Detection and functional analysis of CD8+ T cells specific for PRAME:: a target for T-cell therapy

Purpose: Preferentially expressed antigen on melanomas (PRAME) is an interesting antigen for T-cell therapy because it is frequently expressed in melanomas (95%) and other tumor types. Moreover, due to its role in oncogenic transformation, PRAME-negative tumor cells are not expected to easily arise and escape from T-cell immunity. The purpose of this study is to investigate the usefulness of PRAME as target for anticancer T-cell therapies. Experimental Design: HLA-A*0201-subtyped healthy individuals and advanced melanoma patients were screened for CD8(+) T cells directed against previously identified HLA-A*0201-binding PRAME peptides by IFN-gamma enzyme-linked immunosorbent spot assays and tetramer staining. PRAME-specific T-cell clones were isolated and tested for recognition of melanoma and acute lymphoid leukemia (ALL) cell lines. PRAME mRNA expression was determined by quantitative real-time reverse transcription-PCR. Results: In 30% to 40% of healthy individuals and patients, PRA(100-108)-specific CD8(+) T cells were detected both after in vitro stimulation and directly ex vivo after isolation by magnetic microbeads. Although CD45RA(-) memory PRA(100-108)-specific T cells were found in some individuals, the majority of PRA(100-108)-tetramer(+) T cells expressed CD45RA, suggesting a naive phenotype. PRA(100-108)-tetramer(+) T-cell clones were shown to recognize and lyse HLA-A*0201(+) and PRAME(+) melanoma but not ALL cell lines. Quantitative real-time reverse transcription-PCR showed significantly lower PRAME mRNA levels in ALL than in melanoma cell lines, suggesting that PRAME expression in ALL is below the recognition threshold of our PRA(100-108)-tetramer(+) T cells. Conclusion: These data support the usefulness of PRAME and in particular the PRA(100-108) epitope as target for T-cell therapy of PRAME-overexpressing cancers.