Toll immune signal activates cellular immune response via eicosanoids

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.

Immune responses of the Asian onion moth, Acrolepiopsis sapporensis, and their genetic factors from RNA-Seq analysis

A nonmodel insect, Acrolepiopsis sapporensis, has been analyzed in immune responses. The total hemocytes in the fifth instar larvae were 2.33 × 106  cells/mL. These hemocytes comprised at least five different types and different relative ratios: 47% granulocytes, 26% plasmatocytes, 11% oenocytoid, 8% prohemocytes, and 5% spherulocytes. Upon bacterial challenge, some of the hemocytes exhibited typical hemocyte-spreading behaviors, such as focal adhesion, and filopodial and lamellipodial cytoplasmic extensions. The hemocyte behaviors induced cellular immune responses demonstrated by nodule formation. In addition, the plasma collected from the immune-challenged larvae exhibited humoral immune responses by bacterial growth inhibition along with enhanced phenoloxidase enzyme activity. These cellular and humoral immune responses were further analyzed by determining the immune-associated genes from a transcriptome generated by RNA-Seq. A total of about 12 Gb sequences led to about 218,116 contigs, which were predicted to encode about 46,808 genes. Comparative expression analysis showed 8392 uniquely expressed genes in the immune-challenged larvae. Differentially expressed gene (DEG) analysis among the commonly expressed genes indicated that 782 genes were upregulated and 548 genes were downregulated in the expressions after bacterial challenge. These immune-associated genes included pattern recognition receptors, immune mediation/signaling genes, and various immune effectors. Specifically, the genetic components of the Toll, IMD, and JAK/STAT immune signaling pathways were included in the DEG database. These results demonstrate the immune responses of A. sapporensis larvae and suggest the genes associated with the immune responses in this nonmodel insect.

Mechanisms and roles of the first stage of nodule formation in lepidopteran insects

Nodule formation is a process of cellular immunity in insects and other arthropods with open circulatory systems. Based on histological observations, nodule formation occurs in 2 stages. The first stage occurs immediately after microbial inoculation and includes aggregate formation by granulocytes. The second stage occurs approximately 2-6 h later and involves the attachment of plasmatocytes to melanized aggregates produced during the first stage. The first stage response is thought to play a major role in the rapid capture of invading microorganisms. However, little is known regarding how granulocytes in the hemolymph form aggregates, or how the first stage of the immunological response protects against invading microorganisms. Since the late 1990s, our understanding of the molecules and immune pathways that contribute to nodule formation has improved. The first stage of nodule formation involves a hemocyte-induced response that is triggered by pathogen-associated molecular pattern (PAMP) recognition proteins in the hemolymph regulated by a serine proteinase cascade and cytokine (Spätzle) and Toll signaling pathways. Hemocyte agglutination proceeds through stepwise release of biogenic amine, 5-HT, and eicosanoids that act downstream of the Toll pathway. The first stage of nodule formation is closely linked to melanization and antimicrobial peptide (AMP) production, which is critical for insect humoral immunity. Nodule formation in response to artificial inoculation with millions of microorganisms has long been studied. It has recently been suggested that this system is the original natural immune system, and enables insects to respond to a single invading microorganism in the hemocoel.

Serotonin- and eicosanoid-dependent rapid hemocyte aggregation in the hemolymph is the first step in nodule formation in Bombyx mori larvae

Nodule formation is a well-known process in cellular immunity of insects. However, few studies have investigated the role of hemocytes in rapid aggregation before tissue adhesion. In addition, since nodule formation is usually elicited by injecting large quantities of microbes, it remains unclear whether nodule formation is a natural response. The present study addressed these issues. A small number of nodules adhered to the dorsal vessels 1 min after Saccharomyces cerevisiae injection, while numerous aggregates of hemocytes and S. cerevisiae cells were observed in the hemolymph. The aggregate number decreased sharply after 5 min, corresponding to a rapid increase in the number of nodules. This suggests that aggregates formed in the hemolymph in response to S. cerevisiae injection eventually attached to the tissues. Nodules were induced using conditions that do not occur in nature, i.e., injection of nearly 2,000,000 S. cerevisiae cells. However, many aggregates contained only one S. cerevisiae cell, suggesting that aggregate formation can begin with the invasion of a single cell and that nodule formation is not an unnatural response. Biosynthesis inhibitors of serotonin (5-HT) and eicosanoids inhibited aggregate and nodule formation. In addition, injection of 5-HT and prostaglandin E2 induced hemocyte aggregation within 1 min in the hemolymph, along with hemocytin release. This suggested that 5-HT and eicosanoids induce rapid aggregation in response to invading microorganisms.

Spätzle1 is a constituent of the extracellular signaling pathway that promotes nodule formation, a cell-mediated immune response, in caterpillar hemolymph

The existence of an extracellular signaling pathway that mediates nodule formation, a cell-mediated immune response, has been reported in Bombyx mori larvae. In this pathway, C-type lectins and the hemolymph serine proteinase BmHP-8 function in pathogen associated molecular pattern (PAMPs) recognition and signaling transduction. However, which molecule elicits the cellular response at the end of the pathway is unknown. In this study, the Toll ligand Bombyx mori Spätzel1 was shown to be involved in the pathway by applying anit-Spätzel1 antiserum in an in vitro nodule-like aggregate formation assay and an in vivo nodule formation assay.

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.

Nodule formation in Bombyx mori larvae is regulated by BmToll10-3

Nodule formation is a two-step cell-mediated immune response that is elicited by the cytokine spätzle1. Spätzle1 is activated within 30 s of invasion by microorganisms via an extracellular signaling pathway that consists of pathogen-associated molecular pattern recognition receptors, C-type lectins, and serine proteases. Here, we investigated a hemocyte molecule that is involved in eliciting the first step of nodule formation. BmToll10-3 was one of 14 Toll homologs identified in the silkworm Bombyx mori; it is an ortholog of Spodoptera exigua Toll. Previous research suggested that SeToll elicits nodule formation, but no evidence was presented to indicate whether SeToll elicited the first or second step of nodule formation. Reverse transcription-polymerase chain reaction and immunostaining confirmed that BmToll10-3 is expressed in granulocytes. To determine whether BmToll10-3 is involved in eliciting the first step of nodule formation, we tested an antiserum raised against BmToll10-3 in a nodule formation assay. The antiserum strongly inhibited the first step of nodule formation in B. mori larvae. Next, we tried to knock out BmToll10-3 using genome editing. Strains that were heterozygous for a truncated BmToll10-3 allele were generated, but no strain that was homozygous for truncated BmToll10-3 was generated. Nonetheless, several healthy homozygous larvae were identified before pupation, and we used these larvae in a nodule formation assay. The larvae that were homozygous for truncated BmToll10-3 did not form nodules. These results suggest that BmToll10-3 is involved in a cellular immunity, nodule formation.

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.

PLA2 mediates the innate immune response in Asian corn borer, Ostrinia furnacalis

The eicosanoid signaling pathway mediates insect immune reactions to a wide range of stimuli. This pathway begins with the biosynthesis of arachidonic acid (AA) from the hydrolysis of phospholipids catalyzed by phospholipase A₂ (PLA₂ ). We report here that the PLA₂ inhibitor, dexamethasone (DEX), impaired the innate immune response including nodulation, encapsulation, and melanization in Ostrinia furnacalis larvae, while AA partially reversed these effects of DEX. We cloned a full-length complementary DNA encoding a PLA₂ , designated as OfsPLA₂ , from O. furnacalis. The open reading frame of OfsPLA₂ encodes a 195-amino acid residue protein with a 22-residue signal peptide. Sequence alignment analyses indicated that O. furnacalis PLA₂ might be a Group III secretory PLA₂ . The highest transcript levels of OfsPLA₂ were detected in the fat body, and its transcript levels increased dramatically after infection with Escherichia coli, Micrococcus luteus, or Beauveria bassiana. Recombinant OfsPLA₂ significantly induced prophenoloxidase (PPO) activation in larval hemolymph in the presence of Ca²⁺ and encapsulation of agarose beads. Injection of recombinant OfsPLA₂ into larvae resulted in increased transcript levels of attacin, defencin, and moricin-3 genes. Our results demonstrate the involvement of the eicosanoid signaling pathway in the innate immune response of O. furnacalis larvae and provide new information about the roles of O. furnacalis secretory PLA₂ in activating PPO and antimicrobial peptide production.

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.

A humoral factor, hemolymph proteinase 8, elicits a cellular defense response of nodule formation in Bombyx mori larvae in association with recognition by C-type lectins

Previously, we found that nodule formation, a cellular defense response in insects, is regulated by humoral factors called C-type lectins in the hemolymph. To elucidate the factors that elicit nodule formation following the recognition of microorganisms by C-type lectins, a reproducible quantitative in vitro assay system was constructed. Then, using this system, the inhibitory activities of antisera raised against hemolymph proteases (HPs), serine protease homologues (SPHs), and pathogen-associated molecular pattern (PAMP)-recognition proteins were assessed. Among the antisera raised against HP and SPH, only that against HP8, a terminal proteinase that activates Spätzle, consistently inhibited in-vitro nodule-like aggregate formation in all three tested microorganisms, Micrococcus luteus, Escherichia coli, and Saccharomyces cerevisiae. Antisera raised against C-type lectins, BmLBP, and BmMBP also inhibited nodule-like aggregate formation, while those against β-glucan recognition proteins and peptidoglycan recognition protein-S1 did not. Microorganisms pretreated with hemolymph, which contains HP8 and C-type lectins, also induced nodule-like aggregate formation, indicating that nodulation factors are present on microbial cells. Furthermore, antisera raised against HP8, BmLBP, and BmMBP showed inhibitory activities in the in vivo nodule formation system using Bombyx mori larvae. Thus, two humoral factors in the hemolymph of B. mori larvae, BmHP8 and C-type lectins, were found to play significant roles in eliciting the cellular defense response of nodule formation.

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.

Insulin-Like Peptides and Cross-Talk With Other Factors in the Regulation of Insect Metabolism

The insulin-like peptide (ILP) and insulin-like growth factor (IGF) signalling pathways play a crucial role in the regulation of metabolism, growth and development, fecundity, stress resistance, and lifespan. ILPs are encoded by multigene families that are expressed in nervous and non-nervous organs, including the midgut, salivary glands, and fat body, in a tissue- and stage-specific manner. Thus, more multidirectional and more complex control of insect metabolism can occur. ILPs are not the only factors that regulate metabolism. ILPs interact in many cross-talk interactions of different factors, for example, hormones (peptide and nonpeptide), neurotransmitters and growth factors. These interactions are observed at different levels, and three interactions appear to be the most prominent/significant: (1) coinfluence of ILPs and other factors on the same target cells, (2) influence of ILPs on synthesis/secretion of other factors regulating metabolism, and (3) regulation of activity of cells producing/secreting ILPs by various factors. For example, brain insulin-producing cells co-express sulfakinins (SKs), which are cholecystokinin-like peptides, another key regulator of metabolism, and express receptors for tachykinin-related peptides, the next peptide hormones involved in the control of metabolism. It was also shown that ILPs in Drosophila melanogaster can directly and indirectly regulate AKH. This review presents an overview of the regulatory role of insulin-like peptides in insect metabolism and how these factors interact with other players involved in its regulation.

Immune mediation of HMG-like DSP1 via Toll-Spätzle pathway and its specific inhibition by salicylic acid analogs

Xenorhabdus hominickii, an entomopathogenic bacterium, inhibits eicosanoid biosynthesis of target insects to suppress their immune responses by inhibiting phospholipase A₂ (PLA₂) through binding to a damage-associated molecular pattern (DAMP) molecule called dorsal switch protein 1 (DSP1) from Spodoptera exigua, a lepidopteran insect. However, the signalling pathway between DSP1 and PLA₂ remains unknown. The objective of this study was to determine whether DSP1 could activate Toll immune signalling pathway to activate PLA₂ activation and whether X. hominickii metabolites could inhibit DSP1 to shutdown eicosanoid biosynthesis. Toll-Spätzle (Spz) signalling pathway includes two Spz (SeSpz1 and SeSpz2) and 10 Toll receptors (SeToll1-10) in S. exigua. Loss-of-function approach using RNA interference showed that SeSpz1 and SeToll9 played crucial roles in connecting DSP1 mediation to activate PLA₂. Furthermore, a deletion mutant against SeToll9 using CRISPR/Cas9 abolished DSP1 mediation and induced significant immunosuppression. Organic extracts of X. hominickii culture broth could bind to DSP1 at a low micromolar range. Subsequent sequential fractionations along with binding assays led to the identification of seven potent compounds including 3-ethoxy-4-methoxyphenol (EMP). EMP could bind to DSP1 and prevent its translocation to plasma in response to bacterial challenge and suppress the up-regulation of PLA₂ activity. These results suggest that X. hominickii inhibits DSP1 and prevents its DAMP role in activating Toll immune signalling pathway including PLA₂ activation, leading to significant immunosuppression of target insects.

Immune responses of insects are highly effective in defending various entomopathogens. Xenorhabdus hominickii is an entomopathogenic bacterium that uses a pathogenic strategy of suppressing host insect immunity by inhibiting phospholipase A₂ (PLA₂) which catalyzes the committed step for eicosanoid biosynthesis. Eicosanoids mediate both cellular and humoral immune responses in insects. This study discovers an upstream signalling pathway to activate PLA₂ in response to bacterial challenge. Se-DSP1 is an insect homolog of vertebrate HMGB1 that acts as a damage-associated molecular pattern. Upon bacterial infection, Se-DSP1 is released to the circulatory system to activate Spätzle, an insect cytokine that can bind to Toll receptor. Toll immune signalling pathway can activate antimicrobial peptide gene expression and PLA₂. A deletion mutant against a Toll gene abolished immune responses mediated by Se-DSP1. Indeed, X. hominickii can produce and secrete secondary metabolites including salicylic acid analogs that can strongly bind to Se-DSP1. These bacterial metabolites prevented the release of Se-DSP1, which impaired the activation of PLA₂ and resulted in a significant immunosuppression of target insects against bacterial infection.

Eicosanoid Signaling in Insect Immunology: New Genes and Unresolved Issues

This paper is focused on eicosanoid signaling in insect immunology. We begin with eicosanoid biosynthesis through the actions of phospholipase A₂, responsible for hydrolyzing the C18 polyunsaturated fatty acid, linoleic acid (18:2n-6), from cellular phospholipids, which is subsequently converted into arachidonic acid (AA; 20:4n-6) via elongases and desaturases. The synthesized AA is then oxygenated into one of three groups of eicosanoids, prostaglandins (PGs), epoxyeicosatrienoic acids (EETs) and lipoxygenase products. We mark the distinction between mammalian cyclooxygenases and insect peroxynectins, both of which convert AA into PGs. One PG, PGI₂ (also called prostacyclin), is newly discovered in insects, as a negative regulator of immune reactions and a positive signal in juvenile development. Two new elements of insect PG biology are a PG dehydrogenase and a PG reductase, both of which enact necessary PG catabolism. EETs, which are produced from AA via cytochrome P450s, also act in immune signaling, acting as pro-inflammatory signals. Eicosanoids signal a wide range of cellular immune reactions to infections, invasions and wounding, including nodulation, cell spreading, hemocyte migration and releasing prophenoloxidase from oenocytoids, a class of lepidopteran hemocytes. We briefly review the relatively scant knowledge on insect PG receptors and note PGs also act in gut immunity and in humoral immunity. Detailed new information on PG actions in mosquito immunity against the malarial agent, Plasmodium berghei, has recently emerged and we treat this exciting new work. The new findings on eicosanoid actions in insect immunity have emerged from a very broad range of research at the genetic, cellular and organismal levels, all taking place at the international level.

Deletion mutant of PGE2 receptor using CRISPR-Cas9 exhibits larval immunosuppression and adult infertility in a lepidopteran insect, Spodoptera exigua

Prostaglandins (PGs) mediate various physiological processes in insects and other invertebrates, but there is very little information on PG receptors. This study identified a PGE₂ receptor (SePGE₂R) in the lepidopteran insect, Spodoptera exigua, and addressed its functional association with cellular immunity, development, and reproduction. SePGE₂R is expressed in most developmental stages and tissues. After SePGR₂R expression knock down by RNA interference (RNAi), larval nodule formation (clears bacterial infections from circulating hemolymph) was severely suppressed coupled with reduced F-actin growth in hemocytes. Treating female adults with RNAi prevented nurse cell dumping in follicles and interfered with oocyte development. SePGE₂R was heterologously expressed in Sf9 cells, in which the endogenous S. frugiperda PGE₂R was knocked down by small interfering RNA. This transiently expressed SePGE₂R responded to PGE₂, but not other PGs, with dose-dependent up-regulation of intracellular cAMP concentrations. Treating S. exigua larvae with PGE₂ led to activation of a trimeric Gαs subunit, protein kinase A (PKA), and Rho family small intracellular G proteins in hemocytes. A deletion mutant of SePGE₂R was generated using CRISPR/Cas9 which exhibited severely retarded larval development and adult reproduction. We infer that PGE₂R mediates insect immune and reproductive processes via a PKA signal pathway.

Social Fever or General Immune Response? Revisiting an Example of Social Immunity in Honey Bees

Simple Summary

Behavioral, or social fever in honey bees is frequently cited as a form of social immunity—the behavioral, organizational, and physiological mechanisms that social organisms use to defend against parasites and pathogens to maintain group health. It has been shown previously that colonies elevate brood nest temperature as a response to challenge with the fungal pathogen, Ascosphaera apis, the causative agent of chalkbrood disease in honey bees. Our objective was to test whether we could replicate social fever and its effect on reducing signs of chalkbrood disease in colonies using methods similar to previous reports. We affirmed that honey bees increase the temperature of the brood nest after exposure to A. apis. However, the magnitude of temperature increase was insufficient at preventing infection, as all colonies showed signs of chalkbrood post-exposure. We conducted additional studies to explore alternative hypotheses related to the cause and effect of behavioral fever. We found that challenge with A. apis resulted in an increased immune response of adult bees, but this activation was not due to thermal and other stress, as measured by expression of the heat stress and nutritional genes, Hsp 70Ab-like and vitellogenin, respectively. We proposed additional hypotheses that could be tested.

Abstract

Honey bees use several strategies to protect themselves and the colony from parasites and pathogens. In addition to individual immunity, social immunity involves the cumulative effort of some individuals to limit the spread of parasites and pathogens to uninfected nestmates. Examples of social immunity in honey bees that have received attention include hygienic behavior, or the removal of diseased brood, and the collection and deposition of antimicrobial resins (propolis) on interior nest surfaces. Advances in our understanding of another form of social immunity, social fever, are lacking. Honey bees were shown to raise the temperature of the nest in response to temperature-sensitive brood pathogen, Ascosphaera apis. The increase in nest temperature (−0.6 °C) is thought to limit the spread of A. apis infection to uninfected immatures. We established observation hives and monitored the temperature of the brood nest for 40 days. This observation period was broken into five distinct segments, corresponding to sucrose solution feedings—Pre-Feed, Feed I, Challenge, Feed II, and Post-Feed. Ascosphaera apis was administered to colonies as a 1% solution of ground sporulating chalkbrood mummies in 50% v/v sucrose solution, during the Challenge period. Like previous reports, we observed a modest increase in brood nest temperature during the Challenge period. However, all hives presented signs of chalkbrood disease, suggesting that elevation of the nest temperature was not sufficient to stop the spread of infection among immatures. We also began to explore the molecular mechanisms of temperature increase by exposing adult bees in cages to A. apis, without the presence of immatures. Compared to adult workers who were given sucrose solution only, workers exposed to A. apis showed increased expression of the antimicrobial peptides abaecin (p = 0.07) and hymenoptaecin (p = 0.04), but expression of the heat shock response protein Hsp 70Ab-like (p = 0.76) and the nutritional marker vitellogenin (p = 0.72) were unaffected. These results indicate that adult honey bee workers exposed to a brood pathogen elevate the temperature of the brood nest and initiate an immune response, but the effect of this fever on preventing disease requires further study.

sPLA2 behaves like a prophylactic agent and mediates cellular and humoral immune responses in Plutella xylostella

Most immune effectors are inducible to microbial pathogen infection while some are already present to act as prophylactic immunity against as yet unseen infection. This study identified secretory phospholipase A₂ (sPLA₂ ) as a prophylactic factor in diamondback moth, Plutella xylostella. Western blotting using a polyclonal antibody raised against other lepidopteran sPLA₂ reacted specifically with ∼25 kDa protein, which was present at approximately 0.4 mM in the plasma of naïve larvae. Interrogation of P. xylostella transcriptomes revealed an open-reading frame for sPLA₂ (Px-sPLA₂ ), exhibiting high homology with other Group III sPLA₂ s. Px-sPLA₂ was expressed in all developmental stages. In the larval stage, bacterial challenge induced its expression in hemocytes and fat body but not in gut or epidermis. RNA interference (RNAi) suppressed Px-sPLA₂ messenger RNA level and sPLA₂ activity in plasma. An inhibition zone assay showed that Px-sPLA₂ exhibited antibacterial activities against different species, because specific RNAi knockdown impaired the activity. The RNAi treatment also suppressed the cellular immune response assessed by hemocyte nodule formation and humoral immune response assessed by antimicrobial peptide gene expression. Finally, benzylideneacetone (BZA, a specific sPLA₂ inhibitor) treatment inhibited plasma sPLA₂ activity of naive larvae in a dose-dependent manner. An addition of BZA significantly increased the bacterial virulence of an entomopathogen, Bacillus thuringiensis. These results suggest that Px-sPLA₂ is an immune-associated factor of P. xylostella and its relatively high level of concentration in the plasma of naive larvae strongly suggests its role as a prophylactic factor in defending against pathogens at early infection stages.

Deletion mutant of sPLA2 using CRISPR/Cas9 exhibits immunosuppression, developmental retardation, and failure of oocyte development in legume pod borer, Maruca vitrata

Phospholipase A₂ (PLA₂) catalyzes release of free fatty acids linked to phospholipids at sn-2 position. Some of these released free fatty acids are used to synthesize eicosanoids that mediate various physiological processes in insects. Although a large number of PLA₂s form a superfamily consisting of at least 16 groups, few PLA₂s have been identified and characterized in insects. Furthermore, physiological functions of insect PLA₂s remain unclear. Clustered regularly interspaced short parlindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) has been a useful research tool to validate gene function. This study identified and characterized a secretory PLA₂ (sPLA₂) from legume pod borer, Maruca vitrata (Lepidoptera: Crambidae), and validated its physiological functions using CRISPR/Cas9. An open reading frame of M. vitrata sPLA₂ (Mv-sPLA₂) encoding 192 amino acids contained signal peptide, calcium-binding domain, and catalytic site. Phylogenetic analysis indicated that Mv-sPLA₂ was related to other Group III sPLA₂s. Mv-sPLA₂ was expressed in both larval and adult stages. It was inducible by immune challenge. RNA interference (RNAi) of Mv-sPLA₂ significantly suppressed cellular immunity and impaired larval development. Furthermore, RNAi treatment in female adults prevented oocyte development. These physiological alterations were also observed in a mutant line of M. vitrata with Mv-sPLA₂ deleted by using CRISPR/Cas9. Mv-sPLA₂ was not detected in the mutant line from western blot analysis. Addition of an eicosanoid, PGE₂, significantly rescued oocyte development of females of the mutant line. These results suggest that Mv-sPLA₂ plays crucial role in immune, developmental, and reproductive processes of M. vitrata.

Toll/IMD signal pathways mediate cellular immune responses via induction of intracellular PLA 2 expression

Phospholipase A₂ (PLA₂ ) hydrolyzes fatty acids from phospholipids at the sn-2 position. Two intracellular PLA₂ s, iPLA₂ A and iPLA₂ B, have been found in Spodoptera exigua. Both are calcium-independent cellular PLA₂ . Their orthologs have been found in other insects. These two iPLA₂ s are different in ankyrin motif of N terminal region. The objective of this study was to determine whether Toll/immune deficiency (IMD) signal pathways could mediate cellular immune responses via induction of iPLA₂ expression. Both iPLA ₂ s were expressed in all developmental stages of S. exigua, showing the highest expression in the adult stage. During larval stage, hemocyte is the main tissue showing expression of these iPLA₂ s. Both iPLA₂ s exhibited similar expression patterns after immune challenge with different microbial pathogens such as virus, bacteria, and fungi. Promoter component analysis of orthologs encoded in S. frugiperda indicated nuclear factor-κB- and Relish-responsible elements on their promoters, suggesting their expression in S. exigua under Toll/IMD immune signaling pathways. RNA interference (RNAi) of MyD88 or Pelle under Toll pathway suppressed inducible expression levels of both iPLA₂ s in response to Gram-positive bacteria containing Lys-type peptidoglycan or fungal infection. In contrast, RNAi against Relish under IMD pathway suppressed both iPLA₂ s in response to infection with Gram-negative bacteria. Under RNAi conditions, hemocytes significantly lost cellular immune response measured by nodule formation. However, addition of arachidonic acid (a catalytic product of PLA₂ ) rescued such immunosuppression. These results suggest that Toll/IMD signal pathways can mediate cellular immune responses via eicosanoid signaling by inducing iPLA₂ expression.

Prostaglandins and Other Eicosanoids in Insects: Biosynthesis and Biological Actions

This essay reviews the discoveries, synthesis, and biological significance of prostaglandins (PGs) and other eicosanoids in insect biology. It presents the most current - and growing - understanding of the insect mechanism of PG biosynthesis, provides an updated treatment of known insect phospholipase A2 (PLA2), and details contemporary findings on the biological roles of PGs and other eicosanoids in insect physiology, including reproduction, fluid secretion, hormone actions in fat body, immunity and eicosanoid signaling and cross-talk in immunity. It completes the essay with a prospectus meant to illuminate research opportunities for interested readers. In more detail, cellular and secretory types of PLA2, similar to those known on the biomedical background, have been identified in insects and their roles in eicosanoid biosynthesis documented. It highlights recent findings showing that eicosanoid biosynthetic pathway in insects is not identical to the solidly established biomedical picture. The relatively low concentrations of arachidonic acid (AA) present in insect phospholipids (PLs) (< 0.1% in some species) indicate that PLA2 may hydrolyze linoleic acid (LA) as a precursor of eicosanoid biosynthesis. The free LA is desaturated and elongated into AA. Unlike vertebrates, AA is not oxidized by cyclooxygenase, but by a specific peroxidase called peroxinectin to produce PGH2, which is then isomerized into cell-specific PGs. In particular, PGE2 synthase recently identified converts PGH2 into PGE2. In the cross-talks with other immune mediators, eicosanoids act as downstream signals because any inhibition of eicosanoid signaling leads to significant immunosuppression. Because host immunosuppression favors pathogens and parasitoids, some entomopathogens evolved a PLA2 inhibitory strategy activity to express their virulence.

Overexpression of PGE2 synthase by in vivo transient expression enhances immunocompetency along with fitness cost in a lepidopteran insect

Prostaglandins (PGs) mediate various physiological functions in insects. Especially, PGE2 is known to mediate immunity and egg-laying behavior in the beet armyworm, Spodoptera exigua. A PGE2 synthase 2 (Se-PGES2) has been identified to catalyze the final step to produce PGE2 in S. exigua. Its expression is inducible in response to immune challenge. Inhibition of the gene expression results in immunosuppression. On the other hand, any physiological alteration induced by its uncontrolled overexpression was not recognized in insects. This study used in vivo transient expression (IVTE) technique to induce overexpression and assessed subsequent physiological alteration in S. exigua. Se-PGES2 was cloned into a eukaryotic expression vector and transfected to Sf9 cells to monitor its heterologous expression. The Sf9 cells expressed the recombinant Se-PGES2 (rSe-PGES2) at an expected size (∼47 kDa), which was localized in cytoplasm. The recombinant expression vector was then used to transfect larvae of S. exigua. Hemocytes collected from the larvae treated with IVTE expressed rSe-PGES2 gene for at least 48 h. The larvae treated with IVTE exhibited an enhanced competency in cellular immune response measured by hemocyte nodule formation. In addition, IVTE treatment of Se-PGES2 induced gene expression of antimicrobial peptides without any immune challenge. The larvae treated with IVTE became significantly resistant to infection of an entomopathogenic nematode, Steinernema monticolum or to infection to its symbiotic bacterium, Xenorhabdus hominickii. However, IVTE-treated S. exigua larvae suffered from reduced pupal size and fecundity.

A non-venomous sPLA2 of a lepidopteran insect: Its physiological functions in development and immunity

Eicosanoids are oxygenated C20 polyunsaturated fatty acids that mediate various physiological processes in insects. Eicosanoid biosynthesis begins with a C20 precursor, arachidonic acid (5,8,11,14-eicosatetraenoic acid: AA). AA is usually released from phospholipids at sn-2 position by catalytic activity of phospholipase A₂ (PLA₂). Although various PLA₂s classified into 16 gene families (= Groups) are known in various biological systems, few PLA₂s are known in insects. Only two PLA₂s involved in intracellular calcium independent PLA₂ (iPLA₂) group have been identified in lepidopteran insects with well known eicosanoid physiology. This study reports the first secretory PLA₂ (sPLA₂) in lepidopteran insects. A partial open reading frame (ORF) of PLA₂ was obtained by interrogating Spodoptera exigua transcriptome. Subsequent 3'-RACE resulted in a full ORF (Se-sPLA2A) encoding 194 amino acid sequence containing signal peptide, calcium-binding domain, and catalytic site. Phylogenetic analysis indicated that Se-sPLA₂A was clustered with other Group III sPLA₂s. Se-sPLA₂A was expressed in most larval instars except late last instar. Its expression was inducible by immune challenge and juvenile hormone analog injection. RNA interference of Se-sPLA₂A significantly suppressed cellular immunity and impaired larval development. These results suggest that non-venomous sPLA₂ plays a crucial role in immune and developmental processes in S. exigua, a lepidopteran insect.

An Insect Prostaglandin E2 Synthase Acts in Immunity and Reproduction

Eicosanoids, oxygenated metabolites of C20 polyunsaturated fatty acids (PUFAs), mediate fundamental physiological processes, including immune reactions and reproduction, in insects. Prostaglandins (PGs) make up one group of eicosanoids, of which PGE₂ is a relatively well-known mediator in various insect taxa. While PG biosynthesis has been reported, the specific biosynthetic pathway for PGE₂ is not known in insects. Here, we posed the hypothesis that Se-mPGES2 mediates biosynthesis of physiologically active PGE₂ through its cognate protein. To test this hypothesis, we interrogated a transcriptome of the lepidopteran insect, Spodoptera exigua, to identify a candidate PGE₂ synthase (Se-mPGES2) and analyzed its sequence and expression. Its predicted amino acid sequence contains a consensus thioredoxin homology sequence (Cys-x-x-Cys) responsible for catalytic activity along with an N-terminal membrane-associated hydrophobic domain and C-terminal cytosolic domain. It also shares sequence homology (36.5%) and shares almost overlapping three dimensional structures with a membrane-bound human PGES2 (mPGES2). Se-mPGES2 was expressed in all developmental stages with high peaks during the late larval instar and adult stages. Immune challenge significantly up-regulated its expression levels in hemocytes and fat body. Injecting double-stranded RNA (dsRNA) specific to Se-mPGES2 significantly impaired two cellular immune responses, hemocyte-spreading behavior and nodule formation following bacterial challenge. Humoral immunity was also significantly suppressed, registered as reduced phenoloxidase activity and antimicrobial peptide expression levels. The suppressed immune responses were reversed following PGE₂, but not arachidonic acid (AA), treatments. RNAi treatments also reduced the egg-laying behavior of females. Control females mated with the RNAi-treated males led to substantially reduced egg-laying behavior, which was also reversed following PGE₂ injections into females. These results strongly bolster our hypothesis that Se-mPGES2 acts in the biosynthesis of PGE₂, a crucial biochemical signal mediating immune and reproductive physiology of S. exigua.