Altered thermal preferences of infected or immune-challenged Aedes aegypti and Aedes japonicus mosquitoes.

Temperature is a critical factor shaping physiology, life cycle, and behaviour of ectothermic vector insects, as well as the development and multiplication of pathogens within them. However, the influence of pathogen infections on thermal preferences (behavioural thermoregulation) is not well-understood. The present study examined the thermal preferences of mosquitoes (Aedes aegypti and Ae. japonicus) infected with either Sindbis virus (SINV) or Dirofilaria immitis over 12 days post exposure (p.e.) or injected with a non-pathogenic Sephadex bead over 24 h in a thermal gradient (15-30 °C). SINV-infected Ae. aegypti preferred 5 °C warmer temperatures than non-infected ones at day 6 p.e., probably the time of highest innate immune response. In contrast, D. immitis-infected Ae. japonicus preferred 4 °C cooler temperatures than non-infected ones at day 9 p.e., presumably a stress response during the migration of third instar larvae from their development site to the proboscis. Sephadex bead injection also induced a cold preference in the mosquitoes but to a level that did not differ from control-injections. The cold preference thus might be a strategy to escape the risk of desiccation caused by the wound created by piercing the thorax. Further research is needed to uncover the genetic and physiological mechanisms underlying these behaviours.

N-acetyltransferase 10 regulates alphavirus replication via N4-acetylcytidine (ac4C) modification of the lymphocyte antigen six family member E (LY6E) mRNA.

Epitranscriptomic RNA modifications can regulate the stability of mRNA and affect cellular and viral RNA functions. The N4-acetylcytidine (ac4C) modification in the RNA viral genome was recently found to promote viral replication; however, the mechanism by which RNA acetylation in the host mRNA regulates viral replication remains unclear. To help elucidate this mechanism, the roles of N-acetyltransferase 10 (NAT10) and ac4C during the infection and replication processes of the alphavirus, Sindbis virus (SINV), were investigated. Cellular NAT10 was upregulated, and ac4C modifications were promoted after alphavirus infection, while the loss of NAT10 or inhibition of its N-acetyltransferase activity reduced alphavirus replication. The NAT10 enhanced alphavirus replication as it helped to maintain the stability of lymphocyte antigen six family member E mRNA, which is a multifunctional interferon-stimulated gene that promotes alphavirus replication. The ac4C modification was thus found to have a non-conventional role in the virus life cycle through regulating host mRNA stability instead of viral mRNA, and its inhibition could be a potential target in the development of new alphavirus antivirals.IMPORTANCEThe role of N4-acetylcytidine (ac4C) modification in host mRNA and virus replication is not yet fully understood. In this study, the role of ac4C in the regulation of Sindbis virus (SINV), a prototype alphavirus infection, was investigated. SINV infection results in increased levels of N-acetyltransferase 10 (NAT10) and increases the ac4C modification level of cellular RNA. The NAT10 was found to positively regulate SINV infection in an N-acetyltransferase activity-dependent manner. Mechanistically, the NAT10 modifies lymphocyte antigen six family member E (LY6E) mRNA-the ac4C modification site within the 3'-untranslated region (UTR) of LY6E mRNA, which is essential for its translation and stability. The findings of this study demonstrate that NAT10 regulated mRNA stability and translation efficiency not only through the 5'-UTR or coding sequence but also via the 3'-UTR region. The ac4C modification of host mRNA stability instead of viral mRNA impacting the viral life cycle was thus identified, indicating that the inhibition of ac4C could be a potential target when developing alphavirus antivirals.

TMEΜ45B Interacts with Sindbis Virus Nsp1 and Nsp4 and Inhibits Viral Replication.

Alphavirus infection induces the expression of type I interferons, which inhibit the viral replication by upregulating the expression of interferon-stimulated genes (ISGs). Identification and mechanistic studies of the antiviral ISGs help to better understand how the host controls viral infection and help to better understand the viral replication process. Here, we report that the ISG product TMEM45B inhibits the replication of Sindbis virus (SINV). TMEM45B is a transmembrane protein that was detected mainly in the trans-Golgi network, endosomes, and lysosomes but not obviously at the plasma membrane or endoplasmic reticulum. TMEM45B interacted with the viral nonstructural proteins Nsp1 and Nsp4 and inhibited the translation and promoted the degradation of SINV RNA. TMEM45B overexpression rendered the intracellular membrane-associated viral RNA sensitive to RNase treatment. In line with these results, the formation of cytopathic vacuoles (CPVs) was dramatically diminished in TMEM45B-expressing cells. TMEM45B also interacted with Nsp1 and Nsp4 of chikungunya virus (CHIKV), suggesting that it may also inhibit the replication of other alphaviruses. These findings identified TMEM45B as an antiviral factor against alphaviruses and help to better understand the process of the viral genome replication. IMPORTANCE Alphaviruses are positive-stranded RNA viruses with more than 30 members. Infection with Old World alphaviruses, which comprise some important human pathogens such as chikungunya virus and Ross River virus, rarely results in fatal diseases but can lead to high morbidity in humans. Infection with New World alphaviruses usually causes serious encephalitis but low morbidity in humans. Alphavirus infection induces the expression of type I interferons, which subsequently upregulate hundreds of interferon-stimulated genes. Identification and characterization of host antiviral factors help to better understand how the viruses can establish effective infection. Here, we identified TMEM45B as a novel interferon-stimulated antiviral factor against Sindbis virus, a prototype alphavirus. TMEM45B interacted with viral proteins Nsp1 and Nsp4, interfered with the interaction between Nsp1 and Nsp4, and inhibited the viral replication. These findings provide insights into the detailed process of the viral replication and help to better understand the virus-host interactions.

High-Throughput Platform for Detection of Neutralizing Antibodies Using Flavivirus Reporter Replicon Particles.

Flavivirus outbreaks require fast and reliable diagnostics that can be easily adapted to newly emerging and re-emerging flaviviruses. Due to the serological cross-reactivity among flavivirus antibodies, neutralization tests (NT) are considered the gold standard for sero-diagnostics. Here, we first established wild-type single-round infectious virus replicon particles (VRPs) by packaging a yellow fever virus (YFV) replicon expressing Gaussia luciferase (Gluc) with YFV structural proteins in trans using a double subgenomic Sindbis virus (SINV) replicon. The latter expressed the YFV envelope proteins prME via the first SINV subgenomic promoter and the capsid protein via a second subgenomic SINV promoter. VRPs were produced upon co-electroporation of replicon and packaging RNA. Introduction of single restriction enzyme sites in the packaging construct flanking the prME sequence easily allowed to exchange the prME moiety resulting in chimeric VRPs that have the surface proteins of other flaviviruses including dengue virus 1--4, Zika virus, West Nile virus, and tick-borne encephalitis virus. Besides comparing the YF-VRP based NT assay to a YF reporter virus NT assay, we analyzed the neutralization efficiencies of different human anti-flavivirus sera or a monoclonal antibody against all established VRPs. The assays were performed in a 96-well high-throughput format setting with Gluc as readout in comparison to classical plaque reduction NTs indicating that the VRP-based NT assays are suitable for high-throughput analyses of neutralizing flavivirus antibodies.

Interplay between autophagy and Sindbis virus in cells derived from key arbovirus vectors, Aedes albopictus and Aedes aegypti mosquitoes.

Aedes albopictus and Aedes aegypti are two species of Aedes mosquitoes which transmit multiple arboviruses causing serious diseases in human. Intriguingly, infection of arbovirus in both Aedes mosquitoes does not cause dramatic pathology, indicating that both mosquitoes have evolved mechanisms to tolerate persistent infection and restrict viral replication to nonpathogenic levels. Therefore, understanding how these mosquitoes interact with viruses would help to find targets for controlling the related mosquito-borne diseases. Autophagy is a conserved cellular recycling process functioning in maintenance of cellular homeostasis and recirculation of cytoplasmic materials under stressful conditions. Autophagy also acts as a cellular defense mechanism against viral infection. It is known that autophagy plays important roles in the replication of several Aedes mosquito-borne viruses in mammalian systems. However, little information is available regarding the role of autophagy in replication of those viruses in their primary vector, Aedes mosquitoes. This study found that interaction between autophagy and replication of Sindbis virus (SINV) occurred in Aedes albopictus C6/36 cells and Ae. aegypti Aag2 cells. Moreover, it discovered that the patterns of interaction between autophagy and SINV replication are different in C6/36 cells and Aag2 cells. It was shown that replication of SINV induced complete autophagy in C6/36 cells but suppressed autophagy in Aag2 cells. Moreover, induction of autophagy by rapamycin treatment restricted SINV replication in C6/36 cells but promoted SINV replication in Aag2 cells. Consistent with this, suppression of autophagy by down regulation of Atg8 promoted SINV replication in C6/36 cells but restricted SINV replication in Aag2 cells. It was also found that, in both C6/36 and Aag2 cells, interaction between autophagy and SINV replication occurred after viral entry and prior to viral assembly. Collectively, this work demonstrated that SINV replication manipulated autophagy in Aedes mosquito cells and provided strong evidence of the role autophagy played in viral replication in Aedes mosquitoes. The findings have laid a foundation to elucidate the correlation between autophagy and arbovirus replication in Aedes mosquitoes and could help to understand the difference in viral transmission capacity of the two Aedes mosquitoes, Ae. albopictus and Ae. aegypti.

VLDLR and ApoER2 are receptors for multiple alphaviruses.

Alphaviruses, like many other arthropod-borne viruses, infect vertebrate species and insect vectors separated by hundreds of millions of years of evolutionary history. Entry into evolutionarily divergent host cells can be accomplished by recognition of different cellular receptors in different species, or by binding to receptors that are highly conserved across species. Although multiple alphavirus receptors have been described1-3, most are not shared among vertebrate and invertebrate hosts. Here we identify the very low-density lipoprotein receptor (VLDLR) as a receptor for the prototypic alphavirus Semliki forest virus. We show that the E2 and E1 glycoproteins (E2-E1) of Semliki forest virus, eastern equine encephalitis virus and Sindbis virus interact with the ligand-binding domains (LBDs) of VLDLR and apolipoprotein E receptor 2 (ApoER2), two closely related receptors. Ectopic expression of either protein facilitates cellular attachment, and internalization of virus-like particles, a VLDLR LBD-Fc fusion protein or a ligand-binding antagonist block Semliki forest virus E2-E1-mediated infection of human and mouse neurons in culture. The administration of a VLDLR LBD-Fc fusion protein has protective activity against rapidly fatal Semliki forest virus infection in mouse neonates. We further show that invertebrate receptor orthologues from mosquitoes and worms can serve as functional alphavirus receptors. We propose that the ability of some alphaviruses to infect a wide range of hosts is a result of their engagement of evolutionarily conserved lipoprotein receptors and contributes to their pathogenesis.

Potent and Targeted Sindbis Virus Platform for Immunotherapy of Ovarian Cancer.

Our laboratory has been developing a Sindbis viral (SV) vector platform for treatments of ovarian and other types of cancers. In this study we show that SV.IL-12 combined with an agonistic OX40 antibody can eliminate ovarian cancer in a Mouse Ovarian Surface Epithelial Cell Line (MOSEC) model and further prevent tumors in mice rechallenged with tumor cells after approximately 5 months. Treatment efficacy is shown to be dependent upon T-cells that are transcriptionally and metabolically reprogramed. An influx of immune cells to the tumor microenvironment occurs. Combination of sequences encoding both IL-12 and anti-OX40 into a single SV vector, SV.IgGOX40.IL-12, facilitates the local delivery of immunoregulatory agents to tumors enhancing the anti-tumor response. We promote SV.IgGOX40.IL-12 as a safe and effective therapy for multiple types of cancer.

Wolbachia prevalence in the vector species Culex pipiens and Culex torrentium in a Sindbis virus-endemic region of Sweden.

BACKGROUND: Wolbachia pipientis are endosymbiotic bacteria present in a large proportion of terrestrial arthropods. The species is known to sometimes affect the ability of its host to transmit vector-borne pathogens. Central Sweden is endemic for Sindbis virus (SINV), where it is mainly transmitted by the vector species Culex pipiens and Culex torrentium, with the latter established as the main vector. In this study we investigated the Wolbachia prevalence in these two vector species in a region highly endemic for SINV. METHODS: Culex mosquitoes were collected using CDC light traps baited with carbon dioxide over 9 years at 50 collection sites across the River Dalälven floodplains in central Sweden. Mosquito genus was determined morphologically, while a molecular method was used for reliable species determination. The presence of Wolbachia was determined through PCR using general primers targeting the wsp gene and sequencing of selected samples. RESULTS: In total, 676 Cx. pipiens and 293 Cx. torrentium were tested for Wolbachia. The prevalence of Wolbachia in Cx. pipiens was 97% (95% CI 94.8-97.6%), while only 0.7% (95% CI 0.19-2.45%) in Cx. torrentium. The two Cx. torrentium mosquitoes that were infected with Wolbachia carried different types of the bacteria. CONCLUSIONS: The main vector of SINV in the investigated endemic region, Cx. torrentium, was seldom infected with Wolbachia, while it was highly prevalent in the secondary vector, Cx. pipiens. The presence of Wolbachia could potentially have an impact on the vector competence of these two species. Furthermore, the detection of Wolbachia in Cx. torrentium could indicate horizontal transmission of the endosymbiont between arthropods of different species.

Sindbis virus infection of mosquito species in the wetlands of northwestern Iran and modeling the probable ecological niches of SINV vectors in the country.

Sindbis virus (SINV) and Chikungunya virus (CHIKV) are among the most widely spread mosquito-borne viruses worldwide. Due to the key role of mosquitoes in the transmission cycle of vector-borne diseases, models such as Maximum Entropy (MaxEnt) have been used in recent years to predict the environmental suitability and ecological niches of mosquito vectors. Infection of three mosquito species (Anopheles maculipennis s.l., Culex tritaeniorhynchus, and Culiseta longiareolata) with CHIKV has recently been reported in Iran. However, given the importance of vector-borne diseases in the country, there is a need for extensive studies on the infection of mosquitoes with CHIKV and SINV in different areas of the country. Accordingly, the current research was conducted to investigate the infection of mosquitoes with the two aforementioned viruses in the northwestern part of Iran and also to model the ecological niches of the vectors of these mosquito-borne viruses in the country. In the current study, 4639 mosquito specimens, consisting of 2515 adults and 2124 larvae, were collected from the wetlands of West Azerbaijan Province and identified. Ten species belonging to four genera were identified in this study. The specimens were allocated to 149 pools for the determination of infection with CHIKV and SINV. The amplification pattern of five pools comprising two mosquito species (Culex pipiens complex and Cx. Theileri) corresponded to the reference strain of SINV, and the isolates were sequenced to confirm the presence of SINV genome. No cases of CHIKV infection were found among the 149 examined mosquito pools. Data on the distribution of Cx. Pipiens complex and Cx. Theileri were mapped using ArcMap 10.5. Prediction maps of the presence probability for these species revealed that they are most likely to be found in and spread to the north, northwest, south, and southeastern areas of the country and in areas with abundant water resources. For the first time in Iran, our study investigated the presence probability of SINV vectors using ecological niche modeling. Ecological niche profiling showed that the most suitable habitats for Cx. pipiens are mainly concentrated in the north and northwestern parts of the country, whereas Cx. theileri is mostly located in the northwest and western regions. However, there were some other areas of low suitability for these two species in the country. Further studies in a broader geographical area with more species of mosquitos and the determination of infection with other mosquito-borne viruses can provide a clear understanding of the potential spread of mosquito-borne diseases in various regions of Iran.

Zebrafish as an animal model for the antiviral RNA interference pathway.

The zebrafish (Danio rerio) possesses evolutionarily conserved innate and adaptive immunity as a mammal and has recently become a popular vertebrate model to exploit infection and immunity. Antiviral RNA interference (RNAi) has been illuminated in various model organisms, including Arabidopsis thaliana, Drosophila melanogaster, Caenorhabditis elegans and mice. However, to date, there is no report on the antiviral RNAi pathway of zebrafish. Here, we have evaluated the possible use of zebrafish to study antiviral RNAi with Sindbis virus (SINV), vesicular stomatitis virus (VSV) and Nodamura virus (NoV). We find that SINVs and NoVs induce the production of virus-derived small interfering RNAs (vsiRNAs), the hallmark of antiviral RNAi, with a preference for a length of 22 nucleotides, after infection of larval zebrafish. Meanwhile, the suppressor of RNAi (VSR) protein, NoV B2, may affect the accumulation of the NoV in zebrafish. Furthermore, taking advantage of the fact that zebrafish argonaute-2 (Ago2) protein is naturally deficient in cleavage compared with that of mammals, we provide evidence that the slicing activity of human Ago2 can virtually inhibit the accumulation of RNA virus after being ectopically expressed in larval zebrafish. Thus, zebrafish may be a unique model organism to study the antiviral RNAi pathway.

Sindbis Virus Infection in Non-Blood-Fed Hibernating Culex pipiens Mosquitoes in Sweden.

A crucial, but unresolved question concerning mosquito-borne virus transmission is how these viruses can remain endemic in regions where the transmission is halted for long periods of time, due to mosquito inactivity in, e.g., winter. In northern Europe, Sindbis virus (SINV) (genus alphavirus, Togaviridae) is transmitted among birds by Culex mosquitoes during the summer, with occasional symptomatic infections occurring in humans. In winter 2018-19, we sampled hibernating Culex spp females in a SINV endemic region in Sweden and assessed them individually for SINV infection status, blood-feeding status, and species. The results showed that 35 out of the 767 collected mosquitoes were infected by SINV, i.e., an infection rate of 4.6%. The vast majority of the collected mosquitoes had not previously blood-fed (98.4%) and were of the species Cx. pipiens (99.5%). This is the first study of SINV overwintering, and it concludes that SINV can be commonly found in the hibernating Cx. pipiens population in an endemic region in Sweden, and that these mosquitoes become infected through other means besides blood-feeding. Further studies on mosquito ecology and viral interactions are needed to elucidate the mechanisms of the persistence of these viruses over winter.

Comparative analyses of alphaviral RNA:Protein complexes reveals conserved host-pathogen interactions.

The identification of host / pathogen interactions is essential to both understanding the molecular biology of infection and developing rational intervention strategies to overcome disease. Alphaviruses, such as Sindbis virus, Chikungunya virus, and Venezuelan Equine Encephalitis virus are medically relevant positive-sense RNA viruses. As such, they must interface with the host machinery to complete their infectious lifecycles. Nonetheless, exhaustive RNA:Protein interaction discovery approaches have not been reported for any alphavirus species. Thus, the breadth and evolutionary conservation of host interactions on alphaviral RNA function remains a critical gap in the field. Herein we describe the application of the Cross-Link Assisted mRNP Purification (CLAMP) strategy to identify conserved alphaviral interactions. Through comparative analyses, conserved alphaviral host / pathogen interactions were identified. Approximately 100 unique host proteins were identified as a result of these analyses. Ontological assessments reveal enriched Molecular Functions and Biological Processes relevant to alphaviral infection. Specifically, as anticipated, Poly(A) RNA Binding proteins are significantly enriched in virus specific CLAMP data sets. Moreover, host proteins involved in the regulation of mRNA stability, proteasome mediated degradation, and a number of 14-3-3 proteins were identified. Importantly, these data expand the understanding of alphaviral host / pathogen interactions by identifying conserved interactants.

The Alphavirus Sindbis Infects Enteroendocrine Cells in the Midgut of Aedes aegypti.

Transit of the arthropod-borne-virus (arbovirus) Sindbis (SINV) throughout adult female mosquitoes initiates with its attachment to the gut lumen, entry and amplification in midgut cells, followed by dissemination into the hemolymph. Free-mated adult females, aged day 5-7, were proffered a viremic blood suspension via sausage casings containing SINV-TaV-Green Fluorescent Protein (GFP) at a final titer of 106 PFU/mL. Midguts (MGs) from fully engorged mosquitoes were resected on days 5 and 7 post-bloodmeal, and immunolabeled using FMRFamide antibody against enteroendocrine cells (ECs) with a TX-Red secondary antibody. Following immunolabeling, the organs were investigated via laser confocal microscopy to identify the distribution of GFP and TX-Red. Infection using this reporter virus was observed as multiple GFP expression foci along the posterior midgut (PMG) epithelium and ECs were observed as TX-Red labeled cells scattered along the entire length of the MG. Our results demonstrated that SINVGFP did infect ECs, as indicated by the overlapping GFP and TX-Red channels shown as yellow in merged images. We propose that ECs may be involved in the SINV infection pathway in the mosquito MG. Due to the unique role that ECs have in the exocytosis of secretory granules from the MG and the apical-basolateral position of ECs in the PMG monolayer, we speculate that these cells may assist as a mechanism for arboviruses to cross the gut barriers. These findings suggest that MG ECs are involved in arbovirus infection of the invertebrate host.

Effects of Manipulating Fibroblast Growth Factor Expression on Sindbis Virus Replication In Vitro and in Aedes aegypti Mosquitoes.

Fibroblast growth factors (FGFs) are conserved among vertebrate and invertebrate animals and function in cell proliferation, cell differentiation, tissue repair, and embryonic development. A viral fibroblast growth factor (vFGF) homolog encoded by baculoviruses, a group of insect viruses, is involved in escape of baculoviruses from the insect midgut by stimulating basal lamina remodeling. This led us to investigate whether cellular FGF is involved in the escape of an arbovirus from mosquito midgut. In this study, the effects of manipulating FGF expression on Sindbis virus (SINV) replication and escape from the midgut of the mosquito vector Aedes aegypti were examined. RNAi-mediated silencing of either Ae. aegypti FGF (AeFGF) or FGF receptor (AeFGFR) expression reduced SINV replication following oral infection of Ae. aegypti mosquitoes. However, overexpression of baculovirus vFGF using recombinant SINV constructs had no effect on replication of these viruses in cultured mosquito or vertebrate cells, or in orally infected Ae. aegypti mosquitoes. We conclude that reducing FGF signaling decreases the ability of SINV to replicate in mosquitoes, but that overexpression of vFGF has no effect, possibly because endogenous FGF levels are already sufficient for optimal virus replication. These results support the hypothesis that FGF signaling, possibly by inducing remodeling of midgut basal lamina, is involved in arbovirus midgut escape following virus acquisition from a blood meal.

Viral infection impacts transposable element transcript amounts in Drosophila.

Transposable elements (TEs) are genomic parasites that are found in all genomes, some of which display sequence similarity to certain viruses. In insects, TEs are controlled by the Piwi-interacting small interfering RNA (piRNA) pathway in gonads, while the small interfering RNA (siRNA) pathway is dedicated to TE somatic control and defense against viruses. So far, these two small interfering RNA pathways are considered to involve distinct molecular effectors and are described as independent. Using Sindbis virus (SINV) in Drosophila, here we show that viral infections affect TE transcript amounts via modulations of the piRNA and siRNA repertoires, with the clearest effects in somatic tissues. These results suggest that viral acute or chronic infections may impact TE activity and, thus, the tempo of genetic diversification. In addition, these results deserve further evolutionary considerations regarding potential benefits to the host, the virus, or the TEs.

Development of encoded Broccoli RNA aptamers for live cell imaging of alphavirus genomic and subgenomic RNAs.

Sindbis virus (SINV) can infect neurons and cause encephalomyelitis in mice. Nonstructural proteins are translated from genomic RNA and structural proteins from subgenomic RNA. While visualization of viral proteins in living cells is well developed, imaging of viral RNAs has been challenging. RNA aptamers that bind and activate conditional fluorophores provide a tool for RNA visualization. We incorporated cassettes of two F30-scaffolded dimers of the Broccoli aptamer into a SINV cDNA clone using sites in nsP3 (genomic RNA), the 3'UTR (genomic and subgenomic RNAs) and after a second subgenomic promoter resulting in 4-28 Broccoli copies. After addition of the cell-permeable 3,5-difluoro-4-hydroxybenzylidene imidazolinone (DFHBI-1T) conditional fluorophore and laser excitation, infected cells emitted green fluorescence that correlated with Broccoli copy numbers. All recombinant viruses replicated well in BHK and undifferentiated neural cells but viruses with 14 or more Broccoli copies were attenuated in differentiated neurons and mice. The signal survived fixation and allowed visualization of viral RNAs in differentiated neurons and mouse brain, as well as BHK cells. Subgenomic RNA was diffusely distributed in the cytoplasm with genomic RNA also in perinuclear vesicle-like structures near envelope glycoproteins or mitochondria. Broccoli aptamer-tagging provides a valuable tool for live cell imaging of viral RNA.

Confocal Analysis of the Distribution and Persistence of Sindbis Virus (TaV-GFP) Infection in Midguts of Aedes aegypti Mosquitoes.

Biological transmission of arthropod-borne viruses (arboviruses) to vertebrate hosts by hematophagous insects poses a global threat because such arboviruses can result in a range of serious public health infectious diseases. Sindbis virus (SINV), the prototype Alphavirus, was used to track infections in the posterior midgut (PMG) of Aedes aegypti adult mosquitoes. Females were fed viremic blood containing a virus reporter, SINV [Thosea asigna virus-green fluorescent protein (TaV-GFP)], that leaves a fluorescent signal in infected cells. We assessed whole-mount PMGs to identify primary foci, secondary target tissues, distribution, and virus persistence. Following a viremic blood meal, PMGs were dissected and analyzed at various days of post blood-feeding. We report that virus foci indicated by GFP in midgut epithelial cells resulted in a 9.8% PMG infection and a 10.8% dissemination from these infected guts. The number of virus foci ranged from 1 to 3 per individual PMG and was more prevalent in the PMG-middle > PMG-frontal > PMG-caudal regions. SINV TaV-GFP was first observed in the PMG (primary target tissue) at 3 days post blood-feeding, was sequestered in circumscribed foci, replicated in PMG peristaltic muscles (secondary target tissue) following dissemination, and GFP was observed to persist in PMGs for 30 days postinfection.

The Heat Shock Response in the Western Honey Bee (Apis mellifera) is Antiviral.

Honey bees (Apismellifera) are an agriculturally important pollinator species that live in easily managed social groups (i.e., colonies). Unfortunately, annual losses of honey bee colonies in many parts of the world have reached unsustainable levels. Multiple abiotic and biotic stressors, including viruses, are associated with individual honey bee and colony mortality. Honey bees have evolved several antiviral defense mechanisms including conserved immune pathways (e.g., Toll, Imd, JAK/STAT) and dsRNA-triggered responses including RNA interference and a non-sequence specific dsRNA-mediated response. In addition, transcriptome analyses of virus-infected honey bees implicate an antiviral role of stress response pathways, including the heat shock response. Herein, we demonstrate that the heat shock response is antiviral in honey bees. Specifically, heat-shocked honey bees (i.e., 42 °C for 4 h) had reduced levels of the model virus, Sindbis-GFP, compared with bees maintained at a constant temperature. Virus-infection and/or heat shock resulted in differential expression of six heat shock protein encoding genes and three immune genes, many of which are positively correlated. The heat shock protein encoding and immune gene transcriptional responses observed in virus-infected bees were not completely recapitulated by administration of double stranded RNA (dsRNA), a virus-associated molecular pattern, indicating that additional virus-host interactions are involved in triggering antiviral stress response pathways.

Interferon-Gamma Modulation of the Local T Cell Response to Alphavirus Encephalomyelitis.

Infection of mice with Sindbis virus (SINV) provides a model for examining the role of the immune response to alphavirus infection of the central nervous system (CNS). Interferon-gamma (IFN-γ) is an important component of this response, and we show that SINV-infected differentiated neurons respond to IFN-γ in vitro by induction of antiviral genes and suppression of virus replication. To determine the in vivo effects of IFN-γ on SINV clearance and T cell responses, C57BL/6 mice lacking IFN-γ or IFN-γ receptor-1 were compared to wild-type (WT) mice after intracranial SINV infection. In WT mice, IFN-γ was first produced in the CNS by natural killer cells and then by CD4+ and CD8+ T cells. Mice with impaired IFN-γ signaling initiated clearance of viral RNA earlier than WT mice associated with CNS entry of more granzyme B-producing CD8+ T cells. However, these mice established fewer CD8+ tissue-resident memory T (TRM) cells and were more likely to experience reactivation of viral RNA synthesis late after infection. Therefore, IFN-γ suppresses the local development of granzyme B-expressing CD8+ T cells and slows viral RNA clearance but promotes CD8+ TRM cell establishment.

The Capsid Protein of Semliki Forest Virus Antagonizes RNA Interference in Mammalian Cells.

RNA interference (RNAi) is a conserved antiviral immune defense in eukaryotes, and numerous viruses have been found to encode viral suppressors of RNAi (VSRs) to counteract antiviral RNAi. Alphaviruses are a large group of positive-stranded RNA viruses that maintain their transmission and life cycles in both mosquitoes and mammals. However, there is little knowledge about how alphaviruses antagonize RNAi in both host organisms. In this study, we identified that Semliki Forest virus (SFV) capsid protein can efficiently suppress RNAi in both insect and mammalian cells by sequestrating double-stranded RNA and small interfering RNA. More importantly, when the VSR activity of SFV capsid was inactivated by reverse genetics, the resulting VSR-deficient SFV mutant showed severe replication defects in mammalian cells, which could be rescued by blocking the RNAi pathway. Besides, capsid protein of Sindbis virus also inhibited RNAi in cells. Together, our findings show that SFV uses capsid protein as VSR to antagonize RNAi in infected mammalian cells, and this mechanism is probably used by other alphaviruses, which shed new light on the knowledge of SFV and alphavirus.IMPORTANCE Alphaviruses are a genus of positive-stranded RNA viruses and include numerous important human pathogens, such as Chikungunya virus, Ross River virus, Western equine encephalitis virus, etc., which create the emerging and reemerging public health threat worldwide. RNA interference (RNAi) is one of the most important antiviral mechanisms in plants and insects. Accumulating evidence has provided strong support for the existence of antiviral RNAi in mammals. In response to antiviral RNAi, viruses have evolved to encode viral suppressors of RNAi (VSRs) to antagonize the RNAi pathway. It is unclear whether alphaviruses encode VSRs that can suppress antiviral RNAi during their infection in mammals. In this study, we first uncovered that capsid protein encoded by Semliki Forest virus (SFV), a prototypic alphavirus, had a potent VSR activity that can antagonize antiviral RNAi in the context of SFV infection in mammalian cells, and this mechanism is probably used by other alphaviruses.