Figure 2.

Cross-reactive hemagglutinin (HA)-specific antibody responses elicited by vaccination. (A) Breadth of the HA-specific sera IgG antibodies. To examine the breadth of HA-specific sera IgG antibodies induced by vaccination, recombinant HA proteins expressed in insect cells were used as coating antigens in ELISA. The HA proteins tested include five different group 1 HAs from H1N1 (A/California/6/2009), H1N1 (A/Puerto Rico/8/1934), H2N2 (A/Canada/720/2006), H5N1 (A/Indonesia/5/2005), and H9N2 (A/Hong Kong/35820/2009) and two group 2 HAs from H3N2 (A/Sydney/5/1997) and H7N9 (A/Anhui/1/2013) influenza viruses. (B) Antibody titers specific to HA full-length or stalk of pH1N1 or PR8 (H1N1) virus. Using the Escherichia coli-expressed LysRS-HA fusion proteins as coating antigens, sera IgG antibodies specific to the HA full-length or stalk protein were measured by ELISA. Antibody titers were expressed as the reciprocal serum dilution that yielded OD₄₅₀ greater than the mean + 2 SD (SD) of PBS control group. Data are the mean of each cohort (N = 5), and error bars indicate SD. ***P < 0.001; **P < 0.01; *P < 0.05 when comparing the antibody titers between the vaccination group and PBS control group (A) or between two different groups (B).