The 2.0-angstrom resolution crystal structure of a trimeric antibody fragment with noncognate V-H-V-L domain pairs shows are arrangement of V-H CDR3

The 2.0-Angstrom resolution x-ray crystal structure of a novel trimeric antibody fragment, a ''triabody,'' has been determined. The trimer is made up of polypeptides constructed in a manner identical to that previously 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 cyclic, head-to-tail fashion. For the particular structure reported here, the polypeptide was constructed with a V-H domain from one antibody fused to the V-L domain from an unrelated antibody giving rise to ''combinatorial'' 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 conformational changes observed in antibody fragments in response to antigen binding. Combinatorial pairing of V-H and V-L domains constitutes a major component of antibody diversity, Conformationally flexible antigen-binding sites capable of adapting to the specific CDR3 loop context created upon V-H-V-L pairing may be employed by the immune system to maximize the structural diversity of the immune response.