Background: Normal human cells resist the lytic activity of homologous complement (C) by expressing inhibitory molecules on their cell membranes. Recently, it has become increasingly evident that information on C inhibitors on malignant tumor cells is crucial before considering any immunotherapeutic attempts with C-activating antibodies. As one of the most potent inhibitors of C lysis is protectin (CD59), we have examined its expression and function on human breast cancer cells.
Experimental design: Immunofluorescence microscopy was used to detect protectin expression on solid breast tumor samples (N = 12). Using immunoaffinity chromatography, protectin was isolated from the membranes of cultured MCF7 and T47D breast cancer cells. The purified proteins were incorporated into heterologous cells to study their C inhibitory activities. The reactivity of tumor cell protectins with terminal C complexes was examined by sucrose density ultracentrifugation analysis. A chromium release assay was used to study the effects of protectin neutralization on the sensitivity of MCF7 and T47D cells to C-mediated cytotoxicity.
Results: Protectin was found to be strongly expressed by all human breast cancer tumors examined. The affinity-purified protectins had a glycophosphoinositollipid anchor and migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as glycosylated smears of 19 to 25 kilodaltons. Protectin isolated from T47D cells bound to nascent C5b-9 complexes generated in human sera and inhibited C lysis of guinea pig erythrocytes when incorporated into their cell membranes. C-mediated killing of breast cancer cells could be significantly enhanced after treatment of the cells with F(ab')2 fragments of the anti-protectin monoclonal antibody YTH53.1.
Conclusions: Human breast cancer cells resist C membrane attack by expressing protectin on their cell membranes. Neutralization of protectin on the surface of the tumor cells increases their sensitivity to C lysis.