BackgroundA pandemic H5N1 influenza outbreak would be facilitated by an absence of immunity to the avian-derived virus in the human population. Although this condition is likely in regard to hemagglutinin-mediated immunity, the neuraminidase (NA) of H5N1 viruses (avN1) and of endemic human H1N1 viruses (huN1) are classified in the same serotype. We hypothesized that an immune response to huN1 could mediate cross-protection against H5N1 influenza virus infection.Methods and FindingsMice were immunized against the NA of a contemporary human H1N1 strain by DNA vaccination. They were challenged with recombinant A/Puerto Rico/8/34 (PR8) viruses bearing huN1 (PR8-huN1) or avN1 (PR8-avN1) or with H5N1 virus A/Vietnam/1203/04. Additional naïve mice were injected with sera from vaccinated mice prior to H5N1 challenge. Also, serum specimens from humans were analyzed for reactivity with avN1. Immunization elicited a serum IgG response to huN1 and robust protection against the homologous challenge virus. Immunized mice were partially protected from lethal challenge with H5N1 virus or recombinant PR8-avN1. Sera transferred from immunized mice to naïve animals conferred similar protection against H5N1 mortality. Analysis of human sera showed that antibodies able to inhibit the sialidase activity of avN1 exist in some individuals.ConclusionsThese data reveal that humoral immunity elicited by huN1 can partially protect against H5N1 infection in a mammalian host. Our results suggest that a portion of the human population could have some degree of resistance to H5N1 influenza, with the possibility that this could be induced or enhanced through immunization with seasonal influenza vaccines.

Seasonal epidemics caused by influenza virus are driven by antigenic changes (drift) in viral surface glycoproteins that allow evasion from preexisting humoral immunity. Antigenic drift is a feature of not only the hemagglutinin (HA), but also of neuraminidase (NA). We have evaluated the antigenic evolution of each protein in H1N1 and H3N2 viruses used in vaccine formulations during the last 15 y by analysis of HA and NA inhibition titers and antigenic cartography. As previously shown for HA, genetic changes in NA did not always lead to an antigenic change. The noncontinuous pattern of NA drift did not correspond closely with HA drift in either subtype. Although NA drift was demonstrated using ferret sera, we show that these changes also impact recognition by NA-inhibiting antibodies in human sera. Remarkably, a single point mutation in the NA of A/Brisbane/59/2007 was primarily responsible for the lack of inhibition by polyclonal antibodies specific for earlier strains. These data underscore the importance of NA inhibition testing to define antigenic drift when there are sequence changes in NA.

Antibodies to neuraminidase (NA), the second most abundant surface protein on influenza virus, contribute toward protection against influenza. The traditional thiobarbituric acid (TBA) method to quantify NA inhibiting antibodies is cumbersome and not suitable for routine serology. An enzyme-linked lectin assay (ELLA) described by Lambre et al. (1990) is a practical alternative method for measuring NA inhibition (NI) titers. This report describes optimization of the ELLA for measuring NI titers in human sera against influenza A viruses, using H6N1 and H6N2 viruses as antigens. The optimized ELLA is subtype-specific and reproducible. While the titers measured by ELLA are somewhat greater than those measured by a miniaturized TBA method, seroconversion rates are the same, suggesting similarity in assay sensitivity under these optimized conditions. The ELLA described in this report provides a practical format for routine evaluation of human antibody responses to NA.

Control of swine influenza A virus (IAV) in the United States is hindered because inactivated vaccines do not provide robust cross-protection against the multiple antigenic variants cocirculating in the field. Vaccine efficacy can be limited further for vaccines administered to young pigs that possess maternally derived immunity. We previously demonstrated that a recombinant A/sw/Texas/4199-2/1998 (TX98) (H3N2) virus expressing a truncated NS1 protein is attenuated in swine and has potential for use as an intranasal live attenuated influenza virus (LAIV) vaccine. In the present study, we compared 1 dose of intranasal LAIV with 2 intramuscular doses of TX98 whole inactivated virus (WIV) with adjuvant in weanling pigs with and without TX98-specific maternally derived antibodies (MDA). Pigs were subsequently challenged with wild-type homologous TX98 H3N2 virus or with an antigenic variant, A/sw/Colorado/23619/1999 (CO99) (H3N2). In the absence of MDA, both vaccines protected against homologous TX98 and heterologous CO99 shedding, although the LAIV elicited lower hemagglutination inhibition (HI) antibody titers in serum. The efficacy of both vaccines was reduced by the presence of MDA; however, WIV vaccination of MDA-positive pigs led to dramatically enhanced pneumonia following heterologous challenge, a phenomenon known as vaccine-associated enhanced respiratory disease (VAERD). A single dose of LAIV administered to MDA-positive pigs still provided partial protection from CO99 and may be a safer vaccine for young pigs under field conditions, where dams are routinely vaccinated and diverse IAV strains are in circulation. These results have implications not only for pigs but also for other influenza virus host species.

Influenza A virus in swine (IAV-S) is one of the most important infectious disease agents of swine in North America. In addition to the economic burden of IAV-S to the swine industry, the zoonotic potential of IAV-S sometimes leads to serious public health concerns. Adjuvanted, inactivated vaccines have been licensed in the United States for over 20 years, and there is also widespread usage of autogenous/custom IAV-S vaccines. Vaccination induces neutralizing antibodies and protection against infection with very similar strains. However, IAV-S strains are so diverse and prone to mutation that these vaccines often have disappointing efficacy in the field. This scientific review was developed to help veterinarians and others to identify the best available IAV-S vaccine for a particular infected herd. We describe key principles of IAV-S structure and replication, protective immunity, currently available vaccines, and vaccine technologies that show promise for the future. We discuss strategies to optimize the use of available IAV-S vaccines, based on information gathered from modern diagnostics and surveillance programs. Improvements in IAV-S immunization strategies, in both the short term and long term, will benefit swine health and productivity and potentially reduce risks to public health.