Characterization Of An Antibody Idiotope Specific To Ganglioside GD3. Stephen V. Evans*, Marcin J. Kaminski*, Marilyn J. Mooibroek*, Alan N. Houghton+[daggerdbl] and Paul B. Chapman+[daggerdbl]. *Department of Biochemistry, University of Ottawa, Ottawa K1H 8M5 Canada. +Immunology Program and Department of Medicine Memorial Sloan-Kettering Cancer Center, New York, NY, 10021. [daggerdbl]Cornell University Medical College, New York, NY, 10021.

The use of monoclonal antibodies specific to oligosaccharide antigens is an increasingly important therapy for some forms of cancer. One antigen in particular, the liposaccharide disialoganglioside GD3, is displayed on the surface of most melanomas, soft tissue sarcomas and tumors of neuroectodermal origin, while normal tissues display little or no GD3. The monoclonal antibody R24 is specific for ganglioside GD3, and human trials have shown it to induce tumor regression in some cases. The Fab (antigen-binding fragment) of R24 was generated by controlled papain digestion of the intact antibody, purified by ion exchange chromatography, and crystallized. X-ray diffraction data was collected to 2.5 Å resolution. The structure was solved in space group C2, and refined using all data with I Æ 3&(I) to a final R-factor of 18.2%. The binding surface of the antibody displays a cleft formed by the three complementarity determining regions on the heavy chain. This cleft serves to recognize and to bind the terminal sialic acid residue of GD3. The crystal structure provides a basis for understanding the immunolgical behaviour of R24 and R24 mutant antibodies.