The membrane attack complex of complement alters the membrane integrity of cultured endothelial cells: A possible pathophysiology for immune complex vasculitis

Recently, the possibility of direct involvement of the membrane attack complex of complement in the endothelial damage of immune complex vasculitis has been pointed out. However, no studies have so far elucidated this mechanism. The present study investigated the effects of complement on the membrane integrity of endothelial cells, using the fluorescein diacetate and ethidium bromide staining method. Cultured human umbilical vein endothelial cells were maintained in medium containing 10% zymosan-activated normal human serum. Cell detachment began to occur after 3 h of incubation, and the number of fluorescein diacetate-positive adherent cells decreased significantly, whereas that of ethidium bromide-positive detached cells increased significantly. Heat inactivation of the serum or replacement of the complement source with non-activated normal human serum or C5-, C7- or C9-deficient serum resulted in complete inhibition of these effects. These results suggest that complement induces detachment of endothelial cells by altering the cell membrane integrity and support the contention that the membrane attack complex of complement plays a significant role in the mechanisms of endothelial cell damage in immune complex vasculitis.