Activated endothelial cells can directly participate in immune responses by interacting with immunocompetent cells via class II MHC proteins. We show here that, after induction of MHC class II molecule expression by IFN-gamma, rat brain endothelial cells responded to MHC class II ligands, anti-MHC class II Abs, or superantigens by expression of IL-6 transcript and IL-6 secretion. This response was not affected by protein kinase C depletion but was mimicked by the cAMP-elevating agent forskolin and completely blocked by H89, an inhibitor of cAMP-dependent protein kinase (PKA). Involvement of a cAMP/PKA signaling pathway in response to MHC class II ligands was further demonstrated by measure of a dose-dependent increase in cAMP level and phosphorylation of the transcription factor cAMP response element-binding protein (CREB). Our results indicate that MHC class II engagement in brain endothelial cells is directly coupled to IL-6 production via a cAMP/PKA-dependent intracellular pathway.