THE 2.0-ANGSTROM RESOLUTION CRYSTAL-STRUCTURE OF A TRIMERIC ANTIBODY FRAGMENT WITH NONCOGNATE V-H-V-L DOMAIN PAIRS SHOWS ARE ARRANGEMENT OFV-H CDR3

The 2.0-Angstrom resolution x-ray crystal structure of a novel trimeri c antibody fragment, a ''triabody,'' has been determined. The trimer i s made up of polypeptides constructed in a manner identical to that pr eviously described for some ''diabodies'': a V-L domain directly fused to the C terminus of a V-H domain-i.e., without any linker sequence. The trimer has three Fv heads with the polypeptides arranged in a cycl ic, head-to-tail fashion. For the particular structure reported here, the polypeptide was constructed with a V-H domain from one antibody fu sed to the V-L domain from an unrelated antibody giving rise to ''comb inatorial'' Fvs upon formation of the trimer. The structure shows that the exchange of the V-L domain from antibody B1-8, a V-lambda domain, with the V-L domain from antibody NQ11, a V-kappa domain, leads to a dramatic conformational change in the V-H CDR3 loop of antibody B1-8, The magnitude of this change is similar to the largest of the conforma tional changes observed in antibody fragments in response to antigen b inding. Combinatorial pairing of V-H and V-L domains constitutes a maj or component of antibody diversity, Conformationally flexible antigen- binding sites capable of adapting to the specific CDR3 loop context cr eated upon V-H-V-L pairing may be employed by the immune system to max imize the structural diversity of the immune response.