Antibody 27F3 Broadly Targets Influenza A Group 1 and 2 Hemagglutinins through a Further Variation in V_H1-69 Antibody Orientation on the HA Stem

Antibodies that target both group 1 and group 2 influenza A viruses are valuable for therapeutic and vaccine development, but only a few have been reported to date. Here, we describe a new V_H1-69 antibody 27F3 that broadly recognizes heterosubtypic hemagglutinins (HAs) from both group 1 and group 2 influenza A viruses. Structural characterization of 27F3 Fab with A/California/04/2009 (H1N1) hemagglutinin illustrates that 27F3 shares the key binding features observed in other V_H1-69 antibodies to the HA stem. Compared to other V_H1-69 antibodies, the 27F3 V_H domain interacts with the HA stem in a distinct orientation, which alters its epitope and may have influenced its breadth. The diverse rotations of V_H1-69 antibodies on the HA stem epitope highlight the different ways that this antibody family can evolve to broadly neutralize influenza A viruses. These results have important implications for understanding how to elicit broad antibody responses against influenza virus.