CD8+ T-cell Clones Specific for the 5T4 Antigen Target Renal Cell Carcinoma Tumor-initiating Cells in a Murine Xenograft Model

The tumor antigen 5T4 is frequently expressed at high levels on renal cell carcinoma (RCC) and other epithelial carcinomas. Surveys of normal tissues demonstrate abundant 5T4 expression on placental trophoblast cells with limited expression elsewhere. 5T4 is the target for a therapeutic cancer vaccine (MVA-5T4) that elicits 5T4-specific serological, proliferative, and cytotoxic T lymphocyte (CTL) responses. However, the antitumor activity of 5T4-specific CTL has not been extensively characterized. CD8+ T cells from HLA-A2+ healthy donors (n=4) or RCC patients (n=2) were stimulated in vitro with the HLA-A2-binding nonamer peptides 5T417–25 or 5T497–105 and screened by flow cytometry with specific tetramers (TET). CD8+/TET+ T-cell clones specific for 5T417–25 or 5T497–105 peptide were isolated from 4/6 and 1/4 donors, respectively. A subset of clones specific for 5T417–25 was cytolytic for MVA-5T4-infected HLA-A2+ EBV-transformed lymphoblastoid cell line target cells and for constitutively HLA-A2-expressing and 5T4-expressing RCC tumor cell lines (including A498 RCC). In a xenoengraftment assay, the coinoculation of a representative 5T417–25-specific CTL clone with A498 RCC tumors cells into immune-deficient mice completely prevented growth of A498 tumors. Taken together, these data demonstrate high-avidity CD8+ CTL able to recognize the naturally processed 5T417–25 epitope on RCC tumor cells including putative tumor-initiating cells are present in peripheral blood of both healthy donors and RCC patients. CD8+T-cell immunity targeting 5T417–25 is therefore of substantial interest both as a potential target for further development of vaccination or adoptive cellular immunotherapy and for immune monitoring studies in association with nonspecific immunotherapies.