OPTIMAL T-CELL ACTIVATION BY MELANOMA-CELLS DEPENDS ON A MINIMAL LEVEL OF ANTIGEN TRANSCRIPTION

We reported previously that a large fraction of melanoma cell lines in duced a suboptimal activation of specific CTL clones, characterized by good tumor cell lysis but no detectable IL-2 production. Using synthe tic peptides, we demonstrated recently that this was due to expression of subthreshold levels of appropriate MHC-peptide complexes. We measu re here by semiquantitative reverse transcription-PCR the expression o f two melanoma Ag (NA17-A and Melan-A/MART-1) mRNAs in 13 melanoma cel l lines and analyze the responses to these cell lines of specific HLA- A2-restricted CTL clones. In line with the idea that the density of MH C-antigenic peptide complexes on melanoma cells is a direct function o f the Ag's mRNA level, we found that CTL lysis was grossly proportiona l to this level. We also established that a minimal level of transcrip tion is required for melanoma cells to induce IL-2 secretion. interest ingly, all cell lines that expressed the Ag above this minimal level, either spontaneously or after gene transfection, stimulated the secret ion by tumor-infiltrating lymphocyte of IL-2 amounts proportional to A g expression unless they exhibited a defective expression of intracell ular adhesion molecule-1 or LFA-3 molecules or a low expression of the restricting HLA element. These results indicate that optimal activati on and therefore, doubtless, full functionality of melanoma-specific C TL clones critically depend on the mRNA level of the Ag in tumor cells and also on a minimal expression of the HLA restriction element, intr acellular adhesion molecule-1, and LFA-3. These data provide a rationa le for a better selection of patients to be included in Ag-specific im munization protocols.