The mechanism(s) of action responsible for the anti-inflammatory effects mediated by interferon (IFN)-beta are still elusive although suggestions include anti-viral effects, the enhancement of natural killer (NK) or suppressor T cell activity and opposition to the effects of inflammatory cytokines. As vascular endothelial cells are active participants in inflammatory and demyelinating processes, we decided to examine the effects of IFN-beta on the expression of major histocompatibility complex (MHC) gene products and intercellular adhesion molecule (ICAM)-1 on human vascular endothelial cells (ECs). Human umbilical ECs demonstrated constitutive expression of ICAM-1 and MHC class I molecules but did not express MHC class II molecules. Basal expression of ICAM-1 molecules was enhanced by TNF alpha and to a lesser extent by IFN-beta, but was not affected by IFN-gamma. MHC class I expression on ECs was enhanced by IFN-beta, IFN-gamma, and tumor necrosis factor (TNF)-alpha. Furthermore, a synergistic effect was observed to combinations of these interacting cytokines. Incubation of ECs with IFN-gamma, but not IFN-beta, induced class II expression in a dose dependent manner. Moreover, co-incubation of ECs with IFN-beta and IFN-gamma resulted in significant down-regulation of class II molecules expression which was directly dependent on IFN-beta concentration. Northern blot analysis of DR alpha and Beta 2-microglobulin mRNA expression suggested that cytokine-mediated regulation of MHC molecules is at the transcriptional level, while modulation of ICAM-1 expression appears to be at the transcriptional as well as post-transcriptional level. Thus, our study demonstrated that IFN-beta and interacting cytokines exert complex immunoregulatory effects on endothelial cells with differential modulatory effects on various cell surface markers. Understanding the biological significance of these immunomodulatory effects mediated by IFN-beta may have important implications for cytokine-based strategies in the treatment of inflammatory and autoimmune diseases.