Chimeric L6 anti-tumor antibody. Genomic construction, expression, and characterization of the antigen binding site.

We report the cloning of the genomic variable region genes of the human carcinoma reactive murine monoclonal antibody L6 and their genetic linkage to human constant region exons to encode a human IgG1/kappa chimeric antibody. The chimeric protein was produced at levels greater than 20 micrograms/ml (enabling the initiation of clinical trials) and was found to have binding properties identical with that of the murine parent. The nucleic acid sequence of the variable regions was determined and found to be different than that previously reported (1). The deduced amino acid sequence was then used to generate a structural homology based three-dimensional model of the antibody binding site, which was found to share features with antibodies known to interact with a protein surface, but distinct from those that bind to carbohydrate epitopes. Biochemical analysis of binding between antibody and the in vitro-translated product of a cDNA clone that confers L6 immunoreactivity demonstrates that the antibody recognizes a protein epitope encoded by this transcript which requires the presence of membranes, but is unaffected by the removal of carbohydrate side chains.