Development and Characterization of Chimeric Anti‐carcinoembryonic Antigen Monoclonal Antibodies and Their Fab Fragments

In an attempt to reduce the immunogenicity of two different murine anti‐carcinoembryonic antigen (CEA) monoclonal antibodies (MAbs), KM10 and A10, we produced recombinant mouse/human chimeric MAbs and the respective Fab fragments carrying the variable regions of the murine MAbs. Chimeric A10 Fab fragment was expressed in Escherichia coli, and produced in large quantities in a mini‐jar fermentation system. In competitive binding assays, chimeric MAbs and their Fab fragments showed identical specificity to human CEA epitopes, as compared to the parental MAbs or Fab fragments. Both chimeric Fab fragments exhibited strong immunohistochemical reactivity with various gastrointestinal carcinomas and no reactivity with CEA‐related antigens, such as NCA (nonspecific cross‐reacting antigen) or BGPI (biliary glycoprotein I). Furthermore, chimeric KM10 MAb elicited substantially higher antibody‐dependent cellular cytotoxicity than the murine MAb. Complement‐dependent cytotoxicity in vitro was much weaker with chimeric KM10 MAb. These results indicate that chimeric MAbs or Fab fragments could potentially replace the parental murine antibodies or their Fab fragments in therapy or diagnosis of human gastrointestinal carcinomas.