An epitope common to gangliosides O-acetyl-GD3 and GD3 recognized by antibodies in melanoma patients after active specific immunotherapy.

GD3 is 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 GD3 neither purified nor in melanoma cell vaccine (MCV), 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 GD3, O-acetylated GD3 (O-AcGD3), a minor ganglioside expressed on human melanoma cells, and that the antibodies cross-reacted with GD3. Thin layer chromatographic immunostaining revealed that all of the sera containing antibodies against O-AcGD3 also reacted to GD3. None of the other sera responded only to GD3, although the MCV contained 7- to 12-fold higher GD3 than O-AcGD3. Furthermore, the antibody activity was completely abolished by absorption with animal erythrocytes expressing either O-acetyl disialogangliosides or GD3, indicating that the antibodies recognize an epitope commonly shared by GD3 and O-AcGD3. The antibodies bound only to the sialyloligosaccharide moiety but not to the ceramide portion of GD3 after endoglycosylceramidase treatment. The antibodies failed to bind to GD3 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 GD3 but not that to O-AcGD3, revealing that the glycerol side chain of the sialic acids in both GD3s was an important structure of the epitope. The binding of the antibodies to melanoma cell surface gangliosides was confirmed by an absorption with a GD3- and O-AcGD3-positive melanoma cell line. These results in the light of previous reports on the inability of GD3 to elicit immune response in humans suggest that anti-GD3 antibodies found in the melanoma patients were induced by immunization with O-AcGD3 and O-AcGD3 present in the MCV would serve as an antigen source for GD3-targeted active specific immunotherapy of melanoma.