In the previous paper, we reported that Neocarzinostatin (NCS) could bind covalently to rabbit anti-tumor IgG (Immune IgG) without losing its pharmacological activity. In this study, the complement dependent cytotoxicity and capping inhibition of the conjugate were evaluated. When antibody activity was measured by (3)H-TdR uptake inhibition and (51)Cr release tests in the presence of human complement, the cytotoxicity of the conjugate was significantly reduced in contrast to the antibody binding activity tested by immunofluorescence. In addition, an anti-complement immunofluorescent test showed that NCS-immune IgG fixed less complement on NALL-1 cells than immune IgG. These results indicate that complement fixing sites of NCS-immune IgG are damaged during chemical conjugation. The capping experiment demonstrated that NCS-immune IgG interfered with the capping of antigens on the NALL-1 cell surface, and that the capping inhibiting activity was almost the same degree as that of free NCS. This unique advantage of NCS-immune IgG may be useful for inhibiting the immunological escape of tumor cells from antibody attack in vivo.
Complement dependent cytotoxicity