Insect immunity-effects of factors produced by a nematobacterial complex on immunocompetent cells(1)

Non-lethal effects of entomopathogenic nematode infection

Entomopathogenic nematodes are typically considered lethal parasites of insect hosts. Indeed they are employed as such for biological control of insect pests. The effects of exposure to entomopathogenic nematodes are not strictly limited to mortality, however. Here we explore non-lethal effects of exposure to entomopathogenic nematodes by introducing the relatively non-susceptible pupal stage of Delia antiqua to thirteen different strains. We specifically chose to inoculate the pupal stage because it tends to be more resistant to infection, yet resides in the soil where it could come into contact with EPN biological control agents. We find that there is no significant mortality at the pupal stage, but that there are a host of strain-dependent non-lethal effects during and after the transition to adulthood including altered developmental times and changes in risk of death compared to controls. We also find that exposure to specific strains can reduce risk of mortality. These results emphasize the strain-dependent nature of entomopathogenic nematode infection and highlight the positive and negative ramifications for non-lethal effects for biological control of insect pests. Our work emphasizes the need for strain-specific screening of biological control agents before wide-spread adoption.

Partner-specific induction of Spodoptera frugiperda immune genes in response to the entomopathogenic nematobacterial complex Steinernema carpocapsae-Xenorhabdus nematophila

The Steinernema carpocapsae-Xenorhabdus nematophila association is a nematobacterial complex used in biological control of insect crop pests. The infection success of this dual pathogen strongly depends on its interactions with the host's immune system. Here, we used the lepidopteran pest Spodoptera frugiperda to analyze the respective impact of each partner in the induction of its immune responses. First, we used previously obtained RNAseq data to construct the immunome of S. frugiperda and analyze its induction. We then selected representative genes to study by RT-qPCR their induction kinetics and specificity after independent injections of each partner. We showed that both X. nematophila and S. carpocapsae participate in the induction of stable immune responses to the complex. While X. nematophila mainly induces genes classically involved in antibacterial responses, S. carpocapsae induces lectins and genes involved in melanization and encapsulation. We discuss putative relationships between these differential inductions and the pathogen immunosuppressive strategies.

Bacteria of the Genus Xenorhabdus, a Novel Source of Bioactive Compounds

The genus Xenorhabdus of the family Enterobacteriaceae, are mutualistically associated with entomopathogenic nematodes of the genus Steinernema. Although most of the associations are species-specific, a specific Xenorhabdus sp. may infect more than one Steinernema sp. During the Xenorhabdus-Steinernema life cycle, insect larvae are infected and killed, while both mutualists produce bioactive compounds. These compounds act synergistically to ensure reproduction and proliferation of the nematodes and bacteria. A single strain of Xenorhabdus may produce a variety of antibacterial and antifungal compounds, some of which are also active against insects, nematodes, protozoa, and cancer cells. Antimicrobial compounds produced by Xenorhabdus spp. have not been researched to the same extent as other soil bacteria and they may hold the answer to novel antibacterial and antifungal compounds. This review summarizes the bioactive secondary metabolites produced by Xenorhabdus spp. and their application in disease control. Gene regulation and increasing the production of a few of these antimicrobial compounds are discussed. Aspects limiting future development of these novel bioactive compounds are also pointed out.

Translational Control of Host Gene Expression by a Cys-Motif Protein Encoded in a Bracovirus

Translational control is a strategy that various viruses use to manipulate their hosts to suppress acute antiviral response. Polydnaviruses, a group of insect double-stranded DNA viruses symbiotic to some endoparasitoid wasps, are divided into two genera: ichnovirus (IV) and bracovirus (BV). In IV, some Cys-motif genes are known as host translation-inhibitory factors (HTIF). The genome of endoparasitoid wasp Cotesia plutellae contains a Cys-motif gene (Cp-TSP13) homologous to an HTIF known as teratocyte-secretory protein 14 (TSP14) of Microplitis croceipes. Cp-TSP13 consists of 129 amino acid residues with a predicted molecular weight of 13.987 kDa and pI value of 7.928. Genomic DNA region encoding its open reading frame has three introns. Cp-TSP13 possesses six conserved cysteine residues as other Cys-motif genes functioning as HTIF. Cp-TSP13 was expressed in Plutella xylostella larvae parasitized by C. plutellae. C. plutellae bracovirus (CpBV) was purified and injected into non-parasitized P. xylostella that expressed Cp-TSP13. Cp-TSP13 was cloned into a eukaryotic expression vector and used to infect Sf9 cells to transiently express Cp-TSP13. The synthesized Cp-TSP13 protein was detected in culture broth. An overlaying experiment showed that the purified Cp-TSP13 entered hemocytes. It was localized in the cytosol. Recombinant Cp-TSP13 significantly inhibited protein synthesis of secretory proteins when it was added to in vitro cultured fat body. In addition, the recombinant Cp-TSP13 directly inhibited the translation of fat body mRNAs in in vitro translation assay using rabbit reticulocyte lysate. Moreover, the recombinant Cp-TSP13 significantly suppressed cellular immune responses by inhibiting hemocyte-spreading behavior. It also exhibited significant insecticidal activities by both injection and feeding routes. These results indicate that Cp-TSP13 is a viral HTIF.

Insect Immunity to Entomopathogenic Nematodes and Their Mutualistic Bacteria

Entomopathogenic nematodes are important organisms for the biological control of insect pests and excellent models for dissecting the molecular basis of the insect immune response against both the nematode parasites and their mutualistic bacteria. Previous research involving the use of various insects has found distinct differences in the number and nature of immune mechanisms that are activated in response to entomopathogenic nematode parasites containing or lacking their associated bacteria. Recent studies using model insects have started to reveal the identity of certain molecules with potential anti-nematode or antibacterial activity as well as the molecular components that nematodes and their bacteria employ to evade or defeat the insect immune system. Identification and characterization of the genes that regulate the insect immune response to nematode-bacteria complexes will contribute significantly to the development of improved practices to control insects of agricultural and medical importance, and potentially nematode parasites that infect mammals, perhaps even humans.

Modulation of immune responses of Rhynchophorus ferrugineus (Insecta: Coleoptera) induced by the entomopathogenic nematode Steinernema carpocapsae (Nematoda: Rhabditida)

Aim of this study was to investigate relationships between the red palm weevil (RPW) Rhynchophorus ferrugineus (Olivier) and the entomopathogenic nematode Steinernema carpocapsae (EPN); particularly, the work was focused on the immune response of the insect host in naive larvae and after infection with the EPN. Two main immunological processes have been addressed: the activity and modulation of host prophenoloxidase-phenoloxidase (proPO) system, involved in melanization of not-self and hemocytes recognition processes responsible for not-self encapsulation. Moreover, immune depressive and immune evasive strategies of the parasite have been investigated. Our results suggest that RPW possess an efficient immune system, however in the early phase of infection, S. carpocapsae induces a strong inhibition of the host proPO system. In addition, host cell-mediated mechanisms of encapsulation, are completely avoided by the parasite, the elusive strategies of S. carpocapsae seem to be related to the structure of its body-surface, since induced alterations of the parasite cuticle resulted in the loss of its mimetic properties. S. carpocapsae before the release of its symbiotic bacteria, depress and elude RPW immune defenses, with the aim to arrange a favorable environment for its bacteria responsible of the septicemic death of the insect target.

Insect immune responses to nematode parasites

Host innate immunity plays a central role in detecting and eliminating microbial pathogenic infections in both vertebrate and invertebrate animals. Entomopathogenic or insect pathogenic nematodes are of particular importance for the control of insect pests and vectors of pathogens, while insect-borne nematodes cause serious diseases in humans. Recent work has begun to use the power of insect models to investigate host-nematode interactions and uncover host antiparasitic immune reactions. This review describes recent findings on innate immune evasion strategies of parasitic nematodes and host cellular and humoral responses to the infection. Such information can be used to model diseases caused by human parasitic nematodes and provide clues indicating directions for research into the interplay between vector insects and their invading tropical parasites.

A pathogenic parasite interferes with phagocytosis of insect immunocompetent cells

Phagocytosis activity of hemocytes of the host Galleria mellonella (Lepidoptera) was modulated by the infection of the entomopathogenic nematode Steinernema feltiae (Rahbditida) and was found to be correlated with the opsonization of bacteria by hemolymph factors. The presence of nematodes resulted in a significative decrease in phagocytosis of bacteria by host hemocytes, both in in vivo and in in vitro assays. Host interacting proteins (HIPs), which appear to function as opsonic factors and are essential to perform immune responses, were removed by S. feltiae from host hemolymph, by means of its epicuticle binding properties. Host humoral factors sequestered by the parasite have been identified by monodimensional and 2D electrophoretic analysis. The data suggest that S. feltiae, living in association with symbiontic bacteria (Xenorhabdus nematophilus), develop an immune suppressive strategy to support its bacteria, which diminished the effectiveness of immunological surveillance by the host.

Probing the tri-trophic interaction between insects, nematodes and Photorhabdus

SUMMARY

Photorhabdus sp. are entomopathogenic bacteria which, upon experimental infection, interact with the insect immune system, but little is known about the roles of their symbiotic nematode partners Heterorhabditis sp. in natural infections. Here, we investigated the respective contributions of nematodes and bacteria by examining humoral and cellular immune reactions of the model lepidopteran insect Manduca sexta against Heterorhabditis carrying Photorhabdus, nematodes free of bacteria (axenic nematodes) and bacteria alone. Insect mortality was slower following infection with axenic nematodes than when insects were infected with nematodes containing Photorhabdus, or the bacteria alone. Nematodes elicited host immune responses to a lesser extent than bacteria. Transcription of certain recognition and antibacterial genes was lower when insects were naturally infected with nematodes carrying no bacteria compared to insects that received bacteria, either with or without nematodes. Axenic nematodes also did not elicit such high levels of phenoloxidase activity and haemocyte aggregates as did treatments involving Photorhabdus. By contrast, the phagocytic capability of host haemocytes was decreased by both axenic and bacteria-associated nematodes, but not by Photorhabdus alone. These results imply that both bacteria and nematodes contribute separately to the pathogenic modulation of host immune responses during natural infections by the mutualistic Heterorhabdus-Photorhabdus complex.

Social management of LPS-induced inflammation in Formica polyctena ants

Invertebrates, and especially insects, constitute valuable and convenient models for the study of the evolutionary roots of immune-related behaviors. With stable conditions in the nest, high population densities, and frequent interactions, social insects such as ants provide an excellent system for examining the spread of pathogens. The evolutionary success of these species raises questions about the behavioral responses of social insects to an infected nestmate. In this experiment, we tested the behavioral changes of the red wood ant Formica polyctena toward an immune-stimulated nestmate. We used bacterial endotoxin (lipopolysaccharides, LPS) to active the innate immune system of individual worker ants without biasing our observation with possible cues or host-manipulation from a living pathogen. We show that LPS-induced immune activation in ants triggers behavioral changes in nestmates. Contrary to what would be expected, we did not find removal strategies (e.g. agonistic behaviors) or avoidance of the pathogenic source, but rather a balance between a limitation of pathogen dissemination (i.e. decreased trophallaxis and locomotion of the LPS-treated ant), and what could constitute the behavioral basis for a "social vaccination" (i.e. increased grooming). This supports the importance of social interactions in resistance to disease in social insects, and perhaps social animals in general.

Cuticular surface lipids are responsible for disguise properties of an entomoparasite against host cellular responses

Entomopathogenic nematodes are widely used as alternatives to chemicals for the biological control of insects. These endoparasites are symbiotically associated with bacteria that are lethal for the host; however, parasites need to overcome the host immune defences to complete a successful life cycle. The processes parasites employ to escape or depress host immunity are targeted at deceiving non-self recognition as well as inactivating defence reactions. The purpose of this paper is to investigate the interactions between the entomopathogenic nematode Steinernema feltiae and the lepidopteran Galleria mellonella, focusing on the role of the parasite's body-surface compounds in the immunoevasion of host cell-mediated responses. To evaluate host self/non-self discrimination and encapsulation efficiency, we carried out a series of interaction assays between cultured host hemocytes and parasites or isolated cuticles. The data obtained suggest that the parasite cuticular lipids (PCLs) are able to bind a variety of host hemolymph molecules; PCLs attract host proteins from the hemolymph creating a coat around the parasite, thus, enabling Steinernema to disguise itself against hemocytes recognition. The role of parasite lipids in the disguise process was also investigated by simulating the nematode body surface with agarose microbeads covered with purified cuticular components; when the beads were coated with cuticular lipids, host hemocytes were not able to recognize and encapsulate. Results suggest that by means of attracting host hemolymph components onto its cuticular surface, S. feltiae prevents hemocytes attachment to its cuticle and inhibits melanization by depleting hemolymph components.

Friend and foe: the two faces of Xenorhabdus nematophila

Comparisons of mutualistic and pathogenic relationships are necessary to decipher the common language of microorganism-host interactions, as well as the subtle differences in dialect that distinguish types of symbiosis. One avenue towards making such comparisons is to study a single organism that speaks both dialects, such as the gamma-proteobacterium Xenorhabdus nematophila. X. nematophila inhabits and influences the lives of two host animals, helping one to reproduce optimally while killing the other.

Clonal variation in Xenorhabdus nematophila virulence and suppression of Manduca sexta immunity

Virulence of the insect pathogen Xenorhabdus nematophila is attributed in part to its ability to suppress immunity. For example, X. nematophila suppresses transcripts encoding several antimicrobial proteins, even in the presence of Salmonella enterica, an inducer of these transcripts. We show here that virulence and immune suppression phenotypes can be lost in a subpopulation of X. nematophila. Cells that have undergone 'virulence modulation' (vmo) have attenuated virulence and fail to suppress antimicrobial transcript levels, haemocyte aggregation and nodulation in Manduca sexta insects. When plated on certain media, vmo cells have a higher proportion of translucent (versus opaque) colonies compared with non-vmo cells. Like vmo strains, translucent colony isolates are defective in virulence and immune suppression. The X. nematophila genome encodes two 'opacity' genes with similarity to the Ail/PagC/Rck family of outer membrane proteins involved in adherence, invasion and serum resistance. Quantitative polymerase chain reaction analysis shows that RNA levels of one of these opacity genes, opaB, are higher in opaque relative to translucent colonies. We propose that in X. nematophila opaB may be one of several factors involved in immune suppression during infection, and expression of these factors can be co-ordinately eliminated in a subpopulation, possibly through a phase variation mechanism.

The xaxAB Genes Encoding a New Apoptotic Toxin from the Insect Pathogen Xenorhabdus nematophila Are Present in Plant and Human Pathogens

Xenorhabdus nematophila, a member of the Enterobacteriaceae, kills many species of insects by strongly depressing the immune system and colonizing the entire body. A peptide cytotoxin has been purified from X. nematophila broth growth, and the cytolytic effect on insect immunocytes and hemolytic effect on mammalian red blood cells of this toxin have been described (Ribeiro, C., Vignes, M., and Brehélin, M. (2003) J. Biol. Chem. 278, 3030-3039). We show here that this toxin, Xenorhabdus alpha-xenorhabdolysin (Xax), triggers apoptosis in both insect and mammalian cells. We also report the cloning and sequencing of two genes, xaxAB, encoding this toxin in X. nematophila. The expression of both genes in recombinant Escherichia coli led to the production of active cytotoxin/hemolysin. However, hemolytic activity was observed only if the two peptides were added in the appropriate order. Furthermore, we report here that inactivation of xaxAB genes in X. nematophila abolished the major cytotoxic activity present in broth growth, called C1. We also show that these genes are present in various entomopathogenic bacteria of the genera Xenorhabdus and Photorhabdus, in Pseudomonas entomophila, in the human pathogens Yersinia enterocolitica and Proteus mirabilis, and in the plant pathogen Pseudomonas syringae. This toxin cannot be classified in any known family of cytotoxins on the basis of amino acid sequences, locus organization, and activity features. It is, therefore, probably the prototype of a new family of binary toxins.

The cytotoxic fimbrial structural subunit of Xenorhabdus nematophila is a pore-forming toxin

We have purified a fimbrial shaft protein (MrxA) of Xenorhabdus nematophila. The soluble monomeric protein lysed larval hemocytes of Helicoverpa armigera. Osmotic protection of the cells with polyethylene glycol suggested that the 17-kDa MrxA subunit makes pores in the target cell membrane. The internal diameter of the pores was estimated to be >2.9 nm. Electron microscopy confirmed the formation of pores by the fimbrial subunit. MrxA protein oligomerized in the presence of liposomes. Electrophysiological studies demonstrated that MrxA formed large, voltage-gated passive-diffusion channels in lipid bilayers.

Down-regulation of antibacterial peptide synthesis in an insect model induced by the body-surface of an entomoparasite (Steinernema feltiae)

This study focuses on the interaction between an entomopathogenic nematode and an insect model to further investigate the role of parasite body-surface and its compounds (particularly epicuticular lipids) in the immunosuppression of host defences. Our goal was to ascertain the potential interference of the parasite epicuticular lipids with the antimicrobial response mounted by Gram-negative bacteria-challenged hosts. Since the parasite model used in this study (Steinernema feltiae) releases symbiontic bacteria in the host hemocoel during the late phase of infection, the inhibition of the antimicrobial response could be needed to avoid the activity of host factors potentially harmful for the microorganisms. After bacterial challenge, when insect hosts were infected with purified parasite cuticles, we always observed lack of bacterial clearance concurrently with the absence of hemolymph low molecular weight components. The observed effects seem to be related to the interaction of parasite cuticular lipids (PCLs) with specific components of the host hemolymph; these host interacting proteins (HIP17, HIP26 and HIP35) were removed by the parasite, and their absence (or reduction) apparently prevented antimicrobial peptide synthesis. The inhibitory properties were lost when cuticles were pre-treated with compounds (such as lipase or methanol-chloroform) affecting their lipidic moiety. Moreover, the key role of epicuticular lipids was also confirmed by the inhibitory properties of methanol-chloroform extracted lipids, which were comparable to those of parasite whole cuticles. Finally, the involvement of HIPs was assessed by their partial purification followed by injection into the host. When HIPs were co-injected with bacteria into cuticle-inhibited larvae, the antimicrobial activity was completely restored.

Two groups of entomopathogenic bacteria, Photorhabdus and Xenorhabdus, share an inhibitory action against phospholipase A2 to induce host immunodepression

Photorhabdus and Xenorhabdus are two genera of entomopathogenic bacteria having a mutualistic relationship with their respective nematode hosts, Heterorhabditis and Steinernema. One of the pathogenic mechanisms of these bacteria includes host immunodepression, which leads to lethal septicemia. It has been known that X. nematophila inhibits phospholipase A2 (PLA2) to induce host immunodepression. Here, we tested the hypothesis of PLA2 inhibition using another bacterial species involved in other genera. P. temperata subsp. temperata is the intestinal symbiont of an entomopathogenic nematode, H. megidis. The bacteria caused potent pathogenicity in a dose-dependent manner against the fifth instar larvae of a test target insect, Spodoptera exigua, as early as 24 h after the intra-hemocoelic injection. In response to the live bacterial injection, hemocyte nodulation (a cellular immune response) and prophenoloxidase (pPO) activation were inhibited, while the injection of heat-killed bacteria significantly induced both immune reactions. The immunodepression induced by the live bacteria was reversed by the addition of arachidonic acid, the catalytic product of phospholipase A2. In contrast, the addition of dexamethasone, a specific PLA2 inhibitor to the heat-killed bacterial treatment, inhibited both immune capacities. In addition to a previously known PLA2 inhibitory action of X. nematophila, the inhibition of P. temperata temperata on PLA2 suggests that bacteria symbiotic to entomopathogenic nematodes share a common pathogenic target to result in an immunodepressive state of the infected insects. To prove this generalized hypothesis, we used other bacterial species (X. bovienni, X. poinarii, and P. luminescens) involved in these two genera. All our experiments clearly showed that these other bacteria also share their inhibitory action against PLA2 to induce host immunodepression.

Stages of infection during the tripartite interaction between Xenorhabdus nematophila, its nematode vector, and insect hosts

Bacteria of the genus Xenorhabdus are mutually associated with entomopathogenic nematodes of the genus Steinernema and are pathogenic to a broad spectrum of insects. The nematodes act as vectors, transmitting the bacteria to insect larvae, which die within a few days of infection. We characterized the early stages of bacterial infection in the insects by constructing a constitutive green fluorescent protein (GFP)-labeled Xenorhabdus nematophila strain. We injected the GFP-labeled bacteria into insects and monitored infection. We found that the bacteria had an extracellular life cycle in the hemolymph and rapidly colonized the anterior midgut region in Spodoptera littoralis larvae. Electron microscopy showed that the bacteria occupied the extracellular matrix of connective tissues within the muscle layers of the Spodoptera midgut. We confirmed the existence of such a specific infection site in the natural route of infection by infesting Spodoptera littoralis larvae with nematodes harboring GFP-labeled Xenorhabdus. When the infective juvenile (IJ) nematodes reached the insect gut, the bacterial cells were rapidly released from the intestinal vesicle into the nematode intestine. Xenorhabdus began to escape from the anus of the nematodes when IJs were wedged in the insect intestinal wall toward the insect hemolymph. Following their release into the insect hemocoel, GFP-labeled bacteria were found only in the anterior midgut region and hemolymph of Spodoptera larvae. Comparative infection assays conducted with another insect, Locusta migratoria, also showed early bacterial colonization of connective tissues. This work shows that the extracellular matrix acts as a particular colonization site for X. nematophila within insects.

The role of Steinernema feltiae body-surface lipids in host-parasite immunological interactions

Interactions between entomopathogenic nematodes (Steinernema feltiae) and insect host (Galleria mellonella) immune system were investigated. We focused on the immunosuppressive properties of the parasite cuticle and on its interaction with hemolymph humoral components. Effects of parasite cuticle against host proPO system enzymatic cascade were evaluated a short time after infection. The presence of parasite cuticles decreased both normal and LPS-elicited proPO system activity, suggesting that S. feltiae body surface plays a key role in the early parasitation phase, probably interfering with host proPO activation pathways. The data obtained showed that cuticle lipidic compounds are able to interact with host humoral components, removing them from the hemolymph. The depletion of these molecules, arbitrarily named host-interacting proteins (HIPs), seems to be responsible of the drastic decrease in proPO system activity. Moreover, hemolymph HIPs showed LPS-binding properties and parasite cuticle cross-reacted with anti-LPS antibodies. Finally, we also assessed the involvement of parasite body surface on immunoevasion strategies of S. feltiae against host cell-mediated encapsulation processes. We conclude that S. feltiae body surface is responsible for short-term immunosuppression and immunoevasion processes; since it is able to sequester host hemolymph compounds involved in proPO system activation and this process could be responsible for a molecular disguise strategy against cellular encapsulation.

Modes of phagocytosis of Gram-positive and Gram-negative bacteria by Spodoptera littoralis granular haemocytes

Haemocytes are the main immunocompetent cells in insect cellular immune reactions. Here, we show that in Spodoptera littoralis, granular haemocytes are the primary phagocyte haemocytes, both in vivo and in vitro. The "trigger" and "zipper" modes of engulfment known in mammal macrophages are active, in vivo, in S. littoralis granular haemocytes, together with macropinocytosis. Lipopolysaccharide as well as lipoteichoic acid inhibit the binding of both Gram-positive (Corynebacterium xerosis) and Gram-negative (Escherichia coli) bacteria on granular haemocytes. In addition, different ligands can inhibit the binding of E. coli. Most of these inhibitors are known as ligands of scavenger receptors in mammal macrophages and we hypothesise that one of the receptors present on S. littoralis granular haemocytes could be a scavenger-like receptor.

An entomopathogenic bacterium, Xenorhabdus nematophila, inhibits hemocytic phospholipase A2 (PLA2) in tobacco hornworms Manduca sexta

The entomopathogenic bacterium, Xenorhabdus nematophila, induces immunodepression in target insects and finally leads to lethal septicemia of the infected hosts. A hypothesis has been raised that the bacteria inhibit eicosanoid-biosynthesis pathway to interrupt immune signaling of the infected hosts. Here, we show direct evidence that X. nematophila inhibits the activity of phospholipase A2 (PLA2), the initial step in the eicosanoid-biosynthesis pathway. Inhibition of PLA2 was dependent on both incubation time with X. nematophila and the bacterial concentration in in vitro PLA2 preparations of Manduca sexta hemocytes. While living bacteria inhibited PLA2 activity, heat-killed X. nematophila rather increased PLA2 activity. X. nematophila secreted PLA2 inhibitor(s) which were detected in the organic, but not aqueous, extract of the bacterial culture medium. The PLA2 inhibitory activity of the organic extract was lost after heat treatment. These results clearly indicate that X. nematophila inhibits PLA2 activity, and thereby inhibits eicosanoid biosynthesis which leads to immunodepression of the infected hosts.

Xenorhabdus nematophilus inhibits p-bromophenacyl bromide (BPB)-sensitive PLA2 of Spodoptera exigua

Xenorhabdus nematophilus is a Gram-negative symbiotic bacterium of the entomopathogenic nematode, Steinernema carpocapsae. The bacteria delivered into the insect hemocoel by the nematodes cause immunodepression of the target insects to protect host nematodes and themselves from the cellular immune reaction. Previous reports suggest that the immunodepression is caused by inhibition of the eicosanoid pathway that is known to be critically important to mediate cellular immunity. This study focused on the inhibitory effect of X. nematophilus on PLA2 activity of Spodoptera exigua. The PLA2 activity was functionally associated with the activation cascade of prophenoloxidase (pPO). Dexamethasone (DEX), a specific PLA2 inhibitor, inhibited pPO activation completely at the higher doses of approximately 2.4 muM in vitro condition. The inhibitory effect of DEX was reversed by the addition of arachidonic acid, the catalytic product of PLA2. By means of this in vitro PLA2 inhibitor assay system, two different PLA2 inhibitors were used to compare their inhibitory effects on the hemolymph PLA2 of S. exigua. p-Bromophenacyl bromide (BPB), a specific inhibitor of secretory PLA2 (sPLA2), significantly inhibited pPO activation, but methylarachidonyl fluorophosphates (MAFP), a specific inhibitor of cytosolic PLA2 (cPLA2), did not show any inhibitory effect. BPB also inhibited pPO activation of the plasma, though much higher PO activation and its inhibition by BPB was found in the hemocytes. Growth medium of X. nematophilus at the stationary phase had a PLA2 inhibitory effect. Via the in vitro PLA2 inhibitor assay, it was shown that the ethyl ether extract of the medium contained significant PLA2 inhibitor activity. These results indicate that X. nematophilus produces and secretes PLA2 inhibitor, which acts on BPB-susceptible PLA2 of S. exigua.

Suppression of the prophenoloxidase system in Rhodnius prolixus orally infected with Trypanosoma rangeli

Investigations were carried out to compare aspects of the prophenoloxidase (proPO)-activating pathway in Rhodnius prolixus hemolymph in response to oral infection and inoculation of the insects with two developmental forms of Trypanosoma rangeli epimastigotes strain H14. In vivo experiments demonstrated that in control insects fed on uninfected blood, inoculation challenge with short epimastigotes resulted in high phenoloxidase (PO) activity. In contrast, previous feeding on blood containing either short or long epimastigotes was able to suppress the proPO activation induced by thoracic inoculation of the short forms. In vitro assays in the presence of short epimastigotes demonstrated that control hemolymph or hemolymph provided by insects previously fed on blood containing epimastigotes incubated with fat body homogenates from control insects significantly increased the PO activity. However, fat body homogenates from insects previously fed on blood containing epimastigotes, incubated with hemolymph taken from insects fed on control blood or blood infected with epimastigotes, drastically reduced the proPO activation. The proteolytic activity in the fat body homogenates of control insects was significantly higher than in those obtained from fat body extracts of insects previously fed on blood containing epimastigotes. These findings indicate that the reduction of the proteolytic activities in the fat body from insects fed on infected blood no longer allows a significant response of the proPO system against parasite challenge. It also provides a better understanding of T. rangeli infection in the vector and offer novel insights into basic immune processes in their invertebrate hosts.

Xenorhabdus nematophila (enterobacteriacea) secretes a cation-selective calcium-independent porin which causes vacuolation of the rough endoplasmic reticulum and cell lysis

Xenorhabdus nematophila and Photorhabdus luminescens are two related enterobacteriaceae studied for their use in biological control and for synthesis of original virulence factors and new kinds of antibiotics. X. nematophila broth growth exhibits different cytotoxic activities on insect (Spodoptera littoralis, lepidoptera) immunocytes (hemocytes). Here we report the purification of the flhDC-dependent cytotoxin, a 10,790-Da peptide we have called alpha-Xenorhabdolysin (alpha X). We show that plasma membrane of insect hemocytes and of mammal red blood cells is the first target of this toxin. Electrophysiological and pharmacological approaches indicate that the initial effect of alpha X on macrophage plasma membrane is an increase of monovalent cation permeability, sensitive to potassium channel blockers. As a consequence, several events can occur intracellularly, such as selective vacuolation of the endoplasmic reticulum, cell swelling, and cell death by colloid-osmotic lysis. These effects, inhibited by potassium channel blockers, are totally independent of Ca(2+). However, the size of the pores created by alpha X on macrophage or red blood cell plasma membrane increases with toxin concentration, which leads to a rapid cell lysis.

Immune suppression of Galleria mellonella (Insecta, Lepidoptera) humoral defenses induced by Steinernema feltiae (Nematoda, Rhabditida): involvement of the parasite cuticle

Immune depression of Galleria mellonella larvae was evaluated a short time after infection with the entomopathogenic nematode Steinernema feltiae. In the host the activity of the enzymatic cascade known as the proPO system was significantly reduced by the presence of either live or dead parasites. The presence of parasites decreased the LPS-elicited proPO system activity. In addition, this process seems to be related to a decrease in the activity of hemolymph proteases, more than to phenoloxidase damage. proPO inhibition was also achieved by injected isolated cuticle fragments, suggesting that the parasite body surface plays an important role in the early parasitation phase.

Two distinct hemolytic activities in Xenorhabdus nematophila are active against immunocompetent insect cells

Xenorhabdus spp. and Photorhabdus spp. are major insect bacterial pathogens symbiotically associated with nematodes. These bacteria are transported by their nematode hosts into the hemocoel of the insect prey, where they proliferate within hemolymph. In this work we report that wild strains belonging to different species of both genera are able to produce hemolysin activity on blood agar plates. Using a hemocyte monolayer bioassay, cytolytic activity against immunocompetent cells from the hemolymph of Spodoptera littoralis (Lepidoptera: Noctuidae) was found only in supernatants of Xenorhabdus; none was detected in supernatants of various strains of Photorhabdus. During in vitro bacterial growth of Xenorhabdus nematophila F1, two successive bursts of cytolytic activity were detected. The first extracellular cytolytic activity occurred when bacterial cells reached the stationary phase. It also displayed a hemolytic activity on sheep red blood cells, and it was heat labile. Among insect hemocyte types, granulocytes were the preferred target. Lysis of hemocytes by necrosis was preceded by a dramatic vacuolization of the cells. In contrast the second burst of cytolytic activity occurred late during stationary phase and caused hemolysis of rabbit red blood cells, and insect plasmatocytes were the preferred target. This second activity is heat resistant and produced shrinkage and necrosis of hemocytes. Insertional inactivation of flhD gene in X. nematophila leads to the loss of hemolysis activity on sheep red blood cells and an attenuated virulence phenotype in S. littoralis (A. Givaudan and A. Lanois, J. Bacteriol. 182:107-115, 2000). This mutant was unable to produce the early cytolytic activity, but it always displayed the late cytolytic effect, preferably active on plasmatocytes. Thus, X. nematophila produced two independent cytolytic activities against different insect cell targets known for their major role in cellular immunity.

Refractoriness of host haemocytes to parasite immunosuppressive factors as a putative resistance mechanism in the Biomphalaria glabrata-Echinostoma caproni system

In contrast to the growing knowledge accumulated on plant resistance to pathogens, mechanisms of parasite resistance largely remain to be elucidated in animal species. In the present study we investigated mechanisms underlying resistance/susceptibility in the snail-trematode system Biomphalaria glabrata-Echinostoma caproni. In particular, we compared the effect of the parasite excretory-secretory (E-S) products on the defence functions of haemocytes from 2 susceptible and 2 resistant snail strains. In vitro experiments showed that E. caproni E-S products inhibit adhesion and phagocytosis of haemocytes from susceptible snails. A partial biochemical characterization also suggested that the interfering factor(s) is (are) heat-labile glycosylated polypeptides of molecular mass between 10 and 30 kDa. Interestingly, haemocytes from resistant snails remained unaffected by the parasite E-S products, suggesting that a constitutive difference results in their refractoriness to the parasite's immunosuppressive factor(s).

The plcR regulon is involved in the opportunistic properties of Bacillus thuringiensis and Bacillus cereus in mice and insects

Bacillus thuringiensis has been widely used for 40 years as a safe biopesticide for controlling agricultural pests and mosquitoes because it produces insecticidal crystal proteins. However, spores have also been shown to contribute to overall entomopathogenicity. Here, the opportunistic properties of acrystalliferous B. thuringiensis Cry(-) and Bacillus cereus strains were investigated in an insect species, Galleria mellonella, and in a mammal, BALB/c mice. In both animal models, the pathogenicity of the two bacterial species was similar. Mutant strains were constructed in which the plcR gene, encoding a pleiotropic regulator of extracellular factors, was disrupted. In larvae, co-ingestion of 10(6) spores of the parental strain with a sublethal concentration of Cry1C toxin caused 70% mortality whereas only 7% mortality was recorded if spores of the DeltaplcR mutant strain were used. In mice, nasal instillation of 10(8) spores of the parental strain caused 100% mortality whereas instillation with the same number of DeltaplcR strain spores caused much lower or no mortality. Similar effects were obtained if vegetative cells were used instead of spores. The cause of death is unknown and is unlikely to be due to actual growth of the bacteria in mice. The lesions caused by B. thuringiensis supernatant in infected mice suggested that haemolytic toxins were involved. The cytolytic properties of strains of B. thuringiensis and B. cereus, using sheep, horse and human erythrocytes and G. mellonella haemocytes, were therefore investigated. The level of cytolytic activity is highly reduced in DeltaplcR strains. Together, the results indicate that the pathogenicity of B. thuringiensis strain 407 and B. cereus strain ATCC 14579 is controlled by PlcR.

Eicosanoids rescue Spodoptera exigua infected with Xenorhabdus nematophilus, the symbiotic bacteria to the entomopathogenic nematode Steinernema carpocapsae

Xenorhabdus nematophilus is a pathogenic bacterium causing insect haemolymph septicemia, which leads to host insect death. To address the fundamental mechanisms underlying this haemolymph septicemia, or the immunodepressive response of the host insects following bacterial infection, we tested a hypothesis that the insect immune-mediating eicosanoid pathway is blocked by inhibitory action of the bacterium. Haemocoelic injection of the bacteria into the fifth instar larvae of Spodoptera exigua reduced the total number of living haemocytes with postinjection time and resulted in host death in 16 h at 25 degrees C. The lethal efficacy, described by the median lethal bacterial dose (LD(50)), was estimated as 33 colony-forming units per fifth instar larva of S. exigua. The lethal effect of the bacteria on the infected larvae decreased significantly with the addition of exogenous arachidonic acid (10 µg), a precursor of eicosanoids. In comparison, injections of dexamethasone (10 µg), a specific inhibitor of phospholipase A(2), and other eicosanoid biosynthesis inhibitors elevated significantly the bacterial pathogenicity. Live X. nematophilus induced the infected larvae to form less nodules than did the heat-killed bacteria, but the addition of arachidonic acid increased the number of nodules formed significantly in response to live bacterial injection. The treatment with dexamethasone and other inhibitors, however, decreased the nodule formation after injection of heat-killed bacteria. These results indicate that eicosanoids play a role in the immune response of S. exigua, and suggest strongly that X. nematophilus inhibits its eicosanoid pathway, which then results in immunodepressive haemolymph septicemia.

flhDC, the flagellar master operon of Xenorhabdus nematophilus: requirement for motility, lipolysis, extracellular hemolysis, and full virulence in insects

Xenorhabdus is a major insect pathogen symbiotically associated with nematodes of the family Steinernematidae. This motile bacterium displays swarming behavior on suitable media, but a spontaneous loss of motility is observed as part of a phenomenon designated phase variation which involves the loss of stationary-phase products active as antibiotics and potential virulence factors. To investigate the role of one of the transcriptional activators of flagellar genes, FlhDC, in motility and virulence, the Xenorhabdus nematophilus flhDC locus was identified by functional complementation of an Escherichia coli flhD null mutant and DNA sequencing. Construction of X. nematophilus flhD null mutants confirmed that the flhDC operon controls flagellin expression but also revealed that lipolytic and extracellular hemolysin activity is flhDC dependent. We also showed that the flhD null mutant displayed a slightly attenuated virulence phenotype in Spodoptera littoralis compared to that of the wild-type strain. Thus, these data indicated that motility, lipase, hemolysin, or unknown functions controlled by the flhDC operon are involved in the infectious process in insects. Our investigation expands the view of the flagellar regulon as a checkpoint coupled to a major network involving bacterial physiological aspects as well as motility.