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-γ 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.