Expression of a recombinant breast tumor-associated mucin fusion protein in Escherichia coli exposes the tumor-specific epitope.

Mucins are highly immunogenic glycoproteins that are abundantly expressed by breast carcinomas and other carcinomas. The fact that deglycosylated normal mucin can induce tumor-specific monoclonal antibodies indicates that tumor-specific epitopes are hidden in the fully glycosylated form. Using recombinant DNA techniques, a fragment of mucin tandem repeats was inserted into pMal-p, an Escherichia coli expression vector, and resulted in the expression of an unglycosylated maltose-binding protein-mucin fusion protein. This fusion protein has been purified and showed strong affinity to breast tumor-specific monoclonal antibody SM3. The antisera against this recombinant mucin fusion protein recognized all breast tumor cell lines we tested. Competition assay with monoclonal antibody SM3 shows that anti-recombinant mucin fusion protein binds the epitope that SM3 binds. These results confirm the hypothesis that unglycosylated mucin contains a tumor-specific epitope. This leads to the possibility that recombinant mucin may be used to develop vaccines against breast cancer and cytotoxic T-lymphocyte lines for immunotherapy of breast cancer.