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

Abstract

Over 50 million humans live in areas of potential exposure to tick-borne encephalitis virus (TBEV). The disease exhibits an estimated 16,000 cases recorded annually over 30 European and Asian countries. Conventionally, TBEV transmission to Ixodes spp. ticks occurs whilst feeding on viraemic animals. However, an alternative mechanism of non-viraemic transmission (NVT) between infected and uninfected ticks co-feeding on the same transmission-competent host, has also been demonstrated. Here, using laboratory-bred I. ricinus ticks, we demonstrate low and high efficiency NVT for TBEV strains Vasilchenko (Vs) and Hypr, respectively. These virus strains share high sequence similarity but are classified as two TBEV subtypes. The Vs strain is a Siberian subtype, naturally associated with I. persulcatus ticks whilst the Hypr strain is a European subtype, transmitted by I. ricinus ticks. In mammalian cell culture (porcine kidney cell line PS), Vs and Hypr induce low and high cytopathic effects (cpe), respectively. Using reverse genetics, we engineered a range of viable Vs/Hypr chimaeric strains, with substituted genes. No significant differences in replication rate were detected between wild-type and chimaeric viruses in cell culture. However, the chimaeric strain Vs[Hypr str] (Hypr structural and Vs non-structural genomic regions) demonstrated high efficiency NVT in I. ricinus whereas the counterpart Hypr[Vs str] was not transmitted by NVT, indicating that the virion structural proteins largely determine TBEV NVT transmission efficiency between ticks. In contrast, in cell culture, the extent of cpe was largely determined by the non-structural region of the TBEV genome. Chimaeras with Hypr non-structural genes were more cytotoxic for PS cells when compared with Vs genome-based chimaeras.

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

“…It has recently been demonstrated that the structural genes of Eu-TBEV are prime determinants for TBEV replication and non-viremic transmission among co-feeding I. ricinus , whereas the non-structural genes regulate the cytopathogenicity of TBEV [ 24 ]. The genes encoding the structural proteins of JP-296, JP-554 and Mandal-2009 exhibited the highest numbers of SNPs (data not shown), whereas in another study, the NS2A gene of cDNA-derived TBEV exhibited the highest number of SNPs after cultivation in cell culture and in mice [ 8, 25, 26 ].…”

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

“…Non-systemic transmission can also happen when the host is immune to the infection [46]. Therefore, the force of infection for the questing larvae through non-systemic transmission route is …”

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

“…In summary, both amino acid changes in the E protein of the 40 PS probably contribute to lower virulence in vivo, while the mutation in the NS4B protein most likely arose as a consequence of viral adaptation to PS cells. An underlying role of the structural genes in the pathogenicity for mice was reported previously [75]. Small plaque production by 40 IRE might be the consequence of mutations in the prM protein and 5' UTR, while the mutation in the NS4B protein arose most likely as a consequence of viral adaptation to tick cells.…”