Lack of tumor recognition by cytolytic T lymphocyte clones recognizing peptide 195–203 encoded by gene MAGE-A3 and presented by HLA-A24 molecules

Gene MAGE-A3 encodes tumor-specific antigenic peptides recognized by T cells on many tumors. MAGE-A3 peptides presented by HLA class I molecules have been identified using CD8 lymphocytes stimulated with cells that either expressed gene MAGE-A3 or were pulsed with candidate peptides. One antigen identified with the latter method is peptide MAGE-A3_195–203 IMPKAGLLI, presented by HLA-A24 molecules. It has been used to vaccinate advanced cancer patients. Here, we have used HLA/peptide tetramers to detect T cells recognizing this peptide. Their frequency was estimated to be 2 × 10⁻⁸ of the blood CD8 cells in non-cancerous HLA-A24⁺ individuals, which is tenfold lower than the reported frequencies of T cells against other MAGE peptides. In the blood of a patient vaccinated with MAGE-A3, the estimated frequency was 5 × 10⁻⁷. Anti-MAGE-3.A24 cytolytic T cell clones were derived, that lysed peptide-pulsed cells with half-maximal effect at the low concentration of 500 pM. However, these CTL did not recognize a panel of HLA-A24⁺ tumor cells that expressed MAGE-A3 at levels similar to those found in HLA-A1⁺ tumor cells recognized by anti-MAGE-3.A1 CTLs. Furthermore, 293-EBNA cells transfected with MAGE-A3 and HLA-A24 constructs were hardly recognized by the anti-MAGE-3.A24 CTL clones. These results suggest that peptide MAGE-A3_195–203 is poorly processed and is not an appropriate target for cancer immunotherapy.