A Roadmap for Tick-Borne Flavivirus Research in the "Omics" Era

Towards modelling tick-virus interactions using the weakly pathogenic Sindbis virus: Evidence that ticks are competent vectors

Most tick-borne viruses (TBVs) are highly pathogenic and require high biosecurity, which severely limits their study. We found that Sindbis virus (SINV), predominantly transmitted by mosquitoes, can replicate in ticks and be subsequently transmitted, with the potential to serve as a model for studying tick-virus interactions. We found that both larval and nymphal stages of Rhipicephalus haemaphysaloides can be infected with SINV-wild-type (WT) when feeding on infected mice. SINV replicated in two species of ticks (R. haemaphysaloides and Hyalomma asiaticum) after infecting them by microinjection. Injection of ticks with SINV expressing enhanced Green Fluorescent Protein (eGFP) revealed that SINV-eGFP specifically aggregated in the tick midguts for replication. During blood-feeding, SINV-eGFP migrated from the midguts to the salivary glands and was transmitted to a new host. SINV infection caused changes in expression levels of tick genes related to immune responses, substance transport and metabolism, cell growth and death. SINV mainly induced autophagy during the early stage of infection; with increasing time of infection, the level of autophagy decreased, while the level of apoptosis increased. During the early stages of infection, the transcript levels of immune-related genes were significantly upregulated, and then decreased. In addition, SINV induced changes in the transcription levels of some functional genes that play important roles in the interactions between ticks and tick-borne pathogens. These results confirm that the SINV-based transmission model between ticks, viruses, and mammals can be widely used to unravel the interactions between ticks and viruses.

A combination of two resistance mechanisms is critical for tick-borne encephalitis virus escape from a broadly neutralizing human antibody

Tick-borne encephalitis virus (TBEV) is a flavivirus that causes human neuroinfections and represents a growing health problem. The human monoclonal antibody T025 targets envelope protein domain III (EDIII) of TBEV and related tick-borne flaviviruses, potently neutralizing TBEV in vitro and in preclinical models, representing a promising candidate for clinical development. We demonstrate that TBEV escape in the presence of T025 or T028 (another EDIII-targeting human monoclonal antibody) results in virus variants of reduced pathogenicity, characterized by distinct sets of amino acid changes in EDII and EDIII that are jointly needed to confer resistance. EDIII substitution K311N impairs formation of a salt bridge critical for T025-epitope interaction. EDII substitution E230K is not on the T025 epitope but likely induces quaternary rearrangements of the virus surface because of repulsion of positively charged residues on the adjacent EDI. A combination of T025 and T028 prevents virus escape and improves neutralization.

A Powassan virus domain III nanoparticle immunogen elicits neutralizing and protective antibodies in mice

Powassan virus (POWV) is an emerging tick borne flavivirus (TBFV) that causes severe neuroinvasive disease. Currently, there are no approved treatments or vaccines to combat POWV infection. Here, we generated and characterized a nanoparticle immunogen displaying domain III (EDIII) of the POWV E glycoprotein. Immunization with POWV EDIII presented on nanoparticles resulted in significantly higher serum neutralizing titers against POWV than immunization with monomeric POWV EDIII. Furthermore, passive transfer of EDIII-reactive sera protected against POWV challenge in vivo. We isolated and characterized a panel of EDIII-specific monoclonal antibodies (mAbs) and identified several that potently inhibit POWV infection and engage distinct epitopes within the lateral ridge and C-C' loop of the EDIII. By creating a subunit-based nanoparticle immunogen with vaccine potential that elicits antibodies with protective activity against POWV infection, our findings enhance our understanding of the molecular determinants of antibody-mediated neutralization of TBFVs.

An Experimental Murine Model to Study Acquisition Dynamics of Tick-Borne Langat Virus in Ixodes scapularis

Ixodes scapularis ticks acquire several pathogens from reservoir animals and transmit them to humans. Development of an animal model to study acquisition/transmission dynamics of these pathogens into and from ticks, respectively, is challenging due to the fact that in nature ticks feed for a longer duration and on multiple vertebrate hosts. To understand the complex nature of pathogen acquisition/transmission, it is essential to set up a successful tick blood feeding method on a suitable vertebrate host. In this study, we provide evidence that murine model can be successfully used to study acquisition dynamics of Langat virus (LGTV), a member of tick-borne flaviviruses. Mice were inoculated intraperitoneally with LGTV that showed detectable viral loads in blood, skin, and other tissues including the brain. Both larval and nymphal ticks that were allowed to feed on the murine host successfully acquired LGTV loads. Also, we found that after molting, LGTV was transstadially transmitted from larval to nymphal stage. In addition, we noted that LGTV down-regulated IsSMase expression in all groups of ticks possibly for its survival in its vector host. Taken together, we provide evidence for the use of murine model to not only study acquisition dynamics of LGTV but also to study changes in tick gene expression during acquisition of arboviruses into ticks.

Ixodid Tick Dissection and Tick Ex Vivo Organ Cultures for Tick-Borne Virus Research

Tick-borne viruses cause thousands of cases of disease worldwide every year. Specific countermeasures to many tick-borne viruses are not commercially available. Very little is known regarding tick-virus interactions and increasing this knowledge can lead to potential targets for countermeasure development. Virus infection of ex vivo organ cultures from ticks can provide an approach to identify susceptible cell types of tissue to infection. Additionally, these organ cultures can be used for functional genomic studies to pinpoint tick-specific genes involved in the virus lifecycle. Provided here are step-by-step procedures to set up basic tick organ cultures in combination with virus infection and/or functional genomic studies. These procedures can be adapted for future use to characterize other tick-borne pathogen infections as well as tick-specific biological processes. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Loading 96-well plates with gelfoam substrate Basic Protocol 2: Step-by-step aseptic dissection of unfed female/male Ixodes scapularis ticks for multiple organs Basic Protocol 3: Step-by-step aseptic dissection of fed female Ixodes scapularis ticks to remove salivary glands Basic Protocol 4: Metabolic viability analyses of tick organ cultures Basic Protocol 5: Virus infection of tick organ cultures Basic Protocol 6: Functional RNA interference analyses using tick organ cultures.

The antiviral immunity of ticks against transmitted viral pathogens

Ticks, being obligate hematophagous arthropods, are exposed to various blood-borne pathogens, including arboviruses. Consequently, their feeding behavior can readily transmit economically important viral pathogens to humans and animals. With this tightly knit vector and pathogen interaction, the replication and transmission of tick-borne viruses (TBVs) must be highly regulated by their respective tick vectors to avoid any adverse effect on the ticks' biological development and viability. Knowledge about the tick-virus interface, although gaining relevant advances in recent years, is advancing at a slower pace than the scientific developments related to mosquito-virus interactions. The unique and complicated feeding behavior of ticks, compared to that of other blood-feeding arthropods, also limits the studies that would further elaborate the antiviral immunity of ticks against TBVs. Hence, knowledge of molecular and cellular immune mechanisms at the tick-virus interface, will further elucidate the successful viral replication of TBVs in ticks and their effective transmission to human and animal hosts.

Exploration of binary protein-protein interactions between tick-borne flaviviruses and Ixodes ricinus

Background

Louping ill virus (LIV) and tick-borne encephalitis virus (TBEV) are tick-borne flaviviruses that are both transmitted by the major European tick, Ixodes ricinus. Despite the importance of I. ricinus as an arthropod vector, its capacity to acquire and subsequently transmit viruses, known as vector competence, is poorly understood. At the molecular scale, vector competence is governed in part by binary interactions established between viral and cellular proteins within infected tick cells.

Methods

To investigate virus-vector protein–protein interactions (PPIs), the entire set of open reading frames for LIV and TBEV was screened against an I. ricinus cDNA library established from three embryonic tick cell lines using yeast two-hybrid methodology (Y2H). PPIs revealed for each viral bait were retested in yeast by applying a gap repair (GR) strategy, and notably against the cognate protein of both viruses, to determine whether the PPIs were specific for a single virus or common to both. The interacting tick proteins were identified by automatic BLASTX, and in silico analyses were performed to expose the biological processes targeted by LIV and TBEV.

Results

For each virus, we identified 24 different PPIs involving six viral proteins and 22 unique tick proteins, with all PPIs being common to both viruses. According to our data, several viral proteins (pM, M, NS2A, NS4A, 2K and NS5) target multiple tick protein modules implicated in critical biological pathways. Of note, the NS5 and pM viral proteins establish PPI with several tumor necrosis factor (TNF) receptor-associated factor (TRAF) proteins, which are essential adaptor proteins at the nexus of multiple signal transduction pathways.

Conclusion

We provide the first description of the TBEV/LIV-I. ricinus PPI network, and indeed of any PPI network involving a tick-borne virus and its tick vector. While further investigation will be needed to elucidate the role of each tick protein in the replication cycle of tick-borne flaviviruses, our study provides a foundation for understanding the vector competence of I. ricinus at the molecular level. Indeed, certain PPIs may represent molecular determinants of vector competence of I. ricinus for TBEV and LIV, and potentially for other tick-borne flaviviruses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04651-3.

The case for oxidative stress molecule involvement in the tick-pathogen interactions -an omics approach

The blood-feeding behavior of ticks has resulted in them becoming one of the most important vectors of disease-causing pathogens. Ticks possess a well-developed innate immune system to counter invading pathogens. However, the coevolution of ticks with tick-borne pathogens has adapted these pathogens to the tick's physiology and immune response through several mechanisms including transcriptional regulation. The recent development in tick and tick-borne disease research greatly involved the "omics" approach. The omics approach takes a look en masse at the different genes, proteins, metabolomes, and the microbiome of the ticks that could be differentiated during pathogen infection. Data from this approach revealed that oxidative stress-related molecules in ticks are differentiated and possibly being exploited by the pathogens to evade the tick's immune response. In this study, we review and discuss transcriptomic and proteomic data for some oxidative stress molecules differentially expressed during pathogen infection. We also discuss metabolomics and microbiome data as well as functional genomics in order to provide insight into the tick-pathogen interaction.

Dissecting Flavivirus Biology in Salivary Gland Cultures from Fed and Unfed Ixodes scapularis (Black-Legged Tick)

Tick-borne flaviviruses (TBFVs) are responsible for more than 15,000 human disease cases each year, and Powassan virus lineage 2 (POWV-L2) deer tick virus has been a reemerging threat in North America over the past 20 years. Rapid transmission of TBFVs in particular emphasizes the importance of preventing tick bites, the difficulty in developing countermeasures to prevent transmission, and the importance of understanding TBFV infection in tick salivary glands (SGs). Tick blood feeding is responsible for phenomenal physiological changes and is associated with changes in TBFV multiplication within the tick and in SGs. Using SG cultures from Ixodes scapularis female ticks, the primary aims of this study were to identify cellular localization of virus-like particles in acini of infected SGs from fed and unfed ticks, localization of TBFV infection in infected SGs from fed ticks, and a tick transcript (with associated metabolic function) involved in POWV-L2 infection in SG cultures.

The Ixodes scapularis tick transmits a number of pathogens, including tick-borne flaviviruses (TBFVs). In the United States, confirmed human infections with the Powassan virus (POWV) TBFV have a fatality rate of ∼10% and are increasing in incidence. Tick salivary glands (SGs) serve as an organ barrier to TBFV transmission, and little is known regarding the location of TBFV infection in SGs from fed ticks. Previous studies showed I. scapularis vanin (VNN) involved with TBFV infection of I. scapularis ISE6 embryonic cells, suggesting a potential role for this gene. The overall goal of this study was to use SG cultures to compare data on TBFV biology in SGs from fully engorged, replete (fed) ticks and from unfed ticks. TBFV multiplication was higher in SGs from fed ticks than in those from unfed ticks. Virus-like particles were observed only in granular acini of SGs from unfed ticks. The location of TBFV infection of SGs from fed ticks was observed in cells lining lobular ducts and trachea but not observed in acini. Transcript knockdown of VNN decreased POWV multiplication in infected SG cultures from both fed and unfed ticks. This work was the first to identify localization of TBFV multiplication in SG cultures from a fed tick and a tick transcript important for POWV multiplication in the tick SG, an organ critical for TBFV transmission. This research exemplifies the use of SG cultures in deciphering TBFV biology in the tick and as a translational tool for screening and identifying potential tick genes as potential countermeasure targets.

Potential effects of blood meal host on bacterial community composition in Ixodes scapularis nymphs

Tick microbiomes may play an important role in pathogen transmission. However, the drivers of microbiome variation are poorly understood, and this limitation has impeded mechanistic understanding of the functions of microbial communities for pathogen acquisition. The goal of this research was to characterize the role of the blood meal host in structuring the microbiome of Ixodes scapularis, the primary vector of Lyme disease in the eastern United States, and to determine if ticks that fed from different host species harbor distinct bacterial communities. We performed high-throughput 16S rDNA amplicon sequencing on I. scapularis nymphs that fed as larvae from known wildlife hosts: raccoon, Virginia opossum, striped skunk, red squirrel or gray squirrel. Using Analysis of Similarity, we found significant differences in the abundance-weighted Unifrac distance matrix among ticks fed from different host species (p =  0.048) and a highly significant difference in the weighted and unweighted Unifrac matrices for individuals within species (p <  0.01). This finding of associations between the blood meal host and I. scapularis microbiome demonstrates that the blood meal host may be a driver of microbiome variation that should be accounted for in studies of pathogen acquisition by ticks.

The Use of Ex Vivo Organ Cultures in Tick-Borne Virus Research

Each year there are more than 15 000 cases of human disease caused by infections with tick-borne viruses (TBVs). These illnesses occur worldwide and can range from very mild illness to severe encephalitis and hemorrhagic fever. Although TBVs are currently identified as neglected vector-borne pathogens and receive less attention than mosquito-borne viruses, TBVs are expanding into new regions, and infection rates are increasing. Furthermore, effective vaccines, diagnostic tools, and other countermeasures are limited. The application of contemporary technologies to TBV infections presents an excellent opportunity to develop improved, effective countermeasures. Experimental tick and mammal models of infection can be used to characterize determinants of infection, transmission, and virulence and to test candidate countermeasures. The use of ex vivo tick cultures in TBV research provides a unique way to look at infection in specific tick organs. Mammal ex vivo organ slice and, more recently, organoid cultures are additional models that can be used to elucidate direct tissue-specific responses to infection. These ex vivo model systems are convenient for testing methods involving transcript knockdown and small molecules under tightly controlled conditions. They can also be combined with in vitro and in vivo studies to tease out possible host factors and potential vaccine or therapeutic candidates. In this brief perspective, we describe how ex vivo cultures can be combined with modern technologies to advance research on TBV infections.