Melanoma-infiltrating dendritic cells induce protective antitumor responses mediated by T cells.

Dendritic cells are the most potent antigen-presenting cells inducing innate and adaptive immune response. Dendritic cells infiltrate melanomas, but their ability to induce host antitumor immunity remains obscure. In a previous study, we have observed that melanoma-infiltrating dendritic cells have the capacity to process antigens and migrate to lymph nodes to prime T lymphocytes. Here, we observed that melanoma-infiltrating dendritic cells extracted from melanoma without any additional manipulations were able to protect naive mice against a lethal challenge with the tumor. Remarkably, this was achieved with reinjection of 10(5) melanoma-infiltrating dendritic cells, a number that did not exceed the total number of melanoma-infiltrating dendritic cells recovered from one single tumor. Three observations indicate that protection was due to the natural loading of melanoma-infiltrating dendritic cells with tumor antigens. First, the protective effect was not observed with equivalent numbers of bone marrow-derived dendritic cells. Second, the protection induced was specific for the tumor from which the tumor-infiltrating dendritic cells were isolated. Third, depletion experiments indicate that both CD4+ and CD8+ T lymphocytes were required during the effector phase of the antitumor response. Hence, designing strategies aimed at rendering melanoma-infiltrating dendritic cells visible to host T cells may boost spontaneous antitumor immunity.