Immunofocusing via antigen reorientation - towards a universal flu vaccine

A major challenge in vaccine development is the ability to focus the immune response toward evolutionarily conserved antigenic regions to confer broad protection. In the case of influenza, current seasonal vaccines primarily elicit immunity to the variable head region of the viral glycoprotein, hemagglutinin (HA), even though this region (HA-head) exhibits high plasticity and evolves continuously. By contrast, the stem region of HA (HA-stem) contains conserved epitopes and represents a prime target for universal flu-vaccine design.

We introduce an approach to controlling antigen orientation based on the site-specific insertion of short stretches of aspartate residues that facilitates antigen-binding to alum adjuvants. We use this approach to reorient HA in an “upside down” configuration, which we envision increases HA-stem exposure, therefore also improving its immunogenicity compared to HA-head. When applied to an H2 HA, the reoriented H2 HA (reoH2HA) on alum induced a stem-directed antibody response that cross-reacted with both group 1 and 2 influenza A subtypes. Electron microscopy polyclonal epitope mapping further revealed that cross-reactive antibodies elicited by reoH2HA recognized stem and anchor epitopes of group 2 HAs. Our results demonstrate the possibility and benefits of antigen reorientation, which represents a generalizable immunofocusing approach readily applicable for designing epitope-focused vaccine candidates.