Boosting heterosubtypic neutralization antibodies in recipients of 2009 pandemic H1N1 influenza vaccine

Abstract

Background: A mass vaccination has been implemented to prevent the spread of 2009 pandemic influenza virus in China. Highly limited information is available on whether this vaccine induces cross-reactive neutralization antibodies against other subtypes of influenza viruses.

Methods: We employed pseudovirus-based assays to analyze heterosubtypic neutralization responses in serum samples of 23 recipients of 2009 pandemic influenza vaccine.

Results: One dose of pandemic vaccine not only stimulated good neutralization antibodies against cognate influenza virus 2009 influenza A (H1N1), but also raised broad cross-reactive neutralization activities against seasonal H3N2 and highly pathogenic avian influenza virus H5N1 and lesser to H2N2. The cross-reactive neutralization activities were completely abolished after the removal of immunoglobin G (IgG). In contrast, H1N1 vaccination alone in influenza-naive mice elicited only vigorous homologous neutralizing activities but not cross-reactive neutralization activities.

Conclusions: Our data suggest that the cross-reactive neutralization epitopes do exist in this vaccine and could elicit significant cross-reactive neutralizing IgG antibodies in the presence of preexisting responses. The exposure to H1N1 vaccine is likely to modify the hierarchical order of preexisting immune responses to influenza viruses. These findings provide insights into the evolution of human immunity to influenza viruses after experiencing multiple influenza virus infections and vaccinations.

Figure 1.

Homologous immune responses in serum samples of 2009 pandemic vaccine recipients. Twenty-three volunteers received 1 dose of 2009 pandemic vaccine. Serum samples were collected before and 21 days after vaccination. A, The individual IC₅₀ titers (reciprocal 50% inhibitory concentration) before (gray) and after (dark) vaccination. B, Correlation between the IC₅₀ titers and hemagglutination-inhibition (HI) titers in serum samples post vaccination. C, Correlation between the increase of HI titer from baseline (HI post – HI pre) and the change in magnitude of IC₅₀ titer from baseline (IC₅₀ post – IC₅₀ pre).

Figure 2.

Cross-reactive neutralization activities to heterosubtypes of influenza viruses in serum samples of 2009 pandemic vaccine recipients. Neutralization activities against A/Moscow/10/1999(H3N2) (A), A/Brisbane/59/2007(H1N1) (B), A/Japan/305/57(H2N2) (C), and A/Viet Nam/1203/2004(H5N1) (D). Left, individual 50% inhibitory concentration (IC₅₀) titers; middle, correlation between the hemagglutination-inhibition (HI) titers to pandemic influenza and IC₅₀ titers to tested virus in sera postvaccination; right, correlation between the increase of HI titers to pandemic influenza from baseline and the change in magnitude of IC₅₀ titers to tested virus.

Figure 3.

Vaccination with 2009 pandemic influenza–derived hemagglutinin (HA) immunogen mainly elicited homologous neutralization antibodies in influenza-naive mice. Six mice per group bred in specific pathogen-free environment received 4× DNA vaccine-expressing HA of pandemic vaccine, or sham DNA as mock control group. Two weeks after final inoculation, serum samples were collected and combined by group. The neutralization activities against homologous, seasonal, and highly pathogenic avian influenza viruses in both vaccinated mice (▴) and mock-control mice (Δ) were shown in serial 2-fold dilutions, triplicates were set for each dilution, and the mean value/standard error bar were used in plot. Less than 50% inhibition was considered as negative, the final dilution of serum samples able to reach ≥50% inhibition was considered as the serum sample titer. Since serum samples from 6 mice were combined for this assay, the value for each serum sample dilution represents the mean value from 6 mice.