Department of Microbiology and Immunology, University of Melbourne, Victoria 3010, Australia
Correspondence
Margot Anders
emanders{at}unimelb.edu.au
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Many cell types can produce IFN-/
in response to virus infection. Double-stranded RNA (dsRNA) synthesized during the virus replicative cycle (Jacobs & Langland, 1996
) is considered a likely trigger, since synthetic dsRNA, e.g. polyinosinicpolycytidylic acid [poly(I)·poly(C)] is known to be a potent inducer of type 1 IFN (De Clercq, 1981
). A second pathway of IFN-
/
induction that is independent of virus replication or gene expression is seen in the response of certain cells of haemopoietic origin to enveloped viruses. These so-called natural IFN-producing cells (IPCs), found in human and porcine peripheral blood and mouse spleen, produce type 1 IFN in response to physically and chemically inactivated virus, to fixed virus-infected cells, or to cells transfected with particular viral glycoproteins (Fitzgerald-Bocarsly, 1993
; Ito, 1994
). This IFN-inducing activity has been described for a range of enveloped viruses including herpes-, lenti-, rhabdo-, corona-, orthomyxo- and paramyxoviruses and appears to result from a direct interaction of viral glycoproteins with the surface of the IPC (Charley & Laude, 1988
; Fitzgerald-Bocarsly, 1993
; Feldman et al., 1994
). The principal IPC in human blood has been identified as a particular subset of immature DCs, the type 2 DC precursor or plasmacytoid DC (Cella et al., 1999
; Siegal et al., 1999
). Recently, several groups have reported the isolation of a CD11c+ B220+ Gr-1+ cell population from mouse spleen that produces large amounts of IFN-
on stimulation with live (Nakano et al., 2001
) or inactivated (Asselin-Paturel et al., 2001
) influenza virus or with herpes simplex virus (HSV) (Bjorck, 2001
) and appears to be the murine counterpart of the human plasmacytoid DC. The response of this cell to inactivated virus and its failure to respond to poly(I)·poly(C) (O'Keeffe et al., 2002
) is consistent with previous studies indicating that the response to virus is mediated by the viral glycoproteins, rather than by dsRNA.
In examining the response of murine spleen cells to inactivated influenza A virus, we observed a markedly lower response to certain virus strains. In the present study, we have attempted to explore further the nature of the viruscell interaction between influenza virus and the murine IPC and the basis for the difference between virus strains in their IFN-/
-inducing ability.
![]() |
Methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Influenza viruses were propagated in the allantoic cavity of 10-day-old embryonated hens' eggs (Anders et al., 1990). For some experiments, viruses were also grown in MadinDarby canine kidney (MDCK) cells or Vero cells in the presence of trypsin (Kaverin & Webster, 1995
). Mouse-lung-grown stocks of BJx109 influenza virus were prepared as described previously (Reading et al., 1997
). Infectivity titres were determined by plaque assay on MDCK cells (Anders et al., 1994
). For UV-inactivation of virus, virus was placed in a 60 mm Petri dish and irradiated for 30 min at a distance of 15 cm from a 20 W germicidal lamp.
-Propiolactone-inactivated Guangdong/93 virus (BPL Guangdong/93) was provided by Michael Hocart (Influenza Process Development, CSL Ltd, Melbourne, Australia).
For treatment of virus with periodate, one vol. of virus in allantoic fluid was incubated with 3 vols 0·011 M KIO4 for 15 min at room temperature; the KIO4 was then inactivated by the addition of 6 vols 0·22 % (w/v) glycerol in water. For mock treatment, the periodate and glycerol were mixed 15 min before addition of virus.
Media and reagents.
Serum-free (SF) medium was RPMI 1640 (Gibco BRL) supplemented with 2 mM glutamine, 2 mM pyruvate, 30 µg gentamicin ml1, 100 µg streptomycin ml1 and 100 IU penicillin ml1. For culture medium this was supplemented with 5 or 10 % heat-inactivated (56 °C, 30 min) foetal calf serum (FCS; Hyclone) and was referred to as RF5 or RF10, respectively.
Zanamivir (4-guanidino-2,3-dehydro-N-acetylneuraminic acid) was purchased from Glaxo Wellcome Australia. Since this product contains lactose (3·7 mol per mol of zanamivir), the control for zanamivir used in culture was medium containing the appropriate concentration of lactose alone.
Mice.
Inbred BALB/c mice of either sex were used at 68 weeks of age.
Cells.
Spleen cell suspensions were prepared by gently pressing spleen fragments through a stainless steel sieve. The cells were treated with Tris/NH4Cl (0·14 M NH4Cl in 17 mM Tris, pH 7·2) to lyse erythrocytes, and then washed twice in SF medium.
Resident peritoneal macrophages were obtained by lavage of the peritoneal cavity as described previously (Reading et al., 2000) but using RPMI-based medium. Cells (1x106 in 1 ml) were seeded into the wells of 24-well tissue culture plates (Nunc), and after 4 h incubation, non-adherent cells were removed by washing and the adherent macrophages were incubated overnight in RF10.
To prepare macrophages in suspension, 4x107 peritoneal cells in 10 ml RF10 were seeded into 80 cm2 tissue culture flasks (Nunc), incubated for 4 h at 37 °C and non-adherent cells were removed by washing. The adherent cells were eluted from the plastic by incubation with Hanks' balanced salts solution containing 5 mM EDTA for 30 min on ice and washed in RF5 before use.
Induction of IFN by live or inactivated influenza virus.
Spleen cells (1x107) were incubated for 1 h in 0·5 ml SF medium containing either 1x106 to 1x107 p.f.u. influenza virus, the equivalent amount of UV-inactivated virus, or 1001000 haemagglutinating units (HAU) BPL-inactivated virus, or in SF medium alone. After centrifugation and removal of virus, the cells were washed with SF medium, resuspended in 1 ml RF5 and incubated in the wells of a 24-well culture plate at 37 °C overnight. Peritoneal macrophage monolayers in 24-well plates were treated with virus and cultured overnight in a similar manner. Culture fluids were collected and clarified by centrifugation, then absorbed with 5 % (v/v) packed chicken red blood cells for 20 min at 4 °C to remove residual virus. Following centrifugation, the supernatants were removed and stored at 20 °C until assayed for IFN activity.
The effect of zanamivir on IFN induction was tested by pre-incubating influenza virus with 50 µM zanamivir (containing 187 µM lactose), or 187 µM lactose alone, in SF medium for 15 min before addition to spleen cells. After 1 h incubation at 37 °C, the virus was removed and culture of the spleen cells was continued in medium containing the appropriate concentration of zanamivir or lactose.
Induction of IFN by fixed, virus-infected cells.
Confluent monolayers of MDCK cells in 24-well tissue culture plates were infected for 1 h with Guangdong/93 virus at an m.o.i. of 5. Virus was then removed and the cells were incubated for a further 7 h in SF medium. After washing with SF medium, infected and control cell monolayers were fixed with glutaraldehyde (0·025 % in PBS, pH 7·4) for 10 min at room temperature. The monolayers were washed three times with PBS, and 1x107 spleen cells or 3x106 peritoneal macrophages were added in 1 ml RF5. After overnight incubation at 37 °C, culture supernatants were collected and assayed for IFN activity.
IFN bioassay.
IFN was assayed by inhibition of the cytopathic effect (CPE) of Semliki Forest virus (SFV) on murine L929 cells. L929 cell monolayers in 96-well culture plates were incubated with serial twofold dilutions of samples overnight and then challenged with 2·5x104 TCID50 SFV. Controls included L929 cells exposed to SF medium throughout (cell control) or SF medium followed by SFV (virus control). After 2 days, the degree of CPE in individual wells was determined by a colorimetric assay based on that of Mosmann (1983): 10 µl 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT, 5 mg ml1 in PBS; Sigma M-2128) was added directly to each well, the plates were incubated at 37 °C for 8 h and the cells were subsequently solubilized by the addition of 100 µl 10 % (w/v) SDS, 0·1 % HCl in water. The following day, absorbance was read using a test wavelength of 540 nm and a reference wavelength of 690 nm, and the titre of IFN was expressed as the reciprocal of the sample dilution causing 50 % reduction in CPE, corresponding to an absorbance midway between that of cell and virus controls. The data were converted to International Units (IU) by comparison with a laboratory IFN-
/
standard that had been calibrated against the NIH muIFN-
standard Ga02-901-511.
Elution of virus from spleen cells.
Influenza virus (50100 HAU) was incubated with 5x107 spleen cells in 1 ml SF medium supplemented with either 50 µM zanamivir containing 187 µM lactose, or 187 µM lactose alone, on ice. After 20 min, a sample (140 µl) was taken, centrifuged at 10 000 g for 5 min, and the supernatant was taken and stored on ice. Following transfer of the cell mixtures to a 37 °C water bath, further samples were taken at various intervals and treated similarly. The haemagglutinating titre in each supernatant was determined by standard microassay and expressed as a percentage of the haemagglutinating titre of the original virus used.
Statistics.
Statistical analysis of results was performed using the two-tailed Student's t-test.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
To determine whether induction of IFN-/
required the sialic acid-binding function of the viral HA, the effect of horse serum on the response was examined. Horse serum contains a sialylated glycoprotein,
2-macroglobulin, that inhibits the infectivity and haemagglutinating activity of H2 and H3 subtype human influenza viruses by competing with sialylated cell receptors for binding to the receptor-binding pocket of HA (Rogers et al., 1983
). As shown in Fig. 2
, induction of IFN-
/
in spleen cells by Mem/71H-BelN (H3N1) and Victoria/75 (H3N2) viruses was inhibited in the presence of 2.5 % (v/v) horse serum. In contrast, IFN-
/
induction by receptor mutants of these two viruses that are resistant to the horse serum inhibitor (Mem/71H-BelN/HS and Victoria/75/HS; see Methods) was unaffected by horse serum, ruling out a non-specific inhibitory effect of the serum on the IPCs. These data indicate that binding of the viral HA to sialylated cell-surface molecules is a necessary step in the induction of IFN-
/
in splenic IPCs.
|
|
|
|
|
To compare the relative NA activity of PR/8/34 and NWS/33 viruses, virus was adsorbed to spleen cells at 0 °C and the rate of elution following transfer of the cells to 37 °C was measured by assaying haemagglutinating activity in the supernatant sampled at different times. After 1 min at 37 °C, PR/8/34 had completely eluted from splenocytes, whereas 75 % of NWS/33 remained bound to the cells after 30 min (Fig. 6A). As expected, inclusion of the NA inhibitor zanamivir blocked the elution of both viruses. The effect of zanamivir on the ability of PR/8/34 and NWS/33 to induce IFN-
/
from spleen cells was then examined. No effect was seen on the level of IFN induced by NWS/33, but induction by PR/8/34 was substantially increased by the inclusion of zanamivir, to a level similar to that induced by NWS/33 (Fig. 6B
).
|
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Given the established role of sialic acid as the primary receptor for influenza virus, the requirement for binding of the virus to sialic acid for IFN-/
induction was not unexpected. This point needed to be established, however, since IFN-
/
is induced by viruses such as HSV, human immunodeficiency virus type 1 (HIV-1) and vesicular stomatitis virus (Feldman et al., 1994
), which do not bind sialic acid. Furthermore, influenza virus may also interact with cells in other ways. Stray et al. (2000)
have recently reported influenza virus infection of MDCK cells by a sialic acid-independent mechanism that is not yet understood. Binding of enveloped viruses through their glycans to cell membrane C-type lectins also occurs, for example binding of Sendai virus to the asialoglycoprotein receptor on hepatocytes (Markwell et al., 1985
), HIV-1 to DC-SIGN on DCs (Geijtenbeek et al., 2000
) and influenza virus to the mannose receptor on macrophages (Reading et al., 2000
). For IFN-
/
induction, the requirement for sialic acid binding was indicated by two observations: (i) horse serum inhibited IFN-
/
induction by strains of influenza virus known to bind the sialylated
-inhibitor, equine
2-macroglobulin, but had no effect on the response to inhibitor-resistant strains of virus; and (ii) viruses that eluted rapidly from spleen cells due to a very active NA were poor inducers of IFN-
/
in this system but induction was restored in the presence of the NA inhibitor, zanamivir. The latter finding suggests that the signal for IFN-
/
induction is not generated immediately on binding of virus to the cell surface; rather, time at the surface may be required for the virus to encounter and cross-link several molecules of a critical receptor for a signal to be transmitted.
Preferential specificity of the HA molecule for sialic acid linked 2,6 or
2,3 to galactose did not appear to have a marked effect on efficiency of induction of IFN-
/
, since comparable levels of IFN-
/
were induced by the horse serum-sensitive (SA
2,6Gal-specific) and -resistant (SA
2,3Gal-specific) variants of both Mem/71H-BelN (H3N1) and Victoria/75 (H3N2) viruses. This result contrasts with our earlier studies of the mitogenic activity of H3 subtype influenza A viruses for murine B cells (Anders et al., 1986
). In that case, mitogenic activity was restricted to SA
2,6Gal-specific viruses and was proposed to result from interaction with a critical receptor bearing
2,6-linked sialic acid only. In the induction of IFN-
/
, binding to sialic acid may serve simply to concentrate the virus on the cell surface, facilitating its subsequent binding to a second, triggering receptor through a sialic acid-independent interaction. Alternatively, if binding to sialic acid on a specific triggering molecule is necessary, the molecule in question may bear sialic acid linkages of both types.
IFN-/
induction by influenza virus was substantially reduced following oxidation of viral carbohydrate with periodate, to an extent greatly exceeding the reduction in haemagglutinating activity. The efficiency of IFN-
/
induction was also found to be dependent on the host cell type in which the virus was grown, egg-grown virus being a stronger inducer than virus grown in MDCK cells or Vero cells. Here, the difference in IFN-
/
induction was not mediated through a difference in NA activity, since egg-grown and MDCK-grown BJx109 viruses, and periodate-treated and -untreated Guangdong/93 viruses, all displayed a very low rate of elution from spleen cells. Differences in the glycosylation of influenza viruses grown in different host cells are well documented and include differences in both size and composition of viral glycans (Nakamura & Compans, 1979
; Deom & Schulze, 1985
). Taken together, these findings suggested that the carbohydrate moieties decorating the viral glycoproteins might influence the triggering process in some way. Interestingly, an effect of viral glycosylation on IFN-
/
induction in porcine leukocytes by the coronavirus transmissible gastroenteritis virus has previously been reported (Charley et al., 1991
; Laude et al., 1992
). The particularly strong IFN-
/
response to egg-grown influenza virus may represent an effect of egg-derived glycans that is not physiologically relevant to natural infection, although it may be to vaccination. However, we also demonstrated a significant IFN-
/
response of spleen cells to a low dose of inactivated mouse-lung-grown influenza virus, confirming that stimulation also occurs with virus bearing glycans derived from the homologous host.
In one possible scenario for stimulation of the IPC by influenza virus, triggering may result from direct multivalent binding of HA, through sialic acid, to molecules of a particular critical receptor on the splenic IPC. It is known that changes in size or composition of viral glycans on the HA molecule that result from growth of the virus in different host cells can affect the receptor-binding properties of the virus (Crecelius et al., 1984; Deom & Schulze, 1985
; Gambaryan et al., 1998
). Such viruses might therefore differ in the repertoire of sialylated molecules on the IPC with which they interact. Differences in glycosylation may also affect the density of packing of the viral glycoproteins in the viral membrane (Rudd et al., 1999
) and hence the avidity of interaction with critical cell receptors.
Alternatively, triggering of the IPC may require interaction of the virus with a putative second receptor following binding to sialic acid. This second receptor, a pattern recognition receptor (PRR) (Medzhitov & Janeway, 2000), would recognize some molecular pattern displayed by the viral glycoproteins or envelope that is brought into close apposition with the membrane of the IPC through multivalent binding of virus to sialylated receptors. Host-derived differences in carbohydrate moieties on the viral glycoproteins might affect the conformation of the viral epitope in question, or differentially impede its access to the PRR through steric hindrance.
In either model, the observed inhibition of IFN-/
induction by mannan and laminarin may be mediated through delivery of a negative signal via a separate lectin-like receptor on the IPC. DC populations are known to express a number of different C-type lectins on their surface (Figdor et al., 2002
) and precedents for negative regulation of cytokine responses by saccharides or by antibodies to C-type lectins exist in the literature (Dzionek et al., 2001
; Nigou et al., 2001
). Alternatively, the PRR may itself be lectin-like and interact with the viral glycans to trigger the IFN-
/
response, viral glycans derived from different cell types binding with differing avidity depending on their composition. In the latter case, inhibition by mannan and laminarin could conceivably be mediated through competition with the viral glycans for binding to the PRR, although these saccharides themselves do not induce an IFN-
/
response.
The IPC in mouse spleen has recently been identified by others as the plasmacytoid DC precursor (Asselin-Paturel et al., 2001; Bjorck, 2001
; Nakano et al., 2001
). In a collaborative study with M. O'Keeffe, K. Shortman and others, we have confirmed responsiveness of purified splenic plasmacytoid pre-DCs to BPL-inactivated influenza virus and shown a lack of response to poly(I)·poly(C) (O'Keeffe et al., 2002
), consistent with previous studies indicating that the viral stimulus for the splenic IPC is provided by the viral glycoproteins rather than by dsRNA (Fitzgerald-Bocarsly, 1993
; Ito, 1994
). The IFN produced was a mixture of IFN-
and IFN-
, and the yield of IFN-
/
from purified plasmacytoid pre-DCs was sufficient to account for all of the IFN-
/
produced by mouse spleen cells in response to BPL-inactivated influenza virus (J. L. Miller & M. O'Keeffe, unpublished results), indicating that the plasmacytoid pre-DC is the major IFN-
/
producing cell in the spleen responding to this stimulus.
The ability to work with defined populations of murine plasmacytoid pre-DCs should facilitate further study of the triggering receptor(s) and possible inhibitory receptor(s) involved in the IFN-/
response to inactivated influenza virus and the existence of common or distinct pathways of stimulation by other enveloped viruses and microbial stimuli. The involvement of toll-like receptors (TLR) will be of particular interest, since this family of PRRs is known to play a critical role in the recognition of various microbial and viral components and signalling of innate immune responses (Takeda & Akira, 2001
). Two non-viral inducers of type 1 IFN in human plasmacytoid pre-DCs oligodeoxynucleotides containing unmethylated CpG motifs and small imidazoquinoline compounds have been shown to signal through TLR9 and TLR7, respectively (Hemmi et al., 2000
; Kadowaki et al., 2001
; Krug et al., 2001
; Hemmi et al., 2002
; Ito et al., 2002
).
![]() |
ACKNOWLEDGEMENTS |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Anders, E. M., Hartley, C. A. & Jackson, D. C. (1990). Bovine and mouse serum inhibitors of influenza A viruses are mannose-binding lectins. Proc Natl Acad Sci U S A 87, 44854489.[Abstract]
Anders, E. M., Hartley, C. A., Reading, P. C. & Ezekowitz, R. A. (1994). Complement-dependent neutralization of influenza virus by a serum mannose-binding lectin. J Gen Virol 75, 615622.[Abstract]
Asselin-Paturel, C., Boonstra, A., Dalod, M. & 8 other authors (2001). Mouse type I IFN-producing cells are immature APCs with plasmacytoid morphology. Nature Immunol 2, 11441150.[CrossRef][Medline]
Biron, C. A., Nguyen, K. B., Pien, G. C., Cousens, L. P. & Salazar-Mather, T. P. (1999). Natural killer cells in antiviral defense: function and regulation by innate cytokines. Annu Rev Immunol 17, 189220.[CrossRef][Medline]
Bjorck, P. (2001). Isolation and characterization of plasmacytoid dendritic cells from Flt3 ligand and granulocytemacrophage colony-stimulating factor-treated mice. Blood 98, 35203526.
Bogdan, C. (2000). The function of type I interferons in antimicrobial immunity. Curr Opin Immunol 12, 419424.[CrossRef][Medline]
Caton, A. J., Brownlee, G. G., Yewdell, J. W. & Gerhard, W. (1982). The antigenic structure of the influenza virus A/PR/8/34 hemagglutinin (H1 subtype). Cell 31, 417427.[Medline]
Cella, M., Jarrossay, D., Facchetti, F., Alebardi, O., Nakajima, H., Lanzavecchia, A. & Colonna, M. (1999). Plasmacytoid monocytes migrate to inflamed lymph nodes and produce large amounts of type I interferon. Nature Med 5, 919923.[CrossRef][Medline]
Charley, B. & Laude, H. (1988). Induction of interferon by transmissible gastroenteritis coronavirus: role of transmembrane glycoprotein E1. J Virol 62, 811.[Medline]
Charley, B., Lavenant, L. & Delmas, B. (1991). Glycosylation is required for coronavirus TGEV to induce an efficient production of IFN by blood mononuclear cells. Scand J Immunol 33, 435440.[Medline]
Crecelius, D. M., Deom, C. M. & Schulze, I. T. (1984). Biological properties of a hemagglutinin mutant of influenza virus selected by host cells. Virology 139, 164177.[CrossRef][Medline]
De Clercq, E. (1981). Interferon induction by polynucleotides, modified polynucleotides, and polycarboxylates. Methods Enzymol 78, 227236.[Medline]
Deom, C. M. & Schulze, I. T. (1985). Oligosaccharide composition of an influenza virus hemagglutinin with host-determined binding properties. J Biol Chem 260, 1477114774.
Dzionek, A., Sohma, Y. Nagafune, J. & 14 other authors (2001). BDCA-2, a novel plasmacytoid dendritic cell-specific type II C-type lectin, mediates antigen capture and is a potent inhibitor of interferon /
induction. J Exp Med 194, 18231834.
Feldman, S. B., Ferraro, M., Zheng, H. M., Patel, N., Gould-Fogerite, S. & Fitzgerald-Bocarsly, P. (1994). Viral induction of low frequency interferon- producing cells. Virology 204, 17.[CrossRef][Medline]
Figdor, C. G., van Kooyk, Y. & Adema, G. J. (2002). C-type lectin receptors on dendritic cells and Langerhans cells. Nature Rev Immunol 2, 7784.[CrossRef][Medline]
Fitzgerald-Bocarsly, P. (1993). Human natural interferon- producing cells. Pharmacol & Ther 60, 3962.[CrossRef][Medline]
Gallucci, S., Lolkema, M. & Matzinger, P. (1999). Natural adjuvants: endogenous activators of dendritic cells. Nature Med 5, 12491255.[CrossRef][Medline]
Gambaryan, A. S., Marinina, V. P., Tuzikov, A. B., Bovin, N. V., Rudneva, I. A., Sinitsyn, B. V., Shilov, A. A. & Matrosovich, M. N. (1998). Effects of host-dependent glycosylation of hemagglutinin on receptor-binding properties on H1N1 human influenza A virus grown in MDCK cells and in embryonated eggs. Virology 247, 170177.[CrossRef][Medline]
Geijtenbeek, T. B., Kwon, D. S., Torensma, R. & 9 other authors (2000). DC-SIGN, a dendritic cell-specific HIV-1-binding protein that enhances trans-infection of T cells. Cell 100, 587597.[Medline]
Hemmi, H., Takeuchi, O., Kawai, T. & 8 other authors (2000). A Toll-like receptor recognizes bacterial DNA. Nature 408, 740745.[CrossRef][Medline]
Hemmi, H., Kaisho, T., Takeuchi, O. & 7 other authors (2002). Small anti-viral compounds activate immune cells via the TLR7 MyD88-dependent signaling pathway. Nature Immunol 3, 196200.[CrossRef][Medline]
Isaacs, A. & Lindenmann, J. (1957). Virus interference. I. The interferon. Proc R Soc Lond Ser B Biol Sci 147, 258268.
Ito, Y. (1994). Induction of interferon by virus glycoprotein(s) in lymphoid cells through interaction with the cellular receptors via lectin-like action: an alternative interferon induction mechanism. Arch Virol 138, 187198.[Medline]
Ito, Y., Nishiyama, Y., Shimokata, K., Nagata, I., Takeyama, H. & Kunii, A. (1978). The mechanism of interferon induction in mouse spleen cells stimulated with HVJ. Virology 88, 128137.[CrossRef][Medline]
Ito, T., Amakawa, R., Kaisho, T. & 7 other authors (2002). Interferon- and interleukin-12 are induced differentially by Toll-like receptor 7 ligands in human blood dendritic cell subsets. J Exp Med 195, 15071512.
Jacobs, B. L. & Langland, J. O. (1996). When two strands are better than one: the mediators and modulators of the cellular responses to double-stranded RNA. Virology 219, 339349.[CrossRef][Medline]
Kadowaki, N., Ho, S., Antonenko, S., Maleyft, R. W., Kastelein, R. A., Bazan, F. & Liu, Y. J. (2001). Subsets of human dendritic cell precursors express different Toll-like receptors and respond to different microbial antigens. J Exp Med 194, 863869.
Kaverin, N. V. & Webster, R. G. (1995). Impairment of multicycle influenza virus growth in Vero (WHO) cells by loss of trypsin activity. J Virol 69, 27002703.[Abstract]
Krug, A., Rothenfusser, S., Hornung, V., Jahrsdorfer, B., Blackwell, S., Ballas, Z. K., Endres, S., Krieg, A. M. & Hartmann, G. (2001). Identification of CpG oligonucleotide sequences with high induction of IFN-/
in plasmacytoid dendritic cells. Eur J Immunol 31, 21542163.[CrossRef][Medline]
Laude, H., Gelfi, J., Lavenant, L. & Charley, B. (1992). Single amino acid changes in the viral glycoprotein M affect induction of interferon by the coronavirus transmissible gastroenteritis virus. J Virol 66, 743749.[Abstract]
Le Bon, A., Schiavoni, G., D'Agostino, G., Gresser, I., Belardelli, F. & Tough, D. F. (2001). Type 1 interferons potently enhance humoral immunity and can promote isotype switching by stimulating dendritic cells in vivo. Immunity 14, 461470.[CrossRef][Medline]
Luft, T., Pang, K. C., Thomas, E., Hertzog, P., Hart, D. N., Trapani, J. & Cebon, J. (1998). Type I IFNs enhance the terminal differentiation of dendritic cells. J Immunol 161, 19471953.
Markwell, M. A., Portner, A. & Schwartz, A. L. (1985). An alternative route of infection for viruses: entry by means of the asialoglycoprotein receptor of a Sendai virus mutant lacking its attachment protein. Proc Natl Acad Sci U S A 82, 978982.[Abstract]
Medzhitov, R., & Janeway, C., Jr (2000). Innate immune recognition: mechanisms and pathways. Immunol Rev 173, 8997.[CrossRef][Medline]
Mosmann, T. (1983). Rapid colorimetric assay for cellular growth and survival: application to proliferation and cytotoxicity assays. J Immunol Methods 65, 5563.[CrossRef][Medline]
Muller, U., Steinhoff, U., Reis, L. F., Hemmi, S., Pavlovic, J., Zinkernagel, R. M. & Aguet, M. (1994). Functional role of type I and type II interferons in antiviral defense. Science 264, 19181921.[Medline]
Nakamura, K. & Compans, R. W. (1979). Host cell- and virus strain-dependent differences in oligosaccharides of hemagglutinin glycoproteins of influenza A viruses. Virology 95, 823.[CrossRef][Medline]
Nakano, H., Yanagita, M. & Gunn, M. D. (2001). CD11c+ B220+ Gr-1+ cells in mouse lymph nodes and spleen display characteristics of plasmacytoid dendritic cells. J Exp Med 194, 11711178.
Nigou, J., Zelle-Rieser, C., Gilleron, M., Thurnher, M. & Puzo, G. (2001). Mannosylated lipoarabinomannans inhibit IL-12 production by human dendritic cells: evidence for a negative signal delivered through the mannose receptor. J Immunol 166, 74777485.
O'Keeffe, M., Hochrein, H., Vremec, D. & 10 other authors (2002). Mouse plasmacytoid cells: long-lived cells, heterogeneous in surface phenotype and function, that differentiate into CD8+ dendritic cells only after microbial stimulus. J Exp Med 196, 13071319.
Raymond, F. L., Caton, A. J., Cox, N. J., Kendal, A. P. & Brownlee, G. G. (1983). Antigenicity and evolution amongst recent influenza viruses of H1N1 subtype. Nucleic Acids Res 11, 71917203.[Abstract]
Reading, P. C., Morey, L. S., Crouch, E. C. & Anders, E. M. (1997). Collectin-mediated antiviral host defense of the lung: evidence from influenza virus infection of mice. J Virol 71, 82048212.[Abstract]
Reading, P. C., Miller, J. L. & Anders, E. M. (2000). Involvement of the mannose receptor in infection of macrophages by influenza virus. J Virol 74, 51905197.
Rogers, G. N., Pritchett, T. J., Lane, J. L. & Paulson, J. C. (1983). Differential sensitivity of human, avian, and equine influenza A viruses to a glycoprotein inhibitor of infection: selection of receptor specific variants. Virology 131, 394408.[Medline]
Rudd, P. M., Wormald, M. R., Harvey, D. J., Devasahayam, M., McAlister, M. S., Brown, M. H., Davis, S. J., Barclay, A. N. & Dwek, R. A. (1999). Oligosaccharide analysis and molecular modeling of soluble forms of glycoproteins belonging to the Ly-6, scavenger receptor, and immunoglobulin superfamilies expressed in Chinese hamster ovary cells. Glycobiology 9, 443458.
Sawyer, W. (1969). Interaction of influenza virus with leukocytes and its effect on phagocytosis. J Infect Dis 119, 541556.[Medline]
Sedmak, J. J. & Grossberg, S. E. (1973). Comparative enzyme kinetics of influenza neuraminidases with the synthetic substrate methoxyphenylneuraminic acid. Virology 56, 658661.[CrossRef][Medline]
Siegal, F. P., Kadowaki, N., Shodell, M., Fitzgerald-Bocarsly, P. A., Shah, K., Ho, S., Antonenko, S. & Liu, Y. J. (1999). The nature of the principal type 1 interferon-producing cells in human blood. Science 284, 18351837.
Stray, S. J., Cummings, R. D. & Air, G. M. (2000). Influenza virus infection of desialylated cells. Glycobiology 10, 649658.
Takeda, K. & Akira, S. (2001). Roles of Toll-like receptors in innate immune responses. Genes Cells 6, 733742.
Ward, A. C. & de Koning-Ward, T. F. (1995). Changes in the hemagglutinin gene of the neurovirulent influenza virus strain A/NWS/33. Virus Genes 10, 179183.[Medline]
Weis, W., Brown, J. H., Cusack, S., Paulson, J. C., Skehel, J. J. & Wiley, D. C. (1988). Structure of the influenza virus haemagglutinin complexed with its receptor, sialic acid. Nature 333, 426431.[CrossRef][Medline]
Wilson, I. A., Skehel, J. J. & Wiley, D. C. (1981). Structure of the haemagglutinin membrane glycoprotein of influenza virus at 3Å resolution. Nature 289, 366373.[Medline]
Received 21 May 2002;
accepted 30 August 2002.