Analysis of complement C3 activation products in human atherosclerotic lesions

Cleavage of the complement C3 protein is essential for complement activation. Saline extracts of human atherosclerotic lesions were examined by various techniques for the presence of C3 cleavage fragments. Crossed intermediate gel immunoelectrophoresis revealed that native C3 was the predominate C3 protein in extracts and that the C3dg fragment was also detected. SDS-PAGE/Western blot analyses of lesion extracts employing monoclonal antibodies directed at C3c and C3dg fragment determinants demonstrated molecular weight bands corresponding to the known molecular weights of all the physiologic C3 cleavage fragments, except C3b which is known to have a short half-life. After C3, the two most common fragments observed were C3c and C3dg. No bands other than those corresponding to known C3 cleavage fragments were observed and control antibody stains were always negative. In some blots bands with a greater molecular mass than C3 were evident, indicating that some of the C3 in lesions may be covalently bound to an activator. We have previously identified a large (100-500 nm) nonapoprotein containing lipid particle (LCA) as a major complement activating structure in human atherosclerotic lesions. Fractionation of lesion extracts by molecular sieve chromatography and sucrose density gradient centrifugation failed to reveal a concordance between LCA and C3 antigens. The results indicate that complement activation, i.e. C3 convertase formation, takes place in human atherosclerotic lesions and that activated C3 is degraded according to normal complement regulatory mechanisms.