Direct selection of a human antibody fragment directed against the tumor T-cell epitope HLA-A1-MAGE-A1 from a nonimmunized phage-Fab library

Antitumor antibodies with the same specificity as cytotoxic T lymphocytes t hat recognize antigenic peptides encoded by tumor-associated genes and pres ented by MHC class I molecules would be valuable tools to analyze the antig enicity or target tumor cells in vivo. To obtain a human antibody directed against a peptide encoded by gene melanoma-associated antigen (MAGE)-A1 and presented by HLA-A1 molecules, we selected a large phage Fab antibody repe rtoire on a recombinant version of the complex HLA-A1-MAGE-A1 produced by i n vitro refolding. One of the selected phage antibodies shows binding to HL A-A1 complexed with the MAGE-A1 peptide, but does not show binding to HLA-A 1 complexed with a peptide encoded by gene MAGE-A3 and differing from the M AGE-A1 peptide by only three residues. Phages carrying this recombinant ant ibody bind to HLA-A1(+) cells only after in vitro loading with MACE-Al pept ide. These results indicate that nonimmunized phage Fab libraries are a sou rce of antibodies with a T cell antigen receptor-like specificity. The huma n anti-HLA-A1-MAGE-A1 antibody described here may prove very useful for mon itoring the cell surface expression of these complexes, and eventually, as a targeting reagent for the specific immunotherapy of HLA-A1 patients beari ng a MAGE-A1-positive tumor.