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Infection with influenza virus induces antibodies to the viral surface glycoproteins hemagglutinin and neuraminidase, and these responses can be broadly protective. To assess the breadth and magnitude of antibody responses, we sequentially infected mice, guinea pigs and ferrets with divergent H1N1 or H3N2 subtypes of influenza virus. We measured antibody responses by ELISA of an extensive panel of recombinant glycoproteins representing the viral diversity in nature. Guinea pigs developed high titers of broadly cross-reactive antibodies; mice and ferrets exhibited narrower humoral responses. Then, we compared antibody responses after infection of humans with influenza virus H1N1 or H3N2 and found markedly broad responses and cogent evidence for 'original antigenic sin'. This work will inform the design of universal vaccines against influenza virus and can guide pandemic-preparedness efforts directed against emerging influenza viruses.

Between November 2013 and February 2014, China reported three human cases of H10N8 influenza virus infection in the Jiangxi province, two of which were fatal. Using hybridoma technology, we isolated a panel of H10- and N8-directed monoclonal antibodies (MAbs) and further characterized the binding reactivity of these antibodies (via enzyme-linked immunosorbent assay) to a range of purified virus and recombinant protein substrates. The H10-directed MAbs displayed functional hemagglutination inhibition (HI) and neutralization activity, and the N8-directed antibodies displayed functional neuraminidase inhibition (NI) activity against H10N8. Surprisingly, the HI-reactive H10 antibodies, as well as a previously generated, group 2 hemagglutinin (HA) stalk-reactive antibody, demonstrated NI activity against H10N8 and an H10N7 strain; this phenomenon was absent when virus was treated with detergent, suggesting the anti-HA antibodies inhibited neuraminidase enzymatic activity through steric hindrance. We tested the prophylactic efficacy of one representative H10-reactive, N8-reactive, and group 2 HA stalk-reactive antibody in vivo using a BALB/c challenge model. All three antibodies were protective at a high dose (5 mg/kg). At a low dose (0.5 mg/kg), only the anti-N8 antibody prevented weight loss. Together, these data suggest that antibody targets other than the globular head domain of the HA may be efficacious in preventing influenza virus-induced morbidity and mortality. IMPORTANCE Avian H10N8 and H10N7 viruses have recently crossed the species barrier, causing morbidity and mortality in humans and other mammals. Although these reports are likely isolated incidents, it is possible that more cases may emerge in future winter seasons, similar to H7N9. Furthermore, regular transmission of avian influenza viruses to humans increases the risk of adaptive mutations and reassortment events, which may result in a novel virus with pandemic potential. Currently, no specific therapeutics or vaccines are available against the H10N8 influenza virus subtype. We generated a panel of H10- and N8-reactive MAbs. Although these antibodies may practically be developed into therapeutic agents, characterizing the protective potential of MAbs that have targets other than the HA globular head domain will provide insight into novel antibody-mediated mechanisms of protection and help to better understand correlates of protection for influenza A virus infection.

Three cases of influenza A(H10N8) virus infection in humans have been reported; 2 of these infected persons died. Characterization of the receptor binding pattern of H10 hemagglutinin from avian and human isolates showed that both interact weakly with human-like receptors and maintain strong affinity for avian-like receptors.

Current influenza virus vaccines provide solid protection from infection with viruses that are well matched with the vaccine strains. However, they do not protect efficiently against drifted or shifted strains. We developed an antigen based on the conserved stalk domain of the influenza virus hemagglutinin and tested its efficacy as a vaccine in a mouse virus challenge model. Although the antigen lacked the correct conformation of the native stalk domain and was not recognized by a panel of neutralizing stalk-reactive antibodies, it did induce considerable protection against H1N1, H5N1 and H6N1 challenge strains. Protection was enhanced when mice had pre-existing immunity against the stalk domain. Since pre-existing immunity is also present in the human population, we hypothesize that a similar antigen could show efficacy in humans as well.

Influenza virus infections are a major public health concern and cause significant morbidity and mortality worldwide. Current vaccines are effective but strain specific due to their focus on the immunodominant globular head domain of the hemagglutinin (HA). It has been hypothesized that sequential exposure of humans to hemagglutinins with divergent globular head domains but conserved stalk domains could refocus the immune response to broadly neutralizing epitopes in the stalk. Humans have preexisting immunity against H1 (group 1 hemagglutinin), and vaccination with H5 HA (also group 1)—which has a divergent globular head domain but a similar stalk domain—represents one such sequential-exposure scenario. To test this hypothesis, we used novel reagents based on chimeric hemagglutinins to screen sera from an H5N1 clinical trial for induction of stalk-specific antibodies by quantitative enzyme-linked immunosorbent assay (ELISA) and neutralization assays. Importantly, we also investigated the biological activity of these antibodies in a passive transfer in a mouse challenge model. We found that the H5N1 vaccine induced high titers of stalk-reactive antibodies which were biologically active and protective in the passive-transfer experiment. The induced response showed exceptional breadth toward divergent group 1 hemagglutinins but did not extend to group 2 hemagglutinins. These data provide evidence for the hypothesis that sequential exposure to hemagglutinins with divergent globular head domains but conserved stalk domains can refocus the immune response toward the conserved stalk domain. Furthermore, the results support the concept of a chimeric hemagglutinin universal influenza virus vaccine strategy that is based on the same principle. IMPORTANCE Influenza virus vaccines have to be reformulated and readministered on an annual basis. The development of a universal influenza virus vaccine could abolish the need for this cumbersome and costly process and would also enhance our pandemic preparedness. This study addressed the following questions, which are essential for the development of a hemagglutinin stalk-based universal influenza virus vaccine. (i) Can stalk-reactive antibodies be boosted by vaccination with divergent HAs that share conserved epitopes? (ii) How long-lived are these vaccine-induced stalk-reactive antibody responses? (iii) What is the breadth of this reactivity? (iv) Are these antibodies functional and protective? Our results further strengthen the concept of induction of stalk-reactive antibodies by sequential exposure to hemagglutinin immunogens with conserved stalk and divergent head domains. A universal influenza virus vaccine based on the same principles seems possible and might have a significant impact on global human health.

Summary Pathogenic H7N9 avian influenza viruses continue to represent a public health concern, and several candidate vaccines are currently being developed. It is vital to assess if protective antibodies are induced following vaccination and to characterize the diversity of epitopes targeted. Here we characterized the binding and functional properties of twelve H7-reactive human antibodies induced by a candidate A/Anhui/1/2013 (H7N9) vaccine. Both neutralizing and non-neutralizing antibodies protected mice in vivo during passive transfer challenge experiments. Mapping the H7 hemagglutinin antigenic sites by generating escape mutant variants against the neutralizing antibodies identified unique epitopes on the head and stalk domains. Further, the broadly cross-reactive non-neutralizing antibodies generated in this study were protective through Fc-mediated effector cell recruitment. These findings reveal important properties of vaccine-induced antibodies and provide a better understanding of the human monoclonal antibody response to influenza in the context of vaccines.

In the early spring of 2013, Chinese health authorities reported several cases of H7N9 influenza virus infections in humans. Since then the virus has established itself at the human-animal interface in Eastern China and continues to cause several hundred infections annually. In order to characterize the antibody response to the H7N9 virus we generated several mouse monoclonal antibodies against the hemagglutinin of the A/Shanghai/1/13 (H7N9) virus. Of particular note are two monoclonal antibodies, 1B2 and 1H5, that show broad reactivity to divergent H7 hemagglutinins. Monoclonal antibody 1B2 binds to viruses of the Eurasian and North American H7 lineages and monoclonal antibody 1H5 reacts broadly to virus isolates of the Eurasian lineage. Interestingly, 1B2 shows broad hemagglutination inhibiting and neutralizing activity, while 1H5 fails to inhibit hemagglutination and demonstrates no neutralizing activity in vitro. However, both monoclonal antibodies were highly protective in an in vivo passive transfer challenge model in mice, even at low doses. Experiments using mutant antibodies that lack the ability for Fc/Fc-receptor and Fc/complement interactions suggest that the protection provided by mAb 1H5 is, at least in part, mediated by the Fc-fragment of the mAb. These findings highlight that a protective response to a pathogen may not only be due to neutralizing antibodies, but can also be the result of highly efficacious non-neutralizing antibodies not readily detected by classical in vitro neutralization or hemagglutination inhibition assays. This is of interest because H7 influenza virus vaccines induce only low hemagglutination inhibiting antibody titers while eliciting robust antibody titers as measured by ELISA. Our data suggest that these binding but non-neutralizing antibodies contribute to protection in vivo., Author Summary Several hundred human avian H7N9 virus infections with a case fatality rate of approximately 37% have occurred in China since 2013. The emergence of this virus has raised concerns about its pandemic potential and has triggered the development of H7N9 vaccines. Using the traditional correlate of protection for influenza viruses—the hemagglutination inhibition assay—H7N9 vaccines prove to be poorly immunogenic. However, once antibody levels are high enough substantial crossreactivity is observed. Here we characterize several monoclonal antibodies against the H7N9 hemagglutinin. One set of antibodies is active in the hemagglutination inhibition assay across a panel of distant H7 virus strains and the common conserved epitope of these antibodies might explain the broad crossreactivity observed with H7N9 vaccines. The second set of antibodies shows no antiviral activity in vitro but is highly protective in vivo, partially through Fc-mediated functions. Targeting of the non-neutralizing but protective epitope of these antibodies by the immune system might explain the 'low immunogenicity' observed in H7N9 vaccine trials. In conclusion our data suggests that current H7N9 vaccines could protect against divergent H7 strains with pandemic potential in the future and that the traditional correlates of protection for influenza vaccines might not capture the protective immunity to H7 viruses.

Three human cases of H10N8 viruses were reported in China in late 2013 and early 2014, two of which were fatal. This was the first time the H10N8 subtype has been detected in humans and no vaccine candidates or antibody therapy has been developed for these viruses so far. We developed an H10N8 vaccine candidate virus based on A/Jiangxi-Donghu/346/13 that can also be used in a murine challenge model for vaccine and monoclonal antibody research. The vaccine virus is a 6:2 re-assortant virus expressing the surface glycoproteins of A/Jiangxi-Donghu/346/13 on an A/Puerto Rico/8/34 backbone. Vaccination with inactivated challenge virus or recombinant hemagglutinin or neuraminidase derived from this strain protected mice from viral challenge.

Emerging H7N9 influenza virus infections in Asia have once more spurred the development of effective prepandemic H7 vaccines. However, many vaccines based on avian influenza viruses—including H7—are poorly immunogenic, as measured by traditional correlates of protection. Here we reevaluated sera from an H7N1 human vaccine trial performed in 2006. We examined cross-reactive antibody responses to divergent H7 strains, including H7N9, dissected the antibody response into head- and stalk-reactive antibodies, and tested thein vivopotency of these human sera in a passive-transfer H7N9 challenge experiment with mice. Although only a low percentage of vaccinees induced neutralizing antibody responses against the homologous vaccine strain and also H7N9, we detected strong cross-reactivity to divergent H7 hemagglutinins (HAs) in a large proportion of the cohort with a quantitative enzyme-linked immunosorbent assay. Furthermore, H7N1 vaccination induced antibodies to both the head and stalk domains of the HA, which is in sharp contrast to seasonal inactivated vaccines. Finally, we were able to show that both neutralizing and nonneutralizing antibodies improvedin vivovirus clearance in a passive-transfer H7N9 challenge mouse model.

The recent outbreak of H7N9 influenza virus infections in humans in China has raised concerns about the pandemic potential of this strain. Here, we test the efficacy of H3 stalk-based chimeric hemagglutinin universal influenza virus vaccine constructs to protect against H7N9 challenge in mice. Chimeric hemagglutinin constructs protected from viral challenge in the context of different administration routes as well as with a generic oil-in-water adjuvant similar to formulations licensed for use in humans.

To evaluate the antigenic relationship between bat mumps virus (BMV) and the JL5 vaccine strain of mumps virus (MuVJL5), we rescued a chimeric virus bearing the F and HN glycoproteins of BMV in the background of a recombinant JL5 genome (rMuVJL5). Cross-reactivity and cross-neutralization between this chimeric recombinant MuV bearing the F and HN glycoproteins of BMV (rMuVJL5-F/HNBMV) virus and rMuVJL5 were demonstrated using hyperimmune mouse serum samples and a curated panel of human serum. All mouse and human serum samples that were able to neutralize rMuVJL5 infection had cross-neutralizing activity against rMuVJL5-F/HNBMV. Our data suggest that persons who have neutralizing antibodies against MuV might be protected from infection by BMV.