Expansion of Melanoma-specific Cytolytic CD8⁺ T Cell Precursors in Patients with Metastatic Melanoma Vaccinated with CD34⁺ Progenitor-derived Dendritic Cells

Cancer vaccines aim at inducing (a) tumor-specific effector T cells able to reduce/eliminate the tumor mass, and (b) long-lasting tumor-specific memory T cells able to control tumor relapse. We have shown earlier, in 18 human histocompatibility leukocyte antigen (HLA)-A*0201 patients with metastatic melanoma, that vaccination with peptide-loaded CD34–dendritic cells (DCs) leads to expansion of melanoma-specific interferon γ–producing CD8⁺ T cells in the blood. Here, we show in 9 out of 12 analyzed patients the expansion of cytolytic CD8⁺ T cell precursors specific for melanoma differentiation antigens. These precursors yield, upon single restimulation with melanoma peptide–pulsed DCs, cytotoxic T lymphocytes (CTLs) able to kill melanoma cells. Melanoma-specific CTLs can be grown in vitro and can be detected in three assays: (a) melanoma tetramer binding, (b) killing of melanoma peptide–pulsed T2 cells, and (c) killing of HLA-A*0201 melanoma cells. The cytolytic activity of expanded CTLs correlates with the frequency of melanoma tetramer binding CD8⁺ T cells. Thus, CD34-DC vaccines can expand melanoma-specific CTL precursors that can kill melanoma antigen–expressing targets. These results justify the design of larger follow-up studies to assess the immunological and clinical response to peptide-pulsed CD34-DC vaccines.