Modified Project Summary/Abstract Section Most humans are infected with influenza viruses by the time they reach 3 years of age. Our past studies suggest that early childhood influenza infections can leave lifelong immunological ‘imprints’. During the first funding period (4 years) of this grant, we found that human Ab responses against seasonal influenza viruses are typically focused on epitopes that are conserved between contemporary viral strains and viral strains that circulated during each individual donor’s childhood. We found that immune responses generated against contemporary influenza strains are dominated by memory B cells that recognize conserved epitopes present in past viral strains. Our studies have examined how viral infections with one subtype of influenza virus (i.e. H1N1) influence immune responses against an antigenically distinct version of that same subtype (i.e. H1N1). Although different influenza virus subtypes have very different antigenic properties, there are epitopes that are conserved among these viruses. H1N1, H2N2, and H3N2 viruses have circulated at different times in humans over the past 100 years and an individual’s birth year largely predicts the influenza virus subtype that they were initially infected with in childhood. It is important to elucidate how viral infections with one influenza subtype (i.e. H1N1) influence immune responses against a completely different influenza subtype (i.e. H3N2) since multiple influenza virus subtypes currently co-circulate in humans. Further, epidemiological studies suggest that human susceptibility to pandemic H5N1 and H7N9 viruses is influenced by childhood infections with different subtypes of seasonal influenza viruses. We hypothesize that early childhood seasonal influenza virus infections leave long-lived immunological imprints that bias the immune system to preferentially respond efficiently to more closely related influenza subtypes and poorly to more distant influenza subtypes. In this proposal we will use mouse and ferret models to determine how initial seasonal influenza infections shape the specificity and neutralization efficiency of Abs elicited against distinct seasonal and pandemic influenza virus subtypes. We will then examine sera samples collected for a pediatric cohort study to determine how initial childhood H1N1 versus H3N2 infections affect the development of Ab responses against infections with homologous and heterologous influenza virus subtypes. Finally, we will use mouse and ferret models to determine how different influenza pre-exposures shape the specificity and neutralization efficiency of Abs elicited by a leading ‘universal’ influenza vaccine candidate. Collectively, these studies will determine (1) if infections with one influenza virus subtype influence the specificity of Abs elicited against a second influenza virus subtype, (2) the specificity and functionality of Abs elicited in children with different influenza virus exposure histories and (3) how prior influenza exposures influence the effectiveness of a new ‘universal’ influenza vaccine.

Modified Public Health Relevance Section The immune system preferentially mounts immune responses to old influenza strains, as opposed to new immune responses that target emerging viral strains. The goal of this application is to determine how previous influenza infections influence the development of new immune responses elicited by distinct influenza virus subtypes and new ‘universal’ influenza vaccine antigens.