Correlation of humoral immune responses to different SARS-CoV-2 antigens with virus neutralizing antibodies and symptomatic severity in a German COVID-19 cohort

Abstract

Monitoring the humoral protective immune response and its durability after SARS-CoV-2 infections is essential for risk assessment of reinfections, the improvement of diagnostic methods and the evaluation of vaccine trials. We have analyzed neutralizing antibodies and IgG responses specific to different antigens, including the inactivated whole-virion of SARS-CoV-2, the spike subunit 1 protein and its receptor binding domain, as well as the nucleocapsid protein. We show the dynamic developments of the responses from the early convalescent stages up to 9 months post symptoms onset in follow-up samples from 57 COVID-19 patients with varying clinical severity. By correlating antibody signals to neutralizing titres, valid diagnostic markers for the estimation of neutralizing protection could be identified.

Keywords: COVID-19; RBD-ELISA; S1-ELISA; humoral immunity; inactivated SARS-CoV-2; neutralizing antibodies; nucleocapsid-ELISA.

Figure 1.

Antibody characteristics of patient sera with mild and severe COVID-19 outcomes as measured on different time points PSO; A: SARS-CoV-2 focus-reduction-neutralizing titres (FRNT90) and IgG-specific antibodies to SARS-CoV-2 whole-virion, S1, RBD and N protein clustered ≤2 months PSO and 6–9 months PSO; two-way ANOVA with Holm-Sidak's multiple comparison test * = p < 0.05, ** = p < 0.01, *** = p < 0.001, **** = p < 0.0001, B: Individual progression of SARS-CoV-2 FRNT90 and IgG-specific antibodies to SARS-CoV-2 whole-virion, S1, RBD and N protein of mild and severe COVID-19 patients; C: Reduction of SARS-CoV-2 neutralizing titres and IgG antibody signals specific to SARS-CoV-2 whole-virion, S1, RBD and N protein over two follow-up timepoints, calculated as ratio of value on timepoint 1 (≤2 months PSO) to timepoint 2 (6–9 months PSO); D: Seropositivity (%) of study participants for SARS-CoV-2 neutralizing antibodies and IgG on SARS-CoV-2 whole-virion, S1, RBD and N; Abbreviations: ANOVA: 1-way analysis of variance; COVID-19: coronavirus induced disease 19; EI: Euroimmun; FRNT90: Focus reduction neutralization titre 90; N: nucleocapsid protein; nAb: neutralizing antibodies; PSO: post symptom onset; RBD: receptor binding domain; SARS-CoV-2: Severe acute respiratory syndrome coronavirus-2; S1: subunit 1 of SARS-CoV-2 spike protein; VT: Virotech.

Figure 2.

Correlation of SARS-CoV-2 neutralization titres (FRNT90) to A: SARS-CoV-2 whole-virion IgG ELISA; B: S1 IgG ELISA; C: RBD IgG ELISA and D: N IgG ELISA for samples ≤2 months PSO and 6–9 months PSO; Abbreviations: FRNT90: Focus reduction neutralization titre 90; N: nucleocapsid protein; PSO: post symptom onset; r: Spearman's rank correlation coefficient; RBD: receptor binding domain; SARS-CoV-2: Severe acute respiratory syndrome coronavirus-2; S1: subunit 1 of SARS-CoV-2 spike protein; * = p < 0.05, ** = p < 0.01, *** = p < 0.001, **** = p < 0.0001.

Figure 3.

Correlation of SARS-CoV-2 neutralization titres (FRNT90) of A: mild COVID-19 patients and B: severe COVID-19 patients to SARS-CoV-2 whole-virion IgG ELISA, S1 IgG ELISA, RBD IgG ELISA and N IgG ELISA Abbreviations: COVID-19: corona virus induced disease 19; FRNT90: Focus reduction neutralization titre 90; N: nucleocapsid protein; PSO: post symptom onset; r: Spearman's rank correlation coefficient; RBD: receptor binding domain; SARS-CoV-2: Severe acute respiratory syndrome coronavirus-2; S1: subunit 1 of SARS-CoV-2 spike protein; * = p < 0.05, ** = p < 0.01, *** = p < 0.001, **** = p < 0.0001.

Figure 4.

Correlation of SARS-CoV-2 neutralization titres (FRNT90) to RBD IgGAM ELISA for samples ≤2 months PSO and 6–9 months PSO; Abbreviations: FRNT90: Focus reduction neutralization titre 90; PSO: post symptom onset; r: Spearman's rank correlation coefficient; RBD: receptor binding domain; SARS-CoV-2: Severe acute respiratory syndrome coronavirus-2; **** = p < 0.0001.