EXPRESSION AND FUNCTION OF THE COMPLEMENT MEMBRANE ATTACK COMPLEX INHIBITOR PROTECTIN (CD59) ON HUMAN BREAST-CANCER CELLS

BACKGROUND: Normal human cells resist the lytic activity of homologous complement (C) by expressing inhibitory molecules on their cell membr anes. Recently, it has become increasingly evident that information on C inhibitors on malignant tumor cells is crucial before considering a ny immunotherapeutic attempts with C-activating antibodies. As one of the most potent inhibitors of C lysis is protectin (CD59), we have exa mined its expression and function on human breast cancer cells. EXPERI MENTAL DESIGN: Immunofluorescence microscopy was used to detect protec tin expression on solid breast tumor samples (N = 12). Using immunoaff inity chromatography, protectin was isolated from the membranes of cul tured MCF7 and T47D breast cancer cells. The purified proteins were in corporated into heterologous cells to study their C inhibitory activit ies. The reactivity of tumor cell protectins with terminal C complexes was examined by sucrose density ultracentrifugation analysis. A chrom ium release assay was used to study the effects of protectin neutraliz ation on the sensitivity of MCF7 and T47D cells to C-mediated cytotoxi city. RESULTS: Protectin was found to be strongly expressed by all hum an breast cancer tumors examined. The affinity-purified protectins had a glycophosphoinositollipid anchor and migrated in sodium dodecyl sul fate-polyacrylamide gel electrophoresis as glycosylated smears of 19 t o 25 kilodaltons. Protectin isolated from T47D cells bound to nascent C5b-9 complexes generated in human sera and inhibited C lysis of guine a pig erythrocytes when incorporated into their cell membranes. C-medi ated killing of breast cancer cells could be significantly enhanced af ter treatment of the cells with F(ab')(2) fragments of the anti-protec tin monoclonal antibody YTH53.1. CONCLUSIONS: Human breast cancer cell s resist C membrane attack by expressing protectin on their cell membr anes. Neutralization of protectin on the surface of the tumor cells in creases their sensitivity to C lysis.