Immunoreactivity of recombinant carcinoembryonic antigen proteins expressed in Escherichia coli

Immunoreactivities of recombinant carcinoembryonic antigen (CEA) proteins expressed in Escherichia coli (E. coli) were analyzed in relation to the CEA domain structure [domains N, I (A1-B1), II (A2-B2), III (A3-B3) and M]. We reconstructed in a prokaryotic expression vector, pUCPL-cI, the cDNAs fro CEA-N, CEA-I, CEA-II, and CEA-III-M. The latter three were expressed as fusion products with bacterial beta-galactosidase. The recombinant proteins were solubilized by sonication in 1% sodium dodecyl sulfate (SDS) and purified by preparative SDS-polyacrylamide gel electrophoresis followed by electroelution. Their molecular weights judged from Western blotting coincided with those calculated from their cDNA sequences, respectively. By solid-phase enzyme immunoassays, the immunoreactivities of the purified recombinant proteins were tested with 21 distinct anti-CEA monoclonal antibodies (MAbs) which had been found to recognize the peptide epitopes of the CEA molecule and to be reactive with the recombinant CEA proteins expressed in Chinese hamster ovary (CHO) cells. Fourteen of the 21 MAbs reacted with the recombinant CEA proteins expressed in E. coli and confirmed the localization of the epitopes identified by using the recombinant CEA proteins expressed in CHO cells. The reactivities of 5 MAbs with the recombinant proteins expressed in E. coli were remarkably low when compared with those of the proteins expressed in CHO cells but also confirmed the localization of the epitopes identified with the recombinant CEA proteins expressed in CHO cells. The remaining 2 MAbs did not react with any recombinant protein expressed in E. coli. These results indicate that the fusion CEA-proteins expressed in E. coli are useful in the localization of the epitopes on the polypeptide chains when they reacted with the MAbs tested. However, one third of the epitopes of CEA peptides may be profoundly affected by the presence of disulfide bonds and/or sugar chains which do not seem to be formed well in E. coli.