An Epitope Common to Gangliosides 0-Acetyl-GD3 and GD3Recognized by Antibodies in Melanoma Patients after Active Specific Immunotherapy1

GDIis a major ganglioside of human melanoma and was shown to be an effective target for passive immunotherapy with murine monoclonal antibodies. It was noted earlier that G,,.,neither purified nor in melanoma cell vaccine (MC V), could elicit an antibody response in melanoma patients. In this study, we demonstrate that melanoma patients who received MCV had autoantibodies against a derivative of '.•„>, 0-acetylated GDI(6>-Ac(;,,,), a minor ganglioside expressed on human melanoma cells, and that the antibodies cross-reacted with Gnl. Thin layer Chro matographie immunostaining revealed that all of the sera containing antibodies against O-AcGoj also reacted to G,,,. None of the other sera responded only to Gm, although the MCV contained 7to 12-fold higher GDJ than 0-AcGM. Furthermore, the antibody activity was completely abolished by absorption with animal erythrocytes expressing either ()acetyl disialogangliosides or GDI, indicating that the antibodies recognize an epitope commonly shared by (.,,, and O-\cGm. The antibodies bound only to the sialyloligosaccharide moiety but not to the ceramide portion of GDI after endoglycosylceramidase treatment. The antibodies failed to bind to Gi)t after neuraminidase treatment. These results indicate that the sialyloligosaccharides of the gangliosides are important components of the epitope. Periodate oxidation abolished reactivity of the antibodies to GD.Ibut not that to 0-AcGDj, revealing that the glycerol side chain of the sialic acids in both Gi)ts was an important structure of the epitope. The binding of the antibodies to melanoma cell surface gangliosides was confirmed by an absorption with a GDI-and 0-AcGDJ-positive melanoma cell line. These results in the light of previous reports on the inability of GDIto elicit immune response in humans suggest that ¡iiiti-CDIantibodies found in the melanoma patients were induced by immunization with OAcGoj and 0-AcGD.i present in the MCV would serve as an antigen source for Goj-targeted active specific immunotherapy of melanoma.