1976 and 2009 H1N1 Influenza Virus Vaccines Boost Anti-Hemagglutinin Stalk Antibodies in Humans

Rapid antigenic shift and drift of influenza A viruses (IAVs) result in annual epidemics and periodic pandemics that place a major strain on global healthcare systems and pose a significant threat to the global economy [1]. Recent work has focused on characterizing a new class of IAV antibodies that bind to the hemagglutinin (HA) stalk domain [2–11]. These antibodies typically exhibit much broader reactivity and neutralizing activity than antibodies that bind to conventional antigenic sites on the HA head. HA stalk antibodies are thought to be boosted most efficiently in the context of infection when individuals are exposed to HAs whose head domains differ substantially from previous exposures, but whose stalk domains remain conserved [12]. We have recently postulated that the elicitation of such antibodies may have contributed to the extinction of seasonal IAV strains [12]. Indeed, we have demonstrated that individuals infected with pandemic 2009 (p2009) IAV experienced a boost in virus-neutralizing antibodies specific to the HA stalk [13]. This phenomenon has also been recently confirmed in a mouse model of sequential infection [14]. Importantly, other groups have observed naturally occurring HA stem antibodies in individuals who received a seasonal vaccine containing H1 and H3 viruses [15] or an H5N1 vaccine. Likewise, recent studies have shown that monoclonal antibodies and antibody-producing cells specific to the HA stem could be isolated from individuals who received the p2009 IAV vaccine [16, 17]. The HA segment of A/California/04/09 (Cal/09) virus descends from a separate lineage than that of previously circulating seasonal H1N1 strains. However, it is closely related to the HA segments of the A/New Jersey/1976 (NJ/76) virus and to the 1918 “Spanish Flu” [18, 19]. We therefore reasoned that individuals who received the NJ/76 vaccine or the Cal/09 vaccine may have also experienced a boost in cross-neutralizing antibodies specific to the HA stalk. To investigate this possibility, we examined 2 cohorts of subjects for the presence of HA stalk antibodies. One cohort consisted of individuals who received the NJ/76 vaccine as well as the Cal/09 vaccine. The second cohort of age-matched controls received the Cal/09 vaccine only. We demonstrate that individuals who received the NJ/76 vaccine had elevated levels of anti-HA stalk antibodies prior to receiving the Cal/09 vaccine. Vaccination with Cal/09 boosted titers of anti-HA stalk antibodies only in subjects who had not been vaccinated with NJ/76. Importantly, receipt of the NJ/76 vaccine or the Cal/09 vaccine led to an enhanced neutralization response against a virus containing a homologous HA stalk and a heterosubtypic HA head. These findings confirm that anti-HA stalk antibodies were elicited by the NJ/76 and Cal/09 vaccines and enhance our understanding of the mechanisms through which these antibodies are generated naturally. This raises the exciting possibility that generation of vaccine constructs designed to specifically boost titers of HA stalk antibodies may lead to a universal influenza virus vaccine, capable of broad and long-lasting protection against diverse IAV strains.