Langat virus, a prototypic tick‐borne encephalitis virus, impacts IL‐6 signaling by downregulating gp130 expression

Tick-borne flaviviruses make up a distinct and substantial arm of the Flaviviridae family, currently consisting of 12 members of which eight can cause human disease.1-3 Five members (tick-borne encephalitis virus [TBEV], louping ill virus [LIV], Kyasanur Forest diseases virus [KFDV], Powassan virus [POWW] and Omsk hemorrhagic fever virus [OHFV]) comprise the human "tick-borne encephalitis (TBE) serocomplex," causing an estimated 12 000 annual cases of TBE in Europe and Asia.4 TBEV is the largest causative agent of TBE, resulting in disease characterized by an initial nonspecific illness followed by central nervous system (CNS) inflammation and long-term neurological symptoms in up to one half of the cases.5-7 TBEV morbidity and mortality rates vary across its three main subtypes (European, Siberian, and far-eastern), with the far-eastern strain associated with the highest rates of neurological complications and a mortality rate of up to 40%.8 Further study is needed to develop antivirals against TBEV (which are currently not available) and more optimized TBEV vaccines (which currently require three initial doses followed by booster doses at 3–5-year intervals9). However, TBEV research is limited by its classification as a BSL-4/BSL-3 enhanced level pathogen, making the use of attenuated tick-borne flaviviruses a useful tool in characterizing TBEV. The Langat virus (LGTV) is one such model for TBEV, sharing over 80% genetic homology with TBEV10, 11 and was previously tested as an attenuated live-vaccine candidate for TBEV.12-14 Recent work has begun to identify host molecular pathways impacted by LGTV infection. For example, type 1 interferon signaling has been found to impact infection and antiviral immunity in the brain15-17 and to prevent mouse-to-mouse transmission18 (when it is normally transmitted by ticks19). LGTV has also been found to impact universal cellular signaling pathways including the unfolded protein response,20, 21 protein kinase c,22 and protein kinase r.23 A recently accepted article by the Journal of Medical Virology entitled, "Langat virus inhibits the gp130/JAK/STAT signaling by reducing the gp130 protein level"24 addresses another gap in the knowledge of LGTV signaling by investigating its impact on the cellular interleukin (IL)-6 pathway. The IL-6 pathway consists of a family of related cytokines,25 two of which (IL-6 and oncostatin M [OSM]) were utilized in this manuscript. These cytokines bind to a membrane-associated protein complex consisting of their respective receptors and gp130, which result in the phosphorylation and dimerization of STAT3, its translocation into the nucleus, and target gene expression26-28 (Figure 1). Both IL-6 and OSM initiate inflammation and activation of innate and adaptive immune cells.29, 30 Using an in vitro approach, the authors first investigated whether LGTV infection impacted gp130/JAK/STAT signaling and vice-versa. An inhibitory relationship existed in both directions, with OSM treatment limiting LGTV replication contingent upon STAT3 phosphorylation and LGTV infection inhibiting STAT3 phosphorylation. LGTV inhibition of pSTAT3 was dose-dependent and was detectable by 24 h postinfection. Inhibition also happenned with infection alone or with attempted rescue of STAT3 phosphorylation by treatment with OSM or with the expression of the recombinant human herpesvirus 8 (HHV8) viral-encoded IL6 (vIL6) during LGTV infection. The authors then postulated that the mechanism behind LGTV inhibition might lie upstream of STAT3, as LGTV infection was also found to block phosphorylation of JAK1 and downregulate protein levels of gp130 (which was recapitulated with TBEV in vitro infection). Downregulation of gp130 was independent of phosphatase activity and of interferon induced STAT1/2 signaling, and the lack of contribution of STAT1/2 was confirmed by RNAi knockdown. Expression of individual LGTV proteins showed that NS5 protein appeared to be responsible for inhibition of gp130/JAK1/STAT3 signaling. Notably, gp130 downregulation also occurred during TBEV infection, demonstrating the potential conservation of this signaling mechanism across tick-borne flavivirues. In summary, this manuscript provides solid evidence that LGTV inhibits gp130/JAK/STAT signaling by downregulating gp130. The evidence appears to suggest a direct association, with inhibition of gp130 expression occurring independently of interferon signaling and phosphatase activity. This is strengthened by pSTAT3 inhibition also occurring during vIL6 treatment, which can activate gp130 in the absence of the cognate IL-6R receptor. Gp130 downregulation also occurred during TBEV infection, although future studies are needed to fully correlate TBEV and LGTV infections. Further investigation will also be needed to determine the contribution IL-6 inhibition to immune responses raised against LGTV in vivo. Nevertheless, the report provides a potential new target for therapeutic development against TBEV infection and its role in the development of TBE. Morgan Brisse was supported in part by the Intramural Research Program of the NIH. The authors declare no conflict of interest. No new data were included in this article.