Fate of influenza A virion proteins after entry into subcellular fractions of LLC cells and the effect of amantadine

Antiviral activity of an aqueous extract derived from Aloe arborescens Mill. against a broad panel of viruses causing infections of the upper respiratory tract

BACKGROUND

A number of antiviral therapies have evolved that may be effectively administered to treat respiratory viral diseases. But these therapies are very often of limited efficacy or have severe side effects. Therefore there is great interest in developing new efficacious and safe antiviral compounds e.g. based on the identification of compounds of herbal origin.

HYPOTHESIS

Since an aqueous extract of Aloe arborescens Mill. shows antiviral activity against viruses causing infections of the upper respiratory tract in vitro we hypothesised that a product containing it such as Biaron C(®) could have an antiviral activity too.

STUDY DESIGN

Antiviral activity of Bioaron C(®), an herbal medicinal product consisting of an aqueous extract of Aloe arborescens Mill., Vitamin C, and Aronia melanocarpa Elliot. succus, added as an excipient, was tested in vitro against a broad panel of viruses involved in upper respiratory tract infections.

METHODS

These studies included human adenovirus and several RNA viruses and were performed either with plaque reduction assays or with tests for the detection of a virus-caused cytopathic effect.

RESULTS

Our studies demonstrated an impressive activity of Bioaron C(®) against members of the orthomyxoviridae - influenza A and influenza B viruses. Replication of both analysed influenza A virus strains - H1N1 and H3N2 - as well as replication of two analysed influenza B viruses - strains Yamagatal and Beiying - was significantly reduced after addition of Bioaron C(®) to the infected cell cultures. In contrast antiviral activity of Bioaron C(®) against other RNA viruses showed a heterogeneous pattern. Bioaron C(®) inhibited the replication of human rhinovirus and coxsackievirus, both viruses belonging to the family of picornaviridae and both representing non-enveloped RNA viruses. In vitro infections with respiratory syncytial virus and parainfluenza virus, both belonging to the paramyxoviridae, were only poorly blocked by the test substance. No antiviral activity of Bioaron C(®) was detected against adenovirus - a non-enveloped DNA virus.

CONCLUSIONS

These results represent the first proof of a selective antiviral activity of Bioaron C(®) against influenza viruses and create basis for further analyses of type and molecular mechanisms of the antiviral activity of this herbal medicine.

Viral infections of the respiratory tract: prevention and treatment

The rapid discovery of specific viral agents as the cause of many acute respiratory diseases was accompanied by considerable optimism that vaccines or other control measures could be developed quickly. Subsequent experience has demonstrated that effective control of these important public health problems has been an elusive goal. However, recent exciting developments in our understanding of the molecular biology and immunology of these viruses may provide the basis for more effective strategies in the future.

Hemagglutinin 1-specific immunoglobulin G and Fab molecules mediate postattachment neutralization of influenza A virus by inhibition of an early fusion event

In standard neutralization (STAN), virus and antibody are reacted together before inoculation of target cells, and inhibition of almost any of the processes concerned in the early interaction of virus and cell, including inhibition of virus attachment to cell receptors, can be the cause of neutralization by a particular monoclonal antibody (MAb). To simplify the interpretation of antibody action, we carried out a study of postattachment neutralization (PAN), where virus is allowed to attach to target cells before neutralizing antibody is introduced. We used influenza virus A/PR/8/34 (H1N1) and monoclonal immunoglobulin G (IgG) molecules and their Fabs specific to antigenic sites Sb (tip), Ca2 (loop), and Cb (hinge) of the hemagglutinin 1 (HA1) protein. All IgGs and Fabs gave PAN, although with reduced efficiency compared with STAN. Thus, bivalent binding of antibody was not essential for PAN. By definition, none of these MAbs gave PAN by inhibiting virus attachment, and they did not elute attached virus from the target cell or inhibit endocytosis of virus. However, virus-cell fusion, as demonstrated by R18 fluorescence dequenching or hemolysis of red blood cells, was inhibited in direct proportion to neutralization and in a dose-dependent manner and was thus likely to be responsible for the observed neutralization. However, to get PAN, it was necessary to inhibit the activation of the prefusion intermediate, the earliest known form on the fusion pathway that is created when virus is incubated at pH 5 and 4 degrees C. PAN antibodies may act by binding HA trimers in contact with the cell and/or trimers in the immediate vicinity of the virus-cell contact point and so inhibit the recruitment of additional receptor-HA complexes.

A haemagglutinin (HA1)-specific FAb neutralizes influenza A virus by inhibiting fusion activity

H9-D3-4R2 (referred to as H9), a murine monoclonal HA1-specific IgG3, recognizes an epitope within antigenic site Cb of influenza virus A/PR/8/34 (H1N1). At 50% neutralization, inhibition of virus-mediated fusion was responsible for the majority of neutralization but, at higher antibody concentrations, the attachment of virus to target cells was also inhibited and may have contributed to neutralization. H9 FAb was also neutralizing, although the concentration needed was two orders of magnitude greater than for the IgG. Functional affinity of the IgG and affinity of the FAb were almost identical, and it is not clear why the neutralization efficiency of the FAb was so low. Unlike its IgG, H9 FAb had no detectable effect on virus attachment but inhibited virus fusion activity. It thus appears that monovalent binding by this antibody is sufficient to inhibit fusion activity and that this was directly responsible for neutralization of infectivity.

Two influenza A virus-specific Fabs neutralize by inhibiting virus attachment to target cells, while neutralization by their IgGs is complex and occurs simultaneously through fusion inhibition and attachment inhibition

Mabs H36 (IgG2a) and H37 (IgG3) recognize epitopes in antigenic sites Sb and Ca2, respectively, in the HA1 subunit of influenza virus A/PR/8/34 (H1N1). Their neutralization was complex. Our aim here was to investigate the mechanism of neutralization by the IgGs and their Fabs. In MDCK and BHK cells, both IgGs neutralized primarily by inhibiting virus-cell fusion, although at higher IgG concentrations virus attachment to target cells was also inhibited. In contrast, the Fabs neutralized entirely by inhibiting virus attachment, although a higher concentration of Fab than IgG was required to bring this about. Both H36 and H37 exerted a concentration-dependent spectrum of neutralization activity, with virus-cell fusion inhibition and virus-cell attachment inhibition being the predominant mechanisms at low- and high-antibody concentration, respectively, and both mechanisms occurring simultaneously at intermediate concentrations. However, it may be that attachment inhibition was a secondary event, occurring to virus that had already been neutralized through inhibition of its fusion activity. Neutralization by H36 and H37 Fabs was a simple process. Both inhibited virus attachment but required much higher (>100-fold) molar concentrations for activity than did IgG. The functional affinities of the IgGs were high (0.4-0.6 nM) and differences between these and the affinity of their Fabs (H36, nil; H37, 23-fold) were not sufficient to explain the differences observed in neutralization. Similar neutralization data were obtained in two different cell lines. The dose-response curve for neutralization by H36 F(ab')(2) resembled that for IgG, although eightfold more F(ab')(2) was required for 50% neutralization. Overall, neutralization mechanisms of H36 and H37 antibodies were similar, and thus independent of antigenic site, antibody isotype, and target cell.

Chemotherapy of respiratory viruses: prospects and challenges

Billions of people are infected with respiratory viruses annually. Infants and young children, the elderly, immunocompromised individuals and those debilitated by other diseases or nutritional deficiencies are most at risk for serious disease. There are few vaccines available for use against these viruses, and even where there are (influenza, measles and adenovirus), infections remain common. The continued prevalence of respiratory virus infections has lead to renewed efforts to find safe agents effective against the most medically important respiratory viruses: influenza, respiratory syncytial, parainfluenza, measles, rhino- and adenovirus. Copyright 1999 Harcourt Publishers Ltd.

Comparative efficacy of antiviral drugs on human ocular fibroblasts

The effects of several antiviral drugs on fibroblast attachment and proliferation from human Tenon's capsule were investigated. These drugs included purine nucleoside analogs, vidarabine and acyclovir (ACV); pyrimidine nucleoside analog, AZT; and a synthetic cyclic primary amine, amantadine. Fibroblast attachment and proliferation inhibition were determined by Coulter counter, a colorimetric assay of the enzyme hexosaminidase, and a 3H-thymidine uptake assay. Amantadine and AZT inhibited fibroblast attachment at concentrations higher than 6.61 x 10(-4)M and 3.73 x 10(-4) M, respectively. Amantadine and AZT had inhibitory effects on fibroblast proliferation as early as day 1, whereas vidarabine and ACV manifested their inhibitory effects after day three by Coulter counter and hexosaminidase assays. For amantadine, AZT, ACV and vidarabine, the 50% inhibitory dose (ID50) were 4.94 x 10(-5) M, 1.26 x 10(-5) M, 4.60 x 10(-4) M, and 1.52 x 10(-5) M at day 9, respectively, as measured by 3H-thymidine uptake assay. All four antiviral agents tested had inhibitory effects on human ocular fibroblast proliferation and their inhibitory potential decreased in the order of amantadine > or = vidarabine > AZT > or = ACV.

Use of FITC-labeled influenza virus and flow cytometry to assess binding and internalization of virus by monocytes-macrophages and lymphocytes

The binding of influenza virus to the surface of cells and the internalization of virus particles by all or a subset of cells are key points in the pathogenesis of viral infection. The current studies established a method for discrimination of surface-bound from internalized influenza virus. Fluorescein isothiocyanate (FITC) was attached to the viral hemagglutinin and neuroaminidase proteins; the fluorescent virus retained infectivity. A flow cytometric technique was then adapted for study of virus-cell interactions, with addition of ethidium bromide to quench green fluorescence associated with FITC-labeled virus that was cell-bound but remained external. Ethidium bromide was excluded by intact cell membranes, and internalized virions retained green fluorescence. Cells could be examined by fluorescence microscopy or flow cytometry, with flow cytometry allowing rapid, kinetic assessment of large numbers of cells and subsets of virus-exposed cells. The data showed that, whereas a majority of both monocytes-macrophages and lymphocytes bound influenza virus, a large percentage of monocytes-macrophages but only a very small percentage of lymphocytes internalized the virus. This procedure provides a simple and effective method to distinguish surface-bound from internalized influenza virus, and allows precise kinetic analyses on large numbers of cells.

Antiviral therapy: current concepts and practices

Drugs capable of inhibiting viruses in vitro were described in the 1950s, but real progress was not made until the 1970s, when agents capable of inhibiting virus-specific enzymes were first identified. The last decade has seen rapid progress in both our understanding of antiviral therapy and the number of antiviral agents on the market. Amantadine and ribavirin are available for treatment of viral respiratory infections. Vidarabine, acyclovir, ganciclovir, and foscarnet are used for systemic treatment of herpesvirus infections, while ophthalmic preparations of idoxuridine, trifluorothymidine, and vidarabine are available for herpes keratitis. For treatment of human immunodeficiency virus infections, zidovudine and didanosine are used. Immunomodulators, such as interferons and colony-stimulating factors, and immunoglobulins are being used increasingly for viral illnesses. While resistance to antiviral drugs has been seen, especially among AIDS patients, it has not become widespread and is being intensely studied. Increasingly, combinations of agents are being used: to achieve synergistic inhibition of viruses, to delay or prevent resistance, and to decrease dosages of toxic drugs. New approaches, such as liposomes carrying antiviral drugs and computer-aided drug design, are exciting and promising prospects for the future.

Membrane fusion activity of the influenza virus hemagglutinin: interaction of HA2 N-terminal peptides with phospholipid vesicles

We have investigated the interaction of a number of synthetic 20-residue peptides, corresponding to the HA2 N-terminus of the influenza virus hemagglutinin (X31 strain), with phospholipid vesicles and monolayers. Besides the wild-type sequence, two peptides were studied with mutations corresponding to those previously studied in entire HA's expressed in transfected cells [Gething et al., (1986) J. Cell. Biol. 102, 11-23]. These mutations comprised a single Glu replacement for Gly at the N-terminus ("El" mutant) or at position 4 ("E4") of the HA2 subunit and were shown to produce striking alterations in virus-induced hemolysis and syncytia formation, especially for E1. The X31 "wild-type" (wt) peptide and its E4 variant are shown here to have the capacity to insert into phosphatidylcholine (POPC) large unilamellar vesicle (LUV) membranes in a strictly pH-dependent manner, penetration being marginal at pH 7.4 and significant at pH 5.0. Bilayer insertion was evident from a shift in the intrinsic Trp fluorescence of the wt and E4 peptides and from the induction of calcein leakage from POPC LUV and correlated well with the peptides' ability at pH 5.0 to penetrate into POPC monolayers at initial surface pressures higher than 30 mN/m. By contrast, the E1 peptide was found, at pH 5.0, to bind less tightly to vesicles (assessed by a physical separation method) and to cause much less leakage of POPC LUV than the wt, even under conditions where the peptides were bound to approximately the same extent. Consistent with the correlation between leakage and penetration observed for the wt peptide at pH 5 versus 7, the E1 peptide, even at low pH, showed much less lipid-vesicle-induced shift of its Trp fluorescence than wt, caused a much slower rate of leakage of vesicle contents, and did not insert into POPC monolayers at surface pressures beyond 28.5 mN/m. Circular dichroism spectroscopy measurements of peptides in POPC SUV showed that the conformations of all three peptides are sensitive to pH, but only the wt and E4 peptides became predominantly alpha-helical at acid pH.

Inhibition of influenza A virus replication by a kanamycin derivative

We studied the antiviral activity and the mechanism of action of a new antiviral agent and kanamycin derivative, 1-N-eicosanoyl-3"-N-trifluoroacetyl kanamycin A (ETKA), against influenza A virus. From yield reduction assays with VERO cells, ETKA showed a significant antiviral activity with negligible cytotoxic effect. In the presence of 20 micrograms/ml of ETKA at which VERO cell growth was not inhibited, virus titer was suppressed to 11.2% of control, and at 100 micrograms/ml virus production was suppressed to more than 99%. ETKA markedly inhibited viral protein synthesis when cells were pretreated with the drug before infection, but there was no inhibition when the drug was added 15 min post-infection. ETKA did not inhibit virus adsorption and penetration. Nor did it affect the activity of viral RNA polymerase in vitro. We found that the drug had a direct inactivating effect on influenza A virus under acidic conditions. These results suggest that ETKA exerts its antiviral action mainly in the early stage, prior to uncoating by direct inactivation of the virus due to the acidic environment of the endocytic vesicle. Aerosol treatment with the drug protected mice against a lethal influenza A virus infection.

Mechanisms of neutralization of influenza virus in tracheal epithelial and BHK cells vary according to IgG concentration

The interaction of IgG-neutralized type A influenza virus with differentiated epithelial cells of mouse trachea, BHK cells, and chicken erythrocytes was studied using three mouse monoclonal antibodies (IgG2a) each directed against a different antigenic site on the hemagglutinin. At high HIU:HAU ratios virus was neutralized greater than 99%, monodisperse, and attached to tracheal epithelial and BHK cells in normal amounts. The majority (70-80%) of neutralized virus failed to attach to erythrocytes. At low HIU:HAU ratios the virus was aggregated by each of the antibodies, and attachment to tracheal epithelial and BHK cells was inhibited by up to 75%. Combined aggregation and inhibition of attachment could theoretically account for up to 96% loss of infectivity but this corresponded with the observed degree of neutralization with only one of the antibodies. With increasing antibody:virus ratios, aggregation and inhibition of attachment contributed ever diminishingly to the observed neutralization and eventually not at all. Both neutralized and infectious virus attached to neuraminidase-sensitive receptors. After attachment neutralized virus became increasingly resistant to removal by neuraminidase suggesting that it had been internalized by the cell.

The antiviral drug amantadine has a direct inhibitory effect on T-lymphocytes

We investigated the effect of the antiviral drug amantadine (AmTd) on polyclonal activation of thymic-dependent (T) and thymic-independent (B) lymphocytes from normal mice. In the present studies, T-lymphocytes are defined by their response to concanavalin A (Con A) and B-lymphocytes by their response to lipopolysaccharide (LPS). Polyclonal activator-induced lymphocyte proliferation was assessed by quantifying cellular incorporation of tritiated thymidine. The results show that, in a dose-dependent manner, AmTd exhibits at least 2-fold greater inhibitory activity against Con A-responding T-cells than against LPS-responding B-cells. Further, several findings demonstrate that AmTd has a direct inhibitory effect on T-lymphocytes. First, AmTd pulse treatment of isolated T-cells, but not accessory cells, abolished the T-cell response to Con A. Second, AmTd pulse treatment of the cytotoxic T-lymphocyte line, CTLL-2, markedly reduced their ability to undergo IL-2-induced proliferation. Third, proliferation of T-cells which had already undergone activation by ConA was inhibited by AmTd. Further, the finding that addition of IL-1, IL-2 or both to cultures failed to reverse inhibition of the response to ConA argues that AmTd did not interfere with endogenous production of these lymphokines. Possible implications of these findings are discussed.

Binding and entry of animal viruses

Viruses are infectious agents capable of packaging and delivering nucleic acids and proteins to specific populations of cells. To initiate infection, viruses bind to sites, or receptors, on the cell surface and transfer their genome across the limiting membrane of the cell. The mechanisms underlying these events, and viral tropism for particular host cells, are becoming increasingly well understood. Several cell surface proteins have now been identified as viral receptors, and analyses of intact virus particles and sub-viral components are revealing the structures of the binding determinants on the viruses themselves. For many viruses, the events leading to penetration and delivery involve constitutive endocytic properties of the host cell, and the low pH environment in endocytic compartments is a crucial trigger in the penetration process. The knowledge of viral tropism, binding and entry suggests strategies which may be applied to the design of targeted therapeutic agents with appropriate specificities and effective delivery mechanisms.

Virus entry into animal cells

In addition to its many other functions, the plasma membrane of eukaryotic cells serves as a barrier against invading parasites and viruses. It is not permeable to ions and to low molecular weight solutes, let alone to proteins and polynucleotides. Yet it is clear that viruses are capable of transferring their genome and accessory proteins into the cytosol or into the nucleus, and thus infect the cell. While the detailed mechanisms remain unclear for most animal viruses, a general theme is apparent like other stages in the replication cycle; their entry depends on the activities of the host cell. In order to take up nutrients, to communicate with other cells, to control the intracellular ion balance, and to secrete substances, cells have a variety of mechanisms for bypassing and modifying the barrier properties imposed by their plasma membrane. It is these mechanisms, and the molecules involved in them, that viruses exploit.

Mass action kinetics of virus-cell aggregation and fusion

A simple approximate solution for the mass action kinetics of small particles (viruses or vesicles) binding to large particles (cells) and their subsequent fusion has been derived. The solution is evaluated in terms of the measurable fluorescence changes expected when the virus or vesicles are labeled with fluorescent probes, which are diluted into the cellular membrane by fusion. Comparison with numerical integrations shows that the approximate solution is extremely accurate. Analytic simplifications for a variety of special cases of this general problem are also shown.

Antiviral Chemotherapyand Prophylaxis of Viral Respiratory Disease

Several antiviral agents are currently available for the treatment and prophylaxis of viral respiratory disease. These include oral amantadine for influenza A and aerosolized ribavirin for respiratory syncytial virus infections. Additional agents, such as rimantadine and intranasal interferons, and newer approaches, including thе use of combination chemotherapy, offer promise for the improved management of viral respiratory tract infections.

Fusion of influenza virus in an intracellular acidic compartment measured by fluorescence dequenching

The fusion of influenza virus with cultured cells has been investigated. The virus was labelled with the fluorescent probe octadecyl rhodamine B and fusion was monitored as fluorescence dequenching due to dilution of the probe from the viral into a cellular target membrane. Fusion with the plasma membrane does not occur, unless the extracellular pH is temporarily lowered. At neutral pH fusion occurs only after a lag phase of 10-15 min, the time required for virus internalization, and the reaction is inhibited by NH4Cl, indicating that it takes place in an intracellular acidic compartment, most likely the endosome. This suggests that influenza virus infects cells via the endocytic pathway.

Mechanism of entry of human rhinovirus 2 into HeLa cells

Internalized human rhinovirus 2 (HRV2) undergoes a rapid conformational change leading to recognition by the C-determinant-specific monoclonal antibody 2G2. In the presence of the ionophore monensin, the virus accumulates in the cells in its native conformation and infection is strongly inhibited. At 20 degrees but not at 34 degrees the inhibitory effect of monensin can be overcome by a short incubation of the infected cells at low pH as late as 2 hr after inoculation. Incubation of infected cells at 20 degrees prior to addition of monensin permits virus synthesis to occur, depending on the time of preincubation.