A novel polydnavirus protein inhibits the insect prophenoloxidase activation pathway

IMPACT OF IMMUNE RESPONSE OF A PARASITIC BEETLE Dastarcus helophoroides ON ITS HOST BEETLE Monochamus alternatus

Dastarcus helophoroides is an ectoparasitoid beetle of Monochamus alternatus, and the parasitism by D. helophoroides larvae remarkably influenced on the immune responses of M. alternatus larvae in many aspects. The hemolymph melanization reactions in the hosts were inhibited 1 h and 24 h postparasitization. The phenoloxidase activities of hemolymph were significantly stimulated 4 h postparasitization and inhibited 12 h postparasitization, and back to control level. The antibacterial activities of hemolymph in the parasitized hosts were significantly lower than that in the unparasitized ones 1 h postparasitization. By 72 h postparasitism, the total hemocyte numbers of the parasitized larvae declined to not more than one-seconds of the number collected from the unparasitized larvae. All sampled hemolymph held the capability of nodulation, and there were fluctuations in the number of nodules the hemocytes made. However, there were no significant differences between unparasitized and parasitized larvae at each time point in the hemagglutination activity and the ratios of spreading hemocytes. In conclusion, D. helophoroides larvae could regulate M. alternatus immune system and resulted in the changes in host immune responses.

Transcriptomic response of Manduca sexta immune tissues to parasitization by the bracovirus associated wasp Cotesia congregata

During oviposition, Cotesia congregata parasitoid wasps inject into their host, Manduca sexta, some biological factors such as venom, ovarian fluid and a symbiotic polydnavirus (PDV) named Cotesia congregata bracovirus (CcBV). During parasitism, complex interactions occur between wasp-derived factors and host targets that lead to important modifications in host physiology. In particular, the immune response leading to wasp egg encapsulation is inhibited allowing wasp survival. To date, the regulation of host genes during the interaction had only been studied for a limited number of genes. In this study, we analysed the global impact of parasitism on host gene regulation 24 h post oviposition by high throughput 454 transcriptomic analyses of two tissues known to be involved in the host immune response (hemocytes and fat body). To identify specific effects of parasitism on host transcription at this time point, transcriptomes were obtained from non-treated and parasitized larvae, and also from larvae injected with heat-killed bacteria and double stimulated larvae that were parasitized prior to bacterial challenge. Results showed that, immune challenge by bacteria leads to induction of certain antimicrobial peptide (AMP) genes in M. sexta larvae whether they were parasitized or not prior to bacterial challenge. These results show that at 24 h post oviposition pathways leading to expression of AMP genes are not all inactivated suggesting wasps are in an antiseptic environment. In contrast, at this time point genes involved in phenoloxidase activation and cellular immune responses were globally down-regulated after parasitism in accordance with the observed inhibition of wasp egg encapsulation.

Overwintering Is Associated with Reduced Expression of Immune Genes and Higher Susceptibility to Virus Infection in Honey Bees

The eusocial honey bee, Apis mellifera, has evolved remarkable abilities to survive extreme seasonal differences in temperature and availability of resources by dividing the worker caste into two groups that differ in physiology and lifespan: summer and winter bees. Most of the recent major losses of managed honey bee colonies occur during the winter, suggesting that winter bees may have compromised immune function and higher susceptibility to diseases. We tested this hypothesis by comparing the expression of eight immune genes and naturally occurring infection levels of deformed wing virus (DWV), one of the most widespread viruses in A. mellifera populations, between summer and winter bees. Possible interactions between immune response and physiological activity were tested by measuring the expression of vitellogenin and methyl farnesoate epoxidase, a gene coding for the last enzyme involved in juvenile hormone biosynthesis. Our data show that high DWV loads in winter bees correlate with reduced expression of genes involved in the cellular immune response and physiological activity and high expression of humoral immune genes involved in antibacterial defense compared with summer bees. This expression pattern could reflect evolutionary adaptations to resist bacterial pathogens and economize energy during the winter under a pathogen landscape with reduced risk of pathogenic viral infections. The outbreak of Varroa destructor infestation could have overcome these adaptations by promoting the transmission of viruses. Our results suggest that reduced cellular immune function during the winter may have increased honey bee’s susceptibility to DWV. These results contribute to our understanding of honey bee colony losses in temperate regions.

A neuron-specific antiviral mechanism prevents lethal flaviviral infection of mosquitoes

Mosquitoes are natural vectors for many etiologic agents of human viral diseases. Mosquito-borne flaviviruses can persistently infect the mosquito central nervous system without causing dramatic pathology or influencing the mosquito behavior and lifespan. The mechanism by which the mosquito nervous system resists flaviviral infection is still largely unknown. Here we report that an Aedes aegypti homologue of the neural factor Hikaru genki (AaHig) efficiently restricts flavivirus infection of the central nervous system. AaHig was predominantly expressed in the mosquito nervous system and localized to the plasma membrane of neural cells. Functional blockade of AaHig enhanced Dengue virus (DENV) and Japanese encephalitis virus (JEV), but not Sindbis virus (SINV), replication in mosquito heads and consequently caused neural apoptosis and a dramatic reduction in the mosquito lifespan. Consistently, delivery of recombinant AaHig to mosquitoes reduced viral infection. Furthermore, the membrane-localized AaHig directly interfaced with a highly conserved motif in the surface envelope proteins of DENV and JEV, and consequently interrupted endocytic viral entry into mosquito cells. Loss of either plasma membrane targeting or virion-binding ability rendered AaHig nonfunctional. Interestingly, Culex pipien pallens Hig also demonstrated a prominent anti-flavivirus activity, suggesting a functionally conserved function for Hig. Our results demonstrate that an evolutionarily conserved antiviral mechanism prevents lethal flaviviral infection of the central nervous system in mosquitoes, and thus may facilitate flaviviral transmission in nature.

The central nervous system plays a predominant role in organisms associated with cognition and higher-order functions, which is key to their normal behavior and successful survival. Many mosquito-borne flaviviruses particularly invade the central nervous system in vertebrates, resulting in dramatic neural degeneration and damage. As natural vectors, mosquitoes are highly permissive to flaviviral infection that can be persistent in the mosquito nervous system. However, the infection intriguingly does neither lead to significant malignant pathological sequelae, nor dramatically influences mosquito behavior or lifespan, and thus mosquitoes can transmit viruses efficiently. Little is known about the neuron-specific resistant mechanism in viral infection of mosquitoes. Here we report that a neuron-specific factor specifically controls flaviviral replication in the mosquito nervous system by interfering with viral entry, and its activity prevents lethal flaviviral infection of mosquitoes. Our study provides insight into the sophisticated interactions between mosquito-borne viruses and their vectors, and offers an important target for arboviral limitation in nature.

Polydnaviruses: From discovery to current insights

The International Committee on Taxonomy of Viruses (ICTV) recognized the Polydnaviridae in 1991 as a virus family associated with insects called parasitoid wasps. Polydnaviruses (PDVs) have historically received limited attention but advances in recent years have elevated interest because their unusual biology sheds interesting light on the question of what viruses are and how they function. Here, we present a succinct history of the PDV literature. We begin with the findings that first led ICTV to recognize the Polydnaviridae. We then discuss what subsequent studies revealed and how these findings have shaped views of PDV evolution.

Suppression of shrimp melanization during white spot syndrome virus infection

The melanization cascade, activated by the prophenoloxidase (proPO) system, plays a key role in the production of cytotoxic intermediates, as well as melanin products for microbial sequestration in invertebrates. Here, we show that the proPO system is an important component of the Penaeus monodon shrimp immune defense toward a major viral pathogen, white spot syndrome virus (WSSV). Gene silencing of PmproPO(s) resulted in increased cumulative shrimp mortality after WSSV infection, whereas incubation of WSSV with an in vitro melanization reaction prior to injection into shrimp significantly increased the shrimp survival rate. The hemolymph phenoloxidase (PO) activity of WSSV-infected shrimp was extremely reduced at days 2 and 3 post-injection compared with uninfected shrimp but was fully restored after the addition of exogenous trypsin, suggesting that WSSV probably inhibits the activity of some proteinases in the proPO cascade. Using yeast two-hybrid screening and co-immunoprecipitation assays, the viral protein WSSV453 was found to interact with the proPO-activating enzyme 2 (PmPPAE2) of P. monodon. Gene silencing of WSSV453 showed a significant increase of PO activity in WSSV-infected shrimp, whereas co-silencing of WSSV453 and PmPPAE2 did not, suggesting that silencing of WSSV453 partially restored the PO activity via PmPPAE2 in WSSV-infected shrimp. Moreover, the activation of PO activity in shrimp plasma by PmPPAE2 was significantly decreased by preincubation with recombinant WSSV453. These results suggest that the inhibition of the shrimp proPO system by WSSV partly occurs via the PmPPAE2-inhibiting activity of WSSV453.

An insecticidal compound produced by an insect-pathogenic bacterium suppresses host defenses through phenoloxidase inhibition

A bioassay-guided column chromatographic strategy was adopted in the present study to fractionate the culture extract of Photorhabdus temperata M1021 to identify potential insecticidal and antimicrobial compounds. An ethyl acetate (EtOAc) culture extract of P. temperata was assayed against Galleria mellonella larvae through intra-hemocoel injection and exhibited 100% insect mortality within 60 h. The EtOAc fraction and an isolated compound exhibited phenoloxidase (PO) inhibition of up to 60% and 63%, respectively. The compound was identified as 1,2-benzenedicarboxylic acid (phthalic acid, PA) by gas chromatography-mass spectrometry and nuclear magnetic resonance. PA exhibited insecticidal activity against G. mellonella in a dose-dependent manner, and 100% insect mortality was observed at 108 h after injection of 1 M PA. In a PO inhibition assay, 0.5 and 1 M concentrations of PA were found to inhibit PO activity by 74% and 82%, respectively; and in a melanotic nodule formation assay, nodule formation was significantly inhibited (27 and 10 nodules) by PA (0.5 and 1 M, respectively). PA was furthermore found to have substantial antioxidant activity and maximum antioxidant activity was 64.7% for 0.5 M PA as compare to control. Antibacterial activity was assessed by The MIC values ranged from 0.1 M to 0.5 M of PA. This study reports a multifunctional PA, a potential insecticidal agent, could a factor of insect mortality along with other toxins produced by P. temperata M1021.

Antiviral activity of the inducible humoral immunity and its suppression by eleven BEN family members encoded in Cotesia plutellae bracovirus

Upon parasitization by some endoparasitoids, polydnaviruses (PDVs) play a crucial role in inducing host immunosuppression. This study reports a novel immunosuppressive activity against humoral immune responses by BEN family genes encoded in Cotesia plutellae bracovirus (CpBV). A total of 11 BEN family members are encoded in 10 different CpBV DNA segments. When the CpBV segments were individually injected, specific BEN genes were expressed and suppressed the expression of antimicrobial peptide (AMP) and prophenoloxidase genes following bacterial challenge. The suppressive activities of the BEN genes were reversed by injection of the double-stranded RNA (dsRNA) specific to each BEN gene. The suppression of the AMP gene expressions by the BEN genes was also confirmed using an inhibition zone assay against Gram-positive and Gram-negative bacterial growth. The significance of the suppressive activity of BEN genes against humoral immune responses was analyzed in terms of suppression of antiviral activity by the host humoral immunity. When CpBV was incubated with the plasma obtained from the larvae challenged with bacteria, the immunized plasma severely impaired the expression activity of the viral genes. However, an expression of BEN gene significantly rescued the viral gene expression by suppressing humoral immune response. These results suggest that BEN family genes of CpBV play a crucial role in defending the antiviral response of the parasitized Plutella xylostella by inhibiting humoral immune responses.

Insect prophenoloxidase: the view beyond immunity

Insect prophenoloxidase (PPO) is an important innate immunity protein due to its involvement in cellular and humoral defense. It belongs to a group of type-3 copper-containing proteins that occurs in almost all organisms. Insect PPO has been studied for over a century, and the PPO activation cascade is becoming clearer. The insect PPO activation pathway incorporates several important proteins, including pattern-recognition receptors (PGRP, β GRP, and C-type lectins), serine proteases, and serine protease inhibitors (serpins). Due to their complexity, PPO activation mechanisms vary among insect species. Activated phenoloxidase (PO) oxidizes phenolic molecules to produce melanin around invading pathogens and wounds. The crystal structure of Manduca sexta PPO shows that a conserved amino acid, phenylalanine (F), can block the active site pocket. During activation, this blocker must be dislodged or even cleaved at the N-terminal sequence to expose the active site pockets and allow substrates to enter. Thanks to the crystal structure of M. sexta PPO, some domains and specific amino acids that affect PPO activities have been identified. Further studies of the relationship between PPO structure and enzyme activities will provide an opportunity to examine other type-3 copper proteins, and trace when and why their various physiological functions evolved. Recent researches show that insect PPO has a relationship with neuron activity, longevity, feces melanization (phytophagous insects) and development, which suggests that it is time for us to look back on insect PPO beyond the view of immunity in this review.

Functional analysis of an immune gene of Spodoptera littoralis by RNAi

Insect immune defences rely on cellular and humoral responses targeting both microbial pathogens and metazoan parasites. Accumulating evidence indicates functional cross-talk between these two branches of insect immunity, but the underlying molecular mechanisms are still largely unknown. We recently described, in the tobacco budworm Heliothis virescens, the presence of amyloid fibers associated with melanogenesis in immune capsules formed by hemocytes, and identified a protein (P102) involved in their assembly. Non-self objects coated by antibodies directed against this protein escaped hemocyte encapsulation, suggesting that P102 might coordinate humoral and cellular defence responses at the surface of foreign invaders. Here we report the identification of a cDNA coding for a protein highly similar to P102 in a related Lepidoptera species, Spodoptera littoralis. Its transcript was abundant in the hemocytes and the protein accumulated in large cytoplasmic compartments, closely resembling the localization pattern of P102 in H. virescens. RNAi-mediated gene silencing provided direct evidence for the role played by this protein in the immune response. Oral delivery of dsRNA molecules directed against the gene strongly suppressed the encapsulation and melanization response, while hemocoelic injections did not result in evident phenotypic alterations. Shortly after their administration, dsRNA molecules were found in midgut cells, en route to the hemocytes where the target gene was significantly down-regulated. Taken together, our data demonstrate that P102 is a functionally conserved protein with a key role in insect immunity. Moreover, the ability to target this gene by dsRNA oral delivery may be exploited to develop novel technologies of pest control, based on immunosuppression as a strategy for enhancing the impact of natural antagonists.

Transcriptome immune analysis of the invasive beetle Octodonta nipae (Maulik) (Coleoptera: Chrysomelidae) parasitized by Tetrastichus brontispae Ferrière (Hymenoptera: Eulophidae)

The beetle Octodonta nipae (Maulik) (Coleoptera: Chrysomelidae) is a serious invasive insect pest of palm plants in southern China, and the endoparasitoid Tetrastichus brontispae Ferrière (Hymenoptera: Eulophidae) is a natural enemy of this pest that exhibits great ability in the biocontrol of O. nipae. For successful parasitism, endoparasitoids often introduce or secrete various virulence factors to suppress host immunity. To investigate the effects of parasitization by T. brontispae on the O. nipae immune system, the transcriptome of O. nipae pupae was analyzed with a focus on immune-related genes through Illumina sequencing. De novo assembly generated 49,919 unigenes with a mean length of 598 bp. Of these genes, 27,490 unigenes (55.1% of all unigenes) exhibited clear homology to known genes in the NCBI nr database. Parasitization had significant effects on the transcriptome profile of O. nipae pupae, and most of these differentially expressed genes were down-regulated. Importantly, the expression profiles of immune-related genes were significantly regulated after parasitization. Taken together, these transcriptome sequencing efforts shed valuable light on the host (O. nipae) manipulation mechanisms induced by T. brontispae, which will pave the way for the development of novel immune defense-based management strategies of O. nipae, and provide a springboard for further molecular analyses, particularly of O. nipae invasion.

Systematic analysis of a wasp parasitism arsenal

Parasitoid wasps are among the most diverse insects on earth with many species causing major mortality in host populations. Parasitoids introduce a variety of factors into hosts to promote parasitism, including symbiotic viruses, venom, teratocytes and wasp larvae. Polydnavirus-carrying wasps use viruses to globally suppress host immunity and prevent rejection of developing parasites. Although prior results provide detailed insights into the genes viruses deliver to hosts, little is known about other products. RNAseq and proteomics were used to characterize the proteins secreted by venom glands, teratocytes and larvae from Microplitis demolitor, which carries M. demolitor bracovirus (MdBV). These data revealed that venom glands and teratocytes secrete large amounts of a small number of products relative to ovaries and larvae. Venom and teratocyte products exhibited almost no overlap with one another or MdBV genes, which suggested that M. demolitor effector molecules are functionally partitioned according to their source. This finding was well illustrated in the case of MdBV and teratocytes. Many viral proteins have immunosuppressive functions that include disruption of antimicrobial peptide production, yet this study showed that teratocytes express high levels of the antimicrobial peptide hymenoptaecin, which likely compensates for MdBV-mediated immunosuppression. A second key finding was the prevalence of duplications among genes encoding venom and teratocyte molecules. Several of these gene families share similarities with proteins from other species, while also showing specificity of expression in venom glands or teratocytes. Overall, these results provide the first comprehensive analysis of the proteins a polydnavirus-carrying wasp introduces into its host.

Procambarin: a glycine-rich peptide found in the haemocytes of red swamp crayfish Procambarus clarkii and its response to white spot syndrome virus challenge

We cloned a novel glycine-rich peptide, procambarin, from the haemocytes of unchallenged crayfish Procambarus clarkii. The mature peptide (155 residues) has a13.44 KDa molecular mass with a theoretical pI about 12.12. It is characterized by a high level of glycine (57.42%) and a threefold repeated motif GLKPNVGGGGGFGGG. Generally, it belongs to cationic glycine-rich peptide. The transcripts of this peptide were detected in many tissues. The haemocytes showed the highest expression of glycine-rich peptide mRNA, followed by the ovaries, antennal gland and intestine. The gill, hepatopancreas and heart showed little expression of this peptide and no expression was detected in the musculature. There is no intron in the ORF of it. The fluctuation of mRNA expression level of procambarin after WSSV challenge indicates that this peptide participates in the antiviral immune reaction.

Hemolymph melanization in the silkmoth Bombyx mori involves formation of a high molecular mass complex that metabolizes tyrosine

The phenoloxidase (PO) cascade regulates the melanization of blood (hemolymph) in insects and other arthropods. Most studies indicate that microbial elicitors activate the PO cascade, which results in processing of the zymogen PPO to PO. PO is then thought to oxidize tyrosine and o-diphenols to quinones, which leads to melanin. However, different lines of investigation raise questions as to whether these views are fully correct. Here we report that hemolymph from the silkmoth, Bombyx mori, rapidly melanizes after collection from a wound site. Prior studies indicated that in vitro activated PPO hydroxylates Tyr inefficiently. Measurement of in vivo substrate titers, however, suggested that Tyr was the only PO substrate initially present in B. mori plasma and that it is rapidly metabolized by PO. Fractionation of plasma by gel filtration chromatography followed by bioassays indicated that melanization activity was primarily associated with a high mass complex (~670 kDa) that contained PO. The prophenoloxidase-activating protease inhibitor Egf1.0 blocked formation of this complex and Tyr metabolism, but the addition of phenylthiourea to plasma before fractionation enhanced complex formation and Tyr metabolism. Mass spectrometry analysis indicated that the complex contained PO plus other proteins. Taken together, our results indicate that wounding alone activates the PO cascade in B. mori. They also suggest that complex formation is required for efficient use of Tyr as a substrate.

A strong immune response in young adult honeybees masks their increased susceptibility to infection compared to older bees

Honeybees, Apis mellifera, show age-related division of labor in which young adults perform maintenance (“housekeeping”) tasks inside the colony before switching to outside foraging at approximately 23 days old. Disease resistance is an important feature of honeybee biology, but little is known about the interaction of pathogens and age-related division of labor. We tested a hypothesis that older forager bees and younger “house” bees differ in susceptibility to infection. We coupled an infection bioassay with a functional analysis of gene expression in individual bees using a whole genome microarray. Forager bees treated with the entomopathogenic fungus Metarhizium anisopliae s.l. survived for significantly longer than house bees. This was concomitant with substantial differences in gene expression including genes associated with immune function. In house bees, infection was associated with differential expression of 35 candidate immune genes contrasted with differential expression of only two candidate immune genes in forager bees. For control bees (i.e. not treated with M. anisopliae) the development from the house to the forager stage was associated with differential expression of 49 candidate immune genes, including up-regulation of the antimicrobial peptide gene abaecin, plus major components of the Toll pathway, serine proteases, and serpins. We infer that reduced pathogen susceptibility in forager bees was associated with age-related activation of specific immune system pathways. Our findings contrast with the view that the immunocompetence in social insects declines with the onset of foraging as a result of a trade-off in the allocation of resources for foraging. The up-regulation of immune-related genes in young adult bees in response to M. anisopliae infection was an indicator of disease susceptibility; this also challenges previous research in social insects, in which an elevated immune status has been used as a marker of increased disease resistance and fitness without considering the effects of age-related development.

Honeybees have a highly developed form of social biology in which tasks are distributed among workers according to their age, with younger bees performing housekeeping tasks (“house bees”) before switching to foraging duties when they grow older. This division of labor is vital to colony function and survival. Pathogens are known to be partly responsible for the current decline in honeybee populations around the world, but we understand little about the responses of different types of worker bee to infection. In this study, we infected house and forager bees with an insect pathogen. We measured bee survival rate and the expression of genes that regulate the immune system. More immune genes were up regulated in house bees than foragers in response to infection, but foragers were more resistant to the pathogen than house bees. We found that development from the house to forager stages resulted in increased expression of genes that regulate the production of antimicrobial proteins. The inference is that parts of the immune system are activated during development, resulting in greater resistance to infectious disease in forager bees. Our study provides new insights into the functioning of the honeybee immune system and its interaction with social organisation.

Phenoloxidase activity acts as a mosquito innate immune response against infection with Semliki Forest virus

Several components of the mosquito immune system including the RNA interference (RNAi), JAK/STAT, Toll and IMD pathways have previously been implicated in controlling arbovirus infections. In contrast, the role of the phenoloxidase (PO) cascade in mosquito antiviral immunity is unknown. Here we show that conditioned medium from the Aedes albopictus-derived U4.4 cell line contains a functional PO cascade, which is activated by the bacterium Escherichia coli and the arbovirus Semliki Forest virus (SFV) (Togaviridae; Alphavirus). Production of recombinant SFV expressing the PO cascade inhibitor Egf1.0 blocked PO activity in U4.4 cell- conditioned medium, which resulted in enhanced spread of SFV. Infection of adult female Aedes aegypti by feeding mosquitoes a bloodmeal containing Egf1.0-expressing SFV increased virus replication and mosquito mortality. Collectively, these results suggest the PO cascade of mosquitoes plays an important role in immune defence against arboviruses.

Arboviruses are transmitted to vertebrates by arthropod vectors such as mosquitoes. Infection of mosquitoes with arboviruses activates immune defence responses including the RNA interference pathway. Another component of the insect immune system is the phenoloxidase (PO) cascade, which produces melanin that accumulates at wound sites and around invading microorganisms. Some pathogen-associated pattern recognition molecules are known to activate the PO cascade, which results in the proteolytic processing of inactive prophenoloxidase (PPO) to PO. PO then catalyses the formation of compounds that ultimately form melanin. Some of these products are also known to have anti-microbial properties but whether activation of the PO cascade provides any defence against arboviruses is unclear. Using the arbovirus, Semliki Forest virus, we show that this virus activates the PO cascade. By using recombinant Semliki Forest virus expressing an inhibitor of the PO cascade, we also demonstrate that this pathway inhibits virus spread in cell culture. Moreover, inhibition of this pathway leads to higher virus genome levels and higher mortality of infected mosquitoes. In conclusion, Semliki Forest virus activates the PO cascade which exhibits antiviral activity and can be added to the list of mosquito anti-viral defence mechanisms.

Polydnavirus Ank proteins bind NF-κB homodimers and inhibit processing of Relish

Recent studies have greatly increased understanding of how the immune system of insects responds to infection, whereas much less is known about how pathogens subvert immune defenses. Key regulators of the insect immune system are Rel proteins that form Nuclear Factor-κB (NF-κB) transcription factors, and inhibitor κB (IκB) proteins that complex with and regulate NF-κBs. Major mortality agents of insects are parasitoid wasps that carry immunosuppressive polydnaviruses (PDVs). Most PDVs encode ank genes that share features with IκBs, while our own prior studies suggested that two ank family members from Microplitis demolitor bracovirus (MdBV) (Ank-H4 and Ank-N5) behave as IκB mimics. However, the binding affinities of these viral mimics for Rel proteins relative to endogenous IκBs remained unclear. Surface plasmon resonance (SPR) and co-immunoprecipitation assays showed that the IκB Cactus from Drosophila bound Dif and Dorsal homodimers more strongly than Relish homodimers. Ank-H4 and –N5 bound Dif, Dorsal and Relish homodimers with higher affinity than the IκB domain of Relish (Rel-49), and also bound Relish homodimers more strongly than Cactus. Ank-H4 and –N5 inhibited processing of compound Relish and reduced the expression of several antimicrobial peptide genes regulated by the Imd signaling pathway in Drosophila mbn2 cells. Studies conducted in the natural host Pseudoplusia includens suggested that parasitism by M. demolitor also activates NF-κB signaling and that MdBV inhibits this response. Overall, our data provide the first quantitative measures of insect and viral IκB binding affinities, while also showing that viral mimics disable Relish processing.

Central to the study of host-pathogen interactions is understanding how the immune system of hosts responds to infection, and reciprocally how pathogens subvert host defenses. In the case of insects, understanding of how the immune system responds to infection greatly exceeds understanding of pathogen counterstrategies. Parasitoid wasps are key mortality agents of insects. Thousands of wasp species have also evolved a symbiotic relationship with large DNA viruses in the family Polydnaviridae whose primary function is to deliver immunosuppressive virulence genes to the insect hosts that wasps parasitize. The function of most PDV-encoded virulence genes, however, remains unknown. In this article, we investigated the function of two ank gene family members from Microplitis demolitor bracovirus (MdBV). Our results indicate that Ank-H4 and Ank-N5 function as mimics of IκB proteins, which regulate a family of transcription factors called NF-κBs that control many genes of the insect immune system. IκBs and NF-κBs also function as key regulators of the mammalian immune system. Our results thus suggest that viral Ank proteins subvert the immune system of host insects by targeting conserved signaling pathways used by a diversity of organisms.

Functional amyloids in insect immune response

The innate immune system of insects consists of humoural and cellular responses that provide protection against invading pathogens and parasites. Defence reactions against these latter include encapsulation by immune cells and targeted melanin deposition, which is usually restricted to the surface of the foreign invader, to prevent systemic damage. Here we show that a protein produced by haemocytes of Heliothis virescens (Lepidoptera, Noctuidae) larvae, belonging to XendoU family, generates amyloid fibrils, which accumulate in large cisternae of the rough endoplasmic reticulum and are released upon immune challenge, to form a layer coating non-self objects entering the haemocoel. This amyloid layer acts as a molecular scaffold that promotes localised melanin synthesis and the adhesion of immune cells around the non-self intruder during encapsulation response. Our results demonstrate a new functional role for these protein aggregates that are commonly associated with severe human diseases. We predict that insects will offer new powerful experimental systems for studying inducible amyloidogenesis, which will likely provide fresh perspectives for its prevention.

Polydnaviruses of Parasitic Wasps: Domestication of Viruses To Act as Gene Delivery Vectors

Symbiosis is a common phenomenon in which associated organisms can cooperate in ways that increase their ability to survive, reproduce, or utilize hostile environments. Here, we discuss polydnavirus symbionts of parasitic wasps. These viruses are novel in two ways: (1) they have become non-autonomous domesticated entities that cannot replicate outside of wasps; and (2) they function as a delivery vector of genes that ensure successful parasitism of host insects that wasps parasitize. In this review we discuss how these novelties may have arisen, which genes are potentially involved, and what the consequences have been for genome evolution.

Deep sequencing identifies viral and wasp genes with potential roles in replication of Microplitis demolitor Bracovirus

Viruses in the genus Bracovirus (BV) (Polydnaviridae) are symbionts of parasitoid wasps that specifically replicate in the ovaries of females. Recent analysis of expressed sequence tags from two wasp species, Cotesia congregata and Chelonus inanitus, identified transcripts related to 24 different nudivirus genes. These results together with other data strongly indicate that BVs evolved from a nudivirus ancestor. However, it remains unclear whether BV-carrying wasps contain other nudivirus-like genes and what types of wasp genes may also be required for BV replication. Microplitis demolitor carries Microplitis demolitor bracovirus (MdBV). Here we characterized MdBV replication and performed massively parallel sequencing of M. demolitor ovary transcripts. Our results indicated that MdBV replication begins in stage 2 pupae and continues in adults. Analysis of prereplication- and active-replication-stage ovary RNAs yielded 22 Gb of sequence that assembled into 66,425 transcripts. This breadth of sampling indicated that a large percentage of genes in the M. demolitor genome were sequenced. A total of 41 nudivirus-like transcripts were identified, of which a majority were highly expressed during MdBV replication. Our results also identified a suite of wasp genes that were highly expressed during MdBV replication. Among these products were several transcripts with conserved roles in regulating locus-specific DNA amplification by eukaryotes. Overall, our data set together with prior results likely identify the majority of nudivirus-related genes that are transcriptionally functional during BV replication. Our results also suggest that amplification of proviral DNAs for packaging into BV virions may depend upon the replication machinery of wasps.

Transcriptomic profiling of Microplitis demolitor bracovirus reveals host, tissue and stage-specific patterns of activity

The polydnaviruses (PDVs) are a family of DNA viruses that are symbiotically associated with parasitoid wasps. The transcription of particular genes or gene-family members have been reported for several PDVs, but no studies have characterized the spatio-temporal patterns of expression for the entire complement of predicted genes in the encapsidated genome of any PDV isolate. The braconid wasp Microplitis demolitor carries the PDV Microplitis demolitor bracovirus (MdBV) and parasitizes larval stage Pseudoplusia (Chrysodeixis) includens. The encapsidated genome consists of 15 genomic segments with 51 predicted ORFs encoding proteins ≥100 aa. A majority of these ORFs form four multimember gene families (ptp, ank, glc and egf) while the remaining ORFs consist of single copy (orph) genes. Here we used RT-PCR and quantitative real-time PCR methods to profile the encapsidated transcriptome of MdBV in P. includens and M. demolitor. Our results indicate that most predicted genes are expressed in P. includens. Spatial patterns of expression in P. includens differed among genes, but temporal patterns of expression were generally similar, with transcript abundance progressively declining between 24 and 120 h. A subset of ptp, ank and orph genes were also expressed in adult female but not male M. demolitor. Only one encapsidated gene (ank-H4) was expressed in all life stages of M. demolitor, albeit at much lower levels than in P. includens. However, another encapsidated gene (orph-B1) was expressed in adult M. demolitor at similar levels to those detected in P. includens.

Antiviral, anti-parasitic, and cytotoxic effects of 5,6-dihydroxyindole (DHI), a reactive compound generated by phenoloxidase during insect immune response

Phenoloxidase (PO) and its activation system are implicated in several defense responses of insects. Upon wounding or infection, inactive prophenoloxidase (proPO) is converted to active PO through a cascade of serine proteases and their homologs. PO generates reactive compounds such as 5,6-dihydroxyindole (DHI), which have a broad-spectrum antibacterial and antifungal activity. Here we report that DHI and its spontaneous oxidation products are also active against viruses and parasitic wasps. Preincubation of a baculovirus stock with 1.25 mM DHI for 3 h near fully disabled recombinant protein production. The LC₅₀ for lambda bacteriophage and eggs of the wasp Microplitis demolitor were 5.6 ± 2.2 and 111.0 ± 1.6 μM, respectively. The toxicity of DHI and related compounds also extended to cells derived from insects that serve as hosts for several of the aforementioned pathogens. Pretreatment of Sf9 cells with 1.0 mM DHI for 4 h resulted in 97% mortality, and LC₅₀ values of 20.3 ± 1.2 μM in buffer and 131.8 ± 1.1 μM in a culture medium. Symptoms of DHI toxicity in Sf9 cells included DNA polymerization, protein crosslinking, and lysis. Taken together, these data showed that proPO activation and DHI production is strongly toxic against various pathogens but can also damage host tissues and cells if not properly controlled.

The encapsidated genome of Microplitis demolitor bracovirus integrates into the host Pseudoplusia includens

Polydnaviruses (PDVs) are symbionts of parasitoid wasps that function as gene delivery vehicles in the insects (hosts) that the wasps parasitize. PDVs persist in wasps as integrated proviruses but are packaged as circularized and segmented double-stranded DNAs into the virions that wasps inject into hosts. In contrast, little is known about how PDV genomic DNAs persist in host cells. Microplitis demolitor carries Microplitis demolitor bracovirus (MdBV) and parasitizes the host Pseudoplusia includens. MdBV infects primarily host hemocytes and also infects a hemocyte-derived cell line from P. includens called CiE1 cells. Here we report that all 15 genomic segments of the MdBV encapsidated genome exhibited long-term persistence in CiE1 cells. Most MdBV genes expressed in hemocytes were persistently expressed in CiE1 cells, including members of the glc gene family whose products transformed CiE1 cells into a suspension culture. PCR-based integration assays combined with cloning and sequencing of host-virus junctions confirmed that genomic segments J and C persisted in CiE1 cells by integration. These genomic DNAs also rapidly integrated into parasitized P. includens. Sequence analysis of wasp-viral junction clones showed that the integration of proviral segments in M. demolitor was associated with a wasp excision/integration motif (WIM) known from other bracoviruses. However, integration into host cells occurred in association with a previously unknown domain that we named the host integration motif (HIM). The presence of HIMs in most MdBV genomic DNAs suggests that the integration of each genomic segment into host cells occurs through a shared mechanism.

Prophenoloxidase from Pieris rapae: gene cloning, activity, and transcription in response to venom/calyx fluid from the endoparasitoid wasp Cotesia glomerata

Prophenoloxidase (PPO) plays an important role in melanization, necessary for defense against intruding parasitoids. Parasitoids have evolved to inject maternal virulence factors into the host hemocoel to suppress hemolymph melanization for the successful development of their progeny. In this study, the full-length complementary DNA (cDNA) of a Pieris rapae PPO was cloned. Its cDNA contained a 2 076-base pair (bp) open reading frame (ORF) encoding 691 amino acids (aa). Two putative copper-binding sites, a proteolytic activation site, three conserved hemocyanin domains, and a thiol ester motif were found in the deduced amino acid sequence. According to both multiple alignment and phylogenetic analysis, P. rapae PPO gene cloned here is a member of the lepidopteran PPO-2 family. Injection of Cotesia glomerata venom or calyx fluid resulted in reduction of P. rapae hemolymph phenoloxidase activity, demonstrating the ability to inhibit the host's melanization. Real-time reverse transcriptase polymerase chain reaction (RT-PCR) showed that transcripts of P. rapae PPO-2 in the haemocytes from larvae had not significantly changed following venom injection, suggesting that the regulation of PPO messenger RNA (mRNA) expression by venom was not employed by C. glomerata to cause failure of melanization in parasitized host. While decreased P. rapae PPO-2 gene expression was observed in the haemocytes after calyx fluid injection, no detectable transcriptional change was induced by parasitization, indicating that transcriptional down-regulation of PPO by calyx fluid might play a minor role involved in inhibiting the host's melanization.

Identification of an in vitro interaction between an insect immune suppressor protein (CrV2) and G alpha proteins

The protein CrV2 is encoded by a polydnavirus integrated into the genome of the endoparasitoid Cotesia rubecula (Hymenoptera:Braconidae:Microgastrinae) and is expressed in host larvae with other gene products of the polydnavirus to allow successful development of the parasitoid. CrV2 expression has previously been associated with immune suppression, although the molecular basis for this was not known. Here, we have used time-resolved Förster resonance energy transfer (TR-FRET) to demonstrate high affinity binding of CrV2 to Gα subunits (but not the Gβγ dimer) of heterotrimeric G-proteins. Signals up to 5-fold above background were generated, and an apparent dissociation constant of 6.2 nm was calculated. Protease treatment abolished the TR-FRET signal, and the presence of unlabeled CrV2 or Gα proteins also reduced the TR-FRET signal. The activation state of the Gα subunit was altered with aluminum fluoride, and this decreased the affinity of the interaction with CrV2. It was also demonstrated that CrV2 preferentially bound to Drosophila Gα(o) compared with rat Gα(i1). In addition, three CrV2 homologs were detected in sequences derived from polydnaviruses from Cotesia plutellae and Cotesia congregata (including the immune-related early expressed transcript, EP2). These data suggest a potential mode-of-action of immune suppressors not previously reported, which in addition to furthering our understanding of insect immunity may have practical benefits such as facilitating development of novel controls for pest insect species.

Venom proteins from endoparasitoid wasps and their role in host-parasite interactions

Endoparasitoids introduce a variety of factors into their host during oviposition to ensure successful parasitism. These include ovarian and venom fluids that may be accompanied by viruses and virus-like particles. An overwhelming number of venom components are enzymes with similarities to insect metabolic enzymes, suggesting their recruitment for expression in venom glands with modified functions. Other components include protease inhibitors, paralytic factors, and constituents that facilitate/enhance entry and expression of genes from symbiotic viruses or virus-like particles. In addition, the venom gland may itself support replication/production of some viruses or virus-like entities. Overlapping functions and structural similarities of some venom, ovarian, and virus-encoded proteins suggest coevolution of molecules recruited by endoparasitoids to maintain their fitness relative to their host.

Inhibition of melanization by a Nasonia defensin-like peptide: implications for host immune suppression

The parasitic wasp Nasonia vitripennis suppresses host immune mechanisms that include melanization reactions. Melanization is an important immune response of hosts induced by wasp infection and thus its inhibition represents a successful strategy for parasitism. However, the molecular basis associated with such inhibition is largely unknown in N. vitripennis. Here, we report recombinant expression, structural and functional characterization of a Nasonia-derived defensin-like peptide (called nasonin-3) whose recombinant product exerts inhibitory effect on host melanization. The possible role of nasonin-3 in immune suppression is also discussed.

Examining the relationship between hemolymph phenoloxidase and resistance to a DNA virus, Plodia interpunctella granulosis virus (PiGV)

We have a detailed understanding of invertebrate immune responses to bacteria and fungal pathogens, but we know less about how insects respond to virus challenge. Phenoloxidase (PO) functions as an important immune response against many parasites and pathogens and is routinely used as a measure of immune competance. We examine the role of haemolymph PO activity in Plodia interpuncetella's response to its natural granulosis virus (PiGV). Larvae were challenged with virus by both oral inoculation of occluded virus (the natural infection route) and direct intrahaemocoelic injection of budded virus. Haemolymph was collected at time points post-viral challenge using a novel method that allows the volume of haemolymph to be quanitified. The haemolmyph was collected without killing the larvae so that haemolymph samples from individuals that developed viral disease could be distinguished from samples collected from those that fought off infection. The level of haemolymph PO activity in resistant larvae did not differ from control larvae. Therefore we have no evidence that PO is involved in resistance to virus in the haemocoel whether larvae are challenged naturally by oral innoculation or directly by intraheamocoelic injection. Phenoloxidase may therefore not be a relevant metric of immunocompetence for viral infection.

Isolation, gene cloning and expression profile of a pathogen recognition protein: a serine proteinase homolog (Sp-SPH) involved in the antibacterial response in the crab Scylla paramamosain

To identify the frontline defense molecules against microbial infection in the crab Scylla paramamosain, a live crab pathogenic microbe, Vibrio parahaemolyticus, was recruited as an affinity matrix to isolate innate immune factors from crab hemocytes lysate. Interestingly, a serine proteinase homolog (Sp-SPH) was obtained together with an antimicrobial peptide-antilipopolysaccharide factor (Sp-ALF). We then determined the full-length cDNA sequence of Sp-SPH, which contained 1298bp with an open reading frame of 1107bp encoding 369 amino acid residues. Multiple alignment analysis showed that the deduced amino acid sequences of Sp-SPH shared overall identity (83.8%) with those of SPH-containing proteins from other crab species. Tissue distribution analysis indicated that the Sp-SPH transcripts were present in various tissues including eye stalk, subcuticular epidermis, gill, hemocyte, stomach, thorax ganglion, brain and muscle of S. paramamosain. The Sp-SPH was highly expressed in selected different development stages including embryo (I, II, III and V), zoea (I), megalopa, and juvenile. Importantly, the prophenoloxidase was also present in the embryos, zoea, juvenile and adult crabs, but relatively lower in megalopa compared to those of other stages. Furthermore, the Sp-SPH mRNA expression showed a statistically significant increase (P<0.05) in both hemocyte and subcuticular epidermis at 24h, and in gill at 96h after challenge of V. parahaemolyticus determined by quantitative real-time PCR. Taken together, the live-bacterial-binding activity and the acute-phase response against bacterial infection of Sp-SPH suggested that it might function as an innate immune recognition molecule and play a key role in host defense against microbe invasion in the crab S. paramamosain.

Proteolytic cascades and their involvement in invertebrate immunity

Bacteria and other potential pathogens are cleared rapidly from the body fluids of invertebrates by the immediate response of the innate immune system. Proteolytic cascades, following their initiation by pattern recognition proteins, control several such reactions, notably coagulation, melanisation, activation of the Toll receptor and complement-like reactions. However, there is considerable variation among invertebrates and these cascades, although widespread, are not present in all phyla. In recent years, significant progress has been made in identifying and characterizing these cascades in insects. Notably, recent work has identified several connections and shared principles among the different pathways, suggesting that cross-talk between them may be common.

Dengue virus inhibits immune responses in Aedes aegypti cells

The ability of many viruses to manipulate the host antiviral immune response often results in complex host-pathogen interactions. In order to study the interaction of dengue virus (DENV) with the Aedes aegypti immune response, we have characterized the DENV infection-responsive transcriptome of the immune-competent A. aegypti cell line Aag2. As in mosquitoes, DENV infection transcriptionally activated the cell line Toll pathway and a variety of cellular physiological systems. Most notably, however, DENV infection down-regulated the expression levels of numerous immune signaling molecules and antimicrobial peptides (AMPs). Functional assays showed that transcriptional induction of AMPs from the Toll and IMD pathways in response to bacterial challenge is impaired in DENV-infected cells. In addition, Escherichia coli, a Gram-negative bacteria species, grew better when co-cultured with DENV-infected cells than with uninfected cells, suggesting a decreased production of AMPs from the IMD pathway in virus-infected cells. Pre-stimulation of the cell line with Gram-positive bacteria prior to DENV infection had no effect on DENV titers, while pre-stimulation with Gram-negative bacteria resulted in an increase in DENV titers. These results indicate that DENV is capable of actively suppressing immune responses in the cells it infects, a phenomenon that may have important consequences for virus transmission and insect physiology.

Dual function of a bee venom serine protease: prophenoloxidase-activating factor in arthropods and fibrin(ogen)olytic enzyme in mammals

Bee venom contains a variety of peptides and enzymes, including serine proteases. While the presence of serine proteases in bee venom has been demonstrated, the role of these proteins in bee venom has not been elucidated. Furthermore, there is currently no information available regarding the melanization response or the fibrin(ogen)olytic activity of bee venom serine protease, and the molecular mechanism of its action remains unknown. Here we show that bee venom serine protease (Bi-VSP) is a multifunctional enzyme. In insects, Bi-VSP acts as an arthropod prophenoloxidase (proPO)-activating factor (PPAF), thereby triggering the phenoloxidase (PO) cascade. Bi-VSP injected through the stinger induces a lethal melanization response in target insects by modulating the innate immune response. In mammals, Bi-VSP acts similarly to snake venom serine protease, which exhibits fibrin(ogen)olytic activity. Bi-VSP activates prothrombin and directly degrades fibrinogen into fibrin degradation products, defining roles for Bi-VSP as a prothrombin activator, a thrombin-like protease, and a plasmin-like protease. These findings provide a novel view of the mechanism of bee venom in which the bee venom serine protease kills target insects via a melanization strategy and exhibits fibrin(ogen)olytic activity.

Venom proteins of the parasitoid wasp Nasonia vitripennis: recent discovery of an untapped pharmacopee

Adult females of Nasonia vitripennis inject a venomous mixture into its host flies prior to oviposition. Recently, the entire genome of this ectoparasitoid wasp was sequenced, enabling the identification of 79 venom proteins. The next challenge will be to unravel their specific functions, but based on homolog studies, some predictions already can be made. Parasitization has an enormous impact on hosts physiology of which five major effects are discussed in this review: the impact on immune responses, induction of developmental arrest, increases in lipid levels, apoptosis and nutrient releases. The value of deciphering this venom is also discussed.

The role of lysozyme in the prophenoloxidase activation system of Manduca sexta: an in vitro approach

Activation of the prophenoloxidase (proPO) system and synthesis of antimicrobial peptides (including lysozyme) are two key defense mechanisms in arthropods. Activation of proPO involves a cascade of serine proteinases that eventually converts proPO to active phenoloxidase (PO). However, a trade-off between lysozyme/antibacterial activity and PO activity has been observed in some insects, and a mosquito lysozyme can inhibit melanization. It is not clear whether lysozyme can inhibit PO activity and/or proPO activation. In this study, we used in vitro assays to investigate the role of lysozyme in proPO activation in the tobacco hornworm Manduca sexta. We showed that lysozymes from M. sexta, human milk and hen egg white did not inhibit PO activity in the pre-activated naïve plasma of M. sexta larvae, but significantly inhibited proPO activation in the naïve plasma. Western blot analysis showed that direct incubation of M. sexta lysozyme with the naïve plasma prevented conversion of proPO to PO, but stimulated degradation of precursor proteins for serine proteinase homolog-2 (SPH2) and proPO-activating proteinase-1 (PAP1), two key components required for proPO activation. Far-western blot analysis showed that M. sexta lysozyme and proPO interacted with each other. Altogether, our results suggest that lysozymes may inhibit the proPO activation system by preventing conversion of proPO to PO via direct protein interaction with proPO.

Insights into the venom composition of the ectoparasitoid wasp Nasonia vitripennis from bioinformatic and proteomic studies

With the Nasonia vitripennis genome sequences available, we attempted to determine the proteins present in venom by two different approaches. First, we searched for the transcripts of venom proteins by a bioinformatic approach using amino acid sequences of known hymenopteran venom proteins. Second, we performed proteomic analyses of crude N. vitripennis venom removed from the venom reservoir, implementing both an off-line two-dimensional liquid chromatography matrix-assisted laser desorption/ ionization time-of-flight (2D-LC-MALDI-TOF) mass spectrometry (MS) and a two-dimensional liquid chromatography electrospray ionization Founer transform ion cyclotron resonance (2D-LC-ESI-FT-ICR) MS setup. This combination of bioinformatic and proteomic studies resulted in an extraordinary richness of identified venom constituents. Moreover, half of the 79 identified proteins were not yet associated with insect venoms: 16 proteins showed similarity only to known proteins from other tissues or secretions, and an additional 23 did not show similarity to any known protein. Serine proteases and their inhibitors were the most represented. Fifteen nonsecretory proteins were also identified by proteomic means and probably represent so-called 'venom trace elements'. The present study contributes greatly to the understanding of the biological diversity of the venom of parasitoid wasps at the molecular level.

Infection by a symbiotic polydnavirus induces wasting and inhibits metamorphosis of the moth Pseudoplusia includens

Insect pathogens and parasites often affect the growth and development of their hosts, but understanding of these processes is fragmentary. Among the most species-rich and important mortality agents of insects are parasitoid wasps that carry symbiotic polydnaviruses (PDVs). Like many PDV-carrying wasps, Microplitis demolitor inhibits growth and pupation of its lepidopteran host, Pseudoplusia includens, by causing host hemolymph juvenile hormone (JH) titers to remain elevated and preventing ecdysteroid titers from rising. Here we report these alterations only occurred if P. includens was parasitized prior to achieving critical weight, and were fully mimicked by infection with only M. demolitor bracovirus (MdBV). Metabolic assays revealed that MdBV infection of pre-critical weight larvae caused a rapid and persistent state of hyperglycemia and reduced nutrient stores. In vitro ecdysteroid assays further indicated that prothoracic glands from larvae infected prior to achieving critical weight remained in a refractory state of ecdysteroid release, whereas infection of post-critical weight larvae had little or no effect on ecdysteroid release by prothoracic glands. Taken together, our results suggest MdBV causes alterations in metabolic physiology, which prevent the host from achieving critical weight. This in turn inhibits the endocrine events that normally trigger metamorphosis.

Variation of Leptopilina boulardi success in Drosophila hosts: what is inside the black box?

Interactions between Drosophila hosts and parasitoid wasps are among the few examples in which occurrence of intraspecific variation of parasite success has been studied in natural populations. Such variations can originate from three categories of factors: environmental, host and parasitoid factors. Under controlled laboratory conditions, it is possible to focus on the two last categories, and, using specific reference lines, to analyze their respective importance. Parasitoid and host contributions to variations in parasite success have largely been studied in terms of evolutionary and mechanistic aspects in two Drosophila parasitoids, Asobara tabida and, in more details, in Leptopilina boulardi. This chapter focuses on the physiological and molecular aspects of L. boulardi interactions with two Drosophila host species, while most of the evolutionary hypotheses and models are presented in Chapter 11 of Dupas et al.

Variation and covariation in infectivity, virulence and immunodepression in the host-parasite association Gammarus pulex-Pomphorhynchus laevis

Parasites often manipulate host immunity for their own benefit, either by exacerbating or suppressing the immune response and this may directly affect the expression of parasite virulence. However, genetic variation in immunodepression, which is a prerequisite to its evolution, and the relationship between immunodepression and virulence, have rarely been studied. Here, we investigated the variation among sibships of the acanthocephalan parasite, Pomphorhynchus laevis, in infecting and in immunodepressing its amphipod host, Gammarus pulex. We also assessed the covariation between infectivity, parasite-induced immune depression and host mortality (parasite virulence). We found that infectivity, the intensity of immunodepression and virulence were variable among parasite sibships. Infectivity and the level of immunodepression were not correlated across parasite sibships. Whereas infectivity was unrelated to host mortality, we found that gammarids that were exposed to the parasite sibships that immunodepressed their hosts the most survived better. This positive covariation between host survival and immunodepression suggests that gammarids exposed to the less immunodepressive parasites could suffer from damage imposed by a higher activity of the phenoloxidase.

IkB genes encoded in Cotesia plutellae bracovirus suppress an antiviral response and enhance baculovirus pathogenicity against the diamondback moth, Plutella xylostella

An endoparasitoid wasp, Cotesia plutellae, parasitizes larvae of the diamondback moth, Plutella xylostella, with its symbiotic polydnavirus, C. plutellae bracovirus (CpBV). This study analyzed the role of Inhibitor-kB (IkB)-like genes encoded in CpBV in suppressing host antiviral response. Identified eight CpBV-IkBs are scattered on different viral genome segments and showed high homologies with other bracoviral IkBs in their amino acid sequences. Compared to an insect ortholog (e.g., Cactus of Drosophila melanogaster), they possessed a shorter ankyrin repeat domain without any regulatory domains. The eight CpBV-IkBs are, however, different in their promoter components and expression patterns in the parasitized host. To test their inhibitory activity on host antiviral response, a midgut response of P. xylostella against baculovirus infection was used as a model reaction. When the larvae were orally fed the virus, they exhibited melanotic responses of midgut epithelium, which increased with baculovirus dose and incubation time. Parasitized larvae exhibited a significant reduction in the midgut melanotic response, compared to nonparasitized larvae. Micro-injection of each of the four CpBV genome segments containing CpBV-IkBs into the hemocoel of nonparasitized larvae showed the gene expressions of the encoded IkBs and suppressed the midgut melanotic response in response to the baculovirus treatment. When nonparasitized larvae were orally administered with a recombinant baculovirus containing CpBV-IkB, they showed a significant reduction in midgut melanotic response and an enhanced susceptibility to the baculovirus infectivity.

A serpin from the parasitoid wasp Leptopilina boulardi targets the Drosophila phenoloxidase cascade

The insect phenoloxidase (PO) cascade is known to be tightly regulated by serine proteases and serine protease inhibitors of the serpin family. As a key component of the insect immune system, it is also suspected to be inhibited by several endoparasitoid wasps, insects that develop inside other arthropods as hosts. However, the underlying mechanisms of this inhibition are largely undescribed. Here, we report the characterization of a gene encoding a serpin, LbSPNy, highly expressed in the venom of the wasp Leptopilina boulardi (IS(y) type), and we show that either the venom or the recombinant LbSPNy inhibit the PO cascade in the hemolymph of Drosophila yakuba host larva. Altogether, our results identify the first serpin used as a virulence factor by a parasitoid wasp and show that it disrupts the activation pathway of the PO in the Drosophila host.

Purification and characterization of a small cationic protein from the tobacco hornworm Manduca sexta

The prophenoloxidase (proPO) activation system is an important defense mechanism in arthropods, and activation of proPO to active phenoloxidase (PO) involves a serine proteinase cascade. Here, we report the purification and characterization of a small cationic protein CP8 from the tobacco hornworm, Manduca sexta, which can stimulate proPO activation. BLAST search showed that Manduca CP8 is similar to a fungal proteinase inhibitor-1 (AmFPI-1), an inducible serine proteinase inhibitor-1 (ISPI-1), and other small cationic proteins with unknown functions. However, we showed that Manduca CP8 did not inhibit proteinase activity, but stimulated proPO activation in plasma. When small amount (0.1 microg) of purified native CP8 or BSA was added to cell-free plasma samples and incubated for 20 min, low PO activity was observed in both groups. But significantly higher PO activity was observed in the CP8-group than in the BSA-group when more proteins (0.5 microg) were added and incubated for 20 min. However, when the plasma samples were incubated with proteins for 30 min, high PO activity was observed in both the CP8 and BSA groups regardless of the amount of proteins added. Moreover, when PO in the plasma was pre-activated with Micrococcus luteus, addition of CP8 did not have an effect on PO activity, and CP8/bacteria mixture did not stimulate PO activity to a higher level than did BSA/bacteria. These results suggest that CP8 helps activate proPO more rapidly at the initial stage. CP8 mRNA was specifically expressed in fat body and its mRNA level decreased when larvae were injected with saline or bacteria. However, CP8 protein concentration in hemolymph did not change significantly in larvae injected with saline or microorganisms.

Virulence strategies in parasitoid Hymenoptera as an example of adaptive diversity

Parasitoids are mostly insects that develop at the expense of other arthropods, which will die as a result of the interaction. Their reproductive success thus totally depends on their ability to successfully infest their host whose reproductive success relies on its own ability to avoid or overcome parasitism. Such intense selective pressures have resulted in extremely diverse adaptations in parasitoid strategies that ensure parasitism success. For instance, wasp-specific viruses (polydnaviruses) are injected into the host by parasitoid females to modulate its physiology and immunity. This article synthesizes available physiological and molecular data on parasitoid virulence strategies and discusses the evolutionary processes at work.

Semliki Forest virus strongly reduces mosquito host defence signaling

The Alphavirus genus within the Togaviridae family contains several important mosquito-borne arboviruses. Other than the antiviral activity of RNAi, relatively little is known about alphavirus interactions with insect cell defences. Here we show that Semliki Forest virus (SFV) infection of Aedes albopictus-derived U4.4 mosquito cells reduces cellular gene expression. Activation prior to SFV infection of pathways involving STAT/IMD, but not Toll signaling reduced subsequent virus gene expression and RNA levels. These pathways are therefore not only able to mediate protective responses against bacteria but also arboviruses. However, SFV infection of mosquito cells did not result in activation of any of these pathways and suppressed their subsequent activation by other stimuli.

The ontogeny of immunity: development of innate immune strength in the honey bee (Apis mellifera)

Honey bees (Apis mellifera) are of vital economic and ecological importance. These eusocial animals display temporal polyethism, which is an age-driven division of labor. Younger adult bees remain in the hive and tend to developing brood, while older adult bees forage for pollen and nectar to feed the colony. As honey bees mature, the types of pathogens they experience also change. As such, pathogen pressure may affect bees differently throughout their lifespan. We provide the first direct tests of honey bee innate immune strength across developmental stages. We investigated immune strength across four developmental stages: larvae, pupae, nurses (1-day-old adults), and foragers (22-30 days old adults). The immune strength of honey bees was quantified using standard immunocompetence assays: total hemocyte count, encapsulation response, fat body quantification, and phenoloxidase activity. Larvae and pupae had the highest total hemocyte counts, while there was no difference in encapsulation response between developmental stages. Nurses had more fat body mass than foragers, while phenoloxidase activity increased directly with honey bee development. Immune strength was most vigorous in older, foraging bees and weakest in young bees. Importantly, we found that adult honey bees do not abandon cellular immunocompetence as has recently been proposed. Induced shifts in behavioral roles may increase a colony's susceptibility to disease if nurses begin foraging activity prematurely.

Protein tyrosine phosphatase-H2 from a polydnavirus induces apoptosis of insect cells

The family Polydnaviridae is a large group of immunosuppressive insect viruses that are symbiotically associated with parasitoid wasps. The polydnavirus Microplitis demolitor bracovirus (MdBV) causes several alterations that disable the cellular and humoral immune defences of host insects, including apoptosis of the primary phagocytic population of circulating immune cells (haemocytes), called granulocytes. Here, we show that MdBV infection causes granulocytes in the lepidopteran Spodoptera frugiperda to apoptose. An expression screen conducted in the S. frugiperda 21 cell line identified the MdBV gene ptp-H2 as an apoptosis inducer, as indicated by cell fragmentation, annexin V binding, mitochondrial membrane depolarization and caspase activation. PTP-H2 is a classical protein tyrosine phosphatase that has been shown previously to function as an inhibitor of phagocytosis. PTP-H2-mediated death of Sf-21 cells was blocked by the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-(O-methyl) Asp-fluoromethylketone (Z-VAD-FMK), but cells maintained in this inhibitor still exhibited a suppressed phagocytic response. Mutagenesis experiments indicated that the essential catalytic cysteine residue required for the phosphatase activity of PTP-H2 was required for apoptotic activity in Sf-21 cells. Loss of adhesion was insufficient to stimulate apoptosis of Sf-21 cells. PTP-H2 expression, however, did significantly reduce proliferation of Sf-21 cells, which could contribute to the apoptotic activity of this viral gene. Overall, our results indicate that specific genes expressed by MdBV induce apoptosis of certain insect cells and that this activity contributes to immunosuppression of hosts.

The viral protein Egf1.0 is a dual activity inhibitor of prophenoloxidase-activating proteinases 1 and 3 from Manduca sexta

Some pathogens are capable of suppressing the melanization response of host insects, but the virulence factors responsible are largely unknown. The insect pathogen Microplitis demolitor bracovirus encodes the Egf family of small serine proteinase inhibitors. One family member, Egf1.0, was recently shown to suppress melanization of hemolymph in Manduca sexta in part by inhibiting the enzymatic activity of prophenoloxidase activating proteinase 3 (PAP3). However, other experiments suggested this viral protein suppresses melanization by more than one mechanism. Here we report that Egf1.0 inhibited the amidolytic activity of PAP1 and dose-dependently blocked processing of pro-PAP1 and pro-PAP3. Consistent with its PAP inhibitory activity, Egf1.0 also prevented processing of pro-phenoloxidase, serine proteinase homolog (SPH) 1, and SPH2. Isolation of Egf1.0-protein complexes from plasma indicated that Egf1.0 binds PAPs through its C-terminal repeat domain. Egf1.0 also potentially interacts with SPH2 and two other proteins, ferritin and gloverin, not previously associated with the phenoloxidase cascade. Overall, our results indicate that Egf1.0 is a dual activity PAP inhibitor that strongly suppresses the insect melanization response.

The proPO-system: pros and cons for its role in invertebrate immunity

Melanisation is an important immune response in many invertebrates. Recent evidence also strongly implies that the melanisation (prophenoloxidase activating) cascade is intimately associated with the appearance of factors stimulating cellular defence by aiding phagocytosis and encapsulation reactions. However, some controversy exists in the field, and at least in flies and mosquitoes, the successful combat of some pathogens does not seem to be dependent on phenoloxidase activity. This may be because of redundancy among separate immune mechanisms, inappropriate testing, species differences or a combination thereof. Recently, by using RNA interference against phenoloxidase or in specific host-pathogen interactions where the pathogen prevents melanin production by the host, convincing data have confirmed the importance of this cascade in invertebrate innate immunity.