Reassessing the role of defensin in the innate immune response of the mosquito, Aedes aegypti

Activation of immune pathways in common bed bugs, Cimex lectularius, in response to bacterial immune challenges - a transcriptomics analysis

The common bed bug, Cimex lectularius, is an urban pest of global health significance, severely affecting the physical and mental health of humans. In contrast to most other blood-feeding arthropods, bed bugs are not major vectors of pathogens, but the underlying mechanisms for this phenomenon are largely unexplored. Here, we present the first transcriptomics study of bed bugs in response to immune challenges. To study transcriptional variations in bed bugs following ingestion of bacteria, we extracted and processed mRNA from body tissues of adult male bed bugs after ingestion of sterile blood or blood containing the Gram-positive (Gr+) bacterium Bacillus subtilis or the Gram-negative (Gr-) bacterium Escherichia coli. We analyzed mRNA from the bed bugs' midgut (the primary tissue involved in blood ingestion) and from the rest of their bodies (RoB; body minus head and midgut tissues). We show that the midgut exhibits a stronger immune response to ingestion of bacteria than the RoB, as indicated by the expression of genes encoding antimicrobial peptides (AMPs). Both the Toll and Imd signaling pathways, associated with immune responses, were highly activated by the ingestion of bacteria. Bacterial infection in bed bugs further provides evidence for metabolic reconfiguration and resource allocation in the bed bugs' midgut and RoB to promote production of AMPs. Our data suggest that infection with particular pathogens in bed bugs may be associated with altered metabolic pathways within the midgut and RoB that favors immune responses. We further show that multiple established cellular immune responses are preserved and are activated by the presence of specific pathogens. Our study provides a greater understanding of nuances in the immune responses of bed bugs towards pathogens that ultimately might contribute to novel bed bug control tactics.

Response of the mosquito immune system and symbiotic bacteria to pathogen infection

Mosquitoes are the deadliest animal in the word, transmitting a variety of insect-borne infectious diseases, such as malaria, dengue fever, yellow fever, and Zika, causing more deaths than any other vector-borne pathogen. Moreover, in the absence of effective drugs and vaccines to prevent and treat insect-borne diseases, mosquito control is particularly important as the primary measure. In recent decades, due to the gradual increase in mosquito resistance, increasing attention has fallen on the mechanisms and effects associated with pathogen infection. This review provides an overview of mosquito innate immune mechanisms in terms of physical and physiological barriers, pattern recognition receptors, signalling pathways, and cellular and humoral immunity, as well as the antipathogenic effects of mosquito symbiotic bacteria. This review contributes to an in-depth understanding of the interaction process between mosquitoes and pathogens and provides a theoretical basis for biological defence strategies against mosquito-borne infectious diseases.

MicroRNA Expression Prior to Biting in a Vector Mosquito Anticipates Physiological Processes Related to Energy Utilization, Reproduction and Immunity

Understanding the molecular and physiological processes underlying biting behavior in vector mosquitoes has important implications for developing novel strategies to suppress disease transmission. Here, we conduct small-RNA sequencing and qRT-PCR to identify differentially expressed microRNAs (miRNAs) in the head tissues of two subspecies of Culex pipiens that differ in biting behavior and the ability to produce eggs without blood feeding. We identified eight differentially expressed miRNAs between biting C. pipiens pipiens (Pipiens) and non-biting C. pipiens molestus (Molestus); six of these miRNAs have validated functions or predicted targets related to energy utilization (miR8-5-p, miR-283, miR-2952-3p, miR-1891), reproduction (miR-1891), and immunity (miR-2934-3p, miR-92a, miR8-5-p). Although miRNAs regulating physiological processes associated with blood feeding have previously been shown to be differentially expressed in response to a blood meal, our results are the first to demonstrate differential miRNA expression in anticipation of a blood meal before blood is actually imbibed. We compare our current miRNA results to three previous studies of differential messenger RNA expression in the head tissues of mosquitoes. Taken together, the combined results consistently show that biting mosquitoes commit to specific physiological processes in anticipation of a blood meal, while non-biting mosquitoes mitigate these anticipatory costs.

Entomopathogenic nematodes isolated from agricultural areas of Thailand and their activity against the larvae of Aedes aegypti, Aedes albopictus and Culex quinquefasciatus (Diptera: Culicidae)

Entomopathogenic nematodes (EPNs) of the genera Steinernema and Heterorhabditis have been considered to be effective biological control agents for several insects. In this study, we isolated and identified EPNs from soil samples in agricultural areas of northern Thailand and evaluated their efficacy for controlling larvae of three mosquito vector species, Aedes aegypti, Ae. albopictus and Culex quinquefasciatus. A total of 51 of 1,000 soil samples were positive (5.1% prevalence) for EPNs, which were identified through sequencing of the rDNA and ITS to 37 Steinernema isolates (3.7%) and 14 Heterorhabditis isolates (1.4%). For the bioassay, the larvae of mosquitoes were exposed to Steinernema surkhetense (eALN6.3_TH), Steinernema lamjungense (eALN11.5_TH), Heterorhabditis indica (eACM14.2_TH) and Heterorhabditis bacteriophora (eALN18.2_TH). Heterorhabditis bacteriophora showed the highest efficacy against Ae. aegypti and Cx. quinquefasciatus. At 96 h after exposure, the mortality rates were 60.0 and 91.7%, respectively. The EPNs were observed in the dead mosquito larvae, which were mostly found in the thorax followed by the head and abdomen. Some EPNs were dead with melanization, and some were able to survive in the cavity of mosquito larvae. Our results show the low prevalence of EPN in agricultural areas of Thailand. Moreover, H. bacteriophora may be considered an alternative biocontrol agent for managing and controlling these vector mosquitoes.

A zone-of-inhibition assay to screen for humoral antimicrobial activity in mosquito hemolymph

In insects, antibacterial immunity largely depends on the activation of downstream signaling and effector responses, leading to the synthesis and secretion of soluble effector molecules, such as antimicrobial peptides (AMPs). AMPs are acute infection response peptides secreted into the hemolymph upon bacterial stimulation. The transcription of innate immunity genes encoding for AMPs is highly dependent on several signaling cascade pathways, such as the Toll pathway. In the African malaria mosquito, Anopheles gambiae, AMPs hold a special interest as their upregulation have been shown to limit the growth of malaria parasites, bacteria, and fungi. Most of the current knowledge on the regulation of insect AMPs in microbial infection have been obtained from Drosophila. However, largely due to the lack of convenient assays, the regulation of antimicrobial activity in mosquito hemolymph is still not completely understood. In this study, we report a zone of inhibition assay to identify the contribution of AMPs and components of the Toll pathway to the antimicrobial activity of A. gambiae hemolymph. As a proof of principle, we demonstrate that Micrococcus luteus challenge induces antimicrobial activity in the adult female mosquito hemolymph, which is largely dependent on defensin 1. Moreover, by using RNAi to silence Cactus, REL1, and MyD88, we showed that Cactus kd induces antimicrobial activity in the mosquito hemolymph, whereas the antimicrobial activity in REL1 kd and MyD88 kd is reduced after challenge. Finally, while injection itself is not sufficient to induce antimicrobial activity, our results show that it primes the response to bacterial challenge. Our study provides information that increases our knowledge of the regulation of antimicrobial activity in response to microbial infections in mosquitoes. Furthermore, this assay represents an ex vivo medium throughput assay that can be used to determine the upstream regulatory elements of antimicrobial activity in A. gambiae hemolymph.

Beauveria bassiana interacts with gut and hemocytes to manipulate Aedes aegypti immunity

BACKGROUND

Mosquito-borne diseases affect millions of people. Chemical insecticides are currently employed against mosquitoes. However, many cases of insecticide resistance have been reported. Entomopathogenic fungi (EPF) have demonstrated potential as a bioinsecticide. Here, we assessed the invasion of the EPF Beauveria bassiana into Aedes aegypti larvae and changes in the activity of phenoloxidase (PO) as a proxy for the general activation of the insect innate immune system. In addition, other cellular and humoral responses were evaluated.

METHODS

Larvae were exposed to blastospores or conidia of B. bassiana CG 206. After 24 and 48 h, scanning electron microscopy (SEM) was conducted on the larvae. The hemolymph was collected to determine changes in total hemocyte concentration (THC), the dynamics of hemocytes, and to observe hemocyte-fungus interactions. In addition, the larvae were macerated to assess the activity of PO using L-DOPA conversion, and the expression of antimicrobial peptides (AMPs) was measured using quantitative Real-Time PCR.

RESULTS

Propagules invaded mosquitoes through the midgut, and blastopores were detected inside the hemocoel. Both propagules decreased the THC regardless of the time. By 24 h after exposure to conidia the percentage of granulocytes and oenocytoids increased while the prohemocytes decreased. By 48 h, the oenocytoid percentage increased significantly (P < 0.05) in larvae exposed to blastospores; however, the other hemocyte types did not change significantly. Regardless of the time, SEM revealed hemocytes adhering to, and nodulating, blastospores. For the larvae exposed to conidia, these interactions were observed only at 48 h. Irrespective of the propagule, the PO activity increased only at 48 h. At 24 h, cathepsin B was upregulated by infection with conidia, whereas both propagules resulted in a downregulation of cecropin and defensin A. At 48 h, blastospores and conidia increased the expression of defensin A suggesting this may be an essential AMP against EPF.

CONCLUSION

By 24 h, B. bassiana CG 206 occluded the midgut, reduced THC, did not stimulate PO activity, and downregulated AMP expression in larvae, all of which allowed the fungus to impair the larvae to facilitate infection. Our data reports a complex interplay between Ae. aegypti larvae and B. bassiana CG 206 demonstrating how this fungus can infect, affect, and kill Ae. aegypti larvae.

Characterization of New Defensin Antimicrobial Peptides and Their Expression in Bed Bugs in Response to Bacterial Ingestion and Injection

Common bed bugs, Cimex lectularius, can carry, but do not transmit, pathogens to the vertebrate hosts on which they feed. Some components of the innate immune system of bed bugs, such as antimicrobial peptides (AMPs), eliminate the pathogens. Here, we determined the molecular characteristics, structural properties, and phylogenetic relatedness of two new defensins (CL-defensin1 (XP_024085718.1), CL-defensin2 (XP_014240919.1)), and two new defensin isoforms (CL-defensin3a (XP_014240918.1), CL-defensin3b (XP_024083729.1)). The complete amino acid sequences of CL-defensin1, CL-defensin2, CL-defensin3a, and CL-defensin3b are strongly conserved, with only minor differences in their signal and pro-peptide regions. We used a combination of comparative transcriptomics and real-time quantitative PCR to evaluate the expression of these defensins in the midguts and the rest of the body of insects that had been injected with bacteria or had ingested blood containing the Gram-positive (Gr+) bacterium Bacillus subtilis and the Gram-negative (Gr–) bacterium Escherichia coli. We demonstrate, for the first time, sex-specific and immunization mode-specific upregulation of bed bug defensins in response to injection or ingestion of Gr+ or Gr– bacteria. Understanding the components, such as these defensins, of the bed bugs’ innate immune systems in response to pathogens may help unravel why bed bugs do not transmit pathogens to vertebrates.

Transcriptome Analysis of an Aedes albopictus Cell Line Single- and Dual-Infected with Lammi Virus and WNV

Understanding the flavivirus infection process in mosquito hosts is important and fundamental in the search for novel control strategies that target the mosquitoes’ ability to carry and transmit pathogenic arboviruses. A group of viruses known as insect-specific viruses (ISVs) has been shown to interfere with the infection and replication of a secondary arbovirus infection in mosquitoes and mosquito-derived cell lines. However, the molecular mechanisms behind this interference are unknown. Therefore, in the present study, we infected the Aedes albopictus cell line U4.4 with either the West Nile virus (WNV), the insect-specific Lammi virus (LamV) or an infection scheme whereby cells were pre-infected with LamV 24 h prior to WNV challenge. The qPCR analysis showed that the dual-infected U4.4 cells had a reduced number of WNV RNA copies compared to WNV-only infected cells. The transcriptome profiles of the different infection groups showed a variety of genes with altered expression. WNV-infected cells had an up-regulation of a broad range of immune-related genes, while in LamV-infected cells, many genes related to stress, such as different heat-shock proteins, were up-regulated. The transcriptome profile of the dual-infected cells was a mix of up- and down-regulated genes triggered by both viruses. Furthermore, we observed an up-regulation of signal peptidase complex (SPC) proteins in all infection groups. These SPC proteins have shown importance for flavivirus assembly and secretion and could be potential targets for gene modification in strategies for the interruption of flavivirus transmission by mosquitoes.

Transcriptome profiles of Anopheles gambiae harboring natural low-level Plasmodium infection reveal adaptive advantages for the mosquito

Anopheline mosquitoes are the sole vectors for the Plasmodium pathogens responsible for malaria, which is among the oldest and most devastating of human diseases. The continuing global impact of malaria reflects the evolutionary success of a complex vector-pathogen relationship that accordingly has been the long-term focus of both debate and study. An open question in the biology of malaria transmission is the impact of naturally occurring low-level Plasmodium infections of the vector on the mosquito's health and longevity as well as critical behaviors such as host-preference/seeking. To begin to answer this, we have completed a comparative RNAseq-based transcriptome profile study examining the effect of biologically salient, salivary gland transmission-stage Plasmodium infection on the molecular physiology of Anopheles gambiae s.s. head, sensory appendages, and salivary glands. When compared with their uninfected counterparts, Plasmodium infected mosquitoes exhibit increased transcript abundance of genes associated with olfactory acuity as well as a range of synergistic processes that align with increased fitness based on both anti-aging and reproductive advantages. Taken together, these data argue against the long-held paradigm that malaria infection is pathogenic for anophelines and, instead suggests there are biological and evolutionary advantages for the mosquito that drive the preservation of its high vectorial capacity.

Genetic engineering and bacterial pathogenesis against the vectorial capacity of mosquitoes

Mosquitoes are the main vector of multiple diseases worldwide and transmit viral (malaria, chikungunya, encephalitis, yellow fever, as well as dengue fever), as well as bacterial diseases (tularemia). To manage the outbreak of mosquito populations, various management programs include the application of chemicals, followed by biological and genetic control. Here we aimed to focus on the role of bacterial pathogenesis and molecular tactics for the management of mosquitoes and their vectorial capacity. Bacterial pathogenesis and molecular manipulations have a substantial impact on the biology of mosquitoes, and both strategies change the gene expression and regulation of disease vectors. The strategy for genetic modification is also proved to be excellent for the management of mosquitoes, which halt the development of population via incompatibility of different sex. Therefore, the purpose of the present discussion is to illustrate the impact of both approaches against the vectorial capacity of mosquitoes. Moreover, it could be helpful to understand the relationship of insect-pathogen and to manage various insect vectors as well as diseases.

Transcriptomic Analysis of Aedes aegypti Innate Immune System in Response to Ingestion of Chikungunya Virus

Aedes aegypti (L.) is the primary vector of emergent mosquito-borne viruses, including chikungunya, dengue, yellow fever, and Zika viruses. To understand how these viruses interact with their mosquito vectors, an analysis of the innate immune system response was conducted. The innate immune system is a conserved evolutionary defense strategy and is the dominant immune system response found in invertebrates and vertebrates, as well as plants. RNA-sequencing analysis was performed to compare target transcriptomes of two Florida Ae. aegypti strains in response to chikungunya virus infection. We analyzed a strain collected from a field population in Key West, Florida, and a laboratory strain originating from Orlando. A total of 1835 transcripts were significantly expressed at different levels between the two Florida strains of Ae. aegypti. Gene Ontology analysis placed these genes into 12 categories of biological processes, including 856 transcripts (up/down regulated) with more than 1.8-fold (p-adj (p-adjust value) ≤ 0.01). Transcriptomic analysis and q-PCR data indicated that the members of the AaeCECH genes are important for chikungunya infection response in Ae. aegypti. These immune-related enzymes that the chikungunya virus infection induces may inform molecular-based strategies for interruption of arbovirus transmission by mosquitoes.

Mosquito antiviral defense mechanisms: a delicate balance between innate immunity and persistent viral infection

Mosquito-borne diseases are associated with major global health burdens. Aedes spp. and Culex spp. are primarily responsible for the transmission of the most medically important mosquito-borne viruses, including dengue virus, West Nile virus and Zika virus. Despite the burden of these pathogens on human populations, the interactions between viruses and their mosquito hosts remain enigmatic. Viruses enter the midgut of a mosquito following the mosquito’s ingestion of a viremic blood meal. During infection, virus recognition by the mosquito host triggers their antiviral defense mechanism. Of these host defenses, activation of the RNAi pathway is the main antiviral mechanism, leading to the degradation of viral RNA, thereby inhibiting viral replication and promoting viral clearance. However, whilst antiviral host defense mechanisms limit viral replication, the mosquito immune system is unable to effectively clear the virus. As such, these viruses can establish persistent infection with little or no fitness cost to the mosquito vector, ensuring life-long transmission to humans. Understanding of the mosquito innate immune response enables the discovery of novel antivectorial strategies to block human transmission. This review provides an updated and concise summary of recent studies on mosquito antiviral immune responses, which is a key determinant for successful virus transmission. In addition, we will also discuss the factors that may contribute to persistent infection in mosquito hosts. Finally, we will discuss current mosquito transmission-blocking strategies that utilize genetically modified mosquitoes and Wolbachia-infected mosquitoes for resistance to pathogens.

Mosquito Innate Immunity

Mosquitoes live under the endless threat of infections from different kinds of pathogens such as bacteria, parasites, and viruses. The mosquito defends itself by employing both physical and physiological barriers that resist the entry of the pathogen and the subsequent establishment of the pathogen within the mosquito. However, if the pathogen does gain entry into the insect, the insect mounts a vigorous innate cellular and humoral immune response against the pathogen, thereby limiting the pathogen's propagation to nonpathogenic levels. This happens through three major mechanisms: phagocytosis, melanization, and lysis. During these processes, various signaling pathways that engage intense mosquito⁻pathogen interactions are activated. A critical overview of the mosquito immune system and latest information about the interaction between mosquitoes and pathogens are provided in this review. The conserved, innate immune pathways and specific anti-pathogenic strategies in mosquito midgut, hemolymph, salivary gland, and neural tissues for the control of pathogen propagation are discussed in detail.

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.

Mosquito Phenoloxidase and Defensin Colocalize in Melanization Innate Immune Responses

Mosquitoes mount strong humoral and cellular immune responses against foreign organisms. Two components of the mosquito immune response that have received much attention are the phenoloxidase cascade that leads to melanization and antimicrobial peptides. The purpose of the current study was to use immunocytochemistry and transmission electron microscopy to identify the location of the melanization rate-limiting enzyme phenoloxidase and the antimicrobial peptide defensin in innate immune reactions against Escherichia coli and Micrococcus luteus by the mosquito Aedes aegypti. Our results show that both phenoloxidase and defensin are present at the sites of melanin biosynthesis in immune reactions against bacteria. Furthermore, both proteins are often present inside the same melanotic capsules. When hemocytes were analyzed, phenoloxidase was present in the cytosol of oenocytoids, but no significant amounts of defensin were detected inside any hemocytes. In summary, these data show that phenoloxidase and defensin colocalize in melanization reactions against bacteria and argue for further studies into the potential role of defensin in phenoloxidase-based melanization innate immune responses in mosquitoes.

Limited Specificity in the Injury and Infection Priming against Bacteria in Aedes aegypti Mosquitoes

Injury and infection priming has been observed in several insect groups, reported as host immune protection against contact with a pathogen caused by a previous infection with the same. However, the specific response against a pathogen has not been demonstrated in all insect species. Investigating the specific priming response in insects is important because their immune strategies probably reflect particular selective pressures exerted by different pathogens. Here, we determined whether previous infection of Aedes aegypti would enhance survival and/or lead to greater and specific AMP expression after a second exposure to the same or a distinct bacterium. Mosquitoes previously immunized with a low dose of Escherichia coli, but not Staphylococcus aureus, showed increased survival. Although the host protection herein demonstrated was not specific, each bacterium elicited differential AMP expression. These results can be explained by the susceptible-primed-infected (SPI) epidemiological model, which poses that in the evolution of memory-like responses (priming), a pivotal role is played by pathogen virulence, associated host damage, and the host capacity of pathogen recognition.

Different Effects of Metarhizium anisopliae Strains IMI330189 and IBC200614 on Enzymes Activities and Hemocytes of Locusta migratoria L

BACKGROUND

Metarhizium is an important class of entomopathogenic fungi in the biocontrol of insects, but its virulence is affected by insect immunity. To clarify the mechanism in virulence of Metarhizium, we compared the immunological differences in Locusta migratoria L. when exposed to two strains of Metarhizium anisopliae (Ma).

RESULTS

The virulence of Ma IMI330189 was significantly higher than that of Ma IBC200614 to locust, and IMI330189 overcame the hemocytes and began destroying the hemocytes of locust at 72 h after spray, while locust is immune to IBC200614. IMI330189 could overcome the humoral immunity of locust by inhibiting the activities of phenol oxidase (PO), esterases, multi-function oxidases (MFOs) and acetylcholinesterases in locust while increasing the activities of glutathione-S-transferases (GSTs), catalase and aryl-acylamidase (AA). However IBC200614 inhibit the activities of GSTs and AA in locust and increase the activities of MFOs, PO, superoxide dismutase, peroxidase and chitinase in locust. The changes of enzymes activities in period of infection showed that the time period between the 2nd and the 5th day after spray is critical in the pathogenic process.

CONCLUSION

These results found the phenomenon that Ma initiatively broke host hemocytes, revealed the correlation between the virulence of Ma and the changes of enzymes activities in host induced by Ma, and clarified the critical period in the infection of Ma. So, these results should provide guidance for the construction of efficient biocontrol Ma strains.

Bacterial Exposure at the Larval Stage Induced Sexual Immune Dimorphism and Priming in Adult Aedes aegypti Mosquitoes

Gender differences in the immune response of insects are driven by natural selection for females and sexual selection for males. These natural forces entail a multitude of extrinsic and intrinsic factors involved in a genotype-environment interaction that results in sex-biased expression of the genes shared by males and females. However, little is known about how an infection at a particular ontogenetic stage may influence later stages, or how it may impact sexual immune dimorphism. Using Aedes aegypti mosquitoes, the aim of the present study was to analyze the effect of a bacterial exposure at the larval stage on adult immunity in males and females. The parameters measured were phenoloxidase activity, nitric oxide production, antimicrobial activity, and the antimicrobial peptide transcript response. As a measure of the immune response success, the persistence of injected bacteria was also evaluated. The results show that males, as well as females, were able to enhance survival in the adult stage as a result of being exposed at the larval stage, which indicates a priming effect. Moreover, there was a differential gender immune response, evidenced by higher PO activity in males as well as higher NO production and greater antimicrobial activity in females. The greater bacterial persistence in females suggests a gender-specific strategy for protection after a previous experience with an elicitor. Hence, this study provides a primary characterization of the complex and gender-specific immune response of male and female adults against a bacterial challenge in mosquitoes primed at an early ontogenetic stage.

Small mosquitoes, large implications: crowding and starvation affects gene expression and nutrient accumulation in Aedes aegypti

Background

Environmental factors such as temperature, nutrient availability, and larval density determine the outcome of postembryonic development in mosquitoes. Suboptimal temperatures, crowding, and starvation during the larval phase reduce adult mosquito size, nutrient stores and affect vectorial capacity.

Methods

In this study we compared adult female Aedes aegypti, Rockefeller strain, raised under standard laboratory conditions (Large) with those raised under crowded and nutritionally deprived conditions (Small). To compare the gene expression and nutritional state of the major energy storage and metabolic organ, the fat body, we performed transcriptomics using Illumina based RNA-seq and metabolomics using GC/MS on females before and 24 hours following blood feeding.

Results

Analysis of fat body gene expression between the experimental groups revealed a large number of significantly differentially expressed genes. Transcripts related to immunity, reproduction, autophagy, several metabolic pathways; including amino acid degradation and metabolism; and membrane transport were differentially expressed. Metabolite profiling identified 60 metabolites within the fat body to be significantly affected between small and large mosquitoes, with the majority of detected free amino acids at a higher level in small mosquitoes compared to large.

Conclusions

Gene expression and metabolites in the adult fat body reflect the individual post-embryonic developmental history of a mosquito larva. These changes affect nutritional storage and utilization, immunity, and reproduction. Therefore, it is apparent that changes in larval environment due to weather conditions, nutrition availability, vector control efforts, and other factors can affect adult vectorial capacity in the field.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0863-9) contains supplementary material, which is available to authorized users.

Infection barriers and responses in mosquito-filarial worm interactions

As a function of size, migration trajectory through the body and developmental site, filarial worm parasites inflict significant damage on the mosquito host. Some mosquitoes are equipped with physical and physiological barriers that confer a refractory state to parasite infection. In a susceptible host, parasites migrate to a developmental site and achieve an intracellular existence; during this process, worms elicit canonical mosquito immune response elements, particularly melanization and antimicrobial peptide (AMP) production. It is clear now that the response to infection also involves mitigating stress and manipulation of host cell machinery to delay necrosis. This review focuses on mechanisms of refractoriness and resistance to Brugia malayi, Brugia pahangi, and Dirofilaria immitis, with emphasis on infection in the mosquito, Aedes aegypti.

The damage threshold hypothesis and the immune strategies of insects

The insect immune response strategy has generally been considered bipolar: either resistance or death. Lately, a much broader and subtler landscape has emerged: occurrence of tolerance and resistance has been described as a host-regulated immune response. However, little is known about the interplay between the immune response strategy mounted by the insect during infection and the damage produced by the pathogen. Based on the Matzinger model of danger/damage, we propose a quantitative model to explain the occurrence of either resistance or tolerance. We discuss the features to be analyzed and describe the terms of reference by which, with basic models, we distinguish between immune strategies. Pathogen type and mixed infections are also contemplated. We hope this analysis will give new perspective, from an evolutionary ecology standpoint, on immune response measurements in the context of insect infection, and on the importance of (non-self or self) damage.

Patterns of phenoloxidase activity in insecticide resistant and susceptible mosquitoes differ between laboratory-selected and wild-caught individuals

Background

Insecticide resistance has the potential to alter vector immune competence and consequently affect the transmission of diseases.

Methods

Using both laboratory isogenic strains and field-caught Culex pipiens mosquitoes, we investigated the effects of insecticide resistance on an important component of the mosquito immune system: the phenoloxidase (PO) activity. As infection risk varies dramatically with the age and sex of mosquitoes, allocation to PO immunity was quantified across different stages of the mosquito life cycle.

Results

Our results were consistent in showing that larvae have a higher PO activity than adults, females have a higher PO activity than males, and PO activity declines with adult age. We obtained, however, a marked discrepancy between laboratory and field-collected mosquitoes on the effect of insecticide resistance on PO activity. In the laboratory selected strains we found evidence of strong interactions between insecticide resistance and the age and sex of mosquitoes. In particular, 7 and 14 day old esterase-resistant adult females and acetylcholine-esterase resistant males had significantly higher PO activities than their susceptible counterparts. No such effects were, however, apparent in field-caught mosquitoes.

Conclusions

Combined, the field and laboratory-based approaches employed in this study provide a powerful test of the effect of insecticide resistance on PO-mediated immunity. The use of laboratory-selected insecticide-resistant strains is still the most widely used method to investigate the pleiotropic effects of insecticide resistance. Our results suggest that the outcome of these laboratory-selected mosquitoes must be interpreted with caution and, whenever possible, compared with mosquitoes captured from the field.

Complex environmental drivers of immunity and resistance in malaria mosquitoes

Considerable research effort has been directed at understanding the genetic and molecular basis of mosquito innate immune mechanisms. Whether environmental factors interact with these mechanisms to shape overall resistance remains largely unexplored. Here, we examine how changes in mean ambient temperature, diurnal temperature fluctuation and time of day of infection affected the immunity and resistance of Anopheles stephensi to infection with Escherichia coli. We used quantitative PCR to estimate the gene expression of three immune genes in response to challenge with heat-killed E. coli. We also infected mosquitoes with live E. coli and ran bacterial growth assays to quantify host resistance. Both mosquito immune parameters and resistance were directly affected by mean temperature, diurnal temperature fluctuation and time of day of infection. Furthermore, there was a suite of complex two- and three-way interactions yielding idiosyncratic phenotypic variation under different environmental conditions. The results demonstrate mosquito immunity and resistance to be strongly influenced by a complex interplay of environmental variables, challenging the interpretation of the very many mosquito immune studies conducted under standard laboratory conditions.

The impact of insecticide resistance on Culex pipiens immunity

Because of their role as vectors of diseases, the evolution of insecticide resistance in mosquitoes has been intensively investigated. Insecticide resistance is associated to a wide range of pleiotropic effects on several key life-history traits of mosquitoes such as longevity and behavior. However, despite its potential implications in pathogen transmission, the effects of insecticide resistance on mosquito immunity have received little, if any, attention. Here, we investigate the impact of insecticide resistance in Culex pipiens, an epidemiologically important vector of a wide array of pathogens. Using both isogenic laboratory strains and field-caught mosquitoes, we investigate the impact of two main insecticide resistance mechanisms (metabolic detoxification and target site modification) on the relative transcription of several genes involved in the immune response to pathogens, at both their constitutive and inducible levels. Our results show a discrepancy between the isogenic laboratory lines and field-collected mosquitoes: While in the isogenic strains, insecticide-resistant mosquitoes show a drastic increase in immune gene expression, no such effect appears in the field. We speculate on the different mechanisms that may underlie this discrepancy and discuss the risks of making inferences on the pleiotropic effects of insecticide-resistant genes by using laboratory-selected insecticide-resistant lines.

Increased survivorship following bacterial infection by the mosquito Aedes aegypti as compared to Anopheles gambiae correlates with increased transcriptional induction of antimicrobial peptides

Mosquitoes defend themselves from pathogens by mounting cellular and humoral innate immune responses. Bioinformatic analyses have revealed considerable divergence in immune gene repertoires between mosquito species, but interspecies empirical comparisons of immune responses are lacking. Here, we present a comparative analysis of the antimicrobial responses of two distantly related disease vectors: Aedes aegypti and Anopheles gambiae. Survival studies showed that Ae. aegypti are more proficient in surviving a bacterial infection than An. gambiae, and this correlates with Ae. aegypti's superior ability to kill bacteria in their hemocoels. Hemocytes from both species swiftly phagocytose bacteria, but phagocytosis does not explain Ae. aegypti's increased robustness: An. gambiae contain more circulating hemocytes and display a higher phagocytic index, but the phagocytic capacity of individual hemocytes is greater in Ae. aegypti. Then, profiling of 19 immunity genes revealed that transcriptional induction following infection is significantly elevated in Ae. aegypti when compared to An. gambiae, with the largest change seen in the transcription of cecropin and defensin. These data show that Ae. aegypti is better equipped to survive a bacterial infection than An. gambiae, and this correlates with Ae. aegypti's increased transcriptional induction of antimicrobial peptides and other humoral immune factors in response to infection.

Insect immune responses to nematode parasites

Host innate immunity plays a central role in detecting and eliminating microbial pathogenic infections in both vertebrate and invertebrate animals. Entomopathogenic or insect pathogenic nematodes are of particular importance for the control of insect pests and vectors of pathogens, while insect-borne nematodes cause serious diseases in humans. Recent work has begun to use the power of insect models to investigate host-nematode interactions and uncover host antiparasitic immune reactions. This review describes recent findings on innate immune evasion strategies of parasitic nematodes and host cellular and humoral responses to the infection. Such information can be used to model diseases caused by human parasitic nematodes and provide clues indicating directions for research into the interplay between vector insects and their invading tropical parasites.

Regulation of transcription of the Aedes albopictus cecropin A1 gene: A role for p38 mitogen-activated protein kinase

Regulation of the Aedes albopictus cecropin A1 promoter was studied to provide insight into the transcriptional control of this antimicrobial peptide (AMP) gene in mosquitoes. Gene expression levels of cecropin A1 increased in A. albopictus C6/36 cells in response to heat-killed Escherichiacoli. Reporter gene assays incorporating -757 to +32 of the A. albopictus cecropin A1 promoter revealed that E. coli could induce expression in these cells with more pronounced expression than that seen with lipopolysaccharide (LPS). Analysis of deletion constructs demonstrated that the 5' boundary of the regulatory region for the activation of this AMP was located between -173 and -64. Western blotting with anti-phospho-specific antibodies demonstrated that p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun N-terminal kinase (JNK) were activated by LPS, whereas only p38 MAPK was activated by E. coli. Moreover, pharmacological experiments revealed that pre-incubation of cells with the p38 MAPK inhibitor SB203580 resulted in a striking activation of the cecropin A1 promoter following immune challenge, demonstrating that p38 MAPK negatively regulates cecropin A1 promoter activity. Finally the region required for the negative regulation by p38 MAPK was identified as being between -173 and -64. This report is the first to show involvement of the p38 MAPK pathway in the negative regulation of AMP production in a mosquito.

Mosquito immunity

Throughout their lifetime, mosquitoes are exposed to pathogens during feeding, through breaks in their cuticle and following pathogen-driven cuticular degradation. To resist infection, mosquitoes mount innate cellular and humoral immune responses that are elicited within minutes of exposure and can lead to pathogen death via three broadly defined mechanisms: lysis, melanization and hemocyte-mediated phagocytosis. This chapter reviews our current understanding of the mosquito immune system, with an emphasis on the physical barriers that prevent pathogens from entering the body, the organs and tissues that regulate immune responses and the mechanistic and molecular bases of immunity.

RNA-seq analyses of blood-induced changes in gene expression in the mosquito vector species, Aedes aegypti

Background

Hematophagy is a common trait of insect vectors of disease. Extensive genome-wide transcriptional changes occur in mosquitoes after blood meals, and these are related to digestive and reproductive processes, among others. Studies of these changes are expected to reveal molecular targets for novel vector control and pathogen transmission-blocking strategies. The mosquito Aedes aegypti (Diptera, Culicidae), a vector of Dengue viruses, Yellow Fever Virus (YFV) and Chikungunya virus (CV), is the subject of this study to look at genome-wide changes in gene expression following a blood meal.

Results

Transcriptional changes that follow a blood meal in Ae. aegypti females were explored using RNA-seq technology. Over 30% of more than 18,000 investigated transcripts accumulate differentially in mosquitoes at five hours after a blood meal when compared to those fed only on sugar. Forty transcripts accumulate only in blood-fed mosquitoes. The list of regulated transcripts correlates with an enhancement of digestive activity and a suppression of environmental stimuli perception and innate immunity. The alignment of more than 65 million high-quality short reads to the Ae. aegypti reference genome permitted the refinement of the current annotation of transcript boundaries, as well as the discovery of novel transcripts, exons and splicing variants. Cis-regulatory elements (CRE) and cis-regulatory modules (CRM) enriched significantly at the 5'end flanking sequences of blood meal-regulated genes were identified.

Conclusions

This study provides the first global view of the changes in transcript accumulation elicited by a blood meal in Ae. aegypti females. This information permitted the identification of classes of potentially co-regulated genes and a description of biochemical and physiological events that occur immediately after blood feeding. The data presented here serve as a basis for novel vector control and pathogen transmission-blocking strategies including those in which the vectors are modified genetically to express anti-pathogen effector molecules.

Pathogenomics of Culex quinquefasciatus and meta-analysis of infection responses to diverse pathogens

The mosquito Culex quinquefasciatus poses a substantial threat to human and veterinary health as a primary vector of West Nile virus (WNV), the filarial worm Wuchereria bancrofti, and an avian malaria parasite. Comparative phylogenomics revealed an expanded canonical C. quinquefasciatus immune gene repertoire compared with those of Aedes aegypti and Anopheles gambiae. Transcriptomic analysis of C. quinquefasciatus genes responsive to WNV, W. bancrofti, and non-native bacteria facilitated an unprecedented meta-analysis of 25 vector-pathogen interactions involving arboviruses, filarial worms, bacteria, and malaria parasites, revealing common and distinct responses to these pathogen types in three mosquito genera. Our findings provide support for the hypothesis that mosquito-borne pathogens have evolved to evade innate immune responses in three vector mosquito species of major medical importance.

Mosquito transcriptome profiles and filarial worm susceptibility in Armigeres subalbatus

BACKGROUND

Armigeres subalbatus is a natural vector of the filarial worm Brugia pahangi, but it kills Brugia malayi microfilariae by melanotic encapsulation. Because B. malayi and B. pahangi are morphologically and biologically similar, comparing Ar. subalbatus-B. pahangi susceptibility and Ar. subalbatus-B. malayi refractoriness could provide significant insight into recognition mechanisms required to mount an effective anti-filarial worm immune response in the mosquito, as well as provide considerable detail into the molecular components involved in vector competence. Previously, we assessed the transcriptional response of Ar. subalbatus to B. malayi, and now we report transcriptome profiling studies of Ar. subalbatus in relation to filarial worm infection to provide information on the molecular components involved in B. pahangi susceptibility.

METHODOLOGY/PRINCIPAL FINDINGS

Utilizing microarrays, comparisons were made between mosquitoes exposed to B. pahangi, B. malayi, and uninfected bloodmeals. The time course chosen facilitated an examination of key events in the development of the parasite, beginning with the very start of filarial worm infection and spanning to well after parasites had developed to the infective stage in the mosquito. At 1, 3, 6, 12, 24 h post infection and 2-3, 5-6, 8-9, and 13-14 days post challenge there were 31, 75, 113, 76, 54, 5, 3, 13, and 2 detectable transcripts, respectively, with significant differences in transcript abundance (increase or decrease) as a result of parasite development.

CONCLUSIONS/SIGNIFICANCE

Herein, we demonstrate that filarial worm susceptibility in a laboratory strain of the natural vector Ar. subalbatus involves many factors of both known and unknown function that most likely are associated with filarial worm penetration through the midgut, invasion into thoracic muscle cells, and maintenance of homeostasis in the hemolymph environment. The data show that there are distinct and separate transcriptional patterns associated with filarial worm susceptibility as compared to refractoriness, and that an infection response in Ar. subalbatus can differ significantly from that observed in Ae. aegypti, a common laboratory model.

Mosquito infection responses to developing filarial worms

Human lymphatic filariasis is a mosquito-vectored disease caused by the nematode parasites Wuchereria bancrofti, Brugia malayi and Brugia timori. These are relatively large roundworms that can cause considerable damage in compatible mosquito vectors. In order to assess how mosquitoes respond to infection in compatible mosquito-filarial worm associations, microarray analysis was used to evaluate transcriptome changes in Aedes aegypti at various times during B. malayi development. Changes in transcript abundance in response to the different stages of B. malayi infection were diverse. At the early stages of midgut and thoracic muscle cell penetration, a greater number of genes were repressed compared to those that were induced (20 vs. 8). The non-feeding, intracellular first-stage larvae elicited few differences, with 4 transcripts showing an increased and 9 a decreased abundance relative to controls. Several cecropin transcripts increased in abundance after parasites molted to second-stage larvae. However, the greatest number of transcripts changed in abundance after larvae molted to third-stage larvae and migrated to the head and proboscis (120 induced, 38 repressed), including a large number of putative, immunity-related genes (∼13% of genes with predicted functions). To test whether the innate immune system of mosquitoes was capable of modulating permissiveness to the parasite, we activated the Toll and Imd pathway controlled rel family transcription factors Rel1 and Rel2 (by RNA interference knockdown of the pathway's negative regulators Cactus and Caspar) during the early stages of infection with B. malayi. The activation of either of these immune signaling pathways, or knockdown of the Toll pathway, did not affect B. malayi in Ae. aegypti. The possibility of LF parasites evading mosquito immune responses during successful development is discussed.

Filarial worms that cause human lymphatic filariasis (LF) are transmitted by many species of mosquitoes. Within susceptible mosquitoes, Brugia malayi develop from microfilariae (mf) to infective-stage larvae (L3s), in approximately eight days. These nematodes develop as intracellular parasites within mosquito flight muscle cells, in which they ingest cellular material and eventually cause cell death when L3s migrate to the mosquito's proboscis. We examined the effects of B. malayi parasitism on Aedes aegypti by analyzing changes in mosquito gene expression at different stages of parasite development. We found that a few genes were differentially expressed at the RNA level relative to non-infected controls. The majority of changes occurred at two time periods, when the filarial worms began feeding and when the L3s were in the head and proboscis. Many transcriptional changes in the later group concur with documented descriptions of tissue damage, clean-up and repair that occurs in mosquitoes infected with filarial worms. In addition, we activated two innate immunity signaling pathways and observed the effects on filarial worm development. B. malayi seems to be capable of evading these immune responses, because its development was not impeded by the activation of either the Toll or Imd signal pathways in Ae. aegypti.

A haemocyte tropism for an arbovirus

Horizontally transmitted mosquito-borne viruses enter the midgut with a blood meal then disseminate to infect the salivary glands. En route to the salivary glands, these viruses encounter the plasma (haemolymph) and blood cells (haemocytes). Haemocytes respond to a variety of micro-organisms, but their role in virus replication and dissemination has not been described. To look for a potential haemocyte tropism for an arbovirus, a Sindbis virus was injected intrathoracically into four species of mosquito. Virus infects haemocytes as early as 6 h post injection (p.i.) and infection was evident in these cells for as long as 4 days p.i. More than 90 % of haemocytes were infected, most often the phagocytic granulocytes. Virus titres in the haemolymph increased from 24 h p.i. through 60 h p.i. Similar results were found when Aedes aegypti mosquitoes were injected with orally infectious Sindbis. These data prove that an arbovirus infects, and replicates in, haemocytes.

Aedes FADD: a novel death domain-containing protein required for antibacterial immunity in the yellow fever mosquito, Aedes aegypti

Microbial infections in insects activate a series of immune responses that culminate in the production of antimicrobial peptides (AMPs). In Drosophila, two signaling pathways, govern the challenge-dependent expression of AMPs; the Toll and IMD pathways. While AMPs have been the subject of much research in mosquitoes, the regulation of the pathways required for AMP expression remains largely unknown. We report here the identification of Aedes FADD (AeFADD), a death domain protein in Aedes aegypti. AeFadd is expressed in all immune-competent tissues and all developmental stages examined. At the transcriptional level, AeFadd transcripts increased when challenged with Escherichia coli but not Micrococcus luteus. In both cases, we observed the induction of two AMP genes; cecropin and defensin. Loss of AeFadd function by dsRNA interference impaired the inducible expression of both AMPs, and rendered adult mosquitoes susceptible to both types of bacteria. Identifying molecules that regulate mosquito immunity may help elucidate the factors that contribute to the vectorial capacity and provide insights into general mechanisms that regulate innate immunity.

Identification of immune-responsive genes in the mosquito Culex quinquefasciatus infected with the filarial parasite Wuchereria bancrofti

Several antimicrobial/parasitic peptides are known to be upregulated in mosquitoes upon infection with parasites. The aim of this study was to identify immune-responsive genes in the vector mosquito, Culex quinquefasciatus Say (Diptera: Culicidae) against the human lymphatic filarial parasite, Wuchereria bancrofti (Cobbold) (Spirurida: Onchocercidae). Suppression subtractive hybridization was performed using RNA from filarial infected and non-infected mosquitoes to obtain differentially expressed transcripts, and their identities were confirmed through reverse transcriptase polymerase chain reaction (RT-PCR). Out of 23 clones selected from the suppression subtractive library, three corresponded to antimicrobial peptide genes, defensins, and four corresponded to regulatory serpin peptide genes. RT-PCR using defensin-specific primers and sequencing of the product showed a 284-bp defensin cDNA. Sequence alignment with defensins of the mosquitoes Anopheles gambiae s.s. Giles and Aedes aegypti (L.) showed maximum homology with the former. Similarly, that of serpin-specific primers showed a 406-bp cDNA encoding serpins. Sequence alignment showed maximum homology with that of An. gambiae, as in the case of defensins. Hence, this investigation revealed upregulation of defensins and serpins in Cx quinquefasciatus infected with W. bancrofti. Antimicrobial peptide genes such as defensins may have limited or no specific role in regulating parasite development. Serpins may prove to be facilitating molecules, by regulating melanization of the parasite. However, the exact functions of these molecules in the immune system of the vector mosquito are yet to be investigated.

Using RNA interference to determine the role of varisin in the innate immune system of the hard tick Dermacentor variabilis (Acari: Ixodidae)

Defensins are an important component of the innate immune system of ticks. These small peptides are produced by various genera of ticks, and expressed in various tissues. In this study we used RNA interference to silence the expression of the defensin varisin produced by the hemocytes of the American dog tick, Dermacentor variabilis. Ticks were injected with double stranded varisin RNA prior to being placed on a rabbit. After feeding, the ticks were removed, bled, and the hemolymph plasma and hemocytes separated. Hemocytes were screened for the presence (or absence) of both varisin transcript and peptide. Varisin peptide was below detectable levels and the transcript showed a greater than 99% knockdown. The antimicrobial activity of the hemolymph plasma was reduced 2-4 fold compared to that of control injected ticks indicating varisin accounts for a large portion of the antimicrobial activity of the hemolymph.

Expression of defensin, cecropin, and transferrin in Aedes aegypti (Diptera: Culicidae) infected with Wuchereria bancrofti (Spirurida: Onchocercidae), and the abnormal development of nematodes in the mosquito

The temporal expression of defensin, cecropin and transferrin was assessed in Aedes aegypti naturally refractory to Wuchereria bancrofti upon infection with this worm, in parallel to analysis of filarial development in the insect. Compared to controls, transcription of defensin and cecropin was higher in infected mosquitoes as soon as 2h post infection and peaked before 48h. Transferrin transcription was higher in infected mosquitoes at 24h, and at 48h was almost leveled to controls. At 72h and 7 days post infection, levels of all transcripts in infected insects decreased gradually and were similar to controls in most cases. Worm development in A. aegypti was visually abnormal from the beginning of infection. Here, we report, for the first time, the up-regulation of endogenous immune molecules in A. aegypti infected with W. bancrofti and provide a description of the worm development inside the insect. The specificities of A. aegypti-W. bancrofti model compared to other mosquito-filaria systems are discussed.

Transport of sequence-specific RNA interference information between cells

When eukaryotic cells encounter double-stranded RNA, genes of matching sequence are silenced through RNA interference. Surprisingly, in some animals and plants, the same gene is specifically silenced even in cells that did not encounter the double-stranded RNA, due to the transport of a gene-specific silencing signal between cells. This silencing signal likely has an RNA component that gives it sequence-specificity, however its precise identity remains unknown. Studies in the worm Caenorhabditis elegans and in plants have revealed parts of a complex protein machinery that transports this silencing signal. Some of these proteins are conserved in vertebrates, including mammals, raising the possibility that higher animals can communicate gene-specific silencing information between cells. Such communication provides antiviral immunity in plants and perhaps in C. elegans. Identifying the transported silencing signal and deciphering the evolutionarily selected role of the transport machinery are some of the key challenges for the future.

The malaria vector mosquito Anopheles gambiae expresses a suite of larval-specific defensin genes

cDNAs of Anopheles gambiae Defensin 2 (AgDef2), Defensin 3 (AgDef3) and Defensin 4 (AgDef4), identified in the genome sequence, have been characterized and their expression profiles investigated. In contrast to both typical defensins and insect antimicrobial peptides generally, the newly identified defensins were not upregulated with acute-phase kinetics following immune challenge in insects or cell culture. However, mRNA abundance of AgDef2, AgDef3 and AgDef4 increased significantly during the larval stages. Promoter analysis of all three genes failed to identify putative immune response elements previously identified in other mosquito defensin genes. As previous studies failed to identify these larval-specific defensins, it seems likely that further antimicrobial peptide genes with nontypical expression profiles will be identified as more genome sequences become available.

An insight into the sialome of the blood-sucking bug Triatoma infestans, a vector of Chagas' disease

Triatoma infestans is a hemiptera, vector of Chagas' disease that feeds exclusively on vertebrate blood in all life stages. Hematophagous insects' salivary glands (SG) produce potent pharmacological compounds that counteract host hemostasis, including anticlotting, antiplatelet, and vasodilatory molecules. To obtain a further insight into the salivary biochemical and pharmacological complexity of this insect, a cDNA library from its SG was randomly sequenced. Also, salivary proteins were submitted to two-dimensional gel (2D-gel) electrophoresis followed by MS analysis. We present the analysis of a set of 1534 (SG) cDNA sequences, 645 of which coded for proteins of a putative secretory nature. Most salivary proteins described as lipocalins matched peptide sequences obtained from proteomic results.

Mosquito transcriptome changes and filarial worm resistance in Armigeres subalbatus

Background

Armigeres subalbatus is a natural vector of the filarial worm Brugia pahangi, but it rapidly and proficiently kills Brugia malayi microfilariae by melanotic encapsulation. Because B. malayi and B. pahangi are morphologically and biologically similar, the Armigeres-Brugia system serves as a valuable model for studying the resistance mechanisms in mosquito vectors. We have initiated transcriptome profiling studies in Ar. subalbatus to identify molecular components involved in B. malayi refractoriness.

Results

These initial studies assessed the transcriptional response of Ar. subalbatus to B. malayi at 1, 3, 6, 12, 24, 48, and 72 hrs after an infective blood feed. In this investigation, we initiated the first holistic study conducted on the anti-filarial worm immune response in order to effectively explore the functional roles of immune-response genes following a natural exposure to the parasite. Studies assessing the transcriptional response revealed the involvement of unknown and conserved unknowns, cytoskeletal and structural components, and stress and immune responsive factors. The data show that the anti-filarial worm immune response by Ar. subalbatus to be a highly complex, tissue-specific process involving varied effector responses working in concert with blood cell-mediated melanization.

Conclusion

This initial study provides a foundation and direction for future studies, which will more fully dissect the nature of the anti-filarial worm immune response in this mosquito-parasite system. The study also argues for continued studies with RNA generated from both hemocytes and whole bodies to fully expound the nature of the anti-filarial worm immune response.

Profiling infection responses in the haemocytes of the mosquito, Aedes aegypti

Pathogens that infect and/or are transmitted by mosquitoes typically are exposed to the body cavity, and to haemocytes circulating therein, during development or dissemination. Aedes aegypti haemocytes produce a range of immune response-related gene products, and an endpoint response of phagocytosis and/or melanization that is temporally and structurally distinct for the invading pathogen. Expressed sequence tags were generated from haemocyte libraries and then used to design oligonucleotide microarrays. Arrays were screened with haemocyte material collected 1-, 8- and 24-h post-inoculation with Escherichia coli or Micrococcus luteus bacteria. Data from these studies support the discovery of novel immune response-activated genes, provide an expanded understanding of antimicrobial peptide biology and highlight the coordination of immune factors that leads to an endpoint response.

Ribosomal protein L24 is differentially expressed in ovary and testis of the marine shrimp Marsupenaeus japonicus

In order to identify genes involved in oogenesis in shrimp, an ovarian cDNA library of Marsupenaeus japonicus was screened using a suppression-subtraction hybridization (SSH)-enriched probe. More than 20 genes were identified as differentially expressed genes between the ovary and the testis. Unexpectedly, one of these genes is a ribosomal protein that is normally considered a housekeeping gene. Northern blot shows that the shrimp ribosomal protein L24 gene (srpl24) is 0.6 kb in length. The expression level of srpl24 in the ovary is much higher than in the testis. Bioinformatics analyses show that srpl24 encodes a protein of 164 aa with a predicted molecular mass of 18.2 kDa, which is a cytoplasmic ribosomal protein. Real time PCR analyses demonstrated that the relative abundance of srpl24 mRNA in the different organs is: ovary >> testis, hepatopancreas, muscle and eye. The highest expression level of srpl24 in the ovary suggests that srpl24 has an important role in oogenesis. It is the first reported rpl24 in crustaceans and is the first reported rpl24 that is differentially expressed between the ovary and the testis in animals.

Beta 1, 3-glucan recognition protein from the mosquito, Armigeres subalbatus, is involved in the recognition of distinct types of bacteria in innate immune responses

The activation of an immune response to invading microorganisms generally requires recognition by pattern recognition receptors. Beta 1, 3-glucan recognition proteins (GRPs) have specific affinity for beta 1, 3-glucan, a component on the surface of fungi and bacteria. In this study, we show that GRP from Armigeres subalbatus mosquitoes (AsGRP) is able to bind different bacterial species, and that this binding varies from species to species and is independent of Gram type. AsGRP knockdown with double-stranded RNA increases the mortality of mosquitoes to those bacteria that strongly bind AsGRP, but not to bacteria that do not detectably bind AsGRP. This increase in susceptibility is partially evidenced by decreased melanization in Salmonella typhimurium. Furthermore, AsGRP expression is differentially affected by the presence of different species of bacteria. These results demonstrate that AsGRP is selective in its affinity to different bacteria and; therefore, plays a role in the antibacterial immune response of mosquitoes.

Expression of immune responsive genes in cell lines from two different Anopheline species

Malaria infection results in increased expression of immune responsive genes, including those encoding antimicrobial peptides such as Gambicin (Gam1) and Cecropin A (Cec1). Understanding how these genes are regulated will provide insights how the mosquito immune system is activated by Plasmodium. We previously have shown that Cec1 was primarily regulated by the Imd-Relish (REL2) pathway in the Anopheles gambiae Sua1B cell line. We show here that expression of Defensin A (Def1) and Gam1 was reduced after RNA interference against components of the Imd-REL2 pathway in An. gambiae cell lines. Interestingly, promoter reporters of these antimicrobial peptides were expressed at very low level in the cell line MSQ43 from Anopheles stephensi. Surprisingly, over-expression of either NF-kappaB transcription factor REL1 or REL2 alone is sufficient to induce the expression of Cec1, Gam1 and Def1. These results suggest that expression of these antimicrobial peptides (AMP) in vivo may be regulated by both the Toll and Imd pathways. We also show here for the first time that Tep4, a gene encoding a thioester containing protein, is regulated by REL2. Taken together, these results suggest that there are significant overlaps of genes regulated by the Toll-Rel1 and Imd-Rel2 pathways. Further, the different expression patterns in two different Anopheline cell lines provide a platform to identify other key positive and negative regulators of the antimicrobial peptide genes.

Densoviruses for control and genetic manipulation of mosquitoes

Mosquito densoviruses (MDV) are parvoviruses that replicate in the nuclei of mosquito cells and cause the characteristic nuclear hypertrophy (densonucleosis) that gives them their name. Several MDV that differ in pathogenicity both in vitro and in vivo have been isolated. MDV have a number of features that make them potentially attractive as biological control agents for mosquito-borne disease. They are nonenveloped and relatively stable in the environment. They are highly specific for mosquitoes and they infect and kill larvae in a dose dependent manner in the aqueous larval habitat. Infected larvae that survive to become adult mosquitoes exhibit a dose-dependent shortening of lifespan and many do not survive longer than the extrinsic incubation period for arboviruses. Thus they may have a significant impact on transmission of pathogens. Infected females can transmit the virus vertically by laying infected eggs in new oviposition sites. Studies on how MDV affect populations are relatively limited. Population cage studies suggest that they will persist and spread in populations and limited field studies have shown similar preimaginal mortality in wild populations to that seen in laboratory studies. The availability of infectious clones of MDV genomes allows the development of densovirus vectors for expressing genes of interest in mosquito cells and mosquitoes. Recently short hairpin RNA expression cassettes that induce RNA interference have been inserted into densovirus genomes. These expression cassettes should be useful for both research and disease-control applications.

Antimicrobial peptides in the interactions between insects and flagellate parasites

Innate immunity has a key role in the control of microbial infections in both vertebrates and invertebrates. In insects, including vectors that transmit parasites that cause major human and animal diseases, antimicrobial peptides (AMPs) are important components of innate immunity. AMPs are induced upon parasitic infections and can participate in regulating parasite development in the digestive tract and in the hemolymph. This review presents our current knowledge of a field that is in its infancy: the role of innate immunity in different models of insects infected with flagellate parasites, and in particular the potential role of AMPs in regulating these parasitic infections.

Regulatory mechanisms of chitin biosynthesis and roles of chitin in peritrophic matrix formation in the midgut of adult Aedes aegypti

In mosquitoes, the peritrophic matrix is formed in response to blood feeding and can be a physical barrier when pathogens ingested with blood meal attempt to reach and transverse the midgut epithelium. The main components of the peritrophic matrix are chitin-biding-domain containing proteins, glycosylated proteins, and chitin fibrils. Chitin is synthesized from fructose-6-phosphate by a series of five enzymatic reactions. We previously found that blood feeding induces transcriptional up-regulation of glutamine: fructose-6-phosphate amidotransferase-1 (AeGfat-1) and chitin synthase (AeCs), the first and last enzymes of the biosynthetic pathway, respectively, in the midgut of Aedes aegypti. In this study, we demonstrated that formation of the peritrophic matrix is disrupted when the transcript abundance of either gene is knocked-down using RNAi methodologies. We also have shown that enzymatic activity of recombinant AeGFAT-1 is sensitive to feedback inhibition by UDP-N-acetylglucosamine, a substrate of chitin synthase. These findings demonstrate that in the midgut of adult Ae. aegypti, (1) chitin is synthesized de novo in response to blood feeding and is an essential component of the peritrophic matrix, and (2) chitin biosynthesis is negatively regulated, in part, by inhibitory sensitivity of AeGFAT-1 to UDP-N-acetylglucosamine.