Audray K. Harris Stadtman Investigator, Laboratory of Infectious Diseases Chief, Structural Informatics Unit National Institute of Allergy & Infectious Diseases, NIH Towards the Development of More Efficacious Influenza Vaccines by Structural Analyses of Nanoparticles Abstract   Influenza viruses infect millions of people worldwide on an annual basis and are associated with human mortality and morbidity. Influenza displays antigenic variation of the major surface glycoprotein, hemagglutinin (HA), and as a consequence, vaccines must be formulated each year to match both current and predicted future circulating strains. However, one major challenge is to design influenza vaccine immunogens that can elicit antibodies to conserved epitopes.  Interestingly, there are conserved epitopes within the stem region of HA. Currently, there is an increasing interest in the design and use of nanoparticles as vaccine immunogens for various diseases. However, the structure and epitope display of conserved influenza epitopes on vaccine nanoparticles has not been explored in great detail because many structures do not form crystals that can be studied by protein x-ray crystallography. We have used cryo-electron microscopy to study the organization and disposition of conserved HA epitopes on different types of influenza vaccine nanoparticles. These include commercial HA-based subunit vaccines, virus-like particles and designed symmetrical nanoparticles. Our results indicated that conserved HA epitope display is modulated by both particle size and organization. Different types of nanoparticles can maintain the HA is a prefusion state to which stem antibodies bind, but the level of epitope accessible could vary depending on nanoparticle design. Taken together our studies suggest that further analyses of structures of nanoparticles will aid in the design of immunogens for more efficacious seasonal influenza vaccines and aid in universal influenza vaccine development.