Characterization of IgG and IgM Antibodies Induced in Melanoma Patients by Immunization with Purified Gm2 Ganglioside

The ganglioside Gm2 is a differentiation antigen expressed on the cell surface of human malignant melanomas and other cancers of neuroectodermal origin. We have previously reported that immunization with purified Gm2 combined with Bacillus Calmette-Guerin as adjuvant and pretreatment with low-dose cyclophosphamide induced production of antibodies against Gm2 in five of six patients. We have now extended our study and analyzed the induced antibody response in greater detail. Prior to immunization, high-titer antibodies against Gm2 were not detected. After immunization, high-titer IgM antibodies were induced in 17 of 24 patients, and high-titer IgG antibodies in eight cases. Additional treatment of 12 patients with cimetidine, a histamine H2 receptor antagonist reported to have antisuppressor cell activity, had no effect on Gm2 antibody titers. Antibodies against asialo-Gm2 were present in all patients, and antibodies against Gm1 were present in 33% of patients, before and after immunization. Antibodies induced by immunization were specific for Gm2, though some reacted predominantly with N -acetylneuraminic acid Gm2 (Gm2), and some reacted with Gm2 and N -glycolylneuraminic acid Gm2(NeuGcGm2). The pattern of reactivity with Gm2 is consistent with the response to T-cell-independent antigens: both IgM and IgG antibody responses against Gm2 were short lived; peak titers seen after initial and secondary vaccinations were similar; and delayed-type hypersensitivity responses to skin test challenges with Gm2 were not detected in any patients. However, the IgG response typed as predominantly IgG1 and IgG3, not IgG2 as might be expected for carbohydrate antigens (which are generally T-cell-independent antigens). Because IgG1 and IgG3 antibody responses are usually to T-cell-dependent antigens, the humoral immune response elicited by Gm2 vaccination has both T-cell-dependent and T-cell-independent characteristics. These IgM and IgG responses against this neuroectodermal differentiation antigen expressed on melanoma cells have been induced without evidence of neurological or other toxicity.