HA Antibody-Mediated FcγRIIIa Activity Is Both Dependent on FcR Engagement and Interactions between HA and Sialic Acids

Interactions with receptors for the Fc region of IgG (Fc gamma Rs) have been shown to contribute to the in vivo protection against influenza A viruses provided by broadly neutralizing antibodies (bnAbs) that bind to the viral hemagglutinin (HA) stem. In particular, Fc-mediated antibody-dependent cellular cytotoxicity (ADCC) has been shown to contribute to protection by stem-binding bnAbs. Fc-mediated effector functions appear not to contribute to protection provided by strain-specific HA head-binding antibodies. We used a panel of anti-stem and anti-head influenza A and B monoclonal antibodies with identical human IgG1 Fc domains and investigated their ability to mediate ADCC-associated Fc gamma RIIIa activation. Antibodies which do not interfere with sialic acid binding of HA can mediate Fc gamma RIIIa activation. However, the Fc gamma RIIIa activation was inhibited when a mutant HA, unable to bind sialic acids, was used. Antibodies which block sialic acid receptor interactions of HA interfered with Fc gamma RIIIa activation. The inhibition of Fc gamma RIIIa activation by HA head-binding and sialic acid receptor-blocking antibodies was confirmed in plasma samples of H5N1 vaccinated human subjects. Together, these results suggest that in addition to Fc-Fc gamma R binding, interactions between HA and sialic acids on immune cells are required for optimal Fc-mediated effector functions by anti-HA antibodies.