High frequency of autologous anti-melanoma CTL directed against an antigengenerated by a point mutation in a new helicase gene

We have identified an Ag recognized by autologous CTL on the melanoma cells of a patient who enjoyed an unusually favorable clinical evolution. The an tigenic peptide, which is presented by HLA-A28 molecules, is encoded by a m utated sequence in a new gene. This gene, which was named MUM-3, is express ed ubiquitously and shows homology with the RNA helicase gene family. Limit ing dilution analysis indicated that at least 0.15% of the blood CD8 T cell s were tumor-specific CTL precursors. The MUM-3 Ag was recognized by 90% of these CTL, indicating that it is the dominant target Ag of the tumor-speci fic CTL response. The high frequency of anti-MUM-3 CTL was confirmed with t etramers of soluble HLA-A28 molecules loaded with the antigenic peptide. MU M-3 tetramers stained 1.2% of blood CD8 cells, a frequency that has never b een reported for T cells directed against a strictly tumor-specific Ag, To confirm these results, the CDS T cells that were clearly labeled with tetra mers were restimulated in clonal conditions. About 90% of these cells proli ferated, and all the resulting clones proved lytic and MUM-3 specific, By i mproving the conditions used for the in vitro restimulation of CTL precurso rs by the tumor cells, the same frequency could be obtained in limiting dil ution analysis. These results show that some cancer patients have a high fr equency of circulating CTL that are directed against a strictly tumor-speci fic Ag, These CTL are responsive to restimulation in vitro and are easily d etected with tetramers, Such responses may therefore be an achievable goal for therapeutic vaccination with tumor-specific Ags.