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Light Chain Variants of an IgG3 Anti-G_D3 Monoclonal Antibody and the Relationship among Avidity, Effector Functions, Tumor Targeting, and Antitumor Activity¹

Department of Medicine and the Immunology Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021

R24 is an IgG3 mouse monoclonal antibody which recognizes the ganglioside G_D3. Two variants of R24, in which one (V2-R24) or both (V1-R24) light chains were substituted by MOPC-21 light chains, were isolated and characterized. R24 had a 40-fold higher avidity for G_D3 than either variant, suggesting that high avidity binding required the presence of two R24 light chains and, thus, divalency. R24 and both variants mediated antibody-dependent cellular cytotoxicity but antibody-dependent cellular cytotoxicity mediated by variants was weak compared to R24. The presence of at least one R24 light chain was required for complement-dependent cytotoxicity; complement-dependent cytotoxicity was mediated by R24 and weakly by V2-R24 but not by V1-R24. R24, but not V1-R24 or V2-R24, inhibited attachment of melanoma cells to plastic and activated T-lymphocytes, suggesting a threshold of avidity required for these biological effects. In a human melanoma xenograft model in nu/nu mice, radiolabeled R24, variants, and isotype-matched control monoclonal antibodies all appeared to localize in tumors (based on tumor:normal tissue ratios), but specific tumor targeting by R24 was generally 3- to 6-fold higher. R24 prevented melanoma outgrowth in nu/nu mice, while V2-R24 induced partial tumor protection. V1-R24 and the negative control monoclonal antibody did not inhibit tumor outgrowth. Antitumor activity of R24 corresponded to avidity and ability to mediate complement-dependent cytotoxicity in vitro.

¹ This research was supported by grants from the National Cancer Institute (CA 33049, CA 34079), the American Cancer Society (IM 333), the Louis and Anne Abrons Foundation, the Alcoa Foundation, the Charles A. Revson Foundation, and the Charles A. Dana Foundation.

² Supported by a National Cancer Institute Fellowship in Biomedical Research; recipient of the ASCO Young Investigator Award. To whom requests for reprints should be addressed.

³ Recipient of the Cancer Research Institute/Benjamin Jacobson Family Investigator Award.

Received 8/ 7/89. Revised 10/23/89. Accepted 11/29/89.

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