Diverse expansion potential and heterogeneous avidity in tumor-associated antigen-specific T lymphocytes from primary melanoma patients

While tumor-associated antigen (TAA)-specific CD8(+) T lymphocytes have been detected in metastatic melanoma patients, immune response in early disease phases has not yet been carefully evaluated. We looked for circulating cytotoxic T lymphocytes (CTL) directed against Melan-A / MART1, tyrosinase, gp100 and MAGE-3 antigens in patients with a diagnosis of primary cutaneous melanoma by using fluorescent HLA-A2 tetramers. In five out of six cases high numbers of CD8(+)/tetramer(+) cells could be detected by flow cytometry, and in four patients lymphocyte populations specific for two different melanoma antigens (Melan-A/MART1 and tyrosinase) were contemporaneously present. The TAA-specific cells could represent as much as 1/220 T lymphocytes in the circulating CD8(+) population. When tetramers were used to monitor the in vitro expansion of TAA-specific CTL precursors upon antigen-specific stimulation, a diverse expansion potential was evidenced in CTL from the different donors and, more strikingly, in CTL specific for the different TAA. Melan-A/MART1-specific CTL clones derived from two patients exhibited a broad range of avidity. Only the highest avidity clones, representing about 50 % of the cases analyzed, were tumor specific. By correlating tetramer staining with clone avidity, we found that tetramer fluorescence intensity could represent a good indicator of TCR affinity, but not of overall clone avidity.