ALL-D PEPTIDES RECOGNIZED BY AN ANTICARBOHYDRATE ANTIBODY IDENTIFIED FROM A POSITIONAL SCANNING LIBRARY

Monoclonal antibodies recognize antigens with high affinity and specif icity, but the structural basis for molecular mimicry remains unclear. It is often assumed that cross-reactive antigens share some structura l similarity that is specifically recognized by a monoclonal antibody. Recent studies using combinatorial Libraries, which are composed of m illions of sequences, have examined antibody cross-reactivity in a man ner entirely different from traditional epitope mapping approaches. He re, peptide libraries were screened against an anti-carbohydrate monoc lonal antibody for the identification of peptide mimics. Positional sc anning Libraries composed of all-L or all-D hexapeptides were screened for inhibition of monoclonal antibody HGAC 39.G3 binding to an antige n displaying N-acetyl-D-glucosamine (GlcNAc) residues on a polyrhamnos e backbone. Inhibitory activity by mixtures from the all-D hexapeptide library was greater than the activity from the all-L Libraries. The m ost active D-amino acid residues defined in each of the six positions of the library were selected to prepare 27 different individual hexape ptides. The sequence Ac-yryygl-NH2 was specifically recognized by mAb HGAC 39.G3 with a relative affinity of 300 nM when measured in a compe titive binding assay. The contributions to overall specificity of the residues of the all-D peptide (Ac-yryygl-NH2) in binding to mAb HGAC 3 9.G3 were examined with a series of truncation, L and D-amino acid sub stitution, and retro analogs. Dimeric forms of the all-D peptide were recognized with tenfold to 100-fold greater affinities relative to the monomer. The all-D peptide was found to inhibit mAb HGAC 39.G3 bindin g to an anti-idiotype antibody with approximately 1000-fold greater af finity than GlcNAc. As demonstrated here, the study of immune recognit ion using combinatorial chemistry may offer new insights into the mole cular basis of cross-reactivity. (C) 1998 Academic Press.