SEROLOGICAL AND BIOCHEMICAL-CHARACTERIZATION OF RECOMBINANT BACULOVIRUS CARCINOEMBRYONIC ANTIGEN

Carcinoembryonic antigen (CEA), a glycosylated protein of M, 180 kDa, is one of the most widely used human tumor markers. A majority of gast rointestinal cancers as well as breast and nonsmall cell lung carcinom as express CEA. We have previously described a recombinant baculovirus BVCEA-140 expressing the full-length human CEA and a variant, BVCEA-1 6, that encodes only the NH2-terminal domain, as well as a recombinant (BVNCA) expressing the closely related molecule nonspecific cross-rea ctive antigen (NCA). We have now compared a panel of 24 anti-CEA and a nti-NCA monoclonal antibodies (MAbs) for their ability to bind to thes e recombinant CEA and NCA proteins, as well as with a new 60 kDa subge nomic form designated BVCEA-60. The epitope mapping studies indicate t hat all the CEA specific MAbs can recognize BVCEA-140. We also compare d the sugar composition of BVCEA-140 to native CEA, using a lectin-lin ked immunoradiometric assay. The results demonstrated that both the na tive and recombinant baculovirus CEA contain simple high-mannose carbo hydrates as well as biantennary and biantennary hybrid complexes. Howe ver, native CEA also contains triantennary and tetraantennary complex sugars, while the recombinant CEA molecule does not. Immunogenicity of the recombinant CEA molecules was demonstrated in mice. ELISA and Wes tern blot analyses were used to determine the cross-reactivity of the anti-CEA sera. Mice immunized with BVCEA-140 elicit antibodies that ar e reactive to native CEA. When the BVCEA-16 was used as an immunogen, the antisera failed to detect native CEA or BVCEA-140. These studies d emonstrate that minor sugar differences exist between native and bacul ovirus-derived CEA. However, epitope mapping with a panel of 24 anti-C EA MAbs (recognizing at least 10 CEA epitopes) stowed virtual immunolo gic identity between these two molecules. Moreover, BVCEA-140 appears to be a more potent humoral immunogen in mice than native CEA. These p urified recombinant proteins can thus serve as standards in CEA serum assays for the possible detection and characterization of cell-mediate d immune responses to CEA and as a potential source of immunogen (prim ary or for boosting) for active specific immunotherapy protocols of hu man carcinomas.