IDENTIFICATION OF A PEPTIDE WHICH BINDS TO THE CARBOHYDRATE-SPECIFIC MONOCLONAL ANTIBODY-B3

The monoclonal antibody (mAb) B3 recognizes an antigen found on the su rface of many adenocarcinoma cells. While the structure of the cellula r antigen is unknown, epitope mapping using neoglycoproteins with know n carbohydrate moieties indicates that the mAb B3 reacts with the Lewi s(Y) (Le(Y)) antigen [Pastan et al., Cancer Res. 51 (1991) 3781-3787]. We have used mAb B3 to select for peptides that mimic the carbohydrat e structure using libraries of filamentous phage displaying random pep tides on their surface. Phage that were selected coded for the sequenc e APWLYGPA. The corresponding peptide was synthesized and tested for i ts ability to bind to mAb B3. The peptide was found to inhibit specifi cally the binding of In-111-labeled mAb B3 to A431 adenocarcinoma cell s, as well as to inhibit killing of these cells by a B3 immunotoxin. I n addition, the Le(Y) carbohydrate, lactodifucotetraose, was able to c ompete with the phage displaying this peptide for binding to mAb B3. A lanine-scanning mutagenesis of the sequence coding for this peptide in dicates that four residues, PWLY, were critical for binding to the mAb . The sequence is similar to other sequences known to mimic carbohydra te structures.