Interactions of Opc-expressing Neisseria meningitidis with polarized and non-polarized human umbilical vein endothelial cells (Huvecs) were investigated. Metabolic inhibitors and cytochalasin D treatment showed that host cellular and cytoskeletal functions were important for Opc-expressing bacterial association with Huvecs at the apical surface. In addition, this interaction required the presence of serum in the incubation medium whilst association with nonpolarized cells did not require serum. Pre-exposure of Opc-expressing bacteria to serum was sufficient to increase the number of bacterial interactions at the apical surface; B306, a monoclonal antibody (mAb) against Opc, inhibited these interactions, suggesting that Ope binds to serum factor(s) and this in turn increases adherence to Huvecs. The receptors involved in this ‘sandwich’ adherence belong to the integrin family since the interaction was inhibited by peptides containing the amino acid sequence arginine-glycine-aspartic acid (RGD) and the tetrapeptide RGDS (but not the peptide RGES) was inhibitory. Non-polarized cells appeared to expose receptors/sites that bound to Opc-expressing bacteria directly, did not require serum factors and were not inhibited by RGD-containing peptides. Serum-dependent interactions of Opc-expressing bacteria to apical surface was inhibited significantly by severai mAbs against avβ3 integrins. Some mAbs against α5 and β1 caused partial inhibition; antibodies that did not block the function of β1 integrins or the mAbs against α2 integrins were not inhibitory to bacterial interactions with Huvecs. Purified vitronectin supported adherence of Opc-expressing bacteria to Huvecs but not of Opc-bacteria. These interactions were inhibited by mAb B306 against Opc, by RGDS peptides as well as by blocking antibodies directed against αvβ-3 but not antibodies against other integrins. These data suggest that a sequence of molecular events resulting in trimolecular complexes at the endothelial surface may drive neisserial invasion of Huvesc. The expression of Opc appears to enable bacteria to utilize the normal signal-transduction mechanism of host cells via ligands in sera that adhere to endothelial cell integrins.