Extended Data Fig. 4: Comparison between stalk-only and full-length HA construct.

Stalk-only constructs with or without the A388V mutation (See Extended Data Fig. 7b for the amino acid sequence) were produced to measure the level of antibodies recognizing the mutant stalk in human serum while excluding head-binding antibodies (Fig. 3j). To confirm that the stalk-only construct with A388V mutation appropriately represents the natural A388V stalk structure, the level of decrease in broadly neutralizing monoclonal antibodies (bNAbs) binding was compared between the stalk-only construct and the full-length HA. ELISA was performed using serially diluted (a,b) CR6261, (c,d) CR9114, (e,f) FI6V3, (g,h) 70-1F02, (I,j) C179, and (k,l) CT149. (m,n) Anti-StrepTag II antibody was used to show that equal amounts of wild-type and mutant antigen were used for the analysis. The AUC was calculated using GraphPad Prism8 (v.8.3.0). The AUC for the full-length HAs (b,d,f,h,j,l) was calculated using the data from Fig. 3. Graphs show mean and standard deviations from three independent measurements. The summary table shows the mean OD492 values and standard error of mean (s.e.m.) of the three independent measurements. The comparison result (summarized in the table) shows that the A388V stalk-only construct closely represents the natural stalk structure of the full-length A388V HA.