Tick-borne encephalitis virus replication, intracellular trafficking, and pathogenicity in human intestinal Caco-2 cell monolayers

Breast milk transmission and involvement of mammary glands in tick-borne flavivirus infected mice

Tick-borne flaviviruses (TBFs) are transmitted to humans through milk and tick bites. Although a case of possible mother-to-child transmission of tick-borne encephalitis virus (TBEV) through breast milk has been reported, this route has not been confirmed in experimental models. Therefore, in this study, using type I interferon receptor-deficient A129 mice infected with Langat virus (LGTV), we aimed to demonstrate the presence of infectious virus in the milk and mammary glands of infected mice. Our results showed viral RNA of LGTV in the pup's stomach milk clots (SMCs) and blood, indicating that the virus can be transmitted from dam to pup through breast milk. In addition, we observed that LGTV infection causes tissue lesions in the mammary gland, and viral particles were present in mammary gland epithelial cells. Furthermore, we found that milk from infected mice could infect adult mice via the intragastric route, which has a milder infection process, longer infection time, and a lower rate of weight loss than other modes of infection. Specifically, we developed a nano-luciferase-LGTV reporter virus system to monitor the dynamics of different infection routes and observed dam-to-pup infection using in vivo bioluminescence imaging. This study provides comprehensive evidence to support breast milk transmission of TBF in mice and has helped provide useful data for studying TBF transmission routes.IMPORTANCETo date, no experimental models have confirmed mother-to-child transmission of tick-borne flavivirus (TBF) through breastfeeding. In this study, we used a mouse model to demonstrate the presence of infectious viruses in mouse breast milk and mammary gland epithelial cells. Our results showed that pups could become infected through the gastrointestinal route by suckling milk, and the infection dynamics could be monitored using a reporter virus system during breastfeeding in vivo. We believe our findings have provided substantial evidence to understand the underlying mechanism of breast milk transmission of TBF in mice, which has important implications for understanding and preventing TBF transmission in humans.

A Journey to the Central Nervous System: Routes of Flaviviral Neuroinvasion in Human Disease

Many arboviruses, including viruses of the Flavivirus genera, are known to cause severe neurological disease in humans, often with long-lasting, debilitating sequalae in surviving patients. These emerging pathogens impact millions of people worldwide, yet still relatively little is known about the exact mechanisms by which they gain access to the human central nervous system. This review focusses on potential haematogenous and transneural routes of neuroinvasion employed by flaviviruses and identifies numerous gaps in knowledge, especially regarding lesser-studied interfaces of possible invasion such as the blood-cerebrospinal fluid barrier, and novel routes such as the gut-brain axis. The complex balance of pro-inflammatory and antiviral immune responses to viral neuroinvasion and pathology is also discussed, especially in the context of the hypothesised Trojan horse mechanism of neuroinvasion. A greater understanding of the routes and mechanisms of arboviral neuroinvasion, and how they differ between viruses, will aid in predictive assessments of the neuroinvasive potential of new and emerging arboviruses, and may provide opportunity for attenuation, development of novel intervention strategies and rational vaccine design for highly neurovirulent arboviruses.

Alimentary Infections by Tick-Borne Encephalitis Virus

Tick-borne encephalitis virus (TBEV) causes serious the neurological disease, tick-borne encephalitis (TBE). TBEV can be transmitted to humans by ticks as well as by the alimentary route, which is mediated through the consumption of raw milk products from infected ruminants such as sheep, goats, and cows. The alimentary route of TBEV was recognized in the early 1950s and many important experimental studies were performed shortly thereafter. Nowadays, alimentary TBEV infections are recognized as a relevant factor contributing to the overall increase in TBE incidences in Europe. This review aims to summarize the history and current extent of alimentary TBEV infections across Europe, to analyze experimental data on virus secretion in milk, and to review possible alimentary infection preventive measures.

Mother-to-Child Transmission of Arboviruses during Breastfeeding: From Epidemiology to Cellular Mechanisms

Most viruses use several entry sites and modes of transmission to infect their host (parenteral, sexual, respiratory, oro-fecal, transplacental, transcutaneous, etc.). Some of them are known to be essentially transmitted via arthropod bites (mosquitoes, ticks, phlebotomes, sandflies, etc.), and are thus named arthropod-borne viruses, or arboviruses. During the last decades, several arboviruses have emerged or re-emerged in different countries in the form of notable outbreaks, resulting in a growing interest from scientific and medical communities as well as an increase in epidemiological studies. These studies have highlighted the existence of other modes of transmission. Among them, mother-to-child transmission (MTCT) during breastfeeding was highlighted for the vaccine strain of yellow fever virus (YFV) and Zika virus (ZIKV), and suggested for other arboviruses such as Chikungunya virus (CHIKV), dengue virus (DENV), and West Nile virus (WNV). In this review, we summarize all epidemiological and clinical clues that suggest the existence of breastfeeding as a neglected route for MTCT of arboviruses and we decipher some of the mechanisms that chronologically occur during MTCT via breastfeeding by focusing on ZIKV transmission process.

Contact-dependent transmission of Langat and tick-borne encephalitis virus in type I interferon receptor-1 deficient mice

Tick-borne encephalitis virus (TBEV) is primarily transmitted to humans through tick bites or oral consumption of accordingly contaminated unpasteurized milk or milk products. The detection of TBEV RNA in various body fluids in immunosuppressed human patients is documented. However, the risk of direct contact exposure remains unclear. Interferon-alpha receptor-1 deficient (Ifnar1^-/- ) mice, which are lacking the interferon-α/β responses, develop neurologic manifestations after infection with TBEV and Langat virus (LGTV). We showed that subcutaneous, intranasal, and peroral infection of LGTV lead to disease, whereas mice with intragastric application of LGTV showed no disease signs. With LGTV infected mice exhibit seroconversion and significant viral RNA levels was detected in saliva, eye smear, feces and urine. As a result, TBEV and LGTV are transmitted between mice from infected to naïve co-caged sentinel animals. Although intranasal inoculation of LGTV is entirely sufficient to establish the disease in mice, the virus is not transmitted by aerosols. These pooled results from animal models highlight the risks of exposure to TBEV contaminants and the possibility for close contact transmission of TBEV in interferon-alpha receptor-1 deficient laboratory mice.Importance Tick-borne encephalitis is a severe disease of the central nervous system caused by the tick-borne encephalitis virus (TBEV). Every year between 10,000-12,000 people become infected with this flavivirus. The TBEV is usually transmitted to humans via the bite of a tick, but infections due to consumption of infectious milk products are increasingly being reported. Since there is no therapy for an TBEV infection and mechanisms of virus persistence in reservoir animals are unclear, it is important to highlight the risk of exposure to TBEV contaminants and know possible routes of transmission of this virus. The significance of our research is in identifying other infection routes of TBEV and LGTV, and the possibility of close contact transmission.

ADAM15 Participates in Tick-Borne Encephalitis Virus Replication

Tick-borne encephalitis virus (TBEV), a major tick-borne viral pathogen of humans, is known to cause neurological diseases such as meningitis, encephalitis, and meningoencephalitis. However, the life cycle and pathogenesis of TBEV are not well understood. Here, we show that the knockdown or knockout of ADAM15 (a disintegrin and metalloproteinase 15), a host protein involved in neuroblastoma diseases, leads to TBEV replication and assembly defects. We characterized the disintegrin domain in ADAM15 and found that the ADAM15 subcellular localization was changed following TBEV infection. RNA interference (RNAi) screen analysis confirmed ADAM's nonredundant functions and identified a specific role for ADAM15 in TBEV infection. An RNA-sequencing analysis was also conducted to understand the causal link between TBEV infection and the cellular endomembrane network, namely, the generation of replication organelles promoting viral genome replication and virus production. Our data demonstrated that TBEV infection changes ADAM15 cellular localization, which contributes to membrane reorganization and viral replication.IMPORTANCE Tick populations are increasing, and their geographic ranges are expanding. Increases in tick-borne disease prevalence and transmission are important public health issues. Tick-borne encephalitis virus (TBEV) often results in meningitis, encephalitis, and meningoencephalitis. TBEV causes clinical disease in more than 20,000 humans in Europe and Asia per year. An increased incidence of TBE has been noted in Europe and Asia, as a consequence of climate and socioeconomic changes. The need to investigate the mechanism(s) of interaction between the virus and the host factors is apparent, as it will help us to understand the roles of host factors in the life cycle of TBEV. The significance of our research is in identifying the ADAM15 for TBEV replication, which will greatly enhance our understanding of TBEV life cycle and highlight a target for pharmaceutical consideration.

Model System for the Formation of Tick-Borne Encephalitis Virus Replication Compartments without Viral RNA Replication

TBEV infection causes a broad spectrum of symptoms, ranging from mild fever to severe encephalitis. Similar to other flaviviruses, TBEV exploits intracellular membranes to build RCs for viral replication. The viral NS proteins have been suggested to be involved in this process; however, the mechanism of RC formation and the roles of individual NS proteins remain unclear. To study how TBEV induces membrane remodeling, we developed an inducible stable cell system expressing the TBEV NS polyprotein in the absence of viral RNA replication. Using this system, we were able to reproduce RC-like vesicles that resembled the RCs formed in flavivirus-infected cells, in terms of morphology and size. This cell system is a robust tool to facilitate studies of flavivirus RC formation and is an ideal model for the screening of antiviral agents at a lower biosafety level.

Flavivirus is a positive-sense, single-stranded RNA viral genus, with members causing severe diseases in humans such as tick-borne encephalitis, yellow fever, and dengue fever. Flaviviruses are known to cause remodeling of intracellular membranes into small cavities, where replication of the viral RNA takes place. Nonstructural (NS) proteins are not part of the virus coat and are thought to participate in the formation of these viral replication compartments (RCs). Here, we used tick-borne encephalitis virus (TBEV) as a model for the flaviviruses and developed a stable human cell line in which the expression of NS proteins can be induced without viral RNA replication. The model system described provides a novel and benign tool for studies of the viral components under controlled expression levels. We show that the expression of six NS proteins is sufficient to induce infection-like dilation of the endoplasmic reticulum (ER) and the formation of RC-like membrane invaginations. The NS proteins form a membrane-associated complex in the ER, and electron tomography reveals that the dilated areas of the ER are closely associated with lipid droplets and mitochondria. We propose that the NS proteins drive the remodeling of ER membranes and that viral RNA, RNA replication, viral polymerase, and TBEV structural proteins are not required.

IMPORTANCE TBEV infection causes a broad spectrum of symptoms, ranging from mild fever to severe encephalitis. Similar to other flaviviruses, TBEV exploits intracellular membranes to build RCs for viral replication. The viral NS proteins have been suggested to be involved in this process; however, the mechanism of RC formation and the roles of individual NS proteins remain unclear. To study how TBEV induces membrane remodeling, we developed an inducible stable cell system expressing the TBEV NS polyprotein in the absence of viral RNA replication. Using this system, we were able to reproduce RC-like vesicles that resembled the RCs formed in flavivirus-infected cells, in terms of morphology and size. This cell system is a robust tool to facilitate studies of flavivirus RC formation and is an ideal model for the screening of antiviral agents at a lower biosafety level.

Directional entry and release of Zika virus from polarized epithelial cells

Background

Both vector borne and sexual transmission of Zika virus (ZIKV) involve infection of epithelial cells in the initial stages of infection. Epithelial cells are unique in their ability to form polarized monolayers and their barrier function. Cell polarity induces an asymmetry in the epithelial monolayer, which is maintained by tight junctions and specialized sorting machinery. This differential localization can have a potential impact of virus infection. Asymmetrical distribution of a viral receptor can restrict virus entry to a particular membrane while polarized sorting can lead to a directional release of virions. The present study examined the impact of cell polarity on ZIKV infection and release.

Methods

A polarized Caco-2 cell model we described previously was used to assess ZIKV infection. Transepithelial resistance (TEER) was used to assess epithelial cell polarity, and virus infection was measured by immunofluorescence microscopy and qRT-PCR. Cell permeability was measured using a fluorescein leakage assay. Statistical significance was calculated using one-way ANOVA and significance was set at p < 0.05.

Results

Using the Caco-2 cell model for polarized epithelial cells, we report that Zika virus preferentially infects polarized cells from the apical route and is released vectorially through the basolateral route. Our data also indicates that release occurs without disruption of cell permeability.

Conclusions

Our results show that ZIKV has directional infection and egress in a polarized cell system. This mechanism of directional infection may be one of the mechanisms that enables the cross the epithelial barrier effectively without a disruption in cell monolayer integrity. Elucidation of entry and release characteristics of Zika virus in polarized epithelial cells can lead to better understanding of virus dissemination in the host, and can help in developing effective therapeutic interventions.

Electronic supplementary material

The online version of this article (10.1186/s12985-019-1200-2) contains supplementary material, which is available to authorized users.

Exome-wide search and functional annotation of genes associated in patients with severe tick-borne encephalitis in a Russian population

Background

Tick-borne encephalitis (TBE) is a viral infectious disease caused by tick-borne encephalitis virus (TBEV). TBEV infection is responsible for a variety of clinical manifestations ranging from mild fever to severe neurological illness. Genetic factors involved in the host response to TBEV that may potentially play a role in the severity of the disease are still poorly understood. In this study, using whole-exome sequencing, we aimed to identify genetic variants and genes associated with severe forms of TBE as well as biological pathways through which the identified variants may influence the severity of the disease.

Results

Whole-exome sequencing data analysis was performed on 22 Russian patients with severe forms of TBE and 17 Russian individuals from the control group. We identified 2407 candidate genes harboring rare, potentially pathogenic variants in exomes of patients with TBE and not containing any rare, potentially pathogenic variants in exomes of individuals from the control group. According to DAVID tool, this set of 2407 genes was enriched with genes involved in extracellular matrix proteoglycans pathway and genes encoding proteins located at the cell periphery. A total of 154 genes/proteins from these functional groups have been shown to be involved in protein-protein interactions (PPIs) with the known candidate genes/proteins extracted from TBEVHostDB database. By ranking these genes according to the number of rare harmful minor alleles, we identified two genes (MSR1 and LMO7), harboring five minor alleles, and three genes (FLNA, PALLD, PKD1) harboring four minor alleles.

When considering genes harboring genetic variants associated with severe forms of TBE at the suggestive P-value < 0.01, 46 genes containing harmful variants were identified. Out of these 46 genes, eight (MAP4, WDFY4, ACTRT2, KLHL25, MAP2K3, MBD1, OR10J1, and OR2T34) were additionally found among genes containing rare pathogenic variants identified in patients with TBE; and five genes (WDFY4,ALK, MAP4, BNIPL, EPPK1) were found to encode proteins that are involved in PPIs with proteins encoded by genes from TBEVHostDB. Three genes out of five (MAP4, EPPK1, ALK) were found to encode proteins located at cell periphery.

Conclusions

Whole-exome sequencing followed by systems biology approach enabled to identify eight candidate genes (MAP4, WDFY4, ACTRT2, KLHL25, MAP2K3, MBD1, OR10J1, and OR2T34) that can potentially determine predisposition to severe forms of TBE. Analyses of the genetic risk factors for severe forms of TBE revealed a significant enrichment with genes controlling extracellular matrix proteoglycans pathway as well as genes encoding components of cell periphery.

Electronic supplementary material

The online version of this article (10.1186/s12920-019-0503-x) contains supplementary material, which is available to authorized users.

Neuroinflammation During RNA Viral Infections

Neurotropic RNA viruses continue to emerge and are increasingly linked to diseases of the central nervous system (CNS) despite viral clearance. Indeed, the overall mortality of viral encephalitis in immunocompetent individuals is low, suggesting efficient mechanisms of virologic control within the CNS. Both immune and neural cells participate in this process, which requires extensive innate immune signaling between resident and infiltrating cells, including microglia and monocytes, that regulate the effector functions of antiviral T and B cells as they gain access to CNS compartments. While these interactions promote viral clearance via mainly neuroprotective mechanisms, they may also promote neuropathology and, in some cases, induce persistent alterations in CNS physiology and function that manifest as neurologic and psychiatric diseases. This review discusses mechanisms of RNA virus clearance and neurotoxicity during viral encephalitis with a focus on the cytokines essential for immune and neural cell inflammatory responses and interactions. Understanding neuroimmune communications in the setting of viral infections is essential for the development of treatments that augment neuroprotective processes while limiting ongoing immunopathological processes that cause ongoing CNS disease.

Tick-Borne Encephalitis Virus: A Structural View

Tick-borne encephalitis virus (TBEV) is a growing health concern. It causes a severe disease that can lead to permanent neurological complications or death and the incidence of TBEV infections is constantly rising. Our understanding of TBEV’s structure lags behind that of other flaviviruses, but has advanced recently with the publication of a high-resolution structure of the TBEV virion. The gaps in our knowledge include: aspects of receptor binding, replication and virus assembly. Furthermore, TBEV has mostly been studied in mammalian systems, even though the virus’ interaction with its tick hosts is a central part of its life cycle. Elucidating these aspects of TBEV biology are crucial for the development of TBEV antivirals, as well as the improvement of diagnostics. In this review, we summarise the current structural knowledge on TBEV, bringing attention to the current gaps in our understanding, and propose further research that is needed to truly understand the structural-functional relationship of the virus and its hosts.

Transmembrane Pathways and Mechanisms of Rod-like Paclitaxel Nanocrystals through MDCK Polarized Monolayer

Drug nanocrystals (NCs) appear to be favorable to improving oral bioavailability of poorly water-soluble drugs as evidenced by the great success they have had in the market. However, the pathway and mechanism of drug NCs through epithelial membrane are still unclear. In an attempt to clarify their transport features, paclitaxel nanocrystals (PTX-NCs), and paclitaxel hybrid NCs with lipophilic carbocyanine dyes, were prepared and characterized as the models. The endocytosis, intracellular trafficking, paracellular transport, and transcytosis of PTX-NCs were carefully investigated with Förster resonance energy transfer (FRET) analysis, as well as a colocalization assay, flow cytometry, gene silencing, Western-blot, transepithelial electrical resistance (TEER) study and other approaches on MDCK cells. It was found that rod-like PTX-NCs could transport through the monolayer intact, and the process of endocytosis proved to be time and energy dependent. Endoplasmic reticulum (ER) and Golgi complexes were colocalized with PTX-NCs in cells, so the ER-Golgi complexes/Golgi complexes-basolateral membrane pathway may be involved in the intracellular trafficking and transcytosis of PTX-NCs. It was demonstrated here that cav-1, dynamin, and actin filament modulated the endocytosis process, and Cdc 42 regulated the transcytosis process. In addition, no paracellular transport of PTX-NCs was observed. These findings demonstrated that the rod-like nanocrystals not only enhanced the transcytosis of PTX compared with microparticles of raw drug materials but also changed the pathways of drug delivery. This study certainly provides insight for the oral absorption mechanism of nanocrystals of poorly soluble drugs.

Viral receptors of the gut: vector-borne viruses of medical importance

Arthropods transmit several medically important arboviruses that cause diseases in humans. Therapeutic strategies to treat or prevent diseases transmitted by the arthropods are limiting. Understanding the role of arthropod gut receptors in the interactions with various arboviruses would provide important means for the development of a strong anti-vector vaccine. In this review, we summarize some of the potential findings in the field of arthropod gut receptors for tick-borne or mosquito-borne viruses and discuss their relevance in the development of a broad-spectrum transmission-blocking vaccine to treat or control various diseases caused by arboviruses.

Pathogens vectored by the tick, Dermacentor reticulatus, in endemic regions and zones of expansion in Poland

Background

Dermacentor reticulatus plays an important role in the maintenance of pathogens of medical and veterinary importance in the environment. Currently two isolated populations of D. reticulatus are present in Poland –Western and Eastern. The range of the Eastern population covers endemic areas in eastern Poland but this population is expanding westwards creating an expansion zone in the centre of the country. The expansion zone in western Poland is occupied by the recently discovered Western population, spreading eastwards.

Methods

Questing adult ticks (n = 2585) were collected in 2012–2014 in endemic regions of north-eastern (Warmińsko-Mazurskie Voivodeship) and central Poland (Masovian Voivodeship) and in the expansion zones in central and western Poland, in the region between the Vistula River and the western border of the country. Amplification of Babesia, Rickettsia spp. and Borrelia burgdorferi sensu lato DNAs was performed using specific starters. RNA of the TBE virus was detected using RT-PCR and representative PCR products were sequenced and compared with sequences deposited in GenBank.

Results

Of the total 2585 examined ticks, 1197 (46.3 %) were infected with at least one pathogen. Overall prevalence of pathogens was 4.18 % (108/2585) for Babesia spp., 44.10 % (1140/2585) for Rickettsia spp., 0.09 % (1/1107) for Borrelia afzelii and 7.6 % (7/92) for TBEV. Sequence analysis of DNA showed 99.86 % similarity to R. raoulti and 99.81 % to B. canis. One male from north-eastern Poland was infected with B. microti.

Prevalence of R. raoulti was highest in the Western population (52.03 %) and lowest in the Eastern population in north-eastern Poland (34.18 %). Babesia canis was not detected in 592 ticks collected in the Western population, while in the Eastern population overall prevalence was 5.42 %. There were significant differences in the prevalence of B. canis between tick samples from northern (0.68 %), central (1.18 %) and southern (14.8 %) areas of the expansion zone in central Poland.

Conclusions

Our study found significant differences between the range and prevalence of vectored pathogens in D. reticulatus from the endemic areas and newly inhabited expansion zones. The differences were likely associated with the different time of settlement or ‘source’ of ticks populations, the Eastern and the Western one.