Seven-month kinetics of SARS-CoV-2 antibodies and role of pre-existing antibodies to human coronaviruses

Abstract

Unraveling the long-term kinetics of antibodies to SARS-CoV-2 and the individual characteristics influencing it, including the impact of pre-existing antibodies to human coronaviruses causing common cold (HCoVs), is essential to understand protective immunity to COVID-19 and devise effective surveillance strategies. IgM, IgA and IgG levels against six SARS-CoV-2 antigens and the nucleocapsid antigen of the four HCoV (229E, NL63, OC43 and HKU1) were quantified by Luminex, and antibody neutralization capacity was assessed by flow cytometry, in a cohort of health care workers followed up to 7 months (N = 578). Seroprevalence increases over time from 13.5% (month 0) and 15.6% (month 1) to 16.4% (month 6). Levels of antibodies, including those with neutralizing capacity, are stable over time, except IgG to nucleocapsid antigen and IgM levels that wane. After the peak response, anti-spike antibody levels increase from ~150 days post-symptom onset in all individuals (73% for IgG), in the absence of any evidence of re-exposure. IgG and IgA to HCoV are significantly higher in asymptomatic than symptomatic seropositive individuals. Thus, pre-existing cross-reactive HCoVs antibodies could have a protective effect against SARS-CoV-2 infection and COVID-19 disease.

Fig. 1. Kinetics of SARS-CoV-2 antibody levels since onset of symptoms.

Levels (median fluorescence intensity, MFI) of IgA, IgG, and IgM against each antigen (Nucleocapsid full-length protein (N), and its C-terminal domain, the Receptor Binding Domain (RBD), full S protein and its subregions S1 and S2) measured in 235 samples from 76 symptomatic participants collected in up to four time points per participant (paired samples joined by lines). The black solid line represents the fitted curve calculated using the LOESS (locally estimated scatterplot smoothing) method. Shaded areas represent 95% confidence intervals. Dashed line represents the positivity threshold. Participants were grouped based on their antibody levels at M6 compared to the previous visit, individuals were labeled for each isotype-antigen pair as “Decayers” (pink) when the ratio of antibody levels between both visits was <1 and as “Sustainers/Increasers” (light blue) when the ratio was ≥1 and gray when the classification was not applicable.

Fig. 2. Longitudinal antibody neutralizing capacity.

Antibody neutralizing capacity, as a percentage of RBD-ACE2 binding inhibition in plasma samples from 64 symptomatic participants collected in three serial visits (M0, M1, and M6) represented as days after symptom onset. Paired samples are joined by gray lines. The black solid line represents the fitted curve calculated using the LOESS (locally estimated scatterplot smoothing) method. Shaded areas represent 95% confidence intervals.

Fig. 3. Correlations between antibody levels and RBD-ACE2 neutralization capacity.

Spearman’s rank correlation test between levels (median fluorescence intensity, MFI) of IgA, IgG, and IgM against each antigen (Nucleocapsid full-length protein (N), and its C-terminal domain, the Receptor Binding Domain (RBD), full S protein and its subregions S1 and S2) at A baseline visit (M0) and B M6 visit; and plasma neutralization capacity (as a percentage of RBD-ACE2 binding inhibition). Two-sided spearman test was used to calculate the p-values and r_s correlation coefficients are color-coded for each antigen/isotype pair. Colored lines represent the fitted curve calculated using the linear model method. Shaded areas represent 95% confidence intervals.

Fig. 4. The influence of anti-HCoV antibody levels on the antibody response to SARS-CoV-2.

A Differences in baseline levels (median fluorescence intensity, MFI) of IgG against HCoV N proteins between participants who were seronegative during the entire study (COVID-19 Ab-) and participants who seroconverted (COVID-19 Ab+) (n = 468). B Differences in IgA, IgG and IgM levels prior to infection against N of the four HCoVs between symptomatic and asymptomatic participants who seroconverted during the study (n = 33). C Differences in fold-increase of IgG levels against N of the four HCoVs after SARS-CoV-2 seroconversion in symptomatic vs asymptomatic COVID-19 cases (n = 33). D Differences in anti-HCoV N IgG levels at seroconversion visit between symptomatic and asymptomatic SARS-CoV-2 seropositive participants (n = 110). E Differences in anti-SARS-CoV-2 N IgG, IgA and IgM levels in asymptomatic versus symptomatic participants at seroconversion visit (n = 110). The center line of boxes depicts the median values; the lower and upper hinges correspond to the first and third quartiles; the distance between the first and third quartiles corresponds to the interquartile range (IQR); whiskers extend from the hinge to the highest or lowest value within 1.5 × IQR of the respective hinge. Two-sided Wilcoxon rank test was used to assess statistically significant differences in antibody levels between groups.