Influenza Human Monoclonal Antibody 1F1 Interacts with Three Major Antigenic Sites and Residues Mediating Human Receptor Specificity in H1N1 Viruses

Tshidi Tsibane,Jens C Krause,Damian C Ekiert,Christopher F Basler,Osvaldo Martinez,James E Crowe,Ian A Wilson,Andrew Pekosz

doi:10.1371/journal.ppat.1003067

Abstract

Most monoclonal antibodies (mAbs) to the influenza A virus hemagglutinin (HA) head domain exhibit very limited breadth of inhibitory activity due to antigenic drift in field strains. However, mAb 1F1, isolated from a 1918 influenza pandemic survivor, inhibits select human H1 viruses (1918, 1943, 1947, and 1977 isolates). The crystal structure of 1F1 in complex with the 1918 HA shows that 1F1 contacts residues that are classically defined as belonging to three distinct antigenic sites, Sa, Sb and Ca2. The 1F1 heavy chain also reaches into the receptor binding site (RBS) and interacts with residues that contact sialoglycan receptors and determine HA receptor specificity. The 1F1 epitope is remarkably similar to the previously described murine HC63 H3 epitope, despite significant sequence differences between H1 and H3 HAs. Both antibodies potently inhibit receptor binding, but only HC63 can block the pH-induced conformational changes in HA that drive membrane fusion. Contacts within the RBS suggested that 1F1 may be sensitive to changes that alter HA receptor binding activity. Affinity assays confirmed that sequence changes that switch the HA to avian receptor specificity affect binding of 1F1 and a mAb possessing a closely related heavy chain, 1I20. To characterize 1F1 cross-reactivity, additional escape mutant selection and site-directed mutagenesis were performed. Residues 190 and 227 in the 1F1 epitope were found to be critical for 1F1 reactivity towards 1918, 1943 and 1977 HAs, as well as for 1I20 reactivity towards the 1918 HA. Therefore, 1F1 heavy-chain interactions with conserved RBS residues likely contribute to its ability to inhibit divergent HAs.

Highlights

The hemagglutinin (HA) protein of influenza viruses binds to sialic acid receptors on host cells and is the major target of neutralizing antibodies
In a previous study [18], we described five naturally occurring human monoclonal antibodies (mAbs) that potently inhibit the 1918 H1N1 pandemic influenza virus
Crystal structure of mAb 1F1 in complex with 1918 HA To better understand the mechanism of 1F1 neutralization and its ability to cross-react with H1N1 viruses separated by decades of virus evolution, we determined the crystal structure of Fab 1F1 in complex with SC1918 HA using diffraction data that extend to 3.3 A, but we report a nominal resolution of 3.55 Adue to anisotropy

Summary

Introduction

The hemagglutinin (HA) protein of influenza viruses binds to sialic acid receptors on host cells and is the major target of neutralizing antibodies. A cross-reactive antibody to influenza B CR8033 was shown to bind to the head and overlap with the receptor-binding pocket [12]. Extensive epitope mapping with large panels of murine mAbs previously identified five major antigenic sites on the HA head domain of H1N1 viruses, and these have been termed Sa, Sb (residues 186–198), Ca1, Ca2 (residues 140–145, 224–225) and Cb [15,16,17].

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Conclusion