SARS-CoV-2-neutralizing humoral IgA response occurs earlier but modest and diminishes faster compared to IgG response

Abstract

Secretory immunoglobulin A (IgA) plays a crucial role in the mucosal immunity for preventing the invasion of the exogenous antigens, however, little has been understood about the neutralizing activity of serum IgA. Here, to examine the role of IgA antibodies against COVID-19 illnesses, we determined the neutralizing activity of serum/plasma IgG and IgA purified from previously SARS-CoV-2-infected and COVID-19 mRNA-vaccine-receiving individuals. We found that serum/plasma IgA possesses substantial but rather modest neutralizing activity against SARS-CoV-2 compared to IgG with no significant correlation with the disease severity. Neutralizing IgA and IgG antibodies achieved the greatest activity at approximately 25 and 35 days after symptom onset, respectively. However, neutralizing IgA activity quickly diminished and went down below the detection limit approximately 70 days after onset, while substantial IgG activity was observed till 200 days after onset. The total neutralizing activity in sera/plasmas of those with COVID-19 largely correlated with that in purified-IgG and purified-IgA and levels of anti-SARS-CoV-2-S1-binding IgG and anti-SARS-CoV-2-S1-binding IgA. In individuals who were previously infected with SARS-CoV-2 but had no detectable neutralizing IgA activity, a single dose of BNT162b2 or mRNA-1273 elicited potent serum/plasma neutralizing IgA activity but the second dose did not further strengthen the neutralization antibody response. The present data show that the systemic immune stimulation with natural infection and COVID-19 mRNA-vaccines elicit both SARS-CoV-2-specific neutralizing IgG and IgA response in serum, but the IgA response is modest and diminishes faster compared to IgG response.

Author summary: Immunoglobulin A (IgA) is the most abundant type of antibody in the body mostly located on mucosal surfaces as a dimeric secretory IgA. Such secretory IgA plays an important role in preventing the adherence and invasions of foreign objects by its neutralizing activity, while monomeric serum IgA is thought to relate to the phagocytic immune system activation. Here, we report that individuals with the novel coronavirus disease (COVID-19) developed both systemic neutralizing IgG and IgA active against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although the neutralizing IgA response was quick and reached the highest activity 25 days post-symptom-onset, compared to 35 days for IgG response, neutralizing IgA activity was modest and diminished faster than neutralizing IgG response. In individuals, who recovered from COVID-19 but had no detectable neutralizing IgA activity, a single dose of COVID-19 mRNA-vaccine elicited potent neutralizing IgA activity but the second dose did not further strengthen the antibody response. Our study provides novel insights into the role and the kinetics of serum IgA against the viral pathogen both in naturally-infected and COVID-19 mRNA-vaccine-receiving COVID-19-convalescent individuals.

Fig 1.. Kinetics of SARS-CoV-2-neutralizing activity and S1-binding antibodies.

VeroE6^TMPRSS2 cells were exposed to wild-type SARS-CoV-2^05−2N with or without various concentrations of diluted sera/plasmas (A), purified-IgG (B), or purified-IgA (C), and the neutralizing activity and the amounts of S1-binding antibodies were determined. The dashed line denotes the assay limit values (≤40-fold for the panel a and ≥100 μg/mL for panels b and c). Note that the highest viral neutralizing activity of purified-IgG and -IgA was seen around 35 and 25 days after onset, respectively. Furthermore, the neutralizing activity of serum IgA diminished much quicker than that of IgG. The colored line (NT₅₀ for pink, nIgG-EC₅₀ for purple, and nIgA-EC₅₀ for light green) denote the fitted curve. (D) Attenuation rate of the nIgG-EC₅₀ and nIgA-EC₅₀ between the highest neutralizing activity of purified IgG and IgA by day 28, 42, and 56 post-onset and neutralizing activity determined the latest in the study. The kinetics of the amount of S1-binding IgG (E) and IgA (F) were also shown. The amount of S1-binding IgG and IgA increased approximately by day 21 post symptom onset, followed by a gradual decrease. Note that by contrast, substantial amounts of S1-binding-IgG and -IgA persisted around 200 days after the onset, while the decay occurred more rapidly in IgA.

Fig 2.. COVID-19-convalescent individuals possess the greater neutralizing activity and SARS-CoV-2-S1-binding antibody levels than those at the acute phase.

The neutralizing activity of sera/plasmas, purified-IgG, and purified-IgA (A, B, and C, respectively) and the amounts of S1-binding IgG and IgA (D and E, respectively) were compared between the acute phase (less than 14 days post-symptom onset or when the individual required oxygen treatment) and the convalescent phase (14 days post-symptom onset and beyond with no oxygen requirement).

Fig 3.. Correlations of purified-IgG and -IgA neutralizing activities with sera/plasmas neutralizing titers.

The neutralizing activity of purified-IgG (A, nIgG-EC₅₀) and -IgA (B, nIgA-EC₅₀) against sera/plasmas neutralizing activity (NT₅₀) values are plotted. (C) The nIgA-EC₅₀ values are plotted against the nIgG-EC₅₀ values. Note that a high correlation is observed between NT₅₀ values and nIgG-EC₅₀ values (Repeated measured correlation ρ = −0.72 (95%CI; −0.84 to −0.54) (A) and between the NT₅₀ values and NIgA-EC50 values (ρ = −0.78 (95%CI; −0.88 to −0.62) (B), while moderate correlation was observed between nIgA-EC₅₀ and nIgG-EC₅₀ (Repeated measured correlation ρ = 0.42 (95%CI; 0.13 to 0.65) (C). Each symbol denotes the sample from one and the same individual.

Fig 4.. Kinetics of neutralizing activity and S1-binding antibody levels before and after COVID-19 mRNA-vaccination.

The kinetics of neutralizing activity (A, B, and C) and S1-binding antibody levels (D and E) in eight previously COVID-experienced individuals who received the COVID-19 mRNA-vaccine are shown. Note that all the values significantly rose after the first dose of the vaccine. Also note that none of the participants had detectable nIgA-EC₅₀ values (≥100 μg/mL) before vaccination (C). On the other hand, all of them had low to high levels of nIgA-EC₅₀ after a singled dose of the vaccine and such levels quickly decreased (C) and their S1-binding IgA levels persisted after the two doses of vaccine in the study period (E). The dashed line denotes the assay detection limit (≤40-fold dilution for NT₅₀ and ≥100 μg/mL for nIgG-EC₅₀ and nIgA-EC₅₀). Green symbols denote the samples collected before COVID-19 mRNA-vaccination, while yellow and light-blue denote after the 1^st and 2^nd doses, respectively. Each symbol denotes the sample from one and the same individual.