Protein Engineering, Vol. 12, No. 4, 357-362,
April 1999
© 1999 Oxford University Press
Effects of humanization by variable domain resurfacing on the antiviral activity of a single-chain antibody against respiratory syncytial virus
OraVax Inc., 38 Sidney Street, Cambridge, MA 02139, 1 Hercules Inc.,500 Hercules Road, Mail Stop 8136/216, Wilmington, DE 19808, USA, 2 Oxford Molecular Ltd, The Medawar Centre, Oxford Science Park,Oxford OX4 4GA and 3 School of Biology and Biochemistry, University of Bath, Claverton Down, Bath, Avon BA2 7AY, UK
HNK20 is a mouse monoclonal IgA that binds to the F glycoprotein of respiratory syncytial virus (RSV) and neutralizes the virus, both in vitro and in vivo. The single-chain antibody fragment (scFv) derived from HNK20 is equally active and has allowed us to assess rapidly the effect of mutations on affinity and antiviral activity. Humanization by variable domain resurfacing requires that surface residues not normally found in a human Fv be mutated to the expected human amino acid, thereby eliminating potentially immunogenic sites. We describe the construction and characterization of two humanized scFvs, hu7 and hu10, bearing 7 and 10 mutations, respectively. Both molecules show unaltered binding affinities to the RSV antigen (purified F protein) as determined by ELISA and surface plasmon resonance measurements of binding kinetics (Ka 
1x109 M–1). A competition ELISA using captured whole virus confirmed that the binding affinities of the parental scFv and also of hu7 and hu10 scFvs were identical. However, when compared with the original scFv, hu10 scFv was shown to have significantly decreased antiviral activity both in vitro and in a mouse model. Our observations suggest that binding of the scFv to the viral antigen is not sufficient for neutralization. We speculate that neutralization may involve the inhibition or induction of conformational changes in the bound antigen, thereby interfering with the F protein-mediated fusion of virus and cell membranes in the initial steps of infection.
Keywords: antibody/immunotherapy/resurfacing/single-chain Fv
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