Antibodies to the hemagglutinin (HA) and neuraminidase (NA) glycoproteins are the major mediators of protection against influenza virus infection. Here, we report that current influenza vaccines poorly display key NA epitopes and rarely induce NA-reactive B cells. Conversely, influenza virus infection induces NA-reactive B cells at a frequency that approaches (H1N1) or exceeds (H3N2) that of HA-reactive B cells. NA-reactive antibodies display broad binding activity spanning the entire history of influenza A virus circulation in humans, including the original pandemic strains of both H1N1 and H3N2 subtypes. The antibodies robustly inhibit the enzymatic activity of NA, including oseltamivir-resistant variants, and provide robust prophylactic protection, including against avian H5N1 viruses, in vivo. When used therapeutically, NA-reactive antibodies protected mice from lethal influenza virus challenge even 48 hr post infection. These findings strongly suggest that influenza vaccines should be optimized to improve targeting of NA for durable and broad protection against divergent influenza strains.

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Neuraminidase is one of the two surface glycoproteins of influenza A and B viruses. It has enzymatic activity that cleaves terminal sialic acid from glycans, and that activity is essential at several points in the virus life cycle. While neuraminidase is a major target for influenza antivirals, it is largely ignored in vaccine development. Current inactivated influenza virus vaccines might contain neuraminidase, but the antigen quantity and quality are varied and not standardized. While there are data that show a protective role of anti-neuraminidase immunity, many questions remain unanswered. These questions, among others, concern the targeted epitopes or antigenic sites, the potential for antigenic drift, and, connected to that, the breadth of protection, differences in induction of immune responses by vaccination versus infection, mechanisms of protection, the role of mucosal antineuraminidase antibodies, stability, and the immunogenicity of neuraminidase in vaccine formulations. Reagents for analysis of neuraminidase-based immunity are scarce, and assays are not widely used for clinical studies evaluating vaccines. However, efforts to better understand neuraminidase-based immunity have been made recently. A neuraminidase focus group, NAction!, was formed at a Centers of Excellence for Influenza Research and Surveillance meeting at the National Institutes of Health in Bethesda, MD, to promote research that helps to understand neuraminidase-based immunity and how it can contribute to the design of better and broadly protective influenza virus vaccines. Here, we review open questions and knowledge gaps that have been identified by this group and discuss how the gaps can be addressed, with the ultimate goal of designing better influenza virus vaccines.

A live-attenuated, intranasal respiratory syncytial virus (RSV) candidate vaccine, cpts-248/404, was tested in phase 1 trials in 114 children, including 37 1-2-month-old infants-a target age for RSV vaccines. The cpts-248/404 vaccine was infectious at 104 and 105 plaque-forming units in RSV-naive children and was broadly immunogenic in children >6 months old. Serum and nasal antibody responses in 1-2 month olds were restricted to IgA, had a dominant response to RSV G protein, and had no increase in neutralizing activity. Nevertheless, there was restricted virus shedding on challenge with a second vaccine dose and preliminary evidence for protection from symptomatic disease on natural reexposure. The cpts-248/404 vaccine candidate did not cause fever or lower respiratory tract illness. In the youngest infants, however, cpts-248/404 was unacceptable because of upper respiratory tract congestion associated with peak virus recovery. A live attenuated RSV vaccine for the youngest infant will use cpts-248/404 modified by additional attenuating mutations.

SummaryTransgenic mice homozygous for a B2-microglobulin (B2-m) gene disruption and normal mice that had been treated with a CD8-specific mAb were infected intranasally with an H3N2 influenza A virus. Both groups of CD8 T cell-deficient mice eliminated the virus from the infected respiratory tract. Potent CTL activity was detected in lung lavage populations taken from mice with intact CD8 + T cell function, with minimal levels of cytotoxicity being found for inflammatory cells obtained from the antibody-treated and B2-m mutant mice. We therefore conclude that cells infected with an influenza A virus can be cleared from the respiratory tract of mice lacking both functional class I major histocompatibility complex (MHC) glycoproteins and class I MHCrestricted, CD8 + effector T cells.