Tick-Borne Encephalitis Virus Structural Proteins Are the Primary Viral Determinants of Non-Viraemic Transmission between Ticks whereas Non-Structural Proteins Affect Cytotoxicity

Differential susceptibility of geographically distinct Ixodes ricinus populations to tick-borne encephalitis virus and louping ill virus

Tick-borne encephalitis virus (TBEV) is an emerging pathogen in the Netherlands. Multiple divergent viral strains are circulating and the focal distribution of TBEV remains poorly understood. This may, however, be explained by differences in the susceptibility of tick populations for specific viruses and viral strains, and by viral strains having higher infection success in their local tick population. We investigated this hypothesis by exposing Dutch Ixodes ricinus ticks to two different TBEV strains: TBEV-NL from the Netherlands and TBEV-Neudoerfl from Austria. In addition, we exposed ticks to louping Ill virus (LIV), which is endemic to large parts of the United Kingdom and Ireland, but has not been reported in the Netherlands. Ticks were collected from two locations in the Netherlands: one location without evidence of TBEV circulation and one location endemic for the TBEV-NL strain. Ticks were infected in a biosafety level 3 laboratory using an artificial membrane feeding system. Ticks collected from the region without evidence of TBEV circulation had lower infection rates for TBEV-NL as compared to TBEV-Neudoerfl. Vice versa, ticks collected from the TBEV-NL endemic region had higher infection rates for TBEV-NL compared to TBEV-Neudoerfl. In addition, LIV infection rates were much lower in Dutch ticks compared to TBEV, which may explain why LIV is not present in the Netherlands. Our findings show that ticks from two distinct geographical populations differ in their susceptibility to TBEV strains, which could be the result of differences in the genetic background of the tick populations.

Infection of wild-caught wood mice (Apodemus sylvaticus) and yellow-necked mice (A. flavicollis) with tick-borne encephalitis virus

The distribution of tick-borne encephalitis virus (TBEV) is expanding to Western European countries, including the Netherlands, but the contribution of different rodent species to the transmission of TBEV is poorly understood. We investigated whether two species of wild rodents native to the Netherlands, the wood mouse Apodemus sylvaticus and the yellow-necked mouse Apodemus flavicollis, differ in their relative susceptibility to experimental infection with TBEV. Wild-caught individuals were inoculated subcutaneously with the classical European subtype of TBEV (Neudoerfl) or with TBEV-NL, a genetically divergent TBEV strain from the Netherlands. Mice were euthanised and necropsied between 3 and 21 days post-inoculation. None of the mice showed clinical signs or died during the experimental period. Nevertheless, TBEV RNA was detected up to 21 days in the blood of both mouse species and TBEV was also isolated from the brain of some mice. Moreover, no differences in infection rates between virus strains and mouse species were found in blood, spleen, or liver samples. Our results suggest that the wood mouse and the yellow-necked mouse may equally contribute to the transmission cycle of TBEV in the Netherlands. Future experimental infection studies that include feeding ticks will help elucidate the relative importance of viraemic transmission in the epidemiology of TBEV.

Hybrids of Ixodes ricinus and Ixodes persulcatus ticks effectively acquire and transmit tick-borne encephalitis virus

Ixodes rici nus and Ixodes persulcatus ticks are the main vectors of tick-borne encephalitis virus (TBEV), which has three main subtypes connected with certain tick species: the European subtype, associated with I. ricinus, and the Siberian and Far-Eastern subtypes, associated with I. persulcatus. Distribution ranges of these species overlap and form large sympatric areas in the East European Plain and Baltic countries. It has previously been shown that crossing of I. ricinus and I. persulcatus is possible, with the appearance of sterile hybrids. Hybridization of ticks can affect not only the spread of ticks but also the properties of natural foci of arbovirus infections, in particular TBEV. In the present study, we analyzed the effectiveness of virus transmission from infected mice to larvae and nymphs and trans-stadial transmission (from larvae to nymph and adult) in I. ricinus, I. persulcatus, and hybrids. For this purpose, we bred a hybrid generation from the crossing of I. persulcatus females and I. ricinus males, and we used the Siberian and European subtypes of TBEV. We showed that after feeding on infected mice, virus prevalence in engorged ticks decreased over time, and after molting, the opposite was true. In hybrids we observed the highest acquisition effectiveness and RNA copy numbers during Siberian TBEV subtype transmission. The efficiency of trans-stadial transmission of both TBEV subtypes was similar in hybrids and parental species. After the second trans-stadial TBEV transmission, a significant increase in ticks' infection rates was observed only in specific subtype-tick combination. Our data demonstrate the possible features of TBEV circulation in the I. ricinus and I. persulcatus sympatry area.

Activation of Early Proinflammatory Responses by TBEV NS1 Varies between the Strains of Various Subtypes

Tick-borne encephalitis (TBE) is an emerging zoonosis that may cause long-term neurological sequelae or even death. Thus, there is a growing interest in understanding the factors of TBE pathogenesis. Viral genetic determinants may greatly affect the severity and consequences of TBE. In this study, nonstructural protein 1 (NS1) of the tick-borne encephalitis virus (TBEV) was tested as such a determinant. NS1s of three strains with similar neuroinvasiveness belonging to the European, Siberian and Far-Eastern subtypes of TBEV were studied. Transfection of mouse cells with plasmids encoding NS1 of the three TBEV subtypes led to different levels of NS1 protein accumulation in and secretion from the cells. NS1s of TBEV were able to trigger cytokine production either in isolated mouse splenocytes or in mice after delivery of NS1 encoding plasmids. The profile and dynamics of TNF-α, IL-6, IL-10 and IFN-γ differed between the strains. These results demonstrated the involvement of TBEV NS1 in triggering an immune response and indicated the diversity of NS1 as one of the genetic factors of TBEV pathogenicity.

Fitness of mCherry Reporter Tick-Borne Encephalitis Virus in Tick Experimental Models

The tick-borne encephalitis virus (TBEV) causes a most important viral life-threatening illness transmitted by ticks. The interactions between the virus and ticks are largely unexplored, indicating a lack of experimental tools and systematic studies. One such tool is recombinant reporter TBEV, offering antibody-free visualization to facilitate studies of transmission and interactions between a tick vector and a virus. In this paper, we utilized a recently developed recombinant TBEV expressing the reporter gene mCherry to study its fitness in various tick-derived in vitro cell cultures and live unfed nymphal Ixodes ricinus ticks. The reporter virus was successfully replicated in tick cell lines and live ticks as confirmed by the plaque assay and the mCherry-specific polymerase chain reaction (PCR). Although a strong mCherry signal determined by fluorescence microscopy was detected in several tick cell lines, the fluorescence of the reporter was not observed in the live ticks, corroborated also by immunoblotting. Our data indicate that the mCherry reporter TBEV might be an excellent tool for studying TBEV-tick interactions using a tick in vitro model. However, physiological attributes of a live tick, likely contributing to the inactivity of the reporter, warrant further development of reporter-tagged viruses to study TBEV in ticks in vivo.

Identification of Host Factors Differentially Induced by Clinically Diverse Strains of Tick-Borne Encephalitis Virus

Tick-borne encephalitis virus (TBEV) is an important human arthropod-borne virus that causes tick-borne encephalitis (TBE) in humans. TBEV acutely infects the central nervous system (CNS), leading to neurological symptoms of various severity. No therapeutics are currently available for TBEV-associated disease. Virus strains of various pathogenicity have been described, although the basis of their diverse clinical outcome remains undefined. Work with infectious TBEV requires high-level biocontainment, meaning model systems that can recapitulate the virus life cycle are highly sought. Here, we report the generation of a self-replicating, noninfectious TBEV replicon used to study properties of high (Hypr) and low (Vs) pathogenic TBEV isolates. Using a Spinach2 RNA aptamer and luciferase reporter system, we perform the first direct comparison of Hypr and Vs in cell culture. Infectious wild-type (WT) viruses and chimeras of the nonstructural proteins 3 (NS3) and 5 (NS5) were investigated in parallel to validate the replicon data. We show that Hypr replicates to higher levels than Vs in mammalian cells, but not in arthropod cells, and that the basis of these differences map to the NS5 region, encoding the methyltransferase and RNA polymerase. For both Hypr and Vs strains, NS5 and the viral genome localized to intracellular structures typical of positive-strand RNA viruses. Hypr was associated with significant activation of IRF-3, caspase-3, and caspase-8, while Vs activated Akt, affording protection against caspase-mediated apoptosis. Higher activation of stress-granule proteins TIAR and G3BPI were an additional early feature of Vs but not for Hypr. These findings highlight novel host cell responses driven by NS5 that may dictate the differential clinical characteristics of TBEV strains. This highlights the utility of the TBEV replicons for further virological characterization and antiviral drug screening. IMPORTANCE Tick-borne encephalitis virus (TBEV) is an emerging virus of the flavivirus family that is spread by ticks and causes neurological disease of various severity. No specific therapeutic treatments are available for TBE, and control in areas of endemicity is limited to vaccination. The pathology of TBEV ranges from mild to fatal, depending on the virus genotype. Characterization of TBEV isolates is challenging due to the requirement for high-containment facilities. Here, we described the construction of novel TBEV replicons that permit a molecular comparison of TBEV isolates of high and low pathogenicity.

Tick-Borne Encephalitis Virus Adaptation in Different Host Environments and Existence of Quasispecies

A highly virulent strain (Hypr) of tick-borne encephalitis virus (TBEV) was serially subcultured in the mammalian porcine kidney stable (PS) and Ixodes ricinus tick (IRE/CTVM19) cell lines, producing three viral variants. These variants exhibited distinct plaque sizes and virulence in a mouse model. Comparing the full-genome sequences of all variants, several nucleotide changes were identified in different genomic regions. Furthermore, different sequential variants were revealed to co-exist within one sample as quasispecies. Interestingly, the above-mentioned nucleotide changes found within the whole genome sequences of the new variants were present alongside the nucleotide sequence of the parental strain, which was represented as a minority quasispecies. These observations further imply that TBEV exists as a heterogeneous population that contains virus variants pre-adapted to reproduction in different environments, probably enabling virus survival in ticks and mammals.

Tick-borne encephalitis virus: molecular determinants of neuropathogenesis of an emerging pathogen

Tick-borne encephalitis virus (TBEV) is a zoonotic agent causing severe encephalitis. The transmission cycle involves the virus, the Ixodes tick vector, and a vertebrate reservoir, such as small mammals (rodents, or shrews). Humans are accidentally involved in this transmission cycle. Tick-borne encephalitis (TBE) has been a growing public health problem in Europe and Asia over the past 30 years. The mechanisms involved in the development of TBE are very complex and likely multifactorial, involving both host and viral factors. The purpose of this review is to provide an overview of the current literature on TBE neuropathogenesis in the human host and to demonstrate the emergence of common themes in the molecular pathogenesis of TBE in humans. We discuss and review data on experimental study models and on both viral (molecular genetics of TBEV) and host (immune response, and genetic background) factors involved in TBE neuropathogenesis in the context of human infection.

Assessing systemic and non-systemic transmission risk of tick-borne encephalitis virus in Hungary

Estimating the tick-borne encephalitis (TBE) infection risk under substantial uncertainties of the vector abundance, environmental condition and human-tick interaction is important for evidence-informed public health intervention strategies. Estimating this risk is computationally challenging since the data we observe, i.e., the human incidence of TBE, is only the final outcome of the tick-host transmission and tick-human contact processes. The challenge also increases since the complex TBE virus (TBEV) transmission cycle involves the non-systemic route of transmission between co-feeding ticks. Here, we describe the hidden Markov transition process, using a novel TBEV transmission-human case reporting cascade model that couples the susceptible-infected compartmental model describing the TBEV transmission dynamics among ticks, animal hosts and humans, with the stochastic observation process of human TBE reporting given infection. By fitting human incidence data in Hungary to the transmission model, we estimate key parameters relevant to the tick-host interaction and tick-human transmission. We then use the parametrized cascade model to assess the transmission potential of TBEV in the enzootic cycle with respect to the climate change, and to evaluate the contribution of non-systemic transmission. We show that the TBEV transmission potential in the enzootic cycle has been increasing along with the increased temperature though the TBE human incidence has dropped since 1990s, emphasizing the importance of persistent public health interventions. By demonstrating that non-systemic transmission pathway is a significant factor in the transmission of TBEV in Hungary, we conclude that the risk of TBE infection will be highly underestimated if the non-systemic transmission route is neglected in the risk assessment.

Arbovirus-Mosquito Vector-Host Interactions and the Impact on Transmission and Disease Pathogenesis of Arboviruses

Hundreds of viruses, designated as arboviruses, are transmitted by arthropod vectors in complex transmission cycles between the virus, vertebrate host, and the vector. With millions of human and animal infections per year, it is critical to improve our understanding of the interactions between the biological and environmental factors that play a critical role in pathogenesis, disease outcomes, and transmission of arboviruses. This review focuses on mosquito-borne arboviruses and discusses current knowledge of the factors and underlying mechanisms that influence infection and transmission of arboviruses and discusses critical factors and pathways that can potentially become targets for intervention and therapeutics.

The Role of Mammalian Reservoir Hosts in Tick-Borne Flavivirus Biology

Small-to-medium sized mammals and large animals are lucrative sources of blood meals for ixodid ticks that transmit life-threatening tick-borne flaviviruses (TBFVs). TBFVs have been isolated from various organs obtained from wild-caught Myodes and Apodemus species in Europe and Asia. Thus, these rodents are well-established reservoirs of TBFVs. Wild-caught Peromyscus species have demonstrated seropositivity against Powassan virus, the only TBFV known to circulate in North America, suggesting that they may play an important role in the biology of the virus in this geographic region. However, virus isolation from Peromyscus species is yet to be demonstrated. Wild-caught medium-sized mammals, such as woodchucks (Marmota monax) and skunks (Mephitis mephitis) have also demonstrated seropositivity against POWV, and virus was isolated from apparently healthy animals. Despite the well-established knowledge that small-to-medium sized animals are TBFV reservoirs, specific molecular biology addressing host-pathogen interactions remains poorly understood. Elucidating these interactions will be critical for gaining insight into the mechanism(s) of viral pathogenesis and/or resistance.

Tick-Borne Viruses and Biological Processes at the Tick-Host-Virus Interface

Ticks are efficient vectors of arboviruses, although less than 10% of tick species are known to be virus vectors. Most tick-borne viruses (TBV) are RNA viruses some of which cause serious diseases in humans and animals world-wide. Several TBV impacting human or domesticated animal health have been found to emerge or re-emerge recently. In order to survive in nature, TBV must infect and replicate in both vertebrate and tick cells, representing very different physiological environments. Information on molecular mechanisms that allow TBV to switch between infecting and replicating in tick and vertebrate cells is scarce. In general, ticks succeed in completing their blood meal thanks to a plethora of biologically active molecules in their saliva that counteract and modulate different arms of the host defense responses (haemostasis, inflammation, innate and acquired immunity, and wound healing). The transmission of TBV occurs primarily during tick feeding and is a complex process, known to be promoted by tick saliva constituents. However, the underlying molecular mechanisms of TBV transmission are poorly understood. Immunomodulatory properties of tick saliva helping overcome the first line of defense to injury and early interactions at the tick-host skin interface appear to be essential in successful TBV transmission and infection of susceptible vertebrate hosts. The local host skin site of tick attachment, modulated by tick saliva, is an important focus of virus replication. Immunomodulation of the tick attachment site also promotes co-feeding transmission of viruses from infected to non-infected ticks in the absence of host viraemia (non-viraemic transmission). Future research should be aimed at identification of the key tick salivary molecules promoting virus transmission, and a molecular description of tick-host-virus interactions and of tick-mediated skin immunomodulation. Such insights will enable the rationale design of anti-tick vaccines that protect against disease caused by tick-borne viruses.

Detection and genetic characterization of a wide range of infectious agents in Ixodes pavlovskyi ticks in Western Siberia, Russia

BACKGROUND

The Ixodes pavlovskyi tick species, a member of the I. persulcatus/I. ricinus group, was discovered in the middle of the 20th century in the Russian Far East. Limited data have been reported on the detection of infectious agents in this tick species. The aim of this study was to investigate the prevalence and genetic variability of a wide range of infectious agents in I. pavlovskyi ticks collected in their traditional and recently invaded habitats, the Altai Mountains and Novosibirsk Province, respectively, which are both located within the Western Siberian part of the I. pavlovskyi distribution area.

RESULTS

This study reports the novel discovery of Borrelia bavariensis, Rickettsia helvetica, R. heilongjiangensis, R. raoultii, "Candidatus Rickettsia tarasevichiae", Anaplasma phagocytophilum, Ehrlichia muris, "Candidatus Neoehrlichia mikurensis" and Babesia microti in I. pavlovskyi ticks. In addition, we confirmed the previous identification of B. afzelii, B. garinii and B. miyamotoi, as well as tick-borne encephalitis and Kemerovo viruses in this tick species. The prevalence and some genetic characteristics of all of the tested agents were compared with those found in I. persulcatus ticks that were collected at the same time in the same locations, where these tick species occur in sympatry. It was shown that the prevalence and genotypes of many of the identified pathogens did not significantly differ between I. pavlovskyi and I. persulcatus ticks. However, I. pavlovskyi ticks were significantly more often infected by B. garinii and less often by B. bavariensis, B. afzelii, "Ca. R. tarasevichiae", and E. muris than I. persulcatus ticks in both studied regions. Moreover, new genetic variants of B. burgdorferi (sensu lato) and Rickettsia spp. as well as tick-borne encephalitis and Kemerovo viruses were found in both I. pavlovskyi and I. persulcatus ticks.

CONCLUSION

Almost all pathogens that were previously detected in I. persulcatus ticks were identified in I. pavlovskyi ticks; however, the distribution of species belonging to the B. burgdorferi (sensu lato) complex, the genus Rickettsia, and the family Anaplasmataceae was different between the two tick species. Several new genetic variants of viral and bacterial agents were identified in I. pavlovskyi and I. persulcatus ticks.

Deep sequencing analysis of tick-borne encephalitis virus from questing ticks at natural foci reveals similarities between quasispecies pools of the virus

Every year, tick-borne encephalitis virus (TBEV) causes severe central nervous system infection in 10 000 to 15 000 people in Europe and Asia. TBEV is maintained in the environment by an enzootic cycle that requires a tick vector and a vertebrate host, and the adaptation of TBEV to vertebrate and invertebrate environments is essential for TBEV persistence in nature. This adaptation is facilitated by the error-prone nature of the virus’s RNA-dependent RNA polymerase, which generates genetically distinct virus variants called quasispecies. TBEV shows a focal geographical distribution pattern where each focus represents a TBEV hotspot. Here, we sequenced and characterized two TBEV genomes, JP-296 and JP-554, from questing Ixodes ricinus ticks at a TBEV focus in central Sweden. Phylogenetic analysis showed geographical clustering among the newly sequenced strains and three previously sequenced Scandinavian strains, Toro-2003, Saringe-2009 and Mandal-2009, which originated from the same ancestor. Among these five Scandinavian TBEV strains, only Mandal-2009 showed a large deletion within the 3′ non-coding region (NCR), similar to the highly virulent TBEV strain Hypr. Deep sequencing of JP-296, JP-554 and Mandal-2009 revealed significantly high quasispecies diversity for JP-296 and JP-554, with intact 3′NCRs, compared to the low diversity in Mandal-2009, with a truncated 3′NCR. Single-nucleotide polymorphism analysis showed that 40 % of the single-nucleotide polymorphisms were common between quasispecies populations of JP-296 and JP-554, indicating a putative mechanism for how TBEV persists and is maintained within its natural foci.