Humoral immunity against a tandem repeat epitope of human mucin MUC-1 in sera from breast, pancreatic, and colon cancer patients.

Using synthetic peptides 60,80, and 105 residues long, corresponding to 3, 4, and 5.25 tandem repeats of human mucin MUC-1 protein core, as antigens in a solid-phase enzyme-linked immunosorbent assay, we screened sera from 24 breast cancer patients, 10 colon cancer patients, and 12 pancreatic cancer patients, at various stages of disease, for the presence of mucin-specific antibodies. The 105-residue peptide was superior in allowing detection of high levels of anti-mucin antibodies in 10.9% of sera in each cancer group. Another 4.3% showed intermediate reactivity. Lower levels of detection were achieved with the 80-residue peptide, and no specific reactivity was detectable with the 60-residue peptide. Anti-mucin antibodies were previously undetectable when this assay was performed with purified whole mucin or short synthetic peptides. The presence or absence of antibody did not correlate with the levels of circulating mucin or stage of disease. One highly reactive serum sample was used to identify more precisely the epitope on the long synthetic peptide to which the reactivity was directed. The reactivity of this serum specific for the 105-residue peptide was blocked by a 9-residue peptide from the NH2-terminal region of the 20-residue tandem repeat containing the previously identified immunogenic epitope APDTRP. Another 9-residue mucin peptide, from the COOH-terminal region of the tandem repeat which does not contain the APDTRP epitope, had no effect. All the mucin-specific reactivity was found to be of the IgM isotype, indicating a helper T-cell-independent response, unusual for an antibody against a peptide epitope, but not unexpected for tandemly repeated epitopes.