Experimental Assessment of Possible Factors Associated with Tick-Borne Encephalitis Vaccine Failure

The structure of inactivated mature tick-borne encephalitis virus at 3.0 Å resolution

Tick-borne encephalitis virus (TBEV) causes a severe disease, tick-borne encephalitis (TBE), that has a substantial epidemiological importance for Northern Eurasia. Between 10,000 and 15,000 TBE cases are registered annually despite the availability of effective formaldehyde-inactivated full-virion vaccines due to insufficient vaccination coverage, as well as sporadic cases of vaccine breakthrough. The development of improved vaccines would benefit from the atomic resolution structure of the antigen. Here we report the refined single-particle cryo-electron microscopy (cryo-EM) structure of the inactivated mature TBEV vaccine strain Sofjin-Chumakov (Far-Eastern subtype) at a resolution of 3.0 Å. The increase of the resolution with respect to the previously published structures of TBEV strains Hypr and Kuutsalo-14 (European subtype) was reached due to improvement of the virus sample quality achieved by the optimized preparation methods. All the surface epitopes of TBEV were structurally conserved in the inactivated virions. ELISA studies with monoclonal antibodies supported the hypothesis of TBEV protein shell cross-linking upon inactivation with formaldehyde.

Immunogenicity of tick-borne-encephalitis-virus-(TBEV)-vaccination and impact of age on humoral and cellular TBEV-specific immune responses in patients with rheumatoid arthritis

Inactivated vaccines, such as tick-borne-encephalitis-virus-(TBEV) vaccine, have been discussed as less immunogenic in elderly and in immunocompromised patients. In this controlled cross-sectional cohort study, the antibody and cellular responses after TBEV-vaccination were investigated in 36 rheumatoid arthritis (RA) patients and 112 healthy controls (HC) by evaluating IgG-anti-TBEV concentration, neutralization and relative avidity index (RAI). Cellular reactivity was assessed by IFNgamma-producing spot-forming-units (SFU) by ELISPOT assay and flow cytometry. RA patients showed lower IgG-anti-TBEV compared to HC, which were influenced by age at and time since last TBEV vaccination and disease duration. High-responders regarding cellular immunity and avidity were less frequent in RA compared to HC. RA patients who had received booster vaccinations were more likely to demonstrate higher IgG-anti-TBEV responses compared to those who had not. In conclusion, RA patients showed a negative effect of age on anti-TBEV-IgG and immunological benefits of timely booster vaccination are suggested.

Elongation of N6-benzyladenosine scaffold via Pd-catalyzed C-C bond formation leads to derivatives with antiflaviviral activity

Decoration of nucleoside analogues with lipophilic groups often leads to compounds with improved antiviral activity. For example, N6-benzyladenosine derivatives containing elongated lipophilic substituents in the benzyl core efficiently inhibit reproduction of tick-borne encephalitis virus (TBEV), while N6-benzyladenosine itself potently inhibits reproduction of human enterovirus A71 (EV-A71). We have extended a series of N6-benzyladenosine analogues using effective synthetic methods of CC bond formation based on Pd-catalyzed cross-coupling reactions (Sonogashira and Suzuki) in order to study the influence of bulky lipophilic substituents in the N6 position of adenosine on the antiviral activity against flaviviruses, such as TBEV, yellow fever virus (YFV) and West Nile virus (WNV), as well as a panel of enteroviruses including EV-A71, Echovirus 30 (E30), and poliovirus type 2 (PV2). Reproduction of tested flaviviruses appeared to be inhibited by the micromolar concentrations of the compounds, while cytotoxicity in most cases was beyond the detection limit. Time-of-addition studies demonstrated that the hit compounds inhibited the stage of viral RNA synthesis, but not the stages of the viral entry or protein translation. As a result, several new promising antiflaviviral leads have been identified. On the other hand, none of the synthesized compounds inhibited enterovirus reproduction, indicating a possibility of involvement of flavivirus-specific pathways in their mechanism of action.

Inapparent Tick-Borne Orthoflavivirus Infection in Macaca fascicularis: A Model for Antiviral Drug and Vaccine Research

Tick-borne encephalitis virus (TBEV) and Powassan virus (POWV) are neurotropic tick-borne orthoflaviviruses. They cause mostly asymptomatic infections in hosts, but severe forms with CNS involvement can occur. Studying the early stages of viral infections in humans is challenging, and appropriate animal models are essential for understanding the factors determining the disease severity and for developing emergency prophylaxis and treatment options. In this work, we assessed the model of the early stages of TBEV and POWV mono- and co-infections in Macaca fascicularis. Serological, biochemical, and virological parameters were investigated to describe the infection, including its impact on animal behavior. Viremia, neutralizing antibody dynamics, and viral load in organs were chosen as the main parameters distinguishing early-stage orthoflavivirus infection. Levels of IFNα, monocyte count, and cognitive test scores were proposed as additional informative indicators. An assessment of a tick-borne encephalitis vaccine using this model showed that it provided partial protection against POWV infection in Macaca fascicularis without signs of antibody-dependent enhancement of infection.

Recombinant TBEV Protein E of the Siberian Subtype Is a Candidate Antigen in the ELISA Test System for Differential Diagnosis

The tick-borne encephalitis virus (TBEV) is one of the most common members of the Orthoflavivirus genus, which comprises the causative agents of severe diseases in humans and animals. Due to the expanding areas of orthoflavivirus infection, its differential diagnosis is highly demanded. Commercial test kits based on inactivated TBEV may not provide reliable differentiation between flaviviruses because of serological crossover in this genus. Application of recombinant domains (sE and dIII) of the TBEV Sukhar-strain protein E as antigens in an ELISA test system allowed us to identify a wide range of antibodies specific to different TBEV strains. We tested 53 sera from human patients with confirmed TBE diagnosis (the efficacy of our test system based on sE protein was 98%) and 56 sera from patients with other orthoflavivirus infections in which no positive ones were detected using our ELISA test system, thus being indicative of its 100% specificity. We also tested mouse and rabbit sera containing antibodies specific to 17 TBEV strains belonging to different subtypes; this assay exhibited high efficacy and differentiation ability in detecting antibodies against TBEV from other orthoflaviviruses such as Omsk hemorrhagic fever, Powassan, yellow fever, dengue, West Nile, Zika, and Japanese encephalitis viruses.

Tick-Borne Encephalitis-Review of the Current Status

The tick-borne encephalitis virus (TBEV) is the arboviral etiological agent of tick-borne encephalitis (TBE), considered to be one of the most important tick-borne viral diseases in Europe and Asia. In recent years, an increase in the incidence of TBE as well as an increasing geographical range of the disease have been noted. Despite the COVID-19 pandemic and the imposition of restrictions that it necessitated, the incidence of TBE is rising in more than half of the European countries analyzed in recent studies. The virus is transmitted between ticks, animals, and humans. It seems that ticks and small mammals play a role in maintaining TBEV in nature. The disease can also affect dogs, horses, cattle, and small ruminants. Humans are incidental hosts, infected through the bite of an infected tick or by the alimentary route, through the consumption of unpasteurized milk or milk products from TBEV-infected animals. TBEV infections in humans may be asymptomatic, but the symptoms can range from mild flu-like to severe neurological. In Europe, cases of TBE are reported every year. While there is currently no effective treatment for TBE, immunization and protection against tick bites are critical in preventing this disease.

Is There a Future for Traditional Immunogens When We Have mRNA?

As the SARS-CoV-2 pandemic ends and we enter into a post-pandemic world, it is the time to reflect on the lessons learned [...].

Charge-changing point mutations in the E protein of tick-borne encephalitis virus

Introduction of point mutations is one of the forces enabling arboviruses to rapidly adapt in a changing environment. The influence of these mutations on the properties of the virus is not always obvious. In this study, we attempted to clarify this influence using an in silico approach. Using molecular dynamics (MD) simulations, we investigated how the position of charge-changing point mutations influences the structure and conformational stability of the E protein for a set of variants of a single TBEV strain. The computational findings were supported by experimental evaluation of relevant properties of virions, such as binding to heparan sulfate, thermostability, and susceptibility of the viral hemagglutinating activity to detergents. Our results also point to relationships between E protein dynamics and viral neuroinvasiveness.

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.

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.

Delimitation of the Tick-Borne Flaviviruses. Resolving the Tick-Borne Encephalitis virus and Louping-Ill Virus Paraphyletic Taxa

The tick-borne flavivirus (TBFV) group contains at least 12 members where five of them are important pathogens of humans inducing diseases with varying severity (from mild fever forms to acute encephalitis). The taxonomy structure of TBFV is not fully clarified at present. In particular, there is a number of paraphyletic issues of tick-borne encephalitis virus (TBEV) and louping-ill virus (LIV). In this study, we aimed to apply different bioinformatic approaches to analyze all available complete genome amino acid sequences to delineate TBFV members at the species level. Results showed that the European subtype of TBEV (TBEV-E) is a distinct species unit. LIV, in turn, should be separated into two species. Additional analysis of TBEV and LIV antigenic determinant diversity also demonstrate that TBEV-E and LIV are significantly different both from each other and from the other TBEV subtypes. The analysis of available literature provided data on other virus phenotypic particularities that supported our hypothesis. So, within the TBEV+LIV paraphyletic group, we offer to assign four species to get a more accurate understanding of the TBFV interspecies structure according to the modern monophyletic conception.

Effect of immature tick-borne encephalitis virus particles on antiviral activity of 5-aminoisoxazole-3-carboxylic acid adamantylmethyl esters

Tick-borne encephalitis virus (TBEV), a member of the genus Flavivirus, is common in Europe and Asia and causes a severe disease of the central nervous system. A promising approach in the development of therapy for TBEV infection is the search for small molecule antivirals targeting the flavivirus envelope protein E, particularly its β-n-octyl-d-glucoside binding pocket (β-OG pocket). However, experimental studies of candidate antivirals may be complicated by varying amounts and different forms of the protein E in the virus samples. Viral particles with different conformations and arrangements of the protein E are produced during the replication cycle of flaviviruses, including mature, partially mature, and immature forms, as well as subviral particles lacking genomic RNA. The immature forms are known to be abundant in the viral population. We obtained immature virion preparations of TBEV, characterized them by RT-qPCR, and assessed in vivo and in vitro infectivity of the residual mature virions in the immature virus samples. Analysis of the β-OG pocket structure on the immature virions confirmed the possibility of binding of adamantylmethyl esters of 5-aminoisoxazole-3-carboxylic acid in the pocket. We demonstrated that the antiviral activity of these compounds in plaque reduction assay is significantly reduced in the presence of immature TBEV particles.