Our findings illustrate the potential of NA-specific immunity for achieving broader protection against antigenic drift variants or newly emerging viruses carrying the same NA but a different HA subtype

Our findings illustrate the potential of NA-specific immunity for achieving broader protection against antigenic drift variants or newly emerging viruses carrying the same NA but a different HA subtype. IMPORTANCE Despite the availability of vaccines, annual influenza virus epidemics cause 250,000 to 500,000 deaths worldwide. development of more broadly protective influenza vaccines. Such vaccines may also provide partial protection against newly emerging strains with the same NA but a different HA subtype. KEYWORDS: correlates of protection, functional antibodies, influenza A computer virus, neuraminidase protein, VSV replicon vaccine platform ABSTRACT Immune responses induced by currently licensed inactivated influenza vaccines are mainly directed against the hemagglutinin (HA) glycoprotein, the immunodominant antigen of influenza viruses. The producing antigenic drift of HA requires frequent updating of the vaccine composition and annual revaccination. On the other hand, the levels of antibodies directed against the neuraminidase (NA) glycoprotein, the second major influenza computer virus antigen, vary greatly. To investigate the potential Betulin of the more conserved NA protein for the induction of subtype-specific protection, vesicular stomatitis virus-based replicons expressing a panel of N1 proteins from prototypic seasonal and pandemic H1N1 strains and human H5N1 and H7N9 isolates were generated. Immunization of mice and ferrets with the replicon transporting the matched N1 Kl protein resulted in strong humoral and cellular immune responses and guarded against challenge with the homologous influenza computer virus with an efficacy similar to that of the matched HA protein, illustrating the potential of the NA protein as a vaccine antigen. The extent of protection after immunization with mismatched N1 proteins correlated with the level of cross-reactive neuraminidase-inhibiting antibody titers. Passive serum transfer experiments in mice confirmed that these functional antibodies determine subtype-specific cross-protection. Our findings illustrate the potential of NA-specific immunity for achieving broader protection against antigenic drift variants or newly emerging viruses transporting the same NA but a different HA subtype. IMPORTANCE Despite the availability of vaccines, annual influenza computer virus epidemics cause 250,000 to 500,000 deaths worldwide. Currently Betulin licensed inactivated vaccines, which are standardized for the amount of the hemagglutinin (HA) antigen, primarily induce strain-specific antibodies, whereas the immune response to the neuraminidase (NA) antigen, which is also present around the viral surface, is usually low. Using NA-expressing single-cycle vesicular stomatitis computer virus replicons, we show that this NA antigen conferred protection of mice and ferrets against not only the matched influenza computer virus strains but also viruses transporting NA proteins from other strains of the same subtype. The extent of protection correlated with the level of cross-reactive NA-inhibiting antibodies. This highlights the potential of the NA antigen for the development of more broadly protective influenza vaccines. Such vaccines may Betulin also provide partial protection against newly emerging strains with the same NA but a different HA subtype. KEYWORDS: correlates of protection, functional antibodies, influenza A computer virus, neuraminidase protein, VSV replicon vaccine platform INTRODUCTION Influenza A Betulin viruses cause an acute respiratory infection that can affect up to 30% of the population during seasonal epidemics (1,C4). The viruses undergo continuous antigenic drift to escape preexisting immune responses (5), necessitating the regular update of influenza vaccines. The hemagglutinin (HA) protein, which mediates viral attachment to the target cell and subsequent entry (6), is the main target of neutralizing antibodies and thus is usually subject to continuous antigenic drift. Consequently, the majority of antibodies directed against the HA protein are highly strain specific (5, 7). The neuraminidase (NA) protein, which facilitates viral egress, is usually more conserved (8). However, since the frequently used inactivated influenza vaccines are Betulin standardized only for HA protein content (9), the amount and quality of the NA protein are known to vary considerably among vaccine preparations and computer virus strains used (10, 11). There is increasing evidence that NA-specific antibodies can provide protection from influenza viruses, including reports about broad binding and inhibition of NA activity (12,C18). When administered separately, HA and NA proteins induced comparable total antibody titers (19). Immunization with NA alone decreased lung computer virus titers in mice and reduced severe clinical indicators and viral shedding in patients (11, 19,C22). The producing antibody responses cross-reacted with other NA proteins of the same subtype, and NA-specific.