The pathogenesis of pseudorabies in mice following peripheral inoculation

Microglia play an important role in PRV infection-induced immune responses of the central nervous system

Pseudorabies virus (PRV) can infect multiple hosts and lead to fatal encephalitis. There is a significant increase in the number of microglia in the brain of animals infected with PRV. However, whether and how microglia contribute to central nervous system damage in PRV infection remain unknown. In the present study, we elucidated that PRV infection can cause more severe inflammatory cell infiltration, thicker and more numerous vessel sleeve walls, and more severe inflammatory responses in the brains of natural hosts (pigs) than in those of nonnatural hosts (mice). In a mice infection model, activated microglia restricted viral replication in the early stage of infection. Acute neuroinflammation caused by microglia hyperactivation at late-stage of infection. Furthermore, in vitro experiments revealed that microglia restricted viral replication and decreased viral infectivity. This may be associated with the phagocytic ability of microglia because we observed a significant increase in the expression of the membrane receptor TREM2 in microglia, which is closely related to phagocytosis, we observed that depletion of microglia exacerbated neurological symptoms, blood-brain barrier breakdown, and peripheral lymphocyte infiltration. Taken together, we revealed the dual role of microglia in protecting the host and neurons from PRV infection.

Pseudorabies virus hijacks DDX3X, initiating an addictive "mad itch" and immune suppression, to facilitate viral spread

Infections with defined Herpesviruses, such as Pseudorabies virus (PRV) and Varicella zoster virus (VZV) can cause neuropathic itch, referred to as "mad itch" in multiple species. The underlying mechanisms involved in neuropathic "mad itch" are poorly understood. Here, we show that PRV infections hijack the RNA helicase DDX3X in sensory neurons to facilitate anterograde transport of the virus along axons. PRV induces re-localization of DDX3X from the cell body to the axons which ultimately leads to death of the infected sensory neurons. Inducible genetic ablation of Ddx3x in sensory neurons results in neuronal death and "mad itch" in mice. This neuropathic "mad itch" is propagated through activation of the opioid system making the animals "addicted to itch". Moreover, we show that PRV co-opts and diverts T cell development in the thymus via a sensory neuron-IL-6-hypothalamus-corticosterone stress pathway. Our data reveal how PRV, through regulation of DDX3X in sensory neurons, travels along axons and triggers neuropathic itch and immune deviations to initiate pathophysiological programs which facilitate its spread to enhance infectivity.

A Review of Pseudorabies Virus Variants: Genomics, Vaccination, Transmission, and Zoonotic Potential

Pseudorabies virus (PRV), the causative agent of Aujeszky’s disease, has a broad host range including most mammals and avian species. In 2011, a PRV variant emerged in many Bartha K61-vaccinated pig herds in China and has attracted more and more attention due to its serious threat to domestic and wild animals, and even human beings. The PRV variant has been spreading in China for more than 10 years, and considerable research progresses about its molecular biology, pathogenesis, transmission, and host–virus interactions have been made. This review is mainly organized into four sections including outbreak and genomic evolution characteristics of PRV variants, progresses of PRV variant vaccine development, the pathogenicity and transmission of PRV variants among different species of animals, and the zoonotic potential of PRV variants. Considering PRV has caused a huge economic loss of animals and is a potential threat to public health, it is necessary to extensively explore the mechanisms involved in its replication, pathogenesis, and transmission in order to ultimately eradicate it in China.

Pseudorabies virus kinase UL13 phosphorylates H2AX to foster viral replication

The DNA damage response (DDR) pathway is critical for maintaining genomic integrity and sustaining organismal development. Viruses can either utilize or circumvent the DDR to facilitate their replication. Pseudorabies virus (PRV) infection was shown to induce apoptosis via stimulating DDR. However, the underlying mechanisms have not been fully explored to date. This study showed that PRV infection robustly activates the ATM and DNA-PK signaling pathways shortly after infection. However, inhibition of ATM, but not DNA-PK, could dampen PRV replication in cells. Importantly, we found that PRV-encoded serine/threonine kinase UL13 interacts with and subsequently phosphorylates H2AX. Furthermore, we found that UL13 deletion largely attenuates PRV neuroinvasiveness and virulence in vivo. In addtion, we showed that UL13 contributes to H2AX phosphorylation upon PRV infection both in vitro and in vivo, but does not affect ATM phosphorylation. Finally, we showed that knockdown of H2AX reduces PRV replication, while this reduction can be further enhanced by deletion of UL13. Taken together, we conclude that PRV-encoded kinase UL13 regulates DNA damage marker γH2AX and UL13-mediated H2AX phosphorylation plays a pivotal role in efficient PRV replication and progeny production.

Histopathological Analysis of Adrenal Glands after Simian Varicella Virus Infection

Latent varicella zoster virus (VZV) has been detected in human adrenal glands, raising the possibility of virus-induced adrenal damage and dysfunction during primary infection or reactivation. Rare cases of bilateral adrenal hemorrhage and insufficiency associated with VZV reactivation have been reported. Since there is no animal model for VZV infection of adrenal glands, we obtained adrenal glands from two non-human primates (NHPs) that spontaneously developed varicella from primary simian varicella virus (SVV) infection, the NHP VZV homolog. Histological and immunohistochemical analysis revealed SVV antigen and DNA in the adrenal medulla and cortex of both animals. Adrenal glands were observed to have Cowdry A inclusion bodies, cellular necrosis, multiple areas of hemorrhage, and varying amounts of polymorphonuclear cells. No specific association of SVV antigen with βIII-tubulin-positive nerve fibers was found. Overall, we found that SVV can productively infect NHP adrenal glands, and is associated with inflammation, hemorrhage, and cell death. These findings suggest that further studies are warranted to examine the contribution of VZV infection to human adrenal disease. This study also suggests that VZV infection may present itself as acute adrenal dysfunction with “long-hauler” symptoms of fatigue, weakness, myalgias/arthralgias, and hypotension.

Comparative Pathology of Pseudorabies in Different Naturally and Experimentally Infected Species-A Review

The pseudorabies virus (PRV) is an alphaherpesvirus and the causative agent of Aujeszky’s disease (AD). PRV infects a wide range of animal species including swine as the natural host as well as ruminants, carnivores, rodents and lagomorphs. In these species, except for the pig, PRV infection causes acute, severe disease, characterized by insatiable itching, and is always lethal. Horses, chickens and non-human primates have been shown to be largely resistant to PRV infection, while disease in humans is still controversial. PRV is a pantropic virus, which preferably invades neural tissue, but also infects epithelia of various organs, whereupon multisystemic lesions may result. Although AD is mainly associated with severe pruritus, also known as “mad itch”, there are notable differences regarding infection route, clinical signs, viral distribution and lesion patterns in different animal species. In this comprehensive review, we will present clinico-pathologic findings from different species, which have been either shown to be susceptible to PRV infection or have been tested experimentally.

The Neuropathic Itch Caused by Pseudorabies Virus

Pseudorabies virus (PRV) is an alphaherpesvirus related to varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV1). PRV is the causative agent of Aujeskzy’s disease in swine. PRV infects mucosal epithelium and the peripheral nervous system (PNS) of its host where it can establish a quiescent, latent infection. While the natural host of PRV is the swine, a broad spectrum of mammals, including rodents, cats, dogs, and cattle can be infected. Since the nineteenth century, PRV infection is known to cause a severe acute neuropathy, the so called “mad itch” in non-natural hosts, but surprisingly not in swine. In the past, most scientific efforts have been directed to eradicating PRV from pig farms by the use of effective marker vaccines, but little attention has been given to the processes leading to the mad itch. The main objective of this review is to provide state-of-the-art information on the mechanisms governing PRV-induced neuropathic itch in non-natural hosts. We highlight similarities and key differences in the pathogenesis of PRV infections between non-natural hosts and pigs that might explain their distinctive clinical outcomes. Current knowledge on the neurobiology and possible explanations for the unstoppable itch experienced by PRV-infected animals is also reviewed. We summarize recent findings concerning PRV-induced neuroinflammatory responses in mice and address the relevance of this animal model to study other alphaherpesvirus-induced neuropathies, such as those observed for VZV infection.

Alphaherpesvirus infection of mice primes PNS neurons to an inflammatory state regulated by TLR2 and type I IFN signaling

Pseudorabies virus (PRV), an alphaherpesvirus closely related to Varicella-Zoster virus (VZV) and Herpes simplex type 1 (HSV1) infects mucosa epithelia and the peripheral nervous system (PNS) of its host. We previously demonstrated that PRV infection induces a specific and lethal inflammatory response, contributing to severe neuropathy in mice. So far, the mechanisms that initiate this neuroinflammation remain unknown. Using a mouse footpad inoculation model, we found that PRV infection rapidly and simultaneously induces high G-CSF and IL-6 levels in several mouse tissues, including the footpad, PNS and central nervous system (CNS) tissues. Interestingly, this global increase occurred before PRV had replicated in dorsal root ganglia (DRGs) neurons and also was independent of systemic inflammation. These high G-CSF and IL-6 levels were not caused by neutrophil infiltration in PRV infected tissues, as we did not detect any neutrophils. Efficient PRV replication and spread in the footpad was sufficient to activate DRGs to produce cytokines. Finally, by using knockout mice, we demonstrated that TLR2 and IFN type I play crucial roles in modulating the early neuroinflammatory response and clinical outcome of PRV infection in mice. Overall, these results give new insights into the initiation of virus-induced neuroinflammation during herpesvirus infections.

Herpesviruses are major pathogens worldwide. Pseudorabies virus (PRV) is an alphaherpesvirus related to varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV1). The natural host is the pig, but PRV can infect most mammals. In these non-natural hosts, the virus causes a severe pruritus called the ‘mad itch’. Interestingly, PRV infects the peripheral nervous system (PNS) and induces a specific and lethal inflammatory response in mice, yet little is know about how this neuroinflammatory response is initiated. In this study, we demonstrated for the first time how PNS neurons tightly regulate the inflammatory response during PRV infection and contribute to severe clinical outcome in mice. Our work provides new insights into the process of alphaherpesvirus-induced neuropathies, leading to the development of innovative therapeutic strategies.

Virucidal activity of Garcinia parvifolia leaf extracts in animal cell culture

BACKGROUND

Garcinia species contain bioactive compounds such as flavonoids, xanthones, triterpernoids, and benzophenones with antibacterial, antifungal, anti-inflammatory, and antioxidant activities. In addition, many of these compounds show interesting biological properties such as anti-human immunodeficiency virus activity. Garcinia parvifolia is used in traditional medicine. Currently, the antiviral activity of G. parvifolia is not known.

METHODS

This study was conducted to determine the effects of ethyl acetate (45 L Ea), ethanol (45 L Et), and hexane (45 L H) leaf extracts of G. parvifolia on the infectivity of pseudorabies virus (PrV) in Vero cells. The antiviral effects of the extracts were determined by cytopathic effect (CPE), inhibition, attachment, and virucidal assays.

RESULTS

The 50% cytotoxicity concentration (CC50) values obtained were 237.5, 555.0, and < 1.25 μg/mL for 45 L Ea, 45 L Et, and 45 L H, respectively. The 45 L Ea showed the greatest viral inhibition potency of 75% at 125 μg/mL. Both 45 L Ea and 45 l Et caused 100% residual viral inhibition at 250 μg/mL. The selectivity index values for 45 L Ea, 45 L Et, and 45 L H were 2.65, 1.75, and 0.10 showing that 45 L Ea had the greatest antiviral activity among the three extracts.

CONCLUSION

This study showed that ethyl acetate is the best solvent to be used to obtain extract from G. parvifolia leaves with potent antiviral activities.

Virulent Pseudorabies Virus Infection Induces a Specific and Lethal Systemic Inflammatory Response in Mice

Pseudorabies virus (PRV) is an alphaherpesvirus that infects the peripheral nervous system (PNS). The natural host of PRV is the swine, but it can infect most mammals, including cattle, rodents, and dogs. In these nonnatural hosts, PRV always causes a severe acute and lethal neuropathy called the "mad itch," which is uncommon in swine. Thus far, the pathophysiological and immunological processes leading to the development of the neuropathic itch and the death of the animal are unclear. Using a footpad inoculation model, we established that mice inoculated with PRV-Becker (virulent strain) develop a severe pruritus in the foot and become moribund at 82 h postinoculation (hpi). We found necrosis and inflammation with a massive neutrophil infiltration only in the footpad and dorsal root ganglia (DRGs) by hematoxylin and eosin staining. PRV load was detected in the foot, PNS, and central nervous system tissues by quantitative reverse transcription-PCR. Infected mice had elevated plasma levels of proinflammatory cytokines (interleukin-6 [IL-6] and granulocyte colony-stimulating factor [G-CSF]) and chemokines (Gro-1 and monocyte chemoattractant protein 1). Significant IL-6 and G-CSF levels were detected in several tissues at 82 hpi. High plasma levels of C-reactive protein confirmed the acute inflammatory response to PRV-Becker infection. Moreover, mice inoculated with PRV-Bartha (attenuated, live vaccine strain) did not develop pruritus at 82 hpi. PRV-Bartha also replicated in the PNS, and the infection spread further in the brain than PRV-Becker. PRV-Bartha infection did not induce the specific and lethal systemic inflammatory response seen with PRV-Becker. Overall, we demonstrated the importance of inflammation in the clinical outcome of PRV infection in mice and provide new insights into the process of PRV-induced neuroinflammation.IMPORTANCE Pseudorabies virus (PRV) is an alphaherpesvirus related to human pathogens such as herpes simplex virus 1 and varicella-zoster virus (VZV). The natural host of PRV is the swine, but it can infect most mammals. In susceptible animals other than pigs, PRV infection always causes a characteristic lethal pruritus known as the "mad itch." The role of the immune response in the clinical outcome of PRV infection is still poorly understood. Here, we show that a systemic host inflammatory response is responsible for the severe pruritus and acute death of mice infected with virulent PRV-Becker but not mice infected with attenuated strain PRV-Bartha. In addition, we identified IL-6 and G-CSF as two main cytokines that play crucial roles in the regulation of this process. Our findings give new insights into neuroinflammatory diseases and strengthen further the similarities between VZV and PRV infections at the level of innate immunity.

The pseudorabies virus protein, pUL56, enhances virus dissemination and virulence but is dispensable for axonal transport

Neurotropic herpesviruses exit the peripheral nervous system and return to exposed body surfaces following reactivation from latency. The pUS9 protein is a critical viral effector of the anterograde axonal transport that underlies this process. We recently reported that while pUS9 increases the frequency of sorting of newly assembled pseudorabies virus particles to axons from the neural soma during egress, subsequent axonal transport of individual virus particles occurs with wild-type kinetics in the absence of the protein. Here, we examine the role of a related pseudorabies virus protein, pUL56, during neuronal infection. The findings indicate that pUL56 is a virulence factor that supports virus dissemination in vivo, yet along with pUS9, is dispensable for axonal transport.

Therapeutic Strategy for the Prevention of Pseudorabies Virus Infection in C57BL/6 Mice by 3D8 scFv with Intrinsic Nuclease Activity

3D8 single chain variable fragment (scFv) is a recombinant monoclonal antibody with nuclease activity that was originally isolated from autoimmune-prone MRL mice. In a previous study, we analyzed the nuclease activity of 3D8 scFv and determined that a HeLa cell line expressing 3D8 scFv conferred resistance to herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PRV). In this study, we demonstrate that 3D8 scFv could be delivered to target tissues and cells where it exerted a therapeutic effect against PRV. PRV was inoculated via intramuscular injection, and 3D8 scFv was injected intraperitoneally. The observed therapeutic effect of 3D8 scFv against PRV was also supported by results from quantitative reverse transcription polymerase chain reaction, southern hybridization, and immunohistochemical assays. Intraperitoneal injection of 5 and 10 μg 3D8 scFv resulted in no detectable toxicity. The survival rate in C57BL/6 mice was 9% after intramuscular injection of 10 LD50 PRV. In contrast, the 3D8 scFv-injected C57BL/6 mice showed survival rates of 57% (5 μg) and 47% (10 μg). The results indicate that 3D8 scFv could be utilized as an effective antiviral agent in several animal models.

Efficacy of Intracerebral Immunization against Pseudorabies Virus in Mice

To evaluate the efficacy of intracerebral (IC) immunization, mice were immunized with formalin-inactivated pseudorabies virus (PRV) by either subcutaneous (SC) or IC injection, and then 10(6) plaque-forming units of PRV were introduced into the hindleg of the immunized or non-immunized mice by intramuscular injection. The antibody titer in serum was elevated and boosted by additional immunization via both the SC and IC routes, but was higher after IC immunization. Intracerebrally immunized mice were completely protected from mortality and neurological signs, whereas all the non-immunized and 80% of the subcutaneously immunized mice died after developing neurological signs. In mouse models, IC immunization is more effective at inducing a protective immune response against the transneural spread of PRV than SC immunization.

Imaging the transport dynamics of single alphaherpesvirus particles in intact peripheral nervous system explants from infected mice

ABSTRACT Alphaherpesvirus particles travel long distances in the axons of neurons using host microtubule molecular motors. The transport dynamics of individual virions in neurons have been assessed in cultured neurons, but imaging studies of single particles in tissue from infected mice have not been reported. We developed a protocol to image explanted, infected peripheral nervous system (PNS) ganglia and associated innervated tissue from mice infected with pseudorabies virus (PRV). This ex vivo preparation allowed us to visualize and track individual virions over time as they moved from the salivary gland into submandibular ganglion neurons of the PNS. We imaged and tracked hundreds of virions from multiple mice at different time points. We quantitated the transport velocity, particle stalling, duty cycle, and directionality at various times after infection. Using a PRV recombinant that expressed monomeric red fluorescent protein (mRFP)-VP26 (red capsid) and green fluorescent protein (GFP)-Us9 (green membrane protein), we corroborated that anterograde transport in axons occurs after capsids are enveloped. We addressed the question of whether replication occurs initially in the salivary gland at the site of inoculation or subsequently in the neurons of peripheral innervating ganglia. Our data indicate that significant amplification of infection occurs in the peripheral ganglia after transport from the site of infection and that these newly made particles are transported back to the salivary gland. It is likely that this reseeding of the infected gland contributes to massive invasion of the innervating PNS ganglia. We suggest that this "round-trip" infection process contributes to the characteristic peripheral neuropathy of PRV infection. IMPORTANCE Much of our understanding of molecular mechanisms of alphaherpesvirus infection and spread in neurons comes from studying cultured primary neurons. These techniques enabled significant advances in our understanding of the viral and neuronal components needed for efficient replication and directional spread between cells. However, in vitro systems cannot recapitulate the environment of innervated tissue in vivo with associated defensive properties, such as innate immunity. Therefore, in this report, we describe a system to image the progression of infection by single virus particles in tissue harvested from infected animals. We explanted intact innervated tissue from infected mice and imaged fluorescent virus particles in infected axons of the specific ganglionic neurons. Our measurements of virion transport dynamics are consistent with published in vitro results. Importantly, this system enabled us to address a fundamental biological question about the amplification of a herpesvirus infection in a peripheral nervous system circuit.

Herpesvirus transport to the nervous system and back again

Herpes simplex virus, varicella zoster virus, and pseudorabies virus are neurotropic pathogens of the Alphaherpesvirinae subfamily of the Herpesviridae. These viruses efficiently invade the peripheral nervous system and establish lifelong latency in neurons resident in peripheral ganglia. Primary and recurrent infections cycle virus particles between neurons and the peripheral tissues they innervate. This remarkable cycle of infection is the topic of this review. In addition, some of the distinguishing hallmarks of the infections caused by these viruses are evaluated in terms of their underlying similarities.

Resolving the assembly state of herpes simplex virus during axon transport by live-cell imaging

Neurotropic herpesviruses depend on long-distance axon transport for the initial establishment of latency in peripheral ganglia (retrograde transport) and for viral spread in axons to exposed body surfaces following reactivation (anterograde transport). Images of neurons infected with herpes simplex virus type 1 (HSV-1), acquired using electron microscopy, have led to a debate regarding why different types of viral structures are seen in axons and which of these particles are relevant to the axon transport process. In this study, we applied time-lapse fluorescence microscopy to image HSV-1 virion components actively translocating to distal axons in primary neurons and neuronal cell lines. Key to these findings, only a small fraction of viral particles were engaged in anterograde transport during the egress phase of infection at any given time. By selective analysis of the composition of the subpopulation of actively transporting capsids, a link between transport of fully assembled HSV-1 virions and the neuronal secretory pathway was identified. Last, we have evaluated the seemingly opposing findings made in previous studies of HSV-1 axon transport in fixed cells and demonstrate a limitation to assessing the composition of individual HSV-1 particles using antibody detection methods.

Propagation of swine hemagglutinating encephalomyelitis virus and pseudorabies virus in dorsal root ganglia cells

Swine hemagglutinating encephalomyelitis virus (HEV) causes encephalomyelitis or vomiting and wasting disease in suckling piglets. Neurotoropism of the virus has been demonstrated in previous in vivo studies. In the present study, we investigated the infectivity and propagation of HEV in comparison with those of pseudorabies virus (PRV), another neurotropic virus, using dorsal root ganglia cells of newborn mice containing nerve cells and non-neuronal cells. HEV infected nerve cells but did not infect non-neuronal cells, whereas PRV infected both cell types. By using cytoskeletal inhibitors, it was suggested that propagation of HEV and PRV within and among nerve cells depended on microtubules and intermediate filaments of nerve cells, indicating that the viruses may be transported between the cell body and axonal terminals of neurons by fast axonal flow.

Transport and egress of herpes simplex virus in neurons

The mechanisms of axonal transport of the alphaherpesviruses, HSV and pseudorabies virus (PrV), in neuronal axons are of fundamental interest, particularly in comparison with other viruses, and offer potential sites for antiviral intervention or development of gene therapy vectors. These herpesviruses are transported rapidly along microtubules (MTs) in the retrograde direction from the axon terminus to the dorsal root ganglion and then anterogradely in the opposite direction. Retrograde transport follows fusion and deenvelopment of the viral capsid at the axonal membrane followed by loss of most of the tegument proteins and then binding of the capsid via one or more viral proteins (VPs) to the retrograde molecular motor dynein. The HSV capsid protein pUL35 has been shown to bind to the dynein light chain Tctex1 but is likely to be accompanied by additional dynein binding of an inner tegument protein. The mechanism of anterograde transport is much more controversial with different processes being claimed for PrV and HSV: separate transport of HSV capsid/tegument and glycoproteins versus PrV transport as an enveloped virion. The controversy has not been resolved despite application, in several laboratories, of confocal microscopy (CFM), real-time fluorescence with viruses dual labelled on capsid and glycoprotein, electron microscopy in situ and immuno-electron microscopy. Different processes for each virus seem counterintuitive although they are the most divergent in the alphaherpesvirus subfamily. Current hypotheses suggest that unenveloped HSV capsids complete assembly in the axonal growth cones and varicosities, whereas with PrV unenveloped capsids are only found travelling in a retrograde direction.

Herpes simplex virus type 1 accumulation, envelopment, and exit in growth cones and varicosities in mid-distal regions of axons

The mechanism of anterograde transport of alphaherpesviruses in axons remains controversial. This study examined the transport, assembly, and egress of herpes simplex virus type 1 (HSV-1) in mid- and distal axons of infected explanted human fetal dorsal root ganglia using confocal microscopy and transmission electron microscopy (TEM) at 19, 24, and 48 h postinfection (p.i.). Confocal-microscopy studies showed that although capsid (VP5) and tegument (UL37) proteins were not uniformly present in axons until 24 h p.i., they colocalized with envelope (gG) proteins in axonal varicosities and in growth cones at 24 and 48 h p.i. TEM of longitudinal sections of axons in situ showed enveloped and unenveloped capsids in the axonal varicosities and growth cones, whereas in the midregion of the axons, predominantly unenveloped capsids were observed. Partially enveloped capsids, apparently budding into vesicles, were observed in axonal varicosities and growth cones, but not during viral attachment and entry into axons. Tegument proteins (VP22) were found associated with vesicles in growth cones, either alone or together with envelope (gD) proteins, by transmission immunoelectron microscopy. Extracellular virions were observed adjacent to axonal varicosities and growth cones, with some virions observed in crescent-shaped invaginations of the axonal plasma membrane, suggesting exit at these sites. These findings suggest that varicosities and growth cones are probable sites of HSV-1 envelopment of at least a proportion of virions in the mid- to distal axon. Envelopment probably occurs by budding of capsids into vesicles with associated tegument and envelope proteins. Virions appear to exit from these sites by exocytosis.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.

In vitro demonstration of neural transmission of avian influenza A virus

Neural involvement following infections of influenza viruses can be serious. The neural transport of influenza viruses from the periphery to the central nervous system has been indicated by using mouse models. However, no direct evidence for neuronal infection has been obtained in vitro and the mechanisms of neural transmission of influenza viruses have not been reported. In this study, the transneural transmission of a neurotropic influenza A virus was examined using compartmentalized cultures of neurons from mouse dorsal root ganglia, and the results were compared with those obtained using the pseudorabies virus, a virus with well-established neurotransmission. Both viruses reached the cell bodies of the neurons via the axons. This is the first report on axonal transport of influenza A virus in vitro. In addition, the role of the cytoskeleton (microtubules, microfilaments and intermediate filaments) in the neural transmission of influenza virus was investigated by conducting cytoskeletal perturbation experiments. The results indicated that the transport of avian influenza A virus in the neurons was independent of microtubule integrity but was dependent on the integrity of intermediate filaments, whereas pseudorabies virus needed both for neural spread.

Heterogeneity of a fluorescent tegument component in single pseudorabies virus virions and enveloped axonal assemblies

The molecular mechanisms responsible for long-distance, directional spread of alphaherpesvirus infections via axons of infected neurons are poorly understood. We describe the use of red and green fluorescent protein (GFP) fusions to capsid and tegument components, respectively, to visualize purified, single extracellular virions and axonal assemblies after pseudorabies virus (PRV) infection of cultured neurons. We observed heterogeneity in GFP fluorescence when GFP was fused to the tegument component VP22 in both single extracellular virions and discrete puncta in infected axons. This heterogeneity was observed in the presence or absence of a capsid structure detected by a fusion of monomeric red fluorescent protein to VP26. The similarity of the heterogeneous distribution of these fluorescent protein fusions in both purified virions and in axons suggested that tegument-capsid assembly and axonal targeting of viral components are linked. One possibility was that the assembly of extracellular and axonal particles containing the dually fluorescent fusion proteins occurred by the same process in the cell body. We tested this hypothesis by treating infected cultured neurons with brefeldin A, a potent inhibitor of herpesvirus maturation and secretion. Brefeldin A treatment disrupted the neuronal secretory pathway, affected fluorescent capsid and tegument transport in the cell body, and blocked subsequent entry into axons of capsid and tegument proteins. Electron microscopy demonstrated that in the absence of brefeldin A treatment, enveloped capsids entered axons, but in the presence of the inhibitor, unenveloped capsids accumulated in the cell body. These results support an assembly process in which PRV capsids acquire a membrane in the cell body prior to axonal entry and subsequent transport.

The vagus nerve is one route of transneural invasion for intranasally inoculated influenza a virus in mice

Intranasally inoculated neurotropic influenza viruses in mice infect not only the respiratory tract but also the central nervous system (CNS), mainly the brain stem. Previous studies suggested that the route of invasion of virus into the CNS was via the peripheral nervous system, especially the vagus nerve. To evaluate the transvagal transmission of the virus, we intranasally inoculated unilaterally vagectomized mice with a virulent influenza virus (strain 24a5b) and examined the distribution of the viral protein and genome by immunohistochemistry and in situ hybridization over time. An asymmetric distribution of viral antigens was observed between vagal (nodose) ganglia: viral antigen was detected in the vagal ganglion of the vagectomized side 2 days later than in the vagal ganglion of the intact side. The virus was apparently transported from the respiratory mucosa to the CNS directly and decussately via the vagus nerve and centrifugally to the vagal ganglion of the vagectomized side. The results of this study, thus, demonstrate that neurotropic influenza virus travels to the CNS mainly via the vagus nerve.

Transcriptional response of a common permissive cell type to infection by two diverse alphaherpesviruses

Pseudorabies virus (PRV) and herpes simplex virus type 1 (HSV-1) are distantly related alphaherpesviruses whose natural hosts are pigs and humans, respectively. Adult infections of natural hosts are mild and rarely lethal. However, both viruses are also able to infect other hosts, often with lethal effects. In this report, we use the paradigm of infection of a common permissive cell type and microarray analysis to determine if these two diverse alphaherpesviruses engage similar or different cellular pathways to obtain a common outcome: productive infection. We compared cellular gene expression in growth-arrested, primary rat embryonic fibroblasts that were mock infected or infected with either purified PRV-Becker or HSV-1(F). Infections by either virus affect the transcription of more than 1,500 cellular genes by threefold or more. Few differences are detected early, and the majority of changes occur during the late stages of infection. Remarkably, the transcripts of about 500 genes are regulated in common, while the rest are regulated in a virus-specific manner. Genes whose expression is affected by infection fall into a diverse group of functional classes and cellular pathways. Furthermore, a comparison of the cellular response to HSV-1 infection of primary human and rat fibroblasts revealed unexpected diversity in the transcript profiles.

Influence of tegument proteins of pseudorabies virus on neuroinvasion and transneuronal spread in the nervous system of adult mice after intranasal inoculation

Pseudorabies virus (PrV) is a neurotropic alphaherpesvirus that, after intranasal infection of adult mice, enters peripheral neurons and propagates to the central nervous system. In recent years we have analyzed the contribution of virus-encoded glycoproteins to neuroinvasion and transneuronal spread (reviewed in T. C. Mettenleiter, Virus Res. 92:197-206, 2003). We now extend our studies to analyze the role of tegument proteins in these processes. To this end, PrV mutants unable to express the UL11, UL37, UL46, UL47, and UL48 tegument proteins, as well as the corresponding rescued viruses, were intranasally instilled into 6- to 8-week-old CD1 strain mice. First, mean survival times were determined which showed that mice infected with the UL46 deletion mutant succumbed to the disease as early as wild-type PrV-infected animals. Survival times increased in the order: PrV-DeltaUL47-, PrV-DeltaUL11-, and PrV-DeltaUL48-infected animals, a finding which parallels the growth phenotype of these viruses in cell culture. In contrast, none of the PrV-DeltaUL37-infected animals died. Upon closer histological examination, all viruses except PrV-DeltaUL37 were able to infect the nasal cavity and propagate to first- and second-order neurons as shown by two-color immunofluorescence. However, neuroinvasion was delayed in PrV-DeltaUL47, PrV-DeltaUL11, and PrV-DeltaUL48, a finding that correlated with the extended survival times. Surprisingly, whereas PrV-DeltaUL48 and PrV-DeltaUL37 replicated to similar titers in cell culture which were approximately 500-fold lower than those of wild-type virus, after intranasal infection of mice PrV-DeltaUL48 was able to infect areas of the brain like wild-type PrV, although only after a considerably longer time period. In contrast, PrV-DeltaUL37 was not able to enter neurons and was restricted to the infection of single cells in the nasal respiratory epithelium. Thus, our data demonstrate the importance of herpesviral tegument proteins in neuronal infection and show a different contribution of tegument proteins to the neuroinvasion phenotype of a neurotropic alphaherpesvirus.

In vivo egress of an alphaherpesvirus from axons

Many alphaherpesviruses establish a latent infection in the peripheral nervous systems of their hosts. This life cycle requires the virus to move long distances in axons toward the neuron's cell body during infection and away from the cell body during reactivation. While the events underlying entry of the virion into neurons during infection are understood in principle, no such consensus exists regarding viral egress from neurons after reactivation. In this study, we challenged two different models of viral egress from neurons by using pseudorabies virus (PRV) infection of the rat retina: does PRV egress solely from axon terminals, or can the virus egress from axon shafts as well as axon terminals? We took advantage of PRV gD mutants that are not infectious as extracellular particles but are capable of spreading by cell-cell contact. We observed that both wild-type virus and a PRV gD null mutant are capable of spreading from axons to closely apposed nonneuronal cells within the rat optic nerve after intravitreal infection. However, infection does not spread from these infected nonneuronal cells. We suggest that viral egress can occur sporadically along the length of infected axons and is not confined solely to axon terminals. Moreover, it is likely that extracellular particles are not involved in nonneuronal cell infections. Taking these together with previous data, we suggest a model of viral egress from neurons that unifies previous apparently contradictory data.

Tracking the spread of a lacZ-tagged herpes simplex virus type 1 between the eye and the nervous system of the mouse: comparison of primary and recurrent infection

The spread of herpes simplex virus type 1 (HSV-1) during primary ocular infection and after reactivation of latent infection in the trigeminal ganglion (TG) was examined in the mouse using a genetically modified virus containing the lacZ reporter gene under the control of the immediate-early 110 promoter. Whole tissue mounts of the eye and lids, their sensory nerves, and TG with the attached dorsal root entry zone (DRE) into the central nervous system (CNS) were stained for beta-galactosidase. Sixteen hours after inoculation of the cornea by scarification, staining was found in the scarified epithelium of the cornea and in the unscarified conjunctiva. By 24 h, staining was also seen in a few TG neurons and by 96 h their number had greatly increased and their distribution was more widespread. Stained cells (identified as Schwann cells by their staining for glial fibrillary acidic protein [GFAP] or S-100) in the TG were first seen close to stained neurons at 40 h, and by 48 h lines of such cells extended partway toward the periphery and toward the DRE. By 72 h, these lines had reached the periphery and the DRE where the adjacent CNS was also stained. In the cornea, stained cells with the morphology and arrangement of Schwann cells were seen from 40 to 120 h. After reactivation of latent infection, 10 of 22 samples had positively stained neurons. In eight samples, corneal and lid epithelial cells were stained. No stained Schwann cells were seen in the TG; however, branched networks of such cells were present in the cornea and the lids. This detailed sequential analysis has provided new information on the involvement of Schwann cells in the pathogenesis of primary and recurrent HSV-1 disease in the TG and the cornea.

Directional transneuronal infection by pseudorabies virus is dependent on an acidic internalization motif in the Us9 cytoplasmic tail

The Us9 gene is conserved among most alphaherpesviruses. In pseudorabies virus (PRV), the Us9 protein is a 98-amino-acid, type II membrane protein found in the virion envelope. It localizes to the trans-Golgi network (TGN) region in infected and transfected cells and is maintained in this compartment by endocytosis from the plasma membrane. Viruses with Us9 deleted have no observable defects in tissue culture yet have reduced virulence and restricted spread to retinorecipient neurons in the rodent brain. In this report, we demonstrate that Us9-promoted transneuronal spread in vivo is dependent on a conserved acidic motif previously shown to be essential for the maintenance of Us9 in the TGN region and recycling from the plasma membrane. Mutant viruses with the acidic motif deleted have an anterograde spread defect indistinguishable from that of Us9 null viruses. Transneuronal spread, however, is not dependent on a dileucine endocytosis motif in the Us9 cytoplasmic tail. Through alanine scanning mutagenesis of the acidic motif, we have identified two conserved tyrosine residues that are essential for Us9-mediated spread as well as two serine residues, comprising putative consensus casein kinase II sites, that modulate the rate of PRV transneuronal spread in vivo.

Characterization of transsynaptic tracing with central application of pseudorabies virus

Although transsynaptic tracing with peripheral injection of pseudorabies virus (PRV) has been extensively characterized, several methodological issues related to central application of this tracer have not been addressed. In the present study, we addressed the following three issues by using microinjection of a cocktail containing PRV (Bartha strain) and cholera toxin subunit B (CTb) into different sites in the rat brain. First, we estimated PRV diffusion by examining injection sites at different times after application. Second, we tested whether PRV is taken up by fibers of passage following injections into the olivocerebellar pathway. Third, we developed criteria for leakage of PRV into cerebral ventricles. Our data indicate that (i) centrally injected PRV diffuses very little and produces focal injection sites; (ii) PRV is taken up and transported by fibers of passage, although less prominently than found for Ctb; (iii) PRV produces specific and easily identifiable ependymal cell as well as neuronal labeling following ventricular injection. This labeling can be used as a criterion for determining if labeling obtained was due to injected tracer leaking into brain ventricles. In summary, the present study provides new and important information about using PRV to trace central multisynaptic circuitry.

Molecular mechanisms of neurotropic herpesvirus invasion and spread in the CNS

Pseudorabies virus (PRV) is a herpesvirus in the subfamily alphaherpesvirinae (the alpha herpesviruses). After primary infection at mucosal surfaces, PRV infects the peripheral nervous system in its natural host (swine) with occasional invasion of the central nervous system. When other hosts (including cows and rodents) are infected, the infection almost always gives rise to fatal disease in the CNS as a result of infection of peripheral neurons and subsequent spread to the brain. Part of the ability to cause fatal CNS disease can be attributed to a viral glycoprotein called gE. Viruses lacking gE are thought to be less virulent because they do not spread efficiently from cell to cell. Based on a set of gE mutations we have constructed, we suggest that these two phenotypes of cell-cell spread and virulence reflect separate functions of the gE protein. In this report, we show that viruses carrying these new gE mutations have marked reduction in virulence, yet spread efficiently in defined neural circuits in the rat brain. As such, they offer new insight and opportunities for understanding of viral disease and host response to injury, as well as in the construction of viral tracers of neuronal connections.

Characterization of pseudorabies virus mutants expressing carboxy-terminal truncations of gE: evidence for envelope incorporation, virulence, and neurotropism domains

Glycoprotein E (gE) gene of pseudorabies virus (PRV) is conserved among diverse alphaherpesviruses and therefore is predicted to be important for virus survival. gE contributes to viral spread from cell to cell in a variety of hosts and is responsible, in part, for increased virulence or pathogenesis of the virus. Virulence and spread mediated by gE are thought to be highly correlated. We initiated this study to explore the hypothesis that these two phenotypes might reflect separate functions of the gE protein. We did so by focusing on the role of the gE carboxy terminus in neuronal spread. Viruses harboring nonsense mutations affecting the expression of the gE cytoplasmic domain had several notable phenotypes. First, the truncated gE proteins expressed from these mutants are not found in virion envelopes. Second, the mutants retain the ability to spread to all retinorecipient regions of the rodent brain after retinal infection of rats. Third, the mutants have the reduced virulence phenotype of a gE deletion mutant in rats. Finally, the mutants have distinct plaque-size phenotypes on MDBK cells but not PK15 cells. Based on these observations, we suggest that gE-mediated virulence and spread may reflect separate functions that are not mediated by gE on virus particles.

Demonstration of equine herpesvirus-1 neuronal latency in murine olfactory bulbs using a novel combined in situ PCR and protein synthesis method

Equine herpesvirus-1 (EHV-1) latency in murine olfactory bulbs was demonstrated by a novel combined in situ PCR and in vitro protein synthesis method (in situ PS-PCR). The Escherichia coli lacZ gene replacing a deletion in EHV-1 gene 71 (EUS4) was thus amplified and transcribed/translated in situ followed by enzymatic detection using X-Gal (5-bromo-4-chloro-3-indoyl-beta-D-galactopyranoside). beta-Galactosidase was found to be concentrated over mitral/tufted neurons indicating those to be the sites of latency. Our results suggest that, in common with other alpha-herpesviruses, EHV-1 can establish latency in central nervous system neurons and that the unique membrane glycoprotein encoded by EHV-1 gene 71 is nonessential for infection of neural tissues.

The transneuronal spread phenotype of herpes simplex virus type 1 infection of the mouse hind footpad

The mouse hind footpad inoculation model has served as a standard laboratory system for the study of the neuropathogenesis of herpes simplex virus type 1 (HSV-1) infection. The temporal and spatial distribution of viral antigen, known as the transneuronal spread phenotype, has not previously been described; nor is it understood why mice develop paralysis in an infection that involves sensory nerves. The HSV-as-transneuronal-tracer experimental paradigm was used to define the transneuronal spread of HSV-1 in this model. A new decalcification technique and standard immunocytochemical staining of HSV-1 antigens enabled a detailed analysis of the time-space distribution of HSV-1 in the intact spinal column. Mice were examined on days 3, 4, 5, and 6 postinoculation (p.i.) of a lethal dose of wild-type HSV-1 strain 17 syn+. Viral antigen was traced retrograde into first-order neurons in dorsal root ganglia on day 3 p.i., to the dorsal spinal roots on days 4 and 5 p.i., and to second- and third-order neurons within sensory regions of the spinal cord on days 5 and 6 p.i. HSV-1 antigen distribution was localized to the somatotopic representation of the footpad dermatome within the dorsal root ganglia and spinal cord. Antigen was found in the spinal cord gray and white matter sensory neuronal circuits of nociception (the spinothalamic tract) and proprioception (the dorsal spinocerebellar tract and gracile fasciculus). Within the brain stems and brains of three paralyzed animals examined late in infection (days 5 and 6 p.i.), HSV antigen was restricted to the nucleus subcoeruleus region bilaterally. Since motor neurons were not directly involved, we postulate that hindlimb paralysis may have resulted from intense involvement of the posterior column (gracile fasciculus) in the thoracolumbar spinal cord, a region known to contain the corticospinal tract in rodents.

Effect of chemical sympathectomy on the neural spread of pseudorabies virus in mice

To investigate the routes of neural spread of pseudorabies (Aujeszky's disease) virus (PRV), the effects of chemical sympathectomy by 6-hydroxydopamine (6-OHDA) on clinical signs and viral spread in mice inoculated intraocularly with PRV were examined. Similar to non-treated mice, the treated mice developed pruritus as a major clinical sign, followed by peracute death, but the time of death tended to be slightly delayed in about half of the treated mice. On immunohistological examination, viral antigens in treated mice were found to be markedly reduced in all the ipsilateral retinae; they were detected diffusely in the forebrain area with a localization in the mammillary area. In the treated mice, viral antigens were also reduced in the ipsilateral trigeminal nerve ganglia as well as in its central nuclei. These findings indicate that both the sympathetic nervous system and the trigeminal nervous system play an important role in the neural spread of PRV. Possible involvement of the dopaminergic nervous system, particularly in the eye, as the main site of viral growth was discussed.

MS strain of type 2 herpes simplex virus produces necrotizing retinitis in mice

Intracerebral inoculation of mice with the MS strain of type 2 herpes simplex virus (HSV-2) causes a brief encephalitis associated with multifocal central nervous system demyelination. Many of the mice develop unilateral or bilateral impairment of the pupillary light reflex. We have examined the development of ocular disease in inbred NIH mice inoculated intracerebrally with a low dose (10 pfu) of HSV-2 (MS). The resulting acute encephalitis was fatal in 30-50% of the mice. By 1 month after inoculation, the pupillary response to light was absent or impaired in approximately 80% of the surviving mice. Infectious virus could be isolated from the trigeminal ganglia and optic nerves from day 2 and from the eyes by day 4. Viral antigen was first immunohistochemically detectable in the optic nerves on day 5 and in the retinae on day 6. During the second week after inoculation up to half of the mice developed unilateral or bilateral necrotising retinitis associated with high titres of virus in the eyes and abundant viral antigen in the retinae. Electron microscopy confirmed the presence of viral particles in the retinae, in glia and degenerating neurons. No viral antigen was detected in the corneas and only rarely was antigen found in the ciliary body or iris. Infectious virus persisted longer in the eyes than in the trigeminal ganglia or optic nerves and could still be isolated from a few of the animals 2 weeks after inoculation. By 1 month the titres of virus within the eyes had fallen to undetectable levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Envelope glycoprotein gp50 of pseudorabies virus is essential for virus entry but is not required for viral spread in mice

Phenotypically complemented pseudorabies virus gp50 null mutants are able to produce plaques on noncomplementing cell lines despite the fact that progeny virions are noninfectious. To determine whether gp50 null mutants and gp50+gp63 null mutants are also able to replicate and spread in animals, mice were infected subcutaneously or intraperitoneally. Surprisingly, both gp50 mutants and gp50+gp63 double mutants proved to be lethal for mice. In comparison with the wild-type virus, gp50 mutants were still highly virulent, whereas the virulence of gp50+gp63 mutants was significantly reduced. Severe signs of neurological disorders, notably pruritus, were apparent in animals infected with the wild-type virus or a gp50 mutant but were much less pronounced in animals infected with a gp50+gp63 or gp63 mutant. Immunohistochemical examination of infected animals showed that all viruses were able to reach, and replicate in, the brain. Examination of visceral organs of intraperitoneally infected animals showed that viral antigen was predominantly present in peripheral nerves, suggesting that the viruses reached the central nervous system by means of retrograde axonal transport. Infectious virus could not be recovered from the brains and organs of animals infected with gp50 or gp50+gp63 mutants, indicating that progeny virions produced in vivo are noninfectious. Virions that lacked gp50 in their envelopes, and a phenotypically complemented pseudorabies virus gII mutant (which is unable to produce plaques in tissue culture cells), proved to be nonvirulent for mice. Together, these results show that gp50 is required for the primary infection but not for subsequent replication and viral spread in vivo. These results furthermore indicate that transsynaptic transport of the virus is independent of gp50. Since progeny virions produced by gp50 mutants are noninfectious, they are unable to spread from one animal to another. Therefore, such mutants may be used for the development of a new generation of safer (carrier) vaccines.

Pseudorabies virus envelope glycoprotein gI influences both neurotropism and virulence during infection of the rat visual system

We previously demonstrated that intraocular injections of virulent and attenuated strains of pseudorabies virus (PRV) produce transneuronal infection of functionally distinct central visual circuits in the rat. The virulent Becker strain of PRV induces two temporally separated waves of infection that ultimately target all known retinorecipient neurons; the attenuated Bartha strain only infects a functionally distinct subset of these neurons. In this study, we demonstrate that deletion of a single viral gene encoding glycoprotein gI is sufficient to reproduce both the novel pattern of infectivity and the reduced neurovirulence of the Bartha strain of PRV. Glycoprotein gIII, a major viral membrane protein required for efficient adsorption of virus in cell culture, has no obvious role in determining the pattern of neuronal infectivity, but appears to function with gI to influence neurovirulence. These data suggest that neuroinvasiveness and virulence are the products of an interaction of viral envelope glycoproteins with as yet unidentified cellular receptors.

Pseudorabies virus infections in explants of porcine nasal mucosa

The spread of infection and the morphogenesis of three pseudorabies virus strains were studied in explants of porcine nasal mucosa. Virulent NIA-3 virus was compared with a deletion mutant 2.4N3A, and with a non-virulent Bartha virus. All three virus strains infected nasal epithelial cells. NIA-3 virus particles were enveloped mainly at the inner nuclear membrane; the virus rapidly invaded the stroma, causing widespread necrosis. In contrast, 2.4N3A virus particles were enveloped mainly at the endoplasmic reticulum and the infection spread more slowly. Bartha virus particles were enveloped mainly at the endoplasmic reticulum; the infection spread slowly and remained restricted to the epithelial cells. In situ DNA hybridisation showed an accumulation of Bartha virus DNA in the nucleus 24 hours after inoculation. In nasal mucosa viral virulence seemed directly related to the speed of replication and release of virus from infected cells.

Demonstration of viral antigen and immunoglobulin (IgG and IgM) in brain tissue of pigs experimentally infected with haemagglutinating encephalomyelitis virus

Haemagglutinating encephalomyelitis virus (HEV) was inoculated either orally or intranasally into ten 3-day-old gnotobiotic piglets. All infected pigs showed inappetence and listlessness, but there were no clinical signs of nervous disorder. Severe encephalomyelitis, characterized by neuronophagia, focal gliosis and perivascular cuffing, was observed in the brain stem and cerebral cortex. Nasally infected pigs, in particular, developed lesions in the area of the stria olfactoria and tractus olfactorius. Coincident with the encephalitic changes, HEV antigen was observed first in the trigeminal ganglion cells and then in degenerating neurones. Immunoglobulin (IgG and IgM)-containing cells were also found in perivascular cuffs and glial foci. They appeared at first on PID 7 and after that increased in number. These findings suggest that these encephalitic lesions are a specific immune response to HEV following its multiplication in the central nervous system.

Ocular pathogenicity of herpes simplex virus

As a relatively small, discrete organ that contains a number of widely different cell types the eye provides an intriguing system in which to study fundamental aspects of virus/cell interactions. Such aspects are considered with particular reference to herpes simplex virus and the pivotal role of virus/neuron interactions in the development of ocular disease. Three aspects of this interaction are discussed: the entry of virus into the eye latency in the trigeminal ganglion nerve damage.

Herpesvirus (pseudorabies virus) latency in swine: occurrence and physical state of viral DNA in neural tissues

The occurrence of the pseudorabies virus (PRV, herpes suis 1) genome in various neural tissues of latently infected pigs was investigated. During the latent phase of infection, between 7 and 52 weeks p.i., the average amount of PRV DNA ranged between 0.3 and 0.05 genome copies per cell. The results obtained by in situ cytohybridization and reassociation kinetic experiments indicated that each latently infected cell harbored at least 30 viral genome copies. PRV DNA could be demonstrated in similar frequencies (about 30% of cases) in the trigeminal ganglia, the olfactory bulb, and the medulla oblongata, and less frequently in the brain stem and the spinal cord. Southern blot analysis showed that in general the physical state of the latent genome was linear and nonintegrated. Only in 2 of 15 animals could the presence of circular or concatemeric viral DNA be observed. Thus, we could show that over a period of 13 months after infection the PRV genome persisted both qualitatively and quantitatively in a stable state in different areas of both the peripheral and the central nervous system.

Chemotherapy of Aujeszky's disease (pseudorabies) in the mouse by means of nucleoside analogues: bromovinyldeoxyuridine, acyclovir, and dihydroxypropoxymethylguanine

Pseudorabies virus (PRV) infection was established in mice by means of inoculating the ear flap. The infection was universally fatal once clinical signs appeared. Bromovinyldeoxyuridine (BVDU) was a potent inhibitor of PRV in vitro, but this drug failed to protect mice and produced only marginal reductions in virus titre and slight prolongation of survival. Acyclovir (ACV) and dihydroxypropoxymethylguanine (DHPG) were both less active than BVDU when tested against the virus in BHK cells, yet DHPG therapy was extremely effective in mice; it reduced virus titres markedly and resulted in the long-term survival of mice given a potentially lethal infection. When ACV and DHPG were tested in vitro using murine rather than hamster cells, these compounds, especially DHPG, were shown to be much more active against PRV.

Some characteristics of four attenuated vaccine virus strains and a virulent strain of Aujeszky's disease virus

In the present study four attenuated virus strains, used as vaccines, and a virulent strain of Aujeszky's disease virus (ADV) were compared with respect to their virulence in mice, their ability to induce virus-specified thymidine kinase (TK) in infected cells, and their cleavage profiles of viral DNA's after treatment with the restriction endonuclease KpnI. The survival time of mice inoculated with the B-KAL or the virulent NIA-3 strain was comparable, whereas the Bartha and BUK strains required significantly longer periods to kill mice. Mice were resistant to the MK-25 strain of ADV. The strains were assayed for TK phenotype by plaque autoradiography after 3H-thymidine labelling of infected cells. MK-25 proved to be the only strain defective in induction of TK in pig kidney cells. Restriction endonuclease analysis of viral DNA's revealed that each vaccine strain showed a characteristic fragment pattern that could easily be differentiated from that of other vaccine and field strains of ADV. The present results demonstrate that the mouse virulence test and the TK assay detect differences in biological properties of ADV strains, but that restriction endonuclease analysis is required for unambiguous identification of vaccine and field strains of ADV.

Dissemination of herpes simplex virus in ganglia after footpad inoculation in neurectomized and non-neurectomized mice

After unilateral footpad inoculation with herpes simplex virus (HSV) the infection spreads initially to the ipsilateral and afterwards to the contralateral spinal ganglia. In about 25 percent of the mice the virus also reaches the trigeminal ganglia. Furthermore, we have shown that only a complete severance of the nervous connections can prevent the colonization of ganglia with HSV after footpad inoculation. Results of previous experiments in which only the sectioning of the sciatic nerve was able to prevent the invasion of ganglia, are difficult to explain. It appears also that HSV travels in the nerve toward the ganglia in a non-infectious form, and that the infectious virus detectable in nerves originates not from the peripheral inoculation site, but from the infectious virus pool which accumulates in spinal ganglia. A limited role of the circulatory system in the colonization of sensory ganglia by HSV cannot be excluded, since in a few cases virus was detected in ganglia after sectioning of both the sciatic and the femoral nerve.