Late-phase immune responses limiting oocyst survival are independent of TEP1 function yet display strain specific differences in Anopheles gambiae

Exploring new dimensions of immune cell biology in Anopheles gambiae through genetic immunophenotyping

Mosquito immune cells, or hemocytes, are integral components of the innate immune responses that define vector competence. However, the lack of genetic resources has limited their characterization and our understanding of their functional roles in immune signaling. To overcome these challenges, we engineered transgenic Anopheles gambiae that express fluorescent proteins under the control of candidate hemocyte promoters. Following the characterization of five transgenic constructs through gene expression and microscopy-based approaches, we examine mosquito immune cell populations by leveraging advanced spectral imaging flow cytometry. Our results comprehensively map the composition of mosquito hemocytes, classifying them into twelve distinct populations based on size, granularity, ploidy, phagocytic capacity, and the expression of PPO6, SPARC, and LRIM15 genetic markers. Together, our novel use of morphological properties and genetic markers provides increased resolution into our understanding of mosquito hemocytes, highlighting the complexity and plasticity of these immune cell populations, while providing the foundation for deeper investigations into their roles in immunity and pathogen transmission.

Human Defensin 5 Inhibits Plasmodium yoelii Development in Anopheles stephensi by Promoting Innate Immune Response

Malaria poses a serious threat to human health. Existing vector-based interventions have shortcomings, such as environmental pollution, strong resistance to chemical insecticides, and the slow effects of biological insecticides. Therefore, the need to develop novel strategies for controlling malaria, such as reducing mosquito vector competence, is escalating. Human defensin 5 (HD5) has broad-spectrum antimicrobial activity. To determine its effect on Plasmodium development in mosquitoes, HD5 was injected into Anopheles stephensi at various time points. The infection density of Plasmodium yoelii in An. stephensi was substantially reduced by HD5 treatment administered 24 h prior to infection or 6, 12, or 24 h post-infection (hpi). We found that HD5 treatment upregulated the expression of the innate immune effectors TEP1, MyD88, and Rel1 at 24 and 72 hpi. Furthermore, the RNA interference of MyD88, a key upstream molecule in the Toll signaling pathway, decreased the HD5-induced resistance of mosquitoes against Plasmodium infection. These results suggest that HD5 microinjection inhibits the development of malaria parasites in An. stephensi by activating the Toll signaling pathway.

Activation of complement-like antiparasitic responses in Anopheles mosquitoes

During their development in mosquitoes, malaria parasites undergo massive losses that are in part due to a potent antiparasitic response mounted by the vector. The most efficient and best-characterized response relies on a complement-like system particularly effective against parasites as they cross the mosquito midgut epithelium. While our vision of the molecular and cellular events that lead to parasite elimination is still partial, our understanding of the steps triggering complement activation at the surface of invading parasites has considerably progressed, not only through the identification of novel contributing genes, but also with the recent in-depth characterization of the different mosquito blood cell types, and the ability to track them in live mosquitoes. Here, we propose a simple model based on the time of invasion to explain how parasites may escape complement-like responses during midgut infection.

Distribution of Anopheles gambiae thioester-containing protein 1 alleles along malaria transmission gradients in The Gambia

BACKGROUND

Thioester-containing protein 1 (TEP1) is a highly polymorphic gene playing an important role in mosquito immunity to parasite development and associated with Anopheles gambiae vectorial competence. Allelic variations in TEP1 could render mosquito either susceptible or resistant to parasite infection. Despite reports of TEP1 genetic variations in An. gambiae, the correlation between TEP1 allelic variants and transmission patterns in malaria endemic settings remains unclear.

METHODS

TEP1 allelic variants were characterized by PCR from archived genomic DNA of > 1000 An. gambiae mosquitoes collected at 3 time points between 2009 and 2019 from eastern Gambia, where malaria transmission remains moderately high, and western regions with low transmission.

RESULTS

Eight common TEP1 allelic variants were identified at varying frequencies in An. gambiae from both transmission settings. These comprised the wild type TEP1, homozygous susceptible genotype, TEP1s; homozygous resistance genotypes: TEP1rA and TEP1rB, and the heterozygous resistance genotypes: TEP1srA, TEP1srB, TEP1rArB and TEP1srArB. There was no significant disproportionate distribution of the TEP1 alleles by transmission setting and the temporal distribution of alleles was also consistent across the transmission settings. TEP1s was the most common in all vector species in both settings (allele frequencies: East = 21.4-68.4%. West = 23.5-67.2%). In Anopheles arabiensis, the frequency of wild type TEP1 and susceptible TEP1s was significantly higher in low transmission setting than in high transmission setting (TEP1: Z = - 4.831, P < 0.0001; TEP1s: Z = - 2.073, P = 0.038).

CONCLUSIONS

The distribution of TEP1 allele variants does not distinctly correlate with malaria endemicity pattern in The Gambia. Further studies are needed to understand the link between genetic variations in vector population and transmission pattern in the study settings. Future studies on the implication for targeting TEP1 gene for vector control strategy such as gene drive systems in this settings is also recommended.

The PTEX Pore Component EXP2 Is Important for Intrahepatic Development during the Plasmodium Liver Stage

During vertebrate infection, obligate intracellular malaria parasites develop within a parasitophorous vacuole, which constitutes the interface between the parasite and its hepatocyte or erythrocyte host cells. To traverse this barrier, Plasmodium spp. utilize a dual-function pore formed by EXP2 for nutrient transport and, in the context of the PTEX translocon, effector protein export across the vacuole membrane. While critical to blood-stage survival, less is known about EXP2/PTEX function in the liver stage, although major differences in the export mechanism are suggested by absence of the PTEX unfoldase HSP101 in the intrahepatic vacuole. Here, we employed the glucosamine-activated glmS ribozyme to study the role of EXP2 during Plasmodium berghei liver-stage development in hepatoma cells. Insertion of the glmS sequence into the exp2 3' untranslated region (UTR) enabled glucosamine-dependent depletion of EXP2 after hepatocyte invasion, allowing separation of EXP2 function during intrahepatic development from a recently reported role in hepatocyte invasion. Postinvasion EXP2 knockdown reduced parasite size and largely abolished expression of the mid- to late-liver-stage marker LISP2. As an orthogonal approach to monitor development, EXP2-glmS parasites and controls were engineered to express nanoluciferase. Activation of glmS after invasion substantially decreased luminescence in hepatoma monolayers and in culture supernatants at later time points corresponding to merosome detachment, which marks the culmination of liver-stage development. Collectively, our findings extend the utility of the glmS ribozyme to study protein function in the liver stage and reveal that EXP2 is important for intrahepatic parasite development, indicating that PTEX components also function at the hepatocyte-parasite interface. IMPORTANCE After the mosquito bite that initiates a Plasmodium infection, parasites first travel to the liver and develop in hepatocytes. This liver stage is asymptomatic but necessary for the parasite to transition to the merozoite form, which infects red blood cells and causes malaria. To take over their host cells, avoid immune defenses, and fuel their growth, these obligately intracellular parasites must import nutrients and export effector proteins across a vacuole membrane in which they reside. In the blood stage, these processes depend on a translocon called PTEX, but it is unclear if PTEX also functions during the liver stage. Here, we adapted the glmS ribozyme to control expression of EXP2, the membrane pore component of PTEX, during the liver stage of the rodent malaria parasite Plasmodium berghei. Our results show that EXP2 is important for intracellular development in the hepatocyte, revealing that PTEX components are also functionally important during liver-stage infection.

Malaria oocysts require circumsporozoite protein to evade mosquito immunity

Malaria parasites are less vulnerable to mosquito immune responses once ookinetes transform into oocysts, facilitating parasite development in the mosquito. However, the underlying mechanisms of oocyst resistance to mosquito defenses remain unclear. Here, we show that circumsporozoite protein (CSP) is required for rodent malaria oocysts to avoid mosquito defenses. Mosquito infection with CSPmut parasites (mutation in the CSP pexel I/II domains) induces nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 5 (NOX5)-mediated hemocyte nitration, thus activating Toll pathway and melanization of mature oocysts, upregulating hemocyte TEP1 expression, and causing defects in the release of sporozoites from oocysts. The pre-infection of mosquitoes with the CSPmut parasites reduces the burden of infection when re-challenged with CSPwt parasites by inducing hemocyte nitration. Thus, we demonstrate why oocysts are invisible to mosquito immunity and reveal an unknown role of CSP in the immune evasion of oocysts, indicating it as a potential target to block malaria transmission.

Anopheles gambiae Actively Metabolizes Uric Acid Following Plasmodium Infection to Limit Malaria Parasite Survival

Characterizing the physiological changes that accompany malaria parasite infection of the mosquito host is crucial to our understanding of vectorial capacity in Anopheles mosquitoes, yet has not fully been explored. In this study, we examine the role of uric acid metabolism in the mosquito, Anopheles gambiae, following malaria parasite infection. We demonstrate that levels of uric acid are significantly decreased in the excreta and the mosquito at 24 and 48 h post-Plasmodium infection when compared to controls fed on naïve mouse blood. When we examine the expression of well-known enzymes responsible for uric acid metabolism, we see a significant increase in both urate oxidase (UO) and allatoicase (ALLC) expression following Plasmodium infection. Targeting the essential first step in uric acid metabolism by silencing UO resulted in elevated levels of uric acid, enhancing malaria parasite survival. With implications from other insect systems that bacteria can modulate UO expression, we examined the possibility that the mosquito microbiota and its expansion following blood-feeding may contribute to increased UO levels. However, there was no difference in uric acid metabolism between septic and aseptic mosquitoes, indicating that the mosquito microbiome is not associated with the manipulation of UO expression. Together, our study provides new evidence that Plasmodium infection causes the mosquito host to actively metabolize uric acid by increasing UO expression to limit Plasmodium oocyst survival, suggesting that nitrogen metabolism is an essential pathway in defining mosquito vector competence.

Prostaglandin E2 Signaling Mediates Oenocytoid Immune Cell Function and Lysis, Limiting Bacteria and Plasmodium Oocyst Survival in Anopheles gambiae

Lipid-derived signaling molecules known as eicosanoids have integral roles in mediating immune and inflammatory processes across metazoans. This includes the function of prostaglandins and their cognate G protein-coupled receptors (GPCRs) to employ their immunological actions. In insects, prostaglandins have been implicated in the regulation of both cellular and humoral immune responses, yet in arthropods of medical importance, studies have been limited. Here, we describe a prostaglandin E2 receptor (AgPGE2R) in the mosquito Anopheles gambiae and demonstrate that its expression is most abundant in oenocytoid immune cell populations. Through the administration of prostaglandin E2 (PGE2) and AgPGE2R-silencing, we demonstrate that prostaglandin E2 signaling regulates a subset of prophenoloxidases (PPOs) and antimicrobial peptides (AMPs) that are strongly expressed in populations of oenocytoids. We demonstrate that PGE2 signaling via the AgPGE2R significantly limits both bacterial replication and Plasmodium oocyst survival. Additional experiments establish that PGE2 treatment increases phenoloxidase (PO) activity through the increased expression of PPO1 and PPO3, genes essential to anti-Plasmodium immune responses that promote oocyst killing. We also provide evidence that the mechanisms of PGE2 signaling are concentration-dependent, where high concentrations of PGE2 promote oenocytoid lysis, negating the protective effects of lower concentrations of PGE2 on anti-Plasmodium immunity. Taken together, our results provide new insights into the role of PGE2 signaling on immune cell function and its contributions to mosquito innate immunity that promote pathogen killing.

Single-cell analysis of mosquito hemocytes identifies signatures of immune cell subtypes and cell differentiation

Mosquito immune cells, known as hemocytes, are integral to cellular and humoral responses that limit pathogen survival and mediate immune priming. However, without reliable cell markers and genetic tools, studies of mosquito immune cells have been limited to morphological observations, leaving several aspects of their biology uncharacterized. Here, we use single-cell RNA sequencing (scRNA-seq) to characterize mosquito immune cells, demonstrating an increased complexity to previously defined prohemocyte, oenocytoid, and granulocyte subtypes. Through functional assays relying on phagocytosis, phagocyte depletion, and RNA-FISH experiments, we define markers to accurately distinguish immune cell subtypes and provide evidence for immune cell maturation and differentiation. In addition, gene-silencing experiments demonstrate the importance of lozenge in defining the mosquito oenocytoid cell fate. Together, our scRNA-seq analysis provides an important foundation for future studies of mosquito immune cell biology and a valuable resource for comparative invertebrate immunology.

Additional Feeding Reveals Differences in Immune Recognition and Growth of Plasmodium Parasites in the Mosquito Host

Mosquitoes may feed multiple times during their life span in addition to those times needed to acquire and transmit malaria. To determine the impact of subsequent blood feeding on parasite development in Anopheles gambiae, we examined Plasmodium parasite infection with or without an additional noninfected blood meal. We found that an additional blood meal significantly reduced Plasmodium berghei immature oocyst numbers, yet had no effect on the human parasite Plasmodium falciparum. These observations were reproduced when mosquitoes were fed an artificial protein meal, suggesting that parasite losses are independent of blood ingestion. We found that feeding with either a blood or protein meal compromises midgut basal lamina integrity as a result of the physical distention of the midgut, enabling the recognition and lysis of immature P. berghei oocysts by mosquito complement. Moreover, we demonstrate that additional feeding promotes P. falciparum oocyst growth, suggesting that human malaria parasites exploit host resources provided with blood feeding to accelerate their growth. This is in contrast to experiments with P. berghei, where the size of surviving oocysts is independent of an additional blood meal. Together, these data demonstrate distinct differences in Plasmodium species in evading immune detection and utilizing host resources at the oocyst stage, representing an additional, yet unexplored component of vectorial capacity that has important implications for the transmission of malaria.

IMPORTANCE Mosquitoes must blood feed multiple times to acquire and transmit malaria. However, the impact of an additional mosquito blood meal following malaria parasite infection has not been closely examined. Here, we demonstrate that additional feeding affects mosquito vector competence; namely, additional feeding significantly limits Plasmodium berghei infection, yet has no effect on infection of the human parasite P. falciparum. Our experiments support that these killing responses are mediated by the physical distension of the midgut and by temporary damage to the midgut basal lamina that exposes immature P. berghei oocysts to mosquito complement, while human malaria parasites are able to evade these killing mechanisms. In addition, we provide evidence that additional feeding promotes P. falciparum oocyst growth. This is in contrast to P. berghei, where oocyst size is independent of an additional blood meal. This suggests that human malaria parasites are able to exploit host resources provided by an additional feeding to accelerate their growth. In summary, our data highlight distinct differences in malaria parasite species in evading immune recognition and adapting to mosquito blood feeding. These observations have important, yet previously unexplored, implications for the impact of multiple blood meals on the transmission of malaria.

CRISPRing protozoan parasites to better understand the biology of diseases

Precise gene editing techniques are paramount to gain deeper insights into the biological processes such as host-parasite interactions, drug resistance mechanisms, and gene-function relationships. Discovery of CRISPR-Cas9 system has spearheaded mechanistic understanding of protozoan parasite biology as evident from the number of reports in the last decade. Here, we have described the use of CRISPR-Cas9 in understanding the biology of medically important protozoan parasites such as Plasmodium, Leishmania, Trypanosoma, Babesia and Trichomonas. In spite of intrinsic difficulties in genome editing in these protozoan parasites, CRISPR-Cas9 has acted as a catalyst for faster generation of desired transgenic parasites. Modifications in the CRISPR-Cas9 system for improving the efficiency have been useful in better understanding the molecular mechanisms associated with repair of double strand breaks in the parasites. Moreover, improvement in reagents used for CRISPR mediated gene editing have been instrumental in addressing the issue of non-specificity and toxicity for therapeutic use. These application-based modifications may help in further increasing the efficiency of gene editing in protozoan parasites.

The evolution and history of gene editing technologies

Scientific enquiry must be the driving force of research. This sentiment is manifested as the profound impact gene editing technologies are having in our current world. There exist three main gene editing technologies today: Zinc Finger Nucleases, TALENs and the CRISPR-Cas system. When these systems were being uncovered, none of the scientists set out to design tools to engineer genomes. They were simply trying to understand the mechanisms existing in nature. If it was not for this simple sense of wonder, we probably would not have these breakthrough technologies. In this chapter, we will discuss the history, applications and ethical issues surrounding these technologies, focusing on the now predominant CRISPR-Cas technology. Gene editing technologies, as we know them now, are poised to have an overwhelming impact on our world. However, it is impossible to predict the route they will take in the future or to comprehend the full impact of its repercussions.

Use of Clodronate Liposomes to Deplete Phagocytic Immune Cells in Drosophila melanogaster and Aedes aegypti

The innate immune system is the primary defense response to limit invading pathogens for all invertebrate species. In insects, immune cells are central to both cellular and humoral immune responses, however few genetic resources exist beyond Drosophila to study immune cell function. Therefore, the development of innovative tools that can be widely applied to a variety of insect systems is of importance to advance the study of insect immunity. Here, we have adapted the use of clodronate liposomes (CLD) to deplete phagocytic immune cells in the vinegar fly, Drosophila melanogaster, and the yellow fever mosquito, Aedes aegypti. Through microscopy and molecular techniques, we validate the depletion of phagocytic cell populations in both insect species and demonstrate the integral role of phagocytes in combating bacterial pathogens. Together, these data demonstrate the wide utility of CLD in insect systems to advance the study of phagocyte function in insect innate immunity.

The 20-hydroxyecdysone agonist, halofenozide, promotes anti-Plasmodium immunity in Anopheles gambiae via the ecdysone receptor

Mosquito physiology and immunity are integral determinants of malaria vector competence. This includes the principal role of hormonal signaling in Anopheles gambiae initiated shortly after blood-feeding, which stimulates immune induction and promotes vitellogenesis through the function of 20-hydroxyecdysone (20E). Previous studies demonstrated that manipulating 20E signaling through the direct injection of 20E or the application of a 20E agonist can significantly impact Plasmodium infection outcomes, reducing oocyst numbers and the potential for malaria transmission. In support of these findings, we demonstrate that a 20E agonist, halofenozide, is able to induce anti-Plasmodium immune responses that limit Plasmodium ookinetes. We demonstrate that halofenozide requires the function of ultraspiracle (USP), a component of the canonical heterodimeric ecdysone receptor, to induce malaria parasite killing responses. Additional experiments suggest that the effects of halofenozide treatment are temporal, such that its application only limits malaria parasites when applied prior to infection. Unlike 20E, halofenozide does not influence cellular immune function or AMP production. Together, our results further demonstrate the potential of targeting 20E signaling pathways to reduce malaria parasite infection in the mosquito vector and provide new insight into the mechanisms of halofenozide-mediated immune activation that differ from 20E.

The CLIP-domain serine protease CLIPC9 regulates melanization downstream of SPCLIP1, CLIPA8, and CLIPA28 in the malaria vector Anopheles gambiae

The arthropod melanization immune response is activated by extracellular protease cascades predominantly comprised of CLIP-domain serine proteases (CLIP-SPs) and serine protease homologs (CLIP-SPHs). In the malaria vector, Anopheles gambiae, the CLIP-SPHs SPCLIP1, CLIPA8, and CLIPA28 form the core of a hierarchical cascade downstream of mosquito complement that is required for microbial melanization. However, our understanding of the regulatory relationship of the CLIP-SPH cascade with the catalytic CLIP-SPs driving melanization is incomplete. Here, we report on the development of a novel screen to identify melanization pathway components based on the quantitation of melanotic mosquito excreta, eliminating the need for microdissections or hemolymph enzymatic assays. Using this screen, we identified CLIPC9 and subsequent functional analyses established that this protease is essential for the melanization of both Escherichia coli and the rodent malaria parasite Plasmodium berghei. Mechanistically, septic infection with E. coli promotes CLIPC9 cleavage and both full-length and cleaved CLIPC9 localize to this bacterium in a CLIPA8-dependent manner. The steady state level of CLIPC9 in the hemolymph is regulated by thioester-containing protein 1 (TEP1), suggesting it functions downstream of mosquito complement. In support, CLIPC9 cleavage is inhibited following SPCLIP1, CLIPA8, and CLIPA28 knockdown positioning it downstream of the CLIP-SPH cascade. Moreover, like CLIPA8 and CLIPA28, CLIPC9 processing is negatively regulated by serine protease inhibitor 2 (SRPN2). This report demonstrates how our novel excretion-based approach can be utilized to dissect the complex protease networks regulating mosquito melanization. Collectively, our findings establish that CLIPC9 is required for microbial melanization in An. gambiae and shed light on how the CLIP-SPH cascade regulates this potent immune response.

20-Hydroxyecdysone Primes Innate Immune Responses That Limit Bacterial and Malarial Parasite Survival in Anopheles gambiae

Blood feeding is an integral behavior of mosquitoes to acquire nutritional resources needed for reproduction. This requirement also enables mosquitoes to serve as efficient vectors to acquire and potentially transmit a multitude of mosquito-borne diseases, most notably malaria. Recent studies suggest that mosquito immunity is stimulated following a blood meal, independent of infection status. Since blood feeding promotes production of the hormone 20-hydroxyecdysone (20E), we hypothesized that 20E plays an important role in priming the immune response for pathogen challenge. Here, we examine the immunological effects of priming Anopheles gambiae with 20E prior to pathogen infection, demonstrating a significant reduction in bacteria and Plasmodium berghei survival in the mosquito host. Transcriptome sequencing (RNA-seq) analysis following 20E treatment identifies several known 20E-regulated genes, as well as several immune genes with previously reported function in antipathogen defense. Together, these data demonstrate that 20E influences cellular immune function and antipathogen immunity following mosquito blood feeding, arguing the importance of hormones in the regulation of mosquito innate immune function.IMPORTANCE Blood feeding is required to provide nutrients for mosquito egg production and serves as a mechanism to acquire and transmit pathogens. Shortly after a blood meal is taken, there is a peak in the production of 20-hydroxyecdysone (20E), a mosquito hormone that initiates physiological changes, including yolk protein production and mating refractoriness. Here, we examine additional roles of 20E in the regulation of mosquito immunity, demonstrating that priming the immune system with 20E increases mosquito resistance to pathogens. We identify differentially expressed genes in response to 20E treatment, including several involved in innate immune function as well as lipid metabolism and transport. Together, these data argue that 20E stimulates mosquito cellular immune function and innate immunity shortly after blood feeding.

Characterization of the first insect prostaglandin (PGE2) receptor: MansePGE2R is expressed in oenocytoids and lipoteichoic acid (LTA) increases transcript expression

In arthropods, eicosanoids derived from the oxygenated metabolism of arachidonic acid are significant in mediating immune responses. However, the lack of information about insect eicosanoid receptors is an obstacle to completely decipher immune mechanisms underlying both eicosanoid downstream signal cascades and their relationship to immune pathogen-associated molecular patterns (PAMPs). Here, we cloned and sequenced a G protein-coupled receptor (MW 46.16 kDa) from the model lepidopteran, Manduca sexta (Sphingidae). The receptor shares similarity of amino acid motifs to human prostaglandin E₂ (PGE₂) receptors, and phylogenetic analysis supports its classification as a prostaglandin receptor. In agreement, the recombinant receptor was activated by PGE₂ resulting in intracellular cAMP increase, and therefore designated MansePGE₂R. Expression of MansePGE₂R in Sf9 cells in which the endogenous orthologous receptor had been silenced showed similar cAMP increase upon PGE₂ challenge. Receptor transcript expression was identified in various tissues in larvae and female adults, including Malpighian tubules, fat body, gut and hemocytes, and in female ovaries. In addition to the cDNA cloned that encodes the functional receptor, an mRNA was found featuring the poly-A tail but lacking the predicted transmembrane (TM) regions 2 and 3, suggesting the possibility that internally deleted receptor proteins exist in insects. Immunocytochemistry and in situ hybridization revealed that among hemocytes, the receptor was exclusively localized in the oenocytoids. Larval immune challenges injecting bacterial components showed that lipoteichoic acid (LTA) increased MansePGE₂R expression in hemocytes. In contrast, injection of LPS or peptidoglycan did not increase MansePGE₂R transcript levels in hemocytes, suggesting the LTA-associated increase in receptor transcript is regulated through a distinct pathway. This study provides the first characterization of an eicosanoid receptor in insects, and paves the way for establishing the hierarchy in signaling steps required for establishing insect immune responses to infections.

Developmental and comparative perspectives on mosquito immunity

Diseases spread by mosquitoes have killed more people than those spread by any other group of arthropod vectors and remain an important factor in determining global health and economic stability. The mosquito innate immune system can act to either modulate infection with human pathogens or fight off entomopathogens and increase the fitness and longevity of infected mosquitoes. While work remains towards understanding the larval immune system and the development of the mosquito immune system, it has recently become clearer that environmental factors heavily shape the developing mosquito immune system and continue to influence the adult immune system as well. The adult immune system has been well-studied and is known to involve multiple tissues and diverse molecular mechanisms. This review summarizes and synthesizes what is currently understood about the development of the mosquito immune system and includes comparisons of immune components unique to mosquitoes among the blood-feeding arthropods as well as important distinguishing factors between the anopheline and culicine mosquitoes. An explanation is included for how mosquito immunity factors into vector competence and vectorial capacity is presented along with a model for the interrelationships between nutrition, microbiome, pathogen interactions and behavior as they relate to mosquito development, immune status, adult female fitness and ultimately, vectorial capacity. Novel discoveries in the fields of mosquito ecoimmunology, neuroimmunology, and intracellular antiviral responses are highlighted.

Inhibitors of Eicosanoid Biosynthesis Reveal that Multiple Lipid Signaling Pathways Influence Malaria Parasite Survival in Anopheles gambiae

Eicosanoids are bioactive signaling lipids derived from the oxidation of fatty acids that act as important regulators of immune homeostasis and inflammation. As a result, effective anti-inflammatory drugs have been widely used to reduce pain and inflammation which target key eicosanoid biosynthesis enzymes. Conserved from vertebrates to insects, the use of these eicosanoid pathway inhibitors offer opportunities to evaluate the roles of eicosanoids in less-characterized insect systems. In this study, we examine the potential roles of eicosanoids on malaria parasite survival in the mosquito Anopheles gambiae. Using Plasmodium oocyst numbers to evaluate parasite infection, general or specific inhibitors of eicosanoid biosynthesis pathways were evaluated. Following the administration of dexamethasone and indomethacin, respective inhibitors of phospholipid A2 (PLA2) and cyclooxygenase (COX), oocyst numbers were unaffected. However, inhibition of lipoxygenase (LOX) activity through the use of esculetin significantly increased oocyst survival. In contrast, 12-[[(tricyclo[3.3.1.13,7]dec-1-ylamino)carbonyl]amino]-dodecanoic acid (AUDA), an inhibitor of epoxide hydroxylase (EH), decreased oocyst numbers. These experiments were further validated through RNAi experiments to silence candidate genes homologous to EH in An. gambiae to confirm their contributions to Plasmodium development. Similar to the results of AUDA treatment, the silencing of EH significantly reduced oocyst numbers. These results imply that specific eicosanoids in An. gambiae can have either agonist or antagonistic roles on malaria parasite survival in the mosquito host.

Chemical depletion of phagocytic immune cells in Anopheles gambiae reveals dual roles of mosquito hemocytes in anti-Plasmodium immunity

Mosquito immunity is composed of both cellular and humoral factors that provide protection from invading pathogens. Immune cells known as hemocytes, have been intricately associated with phagocytosis and innate immune signaling. However, the lack of genetic tools has limited hemocyte study despite their importance in mosquito anti-Plasmodium immunity. To address these limitations, we employ the use of a chemical-based treatment to deplete phagocytic immune cells in Anopheles gambiae, demonstrating the role of phagocytes in complement recognition and prophenoloxidase production that limit the ookinete and oocyst stages of malaria parasite development, respectively. Through these experiments, we also define specific subtypes of phagocytic immune cells in An. gambiae, providing insights beyond the morphological characteristics that traditionally define mosquito hemocyte populations. Together, this study represents a significant advancement in our understanding of the roles of mosquito phagocytes in mosquito vector competence and demonstrates the utility of clodronate liposomes as an important tool in the study of invertebrate immunity.

Transcriptional Profile of Aedes aegypti Leucine-Rich Repeat Proteins in Response to Zika and Chikungunya Viruses

Aedes aegypti (L.) is the primary vector of chikungunya, dengue, yellow fever, and Zika viruses. The leucine-rich repeats (LRR)-containing domain is evolutionarily conserved in many proteins associated with innate immunity in invertebrates and vertebrates, as well as plants. We focused on the AaeLRIM1 and AaeAPL1 gene expressions in response to Zika virus (ZIKV) and chikungunya virus (CHIKV) infection using a time course study, as well as the developmental expressions in the eggs, larvae, pupae, and adults. RNA-seq analysis data provided 60 leucine-rich repeat related transcriptions in Ae. aegypti in response to Zika virus (Accession number: GSE118858, accessed on: August 22, 2018, GEO DataSets). RNA-seq analysis data showed that AaeLRIM1 (AAEL012086-RA) and AaeAPL1 (AAEL009520-RA) were significantly upregulated 2.5 and 3-fold during infection by ZIKV 7-days post infection (dpi) of an Ae. aegypti Key West strain compared to an Orlando strain. The qPCR data showed that LRR-containing proteins related genes, AaeLRIM1 and AaeAPL1, and five paralogues were expressed 100-fold lower than other nuclear genes, such as defensin, during all developmental stages examined. Together, these data provide insights into the transcription profiles of LRR proteins of Ae. aegypti during its development and in response to infection with emergent arboviruses.