FIG 2.

Serum responses of vaccinated ferrets increased after virus challenge. The data shown were collected from ferrets included in the initial transmission experiment (four mock immunized and four stalk immunized). (A) The development of H1 stalk-reactive antibody responses was assessed by ELISAs with baculovirus-produced cH2/1 HA. ELISAs were performed as described before (9, 24). OD, optical density. (B) Activity of sera from mock- and stalk-vaccinated ferrets was measured in an ADCC reporter assay (Promega; as described in reference 12) at a dilution of 1:90. The assay was developed to measure the interaction between human Fc fragments and human FcγRIIIa. This is a caveat of the assay because ferret antibodies might be less effective than human antibodies at activating this receptor. However, no ferret-specific assay is available. *, P ≤ 0.05. Ferret influenza virus microneutralization titers (C), HI titers (D), and VSV neutralization titers (E) in sera from mock- and stalk-vaccinated animals are shown. Statistical analysis of the data in panel B was done by an unpaired t test that was performed in GraphPad Prism.