Circulating tumor-associated antigens detected by monoclonal antibodies against the polypeptide core of mucin--comparison of antigen MUSE11 with CA15-3.

The antigen MUSE11 detected by a monoclonal antibody (MAb) is an adenocarcinoma-associated antigen, while CA15-3 is a representative breast cancer-associated antigen detected by MAbs 115D8 and DF3. MAb MUSE11 showed higher binding activity to a synthetic peptide corresponding to the tandem repeat motif of the mucin core protein than that of MAb DF3, although MAb DF3 also had a significant binding activity indicating that MAbs MUSE11 and DF3 could recognize an identical polypeptide core. The reactivity of MAb DF3 to a breast cancer cell line MRK-neu-1 was completely abolished by neuraminidase treatment whereas that of MAb MUSE11 was partly conserved. The simultaneous measurement of the antigens MUSE11 and CA15-3 in sera from 35 cancer patients demonstrated that the incidence of abnormal serum level of CA15-3 was lower than that of antigen MUSE11. These data suggest that at least a part of the structural basis for the difference between the serum levels of antigen MUSE11 and CA15-3 could be carbohydrate side chains including sialic acids.