Complement killing of human neuroblastoma cells: a cytotoxic monoclonal antibody and its F(ab')2-cobra venom factor conjugate are equally cytotoxic.

Only a few monoclonal antibodies mediate complement lysis of tumor cells, but for several antibodies it has been demonstrated that a complement-activating function can be introduced by covalent coupling of cobra venom factor (CVF), a non-toxic glycoprotein which is a structural and functional homologue of human complement component C3. In this study we compared the efficacy of complement killing of human neuroblastoma cells by the complement-activating monoclonal antibody 3F8 directed against the GD2 ganglioside antigen with that of its F(ab')2-CVF conjugate. At equal numbers bound per cell the 3F8 antibody and the 3F8 F(ab')2-CVF conjugate were found to be equally cytotoxic in the presence of complement from several species including human. Maximal killing reached up to 98%. The kinetics of killing and the bivalent metal requirement confirmed that the cytotoxic activity of the 3F8 antibody is mediated via the classical pathway and that of the 3F8 F(ab')2-CVF conjugate via the alternative pathway. To achieve a comparable degree of killing, an approximately eight-fold higher concentration of the 3F8 F(ab')2-CVF conjugate was required which appears to be a consequence of the approximately eight-fold lower binding activity of the 3F8 F(ab')2-CVF conjugate compared to the intact 3F8 antibody. Our data suggest that the coupling of CVF to non-cytotoxic antibodies allows the generation of conjugates with a cytotoxic activity similar to that of inherently cytotoxic antibodies.