Activation of mosquito complement antiplasmodial response requires cellular immunity

Immune interactions between mosquitoes and microbes during midgut colonization

Mosquitoes encounter diverse microbes during their lifetime, including symbiotic bacteria, shaping their midgut ecosystem. The organization of the midgut supports microbiota persistence while defending against potential pathogens. The influx of nutrients during blood feeding triggers bacterial proliferation, challenging host homeostasis. Immune responses, aimed at controlling bacterial overgrowth, impact blood-borne pathogens such as malaria parasites. However, parasites deploy evasion strategies against mosquito immunity. Leveraging these mechanisms could help engineer malaria-resistant mosquitoes, offering a transformative tool for malaria elimination.

Late sporogonic stages of Plasmodium parasites are susceptible to the melanization response in Anopheles gambiae mosquitoes

The malaria-causing parasites have to complete a complex infection cycle in the mosquito vector that also involves attack by the insect's innate immune system, especially at the early stages of midgut infection. However, Anopheles immunity to the late Plasmodium sporogonic stages, such as oocysts, has received little attention as they are considered to be concealed from immune factors due to their location under the midgut basal lamina and for harboring an elaborate cell wall comprising an external layer derived from the basal lamina that confers self-properties to an otherwise foreign structure. Here, we investigated whether Plasmodium berghei oocysts and sporozoites are susceptible to melanization-based immunity in Anopheles gambiae. Silencing of the negative regulator of melanization response, CLIPA14, increased melanization prevalence without significantly increasing the numbers of melanized oocysts, while co-silencing CLIPA14 with CLIPA2, a second negative regulator of melanization, resulted in a significant increase in melanized oocysts and melanization prevalence. Only late-stage oocysts were found to be melanized, suggesting that oocyst rupture was a prerequisite for melanization-based immune attack, presumably due to the loss of the immune-evasive features of their wall. We also found melanized sporozoites inside oocysts and in the hemocoel, suggesting that sporozoites at different maturation stages are susceptible to melanization. Silencing the melanization promoting factors TEP1 and CLIPA28 rescued oocyst melanization in CLIPA2/CLIPA14 co-silenced mosquitoes. Interestingly, silencing of CTL4, that protects early stage ookinetes from melanization, had no effect on oocysts and sporozoites, indicating differential regulation of immunity to early and late sporogonic stages. Similar to previous studies addressing ookinete stage melanization, the melanization of Plasmodium falciparum oocysts was significantly lower than that observed for P. berghei. In summary, our results provide conclusive evidence that late sporogonic malaria parasite stages are susceptible to melanization, and we reveal distinct regulatory mechanisms for ookinete and oocyst melanization.

TNF signaling mediates cellular immune function and promotes malaria parasite killing in the mosquito Anopheles gambiae

Tumor Necrosis Factor-α (TNF-α) is a proinflammatory cytokine and a master regulator of immune cell function in vertebrates. While previous studies have implicated TNF signaling in invertebrate immunity, the roles of TNF in mosquito innate immunity and vector competence have yet to be explored. Herein, we confirm the identification of a conserved TNF-α pathway in Anopheles gambiae consisting of the TNF-α ligand, Eiger, and its cognate receptors Wengen and Grindelwald. Through gene expression analysis, RNAi, and in vivo injection of recombinant TNF-α, we provide direct evidence for the requirement of TNF signaling in regulating mosquito immune cell function by promoting granulocyte midgut attachment, increased granulocyte abundance, and oenocytoid rupture. Moreover, our data demonstrate that TNF signaling is an integral component of anti-Plasmodium immunity that limits malaria parasite survival. Together, our data support the existence of a highly conserved TNF signaling pathway in mosquitoes that mediates cellular immunity and influences Plasmodium infection outcomes, offering potential new approaches to interfere with malaria transmission by targeting the mosquito host.

Revisiting the determinants of malaria transmission

Malaria parasites have coevolved with humans over thousands of years, mirroring their migration out of Africa. They persist to this day, despite continuous elimination efforts worldwide. These parasites can adapt to changing environments during infection of human and mosquito, and when expanding the geographical range by switching vector species. Recent studies in the human malaria parasite, Plasmodium falciparum, identified determinants governing the plasticity of sexual conversion rates, sex ratio, and vector competence. Here we summarize the latest literature revealing environmental, epigenetic, and genetic determinants of malaria transmission.

Tick hemocytes have a pleiotropic role in microbial infection and arthropod fitness

Uncovering the complexity of systems in non-model organisms is critical for understanding arthropod immunology. Prior efforts have mostly focused on Dipteran insects, which only account for a subset of existing arthropod species in nature. Here we use and develop advanced techniques to describe immune cells (hemocytes) from the clinically relevant tick Ixodes scapularis at a single-cell resolution. We observe molecular alterations in hemocytes upon feeding and infection with either the Lyme disease spirochete Borrelia burgdorferi or the rickettsial agent Anaplasma phagocytophilum. We reveal hemocyte clusters exhibiting defined signatures related to immunity, metabolism, and proliferation. Depletion of phagocytic hemocytes affects hemocytin and astakine levels, two I. scapularis hemocyte markers, impacting blood-feeding, molting behavior, and bacterial acquisition. Mechanistically, astakine alters hemocyte proliferation, whereas hemocytin affects the c-Jun N-terminal kinase (JNK) signaling pathway in I. scapularis. Altogether, we discover a role for tick hemocytes in immunophysiology and provide a valuable resource for comparative biology in arthropods.

Mosquito midgut stem cell cellular defense response limits Plasmodium parasite infection

A novel cellular response of midgut progenitors (stem cells and enteroblasts) to Plasmodium berghei infection was investigated in Anopheles stephensi. The presence of developing oocysts triggers proliferation of midgut progenitors that is modulated by the Jak/STAT pathway and is proportional to the number of oocysts on individual midguts. The percentage of parasites in direct contact with enteroblasts increases over time, as progenitors proliferate. Silencing components of key signaling pathways through RNA interference (RNAi) that enhance proliferation of progenitor cells significantly decreased oocyst numbers, while limiting proliferation of progenitors increased oocyst survival. Live imaging revealed that enteroblasts interact directly with oocysts and eliminate them. Midgut progenitors sense the presence of Plasmodium oocysts and mount a cellular defense response that involves extensive proliferation and tissue remodeling, followed by oocysts lysis and phagocytosis of parasite remnants by enteroblasts.

The Anopheles leucine-rich repeat protein APL1C is a pathogen binding factor recognizing Plasmodium ookinetes and sporozoites

Leucine-rich repeat (LRR) proteins are commonly involved in innate immunity of animals and plants, including for pattern recognition of pathogen-derived elicitors. The Anopheles secreted LRR proteins APL1C and LRIM1 are required for malaria ookinete killing in conjunction with the complement-like TEP1 protein. However, the mechanism of parasite immune recognition by the mosquito remains unclear, although it is known that TEP1 lacks inherent binding specificity. Here, we find that APL1C and LRIM1 bind specifically to Plasmodium berghei ookinetes, even after depletion of TEP1 transcript and protein, consistent with a role for the LRR proteins in pathogen recognition. Moreover, APL1C does not bind to ookinetes of the human malaria parasite Plasmodium falciparum, and is not required for killing of this parasite, which correlates LRR binding specificity and immune protection. Most of the live P. berghei ookinetes that migrated into the extracellular space exposed to mosquito hemolymph, and almost all dead ookinetes, are bound by APL1C, thus associating LRR protein binding with parasite killing. We also find that APL1C binds to the surface of P. berghei sporozoites released from oocysts into the mosquito hemocoel and forms a potent barrier limiting salivary gland invasion and mosquito infectivity. Pathogen binding by APL1C provides the first functional explanation for the long-known requirement of APL1C for P. berghei ookinete killing in the mosquito midgut. We propose that secreted mosquito LRR proteins are required for pathogen discrimination and orientation of immune effector activity, potentially as functional counterparts of the immunoglobulin-based receptors used by vertebrates for antigen recognition.

In-depth transcriptomic analysis of Anopheles gambiae hemocytes uncovers novel genes and the oenocytoid developmental lineage

BACKGROUND

Hemocytes are immune cells that patrol the mosquito hemocoel and mediate critical cellular defense responses against pathogens. However, despite their importance, a comprehensive transcriptome of these cells was lacking because they constitute a very small fraction of the total cells in the insect, limiting the study of hemocyte differentiation and immune function.

RESULTS

In this study, an in-depth hemocyte transcriptome was built by extensive bulk RNA sequencing and assembly of hemocyte RNAs from adult A. gambiae female mosquitoes, based on approximately 2.4 billion short Illumina and about 9.4 million long PacBio high-quality reads that mapped to the A. gambiae PEST genome (P4.14 version). A total of 34,939 transcripts were annotated including 4,020 transcripts from novel genes and 20,008 novel isoforms that result from extensive differential splicing of transcripts from previously annotated genes. Most hemocyte transcripts identified (89.8%) are protein-coding while 10.2% are non-coding RNAs. The number of transcripts identified in the novel hemocyte transcriptome is twice the number in the current annotation of the A. gambiae genome (P4.14 version). Furthermore, we were able to refine the analysis of a previously published single-cell transcriptome (scRNAseq) data set by using the novel hemocyte transcriptome as a reference to re-define the hemocyte clusters and determine the path of hemocyte differentiation. Unsupervised pseudo-temporal ordering using the Tools for Single Cell Analysis software uncovered a novel putative prohemocyte precursor cell type that gives rise to prohemocytes. Pseudo-temporal ordering with the Monocle 3 software, which analyses changes in gene expression during dynamic biological processes, determined that oenocytoids derive from prohemocytes, a cell population that also gives rise to the granulocyte lineage.

CONCLUSION

A high number of mRNA splice variants are expressed in hemocytes, and they may account for the plasticity required to mount efficient responses to many different pathogens. This study highlights the importance of a comprehensive set of reference transcripts to perform robust single-cell transcriptomic data analysis of cells present in low abundance. The detailed annotation of the hemocyte transcriptome will uncover new facets of hemocyte development and function in adult dipterans and is a valuable community resource for future studies on mosquito cellular immunity.

Ehrlichia chaffeensis Co-Opts Phagocytic Hemocytes for Systemic Dissemination in the Lone Star Tick, Amblyomma americanum

INTRODUCTION

Hematophagous arthropods can acquire and transmit several pathogens of medical importance. In ticks, the innate immune system is crucial in the outcome between vector-pathogen interaction and overall vector competence. However, the specific immune response(s) elicited by the immune cells known as hemocytes remains largely undefined in Ehrlichia chaffeensis and its competent tick vector, Amblyomma americanum.

METHODS

We utilized injection of clodronate liposome to deplete tick granulocytes combined with infection with E. chaffeensis to demonstrate their essential role in microbial infection.

RESULTS

Here, we show that granulocytes, professional phagocytic cells, are integral in eliciting immune responses against commensal and pathogen infection. The chemical depletion of granulocytes led to decreased phagocytic efficiency of tissue-associated hemocytes. We demonstrate that E. chaffeensis can infect circulating hemocytes, and both cell-free plasma and hemocytes from E. chaffeensis-infected ticks can establish Ehrlichia infection in recipient ticks. Lastly, we provide evidence to show that granulocytes play a dual role in E. chaffeensis infection. Depleting granulocytic hemocytes increased Ehrlichia load in the salivary gland and midgut tissues. In contrast, granulocyte depletion led to a reduced systemic load of Ehrlichia.

CONCLUSION

This study has identified multiple roles for granulocytic hemocytes in the control and systemic dissemination of E. chaffeensis infection.

The heat shock protein Hsc70-3 mediates an anti-apoptotic response critical for Plasmodium evasion of Anopheles gambiae immunity

IMPORTANCE

Malaria transmission by Anopheles gambiae mosquitoes is very effective, in part because the parasite expresses a surface protein called Pfs47 that allows it to evade the mosquito immune system. Here we investigate how this protein changes the response of mosquito midgut epithelial cells to invasion by the parasite. Pfs47 is known to interact with P47Rec, a mosquito midgut receptor. We found that Pf47Rec inhibits caspase-mediated apoptosis by interacting with the Hsc70-3. This disrupts nitration of midgut epithelial cells invaded by the parasite and the release of hemocyte-derived microvesicles, which are critical for effective activation of the mosquito complement system that eliminates the parasite.

Mosquito midgut stem cell cellular defense response limits Plasmodium parasite infection

A novel cellular response of midgut progenitors (stem cells and enteroblasts) to Plasmodium berghei infection was investigated in Anopheles stephensi. The presence of developing oocysts triggers proliferation of midgut progenitors that is modulated by the Jak/STAT pathway, and proportional to the number of oocysts on individual midguts. The percentage of parasites in direct contact with enteroblasts increases over time, as progenitors proliferate. Enhancing proliferation of progenitors significantly decreases oocyst numbers, while limiting proliferation increases oocyst survival. Live imaging revealed that enteroblasts interact directly with oocysts and eliminate them. Midgut progenitors sense the presence of Plasmodium oocysts and mount a cellular defense response that involves extensive proliferation and tissue remodeling, followed by oocysts lysis and phagocytosis of parasite remnants by enteroblasts.

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.

Deceiving and escaping complement - the evasive journey of the malaria parasite

During its life cycle, Plasmodium, the malaria parasite, is exposed to the human and mosquito complement systems. Early experiments demonstrated that activation of complement can pose a serious threat to parasites, but recent studies revealed complement-evasion mechanisms important for parasite survival. Blood-stage parasites and gametes recruit regulators to neutralize human complement activation, while ookinetes inhibit mosquito complement by disrupting epithelial nitration in response to midgut invasion. Here we provide an in-depth overview of the evasion mechanisms currently known and speculate on the existence of others not yet identified. Finally, we discuss how these mechanisms could provide novel targets for urgently needed malaria vaccines and therapeutics.

Anopheles gambiae strain (Ag55) cultured cells originated from Anopheles coluzzii and are phagocytic with hemocyte-like gene expression

Anopheles gambiae and Anopheles coluzzii are closely related species that are predominant vectors of malaria in Africa. Recently, A. gambiae form M was renamed A. coluzzii and we now conclude on the basis of a diagnostic PCR-restriction fragment length polymorphism assay that Ag55 cells were derived from A. coluzzii. We established an Ag55 cell transcriptome, and KEGG pathway analysis showed that Ag55 cells are enriched in phagosome pathway transcripts. The Ag55 transcriptome has an abundance of specific transcripts characteristic of mosquito hemocytes. Functional E. coli bioparticle uptake experiments visualized by fluorescence microscopy and confocal microscopy and quantified by flow cytometry establish the phagocytic competence of Ag55 cells. Results from this investigation of Ag55 cell properties will guide researchers in the use and engineering of the Ag55 cell line to better enable investigations of Plasmodium, other microbes, and insecticidal toxins.

Understanding pathogen survival and transmission by arthropod vectors to prevent human disease

Many endemic poverty-associated diseases, such as malaria and leishmaniasis, are transmitted by arthropod vectors. Pathogens must interact with specific molecules in the vector gut, the microbiota, and the vector immune system to survive and be transmitted. The vertebrate host, in turn, is infected when the pathogen and vector-derived factors, such as salivary proteins, are delivered into the skin by a vector bite. Here, we review recent progress in our understanding of the biology of pathogen transmission from the human to the vector and back, from the vector to the host. We also highlight recent advances in the biology of vector-borne disease transmission, which have translated into additional strategies to prevent human disease by either reducing vector populations or by disrupting their ability to transmit pathogens.

The Role of Mosquito Hemocytes in Viral Infections

Insect hemocytes are the only immune cells that can mount a humoral and cellular immune response. Despite the critical involvement of hemocytes in immune responses against bacteria, fungi, and parasites in mosquitoes, our understanding of their antiviral potential is still limited. It has been shown that hemocytes express humoral factors such as TEP1, PPO, and certain antimicrobial peptides that are known to restrict viral infections. Insect hemocytes also harbor the major immune pathways, such as JAK/STAT, TOLL, IMD, and RNAi, which are critical for the control of viral infection. Recent research has indicated a role for hemocytes in the regulation of viral infection through RNA interference and autophagy; however, the specific mechanism by which this regulation occurs remains uncharacterized. Conversely, some studies have suggested that hemocytes act as agonists of arboviral infection because they lack basal lamina and circulate throughout the whole mosquito, likely facilitating viral dissemination to other tissues such as salivary glands. In addition, hemocytes produce arbovirus agonist factors such as lectins, which enhance viral infection. Here, we summarize our current understanding of hemocytes’ involvement in viral infections.

Malaria parasite evades mosquito immunity by glutaminyl cyclase-mediated posttranslational protein modification

Glutaminyl cyclase (QC) modifies N-terminal glutamine or glutamic acid residues of target proteins into cyclic pyroglutamic acid (pGlu). Here, we report the biochemical and functional analysis of Plasmodium QC. We show that sporozoites of QC-null mutants of rodent and human malaria parasites are recognized by the mosquito immune system and melanized when they reach the hemocoel. Detailed analyses of rodent malaria QC-null mutants showed that sporozoite numbers in salivary glands are reduced in mosquitoes infected with QC-null or QC catalytically dead mutants. This phenotype can be rescued by genetic complementation or by disrupting mosquito melanization or phagocytosis by hemocytes. Mutation of a single QC-target glutamine of the major sporozoite surface protein (circumsporozoite protein; CSP) of the rodent parasite Plasmodium berghei also results in melanization of sporozoites. These findings indicate that QC-mediated posttranslational modification of surface proteins underlies evasion of killing of sporozoites by the mosquito immune system.

Pfs47 as a Malaria Transmission-Blocking Vaccine Target

Transmission-blocking vaccines (TBVs), pioneered by Richard Carter and others, aim to prevent parasite development in the mosquito vector and are a promising new tool for malaria elimination. Pfs47, recently identified as a TBV target, is a three-domain 6-cysteine protein on the surface of Plasmodium falciparum sexual stages. Pfs47 allows the parasite to evade mosquito immunity and is key for P. falciparum infection of the dominant malaria vectors Anopheles gambiae, Anopheles dirus, and Anopheles albimanus. Antibodies against Pfs47 domain 2 (D2) have significant transmission-blocking activity that prevents Plasmodium ookinete development and is independent of human complement. Strong transmission-blocking activity has been mapped to a region of 52 amino acids in Pfs47 D2. Efforts to optimize the immunogenicity of the Pfs47 D2 antigen with a viral-like particle have been successful, and the efficacy of a P47-based TBV was confirmed in vivo with Pbs47, the orthologue of Pfs47 in the mouse malaria parasite Plasmodium berghei. The current evidence warrants further development and clinical testing of a Pfs47-based TBV.

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 CSP_mut 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 CSP_mut parasites reduces the burden of infection when re-challenged with CSP_wt 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.

Circumsporozoite protein (CSP), the major surface protein of Plasmodium sporozoites, is important for parasite targeting to mosquito salivary glands and the mammalian liver. Here, Zhu et al. show that CSP is required for rodent malaria oocysts to evade mosquito immunity by inducing hemocyte nitration and causing subsequent defects in sporozoite-release from oocysts.

Phagocytosis Is the Sole Arm of Drosophila melanogaster Known Host Defenses That Provides Some Protection Against Microsporidia Infection

Microsporidia are obligate intracellular parasites able to infest specifically a large range of species, including insects. The knowledge about the biology of microsporidial infections remains confined to mostly descriptive studies, including molecular approaches such as transcriptomics or proteomics. Thus, functional data to understand insect host defenses are currently lacking. Here, we have undertaken a genetic analysis of known host defenses of the Drosophila melanogaster using an infection model whereby Tubulinosema ratisbonensis spores are directly injected in this insect. We find that phagocytosis does confer some protection in this infection model. In contrast, the systemic immune response, extracellular reactive oxygen species, thioester proteins, xenophagy, and intracellular antiviral response pathways do not appear to be involved in the resistance against this parasite. Unexpectedly, several genes such as PGRP-LE seem to promote this infection. The prophenol oxidases that mediate melanization have different functions; PPO1 presents a phenotype similar to that of PGRP-LE whereas that of PPO2 suggests a function in the resilience to infection. Similarly, eiger and Unpaired3, which encode two cytokines secreted by hemocytes display a resilience phenotype with a strong susceptibility to T. ratisbonensis.

TmIKKε Is Required to Confer Protection Against Gram-Negative Bacteria, E. coli by the Regulation of Antimicrobial Peptide Production in the Tenebrio molitor Fat Body

The inhibitor of nuclear factor-kappa B (NF-κB) kinase (IKK) is the core regulator of the NF-κB pathway against pathogenic invasion in vertebrates or invertebrates. IKKβ, -ε and -γ have pivotal roles in the Toll and immune deficiency (IMD) pathways. In this study, a homolog of IKKε (TmIKKε) was identified from Tenebrio molitor RNA sequence database and functionally characterized for its role in regulating immune signaling pathways in insects. The TmIKKε gene is characterized by two exons and one intron comprising an open reading frame (ORF) of 2,196 bp that putatively encodes a polypeptide of 731 amino acid residues. TmIKKε contains a serine/threonine protein kinases catalytic domain. Phylogenetic analysis established the close homology of TmIKKε to Tribolium castaneum IKKε (TcIKKε) and its proximity with other IKK-related kinases. The expression of TmIKKε mRNA was elevated in the gut, integument, and hemocytes of the last-instar larva and the fat body, Malpighian tubules, and testis of 5-day-old adults. TmIKKε expression was significantly induced by Escherichia coli, Staphylococcus aureus, and Candida albicans challenge in whole larvae and tissues, such as hemocytes, gut, and fat body. The knockdown of the TmIKKε messenger RNA (mRNA) expression significantly reduced the survival of the larvae against microbial challenges. Further, we investigated the induction patterns of 14 T. molitor antimicrobial peptides (AMPs) genes in TmIKKε gene-silencing model after microbial challenges. While in hemocytes, the transcriptional regulation of most AMPs was negatively regulated in the gut and fat body tissue of T. molitor, AMPs, such as TmTenecin 1, TmTenecin 4, TmDefensin, TmColeoptericin A, TmColeoptericin B, TmAttacin 1a, and TmAttacin 2, were positively regulated in TmIKKε-silenced individuals after microbial challenge. Collectively, the results implicate TmIKKε as an important factor in antimicrobial innate immune responses in T. molitor.

Double peroxidase and histone acetyltransferase AgTip60 maintain innate immune memory in primed mosquitoes

Immune priming in Anopheles gambiae is mediated by the systemic release of a hemocyte differentiation factor (HDF), a complex of lipoxin A₄ bound to Evokin, a lipid carrier. HDF increases the proportion of circulating granulocytes and enhances mosquito cellular immunity. Here, we show that Evokin is present in hemocytes and fat-body cells, and messenger RNA (mRNA) expression increases significantly after immune priming. The double peroxidase (DBLOX) enzyme, present in insects but not in vertebrates, is essential for HDF synthesis. DBLOX is highly expressed in oenocytes in the fat-body tissue, and these cells increase in number in primed mosquitoes. We provide direct evidence that the histone acetyltransferase AgTip60 (AGAP001539) is also essential for a sustained increase in oenocyte numbers, HDF synthesis, and immune priming. We propose that oenocytes may function as a population of cells that are reprogrammed, and orchestrate and maintain a broad, systemic, and long-lasting state of enhanced immune surveillance in primed mosquitoes.

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.

Distinct Roles of Hemocytes at Different Stages of Infection by Dengue and Zika Viruses in Aedes aegypti Mosquitoes

Aedes aegypti mosquitoes are vectors for arboviruses of medical importance such as dengue (DENV) and Zika (ZIKV) viruses. Different innate immune pathways contribute to the control of arboviruses in the mosquito vector including RNA interference, Toll and Jak-STAT pathways. However, the role of cellular responses mediated by circulating macrophage-like cells known as hemocytes remains unclear. Here we show that hemocytes are recruited to the midgut of Ae. aegypti mosquitoes in response to DENV or ZIKV. Blockade of the phagocytic function of hemocytes using latex beads induced increased accumulation of hemocytes in the midgut and a reduction in virus infection levels in this organ. In contrast, inhibition of phagocytosis by hemocytes led to increased systemic dissemination and replication of DENV and ZIKV. Hence, our work reveals a dual role for hemocytes in Ae. aegypti mosquitoes, whereby phagocytosis is not required to control viral infection in the midgut but is essential to restrict systemic dissemination. Further understanding of the mechanism behind this duality could help the design of vector-based strategies to prevent transmission of arboviruses.

Transcriptomic analysis reveals pronounced changes in gene expression due to sub-lethal pyrethroid exposure and ageing in insecticide resistance Anopheles coluzzii

BACKGROUND

Malaria control is heavily reliant on the use of insecticides that target and kill the adult female Anopheline vector. The intensive use of insecticides of the pyrethroid class has led to widespread resistance in mosquito populations. The intensity of pyrethroid resistance in some settings in Africa means mosquitoes can contact bednets treated with this insecticide class multiple times with minimal mortality effects. Furthermore, both ageing and diel cycle have been shown to have large impacts on the resistance phenotype. Together, these traits may affect other aspects of vector biology controlling the vectorial capacity or fitness of the mosquito.

RESULTS

Here we show that sublethal exposure of a highly resistant Anopheles coluzzii population originally from Burkina Faso to the pyrethroid deltamethrin results in large and sustained changes to transcript expression. We identify five clear patterns in the data showing changes to transcripts relating to: DNA repair, respiration, translation, behaviour and oxioreductase processes. Further, we highlight differential regulation of transcripts from detoxification families previously linked with insecticide resistance, in addition to clear down-regulation of the oxidative phosphorylation pathway both indicative of changes in metabolism post-exposure. Finally, we show that both ageing and diel cycle have major effects on known insecticide resistance related transcripts.

CONCLUSION

Sub-lethal pyrethroid exposure, ageing and the diel cycle results in large-scale changes in the transcriptome of the major malaria vector Anopheles coluzzii. Our data strongly supports further phenotypic studies on how transcriptional changes such as reduced expression of the oxidative phosphorylation pathway or pyrethroid induced changes to redox state might impact key mosquito traits, such as vectorial capacity and life history traits.

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.

Genetic changes of Plasmodium vivax tempers host tissue-specific responses in Anopheles stephensi

Recently, we showed how an early restriction of gut flora proliferation by Plasmodium vivax favors immune-suppression and Plasmodium survival in the gut lumen (Sharma et al., 2020). Here, we asked post gut invasion how P. vivax interacts with individual tissues such as the midgut, hemocyte, and salivary glands, and manages its survival in the mosquito host. Our data from tissue-specific comparative RNA-Seq analysis and extensive temporal/spatial expression profiling of selected mosquito transcripts in the uninfected and P. vivax infected mosquito’s tissues indicated that (i) a transient suppression of gut metabolic machinery by early oocysts; (ii) enriched expression of nutritional responsive proteins and immune proteins against late oocysts, together may ensure optimal parasite development and gut homeostasis restoration; (iii) pre-immune activation of hemocyte by early gut-oocysts infection via REL induction (p ​< ​0.003); and altered expression of hemocyte-encoded immune proteins may cause rapid removal of free circulating sporozoites from hemolymph; (iv) while a strong suppression of salivary metabolic activities, and elevated expression of salivary specific secretory, as well as immune proteins together, may favor the long-term storage and survival of invaded sporozoites. Finally, our RNA-Seq-based discovery of 4449 transcripts of Plasmodium vivax origin, and their developmental stage-specific expression modulation in the corresponding infected mosquito tissues, predicts a possible mechanism of mosquito responses evasion by P. vivax. Conclusively, our system-wide RNA-Seq analysis provides the first genetic evidence of direct mosquito-Plasmodium interaction and establishes a functional correlation.

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System-wide RNASeq analysis discloses direct mosquito-Plasmodium vivax ​interaction and establishes a functional correlation.

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Discovery of 4449 transcripts specific to ​P. vivax ​developmental stages sheds light on possible immune evasion mechanisms.

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Upregulation of nutritional related transcripts ensures optimal late oocyst development and midgut homeostasis.

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Hemocyte activation by early oocysts ensure removal of free circulatory sporozoites by proliferating immune transcripts.

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Heightened levels of salivary immune transcripts show that an active local immune response restricts salivary invaded sporozoite

The Route of Infection Influences the Contribution of Key Immunity Genes to Antibacterial Defense in Anopheles gambiae

Insect systemic immune responses to bacterial infections have been mainly studied using microinjections, whereby the microbe is directly injected into the hemocoel. While this methodology has been instrumental in defining immune signaling pathways and enzymatic cascades in the hemolymph, it remains unclear whether and to what extent the contribution of systemic immune defenses to host microbial resistance varies if bacteria invade the hemolymph after crossing the midgut epithelium subsequent to an oral infection. Here, we address this question using the pathogenic Serratia marcescens (Sm) DB11 strain to establish systemic infections of the malaria vector Anopheles gambiae, either by septic Sm injections or by midgut crossing after feeding on Sm. Using functional genetic studies by RNAi, we report that the two humoral immune factors, thioester-containing protein 1 and C-type lectin 4, which play key roles in defense against Gram-negative bacterial infections, are essential for defense against systemic Sm infections established through injection, but they become dispensable when Sm infects the hemolymph following oral infection. Similar results were observed for the mosquito Rel2 pathway. Surprisingly, blocking phagocytosis by cytochalasin D treatment did not affect mosquito susceptibility to Sm infections established through either route. Transcriptomic analysis of mosquito midguts and abdomens by RNA-seq revealed that the transcriptional response in these tissues is more pronounced in response to feeding on Sm. Functional classification of differentially expressed transcripts identified metabolic genes as the most represented class in response to both routes of infection, while immune genes were poorly regulated in both routes. We also report that Sm oral infections are associated with significant downregulation of several immune genes belonging to different families, specifically the clip-domain serine protease family. In sum, our findings reveal that the route of infection not only alters the contribution of key immunity genes to host antimicrobial defense but is also associated with different transcriptional responses in midguts and abdomens, possibly reflecting different adaptive strategies of the host.

Mosquito cellular immunity at single-cell resolution

Hemocytes limit the capacity of mosquitoes to transmit human pathogens. Here we profile the transcriptomes of 8506 hemocytes of Anopheles gambiae and Aedes aegypti mosquito vectors. Our data reveal the functional diversity of hemocytes, with different subtypes of granulocytes expressing distinct and evolutionarily conserved subsets of effector genes. A previously unidentified cell type in An. gambiae, which we term "megacyte," is defined by a specific transmembrane protein marker (TM7318) and high expression of lipopolysaccharide-induced tumor necrosis factor-α transcription factor 3 (LL3). Knockdown experiments indicate that LL3 mediates hemocyte differentiation during immune priming. We identify and validate two main hemocyte lineages and find evidence of proliferating granulocyte populations. This atlas of medically relevant invertebrate immune cells at single-cell resolution identifies cellular events that underpin mosquito immunity to malaria infection.

Live In Vivo Imaging of Plasmodium Invasion of the Mosquito Midgut

The mosquito midgut is a critical barrier that Plasmodium parasites must overcome to complete their developmental cycle and be transmitted to a new vertebrate host. Previous confocal studies with fixed infected midguts showed that ookinetes traverse midgut epithelial cells and cause irreversible tissue damage. Here, we investigated the spatiotemporal dynamics of ookinete midgut traversal and the response of midgut cells to invasion. A novel mounting strategy was established, suitable fluorescent dye combinations were identified and protocols optimized to label mosquito tissues in vivo, and live imaging protocols using confocal microscopy were developed. Tracking data showed that ookinetes gliding on the midgut surface travel faster and farther than those that remain in the lumen or those that have invaded the epithelium. Image analysis confirmed that parasite invasion and cell traversal occur within a couple of minutes, while caspase activity in damaged cells, indicative of cellular apoptosis, and F-actin cytoskeletal rearrangements in cells extruded into the gut lumen persist for several hours. This temporal difference highlights the importance of hemocyte-mediated cellular immunity and the mosquito complement system to mount a coordinated and effective antiplasmodial response. This novel in vivo imaging protocol allowed us to continuously observe individual ookinetes in live mosquitoes within the gut lumen and during cell traversal and to capture the subsequent cellular responses to invasion in real time for several hours, without loss of tissue integrity.IMPORTANCE Malaria is one of the most devastating parasitic diseases in humans and is transmitted by anopheline mosquitoes. The mosquito midgut is a critical barrier that Plasmodium parasites must overcome to complete their developmental cycle and be transmitted to a new host. Here, we developed a new strategy to visualize Plasmodium ookinetes as they traverse the mosquito midgut and to follow the response of damaged epithelial cells by imaging live mosquitoes. Understanding the spatial and temporal aspects of these interactions is critical when developing novel strategies to disrupt disease transmission.

PIMMS43 is required for malaria parasite immune evasion and sporogonic development in the mosquito vector

Malaria is transmitted among humans through mosquito bites. Here, we characterize a protein found on the surface of mosquito stages of malaria parasites and reveal that it serves to evade the mosquito immune system and ensure disease transmission. Neutralization of PIMMS43 (Plasmodium Infection of the Mosquito Midgut Screen 43), either by eliminating it from the parasite genome or by preincubating parasites with antibodies that bind to the PIMMS43 protein, inhibits mosquito infection with malaria parasites. Differences in PIMMS43 detected between African malaria parasite populations suggest that these populations have adapted for transmission by different mosquito vectors that are also differentially distributed across the continent. We conclude that targeting PIMMS43 can block malaria parasites inside mosquitoes before they can infect humans.

After being ingested by a female Anopheles mosquito during a bloodmeal on an infected host, and before they can reach the mosquito salivary glands to be transmitted to a new host, Plasmodium parasites must establish an infection of the mosquito midgut in the form of oocysts. To achieve this, they must first survive a series of robust innate immune responses that take place prior to, during, and immediately after ookinete traversal of the midgut epithelium. Understanding how parasites may evade these responses could highlight new ways to block malaria transmission. We show that an ookinete and sporozoite surface protein designated as PIMMS43 (Plasmodium Infection of the Mosquito Midgut Screen 43) is required for parasite evasion of the Anopheles coluzzii complement-like response. Disruption of PIMMS43 in the rodent malaria parasite Plasmodium berghei triggers robust complement activation and ookinete elimination upon mosquito midgut traversal. Silencing components of the complement-like system through RNAi largely restores ookinete-to-oocyst transition but oocysts remain small in size and produce a very small number of sporozoites that additionally are not infectious, indicating that PIMMS43 is also essential for sporogonic development in the oocyst. Antibodies that bind PIMMS43 interfere with parasite immune evasion when ingested with the infectious blood meal and significantly reduce the prevalence and intensity of infection. PIMMS43 genetic structure across African Plasmodium falciparum populations indicates allelic adaptation to sympatric vector populations. These data add to our understanding of mosquito–parasite interactions and identify PIMMS43 as a target of malaria transmission blocking.

Women in immunology: 2020 and beyond

Women have been at the forefront of tremendous achievements in immunology in the past decade. However, disparities still exist, limiting upward potential and further advancements. As four NIH intramural women scientists who care deeply about scientific progress and the progress of women in our field, we review ongoing challenges and discuss potential approaches to help advance the promotion of women in the sciences.

Activation of mosquito immunity blocks the development of transmission-stage filarial nematodes

Mosquito-borne helminth infections are responsible for a significant worldwide disease burden in both humans and animals. Accordingly, development of novel strategies to reduce disease transmission by targeting these pathogens in the vector are of paramount importance. We found that a strain of Aedes aegypti that is refractory to infection by Dirofilaria immitis, the agent of canine heartworm disease, mounts a stronger immune response during infection than does a susceptible strain. Moreover, activation of the Toll immune signaling pathway in the susceptible strain arrests larval development of the parasite, thereby decreasing the number of transmission-stage larvae. Notably, this strategy also blocks transmission-stage Brugia malayi, an agent of human lymphatic filariasis. Our data show that mosquito immunity can play a pivotal role in restricting filarial nematode development and suggest that genetically engineering mosquitoes with enhanced immunity will help reduce pathogen transmission.

Plasmodium falciparum evades immunity of anopheline mosquitoes by interacting with a Pfs47 midgut receptor

The surface protein Pfs47 allows Plasmodium falciparum parasites to survive and be transmitted by making them "undetectable" to the mosquito immune system. P. falciparum parasites express Pfs47 haplotypes compatible with their sympatric vectors, while those with incompatible haplotypes are eliminated by the mosquito. We proposed that Pfs47 serves as a "key" that mediates immune evasion by interacting with a mosquito receptor "the lock," which differs in evolutionarily divergent anopheline mosquitoes. Recombinant Pfs47 (rPfs47) was used to identify the mosquito Pfs47 receptor protein (P47Rec) using far-Western analysis. rPfs47 bound to a single 31-kDa band and the identity of this protein was determined by mass spectrometry. The mosquito P47Rec has two natterin-like domains and binds to Pfs47 with high affinity (17 to 32 nM). P47Rec is a highly conserved protein with submicrovillar localization in midgut cells. It has structural homology to a cytoskeleton-interacting protein and accumulates at the site of ookinete invasion. Silencing P47Rec expression reduced P. falciparum infection, indicating that the interaction of Pfs47 with the receptor is critical for parasite survival. The binding specificity of P47Rec from distant anophelines (Anopheles gambiae, Anopheles dirus, and Anopheles albimanus) with Pfs47-Africa (GB4) and Pfs47-South America (7G8) haplotypes was evaluated, and it is in agreement with the previously documented compatibility between P. falciparum parasites expressing different Pfs47 haplotypes and these three anopheline species. Our findings give further support to the role of Pfs47 in the adaptation of P. falciparum to different vectors.

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.

A mosquito juvenile hormone binding protein (mJHBP) regulates the activation of innate immune defenses and hemocyte development

Insects rely on the innate immune system for defense against pathogens, some aspects of which are under hormonal control. Here we provide direct experimental evidence showing that the juvenile hormone-binding protein (mJHBP) of Aedes aegypti is required for the regulation of innate immune responses and the development of mosquito blood cells (hemocytes). Using an mJHBP-deficient mosquito line generated by means of CRISPR-Cas9 gene editing technology we uncovered a mutant phenotype characterized by immunosuppression at the humoral and cellular levels, which profoundly affected susceptibility to bacterial infection. Bacteria-challenged mosquitoes exhibited significantly higher levels of septicemia and mortality relative to the wild type (WT) strain, delayed expression of antimicrobial peptides (AMPs), severe developmental dysregulation of embryonic and larval hemocytes (reduction in the total number of hemocytes) and increased differentiation of the granulocyte lineage. Interestingly, injection of recombinant wild type mJHBP protein into adult females three-days before infection was sufficient to restore normal immune function. Similarly, injection of mJHBP into fourth-instar larvae fully restored normal larval/pupal hemocyte populations in emerging adults. More importantly, the recovery of normal immuno-activation and hemocyte development requires the capability of mJHBP to bind JH III. These results strongly suggest that JH III functions in mosquito immunity and hemocyte development in a manner that is perhaps independent of canonical JH signaling, given the lack of developmental and reproductive abnormalities. Because of the prominent role of hemocytes as regulators of mosquito immunity, this novel discovery may have broader implications for the understanding of vector endocrinology, hemocyte development, vector competence and disease transmission.

There are many unanswered questions concerning the nature of immune responses of mosquitoes to bacteria, viruses and parasites. This is important because a variety of human pathogens are transmitted by mosquitoes during the process of consuming blood. Much of mosquito physiology is under the control of hormones and we aim to understand a potential role for an important hormone known as juvenile hormone in anti-bacterial immunity. We have produced a strain of the mosquito, Aedes aegypti, that is deficient in the production of a protein that circulates in the blood while carrying juvenile hormone. This strain is shown to have less ability to control bacterial infection, to have lower levels of proteins involved in immunity and to have smaller numbers of blood cells that are known to be important in the mosquito immune response. If the protein is administered to the deficient strain by injection, the immune response and blood cell numbers return to near-normal levels. Other results suggest that the association of the protein with juvenile hormone is important for its ability to function in the immune system. Overall, this study describes an important new protein regulator of mosquito immunity and a potential role of juvenile hormone in this process.

The mosquito melanization response requires hierarchical activation of non-catalytic clip domain serine protease homologs

Serine protease cascades regulate important insect immune responses namely melanization and Toll pathway activation. An important component of these cascades are clip-domain serine protease homologs (cSPHs), which are non-catalytic, but essential for activating the enzyme prophenoloxidase (PPO) in the melanization response during septic infections. The activation of cSPHs requires their proteolytic cleavage, yet factors that control their activation and the complexity of their interactions within these cascades remain unclear. Here, we report the identification of CLIPA28 as a novel immune-related cSPH in the malaria vector Anopheles gambiae. Functional genetic analysis using RNA interference (RNAi) revealed that CLIPA28 is essential for the melanization of Plasmodium berghei parasites in refractory mosquitoes, and for mosquito resistance to fungal infections. We further show, using combined biochemical and genetic approaches, that CLIPA28 is member of a network of at least four cSPHs, whereby members are activated in a hierarchical manner following septic infections. Depletion of the complement-like protein TEP1 abolished the activation of this network after septic infections, whereas, depletion of the serine protease inhibitor 2 (SRPN2) triggered enhanced network activation, even in naïve mosquitoes, culminating in a dramatic reduction in cSPHs hemolymph levels, which paralleled that of PPO. Our data suggest that cSPHs are engaged in complex and multilayered interactions within serine protease cascades that regulate melanization, and identify TEP1 and SRPN2 as two master regulators of the cSPH network.

Melanization is a spectacular immune response of insects that culminates in the deposition of melanin on microbial surfaces leading to their death. Despite more than half a century of investigation of this immune reaction in several insect models, important knowledge gaps, specifically concerning the regulation of this response, remain, especially that its output is tightly regulated by complex genetic and biochemical interactions between members of structurally diverse immune gene families. Here, we identify a new hemolymph protein that is involved in the melanization of malaria parasites and show that it forms, together with other proteins of the same gene family, a network, whereby members are activated in an orderly manner during mosquito septic infections. Furthemore, we show that this network is controlled by two structurally distinct immune proteins which act as master regulators of its activation. Dissecting the complexity of the regulatory mechanisms mediating insect melanization responses, may facilitate fine manipulation of mosquito immunity to reduce vectorial capacity.

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.

A fungal pathogen deploys a small silencing RNA that attenuates mosquito immunity and facilitates infection

Insecticidal fungi represent a promising alternative to chemical pesticides for disease vector control. Here, we show that the pathogenic fungus Beauveria bassiana exports a microRNA-like RNA (bba-milR1) that hijacks the host RNA-interference machinery in mosquito cells by binding to Argonaute 1 (AGO1). bba-milR1 is highly expressed during fungal penetration of the mosquito integument, and suppresses host immunity by silencing expression of the mosquito Toll receptor ligand Spätzle 4 (Spz4). Later, upon entering the hemocoel, bba-milR1 expression is decreased, which avoids induction of the host proteinase CLIPB9 that activates the melanization response. Thus, our results indicate that the pathogen deploys a cross-kingdom small-RNA effector that attenuates host immunity and facilitates infection.

Mosquito Midgut Prostaglandin Release Establishes Systemic Immune Priming

Anopheles gambiae mosquitoes that have been infected with Plasmodium mount a more effective immune response to a subsequent infection. Priming is established when Plasmodium invasion of the mosquito midgut allows contact of the gut microbiota with epithelial cells. This event is followed by a systemic release of a hemocyte differentiation factor (HDF) consisting of Lipoxin A4 bound to Evokin, a lipocalin carrier, which increases the proportion of circulating hemocytes. We show that mosquito midgut cells produce and release prostaglandin E2 (PGE2), which attracts hemocytes to the midgut surface and enhances their patrolling activity. Systemic injection of prostaglandins (PGs) recapitulates the priming response and enhances antiplasmodial immunity by triggering HDF production. Although insects lack cyclooxygenases, two heme peroxidases, HPX7 and HPX8, catalyze essential steps in PG biosynthesis in mosquitoes. Mosquito midgut PGE2 release attracts hemocytes and establishes a long-lasting enhanced systemic cellular immune response to Plasmodium infection.

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Plasmodium invasion or bacterial exposure triggers midgut prostaglandin synthesis

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Prostaglandins attract mosquito hemocytes and increase their patrolling activity

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Two midgut peroxidases, HPX7 and HPX8, catalyze midgut prostaglandin synthesis

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Systemic release of midgut prostaglandins is essential to establish immune priming

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.

Pathogen-insect interaction candidate molecules for transmission-blocking control strategies of vector borne diseases

OBJECTIVE

To analyze the current knowledge of pathogen-insect interactions amenable for the design of molecular-based control strategies of vector-borne diseases.

MATERIALS AND METHODS

We examined malaria, dengue, and Chagas disease pathogens and insect molecules that participate in interactions during their vectors infection.

RESULTS

Pathogen molecules that participate in the insect intestine invasion and induced vector immune molecules are presented, and their inclusion in transmission blocking vaccines (TBV) and in genetically modify insect (GMI) vectors or symbiotic bacteria are discussed.

CONCLUSIONS

Disruption of processes by blocking vector-pathogen interactions provides several candidates for molecular control strategies, but TBV and GMI efficacies are still limited and other secondary effects of GMI (improving transmission of other pathogens, affectation of other organisms) should be discarded.

Vector biology meets disease control: using basic research to fight vector-borne diseases

Human pathogens that are transmitted by insects are a global problem, particularly those vectored by mosquitoes; for example, malaria parasites transmitted by Anopheles species, and viruses such as dengue, Zika and chikungunya that are carried by Aedes mosquitoes. Over the past 15 years, the prevalence of malaria has been substantially reduced and virus outbreaks have been contained by controlling mosquito vectors using insecticide-based approaches. However, disease control is now threatened by alarming rates of insecticide resistance in insect populations, prompting the need to develop a new generation of specific strategies that can reduce vector-mediated transmission. Here, we review how increased knowledge in insect biology and insect-pathogen interactions is stimulating new concepts and tools for vector control. We focus on strategies that either interfere with the development of pathogens within their vectors or directly impact insect survival, including enhancement of vector-mediated immune control, manipulation of the insect microbiome, or use of powerful new genetic tools such as CRISPR-Cas systems to edit vector genomes. Finally, we offer a perspective on the implementation hurdles as well as the knowledge gaps that must be filled in the coming years to safely realize the potential of these novel strategies to eliminate the scourge of vector-borne disease.

Unbiased classification of mosquito blood cells by single-cell genomics and high-content imaging

Mosquito blood cells are immune cells that help control infection by vector-borne pathogens. Despite their importance, little is known about mosquito blood cell biology beyond morphological and functional criteria used for their classification. Here, we combined the power of single-cell RNA sequencing, high-content imaging flow cytometry, and single-molecule RNA hybridization to analyze a subset of blood cells of the malaria mosquito Anopheles gambiae By demonstrating that blood cells express nearly half of the mosquito transcriptome, our dataset represents an unprecedented view into their transcriptional program. Analyses of differentially expressed genes identified transcriptional signatures of two cell types and provide insights into the current classification of these cells. We further demonstrate the active transfer of a cellular marker between blood cells that may confound their identification. We propose that cell-to-cell exchange may contribute to cellular diversity and functional plasticity seen across biological systems.

Antibody targeting of a specific region of Pfs47 blocks Plasmodium falciparum malaria transmission

Transmission-blocking vaccines are based on eliciting antibody responses in the vertebrate host that disrupt parasite development in the mosquito vector and prevent malaria transmission. The surface protein Pfs47 is present in Plasmodium falciparum gametocytes and female gametes. The potential of Pfs47 as a vaccine target was evaluated. Soluble full-length recombinant protein, consisting of three domains, was expressed in E. coli as a thioredoxin fusion (T-Pfs47). The protein was immunogenic, and polyclonal and monoclonal antibodies (mAb) were obtained, but they did not confer transmission blocking activity (TBA). All fourteen mAb targeted either domains 1 or 3, but not domain 2 (D2), and immune reactivity to D2 was also very low in polyclonal mouse IgG after T-Pfs47 immunization. Disruption of the predicted disulfide bond in D2, by replacing cysteines for alanines (C230A and C260A), allowed expression of recombinant D2 protein in E. coli. A combination of mAbs targeting D2, and deletion proteins from this domain, allowed us to map a central 52 amino acid (aa) region where antibody binding confers strong TBA (78-99%). This 52 aa antigen is immunogenic and well conserved, with only seven haplotypes world-wide that share 96-98% identity. Neither human complement nor the mosquito complement-like system are required for the observed TBA. A dramatic reduction in ookinete numbers and ookinete-specific transcripts was observed, suggesting that the antibodies are interacting with female gametocytes and preventing fertilization.

The immune strategies of mosquito Aedes aegypti against microbial infection

Yellow fever mosquito Aedes aegypti transmits many devastating arthropod-borne viruses (arboviruses), such as dengue virus, yellow fever virus, Chikungunya virus, and Zika virus, which cause great concern to human health. Mosquito control is an effective method to block the spread of infectious diseases. Ae. aegypti uses its innate immune system to fight against arboviruses, parasites, and fungi. In this review, we briefly summarize the recent findings in the immune response of Ae. aegypti against arboviral and entomopathogenic infections. This review enriches our understanding of the mosquito immune system and provides evidence to support the development of novel mosquito control strategies.