An influenza HA stalk reactive polymeric IgA antibody exhibits anti-viral function regulated by binary interaction between HA and the antibody

Abstract

IgA antibodies, which are secreted onto the mucosal surface as secretory IgA antibodies (SIgAs), play an important role in preventing influenza virus infection. A recent study reported that anti-hemagglutinin (HA) head-targeting antibodies increase anti-viral functions such as hemagglutination inhibition (HI) and virus neutralization (NT), in addition to HA binding activity (reactivity) via IgA polymerization. However, the functional properties of anti-viral IgA antibodies with mechanisms of action distinct from those of anti-HA head-targeting antibodies remain elusive. Here, we characterized the functional properties of IgG, monomeric IgA, and polymeric IgA anti-HA stalk-binding clones F11 and FI6, and B12 (a low affinity anti-HA stalk clone), as well as Fab-deficient (ΔFab) IgA antibodies. We found that IgA polymerization impacts the functional properties of anti-HA stalk antibodies. Unlike anti-HA head antibodies, the anti-viral functions of anti-HA stalk antibodies were not simply enhanced by IgA polymerization. The data suggest that two modes of binding (Fab paratope-mediated binding to the HA stalk, and IgA Fc glycan-mediated binding to the HA receptor binding site (RBS)) occur during interaction between anti-stalk HA IgA antibodies and HA. In situations where Fab paratope-mediated binding to the HA stalk exceeded IgA Fc glycan-mediated binding to HA RBS, IgA polymerization increased anti-viral functions. By contrast, when IgA Fc glycan-mediated binding to the HA RBS was dominant, anti-viral activity will fall upon IgA polymerization. In summary, the results suggest that coordination between these two independent binding modules determines whether IgA polymerization has a negative or positive effect on the anti-viral functions of anti-HA stalk IgA antibodies.

Structure-Guided Creation of an Anti-HA Stalk Antibody F11 Derivative That Neutralizes Both F11-Sensitive and -Resistant Influenza A(H1N1)pdm09 Viruses

“…To gain insights into the structural impact of F11 binding to HA, we compared fluctuations of the HA ectodomain between the free and F11-bound forms using root mean square fluctuation (RMSF), a quantitative indicator of atomic fluctuation [23] (Figure 4). As expected from previous reports [18,31,32], structural fluctuations were often seen in the loops of the ligand-free HA glycoprotein (Figure 4A, free HA). Notably, the proteolytic cleavage loop in the stalk domain, which is the target of cellular proteases [59][60][61][62], was found to fluctuate most heavily in the HA ectodomain under solution conditions.…”

“…A/Narita/1/2009 (H1N1)pdm09 virus [19] and A/Narita/1/2009 (H1N1)pdm09-derived F11 escape mutants (C1 and G6 [18]) were propagated in MDCK cells.…”

“…Structural fluctuations of biological macromolecules in solution play key roles in molecular interactions [55][56][57][58] and thus in the biological phenotypes of viruses [18,[31][32][33].…”

“…Free and F11 Fab-bound trimeric HA ectodomains were subjected to MD simulations for 200 ns using the Amber 16 program package [23]. RMSF values of the Cα atoms of individual HA residues were calculated using 10,000 snapshots during 180 to 200 ns of MD simulations as described previously [18,31,32] [64], and hydrophobic groove [53,54] are shown in cyan, purple, and orange, respectively. A red arrowhead indicates the cleavage site.…”

“…Recently, we independently isolated an anti-HA stalk antibody, termed F11, from an individual inoculated with an intranasal inactivated influenza vaccine [ 18 ]. In its IgA form, F11 was able to neutralize the H1N1, H3N2, and H5N1 strains of influenza A, but in its IgG form it had lower cross-reactivity [ 18 ]. We also isolated variants of influenza A(H1N1)pdm09 that exhibited reduced susceptibilities to neutralization mediated by the F11 antibody [ 18 ].…”