An extreme example of this phenomenon was the 1968 pandemic where antibodies to the prior H2N2 viruses contributed to protection against H3N2 infection

An extreme example of this phenomenon was the 1968 pandemic where antibodies to the prior H2N2 viruses contributed to protection against H3N2 infection. Antigenic drift of HA is usually observed as low reactivity of a reference ferret antiserum against a circulating strain as well as the reciprocal, that is ferret antiserum raised against the circulating virus inhibits the reference virus poorly. ability to release N-acetyl neuraminic acid from erythrocytes and mucins [2]. It could be separated from other viral proteins after detergent disruption [3], facilitating OSU-03012 studies of OSU-03012 its biochemical activity that provided a foundation for understanding the role of NA in the virus life cycle. It was later identified to be antigenic, eliciting specific antibodies. In the 1960s, NA was shown to be distinct from the hemagglutinin (HA) and to evolve independently. This was clearly documented in the 1968 Hong Kong influenza A(H3N2) pandemic that involved a shift in the HA but not the NA which remained similar to that of previous influenza A(H2N2) viruses [4]. The contribution of the NA to broadened protection during the pandemic was confirmed in a contemporary serologic study, which showed that individuals with higher N2 titers were less likely to be infected [5, 6]. Why then has the contribution of the NA to broad protection received only intermittent attention during the subsequent years? The assay to quantify functional antibody affecting enzymatic activity was difficult to perform safely and reproducibly, and had limited throughput, so immunogenicity studies usually did not evaluate antibody responses to NA. In addition, the amount of NA in various viruses varied, was not easily measured, and was often unstable, resulting in reports of NA immunogenicity that were often in conflict. As a result, it was concluded in 1998 that it was unwise for licensed influenza vaccines to have specifications for NA content [7]. In many ways, it was also the strong role of HA in driving protection, as long as the vaccine virus was closely matched to the circulating strain, that led to ignoring the possible contribution of other OSU-03012 factors in immunity. That role was demonstrated in the first trials of influenza vaccine in 1943 and continued to monopolize thinking even in the regulatory community where new vaccines could be licensed in Europe simply on OSU-03012 demonstration of production of HA antibodies. An exception to SPARC this was the work of Kilbourne and coinvestigators who doggedly pursued an understanding of NA as a potential vaccine antigen [8C12]. The basic concept was that antibodies against NA would not prevent infection, providing permissive immunity that would allow asymptomatic infection and production of a better response to the infecting virus than would vaccination with an inactivated, split virus that induces HA-specific antibodies [13]. The NA vaccine development program recognized that recall responses to HA would easily predominate following exposure to a seasonal influenza virus, providing little opportunity to establish a robust response against NA. For this reason, clinical studies were performed with vaccines comprised of reassortants with an irrelevant HA, usually from an equine virus [13]. Later, purified NA was used [11]. A fundamental flaw in these clinical studies was that the end points to support the permissive infection approach (eg, prevention of symptoms following exposure), were not distinct from measures of vaccine failure. Some animal studies were performed by the group to examine the benefit of supplementing a standard inactivated vaccine with purified NA [9, 14], presaging some of the approaches currently being considered to generate long-lasting, broadened antibody responses. In the course of these studies, it was demonstrated that vaccination with purified NA was safe and produced 4-fold seroconversion at doses OSU-03012 7.7 g in healthy adults [11]. BREADTH OF NA IMMUNITY Separate studies of the antigenic drift of NA demonstrate antigenic changes in HA and NA are independent [15, 16]. NA immunity is therefore highly likely to provide a level of protection when drift in HA occurs. An extreme example of this phenomenon was the 1968 pandemic where antibodies to the prior H2N2 viruses contributed to protection against H3N2 infection. Antigenic drift of HA is usually observed as low reactivity of a reference ferret antiserum against a circulating strain as well as the reciprocal, that is ferret antiserum raised against the circulating virus inhibits the reference virus poorly. In contrast, studies of antigenic change in NA showcase a phenomenon of one-way drift [16], with antiserum against one virus having reduced inhibition of NA activity, but the reciprocal continuing.