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1 Department of Veterinary and Biomedical Sciences, University of Nebraska, Lincoln, Fair Street at East Campus Loop, Lincoln, NE 68583-0905, USA
2 Nebraska Center for Virology, University of Nebraska, Lincoln, Fair Street at East Campus Loop, Lincoln, NE 68583-0905, USA
3 School of Biological Sciences, University of Nebraska, Lincoln, NE 68588, USA
Correspondence
Clinton Jones
cjones{at}unlnotes.unl.edu
| ABSTRACT |
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Present address: Centers for Disease Control, 1600 Clifton Rd, Atlanta, GA 30333, USA. ![]()
A table showing primers used to detect IFN response and viral gene expression is available with the online version of this paper.
| INTRODUCTION |
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Infection of cultured human cells with herpes simplex virus type 1 (HSV-1) leads to the production and secretion of interferon (IFN). ICP0, ICP34.5 and Us11 are HSV-1 genes that inhibit IFN activation post-infection (p.i.) (Lin et al., 2004
; Mossman et al., 2000
, 2001
; Mossman & Smiley, 2002
; Peters, et al., 2002
). The viral glycoprotein gD activates interferon response factor 3 (IRF3) and consequently IFN-
production in mononuclear cells (Katze et al., 2002
). Mice lacking type I and type II IFN receptors in combination with RAG-2 gene deletions die within a few days following BHV-1 infection (Abril et al., 2004
). In contrast, BHV-1 infection of wild-type (wt) mice does not lead to clinical symptoms, confirming that IFN signalling pathways repress productive infection. To date, bICP0 is the only BHV-1 encoded protein known to inhibit IFN responses (Henderson et al., 2005
; Saira et al., 2007
).
The latency-related (LR) gene is abundantly transcribed in trigeminal ganglia (TG) of latently infected calves (Kutish et al., 1990
; Rock et al., 1987
, 1992
) and is antisense with respect to the bICP0 gene (Jones, 1998
, 2003
; Jones et al., 2006
). The LR gene has two open reading frames (ORF-1 and ORF-2), and two reading frames that lack an initiating ATG (RF-B and RF-C) (Fig. 1a
). A mutant BHV-1 virus (LR mutant virus) that contains three stop codons at the beginning of ORF-2 and also lacks 25 bp of wt sequence at the beginning of ORF2 was constructed (Fig. 1b
) (Inman et al., 2001
). The LR mutant virus grows to similar titres to the wt BHV-1 or LR rescued virus in cultured bovine cells, indicating that expression of LR proteins is not necessary for productive infection. When bovine cells are infected with the LR mutant virus, proteins containing all or part of ORF-2 or RF-C are not expressed (Hossain et al., 1995
; Jiang et al., 1998
, 2004
). Calves infected with the LR mutant virus exhibit diminished clinical symptoms and reduced shedding of infectious virus in the eye, tonsil or TG (Inman et al., 2001
, 2002
; Perez et al., 2005
). The LR mutant virus does not reactivate from latency following dexamethasone (DEX) treatment (Inman et al., 2002
), indicating that LR protein expression is crucial for the latency-reactivation cycle. LR gene products inhibit mammalian cell growth by blocking S phase entry (Geiser & Jones, 2005
; Schang et al., 1996
), bICP0 expression (Bratanich et al., 1992
; Geiser et al., 2002
; Schang et al., 1996
) and apoptosis (Ciacci-Zanella et al., 1999
; Henderson et al., 2004
). LR protein expression is necessary for inhibiting apoptosis, in part, because an LR protein binds to two proteins that induce apoptosis, Bid and Cdc42 (Meyer et al., 2007
). In contrast, LR protein expression is not necessary for inhibiting cell growth or bICP0 expression. We predict that non-protein coding functions encoded by LR-RNA cooperate with LR proteins to regulate the latency-reactivation cycle.
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| METHODS |
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Madin-Darby bovine kidney cells (MDBK, ATCC CCL-22) were maintained in Earle's modified Eagle's medium supplemented with 5 % fetal calf serum. To prepare virus stocks, MDBK cells were infected with wt BHV-1, the LR mutant virus or the LR mutant rescued virus at an m.o.i. of 0.01. For the experiments described in this study, an m.o.i. of 5 was used.
Animal studies.
BHV-1-free cross-bred calves (
200 kg) were used for this study. Calves were inoculated with 107 p.f.u. of wt BHV-1, the LR rescued virus or the LR mutant virus into each nostril and conjunctiva for a total of 4x107 p.f.u. per animal as described previously (Inman et al., 2002
; Lovato et al., 2003
; Perez et al., 2005
; Winkler et al., 1999
, 2000
). Calves were housed under strict isolation and given antibiotics before and after BHV-1 infection to prevent secondary bacterial infections. At 60 days post-infection (p.i.), wt BHV-1, the LR rescued virus and the LR mutant-infected calves were injected intravenously with 100 mg DEX. Additional intramuscular injections (25 mg) of DEX were given at 2 and 4 days after the initial intravenous injection to ensure that reactivation occurs. Total RNA prepared from TG and tonsils was from calf studies that were previously described (Inman et al., 2002
; Lovato et al., 2003
; Perez et al., 2005
; Winkler et al., 1999
, 2000
).
RNA extraction.
RNA was extracted from cultured cells or tissue (Chomczynski & Sacchi, 1987
). Tissue from tonsil or TG was first minced into small pieces, placed into 10 ml solution D [4 M guanidine thiocyanate, 25 mM sodium citrate (pH 7.0), 0.5 % sarkosyl and 14 mM β-mercaptoethanol] and homogenized. Two phenol extractions were performed. RNA concentrations were determined spectrophotometrically (260 nm) and RNA was reprecipitated in 3 volumes of ethanol.
DNase treatment and reverse transcription (RT).
Three micrograms of RNA was treated with 1 U RNase-free DNase I (Gibco-BRL) for 15 min at 20 °C in the presence of an RNase inhibitor (RNAsin; Promega). After DNase I treatment, samples were incubated at 65 °C for 7.5 min in the presence of 2 mM EDTA to eliminate DNase I activity. RT reactions were performed with random hexamers or the LR-specific primer 1980 at 65 °C for 7.5 min and chilled on ice. Sixteen microlitres of ice-cold RT mix [20 mM Tris/HCl (pH 8.3), 50 mM KCl, 2.5 mM MgCl2, 100 µg BSA ml–1, 1 mM dithiothreitol, 0.5 mM each deoxynucleotide triphosphate (dNTPs) and 10 U RNAsin] was added. The reaction mixture was incubated for 10 min at 25 °C and then for 50 min at 42 °C. One microlitre of reverse transcriptase was added to each tube and placed at 42 °C in a water bath for 50 min. As a control for DNA contamination in the RNA samples, 3 µg RNA (DNase I treated) was mixed with ice-cold RT mix lacking reverse transcriptase.
PCR.
An aliquot (2 µl) of the RT reaction mixture was used for each PCR, using primers specific for bovine IFN-
, IFN-β, IFN-
, Mx1a and LR genes. Amplification of β-actin was used as an internal control. PCRs were carried out in a total of 50 µl containing 1x commercial PCR buffer, 5 mM MgCl2, 200 µM each dNTP, 1 µM each primer and Taq polymerase. Amplification was carried out for 32 cycles by denaturing at 95 °C for 1 min, annealing at 53 °C (IFN-
1, IFN-
and Mx1a) or 55 °C (IFN-β1, IFN-β2 and IFN-β3) for 1 min and extending at 72 °C for 2 min. Upon completion of the last cycle, the reaction mixtures were further incubated at 72 °C for 7 min to ensure complete extension of the amplified product. PCR products were electrophoresed on 2 % agarose gels and stained with ethidium bromide. Primers used for these studies are shown in Supplementary Table S1 (available in JGV Online).
| RESULTS |
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The LR mutant virus induces higher levels of IFN-β and IFN-responsive genes during productive infection
The finding that LR-RNA expression occurred earlier in MDBK cells infected with the LR mutant virus suggested that a stronger IFN response may occur because LR-RNA has the potential to base pair with bICP0 mRNA and also LR-RNA contains regions that have the potential to form double-stranded structures. To test whether this prediction was true, we compared IFN RNA expression in productively infected MDBK cells following infection with the LR mutant virus or a virus that expresses wt LR gene products. An initial study was performed to prove that MDBK cells were responsive to stimuli that induce IFN production. To this end, MDBK cells were infected with wt BHV-1 or treated with 10 µg imiquimod (Invitrogen) ml–1, a compound that stimulates IFN and cytokine production (Megyeri et al., 1995
). Following treatment with imiquimod, IFN-β promoter activity increased as a function of time for 24 h and was approximately seven times higher compared with untreated cells (Fig. 2
). At 24 or 30 h p.i., IFN-β promoter activity was stimulated five- to sixfold higher than mock-infected cells. Thus, MDBK cells were responsive to factors that induce an IFN response.
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1 RNA was detected at the same time points in MDBK cells following infection with the LR mutant virus, wt BHV-1 (Fig. 3a
following infection with the LR mutant virus for 4, 8 and 12 h p.i. (Fig. 3b
Analysis of IFN in calves infected with BHV-1
In contrast to cultured cells, the LR mutant virus grows less efficiently than wt virus in bovine conjunctiva (Inman et al., 2001
), TG (Inman et al., 2002
) or pharyngeal tonsil (Perez et al., 2005
), suggesting that in certain tissues of infected calves an enhanced IFN response may reduce shedding of the LR mutant virus. To test this possibility, total RNA was prepared from tonsils or TG of acutely infected calves (4 or 6 days p.i.) and the presence of IFN subtypes was detected by RT-PCR. The respective RNA preparations were subjected to cDNA synthesis using random primers and the designated primers described in Supplementary Table S1 were used to amplify cDNA. In general, IFN-
1, the three IFN-β subtypes and IFN-
RNA were consistently detected in tonsils of calves infected with the LR mutant virus for 4 or 6 days (Table 1
). Conversely, these same transcripts were not consistently detected in tonsils of calves acutely infected with wt BHV-1 at 4 or 6 days p.i. Mx1a RNA was detected in total RNA prepared from calves infected with the LR mutant virus or wt BHV-1, as well as uninfected bovine tonsils (data not shown), suggesting this gene is constitutively expressed in tonsils. In contrast to the results obtained in tonsils, IFN RNA was not readily detected in TG of calves infected with the LR mutant virus or wt BHV-1 for 4 or 6 days. Although the main site of BHV-1 latency is TG sensory neurons (Jones, 1998
, 2003
), pharyngeal tonsils are an important site for viral replication and persistence or latency (Perez et al., 2005
; Winkler et al., 1999
, 2000
).
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RNA expression were detected in tonsils prepared from wt-infected calves, but not tonsils of calves infected with the LR mutant virus (Table 2
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| DISCUSSION |
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(Hohle et al., 2005
There appear to be three potential reasons for why the LR mutant virus expressed higher levels of LR-RNA during early phases of productive infection: (i) 25 bp of wt sequences near ORF-2 were deleted, (ii) sequences containing three stop codons and an EcoRI restriction enzyme site were inserted at the 5' terminus of ORF-2 and/or (iii) the LR mutant virus does not synthesize two proteins encoded by the LR gene. LR protein expression is not detected until the late phases of productive infection (Hossain et al., 1995
; Jiang et al., 1998
), suggesting LR proteins do not regulate IFN RNA levels. Although LR-RNA expression was detected at 2 and 4 h p.i. with the LR mutant virus (Fig. 1c and d
), this does not necessarily mean that LR-RNA was expressed under IE conditions. A previous study demonstrated that two BHV-1 early genes (ribonucleotide reductase and thymidine kinase) are expressed 2 h p.i. of bovine cells (Schang & Jones, 1997
), suggesting that early expression of LR-RNA expression can occur as early as 2 h p.i. High levels of bICP0 RNA are expressed throughout productive infection because two promoters drive bICP0 RNA expression: an IE promoter and a separate early promoter (Fraefel et al., 1994
). Consequently, premature expression of LR-RNA would increase the probability that hybridization occurs with bICP0 RNA during the early stages of infection.
Following infection of cattle, LR-RNA (Devireddy & Jones, 1998
), but not other viral genes (unpublished data), are detected in TG at 1 day p.i., suggesting that LR-RNA is the first abundant viral transcript expressed in sensory neurons. Consequently, LR-RNA sequences may promote the early phases of establishing latency by binding to bICP0 mRNA sequences, which would inhibit productive infection by reducing bICP0 levels and inducing an earlier IFN response. Our previous studies also indicate that expression of an LR protein promotes survival of infected neurons by inhibiting apoptosis (Ciacci-Zanella et al., 1999
; Lovato et al., 2003
). In conclusion, we propose that premature expression of LR-RNA by the LR mutant virus, in the absence of LR protein expression, leads to survival of a subset of infected neurons that can establish latency but are unable to reactivate from latency (Inman et al., 2002
).
| ACKNOWLEDGEMENTS |
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Received 26 September 2007;
accepted 17 February 2008.
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