To determine whether nuclear factor (NF)-kappaB is necessary to confer endothelial cell responsiveness to interferon (INF)-gamma in terms of vascular cell adhesion molecule (VCAM)-1 expression and leukocyte adhesion, human endothelial cells were treated with IFN-gamma in the presence of low concentrations (LCs) of interleukin (IL)-1alpha (</=100 pg/mL), which activates NF-kappaB but does not induce VCAM-1 expression. Although IFN-gamma induced major histocompatibility complex class II antigen expression and although a high concentration of IL-1alpha (10 ng/mL) induced leukocyte adhesion and VCAM-1 expression, neither IFN-gamma nor LC IL-1alpha was able to induce VCAM-1 expression or leukocyte adhesion. However, the combination of IFN-gamma and LC IL-1alpha induced VCAM-1 expression and increased leukocyte adhesion (67% and 49% of high-concentration IL-1alpha, respectively). Electrophoretic mobility shift assays and immunoblotting of nuclear extracts showed that IFN-gamma activated signal transducers and activators of transcription (STAT)-1alpha and interferon regulatory factor (IRF)-1 but not NF-kappaB, whereas LC IL-1alpha activated NF-kappaB but not STAT-1alpha or IRF-1. Nuclear run-on studies showed that LC IL-1alpha is necessary but not sufficient for inducing VCAM-1 gene transcription and that the combination of IFN-gamma and LC IL-1alpha is required for full VCAM-1 gene transcription. These findings suggest that factors that activate NF-kappaB can synergize with IFN-gamma in promoting endothelial-leukocyte interaction.