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Type I Interferon response in olfactory bulb, the site of tick-borne flavivirus accumulation, is primarily regulated by IPS-1
Umeå University, Faculty of Medicine, Department of Clinical Microbiology, Virology.
Umeå University, Faculty of Medicine, Department of Clinical Microbiology, Virology.
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2016 (English)In: Journal of Neuroinflammation, ISSN 1742-2094, E-ISSN 1742-2094, Vol. 13, 22Article in journal (Refereed) Published
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Text
Abstract [en]

Background: Although type I interferons (IFNs)—key effectors of antiviral innate immunity are known to be induced via different pattern recognition receptors (PRRs), the cellular source and the relative contribution of different PRRs in host protection against viral infection is often unclear. IPS-1 is a downstream adaptor for retinoid-inducible gene I (RIG-I)-like receptor signaling. In this study, we investigate the relative contribution of IPS-1 in the innate immune response in the different brain regions during infection with tick-borne encephalitis virus (TBEV), a flavivirus that causes a variety of severe symptoms like hemorrhagic fevers, encephalitis, and meningitis in the human host.

Methods: IPS-1 knockout mice were infected with TBEV/Langat virus (LGTV), and viral burden in the peripheral and the central nervous systems, type I IFN induction, brain infiltrating cells, and inflammatory response was analyzed.

Results: We show that IPS-1 is indispensable for controlling TBEV and LGTV infections in the peripheral and central nervous system. Our data indicate that IPS-1 regulates neuropathogenicity in mice. IFN response is differentially regulated in distinct regions of the central nervous system (CNS) influencing viral tropism, as LGTV replication was mainly restricted to olfactory bulb in wild-type (WT) mice. In contrast to the other brain regions, IFN upregulation in the olfactory bulb was dependent on IPS-1 signaling. IPS-1 regulates basal levels of antiviral interferon-stimulated genes (ISGs) like viperin and IRF-1 which contributes to the establishment of early viral replication which inhibits STAT1 activation. This diminishes the antiviral response even in the presence of high IFN-β levels. Consequently, the absence of IPS-1 causes uncontrolled virus replication, in turn resulting in apoptosis, activation of microglia and astrocytes, elevated proinflammatory response, and recruitment of inflammatory cells into the CNS.

Conclusions: We show that LGTV replication is restricted to the olfactory bulb and that IPS-1 is a very important player in the olfactory bulb in shaping the innate immune response by inhibiting early viral replication and viral spread throughout the central nervous system. In the absence of IPS-1, higher viral replication leads to the evasion of antiviral response by inhibiting interferon signaling. Our data suggest that the local microenvironment of distinct brain regions is critical to determine virus permissiveness.

Place, publisher, year, edition, pages
2016. Vol. 13, 22
Keyword [en]
Tick-borne encephalitis, IPS-1, Brain, Olfactory bulb, Type I interferons, Antiviral mechanism, Immune asion
National Category
Microbiology in the medical area
Identifiers
URN: urn:nbn:se:umu:diva-116737DOI: 10.1186/s12974-016-0487-9ISI: 000368886900003PubMedID: 26819220OAI: oai:DiVA.org:umu-116737DiVA: diva2:904830
Available from: 2016-02-19 Created: 2016-02-11 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Interplay between tick-borne encephalitis virus and the host innate immunity
Open this publication in new window or tab >>Interplay between tick-borne encephalitis virus and the host innate immunity
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

Flavivirus finns spridda över hela världen och orsakar miljontals infektioner varje år. Några av de medicinsk mest viktiga flavivirusen är fästingburen encefalit virus (TBEV), West Nile virus (WNV), Japansk encefalit virus (JEV), gula febern (YFV) och Zika virus (ZIKV). Dessa virus kan orsaka olika komplikationer till exempel blödarfeber och hjärninflammation.

Vid en infektion så upptäcker värdcellen virusinfektionen med hjälp av speciella receptorer, så kallade PRRs. Dessa finns i alla celler och känner igen viruskomponenter som normalt inte finns i en oinfekterad cell. När PRRs detekterar en virusinfektion svarar cellen med att tillverka ett signal protein interferon (IFN). IFN skickas ut ur cellen och hämmar virusinfektioner genom att sätta igång ett försvarsprogram i andra celler bestående av hundratals försvarsproteiner som kan motverka virusinfektionen. Vilka PRRs som behövs för att detektera ett virus är olika vid olika virusinfektioner. I första studien fann vi att IPS-1 är av yttersta vikt för skydda mot fästingburna flavivirus. IPS-1 är ett så kallat adapter protein som behövs för att två PRRs, RIG-I och MDA-5, ska kunna förmedla signaler som leder till IFN tillverkning. Med hjälp av möss som saknar IPS-1 fann vi att IPS-1 behövs för att tillverka IFN protein och skydda mot fästingburna flavivirus. IPS-1 var särskilt viktigt för interferon produktion inom luktloben i hjärnan. Därför kunde vi dra slutsatsen att immunresponsen regleras olika inom olika delar av hjärnan.

Ett försvarsprotein som visat sig vara särskilt viktig vid virusinfektion är viperin. Viperin har visat sig kunna hämma en rad olika virus men den specifika rollen av viperin in vivo vid flavivirus infektion var inte fullt känd. Vi fann att viperin behövs för att hämma LGTV i lukloben och storhjärnan men inte i lillhjärnan. Vi kunde bekräfta detta med hjälp av primära nervceller isolerade från dessa hjärnregioner. Vi fann även att viperin var av yttersta vikt för att kontrollera TBEV, WNV och ZIKV infektion i nervceller från hjärnbarken (del av storhjärnan). Därför kunde vi dra slutsatsen att ett enskilt försvarsprotein kan avgöra mottagligheten mot flavivirus inom olika hjärnregioner.

Trots att viperin är så viktig för att skydda mot flavivirus så vet vi inte hur viperin åstadkommer detta. Därför ville vi undersöka hur viperin kan förmedla sin antivirala effekt. Vi fann att viperin kan binda till flera TBEV proteiner, men att viperin specifikt kan bryta ner ett virusprotein som heter NS3. NS3 är väldigt viktigt för att flavivirus ska kunna etablera en infektion och kunna föröka sig. Eftersom vi visste att viperin kan hämma andra flavivirus ville vi veta om viperin även förstör NS3 från JEV, ZIKV och YFV. Vi upptäckte att viperin kunde binda till NS3 hos alla dessa flavivirus men att viperin specifikt förstörde TBEV och ZIKV NS3, intressant nog så kunde viperin endast hämma dessa virus infektioner men inte JEV och YFV.

I den sista studien ville vi karaktärisera en ny TBEV stam som bara orsakar magoch tarmbesvär men inga neurologiska symptom. TBEV har aldrig tidigare visat sig kunna orsaka detta och därför ville vi undersöka saken vidare. Vi fann att denna TBEV stam skiljde sig mot en närbesläktad stam genom att orsaka en starkare immunrespons men mildare sjukdomsförlopp.

Sammanfattningsvis har jag undersökt samspelet mellan fästingburna flavivirus och det medfödda immunförsvaret. Jag har även visat att immunresponsen regleras olika inom olika hjärnregioner, både beträffande IFN inducering och antivirala proteiner. Vidare har jag hittat mekanismen för hur viperin proteinet hämmar TBEV och ZIKV, vilket var genom att förstöra NS3. Dessutom har jag karaktäriserat sjukdomsförloppet hos möss efter infektion med en ovanlig TBEV stam som orsakar mag och tarm besvär.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2017. 51 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1938
Keyword
Virology, Flavivirus, IPS-1, Interferon, Viperin
National Category
Microbiology in the medical area
Research subject
Microbiology
Identifiers
urn:nbn:se:umu:diva-142816 (URN)978-91-7601-821-7 (ISBN)
Public defence
2018-01-19, A5_R0, Building 6F, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2017-12-14 Created: 2017-12-12 Last updated: 2017-12-13Bibliographically approved

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