The envelope protein of tick-borne encephalitis virus influences neuron entry, pathogenicity, and vaccine protection

Evolutionary Patterns and Genotype-Specific Amino Acid Mutations of Tick-Borne Encephalitis Virus

Tick-borne encephalitis virus (TBEV) is a significant tick-borne flavivirus responsible for severe human diseases. Here, we analyzed the genetic diversity and evolutionary dynamics of TBEV using 263 genome sequences from the NCBI database and identified key amino acid mutations. TBEV sequences were classified into five genotypes-Baikalian, European, Far-Eastern, Himalaya, and Siberian-showing ORF nucleotide similarity of 81.5% to 88.0% and amino acid similarity of 93.0% to 96.4%. Extensive recombination between genotypes was not observed. Entropy analyses revealed highly variable sites distributed across the Baikalian (n = 2), European (n = 3), Far-Eastern (n = 5), and Siberian (n = 13) genotypes. Each genotype exhibited specific amino acid mutations. Positive selection analysis identified sites under selection in the full dataset (n = 2), as well as in the European (n = 6), Far-Eastern (n = 7), and Siberian (n = 4) genotypes. By integrating highly variable sites, shared genotype-specific mutations, and positively selected sites, we identified 37 key amino acid positions, primarily located on the surfaces of viral proteins. These positions may have a potential impact on protein function and pathogenicity, though further studies are required to validate and evaluate these effects comprehensively. This study provides the first comprehensive analysis of mutational landscapes across TBEV genotypes, uncovering potential critical mutations that may shape viral biology and pathogenicity, and offers valuable insights for further exploration of TBEV characteristics.

Transcriptional Response to Tick-Borne Flavivirus Infection in Neurons, Astrocytes and Microglia In Vivo and In Vitro

Tick-borne encephalitis virus (TBEV) is a neurotropic member of the genus Orthoflavivirus (former Flavivirus) and is of significant health concern in Europe and Asia. TBEV pathogenesis may occur directly via virus-induced damage to neurons or through immunopathology due to excessive inflammation. While primary cells isolated from the host can be used to study the immune response to TBEV, it is still unclear how well these reflect the immune response elicited in vivo. Here, we compared the transcriptional response to TBEV and the less pathogenic tick-borne flavivirus, Langat virus (LGTV), in primary monocultures of neurons, astrocytes and microglia in vitro, with the transcriptional response in vivo captured by single-nuclei RNA sequencing (snRNA-seq) of a whole mouse cortex. We detected similar transcriptional changes induced by both LGTV and TBEV infection in vitro, with the lower response to LGTV likely resulting from slower viral kinetics. Gene set enrichment analysis showed a stronger transcriptional response in vivo than in vitro for astrocytes and microglia, with a limited overlap mainly dominated by interferon signaling. Together, this adds to our understanding of neurotropic flavivirus pathogenesis and the strengths and limitations of available model systems.

Robust CXCL10/IP-10 and CCL5/RANTES Production Induced by Tick-Borne Encephalitis Virus in Human Brain Pericytes Despite Weak Infection

Tick-borne encephalitis virus (TBEV) targets the central nervous system (CNS), leading to potentially severe neurological complications. The neurovascular unit plays a fundamental role in the CNS and in the neuroinvasion of TBEV. However, the role of human brain pericytes, a key component of the neurovascular unit, during TBEV infection has not yet been elucidated. In this study, TBEV infection of the primary human brain perivascular pericytes was investigated with highly virulent Hypr strain and mildly virulent Neudoerfl strain. We used Luminex assay to measure cytokines/chemokines and growth factors. Both viral strains showed comparable replication kinetics, peaking at 3 days post infection (dpi). Intracellular viral RNA copies peaked at 6 dpi for Hypr and 3 dpi for Neudoerfl cultures. According to immunofluorescence staining, only small proportion of pericytes were infected (3% for Hypr and 2% for Neudoerfl), and no cytopathic effect was observed in the infected cells. In cell culture supernatants, IL-6 production was detected at 3 dpi, together with slight increases in IL-15 and IL-4, but IP-10, RANTES and MCP-1 were the main chemokines released after TBEV infection. These chemokines play key roles in both immune defense and immunopathology during TBE. This study suggests that pericytes are an important source of these signaling molecules during TBEV infection in the brain.

Tick-Borne Encephalitis (TBE): From Tick to Pathology

Tick-borne encephalitis (TBE) is a viral arthropod infection, endemic to large parts of Europe and Asia, and is characterised by neurological involvement, which can range from mild to severe, and in 33-60% of cases, it leads to a post-encephalitis syndrome and long-term morbidity. While TBE virus, now identified as Orthoflavivirus encephalitidis, was originally isolated in 1937, the pathogenesis of TBE is not fully appreciated with the mode of transmission (blood, tick, alimentary), viral strain, host immune response, and age, likely helping to shape the disease phenotype that we explore in this review. Importantly, the incidence of TBE is increasing, and due to global warming, its epidemiology is evolving, with new foci of transmission reported across Europe and in the UK. As such, a better understanding of the symptomatology, diagnostics, treatment, and prevention of TBE is required to inform healthcare professionals going forward, which this review addresses in detail. To this end, the need for robust national surveillance data and randomised control trial data regarding the use of various antivirals (e.g., Galidesivir and 7-deaza-2'-CMA), monoclonal antibodies, and glucocorticoids is required to improve the management and outcomes of TBE.

North American House Sparrows Are Competent for Usutu Virus Transmission

Usutu virus (USUV, Flaviviridae) is an emerging mosquito-borne virus that has been implicated in neuroinvasive disease in humans and epizootic deaths in wild birds. USUV is maintained in an enzootic cycle between ornithophilic mosquitoes, primarily Culex spp., and wild birds, predominantly passerine species. However, limited experimental data exist on the species competent for USUV transmission. Here, we demonstrate that house sparrows are susceptible to multiple USUV strains. Our study also revealed that Culex quinquefasciatus mosquitoes are susceptible to USUV, with a significantly higher infection rate for the Netherlands 2016 USUV strain compared to the Uganda 2012 USUV strain at 50% and 19%, respectively. To assess transmission between avian host and mosquito vector, we allowed mosquitoes to feed on either juvenile chickens or house sparrows inoculated with USUV. Both bird models transmitted USUV to C. quinquefasciatus mosquitoes. Linear regression analyses indicated that C. quinquefasciatus infection rates were positively correlated with avian viremia levels, with 3 to 4 log₁₀ PFU/mL representing the minimum avian viremia threshold for transmission to mosquitoes. Based on the viremia required for transmission, house sparrows were estimated to more readily transmit the Netherlands 2016 strain compared to the Uganda 2012 strain. These studies provide insights on a competent reservoir host of USUV. IMPORTANCE Usutu virus (USUV) is a zoonotic mosquito-borne virus that can cause neuroinvasive disease, including meningitis and encephalitis, in humans and has resulted in hundreds of thousands of deaths in wild birds. The perpetuation of USUV in nature is dependent on transmission between Culex spp. mosquitoes and various avian species. To date, few experimental data exist for determining which bird species are important for the maintenance of USUV. Our studies showed that house sparrows can transmit infectious Usutu virus, indicating their role as a competent host species. By identifying reservoir species of USUV, we can predict areas of USUV emergence and mitigate its impacts on global human and wildlife health.

Size Distribution of Inactivated Tick-Borne Encephalitis Virus Particles Revealed by a Comprehensive Physicochemical Approach

Tick-borne encephalitis virus (TBEV) is an enveloped RNA virus, a member of the genus Flavivirus (family Flaviviridae). Here, we provide a detailed analysis of the size and structure of the inactivated TBEV vaccine strain Sofjin-Chumakov. Four analytical methods were used to analyze individual TBEV particles—negative staining TEM, cryo-EM, atomic force microscopy (AFM), and nanoparticle tracking analysis (NTA). All methods confirmed that the particles were monodisperse and that their mean size was ~50 nm. Cryo-EM data allowed us to obtain a 3D electron density model of the virus with clearly distinguishable E protein molecules. STEM-EELS analysis detected phosphorus in the particles, which was interpreted as an indicator of RNA presence. Altogether, the described analytical procedures can be valuable for the characterization of inactivated vaccine virus samples.

The comparative genomic analysis provides insights into the phylogeny and virulence of tick-borne encephalitis virus vaccine strain Senzhang

Tick-borne encephalitis virus (TBEV) is one of the most dangerous tick-borne viral pathogens for humans. It can cause severe tick-borne encephalitis (TBE), multiple neurological complications, and death. The European subtype (TBEV-Eu), Siberian subtype (TBEV-Sib), and Far-Eastern subtype (TBEV-FE) are three main TBEV subtypes, causing varying clinical manifestations. Though TBEV-FE is the most virulent TBEV subtype, the degree of variation in the amino acid sequence of TBEV polyprotein is not high, leaving an issue without proper explanation. We performed phylogenic analysis on 243 TBEV strains and then took Senzhang strain as a query strain and representative strains of three major TBEV subtypes as reference strains to perform the comparative genomic analysis, including synteny analysis, SNP analysis, InDel analysis, and multiple sequence alignment of their envelope (E) proteins. The results demonstrated that insertions or deletions of large fragments occurred at the 3’ end but not at the 5’ end or in the CDS region of TBEV Senzhang strain. In addition, SNP sites are mainly located in the CDS region, with few SNP sites in the non-coding region. Our data highlighted the insertions or deletions of large fragments at the 3’ end and SNP sites in the CDS region as genomic properties of the TBEV Senzhang strain compared to representative strains with the main subtypes. These features are probably related to the virulence of the TBEV Senzhang strain and could be considered in future vaccine development and drug target screening for TBEV.

Retrospective, matched case-control analysis of tickborne encephalitis vaccine effectiveness by booster interval, Switzerland 2006-2020

OBJECTIVE

To estimate effectiveness of tickborne encephalitis (TBE) vaccination by time interval (<5, 5-10 and 10+years) postvaccination.

DESIGN

A retrospective, matched case-control study PARTICIPANTS: Cases-all adult (age 18-79) TBE cases in Switzerland reported via the national mandatory disease reporting surveillance system from 2006 to 2020 (final n=1868). Controls-community controls from a database of randomly selected adults (age 18-79) participating in a 2018 cross-sectional study of TBE vaccination in Switzerland (final n=4625).

PRIMARY OUTCOME MEASURES

For cases and controls, the number of TBE vaccine doses received and the time since last vaccination were determined. Individuals were classified as being 'unvaccinated' (0 doses), 'incomplete' (1-2 doses) or 'complete' (3+ doses). Individuals with 'complete' vaccination were further classified by time since the last dose was received (<5 years, 5-10 years or 10+ years). A conditional logistic regression model was used to calculate vaccine effectiveness (VE: 100 × [1-OR]) for each vaccination status category.

RESULTS

VE for incomplete vaccination was 76.8% (95% CI 69.0% to 82.6%). For complete vaccination, overall VE was 95.0% (95% CI 93.5% to 96.1%). When the most recent dose was received <5 years prior VE was 91.6% (95% CI 88.4% to 94.0%), 95.2% (95% CI 92.4% to 97.0%) when the most recent dose was received 5-10 years prior, and 98.5% (95% CI 96.8% to 99.2%) when the most recent dose was received 10+ years prior.

CONCLUSIONS

That VE does not decrease among completely vaccinated individuals over 10+ years since last vaccination supports the longevity of the protective response following complete TBE vaccination. Our findings support the effectiveness of 10-year TBE booster intervals currently used in Switzerland.

A cross-sectional study evaluating tick-borne encephalitis vaccine uptake and timeliness among adults in Switzerland

The goal of this study was to evaluate timeliness of Tick-borne Encephalitis vaccination uptake among adults in Switzerland. In this cross-sectional survey, we collected vaccination records from randomly selected adults 18–79 throughout Switzerland. Of 4,626 participants, data from individuals receiving at least 1 TBE vaccination (n = 1875) were evaluated. We determined year and age of first vaccination and vaccine compliance, evaluating dose timeliness. Participants were considered “on time” if they received doses according to the recommended schedule ± a 15% tolerance period. 45% of participants received their first TBE vaccination between 2006 and 2009, which corresponds to a 2006 change in the official recommendation for TBE vaccination in Switzerland. 25% were first vaccinated aged 50+ (mean age 37). More than 95% of individuals receiving the first dose also received the second; ~85% of those receiving the second dose received the third. For individuals completing the primary series, 30% received 3 doses of Encepur, 58% received 3 doses of FSME-Immun, and 12% received a combination. According to “conventional” schedules, 88% and 79% of individuals received their second and third doses “on time”, respectively. 20% of individuals receiving Encepur received their third dose “too early”. Of individuals completing primary vaccination, 19% were overdue for a booster. Among the 31% of subjects receiving a booster, mean time to first booster was 7.1 years. We estimate that a quarter of adults in Switzerland were first vaccinated for TBE aged 50+. Approximately 80% of participants receiving at least one vaccine dose completed the primary series. We further estimate that 66% of individuals completing the TBE vaccination primary series did so with a single vaccine type and adhered to the recommended schedule.

Can the booster interval for the tick-borne encephalitis (TBE) vaccine 'FSME-IMMUN' be prolonged? - A systematic review

Tick-borne encephalitis (TBE) vaccines are effective and well tolerated. However, their acceptance and use by the public in endemic areas are suboptimal. To some extent this is due to the complicated dosing schedule requiring frequent boosters at variable intervals that even change with age. Simplification of the dosing schedule has failed so far as it is debated if the persistence of TBE virus (TBEV) antibodies is the only relevant factor for protection or if immune memory plays a decisive role as well. The objective here is to present the available evidence to determine the need for boosters and their interval after a primary series of three doses of FSME-IMMUN. A systematic literature review was conducted with a focus on serology, particularly seropersistence, immune memory, effectiveness, and vaccine breakthroughs (VB) of FSME-IMMUN. While after a 3-dose primary series seropositivity persisted for more than 10 years in >90% of younger subjects, it dropped to 37.5% in those 60 years or older. In contrast, field effectiveness of FSME-IMMUN remains high in irregularly vaccinated subjects and thus does not correlate well with the percentage of subjects achieving an arbitrarily defined threshold of persisting antibodies. FSME-IMMUN booster doses led to increases in antibody responses within 7 days. VB are rare and remain poorly understood. VB did not increase, and vaccine effectiveness did not significantly decrease with time since completion of the primary vaccination series or with the time since administration of the last vaccine dose. For all these reasons, data identified from this systematic review suggest that seropersistence alone does not explain the high effectiveness of FSME-IMMUN irrespective of the time since the last vaccine dose was administered. Induction of immunological memory characterized by a rapid and sustained secondary immune response is proving to be an alternative mechanism of action for protection against TBE. In this context Switzerland and Finland have adopted a longer booster interval (i.e., 10 years) following the three-dose primary immunization schedule without any evidence of harm at a population level. Longer booster intervals will likely drive up vaccine uptake. There is a lack of data to base an interval recommendation beyond 10 years.

Neurotropic Viruses, Astrocytes, and COVID-19

At the end of 2019, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was discovered in China, causing a new coronavirus disease, termed COVID-19 by the WHO on February 11, 2020. At the time of this paper (January 31, 2021), more than 100 million cases have been recorded, which have claimed over 2 million lives worldwide. The most important clinical presentation of COVID-19 is severe pneumonia; however, many patients present various neurological symptoms, ranging from loss of olfaction, nausea, dizziness, and headache to encephalopathy and stroke, with a high prevalence of inflammatory central nervous system (CNS) syndromes. SARS-CoV-2 may also target the respiratory center in the brainstem and cause silent hypoxemia. However, the neurotropic mechanism(s) by which SARS-CoV-2 affects the CNS remain(s) unclear. In this paper, we first address the involvement of astrocytes in COVID-19 and then elucidate the present knowledge on SARS-CoV-2 as a neurotropic virus as well as several other neurotropic flaviviruses (with a particular emphasis on the West Nile virus, tick-borne encephalitis virus, and Zika virus) to highlight the neurotropic mechanisms that target astroglial cells in the CNS. These key homeostasis-providing cells in the CNS exhibit many functions that act as a favorable milieu for virus replication and possibly a favorable environment for SARS-CoV-2 as well. The role of astrocytes in COVID-19 pathology, related to aging and neurodegenerative disorders, and environmental factors, is discussed. Understanding these mechanisms is key to better understanding the pathophysiology of COVID-19 and for developing new strategies to mitigate the neurotropic manifestations of COVID-19.