Specificity clusters of MUC1-reactive mouse monoclonal antibodies

The ISOBM TD-4 Workshop antibodies 122-177 were grouped according to their reactivity with: (a) monomeric MUC1 peptide (TAP25); (b) the pentameric tandem repeat peptide (TR-5), both unglycosylated or as their GalNAc-substituted derivatives, and (c) the lactation or tumor-associated glycoforms of MUC1. The antibodies were grouped into nine clusters: Clusters 1-4 comprise those antibodies that bind to MUC1 tandem repeat peptide (monomeric) and define sequential epitopes differentially affected by glycosylation (GalNAc substitution) in the peptide motifs VTSA and GSTA. The ISOBM-163 antibody in cluster 5 defines Tn antigen as a site-specific glycotope within the tandem repeat peptide. The antibodies in clusters 6 and 7A recognize peptide epitopes within the tandem repeats, although these seem to be conformationally dependent on complex glycosylation or on oligomeric peptide repetition. Antibodies in clusters 7B bind to a conformational epitope not related to the tandem repeat peptide or to a carbohydrate epitope (cluster 7BC). Antibodies in cluster 8 showed a high selective binding to tumor-associated epitopes on MUC1 from mammary carcinoma cells. Cluster 9 antibodies were not reactive on any of the tested antigens and may be argued to lack MUC1 specificity. Alternatively, these antibodies could have a carbohydrate specificity not expressed by the MUC1 glycoforms tested in our studies.