Reactivity of natural and induced human antibodies to MUC1 mucin with MUC1peptides and N-acetylgalactosamine (GalNAc) peptides

Antibodies (Abs) to MUCI occur naturally in both healthy subjects and cance r patients and can be induced by MUCI peptide vaccination. We compared the specificity of natural and induced MUCI Abs with the objective of defining an effective MUCI vaccine for active immunotherapy of adenocarcinoma patien ts. Serum samples, selected out of a screened population of 492 subjects fo r their high levels of IgG and/or IgM MUCI Abs, were obtained from 55 contr ol subjects and from 26 breast cancer patients before primary treatment, as well as from 19 breast cancer patients immunized with MUCI peptides couple d to keyhole limpet hemocyanin (KLH) and mixed with QS-21, The samples were tested with enzyme-linked immunoassays for reactivity with (I) overlapping hepta- and 20-mer peptides spanning the MUCI tandem repeat sequence; (2) t wo modified 60-mer peptides with substitutions in the PDTR (PDTA) or in the STAPPA (STAAAA) sequence of each tandem repeat; and (3) four 60-mer glycop eptides with each I, 2, 3 and 5 mol N-acetylgalactosamine (GalNAc) per repe at. More than one minimal epitopic sequence could be defined, indicating th at Abs directed to more than one region of the MUCI peptide core can coexis t in one and the same subject. The most frequent minimal epitopic sequence of natural MUCI IgG and IgM Abs was RPAPGS, followed by PPAHGVT and PDTRP, MUCI peptide vaccination induced high titers of IgM and IgG Abs predominant ly directed, respectively, to the PDTRPAP and the STAPPAHGV sequences of th e tandem repeat. Natural MUCI Abs from breast cancer patients reacted more strongly with the N-acetylgalactosamine (GalNAc) peptides than with the nak ed 60-mer peptide, while reactivity with the GalNAc-peptides was significan tly reduced (2-tailed p < 0.0001) in the MUCI IgG and IgM Abs induced by MU CI peptide vaccination. Whereas in cancer patients glycans appear to partic ipate in epitope conformation, the epitope(s) recognized by MUCI Abs induce d by peptide vaccination are already masked by minimal glycosylation, There fore, our results indicate that a MUCI glycopeptide would be a better vacci ne than a naked peptide. (C) 2000 Wiley-Liss, Inc.