Mutations in the receptor binding domain (RBD) (e.g.N501Y) of SARS-CoV2 has resulted in emergence of variants of concern (alpha). Following infection, virus-specific antibodies are generated that can neutralise and clear the virus via Fc effector functions (phagocytosis). The importance of IgG antibodies for protection and control of SARS-CoV2 has been extensively reported. In comparison, other antibody isotypes including IgA have been poorly characterized, especially to variants. Here we endeavoured to determine the functional contribution of plasma IgA from convalescent COVID-19 subjects.
IgA and IgG was purified from the plasma of convalescent COVID-19 patients (n=58) and healthy controls (n=25). IgA and IgA+IgG depleted plasma fractions were also collected. SARS-CoV2-specific antibody responses were characterized via multiplex assay. Neutralization was assessed via a multiplex ACE2-RBD binding inhibition assay. Samples were also characterized for their Fc functional capacity using a THP-1 cell bead-based phagocytosis assay and a cell association assay. Multivariate analysis was used to compare purified antibody binding to different RBD mutants.
Convalescent patients induced SARS-CoV2-specific IgG (100%) and IgA (91.38%) with 85.19% of patients able to inhibit ACE2-RBD binding. IgA depletion from plasma significantly increased neutralization (median=62.12%, p=0.0013) compared to plasma (median=39.62%). Interestingly, purified IgG and IgA exhibited differential antibody binding to 15 RBD mutants e.g. alpha (N501Y) (p<0.05) and neutralization to 5 mutants (p<0.05). Finally, IgA depletion resulted in similar Fc function as plasma, however, IgA+IgG depletion drastically reduced the phagocytosis (p<0.0001) and cell association (p<0.0001) compared to plasma.
We show SARS-CoV2-specific IgA responses are induced in most convalescent COVID-19 individuals, with negligible Fc functional capacity in comparison to IgG. Furthermore, potent IgA neutralisation was observed within a small subset of these individuals. Surprisingly, depletion of IgA from plasma increased neutralizing capacity of plasma in certain individuals, suggesting that IgA may block the binding of other neutralising antibody isotypes. Notably, purified IgG and IgA displayed differential binding to RBD mutants, such as alpha variant, suggesting convalescent antibody class switching could impact the capacity of plasma to neutralise different COVID variants. Understanding the mechanisms behind IgA neutralization and IgA mediated blocking is warranted to provide insights for improved vaccination and antibody therapies.