Analysis of herpes simplex virus ICP0 promoter function in sensory neurons during acute infection, establishment of latency, and reactivation in vivo

iPSC-derived three-dimensional brain organoid models and neurotropic viral infections

Progress in stem cell research has revolutionized the medical field for more than two decades. More recently, the discovery of induced pluripotent stem cells (iPSCs) has allowed for the development of advanced disease modeling and tissue engineering platforms. iPSCs are generated from adult somatic cells by reprogramming them into an embryonic-like state via the expression of transcription factors required for establishing pluripotency. In the context of the central nervous system (CNS), iPSCs have the potential to differentiate into a wide variety of brain cell types including neurons, astrocytes, microglial cells, endothelial cells, and oligodendrocytes. iPSCs can be used to generate brain organoids by using a constructive approach in three-dimensional (3D) culture in vitro. Recent advances in 3D brain organoid modeling have provided access to a better understanding of cell-to-cell interactions in disease progression, particularly with neurotropic viral infections. Neurotropic viral infections have been difficult to study in two-dimensional culture systems in vitro due to the lack of a multicellular composition of CNS cell networks. In recent years, 3D brain organoids have been preferred for modeling neurotropic viral diseases and have provided invaluable information for better understanding the molecular regulation of viral infection and cellular responses. Here we provide a comprehensive review of the literature on recent advances in iPSC-derived 3D brain organoid culturing and their utilization in modeling major neurotropic viral infections including HIV-1, HSV-1, JCV, ZIKV, CMV, and SARS-CoV2.

A Human Stem Cell-Derived Neurosensory–Epithelial Circuitry on a Chip to Model Herpes Simplex Virus Reactivation

Both emerging viruses and well-known viral pathogens endowed with neurotropism can either directly impair neuronal functions or induce physio-pathological changes by diffusing from the periphery through neurosensory–epithelial connections. However, developing a reliable and reproducible in vitro system modeling the connectivity between the different human sensory neurons and peripheral tissues is still a challenge and precludes the deepest comprehension of viral latency and reactivation at the cellular and molecular levels. This study shows a stable topographic neurosensory–epithelial connection on a chip using human stem cell-derived dorsal root ganglia (DRG) organoids. Bulk and single-cell transcriptomics showed that different combinations of key receptors for herpes simplex virus 1 (HSV-1) are expressed by each sensory neuronal cell type. This neuronal–epithelial circuitry enabled a detailed analysis of HSV infectivity, faithfully modeling its dynamics and cell type specificity. The reconstitution of an organized connectivity between human sensory neurons and keratinocytes into microfluidic chips provides a powerful in vitro platform for modeling viral latency and reactivation of human viral pathogens.

HSV Mutant Generation and Dual Detection Methods for Gaining Insight into Latent/Lytic Cycles In Vivo

Two important components of a useful strategy to examine viral gene function, regulation, and pathogenesis in vivo are (1) a highly efficient protocol to generate viral mutants that limits undesired mutation and retains full replication competency in vivo, and (2) an efficient system to detect and quantify viral promoter activity and gene expression in rare cells in vivo and to gain insight into the surrounding tissue environment. Our strategy and protocols for generating, characterizing, and employing HSV viral promoter/reporter mutants in vivo are provided in this two-part chapter.

Targeted Promoter Replacement Reveals That Herpes Simplex Virus Type-1 and 2 Specific VP16 Promoters Direct Distinct Rates of Entry Into the Lytic Program in Sensory Neurons in vivo

Infection and life-long residence in the human nervous system is central to herpes simplex virus (HSV) pathogenesis. Access is gained through innervating axonal projections of sensory neurons. This distinct mode of entry separates the viral genome from tegument proteins, including the potent transactivator of viral IE genes, VP16. This, in turn, promotes a balance between lytic and latent infection which underlies the ability of the virus to invade, disseminate, and set up a large reservoir of latent infections. In the mouse ocular model, TG neurons marked as either “latent” or “lytic” at 48 h postinfection indicated that these programs were selected early and were considered distinct and mutually exclusive. More recently, a temporal analysis of viral program selection revealed a default latent-like state that begins at ~18 h postinfection and in individual neurons, precedes entry into the viral lytic cycle. Studies using refined viral mutants demonstrated that transition out of this latent program depended upon the transactivation function of VP16. Pursuit of the apparent incongruity between the established leaky-late kinetics of VP16 expression with a “preimmediate-early” function led to the discovery of an unrecognized regulatory feature of the HSV-1 VP16 promoter near/downstream of its TATA box. Among three potential sites identified was a putative Egr-1/Sp1 site. Here, we report that a refined mutation of this site, while having no impact on replication in cultured cells or cornea, resulted in ~100-fold reduction in lytic infection in TG in vivo. Notably, the HSV-2 VP16 promoter has 13 direct tandem-repeats upstream of its TATA box forming multiple potential overlapping Egr-1/Sp1 sites. Thus, despite different structures, these promoters might share function in directing the preimmediate-early VP16 protein expression. To test this, the HSV-1 VP16 promoter/5′UTR was replaced by the HSV-2 VP16 promoter/5′UTR in the HSV-1 backbone. Compared to the genomically repaired isolate, the HSV-2 VP16 promoter/5′UTR (1) accelerated the transition into the lytic cycle, and enhanced (2) virulence, and (3) entry into the lytic cycle following a reactivation stressor. These gain-of-function phenotypes support the hypothesis that the VP16 promoter regulates the latent/lytic boundary in neurons and that the HSV-1 and HSV-2 promoter/5′UTRs encode distinct thresholds for this transition.

Compartmented neuronal cultures reveal two distinct mechanisms for alpha herpesvirus escape from genome silencing

Alpha herpesvirus genomes encode the capacity to establish quiescent infections (i.e. latency) in the peripheral nervous system for the life of their hosts. Multiple times during latency, viral genomes can reactivate to start a productive infection, enabling spread of progeny virions to other hosts. Replication of alpha herpesviruses is well studied in cultured cells and many aspects of productive replication have been identified. However, many questions remain concerning how a productive or a quiescent infection is established. While infections in vivo often result in latency, infections of dissociated neuronal cultures in vitro result in a productive infection unless lytic viral replication is suppressed by DNA polymerase inhibitors or interferon. Using primary peripheral nervous system neurons cultured in modified Campenot tri-chambers, we previously reported that reactivateable, quiescent infections by pseudorabies virus (PRV) can be established in the absence of any inhibitor. Such infections were established in cell bodies only when physically isolated axons were infected at a very low multiplicity of infection (MOI). In this report, we developed a complementation assay in compartmented neuronal cultures to investigate host and viral factors in cell bodies that prevent establishment of quiescent infection and promote productive replication of axonally delivered genomes (i.e. escape from silencing). Stimulating protein kinase A (PKA) signaling pathways in isolated cell bodies, or superinfecting cell bodies with either UV-inactivated PRV or viral light particles (LP) promoted escape from genome silencing and prevented establishment of quiescent infection but with different molecular mechanisms. Activation of PKA in cell bodies triggers a slow escape from silencing in a cJun N-terminal kinase (JNK) dependent manner. However, escape from silencing is induced rapidly by infection with UVPRV or LP in a PKA- and JNK-independent manner. We suggest that viral tegument proteins delivered to cell bodies engage multiple signaling pathways that block silencing of viral genomes delivered by low MOI axonal infection.

Alpha herpesvirus infections stay life-long in infected human and animal hosts`nervous systems in a silent state ready to reactivate upon various stress signals. Remarkably, infection of epithelial cells with these viruses results in productive infection whereas infection of peripheral nervous system neurons results in non-productive silent infection (i.e. latency) in the natural hosts. More interestingly, infection of dissociated peripheral neurons in culture also results in productive infection unless DNA replication inhibitors are used. To study the molecular mechanisms of escape from latency, we used primary neurons cultured in compartmented tri-chambers. By this way, we recapitulated the natural route of infection by infecting axons with low dose of virus which resulted in a silent infection in a small number of neuronal cell bodies without the use of any inhibitors. Using these cultures, we developed a new complementation assay to investigate the molecular signals leading to escape from latency and establishment of productive infection. We found two different mechanisms to escape from latency: Cellular stress-mediated slow route and viral tegument mediated-fast route. Furthermore, we showed that the stress-mediated pathway requires protein kinase A and c-Jun N-terminal kinase activity while the viral tegument-mediated fast escape does not require these host cell kinase activities. We also concluded that a general response to DNA virus infection or presence of excess herpesviral genomes in the nucleus to saturate silencing complexes is not enough to escape from latency. Induction of a productive infection requires presence of tegument proteins or activation of PKA and JNK pathway.

Inferred father-to-son transmission of herpes simplex virus results in near-perfect preservation of viral genome identity and in vivo phenotypes

High throughout sequencing has provided an unprecedented view of the circulating diversity of all classes of human herpesviruses. For herpes simplex virus 1 (HSV-1), we and others have previously published data demonstrating sequence diversity between hosts. However the extent of variation during transmission events, or in one host over years of chronic infection, remain unknown. Here we present an initial example of full characterization of viruses isolated from a father to son transmission event. The likely occasion of transmission occurred 17 years before the strains were isolated, enabling a first view of the degree of virus conservation after decades of recurrences, including transmission and adaptation to a new host. We have characterized the pathogenicity of these strains in a mouse ocular model of infection, and sequenced the full viral genomes. Surprisingly, we find that these two viruses have preserved their phenotype and genotype nearly perfectly during inferred transmission from father to son, and during nearly two decades of episodes of recurrent disease in each human host. Given the close genetic relationship of these two hosts, it remains to be seen whether or not this conservation of sequence will occur during non-familial transmission events.

HSV1 latent transcription and non-coding RNA: A critical retrospective

Virologists have invested great effort into understanding how the herpes simplex viruses and their relatives are maintained dormant over the lifespan of their host while maintaining the poise to remobilize on sporadic occasions. Piece by piece, our field has defined the tissues in play (the sensory ganglia), the transcriptional units (the latency-associated transcripts), and the responsive genomic region (the long repeats of the viral genomes). With time, the observed complexity of these features has compounded, and the totality of viral factors regulating latency are less obvious. In this review, we compose a comprehensive picture of the viral genetic elements suspected to be relevant to herpes simplex virus 1 (HSV1) latent transcription by conducting a critical analysis of about three decades of research. We describe these studies, which largely involved mutational analysis of the notable latency-associated transcripts (LATs), and more recently a series of viral miRNAs. We also intend to draw attention to the many other less characterized non-coding RNAs, and perhaps coding RNAs, that may be important for consideration when trying to disentangle the multitude of phenotypes of the many genetic modifications introduced into recombinant HSV1 strains.

De Novo Herpes Simplex Virus VP16 Expression Gates a Dynamic Programmatic Transition and Sets the Latent/Lytic Balance during Acute Infection in Trigeminal Ganglia

The life long relationship between herpes simplex virus and its host hinges on the ability of the virus to aggressively replicate in epithelial cells at the site of infection and transport into the nervous system through axons innervating the infection site. Interaction between the virus and the sensory neuron represents a pivot point where largely unknown mechanisms lead to a latent or a lytic infection in the neuron. Regulation at this pivot point is critical for balancing two objectives, efficient widespread seeding of the nervous system and host survival. By combining genetic and in vivo in approaches, our studies reveal that the balance between latent and lytic programs is a process occurring early in the trigeminal ganglion. Unexpectedly, activation of the latent program precedes entry into the lytic program by 12 -14hrs. Importantly, at the individual neuronal level, the lytic program begins as a transition out of this acute stage latent program and this escape from the default latent program is regulated by de novo VP16 expression. Our findings support a model in which regulated de novo VP16 expression in the neuron mediates entry into the lytic cycle during the earliest stages of virus infection in vivo. These findings support the hypothesis that the loose association of VP16 with the viral tegument combined with sensory axon length and transport mechanisms serve to limit arrival of virion associated VP16 into neuronal nuclei favoring latency. Further, our findings point to specialized features of the VP16 promoter that control the de novo expression of VP16 in neurons and this regulation is a key component in setting the balance between lytic and latent infections in the nervous system.

Herpes simplex virus remains a significant human pathogen associated with extensive acute and chronic disease in humans worldwide. The virus invades the peripheral and central nervous systems where it replicates but also establishes life-long latent infections in neurons. Two distinct viral transcriptional programs support these distinct lifestyles, but how entry into either the lytic or latent programs is regulated in the neuron is not understood. This process is fundamentally important to a virus with the capacity to be extremely virulent, in balancing two objectives, efficient widespread seeding of the nervous system and host survival. In this report, we provide new insight into this regulation and data that support a novel model in which virus transported into the neuron from the body surface enters the latent program by default. In a subset of these, there is a transition into the lytic cycle, which requires VP16 transactivation and is gated by a region in the VP16 promoter. Thus, HSV takes advantage of the anatomy and axonal transport systems in sensory neurons so that VP16 is left behind and latency is favored, while features of the VP16 promoter insure adequate virus spread in the nervous system and maximized latent infections.

Fluorescent Protein Approaches in Alpha Herpesvirus Research

In the nearly two decades since the popularization of green fluorescent protein (GFP), fluorescent protein-based methodologies have revolutionized molecular and cell biology, allowing us to literally see biological processes as never before. Naturally, this revolution has extended to virology in general, and to the study of alpha herpesviruses in particular. In this review, we provide a compendium of reported fluorescent protein fusions to herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV) structural proteins, discuss the underappreciated challenges of fluorescent protein-based approaches in the context of a replicating virus, and describe general strategies and best practices for creating new fluorescent fusions. We compare fluorescent protein methods to alternative approaches, and review two instructive examples of the caveats associated with fluorescent protein fusions, including describing several improved fluorescent capsid fusions in PRV. Finally, we present our future perspectives on the types of powerful experiments these tools now offer.

Herpes simplex virus mutant generation and dual-detection methods for gaining insight into latent/lytic cycles in vivo

Two important components to a useful strategy to examine viral gene regulation in vivo are (1) a highly efficient protocol to generate viral mutants that limits undesired mutation and retains full replication competency in vivo and (2) an efficient system to detect and quantify viral promoter activity in rare cells in vivo. Our strategy and protocols for generating, characterizing, and employing HSV viral promoter/reporter mutants in vivo are provided in this two-part chapter.

The herpes simplex virus type 1 latency associated transcript locus is required for the maintenance of reactivation competent latent infections

Herpes simplex virus (HSV) establishes latent infections in sensory neurons from which it can periodically reactivate and cause recurrent disease and transmission to new hosts. Little is known about the virally encoded mechanisms that influence the maintenance of HSV latent infectious and modulate the frequency of virus reactivation from the latent state. Here, we report that the latency associated transcript locus of HSV-1 is required for long-term maintenance of reactivation competent latent infections.

HSV-1 gene expression from reactivated ganglia is disordered and concurrent with suppression of latency-associated transcript and miRNAs

In cell cultures, HSV-1 replication is initiated by recruitment by virion protein 16 of transcriptional factors and histone-modifying enzymes to immediate early (α) gene promoters. HSV establishes latent infections characterized by suppression of viral gene expression except for latency-associated transcripts (LATs) and miRNAs. The latent virus reactivates in stressed neurons. A fundamental question is how reactivation initiates in the absence of virion protein 16. We report the following findings in the ganglion explant model. (i) Anti-nerve growth factor antibody accelerated the reactivation of latent virus. Viral mRNAs were detected as early as 9 h after explantation. (ii) After explantation the amounts of viral mRNAs increased whereas amounts of miRNAs and LATs decreased. The decrease in miRNAs and LATs required ongoing protein synthesis, raising the possibility that LAT and miRNAs were degraded by a viral gene product. (iii) The expression of viral genes in explanted ganglia was disordered rather than sequentially ordered as in infected cells in culture. These findings suggest that in reactivating ganglia gene expression is totally derepressed and challenge the current models in that establishment of or exit from latency could not be dependent on the suppression or activation of single or small clusters of viral genes. Finally, miRNAs and LATs reached peak levels 9-11 d after corneal inoculation, thus approximating the pattern of virus replication in these ganglia. These findings suggest that the patterns of accumulation of LATs and miRNAs reflect many different stages in the infection of neurons.

An investigation of herpes simplex virus promoter activity compatible with latency establishment reveals VP16-independent activation of immediate-early promoters in sensory neurones

Herpes simplex virus (HSV) type-1 establishes lifelong latency in sensory neurones and it is widely assumed that latency is the consequence of a failure to initiate virus immediate-early (IE) gene expression. However, using a Cre reporter mouse system in conjunction with Cre-expressing HSV-1 recombinants we have previously shown that activation of the IE ICP0 promoter can precede latency establishment in at least 30% of latently infected cells. During productive infection of non-neuronal cells, IE promoter activation is largely dependent on the transactivator VP16 a late structural component of the virion. Of significance, VP16 has recently been shown to exhibit altered regulation in neurones; where its de novo synthesis is necessary for IE gene expression during both lytic infection and reactivation from latency. In the current study, we utilized the Cre reporter mouse model system to characterize the full extent of viral promoter activity compatible with cell survival and latency establishment. In contrast to the high frequency activation of representative IE promoters prior to latency establishment, cell marking using a virus recombinant expressing Cre under VP16 promoter control was very inefficient. Furthermore, infection of neuronal cultures with VP16 mutants reveals a strong VP16 requirement for IE promoter activity in non-neuronal cells, but not sensory neurones. We conclude that only IE promoter activation can efficiently precede latency establishment and that this activation is likely to occur through a VP16-independent mechanism.

Interferon alpha induces establishment of alphaherpesvirus latency in sensory neurons in vitro

Background

Several alphaherpesviruses, including herpes simplex virus 1 (HSV-1) and pseudorabies virus (PRV), establish lifelong latency in neurons of the trigeminal ganglion (TG). Although it is thought that efficient establishment of alphaherpesvirus latency is based on a subtle interplay between virus, neurons and the immune system, it is not clear which immune components are of major importance for the establishment of latency.

Methodology/Principal Findings

Here, using an in vitro model that enables a natural route of infection, we show that interferon alpha (IFNalpha) has the previously uncharacterized capacity to induce a quiescent HSV-1 and PRV infection in porcine TG neurons that shows strong similarity to in vivo latency. IFNalpha induced a stably suppressed HSV-1 and PRV infection in TG neurons in vitro. Subsequent treatment of neurons containing stably suppressed virus with forskolin resulted in reactivation of both viruses. HSV and PRV latency in vivo is often accompanied by the expression of latency associated transcripts (LATs). Infection of TG neurons with an HSV-1 mutant expressing LacZ under control of the LAT promoter showed activation of the LAT promoter and RT-PCR analysis confirmed that both HSV-1 and PRV express LATs during latency in vitro.

Conclusions/Significance

These data represent a unique in vitro model of alphaherpesvirus latency and indicate that IFNalpha may be a driving force in promoting efficient latency establishment.

Therapeutic implications of new insights into the critical role of VP16 in initiating the earliest stages of HSV reactivation from latency

Reactivation of herpes simplex virus (HSV) is a leading cause of fatal encephalitis in the USA and recurrent herpetic keratitis is a major infectious cause of blindness. There is no effective vaccine and no cure for HSV latency. While current antiviral drugs reduce viral replication, none prevent the initiation of reactivation in the nervous system and, thus, chronic inflammatory damage proceeds. The discovery that HSV VP16 is necessary for the exit from latency represents the first potential target for preventing the chronic inflammatory insult associated with HSV reactivation. Blocking VP16 transactivation would reduce the spread of the virus in the population and, importantly, presumably reduce or prevent the pathological long term chronic inflammation in the nervous system.

De novo synthesis of VP16 coordinates the exit from HSV latency in vivo

The mechanism controlling the exit from herpes simplex virus latency (HSV) is of central importance to recurrent disease and transmission of infection, yet interactions between host and viral functions that govern this process remain unclear. The cascade of HSV gene transcription is initiated by the multifunctional virion protein VP16, which is expressed late in the viral replication cycle. Currently, it is widely accepted that VP16 transactivating function is not involved in the exit from latency. Utilizing the mouse ocular model of HSV pathogenesis together with genetically engineered viral mutants and assays to quantify latency and the exit from latency at the single neuron level, we show that in vivo (i) the VP16 promoter confers distinct regulation critical for viral replication in the trigeminal ganglion (TG) during the acute phase of infection and (ii) the transactivation function of VP16 (VP16TF) is uniquely required for the exit from latency. TG neurons latently infected with the VP16TF mutant in1814 do not express detectable viral proteins following stress, whereas viruses with mutations in the other major viral transcription regulators ICP0 and ICP4 do exit the latent state. Analysis of a VP16 promoter/reporter mutant in the background of in1814 demonstrates that the VP16 promoter is activated in latently infected neurons following stress in the absence of other viral proteins. These findings support the novel hypothesis that de novo expression of VP16 regulates entry into the lytic program in neurons at all phases of the viral life cycle. HSV reactivation from latency conforms to a model in which stochastic derepression of the VP16 promoter and expression of VP16 initiates entry into the lytic cycle.

A historical analysis of herpes simplex virus promoter activation in vivo reveals distinct populations of latently infected neurones

Herpes simplex virus type 1 (HSV-1) has the capacity to establish a life-long latent infection in sensory neurones and also to periodically reactivate from these cells. Since mutant viruses defective for immediate-early (IE) expression retain the capacity for latency establishment it is widely assumed that latency is the consequence of a block in IE gene expression. However, it is not clear whether viral gene expression can precede latency establishment following wild-type virus infection. In order to address this question we have utilized a reporter mouse model system to facilitate a historical analysis of viral promoter activation in vivo. This system utilizes recombinant viruses expressing Cre recombinase under the control of different viral promoters and the Cre reporter mouse strain ROSA26R. In this model, viral promoter-driven Cre recombinase mediates a permanent genetic change, resulting in reporter gene activation and permanent marking of latently infected cells. The analyses of HSV-1 recombinants containing human cytomegalovirus major immediate-early, ICP0, gC or latency-associated transcript promoters linked to Cre recombinase in this system have revealed the existence of a population of neurones that have experienced IE promoter activation prior to the establishment of latency.

MicroRNAs expressed by herpes simplex virus 1 during latent infection regulate viral mRNAs

Herpesviruses are characterized by their ability to maintain life-long latent infections in their animal hosts. However, the mechanisms that allow establishment and maintenance of the latent state remain poorly understood. Herpes simplex virus 1 (HSV-1) establishes latency in neurons of sensory ganglia, where the only abundant viral gene product is a non-coding RNA, the latency associated transcript (LAT). Here we show that LAT functions as a primary microRNA (miRNA) precursor that encodes four distinct miRNAs in HSV-1 infected cells. One of these miRNAs, miR-H2-3p, is transcribed in an antisense orientation to ICP0-a viral immediate-early transcriptional activator that is important for productive HSV-1 replication and thought to have a role in reactivation from latency. We show that miR-H2-3p is able to reduce ICP0 protein expression, but does not significantly affect ICP0 messenger RNA levels. We also identified a fifth HSV-1 miRNA in latently infected trigeminal ganglia, miR-H6, which derives from a previously unknown transcript distinct from LAT. miR-H6 shows extended seed complementarity to the mRNA encoding a second HSV-1 transcription factor, ICP4, and inhibits expression of ICP4, which is required for expression of most HSV-1 genes during productive infection. These results may explain the reported ability of LAT to promote latency. Thus, HSV-1 expresses at least two primary miRNA precursors in latently infected neurons that may facilitate the establishment and maintenance of viral latency by post-transcriptionally regulating viral gene expression.

Promyelocytic leukemia-nuclear body proteins: herpesvirus enemies, accomplices, or both?

The promyelocytic leukemia (PML) protein gathers other cellular proteins, such as Daxx and Sp100, to form subnuclear structures termed PML-nuclear bodies (PML-NBs) or ND10 domains. Many infecting viral genomes localize to PML-NBs, leading to speculation that these structures may represent the most efficient subnuclear location for viral replication. Conversely, many viral proteins modify or disrupt PML-NBs, suggesting that viral replication may be more efficient in the absence of these structures. Thus, a debate remains as to whether PML-NBs inhibit or enhance viral replication. Here we review and discuss recent data indicating that for herpesviruses, PML-NB proteins inhibit viral replication in cell types where productive, lytic replication occurs, while at the same time may enhance the establishment of lifelong latent infections in other cell types.

Herpes simplex virus type 1 latently infected neurons differentially express latency-associated and ICP0 transcripts

During the latent phase of herpes simplex virus type 1 (HSV-1) infection, the latency-associated transcripts (LATs) are the most abundant viral transcripts present in neurons, but some immediate-early viral transcripts, such as those encoding ICP0, have also been reported to be transcribed in latently infected mouse trigeminal ganglia (TG). A murine oro-ocular model of herpetic infection was used to study ICP0 gene expression in the major anatomical sites of HSV-1 latency, including the TG, superior cervical ganglion, spinal cord, and hypothalamus. An HSV-1 recombinant strain, SC16 110LacZ, revealed ICP0 promoter activity in several neurons in latently infected ganglia, and following infection with wild-type HSV-1 strain SC16, in situ hybridization analyses identified ICP0 transcripts in the nuclei of neurons at times consistent with the establishment of latency. Reverse transcription (RT)-PCR assays performed on RNA extracted from latently infected tissues indicated that ICP0 transcripts were detected in all anatomical sites of viral latency. Furthermore, quantitative real-time RT-PCR showed that neurons differentially expressed the LATs and ICP0 transcripts, with splicing of ICP0 transcripts being dependent on the anatomical location of latency. Finally, TG neurons were characterized by high-level expression of LATs and detection of abundant unspliced ICP0 transcripts, a pattern markedly different from those of other anatomical sites of HSV-1 latency. These results suggest that LATs might be involved in the maintenance of HSV-1 latency through the posttranscriptional regulation of ICP0 in order to inhibit expression of this potent activator of gene expression during latency.

Evidence that the herpes simplex virus type 1 ICP0 protein does not initiate reactivation from latency in vivo

The stress-induced host cell factors initiating the expression of the herpes simplex virus lytic cycle from the latent viral genome are not known. Previous studies have focused on the effect of specific viral proteins on reactivation, i.e., the production of detectable infectious virus. However, identification of the viral protein(s) through which host cell factors transduce entry into the lytic cycle and analysis of the promoter(s) of this (these) first protein(s) will provide clues to the identity of the stress-induced host cell factors important for reactivation. In this report, we present the first strategy developed for this type of analysis and use this strategy to test the established hypothesis that the herpes simplex virus ICP0 protein initiates reactivation from the latent state. To this end, ICP0 null and promoter mutants were analyzed for the abilities (i) to exit latency and produce lytic-phase viral proteins (initiate reactivation) and (ii) to produce infectious viral progeny (reactivate) in explant and in vivo. Infection conditions were manipulated so that approximately equal numbers of latent infections were established by the parental strains, the mutants, and their genomically restored counterparts, eliminating disparate latent pool sizes as a complicating factor. Following hyperthermic stress (HS), which induces reactivation in vivo, equivalent numbers of neurons exited latency (as evidenced by the expression of lytic-phase viral proteins) in ganglia latently infected with either the ICP0 null mutant dl1403 or the parental strain. In contrast, infectious virus was detected in the ganglia of mice latently infected with the parental strain but not with ICP0 null mutant dl1403 or FXE. These data demonstrate that the role of ICP0 in the process of reactivation is not as a component of the switch from latency to lytic-phase gene expression; rather, ICP0 is required after entry into the lytic cycle has occurred. Similar analyses were carried out with the DeltaTfi mutant, which contains a 350-bp deletion in the ICP0 promoter, and the genomically restored isolate, DeltaTfiR. The numbers of latently infected neurons exiting latency were not different for DeltaTfi and DeltaTfiR. However, DeltaTfi did not reactivate in vivo, whereas DeltaTfiR reactivated in approximately 38% of the mice. In addition, ICP0 was detected in DeltaTfiR-infected neurons exiting latency but was not detected in those neurons exiting latency infected with DeltaTfi. We conclude that while ICP0 is important and perhaps essential for infectious virus production during reactivation in vivo, this protein is not required and appears to play no major role in the initiation of reactivation in vivo.

ICP0 is not required for efficient stress-induced reactivation of herpes simplex virus type 1 from cultured quiescently infected neuronal cells

Viral genes sufficient and required for herpes simplex virus type 1 (HSV-1) reactivation were identified using neuronally differentiated PC12 cells (ND-PC12 cells) in which quiescent infections with wild-type and recombinant strains were established. In this model, the expression of ICP0, VP16, and ICP4 from adenovirus vectors was sufficient to reactivate strains 17+ and KOS. The transactivators induced similar levels of reactivation with KOS; however, 17+ responded more efficiently to ICP0. To identify viral transactivators required for reactivation, we examined quiescently infected PC12 cell cultures (QIF-PC12 cell cultures) established with HSV-1 deletion mutants R7910 (deltaICP0), KD6 (deltaICP4), and in1814, a virus containing an insertion mutation in VP16. Although growth of these mutant viruses was impaired in ND-PC12 cells, R7910 and in1814 reactivated at levels equivalent to or better than their respective parental controls following stress (i.e., heat or forskolin) treatment. After treatment with trichostatin A, in1814 and 17+ reactivated efficiently, whereas the F strain and R7910 reactivated inefficiently. In contrast, KD6 failed to reactivate. In experiments with the recombinant KM100, which contains the in1814 mutation in VP16 and the n212 mutation in ICP0, spontaneous and stress-induced reactivation was observed. However, two strains, V422 and KM110, which lack the acidic activation domain of VP16, did not reactivate above low spontaneous levels after stress. These results demonstrate that in QIF-PC12 cells ICP0 is not required for efficient reactivation of HSV-1, the acidic activation domain of VP16 is essential for stress-induced HSV-1 reactivation, and HSV-1 reactivation is modulated uniquely by different treatment constraints and phenotypes.

The Effect of Ultraviolet Radiation on Human Viral Infections

Exposure to UV radiation is recognized to suppress cell-mediated immunity and therefore could adversely affect the course of a viral infection. Rodent models of viral infection confirm this possibility but the situation in human subjects is not so clear, apart from two exceptions. These are herpes simplex, in which sunlight exposure can cause reactivation, and certain papillomavirus types in which sunlight exposure can lead to the development of nonmelanoma skin cancer. In both cases, there are UV response elements in the viral genomes that alter the normal interactions between the viruses and the host following exposure, and UV-induced effects on the immune response occur in addition. These complex mechanisms are discussed, and the situation regarding UV radiation and viral exanthems plus other viruses, including the retroviruses, summarized. Finally viral vaccination is considered in the context of UV exposure and the importance of the host's genetic background emphasized. Further research is required to evaluate whether sunlight can significantly affect the resistance to common viral infections and vaccines.

Gamma interferon can block herpes simplex virus type 1 reactivation from latency, even in the presence of late gene expression

Herpes simplex virus type 1 (HSV-1)-specific CD8+ T cells and the cytokine gamma interferon (IFN-gamma) are persistently present in trigeminal ganglia (TG) harboring latent HSV-1. We define "latency" as the retention of functional viral genomes in sensory neurons without the production of infectious virions and "reactivation" as a multistep process leading from latency to virion assembly. CD8+ T cells can block HSV-1 reactivation in ex vivo mouse TG cultures and appear to be the sole source of IFN-gamma in these cultures. Here we demonstrate that IFN-gamma alone can block HSV-1 reactivation in some latently infected neurons, and we identify points of intervention in the life cycle of the reactivating virus. Cell suspensions of TG that were latently infected with recombinant RE HSV-1 expressing enhanced green fluorescent protein from the promoter for infected cell protein 0 (ICP0) or glycoprotein C (gC) were depleted of endogenous CD8+ or CD45+ cells and cultured in the presence or absence of IFN-gamma. Our results demonstrate that IFN-gamma acts on latently infected neurons to inhibit (i) HSV-1 reactivation, (ii) ICP0 promoter activity, (iii) gC promoter activity, and (iv) reactivation in neurons in which the ICP0 or gC promoter is active. Interestingly, we detected transcripts for ICP0, ICP4, and gH in neurons that expressed the ICP0 promoter but were prevented by IFN-gamma from reactivation and virion formation. Thus, the IFN-gamma blockade of HSV-1 reactivation from latency in neurons is associated with an inhibition of the expression of the ICP0 gene (required for reactivation) and a blockade of a step that occurs after the expression of at least some viral structural genes.