Novel approach to the characterization of melanoma associated-peptide-specific CTL lines from Japanese metastatic melanoma patients

Melanoma-associated antigens, MART-1, tyrosinase, gp100 and MAGEs, are typical melanoma-specific tumor antigens which can potently induce immune responses in metastatic melanoma patients treated with peptide vaccines. In the present study, we established a dendritic cell (DC)-based HLA-A2 melanoma-associated peptide (MART-1 or gp100)-specific CTL induction method and characterized the CTLs using HLA-A2 tetramer staining in 6 cases of HLA-A2+ melanoma treated with DC vaccines. Peripheral blood mononuclear cells (PBMC) from patients were stimulated twice with MART-1 A2 peptide-pulsed DCs in the presence of a low dose of IL-2. To boost CTL populations, CTL lines were further stimulated twice with MART-1 A2 peptide-pulsed T2 cells. The frequency of MART-1 A2 tetramer-positive CTLs increased from 0.16% (prior to stimulation) to 2.15% (after DC stimulation), and reached 46.5% on average (after additional T2 stimulation) in 4 cases which showed a successful expansion. The absolute numbers of MART-1 A2 tetramer-positive CTLs increased from 187- to 619-fold (average, 415-fold) compared to prior to DC stimulation. CTL assays using MART-1-specific CTL lines demonstrated potent killing activity against MART-1 peptide-pulsed T2 cells or HLA-A2+ melanoma cell lines in accordance with the frequency of tetramer-positive CTLs. Finally, we were successful in identifying melanoma peptide-specific T-cell receptor (TCR) cDNAs in 2 cases for MART-1 and 1 case for gp100 using the anti-TCR MoAb-based sorting as a novel approach instead of a conventional cell cloning, and confirmed peptide-specific IFN-γ production in TCR cDNA-transduced naïve T cells. The results showed that cloned TCR cDNAs were efficient in reconstituting tumor-specific cytotoxicity and good candidates for novel immunotherapy.