by direct contact with astrocytes, by conditioned medium from cocultures or by TGFβ1. Coculture inhibited EC proliferation with no effect on astrocyte proliferation. A neutralizing antibody to TGFβ decreased GGT activity in cocultures and increased cell number. Whereas total TGFβ was not significantly altered by coculture, activated TGFβ increased in astrocyte–EC cocultures. In summary, astrocyte–EC contact induces BNB characteristics in EC and locally activated TGFβ is responsible for part of the induction.