TUMOR-CELL REACTIVITY MEDIATED BY IGM ANTIBODIES IN SERA FROM MELANOMA PATIENTS VACCINATED WITH GM2 GANGLIOSIDE COVALENTLY-LINKED TO KLH ISINCREASED BY IGG ANTIBODIES

Natural IgM antibodies against the melanoma cell-surface ganglioside G M2, and IgM antibodies induced by vaccination with GM2 adherent to bac illus Calmette-Guerin, have been correlated with increased disease-fre e and overall survival in melanoma patients in previous phase I and II clinical trials. A vaccine containing GM2 covalently attached to keyh ole limpet hemocyanin (KLH) plus the immunological adjuvant QS-21 now induces higher-titer, longer-lasting IgM antibodies against GM2 and ha s recently entered phase III clinical trials. For the first time this new vaccine also induces IgG antibodies against GM2 in the majority of immunized patients. With regard to immunity against bacteria, IgM ant ibodies have been described to be 1000-fold more effective than IgG an tibodies at opsonification, complement-mediated cytotoxicity and prote ction from bacterial challenge. Though IgG antibodies have the theoret ical advantage of being able to mediate antibody-directed cell-mediate d cytotoxicity (ADCC), they may inhibit complement mediated IgM effect or mechanisms against melanoma cells. Our goal was to confirm the func tional characteristics of the anti-GM2 IgM and IgG antibodies induced by vaccination and to determine the impact that IgG antibodies might h ave on IgM antibody reactivity with GM2-positive tumor cells. Post-imm unization sera from seven immunized patients were separated by size-ex clusion chromatography into IgM and IgG fractions and a variety of ser ological assays were performed with the individual fractions and their combinations. Assays identifying specific IgM or IgG reactivity demon strated partial inhibition by the opposite fraction. However, when the endpoint was complement-mediated lysis or overall antibody binding, w hich may more faithfully predict in vivo complement-mediated opsonific ation and lysis, the combinations of IgM and IgG fractions consistentl y demonstrated higher reactivity than either fraction alone. In additi on, ADCC was induced in all seven patients. The results were the same whether the sera were obtained after 2 months or 2 years of immunizati ons. These findings suggest that IgG antibodies induced by the GM2-KLH plus QS-21 vaccine will not inhibit and should further augment the cl inical impact of induced IgM antibodies.