With 40% of the world population at risk, infections with dengue virus (DENV) constitute a serious threat to public health. While there is no antiviral therapy available against this potentially lethal disease, the efficacy of the only approved vaccine is not optimal and its safety has been recently questioned. In order to develop better vaccines based on attenuated and/or chimeric viruses, one must consider how the human immune system is engaged during DENV infection. The activation of the innate immunity through the detection of viruses by cellular sensors is the first line of defence against those pathogens. This triggers a cascade of events which establishes an antiviral state at the cell level and leads to a global immunological response. However, DENV has evolved to interfere with the innate immune signalling at multiple levels, hence dampening antiviral responses and favouring viral replication and dissemination. This review elaborates on the interplay between DENV and the innate immune system. A special focus is given on the viral countermeasure mechanisms reported over the last decade which should be taken into consideration during vaccine development.

With no available therapies, infections with Zika virus (ZIKV) constitute a major public health concern as they can lead to congenital microcephaly. In order to generate an intracellular environment favourable to viral replication, ZIKV induces endomembrane remodelling and the morphogenesis of replication factories via enigmatic mechanisms. In this study, we identified the AAA+ type ATPase valosin‐containing protein (VCP) as a cellular interaction partner of ZIKV non‐structural protein 4B (NS4B). Importantly, its pharmacological inhibition as well as the expression of a VCP dominant‐negative mutant impaired ZIKV replication. In infected cells, VCP is relocalised to large ultrastructures containing both NS4B and NS3, which are reminiscent of dengue virus convoluted membranes. Moreover, short treatment with the VCP inhibitors NMS‐873 or CB‐5083 drastically decreased the abundance and size of ZIKV‐induced convoluted membranes. Furthermore, NMS‐873 treatment inhibited ZIKV‐induced mitochondria elongation previously reported to be physically and functionally linked to convoluted membranes in case of the closely related dengue virus. Finally, VCP inhibition resulted in enhanced apoptosis of ZIKV‐infected cells strongly suggesting that convoluted membranes limit virus‐induced cytopathic effects. Altogether, this study identifies VCP as a host factor required for ZIKV life cycle and more precisely, for the maintenance of viral replication factories. Our data further support a model in which convoluted membranes regulate ZIKV life cycle by impacting on mitochondrial functions and ZIKV‐induced death signals in order to create a cytoplasmic environment favourable to viral replication.

The dengue virus (DENV) causes the most prevalent arthropod-borne viral disease worldwide. While its incidence is increasing in many countries, there is no approved antiviral therapy currently available. In infected cells, the DENV induces extensive morphological alterations of the endoplasmic reticulum (ER) to generate viral replication organelles (vRO), which include convoluted membranes (CM) and vesicle packets (VP) hosting viral RNA replication. The viral non-structural protein NS4B localizes to vROs and is absolutely required for viral replication through poorly defined mechanisms, which might involve cellular protein partners. Previous interactomic studies identified the ATPase valosin-containing protein (VCP) as a DENV NS4B-interacting host factor in infected cells. Using both pharmacological and dominant-negative inhibition approaches, we show, in this study, that VCP ATPase activity is required for efficient DENV replication. VCP associates with NS4B when expressed in the absence of other viral proteins while in infected cells, both proteins colocalize within large DENV-induced cytoplasmic structures previously demonstrated to be CMs. Consistently, VCP inhibition dramatically reduces the abundance of DENV CMs in infected cells. Most importantly, using a recently reported replication-independent plasmid-based vRO induction system, we show that de novo VP biogenesis is dependent on VCP ATPase activity. Overall, our data demonstrate that VCP ATPase activity is required for vRO morphogenesis and/or stability. Considering that VCP was shown to be required for the replication of other flaviviruses, our results argue that VCP is a pan-flaviviral host dependency factor. Given that new generation VCP-targeting drugs are currently evaluated in clinical trials for cancer treatment, VCP may constitute an attractive broad-spectrum antiviral target in drug repurposing approaches.

Flaviviridae infections represent a major global health burden. By deciphering mechanistic aspects of hepatitis C virus (HCV)-host interactions, one could discover common strategy for inhibiting the replication of related flaviviruses. By elucidating the HCV interactome, we identified the 17-beta-hydroxysteroid dehydrogenase type 12 (HSD17B12) as a human hub of the very-long-chain fatty acid (VLCFA) synthesis pathway and core interactor. Here we show that HSD17B12 knockdown (KD) impairs HCV replication and reduces virion production. Mechanistically, depletion of HSD17B12 induces alterations in VLCFA-containing lipid species and a drastic reduction of lipid droplets (LDs) that play a critical role in virus assembly. Oleic acid supplementation rescues viral RNA replication and production of infectious particles in HSD17B12 depleted cells, supporting a specific role of VLCFA in HCV life cycle. Furthermore, the small-molecule HSD17B12 inhibitor, INH-12, significantly reduces replication and infectious particle production of HCV as well as dengue virus and Zika virus revealing a conserved requirement across Flaviviridae virus family. Overall, the data provide a strong rationale for the advanced evaluation of HSD17B12 inhibition as a promising broad-spectrum antiviral strategy for the treatment of Flaviviridae infections.

Arthropod-borne diseases currently constitute a source of major health concerns worldwide. They account for about 50% of global infectious diseases and cause nearly 700,000 deaths every year. Their rapid increase and spread constitute a huge challenge for public health, highlighting the need for early detection during epidemics, to curtail the virus spread, and to enhance outbreak management. Here, we compared a standard quantitative polymerase chain reaction (RT-qPCR) and a direct RT-qPCR assay for the detection of Zika (ZIKV), Chikungunya (CHIKV), and Rift Valley Fever (RVFV) viruses from experimentally infected-mosquitoes. The direct RT-qPCR could be completed within 1.5 h and required 1 µL of viral supernatant from homogenized mosquito body pools. Results showed that the direct RT-qPCR can detect 85.71%, 89%, and 100% of CHIKV, RVFV, and ZIKV samples by direct amplifications compared to the standard method. The use of 1:10 diluted supernatant is suggested for CHIKV and RVFV direct RT-qPCR. Despite a slight drop in sensitivity for direct PCR, our technique is more affordable, less time-consuming, and provides a better option for qualitative field diagnosis during outbreak management. It represents an alternative when extraction and purification steps are not possible because of insufficient sample volume or biosecurity issues.

Orthoflaviviruses are RNA viruses responsible for significant diseases in humans, domesticated animals and wildlife. Their NS5 protein is central in viral replication, functioning both as an RNA-dependent RNA polymerase and a methyltransferase, while also modulating cellular processes, including the interferon response. Although viral replication is cytoplasmic, the NS5 protein of several mosquito-borne orthoflaviviruses cycles between the cytoplasm and the nucleus of infected human cells. However, the nuclear localization and function of NS5 of tick-borne orthoflaviviruses, such as tick-borne encephalitis virus (TBEV), remained poorly understood. Microscopy analysis and cell fractionation revealed that the NS5 protein of TBEV localized to both the cytoplasm and nucleoplasm of infected cells. Mutagenesis studies identified critical residues required for its nuclear targeting. Mutating these residues in a TBEV replicon abolished viral replication. Immunoprecipitation-mass spectrometry analyses performed in two human cell lines infected with TBEV recovered 352 NS5 partners. Among them, 187 were nuclear or partially nuclear. By integrating our interactome data with that of Powassan virus (POWV), another tick-borne orthoflavivirus, we refined a list of 20 high-confidence NS5 partners, including splicing factors and chromatin modulators. Functional analysis revealed that seven of these nuclear partners significantly modulated viral replication, further underscoring the importance of nuclear NS5 in the viral life cycle. Our work advances our understanding of the nuclear function of the NS5 proteins of tick-borne orthoflaviviruses. Importance: Tick-borne orthoflaviviruses are emerging globally, spreading across Europe, Asia, and North America, where they infect humans, domesticated animals, and wildlife. These viruses produce a protein called NS5, which drives viral replication and helps evade the innate immune response. We observed that the NS5 protein of tick-borne encephalitis virus (TBEV) localized both in the cytoplasm and nucleoplasm of infected human cells. We identified the specific residues responsible for its nuclear addressing and showed that it interacts with numerous nuclear proteins, including some involved in regulating gene expression. Seven of these nuclear partners significantly influenced viral replication, highlighting the importance of NS5’s nuclear activity. This work sheds light on how tick-borne orthoflaviviruses manipulate host cells, deepening our understanding of their replication strategies.