Tick-borne encephalitis virus delays interferon induction and hides its double-stranded RNA in intracellular membrane vesicles.
Tick-borne encephalitis virus (TBEV) (family Flaviviridae, genus Flavivirus) accounts for approximately 10,000 annual cases of severe encephalitis in Europe and Asia. Here, we investigated the induction of the antiviral type I interferons (IFNs) (alpha/beta IFN [IFN-alpha/beta]) by TBEV. Using strains Neudörfl, Hypr, and Absettarov, we demonstrate that levels of IFN-beta transcripts and viral RNA are strictly correlated. Moreover, IFN induction by TBEV was dependent on the transcription factor IFN regulatory factor 3 (IRF-3). However, even strain Hypr, which displayed the strongest IFN-inducing activity and the highest RNA levels, substantially delayed the activation of IRF-3. As a consequence, TBEV can keep the level of IFN transcripts below the threshold value that would permit the release of IFN by the cell. Only after 24 h of infection have cells accumulated sufficient IFN transcripts to produce detectable amounts of secreted IFNs. The delay in IFN induction appears not to be caused by a specific viral protein, since the individual expressions of TBEV C, E, NS2A, NS2B, NS3, NS4A, NS4B, NS5, and NS2B-NS3, as well as TBEV infection itself, had no apparent influence on specific IFN-beta induction. We noted, however, that viral double-stranded RNA (dsRNA), an important trigger of the IFN response, is immunodetectable only inside intracellular membrane compartments. Nonetheless, the dependency of IFN induction on IFN promoter stimulator 1 (IPS-1) as well as the phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) suggest the cytoplasmic exposure of some viral dsRNA late in infection. Using ultrathin-section electron microscopy, we demonstrate that, similar to other flaviviruses, TBEV rearranges intracellular membranes. Virus particles and membrane-connected vesicles (which most likely represent sites of virus RNA synthesis) were observed inside the endoplasmic reticulum. Thus, apparently, TBEV rearranges internal cell membranes to provide a compartment for its dsRNA, which is largely inaccessible for detection by cytoplasmic pathogen receptors. This delays the onset of IFN induction sufficiently to give progeny particle production a head start of approximately 24 h.
Overby
Anna K
AK
Department of Virology, Institute for Medical Microbiology and Hygiene, University of Freiburg, D-79008 Freiburg, Germany.
Popov
Vsevolod L
VL
Niedrig
Matthias
M
Weber
Friedemann
F
D000818:Animals; D002460:Cell Line; D002522:Chlorocebus aethiops; D006224:Cricetinae; D022162:Cytoplasmic Vesicles / Q000235:genetics / Q000276:immunology / Q000821:virology*; D004669:Encephalitis Viruses, Tick-Borne / Q000235:genetics / Q000502:physiology*; D004675:Encephalitis, Tick-Borne / Q000235:genetics* / Q000276:immunology / Q000821:virology; D006801:Humans; D016898:Interferon-alpha / Q000235:genetics* / Q000276:immunology; D016899:Interferon-beta / Q000235:genetics* / Q000276:immunology; D051379:Mice; D012330:RNA, Double-Stranded / Q000235:genetics* / Q000378:metabolism; D012367:RNA, Viral / Q000235:genetics* / Q000378:metabolism; D015533:Transcriptional Activation; D014709:Vero Cells; D014779:Virus Replication
D016898:Interferon-alpha; D012330:RNA, Double-Stranded; D012367:RNA, Viral; D016899:Interferon-beta