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Effective tumor immunity requires recognition of tumor cells coupled with the activation of host effector responses. Fc receptor (FcR) gamma-/- mice, which lack the activating Fc gamma R types I and III, did not demonstrate protective tumor immunity in models of passive and active immunization against a relevant tumor differentiation antigen, the brown locus protein gp75. In wild-type mice, passive immunization with mAb against gp75 or active immunization against gp75 prevented the development of lung metastases. This protective response was completely abolished in FcR gamma-deficient mice. Immune responses were intact in gamma-/- mice because IgG titers against gp75 develop normally in gamma-/- mice immunized with gp75. However, uncoupling of the Fc gamma R effector pathway from antibody recognition of tumor antigens resulted in a loss of protection against tumor challenge. These data demonstrate an unexpected and critical role for FcRs in mediating tumor cytotoxicity in vivo and suggest that enhancement of Fc gamma R-mediated antibody-dependent cellular cytotoxicity by inflammatory cells is a key step in the development of effective tumor immunotherapeutics.

Recognition of self is emerging as a theme for the immune recognition of human cancer. One question is whether the immune system can actively respond to normal tissue autoantigens expressed by cancer cells. A second but related question is whether immune recognition of tissue autoantigens can actually induce tumor rejection. To address these issues, a mouse model was developed to investigate immune responses to a melanocyte differentiation antigen, tyrosinase-related protein 1 (or gp75), which is the product of the brown locus. In mice, immunization with purified syngeneic gp75 or syngeneic cells expressing gp75 failed to elicit antibody or cytotoxic T-cell responses to gp75, even when different immune adjuvants and cytokines were included. However, immunization with altered sources of gp75 antigen, in the form of either syngeneic gp75 expressed in insect cells or human gp75, elicited autoantibodies to gp75. Immunized mice rejected metastatic melanomas and developed patchy depigmentation in their coats. These studies support a model of tolerance maintained to a melanocyte differentiation antigen where tolerance can be broken by presenting sources of altered antigen (e.g., homologous xenogeneic protein or protein expressed in insect cells). Immune responses induced with these sources of altered antigen reacted with various processed forms of native, syngeneic protein and could induce both tumor rejection and autoimmunity.

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.

The immune system can recognize differentiation antigens that are selectively expressed on malignant cells and their normal cell counterparts. However, it is uncertain whether immunity to differentiation antigens can effectively lead to tumor rejection. The mouse brown locus protein, gp75 or tyrosinase-related protein 1, is a melanocyte differentiation antigen expressed by melanomas and normal melanocytes. The gp75 antigen is recognized by autoantibodies and autoreactive T cells in persons with melanoma. To model autoimmunity against a melanocyte differentiation antigen, mouse antibodies against gp75 were passively transferred into tumor-bearing mice. Passive immunization with a mouse monoclonal antibody against gp75 induced protection and rejection of both subcutaneous tumors and lung metastases in syngeneic C57BL/6 mice, including established tumors. Passive immunity produced coat color alterations but only in regenerating hairs. This system provides a model for autoimmune vitiligo and shows that immune responses to melanocyte differentiation antigens can influence mouse coat color. Immune recognition of a melanocyte differentiation antigen can reject tumors, providing a basis for targeting tissue autoantigens expressed on cancer.