The multiplication of influenza viruses in the fertile egg: A Report to the Medical Research Council

Single-particle measurements of filamentous influenza virions reveal damage induced by freezing

Clinical isolates of influenza virus produce pleiomorphic virions, ranging from small spheres to elongated filaments. The filaments are seemingly adaptive in natural infections, but their basic functional properties are poorly understood and functional studies of filaments often report contradictory results. This may be due to artefactual damage from routine laboratory handling, an issue which has been noted several times without being explored in detail. To determine whether standard laboratory techniques could damage filaments, we used immunofluorescence microscopy to rapidly and reproducibly quantify and characterize the dimensions of filaments. Most of the techniques we tested had minimal impact on filaments, but freezing to -70 °C, a standard storage step before carrying out functional studies on influenza viruses, severely reduced their concentration, median length and the infectivity of the whole virion population. We noted that damage from freezing is likely to have affected most of the functional studies of filaments performed to date, and to address this we show that it can be mitigated by snap-freezing or incorporating the cryoprotectant DMSO. We recommend that functional studies of filaments characterize virion populations prior to analysis to ensure reproducibility, and that they use unfrozen samples if possible and cryoprotectants if not. These basic measures will support the robust functional characterizations of filaments that are required to understand their roles in natural influenza virus infections.

Cryotomography of budding influenza A virus reveals filaments with diverse morphologies that mostly do not bear a genome at their distal end

Influenza viruses exhibit striking variations in particle morphology between strains. Clinical isolates of influenza A virus have been shown to produce long filamentous particles while laboratory-adapted strains are predominantly spherical. However, the role of the filamentous phenotype in the influenza virus infectious cycle remains undetermined. We used cryo-electron tomography to conduct the first three-dimensional study of filamentous virus ultrastructure in particles budding from infected cells. Filaments were often longer than 10 microns and sometimes had bulbous heads at their leading ends, some of which contained tubules we attribute to M1 while none had recognisable ribonucleoprotein (RNP) and hence genome segments. Long filaments that did not have bulbs were infrequently seen to bear an ordered complement of RNPs at their distal ends. Imaging of purified virus also revealed diverse filament morphologies; short rods (bacilliform virions) and longer filaments. Bacilliform virions contained an ordered complement of RNPs while longer filamentous particles were narrower and mostly appeared to lack this feature, but often contained fibrillar material along their entire length. The important ultrastructural differences between these diverse classes of particles raise the possibility of distinct morphogenetic pathways and functions during the infectious process.

Influenza viruses that have been cultivated in the laboratory usually produce particles that are spherical. However, viruses isolated from patients frequently produce long filamentous particles, as well as smaller elliptical particles that we term “bacilliform virions”. Long filaments may be important for cell-to-cell transmission or facilitate release of the smaller particles by disrupting the mucous layer of the respiratory tract. We have used three-dimensional electron microscopy to investigate the structure of influenza virus filaments ‘budding’ from cells. We found that many of the long filaments had a large bulbous head at the end furthest from the cell. Many of these bulbs were empty while some contained tubules that we believe are made of a scaffold-protein M1 that usually lines the inner surface of the viral membrane. Bacilliform virions contain genomes comprised of eight segments of RNA; these are each wrapped up in protein and packaged in an ordered manner. None of the bulb-headed filaments and very few narrower ones had this feature. We hypothesise that the diverse viral structures we have seen suggest distinct assembly pathways and moreover functions. Long filamentous structures that do not appear to contain genomes may combat the immune response or help the smaller virus particles spread.

New millennium antivirals against pandemic and epidemic influenza: the neuraminidase inhibitors

The mushroom shaped outer spike protein of influenza, neuraminidase, was first discovered nearly 60 years ago. Its importance in viral replication was soon recognised both at the point of viral release from the cell and also enabling passage of virus through nasal fluid to reach the cell. The enzyme active site was identified by x-ray crystallography, allowing an atomic study of interaction of enzyme with the sialic acid substrate. Analogues could then be identified and synthesized and became a focused target for antivirals. With the current threat of bioterrorism and the potential for the emergence of a new pandemic strain in the near future, efforts are underway to develop more potent second-generation anti-neuraminidase inhibitors with enhanced protective and therapeutic effects. Here we review older and newer neuraminidase inhibitors and the role that they will play in the fight against influenza in its epidemic and pandemic face.

ENZYMATIC VARIANTS OF INFLUENZA VIRUS. III. FUNCTION OF NEURAMINIDASE IN THE VIRAL GROWTH CYCLE

Padgett, Billie L. (University of Wisconsin, Madison), and Duard L. Walker. Enzymatic variants of influenza virus. III. Function of neuraminidase in the viral growth cycle. J. Bacteriol. 87:363-369. 1964.-Multiplication of a slowly reacting enzymatic variant of influenza B virus, strain LEE, was compared with that of parent virus in ovo. At 35 C, although their final yields were equal, variant virus reached its peak concentration in the allantoic fluids later than parent virus. At 39 C, multiplication of both viruses was slower, parent virus requiring 4 hr and variant virus 8 hr longer to reach infectivity levels comparable with those at 24 hr at 35 C. Variant enzyme activity in vitro can be controlled by altering the temperature and calcium concentration. Growth curves of these viruses in pieces of chorioallantoic membrane (CAM) in culture under conditions in which the variant should be as active as the parent revealed only minor differences between them. Under conditions in which variant enzyme activity would be much slower than the parent, the release of variant virus from the CAM was delayed and the rate of release was slower. Under the most adverse conditions, 39 C and no calcium, formation of infectious variant virus ceased after 5 hr, and the hemagglutination inhibitor in the cells was not degraded although hemagglutinins were produced. These findings are discussed in relation to the function of neuraminidase during viral multiplication.

[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)

Lipids in Viruses

This chapter discusses lipids in viruses. Lipid forms an integral part of many viruses and exists either in the form of a continuous envelope or in lipoprotein complexes that surround a nucleoprotein core or helix. In general, the envelope can be described as a molecular container for the genetic material of the virus. Viruses are obligate intracellular parasites and are not known to carry genetic coding for enzymes involved in lipid synthesis. Hence, they generally contain the same classes of lipid as are found in the host cell or their membrane of assembly. Lipids make up 20–35% by weight of most viruses; however, there are exceptions such as vaccinia virus, which has only 5% lipid despite having a complex multimembrane envelope structure. Naked herpesvirus capsids closely resemble non-lipid-containing viruses such as adenovirus or polyoma virus, which are also assembled in the nucleus but show full infectivity without any envelope. Both naked and enveloped herpesvirus particles are found in infected cells; however, only enveloped particles are found in extracellular fluids.

Lipids in viruses

This chapter discusses lipids in viruses. Lipid forms an integral part of many viruses and exists either in the form of a continuous envelope or in lipoprotein complexes that surround a nucleoprotein core or helix. In general, the envelope can be described as a molecular container for the genetic material of the virus. Viruses are obligate intracellular parasites and are not known to carry genetic coding for enzymes involved in lipid synthesis. Hence, they generally contain the same classes of lipid as are found in the host cell or their membrane of assembly. Lipids make up 20–35% by weight of most viruses; however, there are exceptions such as vaccinia virus, which has only 5% lipid despite having a complex multimembrane envelope structure. Naked herpesvirus capsids closely resemble non-lipid-containing viruses such as adenovirus or polyoma virus, which are also assembled in the nucleus but show full infectivity without any envelope. Both naked and enveloped herpesvirus particles are found in infected cells; however, only enveloped particles are found in extracellular fluids.

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.

Myxovirus envelope proteins: a directing influence on the fatty acids of membrane lipids

Acyl chain compositions of the lipids of three strains of influenza virus show differences not anticipated from current theories of myxovirus assembly. Fatty acids of viruses with antigenically related envelope proteins show greater resemblance than those of an unrelated strain, which suggests that these proteins influence the composition of membrane lipids at the site of viral release.