Herpes simplex virus type 1 infection has two separate modes of spread in three-dimensional keratinocyte culture

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 Novel In Vitro Culture Model System to Study Merkel Cell Polyomavirus-Associated MCC Using Three-Dimensional Organotypic Raft Equivalents of Human Skin

Merkel cell polyomavirus (MCPyV) is a human polyomavirus causally linked to the development of Merkel cell carcinoma (MCC), an aggressive malignancy that largely arises within the dermis of the skin. In this study, we recapitulate the histopathology of human MCC tumors in vitro using an organotypic (raft) culture system that is traditionally used to recapitulate the dermal and epidermal equivalents of skin in three dimensions (3D). In the optimal culture condition, MCPyV+ MCC cells were embedded in collagen between the epidermal equivalent comprising human keratinocytes and a dermal equivalent containing fibroblasts, resulting in MCC-like lesions arising within the dermal equivalent. The presence and organization of MCC cells within these dermal lesions were characterized through biomarker analyses. Interestingly, co-culture of MCPyV+ MCC together with keratinocytes specifically within the epidermal equivalent of the raft did not reproduce human MCC morphology, nor were any keratinocytes necessary for MCC-like lesions to develop in the dermal equivalent. This 3D tissue culture system provides a novel in vitro platform for studying the role of MCPyV T antigens in MCC oncogenesis, identifying additional factors involved in this process, and for screening potential MCPyV+ MCC therapeutic strategies.

Quercetin and 3-O-methylquercetin in vitro skin layers permeation/retention from hydrogels: why only a methoxy group difference determines different behaviors?

OBJECTIVES

The present study was designed to verify if quercetin (QCT), a flavonoid with antioxidant and antiviral activity, and 3-O-methylquercetin (3OMQ), a quercetin C3-methoxylated derivative, present differences in their behavior against complexation with β-cyclodextrin (β-CD) and the corresponding permeation/retention trhough porcine ear skin, when incorporated into hydroxypropyl methylcellulose (HPMC) or chitosan (CS) hydrogels.

METHODS

The influence of β-CD on the skin permeation/retention of QCT and 3OMQ from hydrogels is comparatively evaluated for both flavonoids using porcine ear skin in Franz cells model. The properties of the two flavonoids using the semi-empirical method Recife Model was studied.

KEY FINDINGS

Quercetin presented higher skin retention compared with its C3-methoxy derivative 3OMQ. The best permeation/retention of QCT was observed when it was incorporated into CS hydrogel containing 5% β-CD, whereas, for 3OMQ, the HPMC hydrogel containing 5% β-CD was the best formulation. The flavonoids complexation with β-CD in water occurred preferentially with the insertion of the B ring through the secondary OH rim.

CONCLUSIONS

The dynamic molecular modeling revealed that the methyl group at C3 in 3OMQ molecule determined significant difference in its complexation with β-CD, in comparison to its analogous QCT and that difference is coincident with the permeation behavior of these flavonoids, denoting a possible relationship with their molecular dynamics.

Abortigenic but Not Neurotropic Equine Herpes Virus 1 Modulates the Interferon Antiviral Defense

Equine herpesvirus 1 (EHV1) is considered as a major pathogen of Equidae, causing symptoms from mild respiratory disease to late-term abortion and neurological disorders. Different EHV1 strains circulating in the field have been characterized to be of abortigenic or neurovirulent phenotype. Both variants replicate in a plaque-wise manner in the epithelium of the upper respiratory tract (URT), where the abortigenic strains induce more prominent viral plaques, compared to the neurovirulent strains. Considering the differences in replication at the URT, we hypothesized that abortigenic strains may show an increased ability to modulate the type I IFN secretion/signaling pathway, compared to strains that display the neurovirulent phenotype. Here, we analyze IFN levels induced by abortigenic and neurovirulent EHV1 using primary respiratory epithelial cells (EREC) and respiratory mucosa ex vivo explants. Similar levels of IFNα (~70 U/ml) were detected in explants inoculated with both types of EHV1 strains from 48 to 72 hpi. Second, EREC and mucosa explants were treated with recombinant equine IFNα (rEqIFNα) or Ruxolitinib (Rux), an IFN signaling inhibitor, prior to and during inoculation with abortigenic or neurovirulent EHV1. Replication of both EHV1 variants was suppressed by rEqIFNα. Further, addition of Rux increased replication in a concentration-dependent manner, indicating an IFN-susceptibility for both variants. However, in two out of three horses, at a physiological concentration of 100 U/ml of rEqIFNα, an increase in abortigenic EHV1 replication was observed compared to 10 U/ml of rEqIFNα, which was not observed for the neurovirulent strains. Moreover, in the presence of Rux, the plaque size of the abortigenic variants remained unaltered, whereas the typically smaller viral plaques induced by the neurovirulent variants became larger. Overall, our results demonstrate the importance of IFNα in the control of EHV1 replication in the URT for both abortigenic and neurovirulent variants. In addition, our findings support the speculation that abortigenic variants of EHV1 may have developed anti-IFN mechanisms that appear to be absent or less pronounced in neurovirulent EHV1 strains.

In Vitro Replication of Chelonid Herpesvirus 5 in Organotypic Skin Cultures from Hawaiian Green Turtles (Chelonia mydas)

Fibropapillomatosis (FP) is a tumor disease of marine turtles associated with chelonid herpesvirus 5 (ChHV5), which has historically been refractory to growth in tissue culture. Here we show, for the first time, de novo formation of ChHV5-positive intranuclear inclusions in cultured green turtle cells, which is indicative of active lytic replication of the virus. The minimal requirements to achieve lytic replication in cultured cells included (i) either in vitro cultures of ChHV5-positive tumor biopsy specimens (plugs) or organotypic cultures (rafts) consisting of ChHV5-positive turtle fibroblasts in collagen rafts seeded with turtle keratinocytes and (ii) keratinocyte maturation induced by raising raft or biopsy cultures to the air-liquid interface. Virus growth was confirmed by detailed electron microscopic studies that revealed intranuclear sun-shaped capsid factories, tubules, various stages of capsid formation, nuclear export by budding into the perinuclear space, tegument formation, and envelopment to complete de novo virus production. Membrane synthesis was also observed as a sign of active viral replication. Interestingly, cytoplasmic particles became associated with keratin filaments, a feature not seen in conventional monolayer cell cultures, in which most studies of herpesvirus replication have been performed. Our findings draw a rich and realistic picture of ChHV5 replication in cells derived from its natural host and may be crucial not only to better understand ChHV5 circulation but also to eventually complete Koch's postulates for FP. Moreover, the principles described here may serve as a model for culture of other viruses that are resistant to replication in conventional cell culture.IMPORTANCE A major challenge in virology is the study of viruses that cannot be grown in the laboratory. One example is chelonid herpesvirus 5 (ChHV5), which is associated with fibropapillomatosis, a globally distributed, debilitating, and fatal tumor disease of endangered marine turtles. Pathological examination shows that ChHV5 is shed in skin. Here we show that ChHV5 will grow in vitro if we replicate the complex three-dimensional structure of turtle skin. Moreover, lytic virus growth requires a close interplay between fibroblasts and keratinocytes. Finally, the morphogenesis of herpesviral growth in three-dimensional cultures reveals a far richer, and likely more realistic, array of capsid morphologies than that encountered in traditional monolayer cell cultures. Our findings have applications to other viruses, including those of humans.

Three-dimensional cell culture models for investigating human viruses

Three-dimensional (3D) culture models are physiologically relevant, as they provide reproducible results, experimental flexibility and can be adapted for high-throughput experiments. Moreover, these models bridge the gap between traditional two-dimensional (2D) monolayer cultures and animal models. 3D culture systems have significantly advanced basic cell science and tissue engineering, especially in the fields of cell biology and physiology, stem cell research, regenerative medicine, cancer research, drug discovery, and gene and protein expression studies. In addition, 3D models can provide unique insight into bacteriology, virology, parasitology and host-pathogen interactions. This review summarizes and analyzes recent progress in human virological research with 3D cell culture models. We discuss viral growth, replication, proliferation, infection, virus-host interactions and antiviral drugs in 3D culture models.

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.

Antiherpes Activity and Skin/Mucosa Distribution of Flavonoids from Achyrocline satureioides Extract Incorporated into Topical Nanoemulsions

This study investigated the inhibitory effects of Achyrocline satureioides extract (ASE) incorporated into a topical nanoemulsion on Herpes Simplex Virus type 1 (HSV-1/KOS strain) replication, as well as the distribution of the main ASE flavonoids (quercetin, luteolin, and 3-O-methylquercetin) in porcine skin and mucosa. The ASE-loaded nanoemulsion showed more pronounced effects against HSV-1 replication when compared to the ASE or pure quercetin, as determined by the viral plaque number reduction assay. All flavonoids were detected in the skin epidermis (2.2 µg/cm(2)) and the mucosa upper layers (3.0 µg/cm(2)) from ASE-loaded nanoemulsion until 8 h after topical application. A higher amount of flavonoids was detected when these tissues were impaired, especially in deeper mucosa layers (up to 7-fold). Flavonoids were detected in the receptor fluid only when the mucosa was injured. Such results were supported by confocal microscopy images. Overall, these findings suggest that the tested ASE-loaded nanoemulsion has potential to be used topically for herpes infections.

Efficient replication of Epstein-Barr virus in stratified epithelium in vitro

Epstein-Barr virus is a ubiquitous human herpesvirus associated with epithelial and lymphoid tumors. EBV is transmitted between human hosts in saliva and must cross the oral mucosal epithelium before infecting B lymphocytes, where it establishes a life-long infection. The latter process is well understood because it can be studied in vitro, but our knowledge of infection of epithelial cells has been limited by the inability to infect epithelial cells readily in vitro or to generate cell lines from EBV-infected epithelial tumors. Because epithelium exists as a stratified tissue in vivo, organotypic cultures may serve as a better model of EBV in epithelium than monolayer cultures. Here, we demonstrate that EBV is able to infect organotypic cultures of epithelial cells to establish a predominantly productive infection in the suprabasal layers of stratified epithelium, similar to that seen with Kaposi's-associated herpesvirus. These cells did express latency-associated proteins in addition to productive-cycle proteins, but a population of cells that exclusively expressed latency-associated viral proteins could not be detected; however, an inability to infect the basal layer would be unlike other herpesviruses examined in organotypic cultures. Furthermore, infection did not induce cellular proliferation, as it does in B cells, but instead resulted in cytopathic effects more commonly associated with productive viral replication. These data suggest that infection of epithelial cells is an integral part of viral spread, which typically does not result in the immortalization or enhanced growth of infected epithelial cells but rather in efficient production of virus.

Factorial design applied to the optimization of lipid composition of topical antiherpetic nanoemulsions containing isoflavone genistein

The aim of this study was to optimize topical nanoemulsions containing genistein, by means of a 23 full factorial design based on physicochemical properties and skin retention. The experimental arrangement was constructed using oil type (isopropyl myristate or castor oil), phospholipid type (distearoylphosphatidylcholine [DSPC] or dioleylphosphaditylcholine [DOPC]), and ionic cosurfactant type (oleic acid or oleylamine) as independent variables. The analysis of variance showed effect of third order for particle size, polydispersity index, and skin retention of genistein. Nanoemulsions composed of isopropyl myristate/DOPC/oleylamine showed the smallest diameter and highest genistein amount in porcine ear skin whereas the formulation composed of isopropyl myristate/DSPC/oleylamine exhibited the lowest polydispersity index. Thus, these two formulations were selected for further studies. The formulations presented positive ζ potential values (>25 mV) and genistein content close to 100% (at 1 mg/mL). The incorporation of genistein in nanoemulsions significantly increased the retention of this isoflavone in epidermis and dermis, especially when the formulation composed by isopropyl myristate/DOPC/oleylamine was used. These results were supported by confocal images. Such formulations exhibited antiherpetic activity in vitro against herpes simplex virus 1 (strain KOS) and herpes simplex virus 22 (strain 333). Taken together, the results show that the genistein-loaded nanoemulsions developed in this study are promising options in herpes treatment.

An In Vitro HSV-1 Reactivation Model Containing Quiescently Infected PC12 Cells

Advances in the understanding of the infection and reactivation process of herpes simplex type 1 (HSV-1) are generally gained by monolayer cultures or extensive and cost-intensive animal models. So far, no reliable in vitro skin model exists either to investigate the molecular mechanisms involved in controlling latency and virus reactivation or to test pharmaceuticals. Here we demonstrate the first in vitro HSV-1 reactivation model generated by using the human keratinocyte cell line HaCaT grown on a collagen substrate containing primary human fibroblasts. We integrated the unique feature of a quiescently infected neuronal cell line, the rat pheochromocytoma line PC12, within the dermal layer of the three-dimensional skin equivalent. Transmission electron microscopy, a cell-based TCID₅₀ assay, and polymerase chain reaction analysis were used to verify cell latency. Thereby viral DNA could be detected, whereas extracellular as well as intracellular virus activity could not be found. Further, the infected PC12 cells show no spontaneous reactivation within the in vitro skin equivalent. In order to simulate a physiologically comparable HSV-1 infection, we achieved a specific and pointed reactivation of quiescently HSV-1 infected PC12 cells by UVB irradiation at 1000 mJ/cm².

3D-tissue model for herpes simplex virus-1 infections

Infection with herpes simplex virus type 1 (HSV-1) causes the most common skin disease. Various test systems have been established to recapitulate this cyclical pathway of productive infection, latency, and reactivation. Most studies of latency and reactivation are conducted in animal models. However, the small number of neurons which harbor the viral genome, the complexity of the in vivo setting, and ethical constraints place limits on animal studies. So far, no in vitro model which resembles natural latency exists. Here, we describe the first in vitro HSV-1 infection model based on a human skin equivalent. The 3D infection model is generated using the human keratinocyte cell line HaCaT grown on a collagen substrate containing human primary fibroblasts and in addition a quiescently HSV-1 infected neuronal component.

Skin tissue engineering--in vivo and in vitro applications

Significant progress has been made over the years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin or for the establishment of human-based in vitro skin models. This review summarizes these advances in in vivo and in vitro applications of tissue-engineered skin. We further highlight novel efforts in the design of complex disease-in-a-dish models for studies ranging from disease etiology to drug development and screening.

Skin tissue engineering--in vivo and in vitro applications

Significant progress has been made over the years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin or for the establishment of human-based in vitro skin models. This review summarizes these advances in in vivo and in vitro applications of tissue-engineered skin. We further highlight novel efforts in the design of complex disease-in-a-dish models for studies ranging from disease etiology to drug development and screening.

Host responses to herpes simplex virus and herpes simplex virus vectors

Herpes simplex virus (HSV) is a well-known, ubiquitous pathogen of humans. Engineered mutants of HSV can also be exploited as vectors in gene therapy or for virotherapy of tumors. HSV has multiple abilities to evade and modulate the innate and adaptive responses of the host. The increasing knowledge on the mutual interactions of the invading HSV with the host defenses will contribute to our deeper understanding of the relationship between HSV and the host, and thereby lead to future development of more effective and specific HSV vectors for treatment of human diseases. The future advances of HSV vaccines and vaccine vectors are based on the knowlegde of the complex interplay between HSV and the host defenses.

Epithelial raft cultures for investigations of virus growth, pathogenesis and efficacy of antiviral agents

The organotypic epithelial raft cultures, originally developed to study keratinocytes differentiation, represent a novel approach to the study of viruses able to infect epithelial cells. Organotypic epithelial raft cultures accurately reproduce the process of epithelial differentiation in vitro and can be prepared from normal keratinocytes, explanted epithelial tissue, or established cell lines. This culture system permits cells to proliferate and fully differentiate at the air-liquid interface on a dermal-equivalent support. Normal primary human keratinocytes (PHKs) stratify and fully differentiate in a manner similar to the normal squamous epithelial tissues, while transformed cell lines exhibit dysplastic morphologies similar to the (pre)neoplastic lesions seen in vivo. This three-dimensional (3D) culture system provides an essential tool for investigations of virus growth, virus-host cell interactions, for the genetic analysis of viral proteins and regulatory sequences, and for the evaluation of antiviral agents. The 3D epithelial cultures have proven a breakthrough in the research on papillomaviruses, since their life cycle is strictly linked to the differentiation of the host epithelium. In the last years, several reports have shown the usefulness of the 3D epithelial cultures for the study of other viruses that target at least during a part of their life cycles epithelial cells. The 3D epithelial cultures allow the analysis of virus-host cell interactions in stratified epithelia that more closely resemble the in vivo situation. In this review we describe the advances on research on 3D epithelial cultures for the study of virus growth and pathogenesis of different families of viruses, including papilloma-, herpes-, pox-, adeno-, and parvoviruses.

Three-dimensional culture models for human viral diseases and antiviral drug development

Researchers are recognizing the limitations of two-dimensional (2D) cell cultures, given the fact that they do not reproduce the morphology and biochemical features that the cells possess in the original tissue. As an alternative, the three-dimensional (3D) cell culture approach offers researchers the possibility to study cell growth and differentiation under conditions that more closely resemble the in vivo situation with regard to cell shape and cellular environment. Currently, 3D culture models are being employed in many areas of biomedical research because they offer a more realistic milieu than 2D cultures. The era of 2D culture techniques is moving towards a new epoch of culture systems in 3D. The present review is focused on topics of research on 3D cell cultures in virology and their use in antiviral drug development.

Organotypic epithelial raft cultures as a model for evaluating compounds against alphaherpesviruses

The course of herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) and varicella-zoster virus (VZV) infections in squamous epithelial cells cultured in a three-dimensional organotypic raft culture was tested. In these raft cultures, normal human keratinocytes isolated from neonatal foreskins grown at the air-liquid interface stratified and differentiated, reproducing a fully differentiated epithelium. Typical cytopathic changes identical to those found in the squamous epithelium in vivo, including ballooning and reticular degeneration with the formation of multinucleate cells, were observed throughout the raft following infection with HSV and VZV at different times after lifting the cultures to the air-liquid interface. For VZV, the aspects of the lesions depended on the stage of differentiation of the organotypic cultures. The activity of reference antiviral agents, acyclovir (ACV), penciclovir (PCV), brivudin (BVDU), foscarnet (PFA), and cidofovir (CDV), was evaluated against wild-type and thymidine kinase (TK) mutants of HSV and VZV in the raft cultures. ACV, PCV, and BVDU protected the epithelium against cytopathic effect induced by wild-type viruses in a concentration-dependent manner, while treatment with CDV and PFA proved protective against the cytodestructive effects induced by both TK+ and TK- strains. The quantification of the antiviral effects in the rafts were accomplished by measuring viral titers by plaque assay for HSV and by measuring viral DNA load by real-time PCR for VZV. A correlation between the degree of protection as determined by histological examination and viral quantification could be demonstrated The three-dimensional epithelial raft culture represents a novel model for the study of antiviral agents active against HSV and VZV. Since no animal model is available for the evaluation of antiviral agents against VZV, the organotypic cultures may be considered a model to evaluate the efficacy of new anti-VZV antivirals before clinical trials.

Replication of varicella-zoster virus in human skin organ culture

Varicella-zoster virus (VZV) infection is restricted to humans, which hinders studies of its pathogenesis in rodent models of disease. To facilitate the study of VZV skin tropism, we developed an ex vivo system using human fetal skin organ culture (SOC). VZV replication was analyzed by plaque assay, transmission electron microscopy, and histology. The yield of infectious VZV from SOC increased approximately 100-fold over 6 days, virions were abundant, and lesions developed that contained VZV antigens and resembled varicella and zoster lesions. The SOC system for VZV replication has applications for testing virus mutants and antiviral drugs.

Herpes simplex virus type 1 enters human epidermal keratinocytes, but not neurons, via a pH-dependent endocytic pathway

Herpes simplex virus (HSV) enters some laboratory cell lines via a pH-dependent, endocytic mechanism. We investigated whether this entry pathway is used in human cell types relevant to pathogenesis. Three different classes of lysosomotropic agents, which raise endosomal pH, blocked HSV entry into primary and transformed human keratinocytes, but not into human neurons or neuroblastoma lines. In keratinocytes, incoming HSV particles colocalized with markers of endocytic uptake. Treatment with the isoflavone genistein, an inhibitor of protein tyrosine kinases, reduced the delivery of incoming viral particles to the nuclear periphery and virus-induced gene expression in keratinocytes but not neurons. Moreover, in keratinocyte monolayer islets, HSV infected both the inner and outer cells in a genistein-sensitive manner, suggesting viral endocytosis from both basolateral and apical plasma membrane surfaces. Together, the results indicate that HSV enters human epidermal keratinocytes, but not neurons, by a low-pH, endocytic pathway that is dependent on host tyrosine phosphorylation. Thus, HSV utilizes fundamentally different cellular entry pathways to infect important target cell populations.

Factors That Limit the Effectiveness of Herpes Simplex Virus Type 1 for Treatment of Oral Cancer in Mice

Although the growth of experimental oral cancers can be inhibited by infection with the herpes simplex virus type 1 (HSV-1), the effect is incomplete. To define factors that might limit the effectiveness of the virus, we examined the roles of the innate immune system and the replication status of the tumor cells. AT-84 tumors were induced in strains of mice that had specific immune defects and were treated with the virus. Explanted tumors and tumor cells in culture were also infected. No differences in viral replication or in the effect of virus on the tumor were seen between mice with a lack of T or B lymphocytes, natural killer cells, phagocytic spleen cells, or complement. The virus did not replicate significantly more in tumors that were maintained as explants. Immediately after recovery of cells from a tumor the proportion of cells in the S phase was around 18%, and replication of virus in those cells was very limited. After 3 weeks in culture, the proportion in S had increased to 50% and both the recovery of virus from the cells and the toxic effect of the virus on the cells had increased significantly. The innate immune system thus seemed to have a minimal effect on replication of HSV-1 when used as an oncolytic virus for oral cancers in mice. Instead, the fraction of cells in the S phase was important. Because human oral cancers, like mouse tumors, have a low fraction of cells in the S phase, it is likely that the in vivo use of HSV-1 as cancer therapy will be limited by the replication of the virus.

Herpes simplex virus type 1 exhibits a tropism for basal entry in polarized epithelial cells

Herpes simplex virus type 1 (HSV-1) enters its host via epithelia and spreads to neuronal cells where latency is established. Hence, the in vivo route of infection relies on penetration and subsequent passage of HSV-1 through highly polarized cells. Infection studies were performed in both polarized MDCKII cells and primary human keratinocytes to gain insight into the pathway of virus entry into individual epithelial cells. Early viral gene expression was barely detectable in confluent MDCKII cells, even at high m.o.i. However, after wounding the cell layer, infected cells were observed next to the wound, where basolateral membranes were accessible. In subconfluent monolayers, MDCKII cells are organized in islets. After infection, viral capsids and early viral gene expression were detectable in peripheral cells of islets, supporting virus penetration via basolateral membranes. Further infection studies were performed in human keratinocytes, which represent the primary target cells for HSV-1 infection in vivo. In primary keratinocytes grown as monolayer cultures and wounded prior to infection, HSV-1 infection led to early viral gene expression predominantly in cells next to the wound. When stratifying cultures of primary human keratinocytes were infected, early viral gene expression was localized to peripheral cells of basal keratinocytes. Finally, infection of epithelial tissue such as human foreskin epithelia demonstrated HSV-1 entry exclusively via basal cell layers. Staining of the potential coreceptor nectin-1/HveC revealed no correlation of receptor localization and virus entry sites in keratinocytes. These results provide first evidence for a virus entry mechanism that relies on the accessibility to basal surfaces of epithelial tissue and to basolateral membranes, both in MDCKII and primary keratinocytes.

Antivaccinia activities of acyclic nucleoside phosphonate derivatives in epithelial cells and organotypic cultures

Organotypic "raft" cultures of epithelial cells allow the reconstitution of a skin equivalent that is easily infectible with different viruses with cutaneous tropism. Among these, poxvirus and particularly vaccinia virus (VV) are good candidates for use in antiviral tests, giving histological pictures comparable to those observed in humans infected with smallpox. Therefore, we decided to evaluate a series of phosphonate derivatives for their ability to inhibit VV growth in epithelial cell monolayers, and the most powerful derivatives were tested in the organotypic cultures. The most active compound was 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]adenine [(S)-HPMPA], followed by 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]-2,6-diaminopurine, cyclic (S)-HPMPA, 9-(S)-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine [(S)-HPMPC; cidofovir, Vistide], and cyclic (S)-HPMPC. Cidofovir, which is on the market for the treatment of human cytomegalovirus retinitis in immunocompromised patients, is potentially a good candidate for the treatment of a poxvirus outbreak, in the absence of any vaccination.

Equid herpesvirus 1 is neurotropic in mice, but latency from which infectious virus can be reactivated does not occur

Equid herpesvirus 1 (EHV-1) is the most common cause of virus-induced abortion in horses. After primary infection the virus becomes latent predominantly in the respiratory tract lymph nodes and the genome can also be detected in the peripheral nervous system. The role of mouse as a feasible model for the establishment of latency and reactivation of EHV-1 was investigated. Intracerebral and intranasal infections of 3- and 17-day-old mice were made and virus replication was confirmed by virus isolation and detected by indirect immunofluorescence (IIF) in brain. For reactivation studies, the mice were killed 8 weeks post infection and tissues were collected for cocultivation. In mice from both age groups, infectious virus was not detected by cocultivation. Following attempts to reactivate virus in vivo with corticosteroids, the viral antigen was detected at low levels by IIF and the expression of the gB gene by reverse transcription polymerase chain reaction (RT-PCR) in brain, trigeminal ganglia, olfactory lobe, lung and spleen. Virus was also detected by IIF following incubation of tissue explants in the growth medium containing pokeweed mitogen (PWM). These results show the limitations of the mouse model for investigating EHV-1 latency and highlights the issue of 'ineffective reactivation' of virus.

The spread of herpes simplex virus type 1 from trigeminal neurons to the murine cornea: an immunoelectron microscopy study

An animal model has been developed to clarify the mechanism for spread of herpes simplex virus (HSV) from neuron to epithelial cells in herpetic epithelial keratitis. HSV was introduced into the murine trigeminal ganglion via stereotaxic guided injection. After 2 to 5 days, the animals were euthanized. Ganglia and corneas were prepared for light and electron microscopic immunocytochemistry with antisera to HSV. At 2 days, labeled axons were identified in the stromal layer. At 3 days, we could detect immunoreactive profiles of trigeminal ganglion cell axons that contained many vesicular structures. By 3 and 4 days, the infection had spread to all layers of epithelium, and the center of a region of infected epithelium appeared thinned. At 5 day, fewer basal cells appeared infected, although infection persisted in superficial cells where it had expanded laterally. Mature HSV was found in the extracellular space surrounding wing and squamous cells. Viral antigen was expressed in small pits along the apical surfaces of wing and squamous cells but not at the basal surface of these cells or on basal cells. This polarized expression of viral antigen resulted in the spread of HSV to superficial cells and limited lateral spread to neighboring basal cells. The pathogenesis of HSV infection in these mice may serve as a model of the human recurrent epithelial disease in the progression of focal sites of infection and transfer from basal to superficial cells.