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Journal of Virology, October 2001, p. 8909-8916, Vol. 75, No. 19
Medical Research Council Virology Unit,
Glasgow G11 5JR, Scotland,1 and Division
of Virology, Department of Pathology, University of Cambridge,
Cambridge CB2 1QP, England2
Received 27 April 2001/Accepted 11 June 2001
Activation of cellular interferon-stimulated genes (ISGs) after
infection with herpes simplex virus type 1 (HSV-1) or human cytomegalovirus (HCMV) was investigated. The level of ISG54-specific RNA in human fetal lung (HFL) or human foreskin (BJ) fibroblasts increased substantially after infection with either virus in the presence of cycloheximide. HSV-1 particles lacking glycoprotein D or
glycoprotein H failed to induce ISG54-specific RNA synthesis, demonstrating that entry of virus particles rather than binding of
virions to the cell surface was required for the effect. A DNA-binding
complex that recognized an interferon-responsive sequence motif was
induced upon infection with HSV-1 or HCMV in the presence of
cycloheximide, and the complex was shown to contain the cell proteins
interferon response factor 3 (IRF-3) and CREB-binding protein. IRF-3
was modified after infection with HSV-1 or HCMV to a form of lower
electrophoretic mobility, consistent with phosphorylation. De novo
transcription of viral or cellular genes was not required for the
activation of IRF-3, since the effect was not sensitive to inhibition
by actinomycin D. Infection of HFL fibroblasts with HSV-1 under
conditions in which viral replication proceeded normally resulted in
severely reduced levels of the IRF-3-containing complex, defining the
activation of IRF-3 as a target for viral interference with ISG
induction. In BJ fibroblasts, however, significant activation of IRF-3
was detected even when the viral gene expression program progressed to
later stages, demonstrating that the degree of inhibition of the
response was dependent on host cell type. As a consequence of IRF-3
activation, endogenous interferon was released from BJ cells and was
capable of triggering the appropriate signal transduction pathway in
both infected and uninfected cells. Activation of ISG54-specific RNA
synthesis was not detected after infection of human U-373MG glioblastoma cells, showing that the induction of the response by
infection is cell type dependent.
Alpha/beta interferons
(IFN- The signaling pathway for IFN-
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.19.8909-8916.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Activation of Interferon Response Factor-3 in Human
Cells Infected with Herpes Simplex Virus Type 1 or Human
Cytomegalovirus
ABSTRACT
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
INTRODUCTION
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
/
) are important components of cellular antiviral
responses. The effects of the interferons are mediated by
interferon-stimulated gene (ISG) products, which include proteins with
defined activities, such as 2',5'-oligoadenylate synthetase as well as
many others with unknown functions (reviewed in reference
39). Most studies on the antiviral activities of IFN-/ have focused on the inhibition of RNA virus replication; the effects on DNA virus replication are less well understood. Herpes
simplex virus type 1 (HSV-1) replication is inhibited by interferon
pretreatment of cells due to a block at the level of immediate-early
(IE) gene transcription and additional effects on protein
synthesis that are manifested at later times in infected cells
(2, 25, 30, 32). In mice, IFN-/ are important host
defenses, since attenuated HSV-1 mutants are much more virulent in
animals lacking interferon receptors (21, 22).
/ is well characterized
(39) (Fig. 1). Binding of
interferons to their cellular receptor results in the phosphorylation
of Janus kinases (JAKs) JAK1 and Tyk2 and the consequent
phosphorylation of signal transducers and activators of transcription
(STATs) 1 and 2. The latter proteins are translocated to the nucleus,
where they are recruited by interferon regulatory factor (IRF) 9 (IRF-9; also known as p48) to form a complex, named ISG factor 3 (ISGF3), on interferon-stimulated regulatory elements (ISREs)
within ISG promoters. The acetylases p300 and CREB-binding
protein (CBP) associate with STATs 1 and 2 and are probably important
in mediating transcriptional activation.
View larger version (22K):
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FIG. 1.
Mechanisms of ISG induction. The pathway activated by
IFN- / is depicted on the left side of the diagram, leading to
formation of the complex known as ISGF3 (phosphorylated STATs 1 and 2 plus IRF-9). The route of virus activation of IRF-3 is shown on the
right, leading to a complex containing phosphorylated IRF-3 plus CBP.
The induction of ISGs is mediated by either complex; in addition,
activated IRF-3 can induce the IFN- gene, resulting in the
production of IFN-, which can engage the IFN-/ receptor to
activate the pathway leading to the formation of ISGF3.
Cells commonly produce interferon in response to virus infection, and
significant progress has been made in understanding the cellular
pathways activated upon infection with RNA viruses. Upon infection of
human cells, the cellular factors NF-
B, ATF-2/c-Jun, IRF-3, and/or
IRF-7 combine with high-mobility-group protein HMGI(Y) on the promoter
controlling the IFN- gene to form a stable transcription complex,
the enhanceosome (41, 42). IRF-3 is a cytoplasmic protein
that is phosphorylated in response to infection, resulting in
translocation to the nucleus and binding to elements PRDI and PRDIII in the IFN- promoter (4, 23, 42, 46). In
addition, the activation of IRF-3 results in the induction of
ISG-specific RNA synthesis through a mechanism that is independent of
interferon or ISGF3, since ISREs have homology to the PRDI and PRDIII
sequences found within ISG promoters. In this pathway, phosphorylated
IRF-3 recruits CBP and binds to ISREs, thereby inducing ISG
expression directly (Fig. 1). For RNA viruses, many lines of evidence
suggest that double-stranded RNA is the trigger for the activation of IRF-3 (11, 38, 40, 44).
Typically, viruses encode products that antagonize various aspects of the interferon response in order to overcome host defenses and facilitate virus replication (1, 13, 14, 39). In some situations, IRF-3 is the target for the virus. The influenza virus nonstructural protein NS1 blocks the activation of IRF-3 by binding and sequestering double-stranded RNA (40), whereas the human papillomavirus type 16 protein E6 abolishes IRF-3 activity by interacting with the protein (35). In a further strategy, human herpesvirus 8 encodes IRF homologs that act as decoys to compete with IRF-3 for binding to CBP (10).
Although most studies on direct induction of ISGs have focused on RNA viruses, it was shown that infection with human cytomegalovirus (HCMV) resulted in the stimulation of ISG synthesis and that an early event occurring prior to the translation of viral transcripts was responsible (47, 48). Treatment of cells with purified HCMV glycoprotein B (gB) also induced ISGs, suggesting that the binding of virus particles to the cellular receptor is sufficient to trigger the response (8). In common with the response to RNA virus infection, IRF-3 is activated following infection with HCMV, resulting in the formation of an ISRE-specific complex containing CBP (29). Similar investigations with HSV-1 demonstrated that infection resulted in the synthesis of ISG-specific RNAs, but only under conditions in which viral gene expression was blocked at an early stage (27, 31). Gene array analysis identified ISGs as the predominant set of cellular genes to be induced under these infection conditions (27). In contrast to the situation for HCMV, however, induction was not detected during normal infection, and this finding led to the conclusions that HSV-1 induces ISG synthesis shortly after the penetration of virus and that a viral gene product(s) disarms the response (27, 31). Viruses with mutations in individual IE genes all inhibited the production of ISG-specific RNAs, suggesting that the effect was not due solely to a single viral gene product (27).
The induction of ISG synthesis by HSV-1 may be relevant to the events
that occur after infection of cells under conditions in which viral
gene expression is severely limited. Upon infection with HSV-1 mutants
containing multiple mutations in IE genes, especially that encoding
ICP0, cells are not killed and the viral genome is retained in a
nonreplicating "quiescent" state (26, 33, 36). During
the early stages of such interactions, the viral genome is converted to
an inactive state in which promoters, either homologous or
heterologous, are repressed (33, 36). An apparently
analogous repression occurs when IE gene transcription is inhibited by
pretreatment of cells with IFN-/, suggesting that induced ISG
products may play a role in attainment of the quiescent state
(28, 30). This possibility is supported by other
observations. An antiviral state is attained in cells infected with an
IE gene-impaired mutant, since cultures treated in this way become
resistant to superinfection with HSV-1 or RNA viruses (27). Furthermore, ISG-specific RNA is not induced in
human osteosarcoma U2OS cells, a line in which repression of the viral genome does not occur, as judged by the fact that ICP0-deficient mutants replicate as efficiently as wild-type HSV-1 (27,
45).
We have investigated the signaling pathways activated by HSV-1 and HCMV during the early stages of infection and have investigated further the block to the response at later stages in the replication cycle. The results reveal novel interactions of these viruses with the host and demonstrate significant differences in responses depending on the nature of the host cells.
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MATERIALS AND METHODS |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Cells and viruses. Human fetal lung (HFL) fibroblasts were obtained from Flow Laboratories as Flow 2002 cells and propagated in Dulbecco's modified Eagle medium supplemented with 5% (vol/vol) fetal calf serum, 5% (vol/vol) newborn calf serum, 100 U of penicillin, and 100 µg of streptomycin per ml. Human foreskin fibroblasts transfected with the telomerase coding sequences (BJ cells) (6) were obtained from Geron Corporation and propagated in Dulbecco's modified Eagle medium containing 10% (vol/vol) medium 199 (Life Technologies),10% (vol/vol) fetal calf serum,1 mM sodium pyruvate, 100 U of penicillin, and 100 µg of streptomycin per ml. U-373MG cells were cultured in the same medium (and constituents) as HFL cells.
Wild-type HSV-1 was strain 17 or HFEM, and HCMV was strain AD169. HSV-1 mutant SC16gD.del.Z, lacking gD, was propagated in Vero gD+/19 cells, which express gD (5), and HFEM lacking gH was propagated in CR1 cells, which provide gH (7). To produce stocks lacking glycoproteins, Vero cells were infected with the mutants at a multiplicity of infection (MOI) of 10. After adsorption for 90 min, cells were washed with pH 3 buffer and incubated at 37oC for 24 h as described previously (34). Particles released into the growth medium were purified with Ficoll gradients as described by Rodger et al. (34), except that the gradients were 5 to 15% instead of 15 to 30% Ficoll. Virus particle numbers were estimated by comparison with latex particles of known concentrations using negatively stained preparations as described by Watson et al. (43).Medium transfer.
Culture medium from infected monolayers was
removed and centrifuged at 25,000 × g for 1 h at
4°C. The medium was applied to fresh monolayers after the addition of
cycloheximide to 100 µg/ml (except when cycloheximide was already
present). Recombinant human IFN- was obtained from Sigma, and
anti-IFN- antibody was obtained from CN Biosciences.
EMSA.
For electrophoretic mobility shift assays (EMSA),
whole-cell extracts were prepared as described by Navarro et al.
(29), with the exception that Triton X-100 was added to
the lysis buffer at 0.2% (vol/vol) instead of 1% (vol/vol) and a
mammalian protease inhibitor cocktail (Sigma P-8340) was added at the
concentration recommended by the manufacturer. Extracts were incubated
at 15°C for 5 min in a buffer containing 20 mM HEPES (pH 7.0), 40 mM
KCl, 20 mM NaCl, 10 mM NaF, 1 mM MgCl2, 2 mM
-glycerophosphate, 0.5 mM dithiothreitol, 0.2 mM sodium vanadate,
0.2 mM phenylmethylsulfonyl fluoride, 0.1 mM EDTA, 4% (vol/vol)
Ficoll, 0.08% (vol/vol) Triton X-100, 2 µg of poly(dI)-poly(dC), and
approximately 0.1 ng of 32P-3'-end-labeled
oligonucleotide, with the cell extract constituting 20% of the
reaction volume. Mixtures were loaded onto a 6% acrylamide-0.2% N,N'-methylenebisacrylamide polyacrylamide gel.
After electrophoresis at 7 V/cm for 4 h, the gel was dried and
exposed for autoradiography. Double-stranded oligonucleotides
representing the ISG15 ISRE were (top strand)
5'-GATCGGGAAAGGGAAACCGAAACTGAAGCCA and a mutated version that was identical except for the substitution of C for the
underlined G (44). Extracts were preincubated at 4°C in a reaction mixture containing antibodies specific for IRF-3 (Santa Cruz
sc-9082X) for 5 min or antibodies specific for CBP (Santa Cruz sc-369X)
for 60 min prior to the addition of the radiolabeled oligonucleotide.
Quantification was achieved by excising bands and measuring
incorporated radioactivity.
Detection of IRF-3. Whole-cell extracts prepared as described above were analyzed by protein blotting with an anti-IRF-3 polyclonal antibody using methods described previously (18).
RNA analysis. Cytoplasmic RNA was extracted and analyzed by electrophoresis and hybridization with ISG54- or glyceraldehyde-3-phosphate dehydrogenase (GAPDH)-specific probes as described previously (31). Quantification was achieved by use of a Bio-Rad Molecular Imager and associated software.
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RESULTS |
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Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Activation of ISG expression requires penetration of HSV-1.
The response of cellular ISGs to infection with HSV-1 or HCMV was
investigated with two human fibroblast lines, the HFL fibroblast line
routinely used in our laboratory for investigation of repression of
viral gene expression and BJ fibroblasts that had been transformed with
the human telomerase coding sequences to confer an extended life span
in cultures. HFL and BJ cells were treated with gradient-purified virions of HSV-1 or with virions lacking gD or gH and incubated at
37°C for 5 h in the presence of cycloheximide. As expected from
previous studies (31), treatment of HFL cells with 200 or
40 particles of wild-type HSV-1 per cell or of BJ cells with 100 particles per cell resulted in the production of ISG54-specific RNA
(Fig. 2). The addition of
glycoprotein-deficient virions, however, failed to elicit this response
in either cell type. Analysis of infected cells immediately after the
adsorption period by Southern hybridization demonstrated that
approximately equivalent amounts of viral DNAs were associated with
cells treated with wild-type, gD-negative, or gH-negative preparations,
confirming that the glycoprotein-deficient virions bound efficiently to
the cell surface (results not shown). The experiment in Fig. 2
demonstrates that the entry of virus particles is required for the
induction of ISG54-specific RNA, confirming and extending the
observations of Mossman et al., who showed that virus particles lacking
gD or gB did not induce ISG expression (27). The results
also make an important distinction between the modes of action of HSV-1 and HCMV particles, since induction by the latter is thought to be a
consequence of glycoprotein interaction with the cell membrane rather
than entry of virions (8).
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), or gH-deficient virions
(HActivation of IRF-3 by HSV-1.
In view of the differences in
the modes of action of HSV-1 and HCMV and to characterize the mechanism
of induction by HSV-1, the activation of IRF-3 was investigated.
Lysates of HFL or BJ cells were analyzed on protein blots using an
IRF-3-specific antibody (Fig. 3A). For
both cell types, infection with HSV-1 or HCMV in the presence of
cycloheximide resulted in a small but discernible decrease in the
electrophoretic mobility of IRF-3, consistent with the change observed
by others in response to phosphorylation (23, 46).
Evidence for the activation of IRF-3 was also obtained by EMSA using
extracts prepared after infection with HSV-1 or HCMV in the presence of
cycloheximide (Fig. 3B). A novel complex was formed after incubation of
infected cell extracts with a radiolabeled probe containing an ISRE.
The complex was present in both cell types, but extracts of BJ cells
routinely contained larger amounts of it and formed fewer
"nonspecific" complexes in the upper part of the gel. In all other
EMSA analyses presented here, only the top portion of the
autoradiograph is shown.
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Effects of HSV-1 during normal infection.
Previous studies
showed that ISG-specific RNA synthesis was not induced during normal
infection of HFL cells with wild-type HSV-1 or with many mutants that
express viral gene products, suggesting that the potential induction by
virion components was not realized due to the production of
virus-specified products that blocked the response (27,
31). To investigate the stage at which inhibition of the
response occurs and to examine the kinetics of IRF-3 activation, production of the complex was examined after infection of HFL or BJ
cells in the presence or absence of cycloheximide (Fig. 4). In HFL cells with cycloheximide
present, the novel complex was detectable by 2 h after infection,
and the level rose during the next 3 h. Without cycloheximide
present, the complex was detectable by 2 h after infection, but
the level did not increase greatly during infection and declined after
3.5 h (Fig. 4A). Quantification revealed that the amount of
radiolabeled probe in the complex at 3.5 h after infection was
between 5 and 8% that present when extracts from cycloheximide-treated
cells were analyzed (results not shown). In BJ cells, however,
activation of IRF-3 was detectable during normal infection, reaching 30 to 50% the amount present in extracts made after infection in the
presence of cycloheximide (Fig. 4B). Furthermore, two slower-migrating
complexes were formed by extracts prepared at 3.5, 5, and 6.5 h
after infection, and these species comigrated with the
well-characterized ISGF3 produced in response to IFN-/. This
experiment therefore suggests that interferon is produced in infected
BJ cells.
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was synthesized, presumably following the pathway in
which activated IRF-3, in conjunction with other cellular factors,
forms the enhanceosome (41, 42). Previous studies
(27; C. M. Preston, unpublished observations),
however, concluded that interferon was not produced during infection of HFL cells with wild-type HSV-1 or with mutants blocked such that IE
gene products were not expressed, based on the finding that medium from
cells infected under these conditions did not inhibit virus replication
and did not induce ISG-specific RNA synthesis when applied to fresh
cells. To reconcile these observations with the data in Fig. 4, which
implies the induction of IFN- synthesis in BJ cells, ISG-specific
RNA was investigated using cells infected with wild-type HSV-1; in
addition, the medium from infected cells was centrifuged to remove
virus and was applied to fresh monolayers to test for the induction of
ISG-specific RNA. As was found previously, at an MOI of 5 no
ISG54-specific RNA was detected in HFL cells infected without
cycloheximide (Fig. 5A, lane 3), and the
cell culture medium did not induce ISG54-specific RNA synthesis when applied to fresh cells (Fig. 5A). As expected, infection in the presence of cycloheximide induced ISG54-specific RNA synthesis (Fig.
5A, lane 2). In contrast, ISG54-specific RNA was detected after
infection of BJ cells without cycloheximide (Fig. 5A, lane 5), and the
medium from the cells contained interferon, since it caused the
accumulation of ISG54-specific RNA when added to fresh monolayers (Fig.
5A). Equivalent results were obtained when the medium from HFL cells
was applied to fresh BJ cell monolayers and vice versa (results not
shown).
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was confirmed by the
observation that anti-IFN- reduced the ISG54-specific RNA levels
provoked by infected BJ cell medium but did not affect the potency of
added IFN- (Fig. 5B). Figure 5A also shows that infection of cells
at an MOI of 5 resulted in a reduction in the levels of cellular GAPDH
RNAs in both cell types at 5 h after infection, presumably due to
mRNA degradation through the virion host shutoff function and the later
inhibition of protein synthesis that occurs upon infection with HSV-1
(12, 16, 19). When a lower MOI was used to reduce the
degree of mRNA destruction, limited production of ISG54-specific RNA
was detected in HFL cell monolayers, amounting to 8% (MOI, 0.6), 8%
(MOI, 2.5), and less than 1% (MOI, 10) the levels in
cycloheximide-treated cells (Fig. 5C). In BJ cells, the levels of
ISG54-specific RNA were 67% (MOI, 0.6), 69% (MOI, 2.5), and 37%
(MOI, 10) of those in cycloheximide-treated cells. Therefore,
significant production of ISG54-specific RNA and endogenous IFN-
occurs during normal infection of BJ cells with HSV-1 and, presumably,
HCMV, whereas in HFL cells the level of production of ISG54-specific
RNA is lower and is observed reliably only after infection at a low
MOI. The observations are in accord with the implications from the
activation of IRF-3 and the formation of ISGF3 shown in Fig. 4.
RNA synthesis is not required for the activation of IRF-3.
In
many situations, the activation of IRF-3 is thought to be due to the
intracellular production of viral double-stranded RNA; thus, it was
possible that HSV-1-specific IE RNA was involved in ISG induction. The
effects of adding actinomycin D, an inhibitor of RNA synthesis, on
IRF-3 activation were therefore examined (Fig.
6). During infection in the presence of
cycloheximide, actinomycin D had no effect on the amount of the
IRF-3-specific complex formed (Fig. 6, lanes 3 and 4). Without
cycloheximide, actinomycin D increased the amount of the complex to a
level equivalent to that found in cycloheximide-treated cells and
prevented the production of ISGF3 (lanes 1 and 2). Actinomycin D did
not stimulate complex formation in uninfected cells (lanes 5 and 6).
This experiment shows that newly synthesized RNA does not contribute to
the activation of IRF-3. It also confirms that the reduction in
activation during infection in the absence of cycloheximide, compared
with infection in the presence of the inhibitor, is dependent upon
transcription. Finally, the production of ISGF3 was inhibited by
actinomycin D, suggesting that RNA synthesis is required for the
induction of IFN- production that occurs during infection of BJ
cells with HSV-1.
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Induction of ISG-specific RNA in U-373MG cells.
Mossman et al.
(27) demonstrated that the human osteosarcoma line U2OS
failed to produce ISG-specific RNA after infection with UV-inactivated
HSV-1, in contrast to human lung fibroblasts, which responded strongly,
suggesting that the lack of a requirement for ICP0 correlates with
insensitivity to the induction of ISGs by the virus. The generality of
the response to HSV-1 was therefore investigated. The human cell line
U-373MG is restricted for the replication of ICP0-deficient mutants,
approximately as stringently as human fibroblasts (Preston,
unpublished). Nonetheless, ISG54-specific RNA was not detected after
infection in the presence of cycloheximide, even though IFN- itself
was active in this cell line (Fig. 7). Therefore, the induction of ISG54-specific RNA synthesis does not
strictly correlate with permissiveness for ICP0-deficient mutants.
Similarly, the induction of ISG54-specific RNA synthesis was not
observed for HEC-1B (human) or Vero (monkey) cells, even though the
replication of ICP0-deficient mutants was restricted (results not
shown).
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DISCUSSION |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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We investigated the cellular pathways involved in the induction
and abrogation of ISG synthesis during HSV-1 and HCMV infections. In
contrast to the situation with HCMV, entry of HSV-1 rather than binding
was required for the cellular response, although the downstream
cellular pathways appear to be equivalent for the two viruses. The
induction and disarming of the response are each more complex than
suggested from existing data, with the nature of the host cell being an
important variable. BJ fibroblasts, which respond to infection with
vigorous activation of IRF-3, are able to synthesize ISGs, including
IFN-, during normal infection with HSV-1 or HCMV. The degree of
response shown by human fibroblasts is not exhibited by all human cell types.
The failure of virions lacking gD or gH to induce ISG54-specific RNA, together with the finding that gB-deficient virions are also inactive (27), suggests that virus entry, rather than signal transduction elicited by a glycoprotein interaction with the cell surface, is required for the cellular response. This conclusion is supported by the finding that IRF-3 activation was first detected at 2 h after infection rather than during the first hour, when adsorption takes place. The contrast between the mechanisms used by HSV-1 and HCMV is intriguing, but the differences in the approaches used to reach the conclusions should be noted. Application of purified glycoproteins, as was used for HCMV, may not equate to the events that occur during infection with virus, and it is unfortunate that glycoprotein-deficient mutants of HCMV are not available to allow experiments analogous to those carried out with mutant HSV-1 virions to be performed.
Activation of IRF-3 is a common response to infection with RNA viruses, and in most situations it is thought that double-stranded RNA is the trigger. Indeed, inhibition of Sendai virus replication by the addition of ribavirin or UV irradiation of virions prevented the activation of IRF-3 (38). The ability of UV-irradiated HSV-1 to induce ISG-specific RNA synthesis suggested that de novo synthesis of viral transcripts was not required for induction (27, 31), and the observation that stringent inhibition of RNA synthesis with actinomycin D did not affect the activation of IRF-3 confirmed this conclusion. It has recently been shown that HCMV particles contain viral and host cell RNAs (9, 15), possibly explaining the response to HCMV and to HSV-1, if analogous transcripts exist in its virions. Both viruses, however, contain many proteins in the tegument and capsid structures, including protein kinases, and any of these could have a role in triggering host defenses. If RNA is not required for the effects of HSV-1 and HCMV, IRF-3 activation must proceed through a novel pathway after infection with these viruses.
A major function of IRF-3 is the induction of IFN- synthesis and
hence protection of neighboring uninfected cells, rather than induction
of ISG synthesis in initially infected cells. Our finding that
endogenous IFN- is produced in BJ cells confirms that the pathways
downstream of IRF-3 are intact during the first 6.5 h after
infection with HSV-1 and HCMV. Furthermore, since virtually every cell
is infected at an MOI of 5, the presence of ISGF3 shows that the
JAK/STAT pathway remains functional at least up to 5 h after
infection with HSV-1 or HCMV. At later times after infection with HCMV,
many components of the IFN- signal transduction pathway are
inhibited (24). The fact that HSV-1 can induce IFN-
synthesis through a signal that requires the entry of virions defines a
new and unexpected way in which the virus can induce this cellular
response, in contrast to previous reports. For peripheral blood
mononuclear cells (PBMCs), the presentation of soluble gD or of cells
expressing the protein was able to induce interferon synthesis
(3, 20). In HFL or BJ cells, no induction of
ISG54-specific RNA was detected even after the addition of 1 µM
soluble gD, a concentration that gave maximum induction in PBMCs
(3; Preston, unpublished). The production of
IFN--specific RNA was detected in HSV-1-infected mouse embryo
fibroblasts, but not until 12 h after infection, much later than
what we observed for BJ cells (37).
Previous studies showed that the induction of ISG-specific RNA synthesis was not detectable at 6 h (31) or 24 h (27) after normal infection with HSV-1, although in both studies a general reduction in cellular transcript levels was also observed. Many HSV-1 mutants lacking intact IE genes also disarmed the response, although in these instances the effects on cellular RNAs were smaller than that seen with wild-type virus (27). By analogy to other viruses, it appeared that HSV-1 encoded one or more products dedicated to ablating the response and hence overcoming this aspect of host antiviral defense. The results presented here show that this aspect of the virus-host interaction is more complex and probably less specific than suggested by the initial observations. It is clear that IRF-3 is activated much less efficiently in HFL cells than in BJ cells after infection with HSV-1 without cycloheximide; therefore, IRF-3 is a possible target for viral inhibition of ISG synthesis. Even in BJ cells, the activation of IRF-3 is reduced in comparison with the levels that can be attained in the presence of actinomycin D or cycloheximide.
The way in which HSV-1 may interfere with IRF-3 or the pathways leading to its phosphorylation are unclear at present. The similarities in the responses to infection with HSV-1 and HCMV without cycloheximide are surprising, in view of the different time scales of the HSV-1 and HCMV replication cycles. As in HSV-1-infected cells, IRF-3 was activated poorly in HCMV-infected HFL cells and more strongly, but not to the levels seen with cycloheximide present, in BJ cells. Two main possibilities exist to reconcile these observations. HSV-1 and HCMV may each inhibit the activation of IRF-3 in the same way, due to the production of specific viral proteins that have functional homologies, with the same timing after infection. Alternatively, the similarity in cell responses may signify the operation of normal cellular regulation of IRF-3 activity and may be unrelated to virus infection. If the latter possibility is valid, it follows that infection with HSV-1 does not specifically block ISG synthesis, but rather that the only negative effect attributable to the virus is the overall increase in the degradation of cellular mRNAs.
Even a relatively small change in host cell type, from human lung fibroblasts to human foreskin fibroblasts, results in significantly different responses to infection. This is a surprising finding, since both cell types are fibroblasts derived from human material. The life-extended status of the BJ cells used here does not appear to be an important factor, as a secondary human foreskin fibroblast line (HFFF2) responded in a manner similar to that of BJ cells, with IRF-3- and ISGF3-specific complexes being present in extracts made at 5 h after infection without cycloheximide (Preston, unpublished). Given the strong agreement between our findings with HFL cells and those of Mossman et al. (27) using human embryo lung fibroblasts from a different source, it appears that the tissue origin of the cells used may be the relevant variable. The basis for the cell type difference in response is largely attributable to the stronger activation of IRF-3 in BJ cells, but at present the kinases responsible for the activation of IRF-3 are unknown; thus, it is not possible to speculate on the identities of the cellular factors that respond to infection with HSV-1 (38).
Induction of ISG54-specific RNA synthesis does not correlate with the
requirement for ICP0. The finding that ISG54-specific RNA is not
produced in response to infection of U-373MG cells and other cells
shows that this aspect of the virus-cell interaction is dependent upon
the nature of the host cell. The correlation between the induction of
ISGs and the dependence on ICP0 does not hold invariably, but it is
possible that there are subtle variations in responses to infection and
that ISGs other than ISG54 may indeed be switched on in a broader range
of cell types. The production of IFN- itself does not significantly
influence the virus infection program in the infected cell, since the
replication of HSV-1 is equivalent in HFL and BJ cells (Preston,
unpublished). It would be expected that the antiviral effects of
IFN- rely on the activation of protein kinase R, and the work of He
et al. (17) shows that viral protein ICP34.5 negates this
event by activating a cellular phosphatase.
The results presented here extend current knowledge of the way in which HSV-1 and HCMV interact with host cells. Although the induction of ISG synthesis does not directly correlate with the lack of dependence on ICP0, the activation of IRF-3 can result in the secretion of interferons from human fibroblasts even in the context of a normal productive infection. The induction depends on viral entry rather than the action of viral glycoproteins binding to cells, in contrast to the proposed mechanisms for the responses of HCMV-infected fibroblasts or HSV-1-infected PBMCs. The inducing component is probably not RNA, suggesting a mechanism different from that which occurs in RNA virus-infected cells. Counteraction of the cellular antiviral response is partly due to a general degradation of cellular mRNAs, and although the reduction in the activation of IRF-3 during normal infection with HSV-1 accounts for the lower ISG-specific RNA levels, the effect may be due to normal cellular regulatory pathways rather than virus-specified products. Further investigation of the interactions of HSV-1 and HCMV with different cell types, particularly in vivo, may shed light on the biological significance of the cellular responses to infection that we report here.
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ACKNOWLEDGMENTS |
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We thank Tony Minson for help in the preparation of virions and for interest in this work.
A. N. Harman was supported by the Wellcome Trust.
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FOOTNOTES |
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* Corresponding author. Mailing address: Medical Research Council Virology Unit, Church St., Glasgow G11 5JR, Scotland. Phone: 44 141 330 3921. Fax: 44 141 337 2236. E-mail: c.preston{at}vir.gla.ac.uk.
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REFERENCES |
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Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
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Journal of Virology, October 2001, p. 8909-8916, Vol. 75, No. 19
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.19.8909-8916.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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