Melanoma vaccines: The problems of local immunosuppression

The incidence of cutaneous melanoma in Europe is rising, and the disease is incurable once metastases occur. Because melanoma expresses antigens that can be specifically recognized by the immune system, and because this disease occasionally undergoes spontaneous regression mediated by anti-tumor immunity, a number of different melanoma vaccines have been developed and tested clinically. Although most such vaccines show efficacy in vitro and an ability to stimulate anti-melanoma immune responses in blood, they have proved disappointing in clinical practice. It has become increasingly clear that the interaction between melanoma and the immune system is determined locally, within the tumor or draining lymph nodes. It is now clear that melanoma cells have the ability to anergize the immune system by inducing an immunosuppressive microenvironment that may explain the inability of systemic vaccines to alter patient outcomes. This subversion of the immune system involves alteration of dendritic cell (DC) function by tumor-derived cytokines, leading to the generation of suppressive and regulatory T lymphocytes. Successful melanoma vaccination probably requires therapeutic neutralization of the immunosuppressive microenvironment, which will require greater understanding of the molecular mechanisms used by the tumor to promote immunosuppression. Nevertheless, if these problems can be overcome, it seems likely that the efficacy of melanoma vaccines could be greatly enhanced.