Genetic dimorphism in influenza viruses: characterization of stably associated hemagglutinin mutants differing in antigenicity and biological properties

Contemporary H3N2 influenza viruses have a glycosylation site that alters binding of antibodies elicited by egg-adapted vaccine strains

H3N2 viruses continuously acquire mutations in the hemagglutinin (HA) glycoprotein that abrogate binding of human antibodies. During the 2014-2015 influenza season, clade 3C.2a H3N2 viruses possessing a new predicted glycosylation site in antigenic site B of HA emerged, and these viruses remain prevalent today. The 2016-2017 seasonal influenza vaccine was updated to include a clade 3C.2a H3N2 strain; however, the egg-adapted version of this viral strain lacks the new putative glycosylation site. Here, we biochemically demonstrate that the HA antigenic site B of circulating clade 3C.2a viruses is glycosylated. We show that antibodies elicited in ferrets and humans exposed to the egg-adapted 2016-2017 H3N2 vaccine strain poorly neutralize a glycosylated clade 3C.2a H3N2 virus. Importantly, antibodies elicited in ferrets infected with the current circulating H3N2 viral strain (that possesses the glycosylation site) and humans vaccinated with baculovirus-expressed H3 antigens (that possess the glycosylation site motif) were able to efficiently recognize a glycosylated clade 3C.2a H3N2 virus. We propose that differences in glycosylation between H3N2 egg-adapted vaccines and circulating strains likely contributed to reduced vaccine effectiveness during the 2016-2017 influenza season. Furthermore, our data suggest that influenza virus antigens prepared via systems not reliant on egg adaptations are more likely to elicit protective antibody responses that are not affected by glycosylation of antigenic site B of H3N2 HA.

Assessing the safety of adjuvanted vaccines

Despite the very low risk-to-benefit ratio of vaccines, fear of negative side effects has discouraged many people from getting vaccinated, resulting in reemergence of previously controlled diseases such as measles, pertussis, and diphtheria. Part of this fear stems from the lack of public awareness of the many preclinical and clinical safety evaluations that vaccines must undergo before they are available to the general public, as well as from misperceptions of what adjuvants are or why they are used in vaccines. The resultant "black box" leads to a preoccupation with rare side effects (such as autoimmune diseases) that are speculated, but not proven, to be linked to some vaccinations. The focus of this review article is to open this black box and provide a conceptual framework for how vaccine safety is traditionally assessed. We discuss the strengths and shortcomings of tools that can be and are used preclinically (in animal studies), translationally (in biomarker studies with human sera or cells), statistically (for disease epidemiology), and clinically (in the design of human trials) to help ascertain the risk of the infrequent and delayed adverse events that arise in relation to adjuvanted vaccine administration.

Gene constellation of influenza A virus reassortants with high growth phenotype prepared as seed candidates for vaccine production

Background

Influenza A virus vaccines undergo yearly reformulations due to the antigenic variability of the virus caused by antigenic drift and shift. It is critical to the vaccine manufacturing process to obtain influenza A seed virus that is antigenically identical to circulating wild type (wt) virus and grows to high titers in embryonated chicken eggs. Inactivated influenza A seasonal vaccines are generated by classical reassortment. The classical method takes advantage of the ability of the influenza virus to reassort based on the segmented nature of its genome. In ovo co-inoculation of a high growth or yield (hy) donor virus and a low yield wt virus with antibody selection against the donor surface antigens results in progeny viruses that grow to high titers in ovo with wt origin hemagglutinin (HA) and neuraminidase (NA) glycoproteins. In this report we determined the parental origin of the remaining six genes encoding the internal proteins that contribute to the hy phenotype in ovo.

Methodology

The genetic analysis was conducted using reverse transcription-polymerase chain reaction (RT-PCR) and restriction fragment length polymorphism (RFLP). The characterization was conducted to determine the parental origin of the gene segments (hy donor virus or wt virus), gene segment ratios and constellations. Fold increase in growth of reassortant viruses compared to respective parent wt viruses was determined by hemagglutination assay titers.

Significance

In this study fifty-seven influenza A vaccine candidate reassortants were analyzed for the presence or absence of correlations between specific gene segment ratios, gene constellations and hy reassortant phenotype. We found two gene ratios, 6∶2 and 5∶3, to be the most prevalent among the hy reassortants analyzed, although other gene ratios also conferred hy in certain reassortants.

Anti-ganglioside antibody induction by swine (A/NJ/1976/H1N1) and other influenza vaccines: insights into vaccine-associated Guillain-Barré syndrome

BACKGROUND

Receipt of an A/NJ/1976/H1N1 "swine flu" vaccine in 1976, unlike receipt of influenza vaccines used in subsequent years, was strongly associated with the development of the neurologic disorder Guillain-Barré syndrome (GBS). Anti-ganglioside antibodies (e.g., anti-GM(1)) are associated with the development of GBS, and we hypothesized that the swine flu vaccine contained contaminating moieties (such as Campylobacter jejuni antigens that mimic human gangliosides or other vaccine components) that elicited an anti-GM(1) antibody response in susceptible recipients.

METHODS

Surviving samples of monovalent and bivalent 1976 vaccine, comprising those from 3 manufacturers and 11 lot numbers, along with several contemporary vaccines were tested for hemagglutinin (HA) activity, the presence of Campylobacter DNA, and the ability to induce anti-Campylobacter and anti-GM(1) antibodies after inoculation into C3H/HeN mice.

RESULTS

We found that, although C. jejuni was not detected in 1976 swine flu vaccines, these vaccines induced anti-GM(1) antibodies in mice, as did vaccines from 1991-1992 and 2004-2005. Preliminary studies suggest that the influenza HA induces anti-GM(1) antibodies.

CONCLUSIONS

Influenza vaccines contain structures that can induce anti-GM(1) antibodies after inoculation into mice. Further research into influenza vaccine components that elicit anti-ganglioside responses and the role played by these antibodies (if any) in vaccine-associated GBS is warranted.

Differences in the biological phenotype of low-yielding (L) and high-yielding (H) variants of swine influenza virus A/NJ/11/76 are associated with their different receptor-binding activity

Low- (L) and high-yielding (H) variants of A/sw/NJ/11/76 influenza virus were compared for their growth properties in embryonated chicken eggs and MDCK cells and for their binding affinity for the membrane fractions prepared from cells of the chicken embryo allantoic membrane. MDCK, and swine tracheal cells, as well as for soluble sialic acid containing macromolecules and monovalent sialosides. We have shown, that during infection in MDCK cells and in eggs, the progeny of the L variant remain predominantly cell associated, in contrast to those of H. As a result, accumulation of the L mutant in allantoic or culture fluid is significantly slowed in comparison with the H variant. Visualization of the infectious foci formed by the viruses in MDCK cell monolayers and on the allantoic membrane revealed that L spreads predominantly from cell to cell, while the spread of H involves release of the virus progeny into solution and its rapid distribution over the cell monolayer via convectional flow of the liquid. In the binding assays, L displayed significantly higher binding affinity than H for cellular membranes, gangliosides, and sialylglycoproteins, however, the affinity of the variants for the monovalent sialic acid compounds was comparable. Unlike H. L bound strongly to dextran sulfate. The data obtained suggest that all distinctions of the L and H biological phenotypes reported previously [Kilbourne, E.D., Taylor, A. H. Whitaker, C.W., Sahai, R., and Caton, A (1988) Hemagglutinin polymorphism as the basis for low-and high-yield phenotypes of swine influenza virus. Proc. Natl. Acad. Sci. USA 85, 7782-7785] could be rationally explained by a more avid binding of the L variant to the surface of target cells, and that this effect is mainly due to enhanced electrostatic interactions.

In pursuit of influenza: Fort Monmouth to Valhalla (and back)

In reviewing 50 years of personal research on influenza, I have journeyed, literally and figuratively, from an army camp epidemic in Fort Monmouth NJ in 1947 to a (literal and figurative) Valhalla, where I now conduct my research. Having entered the field as a physician, I have always sought practical applications of my work, yet in every instance, such applications have led me to seek further answers in basic research as new questions arose. I entered the area of influenza virus genetics by the back door through an interest in the effects of corticosteroid hormones on viral replication, used the genetic approach in analyzing the morphological variation of the virus and, in so doing, exploited the finding of a linkage of high-yield growth to spherical morphology. Today, all influenza vaccine viruses are high-yield genetic reassortants. Subsequent study of reassortant viruses facilitated the identification and isolation of the two major antigens of the virus in antigenic hybrids and showed their differing functions in the induction of immunity. In turn, a new approach to influenza vaccination has been discovered and is presently under clinical investigation.

Single amino acid substitutions in the hemagglutinin can alter the host range and receptor binding properties of H1 strains of influenza A virus

We have previously characterized an influenza A (H1N1) virus which has host-dependent growth and receptor binding properties and have shown that a mutation which removes an oligosaccharide from the tip of the hemagglutinin (HA) by changing Asn-129 to Asp permits this virus to grow to high titer in MDBK cells, (C. M. Deom, A. J. Caton, and I. T. Schulze, Proc. Natl. Acad. Sci. USA 83:3771-3775, 1986). We have now isolated monoclonal antibodies specific for the mutant HA and have used escape mutants to identify alterations in HA sequence which reduce virus yields from MDBK cells without reducing those from chicken embryo fibroblasts. Two types of escape mutants which grow equally well in chicken embryo fibroblasts were obtained. Those with the parent phenotype contain Asn at residue 129 and are glycosylated at that site. Those with the mutant phenotype are unchanged at residue 129 but have a Gly to Glu substitution at residue 158, which is close to residue 129 on the HA1 subunit. Binding assays with neoglycoproteins containing N-acetylneuraminic acid in either alpha 2,3 or alpha 2,6 linkage to galactose showed that the MDBK-synthesized oligosaccharides at Asn-129 reduce binding to both of these receptors, leaving the HA's preference for alpha 2,6 linkages unchanged. Glu at residue 158 greatly reduces binding to both receptors without reducing virus yields from MDBK cells. We conclude that changes in the receptor binding properties of the HA can result either from direct alteration of the HA protein by host cell glycosylation or from mutations in the HA gene and that these changes generate heterogeneity that can contribute to the survival of influenza A virus populations in nature.

Sequence of an influenza virus hemagglutinin determined directly from a clinical sample

The sequence of the HA1 region of the hemagglutinin gene of an influenza virus has been determined without growing the virus in eggs or in cultured cells. The virus used was an H1 strain of influenza A from a clinical specimen taken from a patient in 1987. RNA was extracted directly from virus that had been sedimented out of the transport medium in which the sample had been stored. DNA copies of the hemagglutinin gene, obtained by reverse transcription, were then amplified by the polymerase chain reaction and were sequenced by the dideoxy termination method. The deduced amino acid sequence is highly similar to that of other H1 viruses that had been isolated at about the same time and cultured for a limited number of passages in eggs. Furthermore, the HA1 sequence of progeny virus from this isolate obtained after one passage in chicken embryos is identical to that of the virus obtained directly from the nasopharynx. The results suggest that H1 isolates that have been grown for a limited number of passages in embryonated eggs have HA1 subunits that faithfully represent the virus population in the clinical samples from which they were derived.

M protein (M1) of influenza virus: antigenic analysis and intracellular localization with monoclonal antibodies

A panel of 16 monoclonal antibodies recognizing M protein (M1) of influenza virus was generated. Competition analyses resulted in localization of 14 monoclonal antibodies to three antigenic sites. Three monoclonal antibodies localized to site 1B recognized a peptide synthesized to M1 (residues 220 to 236) with enzyme-linked immunosorbent assay titers equivalent to or greater than that seen with purified M1; therefore, site 1B is located near the C terminus of M1. Sites 2 and 3 localize to the N-terminal half of M1. Antigenic variation of M proteins was seen when the monoclonal antibodies were tested against 14 strains of type A influenza viruses. Several monoclonal antibodies showed specific recognition of A/PR/8/34 and A/USSR/90/77 M proteins and little or no reactivity for all other strains tested. Immunofluorescence analysis with the monoclonal antibodies showed migration of M protein to the nucleus during the replicative cycle and demonstrated association of M protein with actin filaments in the cytoplasm. Use of a vaccinia virus recombinant containing the M-protein gene demonstrated migration of M protein to the nucleus in the absence of synthesis of gene products from other influenza virus RNA segments.

Evolution to predominance of swine influenza virus hemagglutinin mutants of predictable phenotype during single infections of the natural host

L and H2 mutants of the A/NJ/11/76 H1N1 strain of swine influenza virus differ by having either a lysine or a glutamic acid at position 153 of the hemagglutinin glycoprotein of the virus. In two separate experiments, experimental infection of swine with various doses of the H2 mutant resulted in the emergence in 11 of 20 animals of virus with the L phenotype. All evidence indicates that the H2----L mutation, selection, and evolution to predominance occurred within the 7-day span of individual infections. L and H2 mutations appear to act as alleles in the adaptation of virus, respectively, to natural and laboratory hosts. Although the gradual evolution of mutants during sequential infections is commonplace, the present recognition of rapid and predictable evolution of mutants of increased replication efficiency and specific phenotype in the natural host, to our knowledge, is unprecedented.

Hemagglutinin polymorphism as the basis for low- and high-yield phenotypes of swine influenza virus

Single amino acid substitutions at the rim of the receptor binding site of the hemagglutinin molecule of swine influenza virus markedly influence the replicative capacity of the virus in chicken embryos, Madin-Darby canine kidney cells (MDCK), and swine as well as its antigenic phenotype. Mutants of low-yield (L) phenotype replicate poorly in chicken embryos and produce small plaques in MDCK cells but are highly infective for swine. Such mutants have lysine at position 153 and glycine at position 155 of the hemagglutinin (residues 156 and 158 in the H3 model). High-yield (H) mutants have the converse replicative characteristics and can be antigenically distinguished from L mutants (and from each other) based on their differential reactivity with two monoclonal antibodies, 9C8 and Sa-13. H mutants differ from L mutants in that the H mutants express glutamic acid at either position 153 or 155. L and H mutants act in an allelic fashion in effecting predictable one-step adaptation to different hosts. Selection for replication (e.g., high-yielding) phenotype results in concordant pleiotropic change in antigenic phenotype and in genotype. Conversely, immunoselection leads to change in replicative phenotype. Although the mechanism by which these mutations affect viral replication has not yet been defined, they may reflect differences in the affinity of each mutant for different host receptors.

Antibody response in humans to influenza virus type B host-cell-derived variants after vaccination with standard (egg-derived) vaccine or natural infection

Hemagglutination inhibition (HI) and neutralization tests were used to determine antibody responses to egg-derived and Madin-Darby canine kidney (MDCK)-derived influenza B virus (B/England/222/82) in paired sera from persons naturally infected with influenza B and in persons vaccinated with standard egg-derived inactivated influenza vaccine. When tested by HI, the MDCK-derived antigen gave significantly higher (8- to 12-fold) geometric mean titers (GMT) in convalescent-phase sera from persons naturally infected during community outbreaks, as well as more 4-fold titer rises, than did tests with egg-derived antigen. When tested by neutralization, however, the convalescent-phase sera GMTs were only threefold higher with the MDCK-derived antigen and an equivalent number of fourfold titer rises were detected with both antigens. With postvaccine sera, the MDCK-derived antigen gave GMTs that were threefold higher than those obtained with egg-derived antigen in both the HI and neutralization tests and both antigens detected an equivalent number of fourfold titer rises in HI and neutralization tests. Sucrose gradient-fractionated egg-derived antigen showed a single peak of hemagglutinin activity corresponding to whole virions, whereas MDCK-derived antigen contained two distinct peaks of hemagglutinin activity, one of which had a lower sedimentation rate. The overall findings indicate that the egg-derived antigen in the vaccine induced HI and neutralizing antibody to both egg- and MDCK-derived variants and suggest that titers of antibody to MDCK-derived virus may be affected by the physical form of the hemagglutinin antigen.

The molecular biology of influenza virus pathogenicity

It is an accepted concept that the pathogenicity of a virus is of polygenic nature. Because of their segmented genome, influenza viruses provide a suitable system to prove this concept. The studies employing virus mutants and reassortants have indicated that the pathogenicity depends on the functional integrity of each gene and on a gene constellation optimal for the infection of a given host. As a consequence, virtually every gene product of influenza virus has been reported to contribute to pathogenicity, but evidence is steadily growing that a key role has to be assigned to hemagglutinin. As the initiator of infection, hemagglutinin has a double function: (1) promotion of adsorption of the virus to the cell surface, and (2) penetration of the viral genome through a fusion process among viral and cellular membranes. Adsorption is based on the binding to neuraminic acid-containing receptors, and different virus strains display a distinct preference for specific oligosaccharides. Fusion capacity depends on proteolytic cleavage by host proteases, and variations in amino acid sequence at the cleavage site determine whether hemagglutinin is activated in a given cell. Differences in cleavability and presumably also in receptor specificity are important determinants for host tropism, spread of infection, and pathogenicity. The concept that proteolytic activation is a determinant for pathogenicity was originally derived from studies on avian influenza viruses, but there is now evidence that it may also be relevant for the disease in humans because bacterial proteases have been found to promote the development of influenza pneumonia in mammals.

Antigenic heterogeneity within influenza A (H3N2) virus strains

On the basis of their antigenic properties, influenza virus strains are classified into types and subtypes, which are further subdivided into variants that differ to various degrees in haemagglutination-inhibition assays. Evidence is presented that during infection with an influenza A(H3N2) virus the respiratory tract of a human patient often harbours more than one antigenic virus variant. These variants are frequently propagated by embryonated fowl eggs and monkey cells with different efficiencies, and this may lead to the selection of different variants by either of these host systems. Also, passage of virus by a given host is sometimes attended by changes in reactivity in haemagglutination-inhibition tests. In some cases the heterogeneity described also affects the specific immunogenicity of the virus in ferrets. Virus strains cloned in monkey kidney cell cultures gave variants that were stable upon further passage. These results may have implications for antigenic and biochemical investigations of epidemiologically relevant virus variants. It is argued that the antigenic drift of influenza A(H3N2) viruses is best characterized by analyses, both with post-infection ferret antisera and with panels of monoclonal antibodies, of virus strains isolated and passaged in monkey kidney cell cultures only.

Serological studies with influenza A(H1N1) viruses cultivated in eggs or in a canine kidney cell line (MDCK)

Pairs of influenza A(H1N1) viruses cultivated from the same clinical specimen in canine kidney (MDCK) cells or in embryonated hens' eggs can frequently be distinguished by their reactions with monoclonal antibodies to haemagglutinin and with antibodies in ferret or human sera. Egg-adapted virus, further passaged in MDCK cultures remained "egg-like" in serological characteristics indicating that the differences in their serological reactions were not a direct result of host cell-dependent glycosylation of the haemagglutinin. Haemagglutination-inhibiting (HI) or virus neutralizing antibodies in human sera can be detected more frequently, and to higher titre, in tests employing virus grown exclusively in MDCK cells than in tests with virus adapted to growth in embryonated eggs. Striking differences were detected in the serological reactions in HI tests when sera from ferrets infected with egg-grown virus were tested against a series of strains of influenza A(H1N1) virus isolated in 1983 and adapted to growth in eggs. In contrast, sera from ferrets infected with MDCK-derived virus failed to distinguish serologically between the same viruses that had been passaged exclusively in MDCK cells and also revealed relatively small differences between their egg-adapted counterparts.It was concluded that the cell substrate used for virus isolation and cultivation is a factor that should be considered when interpreting the results of strain characterization of influenza A(H1N1) isolates and in sero-surveys using these viruses.

Analysis of antigenic determinants on internal and external proteins of influenza virus and identification of antigenic subpopulations of virions in recent field isolates using monoclonal antibodies and immunogold labelling

An electron microscopic immunogold labelling technique employing monoclonal antibodies has been applied to the antigenic analysis of influenza A and B viruses. Reassortant influenza A H3N2 viruses containing haemagglutinin molecules from viruses isolated between 1968 and 1982 were analysed with a panel of monoclonal antibodies raised against viruses which appeared over the same period. The immunogold labelling technique clearly demonstrated the antigenic drift in the haemagglutinin molecule that occurred between 1968 and 1982. When the technique was applied to the examination of viruses from a more geographically restricted influenza epidemic in a semi-closed community, antigenic variants were found. Furthermore the technique enabled the identification of distinct antigenic variant subpopulations within a single clinical isolate. Analysis of the HA of MDCK cell or egg grown virus by this procedure provided data to support the hypothesis that the host cell exerts selective pressure on subpopulations of virus resulting in the emergence of antigenic variants.

Antigenic heterogeneity among influenza A(H3N2) field isolates during an outbreak in 1982/83, estimated by methods of numerical taxonomy

Fourteen influenza A(H3N2) field isolates, mainly obtained during the first weeks of the 1982/83 influenza epidemic in The Netherlands, and nine influenza A(H3N2) reference strains were examined by means of haemagglutination inhibition (HI) tests with 23 polyclonal ferret sera. The resulting HI patterns were subjected to various methods of numerical taxonomy using, among others, taxonomic distance and correlation between strains for resemblance coefficients. Marked differences between distance and correlation coefficients were found in strains which differed in avidity only. The field isolates could be divided into four groups in respect of their taxonomic resemblance to the reference strains. The same grouping was found for five of the field isolates by testing these against 200 human sera.

Antigenic characterization of influenza A virus matrix protein with monoclonal antibodies

Monoclonal antibodies were used to study antigenic variation in three distinct epitopes on the matrix protein of influenza A viruses. We found that two of these epitopes underwent antigenic variation, but in a very limited number of virus strains. A third epitope appeared to be an invariant type-specific determinant for influenza A viruses. Competitive antibody binding assays and Western blot analysis of proteolytically digested matrix protein indicated that at least two of the three epitopes are located in nonoverlapping domains on the matrix protein molecule.

Antigenic comparisons of swine-influenza-like H1N1 isolates from pigs, birds and humans: an international collaborative study

The objective of this international collaborative study was to compare recent swine isolates of influenza viruses and determine whether significant antigenic differences among isolates from different areas of the world could be detected. H1N1 viruses isolated from pigs, birds and humans in 12 different countries were compared in haemagglutination-inhibition assays with post-infection ferret sera and monoclonal antibodies to H1N1 strains. Using A/NJ/8/76 as the reference strain, we found that recent swine isolates from Hong Kong, Italy, Japan, and the USA possess a haemagglutinin virtually indistinguishable from that of viruses typically associated with pigs, i.e., A/NJ/8/76. In contrast, recent swine isolates from several European countries (Belgium, Denmark, France, Federal Republic of Germany, and Spain) were distinguishable from A/NJ/8/76, as demonstrated by tests in the various laboratories. These studies suggest that the H1N1 viruses in pigs are antigenically heterogeneous and that the circulation of particular variants is associated with the geographical location of the animals. These results raise the question of whether these viruses originated from the same source, i.e., pigs, and have undergone antigenic drift or, alternatively, were introduced from other hosts, such as birds.

Hemagglutinin of swine influenza virus: a single amino acid change pleiotropically affects viral antigenicity and replication

The complete nucleotide sequence has been obtained of the H1 hemagglutinin (HA) gene of a high-yielding (H) mutant of the A/NJ/11/76(H1N1) strain of swine influenza virus in studies of a viral reassortant (X-53a) bearing this gene. This determination has permitted comparison with human influenza H1N1 prototype viruses A/WSN/33 and A/PR/8/34, with which 80% and 94% amino acid homology was found between HA1 and HA2, respectively. Partial sequences have been determined for other viral reassortants containing either H or L (low-yielding phenotype) genes derived from A/NJ/11/76. Sequence of the HA1 region of an L mutant prototype was virtually completed and differed from that of the H mutant by only four amino acid changes. Sequence analysis of four other viruses was restricted to regions of the HA with which monoclonal antibodies capable of distinguishing L and H mutants are presumed to react. Therefore, changes in these sequences are relevant to changes in viral phenotype. Change at residue 155 from Gly to Glu is associated with change from L to H HA phenotype. This site, structurally equivalent to amino acid 158 on the Wiley et al. HA model [Wiley, D. C., Wilson, I. A. & Skehel, J. J. (1981) Nature (London) 289, 373-378] is near the tip of the HA monomer adjacent to the proposed receptor binding site and therefore credibly could influence both viral antigenicity and replication. Because both L and H variants exist in nature and because revertants may be selected in the laboratory as replication variants in the absence of immunoselection, these studies provide evidence for fortuitous antigenic change in association with change in biological function, which is determined by a single base change.

Monoclonal antibodies to the hemagglutinin Sa antigenic site of a/pr/8/34 influenza virus distinguish biologic mutants of swine influenza virus

The dimorphic L and H hemagglutinin mutants of A/NJ/11/76(H1N1) (swine) influenza virus differ pleiotropically in their replication and virulence characteristics and in their antigenicity. L mutants replicate less well in chicken embryos and Madin-Darby canine kidney cells and are more infective for swine than are H mutants. L and H mutants are not antigenically distinguishable in cross-neutralization tests with homotypic antisera, but they can be identified with certain heterotypic heterogeneous antisera. The present studies demonstrate that two monoclonal antibodies (Sa-5 and Sa-13) to the Sa antigenic site of the hemagglutinin of A/PR/8/34H1N1 influenza virus react with mutants and viral reassortants containing the H hemagglutinin in radioimmunoassay, neutralization, and hemagglutination-inhibition tests but to a lesser degree or not at all with L mutants and reassortants. Conversely, monoclonal antibody (9C8) to the L mutant does not react with H mutants. L to H and H to L revertants, whether or not selected with monoclonal antibody, demonstrate concomitant change in biological and antigenic phenotype. Reactivity of H mutants with Sa monoclonal antibodies localizes the mutational site to a position on the hemagglutinin near the receptor binding site--a position in which single amino acid changes could readily influence both antigenic and biologic activity.

Presentation of Academy Medal to Edwin D. Kilbourne, M.D

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Evidence for host-cell selection of influenza virus antigenic variants

Extensive antigenic variability and a capricious epidemiology are characteristics of influenza A and B viruses of man. The haemagglutinin (HA) undergoes frequent and progressive antigenic drift as a result of selection, under immunological pressure, of viruses possessing alterations in the amino acid sequences at specific sites in the molecule. Here we present evidence for an additional selection mechanism for antigenic variants of influenza virus that depends on differing host cell tropisms of virus subpopulations. These studies were initiated after earlier observations of the occurrence of a marked degree of antigenic variation during passage of laboratory strains of influenza virus in eggs and cell cultures (J.C.J., in preparation). We have now shown that cultivation of influenza B viruses in eggs selects subpopulations which are antigenically distinct from virus from the same source grown in mammalian cell cultures. As antigenic characterization of influenza virus strains for epidemiological purposes and for the preparation of influenza vaccines conventionally relies on the cultivation of virus in eggs, our findings may have important practical implications for vaccine design and efficacy.

Characterization of an antigenic determinant of the glycoprotein that correlates with pathogenicity of rabies virus

The pathogenicity of fixed rabies virus strains for adult mice depends on the presence of an antigenic determinant on the viral glycoprotein. Two virus-neutralizing monoclonal antibodies have been used to identify this determinant. All pathogenic strains of fixed rabies virus bind to these antibodies and are neutralized by them, whereas nonpathogenic strains fail to react with these monoclonal antibodies and are not neutralized by them. Antigenic variants of the rabies virus with altered glycoprotein were selected by growing virus in the presence of one monoclonal antibody, 194-2. All variants that lost their ability to react with this antibody and an additional antibody, 248-8, were found to be nonpathogenic for adult mice. Analysis of tryptic peptides of the glycoproteins of pathogenic parent virus and nonpathogenic variants and the amino acid sequence of a specific variant tryptic peptide revealed that the change in pathogenicity corresponded to an amino acid substitution at position 333 of the glycoprotein molecule. The nucleotide sequence of the nonpathogenic variant glycoprotein gene contained a base change that confirmed the single amino acid substitution in the tryptic peptide replacing arginine-333 in the parental glycoprotein. We conclude that arginine-333 is essential for the integrity of an antigenic determinant and for the ability of rabies viruses to produce lethal infection in adult mice.

Detection of antibodies to influenza virus M protein by an enzyme-linked immunosorbent assay

An enzyme-linked immunosorbent assay test system was developed in which purified influenza virus M protein was used for the detection of M antibody in human sera. Antibody levels to influenza A virus M protein were monitored in sera from a vaccine study population by using an enzyme-linked immunosorbent assay technique with purified M protein as the adsorbent antigen. A 10-fold variation in titers of preexisting M antibody was observed in this population of young adults. Increases of anti-M titer of 7- to 24-fold were observed upon immunization with Formalin-inactivated vaccine or after natural infection. The antibody response to M protein was dissociated from the response to the hemagglutinin or neuraminidase antigens. The M antibody response preceded or was coincident with the antibody response to H1 hemagglutinin upon natural exposure to circulating virus.

Plaquing characteristics of influenza A virus recombinants of defined genetic composition. Brief report

The plaque size and morphology of twenty-two influenza A virus recombinants representing seven distinct families were analyzed on MDCK cells. By examination of the genetic composition of the recombinants no relationship could be established between any gene, including those coding for the surface antigens, and the plaque size and morphology.

Antigenicity in hamsters of inactivated vaccines prepared from recombinant influenza viruses

Inactivated vaccines prepared form influenza virus strains obtained by the recombination of A/PR/8/34 (H1N1) or A/FM/1/47 (H1N1) viruses with A/Victoria/3/75 (H3N2) virus, were tested for their antigenicity in hamsters. The parental origin of the genes of each cloned recombinant virus was determined by polyacrylamide gel electrophoresis, and vaccines prepared from each strain by concentration, purification on sucrose density gradients and inactivation with formalin. All the recombinant strains used in these studies possessed surface haemagglutinin and neuraminidase antigens derived from the A/Victoria/75 parent strain. On inoculation into hamsters, at equivalent concentrations, these vaccines varied in their ability to induce haemagglutination-inhibiting (HI) antibodies in the serum. This variation was not dependent on concentration and was observed using neutralization and single radial haemolysis, as well as HI. The possible reasons for the findings are discussed.

Frequency of naturally occurring antibody to influenza virus antigenic variants selected in vitro with monoclonal antibody

Antigenic variants of A/Texas/77 (H3N2) virus were selected in vitro using monoclonal antibody to virus haemagglutinin (HA). The antigenic variants and parental A/Texas/77 viruses were used to to evaluate the frequency of anti-HA antibodies in the sera of children and adults using single-radial-haemolysis (SRH) tests. Twenty to 41% of selected sera from adults, which contained antibody to the parental A/Texas/77 virus, failed to react with the different antigenic mutant viruses. A higher proportion of sera from children (37-58%) failed to react with the antigenic variants. Certain human sera and particularly those of children would appear to possess a more limited antibody repertoire to influenza HA, potentially allowing new antigenic variants to escape neutralization and spread in the community.

Recent contributions of molecular biology to the clinical virology of myxoviruses

Recent advances in the clinical virology of influenza are based on non-pragmatically oriented research on the genetics and biochemistry of the influenza virus. Antigenically hybrid recombinant viruses can be tailored to provide monospecific reagents for serological studies. Basic research on viral structure and the mechanism of viral replication has directly influenced the establishment of a cell culture system suitable for the isolation of most influenza viruses. Identification of viral genotype by RNA gel electrophoresis and mapping of oligonucleotides of viral RNA has already facilitated epidemiologic investigations. The clinical virologist of the future must have an understanding of the potential limitations of these techniques for specific strain identification.

Hemagglutinin mutants of swine influenza virus differing in replication characteristics in their natural host

In two mutant clones (L and H) of A/NJ/11/76 (Hsw 1N1) influenza viruses which differ slightly antigenically and markedly in replication characteristics in chicken embryos and Madin Darby canine kidney cells, these pleiotropic differences are mediated by mutation in the hemagglutinin gene (E. D. Kilbourne, Proc. Natl. Acad. Sci. U.S.A. 75:6258--6262, 1978). Experimental infection of swine with either the mutant L and H clones or recombinant viruses differing genetically only with respect to the presence of L or H hemagglutinin demonstrated greater infectivity for the natural host of viruses bearing the L hemagglutinin. Introduction of the L but not the H hemagglutinin gene into the human influenza virus A/PR/8/34 rendered it infective for swine. Both L and H variants were isolated from pigs naturally infected with contemporary swine influenza viruses when selective conditions for the suppression of the more prevalent L mutant were employed. The L and H mutants of swine influenza virus are yet another example of viral dimorphism in nature and probably are not mere artifacts of laboratory selection. In any event, the frequent apparent allelic appearance of the two forms suggests frequent mutation and/or reversion involving a point mutation in the hemagglutinin gene. The present studies demonstrate the importance of a single gene in the pathogenesis of an influenza viral infection in its natural host.

Probable association of plaque size with neuraminidase subtype among H3N2 influenza A viruses. Brief report

The present study demonstrates that the plaque size of certain influenza A H3N2 virus recombinants is dependent on their containing a specific neuraminidase glycoprotein, but is independent of the genes coding for the HA, P3, and NP proteins.