A precursor glycoprotein in influenza C virus

Host Range, Biology, and Species Specificity of Seven-Segmented Influenza Viruses-A Comparative Review on Influenza C and D

Other than genome structure, influenza C (ICV), and D (IDV) viruses with seven-segmented genomes are biologically different from the eight-segmented influenza A (IAV), and B (IBV) viruses concerning the presence of hemagglutinin-esterase fusion protein, which combines the function of hemagglutinin and neuraminidase responsible for receptor-binding, fusion, and receptor-destroying enzymatic activities, respectively. Whereas ICV with humans as primary hosts emerged nearly 74 years ago, IDV, a distant relative of ICV, was isolated in 2011, with bovines as the primary host. Despite its initial emergence in swine, IDV has turned out to be a transboundary bovine pathogen and a broader host range, similar to influenza A viruses (IAV). The receptor specificities of ICV and IDV determine the host range and the species specificity. The recent findings of the presence of the IDV genome in the human respiratory sample, and high traffic human environments indicate its public health significance. Conversely, the presence of ICV in pigs and cattle also raises the possibility of gene segment interactions/virus reassortment between ICV and IDV where these viruses co-exist. This review is a holistic approach to discuss the ecology of seven-segmented influenza viruses by focusing on what is known so far on the host range, seroepidemiology, biology, receptor, phylodynamics, species specificity, and cross-species transmission of the ICV and IDV.

Epidemiology and Clinical Characteristics of Influenza C Virus

Influenza C virus (ICV) is a common yet under-recognized cause of acute respiratory illness. ICV seropositivity has been found to be as high as 90% by 7-10 years of age, suggesting that most people are exposed to ICV at least once during childhood. Due to difficulty detecting ICV by cell culture, epidemiologic studies of ICV likely have underestimated the burden of ICV infection and disease. Recent development of highly sensitive RT-PCR has facilitated epidemiologic studies that provide further insights into the prevalence, seasonality, and course of ICV infection. In this review, we summarize the epidemiology and clinical characteristics of ICV.

Genetic Evolution and Molecular Selection of the HE Gene of Influenza C Virus

Influenza C virus (ICV) was first identified in humans and swine, but recently also in cattle, indicating a wider host range and potential threat to both the livestock industry and public health than was originally anticipated. The ICV hemagglutinin-esterase (HE) glycoprotein has multiple functions in the viral replication cycle and is the major determinant of antigenicity. Here, we developed a comparative approach integrating genetics, molecular selection analysis, and structural biology to identify the codon usage and adaptive evolution of ICV. We show that ICV can be classified into six lineages, consistent with previous studies. The HE gene has a low codon usage bias, which may facilitate ICV replication by reducing competition during evolution. Natural selection, dinucleotide composition, and mutation pressure shape the codon usage patterns of the ICV HE gene, with natural selection being the most important factor. Codon adaptation index (CAI) and relative codon deoptimization index (RCDI) analysis revealed that the greatest adaption of ICV was to humans, followed by cattle and swine. Additionally, similarity index (SiD) analysis revealed that swine exerted a stronger evolutionary pressure on ICV than humans, which is considered the primary reservoir. Furthermore, a similar tendency was also observed in the M gene. Of note, we found HE residues 176, 194, and 198 to be under positive selection, which may be the result of escape from antibody responses. Our study provides useful information on the genetic evolution of ICV from a new perspective that can help devise prevention and control strategies.

Analyses of Evolutionary Characteristics of the Hemagglutinin-Esterase Gene of Influenza C Virus during a Period of 68 Years Reveals Evolutionary Patterns Different from Influenza A and B Viruses

Infections with the influenza C virus causing respiratory symptoms are common, particularly among children. Since isolation and detection of the virus are rarely performed, compared with influenza A and B viruses, the small number of available sequences of the virus makes it difficult to analyze its evolutionary dynamics. Recently, we reported the full genome sequence of 102 strains of the virus. Here, we exploited the data to elucidate the evolutionary characteristics and phylodynamics of the virus compared with influenza A and B viruses. Along with our data, we obtained public sequence data of the hemagglutinin-esterase gene of the virus; the dataset consists of 218 unique sequences of the virus collected from 14 countries between 1947 and 2014. Informatics analyses revealed that (1) multiple lineages have been circulating globally; (2) there have been weak and infrequent selective bottlenecks; (3) the evolutionary rate is low because of weak positive selection and a low capability to induce mutations; and (4) there is no significant positive selection although a few mutations affecting its antigenicity have been induced. The unique evolutionary dynamics of the influenza C virus must be shaped by multiple factors, including virological, immunological, and epidemiological characteristics.

The role of stearate attachment to the hemagglutinin-esterase-fusion glycoprotein HEF of influenza C virus

The only spike of influenza C virus, the hemagglutinin-esterase-fusion glycoprotein (HEF) combines receptor binding, receptor hydrolysis and membrane fusion activities. Like other hemagglutinating glycoproteins of influenza viruses HEF is S-acylated, but only with stearic acid at a single cysteine located at the cytosol-facing end of the transmembrane region. Previous studies established the essential role of S-acylation of hemagglutinin for replication of influenza A and B virus by affecting budding and/or membrane fusion, but the function of acylation of HEF was hitherto not investigated. Using reverse genetics we rescued a virus containing non-stearoylated HEF, which was stable during serial passage and showed no competitive fitness defect, but the growth rate of the mutant virus was reduced by one log. Deacylation of HEF does neither affect the kinetics of its plasma membrane transport nor the protein composition of virus particles. Cryo-electron microscopy showed that the shape of viral particles and the hexagonal array of spikes typical for influenza C virus were not influenced by this mutation indicating that virus budding was not disturbed. However, the extent and kinetics of haemolysis were reduced in mutant virus at 37°C, but not at 33°C, the optimal temperature for virus growth, suggesting that non-acylated HEF has a defect in membrane fusion under suboptimal conditions.

Hemagglutinin-esterase-fusion (HEF) protein of influenza C virus

Influenza C virus, a member of the Orthomyxoviridae family, causes flu-like disease but typically only with mild symptoms. Humans are the main reservoir of the virus, but it also infects pigs and dogs. Very recently, influenza C-like viruses were isolated from pigs and cattle that differ from classical influenza C virus and might constitute a new influenza virus genus. Influenza C virus is unique since it contains only one spike protein, the hemagglutinin-esterase-fusion glycoprotein HEF that possesses receptor binding, receptor destroying and membrane fusion activities, thus combining the functions of Hemagglutinin (HA) and Neuraminidase (NA) of influenza A and B viruses. Here we briefly review the epidemiology and pathology of the virus and the morphology of virus particles and their genome. The main focus is on the structure of the HEF protein as well as on its co- and post-translational modification, such as N-glycosylation, disulfide bond formation, S-acylation and proteolytic cleavage into HEF1 and HEF2 subunits. Finally, we describe the functions of HEF: receptor binding, esterase activity and membrane fusion.

A seven-segmented influenza A virus expressing the influenza C virus glycoprotein HEF

Influenza viruses are classified into three types: A, B, and C. The genomes of A- and B-type influenza viruses consist of eight RNA segments, whereas influenza C viruses only have seven RNAs. Both A and B influenza viruses contain two major surface glycoproteins: the hemagglutinin (HA) and the neuraminidase (NA). Influenza C viruses have only one major surface glycoprotein, HEF (hemagglutinin-esterase fusion). By using reverse genetics, we generated two seven-segmented chimeric influenza viruses. Each possesses six RNA segments from influenza virus A/Puerto Rico/8/34 (PB2, PB1, PA, NP, M, and NS); the seventh RNA segment encodes either the influenza virus C/Johannesburg/1/66 HEF full-length protein or a chimeric protein HEF-Ecto, which consists of the HEF ectodomain and the HA transmembrane and cytoplasmic regions. To facilitate packaging of the heterologous segment, both the HEF and HEF-Ecto coding regions are flanked by HA packaging sequences. When introduced as an eighth segment with the NA packaging sequences, both viruses are able to stably express a green fluorescent protein (GFP) gene, indicating a potential use for these viruses as vaccine vectors to carry foreign antigens. Finally, we show that incorporation of a GFP RNA segment enhances the growth of seven-segmented viruses, indicating that efficient influenza A viral RNA packaging requires the presence of eight RNA segments. These results support a selective mechanism of viral RNA recruitment to the budding site.

Structure, function and evolution of the hemagglutinin-esterase proteins of corona- and toroviruses

Virus attachment to host cells is mediated by dedicated virion proteins, which specifically recognize one or, at most, a limited number of cell surface molecules. Receptor binding often involves protein-protein interactions, but carbohydrates may serve as receptor determinants as well. In fact, many different viruses use members of the sialic acid family either as their main receptor or as an initial attachment factor. Sialic acids (Sias) are 9-carbon negatively-charged monosaccharides commonly occurring as terminal residues of glycoconjugates. They come in a large variety and are differentially expressed in cells and tissues. By targeting specific Sia subtypes, viruses achieve host cell selectivity, but only to a certain extent. The Sia of choice might still be abundantly present on non-cell associated molecules, on non-target cells (including cells already infected) and even on virus particles themselves. This poses a hazard, as high-affinity virion binding to any of such "false'' receptors would result in loss of infectivity. Some enveloped RNA viruses deal with this problem by encoding virion-associated receptor-destroying enzymes (RDEs). These enzymes make the attachment to Sia reversible, thus providing the virus with an escape ticket. RDEs occur in two types: neuraminidases and sialate-O-acetylesterases. The latter, originally discovered in influenza C virus, are also found in certain nidoviruses, namely in group 2 coronaviruses and in toroviruses, as well as in infectious salmon anemia virus, an orthomyxovirus of teleosts. Here, the structure, function and evolution of viral sialate-O-acetylesterases is reviewed with main focus on the hemagglutinin-esterases of nidoviruses.

Use of influenza C virus glycoprotein HEF for generation of vesicular stomatitis virus pseudotypes

Influenza C virus contains two envelope glycoproteins: CM2, a putative ion channel protein; and HEF, a unique multifunctional protein that performs receptor-binding, receptor-destroying and fusion activities. Here, it is demonstrated that expression of HEF is sufficient to pseudotype replication-incompetent vesicular stomatitis virus (VSV) that lacks the VSV glycoprotein (G) gene. The pseudotyped virus showed characteristic features of influenza C virus with respect to proteolytic activation, receptor usage and cell tropism. Chimeric glycoproteins composed of HEF ectodomain and VSV-G C-terminal domains were efficiently incorporated into VSV particles and showed receptor-binding and receptor-destroying activities but, unlike authentic HEF, did not mediate efficient infection, probably because of impaired fusion activity. HEF-pseudotyped VSV efficiently infected polarized Madin-Darby canine kidney cells via the apical plasma membrane, whereas entry of VSV-G-complemented virus was restricted to the basolateral membrane. These findings suggest that pseudotyping of viral vectors with HEF might be useful for efficient apical gene transfer into polarized epithelial cells and for targeting cells that express 9-O-acetylated sialic acids.

The sialate-4-O-acetylesterases of coronaviruses related to mouse hepatitis virus: a proposal to reorganize group 2 Coronaviridae

Group 2 coronaviruses are characterized within the order Nidovirales by a unique genome organization. A characteristic feature of group 2 coronaviruses is the presence of a gene encoding the haemagglutinin-esterase (HE) protein, which is absent in coronaviruses of groups 1 and 3. At least three coronavirus strains within group 2 expressed a structural protein with sialate-4-O-acetylesterase activity, distinguishing them from other members of group 2, which encode an enzyme specific for 5-N-acetyl-9-O-acetylneuraminic acid. The esterases of mouse hepatitis virus (MHV) strains S and JHM and puffinosis virus (PV) specifically hydrolysed 5-N-acetyl-4-O-acetylneuraminic acid (Neu4,5Ac2) as well as the synthetic substrates p-nitrophenyl acetate, 4-methylumbelliferyl acetate and fluorescein diacetate. The K(m) values of the MHV-like esterases for the latter substrates were two- to tenfold lower than those of the sialate-9-O-acetylesterases of influenza C viruses. Another unspecific esterase substrate, alpha-naphthyl acetate, was used for the in situ detection of the dimeric HE proteins in SDS-polyacrylamide gels. MHV-S, MHV-JHM and PV bound to horse serum glycoproteins containing Neu4,5Ac2. De-O-acetylation of the glycoproteins by alkaline treatment or incubation with the viral esterases resulted in a complete loss of recognition, indicating a specific interaction of MHV-like coronaviruses with Neu4,5Ac2. Combined with evidence for distinct phylogenetic lineages of group 2 coronaviruses, subdivision into subgroups 2a (MHV-like viruses) and 2b (bovine coronavirus-like viruses) is suggested.

Cloning and identification of the infectious salmon anaemia virus haemagglutinin

Infectious salmon anaemia virus (ISAV) is an orthomyxo-like virus that causes serious disease in Atlantic salmon (Salmo salar). Like the orthomyxoviruses, ISAV has been shown to possess haemagglutinin (HA) activity. This study presents the cloning, expression and identification of the ISAV HA gene, which was isolated from a cDNA library by immunoscreening. The HA gene contained an ISAV-specific conserved nucleotide motif in the 5' region and a 1167 bp open reading frame encoding a protein with a predicted molecular mass of 42.4 kDa. The HA gene was expressed in a baculovirus system. A monoclonal antibody (MAb) shown previously to be directed against the ISAV HA reacted with insect cells infected with recombinant baculovirus. Salmon erythrocytes also adsorbed to these cells and adsorption was inhibited by the addition of either the ISAV-specific MAb or a polyclonal rabbit serum prepared against purified virus, confirming the virus specificity of the reaction. Immunoblot analyses indicated that ISAV HA, in contrast to influenza virus HA, is not posttranslationally cleaved. Sequence comparisons of the HA gene from five Norwegian, one Scottish and one Canadian isolate revealed a highly polymorphic region that may be useful in epidemiological studies.

Cell surface expression of biologically active influenza C virus HEF glycoprotein expressed from cDNA

The hemagglutinin, esterase, and fusion (HEF) glycoprotein of influenza C virus possesses receptor binding, receptor destroying, and membrane fusion activities. The HEF cDNAs from influenza C/Ann Arbor/1/50 (HEF-AA) and influenza C/Taylor/1223/47 (HEF-Tay) viruses were cloned and expressed, and transport of HEF to the cell surface was monitored by susceptibility to cleavage by exogenous trypsin, indirect immunofluorescence microscopy, and flow cytometry. Previously it has been found in studies with the C/Johannesburg/1/66 strain of influenza C virus (HEF-JHB) that transport of HEF to the cell surface is severely inhibited, and it is thought that the short cytoplasmic tail, Arg-Thr-Lys, is involved in blocking HEF cell surface expression (F. Oeffner, H.-D. Klenk, and G. Herrler, J. Gen. Virol. 80:363-369, 1999). As the cytoplasmic tail amino acid sequences of HEF-AA and HEF-Tay are identical to that of HEF-JHB, the data indicate that cell surface expression of HEF-AA and HEF-Tay is not inhibited by this amino acid sequence. Furthermore, the abundant cell surface transport of HEF-AA and HEF-Tay indicates that their cell surface expression does not require coexpression of another viral protein. The HEF-AA and HEF-Tay HEF glycoproteins bound human erythrocytes, promoted membrane fusion in a low-pH and trypsin-dependent manner, and displayed esterase activity, indicating that the HEF glycoprotein alone mediates all three known functions at the cell surface.

Location of a linear epitope recognized by monoclonal antibody S16 on the hemagglutinin-esterase glycoprotein of influenza C virus

We reported previously that monoclonal antibody S16, which had been raised against the hemagglutinin-esterase (HE) glycoprotein of influenza C/Ann Arbor/1/50 (AA/50) virus, recognizes a linear epitope present on the HE molecules of all influenza C viruses examined except for viruses belonging to a lineage represented by Aichi/1/81 (AI/81). Comparison of the deduced amino acid sequence of HE between viruses on the AI/81-related lineage and those on the others suggests that the epitope recognized by S16 is located in a region containing amino acid residue 403 and that a change from Glu to Lys at this position causes the loss of reactivity with the antibody. To prove it, the wild type (WT) HEs of AA/50 and AI/81 as well as their mutants with an amino acid substitution at residue 403 were expressed in CV-1 cells from the recombinant simian virus 40 (SV40) and tested for reactivity with S16 by immunoprecipitation. The results showed that the AA/50 virus WT and AI/81 virus mutant HEs (both having Glu at residue 403) were reactive with S16 whereas the AI/81 virus WT and AA/50 virus mutant HEs (both having Lys at residue 403) were not. Furthermore, we examined the reactivity of S16 with two synthetic peptides (corresponding to residues 397-409) that possess Glu and Lys at position 403, respectively, by enzyme-linked immunosorbent assays. The results demonstrated that the former peptide but not the latter was reactive with S16. These observations support strongly the notion described above. During this study it was also found that S16 cross-reacts with large T antigen of SV40.

Differential fatty acid selection during biosynthetic S-acylation of a transmembrane protein (HEF) and other proteins in insect cells (Sf9) and in mammalian cells (CV1)

The transmembrane glycoprotein HEF and its acylation deficient mutant M1 were expressed in Sf9 insect cells infected with recombinant baculovirus and in CV1 mammalian cells using the vaccinia T7 system. In insect cells (Sf9), both wild type HEF and HEF(M1) are synthesized in their precursor form HEF0, which appears as a double band in SDS gels. Digestion with glycopeptidase F and endoglycosidase H reveals that the larger 84-kDa form is modified by the attachment of unprocessed carbohydrates of the high mannose type whereas the smaller 76-kDa form is non-glycosylated. As revealed by in vitro labeling experiments with palmitic acid another modification of HEF is the attachment of a long chain fatty acid to cysteine residue Cys-652 which is located at the internal border of the cytoplasmic membrane. After labeling with [3H]palmitic acid in both systems only HEF(WT) is acylated, whereas HEF(M1) is not. High performance liquid chromatography analysis of the fatty acids bound to HEF(WT) expressed in Sf9 insect cells reveals nearly 80% of palmitic acid. In contrast to this finding, the acylation pattern of HEF expressed in CV1 cells shows nearly the same amounts of stearic and palmitic acid (40%). Since the interconversion of the input [3H]palmitic acid to stearic acid is even lower in CV1 cells than in insect cells, it follows that only HEF expressed in mammalian, but not in insect cells selects for stearic acid during its biosynthetic acylation. We extended our study to acylation of endogenous proteins in Sf9 cells. In finding only palmitate linked to protein we present evidence that, in contrast to mammalian cells, insect cells (Sf9) cannot transfer stearic acid to polypeptide. This finding favors the hypothesis of enzymatic acylation over non-enzymatic mechanisms of acyl transfer to protein.

Comparison of receptor-binding properties among influenza C virus isolates

A total of 10 influenza C virus strains isolated recently in Yamagata City, Japan and shown to belong to the same lineage was compared for the ability to agglutinate chicken and mouse erythrocytes under various conditions. C/Yamagata/10/89 was unique in lacking the ability to agglutinate chicken erythrocytes at a temperature > or = 4 degrees C. This isolate also agglutinated native mouse erythrocytes only very inefficiently, although the high agglutination titer was obtained with the glutaraldehyde-fixed cells. Furthermore, it was found that C/Yamagata/4/88, unlike the other isolates, agglutinated erythrocytes from chickens to lower titers than those from mice, even when assayed at 0 degree C. Comparison of the deduced amino acid sequence of hemagglutinin-esterase among the 6 representative strains including two older isolates, C/Yamagata/26/81 and C/Nara/2/85, suggested that the failures of C/Yamagata/10/89 to agglutinate chicken erythrocytes at > or = 4 degrees C and unfixed mouse erythrocytes to high titers may be due to amino acid changes at residues 337 (Glu-->Lys) and 340 (Thr-->Tyr), respectively, and that a change at residue 347 (Leu-->Ser) may be responsible for the decreased ability of C/Yamagata/4/88 to agglutinate chicken erythrocytes.

Identification of influenza C virus phosphoproteins

The HMV-II cells infected with influenza C virus were labeled with inorganic [32P]phosphate to identify phosphorylated proteins. Analysis by radioimmunoprecipitation with antiviral serum or monoclonal antibodies revealed that three major structural proteins of the virus, hemagglutinin-esterase (HE), nucleoprotein (NP), and matrix protein (M1) are all phosphorylated in both infected cells and virions. It was also observed that, in the presence of trypsin (10 micrograms/ml), the unphosphorylated form of the HE glycoprotein was cleaved efficiently whereas the phosphorylated form was not, raising the possibility that phosphorylation of HE may influence its susceptibility to degradation by proteolytic enzymes.

MDCK cell cultures supplemented with high concentrations of trypsin exhibit remarkable susceptibility to influenza C virus

Multiplication of influenza C virus in MDCK cell cultures increased with increasing concentrations of trypsin up to 160 micrograms/ml, whereas maximum growth of influenza A virus in the same culture was observed at a concentration of 10 micrograms/ml. In the presence of 160 micrograms of trypsin per ml MDCK cells showed the same or even higher susceptibility to various strains of influenza C virus compared with HMV-II cells, a human melanoma cell line that has been reported to have high susceptibility to the virus. Complete cleavage of the HE precursor protein of MDCK-grown influenza C virus into HE1 and HE2 subunits was achieved by trypsin at a concentration of 160 micrograms/ml, whereas only partial cleavage was observed at 10 micrograms/ml. The present results thus demonstrate that MDCK cell cultures supplemented with trypsin at a concentration of 160 micrograms/ml become highly susceptible to influenza C virus.

Surface glycoprotein of influenza C virus: inactivation and restoration of the acetylesterase activity on nitrocellulose

The influenza C glycoprotein HEF was analyzed for acetylesterase activity after SDS-polyacrylamide gel electrophoresis and transfer to nitrocellulose membranes. Using a histological esterase assay, the glycoprotein was detected as a colored band indicating that it is enzymatically active. The enzyme activity was not affected by low pH, but was abolished after denaturation by SDS as well as after breaking the disulfide bonds by reducing agents. Glycoprotein inactivated by SDS regained its enzyme activity if the ionic detergent was displaced by either bovine serum albumin or a nonionic detergent. The stability of the enzyme combined with the color assay provides a convenient tool to study the acetylesterase activity of the influenza C virus glycoprotein.

Increased influenza A virus sialidase activity with N-acetyl-9-O-acetylneuraminic acid-containing substrates resulting from influenza C virus O-acetylesterase action

Influenza virus type C (Johannesburg/1/66) was used as a source for the enzyme O-acetylesterase (EC 3.1.1.53) with several natural sialoglycoconjugates as substrates. The resulting products were immediately employed as substrates using influenza virus type A [(Singapore/6/86) (H1N1) or Shanghai/11/87 (H3N2)] as a source for sialidase (neuraminidase, EC 3.2.1.18). A significant increase in the percentage of sialic acid released was found when the O-acetyl group was cleaved by O-acetylesterase activity from certain substrates (bovine submandibular gland mucin, rat serum glycoproteins, human saliva glycoproteins, mouse erythrocyte stroma, chick embryonic brain gangliosides and bovine brain gangliosides). A common feature of all these substrates is that they contain N-acetyl-9-O-acetylneuraminic acid residues. By contrast, no significant increase in the release of sialic acid was detected when certain other substrates could not be de-O-acetylated by the action of influenza C esterase, either because they lacked O-acetylsialic acid (human glycophorin A, alpha 1-acid glycoprotein from human serum, fetuin and porcine submandibular gland mucin) or because the 4-O-acetyl group was scarcely cleaved by the viral O-acetylesterase (equine submandibular gland mucin). The biological significance of these facts is discussed, relative to the infective capacity of influenza C virus.

Location of neutralizing epitopes on the hemagglutinin-esterase protein of influenza C virus

Neutralization-resistant variants of influenza C/Ann Arbor/1/50 virus were selected with monoclonal antibodies against four different antigenic sites on the hemagglutinin-esterase (HE) glycoprotein, and their HE genes were sequenced to identify amino acid residues important for the integrity of each site. Twelve different amino acid substitutions in a total of 18 antigenic variants were all located on the HE1 subunit. Although variants for antigenic site A-2 had a change at position 367, all substitutions in the variants for sites A-1, A-3, and A-4 occurred in the central region of the HE1 spanning amino acid positions 178 to 283. Furthermore, it was found that many of the substitutions in the variants selected with antibodies to sites A-1 and A-3 were clustered within or near one of the three variable regions revealed previously by comparing amino acid sequences of the HEs among various influenza C isolates (Buonagurio, D. A., Nakada, S., Fitch, W. M., and Palese, P., Virology 146, 221-232, 1985). The antigenic variants were also examined for their ability to agglutinate chicken and human erythrocytes in order to obtain information concerning the receptor-binding site on the HE molecule. The results suggested that the amino acid changes at residues 178, 186, 187, 190, 206, 212, and 226 decreased the hemagglutinating activity whereas those at residues 245, 266, and 283 produced an opposite effect.

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.

Structure and function of the HEF glycoprotein of influenza C virus

Soon after the first isolation of an influenza C virus from a patient, it became obvious that this virus differs from other myxoviruses in several aspects. Pronounced differences have been observed in the interactions between the virus and cell surfaces, suggesting that influenza C virus attaches to the receptors different from those recognized by other myxoviruses. While influenza A and B viruses agglutinate erythrocytes from many species, including humans, the spectrum of erythrocytes agglutinated by influenza C virus is much more restricted. Erythrocytes from rats, mice, and adult chickens are suitable for hemagglutination and hemadsorption tests; cells from other species, however, react not at all or only poorly with influenza C virus. Differences are also observed so far as hemagglutination inhibitors are concerned. A variety of glycoproteins have been shown to prevent influenza A and B viruses from agglutinating erythrocytes. In the case of influenza C virus, rat serum was for a long time the only known hemagglutination inhibitor. A difference in the receptors for influenza C virus and other myxo-viruses was also suggested by studies on the receptor-destroying enzyme. The ability of influenza C virus to inactivate its own receptors was reported soon after the first isolation of this virus from a patient. However, the influenza C enzyme did not affect the receptors of other myxoviruses and, conversely, the receptor-destroying enzyme of either of the latter viruses was unable to inactivate the receptors for influenza C virus on erythrocytes. While the enzyme of influenza A and B virus was characterized as a neuraminidase in the 1950s, even with refined methodology no such activity was detectable with influenza C virus. It is now known that both the receptor-binding and receptor-destroying activities, as well as the fusion activity of influenza C virus are mediated by the only glycoprotein present on the surface of the virus particle. The structure and functions of this protein, which is designated as HEF, are reviewed in this chapter.

Characterization of the cord-like structures emerging from the surface of influenza C virus-infected cells

When HMV-II cells (a human malignant melanoma cell line) infected with a newly isolated influenza C strain (Yamagata/1/88) were examined by simple light microscopy, it was found that a large number of cord-like structures which had lengths up to about 500 microns or greater were emerging from the cell surface. The existence of viral glycoproteins (hemagglutinin-esterase, HE) on the surface of these huge structures was confirmed by hemadsorption experiments with erythrocytes from a variety of species as well as by immunofluorescent staining with anti-HE monoclonal antibody. Furthermore, electron microscopy revealed that numerous filamentous particles in the process of budding, each covered with a layer of surface projections approximately 13 nm in length, aggregated with their long axes parallel to form a cord-like structure visible under a light microscope. An electron-dense layer, which presumably consists of membrane protein (M), was seen in cross-sections of all filamentous virions whereas internal nucleocapsids were rarely seen. SDS-polyacrylamide gel electrophoresis of the purified cords also showed that they contained HE and M polypeptides but not nucleoprotein, confirming that long filamentous particles are mostly devoid of nucleocapsids. The emergence of cords on the cell surface was observed in various cell cultures infected with C/Yamagata/1/88 though their number and length varied markedly depending on cell type. The production of cord-like structures was also evident in HMV-II cells infected with any of several different influenza C strains, which suggests that the cord formation is a common feature of influenza C virus group.

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.

Developmentally expressed O-acetyl ganglioside GT3 in fetal rat cerebral cortex

Monoclonal antibody M6704, established against the chick neural tube, was shown to recognize a trisialosyl residue, NeuAc alpha 2-8NeuAc alpha 2-8NeuAc alpha 2-3-R of C-series gangliosides. Using this antibody, the developmental changes of C-series gangliosides in fetal rat cerebral cortex have been examined. Two dimensional thin layer chromatography (TLC) enzyme-immunostaining analysis revealed that alkali treatment resulted in a great increase in GT3 that amounted to more than 85% of the total GT3 detected. The alkalilabile form was easily degraded to form GT3 by the action of the receptor-destroying enzyme of influenza C virus, sialate O-acetylesterase, indicating that the antigen was most probably 9-O-acetyl-NeuAc containing GT3. The ganglioside was highly enriched at the 14th gestation day, gradually decreased, and was not detected in adult rat cerebral cortex.

Evidence that the matrix protein of influenza C virus is coded for by a spliced mRNA

In contrast to influenza A and B viruses, which encode their matrix (M) proteins via an unspliced mRNA, the influenza C virus M protein appears to be coded for by a spliced mRNA from RNA segment 6. Although an open reading frame in RNA segment 6 of influenza C/JJ/50 virus could potentially code for a protein of 374 amino acids, a splicing event results in an mRNA coding for a 242-amino-acid M protein. The message for this protein represents the major M gene-specific mRNA species in C virus-infected cells. Despite the difference in coding strategies, there are sequence homologies among the M proteins of influenza A, B, and C viruses which confirm the evolutionary relationship of the three influenza virus types.

Isolation of the influenza C virus glycoprotein in a soluble form by bromelain digestion

The spike glycoprotein of influenza C/Johannesburg/1/66 was isolated in a soluble form by digestion of MDCK cell-grown virions with bromelain. The whole ectodomain of the glycoprotein could be recovered with an apparent molecular weight of 75,000 daltons determined in SDS-PAGE. Comparison to Triton X-100-isolated glycoprotein revealed that a C-terminal peptide of 3000-4500 daltons must have remained in the viral membrane. When purified by sucrose density gradient centrifugation the glycoprotein sedimented with a sedimentation coefficient of 10 S, indicating a molecular weight of 206,000 daltons, which is consistent with a trimeric structure of the spike molecule. The trimeric form was stabilized in sucrose gradients by Ca2+ ions. Bromelain digestion of virions with uncleaved glycoprotein, grown in MDCK cells without trypsin, produced two disulphide-linked subunits with similar electrophoretic mobilities in SDS-PAGE to the biologically active glycoprotein. The smaller subunit differed from the product cleaved in vivo (gp 30) by the presence of an additional arginine residue at the N-terminus. The soluble glycoprotein appears to possess both receptor-binding and receptor-destroying enzyme activities, as isolated glycoprotein inhibited hemagglutination of intact influenza C virions and showed RDE activity in an in vitro test. Glycoprotein exposed to low pH, which was sensitive to trypsin digestion, also demonstrated both these biological activities. Glycoprotein-mediated hemolysis could not be observed.

Serine 71 of the glycoprotein HEF is located at the active site of the acetylesterase of influenza C virus

The acetylesterase of influenza C virus has been reported recently to be inhibited by diisopropylfluorophosphate (DFP) [Muchmore EA, Varki A (1987) Science 236: 1293-1295]. As this inhibitor is known to bind covalently to the serine in the active site of serine esterases, we attempted to determine the serine in the active site of the influenza C acetylesterase. Incubation of purified influenza C virus with 3H-DFP resulted in the selective labelling of the influenza C glycoprotein HEF. The labelled glycoprotein was isolated from a SDS-polyacrylamide gel. Following reduction and carboxymethylation, tryptic peptides of HEF were prepared and analyzed by reversed phase HPLC. The peptide containing the 3H-DFP was subjected to sequence analysis. The amino acids determined from the NH2-terminus were used to locate the peptide on the HEF polypeptide. Radiosequencing revealed that 3H-DFP is attached to amino acid 17 of the tryptic peptide. These results indicate that serine 71 is the active-site serine of the acetylesterase of influenza C virus.

The influenza C virus glycoprotein (HE) exhibits receptor-binding (hemagglutinin) and receptor-destroying (esterase) activities

A cDNA copy of RNA segment 4 of influenza C/Cal/78 virus was cloned into an SV40 vector and expressed in CV-1 cells. The gene product expressed from the SV40 recombinant virus was immunoprecipitated by monoclonal antibodies directed against the influenza C virus glycoprotein. Cells infected with the recombinant virus also exhibited C virus-specific hemagglutinin and O-acetylesterase activity. This suggests that the same C virus protein is associated with receptor-binding as well as receptor-destroying activity. The latter viral activity was measured using as substrates bovine submaxillary mucin or a low molecular weight compound p-nitrophenylacetate. In analogy to the parainfluenza virus HN protein, the influenza C virus glycoprotein was termed HE, because it possesses hemagglutinin and esterase (receptor-destroying) activity.

Complete nucleotide sequence of the tick-borne, orthomyxo-like Dhori/Indian/1313/61 virus nucleoprotein gene

The complete nucleotide sequence of the fifth largest segment of single-stranded RNA of the tick-borne, orthomyxo-like Dhori/Ind/1313/61 virus was determined by using cloned cDNA derived from infected cell mRNA and dideoxynucleotide sequencing of viral RNA. The fifth RNA contains 1479 nucleotides and can code for a protein of 477 amino acids with a molecular weight of 53,679 Da. The RNA 5 protein of the Dhori/Ind/1313/61 virus possesses five short regions (16-26 amino acids) which share a high degree (50-59%) of amino acid sequence homology with a computer-aligned consensus sequence of the influenza nucleoprotein gene family. These and other structural features of the RNA 5 protein suggest that RNA 5 of Dhori viruses codes for the nucleoprotein. The data also suggest that Dhori viruses are orthomyxoviruses, but that they are more distantly related to the influenza viruses than type A, B, and C viruses are to each other.

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 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.

The functions of oligosaccharide chains associated with influenza C viral glycoproteins. II. The role of carbohydrates in the antigenic properties of influenza C viral glycoproteins

The antigenic properties of influenza C viral glycoprotein gp88 were compared with those of its nonglycosylated counterpart T76 synthesized in infected cells treated with tunicamycin. Radioimmunoprecipitation experiments with three different monoclonal antibodies against gp88 revealed that an antibody designated Q-5 precipitated gp88 but not T76, indicating the requirement for glycosylation for the binding of this antibody to gp88. It is unlikely, however, that the antigenic determinant recognized by Q-5 is carbohydrate moiety since the ability of the antibody to bind to gp88 varied depending on the virus strain, and trypsin-treatment of gp88 eliminated its reactivity with Q-5. Gel electrophoretic analysis under nonreducing conditions showed that T76 underwent the formation of disulfide-linked multimers in the absence of reducing agent while gp88 behaved as monomers, suggesting that glycosylation is required for gp88 molecules to attain an appropriate conformation. These observations, altogether, suggests that glycosylation is important in determining the immunological specificity of gp88 presumably by influencing the folding of this glycoprotein.

Structure and variation of the influenza C glycoprotein

The amino acid sequence of the influenza C/JHB/1/66 glycoprotein was predicted from sequence analysis of cloned DNA. The glycoprotein exhibited several similarities to the haemagglutinin (HA) glycoproteins of influenza A and B viruses, although its overall homology to these glycoproteins was low. A comparison between the sequences of C/JHB/1/66 and C/Cal/78 strains revealed 96% homology. The nucleotide and amino acid changes appeared to be non-random, with a high proportion occurring between amino acid positions 182-212. A comparison between the JHB/1/66 glycoprotein and the influenza A/X31 HA sequence suggested that this region may be structurally analogous to the 'A' antigenic site of the HA glycoprotein.

Rat alpha 1 macroglobulin inhibits hemagglutination by influenza C virus

Purified alpha 1-macroglobulin (RMG) isolated from rat plasma was found to be a potent inhibitor of hemagglutination by influenza C virus. Neuraminidase treatment of purified RMG reduced its inhibitory activity by more than 80% indicating that sialic acid is required for maximal HI-activity. The inhibitory activity of RMG was shown to be sensitive to the receptor-destroying activity (RDA) of influenza C virus. Methylation analysis of the glycopeptides of RMG indicated the presence of only one major type of oligosaccharide which is a complex N-linked oligosaccharide with a biantennary structure. Comparison of the glycopeptides before and after neuraminidase treatment revealed that the oligosaccharides are terminated by sialic acid residues attached to galactose residues at position C-6. Methylation analysis was also performed on RMG which had lost its inhibitory activity upon incubation with RDA of influenza C virus. No difference between the glycopeptides of native and inactive RMG could be detected. Galactose was found to be substituted at position C-6 in both samples, indicating that also the oligosaccharides of inactive RMG are terminated by sialic acid. The implications of these results are discussed.

An assay for the receptor-destroying activity of influenza C virus

We have developed a convenient method for assaying the receptor-destroying enzyme (RDE) activity of influenza C virus. This method measures the ability of the RDE to destroy the hemagglutination-inhibition activity of a potent inhibitor present in rat serum. Some physico-chemical properties of the RDE of influenza C virus were investigated by using this method. The temperature optimum for maximal activity of this enzyme was found to be 45 C to 53 C. There was little difference in thermostability between the RDE and hemagglutinating activities of influenza C virus. When influenza C virions were treated with various concentrations of trypsin, the RDE activity decreased in parallel with the decrease in the amount of residual gp88 glycoprotein, suggesting association of RDE with this glycoprotein.

Complete nucleotide sequence of the influenza C/California/78 virus nucleoprotein gene

The complete nucleotide sequence of RNA segment 5 of the influenza C/California/78 (C/Cal/78) virus was determined by using cloned cDNA derived from viral RNA. The gene contains 1809 nucleotides and can code for a protein of 565 amino acids with a molecular weight of 63 525. The RNA 5 protein of the influenza C/Cal/78 virus possesses two short regions which share a high degree (60-83%) of sequence homology with the nucleoproteins of influenza A and B viruses. These and other structural features of the RNA 5 protein suggest that RNA 5 of influenza C viruses codes for the nucleoprotein. The data also suggest that influenza C viruses are orthomyxoviruses, but that they are more distantly related to either type A or type B viruses than are influenza A and B viruses to each other.

Effect of urea and uric acid on influenza virus type C. Brief report

The replication of influenza type C/1233 virus following inoculation into the amniotic sac of embryonated hen's eggs is significantly greater than that seen in the allantoic cavity. However, optimal growth condition for amniotically propagated C/1233 occur in the allantois of 8-day old embryos incubated post-inoculation at 32 degrees C. Differences in pH levels between the amniotic and allantoic cavities did not account for the variable growth ability of C/1233 virus, but in both in vitro and in ovo studies, levels of urea and uric acid, or urea-uric acid mixtures caused significant reductions in virus yield.

3'-Terminal sequences of influenza C virion RNA. Brief report

The 3'-terminal nucleotides of the genome segments of influenza C/Taylor/47. C/Bavaria/79 and C/Johannesburg/1/66 were identified by two RNA sequencing techniques. These comprised 11 nucleotides (3' UCGUCUUCGUC) which were found to be conserved among the genome segments of each virus.

Low resolution structure of the influenza C glycoprotein determined by electron microscopy

The influenza C glycoprotein is clearly seen to be a trimer in specimens prepared with uranyl stains. Three-dimensional reconstructions from naturally occurring hexagonal arrays show that at low resolution (approximately 30 A) the influenza C glycoprotein exhibits similar features to the haemagglutinin glycoprotein of influenza A. Both have a triangular stalk near the membrane. Further from the membrane, the stalk becomes broader and the monomers more separated, leaving an open centre. The molecule narrows at the top. The regions of greatest contact between adjacent trimers in the arrays are situated nearer the distal end of the molecule. These contact zones can be related to equivalent zones on the influenza A haemagglutinin. Differences between the structure of the influenza A haemagglutinin glycoprotein determined by X-ray analysis and reconstructions of the influenza C glycoprotein are greatest at either end of the molecule, where the reconstructions are least reliable. Ordered glycoprotein arrays have not been observed on influenza C virions incubated at low pH. The staining patterns of glycoproteins on intact virions are essentially determined by the pH at which the virus is incubated, and the stain type, but not the pH of the stain.

Structure of the influenza C glycoprotein gene as determined from cloned DNA

The complete nucleotide sequence of RNA segment 4 of influenza C/JHB/1/66 virus has been determined, utilizing cloned cDNA derived from the viral RNA segment. The gene is 2073 nucleotides in length, and can code for a polypeptide of 655 amino acids, which corresponds to the viral glycoprotein. The predicted polypeptide has a molecular weight of 72 063, not including oligosaccharides linked to eight predicted glycosylation sites. The influenza C glycoprotein shares structural features with hemagglutinin (HA) glycoproteins of influenza A and B viruses, including three stretches of hydrophobic amino acids believed to function as a signal sequence, a fusion function, and a membrane anchor. However, a substantial part of the protein lacks direct sequence homology to these HA glycoproteins. The results suggest a more distant evolutionary relationship between influenza C virus and influenza A and B viruses, compared to that between influenza A and B themselves.

The synthesis of polypeptides in influenza C virus-infected cells

The synthesis of virus-specific polypeptides was analyzed in MDCK cells infected with the JJ/50 strain of influenza C virus. In addition to three major structural proteins gp88, NP, and M, the synthesis of five polypeptides with molecular weights of 29,500 (C1), 27,500 (C2), 24,000 (C3), 19,000 (C4), and 14,000 (C5) was found in infected cells. None of these polypeptides were detected either in virions or in immunoprecipitates obtained after treatment of infected cell lysates with antiviral serum, suggesting that they are not viral structural proteins. Polypeptides C1-C5 were found to be synthesized in MDCK cells infected with different influenza C virus strains as well as in different host cell types infected with C/JJ/50. Further, it was observed that cellular protein synthesis was greatly reduced under hypertonic conditions, whereas the synthesis of C1-C5 was relatively unaffected. These results suggest that polypeptides C1-C5 are virus coded rather than host cell coded. Peptide mapping studies showed that each of polypeptides C3, C4, and C5 had a peptide composition similar to the M protein. The amount of C2 synthesized in infected cells was insufficient for mapping. This polypeptide was, however, found to rapidly disappear in pulse-chase experiments, suggesting that C2 is probably not unique but biosynthetically related to one of the other proteins. In contrast to these polypeptides, polypeptide C1 showed a map which is largely different from any major structural polypeptide. It therefore appears likely that C1 is a nonstructural protein of influenza C virus similar to the NS1 protein of influenza A and B viruses.

Posttranslational modification and intracellular transport of mumps virus glycoproteins

Analysis of the pronase-derived glycopeptides of isolated mumps virus glycoproteins revealed the presence of both complex and high-mannose-type oligosaccharides on the HN and F1 glycoproteins, whereas only high-mannose-type glycopeptides were detected on F2. Endoglycosidase F, a newly described glycosidase that cleaves N-linked high mannose as well as complex oligosaccharides, appeared to completely cleave the oligosaccharides linked to HN and F2, whereas F1 was resistant to the enzyme. Two distinct cleavage products of F2 were observed, suggesting the presence of two oligosaccharide side chains. Tunicamycin was found to reduce the infectious virus yield and inhibit mumps virus particle formation. The two glycoproteins, HN and F, were not found in the presence of the glycosylation inhibitor. However, two new polypeptides were detected, with molecular weights of 63,000 (HNT) and 53,000 (FT), respectively, which may represent nonglycosylated forms of the glycoproteins. Synthesis of the nonglycosylated virus-coded proteins (L, NP, P, M, pI, and pII) was not affected by tunicamycin. The formation of HN oligomers and the proteolytic cleavage of the F protein were found to occur with the same kinetics. Analysis of the time course of appearance of mumps virus glycoproteins on the cell surface suggested that dimerization of HN and cleavage of F occur immediately after their exposure on the plasma membrane.

Evidence of a soluble substrate for the receptor-destroying enzyme of influenza C virus

Influenza C virus contains a hemagglutinin and a receptor-destroying enzyme (RDE) whose specificities remain undetermined. In rat serum, there is a molecule that binds specifically to C virus, inhibiting its hemagglutinin. The complex between C virus and the rat serum inhibitor (RSI) was determined to be stable at 4 degrees C, but was disrupted within 20 to 90 min at 23 or 37 degrees C. Virus emerged from the complex with numerous functions intact, whereas the RSI at this point was inactivated, i.e., incapable of further inhibitory reactions with C virus. RSI could not be inactivated at these temperatures by nonviral components of allantoic fluid of infected chicken embryos; however, RSI inactivation was achieved by preparations of sucrose gradient-purified virus. Neutralization of viral hemagglutination activity with antiviral antibody protected the RSI from inactivation. RSI inactivation occurred at temperatures at which the viral RDE was active, and inhibition of viral RDE by periodate treatment sharply reduced the ability of virus to inactivate the RSI. One interpretation of the data suggests that RSI is a receptor analog reactive with both the hemagglutinin and RDE of C virus and that RSI inactivation is an assay of influenza C viral RDE.

Demonstration of hemolytic and fusion activities of influenza C virus

Influenza C virus showed a marked hemolytic activity when incubated with murine erythrocytes at 37 degrees C in acidic medium. The virus-specific hemolysis was most efficient at pH 5.0. Extensive cell fusion also occurred when the erythrocytes were treated with the virus at acidic pH. When propagated in MDCK cells, the virus had an extremely low infectivity and did not display hemolytic activity in any pH range. When the inactive virus was subjected to mild trypsin treatment, hemolytic activity was drastically manifested, accompanying a drastic increase in infectivity. The glycoprotein in the inactive virus was cleaved into smaller components by trypsin treatments. These results indicated that the envelope of influenza C virus can fuse with the cellular membrane under acidic conditions and that the activation of influenza C virus by cleavage was due to the appearance of this envelope fusion activity.

Oligonucleotide fingerprint analyses of influenza C virion RNA recovered from five different isolates

Five different isolates of influenza C virus which were isolated over a period of 32 years and from four different continents were compared by RNA genome oligonucleotide fingerprinting analyses. The earliest isolate of influenza C virus was reported in 1949 by Taylor (19) and served as a reference strain for this study. The results obtained using this technique of comparing relatedness between viruses clearly showed that all strains are distinct. However, the similarities in the pattern of the oligonucleotide fingerprints are marked for the more recent virus isolates (1966-1979), whereas the reference strain C/Taylor shows more pronounced differences. The results are consistent with the high degree of serological crossreaction amongst influenza C viruses isolated over a long period of time, a property which sets this group of viruses apart from type A and B members of the orthomyxoviridae.