Viral proteogenomic and expression profiling during productive replication of a skin-tropic herpesvirus in the natural host

Efficient transmission of herpesviruses is essential for dissemination in host populations; however, little is known about the viral genes that mediate transmission, mostly due to a lack of natural virus-host model systems. Marek's disease is a devastating herpesviral disease of chickens caused by Marek's disease virus (MDV) and an excellent natural model to study skin-tropic herpesviruses and transmission. Like varicella zoster virus that causes chicken pox in humans, the only site where infectious cell-free MD virions are efficiently produced is in epithelial skin cells, a requirement for host-to-host transmission. Here, we enriched for heavily infected feather follicle epithelial skin cells of live chickens to measure both viral transcription and protein expression using combined short- and long-read RNA sequencing and LC/MS-MS bottom-up proteomics. Enrichment produced a previously unseen breadth and depth of viral peptide sequencing. We confirmed protein translation for 84 viral genes at high confidence (1% FDR) and correlated relative protein abundance with RNA expression levels. Using a proteogenomic approach, we confirmed translation of most well-characterized spliced viral transcripts and identified a novel, abundant isoform of the 14 kDa transcript family via IsoSeq transcripts, short-read intron-spanning sequencing reads, and a high-quality junction-spanning peptide identification. We identified peptides representing alternative start codon usage in several genes and putative novel microORFs at the 5' ends of two core herpesviral genes, pUL47 and ICP4, along with strong evidence of independent transcription and translation of the capsid scaffold protein pUL26.5. Using a natural animal host model system to examine viral gene expression provides a robust, efficient, and meaningful way of validating results gathered from cell culture systems.

The alphaherpesvirus conserved pUS10 is important for natural infection and its expression is regulated by the conserved Herpesviridae protein kinase (CHPK)

Conserved Herpesviridae protein kinases (CHPK) are conserved among all members of the Herpesviridae. Herpesviruses lacking CHPK propagate in cell culture at varying degrees, depending on the virus and cell culture system. CHPK is dispensable for Marek's disease herpesvirus (MDV) replication in cell culture and experimental infection in chickens; however, CHPK-particularly its kinase activity-is essential for horizontal transmission in chickens, also known as natural infection. To address the importance of CHPK during natural infection in chickens, we used liquid chromatography-tandem mass spectrometry (LC-MS/MS) based proteomics of samples collected from live chickens. Comparing modification of viral proteins in feather follicle epithelial (FFE) cells infected with wildtype or a CHPK-null virus, we identified the US10 protein (pUS10) as a potential target for CHPK in vivo. When expression of pUS10 was evaluated in cell culture and in FFE skin cells during in vivo infection, pUS10 was severely reduced or abrogated in cells infected with CHPK mutant or CHPK-null viruses, respectively, indicating a potential role for pUS10 in transmission. To test this hypothesis, US10 was deleted from the MDV genome, and the reconstituted virus was tested for replication, horizontal transmission, and disease induction. Our results showed that removal of US10 had no effect on the ability of MDV to transmit in experimentally infected chickens, but disease induction in naturally infected chickens was significantly reduced. These results show CHPK is necessary for pUS10 expression both in cell culture and in the host, and pUS10 is important for disease induction during natural infection.

Coinfection in the host can result in functional complementation between live vaccines and virulent virus

One of the greatest achievements of the last century is the development of vaccines against viral diseases. Vaccines are essential for battling infectious diseases and many different formulations are available, including live attenuated vaccines. However, the use of live attenuated vaccines has the potential for adverse effects, including reversion of pathogenicity, recombination, and functional complementation in the host. Marek’s disease is a serious disease in poultry controlled by live attenuated vaccines that has resulted in increased virulence over the decades. Recombination between circulating field viruses or vaccines is a proposed mechanism for the increase in virulence, however, complementation between vaccines and field strains has not been demonstrated in chickens. Here, we describe functional complementation of vaccines with virulent virus to functionally complement transmission and spread in the host. Using the natural virus-host model of Marek’s disease in chickens, our results show dual infection of target cells in chickens with vaccine and virulent virus providing the opportunity for recombination or complementation to transpire. Interestingly, our controlled results showed no evidence of recombination between vaccine and virulent virus, but functional complementation occurred in two independent experiments providing proof for complementation during natural infection in vaccinated individuals. These results suggest complementation as a potential mechanism for vaccine-mediated viral evolution and the potential for complementation should be taken into consideration when developing novel vaccines.

ICTV Virus Taxonomy Profile: Herpesviridae 2021

Members of the family Herpesviridae have enveloped, spherical virions with characteristic complex structures consisting of symmetrical and non-symmetrical components. The linear, double-stranded DNA genomes of 125–241 kbp contain 70–170 genes, of which 43 have been inherited from an ancestral herpesvirus. In general, herpesviruses have coevolved with and are highly adapted to their hosts, which comprise many mammalian, avian and reptilian species. Following primary infection, they are able to establish lifelong latent infection, during which there is limited viral gene expression. Severe disease is usually observed only in the foetus, the very young, the immunocompromised or following infection of an alternative host. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the family Herpesviridae, which is available at ictv.global/report/herpesviridae.

Marek's disease alphaherpesvirus (MDV) RLORF4 is not required for expression of glycoprotein C and interindividual spread

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus that causes Marek's disease (MD) in chickens. RLORF4 is a MDV-specific gene that is systematically deleted during attenuation of MDV in vitro. Concomitantly, the expression of glycoprotein C (gC) is diminished during attenuation, suggesting these two changes may be linked. Original studies in which RLORF4 was deleted utilized an infectious clone that lacked gC expression due to a frame-shift mutation within the gC open reading frame (UL44). Here, we utilized an infectious clone in which gC expression was restored to test our hypothesis that RLORF4 is important for expression of MDV gC, and subsequently, interindividual spread. Contrary to our hypothesis, gC expression was unaltered during both in vitro and in vivo replication of RLORF4-null MDV and was able to efficiently transmit from chicken to chicken, conclusively showing that RLORF4 does not regulate gC expression and is not required for horizontal transmission.

Cellular Stress Response to Varicella-Zoster Virus Infection of Human Skin Includes Highly Elevated Interleukin-6 Expression

Background

The infectious cycle of varicella-zoster virus (VZV) after reactivation from the dorsal root ganglia includes replication and assembly of complete enveloped virions in the human skin to cause the characteristic herpes zoster (shingles).

Methods

To pursue studies of innate immunity to VZV infection, we have adapted a fetal skin organ culture model to a human neonatal foreskin explant model.

Results

Abundant expression of VZV IE62, gE, and gC was visualized by confocal microscopy while numerous enveloped virions were observed by electron microscopy in infected skin organ cultures. Microarray experiments demonstrated that the patterns of upregulated transcripts differed between VZV-infected cells and VZV-infected skin explants. One result stood out, namely a >30-fold elevated interleukin (IL)-6 level in the infected skin explant that was not present in the infected monolayer culture. The IL-6 results in the polyermase chain reaction (PCR) assay were reproduced by quantitative PCR testing with newly designed primers. To determine if increased transcription was accompanied by increased IL-6 expression, we quantitated the levels of IL-6 protein in the explant media at increasing intervals after infection. We found a statistically significant increase in IL-6 protein levels secreted into the media from VZV-infected skin explants as compared with mock-infected explants.

Conclusions

The cellular stress response to VZV infection in neonatal skin explants included highly elevated levels of IL-6 transcription and expression. This skin organ model could be adapted to other viruses with a skin tropism, such as herpes simplex virus.

Interindividual Spread of Herpesviruses

Interindividual spread of herpesviruses is essential for the virus life cycle and maintenance in host populations. For most herpesviruses, the virus-host relationship is close, having coevolved over millions of years resulting in comparatively high species specificity. The mechanisms governing interindividual spread or horizontal transmission are very complex, involving conserved herpesviral and cellular proteins during the attachment, entry, replication, and egress processes of infection. Also likely, specific herpesviruses have evolved unique viral and cellular interactions during cospeciation that are dependent on their relationship. Multiple steps are required for interindividual spread including virus assembly in infected cells; release into the environment, followed by virus attachment; and entry into new hosts. Should any of these steps be compromised, transmission is rendered impossible. This review will focus mainly on the natural virus-host model of Marek's disease virus (MDV) in chickens in order to delineate important steps during interindividual spread.

Differential expression of Marek's disease virus (MDV) late proteins during in vitro and in situ replication: role for pUL47 in regulation of the MDV UL46-UL49 gene locus

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus that replicates in a highly cell-associated manner in vitro. Production of infectious cell-free virus only occurs in feather follicle epithelial (FFE) cells of infected chicken skins. Previously, we described differential expression for a core alphaherpesvirus protein, pUL47 that was found to be abundantly expressed in FFE cells of infected chickens, while barely detectable during in vitro propagation. Here, we further examined the dynamics of expression of four tegument proteins within the UL46-49 locus during in vitro and in situ replication. All four proteins examined were expressed abundantly in situ, whereas both pUL47 and pUL48 expression were barely detectable in vitro. Replacement of the putative UL47 and UL48 promoters with the minimal cytomegalovirus promoter enhanced mRNA and protein expression in vitro. Interestingly, enhanced expression of pUL47 resulted in increased UL46, UL48, and UL49 transcripts that resulted in increased pUL46 and pUL48 expression.

Expression of fluorescent proteins within the repeat long region of the Marek's disease virus genome allows direct identification of infected cells while retaining full pathogenicity

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus and causes Marek's disease (MD) in chickens. RLORF4 is an MDV-specific gene located in the repeat long (RL) regions of the genome and is directly involved in attenuation. In this report, we generated recombinant (r)MDVs in which eGFP or mRFP was inserted in-frame of the 3' end of the RLORF4 gene. In vitro growth was unaffected and infected cells could be identified by using fluorescent microscopy. Interestingly, though inserted in-frame with RLORF4, eGFP and mRFP were expressed alone, confirming mRNA expression and splicing within the RL of MDV is complex. In vivo, rMDVs expressing mRFP or eGFP caused tumors similar to wild-type MDV. Fluorescent protein expression could be seen in spleen, tumor, and feather follicle epithelial cells. These results show that expression of fluorescent proteins within the RL region results in fluorescent rMDVs that still maintains full pathogenicity in the chicken.

A deletion in the glycoprotein L (gL) gene of U.S. Marek's disease virus (MDV) field strains is insufficient to confer increased pathogenicity to the bacterial artificial chromosome (BAC)-based strain, RB-1B

Marek's disease (MD) is a highly transmissible, herpesvirus-associated malignancy of chickens and turkeys caused by Marek's disease virus (MDV). MD is currently controlled through the use of nonsterilizing vaccines composed of antigenically related, apathogenic herpesviruses Mardivirus 2 (MDV-2), Meleagrid herpesvirus 1 (herpesvirus of turkeys, HVT), or attenuated MDV-1 strain CVI988 (Rispens). Since the mid-1960s, field strains of MDV have increased in virulence, due, in part, to the widespread use of vaccines since the early 1970s. One mutation that we have identified common to very virulent field strains (vv and vv+MDVs) since the 1990s has been a mutation in the UL1 gene, encoding glycoprotein L (gL). This mutation, a 12-nucleotide (nt) deletion in the signal peptide of gL, has been associated with increased virulence and decreased vaccine protection in the context of challenge with a vv+MDV, strain TK. To determine whether this mutation alone was sufficient to confer increased virulence, we introduced this mutation into the transmission-competent pRB-1B bacterial artificial chromosome (BAC) using two-step, Red-mediated recombination. The resulting mutant, pRB-1BgLdelta, was tested for changes in replication in cell culture using multistep growth curves, plaque size analysis, viral burst analysis, and the ability to compete with the parental virus when co-transfected at different ratios and sequentially passaged. In addition, we examined this mutant for changes in pathogenicity in inoculated and contact-exposed unvaccinated and vaccinated chickens. Our data show minor differences in plaque sizes in cell culture, but no discernible changes in the infection of specific-pathogen-free (SPF) leghorn chickens. We therefore conclude that although this mutation is indeed common to MDV field strains isolated in the eastern United States, it is insufficient to confer increased virulence or loss of vaccine protection previously observed for a vv+MDV strain having this mutation.

Marek's disease virus late protein expression in feather follicle epithelial cells as early as 8 days postinfection

Marek's disease virus (MDV) or Gallid herpesvirus 2 (GaHV-2) is a lymphotropic alphaherpesvirus and causes Marek's disease. Former studies have demonstrated that MDV is spread from chicken to chicken about 2 wk postexposure as infectious dander shed from infected chickens. More recent reports, using highly sensitive quantitative PCR analyses of dander from infected chickens, suggested that MDV replicates and is shed from the chicken much earlier (5-7 days). However, detection of viral DNA in chicken dander does not indicate whether fully infectious virus is present. To determine if viral replication is present in the skin of infected chickens at these early times, expression of a late viral protein indicative of fully productive virus replication was evaluated using fluorescent microscopy. To do this, highly virulent and attenuated recombinant (r)MDV was generated that abundantly expresses the monomeric red fluorescent protein fused to the late UL47 (VP13/14) protein in feather follicle epithelial cells. Detection of viral DNA could be detected in the skin of infected chickens as early as 6 days postinfection (p.i.), consistent with previous reports detecting viral DNA in dander shed from infected chickens. Replication of virulent rMDV was evident in the feather follicles as early as 8 days p.i., while attenuated rMDV replication in the feather follicles was delayed 1-2 days. Former studies, using less sensitive techniques, suggested viral protein expression to occur about 10-12 days p.i. Undoubtedly differences in time of detection can partly be explained by multiple factors including the pathotype of virus, the route of infection, and the age and genetic line of the infected chickens used in different studies. In summary, though viral DNA can be detected as early as 6 days p.i., late viral protein expression, indicative of infectious virus production, occurs 2-3 days after DNA detection, but earlier than previously thought.

Marek's disease virus (MDV) ubiquitin-specific protease (USP) performs critical functions beyond its enzymatic activity during virus replication

Marek's disease virus (MDV) encodes an ubiquitin-specific protease (USP) within its UL36 gene. USP is highly conserved among herpesviruses and was shown to be important for MDV replication and pathogenesis in MDV's natural host, the chicken. To further investigate the role of MDV USP, several recombinant (r) MDVs were generated and their in vitro phenotypes were evaluated using plaque size and growth kinetics assays. We discovered that the N-terminus of pUL36 is essential for MDV replication and could not be complemented by ectopic expression of MDV USP. In addition, we demonstrated that the region located between the conserved glutamine (Q85) and leucine (L106) residues comprising the active site cysteine (C98) is also essential for MDV replication. Based on the analyses of the rMDVs generated here, we concluded that MDV USP likely contributes to the structure and/or stability of pUL36 and affects replication and oncogenesis of MDV beyond its enzymatic activity.

Marek's disease virus expresses multiple UL44 (gC) variants through mRNA splicing that are all required for efficient horizontal transmission

Marek's disease (MD) is a devastating oncogenic viral disease of chickens caused by Gallid herpesvirus 2, or MD virus (MDV). MDV glycoprotein C (gC) is encoded by the alphaherpesvirus UL44 homolog and is essential for the horizontal transmission of MDV (K. W. Jarosinski and N. Osterrieder, J. Virol. 84:7911-7916, 2010). Alphaherpesvirus gC proteins are type 1 membrane proteins and are generally anchored in cellular membranes and the virion envelope by a short transmembrane domain. However, the majority of MDV gC is secreted in vitro, although secondary-structure analyses predict a carboxy-terminal transmembrane domain. In this report, two alternative mRNA splice variants were identified by reverse transcription (RT)-PCR analyses, and the encoded proteins were predicted to specify premature stop codons that would lead to gC proteins that lack the transmembrane domain. Based on the size of the intron removed for each UL44 (gC) transcript, they were termed gC104 and gC145. Recombinant MDV viruses were generated in which only full-length viral gC (vgCfull), gC104 (vgC104), or gC145 (vgC145) was expressed. Predictably, gCfull was expressed predominantly as a membrane-associated protein, while both gC104 and gC145 were secreted, suggesting that the dominant gC variants expressed in vitro are the spliced variants. In experimentally infected chickens, the expression of each of the gC variants individually did not alter replication or disease induction. However, horizontal transmission was reduced compared to that of wild-type or revertant viruses when the expression of only a single gC was allowed, indicating that all three forms of gC are required for the efficient transmission of MDV in chickens.

Dual infection and superinfection inhibition of epithelial skin cells by two alphaherpesviruses co-occur in the natural host

Hosts can be infected with multiple herpesviruses, known as superinfection; however, superinfection of cells is rare due to the phenomenon known as superinfection inhibition. It is believed that dual infection of cells occurs in nature, based on studies examining genetic exchange between homologous alphaherpesviruses in the host, but to date, this has not been directly shown in a natural model. In this report, gallid herpesvirus 2 (GaHV-2), better known as Marek's disease virus (MDV), was used in its natural host, the chicken, to determine whether two homologous alphaherpesviruses can infect the same cells in vivo. MDV shares close similarities with the human alphaherpesvirus, varicella zoster virus (VZV), with respect to replication in the skin and exit from the host. Recombinant MDVs were generated that express either the enhanced GFP (eGFP) or monomeric RFP (mRFP) fused to the UL47 (VP13/14) herpesvirus tegument protein. These viruses exhibited no alteration in pathogenic potential and expressed abundant UL47-eGFP or -mRFP in feather follicle epithelial cells in vivo. Using laser scanning confocal microscopy, it was evident that these two similar, but distinguishable, viruses were able to replicate within the same cells of their natural host. Evidence of superinfection inhibition was also observed. These results have important implications for two reasons. First, these results show that during natural infection, both dual infection of cells and superinfection inhibition can co-occur at the cellular level. Secondly, vaccination against MDV with homologous alphaherpesvirus like attenuated GaHV-2, or non-oncogenic GaHV-3 or meleagrid herpesvirus (MeHV-1) has driven the virus to greater virulence and these results implicate the potential for genetic exchange between homologous avian alphaherpesviruses that could drive increased virulence. Because the live attenuated varicella vaccine is currently being administered to children, who in turn could be superinfected by wild-type VZV, this could potentiate recombination events of VZV as well.

Herpesvirus telomerase RNA (vTR) with a mutated template sequence abrogates herpesvirus-induced lymphomagenesis

Telomerase reverse transcriptase (TERT) and telomerase RNA (TR) represent the enzymatically active components of telomerase. In the complex, TR provides the template for the addition of telomeric repeats to telomeres, a protective structure at the end of linear chromosomes. Human TR with a mutation in the template region has been previously shown to inhibit proliferation of cancer cells in vitro. In this report, we examined the effects of a mutation in the template of a virus encoded TR (vTR) on herpesvirus-induced tumorigenesis in vivo. For this purpose, we used the oncogenic avian herpesvirus Marek's disease virus (MDV) as a natural virus-host model for lymphomagenesis. We generated recombinant MDV in which the vTR template sequence was mutated from AATCCCAATC to ATATATATAT (vAU5) by two-step Red-mediated mutagenesis. Recombinant viruses harboring the template mutation replicated with kinetics comparable to parental and revertant viruses in vitro. However, mutation of the vTR template sequence completely abrogated virus-induced tumor formation in vivo, although the virus was able to undergo low-level lytic replication. To confirm that the absence of tumors was dependent on the presence of mutant vTR in the telomerase complex, a second mutation was introduced in vAU5 that targeted the P6.1 stem loop, a conserved region essential for vTR-TERT interaction. Absence of vTR-AU5 from the telomerase complex restored virus-induced lymphoma formation. To test if the attenuated vAU5 could be used as an effective vaccine against MDV, we performed vaccination-challenge studies and determined that vaccination with vAU5 completely protected chickens from lethal challenge with highly virulent MDV. Taken together, our results demonstrate 1) that mutation of the vTR template sequence can completely abrogate virus-induced tumorigenesis, likely by the inhibition of cancer cell proliferation, and 2) that this strategy could be used to generate novel vaccine candidates against virus-induced lymphoma.

Herpesvirus telomeric repeats facilitate genomic integration into host telomeres and mobilization of viral DNA during reactivation

Herpesvirus telomeric repeats facilitate virus integration into host telomeres, a process which is required for the establishment of virus latency.

Some herpesviruses, particularly lymphotropic viruses such as Marek’s disease virus (MDV) and human herpesvirus 6 (HHV-6), integrate their DNA into host chromosomes. MDV and HHV-6, among other herpesviruses, harbor telomeric repeats (TMRs) identical to host telomeres at either end of their linear genomes. Using MDV as a natural virus-host model, we show that herpesvirus TMRs facilitate viral genome integration into host telomeres and that integration is important for establishment of latency and lymphoma formation. Integration into host telomeres also aids in reactivation from the quiescent state of infection. Our results and the presence of TMRs in many herpesviruses suggest that integration mediated by viral TMRs is a conserved mechanism, which ensures faithful virus genome maintenance in host cells during cell division and allows efficient mobilization of dormant viral genomes. This finding is of particular importance as reactivation is critical for virus spread between susceptible individuals and is necessary for continued herpesvirus evolution and survival.

Further analysis of Marek's disease virus horizontal transmission confirms that U(L)44 (gC) and U(L)13 protein kinase activity are essential, while U(S)2 is nonessential

Marek's disease virus (MDV) causes a devastating disease in chickens characterized by the development of lymphoblastoid tumors in multiple organs and is transmitted from the skin of infected chickens. We have previously reported that the U(S)2, U(L)44 (glycoprotein C [gC]), and U(L)13 genes are essential for horizontal transmission of MDV in gain-of-function studies using an a priori spread-deficient virus that was based on an infectious clone from the highly virulent RB-1B virus (pRB-1B). To precisely determine the importance of each individual gene in the process of chicken-to-chicken transmission, we used the transmission-restored clone that readily transmits horizontally and mutated each individual gene in loss-of-function experiments. Two independent U(S)2-negative mutants transmitted horizontally, eliminating U(S)2 as being essential for the process. In contrast, the absence of gC expression or mutating the invariant lysine essential for U(L)13 kinase activity abolished horizontal spread of MDV between chickens.

Down-regulation of MHC class I by the Marek's disease virus (MDV) UL49.5 gene product mildly affects virulence in a haplotype-specific fashion

Marek's disease is a devastating neoplastic disease of chickens caused by Marek's disease virus (MDV). MDV down-regulates surface expression of MHC class I molecules, although the mechanism has remained elusive. MDV harbors a UL49.5 homolog that has been shown to down-regulate MHC class I expression in other Varicelloviruses. Using in vitro assays, we showed that MDV pUL49.5 down-regulates MHC class I directly and identified its cytoplasmic tail as essential for this function. In vivo, viruses lacking the cytoplasmic tail of pUL49.5 showed no differences in MD pathogenesis compared to revertant viruses in highly susceptible chickens of the B(19)B(19) MHC class I haplotype, while there was a mild reduction in pathogenic potential of the deletion viruses in chickens more resistant to MD pathogenesis (MHC:B(21)B(21)). We concluded that the pathogenic effect of MHC class I down-regulation mediated by pUL49.5 is small because virus immune evasion possibly requires more than one viral protein.

Viral control of vTR expression is critical for efficient formation and dissemination of lymphoma induced by Marek's disease virus (MDV)

Marek's disease virus (MDV) is an alphaherpesvirus that causes lethal T-cell lymphomas in chickens. MDV is unique in that it harbors two copies of a viral telomerase RNA subunit (vTR) in its genome exhibiting 88% sequence identity to the chicken orthologue, chTR. The minimal telomerase ribonucleoprotein complex consists of a protein subunit with reverse transcriptase activity (TERT) and TR. Physiologically, the complex compensates for the progressive telomere shortening that occurs during mitosis and is involved in the process of cellular immortalization. Previous studies showed that MDV vTR performes an auxiliary function during oncogenesis. Comparative in vitro analysis of the viral and chicken TR promoters revealed that the vTR promoter (PvTR) was up to 3-fold more efficient than the chTR promoter (PchTR) in avian cells and that the stronger transcriptional activity of PvTR resulted largely from an E-box located two nucleotides downstream of the transcriptional start site of the vTR gene. To test the hypothesis that PvTR is required for vTR expression and, hence, efficient tumor formation, we generated a recombinant virus, vPchTR+/+, in which the vTR promoter was replaced by that of chTR. In vivo, growth of vPchTR+/+ was indistinguishable from that of parental virus; however, tumor induction was reduced by >50% and lymphomas were smaller and less disseminated when compared to those induced by parental virus. We concluded that PvTR is not required for lytic replication in vivo but is essential for efficient transcription of vTR and thereby critical for efficient MDV lymphoma formation.

Effective treatment of respiratory alphaherpesvirus infection using RNA interference

BACKGROUND

Equine herpesvirus type 1 (EHV-1), a member of the Alphaherpesvirinae, is spread via nasal secretions and causes respiratory disease, neurological disorders and abortions. The virus is a significant equine pathogen, but current EHV-1 vaccines are only partially protective and effective metaphylactic and therapeutic agents are not available. Small interfering RNAs (siRNA's), delivered intranasally, could prove a valuable alternative for infection control. siRNA's against two essential EHV-1 genes, encoding the viral helicase (Ori) and glycoprotein B, were evaluated for their potential to decrease EHV-1 infection in a mouse model. METHODOLOGY/PRINCIPAL FNDINGS: siRNA therapy in vitro significantly reduced virus production and plaque size. Viral titers were reduced 80-fold with 37.5 pmol of a single siRNA or with as little as 6.25 pmol of each siRNA when used in combination. siRNA therapy in vivo significantly reduced viral replication and clinical signs. Intranasal treatment did not require a transport vehicle and proved effective when given up to 12 h before or after infection.

CONCLUSIONS/SIGNIFICANCE

siRNA treatment has potential for both prevention and early treatment of EHV-1 infections.

Negative modulation of the chicken infectious anemia virus promoter by COUP-TF1 and an E box-like element at the transcription start site binding δEF1

Expression of enhanced green fluorescent protein (EGFP) under control of the promoter-enhancer of chicken infectious anemia virus (CAV) is increased in an oestrogen receptor-enhanced cell line when treated with oestrogen and the promoter-enhancer binds unidentified proteins that recognize a consensus oestrogen response element (ERE). Co-transfection assays with the CAV promoter and the nuclear receptor chicken ovalbumin upstream promoter transcription factor 1 (COUP-TF1) showed that expression of EGFP was decreased by 50 to 60 % in DF-1 and LMH cells. The CAV promoter that included sequences at and downstream of the transcription start point had less expression than a short promoter construct. Mutation of a putative E box at this site restored expression levels. Electromobility shift assays showed that the transcription regulator delta-EF1 (δEF1) binds to this E box region. These findings indicate that the CAV promoter activity can be affected directly or indirectly by COUP-TF1 andδEF1.

Horizontal transmission of Marek's disease virus requires US2, the UL13 protein kinase, and gC

Marek's disease virus (MDV) causes a general malaise in chickens that is mostly characterized by the development of lymphoblastoid tumors in multiple organs. The use of bacterial artificial chromosomes (BACs) for cloning and manipulation of the MDV genome has facilitated characterization of specific genes and genomic regions. The development of most MDV BACs, including pRB-1B-5, derived from a very virulent MDV strain, involved replacement of the US2 gene with mini-F vector sequences. However, when reconstituted viruses based on pRB-1B were used in pathogenicity studies, it was discovered that contact chickens housed together with experimentally infected chickens did not contract Marek's disease (MD), indicating a lack of horizontal transmission. Staining of feather follicle epithelial cells in the skins of infected chickens showed that virus was present but was unable to be released and/or infect susceptible chickens. Restoration of US2 and removal of mini-F sequences within viral RB-1B did not alter this characteristic, although in vivo viremia levels were increased significantly. Sequence analyses of pRB-1B revealed that the UL13, UL44, and US6 genes encoding the UL13 serine/threonine protein kinase, glycoprotein C (gC), and gD, respectively, harbored frameshift mutations. These mutations were repaired individually, or in combination, using two-step Red mutagenesis. Reconstituted viruses were tested for replication, MD incidence, and their abilities to horizontally spread to contact chickens. The experiments clearly showed that US2, UL13, and gC in combination are essential for horizontal transmission of MDV and that none of the genes alone is able to restore this phenotype.

Isolation and molecular characterization of a new Muscovy duck parvovirus from Muscovy ducks in the USA

Between 1997 and 1999 several cases of a new disease in Muscovy ducks were reported in Pennsylvania, USA. The cases were characterized by locomotor dysfunction, weakness, recumbency, 40 to 60% morbidity and 10 to 40% mortality. The most characteristic microscopic lesions were moderate to severe degenerative rhabodomyopathy. In order to characterize the aetiological agent, virus isolation was attempted from the spleen, liver, heart, skeletal muscle and intestine by inoculation of 14-day-old Muscovy duck embryos with tissue homogenates. Deaths occurred on the second egg passage and parvoviruses were isolated by serial passage of allantoic fluid from dead embryos and then in Muscovy duck embryo fibroblast (MDEF) cultures. Parvovirus particles were observed in allantoic fluids and supernatants of MDEF cultures by transmission electron microscopy. Two genomic fragments, comprising 1108 nucleotides of the right open reading frame that codes for the structural viral proteins 1, 2 and 3, were amplified by polymerase chain reaction from one of the isolates, Muscovy duck parvovirus (MDPV)/PSU-31010. Comparison of this fragment with available sequences of other MDPV and related goose parvovirus (GPV) isolates showed that it had only 84.5% sequence identity with other MDPV isolates and 84.6% identity with the GPV isolates. This region shares over 99% identity among previously sequenced MDPV isolates and 95% identity among the related GPV isolates. This suggests that MDPV/PSU-31010 is divergent from all other sequenced MDPV and GPV isolates, and may represent a new group of avian parvoviruses.

Marek's disease virus: lytic replication, oncogenesis and control

Marek's disease (MD) is caused by a ubiquitous, lymphotropic alphaherpesvirus, MD virus (MDV). MD has been a major concern in the poultry industry owing to the emergence of increasingly virulent strains over the last few decades that were isolated in the face of comprehensive vaccination. The disease is characterized by a variety of clinical signs; among them are neurological symptoms, chronic wasting and, most notably, the development of multiple lymphomas that manifest as solid tumors in the viscera and musculature. Much work has been devoted to study MD-induced oncogenesis and the genes involved in this process. Among the many genes encoded by MDV, a number have been shown recently to affect the development of tumors in chickens, one protein directly causing transformation of cells (Meq) and another being involved in maintaining transformed cells (vTR). Other MDV gene products modulate and are involved in early lytic in vivo replication, thereby increasing the chance of transformation occurring. In this review, we will summarize specific genes encoded by MDV that are involved in the initiation and/or maintenance of transformation and will focus mostly on current vaccination and control strategies against MD, particularly how modern molecular biological methods may be used to improve strategies to combat the disease in the future.

Expression of Marek's disease virus phosphorylated polypeptide pp38 produces splice variants and enhances metabolic activity

The phosphorylated polypeptide (pp)38 of oncogenic Marek's disease (MD) herpesvirus (MDV) is expressed during lytic infections in vivo and in vitro, but its functions have not been fully elucidated. The quail cell line QT-35, latently infected with MDV, was used to generate QTP32 in which pp38 is expressed under control of a tetracycline controlled promoter to examine possible functions of pp38. Induction of pp38 did not influence late MDV genes expression, but it enhanced mitochondrial dehydrogenase activity significantly. Two new pp38 splice variants were found in induced QTP32 cells, in additional in vitro systems and MDV-infected chickens. Differential expression of full-length pp38 and splice variants suggests that the splice variants are important during latency and perhaps transformation. Polypeptides of 40 and 20kDa were detected by Western blot using monoclonal antibody H19. These polypeptides were also produced in DF-1 cells transfected with a pp38 construct in which the splice acceptor sites had been mutated. Our results add important new information to the role of pp38 in the pathogenesis of MD. The data suggest that pp38 and the two newly described splice variants may influence metabolic activity, which may have important consequences for the understanding of latency and tumor development.

Attenuation of Marek's disease virus by deletion of open reading frame RLORF4 but not RLORF5a

Marek's disease (MD) in chickens is caused by the alphaherpesvirus MD virus (MDV) and is characterized by the development of lymphoblastoid tumors in multiple organs. The recent identification and cloning of RLORF4 and the finding that four of six attenuated strains of MDV contained deletions within RLORF4 suggested that it is involved in the attenuation process of MDV. To assess the role of RLORF4 in MD pathogenesis, its coding sequence was deleted in the pRB-1B bacterial artificial chromosome clone. Additionally, RLORF5a was deleted separately to examine its importance for oncogenesis. The sizes of plaques produced by MDV reconstituted from pRB-1BdeltaRLORF5a (rRB-1BdeltaRLORF5a) were similar to those produced by the parental pRB-1B virus (rRB-1B). In contrast, virus reconstituted from pRB-1BDeltaRLORF4 (rRB-1BdeltaRLORF4) produced significantly larger plaques. Replication of the latter virus in cultured cells was higher than that of rRB-1B or rRB-1BdeltaRLORF5a using quantitative PCR (qPCR) assays. In vivo, both deletion mutants and rRB-1B replicated at comparable levels at 4, 7, and 10 days postinoculation (p.i.), as determined by virus isolation and qPCR assays. At 14 days p.i., the number of PFU of virus isolated from chickens infected with rRB-1BdeltaRLORF4 was comparable to that from chickens infected with highly attenuated RB-1B and significantly lower than that from rRB-1B-infected birds. The number of tumors and kinetics of tumor production in chickens infected with rRB-1BdeltaRLORF5a were similar to those of P2a chickens infected with rRB-1B. In stark contrast, none of the chickens inoculated with rRB-1BdeltaRLORF4 died up to 13 weeks p.i.; however, two chickens had tumors at the termination of the experiment. The data indicate that RLORF4 is involved in attenuation of MDV, although the function of RLORF4 is still unknown.

Positive and negative regulation of chicken anemia virus transcription

Chicken anemia virus (CAV) is a small circular single-stranded DNA virus with a single promoter-enhancer region containing four consensus cyclic AMP response element sequences (AGCTCA), which are similar to the estrogen response element (ERE) consensus half-sites (A)GGTCA. These sequences are arranged as direct repeats, an arrangement that can be recognized by members of the nuclear receptor superfamily. Transient-transfection assays which use a short CAV promoter construct that ended at the transcription start site and drive expression of enhanced green fluorescent protein (EGFP) showed high basal activity in DF-1, LMH, LMH/2A, and primary theca and granulosa cells. The estrogen receptor-enhanced cell line, LMH/2A, had significantly greater expression than LMH cells, and this expression was significantly increased with estrogen treatment. A long promoter construct which included GGTCA-like sequences downstream of the first CAV protein translation start site was found to have significantly less EGFP expression in DF-1 cells than the short promoter, which was largely due to decreased RNA transcription. DNA-protein binding assays indicated that proteins recognizing a consensus ERE palindrome also bind GGTCA-like sequences in the CAV promoter. Estrogen receptor and other members of the nuclear receptor superfamily may provide a mechanism to regulate CAV activity in situations of low virus copy number.

Pro-inflammatory Responses in Chicken Spleen and Brain Tissues after Infection with Very Virulent Plus Marek's Disease Virus

In chickens infected with virulent (v) or very virulent (vv) Marek's disease (MD) virus (MDV) strains, small to moderate increases in plasma nitric oxide (NO) levels are seen, respectively, whereas very virulent plus (vv+) strains induce very high levels in vivo. The data presented in this report show that chickens presenting with clinical neurological disease following infection with the vv+ RK-1 strain have significantly higher in vivo NO levels compared to RK-1-infected non-symptomatic chickens. Using quantitative real-time PCR (qPCR) assays, DNA was used to measure MDV copy numbers in the spleen and brain of P2a (MD-susceptible) and N2a (MD-resistant) chickens following infection with the JM-16 (v) or RK-1 (vv+) strains. RNA was used to measure inducible NO synthase (iNOS), interferon-gamma (IFN-gamma), interleukin (IL)-1beta, IL-6, and IL-8 mRNA levels, in addition to MDV-specific mRNA expression using quantitative RT-PCR (qRT-PCR) assays. Viral DNA loads were found to be considerably higher in RK-1-infected chickens than JM-16-infected chickens at most time points in both organs, with viral copy numbers being two to four logs lower in the brain. Large increases in iNOS, IFN-alpha, IL-1beta, IL-6, and IL-8 were seen in the brains of RK-1-infected chickens. These data strongly support the hypothesis that pro-inflammatory responses, including high levels of iNOS/NO, IFN-alpha, and pro-inflammatory cytokine expression in the chicken brain, may play a major role in the neurological diseases associated with vv+MDV strains.

Association between rate of viral genome replication and virulence of Marek's disease herpesvirus strains

The early pathogenesis of Marek's disease virus (MDV) infection is characterized by a lytic infection followed by the induction of latency. Genetically resistant N2a and susceptible P2a chickens were infected with the less virulent JM-16 or the very virulent plus (vv+) RK-1 MDV strains to examine the relationship between virulence and resistance on virus replication during 1-10 days postinfection (dpi) using real-time quantitative polymerase chain reaction (qPCR) and quantitative reverse transcriptase (qRT)-PCR assays. The numbers of copies of the viral DNA or transcripts amplified by these assays were normalized relative to cellular controls and subjected to three-way ANOVA. Viral DNA but not RNA was present in spleens at 1-3 dpi in decreasing quantities, and at 4 dpi, viral DNA started to increase concomitant with the initiation of viral transcription independently of virus strain and genetic resistance. At 6 dpi, JM-16 became latent in resistant N2a and susceptible P2a chickens with low levels of viral transcripts, but transcriptional activity increased in susceptible P2a chickens at 9 and 10 dpi. In contrast, infection with vv+ RK-1 never went into latency in both chicken lines. Viral transcripts were present from 4 to 10 dpi showing a higher and more persistent viral activity that may lead to severe damage to the lymphoid organs resulting in increased immunosuppression and increased incidence of MD. The use of qPCR and qRT-PCR to determine viral DNA load and transcriptional activity may offer an alternative to the current system of pathotyping to characterize new MDV isolates.

Influence of genetic resistance of the chicken and virulence of Marek's disease virus (MDV) on nitric oxide responses after MDV infection

Nitric oxide (NO), a free radical produced by the enzyme NO synthase (NOS), is a potent antiviral agent in addition to having immune regulating functions. Recently, it was reported that chickens resistant (N2a, MHC: B21B21) to the development of Marek's disease (MD) had a greater potential to produce NO than MD-susceptible chickens (P2a, MHC: B19B19). This difference was shown by measuring NO levels in chick embryo fibroblast cultures obtained from these chickens after treatment with lipopolysaccharide and recombinant chicken interferon-gamma (IFN-gamma). To extend these results, the levels of NO in blood plasma from N2a and P2a chickens inoculated with the nonattenuated JM-16 strain of MD virus (MDV) were examined. In four out of five experiments, N2a chickens had increased NO levels at 7 days postinoculation (DPI). In contrast, P2a chickens challenged with JM-16 had a significant increase in NO in only one of four experiments, and in that experiment the increase was delayed (10 DPI) compared with N2a chickens. Attenuation abrogated MDV-induced NO in chickens. Inoculation with MDV strains ranging from mild to very virulent plus showed that the more virulent strains induced the highest level of NO in blood plasma, suggesting a role of NO in the pathogenesis of MD with more virulent strains. On the basis of quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) assays for analysis of mRNA expression, IFN-gamma does not appear to be the primary inducer of inducible (i)NOS gene expression during MDV infection. iNOS gene expression and NO production are mediated during the cytolytic phase of MDV infection on the basis of real-time RT-PCR assays with primers specific for glycoprotein B, a late gene expressed only during the cytolytic phase of MDV infection. These findings implicate NO as a factor potentially involved in increasing virulence of MDV, possibly through immune suppression.

Interferon regulatory factor-1 is required for interferon-gamma-induced MHC class I genes in astrocytes

Recent studies have shown that the role of the transcription factor interferon regulatory factor-1 (IRF-1) in the expression of major histocompatibility complex (MHC) class I molecules is tissue-specific. Our previous studies indicated a role for IRF-1 in expression of MHC class I genes in cultured astrocytes in response to interferon-gamma (IFN-gamma). However, the requirement for IRF-1 in MHC class I expression has not been directly analyzed in neural tissue. To further ascertain the importance of IRF-1 in the induction of MHC class I genes in astrocytes in response to IFN-gamma, we analyzed astrocytes from mice with a targeted disruption of the IRF-1 gene (IRF-1(-/-) mice). As expected, astrocytes from wild type (IRF-1(+/+)) mice showed a coordinate increase in both IRF-1 and MHC class I gene expression in response to IFN-gamma. To the contrary, astrocytes from IRF-1(-/-) mice had greatly reduced MHC class I mRNA expression. MHC class I gene promoter activity in astrocytes was controlled entirely through a single enhancer, the MHC-IRF-E, to which IRF-1 bound in response to IFN-gamma in wild type but not in IRF-1(-/-) mouse astrocytes. In vivo, astrocytes in brains of wild type mice readily responded to IFN-gamma to express MHC class I molecules. This correlated with increased MHC class I mRNA in the brain. In contrast, brains of IRF-1(-/-) mice showed no MHC class I gene induction following exposure to IFN-gamma indicating that all cells in the central nervous system (CNS) including astrocytes with the potential to express MHC class I molecules were dependent on IRF-1. These studies conclusively demonstrate a major role for IRF-1/MHC-IRF-E interactions in controlling MHC class I gene expression in astrocytes in response to IFN-gamma.

Specific deficiency in nuclear factor-kappaB activation in neurons of the central nervous system

The expression and activation of nuclear factor-kappaB (NF-kappaB) in neurons and glia of the central nervous system (CNS) has been intensely investigated because of its potential importance in understanding how this multifunctional transcription factor controls developmental and pathological processes. In particular, there has been interest in how NF-kappaB may be differentially regulated in these two major functional subgroups of cells in the CNS to provide for specific responses to various stimuli. Of special interest are responses to both proinflammatory cytokines and microbial products that signal from specific cell receptors to activate NF-kappaB. In the present studies, both neurons and glia (astrocytes) in vivo expressed latent cytoplasmic NF-kappaB analyzed by immunofluorescence microscopy and electrophoretic mobility shift analysis. In vitro, neurons and astrocytes expressed comparable levels of latent NF-kappaB molecules, but NF-kappaB nuclear localization stimulated by proinflammatory cytokines or microbial products was markedly deficient in neurons. In accord with this finding, the rapid degradation of inhibitor of NF-kappaB alpha (IkappaBalpha) that is seen in astrocytes did not occur in neurons in response to these agents. However, long-term exposure to translational inhibitors resulted in IkappaBalpha decay and activation of latent NF-kappaB in neurons, indicating potential NF-kappaB activity in these cells. Analysis of NF-kappaB-responsive interferon regulatory factor-1 gene expression indicated that increased nuclear NF-kappaB in neurons had transcriptional potential. We conclude that mechanisms responsible for inducible targeting of IkappaBalpha are uniquely regulated in neurons and account for the hypo-responsiveness of these cells to signals generated during microbial infections in the CNS. Thus, modulation of signals that target IkappaBalpha degradation may be unique and a key component of specific NF-kappaB regulation in neurons.

Cellular responses in chickens treated with IFN-alpha orally or inoculated with recombinant Marek's disease virus expressing IFN-alpha

Mammalian type I interferons (IFN-alpha/beta) are potent mediators of innate antiviral immune responses, in particular through enhancement of natural killer (NK) cell cytotoxicity. Recently, chicken IFN-alpha (ChIFN-alpha) has been identified and shown to ameliorate Newcastle disease virus (NDV) infection when given to chickens at relatively high concentrations in the drinking water. In this report, the effect of recombinant ChIFN-alpha (rChIFN-alpha) on NK cell cytotoxicity was examined using (51)Cr-release assays. NK cell cytotoxic activity was also analyzed following inoculation with attenuated Marek's disease virus (MDV) serotype 1 strain R2/23 and a recombinant MDV (parent strain R2/23)-expressing ChIFN-alpha [rMDV(IFN-alpha)]. Treatment of chickens with high doses of rChIFN-alpha in the drinking water significantly decreased NK cell cytotoxicity compared with untreated chickens over a 7-day period. Inoculation of chickens with R2/23 significantly decreased NK cell cytotoxicity as well, whereas the rMDV(IFN-alpha) had no effect on NK cell cytotoxicity. Treatment of chicken embryo cell cultures with rChIFN-alpha inhibited replication of the very virulent MDV RB-1B strain in vitro, and oral treatment of chickens with rChIFN-alpha reduced MDV R2/23 replication in vivo.

Expression and function of the protein tyrosine phosphatase SHP-1 in oligodendrocytes

Previous studies in this laboratory have shown that the SH-2 domain-containing protein tyrosine phosphatase SHP-1 is expressed in CNS glia and functions to modulate cytokine activities in these cells. The present study demonstrates that SHP-1 is expressed within multiple regions of the CNS in vivo, especially in white matter. Interestingly, we show that mice genetically lacking in SHP-1 (motheaten mice) in the CNS displayed dysmyelination. We therefore examined the expression of SHP-1 in the myelin-forming oligodendrocytes. Oligodendrocytes present in either mixed glial cultures or pure cultures expressed high levels of SHP-1 in the cytoplasm of cell bodies and processes. Oligodendrocytes isolated from motheaten mice did not express SHP-1. To test possible functions for SHP-1 in oligodendrocytes in controlling cytokine signaling, we compared the responsiveness of oligodendrocytes isolated from either motheaten or normal littermate mice with IL-6. IL-6 induced higher levels of STAT3 phosphorylation and STAT3-responsive c-fos gene expression in pure oligodendrocyte cultures of motheaten compared with normal littermate mice. These studies demonstrate that oligodendrocytes express SHP-1 and that SHP-1 functions to control IL-6 signaling. SHP-1 may therefore be a critical regulator of oligodendrocyte differentiation in response to IL-6 family cytokines. Further, these findings may relate to dysmyelination in mice lacking SHP-1.

A mechanism for selective induction of 2'-5' oligoadenylate synthetase, anti-viral state, but not MHC class I genes by interferon-beta in neurons

Interferon-inducible expression of major histocompatibility class I genes has previously been found to be quantitatively and functionally deficient in neurons compared to other somatic cells or other neural cell types including astrocytes. This deficiency is a key component of neuronal immunoprivilege during viral infections of the CNS. To the contrary, in the present study, induction of functional antiviral state by IFN-beta in neurons compared to astrocytes was found to be highly efficient with respect to both viral replication and protection from cytopathic effects. A candidate antiviral state gene found to be efficiently induced in neurons by IFN-beta was the 2'-5'-oligoadenylate synthetase (OAS) gene. Unlike MHC class I genes, induction of OAS was comparable in neurons and astrocytes indicating differential expression in these neural cell types. Analysis of OAS gene promoter activity indicated that induction of the OAS gene by IFN-beta was dependent on a region containing the interferon stimulated responsive element (ISRE). In contrast, a construct containing the MHC class I-ISRE responsible for induction by IFN-beta in astrocytes was not responsive to IFN-beta in neurons. Therefore, transcription factor binding to the OAS- and MHC-ISREs was analyzed. While the OAS and MHC Class I site bound equal amounts of the transcriptional repressor IRF-2, the OAS-ISRE preferentially interacted with the transcriptional activator ISGF3 in response to IFN-beta. Further, unlike neurons, upregulation of MHC class I genes in astrocytes was related to binding of IRF-1 instead of IRF-2 to the MHC-ISRE. It is proposed that selective activation of anti-viral state genes compared to MHC class I genes by IFN-beta in neurons is mediated by preferential induction and binding of ISGF3 to anti-viral state gene ISREs but not the MHC-ISRE.