Adjuvant strategies for universal and multiseasonal influenza vaccine candidates in the context of pre-existing immunity

Project Summary/Abstract The burden of influenza remains high, with seasonal epidemics resulting in 3-5 million cases of severe illness globally and ~650,000 fatalities/year. While immunity to influenza virus acquired through natural infection is relatively broad and long-lived, immune responses induced through vaccination with current vaccines are short- lived and narrow, requiring annual reformulation and revaccination. It is unclear what causes such dissimilarities between infection- and vaccine-induced immunity, however major differences include route of exposure (respiratory vs. parenteral), antigen type (live vs. inactivated virus/protein), and the robust activation of viral- specific innate immune responses during infection. Current influenza vaccines elicit immune responses primarily directed towards epitopes in the highly variable immunodominant head domain of the viral hemagglutinin (HA). While antibodies directed towards the more conserved HA stalk domain provide cross-strain protection, this domain is poorly immunogenic. HA head dominance compounded with the rapid pace of antigenic drift severely limits cross-protection. Broad-based cellular immunity and mucosal immunity are also key to heterosubtypic protection, however current parenteral vaccines do not induce these sufficiently. Thus, development of more effective adjuvants and vaccination strategies targeting the mucosal route are clearly needed. Natural infection with influenza virus stimulates strong adaptive immune responses through activation of Toll-, RIG-I-, and NOD- like receptors (TLRs, RLRs, NLRs). As induction of appropriate innate responses is crucial for long-lasting adaptive immunity and for shaping the correct types of immune responses, we will employ a rationally designed intranasal combination adjuvant to activate all three pathways through the mucosal route to more closely mimic natural infection, which we hypothesize will lead to more robust and durable vaccine responses. This adjuvant consists of a nanoemulsion adjuvant (NE) that activates TLRs and NLRP3 and an RNA agonist of RIG-I derived from the defective interfering RNA of Sendai virus (IVT). NE/IVT induces synergistic immune responses against a variety of viral antigens, with magnified TH1-biased cellular immunity and mucosal immune responses. Here, we will compare this adjuvant to two adjuvants licensed for parenteral vaccination with influenza vaccines (AS03 and MF59) to improve immune responses to two different antigen systems that have broadened heterosubtypic protection in preclinical and clinical settings: (1) a sequential regimen utilizing split viruses containing chimeric HAs with different “exotic” avian HA head domains all with the same stalk domain to guide immune responses to epitopes in the stalk, and (2) a recombinant tetrameric neuraminidase. Human immune responses to influenza vaccines are heavily influenced by pre-existing immunity acquired through past infections and/or immunizations. We will further determine whether the immune responses driven by mucosal administration of NE/IVT with these antigen systems can better take advantage of such pre-existing immune responses compared to parenteral vaccination with AS03 by redirecting memory responses to more conserved epitopes.