Hemagglutinin-specific antibody responses following 2009 pandemic H1N1 influenza vaccination in healthcare workers
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Influenza, a respiratory virus, has lead to a substantial morbidity and mortality in humans. The most reliable way of preventing the infection is through vaccination. Due to the high rate of change in influenza surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), vaccination does not provide broad and long-term immunity. Generally, antibodies that are directed to the head domain of hemagglutinin are measured as surrogate correlates of protection in the hemagglutination inhibition (HI) assay. Seasonal influenza vaccines did not induce protection against the novel pandemic influenza H1N1 virus that started circulating in 2009 (A(H1N1)pdm09). A(H1N1)pdm09 vaccines were shown to induce HA stalk-specific antibodies that are broadly protective, in contrast to seasonal vaccines that induce strain-specific antibodies directed to the HA head domain and little or no HA stalk-specific antibodies.
The purpose of this study was to evaluate in detail the HA head and stalk specific antibody responses to vaccination with AS03 adjuvanted A(H1N1)pdm09 vaccine Pandemrix® among 57 healthcare workers at Haukeland University Hospital (HUH) (Bergen, Norway). Serum samples were taken prior to and 21 days post vaccination. The antibody responses were studied using traditional assays, HI and microneutralization (MN), which measure HA receptor binding antibodies and neutralizing antibodies, respectively. The IgG avidity to the conserved HA stalk and the more variable HA head domains were assessed in avidity enzyme linked immunosorbent assay (avidity ELISA) assay. To dissect the IgG response towards the HA stalk and HA head domains, specific IgG1, IgG2, IgG3 and IgG4 subclass responses were quantified in ELISA. We also assessed the functional ability of the HA stalk antibodies to activate Natural Killer (NK) cells in antibody dependent cellular cytotoxicity (ADCC) assay.
Based on HI titer prior to vaccination individuals were assigned to two groups based on whether they had seroprotective HI titer (HI titer ≥40). Group 1 (20 individuals) had baseline protective HI titer (HI titer ≥40) while group 2 (37 individuals) had baseline HI titer below the protective level. Consistent with the HI results, group 1 showed higher baseline MN titers and HA head and stalk specific IgG1 and IgG3 in ELISA. However, post-vaccination, the HI and MN titers increased did not differ significantly between the two groups. This was also the case with the concentration of IgG1 and IgG3. The IgG1 and IgG3 response was HA stalk dominant pre vaccination in both groups. However, post vaccination, the response remained HA stalk dominant for IgG3 but became HA head dominant for IgG1 in group 1.
Of interest, the baseline avidity of HA head and HA stalk specific IgG was lower in group 1 compared to group 2. Furthermore, vaccination did not significantly increase the avidity of HA stalk and HA head specific antibodies. However, the stalk specific antibodies had higher avidity levels compared to the HA head specific antibodies at both time-points.
In ADCC, NK cell activation was measured by the expression of CD107a and INF-γ in response to activation by HA stalk-specific antibodies. NK cell activation was not significantly different between the two groups, both at baseline and post vaccination levels. However, post vaccination, the percentage of NK cells expressing CD107a and INF-γ was significantly increased in both groups. This provides evidence that the HA stalk-specific antibodies are indeed available regardless of the extent to which the mainly neutralizing antibodies are present, but their importance might not be reflected in more conventional serological assays (i.e. HI).
The results of the present study have implications for vaccination strategies aimed at inducing higher levels of high avidity HA stalk-specific antibodies that may provide protection through non-neutralizing functions such as ADCC. These HA stalk-specific antibodies may provide a broader protection against different strains of seasonal and pandemic influenza viruses.