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. 2021 Jan 7;16(1):e0245244. doi: 10.1371/journal.pone.0245244

Fig 1. IgA polymers mask the hemagglutinin (HA) receptor binding site.

Fig 1

(A) The HI titer for each IgG1, monomeric (M) or polymeric (P) IgA1 (A1), and IgA2m2 (A2m2) antibody derived from clones F11 and FI6, tested against virus strain A/New Caledonia/20/1999 (H1N1, NC20). HI activity is presented in the scatter plots as the geometric mean, with the geometric standard deviation (SD) of six technical replicates. Y-axis values represent the HI titer, calculated as the 100,000/minimum concentration (μg/ml), which represents HI activity. Dotted lines represent the detection limit (y = 200; 500 μg/mL). **p < 0.01, comparing monomers with polymers (Mann–Whitney U test). ☨☨p < 0.01 and ☨☨☨☨p < 0.0001, comparing IgA monomers/polymers with IgG (Kruskal–Wallis test, followed by Dunn’s multiple comparison test). For statistical analysis, a provisional minimum HI activity value (y = 100; 1000 μg/mL) was applied to samples in which the HI activity was below the detection limit. (B) Schematic diagram showing RBS blockade, as assessed by SPR. First, recombinant HA from virus strain A/New Caledonia/20/1999 (H1N1) was bound to IgG1 or to monomeric or polymeric F11 IgA2m2. The amount of F045-092 IgG1 binding to F11-pre-bound HA was then measured to evaluate RBS blocking by F11. The relative response (RU) was acquired immediately after addition of F045-092 IgG1 (binding stability). The maximum stability value was acquired by measuring the RU immediately following binding of F045-092 IgG1 to free HA. (C) Binding of F045-092 IgG1 to F11 IgG1-, IgA2m2-, or IgA2m2-pre-bound HA. Y-axis values represent the ratio of F045-092 IgG1 binding to the maximum stability value. **p < 0.01, ***p < 0.001, and ****p < 0.0001 (two-way ANOVA).