Although TNF-alpha and several products of the activated complement system (e.g., C3b, iC3b, and C5a) are known to modulate endothelial cell function in vitro, relatively little is known about the potential modulatory role of the membrane attack complex (MAC) in endothelial cell activation. Using an in vitro neutrophil-endothelial adhesion assay and a quantitative whole cell ELISA to measure endothelial E-selectin and intracellular adhesion molecule-1 (ICAM-1) expression, we examined the modulatory role of the MAC in TNF-alpha-induced neutrophil-endothelial cell adhesive interactions. Activation of quiescent human umbilical vein endothelial cells (HUVECs) with TNF-alpha results in a concentration-dependent increase in neutrophil adhesion measured at 4 h. Assembly of sublytic concentrations of the MAC on endothelial cells did not result in changes in neutrophil-HUVEC adhesion measured at 4 h. Activation of HUVECs with TNF-alpha followed by assembly of the MAC resulted in a marked increase in neutrophil binding as compared with that observed in cells treated with TNF-alpha alone. Blocking studies of mAb revealed that in either TNF-alpha-stimulated or TNF-alpha and MAC-activated endothelial cells enhanced neutrophil binding was nearly entirely attributable to E-selectin and ICAM-1. This conclusion was further supported by a whole-cell ELISA, which provided evidence that the MAC augments TNF-alpha-induced up-regulation of both E-selectin and ICAM-1. This study provides data that support the conclusion that the distal complement system (MAC) can enhance TNF-alpha-induced proinflammatory endothelial cell functions.