Abstract Novel vaccines are needed that have higher efficacy and that provide long-lasting protection against emerging influenza viruses. An attractive approach towards developing a “universal” vaccine is focused on eliciting antibodies that target conserved regions of the hemagglutinin (HA) protein on the surface of the virus. The receptor binding site and the stem are two such HA regions, and the elicitation of humoral responses focused on these sites has been the subject of multiple studies. Recently, a new class of antibodies has been discovered that targets the highly conserved HA head trimer interface. Antibodies against this epitope were found in multiple individuals, have broad heterosubtypic cross-reactivity and provided protection against viral challenges in animal models. Therefore, induction of these type of antibodies as part of a universal influenza vaccine would be valuable. The head trimer interface epitope is occluded in crystal structures of HAs, and likely becomes exposed just transiently through molecular “breathing”. This limited accessibility may explain at least in part why natural humoral responses against this site are rare. In this proposal, we will engineer immunogens that present unobstructed the HA head trimer interface epitope in molecular contexts devoid of other immunodominant sites. Using computational protein modeling and high throughput library screening, HA derived as well as non-influenza based immunogens will be developed that: 1) expose the HA head trimer interface to facilitate immune recognition; 2) preferentially interact with broadly cross-reactive, but not with strain specific, antibodies that target this site and 3) occlude immunodominant regions to focus the immune responses on the epitope. Designed immunogens will be used to vaccinate small animals to assess their ability to protect against live virus. Detailed analysis of the B cell receptor repertoires of vaccinated animals will reveal the developmental pathways that lead to the activation and maturation of humoral responses against the conserved HA head trimer interface. This proposal will provide candidate immunogen towards the development of a universal influenza vaccine, lead to a better understanding of molecular features that control HA conformation and immunogenicity, and establish general approaches for immunogen design.