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Short Communication |
1 Virology Unit, Department of Public Health, University of Florence, Viale Morgagni 48, I-50134 Firenze, Italy
2 Department of Infectious, Parasitic and Immune-Mediated Diseases, Istituto Superiore Sanità, Rome, Italy
3 Istituto Nazionale per la Fauna Selvatica, Ozzano Emilia, Bologna, Italy
Correspondence
Simone Giannecchini
simone.giannecchini{at}unifi.it
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ABSTRACT |
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These authors contributed equally to this work. 
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MAIN TEXT |
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; Lipatov et al., 2004). This ability is likely to depend on the functional integrity and optimal combination of each genome constellation (Horimoto & Kawaoka, 2001). In this context, a balance in haemagglutinin (HA) and neuraminidase (NA) viral glycoprotein activities plays a role and may also be linked to the pathogenicity of avian influenza viruses (Mitnaul et al., 2000; Hulse et al., 2004). In spite of considerable recent advances on the structural and functional features involved in virus–cell interaction during cell entry (Skehel & Wiley, 2000; Huang et al., 2003; Russell et al., 2004), the determinants of avian influenza virus interspecies transmission and emergence of potentially pandemic influenza viruses remains poorly understood.
In the presence of limited indications of influenza virus transmission to humans directly from the wild bird reservoir (Kurtz et al., 1996), it is hypothesized that terrestrial poultry (chickens, quail, etc.) could act as an intermediate host where virus from wild waterfowl may acquire mutations that render it more able to transmit to humans (Perez et al., 2003). Recently, a very close relationship has been described between H7N3 viruses isolated from wild ducks in 2001 and H7N3 viruses circulating since the autumn 2002 in turkeys and chickens in a large number of poultry farms in Northern Italy (Capua et al., 2002a, b). Circumstantial evidence suggested direct ‘in toto’ derivation of the low pathogenicity H7N3 turkey viruses from the avian influenza strains circulating in wild waterfowl 1 year earlier. Moreover, serological evidence in humans indicated that the same poultry strains were able to cause infection in poultry workers during the 2002–2003 avian epidemics (Puzelli et al., 2005). Sequence comparison of HA and NA genes of viruses isolated in embryonated fowl's eggs had shown only 2 aa differences at positions 261 (R
S) in the HA1, corresponding to position 271 on the H3 molecule, and 161 (KR) in the HA2, and few amino acid differences as well as a 23 aa deletion in the NA gene, between the duck and poultry viruses, respectively (Campitelli et al., 2004). Therefore, it seemed important to compare the two groups of duck and turkey viruses, each group with identical HA and NA sequences, with regards to their receptor binding and NA activities, as well as virus replication, with specific attention to virus–cell fusion.
Here, the receptor-binding properties of these H7N3 duck and turkey viruses were first investigated, even though neither of the two HA amino acid changes were located in the receptor-binding region (Campitelli et al., 2004). Before testing, confirmation of amino acid differences previously described and absence of additional mutations were assessed for all viral samples used. The two H7N3 duck viruses (A/Mallard/IT/33/01 and A/Mallard/IT/43/01) and the two H7N3 turkey viruses (A/Turkey/IT/214845/02 and A/Turkey/IT/220158/02), grown in 10-day-old embryonated fowl's eggs, were titrated on Madin–Darby canine kidney (MDCK) cells by observing cytopathic effect at 3 days after infection, and 100 TCID50 of allantoic fluid was analysed in haemagglutination tests using human and chicken red blood cells (RBC). All four viruses had similar HA activity with both types of substrates, and titres ranged from 128 to 256. Furthermore, when viruses were normalized by HA unit and an ELISA assay was carried out using fetuin, which possesses features resembling influenza virus receptor-analogues (Gambaryan & Matrosovich, 1992), no differences were observed (data not shown). Together, these results showed that the two groups of viruses had similar receptor-binding properties under the experimental conditions used.
Of the two HA amino acid differences observed between the duck and turkey viruses, the one at position 271 located within the ‘hinge region’ on the HA1 stalk, is in close contact with aa 90 and 91 in the globular head and also to residue 284 in the HA stalk (H3 numbering is used throughout). Because the substitution RS changes both the charge and size of aa 271 it could affect atomic interactions between the HA head and stalk in this region during fusion activation within the cellular endosome. To investigate the effect of endosomal acidification during virus replication in MDCK cells, bafilomycin A (Alexis Biochemicals), a selective inhibitor of the vacuolar-type proton-ATPase, was used. Here, the replication ability of the two groups of viruses was tested in MDCK cells in the presence of different concentrations of bafilomycin A (Fig. 1). Fifty TCID50 of each virus was inoculated in quadruplicate onto the wells of 96-well flat-bottom plates containing MDCK cells with or without bafilomycin A in modified Eagle's medium. After 2 h at 37 °C, the inocula were removed and replaced with fresh medium supplemented with TPCK (L-1-tosylamide-2-phenylethyl chloromethyl ketone) treated trypsin (2 µg ml–1; Sigma) in the presence or absence of the inhibitor. Infected cells were analysed on day 4, when HA production in control wells inoculated with virus alone was evident by the ELISA fetuin-binding assay (Gambaryan & Matrosovich, 1992). As shown in Fig. 1(a), at increasing concentrations of bafilomycin A the growth of all four viruses was profoundly impaired compared with that of the control, confirming previous observations (Guinea & Carrasco, 1995). However, duck viruses required a concentration of bafilomycin A 10-fold higher than that necessary for the turkey viruses to achieve the same level of inhibition. Bafilomycin A had no detectable effects on MDCK cell viability up to 500 nM concentration under the experimental conditions used (data not shown). Subsequently, experiments performed in the same manner as described above, but maintaining the antibiotic only during the 2 h of the virus cell adsorption, were carried out to restrict the effects of bafilomycin A to the early steps of virus replication. Experiments with the initial incubation time of 2 h at 4 °C, a temperature that allows virus cell adsorption but not entry, were also performed to investigate the inhibitory effect on binding to or internalization into cells. When treatment of cells with bafilomycin A was carried out at 4 °C during the initial infection of the cells, virus growth of both turkey and duck viruses was only partially affected by bafilomycin A as measured following 4 days of incubation at 37 °C. However, turkey viruses showed a significant, temperature related, increased inhibition when incubation with bafilomycin A was carried out at 37 °C, whereas the duck viruses displayed no difference in their virus replication whether the bafilomycin A treatment of cells was at 37 or 4 °C (Fig. 1b and c). Overall, these results indicate that the two groups of H7N3 viruses differ in the replication properties restricted to the early steps of virus entry into a cell immediately after cell adsorption.
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; Skehel & Wiley, 2000). Since no amino acid differences were observed in the M2 trans-membrane region (the ion-channel domain) of the duck and turkey H7N3 viruses (L. Campitelli, unpublished data), we investigated whether the differences in the susceptibility of H7N3 duck and turkey viruses to the bafilomycin A inhibitor were related to their HA fusion properties. A fusion assay, based on fluorescence dequenching of octadecyl rhodamine (R18)-labelled (Molecular Probes) purified virus was performed as described using MDCK cells (Takeda et al., 2003; Chu & Whittaker, 2004). One hundred microlitres of labelled gradient-purified virus (100 µg ml–1) was added to 2x106 MDCK cells at 4 °C for 1 h in binding buffer constituted of PBS with 0·2 % BSA. Unbound virus was removed by washing with PBS, and cells were resuspended in binding buffer with 5 mM HEPES pH 7·0. Fusion of virus with the cell membrane was triggered by adding a predetermined amount of 250 mM HCl to obtain a final pH of 6·0 and 5·0, and then incubated for 10 min at 37 °C. Then, after washing with PBS, cells were resuspended in PBS and fluorescence dequenching was measured by using a FACScan flow cytometer (Becton Dickinson). Fig. 2 shows that the two groups of viruses exerted a similar fusion activity at pH 5·0, i.e. the experimental conditions that correspond to the physiological environment in which virus fusion is presumed to occur inside the endosome. However, when the experiments were performed at a higher pH, pH 6·0, the two H7N3 duck viruses maintained a significant fusion activity, whereas the turkey viruses did not. In particular, cell fusion activity of duck viruses at pH 6·0 was fivefold higher than that exerted by turkey viruses. As expected, none of the four viruses had significant fusion activity at pH 7·0.
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indicate that turkey viruses were not able to elute from either type of RBC during an overnight incubation at 37 °C, whereas duck viruses eluted completely within 1 h. When the elution experiments were performed entirely at 4 °C, a temperature at which NA activity is blocked, none of the viruses were able to elute, even after an overnight incubation. Conversely, when the experiments were performed at 37 °C in the presence of receptor-destroying enzyme (Sigma), all viruses eluted, beginning from 30 min incubation (data not shown). Furthermore, elution of the duck viruses was inhibited during incubation at 37 °C in presence of Zanamivir (Table 1), thus demonstrating that the observed effect was specifically determined by the NA activity. Finally, because the type of cell substrates on which influenza virus is grown may affect some molecular characteristics of the virus, a similar experiment was conducted using virus passed on MDCK cells. As shown in Table 1, impairment of NA activity on turkey virus did not depend on the cells in which the virus was propagated.
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; Doms et al., 1986; Grambas & Hay, 1992). The reason for selection of HA mutants that mediate membrane fusion at elevated pH has not been established yet, but it is hypothesized that may be a consequence of differences in endosomal pH within different cell types (Lin et al., 1997). On the other hand, very little is known about the appropriate pH requirements of influenza viruses within different tissues of birds such as ducks and turkeys. Tissue tropism of influenza viruses in such hosts can be quite different, since duck viruses grow typically on the intestine epithelium, whereas in turkeys the main target organ is the respiratory apparatus, although virus is also excreted in faeces (Swayne et al., 1992). Whether the in vitro characteristics we observed could be related to different tropism requirements implying host adaptation is not known. It should be noted that, although the amino acid R and S found at position 271 is unique within the entire H7 HA database, this position in duck viruses is often occupied by polar charge residues compared with aliphatic uncharged residues usually found in turkey viruses.
The H7N3 turkey viruses showed a reduced ability to release virions from the cell substrate, in keeping with the presence of a short-stalked NA. An NA stalk deletion appears to be an early phenomenon of adaptation of duck viruses to domestic poultry, but the biological significance of this is unclear (Castrucci & Kawaoka, 1993; Wagner et al., 2000; Banks et al., 2001). In fact, this feature is often associated to changes in HA protein that in turn reduce HA receptor affinity to counterbalance the reduced NA activity (Baigent & McCauley, 2001). In our study, the absence of additional glycosylation sites as well as other amino acid changes on the HA globular head of the turkey viruses, isolated not only at the time of the presumed initial virus introduction into poultry but also 2, 5, 6 and 7 months later (L. Campitelli, unpublished data), suggest the existence of an alternative mechanism of compensation. Proving this latter aspect, as well as the involvement of amino acid positions 271 of the HA1 and 161 of HA2 in the fusion process, will require development of HA and NA reassortant and mutagenized viruses.
Also in the light of recent serological evidence of human infection with the low pathogenicity H7N3 turkey viruses in Italy (Puzelli et al., 2005), expanding our understanding of the molecular factors that influence replication properties of duck and turkey viruses in different avian species could provide a better insight into some aspects involved in the interspecies transmission.
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ACKNOWLEDGEMENTS |
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Received 17 May 2005;
accepted 27 September 2005.
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