The structure of influenza viruses

Neuraminidase, the Forgotten Surface Antigen, Emerges as an Influenza Vaccine Target for Broadened Protection

For 50 years it has been known that antibodies to neuraminidase (NA) protect against infection during seasonal and pandemic influenza outbreaks. However, NA is largely ignored in the formulation and standardization of our current influenza vaccines. There are a number of factors that contributed to this antigen being forgotten, including the lack of an easily performed test to measure NA antibody. With the availability of that test, it has been possible to show its independent contribution to protection in various situations. The challenge now is to make it possible to include known amounts of NA in investigational vaccines or to routinely measure NA content in licensed vaccines. Vaccines containing optimal amounts of NA may be particularly useful when there are antigenic changes, either drift or shift, in the hemagglutinin because NA immunity offers broad protection. It is now time to remember the NA as we work toward improved influenza vaccines.

Antiviral drugs for influenza: Tamiflu® past, present and future

Two antiviral drugs that are currently available for the treatment of influenza are effective against all strains of the virus, if used correctly. These are the neuraminidase inhibitors, zanamivir (Relenza®) and oseltamivir (Tamiflu®). These drugs are the result of basic research performed over a 60-year period by many people around the world. They were deliberately synthesized from a knowledge of the x-ray crystal structure of influenza virus neuraminidase. This article provides a brief historical account of some of the scientific events that lead to their creation.

Pandemic influenza: its origin and control

A "new" influenza virus will appear at some time in the future. This virus will arise by natural processes, which we do not fully understand, or it might be created by some bioterrorist. The world's population will have no immunity to the new virus, which will spread like wild-fire, causing much misery, economic disruption and many deaths. Vaccines will take time to develop and the only means of control, at least in the early stages of the epidemic, are anti-viral drugs, of which the neuraminidase inhibitors currently seem the most effective.

Parainfluenza virus type 1 reduces the affinity of agonists for muscarinic receptors in guinea-pig lung and heart

Membrane preparations of guinea-pig lung (containing multiple muscarinic receptor subtypes) and heart (containing M2 receptors only) were incubated with either neuraminidase, parainfluenza virus (which contains neuraminidase), or virus plus 2,3-dehydro-2-deoxy-N-acetylneuraminic acid, a neuraminidase inhibitor. None of these treatments affected [3H]quinuclidinyl benzilate [( 3H]QNB) binding. In the lung and heart, carbachol displaced 0.2 nM [3H]QNB from two sites. After treatment with either neuraminidase or virus the high affinity site was shifted to the right, and carbachol displaced QNB from one site with low affinity in the lung. In contrast, neuraminidase or virus decreased the affinity of carbachol for both sites in the heart. The neuraminidase inhibitor completely blocked virus-induced changes in carbachol affinity in both tissues. These results suggest that parainfluenza virus decreases the affinity of agonists for some of the muscarinic receptors in the lung, and for all of the muscarinic receptors in the heart due to its neuraminidase activity, which results in removal of sialic acid. The decreased agonist affinity in the lung may be responsible for the increased vagally induced bronchoconstriction seen in viral respiratory infections.

Action of influenza virus neuraminidase on gangliosides. Haemagglutinin inhibits viral neuraminidase

The action of partly purified neuraminidase (NA) of influenza A virus, a mixture of detergent solubilized NA and haemagglutinin (HA) and of intact virions on gangliosides GT1b, GD1a, GD1b, GM1 was studied. The viral NA transformed GT1b mainly into GD1b with formation of only minor amounts of GM1. HA was found to inhibit the hydrolysis activity of viral NA. At the same time viral NA transformed GD1a quantitatively into GM1 which was not hydrolyzed by the enzyme. These results suggest that the function of NA is to transfer the 'primary' receptor (such as GT1b) into the proper carbohydrate sequence (GD1b-like) which is proposed to serve as the minimal structure required for influenza virus reception.

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.

Folding of the mitochondrial proton adenosinetriphosphatase proteolipid channel in phospholipid vesicles

The mitochondrial H+-ATPase proteolipid from Neurospora crassa was incorporated into small unilamellar dimyristoylphosphatidylcholine vesicles and its conformation determined by circular dichroism spectroscopy (CD). While the largely alpha-helical conformation is relatively independent of the method of incorporation into vesicles, i.e., rehydration, detergent dialysis, or detergent dilution, the proteolipid conformation was significantly different in detergent micelles and in organic solvents. Only very slight changes in the CD spectrum were observed upon binding of the H+-ATPase inhibitor dicyclohexylcarbodiimide to the proteolipid in vesicles, thus suggesting that the inhibitor acts either by blocking the channel or by masking an essential charge group, rather by than causing an overall conformational change in the channel. Additionally, very similar CD spectra were obtained for vesicles with different lipid/protein mole ratios, indicating either that no substantial conformational differences exist between monomer and multimers or that monomers self-associate to form stable complexes during incorporation into vesicles. This study has provided a physical basis for model-building studies of the proteolipid channel structure.

Preparation of projection-less particles from influenza virus and their messenger activities in prokaryotic and eukaryotic systems

A fraction of projection-less particles was prepared from influenza A/Dunedin/4/73 and A/Victoria/3/75 (X-47) (H3N2) by detergent treatment and extraction into ether at 0 degrees C. The activity of this material in stimulating protein synthesis in vitro was studied and compared with that of isolated virion RNA using a) an RNA-dependent E. coli system, and b) a wheat germ system. In the bacterial system the purified RNA had the highest template activity, while in the eukaryotic system the disrupted particle preparation was by far the most active. Translation products were formed with immunological and electrophoretic properties similar to those of several influenza virion proteins. The experiments indicate that, when added in the form of disrupted projection-less particles, RNA from influenza A2 virus is utilized as a template by eukaryotic ribosomes.

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.

Purification of neuraminidase from influenza viruses by affinity chromatography

The neuraminidase (acylneuraminyl hydrolase, EC 3.2.1.18) of the influenza virus recombinant strain (HON2) was solubilized with detergents and isolated by affinity chromatography. The neuraminidase could be purified to a single high molecular weight glycoprotein when assayed under non-reducing conditions on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The enzyme showed an increase in specific activity from 2.46 to 189 micronM N-acetylneuraminic acid released per min per mg protein and the recovery represented 123% of the activity of intact virus particles. The enzyme could be purified from laboratory preparations of virus or from outdated influenza virus vaccine. Viral neuraminidases purified by this technique were stable at pH 6.0 for several hours.

Effect of Ca++ on the stability of influenza virus neuraminidase

The neuraminidases of different strains of influenza virus varied in their stability at 37 degrees C. The enzymes of the strains with N1 neuraminidases were found to be unstable during incubation at 37 degrees C whereas the enzymes of the strains with the N2 neuraminidases were stable. Among the strains with N2 neuraminidases, the enzymes of some strains were inactivated during dialysis at 37 degrees C whereas the enzymes of others were stable. This observed loss of enzyme activity during dialysis at 37 degrees C was not restricted to a single substrate as the same loss of enzyme activity was observed irrespective of the size of the substrate used in the assay. The enzymically inactive neuraminidase was found to be non-antigenic and non-immunogenic. The inactivation of the enzyme could be prevented by the addition of Ca++ but not Mg++. Out results suggest that Ca++ is essential for the stability of the enzyme at 37 degrees C. The results would also suggest that the enzymic, antigenic and immunogenic sites are either the same or very closely situated on the surface of the neuraminidase molecule.

The activity of Newcastle disease virus-envelope proteins after treatment with detergents

Treatment of NDV with anionic detergents or lipid solvents destroys the activities of hemagglutinin and neuraminidase. After disruption of the virus with non-ionic detergents, the activities of envelope proteins remain unchanged. It is suggested that the phosholipids are very important for the biological activity of NDV-envelope proteins.

The role of serum haemagglutination-inhibiting antibody in protection against challenge infection with influenza A2 and B viruses

The intranasal inoculation of volunteers with living partially attenuated strains of influenza A and B viruses offers a new opportunity to determine the protective effect of serum haemagglutin-inhibiting antibody against a strictly homologous virus, under conditions where the time and dosage of the infective challenge can be controlled, the scoring of proven infections can be more precise and higher rates of infection can be achieved than in most natural epidemics.In 1032 adult volunteers, whose serum HI antibody titre was determined immediately before virus challenge, there was a consistent inverse quantitative relationship between the HI titre and the likelihood of infection. The PD 50 (50% protective dose) of HI antibody was 1/18-1/36, but an unusual finding was that volunteers with no detectable pre-challenge antibody often seem to be less susceptible to infection than those with pre-challenge antibody in low titre.In one group of volunteers challenged with an influenza B strain there was no evidence that pre-challenge antibody titres against viral neuraminidase had any significant protective effect against challenge infection.

Degradation of influenza virus by non-ionic detergent

Preparations of influenza virus A0 PR8/34 and A2 Malaysia/68 have been studied in the electron microscope. They were similar in appearance to preparations made by others. Each preparation was degraded by Triton N 101. The process of degradation appeared to be different from that observed using ether and, by inference, a number of other agents.

Soluble antigens obtained from influenza virus by treatment with non-ionic detergent

Highly purified influenza virus was degraded using anionic and non-ionic detergents. Best results were obtained using the non-ionic detergent Triton N 101. Tests showed that virus extracts contained neuraminidase and a substance that reacted specifically with rabbit antibody to virus haemagglutinin (specific serum blocking substance). Haemagglutination-inhibiting antibody was produced when virus extracts were inoculated into guinea-pigs. Immunodiffusion tests showed that extracts were complex. Host-specific material was regularly found. Under appropriate conditions S-antigen was detected as a single line pattern component. Two or more virus-specific materials were also present. One of these was probably neuraminidase and the other the specific serum blocking substance.