Evolution of Anti-RBD IgG Avidity following SARS-CoV-2 Infection

Abstract

SARS-CoV-2 infection rapidly elicits anti-Spike antibodies whose quantity in plasma gradually declines upon resolution of symptoms. This decline is part of the evolution of an immune response leading to B cell differentiation into short-lived antibody-secreting cells or resting memory B cells. At the same time, the ongoing class switch and antibody maturation processes occurring in germinal centers lead to the selection of B cell clones secreting antibodies with higher affinity for their cognate antigen, thereby improving their functional activity. To determine whether the decline in SARS-CoV-2 antibodies is paralleled with an increase in avidity of the anti-viral antibodies produced, we developed a simple assay to measure the avidity of anti-receptor binding domain (RBD) IgG elicited by SARS-CoV-2 infection. We longitudinally followed a cohort of 29 convalescent donors with blood samples collected between 6- and 32-weeks post-symptoms onset. We observed that, while the level of antibodies declines over time, the anti-RBD avidity progressively increases and correlates with the B cell class switch. Additionally, we observed that anti-RBD avidity increased similarly after SARS-CoV-2 mRNA vaccination and after SARS-CoV-2 infection. Our results suggest that anti-RBD IgG avidity determination could be a surrogate assay for antibody affinity maturation and, thus, suitable for studying humoral responses elicited by natural infection and/or vaccination.

A boost with SARS-CoV-2 BNT162b2 mRNA vaccine elicits strong humoral responses independently of the interval between the first two doses

“…Consistent with the proposed use of the RBD-avidity index as a surrogate for antibody maturation, the kinetics differed from those of the regular ELISA that only measure total levels of anti-RBD Abs. For example, 12 weeks after the first dose in the LI regimen, there was a significant decline in anti-RBD IgG levels but the affinity of anti-RBD Abs likely improved as shown by an increase in their avidity (Figure 4B and D), consistent with recent results (Tauzin et al, 2022b). We previously reported that several humoral responses, including RBD-avidity was lower in individuals receiving a SI (Tauzin et al, CHM 2022).…”

“…Collection time points did not perfectly match between the two cohorts rendering perilous a side-by-side comparison using assays that only measure antibody quantities rather than their quality. We therefore decided to measure the avidity for the RBD of the induced IgG by a previously described assay (Björkman et al, 1999; Fialová et al, 2017; Tauzin et al, 2022b). This assay consists of parallel ELISAs with washing buffer having or not a chaotropic agent (8M urea).…”

“…This process is important for vaccine efficacy since it allows the immune system to generate antibodies with greater potency for viral neutralization and Fc-effector functions. We recently developed a high-throughput assay that could be used as a surrogate for antibody maturation (Tauzin et al, 2022b). Using this assay, we observed that the avidity of anti-RBD Abs was significantly higher in the LI after the second dose compared to the SI, suggesting that increasing the time between exposure to the antigen led to a better maturation of the B cells and so to Abs with higher avidity (Tauzin et al, 2022a).…”

“…We therefore decided to measure the avidity for the RBD of the induced IgG by a previously described assay (Björkman et al, 1999;Fialová et al, 2017;Tauzin et al, 2022b). This assay consists of parallel ELISAs with washing buffer having or not a chaotropic agent (8M urea).…”

“…To calculate the RBD-avidity index, we performed in parallel a stringent ELISA, where the plates were washed with a chaotropic agent, 8M of urea, added of the washing buffer. This assay was previously described (Tauzin et al, 2022b). HRP enzyme activity was determined after the addition of a 1:1 mix of Western Lightning oxidizing and luminol reagents (Perkin Elmer Life Sciences).…”