Maturation of dendritic cells is a prerequisite for inducing immune responses in advanced melanoma patients.

We have investigated the capacity of immature and mature monocyte-derived DCs pulsed with melanoma-associated peptides (gp100 and tyrosinase) to induce a primary cytotoxic T-lymphocyte response in vivo. Advanced HLA-A2.1(+) melanoma patients were vaccinated with peptide- and keyhole limpet hemocyanin (KLH)-pulsed DCs, either immature (9 patients) or matured by monocyte-conditioned medium/tumor necrosis factor alpha/prostaglandin E(2) (10 patients). All patients vaccinated with mature DCs showed a pronounced proliferative T-cell and humoral response against KLH. By contrast, KLH responses were absent in most of the patients vaccinated with immature DCs. Delayed-type hypersensitivity (DTH) reactions against antigen-pulsed DCs were only observed in patients vaccinated with mature DCs and not in patients vaccinated with immature DCs. MHC-peptide tetramer staining of DTH-derived T cells revealed the presence of specific T cells recognizing the melanoma-associated peptides in 1 patient. In a second patient, DTH-derived T cells showed specific lysis of tumor cells expressing the antigens used for DC pulsing. Only patients vaccinated with mature DCs showed objective clinical responses. Interestingly, both patients with long-term progression-free survival (22 and >40 months) were both vaccinated with mature DCs and demonstrated antigen-specific T-cell reactivity of DTH-derived T cells. We conclude that mature DC are superior to immature DC in the induction of immunological responses in melanoma patients, which may translate into improved clinical results.