Entry mechanisms of herpes simplex virus 1 into murine epidermis: involvement of nectin-1 and herpesvirus entry mediator as cellular receptors

Small Animal Models to Study Herpes Simplex Virus Infections

Herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2) are two of the most prevalent human viruses worldwide. They are known to cause a variety of diseases including genital herpes, meningitis, encephalitis, cold sores and herpes stromal keratitis. The seropositive rate for HSV-1 is around 90%, whereas for HSV-2 it remains around 20-25% for the general adult population. The infections caused by these viruses remain difficult to study because a large proportion of infected individuals are asymptomatic. Furthermore, given the neurotropic characteristics of the virus, studies aimed at understanding the complex pathogenesis in humans is difficult. As a result, animal models have been developed to understand several characteristics of HSV biology, pathogenesis, disease and host responses to infection. These models are also commonly used as the first evaluation of new drugs and vaccines. There are several well-established animal models to study infection with HSV, including mice, guinea pigs and rabbits. Variables within the animal models depend on the species of animal, route of infection, viral strain, dosage, etc. This review aims at summarizing the most commonly used animal models to study HSV pathogenesis and therapies.

Herpes simplex virus spreads rapidly in human foreskin, partly driven by chemokine-induced redistribution of Nectin-1 on keratinocytes

HSV infects keratinocytes in the epidermis of skin via nectin-1. We established a human foreskin explant infection model to investigate HSV entry and spread. HSV1 entry could only be achieved by the topical application of virus via high density microarray projections (HD-MAPs) to the epidermis, which penetrated beyond one third of its thickness, simulating in vivo microtrauma. Rapid lateral spread of HSV1 to a mean of 13 keratinocytes wide occurred after 24 hours and free virus particles were observed between keratinocytes, consistent with an intercellular route of spread. Nectin-1 staining was markedly decreased in foci of infection in the epidermis and in the human keratinocyte HaCaT cell line. Nectin-1 was redistributed, at the protein level, in adjacent uninfected cells surrounding infection, inducible by CCL3, IL-8 (or CXCL8), and possibly CXCL10 and IL-6, thus facilitating spread. These findings provide the first insights into HSV1 entry and spread in human inner foreskin in situ.

Anti-HSV activity of nectin-1-derived peptides targeting HSV gD: an in-silico and in-vitro approach

Herpes simplex virus (HSV) infections affect a wide range of the global population. The emergence of resistance to the existing anti-HSV therapy highlights the necessity for an innovative strategy. The interaction of HSV gD with its main host receptor nectin-1 is a potential target for new antiviral drugs. The aim of this study was to develop a peptide derived from nectin-1 targeting HSV gD using the in-silico method and evaluate them for anti-HSV activity. Residues 59-133 of the Nectin-1 V-domain constitute the interaction interface with HSV gD. Bioinformatic tools viz., PEP-FOLD3, ClusPro 2.0, HawkDock and Desmond were used to model the peptide and confirm its binding specificity with HSV gD protein. The peptides with potential interactions were custom synthesized and anti-HSV activity was evaluated in vitro against HSV-1 and HSV-2 by CPE inhibition assay. Five peptide sequences were identified as exhibiting good interaction with HSV-gD proteins. Among them, peptide N1 (residues 76-90) offered maximum protection against HSV-1 (66.57%) and HSV-2 (71.12%) infections. Modification of the identified peptide through peptidomimetic approaches may further enhance the activity and stability of the identified peptide.Communicated by Ramaswamy H. Sarma.

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.

Ex Vivo Infection of Human Skin Models with Herpes Simplex Virus 1: Accessibility of the Receptor Nectin-1 during Formation or Impairment of Epidermal Barriers Is Restricted by Tight Junctions

Herpes simplex virus 1 (HSV-1) must overcome epidermal barriers to reach its receptors on keratinocytes and initiate infection in human skin. The cell-adhesion molecule nectin-1, which is expressed in human epidermis, acts as an efficient receptor for HSV-1 but is not within reach of the virus upon exposure of human skin under nonpathological conditions. Atopic dermatitis skin, however, can provide an entry portal for HSV-1 emphasizing the role of impaired barrier functions. Here, we explored how epidermal barriers impact HSV-1 invasion in human epidermis and influence the accessibility of nectin-1 for the virus. Using human epidermal equivalents, we observed a correlation of the number of infected cells with tight-junction formation, suggesting that mature tight junctions prior to formation of the stratum corneum prevent viral access to nectin-1. Consequently, impaired epidermal barriers driven by Th2-inflammatory cytokines interleukin 4 (IL-4) and IL-13 as well as the genetic predisposition of nonlesional atopic dermatitis keratinocytes correlated with enhanced infection supporting the impact of functional tight junctions for preventing infection in human epidermis. Comparable to E-cadherin, nectin-1 was distributed throughout the epidermal layers and localized just underneath the tight-junctions. While nectin-1 was evenly distributed on primary human keratinocytes in culture, the receptor was enriched at lateral surfaces of basal and suprabasal cells during differentiation. Nectin-1 showed no major redistribution in the thickened atopic dermatitis and IL-4/IL-13-treated human epidermis in which HSV-1 can invade. However, nectin-1 localization toward tight junction components changed, suggesting that defective tight-junction barriers make nectin-1 accessible for HSV-1 which enables facilitated viral penetration. IMPORTANCE Herpes simplex virus 1 (HSV-1) is a widely distributed human pathogen which productively infects epithelia. The open question is which barriers of the highly protected epithelia must the virus overcome to reach its receptor nectin-1. Here, we used human epidermal equivalents to understand how physical barrier formation and nectin-1 distribution contribute to successful viral invasion. Inflammation-induced barrier defects led to facilitated viral penetration strengthening the role of functional tight-junctions in hindering viral access to nectin-1 that is localized just underneath tight junctions and distributed throughout all layers. We also found nectin-1 ubiquitously localized in the epidermis of atopic dermatitis and IL-4/IL-13-treated human skin implying that impaired tight-junctions in combination with a defective cornified layer allow the accessibility of nectin-1 to HSV-1. Our results support that successful invasion of HSV-1 in human skin relies on defective epidermal barriers, which not only include a dysfunctional cornified layer but also depend on impaired tight junctions.

HSV-1 Glycoprotein D and Its Surface Receptors: Evaluation of Protein-Protein Interaction and Targeting by Triazole-Based Compounds through In Silico Approaches

Protein-protein interactions (PPI) represent attractive targets for drug design. Thus, aiming at a deeper insight into the HSV-1 envelope glycoprotein D (gD), protein-protein docking and dynamic simulations of gD-HVEM and gD-Nectin-1 complexes were performed. The most stable complexes and the pivotal key residues useful for gD to anchor human receptors were identified and used as starting points for a structure-based virtual screening on a library of both synthetic and designed 1,2,3-triazole-based compounds. Their binding properties versus gD interface with HVEM and Nectin-1 along with their structure-activity relationships (SARs) were evaluated. Four [1,2,3]triazolo[4,5-b]pyridines were identified as potential HSV-1 gD inhibitors, for their good theoretical affinity towards all conformations of HSV-1 gD. Overall, this study suggests promising basis for the design of new antiviral agents targeting gD as a valuable strategy to prevent viral attachment and penetration into the host cell.

Herpes Simplex Virus 1 Can Bypass Impaired Epidermal Barriers upon Ex Vivo Infection of Skin from Atopic Dermatitis Patients

To infect its human host, herpes simplex virus 1 (HSV-1) must overcome the protective barriers of skin and mucosa. Here, we addressed whether pathological skin conditions can facilitate viral entry via the skin surface and used ex vivo infection studies to explore viral invasion in atopic dermatitis (AD) skin characterized by disturbed barrier functions. Our focus was on the visualization of the onset of infection in single cells to determine the primary entry portals in the epidermis. After ex vivo infection of lesional AD skin, we observed infected cells in suprabasal layers indicating successful invasion in the epidermis via the skin surface which was never detected in control skin where only sample edges allowed viral access. The redistribution of filaggrin, loricrin, and tight-junction components in the lesional skin samples suggested multiple defective mechanical barriers. To dissect the parameters that contribute to HSV-1 invasion, we induced an AD-like phenotype by adding the Th2 cytokines interleukin 4 (IL-4) and IL-13 to healthy human skin samples. Strikingly, we detected infected cells in the epidermis, implying that the IL-4/IL-13-driven inflammation is sufficient to induce modifications allowing HSV-1 to penetrate the skin surface. In summary, not only did lesional AD skin facilitate HSV-1 penetration but IL-4/IL-13 responses alone allowed virus invasion. Our results suggest that the defective epidermal barriers of AD skin and the inflammation-induced altered barriers in healthy skin can make receptors accessible for HSV-1.

IMPORTANCE Herpes simplex virus 1 (HSV-1) can target skin to establish primary infection in the epithelium. While the human skin provides effective barriers against viral invasion under healthy conditions, a prominent example of successful invasion is the disseminated HSV-1 infection in the skin of atopic dermatitis (AD) patients. AD is characterized by impaired epidermal barrier functions, chronic inflammation, and dysbiosis of skin microbiota. We addressed the initial invasion process of HSV-1 in atopic dermatitis skin to understand whether the physical barrier functions are sufficiently disturbed to allow the virus to invade skin and reach its receptors on skin cells. Our results demonstrate that HSV-1 can indeed penetrate and initiate infection in atopic dermatitis skin. Since treatment of skin with IL-4 and IL-13 already resulted in successful invasion, we assume that inflammation-induced barrier defects play an important role for the facilitated access of HSV-1 to its target cells.

Ex vivo Human Skin Infection with Herpes Simplex Virus 1

Although herpes simplex virus 1 (HSV-1) is a well-studied virus, how the virus invades its human host via skin and mucosa to reach its receptors and initiate infection remains an open question. For studies of HSV-1 infection in skin, mice have been used as animal models. Murine skin infection can be induced after injection or scratching of the skin, which provides insights into disease pathogenesis but is clearly distinct from the natural entry route in human tissue. To explore the invasion route of HSV-1 on the tissue level, we established an ex vivo infection assay using skin explants. Here, we detail a protocol allowing the investigation of how the virus overcomes mechanical barriers in human skin to penetrate in keratinocytes and dermal fibroblasts. The protocol includes the preparation of total skin samples, skin shaves, and of separated epidermis and dermis, which is followed by incubation in virus suspension. The ex vivo infection assay allows the visualization, quantification, and characterization of single infected cells in the epidermis and dermis prior to viral replication and the virus-induced tissue damage. Hence, this experimental approach enables the identification of primary viral entry portals. Graphical abstract.

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.

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.

Cell-to-cell transmission of HSV1 in human keratinocytes in the absence of the major entry receptor, nectin1

Herpes simplex virus 1 (HSV1) infects the stratified epithelia of the epidermis, oral or genital mucosa, where the main cell type is the keratinocyte. Here we have used nTERT human keratinocytes to generate a CRISPR-Cas9 knockout (KO) of the primary candidate HSV1 receptor, nectin1, resulting in a cell line that is refractory to HSV1 entry. Nonetheless, a small population of KO cells was able to support infection which was not blocked by a nectin1 antibody and hence was not a consequence of residual nectin1 expression. Strikingly at later times, the population of cells originally resistant to HSV1 infection had also become infected. Appearance of this later population was blocked by inhibition of virus genome replication, or infection with a ΔUL34 virus defective in capsid export to the cytoplasm. Moreover, newly formed GFP-tagged capsids were detected in cells surrounding the initial infected cell, suggesting that virus was spreading following replication in the original susceptible cells. Additional siRNA depletion of the second major HSV1 receptor HVEM, or PTP1B, a cellular factor shown elsewhere to be involved in cell-to-cell transmission, had no effect on virus spread in the absence of nectin1. Neutralizing human serum also failed to block virus transmission in nectin1 KO cells, which was dependent on the receptor binding protein glycoprotein D and the cell-to-cell spread glycoproteins gI and gE, indicating that virus was spreading by direct cell-to-cell transmission. In line with these results, both HSV1 and HSV2 formed plaques on nectin1 KO cells, albeit at a reduced titre, confirming that once the original cell population was infected, the virus could spread into all other cells in the monolayer. We conclude that although nectin1 is required for extracellular entry in to the majority of human keratinocytes, it is dispensable for direct cell-to-cell transmission.

Herpes simplex virus 1 (HSV1) infects the epithelia of the epidermis, oral or genital mucosa to cause cold sores, genital herpes, or more serious outcomes such as keratitis and neonatal herpes. Like many viruses, HSV1 can spread through the extracellular environment or by direct cell-to-cell transmission, with the latter mechanism being important for avoiding antibody responses in the host. Here we have studied HSV1 entry and transmission in the human keratinocyte, the main cell type in the target epithelia, by generating a CRISPR-Cas9 knockout of the primary candidate virus receptor, nectin1. While HSV1 was unable to infect the majority of nectin1 knockout keratinocytes, a small population of these nectin1 KO cells remained susceptible to virus entry, and once infected, the virus was able to spread into the rest of the monolayer. This spread continued in the presence of neutralising serum which blocks extracellular virus, and required glycoprotein D, the main virus receptor-binding protein, and glycoproteins gE and gI which are known to be involved in cell-to-cell spread. Hence, while nectin1 is required for virus entry into the majority of human keratinocyte cells, it is dispensable for cell-to-cell transmission of the virus. These data have implications for the mechanism of HSV1 epithelial spread and pathogenesis.

Ex vivo infection of human skin with herpes simplex virus 1 reveals mechanical wounds as insufficient entry portals via the skin surface

Herpes simplex virus 1 (HSV-1) enters its human host via the skin and mucosa. The open question is how the virus invades this highly protective tissue in vivo to approach its receptors in the epidermis and initiate infection. Here, we performed ex vivo infection studies in human skin to investigate how susceptible the epidermis and dermis are to HSV-1 and whether wounding facilitates viral invasion. Upon ex vivo infection of complete skin, only sample edges with integrity loss demonstrated infected cells. After removal of the dermis, HSV-1 efficiently invaded the basal layer of the epidermis, and from there, gained access to suprabasal layers. This finding supports a high susceptibility of all epidermal layers which correlated with the surface expression of the receptors nectin-1 and herpesvirus entry mediator (HVEM). In contrast, only single infected cells were detected in the separated dermis where minor expression of the receptors was found. Interestingly, after wounding, nearly no infection of the epidermis was observed via the skin surface. However, if the wounding of the skin samples led to breaks through the dermis, HSV-1 infected mainly keratinocytes via the damaged dermal layer. The application of latex beads revealed only occasional entry via the wounded dermis, however, facilitated penetration via the wounded skin surface. Thus, we suggest that although the wounded human skin surface allows particle penetration, the skin still provides barriers that prevent HSV-1 from reaching its receptors. Importance The human pathogen HSV-1 invades its host via the skin and mucosa which leads to primary infection of the epithelium. As the various epithelial barriers effectively protect the tissue against viral invasion, successful infection most likely depends on tissue damage. We addressed the initial invasion process in human skin by ex vivo infection to understand how HSV-1 overcomes physical skin barriers and reaches its receptors to enter skin cells. Our results demonstrate that intact skin samples allow viral access only from the edges, while the epidermis is highly susceptible once the basal epidermal layer serves as initial entry portal. Surprisingly, mechanical wounding did not facilitate HSV-1 entry via the skin surface although latex beads still penetrated via the lesions. Our results imply that successful invasion of HSV-1 depends on how well the virus can reach its receptors which was not accomplished by skin lesions under ex vivo conditions.

Characterisation of Antiviral Activity of Cathelicidins from Naked Mole Rat and Python bivittatus on Human Herpes Simplex Virus 1

Hg-CATH and Pb-CATH4 are cathelicidins from Heterocephalus glaber and Python bivittatus that have been previously identified as potent antibacterial peptides. However, their antiviral properties were not previously investigated. In this study, their activity against the herpes simplex virus (HSV)-1 was evaluated during primary human keratinocyte infection. Both of them significantly reduced HSV-1 DNA replication and production of infectious viral particles in keratinocytes at noncytotoxic concentrations, with the stronger activity of Pb-CATH4. These peptides did not show direct virucidal activity and did not exhibit significant immunomodulatory properties, except for Pb-CATH4, which exerted a moderate proinflammatory action. All in all, our results suggest that Hg-CATH and Pb-CATH4 could be potent candidates for the development of new therapies against HSV-1.

Peptide Inhibitor of Complement C1, RLS-0071, Reduces Zosteriform Spread of Herpes Simplex Virus Type 1 Skin Infection and Promotes Survival in Infected Mice

Herpes simplex virus type 1 (HSV-1) is a prevalent human pathogen primarily transmitted through skin-to-skin contact, especially on and around mucosal surfaces where there is contact with contaminated saliva during periods of viral shedding. It is estimated that 90% of adults worldwide have HSV-1 antibodies. Cutaneous HSV-1 infections are characterized by a sensation of tingling or numbness at the initial infection site followed by an eruption of vesicles and then painful ulcers with crusting. These symptoms can take ten days to several weeks to heal, leading to significant morbidity. Histologically, infections cause ballooning degeneration of keratinocytes and formation of multinucleated giant cells, ultimately resulting in a localized immune response. Commonly prescribed treatments against HSV-1 infections are nucleoside analogs, such as acyclovir (ACV). However, the emergence of ACV-resistant HSV (ACV^R-HSV) clinical isolates has created an urgent need for the development of compounds to control symptoms of cutaneous infections. RLS-0071, also known as peptide inhibitor of complement C1 (PIC1), is a 15-amino-acid anti-inflammatory peptide that inhibits classical complement pathway activation and modulates neutrophil activation. It has been previously shown to aid in the healing of chronic diabetic wounds by inhibiting the excessive activation of complement component C1 and infiltration of leukocytes. Here, we report that treatment of cutaneous infections of HSV-1 and ACV^R-HSV-1 in BALB/cJ mice with RLS-0071 significantly reduced the rate of mortality, decreased zosteriform spread, and enhanced the healing of the infection-associated lesions compared to control-treated animals. Therefore, RLS-0071 may work synergistically with other antiviral drugs to aid in wound healing of HSV-1 cutaneous infection and may potentially aid in rapid wound healing of other pathology not limited to HSV-1.

Oncolytic herpes simplex virus infects myeloma cells in vitro and in vivo

Because most patients with multiple myeloma (MM) develop resistance to current regimens, novel approaches are needed. Genetically modified, replication-competent oncolytic viruses exhibit high tropism for tumor cells regardless of cancer stage and prior treatment. Receptors of oncolytic herpes simplex virus 1 (oHSV-1), NECTIN-1, and HVEM are expressed on MM cells, prompting us to investigate the use of oHSV-1 against MM. Using oHSV-1-expressing GFP, we found a dose-dependent increase in the GFP⁺ signal in MM cell lines and primary MM cells. Whereas NECTIN-1 expression is variable among MM cells, we discovered that HVEM is ubiquitously and highly expressed on all samples tested. Expression of HVEM was consistently higher on CD138⁺/CD38⁺ plasma cells than in non-plasma cells. HVEM blocking demonstrated the requirement of this receptor for infection. However, we observed that, although oHSV-1 could efficiently infect and kill all MM cell lines tested, no viral replication occurred. Instead, we identified that oHSV-1 induced MM cell apoptosis via caspase-3 cleavage. We further noted that oHSV-1 yielded a significant decrease in tumor volume in two mouse xenograft models. Therefore, oHSV-1 warrants exploration as a novel potentially effective treatment option in MM, and HVEM should be investigated as a possible therapeutic target.

Herpes Simplex Virus and Pattern Recognition Receptors: An Arms Race

Herpes simplex viruses (HSVs) are experts in establishing persistent infection in immune-competent humans, in part by successfully evading immune activation through diverse strategies. Upon HSV infection, host deploys pattern recognition receptors (PRRs) to recognize various HSV-associated molecular patterns and mount antiviral innate immune responses. In this review, we describe recent advances in understanding the contributions of cytosolic PRRs to detect HSV and the direct manipulations on these receptors by HSV-encoded viral proteins as countermeasures. The continuous update and summarization of these mechanisms will deepen our understanding on HSV-host interactions in innate immunity for the development of novel antiviral therapies, vaccines and oncolytic viruses.

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.

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.

Antagonism of interferon signaling by fibroblast growth factors promotes viral replication

Fibroblast growth factors (FGFs) play key roles in the pathogenesis of different human diseases, but the cross‐talk between FGFs and other cytokines remains largely unexplored. We identified an unexpected antagonistic effect of FGFs on the interferon (IFN) signaling pathway. Genetic or pharmacological inhibition of FGF receptor signaling in keratinocytes promoted the expression of interferon‐stimulated genes (ISG) and proteins in vitro and in vivo. Conversely, FGF7 or FGF10 treatment of keratinocytes suppressed ISG expression under homeostatic conditions and in response to IFN or poly(I:C) treatment. FGF‐mediated ISG suppression was independent of IFN receptors, occurred at the transcriptional level, and required FGF receptor kinase and proteasomal activity. It is not restricted to keratinocytes and functionally relevant, since FGFs promoted the replication of herpes simplex virus I (HSV‐1), lymphocytic choriomeningitis virus, and Zika virus. Most importantly, inhibition of FGFR signaling blocked HSV‐1 replication in cultured human keratinocytes and in mice. These results suggest the use of FGFR kinase inhibitors for the treatment of viral infections.

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.

Histopathological and Immunohistochemical Characteristics of Measles Exanthema: A Study of a Series of 13 Adult Cases and Review of the Literature

Despite available vaccination, measles is one of the leading causes of death among young children in developing countries. In clinical practice, the spectrum of differential diagnoses of morbilliform exanthemas associated with fever is wide, and it can be hard to differentiate from other infectious eruptions, especially in adults or in atypical courses in immunocompromised patients. The goal of our study was to identify characteristic histomorphological and immunohistochemical patterns of measles exanthema through the study of 13 skin biopsy specimens obtained from 13 patients with this disease and a review of cases in the literature. Histopathological features of measles exanthema are quite distinctive and characterized by a combination of multinucleated keratinocytes, and individual and clustered necrotic keratinocytes in the epidermis with pronounced folliculosebaceous as well as acrosyringeal involvement. Immunohistochemical staining of skin biopsies with anti-measles virus (MeV) nucleoprotein and anti-MeV phosphoprotein can be of great value in confirming the diagnosis of measles. Both methods can serve as quick additional diagnostic tools for prompt implementation of quarantine measures and for providing medical assistance, even in patients in whom the clinician did not consider measles as a differential diagnosis of the rash due to the rarity of the disease in a putatively vaccinated community.

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.

Saliva enhances infection of gingival fibroblasts by herpes simplex virus 1

Oral herpes is a highly prevalent infection caused by herpes simplex virus 1 (HSV-1). After an initial infection of the oral cavity, HSV-1 remains latent in sensory neurons of the trigeminal ganglia. Episodic reactivation of the virus leads to the formation of mucocutaneous lesions (cold sores), but asymptomatic reactivation accompanied by viral shedding is more frequent and allows virus spread to new hosts. HSV-1 DNA has been detected in many oral tissues. In particular, HSV-1 can be found in periodontal lesions and several studies associated its presence with more severe periodontitis pathologies. Since gingival fibroblasts may become exposed to salivary components in periodontitis lesions, we analyzed the effect of saliva on HSV-1 and -2 infection of these cells. We observed that human gingival fibroblasts can be infected by HSV-1. However, pre-treatment of these cells with saliva extracts from some but not all individuals led to an increased susceptibility to infection. Furthermore, the active saliva could expand HSV-1 tropism to cells that are normally resistant to infection due to the absence of HSV entry receptors. The active factor in saliva was partially purified and comprised high molecular weight complexes of glycoproteins that included secretory Immunoglobulin A. Interestingly, we observed a broad variation in the activity of saliva between donors suggesting that this activity is selectively present in the population. The active saliva factor, has not been isolated, but may lead to the identification of a relevant biomarker for susceptibility to oral herpes. The presence of a salivary factor that enhances HSV-1 infection may influence the risk of oral herpes and/or the severity of associated oral pathologies.

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.

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.

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.

Mechanical Barriers Restrict Invasion of Herpes Simplex Virus 1 into Human Oral Mucosa

Oral mucosa is one of the main target tissues of the human pathogen herpes simplex virus 1 (HSV-1). How the virus overcomes the protective epithelial barriers and penetrates the tissue to reach its receptors and initiate infection is still unclear. Here, we established an ex vivo infection assay with human oral mucosa that allows viral entry studies in a natural target tissue. The focus was on the susceptibility of keratinocytes in the epithelium and the characterization of cellular receptors that mediate viral entry. Upon ex vivo infection of gingiva or vestibular mucosa, we observed that intact human mucosa samples were protected from viral invasion. In contrast, the basal layer of the oral epithelium was efficiently invaded once the connective tissue and the basement membrane were removed. Later during infection, HSV-1 spread from basal keratinocytes to upper layers, demonstrating the susceptibility of the stratified squamous epithelium to HSV-1. The analysis of potential receptors revealed nectin-1 on most mucosal keratinocytes, whereas herpesvirus entry mediator (HVEM) was found only on a subpopulation of cells, suggesting that nectin-1 acts as primary receptor for HSV-1 in human oral mucosa. To mimic the supposed entry route of HSV-1 via microlesions in vivo, we mechanically wounded the mucosa prior to infection. While we observed a limited number of infected keratinocytes in some wounded mucosa samples, other samples showed no infected cells. Thus, we conclude that mechanical wounding of mucosa is insufficient for the virus to efficiently overcome epithelial barriers and to make entry-mediating receptors accessible.IMPORTANCE To invade the target tissue of its human host during primary infection, herpes simplex virus (HSV) must overcome the epithelial barriers of mucosa, skin, or cornea. For most viruses, the mechanisms underlying the invasion into the target tissues of their host organism are still open. Here, we established an ex vivo infection model of human oral mucosa to explore how HSV can enter its target tissue. Our results demonstrate that intact mucosa samples and even compromised tissue allow only very limited access of HSV to keratinocytes. Detailed understanding of barrier functions is an essential precondition to unravel how HSV bypasses the barriers and approaches its receptors in tissue and why it is beneficial for the virus to use a cell-cell adhesion molecule, such as nectin-1, as a receptor.

Scaffolding proteins in the development and maintenance of the epidermal permeability barrier

The skin of mammals and other terrestrial vertebrates protects the organism against the external environment, preventing heat, water and electrolyte loss, as well as entry of chemicals and pathogens. Impairments in the epidermal permeability barrier function are associated with the genesis and/or progression of a variety of pathological conditions, including genetic inflammatory diseases, microbial and viral infections, and photodamage induced by UV radiation. In mammals, the outside-in epidermal permeability barrier is provided by the joint action of the outermost cornified layer, together with assembled tight junctions in granular keratinocytes found in the layers underneath. Tight junctions serve as both outside-in and inside-out barriers, and impede paracellular movements of ions, water, macromolecules and microorganisms. At the molecular level, tight junctions consist of integral membrane proteins that form an extracellular seal between adjacent cells, and associate with cytoplasmic scaffold proteins that serve as links with the actin cytoskeleton. In this review, we address the roles that scaffold proteins play specifically in the establishment and maintenance of the epidermal permeability barrier, and how various pathologies alter or impair their functions.

Petri Net computational modelling of Langerhans cell Interferon Regulatory Factor Network predicts their role in T cell activation

Langerhans cells (LCs) are able to orchestrate adaptive immune responses in the skin by interpreting the microenvironmental context in which they encounter foreign substances, but the regulatory basis for this has not been established. Utilising systems immunology approaches combining in silico modelling of a reconstructed gene regulatory network (GRN) with in vitro validation of the predictions, we sought to determine the mechanisms of regulation of immune responses in human primary LCs. The key role of Interferon regulatory factors (IRFs) as controllers of the human Langerhans cell response to epidermal cytokines was revealed by whole transcriptome analysis. Applying Boolean logic we assembled a Petri net-based model of the IRF-GRN which provides molecular pathway predictions for the induction of different transcriptional programmes in LCs. In silico simulations performed after model parameterisation with transcription factor expression values predicted that human LC activation of antigen-specific CD8 T cells would be differentially regulated by epidermal cytokine induction of specific IRF-controlled pathways. This was confirmed by in vitro measurement of IFN-γ production by activated T cells. As a proof of concept, this approach shows that stochastic modelling of a specific immune networks renders transcriptome data valuable for the prediction of functional outcomes of immune responses.

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.

Herpes simplex virus glycoprotein D relocates nectin-1 from intercellular contacts

Herpes simplex virus (HSV) uses the cell adhesion molecule nectin-1 as a receptor to enter neurons and epithelial cells. The viral glycoprotein D (gD) is used as a non-canonical ligand for nectin-1. The gD binding site on nectin-1 overlaps with a functional adhesive site involved in nectin-nectin homophilic trans-interaction. Consequently, when nectin-1 is engaged with a cellular ligand at cell junctions, the gD binding site is occupied. Here we report that HSV gD is able to disrupt intercellular homophilic trans-interaction of nectin-1 and induce a rapid redistribution of nectin-1 from cell junctions. This movement does not require the receptor's interaction with the actin-binding adaptor afadin. Interaction of nectin-1 with afadin is also dispensable for virion surfing along nectin-1-rich filopodia. Cells seeded on gD-coated surfaces also fail to accumulate nectin-1 at cell contact. These data indicate that HSV gD affects nectin-1 locally through direct interaction and more globally through signaling.

Herpes simplex virus type-1: replication, latency, reactivation and its antiviral targets

Infection by herpes simplex virus type-1 (HSV-1) causes several diseases, ranging from cutaneous, oral and genital infections to fatal encephalitis. Despite the availability of antiviral therapies on the market, their efficacies are incomplete, and new cases of resistant strains arise, mainly in the immunocompromised, but also recently documented in immunocompetent patients. Over the last decades a lot has been discovered about the molecular basis of infection which has been of great benefit to the investigation of new anti-HSV-1 molecules. In this review we summarize replication, latency and reactivation highlighting potential antiviral targets and new molecules described in the past several years in the literature.

Ex Vivo Infection of Murine Epidermis with Herpes Simplex Virus Type 1

To enter its human host, herpes simplex virus type 1 (HSV-1) must overcome the barrier of mucosal surfaces, skin, or cornea. HSV-1 targets keratinocytes during initial entry and establishes a primary infection in the epithelium, which is followed by latent infection of neurons. After reactivation, viruses can become evident at mucocutaneous sites that appear as skin vesicles or mucosal ulcers. How HSV-1 invades skin or mucosa and reaches its receptors is poorly understood. To investigate the invasion route of HSV-1 into epidermal tissue at the cellular level, we established an ex vivo infection model of murine epidermis, which represents the site of primary and recurrent infection in skin. The assay includes the preparation of murine skin. The epidermis is separated from the dermis by dispase II treatment. After floating the epidermal sheets on virus-containing medium, the tissue is fixed and infection can be visualized at various times postinfection by staining infected cells with an antibody against the HSV-1 immediate early protein ICP0. ICP0-expressing cells can be observed in the basal keratinocyte layer already at 1.5 hr postinfection. With longer infection times, infected cells are detected in suprabasal layers, indicating that infection is not restricted to the basal keratinocytes, but the virus spreads to other layers in the tissue. Using epidermal sheets of various mouse models, the infection protocol allows determining the involvement of cellular components that contribute to HSV-1 invasion into tissue. In addition, the assay is suitable to test inhibitors in tissue that interfere with the initial entry steps, cell-to-cell spread and virus production. Here, we describe the ex vivo infection protocol in detail and present our results using nectin-1- or HVEM-deficient mice.

Invasion of Herpes Simplex Virus Type 1 into Murine Epidermis: An Ex Vivo Infection Study

Herpes simplex virus type 1 (HSV-1) invades its human host via the skin or mucosa. We aim to understand how HSV-1 overcomes the barrier function of the host epithelia, and for this reason, we established an ex vivo infection assay initially with murine skin samples. Here, we report how tissue has to be prepared to be susceptible to HSV-1 infection. Most efficient infection of the epidermis was achieved by removing the dermis. HSV-1 initially invaded the basal epidermal layer, and from there, spreading to the suprabasal layers was observed. Strikingly, in resting stage hair follicles, only the hair germ was infected, whereas the quiescent bulge stem cells (SCs) were resistant to infection. However, during the growth phase, infected cells were also detected in the activated bulge SCs. We demonstrated that cell proliferation was not a precondition for HSV-1 invasion, but SC activation was required as shown by infection of aberrantly activated bulge SCs in integrin-linked kinase (ILK)-deficient hair follicles. These results suggest that the status of the bulge SCs determines whether HSV-1 can reach its receptors, whereas the receptors on basal keratinocytes are accessible irrespective of their proliferation status.

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

The cellular proteins nectin-1 and herpesvirus entry mediator (HVEM) can both mediate the entry of herpes simplex virus 1 (HSV-1). We have recently shown how these receptors contribute to infection of skin by investigating HSV-1 entry into murine epidermis. Ex vivo infection studies reveal nectin-1 as the primary receptor in epidermis, whereas HVEM has a more limited role. Although the epidermis represents the outermost layer of skin, the contribution of nectin-1 and HVEM in the underlying dermis is still open. Here, we analyzed the role of each receptor during HSV-1 entry in murine dermal fibroblasts that were deficient in expression of either nectin-1 or HVEM or both receptors. Because infection was not prevented by the absence of either nectin-1 or HVEM, we conclude that they can act as alternative receptors. Although HVEM was found to be highly expressed on fibroblasts, entry was delayed in nectin-1-deficient cells, suggesting that nectin-1 acts as the more efficient receptor. In the absence of both receptors, entry was strongly delayed leading to a much reduced viral spread and virus production. These results suggest an unidentified cellular component that acts as alternate but inefficient receptor for HSV-1 on dermal fibroblasts. Characterization of the cellular entry mechanism suggests that HSV-1 can enter dermal fibroblasts both by direct fusion with the plasma membrane and via endocytic vesicles and that this is not dependent on the presence or absence of nectin-1. Entry was also shown to require dynamin and cholesterol, suggesting comparable entry pathways in keratinocytes and dermal fibroblasts.

IMPORTANCE

Herpes simplex virus (HSV) is a human pathogen which infects its host via mucosal surfaces or abraded skin. To understand how HSV-1 overcomes the protective barrier of mucosa or skin and reaches its receptors in tissue, it is essential to know which receptors contribute to the entry into individual skin cells. Previously, we have explored the contribution of nectin-1 and herpesvirus entry mediator (HVEM) as receptors for HSV-1 entry into murine epidermis, where keratinocytes form the major cell type. Since the underlying dermis consists primarily of fibroblasts, we have now extended our study of HSV-1 entry to dermal fibroblasts isolated from nectin-1- or HVEM-deficient mice or from mice deficient in both receptors. Our results demonstrate a role for both nectin-1 and HVEM as receptors and suggest a further receptor which appears much less efficient.