Heterosubtypic IgA1 or IgA2 antibodies neutralize virus much more potently than IgG1
Sialic acid in IgA’s C-terminal tail competes with viral receptor binding
This may represent an innate line of defense against viral pathogens
Immunoglobulin A (IgA) plays an important role in protecting our mucosal surfaces from viral infection, in maintaining a balance with the commensal bacterial flora, and in extending maternal immunity via breast feeding. Here, we report an additional innate immune effector function of human IgA molecules in that we demonstrate that the C-terminal tail unique to IgA molecules interferes with cell-surface attachment of influenza A and other enveloped viruses that use sialic acid as a receptor. This antiviral activity is mediated by sialic acid found in the complex N-linked glycans at position 459. Antiviral activity was observed even in the absence of classical antibody binding via the antigen binding sites. Our data, therefore, show that the C-terminal tail of IgA subtypes provides an innate line of defense against viruses that use sialic acid as a receptor and the role of neuraminidases present on these virions.
Vaccination against influenza A viruses relies on the induction of strain-specific neutralizing immunoglobulin G (IgG) and has to be repeated annually. Rare heterosubtypic antibodies, i.e., antibodies that are able to neutralize multiple strains and subtypes of influenza virus, can be found in most individuals (Corti et al., 2010, Kohler et al., 2014) but currently cannot specifically be induced by immunization.
Immunoglobulin A (IgA) exists as monomeric molecules in serum or as dimeric secretory IgA on mucosal surfaces. Although roughly three quarters of the daily antibody production (i.e., about 3 mg) are of the IgA isotype (Delacroix et al., 1982), IgA deficiency is frequent (∼1 in 300–600 individuals) and mostly asymptomatic (Janzi et al., 2009). IgA-deficient mice exhibit an enhanced susceptibility to influenza A virus infection and display impaired T helper cell priming (Arulanandam et al., 2001). In humans and great apes, two IgA subtypes are found: IgA1 that is characterized by a 23-amino-acid (aa)-long and heavily O-glycosylated hinge region, and a conserved C-terminal tail of 19 amino acids that interacts with the J chain and secretory chain to mediate dimerization (Correa et al., 2013) and that contains a N-linked glycosylation site. IgA2 more closely resembles the IgA isotypes of other vertebrates (Rogers et al., 2008), and it also possesses a C-terminal tail, but its shorter hinge region is devoid of O-linked glycosylation.
All Ig isotypes contain complex N-linked glycosylation: while about 10% of the N-linked glycans in IgG1 contain sialic acid (Wuhrer et al., 2007) over 90% of the N-linked glycans are sialylated in IgA1. Monomeric and secretory IgA1 glycosylation of CH2 at position 263 is predominantly of the biantennary type with α2–6-linked sialic acids, while CH3 glycans at position 459 are of the triantennary type, with α2–6 and α2–3 linkage in their sialic acids (Mattu et al., 1998, Royle et al., 2003).
Recent reports also suggest that heterosubtypic IgA has a more potent antiviral activity against influenza viruses than IgG (He et al., 2015, Muramatsu et al., 2014, Yu et al., 2013). Some of these observations could be explained by immune-geography, i.e., IgA’s preferred secretion into the pulmonary lumen and increased avidity due to multimerization. Moreover, it has been shown that both IgA and secretory component are important mediators of innate immunity against various bacterial pathogens: sialic acids on secretory IgA (sIgAs) inhibit attachment of S-fimbriated Escherichia coli (Schroten et al., 1998), while N-linked glycosylation of secretory component (alone or as part of sIgA) has been shown to compete with Helicobacter pylori for receptors (Borén et al., 1993) and was shown to bind to Escherichia coli (de Oliveira et al., 2001, Wold et al., 1990), toxin A from Clostridium difficile (Dallas and Rolfe, 1998), and Streptococcus pneumonia (Hammerschmidt et al., 1997, Zhang et al., 2000).
To assess the impact of glycosylation and hinge length on the activity of heterosubtypic antibodies to influenza A virus, monoclonal antibodies (mAbs) 1.12 (Wyrzucki et al., 2015) and 3.1 (Wyrzucki et al., 2014) were recombinantly expressed as human IgG1, IgG3, IgA1, and IgA2 molecules, and their in vitroantiviral activity was characterized.