Effect of influenza A(H5N1) vaccine prepandemic priming on CD4+ T-cell responses.

Introduction: Previous priming with avian influenza vaccines results in more rapid and more robust neutralizing antibody responses upon revaccination, but the role CD4(+) T cells play in this process is not currently known.
Methods: Human subjects previously enrolled in trials of inactivated influenza A(H5N1) vaccines and naive subjects were immunized with an inactivated subunit influenza A/Indonesia/5/05(H5N1) vaccine. Neutralizing antibody responses were measured by a microneutralization assay, and hemagglutinin (HA)-specific and nucleoprotein (NP)-specific CD4(+) T-cell responses were quantified using interferon γ enzyme-linked immunosorbent spot assays.
Results: While vaccination induced barely detectable CD4(+) T-cell responses specific for HA in the previously unprimed group, primed subjects had readily detectable HA-specific memory CD4(+) T cells at baseline and mounted a more robust response to HA-specific epitopes after vaccination. There were no differences between groups when conserved NP-specific CD4(+) T-cell responses were examined. Interestingly, neutralizing antibody responses following revaccination were significantly higher in individuals who mounted a CD4(+) T-cell response to the H5 HA protein, a correlation not observed for NP-specific responses.
Conclusions: These findings suggest that prepandemic vaccination results in an enriched population of HA-specific CD4(+) T cells that are recruited on rechallenge with a drifted vaccine variant and contribute to more robust and more rapid neutralizing antibody responses.
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