Fast type I interferon response protects astrocytes from flavivirus infection and virus-induced cytopathic effects

Lindqvist, Richard; Mundt, Filip; Gilthorpe, Jonathan D.; Wölfel, Silke; Gekara, Nelson O.; Kröger, Andrea; Överby, Anna K.

In this study, we show that, although primary astrocytes are infected in a comparable manner to MEF initially, viral replication and spread is dramatically inhibited, indicating that astrocytes are abortively infected with neurotropic flavivirus similar to La Crosse virus infections [73] . This phenotype is dependent on IFNAR expression, since viral replication is uncontrolled in IFNAR-deficient astrocytes. Consistent with previous results for WNV, we now show for all three subtypes of TBEV, JEV, and ZIKV that the rapidly produced type I IFN in astrocytes after neurotropic flavivirus infection limits the viral spread and prevent virus-induced killing of the cells. showing upstream regulators that are predicted to be activated (>2 activation score) or inhibited (<−2 activation score) according to experimental and literature findings. Y-axis show upstream regulators predicted from supernatant-treated astrocytes, and x-axis show upstream regulators predicted from IFNαB/D-treated astrocytes. Five upstream regulators show an idiosyncratic activation specific for the supernatant-treated cells, aligning the y-axis and with an activations score of >2. b An example of predicted activators by IPA. The circle of markers is upregulated transcripts (red), and the center is a regulator (IFN alpha/beta), predicted to be active based on information (orange arrows) in the IPA knowledge-database. Each black dot in a is representing a similar circle of regulated markers. c Genes that, when overexpressed, lead to a similar expression pattern as either supernatant treatment (y-axis) or IFNαB/D treatment (x-axis) according to the connectivity map database (CMAP). Genes predicted by both treatments are in the top right corner (e.g., IFNG and IFNB1) while effects exclusive to supernatant treatment are high (>90) on the y-axis and low (<90) on the x-axis Weak IFN signals, transmitted independently of viral infection, could be crucial for predisposing cells to amplify their IFN production in response to viral infection and enhance their response to other cytokines [74, 75] . Intriguingly, WT astrocytes were in an antiviral state with higher levels of ISGs compared to MEFs, and these were rapidly induced to even higher levels after TBEV infection. Because no difference in basal levels of IFNβ or IFNα2 was observed between MEFs and WT astrocytes (data not shown), a different mechanism compared to IFN priming may exist [76] . However, it seems that the responsiveness to viral infection is determined by basal levels of innate immune components. We have reported previously that TBEV is able to delay IFNβ induction by hiding its dsRNA within replication vesicles in A549 cells [42, 77] and a similar observation was made in the MEFs in this study. No delay in type I IFN induction was observed in astrocytes, clearly indicating that if certain antiviral factors are present in the cell at a basal level the virus is unable to delay the IFN response. High basal expression of ISGs have also been linked to viral resistance to influenza A infection in bronchial epithelial cells [78] and contribute to neuronal tropism of WNV [19] . Furthermore, we previously showed that viperin is a strong inhibitor of TBEV infection [50] . Therefore, the increased basal expression of some key ISGs could contribute to lower flavivirus replication in astrocytes. However, the basal expression was not enough by itself to restrict TBEV replication or spread as neutralization of IFNAR with antibodies