EFFICACY OF TUMOR-CELL VACCINE AFTER INCORPORATING MONOPHOSPHORYL LIPID-A (MPL) IN TUMOR-CELL MEMBRANES CONTAINING TURNER-ASSOCIATED GANGLIOSIDE

Murine B16 melanoma expresses the ganglioside GM(3). GM(3) shed from tumor cells is immunosuppressive and promotes tumor growth(1). Reduction or elimination of the shed GM(3) could be therapeutic, and the anti-GM(3) antibodies may reduce and clear the shed ganglioside. To test this hypothesis, mice were challenged with tumor cells, with or without inducing anti-GM(3) antibody response. Since gangliosides are poor immunogens and T-cell independent antigens, an adjuvant (monophosphoryl lipid A (MPL), a non-toxic lipid A of Salmonella), directed against B-cells, was employed. MPL was incorporated onto liposomes and into the surface membrane of B16 mouse melanoma cells; both are rich in GM(3). C57BL/6J mice immunized with MPL-liposomes or MPL-B16 cells responded with elevated levels of anti-GM(3) IgM. Non-immunized mice or mice immunized with B16 cells alone or ganglioside GM(3) alone (without MPL) elicited poor anti-GM(3) IgM response, confirming the GM(3)'s immunologic crypticity and MPL's immunopotentiating effect. MPL's immunopotentiating effect was improved by coupling it to melanoma cell membranes. C57BL/6J mice were immunized with irradiated B16 alone or MPL alone or MPL-conjugated irradiated B16. After three weekly immunizations, each mouse received a challenge dose of viable syngeneic B16. Neither MPL alone nor B16 alone had a significant effect on tumor growth or host survival; however, administration of MPL-conjugated B16 cells significantly prevented tumor growth and prolonged survival. Our results indicate that MPL-incorporated B16 cells augment the anti-GM(3) IgM response, which may reverse GM(3)-induced immunosuppression by eliminating tumor-derived GM(3), and restore immunocompetence.