Eicosanoid Signaling in Insect Immunology: New Genes and Unresolved Issues

Immune modulation by dexketoprofen trometamol, a selective eicosanoid biosynthesis inhibitor of cellular immune response and phenoloxidase reaction in response to viral infection in Pimpla turionellae adults

Nodulation is the first immune defence mechanism related to melanisation in response to microbial infections in insects. Adult parasitoid insects have been hypothesised to produce nodules with melanisation in response to viral infections and, eicosanoids, to mediate nodulation reactions and phenoloxidase (PO) activation in this type of infections. To test this hypothesis, endoparasitoid Pimpla turionellae adults were first inoculated with a novel generation nonsteroidal anti-inflammatory drug (NSAID) dexketoprofen trometamol (DT) (5 μg/adult), which is a selective cyclooxygenase-1 (COX-1) inhibitor. These adults were then immediately injected with intrahaemocoelic injection of Bovine herpes simplex virus-1 (BHSV-1) as a model insect-virus interaction. Additionally, adults were fed on artificial diet with increasing concentrations of DT (0.001, 0.01, or 0.1 g/100 ml diet) per os prior to intrahaemocoelic injection of BHSV-1 (2 × 103 PFU/adult) and nodulation and PO activity were recorded at 2 h post inoculation (PI). BHSV-1-treated newly emerged adults fed with inhibitors showed low levels of nodulation and increased PO enzyme activity. DT-treated Pimpla adults produced significantly fewer nodules (approximately nine nodules/adult), whereas viral infection provoked nodules (approximately 33 nodules/adult) in comparison with needle (vehicle)-treated controls (approximately five nodules/adult). Increasing dietary dexketoprofen trometamol concentrations decreased nodulation (by 12-fold at the highest concentration) and increased PO reactions (by approximately 3-fold at the highest concentration) to BHSV-1 injection. Compared with control adults, adults orally fed on the lowest DT concentration (0.001 %) significantly increased PO activity (1.22 ± 0.23-2.74 ± 0.31 unit/min/mg protein) while nodules significantly decreased (43.19 ± 4.26-17.84 ± 2.19) in response to virus infections. These findings suggest that eicosanoid biosynthesis, at least in the context of prostaglandins (PGs) formed by COX-1, mediates nodulation reactions and PO activation in response to viral infection in adults of this endoparasitoid. This is the first demonstration that the immune response of P. turionellae adults to viral pathogens is modulated by DT, which initiates haemolymph PO activation.

Apolipoprotein D3 and LOX product play a role in immune-priming of a lepidopteran insect, Spodoptera exigua

Immune-priming occurs in insects after a prior pathogen exposure. However, its underlying mechanism in insects remains elusive. In the present work, immune-priming was detected in a lepidopteran insect, Spodoptera exigua. Specifically, a prior infection with a heat-killed pathogenic bacterium, Escherichia coli, led to increased survival upon the second infection of different pathogens. Plasma collected from larvae with the prior infection possessed the immune-priming factor(s) that significantly up-regulated cellular and humoral immune responses of naïve larvae. Our study also finds that variations in the timing of plasma collection for priming larvae resulted in distinct impacts on both cellular and humoral responses. However, when the active plasma exhibiting the immune-priming was heat-treated, it lost this priming activity, therefore suggesting that protein factor(s) play a role in this immune-priming. An immunofluorescence assay showed that the hemocytes collected from the immune-primed larvae highly reacted to a polyclonal antibody specific to a vertebrate lipocalin, apolipoprotein D (ApoD). Among 27 ApoD genes (Se-ApoD1 ∼ Se-ApoD27) of S. exigua, Se-ApoD3 was found to be highly induced during the immune-priming, in which it was shown to be expressed in hemocytes and fat body from a fluorescence in situ hybridization analysis. RNA interference of Se-ApoD3 expression significantly impaired the immune-priming of S. exigua larvae. Moreover, the inhibition of eicosanoid biosynthesis suppressed the immune-priming, in which treatment with a lipoxygenase (LOX) inhibitor-and not treatment with a cyclooxygenase inhibitor-suppressed immune-priming. Further, an addition of LOX product such as lipoxin A4 or lipoxin B4 significantly rescued the lost immune-priming activity. Taken together, these results suggest that a complex of ApoD3 and LOX product mediates the immune-priming activity of S. exigua.

Polyunsaturated fatty acids stimulate immunity and eicosanoid production in Drosophila melanogaster

Eicosanoids are a class of molecules derived from C20 polyunsaturated fatty acids (PUFAs) that play a vital role in mammalian and insect biological systems, including development, reproduction, and immunity. Recent research has shown that insects have significant but lower levels of C20 PUFAs in circulation in comparison to C18 PUFAs. It has been previously hypothesized in insects that eicosanoids are synthesized from C18 precursors, such as linoleic acid (LA), to produce downstream eicosanoids. In this study, we show that introduction of arachidonic acid (AA) stimulates production of cyclooxygenase, lipoxygenase, and cytochrome P450-derived eicosanoids. Downstream immune readouts showed that LA stimulates phagocytosis by hemocytes, while both LA and AA stimulate increased antimicrobial peptide production when D. melanogaster is exposed to a heat-killed bacterial pathogen. In totality, this work identifies PUFAs that are involved in insect immunity and adds evidence to the notion that Drosophila utilizes immunostimulatory lipid signaling to mitigate bacterial infections. Our understanding of immune signaling in the fly and its analogies to mammalian systems will increase the power and value of Drosophila as a model organism in immune studies.

Role of eicosanoids in insect immunity: new insights and recent advances

Viruses, bacteria, fungus, protozoans, and different metazoan parasites and parasitoids present a constant threat to insects. Insect immunity has two components: humoral and cell mediated. Humoral immunity can be achieved by various antimicrobial proteins, namely, cecropins, sarcotoxin, defensin, attacin, etc. The cell-mediated immunity comprises various cells having immune functions fostering nodulation, phagocytosis, microaggregation, encapsulation etc. Eicosanoids play a crucial role in insect immunity comparable to other animals. The above-mentioned are signaling molecules derived from polyunsaturated fatty acids and they exert numerous physiological effects, namely, inflammation, immune modulation, and regulation of cellular processes. The review article elucidates various roles of eicosanoids, namely, nodulation reaction, Toll signaling pathway, nitric oxide (NO) generation, Ca2+ mobilization, production of reactive oxygen species (ROS), actin polymerization and aquaporin activation. Eicosanoids can function in immune priming in insects drawing hemocytes. An agent named Duox was also identified serving as ROS generator in insect gut. Moreover, role of Repat gene in insect immunity was also studied. However, recently the role of prostacyclin (PGI2) was found to be negative as it inhibits platelet aggregation. In this brief review, we have tried to shed light on the various functions of eicosanoids in immunity of insect those have been discovered recently. This concise study will allow to decipher eicosanoids' function in insect immunity in a nutshell, and it will pave the way for more researches to understand the key players of insect immunity which may eventually help to develop novel vector and pest control strategies in near future.

Enhanced baculoviral virulence by suppressing the degradation of an insect immune resolvin, epoxyoctadecamonoenoic acid, in three lepidopteran insects

Epoxyoctadecamonoenoic acids (EpOMEs) are produced from linoleic acid by a cytochrome P450 monooxygenase (CYP) and play a crucial role in terminating excessive and unnecessary immune responses during the late infection stage in insects. This suggests that an increase in the EpOME level may enhance the virulence of insect pathogens against pests. This study tested this hypothesis using a specific inhibitor against soluble epoxide hydrolase (sEH) to degrade EpOMEs, which leads to elevated endogenous EpOME levels. A baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), was used to infect three different lepidopteran insects (Spodoptera exigua, Maruca vitrata, and Plutella xylostella) by oral feeding or hemocoelic injection treatments. Within one hour, the viral infection induced the expression of three different phospholipase A2 (PLA2) genes and, after 12 h, up-regulated the expressions of CYP and sEH genes in Spodopera exigua. As expected, AcMNPV virulence was suppressed by the addition of arachidonic acid (a catalytic product of PLA2) but was enhanced by the addition of either of the EpOME regioisomers. In addition, treatment with a specific sEH inhibitor (AUDA) increased AcMNPV virulence against three different lepidopteran insects, presumably by increasing endogenous EpOME levels. This enhanced effect of EpOMEs on virulence was further supported by specific RNA interference (RNAi), in which RNAi specific to CYP expression decreased AcMNPV virulence while a specific RNAi against sEH expression significantly enhanced virulence. In response to AcMNPV infection, TUNEL assay results showed that S. exigua larvae exhibited apoptosis in the midgut, fat body, and epidermis. Inhibition of apoptosis by a pan-caspase inhibitor, Z-VAD-FMK, significantly increased virulence. Similarly, the addition of AUDA to the viral treatment suppressed the gene expression of five inducible caspases and cytochrome C to suppress apoptosis, which led to a significant increase in the tissue viral titers. These results indicate that EpOMEs play a role in terminating excessive and unnecessary immune responses against viral infection during the late stage by down-regulating antiviral apoptosis in lepidopteran insects.

Insect immune resolution with EpOME/DiHOME and its dysregulation by their analogs leading to pathogen hypersensitivity

Upon immune challenge, recognition signals trigger insect immunity to remove the pathogens through cellular and humoral responses. Various immune mediators propagate the immune signals to nearby tissues, in which polyunsaturated fatty acid (PUFA) derivatives play crucial roles. However, little was known on how the insects terminate the activated immune responses after pathogen neutralization. Interestingly, C20 PUFA was detected at the early infection stage and later C18 PUFAs were induced in a lepidopteran insect, Spodoptera exigua. This study showed the role of epoxyoctadecamonoenoic acids (EpOMEs) in the immune resolution at the late infection stage to quench the excessive and unnecessary immune responses. In contrast, dihydroxy-octadecamonoenoates (DiHOMEs) were the hydrolyzed and inactive forms of EpOMEs. The hydrolysis is catalyzed by soluble epoxide hydrolase (sEH). Inhibitors specific to sEH mimicked the immunosuppression induced by EpOMEs. Furthermore, the inhibitor treatments significantly enhanced the bacterial virulence of Bacillus thuringiensis against S. exigua. This study proposes a negative control of the immune responses using EpOME/DiHOME in insects.

Four phospholipase A2 genes encoded in the western flower thrips genome and their functional differentiation in mediating development and immunity

Eicosanoids are synthesized from phospholipids by the catalytic activity of phospholipase A2 (PLA2). Even though several PLA2s are encoded in the genome of different insect species, their physiological functions are not clearly discriminated. This study identified four PLA2 genes encoded in the western flower thrips, Frankliniella occidentalis. Two PLA2s (Fo-PLA2C and Fo-PLA2D) are predicted to be secretory while the other two PLA2s (Fo-PLA2A and Fo-PLA2B) are intracellular. All four PLA2 genes were expressed in all developmental stages, of which Fo-PLA2B and Fo-PLA2C were highly expressed in larvae while Fo-PLA2A and Fo-PLA2D were highly expressed in adults. Their expressions in different tissues were also detected by fluorescence in situ hybridization. All four PLA2s were detected in the larval and adult intestines and the ovary. Feeding double-stranded RNAs specific to the PLA2 genes specifically suppressed the target transcript levels. Individual RNA interference (RNAi) treatments led to significant developmental retardation, especially in the treatments specific to Fo-PLA2B and Fo-PLA2D. The RNAi treatments also showed that Fo-PLA2B and Fo-PLA2C expressions were required for the induction of immune-associated genes, while Fo-PLA2A and Fo-PLA2D expressions were required for ovary development. These results suggest that four PLA2s are associated with different physiological processes by their unique catalytic activities and expression patterns.

Ligands of HMG-like dorsal switch protein 1 of Spodoptera exigua leads to mortality in diamondback moth, Plutellaxylostella

HMG-like dorsal switch protein 1 (DSP1) is the insect homolog of the high mobility group box 1 (HMGB1) protein of the vertebrates. Previous studies confirmed DSP1 in Spodoptera exigua, Tenebrio molitor, and Aedes albopictus, and were analyzed for their immune roles, survivability, and binding affinity with entomopathogenic bacterial metabolites. The present study aimed to predict, and confirm DSP1 in diamondback moth, Plutella xylostella along with the effect of Spodoptera exigua DSP1 ligands in the survivability of this insect. DSP1 of Plutella xylostella (Px-DSP1) consists of 465 amino acids (AA). Phylogeny analysis showed that Px-DSP1 clustered with other Lepidopteran insects where each insect order clustered separately. Domain analysis showed that like other insects, Px-DSP1 contains two HMG boxes (Box A and Box B), one coiled-coil (CC), five Q-rich low complexity (LC), and an acidic tail (AT). Px-DSP1 was expressed in each developmental stage and tissue. The highest expression was in L4 larvae and fat body tissues. Thermal shift assay (TSA) showed the binding affinity of 3-Ethoxy-4-Methoxyphenol (EMP), Phthalimide (PM), and o-Cyanobenzoic acid (CBA) to rDSP1 of Spodoptera exigua. Mortality bioassay showed that all these metabolites were toxic against P. xylostella larvae. Among these, EMP was more toxic providing more than 65% mortality at 500 ppm concentration. However, PM and CBA also showed more than 60 and 50% mortality, respectively at 500 ppm concentration. We assume that like Se-DSP1, these compounds also bind with Px-DSP1 which leads to the inhibition of DSP1-mediated immunity and impose the mortality of Plutella xylostella larvae.

Enhancement of an entomopathogenic fungal virulence against the seedcorn maggot, Delia platura, by suppressing immune responses with a bacterial culture broth of Photorhabdus temperata subsp. temperata

In Korea, there are two maggot species in the Delia genus that commonly infest the roots and stems of the Welsh onion, thus causing serious economic damage on the crop at the seedling stage. In this study, the seedcorn maggot (Delia platura) was detected in onion fields in two different localities in Korea. After overwintering, maggot infestations occurred throughout the entire growing seasons from transplantation to harvest, but their specific patterns of occurrence varied in the two localities examined. Entomopathogenic fungi induced significant virulence against the maggot larvae, in which a strain of Beauveria bassiana was effective, though it exhibited limited mortality in its insecticidal activity. To enhance this insecticidal activity, a culture broth from an entomopathogenic bacterium, Photorhabdus temperata temperata (Ptt), was added to B. bassiana treatment. The addition of Ptt broth significantly increased the insecticidal activity of B. bassiana in a dose-dependent manner. To elucidate this enhancement in insecticidal activity, the immunosuppressive activity of Ptt broth was assessed by identifying the immune responses of the seedcorn maggots. The seedcorn maggots possessed at least three different hemocytes with plasmatocytes, crystal cells, and lamellocytes. These hemocytes exhibited nodule formation in response to the fungal infection. In addition to the cellular immunity, the maggots exhibited inducible expressions of antimicrobial peptide (AMP) genes such as cecropin and defensin. The addition of Ptt broth suppressed the nodule formation and the AMP expressions in response to the fungal infection. Altogether, this study demonstrated the innate immune responses in a non-model insect, D. platura, along with the application of immunosuppression to develop a highly efficient biological control by enhancing the virulence of B. bassiana.

Phospholipase A2 activity is required for immune defense of European (Apis mellifera) and Asian (Apis cerana) honeybees against American foulbrood pathogen, Paenibacillus larvae

Honeybees require an efficient immune system to defend against microbial pathogens. The American foulbrood pathogen, Paenibacillus larvae, is lethal to honeybees and one of the main causes of colony collapse. This study investigated the immune responses of Apis mellifera and Apis cerana honeybees against the bacterial pathogen P. larvae. Both species of honeybee larvae exhibited significant mortality even at 102 103 cfu/mL of P. larvae by diet-feeding, although A. mellifera appeared to be more tolerant to the bacterial pathogen than A. cerana. Upon bacterial infection, the two honeybee species expressed both cellular and humoral immune responses. Hemocytes of both species exhibited characteristic spreading behaviors, accompanied by cytoskeletal extension along with F-actin growth, and formed nodules. Larvae of both species also expressed an antimicrobial peptide called apolipophorin III (ApoLpIII) in response to bacterial infection. However, these immune responses were significantly suppressed by a specific inhibitor to phospholipase A2 (PLA2). Each honeybee genome encodes four PLA2 genes (PLA2A ~ PLA2D), representing four orthologous combinations between the two species. In response to P. larvae infection, both species significantly up-regulated PLA2 enzyme activities and the expression of all four PLA2 genes. To determine the roles of the four PLA2s in the immune responses, RNA interference (RNAi) was performed by injecting gene-specific double stranded RNAs (dsRNAs). All four RNAi treatments significantly suppressed the immune responses, and specific inhibition of the two secretory PLA2s (PLA2A and PLA2B) potently suppressed nodule formation and ApoLpIII expression. These results demonstrate the cellular and humoral immune responses of A. mellifera and A. cerana against P. larvae. This study suggests that eicosanoids play a crucial role in mediating common immune responses in two closely related honeybees.

Evolution of the Major Components of Innate Immunity in Animals

Innate immunity is present in all animals. In this review, we explore the main conserved mechanisms of recognition and innate immune responses among animals. In this sense, we discuss the receptors, critical for binding to pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs); the downstream signaling proteins; and transcription factors that govern immune responses. We also highlight conserved inflammatory mediators that are induced after the recognition of DAMPs and PAMPs. At last, we discuss the mechanisms that are involved in the regulation and/or generation of reactive oxygen species (ROS), influencing immune responses, like heme-oxygenases (HOs).

Indomethacin and 20-hydroxyecdysone influence protein expression in a Spodoptera frugiperda nervous system cell line

Insecticide mode of action studies provide insights into how new insecticidal actives function and contribute to assessing safety to humans and nontarget organisms. Insect cell lines that express potential target sites can serve as valuable tools in this effort. In this paper, we report on the influence of two signaling molecules on protein expression in a nervous system cell line established from Spodoptera frugiperda (Bayer/BCIRL-SfNS2-0714-TR). We selected this line because we established it in our laboratory and we are experienced in using it. Cells were exposed to the insect developmental hormone (1 µg/mL 20-hydroxyecdysone, 20E) and/or a cyclooxygenase (COX) inhibitor (25 μM indomethacin, INDO; inhibits prostaglandin [PG] biosynthesis) for 24 h (Day 2), 72 h (Day 4), or 120 h (Day 6). We selected a PG biosynthesis inhibitor because PGs act in many aspects of insect biology, such as embryonic development, immunity, and protein phosphorylation. We selected the developmental hormone, 20E, because it also acts in fundamental aspects of insect biology. We identified specific proteins via in silico analysis. Changes in protein expression levels were determined using liquid chromatography-mass spectrometry (MS) + MS-MS. The largest number of changes in protein expression occurred on Day 2. The combination of 20E plus INDO led to 222 differentially expressed proteins, which documents the deep significance of PGs and 20E in insect biology. 20E and, separately, INDO led to changes in 30 proteins each (p value < 0.01; >2X or <0.5X-fold changes). We recorded changes in the expression of 9 or 12 proteins (20E), 10 or 6 proteins (INDO), and 21 or 20 proteins (20E + INDO) on D4 and D6, respectively. While the cell line was established from neuronal tissue, the differentially expressed proteins act in a variety of fundamental cell processes. In this paper, we moved beyond a list of proteins by providing detailed, Gene Ontology term analyses and enrichment, which offers an in-depth understanding of the influence of these treatments on the SfNS2 cells. Because proteins are active components of cell physiology in their roles as enzymes, receptors, elements of signaling transduction pathways, and cellular structures, changes in their expression levels under the influence of signaling molecules provide insights into their function in insect cell physiology.

Screening of insect immune suppressors using a recombinant phospholipase A2 of a lepidopteran insect

Phospholipase A2 (PLA2 ) catalyzes phospholipids at the sn-2 position to release free fatty acids, including arachidonic acid (AA) or its precursor. The free AA is then oxygenated into different eicosanoids, which mediate the diverse physiological processes in insects. Any inhibition of the PLA2 catalysis would give rise to serious malfunctioning in insect growth and development. An onion moth, Acrolepiopsis sapporensis, encodes four different PLA2 genes (As-PLA2 A-As-PLA2 D), in which As-PLA2 A is dominantly expressed at all developmental stages and in different larval tissues. RNA interference of the As-PLA2 A expression significantly reduced the PLA2 activity of A. sapporensis, which suffered from immunosuppression. A recombinant As-PLA2 A protein was purified from a bacterial expression system, which exhibited a typical Michaelis-Menten kinetics and hence susceptible to a specific inhibitor to sPLA2 and dithiothreitol. A total of 19 bacterial metabolites derived from Xenorhabdus and Photorhabdus were screened against the recombinant As-PLA2 A. Five potent metabolites were highly inhibitory and followed a competitive enzyme inhibition. These five inhibitors suppressed the immune responses of A. sapporensis by inhibiting hemocyte-spreading behavior and phenoloxidase activity. However, an addition of AA could significantly rescue the immunosuppression induced by the selected inhibitors. These studies suggest that the recombinant As-PLA2 A protein can be applied for high-throughput screening of insect immunosuppressive compounds.

Trade-Offs among Immune Mechanisms: Bacterial-Challenged Spodoptera frugiperda Larvae Reduce Nodulation Reactions during Behavioral Fever

Insect innate immunity is composed of cellular and humoral reactions, the former acting via circulating hemocytes and the latter via immune signaling that lead to the production of antimicrobial peptides and phenol oxidase-driven melanization. Cellular immunity involves direct interactions between circulating hemocytes and invaders; it includes internalization and killing microbes (phagocytosis) and formation of bacterial-laden microaggregates which coalesce into nodules that are melanized and attached to body walls or organs. Nodulation can entail investing millions of hemocytes which must be replaced. We hypothesized that biologically costly hemocyte-based immunity is traded off for behavioral fevers in infected larvae of fall armyworms, Spodoptera frugiperda, that were allowed to fever. We tested our hypothesis by infecting larvae with the Gram-negative bacterium, Serratia marcescens, placing them in thermal gradients (TGs) and recording their selected body temperatures. While control larvae selected about 30 °C, the experimental larvae selected up 41 °C. We found that 4 h fevers, but not 2, 6 or 24 h fevers, led to increased larval survival. Co-injections of S. marcescens with the prostaglandin (PG) biosynthesis inhibitor indomethacin (INDO) blocked the fevers, which was reversed after co-injections of SM+INDO+Arachidonic acid, a precursor to PG biosynthesis, confirming that PGs mediate fever reactions. These and other experimental outcomes support our hypothesis that costly hemocyte-based immunity is traded off for behavioral fevers in infected larvae under appropriate conditions.

Prostaglandin E2 mediates chorion formation of the Asian tiger mosquito, Aedes albopictus, at late oogenesis

Chorion-i.e., the eggshell-is formed during the late stage of oogenesis by follicular epithelium in the ovary. Although the endocrine signal(s) driving choriogenesis remain unclear in mosquitoes, this process in other insects has been suspected to involve the mediation of prostaglandins (PGs). This study tested the role of PG in the choriogenesis of the Asian tiger mosquito, Aedes albopictus, and its influence on controlling the expressions of genes associated with chorion formation by a transcriptome analysis. An immunofluorescence assay showed that PGE2 is localised in follicular epithelium. With the treatment of aspirin, an inhibitor of PG biosynthesis, at mid oogenesis, the PGE2 signal disappeared in the follicular epithelium led to significantly inhibited chorion formation along with a malformed eggshell. Ovary transcriptomes were assessed by RNASeq at the mid and late ovarian developmental stages. Differentially expressed genes (DEGs) exhibiting more than twofold changes in expression levels included 297 genes at mid stage and 500 genes at late stage. These DEGs at these two developmental stages commonly included genes associated with egg and chorion proteins of Ae. albopictus. Most chorion-associated genes were clustered in the 168 Mb region on a chromosome and exhibited significantly induced expressions at both ovarian developmental stages. The inhibition of PG biosynthesis significantly suppressed the expression of the chorion-associated genes while the addition of PGE2 rescued the gene expressions and led to recovery of choriogenesis. These results suggest that PGE2 mediates the choriogenesis of Ae. albopictus.

Insect immune resolution with EpOME/DiHOME and its dysregulation by their analogs leading to pathogen hypersensitivity

Epoxyoctadecamonoenoic acids (EpOMEs) are epoxide derivatives of linoleic acid (9,12-octadecadienoic acid: LA). They are metabolized into dihydroxyoctadecamonoenoic acids (DiHOMEs) in mammals. Unlike in mammals where they act as adipokines or lipokines, EpOMEs act as immunosuppressants in insects. However, the functional link between EpOMEs and pro-immune mediators such as PGE2 is not known. In addition, the physiological significance of DiHOMEs is not clear in insects. This study analyzed the physiological role of these C18 oxylipins using a lepidopteran insect pest, Spodoptera exigua. Immune challenge of S. exigua rapidly upregulated the expression of the phospholipase A2 gene to trigger C20 oxylipin biosynthesis, followed by the upregulation of genes encoding EpOME synthase (SE51385) and a soluble epoxide hydrolase (Se-sEH). The sequential gene expression resulted in the upregulations of the corresponding gene products such as PGE2, EpOMEs, and DiHOMEs. Interestingly, only PGE2 injection without the immune challenge significantly upregulated the gene expression of SE51825 and Se-sEH. The elevated levels of EpOMEs acted as immunosuppressants by inhibiting cellular and humoral immune responses induced by the bacterial challenge, in which 12,13-EpOME was more potent than 9,10-EpOME. However, DiHOMEs did not inhibit the cellular immune responses but upregulated the expression of antimicrobial peptides selectively suppressed by EpOMEs. The negative regulation of insect immunity by EpOMEs and their inactive DiHOMEs were further validated by synthetic analogs of the linoleate epoxide and corresponding diol. Furthermore, inhibitors specific to Se-sEH used to prevent EpOME degradation significantly suppressed the immune responses. The data suggest a physiological role of C18 oxylipins in resolving insect immune response. Any immune dysregulation induced by EpOME analogs or sEH inhibitors significantly enhanced insect susceptibility to the entomopathogen, Bacillus thuringiensis.

Identification of four secretory phospholipase A2s in a lepidopteran insect, Acrolepiopsis sapporensis, and their functional association with cellular immune responses

Background

Eicosanoids are a group of the oxygenated C20 polyunsaturated fatty acids and play crucial roles in mediating various insect physiological processes. Catalytic activity of phospholipase A₂ (PLA₂) provides an initial substrate, arachidonic acid (AA), for subsequent eicosanoid biosynthesis.

Results

This study identified four different secretory PLA₂ (As-PLA₂A-As-PLA₂D) genes encoded in the Asian onion moth, Acrolepiopsis sapporensis. A phylogenetic analysis indicated that As-PLA₂A and As-PLA₂D are clustered with Group III PLA₂s while As-PLA₂B and As-PLA₂C are clustered with Group XII and Group X PLA₂s, respectively. Expression levels of these PLA₂ genes increased along with larval development, especially in the fat body. A bacterial immune challenge upregulated the basal expression levels of the four PLA₂ genes, which resulted in significant increases of the PLA₂ enzyme activity. The enzyme activity was susceptible to a calcium chelator or reducing agent, suggesting Ca²⁺ dependency and disulfide linkage required for the catalytic activities of the secretory type of PLA₂s. In addition, the PLA₂ activity was also susceptible to bromophenacyl bromide (BPB), a specific inhibitor to sPLA₂, but not to intracellular PLA₂ inhibitors. An addition of BPB to the immune challenge significantly prevented hemocyte-spreading behavior of A. sapporensis. BPB treatment also suppressed a cellular immune response measured by hemocyte nodule formation. However, the immunosuppression was significantly rescued by the AA addition. To determine the PLA₂(s) responsible for the immunity, individual RNA interference (RNAi) treatments specific to each of the four PLA₂s were performed. Injection of gene-specific double-stranded RNAs caused significant reductions in the transcript level in all four PLA₂s. In all four PLA₂s, the RNAi treatments prevented the cellular immune response even after the immune challenge.

Conclusion

This study reports four secretory PLA₂s encoded in A. sapporensis and their function in mediating cellular immunity.

Lipids as a key element of insect defense systems

The relationship between insect pathogenic fungi and their insect hosts is a classic example of a co-evolutionary arms race between pathogen and target host: parasites evolve towards mechanisms that increase their advantage over the host, and the host increasingly strengthens its defenses. The present review summarizes the literature data describing the direct and indirect role of lipids as an important defense mechanism during fungal infection. Insect defense mechanisms comprise anatomical and physiological barriers, and cellular and humoral response mechanisms. The entomopathogenic fungi have the unique ability to digest the insect cuticle by producing hydrolytic enzymes with chitin-, lipo- and proteolytic activity; besides the oral tract, cuticle pays the way for fungal entry within the host. The key factor in insect resistance to fungal infection is the presence of certain types of lipids (free fatty acids, waxes or hydrocarbons) which can promote or inhibit fungal attachment to cuticle, and might also have antifungal activity. Lipids are considered as an important source of energy, and as triglycerides are stored in the fat body, a structure analogous to the liver and adipose tissue in vertebrates. In addition, the fat body plays a key role in innate humoral immunity by producing a range of bactericidal proteins and polypeptides, one of which is lysozyme. Energy derived from lipid metabolism is used by hemocytes to migrate to the site of fungal infection, and for phagocytosis, nodulation and encapsulation. One polyunsaturated fatty acid, arachidonic acid, is used in the synthesis of eicosanoids, which play several crucial roles in insect physiology and immunology. Apolipoprotein III is important compound with antifungal activity, which can modulate insect cellular response and is considered as important signal molecule.

Genome-wide identification, classification, and expression profiling of serine esterases and other esterase-related proteins in the tobacco hornworm, Manduca sexta

Serine esterases (SEs) are hydrolases that catalyze the conversion of carboxylic esters into acids and alcohols. Lipases and carboxylesterases constitute two major groups of SEs. Although over a hundred of insect genomes are known, systematic identification and classification of SEs are rarely performed, likely due to large size and complex composition of the gene family in each species. Considering their key roles in lipid metabolism and other physiological processes, we have categorized 144 M. sexta SEs and SE homologs (SEHs), 114 of which contain a motif of GXSXG. Multiple sequence alignment and phylogenetic tree analysis have revealed 39 neutral lipases (NLs), 3 neutral lipase homologs (NLHs), 11 acidic lipases (ALs), 3 acidic lipase homologs (ALHs), a lipase-3, a triglyceride lipase, a monoglyceride lipase, a hormone-sensitive lipase, and a GDSL lipase. Eighty-three carboxylesterase genes encode 29 α-esterases (AEs), 12 AEHs (e.g., SEH4-1-3), 20 feruloyl esterases (FEs), 2 FEHs, 2 β-esterases (BEs), 2 integument esterases (IEs), 1 IEH, 4 juvenile hormone esterases, 2 acetylcholinesterases, gliotactin, 6 neuroligins, neurotactin, and an uncharacteristic esterase homolog. In addition to these GXSXG proteins, we have identified 26 phospholipases and 13 thioesterases. Expression profiling of these genes in specific tissues and stages has provided insights into their functions including digestion, detoxification, hormone processing, neurotransmission, reproduction, and developmental regulation. In summary, we have established a framework of information on SEs and related proteins in M. sexta to stimulate their research in the model species and comparative investigations in agricultural pests or disease vectors.

A soil fungus confers plant resistance against a phytophagous insect by disrupting the symbiotic role of its gut microbiota

Plants generate energy flows through natural food webs, driven by competition for resources among organisms, which are part of a complex network of multitrophic interactions. Here, we demonstrate that the interaction between tomato plants and a phytophagous insect is driven by a hidden interplay between their respective microbiotas. Tomato plants colonized by the soil fungus Trichoderma afroharzianum, a beneficial microorganism widely used in agriculture as a biocontrol agent, negatively affects the development and survival of the lepidopteran pest Spodoptera littoralis by altering the larval gut microbiota and its nutritional support to the host. Indeed, experiments aimed to restore the functional microbial community in the gut allow a complete rescue. Our results shed light on a novel role played by a soil microorganism in the modulation of plant-insect interaction, setting the stage for a more comprehensive analysis of the impact that biocontrol agents may have on ecological sustainability of agricultural systems.

Beyond Cellular Immunity: On the Biological Significance of Insect Hemocytes

Insect immunity is assorted into humoral and cellular immune reactions. Humoral reactions involve the regulated production of anti-microbial peptides, which directly kill microbial invaders at the membrane and intracellular levels. In cellular immune reactions, millions of hemocytes are mobilized to sites of infection and replaced by hematopoiesis at a high biological cost after the immune defense. Here, we considered that the high biological costs of maintaining and replacing hemocytes would be a better investment if hemocytes carried out meaningful biological actions unrelated to cellular immunity. This idea allows us to treat a set of 10 hemocyte actions that are not directly involved in immunity, some of which, so far, are known only in Drosophila melanogaster. These include (1) their actions in molting and development, (2) in surviving severe hypoxia, (3) producing phenoloxidase precursor and its actions beyond immunity, (4) producing vitellogenin in a leafhopper, (5) recognition and responses to cancer in Drosophila, (6) non-immune actions in Drosophila, (7) clearing apoptotic cells during development of the central nervous system, (8) developing hematopoietic niches in Drosophila, (9) synthesis and transport of a lipoprotein, and (10) hemocyte roles in iron transport. We propose that the biological significance of hemocytes extends considerably beyond immunity.

PGE2 mediation of egg development in Western flower thrip, Frankliniella occidentalis

Eicosanoids mediate various insect physiological processes, including reproduction. Especially, the eicosanoid prostaglandin E₂  (PGE₂ ) is known to mediate oocyte development in some insects. The explosive reproductive potential of the Western flower thrips, Frankliniella occidentalis, damages various agricultural crops. However, little is known about the underlying physiological processes of egg development in this pest. This study found that treatment with aspirin (ASP) (a specific cyclooxygenase (COX) inhibitor) used to inhibit PGE₂ biosynthesis during ovarian development significantly suppressed the reproduction of female F. occidentalis. However, the addition of PGE₂ to ASP-treated females significantly rescued the suppressed reproduction. PGE₂ was detected in growing ovarian follicles in an immunofluorescence assay. The hypothetical biosynthetic machinery of PGE₂ was predicted from the F. occidentalis genome and included phospholipase A₂  (PLA₂ ), COX-like peroxidase (POX), and PGE₂ synthase (PGES). Three specific PLA₂ s were highly expressed in female adults during active oogenesis. Specific POX and PGES genes also showed high expression during active oogenesis. The adverse effect of ASP treatment on oogenesis was observed in follicle formation in the germarium where the follicle numbers in an ovariole were decreased, which resulted in hypotrophied ovaries. This impairment was rescued by the addition of PGE₂ . ASP treatment also significantly inhibited chorion formation and suppressed gene expression associated with choriogenesis, which included chorion protein, mucin, and yellow while it did not inhibit vitellogenin gene expression. However, the addition of PGE₂ induced the expression of the target genes suppressed by ASP treatment and rescued chorion formation. These results suggest that PGE₂ mediated ovarian development by affecting follicle formation and choriogenesis in F. occidentalis.

Damage signal induced by Bacillus thuringiensis infection triggers immune responses via a DAMP molecule in lepidopteran insect, Spodoptera exigua

Insect immunity defends the infection of an insect pathogenic bacterium, Bacillus thuringiensis (Bt). However, it was not clear on the recognition of Bt infection by the insect immune system. This study tested a physiological function of dorsal switch protein 1 (DSP1) in the Bt infection. DSP1 is classified into HMGB1-like damage-associated molecular pattern (DAMP) in insects. Upon Bt infection in a lepidopteran Spodoptera exigua, DSP1 was released from the nuclei of the midgut epithelium and activated immune responses. For this DSP1 release, a functional binding between Bt and its receptors on the midgut epithelium was required because any RNA interference (RNAi) treatments of Bt receptor (cadherin or ABCC) prevented the DSP1 release and became susceptible to the bacterial infection. The DSP1 release was required for the gene induction of Repat33, which is a member of response to pathogen gene family and its gene product mediated cellular and humoral immune responses against pathogen infection in S. exigua. The released DSP1 activated phospholipase A₂ (PLA₂) to produce eicosanoids, which induced the Repat33 expression because a hemocoelic injection of a recombinant DSP1 induced the Repat33 expression without Bt infection. However, any inhibition of PLA₂ activity impaired the DAMP signaling between DSP1 and Repat33. DSP1 also up-regulated two other immune mediators, nitric oxide (NO) and a cytokine called plasmatocyte-spreading peptide (PSP). Either NO or PSP activated PLA₂ to up-regulate Repat33 expression. These results suggest that Bt infection of the insect midgut generates a DAMP signal via DSP1 release, which turns on NO or the cytokine-PLA₂-Repat33 immune signaling pathway.

Cannabidiol (CBD) Supports the Honeybee Worker Organism by Activating the Antioxidant System

In the experiment, we tested the effect of 30% CBD oil on the activity of the antioxidant system (superoxide dismutase, catalase, glutathione peroxidase, glutathione), the level of total antioxidant capacity, and the concentrations of ions (calcium, magnesium, and phosphorus) in honeybee workers in the hive test. For this purpose, we prepared hives containing all stages of the development of honey bees and started the experiment by adding 200 marked, one-day old bees to each colony (intended for hemolymph collection). In the test, we created three groups (two colonies per group): (1) Experimental with CBD oil mixed with sugar syrup (CSy); (2) experimental with CBD oil on textile strips (CSt); and (3) control with pure sugar syrup only (C). Every week, we collected hemolymph from the marked bees. In the experiment, all antioxidant enzyme activities were higher for the experimental groups CSy and CSt compared to group C. The highest concentrations/levels were obtained for the CSy group. Concentrations of calcium, magnesium, and phosphorus ions were also higher for the experimental groups compared to the C group (the highest concentration for the CSy group). We conclude that CBD oil positively contributes to stimulating the antioxidant system of honeybees.

Immune Stimulation via Wounding Alters Chemical Profiles of Adult Tribolium castaneum

Group-living individuals experience immense risk of disease transmission and parasite infection. In social and in some non-social insects, disease control with immunomodulation arises not only via individual immune defenses, but also via infochemicals such as contact cues and (defensive) volatiles to mount a group-level immunity. However, little is known about whether activation of the immune system elicits changes in chemical phenotypes, which may mediate these responses. We here asked whether individual immune experience resulting from wounding or injection of heat-killed Bacillus thuringiensis (priming) leads to changes in the chemical profiles of female and male adult red flour beetles, Tribolium castaneum, which are non-social but gregarious. We analyzed insect extracts using GC-FID to study the chemical composition of (1) cuticular hydrocarbons (CHCs) as candidates for the transfer of immunity-related information between individuals via contact, and (2) stink gland secretions, with analysis of benzoquinones as main active compounds regulating 'external immunity'. Despite a pronounced sexual dimorphism in CHC profiles, wounding stimulation led to similar profile changes in males and females with increases in the proportion of methyl-branched alkanes compared to naïve beetles. While changes in the overall secretion profiles were less pronounced, absolute amounts of benzoquinones were transiently elevated in wounded compared to naïve females. Responses to priming were insignificant in CHCs and secretions. We suggest that changes in different infochemicals after wounding may mediate immune status signaling in the context of both internal and external immune responses in groups of this non-social insect, thus showing parallels to social immunity.

Aspirin Inhibition of Prostaglandin Synthesis Impairs Mosquito Egg Development

Several endocrine signals mediate mosquito egg development, including 20-hydroxyecdysone (20E). This study reports on prostaglandin E₂ (PGE₂) as an additional, but core, mediator of oogenesis in a human disease-vectoring mosquito, Aedes albopictus. Injection of aspirin (an inhibitor of cyclooxygenase (COX)) after blood-feeding (BF) inhibited oogenesis by preventing nurse cell dumping into a growing oocyte. The inhibitory effect was rescued by PGE₂ addition. PGE₂ was found to be rich in nurse cells and follicular epithelium after BF. RNA interference (RNAi) treatments of PG biosynthetic genes, including PLA₂ and two COX-like peroxidases, prevented egg development. Interestingly, 20E treatment significantly increased the expressions of PG biosynthetic genes, while the RNAi of Shade (which is a 20E biosynthetic gene) expression prevented inducible expressions after BF. Furthermore, RNAi treatments of PGE₂ receptor genes suppressed egg production, even under PGE₂. These results suggest that a signaling pathway of BF-20E-PGE₂ is required for early vitellogenesis in the mosquito.

Dorsal switch protein 1 as a damage signal in insect gut immunity to activate dual oxidase via an eicosanoid, PGE2

Various microbiota including beneficial symbionts reside in the insect gut. Infections of pathogens cause dysregulation of the microflora and threaten insect survival. Reactive oxygen species (ROS) have been used in the gut immune responses, in which its production is tightly regulated by controlling dual oxidase (Duox) activity via Ca2+ signal to protect beneficial microflora and gut epithelium due to its high cytotoxicity. However, it was not clear how the insects discriminate the pathogens from the various microbes in the gut lumen to trigger ROS production. An entomopathogenic nematode (Steinernema feltiae) infection elevated ROS level in the gut lumen of a lepidopteran insect, Spodoptera exigua. Dorsal switch protein 1 (DSP1) localized in the nucleus in the midgut epithelium was released into plasma upon the nematode infection and activated phospholipase A2 (PLA2). The activated PLA2 led to an increase of PGE2 level in the midgut epithelium, in which rising Ca2+ signal up-regulated ROS production. Inhibiting DSP1 release by its specific RNA interference (RNAi) or specific inhibitor, 3-ethoxy-4-methoxyphenol, treatment failed to increase the intracellular Ca2+ level and subsequently prevented ROS production upon the nematode infection. A specific PLA2 inhibitor treatment also prevented the up-regulation of Ca2+ and subsequent ROS production upon the nematode infection. However, the addition of PGE2 to the inhibitor treatment rescued the gut immunity. DSP1 release was not observed at infection with non-pathogenic pathogens but detected in plasma with pathogenic infections that would lead to damage to the gut epithelium. These results indicate that DSP1 acts as a damage-associated molecular pattern in gut immunity through DSP1/PLA2/Ca2+/Duox.

Integrated Biological Control Using a Mixture of Two Entomopathogenic Bacteria, Bacillus thuringiensis and Xenorhabdus hominickii, against Spodoptera exigua and Other Congeners

Insect immunity defends against the virulence of various entomopathogens, including Bacillus thuringiensis (Bt). This study tested a hypothesis that any suppression of immune responses enhances Bt virulence. In a previous study, the entomopathogenic bacterium, Xenorhabdus hominickii (Xh), was shown to produce secondary metabolites to suppress insect immune responses. Indeed, the addition of Xh culture broth (XhE) significantly enhanced the insecticidal activity of Bt against S. exigua. To analyze the virulence enhanced by the addition of Xh metabolites, four bacterial secondary metabolites were individually added to the Bt treatment. Each metabolite significantly enhanced the Bt insecticidal activity, along with significant suppression of the induced immune responses. A bacterial mixture was prepared by adding freeze-dried XhE to Bt spores, and the optimal mixture ratio to kill the insects was determined. The formulated bacterial mixture was applied to S. exigua larvae infesting Welsh onions in a greenhouse and showed enhanced control efficacy compared to Bt alone. The bacterial mixture was also effective in controlling other Spodopteran species such as S. litura and S. frugiperda but not other insect genera or orders. This suggests that Bt+XhE can effectively control Spodoptera-associated pests by suppressing the immune defenses.

Cellular immune responses of the yellow peach moth, Conogethes punctiferalis (Lepidoptera: Crambidae), to the entomopathogenic fungus, Beauveria bassiana (Hypocreales: Cordycipitaceae)

The yellow peach moth (YPM), Conogethes punctiferalis, is a destructive insect pest of maize in eastern China and adapts to diverse environments, especially against pathogens. In insects, innate immunity comprising both humoral and cellular defense responses, is the primary defense against invading microbial pathogens. In this study, we identified five types of circulating hemocytes from the hemolymph of YPM larvae and analyzed their alterations and functions in immune responses to the infection of Beauveria bassiana, an entomopathogenic fungus infesting many lepidopteran species. The identified hemocytes included prohemocytes, plasmatocytes, granulocytes, spherulocytes and oenocytoids. Significant decreases of total and differential hemocyte counts were recorded over time in larvae, after they were injected with B. bassiana conidia. Additionally, hemocyte-mediated phagocytosis and nodulation were initiated in the hemolymph of larvae from the B. bassiana conidia challenge. The introduction of DEAE-Sepharose Fast Flow beads stained with Congo red also induced a strong encapsulation response in the larval hemolymph. Our observations unravel the occurrence of phagocytosis, nodulation and encapsulation in the hemocoel of YPM larvae to fight against the fungal infection, and offer the first insight into the YPM immune system.

Immune Function of Endothelial Cells: Evolutionary Aspects, Molecular Biology and Role in Atherogenesis

Atherosclerosis is one of the key problems of modern medicine, which is due to the high prevalence of atherosclerotic cardiovascular diseases and their significant share in the structure of morbidity and mortality in many countries. Atherogenesis is a complex chain of events that proceeds over many years in the vascular wall with the participation of various cells. Endothelial cells are key participants in vascular function. They demonstrate involvement in the regulation of vascular hemodynamics, metabolism, and innate immunity, which act as leading links in the pathogenesis of atherosclerosis. These endothelial functions have close connections and deep evolutionary roots, a better understanding of which will improve the prospects of early diagnosis and effective treatment.

The larval saliva of an endoparasitic wasp, Pteromalus puparum, suppresses host immunity

In the lengthy co-evolution between insects and their animal or plant hosts, insects have evolved a wide range of salivary strategies to help evade host defenses. Although there is a very large literature on saliva of herbivorous and hematophagous insects, little attention has been focused on the saliva of parasitoid wasps. Some parasitoid species are natural enemies that effectively regulate insect population sizes in nature that they are applied for biological control of agricultural pests. Here, we demonstrate the influence of the endoparasitoid, Pteromalus puparum, larval saliva on the cellular and humoral immunity of its host. Larval saliva increases mortality of hemocytes, and inhibits hemocyte spreading, a specific cellular immune action. We report that high saliva concentrations inhibit host cellular encapsulation of foreign invaders. The larval saliva also inhibits melanization in host hemolymph. The saliva inhibits the growth of some bacterial species, Bacillus subtilis, Staphylococcus aureus and Pseudomonas aeruginosa in vitro. This may promote larvae fitness by protecting them from infections. Insight into such functions of parasitic wasp saliva provides a new insight into host-parasitoid relationships and possibly leads to new agricultural pest management technologies.

Phurealipids, produced by the entomopathogenic bacteria, Photorhabdus, mimic juvenile hormone to suppress insect immunity and immature development

Phurealipids (Photorhabdus urea lipids) are synthesized from Photorhabdus bacteria that are symbiotic to entomopathogenic nematodes. Their chemical structures are similar to that of juvenile hormone (JH) and have been suspected to mimic JH signaling in immunity and the development of insects. This study investigated the physiological roles of phurealipids with respect to their contribution to bacterial pathogenicity using four natural (HB13, HB69, HB416, and HB421) and one derivative (HB27) compound. First, phurealipids like JH suppressed insect immune responses. Overall, phurealipids showed JH like immunosuppressive behavior in a lepidopteran insect Spodoptera exigua larvae. More specifically, phurealipids significantly suppressed the hemocyte spreading behavior which is a key immune response upon immune challenge. Interestingly, the methyl urea derivatives (HB13, HB27, and HB69) were more potent than the unmethylated forms (HB416 and HB421). The inhibitory activity of phurealipids prevented the cellular immune response measured by hemocytic nodule formation in response to the bacterial challenge. Phurealipids also suppressed the expression of cecropin and gallerimycin, which are two highly inducible antimicrobial peptides, in S. exigua upon immune challenge. The immunosuppressive activity of the phurealipids significantly enhanced the bacterial pathogenicity of Bacillus thuringiensis against S. exigua. Second, phurealipids like JH prevented insect metamorphosis. Especially, the methylated urea derivatives of the phurealipids showed the JH-like function by inducing the expression of S. exigua Kr-h1, a transcriptional factor. At the pupal stage, exhibiting the lowest expression of Kr-h1, phurealipid treatments elevated the expression level of Kr-h1 and delayed the pupa-to-adult metamorphosis. These results suggest that phurealipids play crucial roles in Photorhabdus pathogenicity by suppressing host immune defenses and delaying host metamorphosis.

Identification and function analysis of BmPxtA in the immune response regulated by PGE2 of silkworm, Bombyx mori

Prostaglandins (PGs) can mediate the immune response of insects to infection. Mammalian cyclooxygenase (COXs) is a key enzyme in the synthesis of PGs, and Pxt may be its homologous gene in some sequenced insect genomes. As a representative of Lepidoptera, the silkworm also contains PGs, but the biosynthetic source of PGs is still unclear. In this study, Sequence analysis showed that peroxinectin (BmPxtA) gene of silkworm was closely related to human COX gene, and its homologous protein had conserved domains corresponding to human COX. The expression of BmPxtA gene was the highest in the hemocytes and was induced by Nuclear Polyhedrosis Virus (NPV) challenge in the detected tissues. The quantitative polymerase chain reaction (qPCR) results showed that silencing BmPxtA mediated by RNA interference (RNAi) inhibited the expression of immune-related pathway genes, and specifically suppressed hemocyte-spreading and nodule formation in silkworm; Hemocyte-spreading and nodule formation were also inhibited by aspirin, a COX inhibitor. Treatment by PGE₂ but not arachidonic acid (AA) rescued the immunosuppression; PGs concentrations was also inhibited by aspirin. PGE₂, but not AA, treatment rescued the PGs concentrations. These results suggest that BmPxtA gene is associated with PG biosynthesis in silkworm and the immune response of silkworm was affected by regulating the concentrations of PGs.

HMG-Like DSP1 Mediates Immune Responses of the Western Flower Thrips (Frankliniella occidentalis) Against Beauveria bassiana, a Fungal Pathogen

Western flower thrips, Frankliella occidentalis, is a serious pest by directly infesting host crops. It can also give indirect damage to host crops by transmitting a plant virus called tomato spotted wilt virus. A fungal pathogen, Beauveria bassiana, can infect thrips. It has been used as a biopesticide. However, little is known on the defense of thrips against this fungal pathogen. This study assessed the defense of thrips against the fungal infection with respect to immunity by analyzing immune-associated genes of F. occidentalis in both larvae and adults. Immunity-associated genes of western flower thrips were selected from three immunity steps: nonself recognition, mediation, and immune responses. For the pathogen recognition step, dorsal switch protein 1 (DSP1) was chosen. For the immune mediation step, phospholipase A₂ (PLA₂) and prostaglandin E₂ synthase were also selected. For the step of immune responses, two phenoloxidases (PO) genes and four proPO-activating peptidase genes involved in melanization against pathogens were chosen. Dual oxidase gene involved in the production of reactive oxygen species and four antimicrobial peptide genes for executing humoral immune responses were selected. All immunity-associated genes were inducible to the fungal infection. Their expression levels were induced higher in adults than in larvae by the fungal infections. However, inhibitor treatments specific to DSP1 or PLA₂ significantly suppressed the inducible expression of these immune-associated genes, leading to significant enhancement of fungal pathogenicity. These results suggest that immunity is essential for thrips to defend against B. bassiana, in which DSP1 and eicosanoids play a crucial role in eliciting immune responses.

Thromboxane Mobilizes Insect Blood Cells to Infection Foci

Innate immune responses are effective for insect survival to defend against entomopathogens including a fungal pathogen, Metarhizium rileyi, that infects a lepidopteran Spodoptera exigua. In particular, the fungal virulence was attenuated by cellular immune responses, in which the conidia were phagocytosed by hemocytes (insect blood cells) and hyphal growth was inhibited by hemocyte encapsulation. However, the chemokine signal to drive hemocytes to the infection foci was little understood. The hemocyte behaviors appeared to be guided by a Ca²⁺ signal stimulating cell aggregation to the infection foci. The induction of the Ca²⁺ signal was significantly inhibited by the cyclooxygenase (COX) inhibitor. Under the inhibitory condition, the addition of thromboxane A₂ or B₂ (TXA₂ or TXB₂) among COX products was the most effective to recover the Ca²⁺ signal and hemocyte aggregation. TXB₂ alone induced a microaggregation behavior of hemocytes under in vitro conditions. Indeed, TXB₂ titer was significantly increased in the plasma of the infected larvae. The elevated TXB₂ level was further supported by the induction of phospholipase A₂ (PLA₂) activity in the hemocytes and subsequent up-regulation of COX-like peroxinectins (SePOX-F and SePOX-H) in response to the fungal infection. Finally, the expression of a thromboxane synthase (Se-TXAS) gene was highly expressed in the hemocytes. RNA interference (RNAi) of Se-TXAS expression inhibited the Ca²⁺ signal and hemocyte aggregation around fungal hyphae, which were rescued by the addition of TXB₂. Without any ortholog to mammalian thromboxane receptors, a prostaglandin receptor was essential to mediate TXB₂ signal to elevate the Ca²⁺ signal and mediate hemocyte aggregation behavior. Specific inhibitor assays suggest that the downstream signal after binding TXB₂ to the receptor follows the Ca²⁺-induced Ca²⁺ release pathway from the endoplasmic reticulum of the hemocytes. These results suggest that hemocyte aggregation induced by the fungal infection is triggered by TXB₂via a Ca²⁺ signal through a PG receptor.

Infection of Galleria mellonella (Lepidoptera) Larvae With the Entomopathogenic Fungus Conidiobolus coronatus (Entomophthorales) Induces Apoptosis of Hemocytes and Affects the Concentration of Eicosanoids in the Hemolymph

Apoptosis and autophagy, the mechanisms of programmed cell death, play critical roles in physiological and pathological processes in both vertebrates and invertebrates. Apoptosis is also known to play an important role in the immune response, particularly in the context of entomopathogenic infection. Of the factors influencing the apoptotic process during infection, two of the lesser known groups are caspases and eicosanoids. The aim of this study was to determine whether infection by the entomopathogenic soil fungus Conidiobolus coronatus is associated with apoptosis and changes in caspase activity in the hemocytes of Galleria mellonella larvae, and to confirm whether fungal infection may affect eicosanoid levels in the host. Larvae were exposed for 24 h to fully grown and sporulating fungus. Hemolymph was collected either immediately after termination of exposure (F24 group) or 24 h later (F48 group). Apoptosis/necrosis tests were performed in hemocytes using fluorescence microscopy and flow cytometry, while ELISA tests were used to measure eicosanoid levels. Apoptosis and necrosis occurred to the same degree in F24, but necrosis predominated in F48. Fungal infection resulted in caspase activation, increased PGE1, PGE2, PGA1, PGF2α, and 8-iso-PGF2α levels and decreased TXB2 levels, but had no effect on TXA2 or 11-dehydro-TXB2 concentrations. In addition, infected larvae demonstrated significantly increased PLA2 activity, known to be involved in eicosanoid biosynthesis. Our findings indicate that fungal infection simultaneously induces apoptosis in insects and stimulates general caspase activity, and this may be correlated with changes in the concentrations of eicosanoids.

Involvement of Fatty Acids and Their Metabolites in the Development of Inflammation in Atherosclerosis

Despite all the advances of modern medicine, atherosclerosis continues to be one of the most important medical and social problems. Atherosclerosis is the cause of several cardiovascular diseases, which are associated with high rates of disability and mortality. The development of atherosclerosis is associated with the accumulation of lipids in the arterial intima and the disruption of mechanisms that maintain the balance between the development and resolution of inflammation. Fatty acids are involved in many mechanisms of inflammation development and maintenance. Endothelial cells demonstrate multiple cross-linkages between lipid metabolism and innate immunity. In addition, these processes are linked to hemodynamics and the function of other cells in the vascular wall, highlighting the central role of the endothelium in vascular biology.

The metabolism and role of free fatty acids in key physiological processes in insects of medical, veterinary and forensic importance

Insects are the most widespread group of organisms and more than one million species have been described. These animals have significant ecological functions, for example they are pollinators of many types of plants. However, they also have direct influence on human life in different manners. They have high medical and veterinary significance, stemming from their role as vectors of disease and infection of wounds and necrotic tissue; they are also plant pests, parasitoids and predators whose activities can influence agriculture. In addition, their use in medical treatments, such as maggot therapy of gangrene and wounds, has grown considerably. They also have many uses in forensic science to determine the minimum post-mortem interval and provide valuable information about the movement of the body, cause of the death, drug use, or poisoning. It has also been proposed that they may be used as model organisms to replace mammal systems in research. The present review describes the role of free fatty acids (FFAs) in key physiological processes in insects. By focusing on insects of medical, veterinary significance, we have limited our description of the physiological processes to those most important from the point of view of insect control; the study examines their effects on insect reproduction and resistance to the adverse effects of abiotic (low temperature) and biotic (pathogens) factors.

Toll signal pathway activating eicosanoid biosynthesis shares its conserved upstream recognition components in a lepidopteran Spodoptera exigua upon infection by Metarhizium rileyi, an entomopathogenic fungus

Eicosanoids play crucial roles in mediating immune responses in insects. Upon a fungal infection, Toll signal pathway can mediate immune responses of Spodoptera exigua, a lepidopteran insect, by activating eicosanoid biosynthesis. However, upstream signal components of the Toll signal pathway activating eicosanoid biosynthesis remain unclear. This study predicted pattern recognition receptors (PRRs) and serine proteases (SPs) as upstream components of the Toll pathway with reference to known signal components of Manduca sexta, another lepidopteran insect. S. exigua infected with Metarhizium rileyi, an entomopathogenic fungus, activated phospholipase A₂ (PLA₂) and phenoloxidase (PO) enzymes along with marked increases of expression levels of genes encoding three specific antimicrobial peptides, cecropin, gallerimycin, and hemolin. Among ten Toll receptors encoded in the genome of S. exigua, seven Toll genes were associated with immune responses against fungal infection by M. rileyi through individual RNA interference (RNAi) screening. In addition, two Spätzles (ligands of Toll receptor) were required for Toll signaling against the fungal infection. All predicted upstream components of the Toll pathway were inducible by the fungal infection. Individual RNAi screening showed that three PRRs (βGRP-1, βGRP-2, and GNBP3) and five SPs (ModSP, HP21, HP5, HP6, and HP8) were required for immune responses of S. exigua mediated by Toll signal pathway against the fungal infection. However, two PO-activating proteases (PAP1 and PAP3) were not required for PLA₂ activation, although they were required for PO activation. These results suggest that PRRs and SPs conserved as upstream components in Toll signal pathway play crucial roles in triggering eicosanoid biosynthesis of S. exigua to mediate various immune responses against fungal infection.

Prostaglandin A2 induces apoptosis in three cell lines derived from the fall armyworm, Spodoptera frugiperda

Animals maintain homeostasis of cell numbers, constantly creating new cells and eliminating others. Programmed cell death, apoptosis, is a mechanism of cell elimination and it acts in many aspects of animal biology. Drawing on the biomedical background, several signals launch the apoptosis mechanisms, including prostaglandins (PGs). Based on this information, we posed the hypothesis that PGs similarly induce apoptosis in insect cell lines. We used three Spodoptera frugiperda cell lines, including two newly established, BCIRL-SfNS-0518B-YL derived from the central nervous system and BCIRL-Sf4FB-0614-SGS derived from fat body, and the commercially available Sf9 cells. Using a kinetic apoptosis kit, we found treating SfNS cells for 18 h with 15 or 20 μM PGA2 led to decreases in cell numbers, coupled with increased numbers of apoptotic and dead cells. Similar exposures to 10 μM PGA2 (24 h) led to substantial increases in apoptotic cells, confirmed by a terminal deoxynucleotidyl transferase dUTP nick end labeling assay on a flow cytometer. The influence of PGA2 treatments increased with dosage, as we recorded about 20% apoptosis at 24 h post-PGA2 treatments (10 μM) and about 34% apoptosis at 24 h post-30 μM treatments. PGA2 treatments led to 10- to 30-fold increases in messenger RNAs (mRNAs) encoding apoptosis-specific caspases-1, -2, -3, and -5 at 12 h and 40- to 60-fold increases in mRNAs encoding caspases-1 and -2, 10-fold increases for caspases-3 and -5 at 24 h. These findings strongly support our hypothesis that PGs induce apoptosis in an insect cell line and confirm an additional PG action in insect biology.

Cell Line Platforms Support Research into Arthropod Immunity

Simple Summary

Many insect and tick species are serious pests, because insects damage crop plants and, along with ticks, transmit a wide range of human and animal diseases. One way of controlling these pests is by impairing their immune system, which protects them from bacterial, fungal, and viral infections. An important tool for studying immunity is using long-lasting cell cultures, known as cell lines. These lines can be frozen and thawed at will to be used in automated tests, and they provide consistent results over years. Questions that can be asked using cell lines include: How do insects or ticks recognize when they have been infected and by what organism? What kinds of defensive strategies do they use to contain or kill infectious agents? This article reviews research with insect or tick cell lines to answer these questions, as well as other questions relating to immunity. This review also discusses future research strategies for working with cell lines.

Abstract

Innate immune responses are essential to maintaining insect and tick health and are the primary defense against pathogenic viruses, bacteria, and fungi. Cell line research is a powerful method for understanding how invertebrates mount defenses against pathogenic organisms and testing hypotheses on how these responses occur. In particular, immortal arthropod cell lines are valuable tools, providing a tractable, high-throughput, cost-effective, and consistent platform to investigate the mechanisms underpinning insect and tick immune responses. The research results inform the controls of medically and agriculturally important insects and ticks. This review presents several examples of how cell lines have facilitated research into multiple aspects of the invertebrate immune response to pathogens and other foreign agents, as well as comments on possible future research directions in these robust systems.

HMGB1-like dorsal switch protein 1 of the mealworm, Tenebrio molitor, acts as a damage-associated molecular pattern

High-mobility group box 1 (HMGB1) is a nuclear protein highly conserved in eukaryotes and ubiquitously expressed to regulate transcription and chromatin remodeling. Dorsal switch protein 1 (DSP1) is its insect homolog. A lepidopteran DSP1 acts as a damage-associated molecular pattern (DAMP) in response to immune challenge. The objective of this study was to determine the role of DAMP in the mealworm beetle, Tenebrio molitor, a coleopteran insect. DSP1 of T. molitor (Tm-DSP1) encodes 536 amino acids and shares sequence similarities with Homo sapiens HMGB1 (56.3%) and Spodoptera exigua DSP1 (59.2%). An antisera raised against S. exigua DSP1 was cross-reactive to Tm-DSP1. Like other insect DSPs, Tm-DSP1 has a relatively long N-terminal extension in addition to two conserved HMG box domains. It was expressed in all developmental stages of T. molitor and different larval tissues. Upon immune challenge, its expression level was upregulated. Its RNA interference (RNAi) treatment resulted in a significant reduction in immune responses measured by hemocyte nodule formation against bacterial infection. In addition, the induction of some antimicrobial peptide genes to the immune challenge was suppressed by its RNAi treatment. Interestingly, phospholipase A₂ associated with eicosanoid biosynthesis was significantly suppressed in its catalytic activity by the RNAi treatment specific to Tm-DSP1 expression. Without any pathogen infection, injection of a lepidopteran DSP1 induced both cellular and humoral immune responses. These results suggest that Tm-DSP1 in T. molitor can act as a DAMP molecule and mediate immune responses upon immune challenge.

Fat Body-Multifunctional Insect Tissue

Simple Summary

Efficient and proper functioning of processes within living organisms play key roles in times of climate change and strong human pressure. In insects, the most abundant group of organisms, many important changes occur within their tissues, including the fat body, which plays a key role in the development of insects. Fat body cells undergo numerous metabolic changes in basic energy compounds (i.e., lipids, carbohydrates, and proteins), enabling them to move and nourish themselves. In addition to metabolism, the fat body is involved in the development of insects by determining the time an individual becomes an adult, and creates humoral immunity via the synthesis of bactericidal proteins and polypeptides. As an important tissue that integrates all signals from the body, the processes taking place in the fat body have an impact on the functioning of the entire body.

Abstract

The biodiversity of useful organisms, e.g., insects, decreases due to many environmental factors and increasing anthropopressure. Multifunctional tissues, such as the fat body, are key elements in the proper functioning of invertebrate organisms and resistance factors. The fat body is the center of metabolism, integrating signals, controlling molting and metamorphosis, and synthesizing hormones that control the functioning of the whole body and the synthesis of immune system proteins. In fat body cells, lipids, carbohydrates and proteins are the substrates and products of many pathways that can be used for energy production, accumulate as reserves, and mobilize at the appropriate stage of life (diapause, metamorphosis, flight), determining the survival of an individual. The fat body is the main tissue responsible for innate and acquired humoral immunity. The tissue produces bactericidal proteins and polypeptides, i.e., lysozyme. The fat body is also important in the early stages of an insect’s life due to the production of vitellogenin, the yolk protein needed for the development of oocytes. Although a lot of information is available on its structure and biochemistry, the fat body is an interesting research topic on which much is still to be discovered.