Prostate-specific antigen-reactive cytotoxic T lymphocyte precursors in colon cancer patients

Prostate-specific antigen (PSA) is a representative of the prostate-related antigens, and has been considered to be a tumor marker of prostate cancer. However, some studies suggest that PSA could be produced by several types of tumors. In the present study, we attempted to determine whether or not PSA could be a target molecule in specific immunotherapy for patients with colon cancer. Five colon cancer cell lines were examined for their PSA expression at the mRNA and protein levels by RT-PCR and immunocytostaining, respectively. As a result, four cell lines were found to be positive for PSA at both the mRNA and protein levels. We also attempted to determine whether PSA-reactive cytotoxic T lymphocytes (CTLs) could be induced from the peripheral blood mononuclear cells (PBMCs) of HLA-A24+ colon cancer patients by in vitro stimulation with PSA-derived peptides. As a consequence, PSA peptide-specific CTLs could be generated from the PBMCs of male and female colon cancer patients. Their cytotoxicity against HLA-A24+ PSA-expressing colon cancer cells was dependent on HLA class I-restricted and CD8+ T cells. These findings indicate that PSA-reactive CTL precursors are present in the periphery of colon cancer patients, and that PSA could be a target molecule in specific immunotherapy to colon cancer.