Recognition of an epitope of a breast cancer antigen by human antibody

A novel tumor-associated peptide epitope KASIFLK expressed preferentially b y breast cancer cells was identified using an IgG antibody from a breast ca ncer patient. A cDNA library from a MCF-7 breast cancer cell line was scree ned to isolate three cDNA clones that were immunoreactive with this antibod y. KASIFLK was located in clones 27 and 40, both of which were identical to the cDNA and protein sequence of retinoblastoma binding protein 1 (RBP1, 2 50-256). An affinity-purified IgG antibody against the peptide epitope was completely absorbed by cytoplasmic extracts of MCF-7 cells. Immunohistochem ical staining using this antibody revealed the antigen in MCF-7 cells and i n 12 of 15 primary breast cancer tissues and 3 of 34 other cancer tissues, but in none of 6 normal breast tissues. Anti-KASIFLK antibody titers were s ignificantly higher in sera of 55 breast cancer patients than in sera from 30 normal healthy donors (P > 0.001). These results suggest that KASIFLK or its cross-reactive epitope is a breast cancer antigen and is immunogenic i n humans.