Dissociation of the haemagglutination inhibition and the infectivity neutralization in the reactions of influenza A/USSR/90/77 (H1N1) virus variants with monoclonal antibodies

Comparison of hemagglutination inhibition and hemagglutinin pseudovirus neutralization titres in relation to protection against influenza in a mouse model

The hemagglutination inhibition (HI) test has long been used as a standard measure of antibody response for inactivated influenza vaccines. However, the HI test has limitations, such as insensitivity when using some H3N2 virus strains and failure to detect neutralizing antibodies that target regions distant from the receptor binding site. We therefore examined a hemagglutinin pseudovirus neutralization (PVN) test as a possible supplement or alternative to the HI test. We evaluated the association of HI or PVN titres with protection against influenza infection in mice based on morbidity (where the illness was defined as 25% body weight loss). We assessed this relationship using dose-response models incorporating HI or PVN titres as a variable. The morbidity was correlated with the pre-exposure titres, and such a correlation was well described by a modified dose-response model. The mathematical modelling suggests that PVN titres consistently show a stronger association with in vivo protection as compared to HI titres in mice. Given our findings, the PVN test warrants further investigation as a tool for evaluating antibody responses to influenza vaccines containing hemagglutinin. The resulting models may also be useful for analyzing human clinical data to identify potentially protective antibody titres against influenza illness.

Preexisting Immunity, More Than Aging, Influences Influenza Vaccine Responses

Background.  Influenza disproportionately impacts older adults while current vaccines have reduced effectiveness in the older population. Methods.  We conducted a comprehensive evaluation of cellular and humoral immune responses of adults aged 50 years and older to the 2008-2009 seasonal trivalent inactivated influenza vaccine and assessed factors influencing vaccine response. Results.  Vaccination increased hemagglutination inhibition and neutralizing antibody; however, 66.3% of subjects did not reach hemagglutination inhibition titers ≥ 40 for H1N1, compared with 22.5% for H3N2. Increasing age had a minor negative impact on antibody responses, whereas prevaccination titers were the best predictors of postvaccination antibody levels. Preexisting memory B cells declined with age, especially for H3N2. However, older adults still demonstrated a significant increase in antigen-specific IgG(+) and IgA(+) memory B cells postvaccination. Despite reduced frequency of preexisting memory B cells associated with advanced age, fold-rise in memory B cell frequency in subjects 60+ was comparable to subjects age 50-59. Conclusions.  Older adults mounted statistically significant humoral and cell-mediated immune responses, but many failed to reach hemagglutination inhibition titers ≥40, especially for H1N1. Although age had a modest negative effect on vaccine responses, prevaccination titers were the best predictor of postvaccination antibody levels, irrespective of age.

Immune escape mutants of Highly Pathogenic Avian Influenza H5N1 selected using polyclonal sera: identification of key amino acids in the HA protein

Evolution of Avian Influenza (AI) viruses--especially of the Highly Pathogenic Avian Influenza (HPAI) H5N1 subtype--is a major issue for the poultry industry. HPAI H5N1 epidemics are associated with huge economic losses and are sometimes connected to human morbidity and mortality. Vaccination (either as a preventive measure or as a means to control outbreaks) is an approach that splits the scientific community, due to the risk of it being a potential driving force in HPAI evolution through the selection of mutants able to escape vaccination-induced immunity. It is therefore essential to study how mutations are selected due to immune pressure. To this effect, we performed an in vitro selection of mutants from HPAI A/turkey/Turkey/1/05 (H5N1), using immune pressure from homologous polyclonal sera. After 42 rounds of selection, we identified 5 amino acid substitutions in the Haemagglutinin (HA) protein, most of which were located in areas of antigenic importance and suspected to be prone to selection pressure. We report that most of the mutations took place early in the selection process. Finally, our antigenic cartography studies showed that the antigenic distance between the selected isolates and their parent strain increased with passage number.

The antigenic evolution of influenza: drift or thrift?

It is commonly assumed that antibody responses against the influenza virus are polarized in the following manner: strong antibody responses are directed at highly variable antigenic epitopes, which consequently undergo 'antigenic drift', while weak antibody responses develop against conserved epitopes. As the highly variable epitopes are in a constant state of flux, current antibody-based vaccine strategies are focused on the conserved epitopes in the expectation that they will provide some level of clinical protection after appropriate boosting. Here, we use a theoretical model to suggest the existence of epitopes of low variability, which elicit a high degree of both clinical and transmission-blocking immunity. We show that several epidemiological features of influenza and its serological and molecular profiles are consistent with this model of 'antigenic thrift', and that identifying the protective epitopes of low variability predicted by this model could offer a more viable alternative to regularly update the influenza vaccine than exploiting responses to weakly immunogenic conserved regions.

Viva la revolución: rethinking influenza a virus antigenic drift

Rapid antigenic evolution of the influenza A virus hemagglutinin has precluded developing vaccines that provide durable protection. The yearly costs of influenza (circa $10(11) in the USA alone) easily justify investments in better understanding the interaction of influenza with antibodies and other inducible elements of the immune system that potentially limit or circumvent antigenic variation. Here, I summarize exciting new findings that offer the possibility of a quantum improvement in vaccine efficacy, focusing on studies clearly documenting robust neutralizing antibody responses to the conserved stem region of the hemagglutinin.

Specific sites of N-linked glycosylation on the hemagglutinin of H1N1 subtype influenza A virus determine sensitivity to inhibitors of the innate immune system and virulence in mice

Oligosaccharides on the hemagglutinin (HA) and neuraminidase of influenza A virus (IAV) are a target for recognition by lectins of the innate immune system, including soluble surfactant protein-D and the macrophage mannose receptor on airway macrophages. Glycans attached to the head of H1 subtype of IAV differ markedly in number and location. A reverse genetic approach was used to define the importance of particular N-glycosylation sites on H1 in determining sensitivity to innate immune defenses and virulence in mice. The HA of A/PR/8/34 (PR8, H1N1) and A/Brazil/11/78 (Brazil, H1N1) express zero and four glycosylation sites on the head of HA, respectively. Site-directed mutagenesis was used to add (PR8) or delete (Brazil) glycosylation sites, and IAV expressing wild-type or mutant HA were generated on a PR8 backbone. Addition or removal of particular glycans modulated sensitivity to mouse lung fluids but was not a major factor determining susceptibility of airway macrophages to infection. PR8 is a mouse-adapted virus, and mutations in multiple IAV genes have been shown to contribute to virulence, yet addition of glycosylation to PR8 HA was sufficient to attenuate disease. In contrast, removal of glycans from Brazil HA resulted in severe disease and death. These studies provide insight regarding the mechanisms by which IAV can induce disease in mice. Moreover, reduced glycosylation of HA is likely to be an important factor associated with adaptation of human IAV to growth in mouse lung.

Comparison of hemagglutination inhibition assay, an ELISA-based micro-neutralization assay and colorimetric microneutralization assay to detect antibody responses to vaccination against influenza A H1N1 2009 virus

The hemagglutination inhibition (HI) assay has been the main method used to investigate immune responses to vaccination against influenza H1N1 (2009) virus. However microneutralization tests (MNT) have been shown to be more sensitive and more specific. In this study, the three methods of choice: (i) the HI assay, (ii) an ELISA-based conventional MNT and (iii) a colorimetric MNT in terms of their ability to detect antibody responses in serum pairs collected from 43 healthy individuals before and 21 days after vaccination were compared. The colorimetric MNT was established yielding intra- and inter-run imprecisions of 7.5% and 12.4%, respectively. Testing of antisera to seasonal influenza viruses demonstrated the assay to be specific for antibodies to influenza H1N1 (2009) virus. A good correlation between the three methods was found, being highest for the ELISA-MNT and the colorimetric MNT (r=0.714 for geometric mean titers (GMT) and r=0.695 for titer increases). Similar rates of fourfold titer increases were detected: 95.3% in the ELISA-MNT vs. 93.0% in colorimetric MNT and 95.3% in HI assay. The ELISA-based MNT demonstrated the highest titer range leading to the highest postvaccination GMT and the highest titer increase (>50-fold). The lowest GMTs were measured with the HI assay, while the colorimetric MNT detected the highest GMT in prevaccination sera. Taken together, similar seroconversion rates were obtained with the three assays. The ELISA-MNT appeared to be the best method to compare absolute pre- and postvaccination GMTs. The colorimetric MNT, being less labour-intensive than the ELISA-MNT, seems to be a suitable tool in vaccination studies.

Influenza outbreaks

Influenza is a virus that causes considerable morbidity and mortality in human populations every year. This fact, coupled with its perceived pandemic potential, means that influenza features prominently in both scientific literature and the media. In this review we focus on the biological assumptions behind theoretical attempts to understand the seasonal and evolutionary dynamics of influenza through mathematical modelling and suggest that the largely unchallenged dogma upon which most efforts are currently based is sorely lacking.

Up to new tricks - a review of cross-species transmission of influenza A viruses

Influenza is a highly contagious disease that has burdened both humans and animals since ancient times. In humans, the most dramatic consequences of influenza are associated with periodically occurring pandemics. Pandemics require the emergence of an antigenically novel virus to which the majority of the population lacks protective immunity. Historically, influenza A viruses from animals have contributed to the generation of human pandemic viruses and they may do so again in the future. It is, therefore, critical to understand the epidemiological and molecular mechanisms that allow influenza A viruses to cross species barriers. This review summarizes the current knowledge of influenza ecology, and the viral factors that are thought to determine influenza A virus species specificity.

Restoration of virulence of escape mutants of H5 and H9 influenza viruses by their readaptation to mice

Antigenic mapping of the haemagglutinin (HA) molecule of H5 and H9 influenza viruses by selecting escape mutants with monoclonal anti-HA antibodies and subjecting the selected viruses to immunological analysis and sequencing has previously been performed. The viruses used as wild-type strains were mouse-adapted variants of the original H5 and H9 isolates. Phenotypic characterization of the escape mutants revealed that the amino acid change in HA that conferred resistance to a monoclonal antibody was sometimes associated with additional effects, including decreased virulence for mice. In the present study, the low-virulence H5 and H9 escape mutants were readapted to mice. Analysis of the readapted variants revealed that the reacquisition of virulence was not necessarily achieved by reacquisition of the wild-type HA gene sequence, but was also associated either with the removal of a glycosylation site (the one acquired previously by the escape mutant) without the exact restoration of the initial wild-type amino acid sequence, or, for an H5 escape mutant that had no newly acquired glycosylation sites, with an additional amino acid change in a remote part of the HA molecule. The data suggest that such 'compensating' mutations, removing the damaging effects of antibody-selected amino acid changes, may be important in the course of influenza virus evolution.

Structural differences among hemagglutinins of influenza A virus subtypes are reflected in their antigenic architecture: analysis of H9 escape mutants

We used a panel of monoclonal antibodies to H9 hemagglutinin to select 18 escape mutants of mouse-adapted influenza A/Swine/Hong Kong/9/98 (H9N2) virus. Cross-reactions of the mutants with the antibodies and the sequencing of hemagglutinin genes revealed two minimally overlapping epitopes. We mapped the amino acid changes to two areas of the recently reported three-dimensional structure of A/Swine/Hong Kong/9/98 hemagglutinin. The grouping of the antigenically relevant amino acid positions in H9 hemagglutinin differs from the pattern observed in H3 and H5 hemagglutinins. Several positions in site B of H3 hemagglutinin are distributed in two sites of H9 hemagglutinin. Unlike any subtype analyzed so far, H9 hemagglutinin does not contain an antigenic site corresponding to site A in H3 hemagglutinin. Positions 145 and 193 (H3 numbering), which in H3 hemagglutinin belong to sites A and B, respectively, are within one site in H9 hemagglutinin. This finding is consistent with the peculiarity of the three-dimensional structure of the H9 molecule, that is, the absence from H9 hemagglutinin of the lateral loop that forms site A in H3 and the equivalent site in H5 hemagglutinins. The escape mutants analyzed displayed phenotypic variations, including decreased virulence for mice and changes in affinity for sialyl substrates. Our results demonstrate a correlation between intersubtype differences in three-dimensional structure and variations among subtypes in the distribution of antigenic areas. Our findings also suggest that covariation and pleiotropic effects of antibody-selected mutations may be important in the evolution of H9 influenza virus, a possible causative agent of a future pandemic.

Structure of antigenic sites on the haemagglutinin molecule of H5 avian influenza virus and phenotypic variation of escape mutants

To elucidate the structure of the antigenic sites of avian H5 influenza virus haemagglutinin (HA) we analysed escape mutants of a mouse-adapted variant of the H5N2 strain A/Mallard/Pennsylvania/10218/84. A panel of five anti-H5 monoclonal antibodies (mAbs) was used to select 16 escape mutants. The mutants were tested by ELISA and haemagglutination inhibition with this panel of anti-H5 mAbs and the HA genes of the mutants were sequenced. The sequencing demonstrated that the amino acid changes were grouped in two antigenic sites. One corresponded to site A in the H3 HA. The other contained areas that are separated in the amino acid sequence but are topographically close in the three-dimensional structure and partially overlap in the reactions with mAbs. This site corresponds in part to site B in the H3 structure; it also includes a region not involved in site B that partially overlaps site Sa in the H1 HA and an antigenic area in H2 HA. Mutants with the amino acid change K152N, as well as those with the change D126N, showed reduced lethality in mice. The substitution D126N, creating a new glycosylation site, was accompanied by an increase in the sensitivity of the mutants to normal mouse serum inhibitors. Several amino acid changes in the H5 escape mutants occurred at the positions of reported changes in H2 drift variants. This coincidence suggests that the antigenic sites described and analysed here may be important for drift variation if H5 influenza virus ever appears as a pathogen circulating in humans.

Rapid antibody response to influenza vaccination in "at risk" groups

Persons attending for routine influenza vaccination in an urban practice each provided three specimens of blood for evaluating their immunological response. 138 (67%) of the 206 persons were defined as "at risk" by reason of morbidity as given in the guidelines published by the Chief Medical Officer. The mean age was 67 yr and 65% were aged 65 yr or more. By day 7, 71% of 31 persons had protective H(1)N(1) titres, 61% H(3)N(2) and 42% B. These proportions were similar to those found at day 14 and at day 21 based on 159 persons. These findings suggest that an effective immune response is mounted within seven days of vaccination indicating that the vaccination of persons "at risk" is worthwhile even after an epidemic has established itself. This is not a reason to modify present policy of routine vaccination in early winter well before epidemics are likely to occur.

Effects of host-dependent glycosylation of hemagglutinin on receptor-binding properties on H1N1 human influenza A virus grown in MDCK cells and in embryonated eggs

There is growing evidence that the receptor-binding characteristics of influenza viruses are affected by the host-dependent glycosylation of viral hemagglutinin (HA). To better understand these effects, we propagated two variants of the human influenza virus USSR/90/77 (which differed by the mutation Asn131 reversible Asp131 in the glycosylation sequon of their HA) in either embryonated chicken eggs or MDCK cell. Those variants were then compared for their ability to bind soluble receptor analogs and to attach to receptors represented on a solid phase. The carbohydrate chain at position 131 of the HA (CHO 131) interfered with virus binding to soluble Sia2-6Gal-containing macromolecular receptors, but had little or no effect on its binding to Sia2-3Gal-containing macromolecules. This specificity could be explained by the different orientation of the asialic parts of the 2-3-linked sialosides versus 2-6-linked sialosides with respect to the receptor-binding site (Eisen et al., 1997, Virology 232, 19-31). In the case of virus attachment to solid-phase immobilized receptors, MDCK-grown viruses bound substantially more weakly than their egg-grown counterparts to receptors of avian origin, whereas binding to mammalian cell membranes was only marginally affected by differences in host-specific glycosylation of the virus. Our data indicated that the effects of the carbohydrate side chain of HA on virus receptor-binding activity are dependent on both the cells in which the virus was grown and the nature of the cellular receptors or intercellular inhibitors to which the virus binds.

Virulence of influenza A virus for mouse lung

The experimental infection of mouse lung with influenza A virus has proven to be an invaluable model for studying the mechanisms of viral adaptation and virulence. These investigations have identified critical roles for the haemagglutinin (HA) and matrix (M) genes of the virus in determining virulence for mouse lung. For the HA gene, the loss of glycosylation sites from the encoded polypeptide or changes which may affect the pH of HA-mediated endosome fusion have been observed following adaptation. These alterations also have the potential to impact on receptor specificity, beta inhibitor sensitivity and activation cleavage which may act in concert to account for the increased virulence of adapted strains. For the M gene, two specific changes in the M1 protein have been identified in strains adapted to, or virulent for, mouse lung. These changes are likely to affect pH-dependent association/dissociation of M1 with the viral ribonucleoprotein, and control virulence as well as growth. The role of other genes in mouse lung virulence remains unknown.

Changes in the hemagglutinin gene of the neurovirulent influenza virus strain A/NWS/33

The neurovirulent strain of influenza A virus, A/NWS/33, is able to infect a large range of cell types, including mouse brain cells, which are not infected by its parent, A/WS/33. This seems to be largely due to the hemagglutinin of A/NWS/33. The complete nucleotide sequence of the HA genes of both strains has been determined and a comparison revealed a number of changes. Analysis showed that the virulence capabilities of the NWS HA involve at least three different mechanisms: (a) loss of a glycosylation site; (b) a change at the cleavage site; and (c) a substitution in HA2, which may increase the pH of fusion.

Neutralization enzyme immunoassay for influenza virus

A neutralization enzyme immunoassay (N-EIA) was developed for the detection of antibody titer rises in sera of patients infected with influenza A (H3N2) virus. In this N-EIA, a selected strain of influenza A (H3N2) virus was added to monolayers of LLC-MK2 cells in microtiter plates. After 24 h, the replicated virus could be demonstrated with a virus-specific enzyme-labeled monoclonal antibody. Preincubation of the influenza virus with convalescent-phase sera of patients infected with influenza A (H3N2) virus resulted 1 day later in decreased absorbance values that could be used for calculation of neutralization titers. From use of paired serum samples from 10 patients with a history of flu-like symptoms, the results obtained with N-EIA correlated well (r = 0.83) with those of the standard hemagglutination inhibition test.

Neutralization map of the hemagglutinin-neuraminidase glycoprotein of Newcastle disease virus: domains recognized by monoclonal antibodies that prevent receptor recognition

Monoclonal antibodies (MAbs) to the hemagglutinin-neuraminidase (HN) glycoprotein of Newcastle disease virus delineate seven overlapping antigenic sites which form a continuum on the surface of the molecule. Antibodies to five of these sites neutralize viral infectivity principally by preventing attachment of the virion to cellular receptors. Through the identification of single amino acid substitutions in variants which escape neutralization by MAbs to these five antigenic sites, a neutralization map of HN was constructed, identifying several residues that contribute to the epitopes recognized by MAbs which block the attachment function of the molecule. These epitopes are defined, at least in part, by three domains on HN: residues 193 to 201; 345 to 353 (which include the only linear epitope we have identified in HN); and a C-terminal domain composed of residues 494, 513 to 521, and 569. To identify HN residues directly involved in receptor recognition, each of the variants was tested for its ability to agglutinate periodate-modified chicken erythrocytes. One variant with a single amino acid substitution at residue 193 was 2.5- to 3-fold more resistant to periodate treatment of erythrocytes than the wild-type virus, suggesting that this residue influences the binding of virus to a sialic acid-containing receptor(s) on the cell surface.

Insights into neutralization of animal viruses gained from study of influenza virus

It has long been known that the binding of antibodies to viruses can result in a loss of infectivity, or neutralization, but little is understood of the mechanism or mechanisms of this process. This is probably because neutralization is a multifactorial phenomenon depending upon the nature of the virus itself, the particular antigenic site involved, the isotype of immunoglobulin and the ratio of virus to immunoglobulin (see below). Thus not only is it likely that neutralization of one virus will differ from another but that changing the circumstances of neutralization can change the mechanism itself. To give coherence to the topic we are concentrating this review on one virus, influenza type A which is itself well studied and reasonably well understood [1–3]. Reviews of the older literature can be found in references 4 to 7.

Bovine and mouse serum beta inhibitors of influenza A viruses are mannose-binding lectins

Normal bovine and mouse sera contain a component, termed beta inhibitor, that inhibits the infectivity and hemagglutinating activity of influenza A viruses of the H1 and H3 subtypes. To investigate the nature of the interaction of beta inhibitors with influenza A viruses we isolated a mutant of the virus Mem71H-BelN (H3N1) that could grow in the presence of bovine serum. The mutant virus was resistant to hemagglutination inhibition by mouse serum as well as by bovine serum and had undergone changes in the receptor-binding and the antigenic properties of its hemagglutinin (HA) molecule. Sequence analysis of the HA genes of parent and mutant viruses revealed a single nucleotide change in the mutant, resulting in the substitution Thr----Asn at residue 167 of the HA1 chain of HA. This change leads to loss of the potential glycosylation site Asn-165-Val-166-Thr-167 at the tip of the HA spike, which in viruses of the H3 subtype is known to bear a high-mannose (type II) carbohydrate side chain N-linked to Asn-165. The association of beta inhibitor resistance with loss of this carbohydrate side chain suggested that beta inhibitors may be lectins. In support of this hypothesis, treatment of the beta inhibitor-sensitive parent virus Mem71H-BelN with periodate converted it to the resistant state. Furthermore, the inhibitory activity of both bovine and mouse sera for the parental virus was abrogated by D-mannose. We conclude that the beta inhibitors in bovine and mouse sera are mannose-binding lectins that inhibit hemagglutination and neutralize virus infectivity by binding to carbohydrate at the tip of the HA spike, blocking access of cell-surface receptors to the receptor-binding site on HA.

Rapid focus reduction neutralization test of influenza A and B viruses in microtiter system

A rapid neutralization test for influenza A and B viruses was developed. In this method, a 96-well tissue culture plate was used for the preparation of cell monolayers and the peroxidase-antiperoxidase staining technique was used for the visualization of foci infected with these viruses. In the presence of trypsin and tragacanth gum, clear foci developed 1 day after infection. A linear relationship between virus dilutions and numbers of foci was observed. When neutralizing antibodies in some test sera were assayed, a good correlation was observed between the titers obtained by the focus method and those obtained by the ordinary plaque method. In addition, many serum specimens were investigated by the neutralization test, and it was demonstrated that the test is useful for serological studies of influenza.

Studies on the genetic basis of human influenza A virus adaptation to mice: degrees of virulence of reassortants with defined genetic content

A highly virulent mouse-adapted variant of influenza virus A/Aichi/2/68 (H3N2) was crossed either with the original A/USSR/90/77 (H1N1) influenza virus strain or with its mouse-adapted, moderately mouse virulent variant. The reassortants were characterized with respect to their genetic content and pneumovirulence for mice. The reassortants fell into three categories: avirulent, highly virulent (resembling in this respect the parent A/Aichi/2/68 virus) and moderately virulent (resembling the mouse-adapted A/USSR/90/77 parent virus). The analysis of the parental origin of the genes of 6 reassortants allowed to suggest that changes in the HA gene and in a polymerase gene (most likely, PB1) were necessary for the acquisition of virulence by the A/USSR/90/77 virus in the course of adaptation to mice, whereas the changes in two other polymerase genes as well as in the genes NA and NS were not involved. The low degree of pathogenicity characteristic of the mouse-adapted A/USSR/90/77 virus was determined by gene(s) other than HA.