Elucidating the Autoimmune and Antitumor Effector Mechanisms of a Treatment Based on Cytotoxic T Lymphocyte Antigen-4 Blockade in Combination with a B16 Melanoma Vaccine

We have previously shown that small B16 melanomas can be successfully treated using a combination of anti–cytotoxic T lymphocyte antigen (CTLA)-4 monoclonal antibody with a granulocyte/macrophage colony-stimulating factor (GM-CSF) producing irradiated tumor cell vaccine. Regression of tumors results in long-lasting immunity and is frequently accompanied by autoimmune depigmentation. Here we examine the cellular and molecular mechanisms of this combined treatment. Histological examination of depigmented lesions revealed infiltration of polymorphonuclear cells and deposition of antibody. The combination therapy also induced tumor rejection and skin depigmentation in B cell–deficient and in CD4+ T cell–depleted mice. Both effects of the treatment absolutely required CD8+ T cells. Analysis of the response in successfully treated mice revealed elevated levels of CD8+ T cells specific for a nonameric peptide consisting of residues 180–188 of the melanocyte differentiation antigen tyrosinase-related protein (TRP)2. There was no evidence of reactivity to the melanocyte antigens gp100, tyrosinase, Mart1/MelanA, or TRP1. Fas–FasL interactions and perforin played a role in mounting the effect