Wolbachia-mediated antiviral protection is cell-autonomous

Antiviral Wolbachia strains associate with Aedes aegypti endoplasmic reticulum membranes and induce lipid droplet formation to restrict dengue virus replication

Wolbachia are a genus of insect endosymbiotic bacteria which includes strains wMel and wAlbB that are being utilized as a biocontrol tool to reduce the incidence of Aedes aegypti-transmitted viral diseases like dengue. However, the precise mechanisms underpinning the antiviral activity of these Wolbachia strains are not well defined. Here, we generated a panel of Ae. aegypti-derived cell lines infected with antiviral strains wMel and wAlbB or the non-antiviral Wolbachia strain wPip to understand host cell morphological changes specifically induced by antiviral strains. Antiviral strains were frequently found to be entirely wrapped by the host endoplasmic reticulum (ER) membrane, while wPip bacteria clustered separately in the host cell cytoplasm. ER-derived lipid droplets (LDs) increased in volume in wMel- and wAlbB-infected cell lines and mosquito tissues compared to cells infected with wPip or Wolbachia-free controls. Inhibition of fatty acid synthase (required for triacylglycerol biosynthesis) reduced LD formation and significantly restored ER-associated dengue virus replication in cells occupied by wMel. Together, this suggests that antiviral Wolbachia strains may specifically alter the lipid composition of the ER to preclude the establishment of dengue virus (DENV) replication complexes. Defining Wolbachia's antiviral mechanisms will support the application and longevity of this effective biocontrol tool that is already being used at scale.IMPORTANCEAedes aegypti transmits a range of important human pathogenic viruses like dengue. However, infection of Ae. aegypti with the insect endosymbiotic bacterium, Wolbachia, reduces the risk of mosquito to human viral transmission. Wolbachia is being utilized at field sites across more than 13 countries to reduce the incidence of viruses like dengue, but it is not well understood how Wolbachia induces its antiviral effects. To examine this at the subcellular level, we compared how different strains of Wolbachia with varying antiviral strengths associate with and modify host cell structures. Strongly antiviral strains were found to specifically associate with the host endoplasmic reticulum and induce striking impacts on host cell lipid droplets. Inhibiting Wolbachia-induced lipid redistribution partially restored dengue virus replication demonstrating this is a contributing role for Wolbachia's antiviral activity. These findings provide new insights into how antiviral Wolbachia strains associate with and modify Ae. aegypti host cells.

Wolbachia-Virus interactions and arbovirus control through population replacement in mosquitoes

Following transfer into the primary arbovirus vector Aedes aegypti, several strains of the intracellular bacterium Wolbachia have been shown to inhibit the transmission of dengue, Zika, and chikungunya viruses, important human pathogens that cause significant morbidity and mortality worldwide. In addition to pathogen inhibition, many Wolbachia strains manipulate host reproduction, resulting in an invasive capacity of the bacterium in insect populations. This has led to the deployment of Wolbachia as a dengue control tool, and trials have reported significant reductions in transmission in release areas. Here, we discuss the possible mechanisms of Wolbachia-virus inhibition and the implications for long-term success of dengue control. We also consider the evidence presented in several reports that Wolbachia may cause an enhancement of replication of certain viruses under particular conditions, and conclude that these should not cause any concerns with respect to the application of Wolbachia to arbovirus control.

Investigating Wolbachia symbiont-mediated host protection against a bacterial pathogen using a natural Wolbachia nuclear insert

Wolbachia bacterial endosymbionts provide protection against pathogens in various arthropod species but the underlying mechanisms remain misunderstood. By using a natural Wolbachia nuclear insert (f-element) in the isopod Armadillidium vulgare, we explored whether Wolbachia presence is mandatory to observe protection in this species or the presence of its genes is sufficient. We assessed survival of closely related females carrying or lacking the f-element (and lacking Wolbachia) challenged with the bacterial pathogen Salmonella enterica. Despite marginal significant effects, the f-element alone did not appear to confer survival benefits to its host, suggesting that Wolbachia presence in cells is crucial for protection.

Wolbachia RNase HI contributes to virus blocking in the mosquito Aedes aegypti

The endosymbiotic bacterium Wolbachia pipientis blocks replication of several arboviruses in transinfected Aedes aegypti mosquitoes. However, the mechanism of virus blocking remains poorly understood. Here, we characterized an RNase HI gene from Wolbachia, which is rapidly induced in response to dengue virus (DENV) infection. Knocking down w RNase HI using antisense RNA in Wolbachia-transinfected mosquito cell lines and A. aegypti mosquitoes led to increased DENV replication. Furthermore, overexpression of wRNase HI, in the absence of Wolbachia, led to reduced replication of a positive sense RNA virus, but had no effect on a negative sense RNA virus, a familiar scenario in Wolbachia-infected cells. Altogether, our results provide compelling evidence for the missing link between early Wolbachia-mediated virus blocking and degradation of viral RNA. These findings and the successful pioneered knockdown of Wolbachia genes using antisense RNA in cell line and mosquitoes enable new ways to manipulate and study the complex endosymbiont-host interactions.

High Temperature Cycles Result in Maternal Transmission and Dengue Infection Differences Between Wolbachia Strains in Aedes aegypti

Environmental factors play a crucial role in the population dynamics of arthropod endosymbionts, and therefore in the deployment of Wolbachia symbionts for the control of dengue arboviruses. The potential of Wolbachia to invade, persist, and block virus transmission depends in part on its intracellular density. Several recent studies have highlighted the importance of larval rearing temperature in modulating Wolbachia densities in adults, suggesting that elevated temperatures can severely impact some strains, while having little effect on others. The effect of a replicated tropical heat cycle on Wolbachia density and levels of virus blocking was assessed using Aedes aegypti lines carrying strains wMel and wAlbB, two Wolbachia strains currently used for dengue control. Impacts on intracellular density, maternal transmission fidelity, and dengue inhibition capacity were observed for wMel. In contrast, wAlbB-carrying Ae. aegypti maintained a relatively constant intracellular density at high temperatures and conserved its capacity to inhibit dengue. Following larval heat treatment, wMel showed a degree of density recovery in aging adults, although this was compromised by elevated air temperatures. IMPORTANCE In the past decades, dengue incidence has dramatically increased all over the world. An emerging dengue control strategy utilizes Aedes aegypti mosquitoes artificially transinfected with the bacterial symbiont Wolbachia, with the ultimate aim of replacing wild mosquito populations. However, the rearing temperature of mosquito larvae is known to impact on some Wolbachia strains. In this study, we compared the effects of a temperature cycle mimicking natural breeding sites in tropical climates on two Wolbachia strains, currently used for open field trials. When choosing the Wolbachia strain to be used in a dengue control program it is important to consider the effects of environmental temperatures on invasiveness and virus inhibition. These results underline the significance of understanding the impact of environmental factors on released mosquitoes, in order to ensure the most efficient strategy for dengue control.

Designing effective Wolbachia release programs for mosquito and arbovirus control

Mosquitoes carrying endosymbiotic bacteria called Wolbachia are being released in mosquito and arbovirus control programs around the world through two main approaches: population suppression and population replacement. Open field releases of Wolbachia-infected male mosquitoes have achieved over 95% population suppression by reducing the fertility of wild mosquito populations. The replacement of populations with Wolbachia-infected females is self-sustaining and can greatly reduce local dengue transmission by reducing the vector competence of mosquito populations. Despite many successful interventions, significant questions and challenges lie ahead. Wolbachia, viruses and their mosquito hosts can evolve, leading to uncertainty around the long-term effectiveness of a given Wolbachia strain, while few ecological impacts of Wolbachia releases have been explored. Wolbachia strains are diverse and the choice of strain to release should be made carefully, taking environmental conditions and the release objective into account. Mosquito quality control, thoughtful community awareness programs and long-term monitoring of populations are essential for all types of Wolbachia intervention. Releases of Wolbachia-infected mosquitoes show great promise, but existing control measures remain an important way to reduce the burden of mosquito-borne disease.

Using Wolbachia to Eliminate Dengue: Will the Virus Fight Back?

Recent field trials have demonstrated that dengue incidence can be substantially reduced by introgressing strains of the endosymbiotic bacterium Wolbachia into Aedes aegypti mosquito populations. This strategy relies on Wolbachia reducing the susceptibility of Ae. aegypti to disseminated infection by positive-sense RNA viruses like dengue. However, RNA viruses are well known to adapt to antiviral pressures. Here, we review the viral infection stages where selection for Wolbachia-resistant virus variants could occur. We also consider the genetic constraints imposed on viruses that alternate between vertebrate and invertebrate hosts, and the likely selection pressures to which dengue virus might adapt in order to be effectively transmitted by Ae. aegypti that carry Wolbachia. While there are hurdles to dengue viruses developing resistance to Wolbachia, we suggest that long-term surveillance for resistant viruses should be an integral component of Wolbachia-introgression biocontrol programs.

Current State and Promising Opportunities on Pharmaceutical Approaches in the Treatment of Polymicrobial Diseases

In recent years, the emergence of newly identified acute and chronic infectious disorders caused by diverse combinations of pathogens, termed polymicrobial diseases, has had catastrophic consequences for humans. Antimicrobial agents have been clinically proven to be effective in the pharmacological treatment of polymicrobial diseases. Unfortunately, an increasing trend in the emergence of multi-drug-resistant pathogens and limited options for delivery of antimicrobial drugs might seriously impact humans' efforts to combat polymicrobial diseases in the coming decades. New antimicrobial agents with novel mechanism(s) of action and new pharmaceutical formulations or delivery systems to target infected sites are urgently required. In this review, we discuss the prospective use of novel antimicrobial compounds isolated from natural products to treat polymicrobial infections, mainly via mechanisms related to inhibition of biofilm formation. Drug-delivery systems developed to deliver antimicrobial compounds to both intracellular and extracellular pathogens are discussed. We further discuss the effectiveness of several biofilm-targeted delivery strategies to eliminate polymicrobial biofilms. At the end, we review the applications and promising opportunities for various drug-delivery systems, when compared to conventional antimicrobial therapy, as a pharmacological means to treat polymicrobial diseases.

Wolbachia strain wAlbB maintains high density and dengue inhibition following introduction into a field population of Aedes aegypti

Aedes aegypti mosquitoes carrying the wAlbB Wolbachia strain show a reduced capacity to transmit dengue virus. wAlbB has been introduced into wild Ae. aegypti populations in several field sites in Kuala Lumpur, Malaysia, where it has persisted at high frequency for more than 2 years and significantly reduced dengue incidence. Although these encouraging results indicate that wAlbB releases can be an effective dengue control strategy, the long-term success depends on wAlbB maintaining high population frequencies and virus transmission inhibition, and both could be compromised by Wolbachia-host coevolution in the field. Here, wAlbB-carrying Ae. aegypti collected from the field 20 months after the cessation of releases showed no reduction in Wolbachia density or tissue distribution changes compared to a wAlbB laboratory colony. The wAlbB strain continued to induce complete unidirectional cytoplasmic incompatibility, showed perfect maternal transmission under laboratory conditions, and retained its capacity to inhibit dengue. Additionally, a field-collected wAlbB line was challenged with Malaysian dengue patient blood, and showed significant blocking of virus dissemination to the salivary glands. These results indicate that wAlbB continues to inhibit currently circulating strains of dengue in field populations of Ae. aegypti, and provides additional support for the continued scale-up of Wolbachia wAlbB releases for dengue control. This article is part of the theme issue 'Novel control strategies for mosquito-borne diseases'.

Viral RNA is a target for Wolbachia-mediated pathogen blocking

The ability of the endosymbiont Wolbachia pipientis to restrict RNA viruses is presently being leveraged to curb global transmission of arbovirus-induced diseases. Past studies have shown that virus replication is limited early in arthropod cells colonized by the bacterium, although it is unclear if this phenomenon is replicated in mosquito cells that first encounter viruses obtained through a vertebrate blood meal. Furthermore, these cellular events neither explain how Wolbachia limits dissemination of viruses between mosquito tissues, nor how it prevents transmission of infectious viruses from mosquitoes to vertebrate host. In this study, we try to address these issues using an array of mosquito cell culture models, with an additional goal being to identify a common viral target for pathogen blocking. Our results establish the viral RNA as a cellular target for Wolbachia-mediated inhibition, with the incoming viral RNA experiencing rapid turnover following internalization in cells. This early block in replication in mosquito cells initially infected by the virus thus consequently reduces the production of progeny viruses from these same cells. However, this is not the only contributor to pathogen blocking. We show that the presence of Wolbachia reduces the per-particle infectivity of progeny viruses on naïve mosquito and vertebrate cells, consequently limiting virus dissemination and transmission, respectively. Importantly, we demonstrate that this aspect of pathogen blocking is independent of any particular Wolbachia-host association and affects viruses belonging to Togaviridae and Flaviviridae families of RNA viruses. Finally, consistent with the idea of the viral RNA as a target, we find that the encapsidated virion RNA is less infectious for viruses produced from Wolbachia-colonized cells. Collectively, our findings present a common mechanism of pathogen blocking in mosquitoes that establish a link between virus inhibition in the cell to virus dissemination and transmission.