Purification of 1F5 antigen that prevents complement attack on homologous cell membranes.

A mAb, 1F5, has the ability to cause hemolysis by human serum of human E treated with neuraminidase via the alternative C pathway. By Western blotting, this mAb reacts with a glycoprotein having a molecular mass of 20 kDa (1F5Ag). 1F5Ag was isolated from human E by affinity chromatography with mAb-coupled Sepharose. Purified 1F5Ag was then adsorbed to guinea pig E rendering them resistant to human C attack by both the classical and the alternative pathways. Furthermore, experiments with isolated C components revealed that 1F5Ag interferes with both homologous human C8 and C9 in the terminal stage of the C reaction, whereas it has little effect on hemolysis by rabbit C8 and C9. Therefore, 1F5Ag can be called HRF20, which stands for homologous restriction factor of 20 kDa.