Affinity of anti-spike antibodies in SARS-CoV-2 patient plasma and its effect on COVID-19 antibody assays

Abstract

Background Measuring anti-spike protein antibodies in human plasma or serum is commonly used to determine prior exposure to SARS-CoV-2 infection and to assess the anti-viral protection capacity. According to the mass-action law, a lesser concentration of tightly binding antibody can produce the same quantity of antibody-antigen complexes as higher concentrations of lower affinity antibody. Thus, measurements of antibody levels reflect both affinity and concentration. These two fundamental parameters cannot be disentangled in clinical immunoassays, and so produce a bias which depends on the assay format. Methods To determine the apparent affinity of anti-spike protein antibodies, a small number of antigen-coated magnetic microparticles were imaged by fluorescence microscopy after probing antigen-antibody equilibria directly in patient plasma. Direct and indirect anti-SARS-CoV-2 immunoassays were used to measure antibody levels in the blood of infected and immunised individuals. Findings We observed affinity maturation of antibodies in convalescent and vaccinated individuals, showing that higher affinities are achieved much faster by vaccination. We demonstrate that direct and indirect immunoassays for measuring anti-spike protein antibodies depend differently on antibody affinity which, in turn, affects accurate interpretation of the results. Interpretation Direct immunoassays show substantial antibody affinity dependence. This makes them useful for identifying past SARS-CoV-2 exposure. Indirect immunoassays provide more accurate quantifications of anti-viral antibody levels. Funding The authors are all full-time employees of Abbott Laboratories. Abbott Laboratories provided all operating funds. No external funding sources were used in this study.

Prevalence and predictors of anti-SARS-CoV-2 serology in a highly vulnerable population of Rio de Janeiro: A population-based serosurvey

“…Both assays are automated and commercially available and have high sensitivity and specificity according to the manufacturer, suggesting that the discrepancies derive from antibody kinetics after natural infection and affinity to specific variants. 60 , 61 , 62 , 63 , 64 Antibodies targeting different components of the SARS-CoV-2 (i.e., nucleocapsid or spike proteins) have different dynamics. Anti-N IgG antibodies are detected earlier in the course of the infection and wane fastest (half-life of 63-85 days), followed by anti-RBD (half-life 83-126 days) and lastly by anti-S IgG (half-life 126-229 days).…”

Affinity of anti-spike antibodies to three major SARS-CoV-2 variants in recipients of three major vaccines

“…As we showed in a previous publication 16 , we developed a method to determine the apparent affinity of anti-viral antibody to its antigen by performing binding titrations directly in patient serum. It is important to underscore that the apparent affinity value obtained by this method is independent of antibody concentration and is a measure of the functional binding strength of the polyclonal antibodies.…”

“…However, the neutralization efficiency includes a convolution of both antibody affinity and concentration, which precludes quantification of either property. In our study, we diluted each antibody sample below the expected dissociation constant to ensure that the obtained affinity was independent of concentration 16 . We then probed the titration reaction with a minute number of microparticles to minimize perturbation of the solution-phase equilibrium.…”

“…In our previous publication 16 , we presented a simple imaging-based method for measuring antibody affinities independently of concentration. This was done directly in human blood serum or plasma without purification or labeling and used to follow the development of the humoral immune response in convalescent COVID-19 patients.…”

Exploring the Role of Serology Testing to Strengthen Vaccination Initiatives and Policies for COVID-19 in Asia Pacific Countries and Territories: A Discussion Paper

“…The following steps might be necessary to achieve consensus on the correlates of protection [281][282][283]:…”