Immunohistochemical localization of the terminal C5b-9 complement complex in human aortic fibrous plaque

The terminal C5b-9 complex of the complement system was localized in 26 aortic, 3 iliac and 4 femoral human fibrous plaques using indirect immunofluorescence and immunoperoxidase. IgG, IgA, IgM, Clq, C3c, C4, C9 and fibrinogen were investigated simultaneously. All the fibrous plaques presented C5b-9 deposits appearing like thin threads in the fibrous cap and masses and spots in the amorphous areas. The extent and intensity were in agreement with the size of the fibrous plaques. The intimal thickenings presented less intense deposits which were absent in atherosclerosis-free samples. The C5b-9 deposits were frequently associated with immunoglobulins and complement components in the same areas. Whereas the demonstration of complement components reflected only a nonspecific trapping, the presence of assembled C5b-9 in the damaged tissues is more indicative of the involvement of complement activation in the tissue injury. The absence of C5b-9 in the atherosclerosis-free intima and its presence at lower intensity in the intimal thickenings than in the fibrous plaques suggest a pathogenic involvement in the chronic progression of the atherosclerotic lesion.