Abstract only 3040 Background: Vaccination with dendritic cells (DC) is still a valid experimental option for metastatic melanoma (MM). However, only a few patients experience long-lasting objective responses and the majority of clinical responders afterwards relapse and die. Which mechanisms are actually responsible for this “secondary resistance” to whole tumor antigens-loaded DC vaccines is largely unknown. It has been hypothesized that suppressive immune cell subpopulations, regulatory T cells in particular, may progressively accumulate in tumor tissues thus hampering therapy-induced antitumor immune responses along time. To elucidate this issue we evaluated changes induced by immunologically effective DC vaccination in the composition of tumor-associated T cell subpopulations. Methods: 12 patients with MM previously enrolled in a phase I/II DC vaccine trial and for which tumor tissue taken before and after at least 4 induction vaccine doses were available, were included in the study. Intratumoral lymphocytes were evaluated by CD3, CD4, CD8, FoxP3 and GrB immunostainings, and quantified by a computer-assisted method. A nonparametric two-tailed Wilcoxon signed-rank test was utilized for evaluating differences in the distribution of the number of cell positive for each marker on the total cell counts in pre- and post-vaccine biopsies. Results: Our data showed a considerable and statistically significant decrease of intratumoral FoxP3 + regulatory T cells in melanoma tissues after DC vaccination. In addition, the concurrent increase of intratumoral activated cytotoxic T lymphocytes, as shown by CD8 and Granzyme B stainings, indicated that this decrease has likely a functional relevance. Conclusions: Our findings that vaccination with DC loaded with autologous tumor lysate strongly reduces the intratumoral content of regulatory T cells add strength to the rationale for the development of potentially more effective combination schedules where whole tumor antigen-loaded DC vaccine prime and partially activate tumor-specific low-affinity T cells in a first tumor antigen-focusing step, followed by boosting with non-maximal doses of anti-CTLA-4 antibodies.