Studies on antigenic variation in influenza virus. Evidence for multiple antigenic determinants on the hemagglutinin subunits of A-Hong Kong-68 (H3 N2) virus and the A-England-72 strains

Amino acid substitutions in antigenic region B of hemagglutinin play a critical role in the antigenic drift of subclade 2.3.4.4 highly pathogenic H5NX influenza viruses

As one of the important control strategies for highly pathogenic avian influenza (HPAI) in China, vaccination has been implemented compulsively in poultry flocks since 2004. However, the emergence and dominance of the circulating antigenic variants require the update of vaccines periodically. In order to investigate the key molecular sites responsible for the antigenic drift, a total of 13 amino acid positions divergent between clade 2.3.4 H5 viruses and their descendent subclade 2.3.4.4 variants in or around the recognized antigenic epitopes A-E were initially identified through inspecting a comprehensive HA sequence alignment of the H5 subtype HPAI viruses. Subsequently, a panel of single-site or multi-site HA mutants was constructed by reverse genetics with two H5N1 viruses of S (clade 2.3.4) and QD1 (subclade 2.3.4.4) as the HA backbone to study their antigenic variations, respectively. The hemagglutination-inhibition assay revealed an evident impact of mutations at sites 88, 156, 205, 208, 239 and 289 to the HA antigenicity and highlighted that the amino acid substitutions located in the antigenic region B, especially the combined mutations at sites 205 and 208, were the major antigenic determinant which was also consistent with results from flow cytometry and antigenic mapping. Our findings provided more insights into the molecular mechanism of antigenic drift of the H5 subtype HPAI virus, which would be helpful for the selection of vaccine candidates and accordingly for the prevention and control of this devastating viral agent.

Comparative investigation of the hemagglutinin epitopes of influenza virus A/Brazil/11/78 (H1N1). and its escape variants

A method described previously for determining the concentration of influenza virus antihemagglutinin antibody molecules, the number of epitopes per virus particle and the equilibrium constant of virus antibody interaction was adapted to the use with escape variants (EVs), produced by multiplication of influenza virus A/Brazil (H1N1) in presence of monoclonal antibody directed to each of the four hemagglutinin sites (Sa, Sb, Ca and Cb). The EVs were found to possess an altered antigenic site, which was both antigenic and immunogenic. By use of selected EVs and antibody preparations, the number of epitopes per antigenic site was determined and it was found that each of the four sites was represented by about 390 epitopes per virus particle, suggesting that each of the about 400 hemagglutinin spikes per virion possessed one epitope of the specificity Sa, Sb, Ca and Cb. Alteration of site Sa but not of site Ca increased the avidity of antibody to react with the unchanged sites.

Isolation of the influenza C virus glycoprotein in a soluble form by bromelain digestion

The spike glycoprotein of influenza C/Johannesburg/1/66 was isolated in a soluble form by digestion of MDCK cell-grown virions with bromelain. The whole ectodomain of the glycoprotein could be recovered with an apparent molecular weight of 75,000 daltons determined in SDS-PAGE. Comparison to Triton X-100-isolated glycoprotein revealed that a C-terminal peptide of 3000-4500 daltons must have remained in the viral membrane. When purified by sucrose density gradient centrifugation the glycoprotein sedimented with a sedimentation coefficient of 10 S, indicating a molecular weight of 206,000 daltons, which is consistent with a trimeric structure of the spike molecule. The trimeric form was stabilized in sucrose gradients by Ca2+ ions. Bromelain digestion of virions with uncleaved glycoprotein, grown in MDCK cells without trypsin, produced two disulphide-linked subunits with similar electrophoretic mobilities in SDS-PAGE to the biologically active glycoprotein. The smaller subunit differed from the product cleaved in vivo (gp 30) by the presence of an additional arginine residue at the N-terminus. The soluble glycoprotein appears to possess both receptor-binding and receptor-destroying enzyme activities, as isolated glycoprotein inhibited hemagglutination of intact influenza C virions and showed RDE activity in an in vitro test. Glycoprotein exposed to low pH, which was sensitive to trypsin digestion, also demonstrated both these biological activities. Glycoprotein-mediated hemolysis could not be observed.

Electron microscopic method for determining the concentration of influenza virus antihemagglutinin antibodies of the IgG class

A novel technique for the determination of the concentration of influenza virus antihemagglutinin antibody of the IgG class is described which is based on the following principle: graded doses of purified antibody are allowed to react with homologous virus, resulting in binding of a portion of antibody. The virus-bound antibody is electron microscopically counted after labeling with protein A-gold. Since Freundlich's lambda-formula is valid for describing the relationship between antibody bound per virus particle and the corresponding concentration of unbound antibody, the values of bound antibody recorded can be used for calculating the concentration of unbound antibody. The antibody concentration of the test sample is obtained as sum of bound and unbound antibody. The antibody concentrations measured by use of this technique agreed well with chemically determined values.

Split virus influenza vaccination in children: an evaluation of efficacy

Twenty-seven children aged between 4 and 13 years were given two injections of a split virus influenza vaccine (MFV-Ject, Institut Merieux) at a dose of 0.5 ml and an interval of 1 month. Each vaccination contained A/Philippines/2/82 (H3 N2) 10 micrograms HA, A/Brazil/11/78 (H1 N1) 10 micrograms HA, B/Singapore/222/79 15 micrograms HA. The overall serological response was good although the B group virus produced slightly less response; however, no children were seropositive to B prior to vaccination, reflecting a low previous exposure. Children aged over 10 years showed a generally higher serological response. Side-effects were minimal. A favourable reactogenicity/efficacy ratio was found.

Influenza virus neuraminidase with hemagglutinin activity

Isolated intact influenza virus neuraminidase (NA) molecules of the N9 subtype have been found to possess hemagglutinin (HA) activity which, at equivalent protein concentration, was fourfold higher than that of isolated hemagglutinin molecules of the H3 subtype. The amino-terminal sequence of the N9 NA is the same as in neuraminidases of the eight other influenza A virus NA subtypes previously reported. Viruses possessing N9 NA therefore have two different HA activities and antibody to either HA or NA alone was incapable of inhibiting hemagglutination by the virus. However, antibody to the HA of an H1N9 virus neutralized its infectivity as effectively as it neutralized H1N1 or H1N2 viruses whose neuraminidases have no HA activity. (Antibodies to N9 NA did not neutralize the infectivity of viruses with N9 neuraminidase). 2-deoxy-2,3-dehydro-N-acetyl-neuraminic acid inhibited N9 NA activity but had no effect on the HA activity of the isolated N9 NA. One interpretation of this result would be that the HA and NA activities are located in separate sites. Pronase-released N9 NA heads form crystals suitable for X-ray diffraction studies and preliminary data to 2.9 A establish the space group as cubic, I432 with cell dimension a = 184 A. Data extend to beyond 1.9 A resolution, and these will be collected in the future.

Detection of antibodies to strain-specific and cross-reactive antigenic determinants on the haemagglutinin of influenza viruses A/Texas/1/77 and A/Bangkok/1/79 in human sera. Brief report

The antibody response to antigenic determinants on the haemagglutinin molecule of A/Texas/77 and A/Bangkok/79 strains was analysed by SRH of virus-absorbed sera, in human sera collected from persons of different ages following natural infection or immunization. The results have shown, as expected, low and high frequencies of strain-specific antibodies in sera of adults and young children respectively. However, in the latter group antibodies to cross-reactive antigens were also detected.

Structural analysis of the variable major proteins of Borrelia hermsii

Borrelia hermsii undergoes spontaneous antigenic variation in vivo and in vitro. Serotype specificity is associated with expression of one of a family of molecular weight-variable proteins, the pI proteins. We studied the structure of the pI proteins as well as the molecular weight-invariable pII proteins of three serotypes of B. hermsii HS1: C, 7, and 21. The techniques used were one-dimensional (1-D) mapping of Staphylococcus aureus V8 protease-generated peptides and two-dimensional (2-D) mapping of alpha-chymotrypsin-generated peptides. The pI and pII proteins were isolated by excision of polypeptides from stained polyacrylamide gel electropherograms. The 1-D peptide patterns were visualized by fluorography of intrinsically [14C]leucine-labeled proteins or by silver stain. Before 2-D mapping, polypeptides in excised gel fragments were labeled with 125I in the presence of chloramine-T. We also compared the 2-D peptide maps of pI proteins, pI7 and pI21, after their surface-exposed portions were radioiodinated using 1,3,4,6-tetrachloro-3 alpha,6 alpha-diphenylglycoluril (Iodogen). The I-D and 2-D peptide maps demonstrated the following: (a) pI proteins of the three serotypes have few V8 protease- or chymotrypsin-generated peptides in common, and (b) pI proteins of each serotype appear to be identical. The findings suggest that pI protein variability derives from extensive differences in the amino acid sequences of these proteins.

Type-specific and cross-reactive antigenicity of capsid proteins VP1 and VP2 of echovirus type 7

After disruption of echovirus type 7 virions with urea and heat, VP1 and VP2 were separated by isoelectric focusing in urea-containing sucrose gradients. Antisera to these two polypeptides were produced in guinea pigs. In complement fixation, antiserum to VP1 reacted with native and heated virions (N and H antigens, respectively) of homologous virus, and also cross-reacted with heated virions of some other enteroviruses used. Antiserum to VP2 was reactive only with heated virions of homologous and heterologous viruses. Interestingly, the anti-VP2 serum reacted neither with native nor even with heated procapsids (naturally-occurring empty capsids). Antiserum to VP1, but not VP2, showed neutralizing and hemagglutination-inhibiting activities. These results suggest that 1) both VP1 and VP2 possess cross-reactive antigenic determinants which are exposed on the surface of heated virions, and 2) type-specific determinants of VP1 are located on the surface of native virions.

[Influenza]

Influenza is the last great uncontrolled plague of mankind. Pandemics and epidemics occur at regular time intervals. The influenza viruses are divided into the types A, B and C and show unique variability of their surface antigens (hemagglutinin and neuraminidase). Influenza viruses of type A show the largest degree of antigenic variation which, in turn, resulted in the definition of a number of subtypes, each comprising many strains. By comparison, influenza viruses of types B and C exhibit much less variation of their surface antigens. As a consequence, no subtypes but many different strains have been recognized. The degree of antigenic variation correlates with the epidemiologic significance of the virus types, type A being the most and type C the least important. Two different kinds of antigenic variation have been recognized: In the case of minor variation of one or both surface antigens, the term "antigenic drift" is employed. Antigenic drift occurs with all three types of virus, it is caused by point mutations which increase the chance of survival of mutants in the diseased host. In addition, influenza A viruses show sudden and complete changes of their surface antigens in regular time intervals, resulting in the appearance of new subtypes. This event is called "antigenic shift". The mechanisms responsible for antigenic shift are poorly understood, only. In addition to the recycling of preceding subtypes, reassortment resulting from double infection of cells with strains of human and animal origin are considered possible explanations. By use of modern DNA recombinant technology, the base sequences of a series of virus genes and, as a consequence, the amino acid sequence of the corresponding antigens have been determined. By means of monoclonal antibodies, the antigenic structure of many influenza antigens has been further elucidated. It can be expected that further research on the molecular basis of antigenic variation could finally result in an understanding of the causal mechanisms. It is an outstanding feature of the epidemiology of influenza A viruses that a family of related strains prevails for a certain period of time and disappears abruptly as a new subtype emerges.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of human serum strain-specific antihemagglutinin antibody to A/Port Chalmers/73 (H3N2) influenza virus by radioimmunoprecipitation assays

We performed radioimmunoprecipitation assays in which iodinated preparations of A/Port Chalmers/73 (A/PC/73) hemagglutinin were used as the test antigens and high concentrations of unlabeled A/Hong Kong/68 viral protein were used to inhibit the binding of cross-reactive antibodies to quantitate strain-specific antibody responses in postvaccination sera. Strain-specific antibodies comprised 8 to 48% (mean, 20%) of the total A/PC/73 antigen-binding capacity of the sera tested. Competition radioimmunoprecipitation assays in which disrupted preparations of purified whole virus representative of several of the H3N2 variants were used indicated that the A/PC/73 strain-specific antibody that was present after adsorption of serum by A/Hong Kong/68 antigen was capable of reacting with A/England/72 and A/Victoria/75 hemagglutinins, but generally with lower avidity than with A/PC/73 hemagglutinin. A comparison of the A/PC/73 antibody titers measured by radioimmunoprecipitation and hemagglutination inhibition tests before and after adsorption with A/Hong Kong/68 whole virus suggested that cross-reactive and strain-specific antibodies were comparable in efficiency of inhibiting viral hemagglutination. These data indicated that vaccines containing later variants within a subtype could induce antihemagglutinin antibodies of restricted specificity, but that these antibodies may not be directed against unique antigenic determinant(s).

Preparation and characterization of antisera to electrophoretically purified SA11 virus polypeptides

Antisera to SA11 virus proteins were prepared by immunizing rabbits with individual polypeptides separated by polyacrylamide gel electrophoresis under reducing or nonreducing conditions; the resulting antisera were characterized by four immunological methods. Results of complement fixation tests with double-shelled rotavirus particles and sera raised against reduced or unreduced proteins of the outer shell of the virus suggested the presence of common antigenic determinants in the outer capsid layers of SA11 and the Northern Ireland strain of calf rotavirus. In this test, antisera to outer shell polypeptides gp34 (O2) and gp25 (O4) cross-reacted with calf rotavirions, whereas those to p62 (O1) and p26 (O3) reacted only with the homologous virus. Antisera to the reduced outer shell proteins of the virus did not neutralize viral infectivity, nor did they possess hemagglutination inhibition activity. Evidence suggesting the presence of type-specific antigenic determinant(s) in the major inner protein p42 (I4) of SA11 virus, capable of inducing neutralizing antibody, is presented and discussed. Antisera produced against unreduced gp34 and p26 polypeptides of the virus contained type-specific neutralizing antibodies. Polypeptide gp34 was also capable of inducing hemagglutination inhibiting antibody. All of the antisera to unreduced polypeptides had agglutinating activity against double-shelled particles of homologous and heterologous rotaviruses.

Antigenic structure of influenza virus haemagglutinin defined by hybridoma antibodies

The recurrence of influenza virus infection in man is attributed primarily to changes occurring in the antigenic structure of the viral surface glycoproteins, especially of the haemagglutinin (HA) molecule. Comparative antigenic analysis of epidemic influenza virus strains has allowed the description of 'strain-specific' and 'cross-reactive' antigenic determinants. However, the interpretation of these findings remained ambiguous, because the specificity of the applied antisera was insufficiently defined and because the antigenic differences among the HA molecules of various epidemic virus strains resulted presumably from a large number of amino acid substitutions. Thus, in characterizing the antigenic structure of the HA molecule, our approach has been (1) to generate a panel of monoclonal anti-HA hybridoma antibodies, (2) to use some of these antibodies to select mutants of the influenza A/PR/8/34 (PR8) virus expressing antigenically altered HA molecules, and (3) to construct an operational antigenic map of the HA molecule by comparative antigenic analysis of the mutant viruses with the monoclonal antibodies. As we report here, analysis of the 34 mutant viruses selected has enabled us to define four antigenic sites on the HA molecule. Our observation that these sites have undergone antigenic drift to a different extent in nature implies that the mechanisms responsible for antigenic drift act selectively on distinct structures of the HA molecule.

The detection of influenza A virus antigens in cultured cells by enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay (ELISA) was employed to investigate the expression of influenza A/Hong Kong/68 (H3N3) virus structural proteins on the surface of infected MDCK cells, and to detect viral antigens in culture media and cell extracts. Infected cells were fixed with 0.1 per cent glutaraldehyde before being examined for the presence of cell-surface antigens. Viral antigens were first observed on the surface of cells 4 hours after infection and reached a maximum 10-12 hours after infection, when measured by haemadsorption with chicken erythrocytes and by ELISA and immunofluorescence with hyperimmune antiserum to Hong Kong virus. A good correlation was found between the three assay systems. The presence of individual virion structural proteins on the cell surface was determined by ELISA using specific antibodies purified by differential affinity chromatography. Either or both or the internal matrix and nucleoprotein antigens were expressed from 2 to 6 hours after infection, with maximum expression after 2 hours, and the strain-specific and common antigenic determinants of haemagglutinin were observed on the cell surface from 4 hours after infection, and reached a maximum 8 to 10 hours after infection. Low levels of neuraminidase were detected between 4 and 8 hours after infection. Culture media and cell extracts were titrated by infectivity and haemagglutination assays, and by ELISA. Titres obtained from the culture media showed a close correlation between the three assay methods, with peak titres being attained 24 hours after infection. Viral antigens were first observed in cell extracts by ELISA 4 hours after infection, and infectious virions and haemagglutinin 2 hours later, but whereas maximum titres of infectious virus and haemagglutinin were found 10 hours after infection, the ELISA titre continued to rise until 24 hours after infection, which suggested that virus structural proteins were being accumulated in the cells after most of the progeny virions had been released. The results are discussed in terms of the potential use of ELISA in rapid virus diagnosis.

Amino acid sequence and oligosaccharide distribution of the haemagglutinin from an early Hong Kong influenza virus variant A/Aichi/2/68 (X-31)

The amino acid sequence and oligosaccharide distribution for the haemagglutinin from the early Hong Kong influenza virus A/Aichi/2/68 (X-31) was investigated. The two polypeptide chains, HA1 and HA2, were fragmented by CNBr and enzymic digestion, and the amino acid sequence of each small peptide was deduced by comparing its chromatographic behaviour, electrophoretic mobility, amino acid composition and N-terminus with that of the corresponding peptide of the haemagglutinin of known structure from the influenza-virus variant A/Memphis/102/72. Those peptides in which changes were detected were sequenced fully. The complete amino acid sequence of the haemagglutinin HA1 chain (328 residues) and 188 of the 221 residues of the HA2 chain were established by this approach, and revealed only twelve differences between the amino acid sequences of variant-A/Aichi/68 and -A/Memphis/72 haemagglutinins. These occurred at positions 2, 3, 122, 144, 155, 158, 188, 207, 242 and 275 in the HA1 chain and 150 and 216 in the HA2 chain. The highly aggregated hydrophobic region (residues 180-121) near the C-terminal end of the HA2 chain was not resolved by peptide sequencing. The oligosaccharide distribution in variant-A/Aichi/68 haemagglutinin was identical with that found in that of A/Memphis/72, with sugar units attached at asparagine residues 8, 22 38, 81, 165 and 285 in the HA1 chain and 154 on the HA2 chain. The monosaccharide compositions of the individual carbohydrate units on variant-A/Aichi/68 haemagglutinin differed from those of the corresponding units in variant-A/Memphis/72 haemagglutinin, and evidence was found for heterogeneity in the oligosaccharide units attached at single glycosylation sites.

Strain specificity of serum antibody to the haemagglutinin of influenza A (H3N2) viruses in children following immunization or natural infection

The specificity of serum anti-HA antibody from children immunized or infected with A/Victoria/75 (H3N2 or A/Texas/77 (H3N2) virus was examined using the single radial haemolysis test together with adsorption of antibody with three antigenic variants A/Hong Kong/68 (H3N2), A/Port Chalmers/73 (H3N2) and A/Victoria/75 (H3N2). The majority of young children reacted to vaccination or infection by producing strain-specific (SS) antibody to the homologous virus. A small proportion of children's sera contained cross-reacting (CR) antibodies capable of reacting with the haemagglutinins of all antigenic variants of the sub-type including A/HK/1/68. In contrast, most adults reacted immunologically to either vaccination or infection by producing CR antibody, reacting with all variants of the antigenic subtype including the prototype virus A/HK/1/68 (H3N2).

Antibodies to the strain-specific and cross-reactive determinants of the haemagglutinin of influenza H3N2 viruses. Antiviral activities of the antibodies in biological systems

Antibodies to the strain-specific (SS) and cross-reactive (CR) antigenic determinants of the haemagglutinin (HA) of influenza H3N2 viruses were prepared, and their antiviral properties in experimentally infected mice and in eggs were investigated. The SS antibodies were considerably more efficient than the CR antibodies in direct virus neuralization tests. The difference in the activities of the two types of antibody preparations was less pronounced in virus growth inhibition studies in the allantois-on-shell system and in passive protection studies in mice give antibody before challenge with virus. Nevertheless, even in these tests SS antibody was approximately four-fold more effective than was CR antibody in protection against fatal infection. Passive administration of CR or SS antibody preparations to mice challenged with sublethal virus doses, reduced the peak pulmonary titres of virus on day three post-infection compared with the control animals. The degree of reduction in titre exerted by the SS antibodies was greater than for the CR antibodies. In contrast, the rate of clearance of pulmonary virus 9-12 days after infection in the mice treated with SS antibody was slower than for the CR treated animals. Also, in SS antibody treated animals viruses isolated from mouse lungs showed evidence of minor antigenic variation away from the parent virus.

The mechanism of antigenic drift in influenza viruses: analysis of Hong Kong (H3N2) variants with monoclonal antibodies to the hemagglutinin molecule

Monoclonal antibodies to the hemagglutinin molecule of the Hong Kong variant, Mem/1/71 (H3N2), influenza virus were used to study antigenic drift in the H3N2 subtype of influenza viruses. Antigenic variants of Mem/1/71 (H3N2) were selected after a single passage of the virus in chick embryos in the presence of monoclonal antibody. The variants showed a marked reduction in the ability to react with the monoclonal antibody used in selection. The monoclonal antibodies could be divided into three groups based on their reactions with the variants, providing evidence for at least three nonoverlapping antigenic areas on the hemagglutinin molecule. Amino acid analysis of tryptic peptides of the hemagglutinin from these variants showed that a single amino acid substitution in the heavy polypeptide chain (HA1) of the hemagglutinin molecule accounted for the reduced antibody interactions, and that variants from each group exhibited sequence changes in different areas of the molecule. Sequence changes were also detected in the HA1 polypeptides of naturally occurring H3N2 variants, but in most cases the changes in the monoclonal antibody selected variants were different from the field strains. Antigenic analysis showed that most of the variants selected with monoclonal antibody could not be distinguished from parental viruses with heterogeneous sera, suggesting that they are probably epidemiologically irrelevant. One variant, however, could be distinguished from parental virus with heterogeneous sera. This variant showed a change in sequence at residue 144 of the HA1 polypeptide from glycine in the parent to aspartic acid in the variant. Similar substitutions have been found in naturally occurring variants at this position. These studies suggest that some amino acid substitutions are more important than other for producing viruses with epidemiological potential. Antigenic analysis of naturally occurring H3N2 strains with monoclonal antibodies established that two variants co-circulated in 1968; Hong Kong/1/68 being distinguishable from Aichi/2/68 in at least two antigenic areas. It would appear that there may have been two separate lineages of H3N2 viruses, Hong Kong/1/68 giving rise to variants in England and Aichi/2/68 to variants in USA and Australia.

A mechanism for influenza subtype disappearance

We propose that changes in the antigenic region/s of influenza haemagglutinin can also cause changes in the binding site for host cells. If these changes have an overall deleterious effect on virus binding and hence on viral infectivity and reproduction one has a simple mechanism to explain the limited time span of influenza subtypes. Evidence is presented from the published results of other workers to support this hypothesis.

Antineuraminidase antibody response to vaccination of chickens with intact virus and different submit preparations of the influenza virus strains A/Sing/1/57 (H2N2), A/Hong Kong/1/68 (H3N2) and A/Port Chalmers/1/73 (H3N2)

The antineuraminidase (AN) antibody response to vaccination of chickens with intact virus and different subunit preparations of the influenza virus strains A/Sing/1/57 (H2N2), A/Hong Kong/1/68 (H3N2) and A/Pt. Chalmers/1/73 (H3N2) was tested comparatively. Using a photometric method capable of analysing mixtures of AN antibodies against antigenically different N2 neuraminidases, it was concluded that vaccination with subunits produced by treatment with bromelain and Sarkosyl can yield AN antibody response against heterologous neuraminidase. By contrast, vaccination with intact and ether-treated virus gave AN antibody response against homologous neuraminidases. The conclusion was reached that the NA's of the strains A/Sing/1/57 and A/Pt. Chalmers/1/73 share antigenic determinants and that the NA of the strain A/Hong Kong/1/68 shares antigenic determinants with that of the strains A/Sing/1/57 and A/Pt. Chalmers/1/73.

Serology and energetics of cross-reactions among the H3 antigens of influenza viruses

Reciprocal hemagglutination inhibition titrations were carried out with viruses and antisera of eight field strains of the A3 subtype of influenza A, covering the period from 1968 to 1975. The earlier strains (1968 through 1972) showed asymmetric cross-reactions, with antisera exhibiting more cross-reactions with antecedent strains than with subsequent ones. The later strains, although all were asymmetrically cross-reactive with earlier strains, tended to exhibit distant and variable cross-reactions with each other. The numbers and average affinities of antibody molecules capable of taking part in cross-reactions were calculated from equilibrium filtration experiments. It was found that all the antibody molecules in sera raised against the late strains could combine with earlier viruses, but with reduced affinity. Conversely, only a subset of the antibody molecules in sera raised against early strains could combine with later viruses. The results are discussed in the light of different theories concerning the nature and number of antigenic determinants on the hemagglutinin molecule. They support the existence of a single antigenic area to which all antibody molecules are directed, with differing affinities, rather than the existence of both "common" and "specific" determinants. Thermodynamic measurements on the homologous antigen-antibody reactions indicated that combination was mostly entropy driven. This suggested hydrophobic interaction as the mechanism of combination, i.e., that the complementary regions of antigen and antibody were made up largely or entirely of amino acids with hydrophobic side chains. There was no statistical difference in the magnitude of the entropy term (i.e., the average firmness of binding) among the different virus strains.

Amino acid sequence changes in the haemagglutinin of A/Hong Kong (H3N2) influenza virus during the period 1968--77

Haemagglutinin molecules from nine strains of A/Hong Kong/68 (H3N2) influenza virus, isolated between 1968 and 1977, were examined for changes in amino acid sequences. At least 18 changes, 9 of which were located precisely, occurred in the soluble tryptic peptides of the large haemagglutinin polypeptide (HA1) during this period. These peptides contained 262 residues (82% of HA1). In HA2, only two changes in 129 residues (58% of HA2) were detected. Sequential changes at a particular locus were not found; and as far as we can tell, once an amino acid changed, it did not change again in any subsequent variant examined.

Antibodies to the strain-specific and cross-reactive determinants of the haemagglutinin of influenza H3N2 viruses. 1. Preparation of antibodies and in vitro studies

The serological analysis of antibodies to the haemagglutinin (HA) of influenza A viruses of the Hong Kong (H3N2) subtype is described, using haemagglutination-inhibition, immuno-double-diffusion and single-radial-diffusion techniques. By cross-absorption of antisera to purified HA antigens, different populations of antibody molecules were obtained, which are designated strain-specific and cross-reactive and characterized in terms of their antigenic specificities for HA antigens of the homologous and antigenically variant H3N2 viruses. A narrowly strain-specific population of antibodies (SS"HK) was obtained as the residual antibody in antiserum to A/Hong Kong/1/68 HA after absorption with the closely related A/England/42/72 virus, whilst a contrasting broadly cross-reactive population (CR'HK) was obtained by absorption of the anti-A/Hong Kong/1/68 HA serum with the more distantly related strain A/Victoria/3/75 and eluting the cross-reactive antibodies from the absorbing virus. Similarly, specific and cross-reactive antibodies were derived from antiserum to A/Victoria/3/75 HA antigen by absorption with A/Hong Kong/1/68 virus. Single-radial-diffusion tests were performed, involving sequential application of different antibody preparations in the same wells in immunoplates containing intact virus particles. The cross-reactive and strain-specific antibodies differed in their property of mutual interference of attachment ot antigen. The results suggested that the cross-reactive antigenic determinants on the HA subunit may be located closer to the distal end of the molecule than the strain-specific determinants. Further tests employing single-radial-diffusion showed that there are more cross-reactive than strain-specific sites available for antibody in the intact virus particle. The strain-specific antibodies also gave higher haemagglutination-inhibition titres per microgram IgG than the cross-reactive antibodies.

Antigenic determinants in influenza virus hemagglutinin

Three antigenic determinants were revealed in H3 hemagglutinin of influenza A viruses isolated from 1968 to 1975. One of them was common for all viruses, and two others specified differences between the viruses possessing H3 hemagglutinin.

The influenza virus: Antigenic composition and immune response

The architecture and chemical composition of the influenza virus particle is described with particular reference to the protein constituents and their genetic control. The dominant role in infection of the surface proteins - haemagglutinins and neuraminidases - acting as antigens and undergoing variation in time known as antigenic drift and shift is explained. The immuno-diffusion technique has illuminated the interrelationships of the haemagglutinins of influenza A viruses recovered over long periods of time. The H0 and H1 haemagglutinins are now regarded as a single sub-type with H2 and H3 representing the haemagglutinins of the 1957 and 1968 sub-types. Animal influenza viruses of pigs, horses and birds are described. A relation to human influenza strains has been shown to exist in certain instances as is the capacity of some human strains to pass to the animal kingdom.

The specificity of the anti-haemagglutinin antibody response induced in man by inactivated influenza vaccines and by natural infection

The anti-haemagglutinin antibody response in adult human volunteers to inactivated whole virus or tween ether split influenza A/Victoria/75 (H3N2) and A/Scotland/74 (H3N2) virus vaccines was investigated using antibody absorption and single-radial-haemolysis (SRH) techniques. The concentrations of haemagglutinin (HA), nucleoprotein (NP) and matrix (M) antigens measured by single radial diffusion (SRD) and rocket immunoelectrophoresis were similar for both the whole virus and split vaccines. Whole virus and split vaccines induced crossreactive (CR) antibody in 87% of vaccinees. Strain specific (SS) antibody to A/Hong Kong/1/68 of the homologous virus was induced less frequently than CR antibody. Higher anti-haemagglutinin antibody titres were detected in persons receiving the split virus vaccines than in those receiving the whole virus vaccines. No antibody to the type-specific matrix protein was detectable, but 33% of volunteers developed an antibody rise to type-specific nucleoprotein antigen. The specificity of the anti-haemagglutinin antibody response in human adults to natural infection with A/Port Chalmers/73 (H3N2) virus was similar to that induced by inactivated vaccines in that a high proportion of subjects developed CR anti-haemagglutinin antibody, which reacted with A/Hong Kong/68 virus and the homologous A/Port Chalmers/73 virus, and SS antibody for A/Hong Kong/68 virus but SS antibody for A/Port Chalmers/73 virus was infrequently stimulated by natural infection.

Immunity to challenge in volunteers vaccinated with an inactivated current or earlier strain of influenza A(H3N2)

Volunteers were inoculated with vaccine made from the 30c mutant, A/Port Chalmers/73 or B/Hong Kong/8/73. Preliminary experiments showed that the 30 c strain was antigenically quite close to A/HK/8/68. Volunteers given 30c developed haemagglutination inhibiting antibodies against the 'current' 1973 serotypes (as well as to the vaccine virus) but the titres were less than those after the A/PC/73 vaccine. Volunteers were then challenged with a live attenuated virus, WRL 105, with A/Finland/4/74 antigens, by intranasal inoculation. The rates of infection were 43% after B/Hong Kong/8/73, 20% after 30c and 5% after A/PC/73. This indicated that the 30c gave some protection but that the vaccine prepared from the current strain gave more.

Radioimmunoprecipitation assay for quantitation of serum antibody to the hemagglutinin of type A influenza virus

A double-antibody radioimmunoprecipitation (RIP) assay has been developed to provide a sensitive and specific measure of antibody to hemagglutinins of H3N2 influenza viruses. Chloramine T was used to radiolabel purified hemagglutinins to high specific activity without loss of antigenicity. The purity of the labeled hemagglutinin was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis, which also established that both the HA(1) and HA(2) polypeptides were iodinated. Radiolabeled hemagglutinins with a specific activity that did not exceed 12 muCi/mug of protein could be maintained for up to 30 days at -70 degrees C in the presence of supplemental protein. The RIP assay was compared with conventional methods, hemagglutination inhibition and viral neutralization tests, using H3N equine 1 hybrid viruses for determining serum antihemagglutinin antibody titers. The geometric mean titers for human convalescent sera after infection with A/England/72 virus were 118, 161, and 18,822 for hemagglutination inhibition, viral neutralization, and RIP tests, respectively, and the three tests demonstrated significant rises in antihemagglutinin antibody titers with equal efficiency. In general, a positive correlation existed between antihemagglutinin antibody titers determined by these three procedures; however, the antibody level determined by RIP assay for each individual could not be related to hemagglutination inhibition or viral neutralization titers by a constant factor. A similar lack of a constant relationship was found by using hyperimmune guinea pig antisera, which suggests that the RIP assay can detect antibody populations that exhibit differing efficiencies for inhibition of viral hemagglutination and replication.

Direct solid-phase radioimmunoassay for measuring antigenic differences between the hemagglutinins of influenza viruses

A direct solid-phase radioimmunoassay, which involves binding of purified radio-labeled IgG to influenza virus hemagglutinins immobilized on Sepharose 4B gel, is described.

Preparation-conditioned changes of the antigenicity of influenza virus neuraminidases

The influenza virus strains A/Sing/1/57 (H2N2), A/Bel/42 (H0N1) and A/Bel/42 (HO)-A/Sing/1/57 (N2) were treated with bromelain under reducing conditions and with reducing agent alone, and the antigenicity of the neuraminidase (NA) of intact virus and of the split products was tested comparatively. It was found that the antigenicity of NA was influenced quantitatively and qualitatively by the preparation procedure. Antineuraminidase (AN) antibodies obtained after vaccination of guinea pigs with intact virus and with split products differed in their cross-reactivity with heterologous neuraminidases. In several cases, the quantity of AN antibody formation depended on the hemagglutinin (HA) dose present in the vaccines. The N2 NA on the recombinant virus was significantly more sensitive to treatment with reducing agent than was the N2 NA on the parent virus. AN antibodies directed against N2 NA on the recombinant differed qualitatively from that directed against N2 NA of parent virus. The results warrant the conclusion that the antigenicity of isolated NA or of NA on recombinant virus can differ from that of the NA on intact homologous virus and that such alterations could influence the determination of antigenic relationship between neuraminidases.

Isolation of an influenza virus, similar to A/Port Chalmers/1/73 (H3N2) from a common murre at Sakhalin Island in U.S.S.R. (Strain A/common murre/Sakhalin/1/74)

An influenza A virus isolated from the cloaca of a common murre was characterized antigenically as H3N2 and was most closely related to the A/Port Chalmers/1/73 strain. Serological studies of sera collected from common murres in the area of virus isolation showed that 21 per cent of the birds had antibodies to Hong Kong influenza virus.