Isolation of a panel of human anti-CEA single chain Fv from a large phage display library

A phage display library of human single chain Fv (scFv) has been selected u sing immobilised human carcinoembryonic antigen (CEA) in its fully glycosyl ated state. Six different specific clones have been identified, which vary both in their affinity for native CEA and in their tumour specificity as ju dged by immunocytochemical staining. One clone, CEA6, has a dissociation co nstant for CEA of 7.7 x 10(-9) M and stains human colorectal adenocarcinoma with a pattern typical of CEA-specific monoclonal anti-bodies (mAb), In co ntrast, the remaining five scFv have dissociation constants in the micromol ar range and show a heterogeneous staining pattern on CEA positive human tu mours, Fine specificity analysis of the isolated clones has revealed that C EA6 recognises the polypeptide core of CEA, while the remaining five recogn ise a structure formed at least in part by terminal sialic acid residues on the CEA molecule. The bias towards CEA-specific carbohydrate binders is in marked contrast to the specificity of the majority of mAbs derived from im munised rodents. These results illustrate how antibodies derived from phage display libraries may exhibit specificities which are under-represented in immunised rodent repertoires and may also allow the generation of tumour t argeting antibodies with novel specificities.