A long and complex enhancer activates transcription of the gene coding for the highly abundant immediate early mRNA in murine cytomegalovirus

Enhancer Function in the 3D Genome

In this review, we consider various aspects of enhancer functioning in the context of the 3D genome. Particular attention is paid to the mechanisms of enhancer-promoter communication and the significance of the spatial juxtaposition of enhancers and promoters in 3D nuclear space. A model of an activator chromatin compartment is substantiated, which provides the possibility of transferring activating factors from an enhancer to a promoter without establishing direct contact between these elements. The mechanisms of selective activation of individual promoters or promoter classes by enhancers are also discussed.

An optimized reporter of the transcription factor hypoxia-inducible factor 1α reveals complex HIF-1α activation dynamics in single cells

Immune cells adopt a variety of metabolic states to support their many biological functions, which include fighting pathogens, removing tissue debris, and tissue remodeling. One of the key mediators of these metabolic changes is the transcription factor hypoxia-inducible factor 1α (HIF-1α). Single-cell dynamics have been shown to be an important determinant of cell behavior; however, despite the importance of HIF-1α, little is known about its single-cell dynamics or their effect on metabolism. To address this knowledge gap, here we optimized a HIF-1α fluorescent reporter and applied it to study single-cell dynamics. First, we showed that single cells are likely able to differentiate multiple levels of prolyl hydroxylase inhibition, a marker of metabolic change, via HIF-1α activity. We then applied a physiological stimulus known to trigger metabolic change, interferon-γ, and observed heterogeneous, oscillatory HIF-1α responses in single cells. Finally, we input these dynamics into a mathematical model of HIF-1α-regulated metabolism and discovered a profound difference between cells exhibiting high versus low HIF-1α activation. Specifically, we found cells with high HIF-1α activation are able to meaningfully reduce flux through the tricarboxylic acid cycle and show a notable increase in the NAD+/NADH ratio compared with cells displaying low HIF-1α activation. Altogether, this work demonstrates an optimized reporter for studying HIF-1α in single cells and reveals previously unknown principles of HIF-1α activation.

Flow cytometric reporter assays provide robust functional analysis of signaling complexes

Conventional assays to probe signaling protein interactions and function involve measurement of luciferase reporter expression within the bulk cell population, with lack of control over target-protein expression level. To address this issue, we have developed a rapid and robust flow cytometric assay for analysis of signaling protein function. A fluorescent reporter and fluorescent tagging of the target protein enables simultaneous assessment of protein expression and signaling within individual cells. We have applied our technique to the analysis of variants of the lipopolysaccharide receptor Toll-like receptor 4 (TLR4) and its adapter protein MyD88, using a NF-кB-responsive promoter driving mScarlet-I expression. The assay enables exclusion of nontransfected cells and overexpressing cells that signal spontaneously. Additionally, our assay allows the identification of protein variants that fail to express. We found that the assays were highly sensitive, with cells expressing an appropriate level of GFP-MyD88 showing approximately 200-fold induction of mScarlet-I by lipopolysaccharide, and we can detect subtle protein concentration-dependent effects of mutations. Importantly, the assay is adaptable to various signaling pathways.

Human cytomegalovirus IE2 may impair the cognitive ability of the hippocampus through the GluNRs/CaMKIIα/CREB signaling pathway in the Rosa26-LSL-IE2/Cre mouse

Nowadays, there are few studies in vivo to explore the effects of Human Cytomegalovirus (HCMV) single gene such as immediate early protein 2 (IE2) on the nervous system, let alone the mechanism that IE2 causes cognitive impairment. In this study, the Rosa26-LSL-IE2/Cre mouse was used to show the effects of IE2 on the cognitive ability and the GluNRs/CaMKIIα/CREB signaling pathway in the hippocampus. We divided the mice into experimental and control groups based on the results of PCR firstly. After that, the cognitive abilities of the two groups were compared through new object recognition (NOR) and Morris water maze (MWM) tests. The results of the behavioral tests showed that the cognitive ability of the experimental mice was lower than that of the control group. It is known that changes in the expression levels of N-methyl D-aspartate receptor 1, 2A, and 2B (GluN1, GluN2A, GluN2B) affect synaptic plasticity and cause cognitive changes. Finally, we analyzed the expression levels of GluN1, GluN2A, GluN2B, and related signaling pathway molecules by qPCR and western blot. We found that the expression levels of the GluNRs/CaMKIIα/CREB signaling pathway were decreased in the experimental group. These results indicated that IE2 could affect the expression levels of GluNRs/CaMKIIα/CREB signaling pathway, which was closely related to the cognitive impairment of the experimental group. In summary, we used this novel mouse model to show that IE2 could cause cognitive impairment in the hippocampus, which might be related to the GluNRs/CaMKIIα/CREB signaling pathway. It is helpful to further understand the mechanism of the cognitive impairment induced by HCMV IE2.

Immunodominant Cytomegalovirus Epitopes Suppress Subdominant Epitopes in the Generation of High-Avidity CD8 T Cells

CD8⁺ T-cell responses to pathogens are directed against infected cells that present pathogen-encoded peptides on MHC class-I molecules. Although natural responses are polyclonal, the spectrum of peptides that qualify for epitopes is remarkably small even for pathogens with high coding capacity. Among those few that are successful at all, a hierarchy exists in the magnitude of the response that they elicit in terms of numbers of CD8⁺ T cells generated. This led to a classification into immunodominant and non-immunodominant or subordinate epitopes, IDEs and non-IDEs, respectively. IDEs are favored in the design of vaccines and are chosen for CD8⁺ T-cell immunotherapy. Using murine cytomegalovirus as a model, we provide evidence to conclude that epitope hierarchy reflects competition on the level of antigen recognition. Notably, high-avidity cells specific for non-IDEs were found to expand only when IDEs were deleted. This may be a host’s back-up strategy to avoid viral immune escape through antigenic drift caused by IDE mutations. Importantly, our results are relevant for the design of vaccines based on cytomegaloviruses as vectors to generate high-avidity CD8⁺ T-cell memory specific for unrelated pathogens or tumors. We propose the deletion of vector-encoded IDEs to avoid the suppression of epitopes of the vaccine target.

Multiplexed engineering glycosyltransferase genes in CHO cells via targeted integration for producing antibodies with diverse complex-type N-glycans

Therapeutic antibodies are decorated with complex-type N-glycans that significantly affect their biodistribution and bioactivity. The N-glycan structures on antibodies are incompletely processed in wild-type CHO cells due to their limited glycosylation capacity. To improve N-glycan processing, glycosyltransferase genes have been traditionally overexpressed in CHO cells to engineer the cellular N-glycosylation pathway by using random integration, which is often associated with large clonal variations in gene expression levels. In order to minimize the clonal variations, we used recombinase-mediated-cassette-exchange (RMCE) technology to overexpress a panel of 42 human glycosyltransferase genes to screen their impact on antibody N-linked glycosylation. The bottlenecks in the N-glycosylation pathway were identified and then released by overexpressing single or multiple critical genes. Overexpressing B4GalT1 gene alone in the CHO cells produced antibodies with more than 80% galactosylated bi-antennary N-glycans. Combinatorial overexpression of B4GalT1 and ST6Gal1 produced antibodies containing more than 70% sialylated bi-antennary N-glycans. In addition, antibodies with various tri-antennary N-glycans were obtained for the first time by overexpressing MGAT5 alone or in combination with B4GalT1 and ST6Gal1. The various N-glycan structures and the method for producing them in this work provide opportunities to study the glycan structure-and-function and develop novel recombinant antibodies for addressing different therapeutic applications.

Regulation of the MIE Locus During HCMV Latency and Reactivation

Human cytomegalovirus (HCMV) is a ubiquitous herpesviral pathogen that results in life-long infection. HCMV maintains a latent or quiescent infection in hematopoietic cells, which is broadly defined by transcriptional silencing and the absence of de novo virion production. However, upon cell differentiation coupled with immune dysfunction, the virus can reactivate, which leads to lytic replication in a variety of cell and tissue types. One of the mechanisms controlling the balance between latency and reactivation/lytic replication is the regulation of the major immediate-early (MIE) locus. This enhancer/promoter region is complex, and it is regulated by chromatinization and associated factors, as well as a variety of transcription factors. Herein, we discuss these factors and how they influence the MIE locus, which ultimately impacts the phase of HCMV infection.

Mouse Cytomegalovirus M34 Encodes a Non-essential, Nuclear, Early-Late Expressed Protein Required for Efficient Viral Replication

Human cytomegalovirus (HCMV) is a prototypic betaherpesvirus which causes severe manifestations in individuals with impaired or immature immunity. To investigate cytomegalovirus-induced pathogenesis and virus-specific immune responses, mouse cytomegalovirus (MCMV) infections in mice are employed as accepted small animal model. MCMV and HCMV share co-linear genomes and encode several homologous proteins. Due to the size and complexity of CMV genomes, the molecular functions of numerous cytomegaloviral gene products remain to be elucidated. While the essential nature of viral genes highlights their biological relevance, it renders functional studies particularly cumbersome by precluding experiments in the infection context. The HCMV-encoded protein pUL34 binds the HCMV genome and regulates viral gene expression (e.g., of US3). Several groups provided compelling evidence that UL34 is essential for HCMV replication. MCMV encodes the homologous protein pM34 (34% identical and 55% similar). Based on unsuccessful attempts to reconstitute M34-deficient virus from a bacterial artificial chromosome (BAC), M34 was previously classified as essential for MCMV replication. To characterize pM34 during viral infection, we engineered and analyzed an MCMV mutant expressing an HA-epitope-tagged pM34 which was expressed with early-late kinetics and localized in the nucleus. Additionally, we generated an M34-deficient (“ΔM34”) MCMV-BAC by replacing the entire M34 coding sequence by a kanamycin resistance cassette. The deletion of M34 was confirmed by Southern blot and PCR. Unexpectedly, we could reconstitute replicating ΔM34-MCMV upon transfection of the BAC DNA into mouse embryonic fibroblasts. The absence of M34 from the genome of the replicating ΔM34-MCMV was also confirmed. Accordingly, a ΔM34-MCMV, in which the kanamycin cassette was excised by frt/Flp-mediated recombination, was also replication competent. In order to corroborate the absence of pM34 protein, the M34 deletion was recapitulated on the background of M34HA, which yielded replicating virus devoid of detectable pM34HA protein. The replication of MCMVs lacking M34 was found to be 10- to 100-fold reduced as compared to wt-MCMV which might explain previous unsuccessful reconstitution attempts conducted by others. Taken together, our findings reveal that MCMV remains replication competent despite the absence of M34, enabling functional studies in the infection context.

Cytomegaloviruses and Macrophages-Friends and Foes From Early on?

Starting at birth, newborn infants are exposed to numerous microorganisms. Adaptation of the innate immune system to them is a delicate process, with potentially advantageous and harmful implications for health development. Cytomegaloviruses (CMVs) are highly adapted to their specific mammalian hosts, with which they share millions of years of co-evolution. Throughout the history of mankind, human CMV has infected most infants in the first months of life without overt implications for health. Thus, CMV infections are intertwined with normal immune development. Nonetheless, CMV has retained substantial pathogenicity following infection in utero or in situations of immunosuppression, leading to pathology in virtually any organ and particularly the central nervous system (CNS). CMVs enter the host through mucosal interfaces of the gastrointestinal and respiratory tract, where macrophages (MACs) are the most abundant immune cell type. Tissue MACs and their potential progenitors, monocytes, are established target cells of CMVs. Recently, several discoveries have revolutionized our understanding on the pre- and postnatal development and site-specific adaptation of tissue MACs. In this review, we explore experimental evidences and concepts on how CMV infections may impact on MAC development and activation as part of host-virus co-adaptation.

Specific transcriptional and post-transcriptional regulation of the major immediate early ICP4 gene of GaHV-2 during the lytic, latent and reactivation phases

Transcriptional and post-transcriptional mechanisms are involved in the switch between the lytic, latent and reactivation phases of the viral cycle in herpesviruses. During the productive phases, herpesvirus gene expression is characterized by a temporally regulated cascade of immediate early (IE), early (E) and late (L) genes. In alphaherpesviruses, the major product of the IE ICP4 gene is a transcriptional regulator that initiates the cascade of gene expression that is essential for viral replication. In this study, we redefine the infected cell protein 4 (ICP4) gene of the oncogenic Marek's disease virus (MDV or gallid herpesvirus 2) as a 9438 nt gene ended with four alternative poly(A) signals and controlled by two alternative promoters containing essentially ubiquitous functional response elements (GC, TATA and CCAAT boxes). The distal promoter is associated with ICP4 gene expression during the lytic and the latent phases, whereas the proximal promoter is associated with the expression of this gene during the reactivation phase. Both promoters are regulated by DNA methylation during the viral cycle and are hypermethylated during latency. Transcript analyses showed ICP4 to consist of three exons and two introns, the alternative splicing of which is associated with five predicted nested ICP4ORFs. We show that the ICP4 gene is highly and specifically regulated by transcriptional and post-transcriptional mechanisms during the three phases of the GaHV-2 viral cycle, with a clear difference in expression between the lytic phase and reactivation from latency in our model.

Human Cytomegalovirus IE2 Protein Disturbs Brain Development by the Dysregulation of Neural Stem Cell Maintenance and the Polarization of Migrating Neurons

Despite the high incidence of severe defects in the central nervous system caused by human cytomegalovirus (HCMV) congenital infection, the mechanism of HCMV neuropathogenesis and the roles of individual viral genes have not yet been fully determined. In this study, we show that the immediate-early 2 (IE2) protein may play a key role in HCMV-caused neurodevelopmental disorders. IE2-transduced neural progenitor cells gave rise to neurospheres with a lower frequency and produced smaller neurospheres than control cells in vitro, indicating reduction of self-renewal and expansion of neural progenitors by IE2. At 2 days after in utero electroporation into the ventricle of the developing brain, a dramatically lower percentage of IE2-expressing cells was detected in the ventricular zone (VZ) and cortical plate (CP) compared to control cells, suggesting that IE2 concurrently dysregulates neural stem cell maintenance in the VZ and neuronal migration to the CP. In addition, most IE2⁺ cells in the lower intermediate zone either showed multipolar morphology with short neurites or possessed nonradially oriented processes, whereas control cells had long, radially oriented monopolar or bipolar neurites. IE2⁺ callosal axons also failed to cross the midline to form the corpus callosum. Furthermore, we provide molecular evidence that the cell cycle arrest and DNA binding activities of IE2 appear to be responsible for the increased neural stem cell exit from the VZ and cortical migrational defects, respectively. Collectively, our results demonstrate that IE2 disrupts the orderly process of brain development in a stepwise manner to further our understanding of neurodevelopmental HCMV pathogenesis.IMPORTANCE HCMV brain pathogenesis has been studied in limited experimental settings, such as in vitro HCMV infection of neural progenitor cells or in vivo murine CMV infection of the mouse brain. Here, we show that IE2 is a pivotal factor that contributes to HCMV-induced abnormalities in the context of the embryonic brain using an in utero gene transfer tool. Surprisingly, IE2, but not HCMV IE1 or murine CMV ie3, interferes pleiotropically with key neurodevelopmental processes, including neural stem cell regulation, proper positioning of migrating neurons, and the callosal axon projections important for communication between the hemispheres. Our data suggest that the wide spectrum of clinical outcomes, ranging from mental retardation to microcephaly, caused by congenital HCMV infection can be sufficiently explained in terms of IE2 action alone.

Selection of differently temporally regulated African swine fever virus promoters with variable expression activities and their application for transient and recombinant virus mediated gene expression

African swine fever virus threatens pig production worldwide due to the lack of vaccines, for which generation of both deletion and insertion mutants is considered. For development of the latter, operational ASFV promoters of different temporal regulation and strengths are desirable. We therefore compared the capacities of putative promoter sequences from p72, CD2v, p30, viral DNA polymerase and U104L genes to mediate expression of luciferase from transfected plasmids after activation in trans, or p30-, DNA polymerase- and U104L promoters in cis, using respective ASFV recombinants. We identified sequences with promoter activities upstream the viral ORFs, and showed that they differ in both their expression intensity regulating properties and in their temporal regulation. In summary, p30 and DNA polymerase promoters are recommended for high level early regulated transgene expression. For late expression, the p72, CD2v and U104L promoter are suitable. The latter however, only if low level transgene expression is aimed.

Refining human T-cell immunotherapy of cytomegalovirus disease: a mouse model with 'humanized' antigen presentation as a new preclinical study tool

With the cover headline 'T cells on the attack,' the journal Science celebrated individualized cancer immunotherapy by adoptive transfer of T cells as the 'Breakthrough of the Year' 2013 (J. Couzin-Frankel in Science 342:1432-1433, 2013). It is less well recognized and appreciated that individualized T cell immunotherapy of cytomegalovirus (CMV) infection is approaching clinical application for preventing CMV organ manifestations, interstitial CMV pneumonia in particular. This coincident medical development is particularly interesting as reactivated CMV infection is a major viral complication in the state of transient immunodeficiency after the therapy of hematopoietic malignancies by hematopoietic cell transplantation (HCT). It may thus be attractive to combine T cell immunotherapy of 'minimal residual disease/leukemia (MRD)' and CMV-specific T cell immunotherapy to combat both risks in HCT recipients simultaneously, and ideally with T cells derived from the respective HLA-matched HCT donor. Although clinical trials of human CMV-specific T cell immunotherapy were promising in that the incidence of virus reactivation and disease was found to be reduced with statistical significance, animal models are still instrumental for providing 'proof of concept' by directly documenting the prevention of viral multiple-organ histopathology and organ failure under controlled conditions of the absence versus presence of the therapy, which obviously is not feasible in an individual human patient. Further, animal models can make predictions regarding parameters that determine the efficacy of T cell immunotherapy for improved study design in clinical investigations, and they allow for manipulating host and virus genetics. The latter is of particular value as it opens the possibility for epitope specificity controls that are inherently missing in clinical trials. Here, we review a recently developed new mouse model that is more approximated to human CMV-specific T cell immunotherapy by 'humanizing' antigen presentation using antigenically chimeric CMV and HLA-transgenic mice to allow for an in vivo testing of the antiviral function of human CMV-specific T cells. As an important new message, this model predicts that T cell immunotherapy is most efficient if CD4 T cells are equipped with a transduced TCR directed against an epitope presented by MHC/HLA class-I for local delivery of 'cognate' help to CD8 effector T cells at infected MHC/HLA class-II-negative host tissue cells.

Transplant-induced reactivation of murine cytomegalovirus immediate early gene expression is associated with recruitment of NF-κB and AP-1 to the major immediate early promoter

Reactivation of latent human cytomegalovirus is a significant infectious complication of organ transplantation and current therapies target viral replication once reactivation of latent virus has already occurred. The specific molecular pathways that activate viral gene expression in response to transplantation are not well understood. Our studies aim to identify these factors, with the goal of developing novel therapies that prevent transcriptional reactivation in transplant recipients. Murine cytomegalovirus (MCMV) is a valuable model for studying latency and reactivation of CMV in vivo. We previously demonstrated that transplantation of MCMV-latently infected kidneys into allogeneic recipients induces reactivation of immediate early (IE) gene expression and epigenetic reprogramming of the major immediate early promoter (MIEP) within 48 h. We hypothesize that these events are mediated by activation of signalling pathways that lead to binding of transcription factors to the MIEP, including AP-1 and NF-κB. Here we show that transplantation induces rapid activation of several members of the AP-1 and NF-κB transcription factor family and we demonstrate that canonical NF-κB (p65/p50), the junD component of AP-1, and nucleosome remodelling complexes are recruited to the MIEP following transplantation. Proteomic analysis of recipient plasma and transcriptome analysis of kidney RNA identified five extracellular ligands, including TNF, IL-1β, IL-18, CD40L and IL-6, and three intracellular signalling pathways associated with reactivation of IE gene expression. Identification of the factors that mediate activation of these signalling pathways may eventually lead to new therapies to prevent reactivation of CMV and its sequelae.

Promoter and Terminator Discovery and Engineering

Control of gene expression is crucial to optimize metabolic pathways and synthetic gene networks. Promoters and terminators are stretches of DNA upstream and downstream (respectively) of genes that control both the rate at which the gene is transcribed and the rate at which mRNA is degraded. As a result, both of these elements control net protein expression from a synthetic construct. Thus, it is highly important to discover and engineer promoters and terminators with desired characteristics. This chapter highlights various approaches taken to catalogue these important synthetic elements. Specifically, early strategies have focused largely on semi-rational techniques such as saturation mutagenesis to diversify native promoters and terminators. Next, in an effort to reduce the length of the synthetic biology design cycle, efforts in the field have turned towards the rational design of synthetic promoters and terminators. In this vein, we cover recently developed methods such as hybrid engineering, high throughput characterization, and thermodynamic modeling which allow finer control in the rational design of novel promoters and terminators. Emphasis is placed on the methodologies used and this chapter showcases the utility of these methods across multiple host organisms.

A temporal gate for viral enhancers to co-opt Toll-like-receptor transcriptional activation pathways upon acute infection

Viral engagement with macrophages activates Toll-Like-Receptors (TLRs) and viruses must contend with the ensuing inflammatory responses to successfully complete their replication cycle. To date, known counter-strategies involve the use of viral-encoded proteins that often employ mimicry mechanisms to block or redirect the host response to benefit the virus. Whether viral regulatory DNA sequences provide an opportunistic strategy by which viral enhancer elements functionally mimic innate immune enhancers is unknown. Here we find that host innate immune genes and the prototypical viral enhancer of cytomegalovirus (CMV) have comparable expression kinetics, and positively respond to common TLR agonists. In macrophages but not fibroblasts we show that activation of NFκB at immediate-early times of infection is independent of virion-associated protein, M45. We find upon virus infection or transfection of viral genomic DNA the TLR-agonist treatment results in significant enhancement of the virus transcription-replication cycle. In macrophage time-course infection experiments we demonstrate that TLR-agonist stimulation of the viral enhancer and replication cycle is strictly delimited by a temporal gate with a determined half-maximal time for enhancer-activation of 6 h; after which TLR-activation blocks the viral transcription-replication cycle. By performing a systematic siRNA screen of 149 innate immune regulatory factors we identify not only anticipated anti-viral and pro-viral contributions but also new factors involved in the CMV transcription-replication cycle. We identify a central convergent NFκB-SP1-RXR-IRF axis downstream of TLR-signalling. Activation of the RXR component potentiated direct and indirect TLR-induced activation of CMV transcription-replication cycle; whereas chromatin binding experiments using wild-type and enhancer-deletion virus revealed IRF3 and 5 as new pro-viral host transcription factor interactions with the CMV enhancer in macrophages. In a series of pharmacologic, siRNA and genetic loss-of-function experiments we determined that signalling mediated by the TLR-adaptor protein MyD88 plays a vital role for governing the inflammatory activation of the CMV enhancer in macrophages. Downstream TLR-regulated transcription factor binding motif disruption for NFκB, AP1 and CREB/ATF in the CMV enhancer demonstrated the requirement of these inflammatory signal-regulated elements in driving viral gene expression and growth in cells as well as in primary infection of neonatal mice. Thus, this study shows that the prototypical CMV enhancer, in a restricted time-gated manner, co-opts through DNA regulatory mimicry elements, innate-immune transcription factors to drive viral expression and replication in the face of on-going pro-inflammatory antiviral responses in vitro and in vivo and; suggests an unexpected role for inflammation in promoting acute infection and has important future implications for regulating latency.

Here we discover how inflammatory signalling may unintentionally promote infection, as a result of viruses evolving DNA sequences, known as enhancers, which act as a bait to prey on the infected cell transcription factors induced by inflammation. The major inflammatory transcription factors activated are part of the TLR-signalling pathway. We find the prototypical viral enhancer of cytomegalovirus can be paradoxically boosted by activation of inflammatory “anti-viral” TLR-signalling independent of viral structural proteins. This leads to an increase in viral gene expression and replication in cell-culture and upon infection of mice. We identify an axis of inflammatory transcription factors, acting downstream of TLR-signalling but upstream of interferon inhibition. Mechanistically, the central TLR-adapter protein MyD88 is shown to play a critical role in promoting viral enhancer activity in the first 6h of infection. The co-option of TLR-signalling exceeds the usage of NFκB, and we identify IRF3 and 5 as newly found viral-enhancer interacting inflammatory transcription factors. Taken together this study reveals how virus enhancers, employ a path of least resistance by directly harnessing within a short temporal window, the activation of anti-viral signalling in macrophages to drive viral gene expression and replication to an extent that has not been recognised before.

Enhancers, enhancers – from their discovery to today’s universe of transcription enhancers

Transcriptional enhancers are short (200–1500 base pairs) DNA segments that are able to dramatically boost transcription from the promoter of a target gene. Originally discovered in simian virus 40 (SV40), a small DNA virus, transcription enhancers were soon also found in immunoglobulin genes and other cellular genes as key determinants of cell-type-specific gene expression. Enhancers can exert their effect over long distances of thousands, even hundreds of thousands of base pairs, either from upstream, downstream, or from within a transcription unit. The number of enhancers in eukaryotic genomes correlates with the complexity of the organism; a typical mammalian gene is likely controlled by several enhancers to fine-tune its expression at different developmental stages, in different cell types and in response to different signaling cues. Here, I provide a personal account of how enhancers were discovered more than 30 years ago, and also address the amazing development of the field since then.

Protection of pigs against pandemic swine origin H1N1 influenza A virus infection by hemagglutinin- or neuraminidase-expressing attenuated pseudorabies virus recombinants

Influenza is an important respiratory disease of pigs, and may lead to novel human pathogens like the 2009 pandemic H1N1 swine-origin influenza virus (SoIV). Therefore, improved influenza vaccines for pigs are required. Recently, we demonstrated that single intranasal immunization with a hemagglutinin (HA)-expressing pseudorabies virus recombinant of vaccine strain Bartha (PrV-Ba) protected pigs from H1N1 SoIV challenge (Klingbeil et al., 2014). Now we investigated enhancement of efficacy by prime-boost vaccination and/or intramuscular administration. Furthermore, a novel PrV-Ba recombinant expressing codon-optimized N1 neuraminidase (NA) was included. In vitro replication of this virus was only slightly affected compared to parental virus. Unlike HA, the abundantly expressed NA was efficiently incorporated into PrV particles. Immunization of pigs with the two PrV recombinants, either singly or in combination, induced B cell proliferation and the expected SoIV-specific antibodies, whose titers increased substantially after boost vaccination. After immunization of animals with either PrV recombinant H1N1 SoIV challenge virus replication was significantly reduced compared to PrV-Ba vaccinated or naïve controls. Protective efficacy of HA-expressing PrV was higher than of NA-expressing PrV, and not significantly enhanced by combination. Despite higher serum antibody titers obtained after intramuscular immunization, transmission of challenge virus to naïve contact animals was only prevented after intranasal prime-boost vaccination with HA-expressing PrV-Ba.

High-level protein expression following single and dual gene cloning of infectious bronchitis virus N and S genes using baculovirus systems

Baculovirus is an efficient system for the gene expression that can be used for gene transfer to both insect and different vertebrate hosts. The nucleocapsid gene (N) of the infectious bronchitis virus was cloned in a baculovirus expression system for insect cell expression. Dual expression vectors containing IBV N and spike (S) proteins of the avian infectious bronchitis virus were engineered under the control of human and murine cytomegalovirus immediate-early enhancer/promoter elements in combination with the baculoviral polyhedrin and p10 promoters for simultaneous expression in both vertebrate and insect cells. Transduction of the N gene in the insect Sf9 cells revealed a high level of protein expression. The expressed protein, used in ELISA, effectively detected chicken anti-IBV antibodies with high specificity. Transduction of mammalian and avian cells with BacMam viruses revealed that dual expression cassettes yielded high levels of protein from both transcription units.

Lack of XBP-1 impedes murine cytomegalovirus gene expression

The unfolded protein response (UPR) is an endoplasmic reticulum (ER)-to-nucleus signaling cascade induced in response to ER stress. The UPR aims at restoring homeostasis, but can also induce apoptosis if stress persists. Infection by human and murine cytomegaloviruses (CMVs) provokes ER stress and induces the UPR. However, both CMVs manipulate the UPR to promote its prosurvival activity and delay apoptosis. The underlying mechanisms remain largely unknown. Recently, we demonstrated that MCMV and HCMV encode a late protein to target IRE1 for degradation. However, the importance of its downstream effector, X Box binding protein 1 (XBP-1), has not been directly studied. Here we show that deletion of XBP-1 prior to or early after infection confers a transient delay in viral propagation in fibroblasts that can be overcome by increasing the viral dose. A similar phenotype was demonstrated in peritoneal macrophages. In vivo, acute infection by MCMV is reduced in the absence of XBP-1. Our data indicate that removal of XBP-1 confers a kinetic delay in early stages of MCMV infection and suggest that the late targeting of IRE1 is aimed at inhibiting activities other than the splicing of XBP-1 mRNA.

Increased expression of capsid protein in trans enhances production of single-round infectious particles by West Nile virus DNA vaccine candidate

West Nile virus (WNV; genus Flavivirus, family Flaviviridae) is an emerging pathogenic arbovirus responsible for outbreaks of encephalitis around the world. Whilst no vaccines are currently available to prevent WNV infection of humans, the use of cDNA copies of flavivirus RNA genomes with large internal deletions within the capsid (C) appears promising. C-deleted vaccines are able to replicate and secrete large amounts of non-infectious immunogenic subviral particles (SVPs) from transfected cells. We have previously generated a WNV DNA vaccine candidate pKUNdC/C where C-deleted WNV cDNA was placed under the control of one copy of the cytomegalovirus (CMV) promoter and the C gene was placed under the control of a second copy of the CMV promoter in the same plasmid DNA. This DNA was shown to generate single-round infectious particles (SRIPs) capable of delivering self-replicating C-deleted RNA producing SVPs to surrounding cells, thus enhancing the vaccine potential. However, the amounts of both SRIPs and SVPs produced from pKUNdC/C DNA were relatively low. In this investigation, we aimed at increasing SRIP production by optimizing trans-C expression via incorporating different forms of C and the use of a more powerful promoter. The construct containing an elongation factor EF1α promoter encoding an extended form of C was demonstrated to produce the highest titres of SRIPs and was immunogenic in mice. Additionally, SRIP and SVP titres were further improved via incorporation of a glycosylation motif in the envelope protein. The optimized DNA yielded ~100-fold greater titres of SRIPs than the original construct, thus providing a promising candidate for further vaccine evaluation.

Immunization of pigs with an attenuated pseudorabies virus recombinant expressing the haemagglutinin of pandemic swine origin H1N1 influenza A virus

Pigs can be severely harmed by influenza, and represent important reservoir hosts, in which new human pathogens such as the recent pandemic swine-origin H1N1 influenza A virus can arise by mutation and reassortment of genome segments. To obtain novel, safe influenza vaccines for pigs, and to investigate the antigen-specific immune response, we modified an established live-virus vaccine against Aujeszky's disease of swine, pseudorabies virus (PrV) strain Bartha (PrV-Ba), to serve as vector for the expression of haemagglutinin (HA) of swine-origin H1N1 virus. To facilitate transgene insertion, the genome of PrV-Ba was cloned as a bacterial artificial chromosome. HA expression occurred under control of the human or murine cytomegalovirus immediate early promoters (P-HCMV, P-MCMV), but could be substantially enhanced by synthetic introns and adaptation of the codon usage to that of PrV. However, despite abundant expression, the heterologous glycoprotein was not detectably incorporated into mature PrV particles. Replication of HA-expressing PrV in cell culture was only slightly affected compared to that of the parental virus strain. A single immunization of pigs with the PrV vector expressing the codon-optimized HA gene under control of P-MCMV induced high levels of HA-specific antibodies. The vaccinated animals were protected from clinical signs after challenge with a related swine-origin H1N1 influenza A virus, and challenge virus shedding was significantly reduced.

Murine cytomegalovirus immediate-early 1 gene expression correlates with increased GVHD after allogeneic hematopoietic cell transplantation in recipients reactivating from latent infection

The success of allogeneic (allo) hematopoietic cell transplantation (HCT) is limited by its treatment related complications, mostly graft versus host disease (GVHD) and fungal and viral infections. CMV reactivation after HCT has been associated with increased morbidity and mortality, and a causal relation between GVHD, immunosuppressive therapy and vice versa has been postulated. Using a low GVHD severity murine HCT model, we assessed the role of MCMV reactivation and GVHD development. BALB/c mice were infected with either murine CMV (MCMV) or mock and monitored for 25 weeks to establish latency, followed by sublethal irradiation conditioning and infusion of bone marrow plus splenocytes from either syngeneic (syn) BALB/c or allo B10.D2 donors. Engraftment of allo donor cells was confirmed by PCR for D2Mit265 gene product size. Day+100 mortality and overall GVHD severity in allo MCMV pre-infected recipients was higher than in allo mock controls. Pathologic changes of lung and liver GVHD in immediate-early gene 1 (IE1) positive recipients were significantly increased compared to mock controls, and were only slightly increased in IE1 negative. No significant gut injury was seen in any group. Aggravated lung injury in IE1 positive recipients correlated with higher BAL cell counts both for total cells and for CD4+ T cells when compared with mock controls, and also with protein expression of lung IFN-gamma and liver TNF. No evidence for CMV specific morphologic changes was seen on histopathology in any organ of IE1 positive recipients, suggesting that CMV reactivation is related to increased GVHD severity but does not require active CMV disease, strengthening the concept of a reciprocal relationship between CMV and GVHD.

Viral latency drives 'memory inflation': a unifying hypothesis linking two hallmarks of cytomegalovirus infection

Low public awareness of cytomegalovirus (CMV) results from the only mild and transient symptoms that it causes in the healthy immunocompetent host, so that primary infection usually goes unnoticed. The virus is not cleared, however, but stays for the lifetime of the host in a non-infectious, replicatively dormant state known as 'viral latency'. Medical interest in CMV results from the fact that latent virus can reactivate to cytopathogenic, tissue-destructive infection causing life-threatening end-organ disease in immunocompromised recipients of solid organ transplantation (SOT) or hematopoietic cell transplantation (HCT). It is becoming increasingly clear that CMV latency is not a static state in which the viral genome is silenced at all its genetic loci making the latent virus immunologically invisible, but rather is a dynamic state characterized by stochastic episodes of transient viral gene desilencing. This gene expression can lead to the presentation of antigenic peptides encoded by 'antigenicity-determining transcripts expressed in latency (ADTELs)' sensed by tissue-patrolling effector-memory CD8 T cells for immune surveillance of latency [In Reddehase et al., Murine model of cytomegalovirus latency and reactivation, Current Topics in Microbiology and Immunology, vol 325. Springer, Berlin, pp 315-331, 2008]. A hallmark of the CD8 T cell response to CMV is the observation that with increasing time during latency, CD8 T cells specific for certain viral epitopes increase in numbers, a phenomenon that has gained much attention in recent years and is known under the catchphrase 'memory inflation.' Here, we provide a unifying hypothesis linking stochastic viral gene desilencing during latency to 'memory inflation.'

Simian virus 40 strains with novel properties generated by replacing the viral enhancer with synthetic oligonucleotides

Typical enhancers of viral or cellular genes are approximately 100 to 400 bp long and contain several transcription factor binding sites. Previously, we have shown that simian virus 40 (SV40) genomic DNA that lacks its own enhancer can be used as an "enhancer trap" since it reacquires infectivity upon incorporation of heterologous enhancers. Here, we show that SV40 infectivity can be restored with synthetic enhancers that are assembled by the host cell. We found that several oligonucleotides, cotransfected with enhancerless SV40 DNA into host cells, were incorporated into the viral genome via cellular DNA end joining. The oligonucleotides tested included metal response elements (MREs), the binding sites for the transcription factor MTF-1, which induces gene activity in response to heavy metals. These recombinant SV40 strains showed preferential growth on cells overloaded with zinc or cadmium. We also cotransfected enhancerless SV40 DNA with oligonucleotides corresponding to enhancer motifs of human and mouse cytomegalovirus (HCMV and MCMV, respectively). In contrast to SV40 wild type, the viruses with cytomegalovirus-derived patchwork enhancers strongly expressed T-antigen in human HEK293 cells, accompanied by viral DNA replication. Occasionally, we also observed the assembly of functional viral genomes by incorporation of fragments of bovine DNA, an ingredient of the fetal calf serum in the medium. These fragments contained, among other sites, binding sites for AP-1 and CREB transcription factors. Taken together, our studies show that viruses with novel properties can be generated by intracellular incorporation of synthetic enhancer DNA motifs.

The activator protein 1 binding motifs within the human cytomegalovirus major immediate-early enhancer are functionally redundant and act in a cooperative manner with the NF-{kappa}B sites during acute infection

Human cytomegalovirus (HCMV) infection causes a rapid induction of c-Fos and c-Jun, the major subunits of activator protein 1 (AP-1), which in turn have been postulated to activate the viral immediate-early (IE) genes. Accordingly, the major IE promoter (MIEP) enhancer, a critical control region for initiating lytic HCMV infection and reactivation from the latent state, contains one well-characterized AP-1 site and a second candidate interaction site. In this study we explored the role of these AP-1 elements in the context of the infection. We first show that the distal candidate AP-1 motif binds c-Fos/c-Jun heterodimers (AP-1 complex) and confers c-Fos/c-Jun-mediated activity to a core promoter. Site-directed mutagenesis studies indicate that both AP-1 response elements are critical for 12-O-tetradecanoylphorbol-13-acetate (TPA)-enhanced MIEP activity in transient-transfection assays. In marked contrast to the results obtained with the isolated promoter, disruption of the AP-1 recognition sites of the MIEP in the context of the infectious HCMV genome has no significant influence on the expression of the MIE protein IE1 or viral replication in different cell types. Moreover, a chimeric murine CMV driven by the HCMV MIEP (hMCMV-ES) with the two AP-1 binding sites mutated is not compromised in virulence, is able to grow and disseminate to different organs of the newborn mice as efficiently as the parental virus, and is competent in reactivation. We show, however, that combined inactivation of the enhancer AP-1 and NF-κB recognition sites leads to an attenuation of the hMCMV-ES in the neonatal murine infection model, not observed when each single element is abolished. Altogether, these results underline the functional redundancy of the MIEP elements, highlighting the plasticity of this region, which probably evolved to ensure maximal transcriptional performance across many diverse environments.

Enhancerless cytomegalovirus is capable of establishing a low-level maintenance infection in severely immunodeficient host tissues but fails in exponential growth

Major immediate-early transcriptional enhancers are genetic control elements that act, through docking with host transcription factors, as a decisive regulatory unit for efficient initiation of the productive virus cycle. Animal models are required for studying the function of enhancers paradigmatically in host organs. Here, we have sought to quantitatively assess the establishment, maintenance, and level of in vivo growth of enhancerless mutants of murine cytomegalovirus in comparison with those of an enhancer-bearing counterpart in models of the immunocompromised or immunologically immature host. Evidence is presented showing that enhancerless viruses are capable of forming restricted foci of infection but fail to grow exponentially.

Effect of foot-and-mouth disease virus capsid precursor protein and 3C protease expression on bovine herpesvirus 1 replication

Several reports have previously shown that expression of the foot-and-mouth disease virus (FMDV) capsid precursor protein encoding region P1-2A together with the 3C protease (P1-2A/3C) results in correct processing of the capsid precursor into VP0, VP1 and VP3 and formation of FMDV capsid structures that are able to induce a protective immune response against FMDV challenge after immunization using naked DNA constructs or recombinant viruses. To elucidate whether bovine herpesvirus 1 (BHV-1) might also be suitable as a viral vector for empty capsid generation, we aimed to integrate a P1-2A/3C expression cassette into the BHV-1 genome, which, however, failed repeatedly. In contrast, BHV-1 recombinants that expressed an inactive 3C protease or the P1-2A polyprotein alone could be easily generated, although the recombinant that expressed P1-2A exhibited a defect in direct cell-cell spread and release of infectious particles. These results suggested that expression of the original, active FMDV 3C protease is not compatible with BHV-1 replication. This conclusion is supported by the isolation of recombinant BHV-1/3C*, which contained mutations within the 3C ORF (3C* ORF)--probably introduced spontaneously during generation of BHV-1/3C*--instead of the authentic 3C ORF contained in the transfer plasmids. Within the 3C* ORF, the codons for glycine 38 and phenylalanine 48 were both substituted by codons for serine. The resulting 3C* protease exhibits a highly reduced activity for proteolytic processing of the P1-2A polyprotein and thus might be a good candidate for the generation of live attenuated FMDV variants.

Biphasic recruitment of transcriptional repressors to the murine cytomegalovirus major immediate-early promoter during the course of infection in vivo

Our previous studies showed that establishment of murine cytomegalovirus (MCMV) latency in vivo is associated with repression of immediate-early gene expression, deacetylation of histones bound to the major immediate-early promoter (MIEP), changes in patterns of methylation of histones, and recruitment of cellular repressors of transcription to the MIEP. Here, we have quantitatively analyzed the kinetics of changes in viral RNA expression, DNA copy number, and recruitment of repressors and activators of transcription to viral promoters during the course of infection. Our results show that changes in viral gene expression correlate with changes in recruitment of RNA polymerase and acetylated histones to viral promoters. Binding of the transcriptional repressors histone deacetylase type 2 (HDAC2), HDAC3, YY1, CBF-1/RBP-Jk, Daxx, and CIR to the MIEP and HDACs to other promoters showed a biphasic pattern: some binding was detectable prior to activation of viral gene expression, then decreased with the onset of transcription and increased again as repression of viral gene expression occurred. Potential binding sites for CBF-1/RBP-Jk and YY1 in the MIEP and for YY1 in the M100 promoter (M100P) were identified by in silico analysis. While recruitment of HDACs was not promoter specific, binding of CBF-1/RBP-Jk and YY1 was restricted to promoters with their cognate sites. Our results suggest that sequences within viral promoters may contribute to establishment of latency through recruitment of transcriptional repressors to these genes. The observation that repressors are bound to the MIEP and other promoters immediately upon infection suggests that latency may be established in some cells very early in infection.

Synergism between the components of the bipartite major immediate-early transcriptional enhancer of murine cytomegalovirus does not accelerate virus replication in cell culture and host tissues

Major immediate-early (MIE) transcriptional enhancers of cytomegaloviruses are key regulators that are regarded as determinants of virus replicative fitness and pathogenicity. The MIE locus of murine cytomegalovirus (mCMV) shows bidirectional gene-pair architecture, with a bipartite enhancer flanked by divergent core promoters. Here, we have constructed recombinant viruses mCMV-ΔEnh1 and mCMV-ΔEnh2 to study the impact of either enhancer component on bidirectional MIE gene transcription and on virus replication in cell culture and various host tissues that are relevant to CMV disease. The data revealed that the two unipartite enhancers can operate independently, but synergize in enhancing MIE gene expression early after infection. Kick-start transcription facilitated by the bipartite enhancer configuration, however, did not ultimately result in accelerated virus replication. We conclude that virus replication, once triggered, proceeds with a fixed speed and we propose that synergism between the components of the bipartite enhancer may rather increase the probability for transcription initiation.

TNF-alpha signaling is not required for in vivo transcriptional reactivation of latent murine cytomegalovirus

BACKGROUND

Reactivation of cytomegalovirus (CMV) is frequently observed in recipients of solid organs and bone marrow transplants and is associated with increased risk of acute and chronic allograft rejection, opportunistic infection, graft failure, and patient mortality. The molecular mechanisms by which reactivation occurs are not well understood. Previous studies have suggested that tumor necrosis factor (TNF)-alpha, which is induced by allogeneic transplantation, may have a role in reactivation of CMV through activation of nuclear factor kappa-light-chain-enhancer of activated B cells and subsequent transcriptional reactivation of immediate early (ie) gene expression.

METHODS AND RESULTS

We have tested the role of TNF-alpha in the reactivation of CMV directly by testing whether TNF-alpha is required to initiate transcription of ie gene expression in a murine model of allogeneic transplantation of kidneys latently infected with mouse CMV.

CONCLUSIONS

Our studies show that although TNF-alpha seems to be sufficient, it is not required for initiating transcription of ie gene expression in this model, suggesting that both TNF-alpha-dependent and -independent pathways play an important role in the reactivation of latent CMV infection.

Properties of virion transactivator proteins encoded by primate cytomegaloviruses

Background

Human cytomegalovirus (HCMV) is a betaherpesvirus that causes severe disease in situations where the immune system is immature or compromised. HCMV immediate early (IE) gene expression is stimulated by the virion phosphoprotein pp71, encoded by open reading frame (ORF) UL82, and this transactivation activity is important for the efficient initiation of viral replication. It is currently recognized that pp71 acts to overcome cellular intrinsic defences that otherwise block viral IE gene expression, and that interactions of pp71 with the cell proteins Daxx and ATRX are important for this function. A further property of pp71 is the ability to enable prolonged gene expression from quiescent herpes simplex virus type 1 (HSV-1) genomes. Non-human primate cytomegaloviruses encode homologs of pp71, but there is currently no published information that addresses their effects on gene expression and modes of action.

Results

The UL82 homolog encoded by simian cytomegalovirus (SCMV), strain Colburn, was identified and cloned. This ORF, named S82, was cloned into an HSV-1 vector, as were those from baboon, rhesus monkey and chimpanzee cytomegaloviruses. The use of an HSV-1 vector enabled expression of the UL82 homologs in a range of cell types, and permitted investigation of their abilities to direct prolonged gene expression from quiescent genomes. The results show that all UL82 homologs activate gene expression, and that neither host cell type nor promoter target sequence has major effects on these activities. Surprisingly, the UL82 proteins specified by non-human primate cytomegaloviruses, unlike pp71, did not direct long term expression from quiescent HSV-1 genomes. In addition, significant differences were observed in the intranuclear localization of the UL82 homologs, and in their effects on Daxx. Strikingly, S82 mediated the release of Daxx from nuclear domain 10 substructures much more rapidly than pp71 or the other proteins tested. All UL82 homologs stimulated the early release of ATRX from nuclear domain 10.

Conclusion

All of the UL82 homolog proteins analysed activated gene expression, but surprising differences in other aspects of their properties were revealed. The results provide new information on early events in infection with cytomegaloviruses.

Novel vectors for simultaneous high-level dual protein expression in vertebrate and insect cells by recombinant baculoviruses

Gene transfer into cells of mammalian, avian or piscine origin by baculoviruses carrying expression cassettes active in vertebrate cells (BacMam method) is an attractive alternative to chemical or physical transfection methods or to the use of vectors originating from viruses of vertebrates. For simultaneous high-level expression of two proteins from recombinant baculoviruses we constructed novel dual expression vectors containing human and murine cytomegalovirus immediate-early enhancer/promoter elements in combination with the baculoviral polyhedrin and p10 promoters for simultaneous expression in vertebrate and insect cells. Transduction of ruminant cells with BacMam viruses containing the green fluorescent protein open reading frame downstream from the respective enhancer/promoter elements revealed that a dual expression cassette combining the murine cytomegalovirus immediate-early 1 sequence with the immediate early enhancer/promoter of human cytomegalovirus yields high levels of protein from both transcription units. Protein expression directed by several cytomegalovirus/baculovirus hybrid promoters proceeded efficiently in insect cells infected with the respective recombinants. However, for expression in vertebrate cells the murine ie1 enhancer/promoter upstream the baculoviral p10 promoter was most efficient.

In vivo competence of murine cytomegalovirus under the control of the human cytomegalovirus major immediate-early enhancer in the establishment of latency and reactivation

The human cytomegalovirus (HCMV) major immediate-early enhancer has been postulated to play a pivotal role in the control of latency and reactivation. However, the absence of an animal model has obstructed a direct test of this hypothesis. Here we report on the establishment of an in vivo, experimentally tractable system for quantitatively investigating physiological functions of the HCMV enhancer. Using a neonate BALB/c mouse model, we show that a chimeric murine CMV under the control of the HCMV enhancer is competent in vivo, replicating in key organs of mice with acute CMV infection and exhibiting latency/reactivation features comparable for the most part to those of the parental and revertant viruses.

Establishment of murine cytomegalovirus latency in vivo is associated with changes in histone modifications and recruitment of transcriptional repressors to the major immediate-early promoter

Human cytomegalovirus (CMV) is a ubiquitous herpesvirus with the ability to establish a lifelong latent infection. The mechanism by which this occurs is not well understood. Regulation of, for example, immediate-early (IE) gene expression is thought to be a critical control point in transcriptional control of the switch between latency and reactivation. Here, we present evidence that supports previous studies showing that the majority of genomes are quiescent with respect to gene expression. To study the possible role of epigenetic factors that may be involved in repression of ie gene expression in latency, we have analyzed changes in the patterns of modifications of histones bound to the major IE promoter (MIEP) in the kidneys of acutely and latently infected mice. Our studies show that, like herpes simplex virus, murine CMV genomes become relatively enriched in histones in latent infection. There are dramatic changes in modifications of histones associated with the MIEP when latency is established: H3 and H4 become hypoacetylated and H3 is hypomethylated at lysine 4, while H3 lysine 9 is hypermethylated in latently infected mice. These changes are accompanied by a relative loss of RNA polymerase and gain of heterochromatin protein 1gamma and Yin-Yang 1 bound to the MIEP. Our studies suggest that, in the majority of cells, CMV establishes a true latent infection, defined as the lack of expression of genes associated with productive infection, and that this occurs through changes in histone modifications and recruitment of transcriptional silencing factors to the MIEP.

Transactivation of cellular genes involved in nucleotide metabolism by the regulatory IE1 protein of murine cytomegalovirus is not critical for viral replicative fitness in quiescent cells and host tissues

Despite its high coding capacity, murine CMV (mCMV) does not encode functional enzymes for nucleotide biosynthesis. It thus depends on cellular enzymes, such as ribonucleotide reductase (RNR) and thymidylate synthase (TS), to be supplied with deoxynucleoside triphosphates (dNTPs) for its DNA replication. Viral transactivation of these cellular genes in quiescent cells of host tissues is therefore a parameter of viral fitness relevant to pathogenicity. Previous work has shown that the IE1, but not the IE3, protein of mCMV transactivates RNR and TS gene promoters and has revealed an in vivo attenuation of the mutant virus mCMV-DeltaIE1. It was attractive to propose the hypothesis that lack of transactivation by IE1 and a resulting deficiency in the supply of dNTPs are the reasons for growth attenuation. Here, we have tested this hypothesis with the mutant virus mCMV-IE1-Y165C expressing an IE1 protein that selectively fails to transactivate RNR and TS in quiescent cells upon transfection while maintaining the capacity to disperse repressive nuclear domains (ND10). Our results confirm in vivo attenuation of mCMV-DeltaIE1, as indicated by a longer doubling time in host organs, whereas mCMV-IE1-Y165C replicated like mCMV-WT and the revertant virus mCMV-IE1-C165Y. Notably, the mutant virus transactivated RNR and TS upon infection of quiescent cells, thus indicating that IE1 is not the only viral transactivator involved. We conclude that transactivation of cellular genes of dNTP biosynthesis is ensured by redundancy and that attenuation of mCMV-DeltaIE1 results from the loss of other critical functions of IE1, with its function in the dispersal of ND10 being a promising candidate.

Role of the cytomegalovirus major immediate early enhancer in acute infection and reactivation from latency

The cytomegalovirus (CMV) major immediate early (MIE) enhancer-containing promoter regulates the expression of the downstream MIE genes, which have critical roles in reactivation from latency and acute infection. The enhancer consists of binding sites for cellular transcription factors that are repeated multiple times. The primate and nonprimate CMV enhancers can substitute for one another. The enhancers are not functionally equivalent, but they do have overlapping activities. The CMV MIE enhancers are located between divergent promoters where the leftward genes are critical and essential for reactivation from latency and acute infection and the rightward gene is nonessential. The rightward transcription unit is controlled by an enhancer for murine CMV. In contrast, human CMV has a set of repressor elements that prevents enhancer effects on the rightward viral promoter. The human CMV enhancer that controls the leftward transcription unit has a distal component that is nonessential at high multiplicity of infection (MOI), but has a significant impact on the MIE gene expression at low MOI. The proximal enhancer influences directly the level of transcription of the MIE genes and contains an essential Sp-1 site. The MIE promoter has a site adjacent to the transcription start site that is essential at the earliest stage of infection. The MIE enhancer-containing promoter responds to signal transduction events and to cellular differentiation. The role of the CMV MIE enhancer-containing promoter in acute infection and reactivation from latency are reviewed.

Murine model of cytomegalovirus latency and reactivation

Efficient resolution of acute cytopathogenic cytomegalovirus infection through innate and adaptive host immune mechanisms is followed by lifelong maintenance of the viral genome in host tissues in a state of replicative latency, which is interrupted by episodes of virus reactivation for transmission. The establishment of latency is the result of aeons of co-evolution of cytomegaloviruses and their respective host species. Genetic adaptation of a particular cytomegalovirus to its specific host is reflected by private gene families not found in other members of the cytomegalovirus group, whereas basic functions of the viral replicative cycle are encoded by public gene families shared between different cytomegaloviruses or even with herpesviruses in general. Private genes include genes coding for immunoevasins, a group of glycoproteins specifically dedicated to dampen recognition by the host's innate and adaptive immune surveillance to protect the virus against elimination. Recent data in the mouse model of cytomegalovirus latency have indicated that viral replicative latency established in the immunocompetent host is a dynamic state characterized by episodes of viral gene desilencing and immune sensing of reactivated presentation of antigenic peptides at immunological checkpoints by CD8 T cells. This sensing maintains viral replicative latency by triggering antiviral effector functions that terminate the viral gene expression program before infectious viral progeny are assembled. According to the immune sensing hypothesis of latency control, immunological checkpoints are unique for each infected individual in reflection of host MHC (HLA) polymorphism and the proteome(s) of the viral variant(s) harbored in latency.

Transcriptional reactivation of murine cytomegalovirus ie gene expression by 5-aza-2'-deoxycytidine and trichostatin A in latently infected cells despite lack of methylation of the major immediate-early promoter

We have used a spleen explant model to investigate mechanisms of murine cytomegalovirus latency and reactivation. Induction of immediate-early (ie) gene expression occurs in explants after approximately 9 days in culture and virus reactivation follows induction of ie gene expression with kinetics similar to that of productive infection in vitro. This occurs independently of TNF receptor signalling. Treatment with the DNA methylation inhibitor 5-aza-2'-deoxycytidine and the histone deacetylase inhibitor trichostatin A results in more rapid induction of ie gene expression and reactivation of virus. Despite these results, which suggest a role for DNA methylation in maintenance of viral latency, we find that the major immediate-early promoter/enhancer is not methylated in latently infected mice. Our results support the hypothesis that latency is maintained by epigenetic control of ie gene expression, and that induction of ie gene expression leads to reactivation of virus, but suggest that these are not controlled by DNA methylation.

Murine cytomegalovirus major immediate-early enhancer region operating as a genetic switch in bidirectional gene pair transcription

Enhancers are defined as DNA elements that increase transcription when placed in any orientation relative to a promoter. The major immediate-early (MIE) enhancer region of murine cytomegalovirus is flanked by transcription units ie1/3 and ie2, which are transcribed in opposite directions. We have addressed the fundamental mechanistic question of whether the enhancer synchronizes transcription of the bidirectional gene pair (synchronizer model) or whether it operates as a genetic switch, enhancing transcription of either gene in a stochastic alternation (switch model). Clonal analysis of cytokine-triggered, transcription factor-mediated MIE gene expression from latent viral genomes provided evidence in support of the switch model.

The IE2 promoter/enhancer region from mouse CMV provides high levels of therapeutic protein expression in mammalian cells

Protein expression in mammalian cells is key for the production and manufacturing of bio-therapeutics with human-like properties and activities. As a molecular basis for reaching high protein expression levels, efficient promoter/enhancer systems are a prerequisite. Here we identify a novel enhancer from the mouse cytomegalovirus (CMV) immediate early 2 (IE2) region as a strong expression-promoting element. We further demonstrate its activity in bi-directional promoter architecture and apply it to generate production clones for IL-18BP, a protein with therapeutic indications in autoimmune diseases. These data show that the IE region from mouse CMV, and the IE2 enhancer/promoter in particular, have a broad potential for application in novel gene expression systems for research, development, and manufacturing of protein drugs.

Lipopolysaccharide, tumor necrosis factor alpha, or interleukin-1beta triggers reactivation of latent cytomegalovirus in immunocompetent mice

We have previously shown that cytomegalovirus (CMV) can reactivate in lungs of nonimmunosuppressed patients during critical illness. Our recent work has shown that polymicrobial bacterial sepsis can trigger reactivation of latent murine CMV (MCMV). We hypothesize that MCMV reactivation following bacterial sepsis may be caused by inflammatory mediators. To test this hypothesis, BALB/c mice latently infected with Smith strain MCMV received sublethal intraperitoneal doses of lipopolysaccharide (LPS), tumor necrosis factor alpha (TNF-alpha), interleukin-1beta (IL-1beta), or saline. Lung tissue homogenates were evaluated for viral reactivation 3 weeks after mediator injection. Because LPS is known to signal via Toll-like receptor 4 (TLR-4) in mice, further studies blocking this signaling mechanism were performed using monoclonal MTS510. Finally, mice were tested with intravenous TNF-alpha to determine whether this would cause reactivation. All mice receiving sublethal intraperitoneal doses of LPS, TNF-alpha, or IL-1beta had pulmonary reactivation of latent MCMV 3 weeks following injection, and LPS caused MCMV reactivation with kinetics similar to those for sepsis. When TLR-4 signaling was blocked, exogenous LPS did not reactivate latent MCMV. Intravenous TNF-alpha administration at near-lethal doses did not reactivate MCMV. Exogenous intraperitoneal LPS, TNF-alpha, and IL-1beta are all capable of reactivating CMV from latency in lungs of previously healthy mice. LPS reactivation of MCMV appears dependent on TLR-4 signaling. Interestingly, intravenous TNF-alpha did not trigger reactivation, suggesting possible mechanistic differences that are discussed. We conclude that inflammatory disease states besides sepsis may be capable of reactivating CMV from latency.

High levels of protein expression using different mammalian CMV promoters in several cell lines

With the recent completion of the human genome sequencing project, scientists are faced with the daunting challenge of deciphering the function of these newly found genes quickly and efficiently. Equally as important is to produce milligram quantities of the therapeutically relevant gene products as quickly as possible. Mammalian expression systems provide many advantages to aid in this task. Mammalian cell lines have the capacity for proper post-translational modifications including proper protein folding and glycosylation. In response to the needs described above, we investigated the protein expression levels driven by the human CMV in the presence or absence of intron A, the mouse and rat CMV promoters with intron A, and the MPSV promoter in plasmid expression vectors. We evaluated the different promoters using an in-house plasmid vector backbone. The protein expression levels of four genes of interest driven by these promoters were evaluated in HEK293EBNA and CHO-K1 cells. Stable and transient transfected cells were utilized. In general, the full-length human CMV, in the presence of intron A, gave the highest levels of protein expression in transient transfections in both cell lines. However, the MPSV promoter resulted in the highest levels of stable protein expression in CHO-K1 cells. Using the CMV driven constitutive promoters in the presence of intron A, we have been able to generate >10 microg/ml of recombinant protein using transient transfections.

Reducing the immunostimulatory activity of CpG-containing plasmid DNA vectors for non-viral gene therapy

The mammalian innate immune system has the ability to recognise and direct a response against incoming foreign DNA. The primary signal that triggers this response is unmethylated CpG motifs present in the DNA sequence of various disease-causing pathogens. These motifs are rare in vertebrate DNA, but abundant in bacterial and some viral DNAs. Because gene therapy generally involves the delivery of DNA from either plasmids of bacterial origin or recombinant viruses, an acute inflammatory response of variable severity inevitably results. The response is most serious for non-viral gene delivery vectors composed of cationic lipid-DNA complexes, producing adverse effects at lower doses and lethality at higher doses of complex. This review examines the role of immunostimulatory CpG motifs in the acute inflammatory response to non-viral gene therapy vectors. Strategies to neutralise or eliminate CpG motifs within plasmid DNA vectors, and the existing limitations of CpG reduction on improving the safety profile of non-viral vectors, will be discussed.

In vivo cutaneous interferon-gamma gene delivery using novel dicationic (gemini) surfactant-plasmid complexes

BACKGROUND

Localized scleroderma (morphea and linear scleroderma) is a connective tissue disease, accompanied by excessive proliferation and deposition of collagen within the skin, inflammation, vasculopathy and a deranged immune system. Interferon gamma (IFNgamma), an inhibitor of collagen synthesis and an immunomodulator, could be a potential therapeutic agent if it could be delivered into or expressed locally in affected skin in a non-invasive manner. In this study, the feasibility of topical delivery of the IFNgamma gene and expression of IFNgamma were investigated in mice.

METHODS

Novel dicationic (gemini) surfactant (spacer length n=2-16; alkyl chain m=12 or 16)-DNA complexes were formulated and characterized by circular dichroism and atomic force microscopy to select gemini analogues with the highest transfection efficiency (TE). Transfection and cellular expression of IFNgamma from the bicistronic pGTmCMV.IFN-GFP plasmid were evaluated in PAM 212 keratinocyte culture by ELISA and fluorescence microscopy. Topical delivery of plasmid using liposomal and nanoemulsion systems, based on gemini surfactant 16-3-16, was evaluated in mice by IFNgamma expression analysis.

RESULTS

In vitro TE was found to be dependent on the spacer length of the gemini surfactant, with the C3 spacer showing the highest activity (both 12-3-12 and 16-3-16). Both gemini cationic liposomes and gemini nanoemulsion (3x25 microg DNA/animal) produced significantly higher levels of IFNgamma in the skin (359.4 and 607.24 pg/cm2) compared to naked DNA (135.69 pg/cm2) or a liposomal Dc-chol formulation (82.15 pg/cm2). IFNgamma expression in the lymph nodes was higher in the animals treated with gemini liposomes (422.74 pg/animal) compared to the nanoemulsion formulation (131.27 pg/animal) or the Dc-chol formulation (82pg/animal).

CONCLUSIONS

The feasibility of topical delivery of pGTmCMV.IFN-GFP plasmid in mice using gemini cationic surfactant based delivery systems was demonstrated. IFNgamma expression after treatment with gemini-DNA formulations in the skin was 3-5-fold higher compared to the treatment with naked DNA (p<0.05), and 4-6-fold higher than the Dc-chol-DNA complex, indicating a significant advance in topical DNA delivery across intact skin in vivo.