Abstract
To assess the response to vaccination, quantity (concentration) and quality (avidity) of neutralizing antibodies are the most important parameters. Specifically, an increase in avidity indicates germinal center formation, which is required for establishing long-term protection. For influenza, the classical hemagglutination inhibition (HI) assay, however, quantifies a combination of both, and to separately determine avidity requires high experimental effort. We developed from first principles a biophysical model of hemagglutination inhibition to infer IgG antibody avidities from measured HI titers and IgG concentrations. The model accurately describes the relationship between neutralizing antibody concentration/avidity and HI titer, and explains quantitative aspects of the HI assay, such as robustness to pipetting errors and detection limit. We applied our model to infer avidities against the pandemic 2009 H1N1 influenza virus in vaccinated patients (n = 45) after hematopoietic stem cell transplantation (HSCT) and validated our results with independent avidity measurements using an enzyme-linked immunosorbent assay with urea elution. Avidities inferred by the model correlated with experimentally determined avidities (ρ = 0.54, 95% CI = [0.31, 0.70], P < 10-4). The model predicted that increases in IgG concentration mainly contribute to the observed HI titer increases in HSCT patients and that immunosuppressive treatment is associated with lower baseline avidities. Since our approach requires only easy-to-establish measurements as input, we anticipate that it will help to disentangle causes for poor vaccination outcomes also in larger patient populations. This study demonstrates that biophysical modelling can provide quantitative insights into agglutination assays and complement experimental measurements to refine antibody response analyses.
Highlights
To assess influenza vaccine efficacy, hemagglutination inhibition (HI) titers are traditionally used as a surrogate for the influenza-neutralization capacity of vaccine-induced antibodies in serum [1, 2]
An increase in avidity indicates a successful germinal center reaction, which is required for establishing long-term protection
Since patients with low antibody avidities are at risk for fatal infections, we investigated the association of inferred avidities with three indicators of immunocompromised status as defined by CDC [11], known to be associated with immune cell proliferation, affinity maturation and antibody production [12]: first two years post transplantation, immunosuppressive treatment, and chronic graft-versus-host disease grade according to NIH criteria [13]
Summary
To assess influenza vaccine efficacy, hemagglutination inhibition (HI) titers are traditionally used as a surrogate for the influenza-neutralization capacity of vaccine-induced antibodies in serum [1, 2]. In the presence of influenza-binding antibodies that block RBC binding sites, hemagglutination is inhibited. This allows quantifying the neutralization capacity of serum antibodies in dilution experiments: the highest serum dilution that fully inhibits hemagglutination is determined, and its dilution factor is reported as the HI titer [4]. The HI titer measures a combination of both antibody concentration and avidity, where avidity quantifies the overall strength of a multivalent antibody binding to hemagglutinin epitopes involved in virus-RBC interaction (neutralizing binding). It is important to distinguish between antibody concentration and avidity. No increase in avidity following vaccination indicates a hampered formation of germinal centers (GCs) where B cells undergo affinity maturation and proliferate to long-lived B cells, providing long-term protection [5, 6]
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