Figure 2: Enhanced hAb responses correlated with CD4⁺ T cell cross-reactivity.

C57BL/6 (n = 5 per group) and CB6F1 mice were intranasally infected with either A/PR8 or A/CA. Day 14 post infection spleens were harvested and individually stimulated with pooled peptides (1–10) of A/CA HA. Cells were then stained for analysis by flow cytometry and CD4⁺CD44^hiIFNγ⁺ cells frequency of parent percentage (FoP %) were quantified with in panel (A). Error bars indicate ± SD and data show results from one of two experiments with similar results (n = 5 animals per experiment). (*p<0.05, **p<0.01, ***p<0.001, ****p<0.0001, Two-way ANOVA). Flow cytometry gating strategy and representative flow plots are shown in Supplementary Figure 1. B10.D2 (n = 10 per group) mice were infected with A/CA or mock infected with PBS then 28 days later immunized with A/PR8 HA (1 μg). HAI titer to A/PR8 in panel (B) were evaluated in serum samples 21 days post drifted A/PR8 rHA i.m. immunization. (**p<0.01, One-way ANOVA). Data show results from one of two independent experiments with similar results (n = 10 animals per experiment).