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: Comparison of prophylaxis and therapy

We have previously shown that small B16 melanomas can be successfully treat ed using a combination of anti-cytotoxic T lymphocyte antigen (CTLA)-4 mono clonal 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 depig mentation. Here we examine the cellular and molecular mechanisms of this co mbined treatment. Histological examination of depigmented lesions revealed infiltration of polymorphonuclear cells and deposition of antibody. The com bination therapy also induced tumor rejection and skin depigmentation in B cell-deficient and in CD4(+) T cell-depleted mice. Both effects of the trea tment absolutely required CD8(+) T cells. Analysis of the response in succe ssfully treated mice revealed elevated levels of CD8(+) T cells specific fo r a nonameric peptide consisting of residues 180-188 of the melanocyte diff erentiation antigen tyrosinase-related protein (TRP)2. There was no evidenc e of reactivity to the melanocyte antigens gp100, tyrosinase, Mart1/MelanA, or TRP1. Fas-FasL interactions and perforin played a role in mounting the effector response, whereas the tumor necrosis factor pathway was not requir ed. The cellular requirements for tumor rejection in this therapeutic setti ng were strikingly different from those in a prophylactic setting. In parti cular, if mice received a prophylactic vaccine consisting of anti-CTLA-4 an d B16-GM-CSF before tumor challenge, full protection was obtained even in t he absence of CD8(+) T cells. Our data demonstrate that therapeutic autorea ctive CD8(+) T cell responses can effectively be generated in tumor-bearing mice and stresses the value of studying tumor immunity in a therapeutic ra ther than a prophylactic setting.