Estimation of the number of surface projections on myxo- and paramyxoviruses

Conserved and host-specific features of influenza virion architecture

Viruses use virions to spread between hosts, and virion composition is therefore the primary determinant of viral transmissibility and immunogenicity. However, the virions of many viruses are complex and pleomorphic, making them difficult to analyse in detail. Here we address this by identifying and quantifying virion proteins with mass spectrometry, producing a complete and quantified model of the hundreds of viral and host-encoded proteins that make up the pleomorphic virions of influenza viruses. We show that a conserved influenza virion architecture is maintained across diverse combinations of virus and host. This ‘core’ architecture, which includes substantial quantities of host proteins as well as the viral protein NS1, is elaborated with abundant host-dependent features. As a result, influenza virions produced by mammalian and avian hosts have distinct protein compositions. Finally we note that influenza virions share an underlying protein composition with exosomes, suggesting that influenza virions form by subverting microvesicle production.

CD4 T cell-independent antibody response promotes resolution of primary influenza infection and helps to prevent reinfection

It is generally believed that the production of influenza-specific IgG in response to viral infection is dependent on CD4 T cells. However, we previously observed that CD40-deficient mice generate influenza-specific IgG during a primary infection, suggesting that influenza infection may elicit IgG responses independently of CD4 T cell help. In the present study, we tested this hypothesis and show that mice lacking CD40 or CD4 T cells produce detectable titers of influenza-specific IgG and recover from influenza infection in a manner similar to that of normal mice. In contrast, mice completely lacking B cells succumb to influenza infection, despite the presence of large numbers of functional influenza-specific CD8 effector cells in the lungs. Consistent with the characteristics of a T-independent Ab response, long-lived influenza-specific plasma cells are not found in the bone marrow of CD40-/- and class II-/- mice, and influenza-specific IgG titers wane within 60 days postinfection. However, despite the short-lived IgG response, CD40-/- and class II-/- mice are completely protected from challenge infection with the same virus administered within 30 days. This protection is mediated primarily by B cells and Ab, as influenza-immune CD40-/- and class II-/- mice were still resistant to challenge infection when T cells were depleted. These data demonstrate that T cell-independent influenza-specific Ab promotes the resolution of primary influenza infection and helps to prevent reinfection.

Quantitative relationships between an influenza virus and neutralizing antibody

In this quantitative study of the interaction of influenza virus with neutralizing antibody we have determined the maximum number of antibody molecules which can bind to the haemagglutinin (HA) of native influenza A/FPV/Rostock/34 (H7N1) particles in aqueous suspension and the minimum number which is required to cause neutralization. Using radiolabelled immunoglobulins approximately one IgG molecule, whether of monoclonal or polyclonal origin, binds per HA spike under conditions of antibody saturation. In the same manner, we have determined that when infectivity is neutralized by 63% (1/e) about 70 molecules of monoclonal IgGs HC2 and HC10 were bound per virus particle and this is supported by independent evidence from electron microscopy. However, the kinetics of neutralization were single-hit or at most, under critical conditions of low temperature (4 degrees) and minimal neutralizing concentrations of antibody, two-hit. This apparent conflict is reconciled by a hypothesis which proposes that neutralization occurs only when antibody binds to certain "neutralization relevant" HA spikes which are in the minority. It is suggested that these only differ from the majority of "neutralization irrelevant" HA spikes by their transmembrane interaction with the core of the virion.

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.

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.

Location of ferritin-labeled concanavalin A binding to influenza virus and tumor cell surfaces

Concanavalin A (Con-A) was linked to ferritin with glutaraldehyde and chromatographed on Sepharose 6B to separate unconjugated Con-A and ferritin from covalently cross-linked molecules. Ehrlich ascites tumor cells were infected with WSA influenza virus, stained at intervals with the ferritin-labeled Con-A and examined by electron microscopy. The surfaces of most mature viruses were specifically stained, providing direct evidence that influenza viruses maturing in this cell type have exposed Con-A receptor sites. The ferritin cores of the staining reagent were found at an average distance of 21.3 nm from the virus membrane and 10.8 nm from the uninfected cell membrane. This finding was interpreted to mean that the population of Con-A receptor sites on influenza virus particles is located at an average distance from the virus membrane twice that of the population of Con-A receptor sites found on uninfected cells. The structural elements of viral membranes can provide a reliable means for evaluating electron microscopy staining reagents, thereby enhancing their usefulness as probes for the study of membrane relationships.

Model for vesicular stomatitis virus

Vesicular stomatitis virus contains single-stranded ribonucleic acid of molecular weight 3.6 x 10(6) and three major proteins with molecular weights of 75 x 10(3), 57 x 10(3), and 32.5 x 10(3). The proteins have been shown to be subunits of the surface projections, ribonucleoprotein, and matrix protein, respectively. From these values and from estimates of the proportions of the individual proteins, it has been calculated that the virus has approximately 500 surface projections, 1,100 protein units on the ribonucleoprotein strand, and 1,600 matrix protein units. Possible models of the virus are proposed in which the proteins are interrelated.