Dendritic cell vaccines in melanoma: from promise to proof?

Dendritic cells (DC) are the directors of the immune system, capable of inducing tumour antigen-specific T- and B-cell responses. As such, they are currently applied in clinical studies in cancer patients. Early small clinical trials showed promising results, with frequent induction of anti-cancer immune reactivity and clinical responses. In recent years, additional trials have been carried out in melanoma patients, and although immunological responses are often reported, objective clinical responses remain anecdotal with objective response rates not exceeding 5-10%. Thus, DC vaccination research has now entered a stage in between 'proof of principle' and 'proof of efficacy' trials. Crucial questions to answer at this moment are why the clinical responses remain scarce and what can be done to improve the efficacy of vaccination. The answers to these questions probably lie in the preparation and administration of the DC vaccines. Predominantly, cytokine-matured DC are used in clinical studies, while from preclinical studies it is evident that DC that are activated by pathogen-associated molecules are much more potent T cell activators. For sake of easy accessibility monocyte-derived DC are often used, but are these cells also the most potent type of DC? Other yet unsettled issues include the optimal antigen-loading strategy and route of administration. In addition, trials are needed to investigate the value of manipulating tolerizing mechanisms, such as depletion of regulatory T cells or blockade of the inhibitory T cell molecule CTLA-4. These issues need to be addressed in well-designed comparative clinical studies with biological endpoints in order to determine the optimal vaccine characteristics. DC vaccination can then be put to the ultimate test of randomized clinical trials. Here, we review the immunobiology of DC with emphasis on the different aspects that are most relevant for the induction of anti-tumour responses in vivo. The different variables in preparing and administering DC vaccines are discussed in this context and the immunological and clinical results of studies with DC vaccines in melanoma patients are summarized.