Tissue Barriers to Arbovirus Infection in Mosquitoes

A novel p53 paralogue mediates antioxidant defense of mosquito cells to survive dengue virus replication

Mosquito cells allow dengue viruses (DENVs) to undergo replication without causing serious deleterious effects on the cells, leading to advantages for dissemination to other cells. Despite this, increased accumulation of reactive oxygen species (ROS) is usually detected in C6/36 cells with DENV2 infection as shown in mammalian cells. Uniquely, oxidative stress caused by the ROS is alleviated by eliciting antioxidant defense which leads to protection of mosquito cells from the infection. In the present study, a novel p53 paralogue (p53-2) was identified and proved to be regulated in C6/36 cells with DENV2 infection. With a gene-knockdown technique, p53-2 was demonstrated to transcribe catalase which plays a critical role in reducing ROS accumulation and the death rate of infected cells. Ecologically, a higher survival rate of mosquito cells is a prerequisite for prosperous production of viral progeny, allowing infected mosquitoes to remain healthy and active for virus transmission.

The redox-sensing gene Nrf2 affects intestinal homeostasis, insecticide resistance, and Zika virus susceptibility in the mosquito Aedes aegypti

Production and degradation of reactive oxygen species (ROS) are extensively regulated to ensure proper cellular responses to various environmental stimuli and stresses. Moreover, physiologically generated ROS function as secondary messengers that can influence tissue homeostasis. The cap'n'collar transcription factor known as nuclear factor erythroid-derived factor 2 (Nrf2) coordinates an evolutionarily conserved transcriptional activation pathway that mediates antioxidant and detoxification responses in many animal species, including insects and mammals. Here, we show that Nrf2-mediated signaling affects embryo survival, midgut homeostasis, and redox biology in Aedes aegypti, a mosquito species vector of dengue, Zika, and other disease-causing viruses. We observed that AeNrf2 silencing increases ROS levels and stimulates intestinal stem cell proliferation. Because ROS production is a major aspect of innate immunity in mosquito gut, we found that a decrease in Nrf2 signaling results in reduced microbiota growth and Zika virus infection. Moreover, we provide evidence that AeNrf2 signaling also controls transcriptional adaptation of A. aegypti to insecticide challenge. Therefore, we conclude that Nrf2-mediated response regulates assorted gene clusters in A. aegypti that determine cellular and midgut redox balance, affecting overall xenobiotic resistance and vectorial adaptation of the mosquito.

Establishment of a mouse model for the complete mosquito-mediated transmission cycle of Zika virus

Zika virus (ZIKV) is primarily transmitted by Aedes mosquitoes in the subgenus Stegomyia but can also be transmitted sexually and vertically in humans. STAT1 is an important downstream factor that mediates type I and II interferon signaling. In the current study, we showed that mice with STAT1 knockout (Stat1^-/-) were highly susceptible to ZIKV infection. As low as 5 plaque-forming units of ZIKV could cause viremia and death in Stat1^-/- mice. ZIKV replication was initially detected in the spleen but subsequently spread to the brain with concomitant reduction of the virus in the spleen in the infected mice. Furthermore, ZIKV could be transmitted from mosquitoes to Stat1^-/- mice back to mosquitoes and then to naïve Stat1^-/- mice. The 50% mosquito infectious dose of viremic Stat1^-/- mouse blood was close to 810 focus-forming units (ffu)/ml. Our further studies indicated that the activation of macrophages and conventional dendritic cells were likely critical for the resolution of ZIKV infection. The newly developed mouse and mosquito transmission models for ZIKV infection will be useful for the evaluation of antiviral drugs targeting the virus, vector, and host.

Zika virus (ZIKV) is transmitted mainly by mosquito bites and can also be transmitted between humans by sex or from pregnant women to their babies. ZIKV infection causes damage in many tissues including the brain in adults and newborns, making ZIKV infection an important health issue globally. To develop new tools for ZIKV research, we determined that a genetically modified mouse strain, Stat1^-/-, was highly sensitive to ZIKV infection. We also demonstrated that ZIKV could be delivered to mice by mosquito bites and transmitted back to Stat1^-/- mice. The newly developed mouse model will be useful for developing new strategies to treat ZIKV infection and for studying mechanisms to reduce mosquito-mediated transmission.

Shared larval rearing environment, sex, female size and genetic diversity shape Ae. albopictus bacterial microbiota

The Asian tiger mosquito Aedes albopictus became of public health concern as it can replicate and transmit viral and filarial pathogens with a strong invasive success over the world. Various strategies have been proposed to reduce mosquito population's vectorial capacity. Among them, symbiotic control of mosquito borne disease offers promising perspectives. Such method is likely to be affected by the dynamics of mosquito-associated symbiotic communities, which might in turn be affected by host genotype and environment. Our previous study suggested a correlation between mosquitoes’ origin, genetic diversity and midgut bacterial diversity. To distinguish the impact of those factors, we have been studying the midgut bacterial microbiota of two Ae. albopictus populations from tropical (La Réunion) and temperate (Montpellier) origins under controlled laboratory conditions. the two populations experienced random mating or genetic bottleneck. Microbiota composition did not highlight any variation of the α and β-diversities in bacterial communities related to host’s populations. However, sizes of the mosquitoes were negatively correlated with the bacterial α-diversity of females. Variations in mosquito sex were associated with a shift in the composition of bacterial microbiota. The females’ mosquitoes also exhibited changes in the microbiota composition according to their size and after experiencing a reduction of their genetic diversity. These results provide a framework to investigate the impact of population dynamics on the symbiotic communities associated with the tiger mosquito.

Model-based inference from multiple dose, time course data reveals Wolbachia effects on infection profiles of type 1 dengue virus in Aedes aegypti

Infection is a complex and dynamic process involving a population of invading microbes, the host and its responses, aimed at controlling the situation. Depending on the purpose and level of organization, infection at the organism level can be described by a process as simple as a coin toss, or as complex as a multi-factorial dynamic model; the former, for instance, may be adequate as a component of a population model, while the latter is necessary for a thorough description of the process beginning with a challenge with an infectious inoculum up to establishment or elimination of the pathogen. Experimental readouts in the laboratory are often static, snapshots of the process, assayed under some convenient experimental condition, and therefore cannot comprehensively describe the system. Different from the discrete treatment of infection in population models, or the descriptive summarized accounts of typical lab experiments, in this manuscript, infection is treated as a dynamic process dependent on the initial conditions of the infectious challenge, viral growth, and the host response along time. Here, experimental data is generated for multiple doses of type 1 dengue virus, and pathogen levels are recorded at different points in time for two populations of mosquitoes: either carrying endosymbiont bacteria Wolbachia or not. A dynamic microbe/host-response mathematical model is used to describe pathogen growth in the face of a host response like the immune system, and to infer model parameters for the two populations of insects, revealing a slight—but potentially important—protection conferred by the symbiont.

Infection is usually assayed as a static observation of a pathogen within a host; it is, nevertheless, a dynamic process that cannot be described from a single time point and arbitrary conditions. Results based on the usual methods are a snapshot of a convenient laboratory condition; a more comprehensive data set is required to describe the entire process of infection from inoculation of the host with a microorganism to establishment of a systemic infection, or elimination of the threat by the host. We design an experiment that takes into account increasing pathogen challenges to a mosquito host and viral levels along time; we use a dynamic mathematical model to analyze the resulting data set. The entire framework is used to compare susceptibility to dengue virus of Aedes aegypti mosquitoes either carrying the Wolbachia symbiont or not. Instead of a simple pairwise comparison, we are able to compare infection profiles and parameters associated to host immune processes in this insect-symbiont-virus system.

Natural Variation in Resistance to Virus Infection in Dipteran Insects

The power and ease of Drosophila genetics and the medical relevance of mosquito-transmitted viruses have made dipterans important model organisms in antiviral immunology. Studies of virus-host interactions at the molecular and population levels have illuminated determinants of resistance to virus infection. Here, we review the sources and nature of variation in antiviral immunity and virus susceptibility in model dipteran insects, specifically the fruit fly Drosophila melanogaster and vector mosquitoes of the genera Aedes and Culex. We first discuss antiviral immune mechanisms and describe the virus-specificity of these responses. In the following sections, we review genetic and microbiota-dependent variation in antiviral immunity. In the final sections, we explore less well-studied sources of variation, including abiotic factors, sexual dimorphism, infection history, and endogenous viral elements. We borrow from work on other pathogen types and non-dipteran species when it parallels or complements studies in dipterans. Understanding natural variation in virus-host interactions may lead to the identification of novel restriction factors and immune mechanisms and shed light on the molecular determinants of vector competence.

Altered vector competence in an experimental mosquito-mouse transmission model of Zika infection

Few animal models of Zika virus (ZIKV) infection have incorporated arthropod-borne transmission. Here, we establish an Aedes aegypti mosquito model of ZIKV infection of mice, and demonstrate altered vector competency among three strains, (Orlando, ORL, Ho Chi Minh, HCM, and Patilas, PAT). All strains acquired ZIKV in their midguts after a blood meal from infected mice, but ZIKV transmission only occurred in mice fed upon by HCM, and to a lesser extent PAT, but not ORL, mosquitoes. This defect in transmission from ORL or PAT mosquitoes was overcome by intrathoracic injection of ZIKV into mosquito. Genetic analysis revealed significant diversity among these strains, suggesting a genetic basis for differences in ability for mosquito strains to transmit ZIKV. The intrathoracic injection mosquito-mouse transmission model is critical to understanding the influence of mosquitoes on ZIKV transmission, infectivity and pathogenesis in the vertebrate host, and represents a natural transmission route for testing vaccines and therapeutics.

Zika virus (ZIKV), an emerging flavivirus, is associated with severe clinical outcomes, including Guillain-Barre syndrome and birth defects. Transmission of ZIKV is primarily mosquito-borne, but the complete transmission cycle from mammalian host to mosquito and back to mammalian host has not yet been demonstrated experimentally. Here, for the first time, we establish a transmission model of ZIKV between mice lacking both interferon α and interferon γ receptor (AG129) and the Aedes aegypti mosquito, the main vector of ZIKV. We also highlight differences in vector competence among a common laboratory strain, Orlando (ORL), and two wild-caught strains, Ho Chi Minh (HCM) and Patillas (PAT), of the Aedes aegypti mosquito using oral feeding on infected mice and intrathoracic inoculation of ZIKV. Our results suggest different ZIKV susceptibility among these strains, which could be relevant as this virus spreads throughout the globe. In addition, the mosquito transmission model demonstrated here can be used to better understand the effect mosquitoes have on replication and pathogenesis of this virus.

Mosquitoes as Suitable Vectors for Alphaviruses

Alphaviruses are arthropod-borne viruses and are predominantly transmitted via mosquito vectors. This vector preference by alphaviruses raises the important question of the determinants that contribute to vector competence. There are several tissue barriers of the mosquito that the virus must overcome in order to establish a productive infection. Of importance are the midgut, basal lamina and the salivary glands. Infection of the salivary glands is crucial for virus transmission during the mosquito’s subsequent bloodfeed. Other factors that may contribute to vector competence include the microflora and parasites present in the mosquito, environmental conditions, the molecular determinants of the virus to adapt to the vector, as well as the effect of co-infection with other viruses. Though mosquito innate immunity is a contributing factor to vector competence, it will not be discussed in this review. Detailed understanding of these factors will be instrumental in minimising transmission of alphaviral diseases.

Mosquito Immunobiology: The Intersection of Vector Health and Vector Competence

As holometabolous insects that occupy distinct aquatic and terrestrial environments in larval and adult stages and utilize hematophagy for nutrient acquisition, mosquitoes are subjected to a wide variety of symbiotic interactions. Indeed, mosquitoes play host to endosymbiotic, entomopathogenic, and mosquito-borne organisms, including protozoa, viruses, bacteria, fungi, fungal-like organisms, and metazoans, all of which trigger and shape innate infection-response capacity. Depending on the infection or interaction, the mosquito may employ, for example, cellular and humoral immune effectors for septic infections in the hemocoel, humoral infection responses in the midgut lumen, and RNA interference and programmed cell death for intracellular pathogens. These responses often function in concert, regardless of the infection type, and provide a robust front to combat infection. Mosquito-borne pathogens and entomopathogens overcome these immune responses, employing avoidance or suppression strategies. Burgeoning methodologies are capitalizing on this concerted deployment of immune responses to control mosquito-borne disease.

How Do Virus-Mosquito Interactions Lead to Viral Emergence?

Arboviruses such as West Nile, Zika, chikungunya, dengue, and yellow fever viruses have become highly significant global pathogens through unexpected, explosive outbreaks. While the rapid progression and frequency of recent arbovirus outbreaks is associated with long-term changes in human behavior (globalization, urbanization, climate change), there are direct mosquito-virus interactions which drive shifts in host range and alter virus transmission. This review summarizes how virus-mosquito interactions are critical for these viruses to become global pathogens at molecular, physiological, evolutionary, and epidemiological scales. Integrated proactive approaches are required in order to effectively manage the emergence of mosquito-borne arboviruses, which appears likely to continue into the indefinite future.

Individual co-variation between viral RNA load and gene expression reveals novel host factors during early dengue virus infection of the Aedes aegypti midgut

Dengue virus (DENV) causes more human infections than any other mosquito-borne virus. The current lack of antiviral strategies has prompted genome-wide screens for host genes that are required for DENV infectivity. Earlier transcriptomic studies that identified DENV host factors in the primary vector Aedes aegypti used inbred laboratory colonies and/or pools of mosquitoes that erase individual variation. Here, we performed transcriptome sequencing on individual midguts in a field-derived Ae. aegypti population to identify new candidate host factors modulating DENV replication. We analyzed the transcriptomic data using an approach that accounts for individual co-variation between viral RNA load and gene expression. This approach generates a prediction about the agonist or antagonist effect of candidate genes on DENV replication based on the sign of the correlation between gene expression and viral RNA load. Using this method, we identified 39 candidate genes that went undetected by conventional pairwise comparison of gene expression levels between DENV-infected midguts and uninfected controls. Only four candidate genes were detected by both methods, emphasizing their complementarity. We demonstrated the value of our approach by functional validation of a candidate agonist gene encoding a sterol regulatory element-binding protein (SREBP), which was identified by correlation analysis but not by pairwise comparison. We confirmed that SREBP promotes DENV infection in the midgut by RNAi-mediated gene knockdown in vivo. We suggest that our approach for transcriptomic analysis can empower genome-wide screens for potential agonist or antagonist factors by leveraging inter-individual variation in gene expression. More generally, this method is applicable to a wide range of phenotypic traits displaying inter-individual variation.

Flaviviruses Produce a Subgenomic Flaviviral RNA That Enhances Mosquito Transmission

Mosquito-borne flaviviruses (MBFVs) are a global public health burden. MBFVs have several unique 3'UTR structures that inhibit the host RNA decay machinery to produce subgenomic flaviviral RNAs (sfRNAs). Number of sfRNA species and their relative quantities are dependent on the 3'UTR tertiary structures and can vary between tissues. Two recent in vivo studies demonstrated that sfRNA enhances mosquito transmission, resulting in increased infection rate of saliva. Transmission efficiency is determined by the immune response. First evidence points to sfRNA interference with the Toll and RNAi immune pathways. However, a more complex picture that includes flexibility in sfRNA production and interaction with immune-related proteins remains to be explored.

Rift Valley fever virus and European mosquitoes: vector competence of Culex pipiens and Stegomyia albopicta (= Aedes albopictus)

Rift Valley fever (RVF) is a mosquito-borne disease caused by the Rift Valley fever virus (RVFV). Rift Valley fever affects a large number of species, including human, and has severe impact on public health and the economy, especially in African countries. The present study examined the vector competence of three different European mosquito species, Culex pipiens (Linnaeus, 1758) form molestus (Diptera: Culicidae), Culex pipiens hybrid form and Stegomyia albopicta (= Aedes albopictus) (Skuse, 1894) (Diptera: Culicidae). Mosquitoes were artificially fed with blood containing RVFV. Infection, disseminated infection and transmission efficiency were evaluated. This is the first study to assess the transmission efficiency of European mosquito species using a virulent RVFV strain. The virus disseminated in Cx. pipiens hybrid form and in S. albopicta. Moreover, infectious viral particles were isolated from saliva of both species, showing their RVFV transmission capacity. The presence of competent Cx. pipiens and S. albopicta in Spain indicates that an autochthonous outbreak of RVF may occur if the virus is introduced. These findings provide information that will help health authorities to set up efficient entomological surveillance and RVFV vector control programmes.

Dynamics of epizootic hemorrhagic disease virus infection within the vector, Culicoides sonorensis (Diptera: Ceratopogonidae)

Culicoides sonorensis biting midges are confirmed vectors of epizootic hemorrhagic disease virus (EHDV), which causes mortality in white-tailed deer and ruminant populations. Currently, of the seven EHDV serotypes, only 1, 2, and 6 are detected in the USA, and very few studies have focused on the infection time course of these serotypes within the midge. The objective of this current research was to characterize EHDV-2 infection within the midge by measuring infection prevalence, virus dissemination, and viral load over the course of infection. Midges were fed a blood meal containing 106.9 PFU/ml EHDV-2, collected every 12 h from 0-2 days post feeding (dpf) and daily from 3-10 dpf, and cohorts of 20 C. sonorensis were processed using techniques that assessed EHDV infection and dissemination. Cytopathic effect assays and quantitative (q)PCR were used to determine infection prevalence, revealing a 50% infection rate by 10 dpf using both methods. Using immunohistochemistry, EHDV-2 infection was detectable at 5 dpf, and shown to disseminate from the midgut to other tissues, including fat body, eyes, and salivary glands by 5 dpf. Stain intensity increased from 5-8 dpf, indicating replication of EHDV-2 in secondary infection sites after dissemination. This finding is also supported by trends in viral load over time as determined by plaque assays and qPCR. An increase in titer between 4-5 dpf correlated with viral replication in the midgut as seen with staining at day 5, while the subsequent gradual increase in viral load from 8-10 dpf suggested viral replication in midges with disseminated infection. Overall, the data presented herein suggest that EHDV-2 disseminates via the hemolymph to secondary infection sites throughout the midge and demonstrate a high potential for transmission at five days at 25°C after an infective blood-meal.

Vertical transmission of positive-sense single-stranded RNA viruses in plants as a model for arboviral induced teratogenesis

Teratogenic viruses have increased public health importance with the emergence of Zika virus and a recent decline in rubella virus vaccination. Of the seven viruses known to cause birth defects in humans, three are mosquito-borne pathogens. Ethical oversight, compliance, rising costs, and the need for specialized training slow the pace of study of these human pathogens compared to study of similar teratogenic viruses in plants. Plant viruses have served as models for human viruses which can be applied to animal systems. This review describes the similar features of plant and animal teratogenic arboviruses and the common systems and barriers that are encountered during vertical transmission in the host.

Zika virus: An updated review of competent or naturally infected mosquitoes

Zika virus (ZIKV) is an arthropod-borne virus (arbovirus) that recently caused outbreaks in the Americas. Over the past 60 years, this virus has been observed circulating among African, Asian, and Pacific Island populations, but little attention has been paid by the scientific community until the discovery that large-scale urban ZIKV outbreaks were associated with neurological complications such as microcephaly and several other neurological malformations in fetuses and newborns. This paper is a systematic review intended to list all mosquito species studied for ZIKV infection or for their vector competence. We discuss whether studies on ZIKV vectors have brought enough evidence to formally exclude other mosquitoes than Aedes species (and particularly Aedes aegypti) to be ZIKV vectors. From 1952 to August 15, 2017, ZIKV has been studied in 53 mosquito species, including 6 Anopheles, 26 Aedes, 11 Culex, 2 Lutzia, 3 Coquillettidia, 2 Mansonia, 2 Eretmapodites, and 1 Uranotaenia. Among those, ZIKV was isolated from 16 different Aedes species. The only species other than Aedes genus for which ZIKV was isolated were Anopheles coustani, Anopheles gambiae, Culex perfuscus, and Mansonia uniformis. Vector competence assays were performed on 22 different mosquito species, including 13 Aedes, 7 Culex, and 2 Anopheles species with, as a result, the discovery that A. aegypti and Aedes albopictus were competent for ZIKV, as well as some other Aedes species, and that there was a controversy surrounding Culex quinquefasciatus competence. Although Culex, Anopheles, and most of Aedes species were generally observed to be refractory to ZIKV infection, other potential vectors transmitting ZIKV should be explored.

Vector competence of European mosquitoes for West Nile virus

West Nile virus (WNV) is an arthropod-borne flavivirus of high medical and veterinary importance. The main vectors for WNV are mosquito species of the Culex genus that transmit WNV among birds, and occasionally to humans and horses, which are ‘dead-end’ hosts. Recently, several studies have been published that aimed to identify the mosquito species that serve as vectors for WNV in Europe. These studies provide insight in factors that can influence vector competence of European mosquito species for WNV. Here, we review the current knowledge on vector competence of European mosquitoes for WNV, and the molecular knowledge on physical barriers, anti-viral pathways and microbes that influence vector competence based on studies with other flaviviruses. By comparing the 12 available WNV vector competence studies with European mosquitoes we evaluate the effect of factors such as temperature, mosquito origin and mosquito biotype on vector competence. In addition, we propose a standardised methodology to allow for comparative studies across Europe. Finally, we identify knowledge gaps regarding vector competence that, once addressed, will provide important insights into WNV transmission and ultimately contribute to effective strategies to control WNV.

Full-genome dengue virus sequencing in mosquito saliva shows lack of convergent positive selection during transmission by Aedes aegypti

Like other pathogens with high mutation and replication rates, within-host dengue virus (DENV) populations evolve during infection of their main mosquito vector, Aedes aegypti. Within-host DENV evolution during transmission provides opportunities for adaptation and emergence of novel virus variants. Recent studies of DENV genetic diversity failed to detect convergent evolution of adaptive mutations in mosquito tissues such as midgut and salivary glands, suggesting that convergent positive selection is not a major driver of within-host DENV evolution in the vector. However, it is unknown whether this conclusion extends to the transmitted viral subpopulation because it is technically difficult to sequence DENV genomes in mosquito saliva. Here, we achieved DENV full-genome sequencing by pooling saliva samples collected non-sacrificially from 49 to 163 individual Ae. aegypti mosquitoes previously infected with one of two DENV-1 genotypes. We compared the transmitted viral subpopulations found in the pooled saliva samples collected in time series with the input viral population present in the infectious blood meal. In all pooled saliva samples examined, the full-genome consensus sequence of the input viral population was unchanged. Although the pooling strategy prevents analysis of individual saliva samples, our results demonstrate the lack of strong convergent positive selection during a single round of DENV transmission by Ae. aegypti. This finding reinforces the idea that genetic drift and purifying selection are the dominant evolutionary forces shaping within-host DENV genetic diversity during transmission by mosquitoes.

Blood meal acquisition enhances arbovirus replication in mosquitoes through activation of the GABAergic system

Mosquitoes are hematophagous insects that carry-on and transmit many human viruses. However, little information is available regarding the common mechanisms underlying the infection of mosquitoes by these viruses. In this study, we reveal that the hematophagous nature of mosquitoes contributes to arboviral infection after a blood meal, which suppresses antiviral innate immunity by activating the GABAergic pathway. dsRNA-mediated interruption of the GABA signaling and blockage of the GABA_A receptor by the specific inhibitors both significantly impaired arbovirus replication. Consistently, inoculation of GABA enhanced arboviral infection, indicating that GABA signaling facilitates the arboviral infection of mosquitoes. The ingestion of blood by mosquitoes resulted in robust GABA production from glutamic acid derived from blood protein digestion. The oral introduction of glutamic acid increased virus acquisition by mosquitoes via activation of the GABAergic system. Our study reveals that blood meals enhance arbovirus replication in mosquitoes through activation of the GABAergic system.

Transmission of many human viruses depends on replication in their mosquito vectors. Here, Zhu et al. show that glutamic acid digested from the blood meal activates GABA signaling, resulting in suppression of antiviral innate immunity and increased virus replication in mosquitoes.

Chikungunya virus dissemination from the midgut of Aedes aegypti is associated with temporal basal lamina degradation during bloodmeal digestion

In the mosquito, the midgut epithelium is the initial tissue to become infected with an arthropod-borne virus (arbovirus) that has been acquired from a vertebrate host along with a viremic bloodmeal. Following its replication in midgut epithelial cells, the virus needs to exit the midgut and infect secondary tissues including the salivary glands before it can be transmitted to another vertebrate host. The viral exit mechanism from the midgut, the midgut escape barrier (MEB), is poorly understood although it is an important determinant of mosquito vector competence for arboviruses. Using chikungunya virus (CHIKV) as a model in Aedes aegypti, we demonstrate that the basal lamina (BL) of the extracellular matrix (ECM) surrounding the midgut constitutes a potential barrier for the virus. The BL, predominantly consisting of collagen IV and laminin, becomes permissive during bloodmeal digestion in the midgut lumen. Bloodmeal digestion, BL permissiveness, and CHIKV dissemination are coincident with increased collagenase activity, diminished collagen IV abundance, and BL shredding in the midgut between 24–32 h post-bloodmeal. This indicates that there may be a window-of-opportunity during which the MEB in Ae. aegypti becomes permissive for CHIKV. Matrix metalloproteinases (MMPs) are the principal extracellular endopeptidases responsible for the degradation/remodeling of the ECM including the BL. We focused on Ae. aegypti (Ae)MMP1, which is expressed in midgut epithelial cells, is inducible upon bloodfeeding, and shows collagenase (gelatinase) activity. However, attempts to inhibit AeMMP activity in general or specifically that of AeMMP1 did not seem to affect its function nor produce an altered midgut escape phenotype. As an alternative, we silenced and overexpressed the Ae. aegyptitissue inhibitor of metalloproteinases (AeTIMP) in the mosquito midgut. AeTIMP was highly upregulated in the midgut during bloodmeal digestion and was able to inhibit MMP activity in vitro. Bloodmeal-inducible, midgut-specific overexpression of AeTIMP or its expression via a recombinant CHIKV significantly increased midgut dissemination rates of the virus. Possibly, AeTIMP overexpression affected BL degradation and/or restoration thereby increasing the midgut dissemination efficiency of the virus.

The biological nature of the midgut escape barrier in insects for arthropod-borne viruses has been a mystery for decades. Here we show that the basal lamina (BL) surrounding the mosquito midgut acts as a barrier for chikungunya virus, an alphavirus, which has emerged in the New World hemisphere around three years ago. The barrier became permissive for the virus during digestion of a viremic bloodmeal inside the midgut lumen. Concurrent with BL permissiveness, we observed that collagen IV, a major component of the BL became temporally degraded while the BL was visibly damaged. Based on previous findings, we hypothesized that matrix metalloproteinases such as Ae. aegypti (Ae)MMP1 may be involved in BL degradation. We confirmed that recombinant AeMMP1 exhibited strong gelatinase activity, which was profoundly reduced when recombinant AeMMP1 interacted in vitro with the recombinant Ae. aegypti tissue inhibitor of metalloproteinases (AeTIMP). When transgenically overexpressing AeTIMP in an attempt to temporally inhibit general MMP activity in the mosquito midgut, we observed that the dissemination efficiency of chikungunya virus became significantly increased, while its midgut infection was not affected. It is possible that AeTIMP overexpression affected BL degradation/restoration permitting increased quantities of virus to escape from the midgut.

Functional RNA during Zika virus infection

Zika virus (ZIKV; family Flaviviridae; genus Flavivirus) is a pathogenic mosquito-borne RNA virus that currently threatens human health in the Americas, large parts of Asia and occasionally elsewhere in the world. ZIKV infection is often asymptomatic but can cause severe symptoms including congenital microcephaly and Guillain-Barré syndrome. The positive single-stranded RNA genome of the mosquito-borne ZIKV requires effective replication in two evolutionary distinct hosts - mosquitoes and primates. In addition to some of the viral proteins, the ZIKV genomic RNA and functional RNAs produced thereof aid in the establishment of productive infection and the evasion of host cell antiviral responses. ZIKV has evolved to contain a nucleotide composition and RNA modifications, such as methylation and the formation of G-quadruplexes that allow effective replication in both hosts. Furthermore, a number of host factors interact with the viral genome to modulate RNA replication. Importantly, the ZIKV genome produces non-coding subgenomic flavivirus RNA (sfRNA) due to stalling of host 5'- 3' ribonucleases on viral RNA structures in the 3' untranslated region (UTR). This sfRNA (sfRNA) exerts important proviral functions such as antagonizing the innate interferon response and RNA interference. Here, we discuss the ZIKV genomic RNA and functional RNAs thereof to assess their significance during ZIKV infection. Understanding the details of the ZIKV infection cycle will aid in the development of effective antiviral strategies and safe vaccines.

Differential Vector Competency of Aedes albopictus Populations from the Americas for Zika Virus

To evaluate the potential role of Aedes albopictus (Skuse) as a vector of Zika virus (ZIKV), colonized mosquitoes of low generation number (≤ F5) from Brazil, Houston, and the Rio Grande Valley of Texas engorged on viremic mice infected with ZIKV strains originating from Senegal, Cambodia, Mexico, Brazil, or Puerto Rico. Vector competence was established by monitoring infection, dissemination, and transmission potential after 3, 7, and 14 days of extrinsic incubation. Positive saliva samples were assayed for infectious titer. Although all three mosquito populations were susceptible to all ZIKV strains, rates of infection, dissemination, and transmission differed among mosquito and virus strains. Aedes albopictus from Salvador, Brazil, were the least efficient vectors, demonstrating susceptibility to infection to two American strains of ZIKV but failing to shed virus in saliva. Mosquitoes from the Rio Grande Valley were the most efficient vectors and were capable of shedding all three tested ZIKV strains into saliva after 14 days of extrinsic incubation. In particular, ZIKV strain DakAR 41525 (Senegal 1954) was significantly more efficient at dissemination and saliva deposition than the others tested in Rio Grande mosquitoes. Overall, our data indicate that, while Ae. albopictus is capable of transmitting ZIKV, its competence is potentially dependent on geographic origin of both the mosquito population and the viral strain.

Mosquito-specific and mosquito-borne viruses: evolution, infection, and host defense

Recent virus discovery programs have identified an extensive reservoir of viruses in arthropods. It is thought that arthropod viruses, including mosquito-specific viruses, are ancestral to vertebrate-pathogenic arboviruses. Mosquito-specific viruses are restricted in vertebrate cells at multiple levels, including entry, RNA replication, assembly, and by the inability to replicate at high temperatures. Moreover, it is likely that the vertebrate immune system suppresses replication of these viruses. The evolution from single to dual-host tropism may also require changes in the course of infection in the mosquito host. In this review we explore the adaptive changes required for a switch from a mosquito-specific to a mosquito-borne transmission cycle.

Dengue subgenomic flaviviral RNA disrupts immunity in mosquito salivary glands to increase virus transmission

Globally re-emerging dengue viruses are transmitted from human-to-human by Aedes mosquitoes. While viral determinants of human pathogenicity have been defined, there is a lack of knowledge of how dengue viruses influence mosquito transmission. Identification of viral determinants of transmission can help identify isolates with high epidemiological potential. Additionally, mechanistic understanding of transmission will lead to better understanding of how dengue viruses harness evolution to cycle between the two hosts. Here, we identified viral determinants of transmission and characterized mechanisms that enhance production of infectious saliva by inhibiting immunity specifically in salivary glands. Combining oral infection of Aedes aegypti mosquitoes and reverse genetics, we identified two 3’ UTR substitutions in epidemic isolates that increased subgenomic flaviviral RNA (sfRNA) quantity, infectious particles in salivary glands and infection rate of saliva, which represents a measure of transmission. We also demonstrated that various 3’UTR modifications similarly affect sfRNA quantity in both whole mosquitoes and human cells, suggesting a shared determinism of sfRNA quantity. Furthermore, higher relative quantity of sfRNA in salivary glands compared to midgut and carcass pointed to sfRNA function in salivary glands. We showed that the Toll innate immune response was preferentially inhibited in salivary glands by viruses with the 3’UTR substitutions associated to high epidemiological fitness and high sfRNA quantity, pointing to a mechanism for higher saliva infection rate. By determining that sfRNA is an immune suppressor in a tissue relevant to mosquito transmission, we propose that 3’UTR/sfRNA sequence evolution shapes dengue epidemiology not only by influencing human pathogenicity but also by increasing mosquito transmission, thereby revealing a viral determinant of epidemiological fitness that is shared between the two hosts.

Dengue is a re-emerging global disease transmitted from human-to-human by mosquitoes. While environmental and host immune factors are important, viral determinants of mosquito transmission also shape the epidemiology of dengue. Understanding how dengue viruses influence transmission will help identify isolates with high epidemic potential and untangle the evolutionary pressures at play in the dual-host cycle. Here, we identified 2 substitutions in the 3’UTR of epidemic isolates that increase transmission through immune suppression in the salivary glands. Using oral infection of Aedes aegypti mosquitoes, we reported that epidemic isolates produced more subgenomic flaviviral RNA (sfRNA) in salivary glands. SfRNA is generated from the 3’UTR sequence remaining after partial genome degradation by a host nuclease. Using reverse genetics, we identified the two 3’UTR substitutions responsible for the higher sfRNA quantity in salivary glands. We further showed that these substitutions increased dengue virus titer in salivary glands and rate of saliva infection, and suppressed the Toll immune response in salivary glands. Our study identifies the substitutions that determine virus epidemiological fitness and provides a mechanism for sfRNA-mediated enhancement of transmission. Together with previous work demonstrating that sfRNA sequence modification influences dengue virus pathogenicity in human, and that shows variation in sfRNA sequence when the viruses transition from one host to vector and vice versa, our study supports that sfRNA evolution is constrained in the two hosts.

Competence of Aedes aegypti, Ae. albopictus, and Culex quinquefasciatus Mosquitoes as Zika Virus Vectors, China

In China, the prevention and control of Zika virus disease has been a public health threat since the first imported case was reported in February 2016. To determine the vector competence of potential vector mosquito species, we experimentally infected Aedes aegypti, Ae. albopictus, and Culex quinquefasciatus mosquitoes and determined infection rates, dissemination rates, and transmission rates. We found the highest vector competence for the imported Zika virus in Ae. aegypti mosquitoes, some susceptibility of Ae. albopictus mosquitoes, but no transmission ability for Cx. quinquefasciatus mosquitoes. Considering that, in China, Ae. albopictus mosquitoes are widely distributed but Ae. aegypti mosquito distribution is limited, Ae. albopictus mosquitoes are a potential primary vector for Zika virus and should be targeted in vector control strategies.

The Evolutionary Consequences of Stepwise Infection Processes

Molecular and cellular studies reveal that the resistance of hosts to parasites and pathogens is a cascade-like process with multiple steps required to be passed for successful infection. By contrast, much of evolutionary reasoning is based on strongly simplified, one- or two-step infection processes with simple genetics or on resistance being a quantitative trait. Here we attempt a conceptual unification of these two perspectives with the aim of cross-fostering research and filling some of the gaps in our concepts of the ecology and evolution of disease. This conceptual unification has a profound impact on the way we understand the genetics and evolution of host resistance, ecological immunity, evolution of virulence, defence portfolios, and host-pathogen coevolution.

Mosquito co-infection with Zika and chikungunya virus allows simultaneous transmission without affecting vector competence of Aedes aegypti

BACKGROUND

Zika virus (ZIKV) and chikungunya virus (CHIKV) are highly pathogenic arthropod-borne viruses that are currently a serious health burden in the Americas, and elsewhere in the world. ZIKV and CHIKV co-circulate in the same geographical regions and are mainly transmitted by Aedes aegypti mosquitoes. There is a growing number of case reports of ZIKV and CHIKV co-infections in humans, but it is uncertain whether co-infection occurs via single or multiple mosquito bites. Here we investigate the potential of Ae. aegypti mosquitoes to transmit both ZIKV and CHIKV in one bite, and we assess the consequences of co-infection on vector competence.

METHODOLOGY/PRINCIPAL FINDINGS

First, growth curves indicated that co-infection with CHIKV negatively affects ZIKV production in mammalian, but not in mosquito cells. Next, Ae. aegypti mosquitoes were infected with ZIKV, CHIKV, or co-infected via an infectious blood meal or intrathoracic injections. Infection and transmission rates, as well as viral titers of positive mosquitoes, were determined at 14 days after blood meal or 7 days after injection. Saliva and bodies of (co-)infected mosquitoes were scored concurrently for the presence of ZIKV and/or CHIKV using a dual-colour immunofluorescence assay. The results show that orally exposed Ae. aegypti mosquitoes are highly competent, with transmission rates of up to 73% for ZIKV, 21% for CHIKV, and 12% of mosquitoes transmitting both viruses in one bite. However, simultaneous oral exposure to both viruses did not change infection and transmission rates compared to exposure to a single virus. Intrathoracic injections indicate that the selected strain of Ae. aegypti has a strong salivary gland barrier for CHIKV, but a less profound barrier for ZIKV.

CONCLUSIONS/SIGNIFICANCE

This study shows that Ae. aegypti can transmit both ZIKV and CHIKV via a single bite. Furthermore, co-infection of ZIKV and CHIKV does not influence the vector competence of Ae. aegypti.

Rift Valley fever virus: strategies for maintenance, survival and vertical transmission in mosquitoes

Rift Valley fever virus (RVFV) is a mosquito-borne arbovirus causing severe disease in humans and ruminants. Spread of RVFV out of Africa has raised concerns that it could emerge in Europe or the USA. Virus persistence is dependent on successful infection of, replication in, and transmission to susceptible vertebrate and invertebrate hosts, modulated by virus-host and vector-virus interactions. The principal accepted theory for the long-term maintenance of RVFV involves vertical transmission (VT) of virus to mosquito progeny, with the virus surviving long inter-epizootic periods within the egg. This VT hypothesis, however, is yet to be comprehensively proven. Here, evidence for and against the VT of RVFV is reviewed along with the identification of factors limiting its detection in natural and experimental data. The observations of VT for other arboviruses in the genera Alphavirus, Flavivirus and Orthobunyavirus are discussed within the context of RVFV. The review concludes that VT of RVFV is likely but that current data are insufficient to irrefutably prove this hypothesis.

The midgut transcriptome of Aedes aegypti fed with saline or protein meals containing chikungunya virus reveals genes potentially involved in viral midgut escape

BACKGROUND

The mosquito Aedes aegypti is the primary vector for medically important arthropod-borne viruses, including chikungunya virus (CHIKV). Following oral acquisition, an arbovirus has to persistently infect several organs in the mosquito before becoming transmissible to another vertebrate host. A major obstacle an arbovirus has to overcome during its infection cycle inside the mosquito is the midgut escape barrier, representing the exit mechanism arboviruses utilize when disseminating from the midgut. To understand the transcriptomic basis of midgut escape and to reveal genes involved in the process, we conducted a comparative transcriptomic analysis of midgut samples from mosquitoes which had received a saline meal (SM) or a protein meal (PM) (not) containing CHIKV.

RESULTS

CHIKV which was orally acquired by a mosquito along with a SM or PM productively infected the midgut epithelium and disseminated to secondary tissues. A total of 27 RNA-Seq libraries from midguts of mosquitoes that had received PM or SM (not) containing CHIKV at 1 and 2 days post-feeding were generated and sequenced. Fewer than 80 genes responded differentially to the presence of CHIKV in midguts of mosquitoes that had acquired the virus along with SM or PM. SM feeding induced differential expression (DE) of 479 genes at day 1 and 314 genes at day 2 when compared to midguts of sugarfed mosquitoes. By comparison, PM feeding induced 6029 DE genes at day 1 and 7368 genes at day 2. Twenty-three DE genes encoding trypsins, metalloproteinases, and serine-type endopeptidases were significantly upregulated in midguts of mosquitoes at day 1 following SM or PM ingestion. Two of these genes were Ae. aegypti late trypsin (AeLT) and serine collagenase 1 precursor (AeSP1). In vitro, recombinant AeLT showed strong matrix metalloproteinase activity whereas recombinant AeSP1 did not.

CONCLUSIONS

By substituting a bloodmeal for SM, we identified midgut-expressed genes not involved in blood or protein digestion. These included genes coding for trypsins, metalloproteinases, and serine-type endopeptidases, which could be involved in facilitating midgut escape for arboviruses in Ae. aegypti. The presence of CHIKV in any of the ingested meals had relatively minor effects on the overall gene expression profiles in midguts.

Functional Validation of Apoptosis Genes IAP1 and DRONC in Midgut Tissue of the Biting Midge Culicoides sonorensis (Diptera: Ceratopogonidae) by RNAi

Culicoides biting midges transmit multiple ruminant viruses, including bluetongue virus and epizootic hemorrhagic disease virus, causing significant economic burden worldwide. To further enhance current control techniques, understanding vector-virus interactions within the midge is critical. We developed previously a double-stranded RNA (dsRNA) delivery method to induce RNA interference (RNAi) for targeted gene knockdown in adult Culicoides sonorensis Wirth & Jones. Here, we confirm the C. sonorensis inhibitor of apoptosis 1 (CsIAP1) as an anti-apoptotic functional ortholog of IAP1 in Drosophila, identify the ortholog of the Drosophila initiator caspase DRONC (CsDRONC), and demonstrate that injection of dsRNA into the hemocoel can be used for targeted knockdown in the midgut in C. sonorensis. We observed CsIAP1 transcript reduction in whole midges, with highest transcript reduction in midgut tissues. IAP1knockdown (kd) resulted in pro-apoptotic caspase activation in midgut tissues. In IAP1kd midges, midgut tissue integrity and size were severely compromised. This phenotype, as well as reduced longevity, was partially reverted by co-RNAi suppression of CsDRONC and CsIAP1. Therefore, RNAi can be directed to the midgut of C. sonorensis, the initial site of virus infection, using dsRNA injection into the hemocoel. In addition, we provide evidence that the core apoptosis pathway is conserved in C. sonorensis and can be experimentally activated in the midgut to reduce longevity in C. sonorensis. This study thus paves the way for future reverse genetic analyses of midgut-virus interactions in C. sonorensis, including the putative antiviral properties of RNAi and apoptosis pathways.

Catalase protects Aedes aegypti from oxidative stress and increases midgut infection prevalence of Dengue but not Zika

BACKGROUND

Digestion of blood in the midgut of Aedes aegypti results in the release of pro-oxidant molecules that can be toxic to the mosquito. We hypothesized that after a blood meal, the antioxidant capacity of the midgut is increased to protect cells against oxidative stress. Concomitantly, pathogens present in the blood ingested by mosquitoes, such as the arboviruses Dengue and Zika, also have to overcome the same oxidative challenge, and the antioxidant program induced by the insect is likely to influence infection status of the mosquito and its vectorial competence.

METHODOLOGY/PRINCIPAL FINDINGS

We found that blood-induced catalase mRNA and activity in the midgut peaked 24 h after feeding and returned to basal levels after the completion of digestion. RNAi-mediated silencing of catalase (AAEL013407-RB) reduced enzyme activity in the midgut epithelia, increased H2O2 leakage and decreased fecundity and lifespan when mosquitoes were fed H2O2. When infected with Dengue 4 and Zika virus, catalase-silenced mosquitoes showed no alteration in infection intensity (number of plaque forming units/midgut) 7 days after the infectious meal. However, catalase knockdown reduced Dengue 4, but not Zika, infection prevalence (percent of infected midguts).

CONCLUSION/SIGNIFICANCE

Here, we showed that blood ingestion triggers an antioxidant response in the midgut through the induction of catalase. This protection facilitates the establishment of Dengue virus in the midgut. Importantly, this mechanism appears to be specific for Dengue because catalase silencing did not change Zika virus prevalence. In summary, our data suggest that redox balance in the midgut modulates mosquito vectorial competence to arboviral infections.

Aedes-Borne Virus-Mosquito Interactions: Mass Spectrometry Strategies and Findings

Aedes-borne viruses are responsible for high-impact neglected tropical diseases and unpredictable outbreaks such as the ongoing Zika epidemics. Aedes mosquitoes spread different arboviruses such as Dengue virus (DENV), Chikungunya virus (CHIKV), and Zika virus, among others, and are responsible for the continuous emergence and reemergence of these pathogens. These viruses have complex transmission cycles that include two hosts, namely the Aedes mosquito as a vector and susceptible vertebrate hosts. Human infection with arboviruses causes diseases that range from subclinical or mild to febrile diseases, encephalitis, and hemorrhagic fever. Infected mosquitoes do not show detectable signs of disease, even though the virus maintains a lifelong persistent infection. The infection of the Aedes mosquito by viruses involves a molecular crosstalk between cell and viral proteins. An understanding of how mosquito vectors and viruses interact is of fundamental interest, and it also offers novel perspectives for disease control. In recent years, mass spectrometry (MS)-based strategies in combination with bioinformatics have been successfully applied to identify and quantify global changes in cellular proteins, lipids, peptides, and metabolites in response to viral infection. Although the information about proteomics in the Aedes mosquito is limited, the information that has been reported can set up the basis for future studies. This review reflects how MS-based approaches have extended our understanding of Aedes mosquito biology and the development of DENV and CHIKV infection in the vector. Finally, this review discusses future challenges in the field.

Dengue Chikungunya co-infection: A live-in relationship??

Dengue and Chikungunya are viral infections that are a major public health hazard in recent times. Both these infections are caused by RNA viruses termed arboviruses owing to their requirement of an arthropod vector to get transmitted to vertebrate hosts. Apart from sharing a common vector, namely Aedes mosquitoes, these infections are also characterized by overlapping clinical presentations and are known to exist as co-infection. The present review traces the history and evolution of co-infection across the globe and provides specific compilation of the scenario in India. Furthermore, clinical manifestations during co-infection are discussed. Lastly, up-to-date information with respect to vector behaviour during co-infection both under laboratory conditions and in natural Aedes populations is reviewed.

Identification and initial characterization of matrix metalloproteinases in the yellow fever mosquito, Aedes aegypti

Aedes aegypti is a major vector for arboviruses such as dengue, chikungunya and Zika viruses. During acquisition of a viremic bloodmeal, an arbovirus infects mosquito midgut cells before disseminating to secondary tissues, including the salivary glands. Once virus is released into the salivary ducts it can be transmitted to another vertebrate host. The midgut is surrounded by a basal lamina (BL) in the extracellular matrix, consisting of a proteinaceous mesh composed of collagen IV and laminin. BL pore size exclusion limit prevents virions from passing through. Thus, the BL probably requires remodelling via enzymatic activity to enable efficient virus dissemination. Matrix metalloproteinases (MMPs) are extracellular endopeptidases that are involved in remodelling of the extracellular matrix. Here, we describe and characterize the nine Ae. aegypti encoded MMPs, AeMMPs 1-9, which share common features with other invertebrate and vertebrate MMPs. Expression profiling in Ae. aegypti revealed that Aemmp4 and Aemmp6 were upregulated during metamorphosis, whereas expression of Aemmp1 and Aemmp2 increased during bloodmeal digestion. Aemmp1 expression was also upregulated in the presence of a bloodmeal containing chikungunya virus. Using polyclonal antibodies, AeMMP1 and AeMMP2 were specifically detected in tissues associated with the mosquito midgut.

Antiviral effect of compounds derived from the seeds of Mammea americana and Tabernaemontana cymosa on Dengue and Chikungunya virus infections

Background

The transmission of Dengue virus (DENV) and Chikungunya virus (CHIKV) has increased worldwide, due in part to the lack of a specific antiviral treatment. For this reason, the search for compounds with antiviral potential, either as licensed drugs or in natural products, is a research priority. The objective of this study was to identify some of the compounds that are present in Mammea americana (M. americana) and Tabernaemontana cymosa (T. cymosa) plants and, subsequently, to evaluate their cytotoxicity in VERO cells and their potential antiviral effects on DENV and CHIKV infections in those same cells.

Methods

Dry ethanolic extracts of M. americana and T. cymosa seeds were subjected to open column chromatographic fractionation, leading to the identification of four compounds: two coumarins, derived from M. americana; and lupeol acetate and voacangine derived from T. cymosa.. The cytotoxicity of each compound was subsequently assessed by the MTT method (at concentrations from 400 to 6.25 μg/mL). Pre- and post-treatment antiviral assays were performed at non-toxic concentrations; the resulting DENV inhibition was evaluated by Real-Time PCR, and the CHIKV inhibition was tested by the plating method. The results were analyzed by means of statistical analysis.

Results

The compounds showed low toxicity at concentrations ≤ 200 μg/mL. The compounds coumarin A and coumarin B, which are derived from the M. americana plant, significantly inhibited infection with both viruses during the implementation of the two experimental strategies employed here (post-treatment with inhibition percentages greater than 50%, p < 0.01; and pre-treatment with percentages of inhibition greater than 40%, p < 0.01). However, the lupeol acetate and voacangine compounds, which were derived from the T. cymosa plant, only significantly inhibited the DENV infection during the post-treatment strategy (at inhibition percentages greater than 70%, p < 0.01).

Conclusion

In vitro, the coumarins are capable of inhibiting infection by DENV and CHIKV (with inhibition percentages above 50% in different experimental strategies), which could indicate that these two compounds are potential antivirals for treating Dengue and Chikungunya fever. Additionally, lupeol acetate and voacangine efficiently inhibit infection with DENV, also turning them into promising antivirals for Dengue fever.

Complexity of virus-vector interactions

The inter-relationships among viruses, vectors and vertebrate hosts are complex and dynamic and shaped by biotic (e.g., viral strain, vector genetics, host susceptibility) and abiotic (e.g., temperature, rainfall, human land use) factors. It is anticipated that changes in climate, as predicted by the most recent Report of the Intergovernmental Panel on Climate Change, will result in landscape changes and consequent changes in spatiotemporal patterns of arbovirus transmission. To anticipate evolving patterns of virus activity in a dynamically changing environment, it is important to understand how interconnectedness of mosquito and virus biology together with climate influence arbovirus transmission intensity. Vector competence, survivorship, and feeding behavior, among other aspects of vectorial capacity are intrinsically important to estimate risk and design control approaches.

Plant Virus-Insect Vector Interactions: Current and Potential Future Research Directions

Acquisition and transmission by an insect vector is central to the infection cycle of the majority of plant pathogenic viruses. Plant viruses can interact with their insect host in a variety of ways including both non-persistent and circulative transmission; in some cases, the latter involves virus replication in cells of the insect host. Replicating viruses can also elicit both innate and specific defense responses in the insect host. A consistent feature is that the interaction of the virus with its insect host/vector requires specific molecular interactions between virus and host, commonly via proteins. Understanding the interactions between plant viruses and their insect host can underpin approaches to protect plants from infection by interfering with virus uptake and transmission. Here, we provide a perspective focused on identifying novel approaches and research directions to facilitate control of plant viruses by better understanding and targeting virus-insect molecular interactions. We also draw parallels with molecular interactions in insect vectors of animal viruses, and consider technical advances for their control that may be more broadly applicable to plant virus vectors.

Understanding the immune system architecture and transcriptome responses to southern rice black-streaked dwarf virus in Sogatella furcifera

Sogatella furcifera, the white-backed planthopper (WBPH), has become one of the most destructive pests in rice production owing to its plant sap-sucking behavior and efficient transmission of Southern rice black-streaked dwarf virus (SRBSDV) in a circulative, propagative and persistent manner. The dynamic and complex SRBSDV-WBPH-rice plant interaction is still poorly understood. In this study, based on a homology-based genome-wide analysis, 348 immune-related genes belonging to 28 families were identified in WBPH. A transcriptome analysis of non-viruliferous (NVF) and viruliferous groups with high viral titers (HVT) and median viral titers (MVT) revealed that feeding on SRBSDV-infected rice plants has a significant impact on gene expression, regardless of viral titers in insects. We identified 278 up-regulated and 406 down-regulated genes shared among the NVF, MVT, and HVT groups and detected significant down-regulation of primary metabolism-related genes and oxidoreductase. In viruliferous WBPH with viral titer-specific transcriptome changes, 1,906 and 1,467 genes exhibited strict monotonically increasing and decreasing expression, respectively. The RNAi pathway was the major antiviral response to increasing viral titers among diverse immune responses. These results clarify the responses of immune genes and the transcriptome of WBPH to SRBSDV and improve our knowledge of the functional relationship between pathogen, vector, and host.

Genome Investigations of Vector Competence in Aedes aegypti to Inform Novel Arbovirus Disease Control Approaches

Dengue (DENV), yellow fever, chikungunya, and Zika virus transmission to humans by a mosquito host is confounded by both intrinsic and extrinsic variables. Besides virulence factors of the individual arboviruses, likelihood of virus transmission is subject to variability in the genome of the primary mosquito vector, Aedes aegypti. The “vectorial capacity” of A. aegypti varies depending upon its density, biting rate, and survival rate, as well as its intrinsic ability to acquire, host and transmit a given arbovirus. This intrinsic ability is known as “vector competence”. Based on whole transcriptome analysis, several genes and pathways have been predicated to have an association with a susceptible or refractory response in A. aegypti to DENV infection. However, the functional genomics of vector competence of A. aegypti is not well understood, primarily due to lack of integrative approaches in genomic or transcriptomic studies. In this review, we focus on the present status of genomics studies of DENV vector competence in A. aegypti as limited information is available relative to the other arboviruses. We propose future areas of research needed to facilitate the integration of vector and virus genomics and environmental factors to work towards better understanding of vector competence and vectorial capacity in natural conditions.

Noncoding Subgenomic Flavivirus RNA Is Processed by the Mosquito RNA Interference Machinery and Determines West Nile Virus Transmission by Culex pipiens Mosquitoes

Flaviviruses, such as Zika virus, yellow fever virus, dengue virus, and West Nile virus (WNV), are a serious concern for human health. Flaviviruses produce an abundant noncoding subgenomic flavivirus RNA (sfRNA) in infected cells. sfRNA results from stalling of the host 5'-3' exoribonuclease XRN1/Pacman on conserved RNA structures in the 3' untranslated region (UTR) of the viral genomic RNA. sfRNA production is conserved in insect-specific, mosquito-borne, and tick-borne flaviviruses and flaviviruses with no known vector, suggesting a pivotal role for sfRNA in the flavivirus life cycle. Here, we investigated the function of sfRNA during WNV infection of Culex pipiens mosquitoes and evaluated its role in determining vector competence. An sfRNA1-deficient WNV was generated that displayed growth kinetics similar to those of wild-type WNV in both RNA interference (RNAi)-competent and -compromised mosquito cell lines. Small-RNA deep sequencing of WNV-infected mosquitoes indicated an active small interfering RNA (siRNA)-based antiviral response for both the wild-type and sfRNA1-deficient viruses. Additionally, we provide the first evidence that sfRNA is an RNAi substrate in vivo Two reproducible small-RNA hot spots within the 3' UTR/sfRNA of the wild-type virus mapped to RNA stem-loops SL-III and 3' SL, which stick out of the three-dimensional (3D) sfRNA structure model. Importantly, we demonstrate that sfRNA-deficient WNV displays significantly decreased infection and transmission rates in vivo when administered via the blood meal. Finally, we show that transmission and infection rates are not affected by sfRNA after intrathoracic injection, thereby identifying sfRNA as a key driver to overcome the mosquito midgut infection barrier. This is the first report to describe a key biological function of sfRNA for flavivirus infection of the arthropod vector, providing an explanation for the strict conservation of sfRNA production.

IMPORTANCE

Understanding the flavivirus transmission cycle is important to identify novel targets to interfere with disease and to aid development of virus control strategies. Flaviviruses produce an abundant noncoding viral RNA called sfRNA in both arthropod and mammalian cells. To evaluate the role of sfRNA in flavivirus transmission, we infected mosquitoes with the flavivirus West Nile virus and an sfRNA-deficient mutant West Nile virus. We demonstrate that sfRNA determines the infection and transmission rates of West Nile virus in Culex pipiens mosquitoes. Comparison of infection via the blood meal versus intrathoracic injection, which bypasses the midgut, revealed that sfRNA is important to overcome the mosquito midgut barrier. We also show that sfRNA is processed by the antiviral RNA interference machinery in mosquitoes. This is the first report to describe a pivotal biological function of sfRNA in arthropods. The results explain why sfRNA production is evolutionarily conserved.

Dynamics of West Nile virus evolution in mosquito vectors

West Nile virus remains the most common cause of arboviral encephalitis in North America. Since it was introduced, it has undergone adaptive genetic change as it spread throughout the continent. The WNV transmission cycle is relatively tractable in the laboratory. Thus the virus serves as a convenient model system for studying the population biology of mosquito-borne flaviviruses as they undergo transmission to and from mosquitoes and vertebrates. This review summarizes the current knowledge regarding the population dynamics of this virus within mosquito vectors.

Chikungunya virus transmission between Aedes albopictus and laboratory mice

Background

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus associated with epidemics of acute and chronic arthritic disease in humans. Aedes albopictus has emerged as an important new natural vector for CHIKV transmission; however, mouse models for studying transmission have not been developed.

Methods

Aedes albopictus mosquitoes were infected with CHIKV via membrane feeding and by using infected adult wild-type C57BL/6 mice. Paraffin sections of infected mosquitoes were analysed by immunofluorescent antibody staining using an anti-CHIKV antibody. CHIKV-infected mosquitoes were used to infect adult C57BL/6 and interferon response factor 3 and 7 deficient (IRF3/7^-/-) mice.

Results

Feeding mosquitoes on blood meals with CHIKV titres > 5 log₁₀CCID₅₀/ml, either by membrane feeding or feeding on infected mice, resulted in  ≥ 50 % of mosquitoes becoming infected. However, CHIKV titres in blood meals  ≥ 7 log₁₀CCID₅₀/ml were required before salivary glands showed significant levels of immunofluorescent staining with an anti-CHIKV antibody. Mosquitoes fed on blood meals of 7.5 (but not 5.9) log₁₀CCID₅₀/ml were able efficiently to transmit virus to adult C57BL/6 and IRF3/7^-/- mice, with the latter mice showing overt signs of arthritis post-infection.

Conclusions

The results provide a simple in vivo model for studying transmission of CHIKV from mosquitoes to mammals and also argue against a resistance barrier to CHIKV infection in adult mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1838-1) contains supplementary material, which is available to authorized users.

Microbiota-Dependent Priming of Antiviral Intestinal Immunity in Drosophila

Enteric pathogens must overcome intestinal defenses to establish infection. In Drosophila, the ERK signaling pathway inhibits enteric virus infection. The intestinal microflora also impacts immunity but its role in enteric viral infection is unknown. Here we show that two signals are required to activate antiviral ERK signaling in the intestinal epithelium. One signal depends on recognition of peptidoglycan from the microbiota, particularly from the commensal Acetobacter pomorum, which primes the NF-kB-dependent induction of a secreted factor, Pvf2. However, the microbiota is not sufficient to induce this pathway; a second virus-initiated signaling event involving release of transcriptional paused genes mediated by the kinase Cdk9 is also required for Pvf2 production. Pvf2 stimulates antiviral immunity by binding to the receptor tyrosine kinase PVR, which is necessary and sufficient for intestinal ERK responses. These findings demonstrate that sensing of specific commensals primes inflammatory signaling required for epithelial responses that restrict enteric viral infections.

Functional integration of the circulatory, immune, and respiratory systems in mosquito larvae: pathogen killing in the hemocyte-rich tracheal tufts

BACKGROUND

As both larvae and adults, mosquitoes encounter a barrage of immune insults, ranging from microbe-rich communities in larval habitats to ingested blood-borne pathogens in adult blood meals. Given that mosquito adults have evolved an efficient means of eliminating infections in their hemocoel (body cavity) via the coordinated action of their immune and circulatory systems, the goal of the present study was to determine whether such functional integration is also present in larvae.

RESULTS

By fluorescently labeling hemocytes (immune cells), pericardial cells, and the heart, we discovered that fourth instar larvae, unlike adults, contain segmental hemocytes but lack the periostial hemocytes that surround the ostia (heart valves) in abdominal segments 2-7. Instead, larvae contain an abundance of sessile hemocytes at the tracheal tufts, which are respiratory structures that are unique to larvae, are located in the posterior-most abdominal segment, and surround what in larvae are the sole incurrent openings for hemolymph entry into the heart. Injection of fluorescent immune elicitors and bacteria into the larval hemocoel then showed that tracheal tuft hemocytes mount rapid and robust immune responses against foreign insults. Indeed, green fluorescent protein-labeled Escherichia coli flowing with the hemolymph rapidly aggregate exclusively at the tracheal tufts, where they are killed within 24 h post-infection via both phagocytosis and melanization.

CONCLUSION

Together, these findings show that the functional integration of the circulatory, respiratory, and immune systems of mosquitoes varies drastically across life stages.

Viral receptors of the gut: vector-borne viruses of medical importance

Arthropods transmit several medically important arboviruses that cause diseases in humans. Therapeutic strategies to treat or prevent diseases transmitted by the arthropods are limiting. Understanding the role of arthropod gut receptors in the interactions with various arboviruses would provide important means for the development of a strong anti-vector vaccine. In this review, we summarize some of the potential findings in the field of arthropod gut receptors for tick-borne or mosquito-borne viruses and discuss their relevance in the development of a broad-spectrum transmission-blocking vaccine to treat or control various diseases caused by arboviruses.

Viral RNA Silencing Suppression: The Enigma of Bunyavirus NSs Proteins

The Bunyaviridae is a family of arboviruses including both plant- and vertebrate-infecting representatives. The Tospovirus genus accommodates plant-infecting bunyaviruses, which not only replicate in their plant host, but also in their insect thrips vector during persistent propagative transmission. For this reason, they are generally assumed to encounter antiviral RNA silencing in plants and insects. Here we present an overview on how tospovirus nonstructural NSs protein counteracts antiviral RNA silencing in plants and what is known so far in insects. Like tospoviruses, members of the related vertebrate-infecting bunyaviruses classified in the genera Orthobunyavirus, Hantavirus and Phlebovirus also code for a NSs protein. However, for none of them RNA silencing suppressor activity has been unambiguously demonstrated in neither vertebrate host nor arthropod vector. The second part of this review will briefly describe the role of these NSs proteins in modulation of innate immune responses in mammals and elaborate on a hypothetical scenario to explain if and how NSs proteins from vertebrate-infecting bunyaviruses affect RNA silencing. If so, why this discovery has been hampered so far.

Genetics of War and Truce between Mosquitos and Emerging Viruses

Arboviruses have made unexpected reappearances in recent years. Unlike viruses that undergo direct transmission, arboviruses utilize an arthropod vector (e.g., mosquitos, sandflies, and ticks) to spread throughout human populations. Here, we provide a snapshot of mosquito susceptibility to viral infection using flaviviruses, alphaviruses, and bunyaviruses as examples of emerging pathogens of global health relevance.

Infection pattern and transmission potential of chikungunya virus in two New World laboratory-adapted Aedes aegypti strains

Chikungunya virus (CHIKV) is an emerging mosquito-borne virus belonging to the Togaviridae, which is transmitted to humans by Aedes aegypti and Ae. albopictus. We describe the infection pattern of CHIKV in two New World Ae. aegypti strains, HWE and ORL. Both mosquito strains were susceptible to the virus but showed different infection patterns in midguts and salivary glands. Even though acquisition of a bloodmeal showed moderate levels of apoptosis in midgut tissue, there was no obvious additional CHIKV-induced apoptosis detectable during midgut infection. Analysis of expression of apoptosis-related genes suggested that CHIKV infection dampens rather than promotes apoptosis in the mosquito midgut. In both mosquito strains, the virus was present in saliva within two days post-oral infection. HWE and ORL mosquitoes exhibited no salivary gland infection barrier; however, only 60% (HWE) to 65% (ORL) of the females had released the virus in their saliva at one week post-oral acquisition, suggesting a salivary gland escape barrier. CHIKV induced an apoptotic response in salivary glands of HWE and ORL mosquitoes, demonstrating that the virus caused pathology in its natural vector.

Arboviruses and apoptosis: the role of cell death in determining vector competence

A relatively small number of mosquito species transmit arboviruses such as dengue, yellow fever, chikungunya and West Nile viruses to hundreds of millions of people each year, yet we still lack a thorough understanding of the molecular factors that determine vector competence. Apoptosis has been shown to be an important factor in determining the outcome of virus infection for many viruses. However, until recently, it was not clear whether apoptosis plays a role in determining the outcome of arbovirus infections in mosquitoes. Recent work has begun to shed light on the roles of apoptosis in this important process.