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.

A single vaccination with MVA-NP+M1 boosts T-cell responses to conserved influenza antigens in humans. Protection against influenza disease and virus shedding was demonstrated in an influenza virus challenge study.

The generation of strain-specific neutralizing antibodies against influenza A virus is known to confer potent protection against homologous infections. The majority of these antibodies bind to the hemagglutinin (HA) head domain and function by blocking the receptor binding site, preventing infection of host cells. Recently, elicitation of broadly neutralizing antibodies which target the conserved HA stalk domain has become a promising "universal" influenza virus vaccine strategy. The ability of these antibodies to elicit Fc-dependent effector functions has emerged as an important mechanism through which protection is achieved in vivo. However, the way in which Fc-dependent effector functions are regulated by polyclonal influenza virus-binding antibody mixtures in vivo has never been defined. Here, we demonstrate that interactions among viral glycoprotein-binding antibodies of varying specificities regulate the magnitude of antibody-dependent cell-mediated cytotoxicity induction. We show that the mechanism responsible for this phenotype relies upon competition for binding to HA on the surface of infected cells and virus particles. Nonneutralizing antibodies were poor inducers and did not inhibit antibody-dependent cell-mediated cytotoxicity. Interestingly, anti-neuraminidase antibodies weakly induced antibody-dependent cell-mediated cytotoxicity and enhanced induction in the presence of HA stalk-binding antibodies in an additive manner. Our data demonstrate that antibody specificity plays an important role in the regulation of ADCC, and that cross-talk among antibodies of varying specificities determines the magnitude of Fc receptor-mediated effector functions.T he discovery and ongoing characterization of broadly neutralizing antibodies (bnAbs) that bind to the hemagglutinin (HA) stalk domain of influenza A viruses (IAVs) has galvanized promising new efforts to generate a universal influenza virus vaccine (1). A number of studies have now firmly established that stalk-specific bnAbs, normally present in low quantities, can be boosted substantially in humans following exposure to HA subtypes with antigenically foreign head domains (2-8). Sequential vaccination of animals with chimeric HAs or with "headless" vaccine constructs have effectively recapitulated the boosting of bnAbs observed in humans after exposure to foreign HAs, and also are protective against heterologous and heterosubtypic IAV challenge (9-15). These strategies are now being tested as promising "universal" influenza virus vaccine candidates (16).Whereas traditional strain-specific antibodies neutralize virus by inhibiting receptor binding, stalk-binding bnAbs neutralize virus by distinct postbinding mechanisms (17). A direct comparison of in vitro neutralization has revealed that strain-specific mAbs that inhibit receptor binding tend to be more potent at neutralizing virus compared with monoclonal stalk-binding bnAbs (18, 19); however, this difference is minimized in the context of a polyclonal response (18). In addition to virus neutralization,...

The term “original antigenic sin” (OAS) was first used in the 1960s to describe how one’s first exposure to influenza virus shapes the outcome of subsequent exposures to antigenically related strains. In the decades that have passed, OAS-like responses have been shown to play an integral role in both protection from and susceptibility to infections. OAS may also have an important deterministic role in the differential efficacy of influenza vaccine responses observed for various age cohorts across seasons. In this article, we review how the understanding of OAS has progressed from its initial description and highlight important outstanding questions in need of further study.

The aim of candidate universal influenza vaccines is to provide broad protection against influenza A and B viruses. Studies have demonstrated that broadly reactive antibodies require Fc–Fc gamma receptor interactions for optimal protection; however, the innate effector cells responsible for mediating this protection remain largely unknown. Here, we examine the roles of alveolar macrophages, natural killer cells, and neutrophils in antibody-mediated protection. We demonstrate that alveolar macrophages play a dominant role in conferring protection provided by both broadly neutralizing and non-neutralizing antibodies in mice. Our data also reveal the potential mechanisms by which alveolar macrophages mediate protection in vivo, namely antibody-induced inflammation and antibody-dependent cellular phagocytosis. This study highlights the importance of innate effector cells in establishing a broad-spectrum antiviral state, as well as providing a better understanding of how multiple arms of the immune system cooperate to achieve an optimal antiviral response following influenza virus infection or immunization.