Novel use of a bead-based immunoassay to quantify SARS-CoV-2 variant-specific antibody avidity 4661

Description Antibody avidity has previously been used as a surrogate of affinity maturation to determine the timecourse of prior infections. More recently, antibody avidity has been shown to correlate with neutralizing function against SARS-CoV-2. Using chaotropic agents, such as urea, we can dissociate antigens from low avidity bound antibodies, while high avidity antibody species remain bound. In our pilot study we demonstrate a novel use of urea in a bead-based immunoassay for the determination of SARS-CoV-2 antibody avidity. Using human plasma, we generated avidity indices (AI) for antibodies targeting ancestral and Omicron variant SARS-CoV-2 receptor binding domain (RBD) as well as nucleocapsid (N) proteins. Anti-RBD (ancestral) IgG from early time points (< 3 weeks) following infection showed low avidity (AI < 0.1). Consistent with affinity maturation, late time points (> 4 weeks) showed high avidity (AI > 0.5). We also observed high avidity of anti-RBD (Omicron) IgG binding following multiple vaccine exposures, regardless of vaccine immunogen. Anti-N IgG antibodies demonstrated affinity maturation following infection, but did not achieve avidity indices higher than 0.3. Taken together, these results demonstrate the successful adaptation of a multiplexed serological assay for the measurement of antibody avidity. High-throughput methods such as this will be useful to understand antibody avidity on the population scale, and how it impacts protection against emerging SARS-CoV-2 variants.