A Structural and Mathematical Modeling Analysis of the Likelihood of Antibody-Dependent Enhancement in Influenza

Abstract

Broadly neutralizing monoclonal antibodies (bNAbs) for viral infections, such as HIV, respiratory syncytial virus (RSV), and influenza, are increasingly entering clinical development. For influenza, most neutralizing antibodies target influenza virus hemagglutinin. These bNAbs represent an emerging, promising modality for treatment and prophylaxis of influenza due to their multiple mechanisms of antiviral action and generally safe profile. Preclinical work in other viral diseases, such as dengue, has demonstrated the potential for antibody-based therapies to enhance viral uptake, leading to enhanced viremia and worsening of disease. This phenomenon is referred to as antibody-dependent enhancement (ADE). In the context of influenza, ADE has been used to explain several preclinical and clinical phenomena. Using structural and viral kinetics modeling, we assess the role of ADE in the treatment of influenza with a bNAb.

Figure 1

Surface Representation of HA Homotrimer Molecule. Each monomer shown in green, cyan, or light green illustrating the neutralizing epitopes (shown in red) of the corresponding stem-binding broadly neutralizing antibodies. The epitopes were identified using the PISA server based on antibody co-crystal structures with the HA molecule.

Figure 2

Epitopes for Antibodies to HA2 in Pre- and Post-fusion Conformation. Epitope mapping by phage display had shown that the anti-HA2 antibodies bind to residues 32–76 of HA2 molecule [35]. These residues are shown in red on one molecule of the homotrimer in the (a) pre-fusion and (B) post-fusion conformations of the HA2 crystal structure. The fusion loop residues are shown in black on the pre-fusion form of HA. In the post-fusion state, the fusion peptide is not observed in the crystal structure and is therefore shown as a schematic (black line). In the pre-fusion conformation, most of the HA2 subunit residues are covered by the HA1 subunit, for clarity one HA2 subunit is shown in the insert. More specifically, the HA2 subunit residues 58–76 are covered by the HA1 subunit (shown in orange); therefore, these residues are not available for antibody binding. Conversely, in the post-fusion conformation, in addition to the fusion peptide, the stem residues 32 to 76 of HA2 subunit are found on the surface and are available for antibody binding.

Figure 3

Influenza Fusion with the Host Endosomal Membrane. On the virus surface HA molecules in their pre-fusion conformation are found as an ordered array of molecules, oriented to bind to the host cell surface receptor molecules. In the pre-fusion conformation (shown in state A), the HA2 subunit exists in a meta-stable conformation, anchored by the receptor-bound HA1 subunit, and is ready to undergo conformational changes (this is referred to as the spring loaded state). Upon virus internalization into the endosome of the host cell, HA undergoes a significant conformational change where the HA1 subunit first separates from the HA2 subunit (shown in state B) allowing the fusion peptide to be deployed towards the host endosomal membrane to initiate viral-host membrane fusion show in state C). To achieve successful host infection, both the pre- and post-fusion conformations are essential, for example lack of a protease cleavage within HA0 inhibiting formation of HA1 and HA2 prevents viral fusion. Interestingly, when HA1 and HA2 subunit proteins are expressed separately, only the HA1 molecule can be folded into its pre-fusion conformation; the HA2 molecule, expressed either with the viral transmembrane domain or only as a soluble domain (residues 376–481 HA0 numbering or 1–331 HA2 numbering), folds only into its post-fusion conformation.

Figure 4

Comparison of Virus Infection Models with Head- and Stem-Binding Antibodies. (Gray) Target cell-limited model with delayed virus production (Baccam et al.)[58]. (Black) Model with no antibodies (α = β₂ = β₃ = 0) and explicitly modelled virus consumption (Cao et al.)[61]; parameters V₀, p, and c optimized to match Baccam model. (Red) Head-binding antibody added at 1dpi (β₂ = 0.5 × β₁, β₃ = β₁, c₂ = c₃ = 0). (Pink) Head-binding antibody added at 1dpi without accounting for ADE (β₃ = 0). (Blue) Stem-binding antibody added at 1dpi (β₂ = β₃ = β₁, c₂ = k₂, c₃ = k₃). (Cyan) Stem-binding antibody added at 1dpi without accounting for ADE (β₃ = 0).