Date Published: June 4, 2019
Publisher: Public Library of Science
Author(s): Raquel Rodrigues, Katarina Danskog, Anna K. Överby, Niklas Arnberg, Ulrike Gertrud Munderloh.
Tick-borne encephalitis infections have increased the last 30 years. The mortality associated to this viral infection is 0.5 to 30% with a risk of permanent neurological sequelae, however, no therapeutic is currently available. The first steps of virus-cell interaction, such as attachment and entry, are of importance to understand pathogenesis and tropism. Several molecules have been shown to interact with tick-borne encephalitis virus (TBEV) at the plasma membrane surface, yet, no studies have proven that these are specific entry receptors. In this study, we set out to characterize the cellular attachment receptor(s) for TBEV using the naturally attenuated member of the TBEV complex, Langat virus (LGTV), as a model. Inhibiting or cleaving different molecules from the surface of A549 cells, combined with inhibition assays using peptide extracts from high LGTV binding cells, revealed that LGTV attachment to host cells is dependent on plasma membrane proteins, but not on glycans or glycolipids, and suggested that LGTV might use different cellular attachment factors on different cell types. Based on this, we developed a transcriptomic approach to generate a list of candidate attachment and entry receptors. Our findings shed light on the first step of the flavivirus life-cycle and provide candidate receptors that might serve as a starting point for future functional studies to identify the specific attachment and/or entry receptor for LGTV and TBEV.
Tick-borne encephalitis virus (TBEV) is an important, emerging human pathogen and the causative agent of the most important arbovirus-caused disease in Europe and Russia [1,2]. It belongs to the tick-borne virus complex of the Flaviviridae family and Flavivirus genus, which also includes Langat, Kyasanur forest disease, Louping ill, Negishi, Omsk hemorrhagic fever and Powassan viruses , and are closely related to other arthropod-borne flaviviruses, including dengue, West Nile, yellow fever and Japanese encephalitis viruses . Currently, no specific therapeutic exists for TBEV, however, there are several vaccines available as a preventive measure: two european (FSME-Immun/TicoVax, Pfizer Pharma and Encepur, Chiron Behring) and two russian, (TBE Moscow, Chumakov Institute and Encevir, Microgen) . Despite this, each year 6000 to 8000 clinical cases are reported in Russia and 3000 to 4000 cases in other parts of Europe [6–9]. The symptoms of TBEV infection in humans, ranges from mild to severe forms of meningitis, encephalitis, and haemorrhagic fever  with a risk of permanent neurological sequelae of around 40% . The mortality rate ranges from 0,5 to 30%, depending on the subtype. However, the mechanisms underlying this difference in pathogenicity are not completely understood .
Virus-host interactions such as attachment and entry into host cells are essential first steps of the virus life-cycle. A better knowledge of these interactions is crucial in order to better understand virus tropism and pathogenesis. In simple situations, for a virus to enter into a host cell, it attaches to its specific receptor with high affinity, which leads to virus uptake and entry. However, this process can be far more complex, involving the cooperation of several molecules for attachment and entry, either used simultaneously, sequentially or in a cell-type-specific manner.