Abstract
Because of the pathogenicity and low incidence of avian influenza virus infections in humans, the immune correlates of protection for avian influenza vaccines cannot be determined from clinical studies. Here, we used the ferret model to address this for an avian influenza H5N1 vaccine. Using oil-in-water adjuvants, we generated groups of ferrets with undetectable (geometric mean titer [GMT] < 10), low (GMT = 28.3), or high (GMT > 761.1) hemagglutination-inhibition (HAI) titers to the A/Viet Nam/1203/2004 (H5N1) virus. Ferrets were then challenged with the wild-type virus and disease severity and immunologic parameters were studied. The severity of infection and symptom profile were inversely associated with pre-challenge HAI titers in a dose-dependent manner. A vaccinated ferret with no detectable HAI-antibodies but high flu-specific IgG-antibody titers mounted rapid functional antibodies after infection and experienced milder disease compared to other ferrets in the group. Compared to naïve ferrets, all vaccinated ferrets showed improved cellular immunity in the lungs and peripheral blood. High number of IFNγ+ CD8- T cells in the airways was associated with early viral clearance. Thus, while neutralizing antibodies are the best correlate of protection, non-neutralizing antibodies can also be protective. This should be taken into consideration in future avian influenza vaccine trials.
Highlights
Viruses (AIV) are a significant threat to the human population, especially the H5N1 and H7N9 avian influenza viruses
Our vaccination strategy resulted in three groups of ferrets with distinctly different pre-challenge HAI titers, categorized as undetectable, low (GMT = 28.3), and high (GMT = 761.1) that segregated with adjuvant type
Despite virus neutralization (VN) assays being more sensitive, we found that the VN titers were lower than HAI titers, especially in the AS03-group
Summary
Viruses (AIV) are a significant threat to the human population, especially the H5N1 and H7N9 avian influenza viruses. Compared with other avian influenza viruses, the H5N1 virus, the A/Viet Nam/1203/2004 strain used in this study[1], produces a pathogenic phenotype in ferrets[10], enabling us to delineate the association of immune parameters with multiple aspects of disease severity. We used adjuvants to induce different levels of HAI antibodies (the standard measure of inactivated influenza virus vaccine immunogenicity) in ferrets and subsequently challenged these ferrets with homologous wild-type virus to learn how different immunological factors might modulate the ensuing disease. By using an animal model, this study helps bridge the gap left from vaccine trials by providing information on how vaccine-induced immunogenicity could potentially modulate the resulting disease severity
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