Following infection with SARS-CoV-2, virus-specific antibodies are generated that can neutralise and clear the virus via Fc effector functions. The importance of IgG antibodies for protection and control of SARS-CoV-2 has been extensively reported. In comparison, other antibody isotypes including IgA have been poorly characterized. Here we endeavoured to determine the functional contribution of plasma IgA from convalescent COVD-19 individuals.
IgA and IgG was purified from the plasma of convalescent COVID-19 individuals (n=58) and healthy controls (n=25). IgA and IgA+IgG-depleted plasma fractions were also collected. SARS-CoV-2-specific antibody responses were characterized via multiplex assay. Neutralization was assessed via a multiplex ACE2-RBD binding inhibition assay. Samples were characterized for their Fc functional capacity using a THP-1 cell bead-based phagocytosis assay and a cell association assay.
Convalescent individuals induced SARS-CoV-2-specific IgG (100%) and IgA (91.38%) with 93% of individuals inhibiting ACE2-RBD binding. IgA-depletion from plasma significantly increased neutralization (median=62.12%, p=0.0013) compared to plasma (median=39.62%). Purified IgG (median=26.13%; IQR=29.42) and IgA (median=12.45%; IQR=28.23) exhibited heterogenous neutralizing responses, with 60% and 23.73% of individuals inducing neutralization respectively. Interestingly, purified IgG and IgA exhibited reduced ACE2-RBD binding inhibition to the RBD mutant, N501Y. IgA-depleted plasma showed similar Fc function as plasma, however, IgA+IgG-depletion drastically reduced phagocytosis (p<0.0001) and cell association (p<0.0001).
We show SARS-CoV-2-specific-IgA responses are induced in most convalescent COVID-19 individuals, with negligible Fc functional capacity compared to IgG. Furthermore, potent IgA neutralisation occurred within a small subset of individuals. Analogous to published plasma data, purified IgA and IgG reduced neutralisation of N501Y. Surprisingly, IgA-depleted plasma increased neutralizing capacity of plasma in certain individuals, suggesting that IgA may block binding of other neutralising antibody isotypes. Overall, neutralizing IgA responses appear to be highly heterogenous. Understanding the mechanisms behind IgA mediated neutralization and blocking is warranted to provide insight into SARS-CoV-2-specific-IgA responses in reinfection and vaccination.