Adjuvanted H1 stalk immunization induces seroreactivity to heterosubtypic group I but not group II hemagglutinins.

<p>Mice sequentially immunized (cohort III: cH9/1, cH6/1 and H1 soluble protein) are seroreactive to the homologous PR8 stalk (A), the heterosubtypic H5 (B) and H2 subtype stalks (C), but not to the group II subtype H3N2 virus (D). The H3N2 positive control is sera from mice infected with a sublethal dose of X31 (A/Hong Kong/1968 HA and NA in the PR8 background). Error bars represent the standard error of the mean for individual mice (A) or the standard deviation for replicates of pooled serum samples (B-D). *p<0.001. Significant differences in stalk-specific HA antibody titers were observed at the indicated points compared to the unadjuvanted control group.