Primary target cells of herpes simplex virus type 1 in the hippocampus

Vulnerability of the Hippocampus to Insults: Links to Blood-Brain Barrier Dysfunction

The hippocampus is a critical brain substrate for learning and memory; events that harm the hippocampus can seriously impair mental and behavioral functioning. Hippocampal pathophysiologies have been identified as potential causes and effects of a remarkably diverse array of medical diseases, psychological disorders, and environmental sources of damage. It may be that the hippocampus is more vulnerable than other brain areas to insults that are related to these conditions. One purpose of this review is to assess the vulnerability of the hippocampus to the most prevalent types of insults in multiple biomedical domains (i.e., neuroactive pathogens, neurotoxins, neurological conditions, trauma, aging, neurodegenerative disease, acquired brain injury, mental health conditions, endocrine disorders, developmental disabilities, nutrition) and to evaluate whether these insults affect the hippocampus first and more prominently compared to other brain loci. A second purpose is to consider the role of hippocampal blood-brain barrier (BBB) breakdown in either causing or worsening the harmful effects of each insult. Recent research suggests that the hippocampal BBB is more fragile compared to other brain areas and may also be more prone to the disruption of the transport mechanisms that act to maintain the internal milieu. Moreover, a compromised BBB could be a factor that is common to many different types of insults. Our analysis indicates that the hippocampus is more vulnerable to insults compared to other parts of the brain, and that developing interventions that protect the hippocampal BBB may help to prevent or ameliorate the harmful effects of many insults on memory and cognition.

The Adult Neurogenesis Theory of Alzheimer's Disease

Alzheimer's disease starts in neural stem cells (NSCs) in the niches of adult neurogenesis. All primary factors responsible for pathological tau hyperphosphorylation are inherent to adult neurogenesis and migration. However, when amyloid pathology is present, it strongly amplifies tau pathogenesis. Indeed, the progressive accumulation of extracellular amyloid-β deposits in the brain triggers a state of chronic inflammation by microglia. Microglial activation has a significant pro-neurogenic effect that fosters the process of adult neurogenesis and supports neuronal migration. Unfortunately, this "reactive" pro-neurogenic activity ultimately perturbs homeostatic equilibrium in the niches of adult neurogenesis by amplifying tau pathogenesis in AD. This scenario involves NSCs in the subgranular zone of the hippocampal dentate gyrus in late-onset AD (LOAD) and NSCs in the ventricular-subventricular zone along the lateral ventricles in early-onset AD (EOAD), including familial AD (FAD). Neuroblasts carrying the initial seed of tau pathology travel throughout the brain via neuronal migration driven by complex signals and convey the disease from the niches of adult neurogenesis to near (LOAD) or distant (EOAD) brain regions. In these locations, or in close proximity, a focus of degeneration begins to develop. Then, tau pathology spreads from the initial foci to large neuronal networks along neural connections through neuron-to-neuron transmission.

Variations in Aspects of Neural Precursor Cell Neurogenesis in a Human Model of HSV-1 Infection

Encephalitis, the most significant of the central nervous system (CNS) diseases caused by Herpes simplex virus 1 (HSV-1), may have long-term sequelae in survivors treated with acyclovir, the cause of which is unclear. HSV-1 exhibits a tropism toward neurogenic niches in CNS enriched with neural precursor cells (NPCs), which play a pivotal role in neurogenesis. NPCs are susceptible to HSV-1. There is a paucity of information regarding the influence of HSV-1 on neurogenesis in humans. We investigated HSV-1 infection of NPCs from two individuals. Our results show (i) HSV-1 impairs, to different extents, the proliferation, self-renewing, and, to an even greater extent, migration of NPCs from these two subjects; (ii) The protective effect of the gold-standard antiherpetic drug acyclovir (ACV) varies with viral dose and is incomplete. It is also subject to differences in terms of efficacy of the NPCs derived from these two individuals. These results suggest that the effects of HSV-1 may have on aspects of NPC neurogenesis may vary among individuals, even in the presence of acyclovir, and this may contribute to the heterogeneity of cognitive sequelae across encephalitis survivors. Further analysis of NPC cell lines from a larger number of individuals is warranted.

The Hippocampal Vulnerability to Herpes Simplex Virus Type I Infection: Relevance to Alzheimer's Disease and Memory Impairment

Herpes simplex virus type 1 (HSV-1) as a possible infectious etiology in Alzheimer’s disease (AD) has been proposed since the 1980s. The accumulating research thus far continues to support the association and a possible causal role of HSV-1 in the development of AD. HSV-1 has been shown to induce neuropathological and behavioral changes of AD, such as amyloid-beta accumulation, tau hyperphosphorylation, as well as memory and learning impairments in experimental settings. However, a neuroanatomical standpoint of HSV-1 tropism in the brain has not been emphasized in detail. In this review, we propose that the hippocampal vulnerability to HSV-1 infection plays a part in the development of AD and amnestic mild cognitive impairment (aMCI). Henceforth, this review draws on human studies to bridge HSV-1 to hippocampal-related brain disorders, namely AD and aMCI/MCI. Next, experimental models and clinical observations supporting the neurotropism or predilection of HSV-1 to infect the hippocampus are examined. Following this, factors and mechanisms predisposing the hippocampus to HSV-1 infection are discussed. In brief, the hippocampus has high levels of viral cellular receptors, neural stem or progenitor cells (NSCs/NPCs), glucocorticoid receptors (GRs) and amyloid precursor protein (APP) that support HSV-1 infectivity, as well as inadequate antiviral immunity against HSV-1. Currently, the established diseases HSV-1 causes are mucocutaneous lesions and encephalitis; however, this review revises that HSV-1 may also induce and/or contribute to hippocampal-related brain disorders, especially AD and aMCI/MCI.

Herpes simplex virus type 1 infection leads to neurodevelopmental disorder-associated neuropathological changes

Neonatal herpes simplex virus type 1 (HSV-1) infections contribute to various neurodevelopmental disabilities and the subsequent long-term neurological sequelae into the adulthood. However, further understanding of fetal brain development and the potential neuropathological effects of the HSV-1 infection are hampered by the limitations of existing neurodevelopmental models due to the dramatic differences between humans and other mammalians. Here we generated in vitro neurodevelopmental disorder models including human induced pluripotent stem cell (hiPSC)-based monolayer neuronal differentiation, three-dimensional (3D) neuroepithelial bud, and 3D cerebral organoid to study fetal brain development and the potential neuropathological effects induced by the HSV-1 infections. Our results revealed that the HSV-1-infected neural stem cells (NSCs) exhibited impaired neural differentiation. HSV-1 infection led to dysregulated neurogenesis in the fetal neurodevelopment. The HSV-1-infected brain organoids modelled the pathological features of the neurodevelopmental disorders in the human fetal brain, including the impaired neuronal differentiation, and the dysregulated cortical layer and brain regionalization. Furthermore, the 3D cerebral organoid model showed that HSV-1 infection promoted the abnormal microglial activation, accompanied by the induction of inflammatory factors, such as TNF-α, IL-6, IL-10, and IL-4. Overall, our in vitro neurodevelopmental disorder models reconstituted the neuropathological features associated with HSV-1 infection in human fetal brain development, providing the causal relationships that link HSV biology with the neurodevelopmental disorder pathogen hypothesis.

HSV-1 is one of the most prevalent human pathogens that can spread into the fetal central nervous system by maternal-fetal transmission, and thus resulting in long-term neurological sequelae in adult, including cognitive dysfunction and learning disabilities. However, there is a very limited progress in understanding the role of HSV-1 on human fetal brain development due to limited access to fetal human brain tissue as well as the limitations of existing neurodevelopmental and infection models. Here, we generated the in vitro neurodevelopmental disorder models including hiPSC-based monolayer neuronal differentiation, three-dimensional (3D) neuroepithelial bud, and 3D cerebral organoid to study the neurodevelopmental disorder-associated neuropathological changes with HSV-1 infection in human fetal brain development. Our results revealed that HSV-1 infection led to impaired neural differentiation and dysregulated neurogenesis in the fetal neurodevelopment. Additionally, HSV-1 infection impaired neuronal differentiation and dysregulated brain regionalization in our cerebral organoid model. Furthermore, the cerebral organoid model showed that HSV-1 infection led to the abnormal microglial proliferation and activation, accompanied by the induction of inflammatory factors including TNF-α, IL-6, IL-10, and IL-4. Taken together, our study provides novel evidence that HSV-1 infection impaired human brain development and contributed to neurodevelopmental disorder pathogen hypothesis, and would have implications for raising the therapeutic opportunities for targeting of viral reservoirs relevant to neurodevelopmental disorder.

Neuroinflammation is associated with reduced SOCS2 and SOCS3 expression during intracranial HSV-1 infection

Herpes simplex virus type 1 (HSV-1) is the main etiological agent of acute and sporadic encephalitis. Proteins of the suppressor of cytokine signaling (SOCS) family have shown to regulate the inflammation during HSV-1 infection in the brain. However, the effects of SOCS2 and SOCS3 in viral encephalitis remain unclear. The aim of the current study is to investigate the potential association between SOCS2, SOCS3, cytokines, and hippocampal damage, especially neuronal apoptosis, during acute intracranial HSV-1 infection in mice. Male C57BL/6 mice were infected by intracranial route with 10² plaque-forming units (PFU) inoculum of purified HSV-1. At three days post-infection (3 d.p.i.), mice were euthanized and their hippocampi were collected for histopathological analysis, immunohistochemical reaction against active caspase-3 and quantification of SOCS2, SOCS3 and cytokines (tumoral necrosis factor (TNF), interleukin (IL) 1β, IL-6, IL-10; interferon (IFN) -α, IFN-β, IFN-γ) mRNA expression. Infected mice exhibited neuronal loss and hemorrhagic focus in Cornu Ammonis (CA) region. The apoptotic index was higher in infected mice compared to controls. HSV-1 infection was associated with increased hippocampal expression of TNF, IL1-β, IL-6 and IFNα/IFNβ and decreased expression of IL-10, IFN-γ, SOCS2 and SOCS3. Our results suggest that down regulation of SOCS2 and SOCS3 contributes to a pro-inflammatory environment associated with hippocampal damage and neuronal apoptosis during acute HSV-1 infection in mice.

Selective neuronal vulnerability in Alzheimer's disease

Alzheimer's disease (AD) is defined by a deficiency in specific behavioural and/or cognitive domains, pointing to selective vulnerabilities of specific neurons from different brain regions. These vulnerabilities can be compared across neuron subgroups to identify the most vulnerable neuronal types, regions, and time points for further investigation. Thus, the relevant organizational frameworks for brain subgroups will hold great values for a clear understanding of the progression in AD. Presently, the neuronal vulnerability has yet urgently required to be elucidated as not yet been clearly defined. It is suggested that cell-autonomous and non-cell-autonomous mechanisms can affect the neuronal vulnerability to stressors, and in turn modulates AD progression. This review examines cell-autonomous and non-cell-autonomous mechanisms that contribute to the neuronal vulnerability. Collectively, the cell-autonomous mechanisms seem to be the primary drivers responsible for initiating specific stressor-related neuronal vulnerability with pathological changes in certain brain areas, which then utilize non-cell-autonomous mechanisms and result in subsequent progression of AD. In summary, this article has provided a new perspective on the preventative and therapeutic options for AD.

Patterns of Herpes Simplex Virus 1 Infection in Neural Progenitor Cells

This study employed human induced pluripotent stem cells (hiPSCs) to model the interaction of HSV-1 with NPCs, which reside in the neurogenic niches of the CNS and play a fundamental role in adult neurogenesis. Herein, we provide evidence that in NPCs infected at an MOI as low as 0.001, HSV-1 can establish a latent state, suggesting that (i) a variant of classical HSV-1 latency can be established during earlier stages of neuronal differentiation and (ii) neurogenic niches in the brain may constitute additional sites of viral reactivation. Lytic HSV-1 infections impaired NPC migration, which represents a critical step in neurogenesis. A difference in susceptibility to HSV-1 infection between two-dimensional (2D) and three-dimensional (3D) NPC cultures was observed, highlighting the potential value of 3D cultures for modeling host-pathogen interactions. Together, our results are relevant in light of observations relating HSV-1 infection to postencephalitic cognitive dysfunction.

Herpes simplex virus 1 (HSV-1) can induce damage in brain regions that include the hippocampus and associated limbic structures. These neurogenic niches are important because they are associated with memory formation and are highly enriched with neural progenitor cells (NPCs). The susceptibility and fate of HSV-1-infected NPCs are largely unexplored. We differentiated human induced pluripotent stem cells (hiPSCs) into NPCs to generate two-dimensional (2D) and three-dimensional (3D) culture models to examine the interaction of HSV-1 with NPCs. Here, we show that (i) NPCs can be efficiently infected by HSV-1, but infection does not result in cell death of most NPCs, even at high multiplicities of infection (MOIs); (ii) limited HSV-1 replication and gene expression can be detected in the infected NPCs; (iii) a viral silencing mechanism is established in NPCs exposed to the antivirals (E)-5-(2-bromovinyl)-2′-deoxyuridine (5BVdU) and alpha interferon (IFN-α) and when the antivirals are removed, spontaneous reactivation can occur at low frequency; (iv) HSV-1 impairs the ability of NPCs to migrate in a dose-dependent fashion in the presence of 5BVdU plus IFN-α; and (v) 3D cultures of NPCs are less susceptible to HSV-1 infection than 2D cultures. These results suggest that NPC pools could be sites of HSV-1 reactivation in the central nervous system (CNS). Finally, our results highlight the potential value of hiPSC-derived 3D cultures to model HSV-1–NPC interaction.

IMPORTANCE This study employed human induced pluripotent stem cells (hiPSCs) to model the interaction of HSV-1 with NPCs, which reside in the neurogenic niches of the CNS and play a fundamental role in adult neurogenesis. Herein, we provide evidence that in NPCs infected at an MOI as low as 0.001, HSV-1 can establish a latent state, suggesting that (i) a variant of classical HSV-1 latency can be established during earlier stages of neuronal differentiation and (ii) neurogenic niches in the brain may constitute additional sites of viral reactivation. Lytic HSV-1 infections impaired NPC migration, which represents a critical step in neurogenesis. A difference in susceptibility to HSV-1 infection between two-dimensional (2D) and three-dimensional (3D) NPC cultures was observed, highlighting the potential value of 3D cultures for modeling host-pathogen interactions. Together, our results are relevant in light of observations relating HSV-1 infection to postencephalitic cognitive dysfunction.

Herpes Simplex Virus Type-1 Infection Impairs Adult Hippocampal Neurogenesis via Amyloid-β Protein Accumulation

We previously reported that Herpes simplex virus type-1 (HSV-1) infection of cultured neurons triggered intracellular accumulation of amyloid-β protein (Aβ) markedly impinging on neuronal functions. Here, we demonstrated that HSV-1 affects in vitro and in vivo adult hippocampal neurogenesis by reducing neural stem/progenitor cell (NSC) proliferation and their neuronal differentiation via intracellular Aβ accumulation. Specifically, cultured NSCs were more permissive for HSV-1 replication than mature neurons and, once infected, they exhibited reduced proliferation (assessed by 5'-bromo-deoxyuridine incorporation, Ki67 immunoreactivity, and Sox2 mRNA expression) and impaired neuronal differentiation in favor of glial phenotype (evaluated by immunoreactivity for the neuronal marker MAP2, the glial marker glial fibrillary astrocyte protein, and the expression of the proneuronal genes Mash1 and NeuroD1). Similarly, impaired adult neurogenesis was observed in the subgranular zone of hippocampal dentate gyrus of an in vivo model of recurrent HSV-1 infections, that we recently set up and characterized, with respect to mock-infected mice. The effects of HSV-1 on neurogenesis did not depend on cell death and were due to Aβ accumulation in infected NSCs. Indeed, they were: (a) reverted, in vitro, by the presence of either β/γ-secretase inhibitors preventing Aβ production or the specific 4G8 antibody counteracting the action of intracellular Aβ; (b) not detectable, in vivo, in HSV-1-infected amyloid precursor protein knockout mice, unable to produce and accumulate Aβ. Given the critical role played by adult neurogenesis in hippocampal-dependent memory and learning, our results suggest that multiple virus reactivations in the brain may contribute to Alzheimer's disease phenotype by also targeting NSCs. Stem Cells 2019;37:1467-1480.

Herpes Simplex Virus Type 1 Enhances Expression of the Synaptic Protein Arc for Its Own Benefit

Herpes simplex virus type 1 (HSV-1) is a neurotropic virus able to reach the central nervous system (CNS) after primary infection in oronasal mucosa. HSV-1 establishes latency inside neurons due the repression of its gene expression process, which is related to periodic reactivations in response to cellular stress conditions, constituting a risk factor for neurodegenerative diseases such as Alzheimer’s disease (AD). The immediate-early gene Arc plays an essential role in neuronal morphology, synaptic plasticity and memory formation. Arc acts as a hub protein, interacting with components of the endocytic machinery required for AMPA receptor (AMPAR) recycling as well as with proteins of the post-synaptic density and actin cytoskeleton. However, to date, no studies have evaluated whether persistent neurotropic HSV-1 infection modulates the expression or function of Arc protein in brain tissue. Here, we report that neuronal in vivo and in vitro infection of HSV-1 significantly increases Arc protein levels, showing a robust perinuclear distribution in neuronal cell lines, a process that is dependent on an active HSV-1 replication cycle. Finally, we found that silencing Arc protein caused a decrease in HSV-1 proteins and viral progeny, suggesting that Arc is involved in the lifecycle of HSV-1. Our studies strongly suggest that pathogenicity of HSV-1 neuronal reactivations in humans could be mediated in part by Arc neuronal upregulation and its potential role in endocytic trafficking and AMPA-neuronal function impairment. Further studies are necessary to define whether this phenomenon could have repercussions in cognition and learning processes in infected individuals.

Interplay Between the Autophagy-Lysosomal Pathway and the Ubiquitin-Proteasome System: A Target for Therapeutic Development in Alzheimer's Disease

Alzheimer’s disease (AD) is the most common cause of age-related dementia leading to severe irreversible cognitive decline and massive neurodegeneration. While therapeutic approaches for managing symptoms are available, AD currently has no cure. AD associates with a progressive decline of the two major catabolic pathways of eukaryotic cells—the autophagy-lysosomal pathway (ALP) and the ubiquitin-proteasome system (UPS)—that contributes to the accumulation of harmful molecules implicated in synaptic plasticity and long-term memory impairment. One protein recently highlighted as the earliest initiator of these disturbances is the amyloid precursor protein (APP) intracellular C-terminal membrane fragment β (CTFβ), a key toxic agent with deleterious effects on neuronal function that has become an important pathogenic factor for AD and a potential biomarker for AD patients. This review focuses on the involvement of regulatory molecules and specific post-translational modifications (PTMs) that operate in the UPS and ALP to control a single proteostasis network to achieve protein balance. We discuss how these aspects can contribute to the development of novel strategies to strengthen the balance of key pathogenic proteins associated with AD.

Autophagy interaction with herpes simplex virus type-1 infection

More than 50% of the U.S. population is infected with herpes simplex virus type-I (HSV-1) and global infectious estimates are nearly 90%. HSV-1 is normally seen as a harmless virus but debilitating diseases can arise, including encephalitis and ocular diseases. HSV-1 is unique in that it can undermine host defenses and establish lifelong infection in neurons. Viral reactivation from latency may allow HSV-1 to lay siege to the brain (Herpes encephalitis). Recent advances maintain that HSV-1 proteins act to suppress and/or control the lysosome-dependent degradation pathway of macroautophagy (hereafter autophagy) and consequently, in neurons, may be coupled with the advancement of HSV-1-associated pathogenesis. Furthermore, increasing evidence suggests that HSV-1 infection may constitute a gradual risk factor for neurodegenerative disorders. The relationship between HSV-1 infection and autophagy manipulation combined with neuropathogenesis may be intimately intertwined demanding further investigation.

HSV-1 and Alzheimer's disease: more than a hypothesis

Among the multiple factors concurring to Alzheimer’s disease (AD) pathogenesis, greater attention should be devoted to the role played by infectious agents. Growing epidemiological and experimental evidence suggests that recurrent herpes simplex virus type-1 (HSV-1) infection is a risk factor for AD although the underlying molecular and functional mechanisms have not been fully elucidated yet. Here, we review literature suggesting the involvement of HSV-1 infection in AD also briefly mentioning possible pharmacological implications of these findings.

Antiherpes virus-specific treatment and cognition in schizophrenia: a test-of-concept randomized double-blind placebo-controlled trial

OBJECTIVE

To test our hypothesis that valacyclovir, an antiherpes virus-specific medication, added to antipsychotics (APs) would improve cognitive performance and psychopathology among schizophrenia subjects exposed to neurotropic herpes simplex virus, type 1 (HSV1).

METHODS

Using a double-blind placebo-controlled design, we randomized 24 HSV1-seropositive schizophrenia subjects to receive either valacyclovir (n = 12) or placebo (n = 12) for 18 weeks in addition to stable doses of APs. Valacyclovir dose was stabilized at 1.5 g twice daily orally. At each visit, subjects were evaluated for severity of psychopathology and side effects using standardized scales and a study-specific semistructured checklist. A computerized neurocognitive battery validated on both schizophrenia and healthy subjects was administered at baseline and follow-up. Intent-to-treat analysis, using linear regression models that included all randomized subjects, were used to examine differential changes in cognition and psychopathology scores over 18 weeks between valacyclovir and placebo, accounting for placebo response.

RESULTS

Valacyclovir group improved in verbal memory, working memory, and visual object learning compared with placebo group. The effect sizes (Cohen's d) were 0.79 for working memory, 1.14 for immediate verbal memory, and 0.97 for the visual object learning. Psychotic symptom severity did not improve.

CONCLUSIONS

Supplemental valacyclovir may alleviate impairments in cognitive domains that are often observed in schizophrenia but not psychotic symptoms in those exposed to HSV1. If replicated, this approach could provide a novel strategy to treat cognitive impairments in a subgroup of schizophrenia subjects who can be reliably identified using a blood test.

Behavioral characterization of cereblon forebrain-specific conditional null mice: a model for human non-syndromic intellectual disability

A nonsense mutation in the human cereblon gene (CRBN) causes a mild type of autosomal recessive non-syndromic intellectual disability (ID). Animal studies show that crbn is a cytosolic protein with abundant expression in the hippocampus (HPC) and neocortex (CTX). Its diverse functions include the developmental regulation of ion channels at the neuronal synapse, the mediation of developmental programs by ubiquitination, and a target for herpes simplex type I virus in HPC neurons. To test the hypothesis that anomalous CRBN expression leads to HPC-mediated memory and learning deficits, we generated germ-line crbn knock-out mice (crbn(-/-)). We also inactivated crbn in forebrain neurons in conditional knock-out mice in which crbn exons 3 and 4 are deleted by cre recombinase under the direction of the Ca(2+)/calmodulin-dependent protein kinase II alpha promoter (CamKII(cre/+), crbn(-/-)). crbn mRNA levels were negligible in the HPC, CTX, and cerebellum (CRBM) of the crbn(-/-) mice. In contrast, crbn mRNA levels were reduced 3- to 4-fold in the HPC, CTX but not in the CRBM in CamKII(cre/+), crbn(-/-) mice as compared to wild type (CamKII(cre/+), crbn(+/+)). Contextual fear conditioning showed a significant decrease in the percentage of freezing time in CamKII(cre/+), crbn(-/-) and crbn(-/-) mice while motor function, exploratory motivation, and anxiety-related behaviors were normal. These findings suggest that CamKII(cre/+), crbn(-/-) mice exhibit selective HPC-dependent deficits in associative learning and supports the use of these mice as in vivo models to study the functional consequences of CRBN aberrations on memory and learning in humans.

APOBEC1-mediated editing and attenuation of herpes simplex virus 1 DNA indicate that neurons have an antiviral role during herpes simplex encephalitis

APOBEC1 (A1) is a cytidine deaminase involved in the regulation of lipids in the small intestine. Herpes simplex virus 1 (HSV-1) is a ubiquitous pathogen that is capable of infecting neurons in the brain, causing encephalitis. Here, we show that A1 is induced during encephalitis in neurons of rats infected with HSV-1. In cells stably expressing A1, HSV-1 infection resulted in significantly reduced virus replication compared to that in control cells. Infectivity could be restored to levels comparable to those observed for control cells if A1 expression was silenced by specific A1 short hairpin RNAs (shRNA). Moreover, cytidine deaminase activity appeared to be essential for this inhibition and led to an impaired accumulation of viral mRNA transcripts and DNA copy numbers. The sequencing of viral gene UL54 DNA, extracted from infected A1-expressing cells, revealed G-to-A and C-to-T transitions, indicating that A1 associates with HSV-1 DNA. Taken together, our results demonstrate a model in which A1 induction during encephalitis in neurons may aid in thwarting HSV-1 infection.

Infection of neurons and encephalitis after intracranial inoculation of herpes simplex virus requires the entry receptor nectin-1

Multiple entry receptors can mediate infection of cells by herpes simplex virus (HSV), permitting alternative pathways for infection and disease. We investigated the roles of two known entry receptors, herpesvirus entry mediator (HVEM) and nectin-1, in infection of neurons in the CNS and the development of encephalitis. Wild-type, HVEM KO, nectin-1 KO, and HVEM/nectin-1 double KO mice were inoculated with HSV into the hippocampus. The mice were examined for development of encephalitis or were killed at various times after inoculation for immunohistological analyses of brain slices. Nectin-1 KO mice showed no signs of disease after intracranial inoculation, and no HSV antigens were detectable in the brain parenchyma. However, HSV antigens were detected in non-parenchymal cells lining the ventricles. In the double KO mice, there was also no disease and no detectable expression of viral antigens even in non-parenchymal cells, indicating that infection of these cells in the nectin-1 KO mice was dependent on the expression of HVEM. Wild-type and HVEM KO mice rapidly developed encephalitis, and the patterns of HSV replication in the brain were indistinguishable. Thus, expression of nectin-1 is necessary for HSV infection via the intracranial route and for encephalitis; HVEM is largely irrelevant. These results contrast with recent findings that (i) either HVEM or nectin-1 can permit HSV infection of the vaginal epithelium in mice and (ii) nectin-1 is not the sole receptor capable of enabling spread of HSV infection from the vaginal epithelium to the PNS and CNS.