To study the mechanisms involved in the pathogenesis of the blood- brain barrier (BBB) breakdown in autoimmune demyelinating diseases, such as experimental allergic encephalomyelitis (EAE), we investigated the cell interactionin vitrobetween myelin basic protein (MBP)- specific encephalitogenic T cells and normal and inflamed cerebral endothelial cells, and the cytotoxic effect of antigen specific T cell lines on normal and inflamed cerebral endothelial cells. The importance of relationship between cell surface adhesion and cytotoxic T lymphocyte (CTL) was examined by monoclonal antibodies (mAb) against adhesion receptors. The adhesion of encephalitogenic T cells to inflamed endothelial cells was significantly increased as compared with normal endothelial cells (P 0.001). The percentage lysis of inflamed endothelial target cells was significantly increased by incubation with MBP-encephalitogenic T cell lines in the presence of MBP as compared with those of normal endothelial targets (P<0.0001). Intercellular adhesion molecule-1 (ICAM-1) is not involved in T cell adhesion to endothelial cells or cytotoxic endothelial cell lysis. Antibodies against human a 4 integrin (HP 2/1) andβ1 (Al 1B2) inhibited T cell adhesion, but did not block cytotoxic endothelial cell lysis.These results indicate that T cell adhesion to inflamed cerebral endothelial cells and cytotoxicity of T cells for cerebral endothelial cells may play a central role in the breakdown of the BBB and development of inflammatory lesions in the central nervous system(CNS).