Interplay between tick-borne encephalitis virus and the host innate immunity

Sammanfattning: Flaviviruses are important emerging and re-emerging arthropod-borne pathogens that cause significant morbidity and mortality in humans. It consists of globally distributed human pathogens such as tick-borne encephalitis virus (TBEV), West Nile virus (WNV), Japanese encephalitis virus (JEV), yellow fever virus (YFV), dengue virus (DENV), and Zika virus (ZIKV). Depending on type, flaviviruses can cause a variety of symptoms ranging from haemorrhage to neurological disorders.Virus infection is detected by host pattern recognition receptors (PRRs), and through downstream signalling it leads to the production of interferons (IFNs). These IFNs then act in an autocrine or paracrine manner on the cells to induce various IFN-stimulated genes (ISGs), which have antiviral roles. However, the amount of IFN produced depends on the nature of the PRRs used by host cells to detect a particular virus. Although there are many PRRs present in the host cells, their relative contribution in different cell types and against a specific virus may vary. In the first study, we determined the importance of IPS-1 signalling in immunity and pathogenicity of tick-borne flaviviruses. This is an adaptor protein for cytoplasmic RIG-I-like receptors. Using IPS-1-deficient mice, we showed its importance against TBEV and Langat virus (LGTV) infection (the LGTV model virus belongs to the TBEV serogroup). Absence of IPS-1 leads to uncontrolled virus replication in the central nervous system (CNS), but it has only a minor role in shaping the humoral immune response at the periphery. LGTV-infected IPS-1-deficient mice showed apoptosis, activation of microglia and astrocytes, an elevated proinflammatory response, and recruitment of immune cells to the CNS. Interestingly, we also found that IFN-b upregulation after viral infection was dependent on IPS-1 in the olfactory bulb of the brain.  Thus, our results suggest that local immune microenvironment of distinct brain regions is critical for determination of virus permissiveness.Interferons can upregulate several ISGs. Viperin is one such ISG that has a broad-spectrum antiviral action against many viruses. However, the importance of cell type and the significance of viperin in controlling many flavivirus infections in vivo is not known. Using viperin-deficient mice, we found that viperin was necessary for restriction of LGTV replication in the olfactory bulb and cerebrum, but not in the cerebellum. This finding was also confirmed with primary neurons derived from these brain regions. Furthermore, we could also show the particular importance of viperin in cortical neurons against TBEV, WNV, and ZIKV infection. The results suggested that a single ISG can shape the susceptibility and immune response to a flavivirus in different regions of the brain.Although viperin is such an important ISG against flaviviruses, the exact molecular mechanism of action is not known. To understand the mechanism, we performed co-immunoprecipitation screening to identify TBEV proteins that could interact with viperin. While viperin interacted with the prM, E, NS2A, NS2B, and NS3 proteins of TBEV, its interaction with NS3 led to its degradation through the proteosomal pathway. Furthermore, viperin could reduce the stability of other viperin-binding TBEV proteins in an NS3-dependent manner. We screened for viperin activity regarding interaction with NS3 proteins of other flaviviruses. Viperin interacted with NS3 of JEV, ZIKV, and YFV, but selectively degraded NS3 proteins of TBEV and ZIKV, and this activity correlated with its antiviral activity against these viruses.The last study was based on in vivo characterization of the newly isolated MucAr HB 171/11 strain of TBEV which caused unusual gastrointestinal and constitutional symptoms. This strain was compared with another strain, Torö-2003, of the same European subtype of TBEV but isolated from the different focus. Here we found unique differences in their neuroinvasiveness and neurovirulence, and in the immune response to these two strains.In summary, my work shed some light on the interplay between tick-borne flavivirus and the innate immune system. I have shown two examples of CNS region-specific differences in innate immune response regarding both in IFN induction pathways and antiviral effectors. Furthermore, we have investigated the in vivo pathogenesis of a strain of TBEV that caused unusual gastrointestinal and constitutional symptoms.