Abstract
H5N1 avian influenza viruses remain a threat to public health mainly because they can cause severe infections in humans. These viruses are widespread in birds, and they vary in antigenicity forming three major clades and numerous antigenic variants. The most important features of the human monoclonal antibody FLD194 studied here are its broad specificity for all major clades of H5 influenza HAs, its high affinity, and its ability to block virus infection, in vitro and in vivo. As a consequence, this antibody may be suitable for anti-H5 therapy and as a component of stockpiles, together with other antiviral agents, for health authorities to use if an appropriate vaccine was not available. Our mutation and structural analyses indicate that the antibody recognizes a relatively conserved site near the membrane distal tip of HA, near to, but distinct from, the receptor-binding site. Our analyses also suggest that the mechanism of infectivity neutralization involves prevention of receptor recognition as a result of steric hindrance by the Fc part of the antibody. Structural analyses by EM indicate that three Fab fragments are bound to each HA trimer. The structure revealed by X-ray crystallography is of an HA monomer bound by one Fab. The monomer has some similarities to HA in the fusion pH conformation, and the monomer's formation, which results from the presence of isopropanol in the crystallization solvent, contributes to considerations of the process of change in conformation required for membrane fusion.
Highlights
H5N1 avian influenza viruses remain a threat to public health mainly because they can cause severe infections in humans
We have characterized the structure, virus binding, and infectivity-neutralizing specificity of a human monoclonal antibody derived from an H5N1-infected survivor
Our structural analysis of the Fab complex with H5 HA reveals that the Fab binds outside the receptor-binding site to a relatively conserved epitope
Summary
0.020 0.019 0.046 0.013 0.131 0.042 the structure that HA has been shown to assume after exposure to the pH of membrane fusion. The bound antibody molecules form an extra layer above the HA molecules on the virus surface; the white arrows indicate examples of an IgG cross-linking two HAs. escape mutant selected by the antibody 100F4 [9] was mapped to Glu-225, a residue that is contacted by the HCDR3 of FLD194. The crystal structure of the complex formed by the FLD194 Fab with H5 HA shows that the receptor-binding site is not contacted by the antibody (Fig. 4). Similar concentrations of FLD194 IgG are required to block binding of 100 pM of a mutant H5 virus that has measurable affinity for both avian and human receptors [19] (SI Appendix, Fig. 8). The “fusion peptide,” by contrast, is retained in its neutral pH position, implying that neither dissociation nor partial HA1 detachment directly results in the fusion peptide’s extrusion
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
