Divergent Requirement of Fc-Fcγ Receptor Interactions for Protection against Influenza Viruses by Two Pan-H5 Hemagglutinin Antibodies.

Recent studies have shown that Fc-Fcγ receptor (FcγR) interactions are required for protection against influenza viruses by broadly reactive anti-hemagglutinin (HA) stem, but not virus strain-specific, anti-receptor binding site (RBS), antibodies (Abs). Since only a few Abs recognizing epitopes in the head region but outside the RBS have been tested against single-challenge virus strains, it remains unknown whether Fc-FcγR interactions are required for protection by Abs recognizing epitopes outside the RBS and whether the requirement is virus strain specific or epitope specific. In the present study, we therefore investigated the requirements for protection using two pan-H5 Abs, 65C6 and 100F4. We generated chimeric Abs, 65C6/IgG2a and 100F4/IgG2a, which preferentially engage activating FcγRs, and isogenic forms, 65C6/D265A and 100F4/D265A, which do not bind FcγR. Virus neutralizing activity, binding, antibody-dependent cellular cytotoxicity (ADCC), and protection of these Abs were compared using three H5 strains, A/Shenzhen/406H/2006 (SZ06), A/chicken/Shanxi/2/2006 (SX06), and A/chicken/Netherlands/14015526/2014 (NE14). We found that all four chimeric Abs bound and neutralized the SZ06 and NE14 strains but poorly inhibited the SX06 strain. 65C6/IgG2a and 100F4/IgG2a, but not 65C6/D265A and 100F4/D265A, mediated ADCC against target cells expressing HA derived from all three virus strains. Interestingly, both 65C6/IgG2a and 65C6/D265A demonstrated comparable protection against all three virus strains ; however, 100F4/IgG2a, but not 100F4/D265A, showed protection. Thus, we conclude that Fc-FcγR interactions are required for protection by 100F4, but not by 65C6, and therefore, protection is not virus strain specific but epitope specific. Abs play an important role in immune protection against influenza virus infection. Fc-FcγR interactions are required for protection by broadly neutralizing antistem, but not by virus strain-specific, anti-receptor binding site (RBS), Abs. Whether such interactions are necessary for protection by Abs that recognize epitopes outside RBS is not fully understood. In the present study, we investigated protection mechanisms against three H5 strains by two pan-H5 Abs, 65C6 and 100F4. We show that although these two Abs have similar neutralizing, binding, and ADCC activities against all three H5 strains , they have divergent requirements for Fc-FcγR interactions to protect against the three H5 strains The Fc-FcγR interactions are required for protection by 100F4, but not by 65C6. Thus, we conclude that Fc-FcγR interactions for protection by pan-H5 Abs is not strain specific, but epitope specific.