Molecular recognition of the Lewis Y antigen by monoclonal antibodies.

The murine monoclonal antibody BR55-2 is directed against the tumor-associated antigen Lewis Y oligosaccharide. The Lewis Y core antigen is a difucosylated structure consisting of four hexose units. Analysis of binding profiles of lactoseries isomeric structures by BR55-2 suggest that the binding epitope includes the OH-4 and OH-3 groups of the beta-D-galactose unit, the 6-CH3 groups of the two fucose units and the N-acetyl group of the subterminal beta-D-N-acetylglucosamine (beta DGlcNAc). To elucidate the molecular recognition properties of BR55-2 for the Y antigen, BR55-2 was cloned, sequenced and its three-dimensional structure was examined by molecular modeling. The crystal structure of BR96, another anti-Lewis Y antibody, solved in complex with a nonoate methyl ester Lewis Y tetrasaccharide, and the lectin IV protein in complex with a Lewis b tetrasaccharide core were used as a guide to probe the molecular basis for BR55-2 antigen recognition and specificity. Our modeling study shows that BR55-2 shares similar recognition features for the difucosylated type 2 lactoseries Lewis Y structure observed in the BR96-sugar complex. We observe that a major source of specificity for the Lewis Y structure by anti-Y antibodies emanates from interaction with the beta-D-N-acetylglucosamine residue and the nature of the structures extended at the reducing site of the fucosylated lactosoamine.