Eicosanoid biosynthesis inhibitors influence mortality of Pieris brassicae larvae co-injected with fungal conidia

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.

Eicosanoids: Exploiting Insect Immunity to Improve Biological Control Programs

Insects, like all invertebrates, express robust innate, but not adaptive, immune reactions to infection and invasion. Insect immunity is usually resolved into three major components. The integument serves as a physical barrier to infections. Within the hemocoel, the circulating hemocytes are the temporal first line of defense, responsible for clearing the majority of infecting bacterial cells from circulation. Specific cellular defenses include phagocytosis, microaggregation of hemocytes with adhering bacteria, nodulation and encapsulation. Infections also stimulate the humoral component of immunity, which involves the induced expression of genes encoding antimicrobial peptides and activation of prophenoloxidase. These peptides appear in the hemolymph of challenged insects 6-12 hours after the challenge. Prostaglandins and other eicosanoids are crucial mediators of innate immune responses. Eicosanoid biosynthesis is stimulated by infection in insects. Inhibition of eicosanoid biosynthesis lethally renders experimental insects unable to clear bacterial infection from hemolymph. Eicosanoids mediate specific cell actions, including phagocytosis, microaggregation, nodulation, hemocyte migration, hemocyte spreading and the release of prophenoloxidase from oenocytoids. Some invaders have evolved mechanisms to suppress insect immunity; a few of them suppress immunity by targeting the first step in the eicosanoid biosynthesis pathways, the enzyme phospholipase A₂. We proposed research designed to cripple insect immunity as a technology to improve biological control of insects. We used dsRNA to silence insect genes encoding phospholipase A₂, and thereby inhibited the nodulation reaction to infection. The purpose of this article is to place our view of applying dsRNA technologies into the context of eicosanoid actions in insect immunity. The long-term significance of research in this area lies in developing new pest management technologies to contribute to food security in a world with a rapidly growing human population.

Benzylideneacetone, an eicosanoid biosynthesis inhibitor enhances baculovirus pathogenicity in the diamondback moth, Plutella xylostella

Benzylideneacetone (BZA) is a monoterpenoid compound produced by an entomopathogenic bacterium, Xenorhabdus nematophila. BZA inhibits phospholipase A(2) to suppress biosynthesis of eicosanoids that mediate immune responses in insects. In response to per os infection of Autographa californica multiple nucleopolyhedrosis virus (AcMNPV), the diamondback moth, Plutella xylostella, developed red spots on the midgut epithelium. The midgut exhibiting red spot formation suffered abnormal cell integrity, such as genomic DNA fragmentation and condensed spots in the nucleoplasm. The number of red spots increased with viral dose and incubation time after the viral treatment. BZA inhibited the formation of the midgut red spots in a dose-dependent manner. However, the inhibitory effect of BZA on the red spot formation was reversed by addition of arachidonic acid, suggesting that the red spot response may be mediated by eicosanoids. BZA treatment resulted in significant enhancement of AcMNPV occlusion body (OB) pathogenicity to P. xylostella.

Lipid mediators and vector infection: Trypanosoma rangeli inhibits Rhodnius prolixus hemocyte phagocytosis by modulation of phospholipase A2 and PAF-acetylhydrolase activities

In this work we investigated the effects of Trypanosoma rangeli infection through a blood meal on the hemocyte phagocytosis in experiments using the 5th instar larvae of Rhodnius prolixus. Hemocyte phagocytic activity was strongly blocked by oral infection with the parasites. In contrast, hemocyte phagocytosis inhibition caused by T. rangeli infection was rescued by exogenous arachidonic acid (20 microg/insect) or platelet activating factor (PAF; 1 microg/insect) applied by hemocelic injection. Following the oral infection with the protozoan we observed significant attenuation of phospholipase A2 (PLA2) activities in R. prolixus hemocytes (cytosolic PLA2: cPLA2, secreted PLA2: sPLA2 and Ca+2-independent PLA2: iPLA2) and enhancement of sPLA2 activities in cell-free hemolymph. At the same time, the PAF-acetyl hydrolase (PAF-AH) activity in the cell-free hemolymph increased considerably. Our results suggest that T. rangeli infection depresses eicosanoid and insect PAF analogous (iPAF) pathways giving support to the role of PLA2 in the regulation of arachidonic acid and iPAF biosynthesis and of PAF-AH by reducing the concentration of iPAF in R. prolixus. This illustrates the ability of T. rangeli to modulate the immune responses of R. prolixus to favor its own multiplication in the hemolymph.

Drosophila melanogaster as a model host to dissect the immunopathogenesis of zygomycosis

Zygomycosis is an emerging frequently fatal opportunistic mycosis whose immunopathogenesis is poorly understood. We developed a zygomycosis model by injecting Drosophila melanogaster flies with a standardized amount of fungal spores from clinical Zygomycetes isolates to study virulence and host defense mechanisms. We found that, as opposed to most other fungi, which are nonpathogenic in D. melanogaster (e.g., Aspergillus fumigatus), Zygomycetes rapidly infect and kill wild-type flies. Toll-deficient flies exhibited increased susceptibility to Zygomycetes, whereas constitutive overexpression of the antifungal peptide Drosomycin in transgenic flies partially restored resistance to zygomycosis. D. melanogaster Schneider 2 phagocytic cells displayed decreased phagocytosis and caused less hyphal damage to Zygomycetes compared with that to A. fumigatus. Furthermore, phagocytosis-defective eater mutant flies displayed increased susceptibility to Zygomycetes infection. Classic enhancers of Zygomycetes virulence in humans, such as corticosteroids, increased iron supply, and iron availability through treatment with deferoxamine dramatically increased Zygomycetes pathogenicity in our model. In contrast, iron starvation induced by treatment with the iron chelator deferasirox significantly protected flies infected with Zygomycetes. Whole-genome expression profiling in wild-type flies after infection with Zygomycetes vs. A. fumigatus identified genes selectively down-regulated by Zygomycetes, which act in pathogen recognition, immune defense, stress response, detoxification, steroid metabolism, or tissue repair or have unknown functions. Our results provide insights into the factors that mediate host-pathogen interactions in zygomycosis and establish D. melanogaster as a promising model to study this important mycosis.

Blockades of phospholipase A(2) and platelet-activating factor receptors reduce the hemocyte phagocytosis in Rhodnius prolixus: in vitro experiments

The hemocytes phagocytosis in response to microorganisms may play an important role in the cellular immune responses of insects. Here, we have evaluated the effects of the platelet-activating factor (PAF) and eicosanoids in the phagocytosis of hemocyte monolayers of Rhodnius prolixus to the yeast Saccharomyces cerevisiae. Experiments showed that the phagocytosis of yeast cells by Rhodnius hemocytes is very efficient in both controls and cells treated with PAF and arachidonic acid. Phagocytosis of yeast particles is significantly blocked when the specific phopholipase A(2) inhibitor, dexamethasone, is applied on the hemocytes. By contrast, dexamethasone-pretreated hemocyte monolayers exhibit a drastic increase in the quantity of yeast cell-hemocyte internalization when the cells are treated by arachidonic acid. In addition, phagocytosis presents significant reduction in hemocyte monolayers treated with a specific PAF receptor antagonist, WEB 2086. Nevertheless, inhibition of phagocytosis with WEB 2086 is counteracted by the treatment of the hemocyte monolayers with PAF. In conclusion, phagocytosis of yeast cells by hemocytes is related to the activation of PAF receptors and eicosanoid pathways in the bloodsucking bug, R. prolixus.