Influenza C virus susceptibility to antivirals with different mechanisms of action

Antiviral susceptibility of influenza viruses was assessed using a high-content imaging-based neutralization test. Cap-dependent endonuclease inhibitors, baloxavir and AV5116, were superior to AV5115 against type A viruses, and AV5116 was most effective against PA mutants tested. However, these three inhibitors displayed comparable activity (EC50 8-22 nM) against type C viruses from six lineages. Banana lectin and a monoclonal antibody, YA3, targeting the hemagglutinin-esterase protein effectively neutralized some, but not all, type C viruses.

In silico analysis of local RNA secondary structure in influenza virus A, B and C finds evidence of widespread ordered stability but little evidence of significant covariation

Influenza virus is a persistent threat to human health; indeed, the deadliest modern pandemic was in 1918 when an H1N1 virus killed an estimated 50 million people globally. The intent of this work is to better understand influenza from an RNA-centric perspective to provide local, structural motifs with likely significance to the influenza infectious cycle for therapeutic targeting. To accomplish this, we analyzed over four hundred thousand RNA sequences spanning three major clades: influenza A, B and C. We scanned influenza segments for local secondary structure, identified/modeled motifs of likely functionality, and coupled the results to an analysis of evolutionary conservation. We discovered 185 significant regions of predicted ordered stability, yet evidence of sequence covariation was limited to 7 motifs, where 3-found in influenza C-had higher than expected amounts of sequence covariation.

Susceptibility of Influenza A, B, C, and D Viruses to Baloxavir1

Baloxavir showed broad-spectrum in vitro replication inhibition of 4 types of influenza viruses (90% effective concentration range 1.2-98.3 nmol/L); susceptibility pattern was influenza A ˃ B ˃ C ˃ D. This drug also inhibited influenza A viruses of avian and swine origin, including viruses that have pandemic potential and those resistant to neuraminidase inhibitors.

[Resistance in influenza viruses]

BACKGROUND

Influenza virus infection can be prevented and treated with antiviral drugs. The usage of such drugs in Norway has been infrequent, however, they are an important component in our pandemic preparedness planning, as it will probably be difficult to get access to the appropriate vaccine in time before the pandemic reaches the country. The first generation of influenza drugs acquired resistance to a large degree, in contrast to the modern neuraminidase inhibitors that until recently have had minor problems with resistance.

MATERIAL AND METHODS

This review is based on research found in relevant published literature, together with experience from a virology reference laboratory and participation in a national and international surveillance including susceptibility testing.

RESULTS AND INTERPRETATION

While resistance has been a longstanding problem with the use of the "old" influenza drugs amantadine and rimantadine, only during the winter 2007/2008 did it become clear, that a certain type of virus acquired widespread resistance against the neuraminidase inhibitor oseltamivir. Resistance surveillance is crucial for the correct choice of empiric treatment for influenza infection, and will be one of the most important tasks at the National Influenza Centre in certain phases of a pandemic. The current situation with an increasing resistance problem strengthens the need to conduct continuous monitoring of antiviral susceptibility, as well as development of new antiviral drugs and treatment regimes.

[The main pharmacology study of ZhongSheng capsule dosage form changing]

OBJECTIVE

To study pharmacological action variety of ZhongSheng Pill dosage form changing from pill to capsule.

METHODS

Using anti-pathogenic microbe, antipyretic and antipyrotic effect as the pharmacological index,we compared the pharmacological action between Zhongsheng Pill formation and Capsule formation.

RESULTS

Zhongsheng Capsule had various degrees extraneous antagonistic actions against familiar and conditioned pathogenic bacteria and virus of respiratory tract. Furthermore it coud also reduce the death ratio of mouse infected by staphylococcus aureus and influenza virus, remove fever and diminish inflammation. All these effects had the dose-effect relationship. There were no difference between pill fromation and cpasule formation.

CONCLUSION

Zhongsheng Capsule retain the original pharmacological action of Zhongsheng Pill after dosage form changing.

Inhibition of influenza C viruses by human MxA protein

Human MxA protein was analyzed for its ability to inhibit the replication of different influenza C viruses. Three laboratory derivatives of viral strain C/Ann Arbor/1/50 were investigated, namely the parental wild-type virus C/AA-wt, the persistent variant C/AA-pi and the highly cytopathogenic variant C/AA-cyt. In addition, strain C/Paris/214/91 isolated from an influenza patient was used. Multiplication of all four viruses was suppressed in MxA-expressing Vero cells, as indicated by a decrease in viral RNA synthesis, viral protein synthesis, virion production and induction of a cytopathic effect. Inhibition correlated with the level of MxA expression. Furthermore, inhibition was independent of cell clone-specific differences in expression of virus receptors, as demonstrated by receptor reconstitution experiments. Thus, human MxA protein has antiviral activity against influenza C viruses.

Antiviral effects of rhIFN-alpha 1 against seven influenza viruses

AIM

To study the antiviral effects of rhIFN-alpha 1 (Chinese silkworm gene recombinant interferon alpha 1) on 7 influenza viruses in MDCK cells and in mouse pneumonia caused by PR8 virus.

METHODS

100TCID50 virus (H1N1, H2N2, H3N3, type B, type C, clinical A1, and clinical B) were inoculated into MDCK cells, PR8 viruses were dropped nasally in mice, the antiviral effects of rhIFN-alpha 1 were observed.

RESULTS

The minimal effective concentrations of rhIFN-alpha 1 against these 7 influenza viruses were 12.5, 25, 50, 25, 12.5, 25, and 12.5 kU.L-1, respectively. The infectious therapeutic indices of rhIFN-alpha 1 to these viruses in MDCK cells were 8 x 10(3), 4 x 10(3), 2 x 10(3), 4 x 10(3), 8 x 10(3), 4 x 10(3), and 8 x 10(3), respectively. The inhibitory indices of rhIFN-alpha 1 to the 7 influenza viruses in MDCK cells were 3.6, 4.7, 3.5, 3.3, 3.9, 4.6, and 3.5, respectively. The rhIFN-alpha 1 inhibited the intracellular replication of influenza viruses effectively, but did not kill viruses directly. The rhIFN-alpha 1 prolonged the life span of mice infected with pneumonia by influenza virus A strain PR8 to 94.2%-132.7%. It inhibited the inflammation and hyperplasia of interstitial fibers, and decreased the virus titer. The inhibitory rates of rhIFN-alpha 1 to pulmonary-indice were 14.8%-37.4%.

CONCLUSION

rhIFN-alpha 1 inhibited the proliferation of influenza virus and improved the symptom of mouse pneumonia caused by influenza virus.

Interaction between influenza virus proteins and pine cone antitumor substance that inhibits the virus multiplication

Fractions obtained from pine cone extract (PCE) of Pinus parviflora Sieb. et Zucc. have been shown to suppress the growth of influenza virus. The inhibitory effects of one of the fractions, Fraction VII, on the formation of RNA-viral protein complex and the viral RNA synthesis were investigated. The formation of M1-RNA or NP-RNA complex was inhibited when M1 or NP was preincubated with the PCE fraction. The in vitro viral RNA synthesis was inhibited by the PCE fraction, while this inhibitory effect was titrated out by the increasing concentration of M1 protein. These results suggest that the major target of the PCE fraction was M1 protein.

Disassembly of influenza C viruses, distinct from that of influenza A and B viruses requires neutral-alkaline pH

Influenza C (Flu C) viruses comprise an internal ribonucleoprotein (RNP) and an outer lipoprotein envelope with surface spike glycoproteins and the M1 protein matrix. The lipoprotein envelope and spike glycoproteins are solubilized by nonionic detergent in a pH-independent manner. In contrast, disassembly of the M1 protein matrix appears to depend on pH. Treatment of Flu C viruses with nonionic detergent in neutral or alkaline medium (pH 9.0-7.4) results in disintegration of the virion M1 matrix and leads to a significant release of RNP free of the M1 protein. In acidic medium (pH 6.0-5.0), the M1 matrix is not removed and the viral core-like complex of RNP along with the M1 matrix cover is released. Since Flu A and B viruses were characterized by acid-dependent disassembly of the virion M1 matrix, Flu C viruses seem to resemble the paramyxoviruses, which also show a neutral-alkaline pH dependence on matrix disintegration. These observations suggest that uncoating mechanisms of influenza C viruses and paramyxoviruses in target cells may be similar.

The receptor-destroying enzyme of influenza C virus is required for entry into target cells

The hemagglutinin-esterase (HE) protein of influenza C viruses possesses an acetylesterase activity, which appears essential for replication, as determined by reduced infectivity after inhibition of the viral enzyme [Vlasak et al., J. Virol. 63, 2056-2062 (1989)]. Analysis revealed the absence of virus-specific RNA and protein synthesis in infected cells after inhibition of the receptor-destroying enzyme. In addition, hemolytic activity was reduced after incubation of influenza C/JJ/50 virus with diisopropyl-fluorophosphate or 3,4-dichloro-isocoumarin. Further analysis revealed that inhibition of hemolysis depends on virus and erythrocyte concentrations. It is suggested that an active receptor-destroying enzyme is required for entry of influenza C virus into target cells at a step prior to fusion of the viral and cellular membrane. Our data indicate that cleavage of receptors bound to the HE protein is a prerequisite for the low pH-triggered conformational change required for fusion.

[The effect of different factors on the reproduction of influenza viruses and reassortants in cell cultures]

The influence of the maintenance medium, polyethylene glycol (PEG), DEAE-dextran, and low temperature on reproduction of influenza A, B, and C viruses and their reassortants in diploid and continuous cell cultures was determined. Lowering of pH in the maintenance medium to 6.5 was found to decrease reproduction of influenza A (H1N1) and A (H3N2) viruses and increase that of influenza B viruses. Treatment of cells with PEG solution increased the yield of influenza B and C but not A viruses. However, influenza A virus strains proved to be capable of producing infectious progeny in nonpermissive cell lines treated with PEG. Addition of DEAE-dextran to the medium exerted no effect on the infectivity of influenza A and B reassortants. Moreover, infection of MDCK cells after a "cold shock" led to an increase in hemagglutinin titres in influenza A reassortants.

Activity of influenza C virus O-acetylesterase with O-acetyl-containing compounds

Influenza C virus (strain C/Johannesburg/1/66) was grown, harvested, purified and used as source for the enzyme O-acetylesterase (N-acyl-O-acetylneuraminate O-acetylhydrolase; EC 3.1.1.53). This activity was studied and characterized with regard to some new substrates. The pH optimum of the enzyme is around 7.6, its stability at different pH values shows a result similar to that of the pH optimum, and its activity is well maintained in the pH range from 7.0 to 8.5 (all these tests were performed with 4-nitrophenyl acetate as substrate). Remarkable differences were found in the values of both Km and Vmax, with the synthetic substrates 4-nitrophenyl acetate, 2-nitrophenyl acetate, 4-methylumbelliferyl acetate, 1-naphthyl acetate and fluorescein diacetate. The use of 4-nitrophenyl acetate, 4-methylumbelliferyl acetate or 1-naphthyl acetate as substrate seems to be convenient for routine work, but it is better to carry out the measurements in parallel with those on bovine submandibular gland mucin (the latter is a natural and commercially available substrate). It was found that 4-acetoxybenzoic acid, as well as the methyl ester of 2-acetoxybenzoic acid, but not 2-acetoxybenzoic acid itself, are cleaved by this enzyme. Triacetin, di-O-acetyladenosine, tri-O-acetyladenosine, and di-O-acetyl-N-acetyladenosine phosphate, hitherto unreported as substrates for this viral esterase, are hydrolysed at different rates by this enzyme. We conclude that the O-acetylesterase from influenza C virus has a broad specificity towards both synthetic and natural non-sialic acid-containing substrates. Zn2+, Mn2+ and Pb2+ (as their chloride salts), N-acetylneuraminic acid, 4-methyl-umbelliferone and 2-acetoxybenzoic acid (acetylsalicylic acid) did not act as inhibitors.

Flow cytometric analysis of virus-infected cells and its potential use for screening antiviral agents

Virus-infected cells were analyzed using multiparameter flow cytometry. Two virus-cell systems were investigated: HSV-1-infected VF cells and influenza C virus JHB/1/66-infected MDCK cells. Analysis included the measurement of the appearance of virus specific antigens. On individual cells, with polyclonal antibodies, antigens were first detected at 12 h p.i., and the numbers of labeled cells were followed up to 96 h p.i. The efficacy of four antiviral agents was tested with this system. The results were in good agreement with those of plaque reduction tests and indicated that this new method may be extremely useful for the correlation of viral and cellular events with antiviral activity. Finally, it was demonstrated that infected cells in both systems have a considerably greater volume than non-infected cells.

Influenza C virus esterase: analysis of catalytic site, inhibition, and possible function

The active site serine of the acetylesterase of influenza C virus was localized to amino acid 71 of the hemagglutinin-esterase protein by affinity labeling with 3H-labeled diisopropylfluorophosphate. This serine and the adjacent amino acids (Phe-Gly-Asp-Ser) are part of a consensus sequence motif found in serine hydrolases. Since comparative analysis failed to reveal esterase sequence similarities with other serine hydrolases, we suggest that this viral enzyme is a serine hydrolase constituting a new family of serine esterases. Furthermore, we found that the influenza C virus esterase was inhibited by isocoumarin derivatives, with 3,4-dichloroisocoumarin being the most potent inhibitor. Addition of this compound prevented elution of influenza C virus from erythrocytes and inhibited virus infectivity, possibly through inhibition of virus entry into cells.

Comparative activities of several nucleoside analogs against influenza A, B, and C viruses in vitro

A set of 20 nucleoside analogs were examined for their inhibitory effects on the cytopathogenicity and growth of influenza virus type A, B, and C strains in Madin-Darby canine kidney (MDCK) cells. Among the compounds evaluated, pyrazofurin, 3-deazaguanine, ribavirin, carbodine, and cyclopentenyl cytosine inhibited viral cytopathogenicity at concentrations that were lower than those found cytotoxic for the MDCK cells. No differences were observed in the 50% effective doses (based on inhibition of viral cytopathogenicity) of these five compounds for a number of influenza virus type A (subtypes H1N1 and H3N2), B, and C strains. Pyrazofurin showed the lowest 50% effective dose (0.15 microgram/ml), which was about 20- to 30-fold lower than those of the other four compounds. The selectivity indices of the five compounds, calculated as the ratio of the 50% cytotoxic dose (determined by trypan blue exclusion) to the 50% effective dose, were greater than 100. When the selectivity indices were calculated as the ratios of the 50% inhibitory doses for cellular RNA synthesis to the 50% effective doses, they were greater than 100 for ribavirin, pyrazofurin, and 3-deazaguanine but less than 2 for carbodine and cyclopentenyl cytosine. All five compounds inhibited the growth of influenza virus types A and B in MDCK cells at a concentration which was well below their cytotoxicity threshold for MDCK cells and, therefore, deserve further exploration for their potential in the treatment of influenza virus type A, B, and C infections.

Selective inactivation of influenza C esterase: a probe for detecting 9-O-acetylated sialic acids

The influenza C virus (INF-C) hemagglutinin recognizes 9-O-acetyl-N-acetylneuraminic acid. The same protein contains the receptor-destroying enzyme (RDE), which is a 9-O-acetyl-esterase. The RDE was inactivated by the serine esterase inhibitor di-isopropyl fluorophosphate (DFP). [3H]DFP-labeling localized the active site to the heavy chain of the glycoprotein. DFP did not alter the hemagglutination or fusion properties of the protein, but markedly decreased infectivity of the virus, demonstrating that the RDE is important for primary infection. Finally, DFP-treated INF-C bound specifically and irreversibly to cells expressing 9-O-acetylated sialic acids. This provides a probe for a molecule that was hitherto very difficult to study.

[The nature of the influenza C virus receptor and the specificity of the receptor-destroying enzyme]

Bacterial neuraminidases destroy influenza C virus receptors of chick erythrocytes and inactivate hemagglutination inhibitors: rat alpha 1-macroglobulin (RMG) and bovine submaxillary mucin (BSM). These data indicate that neuraminic acid may be a component of influenza C virus receptor. The inhibiting activity of RMG and BSM is also eliminated by the receptor-destroying enzyme (RDE) of influenza C virus. After inactivation, the inhibitors (RMG and BSM) contain a reduced amount of N-acetyl-9-0-acetylneuraminic acid (Neu5, 9Ac2) and a larger amount of N-acetylneuraminic acid (Neu5 Ac). Transformation of Neu5, 9Ac2 into Neu5 Ac may also occur upon incubation of free neuraminic acid with influenza C virus. These data indicate that the RDE of influenza C virus is neuraminate-O-acetylesterase (N-acyl-9 4-O-acetylneuraminate O-acetylhydrolase (EC 3.1.1.53). It was shown that inhibition of influenza C virus hemagglutination by RMG and BSM and, apparently, adhesion of the virus to the cell surface involves binding of influenza C virus with Neu5, 9Ac2.

The functions of oligosaccharide chains associated with influenza C viral glycoproteins. I. The formation of influenza C virus particles in the absence of glycosylation

The effect of a glycosylation inhibitor, tunicamycin (TM) on the replication of influenza C virus was investigated. Incorporation of [3H]-glucosamine into the gp88 glycoproteins of this virus was completely inhibited by TM at the concentrations higher than 0.25 microgram/ml. Under these conditions, the synthesis of internal proteins NP and M was shown in TM-treated cells but the synthesis of gp88 was not. The disappearance of gp88 was however accompanied with the appearance of two new polypeptides with molecular weights of 80,000 (T80) and 76,000 (T76). While T80 was identified by peptide mapping as a host cell protein whose synthesis was enhanced by TM, T76 was shown to correspond to a nonglycosylated form of gp88. Pulse-chase experiments revealed that there was no significant difference in the intracellular stability of T76 and gp88. Although TM depressed the production of infectious progeny virus greater than 100-fold, only a five-fold decrease was observed in the release of noninfectious physical particles, suggesting that glycosylation is not essential for the formation of influenza C virus particles. However, the virions from TM-treated cells had a lower buoyant density in isopycnic sucrose gradients and lacked surface proteins in either glycosylated or nonglycosylated form.

[Mobility study of influenza C virus proteins in cellulose acetate electrophoresis]

Four strains of influenza C virus were studied by cellulose acetate electrophoresis. All of them were found to contain super-capsid proteins destroyed by electrophoresis apparently due to contact with sodium dodecylsulphate. According to Laver's classification, influenza C viruses may be placed with Group 3 viruses. Among major virion proteins of influenza C viruses matrix protein was found to be the most stable forming a separate protein band on cellulose acetate, and it may be readily identified and eluted from paper in preparative amounts.