The host protein viperin is an interferon stimulated gene (ISG) that is up-regulated during a number of viral infections. In this study we have shown that dengue virus type-2 (DENV-2) infection significantly induced viperin, co-incident with production of viral RNA and via a mechanism requiring retinoic acid-inducible gene I (RIG-I). Viperin did not inhibit DENV-2 entry but DENV-2 RNA and infectious virus release was inhibited in viperin expressing cells. Conversely, DENV-2 replicated to higher tires earlier in viperin shRNA expressing cells. The anti-DENV effect of viperin was mediated by residues within the C-terminal 17 amino acids of viperin and did not require the N-terminal residues, including the helix domain, leucine zipper and S-adenosylmethionine (SAM) motifs known to be involved in viperin intracellular membrane association. Viperin showed co-localisation with lipid droplet markers, and was co-localised and interacted with DENV-2 capsid (CA), NS3 and viral RNA. The ability of viperin to interact with DENV-2 NS3 was associated with its anti-viral activity, while co-localisation of viperin with lipid droplets was not. Thus, DENV-2 infection induces viperin which has anti-viral properties residing in the C-terminal region of the protein that act to restrict early DENV-2 RNA production/accumulation, potentially via interaction of viperin with DENV-2 NS3 and replication complexes. These anti-DENV-2 actions of viperin show both contrasts and similarities with other described anti-viral mechanisms of viperin action and highlight the diverse nature of this unique anti-viral host protein.

1 Although endothelial cell (EC) infection is not widespread during dengue virus (DENV) infection in 2vivo, the endothelium is the site of the pathogenic effects seen in severe DENV disease. In this study 3 we investigated DENV infection of primary EC and defined factors that influence infection in this cell 4 type. 5Consistent with in vivo findings where EC infection is infrequent, only 3-15% of EC became 6 productively DENV-2-infected in vitro. This low level infection could not be attributed to inhibition 7 by heparin, EC donor variation, heterogeneity, or biological source. DENV-infection of EC was 8 associated with induction of innate immune responses, including increased STAT1 protein, STAT1-9 phosphorylation, and IFN-β, OAS-1, IFIT-1/ISG56 and viperin mRNA. Antibody blocking of IFN-β 10 inhibited the induction of OAS1, IFIT1/ISG56 and viperin while shRNA knockdown of viperin 11 enhanced DENV-infection in EC. DENV-infection of EC resulted in increased activity of 12 sphingosine kinase 1, a factor important in maintaining vascular integrity, and altered basal and 13 stimulated changes in barrier integrity of DENV-infected EC monolayers. 14 Thus, DENV productively infects only a small percentage of primary EC but this has a major 15 influence on induction of IFN-β driven innate immune responses that can restrict infection while the 16 EC themselves are functionally altered. These changes may have important consequences for the 17 endothelium and are reflective of pathogenic changes associated with vascular leakage, as seen in 18 DENV disease.

Sphingosine kinase 1 (SphK1) is a lipid kinase with important roles including regulation of cell survival. We have previously shown reduced SphK1 activity in cells with an established dengue virus type-2 (DENV-2) infection. In this study, we examined the effect of alterations in SphK1 activity on DENV-2 replication and cell death and determined the mechanisms of the reduction in SphK1 activity. Chemical inhibition or overexpression of SphK1 after established DENV-2 infection had no effect on infectious DENV-2 production, although inhibition of SphK1 resulted in enhanced DENV-2-induced cell death. Reduced SphK1 activity was observed in multiple cell types, regardless of the ability of DENV-2 infection to be cytopathic, and was mediated by a post-translational mechanism. Unlike bovine viral diarrhea virus, where SphK1 activity is decreased by the NS3 protein, SphK1 activity was not affected by DENV-2 NS3 but, instead, was reduced by expression of the terminal 396 bases of the 39 UTR of DENV-2 RNA. We have previously shown that eukaryotic elongation factor 1A (eEF1A) is a direct activator of SphK1 and here DENV-2 RNA co-localized and coprecipitated with eEF1A from infected cells. We propose that the reduction in SphK1 activity late in DENV-2-infected cells is a consequence of DENV-2 out-competing SphK1 for eEF1A binding and hijacking cellular eEF1A for its own replication strategy, rather than a specific host or virus-induced change in SphK1 to modulate viral replication. Nonetheless, reduced SphK1 activity may have important consequences for survival or death of the infected cell.

Sphingosine kinase (SK) 1 is a host kinase that enhances some viral infections. Here we investigated the ability of SK1 to modulate dengue virus (DENV) infection in vitro. Overexpression of SK1 did not alter DENV infection; however, targeting SK1 through chemical inhibition resulted in reduced DENV RNA and infectious virus release. DENV infection of SK1 2/2 murine embryonic fibroblasts (MEFs) resulted in inhibition of infection in an immortalized line (iMEF) but enhanced infection in primary MEFs (18MEFs). Global cellular gene expression profiles showed expected innate immune mRNA changes in DENV-infected WT but no induction of these responses in SK1 2/2 iMEFs. Reverse transciption PCR demonstrated a low-level induction of IFN-b and poor induction of mRNA for the interferon-stimulated genes (ISGs) viperin, IFIT1 and CXCL10 in DENV-infected SK1 2/2 compared with WT iMEFs. Similarly, reduced induction of ISGs was observed in SK1 2/2 18MEFs, even in the face of high-level DENV replication. In both iMEFs and 18MEFs, DENV infection induced production of IFN-b protein. Additionally, higher basal levels of antiviral factors (IRF7, CXCL10 and OAS1) were observed in uninfected SK1 2/2 iMEFs but not 18MEFs. This suggests that, in this single iMEF line, lack of SK1 upregulates the basal levels of factors that may protect cells against DENV infection. More importantly, regardless of the levels of DENV replication, all cells that lacked SK1 produced IFN-b but were refractory to induction of ISGs such as viperin, IFIT1 and CXCL10. Based on these findings, we propose new roles for SK1 in affecting innate responses that regulate susceptibility to DENV infection.

Recent clinical reports indicate that infection with dengue virus (DENV) commonly has ocular manifestations. The most serious threat to vision is dengue retinopathy, including retinal vasculopathy and macular edema. Mechanisms of retinopathy are unstudied, but observations in patients implicate retinal pigment epithelial cells and retinal endothelial cells. Human retinal cells were inoculated with DENV-2 and monitored for up to 72 hours. Epithelial and endothelial cells supported DENV replication and release, but epithelial cells alone demonstrated clear cytopathic effect, and infection was more productive in those cells. Infection induced type I interferon responses from both cells, but this was stronger in epithelial cells. Endothelial cells increased expression of adhesion molecules, with sustained overexpression of vascular adhesion molecule-1. Transcellular impedance decreased for epithelial monolayers, but not endothelial monolayers, coinciding with cytopathic effect. This reduction was accompanied by disorganization of intracellular filamentous-actin and decreased expression of junctional molecules, zonula occludens 1, and catenin-β1. Changes in endothelial expression of adhesion molecules are consistent with the retinal vasculopathy seen in patients infected with DENV; decreases in epithelial junctional protein expression, paralleling loss of integrity of the epithelium, provide a molecular basis for DENV-associated macular edema. These molecular processes present potential therapeutic targets for vision-threatening dengue retinopathy.