Role of Nectin-1 and Herpesvirus Entry Mediator as Cellular Receptors for Herpes Simplex Virus 1 on Primary Murine Dermal Fibroblasts

Early Events after Herpes Simplex Virus-Type 1 Entry Are Necessary for the Release of Gamma-Hydroxybutyrate upon Acute Infection

We reported that gamma-hydroxybutyrate (GHB) is released upon Herpes Simplex Virus Type-1 (HSV-1) acute infection. However, the cellular biochemical processes involved in the production of GHB in infected cells are unclear. This study aims to shed light on the biochemical pathway and the stage within the viral life cycle responsible for the release of GHB in infected cells. UV-inactivation, acyclovir (ACV), and cycloheximide (CHX) treatments were used to inhibit HSV-1 replication at various stages. Vero cells treated with UV-inactivated HSV-1 significantly decreased GHB production. However, ACV or CHX treatments did not affect GHB production. We also showed that inhibition of glycolytic enzyme enolase by sodium fluoride (NaF) significantly reduces GHB production upon infection. This finding suggests that suppression of glycolytic activity negatively affects cellular GHB production. Our data also indicated that succinic semialdehyde dehydrogenase, an enzyme involved in the shunt of the tricarboxylic acid (TCA) cycle to generate succinic acid, was decreased upon infection, suggesting that infection may trigger the accumulation of succinic semialdehyde, causing the production of GHB. Although the precise mechanism has yet to be defined, our results suggest that early events following infection modulates the release of GHB, which is generated through the metabolic pathways of glycolysis and TCA cycle.

Mouse Models for Human Herpesviruses

More than one hundred herpesviruses have been isolated from different species so far, with nine infecting humans. Infections with herpesviruses are characterized by life-long latency and represent a significant challenge for human health. To investigate the consequences of infections and identify novel treatment options, in vivo models are of particular relevance. The mouse has emerged as an economical small animal model to investigate herpesvirus infections. However, except for herpes simplex viruses (HSV-1, HSV-2), human herpesviruses cannot infect mice. Three natural herpesviruses have been identified in mice: mouse-derived cytomegalovirus (MCMV), mouse herpesvirus 68 (MHV-68), and mouse roseolovirus (MRV). These orthologues are broadly used to investigate herpesvirus infections within the natural host. In the last few decades, immunocompromised mouse models have been developed, allowing the functional engraftment of various human cells and tissues. These xenograft mice represent valuable model systems to investigate human-restricted viruses, making them particularly relevant for herpesvirus research. In this review, we describe the various mouse models used to study human herpesviruses, thereby highlighting their potential and limitations. Emphasis is laid on xenograft mouse models, covering the development and refinement of immune-compromised mice and their application in herpesvirus research.

Critical roles of adherens junctions in diseases of the oral mucosa

The oral cavity is directly exposed to a variety of environmental stimuli and contains a diverse microbiome that continuously interacts with the oral epithelium. Therefore, establishment and maintenance of the barrier function of the oral mucosa is of paramount importance for its function and for the body's overall health. The adherens junction is a cell-cell adhesion complex that is essential for epithelial barrier function. Although a considerable body of work has associated barrier disruption with oral diseases, the molecular underpinnings of these associations have not been equally investigated. This is critical, since adherens junction components also possess significant signaling roles in the cell, in addition to their architectural ones. Here, we summarize current knowledge involving adherens junction components in oral pathologies, such as cancer and oral pathogen-related diseases, while we also discuss gaps in the knowledge and opportunities for future investigation of the relationship between adherens junctions and oral diseases.

A Nectin1 Mutant Mouse Model Is Resistant to Pseudorabies Virus Infection

The present study generated nectin1-mutant mice with single amino acid substitution and tested the anti-pseudorabies virus (PRV) ability of the mutant mice, with the aim to establish a model for PRV-resistant livestock. A phenylalanine to alanine transition at position 129 (F129A) of nectin1 was introduced into the mouse genome to generate nectin1 (F129A) mutant mice. The mutant mice were infected with a field-isolated highly virulent PRV strain by subcutaneous injection of virus. We found that the homozygous mutant mice had significantly alleviated disease manifestations and decreased death rate and viral loading in serum and tissue compared with heterozygous mutant and wild-type mice. In addition to disease resistance, the homozygous mutant mice showed a defect in eye development, indicating the side effect on animals by only one amino acid substitution in nectin1. Results demonstrate that gene modification in nectin1 is an effective approach to confer PRV resistance on animals, but the mutagenesis pattern requires further investigation to increase viral resistance without negative effect on animal development.

Identifying HSV-1 Inhibitors from Natural Compounds via Virtual Screening Targeting Surface Glycoprotein D

Herpes simplex virus (HSV) infections are a worldwide health problem in need of new effective treatments. Of particular interest is the identification of antiviral agents that act via different mechanisms compared to current drugs, as these could interact synergistically with first-line antiherpetic agents to accelerate the resolution of HSV-1-associated lesions. For this study, we applied a structure-based molecular docking approach targeting the nectin-1 and herpesvirus entry mediator (HVEM) binding interfaces of the viral glycoprotein D (gD). More than 527,000 natural compounds were virtually screened using Autodock Vina and then filtered for favorable ADMET profiles. Eight top hits were evaluated experimentally in African green monkey kidney cell line (VERO) cells, which yielded two compounds with potential antiherpetic activity. One active compound (1-(1-benzofuran-2-yl)-2-[(5Z)-2H,6H,7H,8H-[1,3] dioxolo[4,5-g]isoquinoline-5-ylidene]ethenone) showed weak but significant antiviral activity. Although less potent than antiherpetic agents, such as acyclovir, it acted at the viral inactivation stage in a dose-dependent manner, suggesting a novel mode of action. These results highlight the feasibility of in silico approaches for identifying new antiviral compounds, which may be further optimized by medicinal chemistry approaches.

Targeting NECTIN-1 Based on CRISPR/Cas9 System Attenuated the Herpes Simplex Virus Infection in Human Corneal Epithelial Cells In Vitro

PURPOSE

Viral keratitis caused by herpes simplex virus 1 (HSV-1) is a lifelong recurring disease and an unignored cause of blindness worldwide. Current antiviral therapy cannot eliminate the transcriptionally silent HSV-1 in latently infected patients. With the explosive applications of the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease (Cas) 9 gene-editing system in recent years, we aim to develop a CRISPR/Cas9 system targeting down the major HSV receptor, NECTIN-1 on human corneal epithelial cells (HCECs), to provide a novel strategy for herpes simplex keratitis (HSK) treatment.

METHODS

The selected single guide RNAs (sgRNAs) targeting human nectin cell adhesion molecule 1 (NECTIN-1), together with Cas-9, were assembled into lentivirus. HCECs were infected with Lenti-Cas9-gRNAs to establish NECTIN-1 knockdown cells. Following HSV-green fluorescent protein (GFP) infection, cell survival and virus infection were determined by fluorescence microscopy and flow cytometry. Relative HSV DNA amount was also compared through quantitative reverse transcriptase-polymerase chain reaction.

RESULTS

Lentivirus packaged with the CRISPR/Cas9 system and the two selected sgRNAs both successfully edited down the protein levels of NECTIN-1 of HCECs. After HSV-GFP infection, the infection rate of HCECs in knockdown groups dramatically decreased, especially in the NECTIN-1 knockdown group 1. In addition, the relative HSV DNA amount of both knockdown groups was only 30% when compared with the control group.

CONCLUSIONS

We successfully knocked down the NECTIN-1 expression in vitro by the CRISPR/Cas9 system, which alleviated the HSV infection in HCECs.

TRANSLATIONAL RELEVANCE

This study offered a promising target for the cure of HSK.

Susceptibility of human and murine dermal fibroblasts to Herpes simplex Virus 1 in the absence and presence of extracellular matrix

Herpes simplex virus 1 (HSV-1) invades its human host via the skin and mucosa and initiates infection in the epithelium. While human and murine epidermis are highly susceptible to HSV-1, we recently observed rare infected cells in the human dermis and only minor infection efficiency in murine dermis upon ex vivo infection. Here, we investigated why cells in the dermis are so inefficiently infected and explored potential differences between murine and human dermal fibroblasts. In principle, primary fibroblasts are highly susceptible to HSV-1, however, we found a delayed infection onset in human compared to murine cells. Intriguingly, only a minor delayed onset of infection was evident in collagen-embedded compared to unembedded human fibroblasts although expression of the receptor nectin-1 dropped after collagen-embedding. This finding is in contrast to previous observations with murine fibroblasts where collagen-embedding delayed infection. The application of latex beads revealed limited penetration in the dermis which was more pronounced in human compared to murine dermis supporting the species-specific differences already observed for HSV-1 invasion. Our results suggest that the distinct organization of human and murine dermis contribute to the presence and accessibility of the HSV-1 receptors as well as to the variable barrier function of the extracellular matrix. These contributions, in turn, give rise to the inefficient viral access to cells in the dermis while dermal fibroblasts in culture are well infected. Importance Dermal fibroblasts are exposed to HSV-1 upon invasion in skin during in vivo infection. Thus, fibroblasts represent a widely used experimental tool to understand virus-host cell interactions and are highly susceptible in culture. The spectrum of fibroblasts' characteristics in their in vivo environment, however, clearly differs from the observations under cell culture conditions implying putative variations in virus-cell interactions. This becomes evident when ex vivo infection studies in murine as well as human dermis revealed the rather inefficient penetration of HSV-1 in the tissue and uptake in the dermal fibroblasts. Here, we initiated studies to explore the contributions of receptor presence and accessibility to efficient infection of dermal fibroblasts. Our results strengthen the heterogeneity of murine and human dermis and imply that the interplay between dermal barrier function and receptor presence determine how well HSV-1 penetrates the dermis.

Harringtonine Inhibits Herpes Simplex Virus Type 1 Infection by Reducing Herpes Virus Entry Mediator Expression

Herpes simplex virus type 1 (HSV-1) infection induces various clinical disorders, such as herpes simplex encephalitis (HSE), herpes simplex keratitis (HSK), and genital herpes. In clinical intervention, acyclovir (ACV) is the major therapeutic drug used to suppress HSV-1; however, ACV-resistant strains have gradually increased. In the present study, harringtonine (HT) significantly inhibited infection of HSV-1 as well as two ACV-resistant strains, including HSV-1 blue and HSV-1 153. Time-of-drug addition assay further revealed that HT mainly reduced the early stage of HSV-1 infection. We also demonstrated that HT mainly affected herpes virus entry mediator (HVEM) expression as shown by qPCR, Western Blot, and Immunofluorescence. Collectively, HT showed antiviral activity against HSV-1 and ACV-resistant strains by targeting HVEM and could be a promising therapeutic candidate for mitigating HSV-1-induced-pathogenesis.

Nectin-1 Expression Correlates with the Susceptibility of Malignant Melanoma to Oncolytic Herpes Simplex Virus In Vitro and In Vivo

Simple Summary

Talimogene laherparepvec (T-VEC), a first-in-class oncolytic herpes simplex virus, improves the outcome of patients suffering from unresectable melanoma, in particular in combination with checkpoint inhibitors. However, a certain percentage of patients does not profit from this treatment, which raises the question of potential biomarkers to predict success or failure of oncolytic herpes viruses. For these purposes, we studied the oncolytic activity of T-VEC in a panel of 20 melanoma cell lines and evaluated the clinical response of 35 melanoma metastases to intralesional T-VEC application. Through these studies, we characterized Nectin-1 as a suitable biomarker predicting 86% and 78% of melanoma regression in vitro and in vivo, respectively. In contrast, other molecules involved in the entry (HVEM) and signal transduction (cGAS, STING) of herpes simplex viruses were not predictive. Altogether, our data support the role of Nectin-1 in pretreatment biopsies to guide clinical decision-making in malignant melanoma and supposedly other tumor entities.

Abstract

Talimogene laherparepvec (T-VEC), an oncolytic herpes simplex virus, is approved for intralesional injection of unresectable stage IIIB/IVM1a melanoma. However, it is still unclear which parameter(s) predict treatment response or failure. Our study aimed at characterizing surface receptors Nectin-1 and the herpes virus entry mediator (HVEM) in addition to intracellular molecules cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) as potential bio-markers for oncolytic virus treatment. In 20 melanoma cell lines, oncolytic activity of T-VEC was correlated with the expression of Nectin-1 but not HVEM, as evaluated via flow cytometry and immunohistochemistry. Knockout using CRISPR/Cas9 technology confirmed the superior role of Nectin-1 over HVEM for entry and oncolytic activity of T-VEC. Neither cGAS nor STING as evaluated by Western Blot and immunohistochemistry correlated with T-VEC induced oncolysis. The role of these biomarkers was retrospectively analyzed for the response of 35 cutaneous melanoma metastases of 21 patients to intralesional T-VEC injection, with 21 (60.0%) of these lesions responding with complete (n = 16) or partial regression (n = 5). Nectin-1 expression in pretreatment biopsies significantly predicted treatment outcome, while the expression of HVEM, cGAS, and STING was not prognostic. Altogether, Nectin-1 served as biomarker for T-VEC-induced melanoma regression in vitro and in vivo.

Crosstalk Between Epithelial Cells, Neurons and Immune Mediators in HSV-1 Skin Infection

Herpes simplex virus type 1 (HSV-1) infection is highly prevalent in humans, with approximately two-thirds of the world population living with this virus. However, only a fraction of those carrying HSV-1, which elicits lifelong infections, are symptomatic. HSV-1 mainly causes lesions in the skin and mucosae but reaches the termini of sensory neurons innervating these tissues and travels in a retrograde manner to the neuron cell body where it establishes persistent infection and remains in a latent state until reactivated by different stimuli. When productive reactivations occur, the virus travels back along axons to the primary infection site, where new rounds of replication are initiated in the skin, in recurrent or secondary infections. During this process, new neuron infections occur. Noteworthy, the mechanisms underlying viral reactivations and the exit of latency are somewhat poorly understood and may be regulated by a crosstalk between the infected neurons and components of the immune system. Here, we review and discuss the immune responses that occur at the skin during primary and recurrent infections by HSV-1, as well as at the interphase of latently-infected neurons. Moreover, we discuss the implications of neuronal signals over the priming and migration of immune cells in the context of HSV-1 infection.

Transcriptional profiling reveals potential involvement of microvillous TRPM5-expressing cells in viral infection of the olfactory epithelium

Background

Understanding viral infection of the olfactory epithelium is essential because the olfactory nerve is an important route of entry for viruses to the central nervous system. Specialized chemosensory epithelial cells that express the transient receptor potential cation channel subfamily M member 5 (TRPM5) are found throughout the airways and intestinal epithelium and are involved in responses to viral infection.

Results

Herein we performed deep transcriptional profiling of olfactory epithelial cells sorted by flow cytometry based on the expression of mCherry as a marker for olfactory sensory neurons and for eGFP in OMP-H2B::mCherry/TRPM5-eGFP transgenic mice (Mus musculus). We find profuse expression of transcripts involved in inflammation, immunity and viral infection in TRPM5-expressing microvillous cells compared to olfactory sensory neurons.

Conclusion

Our study provides new insights into a potential role for TRPM5-expressing microvillous cells in viral infection of the olfactory epithelium. We find that, as found for solitary chemosensory cells (SCCs) and brush cells in the airway epithelium, and for tuft cells in the intestine, the transcriptome of TRPM5-expressing microvillous cells indicates that they are likely involved in the inflammatory response elicited by viral infection of the olfactory epithelium.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07528-y.

Endocytic Internalization of Herpes Simplex Virus 1 in Human Keratinocytes at Low Temperature

Herpes simplex virus 1 (HSV-1) can adopt a variety of pathways to accomplish cellular internalization. In human keratinocytes representing the natural target cell of HSV-1, both direct plasma membrane fusion and endocytic uptake have been found. The impact of either pathway in successful infection, however, remains to be fully understood. To address the role of each internalization mode, we performed infection studies at low temperature as a tool to interfere with endocytic pathways. Interestingly, successful HSV-1 entry in primary human keratinocytes and HaCaT cells was observed even at 7°C, although delayed compared to infection at 37°C. Moreover, ex vivo infection of murine epidermis demonstrated that virus entry at 7°C is not only accomplished in cultured cells but also in tissue. Control experiments with cholera toxin B confirmed a block of endocytic uptake at 7°C. In addition, uptake of dextran by macropinosomes and phagocytic uptake of latex beads was also inhibited at 7°C. Infection of nectin-1-deficient murine keratinocytes affirmed that the entry at 7°C was receptor-dependent. Strikingly, the lysosomotropic agent, ammonium chloride, strongly inhibited HSV-1 entry suggesting a role for endosomal acidification. Ultrastructural analyses in turn revealed free capsids in the cytoplasm as well as virus particles in vesicles after infection at 7°C supporting both plasma membrane fusion and endocytic internalization as already observed at 37°C. Overall, entry of HSV-1 at 7°C suggests that the virus can efficiently adopt nectin-1-dependent unconventional vesicle uptake mechanisms in keratinocytes strengthening the role of endocytic internalization for successful infection.IMPORTANCE The human pathogen herpes simplex virus 1 (HSV-1) relies on multiple internalization pathways to initiate infection. Our focus is on the entry in human keratinocytes, the major in vivo target during primary and recurrent infection. While antivirals reduce the severity of clinical cases, there is no cure or vaccine against HSV. To develop strategies that interfere with virus penetration, we need to understand the various parameters and conditions that determine virus entry. Here, we addressed the impact of virus internalization via vesicles by blocking endocytic processes at low temperature. Intriguingly, we detected entry of HSV-1 even at 7°C which led to infection of primary keratinocytes and epidermal tissue. Moreover, electron microscopy of human keratinocytes at 7°C support that internalization is based on fusion of the viral envelope with the plasma membrane as well as vesicle membranes. These results provide novel insights into conditions that still allow endocytic internalization of HSV-1.

Transcriptional profiling reveals potential involvement of microvillous TRPM5-expressing cells in viral infection of the olfactory epithelium

Background

Understanding viral infection of the olfactory epithelium is essential because the olfactory nerve is an important route of entry for viruses to the central nervous system. Specialized chemosensory epithelial cells that express the transient receptor potential cation channel subfamily M member 5 (TRPM5) are found throughout the airways and intestinal epithelium and are involved in responses to viral infection.

Results

Herein we performed deep transcriptional profiling of olfactory epithelial cells sorted by flow cytometry based on the expression of mCherry as a marker for olfactory sensory neurons and for eGFP in OMP-H2B::mCherry/TRPM5-eGFP transgenic mice ( Mus musculus ). We find profuse expression of transcripts involved in inflammation, immunity and viral infection in TRPM5-expressing microvillous cells.

Conclusion

Our study provides new insights into a potential role for TRPM5-expressing microvillous cells in viral infection of the olfactory epithelium. We find that, as found for solitary chemosensory cells (SCCs) and brush cells in the airway epithelium, and for tuft cells in the intestine, the transcriptome of TRPM5-expressing microvillous cells indicates that they are likely involved in the inflammatory response elicited by viral infection of the olfactory epithelium.

Skin Viral Infections: Host Antiviral Innate Immunity and Viral Immune Evasion

The skin is an active immune organ that functions as the first and largest site of defense to the outside environment. Serving as the primary interface between host and pathogen, the skin’s early immune responses to viral invaders often determine the course and severity of infection. We review the current literature pertaining to the mechanisms of cutaneous viral invasion for classical skin-tropic, oncogenic, and vector-borne skin viruses. We discuss the skin’s evolved mechanisms for innate immune viral defense against these invading pathogens, as well as unique strategies utilized by the viruses to escape immune detection. We additionally explore the roles that demographic and environmental factors, such as age, biological sex, and the cutaneous microbiome, play in altering the host immune response to viral threats.

Resistance to pseudorabies virus by knockout of nectin1/2 in pig cells

Pseudorabies virus (PRV) is a pig pathogen that causes substantial economic losses to the pig industry. Infection of host cells by PRV is mediated by the membrane proteins nectin1 and nectin2, which are presumed to be receptors for PRV infection. Here, we generated nectin1/2 knockout (KO) cells with the aim of establishing a PRV-resistant cell model. Nectin1 and 2 were ablated in PK15 cells by CRISPR/Cas9-mediated gene targeting. PRV infection in either nectin1 or nectin2 KO cells showed a significant reduction in viral growth compared with wild-type (WT) cells. We further simultaneously deleted nectin1 and nectin2 in PK15 cells and found that double KO cells showed no further increase in resistance to PRV compared with single gene-KO cells, despite being more resistant than WT. By investigating the cell entry steps of PRV infection, we found that nectin1 or/and nectin2 KO did not greatly affect virus attachment or internalization to cells but blocked cell-to-cell spread. Our results demonstrate that KO of either nectin1 or nectin2 confers PRV resistance to PK15 cells. This strategy could be applied to establish PRV-resistant pigs with nectin1/2 modifications to benefit the pig industry.

Herpes Simplex Virus Type 1 Interactions with the Interferon System

The interferon (IFN) system is one of the first lines of defense activated against invading viral pathogens. Upon secretion, IFNs activate a signaling cascade resulting in the production of several interferon stimulated genes (ISGs), which work to limit viral replication and establish an overall anti-viral state. Herpes simplex virus type 1 is a ubiquitous human pathogen that has evolved to downregulate the IFN response and establish lifelong latent infection in sensory neurons of the host. This review will focus on the mechanisms by which the host innate immune system detects invading HSV-1 virions, the subsequent IFN response generated to limit viral infection, and the evasion strategies developed by HSV-1 to evade the immune system and establish latency in the host.

Hsv-1 Endocytic Entry into a Human Oligodendrocytic Cell Line is Mediated by Clathrin and Dynamin but Not Caveolin

Endocytosis is a pathway used by viruses to enter cells that can be classified based on the proteins involved, such as dynamin, clathrin or caveolin. Although the entry of herpes simplex type 1 (HSV-1) by endocytosis has been documented in different cell types, its dependence on clathrin has not been described whereas its dependence on dynamin has been shown according to the cell line used. The present work shows how clathrin-mediated endocytosis (CME) is one way that HSV-1 infects the human oligodendroglial (HOG) cell line. Partial dynamin inhibition using dynasore revealed a relationship between decrease of infection and dynamin inhibition, measured by viral titration and immunoblot. Co-localization between dynamin and HSV-1 was verified by immunofluorescence at the moment of viral entry into the cell. Inhibition by chlorpromazine revealed that viral progeny also decreased when clathrin was partially inhibited in our cell line. RT-qPCR of immediately early viral genes, specific entry assays and electron microscopy all confirmed clathrin's participation in HSV-1 entry into HOG cells. In contrast, caveolin entry assays showed no effect on the entry of this virus. Therefore, our results suggest the participation of dynamin and clathrin during endocytosis of HSV-1 in HOG cells.

Invasion of Herpes Simplex Virus 1 into Murine Dermis: Role of Nectin-1 and Herpesvirus Entry Mediator as Cellular Receptors during Aging

Skin is a major target tissue of herpes simplex virus 1 (HSV-1), and we are only beginning to understand how individual receptors contribute to the initiation of infection in tissue. We recently demonstrated the impact of the receptors nectin-1 and herpesvirus entry mediator (HVEM) for entry of HSV-1 into murine epidermis. Here, we focus on viral invasion into the dermis, a further critical target tissue in vivo In principle, murine dermal fibroblasts are highly susceptible to HSV-1, and we previously showed that nectin-1 and HVEM can act as alternative receptors. To characterize their contribution as receptors in dermal tissue, we established an ex vivo infection assay of murine dermis. Only after separation of the epidermis from the dermis, we observed single infected cells in the upper dermis from juvenile mice at 5 h postinfection with increasing numbers of infected cells at later times. While nectin-1-expressing cells were less frequently detected, we found HVEM expressed on most cells of juvenile dermis. The comparison of infection efficiency during aging revealed a strong delay in the onset of infection in the dermis from aged mice. This observation correlated with a decrease in nectin-1-expressing fibroblasts during aging while the number of HVEM-expressing cells remained stable. Accordingly, aged nectin-1-deficient dermis was less susceptible to HSV-1 than the dermis from control mice. Thus, we conclude that the reduced availability of nectin-1 in aged dermis is a key contributor to a decrease in infection efficiency during aging.IMPORTANCE HSV-1 is a prevalent human pathogen which invades skin and mucocutaneous linings. So far, the underlying mechanisms of how the virus invades tissue, reaches its receptors, and initiates infection are still unresolved. To unravel the mechanical prerequisites that limit or favor viral invasion into tissue, we need to understand the contribution of the receptors that are involved in viral internalization. Here, we investigated the invasion process into murine dermis with the focus on receptor availability and found that infection efficiency decreases in aging mice. Based on studies of the expression of the receptors nectin-1 and HVEM, we suggest that the decreasing number of nectin-1-expressing fibroblasts leads to a delayed onset of infection in the dermis from aged compared to juvenile mice. Our results imply that the level of infection efficiency in murine dermis is closely linked to the availability of the receptor nectin-1 and can change during aging.

Use of Modified Clostridium perfringens Enterotoxin Fragments for Claudin Targeting in Liver and Skin Cells

Claudins regulate paracellular permeability in different tissues. The claudin-binding domain of Clostridium perfringens enterotoxin (cCPE) is a known modulator of a claudin subset. However, it does not efficiently bind to claudin-1 (Cldn1). Cldn1 is a pharmacological target since it is (i) an essential co-receptor for hepatitis C virus (HCV) infections and (ii) a key element of the epidermal barrier limiting drug delivery. In this study, we investigated the potential of a Cldn1-binding cCPE mutant (i) to inhibit HCV entry into hepatocytes and (ii) to open the epidermal barrier. Inhibition of HCV infection by blocking of Cldn1 with cCPE variants was analyzed in the Huh7.5 hepatoma cell line. A model of reconstructed human epidermis was used to investigate modulation of the epidermal barrier by cCPE variants. In contrast to cCPEwt, the Cldn1-binding cCPE-S305P/S307R/S313H inhibited infection of Huh7.5 cells with HCV in a dose-dependent manner. In addition, TJ modulation by cCPE variant-mediated targeting of Cldn1 and Cldn4 opened the epidermal barrier in reconstructed human epidermis. cCPE variants are potent claudin modulators. They can be applied for mechanistic in vitro studies and might also be used as biologics for therapeutic claudin targeting including HCV treatment (host-targeting antivirals) and improvement of drug delivery.

Herpes Simplex Virus 1 Can Enter Dynamin 1 and 2 Double-Knockout Fibroblasts

Dynamin GTPases, best known for their role in membrane fission of endocytic vesicles, provide a target for viruses to be exploited during endocytic uptake. Recently, we found that entry of herpes simplex virus 1 (HSV-1) into skin cells depends on dynamin, although our results supported that viral internalization occurs via both direct fusion with the plasma membrane and via endocytic pathways. To further explore the role of dynamin for efficient HSV-1 entry, we utilized conditional dynamin 1 and dynamin 2 double-knockout (DKO) fibroblasts as an experimental tool. Strikingly, HSV-1 entered control and DKO fibroblasts with comparable efficiencies. For comparison, we infected DKO cells with Semliki Forest virus, which is known to adopt clathrin-mediated endocytosis as its internalization pathway, and observed efficient virus entry. These results support the notion that the DKO cells provide alternative pathways for viral uptake. Treatment of cells with the dynamin inhibitor dynasore confirmed that HSV-1 entry depended on dynamin in the control fibroblasts. As expected, dynasore did not interfere with viral entry into DKO cells. Electron microscopy of HSV-1-infected cells suggests viral entry after fusion with the plasma membrane and by endocytosis in both dynamin-expressing and dynamin-deficient cells. Infection at low temperatures where endocytosis is blocked still resulted in HSV-1 entry, although at a reduced level, which suggests that nonendocytic pathways contribute to successful entry. Overall, our results strengthen the impact of dynamin for HSV-1 entry, as only cells that adapt to the lack of dynamin allow dynamin-independent entry.IMPORTANCE The human pathogen herpes simplex virus 1 (HSV-1) can adapt to a variety of cellular pathways to enter cells. In general, HSV-1 is internalized by fusion of its envelope with the plasma membrane or by endocytic pathways, which reflects the high adaptation to differences in its target cells. The challenges are to distinguish whether multiple or only one of these internalization pathways leads to successful entry and, furthermore, to identify the mode of viral uptake. In this study, we focused on dynamin, which promotes endocytic vesicle fission, and explored how the presence and absence of dynamin can influence viral entry. Our results support the idea that HSV-1 entry into mouse embryonic fibroblasts depends on dynamin; however, depletion of dynamin still allows efficient viral entry, suggesting that alternative pathways present upon dynamin depletion can accomplish viral internalization.

A Preclinical Model for Studying Herpes Simplex Virus Infection

Herpes simplex virus (HSV) infections can cause considerable morbidity. Currently, nucleoside analogues such as acyclovir are widely used for treatment. However, HSV infections resistant to these drugs are a clinical problem among immunocompromised patients. To provide more efficient therapy and to counteract resistance, a different class of antiviral compounds has been developed. Pritelivir, a helicase primase inhibitor, represents a promising candidate for improved therapy. Here, we established an HSV-1 infection model on microneedle-pretreated human skin ex vivo. We identified HSV-1–specific histological changes (e.g., cytopathic effects, multinucleated giant cells), down-regulation of nectin-1, nuclear translocation of NF-κB (p65), interferon regulatory factor 3 (IRF3), and signaling of the IFN-inducible protein MxA. Accordingly, this model was used to test the potency of pritelivir compared with the standard drug acyclovir. We discovered that both drugs had a comparable efficacy for inhibiting HSV-1 replication, suggesting that pritelivir could be an alternative therapeutic agent for patients infected with acyclovir-resistant strains. To our knowledge, we present a previously unreported ex vivo HSV-1 infection model with abdominal human skin to test antiviral drugs, thus bridging the gap between in vitro and in vivo drug screening and providing a valuable preclinical platform for HSV research.

Insights into the pathogenesis of herpes simplex encephalitis from mouse models

A majority of the world population is infected with herpes simplex viruses (HSV; human herpesvirus types 1 and 2). These viruses, perhaps best known for their manifestation in the genital or oral mucosa, can also cause herpes simplex encephalitis, a severe and often fatal disease of the central nervous system. Antiviral therapies for HSV are only partially effective since the virus can establish latent infections in neurons, and severe pathological sequelae in the brain are common. A better understanding of disease pathogenesis is required to develop new strategies against herpes simplex encephalitis, including the precise viral and host genetic determinants that promote virus invasion into the central nervous system and its associated immunopathology. Here we review the current understanding of herpes simplex encephalitis from the host genome perspective, which has been illuminated by groundbreaking work on rare herpes simplex encephalitis patients together with mechanistic insight from single-gene mouse models of disease. A complex picture has emerged, whereby innate type I interferon-mediated antiviral signaling is a central pathway to control viral replication, and the regulation of immunopathology and the balance between apoptosis and autophagy are critical to disease severity in the central nervous system. The lessons learned from mouse studies inform us on fundamental defense mechanisms at the interface of host–pathogen interactions within the central nervous system, as well as possible rationales for intervention against infections from severe neuropathogenic viruses.

Entry of Herpes Simplex Virus 1 into Epidermis and Dermal Fibroblasts Is Independent of the Scavenger Receptor MARCO

To enter host cells, herpes simplex virus 1 (HSV-1) initially attaches to cell surface glycosaminoglycans, followed by the requisite binding to one of several cellular receptors, leading to viral internalization. Although virus-receptor interactions have been studied in various cell lines, the contributions of individual receptors to uptake into target tissues such as mucosa, skin, and cornea are not well understood. We demonstrated that nectin-1 acts as a major receptor for HSV-1 entry into murine epidermis, while herpesvirus entry mediator (HVEM) can serve as an alternative receptor. Recently, the macrophage receptor with collagenous structure (MARCO) has been described to mediate adsorption of HSV-1 to epithelial cells. Here, we investigated the impact of MARCO on the entry process of HSV-1 into the two major cell types of skin, keratinocytes in the epidermis and fibroblasts in the underlying dermis. Using ex vivo infection of murine epidermis, we showed that HSV-1 entered basal keratinocytes of MARCO^-/- epidermis as efficiently as those of control epidermis. In addition, entry into dermal fibroblasts was not impaired in the absence of MARCO. When we treated epidermis, primary keratinocytes, or fibroblasts with poly(I), a ligand for class A scavenger receptors, HSV-1 entry was strongly reduced. As we also observed reducing effects of poly(I) in the absence of both MARCO and scavenger receptor A1, we concluded that the inhibitory effects of poly(I) on HSV-1 infection are not directly linked to class A scavenger receptors. Overall, our results support that HSV-1 entry into skin cells is independent of MARCO.IMPORTANCE During entry into its host cells, the human pathogen herpes simplex virus (HSV) interacts with various cellular receptors. Initially, receptor interaction can mediate cellular adsorption, followed by receptor binding that triggers viral internalization. The intriguing question is which receptors are responsible for the various steps during entry into the natural target tissues of HSV? Previously, we demonstrated the role of nectin-1 as a major receptor and that of HVEM as an alternative receptor for HSV-1 to invade murine epidermis. As MARCO has been described to promote infection in skin, we explored the predicted role of MARCO as a receptor that mediates adsorption to epithelial cells. Our infection studies of murine skin cells indicate that the absence of MARCO does not interfere with the efficiency of HSV-1 entry and that the inhibitory effect on viral adsorption by poly(I), a ligand of MARCO, is independent of MARCO.

Innate Immune Mechanisms and Herpes Simplex Virus Infection and Disease

Innate immune responses play a major role in the control of herpes simplex virus (HSV) infections, and a multiplicity of mechanisms have emerged as a result of human evolution to sense and respond to HSV infections. HSV in turn has evolved a number of ways to evade immune detection and to blunt human innate immune responses. In this review, we summarize the major host innate immune mechanisms and the HSV evasion mechanisms that have evolved. We further discuss how disease can result if this equilibrium between virus and host response is disrupted.

Initial Contact: The First Steps in Herpesvirus Entry

The entry process of herpesviruses into host cells is complex and highly variable. It involves a sequence of well-orchestrated events that begin with virus attachment to glycan-containing proteinaceous structures on the cell surface. This initial contact tethers virus particles to the cell surface and results in a cascade of molecular interactions, including the tight interaction of viral envelope glycoproteins to specific cell receptors. These interactions trigger intracellular signaling and finally virus penetration after fusion of the viral envelope with cellular membranes. Based on the engaged cellular receptors and co-receptors, and the subsequent signaling cascades, the entry pathway will be decided on the spot. A number of viral glycoproteins and many cellular receptors and molecules have been identified as players in one or several of these events during virus entry. This chapter will review viral glycoproteins, cellular receptors and signaling cascades associated with the very first interactions of herpesviruses with their target cells.

A comparative review of viral entry and attachment during large and giant dsDNA virus infections

Viruses enter host cells via several mechanisms, including endocytosis, macropinocytosis, and phagocytosis. They can also fuse at the plasma membrane and can spread within the host via cell-to-cell fusion or syncytia. The mechanism used by a given viral strain depends on its external topology and proteome and the type of cell being entered. This comparative review discusses the cellular attachment receptors and entry pathways of dsDNA viruses belonging to the families Adenoviridae, Baculoviridae, Herpesviridae and nucleocytoplasmic large DNA viruses (NCLDVs) belonging to the families Ascoviridae, Asfarviridae, Iridoviridae, Phycodnaviridae, and Poxviridae, and giant viruses belonging to the families Mimiviridae and Marseilleviridae as well as the proposed families Pandoraviridae and Pithoviridae. Although these viruses have several common features (e.g., topology, replication and protein sequence similarities) they utilize different entry pathways to infect wide-range of hosts, including humans, other mammals, invertebrates, fish, protozoa and algae. Similarities and differences between the entry methods used by these virus families are highlighted, with particular emphasis on viral topology and proteins that mediate viral attachment and entry. Cell types that are frequently used to study viral entry are also reviewed, along with other factors that affect virus-host cell interactions.

Study on a 3D Hydrogel-Based Culture Model for Characterizing Growth of Fibroblasts under Viral Infection and Drug Treatment

Three-dimensional (3D) in vitro tissue models provide an approach for the systematic, repetitive, and quantitative study of drugs. In this study, we constructed an in vitro 3D acrylated hyaluronic acid (AHA) hydrogel model encapsulating fibroblasts, performed long-period 3D culture, and tested cellular topological changes and proliferation variation in the presence of herpes simplex virus-1 (HSV-1) as an infecting virus and acyclovir (ACV) as the treatment drug. The AHA hydrogels were formed by using Michael addition chemistry of bis-cysteine containing MMP-degradable cross-linker onto AHA prefunctionalized with cell adhesion peptides (RGD). Cellular structures of 3T3 fibroblasts in hydrogel presented different morphological evolution processes and proliferation rates between different groups, including HSV-1 treated alone, ACV treated alone, HSV-1 and ACV cotreated, and control samples. In AHA hydrogel, ACV blocked HSV-1 infection/replication on fibroblasts. Yet, the proliferation of ACV-treated fibroblasts was slower than that of the control group. A significantly longer period was required for cells in 3D AHA gel to regain a healthy status when compared with cells in two-dimensional (2D) culture. This hydrogel-based 3D culture model potentially lays a foundation for analyzing the response of self-organized 3D tissues to viruses and drugs in a way that is closer to nature.

Herpes Simplex Virus 1 Enters Human Keratinocytes by a Nectin-1-Dependent, Rapid Plasma Membrane Fusion Pathway That Functions at Low Temperature

Herpes simplex virus 1 (HSV-1) infects humans through stratified epithelia that are composed primarily of keratinocytes. The route of HSV-1 entry into keratinocytes has been the subject of limited investigation, but it is proposed to involve pH-dependent endocytosis, requiring the gD-binding receptor nectin-1. Here, we have utilized the nTERT human keratinocyte cell line as a new model for dissecting the mechanism of HSV-1 entry into the host. Although immortalized, these cells nonetheless retain normal growth and differentiation properties of primary cells. Using short interfering RNA (siRNA) depletion studies, we confirm that, despite nTERT cells expressing high levels of the alternative gD receptor HVEM, HSV-1 requires nectin-1, not HVEM, to enter these cells. Strikingly, virus entry into nTERT cells occurred with unusual rapidity, such that maximum penetration was achieved within 5 min. Moreover, HSV-1 was able to enter keratinocytes but not other cell types at temperatures as low as 7°C, conditions where endocytosis was shown to be completely inhibited. Transmission electron microscopy of early entry events at both 37°C and 7°C identified numerous examples of naked virus capsids located immediately beneath the plasma membrane, with no evidence of virions in cytoplasmic vesicles. Taken together, these results imply that HSV-1 uses the nectin-1 receptor to enter human keratinocyte cells via a previously uncharacterized rapid plasma membrane fusion pathway that functions at low temperature. These studies have important implications for current understanding of the relationship between HSV-1 and its relevant in vivo target cell.

IMPORTANCE

The gold standard of antiviral treatment for any human virus infection is the prevention of virus entry into the host cell. In the case of HSV-1, primary infection in the human begins in the epidermis of the skin or the oral mucosa, where the virus infects keratinocytes, and it is therefore important to understand the molecular events involved in HSV-1 entry into this cell type. Nonetheless, few studies have looked specifically at entry into these relevant human cells. Our results reveal a new route for virus entry that is specific to keratinocytes, involves rapid entry, and functions at low temperatures. This may reflect the environmental conditions encountered by HSV-1 when entering its host through the skin and emphasizes the importance of studying virus-host interactions in physiologically relevant cells.

Novel therapeutic investigational strategies to treat severe and disseminated HSV infections suggested by a deeper understanding of in vitro virus entry processes

The global burden of herpes simplex virus (HSV) legitimates the critical need to develop new prevention strategies, such as drugs and vaccines that are able to fight either primary HSV infections or reactivations. Moreover, the ever-growing number of patients receiving transplants increases the number of severe HSV infections that are unresponsive to current therapies. Finally, the high global incidence of genital HSV-2 infection increases the risk of perinatal transmission to newborns, in which disseminated infection or central nervous system (CNS) involvement is frequent, with associated high morbidity and mortality rates. There are several key features shared by novel anti-HSV drugs, from currently available optimized drugs to small molecules able to interfere with various virus replication steps. However, several virological aspects of the disease and associated clinical needs highlight why an ideal anti-HSV drug has yet to be developed.