Analysis and characterization of antitumor T-cell response after administration of dendritic cells loaded with allogeneic tumor lysate to metastatic melanoma patients

The primary goal of cancer vaccines is to induce CD8 T cells specific for tumor-associated antigens (TAA) but the characterization of these cells has been difficult because of the low sensitivity of ex vivo assays. Here, we focused on TAA-specific CD8 T-cell responses in melanoma patients after vaccination with autologous dendritic cells loaded with lysates derived from allogeneic tumor-cell lines (Lysate-DC). Out of 40 patients treated, 16 patients developed immune response to tumor-cell lysate and/or CD8 T cells specific for differentiation and cancer-testis antigens. TAA-specific CD8 T-cell responses were detected by interferon (IFN)-γ enzyme-linked immunospot after in vitro sensitization and were, either transient during the treatment period or delayed, that is, observed after completion of all vaccinations. We could not correlate these immune responses to clinical data as none of the patients achieved an overall objective response according to Response Evaluation Criteria in Solid Tumors criteria. Three patients were reported as stable disease and 10 patients presented evidence of antitumor activity. We found that TAA-specific T cells characterized in 4 patients produced perforin ex vivo, but no IFN-γ in enzyme-linked immunospot. Differential expression of IFN-γ and perforin was also observed for viral-specific T cells. Altogether, our results show that Lysate-DC therapy elicited tumor-specific CD8 T cells nonlimited to human leukocyte antigen-A2 patients, with some T cells secreting perforin ex vivo and IFN-γ only after restimulation. The differential expression of perforin and IFN-γ by antitumor and antiviral CD8 T cells supports that the sole use of IFN-γ production to monitor T cells overlooks functional T-cell subpopulations triggered by vaccines.