Triabodies: single chain Fv fragments without a linker form trivalent trimers.

A single chain Fv fragment (scFv) of the murine monoclonal antibody 11-1G10 was constructed by directly joining the C-terminal residue of the V(H) domain to the N-terminal residue of V(L). 11-1G10 is an anti-idiotype and competes with the antigen, influenza virus neuraminidase (NA), for binding to the NC41 antibody. The scFv formed stable trimers with three active antigen combining sites for NC41 Fab fragments. We propose that trimeric scFvs may be the preferred conformation for directly linked V(H)-V(L) molecules, which contrasts the formation of scFv dimers (diabodies) when the V(H) and V(L) domains are joined by short flexible linkers of between 5-10 residues. BIAcore biosensor binding experiments showed that the trimeric scFv showed an expected increase in binding affinity, due to avidity, compared to the monomeric 15-residue linked scFv. The increase in avidity of scFv trimers offers advantages for imaging and immunotherapy.