Enterovirus D-68 Infection of Primary Rat Cortical Neurons: Entry, Replication, and Functional Consequences.

Enterovirus D68 (EV-D68) is an emerging pathogen associated with mild to severe respiratory disease. Since 2014, EV-D68 is also linked to acute flaccid myelitis (AFM), causing paralysis and muscle weakness in children. However, it remains unclear whether this is due to an increased pathogenicity of contemporary EV-D68 clades or increased awareness and detection of this virus. Here, we describe an infection model of primary rat cortical neurons to study the entry, replication, and functional consequences of different EV-D68 strains, including historical and contemporary strains. We demonstrate that sialic acids are important (co)receptors for infection of both neurons and respiratory epithelial cells. Using a collection of glycoengineered isogenic HEK293 cell lines, we show that sialic acids on either N-glycans or glycosphingolipids can be used for infection. Additionally, we show that both excitatory glutamatergic and inhibitory GABA-ergic neurons are susceptible and permissive to historical and contemporary EV-D68 strains. EV-D68 infection of neurons leads to the reorganization of the Golgi-endomembranes forming replication organelles, first in the soma and later in the processes. Finally, we demonstrate that the spontaneous neuronal activity of EV-D68-infected neuronal network cultured on microelectrode arrays (MEA) is decreased, independent of the virus strain. Collectively, our findings provide novel insights into neurotropism and -pathology of different EV-D68 strains, and argue that it is unlikely that increased neurotropism is a recently acquired phenotype of a specific genetic lineage. IMPORTANCE Acute flaccid myelitis (AFM) is a serious neurological illness characterized by muscle weakness and paralysis in children. Since 2014, outbreaks of AFM have emerged worldwide, and they appear to be caused by nonpolio enteroviruses, particularly enterovirus-D68 (EV-D68), an unusual enterovirus that is known to mainly cause respiratory disease. It is unknown whether these outbreaks reflect a change of EV-D68 pathogenicity or are due to increased detection and awareness of this virus in recent years. To gain more insight herein, it is crucial to define how historical and circulating EV-D68 strains infect and replicate in neurons and how they affect their physiology. This study compares the entry and replication in neurons and the functional consequences on the neural network upon infection with an old "historical" strain and contemporary "circulating" strains of EV-D68.

Noncanonical Transmission of a Measles Virus Vaccine Strain from Neurons to Astrocytes.

Viruses, including members of the herpes-, entero-, and morbillivirus families, are the most common cause of infectious encephalitis in mammals worldwide. During most instances of acute viral encephalitis, neurons are typically the initial cell type that is infected. However, as replication and spread ensue, other parenchymal cells can become viral targets, especially in chronic infections. Consequently, to ascertain how neurotropic viruses trigger neuropathology, it is crucial to identify which central nervous system (CNS) cell populations are susceptible and permissive throughout the course of infection, and to define how viruses spread between distinct cell types. Using a measles virus (MV) transgenic mouse model that expresses human CD46 (hCD46), the MV vaccine strain receptor, under the control of a neuron-specific enolase promoter (NSE-hCD46+ mice), a novel mode of viral spread between neurons and astrocytes was identified. Although hCD46 is required for initial neuronal infection, it is dispensable for heterotypic spread to astrocytes, which instead depends on glutamate transporters and direct neuron-astrocyte contact. Moreover, in the presence of RNase A, astrocyte infection is reduced, suggesting that nonenveloped ribonucleoproteins (RNP) may cross the neuron-astrocyte synaptic cleft. The characterization of this novel mode of intercellular transport offers insights into the unique interaction of neurons and glia and may reveal therapeutic targets to mitigate the life-threatening consequences of measles encephalitis.IMPORTANCE Viruses are the most important cause of infectious encephalitis in mammals worldwide; several thousand people, primarily the very young and the elderly, are impacted annually, and few therapies are reliably successful once neuroinvasion has occurred. To understand how viruses contribute to neuropathology, and to develop tools to prevent or ameliorate such infections, it is crucial to define if and how viruses disseminate among the different cell populations within the highly complex central nervous system. This study defines a noncanonical mode of viral transmission between neurons and astrocytes within the brain.

The Pathogenesis and Immune Evasive Mechanisms of Equine Herpesvirus Type 1.

Equine herpesvirus type 1 (EHV-1) is an alphaherpesvirus related to pseudorabies virus (PRV) and varicella-zoster virus (VZV). This virus is one of the major pathogens affecting horses worldwide. EHV-1 is responsible for respiratory disorders, abortion, neonatal foal death and equine herpes myeloencephalopathy (EHM). Over the last decade, EHV-1 has received growing attention due to the frequent outbreaks of abortions and/or EHM causing serious economical losses to the horse industry worldwide. To date, there are no effective antiviral drugs and current vaccines do not provide full protection against EHV-1-associated diseases. Therefore, there is an urgent need to gain a better understanding of the pathogenesis of EHV-1 in order to develop effective therapies. The main objective of this review is to provide state-of-the-art information on the pathogenesis of EHV-1. We also highlight recent findings on EHV-1 immune evasive strategies at the level of the upper respiratory tract, blood circulation and endothelium of target organs allowing the virus to disseminate undetected in the host. Finally, we discuss novel approaches for drug development based on our current knowledge of the pathogenesis of EHV-1.

An Alphaherpesvirus Exploits Antimicrobial ß-Defensins To Initiate Respiratory Tract Infection.

ß-Defensins protect the respiratory tract against the myriad of microbial pathogens entering the airways with each breath. However, this potentially hostile environment is known to serve as a portal of entry for herpesviruses. The lack of suitable respiratory model systems has precluded understanding of how herpesvirus virions overcome the abundant mucosal ß-defensins during host invasion. We demonstrate how a central alphaherpesvirus, equine herpesvirus type 1 (EHV1), actually exploits ß-defensins to invade its host and initiate viral spread. The equine ß-defensins (eBDs) eBD1, -2, and -3 were produced and secreted along the upper respiratory tract. Despite the marked antimicrobial action of eBD2 and -3 against many bacterial and viral pathogens, EHV1 virions were resistant to eBDs through the action of the viral glycoprotein M envelope protein. Pretreatment of EHV1 virions with eBD2 and -3 increased the subsequent infection of rabbit kidney (RK13) cells, which was dependent on viral N-linked glycans. eBD2 and -3 also caused the aggregation of EHV1 virions on the cell surface of RK13 cells. Pretreatment of primary equine respiratory epithelial cells (EREC) with eBD1, -2, and -3 resulted in increased EHV1 virion binding to and infection of these cells. EHV1-infected EREC, in turn, showed an increased production of eBD2 and -3 compared to that seen in mock- and influenza virus-infected EREC. In addition, these eBDs attracted leukocytes, which are essential for EHV1 dissemination and which serve as latent infection reservoirs. These novel mechanisms provide new insights into herpesvirus respiratory tract infection and pathogenesis.IMPORTANCE How herpesviruses circumvent mucosal defenses to promote infection of new hosts through the respiratory tract remains unknown due to a lack of host-specific model systems. We used the alphaherpesvirus equine herpesvirus type 1 (EHV1) and equine respiratory tissues to decipher this key event in general alphaherpesvirus pathogenesis. In contrast to several respiratory viruses and bacteria, EHV1 resisted potent antimicrobial equine ß-defensins (eBDs) eBD2 and eBD3 by the action of glycoprotein M. Instead, eBD2 and -3 facilitated EHV1 particle aggregation and infection of rabbit kidney (RK13) cells. In addition, virion binding to and subsequent infection of respiratory epithelial cells were increased upon preincubation of these cells with eBD1, -2, and -3. Infected cells synthesized eBD2 and -3, promoting further host cell invasion by EHV1. Finally, eBD1, -2, and -3 recruited leukocytes, which are well-known EHV1 dissemination and latency vessels. The exploitation of host innate defenses by herpesviruses during the early phase of host colonization indicates that highly specialized strategies have developed during host-pathogen coevolution.

Equine herpesvirus 1 infection orchestrates the expression of chemokines in equine respiratory epithelial cells.

The ancestral equine herpesvirus 1 (EHV1), closely related to human herpes viruses, exploits leukocytes to reach its target organs, accordingly evading the immune surveillance system. Circulating EHV1 strains can be divided into abortigenic/neurovirulent, causing reproductive/neurological disorders. Neurovirulent EHV1 more efficiently recruits monocytic CD172a+ cells to the upper respiratory tract (URT), while abortigenic EHV1 tempers monocyte migration. Whether similar results could be expected for T lymphocytes is not known. Therefore, we questioned whether differences in T cell recruitment could be associated with variations in cell tropism between both EHV1 phenotypes, and which viral proteins might be involved. The expression of CXCL9 and CXCL10 was evaluated in abortigenic/neurovirulent EHV1-inoculated primary respiratory epithelial cells (ERECs). The bioactivity of chemokines was tested with a functional migration assay. Replication of neurovirulent EHV1 in the URT resulted in an enhanced expression/bioactivity of CXCL9 and CXCL10, compared to abortigenic EHV1. Interestingly, deletion of glycoprotein 2 resulted in an increased recruitment of both monocytic CD172a+ cells and T lymphocytes to the corresponding EREC supernatants. Our data reveal a novel function of EHV1-gp2, tempering leukocyte migration to the URT, further indicating a sophisticated virus-mediated orchestration of leukocyte recruitment to the URT.

Deoxynivalenol, but not fumonisin B1, aflatoxin B1 or diesel exhaust particles disrupt integrity of the horse's respiratory epithelium and predispose it for equine herpesvirus type 1 infection.

The horse's respiratory tract daily encounters a plethora of respirable hazards including air pollutants, mycotoxins and airborne pathogens. To date, the precise effect of air pollution and mycotoxins on respiratory epithelial integrity and subsequent pathogen invasion in the horse has not been studied. Here, diesel exhaust particles (DEP) and three major mycotoxins (deoxynivalenol [DON], aflatoxin B1 [AFB1] and fumonisin B1 [FB1]) were applied to the apical surfaces of both ex vivo respiratory mucosal explants and in vitro primary equine respiratory epithelial cells (EREC) cultivated at the air-liquid interface, prior to inoculation with equine herpesvirus type 1 (EHV1). DON, but not AFB1, FB1 and DEP affected epithelial integrity in both ex vivo and in vitro systems, as demonstrated by histological changes in respiratory epithelial morphology and a drop in transepithelial electrical resistance across the EREC monolayer. Further, DON-pretreated explants showed on average 6.5 ± 4.5-fold more EHV1 plaques and produced on average 1 log10 more extracellular virus particles compared to control diluent- and FB1-pretreated respiratory mucosal explants. Similarly, EHV1 infection was greatly enhanced in EREC upon pretreatment with DON. Based on our findings, we propose that inhalation of DON predisposes horses for EHV1 infection by affecting respiratory epithelial integrity.

Beyond Gut Instinct: Metabolic Short-Chain Fatty Acids Moderate the Pathogenesis of Alphaherpesviruses.

Short-chain fatty acids (SCFA), such as sodium butyrate (SB), sodium propionate (SPr), and sodium acetate (SAc), are metabolic end-products of the fermentation of dietary fibers. They are linked with multiple beneficial effects on the general mammalian health, based on the sophisticated interplay with the host immune response. Equine herpesvirus 1 (EHV1) is a major pathogen, which primarily replicates in the respiratory epithelium, and disseminates through the body via a cell-associated viremia in leukocytes, even in the presence of neutralizing antibodies. Infected monocytic CD172a+ cells and T-lymphocytes transmit EHV1 to the endothelium of the endometrium or central nervous system (CNS), causing reproductive or neurological disorders. Here, we questioned whether SCFA have a potential role in shaping the pathogenesis of EHV1 during the primary replication in the URT, during the cell-associated viremia, or at the level of the endothelium of the pregnant uterus and/or CNS. First, we demonstrated the expression of SCFA receptors, FFA2 and FFA3, within the epithelium of the equine respiratory tract, at the cell surface of immune cells, and equine endothelium. Subsequently, EHV1 replication was evaluated in the URT, in the presence or absence of SB, SPr, or SAc. In general, we demonstrated that SCFA do not affect the number of viral plaques or virus titer upon primary viral replication. Only SB and SPr were able to reduce the plaque latitudes. Similarly, pretreatment of monocytic CD172a+ cells and T-lymphocytes with different concentrations of SCFA did not alter the number of infected cells. When endothelial cells were treated with SB, SPr, or SAc, prior to the co-cultivation with EHV1-inoculated mononuclear cells, we observed a reduced number of adherent immune cells to the target endothelium. This was associated with a downregulation of endothelial adhesion molecules ICAM-1 and VCAM-1 in the presence of SCFA, which ultimately lead to a significant reduction of the EHV1 endothelial plaques. These results indicate that physiological concentrations of SCFA may affect the pathogenesis of EHV1, mainly at the target endothelium, in favor of the fitness of the horse. Our findings may have significant implications to develop innovative therapies, to prevent the devastating clinical outcome of EHV1 infections.

Pollens destroy respiratory epithelial cell anchors and drive alphaherpesvirus infection.

Pollens are well-known triggers of respiratory allergies and asthma. The pollen burden in today's ambient air is constantly increasing due to rising climate change and air pollution. How pollens interact with the respiratory mucosa remains largely unknown due to a lack of representative model systems. We here demonstrate how pollen proteases of Kentucky bluegrass, white birch and hazel selectively destroy integrity and anchorage of columnar respiratory epithelial cells, but not of basal cells, in both ex vivo respiratory mucosal explants and in vitro primary equine respiratory epithelial cells (EREC). In turn, this pollen protease-induced damage to respiratory epithelial cell anchorage resulted in increased infection by the host-specific and ancestral alphaherpesvirus equine herpesvirus type 1 (EHV1). Pollen proteases of all three plant species were characterized by zymography and those of white birch were fully identified for the first time as serine proteases of the subtilase family and meiotic prophase aminopeptidase 1 using mass spectrometry-based proteomics. Together, our findings demonstrate that pollen proteases selectively and irreversibly damage integrity and anchorage of columnar respiratory epithelial cells. In turn, alphaherpesviruses benefit from this partial loss-of-barrier function, resulting in increased infection of the respiratory epithelium.

Unravelling the first key steps in equine herpesvirus type 5 (EHV5) pathogenesis using ex vivo and in vitro equine models.

Equine herpesvirus type 5 (EHV5) is a ubiquitous, yet obscure pathogen in the horse population and is commonly associated with fatal equine multinodular pulmonary fibrosis (EMPF). To date, little is known about the precise pathogenesis of EHV5. Here, we evaluated the dynamics of EHV5 infection in representative ex vivo and in vitro equine models, using immunofluorescence staining and virus titration. EHV5 was unable to infect epithelial cells lining the mucosa of nasal and tracheal explants. Similarly, primary equine respiratory epithelial cells (EREC) were not susceptible to EHV5 following inoculation at the apical or basolateral surfaces. Upon direct delivery of EHV5 particles to lung explants, few EHV5-positive cell clusters were observed at 72 hours post-inoculation (hpi). These EHV5-positive cells were identified as cytokeratin-positive alveolar cells. Next, we examined the potential of EHV5 to infect three distinct equine PBMC populations (CD172a+ monocytes, CD3+ T lymphocytes and Ig light chain+ B lymphocytes). Monocytes did not support EHV5 replication. In contrast, up to 10% of inoculated equine T and B lymphocytes synthetized intracellular viral antigens 24 hpi and 72 hpi, respectively. Still, the production of mature virus particles was hampered, as we did not observe an increase in extracellular virus titer. After reaching a peak, the percentage of infected T and B lymphocytes decayed, which was partly due to the onset of apoptosis, but not necrosis. Based on these findings, we propose a model for EHV5 pathogenesis in the horse. Uncovering EHV5 pathogenesis is the corner step to finally contain or even eradicate the virus.

Equine Herpesvirus 1 Bridles T Lymphocytes To Reach Its Target Organs.

Equine herpesvirus 1 (EHV1) replicates in the respiratory epithelium and disseminates through the body via a cell-associated viremia in leukocytes, despite the presence of neutralizing antibodies. "Hijacked" leukocytes, previously identified as monocytic cells and T lymphocytes, transmit EHV1 to endothelial cells of the endometrium or central nervous system, causing reproductive (abortigenic variants) or neurological (neurological variants) disorders. In the present study, we questioned the potential route of EHV1 infection of T lymphocytes and how EHV1 misuses T lymphocytes as a vehicle to reach the endothelium of the target organs in the absence or presence of immune surveillance. Viral replication was evaluated in activated and quiescent primary T lymphocytes, and the results demonstrated increased infection of activated versus quiescent, CD4+ versus CD8+, and blood- versus lymph node-derived T cells. Moreover, primarily infected respiratory epithelial cells and circulating monocytic cells efficiently transferred virions to T lymphocytes in the presence of neutralizing antibodies. Albeit T-lymphocytes express all classes of viral proteins early in infection, the expression of viral glycoproteins on their cell surface was restricted. In addition, the release of viral progeny was hampered, resulting in the accumulation of viral nucleocapsids in the T cell nucleus. During contact of infected T lymphocytes with endothelial cells, a late viral protein(s) orchestrates T cell polarization and synapse formation, followed by anterograde dynein-mediated transport and transfer of viral progeny to the engaged cell. This represents a sophisticated but efficient immune evasion strategy to allow transfer of progeny virus from T lymphocytes to adjacent target cells. These results demonstrate that T lymphocytes are susceptible to EHV1 infection and that cell-cell contact transmits infectious virus to and from T lymphocytes.IMPORTANCE Equine herpesvirus 1 (EHV1) is an ancestral alphaherpesvirus that is related to herpes simplex virus 1 and causes respiratory, reproductive, and neurological disorders in Equidae. EHV1 is indisputably a master at exploiting leukocytes to reach its target organs, accordingly evading the host immunity. However, the role of T lymphocytes in cell-associated viremia remains poorly understood. Here we show that activated T lymphocytes efficiently become infected and support viral replication despite the presence of protective immunity. We demonstrate a restricted expression of viral proteins on the surfaces of infected T cells, which prevents immune recognition. In addition, we indicate a hampered release of progeny, which results in the accumulation of nucleocapsids in the T cell nucleus. Upon engagement with the target endothelium, late viral proteins orchestrate viral synapse formation and viral transfer to the contact cell. Our findings have significant implications for the understanding of EHV1 pathogenesis, which is essential for developing innovative therapies to prevent the devastating clinical symptoms of infection.

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.

Access to a main alphaherpesvirus receptor, located basolaterally in the respiratory epithelium, is masked by intercellular junctions.

The respiratory epithelium of humans and animals is frequently exposed to alphaherpesviruses, originating from either external exposure or reactivation from latency. To date, the polarity of alphaherpesvirus infection in the respiratory epithelium and the role of respiratory epithelial integrity herein has not been studied. Equine herpesvirus type 1 (EHV1), a well-known member of the alphaherpesvirus family, was used to infect equine respiratory mucosal explants and primary equine respiratory epithelial cells (EREC), grown at the air-liquid interface. EHV1 binding to and infection of mucosal explants was greatly enhanced upon destruction of the respiratory epithelium integrity with EGTA or N-acetylcysteine. EHV1 preferentially bound to and entered EREC at basolateral cell surfaces. Restriction of infection via apical inoculation was overcome by disruption of intercellular junctions. Finally, basolateral but not apical EHV1 infection of EREC was dependent on cellular N-linked glycans. Overall, our findings demonstrate that integrity of the respiratory epithelium is crucial in the host's innate defence against primary alphaherpesvirus infections. In addition, by targeting a basolaterally located receptor in the respiratory epithelium, alphaherpesviruses have generated a strategy to efficiently escape from host defence mechanisms during reactivation from latency.

Us3 and Us9 proteins contribute to the stromal invasion of bovine herpesvirus 1 in the respiratory mucosa.

Bovine herpesvirus 1 (BHV-1) infection may lead to conjunctivitis, upper respiratory tract problems, pneumonia, genital disorders and abortion. BHV-1 is able to spread quickly in a plaque-wise manner and invade by breaching the basement membrane (BM) barrier in the respiratory mucosa. BHV-1 Us3, a serine/threonine kinase, induces a dramatic cytoskeletal reorganization and BHV-1 Us9, a tail-anchored membrane protein, is required for axonal transport of viruses in neurons. In this study, we investigated the role of Us3 and Us9 during BHV-1 infection in the respiratory mucosa. First, we constructed and characterized BHV-1 Us3 null, Us9 null and revertant viruses. Then, we analysed the viral replication and plaque size (latitude) in Madin-Darby bovine kidney (MDBK) cells and the respiratory mucosa as well as viral penetration depth underneath the BM of the respiratory mucosa when inoculated with these recombinant viruses. Knockout of Us3 resulted in a 1 log10 reduction in viral titre and plaque size (latitude) in MDBK cells and the trachea mucosa. There were no defects in the cell-to-cell spread observed for BHV-1 Us9 null virus. Both BHV-1 Us3 null and Us9 null viruses showed a significant reduction of plaque penetration underneath the BM; however, penetration was not completely inhibited. In conclusion, the current findings demonstrated that Us3 and Us9 play an important role in the invasion of BHV-1 through the BM of the respiratory mucosa, which shows the way forward for research-based attenuation of viruses in order to make safer and better-performing vaccines.

CCL2 and CCL5 driven attraction of CD172a+ monocytic cells during an equine herpesvirus type 1 (EHV-1) infection in equine nasal mucosa and the impact of two migration inhibitors, rosiglitazone (RSG) and quinacrine (QC).

Equine herpesvirus type 1 (EHV-1) causes respiratory disease, abortion and neurological disorders in horses. Besides epithelial cells, CD172a+ monocytic cells become infected with EHV-1 in the respiratory mucosa and transport the virus from the apical side of the epithelium to the lamina propria en route to the lymph and blood circulation. Whether CD172a+ monocytic cells are specifically recruited to the infection sites in order to pick up virus is unknown. In our study, equine nasal mucosa explants were inoculated with EHV-1 neurological strains 03P37 and 95P105 or the non-neurological strains 97P70 and 94P247 and the migration of monocytic cells was examined by immunofluorescence. Further, the role of monokines CCL2 and CCL5 was determined and the effect of migration inhibitors rosiglitazone (RSG) or quinacrine was analyzed. It was shown that with neurological strains but not with the non-neurological strains, CD172a+ cells specifically migrated towards EHV-1 infected regions and that CCL2 and CCL5 were involved. CCL2 started to be expressed in infected epithelial cells at 24 h post-incubation (hpi) and CCL5 at 48 hpi, which corresponded with the CD172a+ migration. RSG treatment of EHV-1-inoculated equine nasal mucosa had no effect on the virus replication in the epithelium, but decreased the migration of CD172a+ cells in the lamina propria. Overall, these findings bring new insights in the early pathogenesis of EHV-1 infections, illustrate differences between neurological and non-neurological strains and show the way for EHV-1 treatment.

Replication of neurovirulent equine herpesvirus type 1 (EHV-1) in CD172a+ monocytic cells.

Equine herpesvirus type 1 (EHV-1) is responsible for respiratory disorders, abortion and myeloencephalopathy (EHM) in horses. Two pathotypes of EHV-1 strains are circulating in the field: neurovirulent (N) and non-neurovirulent (NN). For both strains, CD172a+ monocytic cells are one of the main carrier cells of EHV-1 during primary infection, allowing the virus to invade the horse's body. Recently, we showed that EHV-1 NN strains showed a restricted and delayed replication in CD172a+ cells. Here we characterize the in vitro replication kinetics of two EHV-1N strains in CD172a+ cells and investigate if the replication of these strains is similarly silenced as shown for EHV-1 NN strains. We found that EHV-1N replication was restricted to 7-8% in CD172a+ cells compared to 100% in control RK-13 cells. EHV-1N replication was not delayed in CD172a+ cells but virus production was significant lower (103.0 TCID50/105 inoculated cells) than in RK-13 cells (108.5 TCID50/105 inoculated cells). Approximately 0.04% of CD172a+ cells produced and transmitted infectious EHV-1 to neighbour cells compared to 65% of RK-13 cells. Unlike what we observed for the NN strain, pretreatment of CD172a+ cells with histone deacetylases inhibitors (HDACi) did not influence the replication of EHV-1N strains in these cells. Overall, these results show that the EHV-1 replication of N strains in CD172a+ cells differs from that observed for NN strains, which may contribute to their different pathogeneses in vivo.

Dual infections of equine herpesvirus 1 and equine arteritis virus in equine respiratory mucosa explants.

Equine herpesvirus 1 (EHV-1) and equine arteritis virus (EAV) induce respiratory problems and abortion in horses and are considered as two serious threats to equine industry. Both EHV-1 and EAV misuse patrolling leukocytes in the upper respiratory tract to breach the basement membrane (BM) and to migrate to blood vessels. So far, the behavior and impact of a double infection in the respiratory mucosa of a horse are unknown. In the present study, the outcome of double infections with EHV-1 and the low virulent EAV strain 08P187 (superinfection with an interval of 12h or co-infection) were compared with single infections in fully susceptible RK-13 cells and equine upper respiratory mucosa explants. When RK-13 cells were inoculated with either EHV-1 or EAV 12h prior to the subsequent EAV or EHV-1 inoculation, the latter EAV or EHV-1 infection was clearly suppressed at 24hpi or 36hpi, respectively, without EHV-1 and EAV co-infecting the same RK-13 cells. After simultaneous infection with EHV-1 and EAV, higher numbers of EAV infected cells but similar numbers of EHV-1 infected cells were found compared to the single infections, with a low number of EHV-1 and EAV co-infected RK-13 cells at 48hpi and 72hpi. In the upper respiratory mucosa exposed to EAV 12h prior to EHV-1, the number and size of the EHV-1-induced plaques were similar to those of the EHV-1 single infected mucosa explants. In nasal and nasopharyngeal mucosae, EAV and EHV-1 pre-infections slightly reduced the number of EHV-1 and EAV infected leukocytes compared to the single infections and co-infection. In double EAV and EHV-1 infected explants, no co-infected leukocytes were detected. From these results, it can be concluded that EAV and EHV-1 are only slightly influencing each other's infection and that they do not infect the same mucosal leukocytes.

Entry of equid herpesvirus 1 into CD172a+ monocytic cells.

Equid herpesvirus 1 (EHV-1) causes respiratory disease, abortion and neurological disorders in horses. Cells from the myeloid lineage (CD172a+) are one of the main target cells of EHV-1 during primary infection. Recently, we showed that EHV-1 restricts and delays its replication in CD172a+ cells as part of an immune-evasive strategy to disseminate to target organs. Here, we hypothesize that a low efficiency of EHV-1 binding to and entry in CD172a+ cells is responsible for this restriction. Thus, we characterized EHV-1 binding and entry into CD172a+ cells, and showed that EHV-1 only bound to 15-20 % of CD172a+ cells compared with 70 % of RK-13 control cells. Enzymic removal of heparan sulphate did not reduce EHV-1 infection, suggesting that EHV-1 does not use heparan sulphate to bind and enter CD172a+ cells. In contrast, we found that treatment of cells with neuraminidase (NA) reduced infection by 85-100 % compared with untreated cells, whilst NA treatment of virus had no effect on infection. This shows that sialic acid residues present on CD172a+ cells are essential in the initiation of EHV-1 infection. We found that αVß3 integrins are involved in the post-binding stage of CD172a+ cell infection. Using pharmacological inhibitors, we showed that EHV-1 does not enter CD172a+ cells via a clathrin- or caveolae-dependent endocytic pathway, nor by macropinocytosis, but requires cholesterol, tyrosine kinase, actin, dynamin and endosomal acidification, pointing towards a phagocytic mechanism. Overall, these results show that the narrow tropism of EHV-1 amongst CD172a+ cells is determined by the presence of specific cellular receptors.

Equine Herpesvirus Type 1 Enhances Viral Replication in CD172a+ Monocytic Cells upon Adhesion to Endothelial Cells.

UNLABELLED: Equine herpesvirus type 1 (EHV-1) is a main cause of respiratory disease, abortion, and encephalomyelopathy in horses. Monocytic cells (CD172a(+)) are the main carrier cells of EHV-1 during primary infection and are proposed to serve as a "Trojan horse" to facilitate the dissemination of EHV-1 to target organs. However, the mechanism by which EHV-1 is transferred from CD172a(+) cells to endothelial cells (EC) remains unclear. The aim of this study was to investigate EHV-1 transmission between these two cell types. We hypothesized that EHV-1 employs specific strategies to promote the adhesion of infected CD172a(+) cells to EC to facilitate EHV-1 spread. Here, we demonstrated that EHV-1 infection of CD172a(+) cells resulted in a 3- to 5-fold increase in adhesion to EC. Antibody blocking experiments indicated that α4ß1, αLß2, and αVß3 integrins mediated adhesion of infected CD172a(+) cells to EC. We showed that integrin-mediated phosphatidylinositol 3-kinase (PI3K) and ERK/MAPK signaling pathways were involved in EHV-1-induced CD172a(+) cell adhesion at early times of infection. EHV-1 replication was enhanced in adherent CD172a(+) cells, which correlates with the production of tumor necrosis factor alpha (TNF-α). In the presence of neutralizing antibodies, approximately 20% of infected CD172a(+) cells transferred cytoplasmic material to uninfected EC and 0.01% of infected CD172a(+) cells transmitted infectious virus to neighboring cells. Our results demonstrated that EHV-1 infection induces adhesion of CD172a(+) cells to EC, which enhances viral replication, but that transfer of viral material from CD172a(+) cells to EC is a very specific and rare event. These findings give new insights into the complex pathogenesis of EHV-1. IMPORTANCE: Equine herpesvirus type 1 (EHV-1) is a highly prevalent pathogen worldwide, causing frequent outbreaks of abortion and myeloencephalopathy, even in vaccinated horses. After primary replication in the respiratory tract, EHV-1 disseminates via cell-associated viremia in peripheral blood mononuclear cells (PBMC) and subsequently infects the endothelial cells (EC) of the pregnant uterus or central nervous system, leading in some cases to abortion and/or neurological disorders. Recently, we demonstrated that CD172a(+) monocytic carrier cells serve as a "Trojan horse" to facilitate EHV-1 spread from blood to target organs. Here, we investigated the mechanism underlying the transmission of EHV-1 from CD172a(+) cells to EC. We demonstrated that EHV-1 infection induces cellular changes in CD172a(+) cells, promoting their adhesion to EC. We found that both cell-to-cell contacts and the secretion of soluble factors by EC activate EHV-1 replication in CD172a(+) cells. This facilitates transfer of cytoplasmic viral material to EC, resulting mainly in a nonproductive infection. Our findings give new insights into how EHV-1 may spread to EC of target organs in vaccinated horses.