A melanosomal membrane protein is a cell surface target for melanoma therapy

Differentiation antigens on cancer cells are recognized by the immune system. A prototype set of these autoantigens in melanoma cells are the melanosomal glycoproteins, expressed in both melanomas and normal melanocytes. These are intracellular proteins that can be recognized by both antibodies and T lymphocytes. While one can understand how T cells can respond to intracellular proteins, based on cellular requirements for antigen processing and presentation, it is more difficult to understand how antibody responses to melanosomal proteins could lead to tumor rejection. We demonstrate that gp75 is expressed on the cell surface as well as intracellularly in human and mouse melanomas. The surface expression of gp75 can be augmented by IFN-gamma and during tumor growth in vivo. Surface expression of gp75 on mouse melanoma cells correlates with the ability of a monoclonal antibody (mAb) against gp75 to reject melanomas in syngeneic mice. Antibody-mediated rejection seems to require the Fc portion of the antibody, suggesting a role for Fc receptor-positive effector cells such as natural killer cells. However, although NK1.1(+) cells have been implicated in antibody-induced rejection in vivo, cell surface expression of gp75(+) on melanoma does not lead to susceptibility to antibody-dependent cellular cytotoxicity in vitro. The mAb to gp75 induced tumor rejection in mice carrying both scid and bg/bg traits, showing that neither thymus-dependent T cells nor natural killer cytotoxic activity was required in vivo. Long-term treatment of mice with mAb led to patchy depigmentation in the coat. In summary, an intracellular organellar protein can be expressed at the cell surface and provide an antigenic target for antibody therapy and autoimmunity.