Host hemolymph monophenoloxidase activity in parasitized Manduca sexta larvae and evidence for inhibition by wasp polydnavirus

Laccase 1 gene from Plutella xylostella (PxLac1) and its functions in humoral immune response

Laccase (EC 1.10.3.2) is a phenoloxidase found in many insect species. The Laccase 1 gene from Plutella xylostella (PxLac1) was cloned, and its expression patterns and functions were determined using qPCR and RNAi methods. The results showed that the expression levels of PxLac1 were consistently high in all larval stages, and the most abundant was in the midgut during the 4th instar stage. Moreover, the expression of PxLac1 was up-regulated in response to bacterial infection, and decreased 24 h after being parasitized by Cotesia vestalis. Further analyses indicated that the effect of parasitization on PxLac1 was induced by active C. vestalis Bracovirus (CvBV). Haemocyte-free hemolymph phenoloxidase (PO) activity was suppressed when PxLac1 was treated with RNAi. Our results provide evidence for a connection between the Laccase 1 gene and insect immunity, and revealed that parasitoid polydnavirus suppresses host PO activity via PxLac1 regulation.

The recurrent domestication of viruses: major evolutionary transitions in parasitic wasps

Several lineages of endoparasitoid wasps, which develop inside the body of other insects, have domesticated viruses, used as delivery tools of essential virulence factors for the successful development of their progeny. Virus domestications are major evolutionary transitions in highly diverse parasitoid wasps. Much progress has recently been made to characterize the nature of these ancestrally captured endogenous viruses that have evolved within the wasp genomes. Virus domestication from different viral families occurred at least three times in parasitoid wasps. This evolutionary convergence led to different strategies. Polydnaviruses (PDVs) are viral gene transfer agents and virus-like particles of the wasp Venturia canescens deliver proteins. Here, we take the standpoint of parasitoid wasps to review current knowledge on virus domestications by different parasitoid lineages. Then, based on genomic data from parasitoid wasps, PDVs and exogenous viruses, we discuss the different evolutionary steps required to transform viruses into vehicles for the delivery of the virulence molecules that we observe today. Finally, we discuss how endoparasitoid wasps manipulate host physiology and ensure parasitism success, to highlight the possible advantages of viral domestication as compared with other virulence strategies.

Serpins in arthropod biology

Serpins are the largest known family of serine proteinase inhibitors and perform a variety of physiological functions in arthropods. Herein, we review the field of serpins in arthropod biology, providing an overview of current knowledge and topics of interest. Serpins regulate insect innate immunity via inhibition of serine proteinase cascades that initiate immune responses such as melanization and antimicrobial peptide production. In addition, several serpins with anti-pathogen activity are expressed as acute-phase serpins in insects upon infection. Parasitoid wasps can downregulate host serpin expression to modulate the host immune system. In addition, examples of serpin activity in development and reproduction in Drosophila have also been discovered. Serpins also function in host-pathogen interactions beyond immunity as constituents of venom in parasitoid wasps and saliva of blood-feeding ticks and mosquitoes. These serpins have distinct effects on immunosuppression and anticoagulation and are of interest for vaccine development. Lastly, the known structures of arthropod serpins are discussed, which represent the serpin inhibitory mechanism and provide a detailed overview of the process.

Effects of the endoparasitoid Cotesia chilonis (Hymenoptera: Braconidae) parasitism, venom, and calyx fluid on cellular and humoral immunity of its host Chilo suppressalis (Lepidoptera: Crambidae) larvae

The larval endoparasitoid Cotesia chilonis injects venom and bracoviruses into its host Chilo suppressalis during oviposition. Here we study the effects of the polydnavirus (PDV)-carrying endoparasitoid C. chilonis (Hymenoptera: Braconidae) parasitism, venom and calyx fluid on host cellular and humoral immunity, specifically hemocyte composition, cellular spreading, encapsulation and melanization. Total hemocyte counts (THCs) were higher in parasitized larvae than in unparasitized larvae in the late stages following parasitization. While both plasmatocyte and granulocyte fractions and hemocyte mortality did not differ between parasitized and unparasitized hosts, in vitro spreading behavior of hemocytes was inhibited significantly by parasitism throughout the course of parasitoid development. C. chilonis parasitism suppressed the encapsulation response and melanization in the early stages. Venom alone did not alter cellular immune responses, including effects on THCs, mortality, hemocyte composition, cell spreading and encapsulation, but venom did inhibit humoral immunity by reducing melanization within 6h after injection. In contrast to venom, calyx fluid had a significant effect on cell spreading, encapsulation and melanization from 6h after injection. Dose-response injection studies indicated the effects of venom and calyx fluid synergized, showing a stronger and more persistent reduction in immune system responses than the effect of either injected alone.

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.

Parasitization by Macrocentrus cingulum (Hymenoptera: Braconidae) influences expression of prophenoloxidase in Asian corn borer Ostrinia furnacalis

A prophenoloxidase (PPO) cDNA (OfPPO) was cloned from the Asian corn borer Ostrinia furnacalis. Sequence analysis revealed a full length transcript of the OfPPO cDNA with 2,686 bp, containing a 2,079 bp open reading frame (ORF), a 73-bp 5'-untranslated region, and a 534-bp 3'-untranslated region with a poly(A) signal. The ORF encodes a 693-amino acid polypeptide, containing two distinct copper-binding regions, a plausible thiol ester site, two proteolytic activation sites, and a conserved C-terminal region, but lacks a signal peptide sequence. Expression of the OfPPO transcript in the plasma, hemocytes, fat body and midgut was inhibited by Macrocentrus cingulum at 4 h post-parasitization (pp). In situ hybridization analysis showed that the hemocytes, especially the oenocytoids, hybridized strongly with the DNA probes of the OfPPO gene. No signal was detected in the cuticular epithelium or fat body of the parasitized larvae. Colloidal gold particles were used to visualize the PPO by immunoelectron microscopy. The time course study revealed a decrease in the labeling of the OfPPO at 4, 6, 8, 12, and 1 day pp in the larval integument and midgut parasitized by M. cingulum. We infer from time course studies of OfPPO gene expression and PO enzymatic activity that OfPPO in the integument is released from hemocytes and that the OfPPO expression was influenced at the transcriptional, translational, and then the post-translational level by parasitization challenge.

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.

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 melanization and cytotoxic by-products in the cellular immune responses of Drosophila against parasitic wasps

The cellular innate immune response of several species of Drosophila terminates with the encasement of large foreign objects within melanotic capsules comprised of several layers of adhering blood cells or hemocytes. This reaction is manifested by various Drosophila hosts in response to infection by endoparasitic wasps (i.e., parasitoids). Creditable assessments of the factor(s) causing, or contributing to, parasite mortality have long been considered as cytotoxic elements certain molecules associated with enzyme-mediated melanogenesis. However, observations that warrant additional or alternative considerations are those documenting parasitoid survival despite melanotic encapsulation, and those where parasitoids are destroyed with no evidence of this host response. Recent studies of the production of some reactive intermediates of oxygen and nitrogen during infection provide a basis for proposing that these molecules constitute important components of the immune arsenal of Drosophila. Studies of the virulence factors injected by female wasps during oviposition that suppress the host response will likely facilitate identification of the cytotoxic molecules as well as the cell-signaling pathways that regulate their synthesis.

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.

Prophenoloxidases 1 and 2 from the spruce budworm, Choristoneura fumiferana: molecular cloning and assessment of transcriptional regulation by a polydnavirus

Immune challenge in arthropods is frequently accompanied by melanization of the hemolymph, a reaction triggered by the activation of prophenoloxidase (PPO). Because their immature stages are spent inside the hemocoel of insect larvae, endoparasitoids have evolved strategies to escape or counter melanin formation. Very little molecular information is available on these endoparasitoid counterstrategies. We have sought to shed light on the inhibition of melanization in the spruce budworm, Choristoneura fumiferana, by the parasitic wasp Tranosema rostrale, by cloning two host PPO homologs and studying their transcriptional regulation after parasitization. The two polypeptides are encoded by transcripts of approximately 3.3 kb (for CfPPO1) and 3.0 kb (for CfPPO2) and possess structural features typical of other insect PPOs. While there appears to be a single CfPPO2 gene in the C. fumiferana genome, we detected three CfPPO1 mRNA variants displaying insertions/deletions in the 3' untranslated region, suggesting that there may be more than one CfPPO1 gene copy. Both CfPPO1 and CfPPO2 were expressed at high levels in C. fumiferana 6th instars, and parasitization by T. rostrale had no apparent impact on the level of their transcripts. Injection of a large dose (0.5 female-equivalent) of polydnavirus-laden calyx fluid extracted from T. rostrale, which is known to inhibit melanization in C. fumiferana, only caused a transient decrease in CfPPO1 and CfPPO2 transcript accumulation at 2-3 d post injection. It thus appears that transcriptional downregulation of C. fumiferana PPO by T. rostrale plays a minor role in the inhibition of hemolymph melanization in this host-parasitoid system.

Different defense strategies of Dendrolimus pini, Galleria mellonella, and Calliphora vicina against fungal infection

The resistance of Galleria mellonella, Dendrolimus pini, and Calliphora vicina larvae against infection by the enthomopathogen Conidiobolus coronatus was shown to vary among the studied species. Exposure of both G. mellonella and D. pini larvae to the fungus resulted in rapid insect death, while all the C. vicina larvae remained unharmed. Microscopic studies revealed diverse responses of the three species to the fungal pathogen: (1) the body cavities of D. pini larvae were completely overgrown by fungal hyphae, with no signs of hemocyte response, (2) infected G. mellonella larvae formed melanotic capsules surrounding the fungal pathogen, and (3) the conidia of C. coronatus did not germinate on the cuticle of C. vicina larvae. The in vitro study on the degradation of the insect cuticle by proteases secreted by C. coronatus revealed that the G. mellonella cuticle degraded at the highest rate. The antiproteolytic capacities of insect hemolymph against fungal proteases correlated well with the insects' susceptibility to fungal infection. The antiproteolytic capacities of insect hemolymph against fungal proteases correlated well with the insects' susceptibility to fungal infection. Of all the tested species, only plasmatocytes exhibited phagocytic potential. Exposure to the fungal pathogen resulted in elevated phagocytic activity, found to be the highest in the infected G. mellonella. The incubation of insect hemolymph with fungal conidia and hyphae revealed diverse reactions of hemocytes of the studied insect species. The encapsulation potential of D. pini hemocytes was low. Hemocytes of G. mellonella showed a high ability to attach and encapsulate fungal structures. Incubation of C. vicina hemolymph with C. coronatus did not result in any hemocytic response. Phenoloxidase (PO) activity was found to be highest in D. pini hemolymph, moderate in G. mellonella, and lowest in the hemolymph of C. vicina. Fungal infection resulted in a significant decrease of PO activity in G. mellonela larvae, while that in the larvae of D. pini remained unchanged. PO activity in C. vicina exposed to fungus slightly increased. The lysozyme-like activity increased in the plasma of all three insect species after contact with the fungal pathogen. Anti E. coli activity was detected neither in control nor in infected D. pini larvae. No detectable anti E. coli activity was found in the control larvae of G. mellonella; however, its exposure to C. coronatus resulted in an increase in the activity to detectable level. In the case of C. vicina exposure to the fungus, the anti E. coli activity was significantly higher than in control larvae. The defense mechanisms of D. pini (species of economic importance in Europe) are presented for the first time.

Effects of venom/calyx fluid from the endoparasitic wasp Cotesia plutellae on the hemocytes of its host Plutella xylostella in vitro

Crude venom and calyx fluid from Cotesia plutellae (Hymenoptera Braconidae) were assayed for biological activity toward hemocytes of Plutella xylostella (Lepidoptera Plutellidae). Venom from C. plutellae displayed high activity toward the spreading of plasmatocytes of P. xylostella early in the incubation period, and the inhibition was more severe as the concentration of venom increased. However, most inhibited hemocytes spread normally after being incubated for 4h. No effects were found toward granular cells from the host. Additionally, the venom from C. plutellae had some lethal effects on hemocytes of P. xylostella at high concentrations. In contrast, when incubated with different concentrations of calyx fluid, the spreading of some hemocytes was inhibited, some began to disintegrate, and some were badly damaged with only the nucleus left. After 4h, the majority of hemocytes died. The same results were observed when hemocytes were incubated in calyx fluid together with venom. These results show that calyx fluid from C. plutellae may play a major role in the suppression of the host immune system, whereas venom from C. plutellae has a limited effect on hemocytes and probably synergizes the effect of calyx fluid or polydnavirus.

Evolution of developmental strategies in parasitic hymenoptera

Parasitoid wasps have evolved a wide spectrum of developmental interactions with hosts. In this review we synthesize and interpret results from the phylogenetic, ecological, physiological, and molecular literature to identify factors that have influenced the evolution of parasitoid developmental strategies. We first discuss the origins and radiation of the parasitoid lifestyle in the Hymenoptera. We then summarize how parasitoid developmental strategies are affected by ecological interactions and assess the inventory of physiological and molecular traits parasitoids use to successfully exploit hosts. Last, we discuss how certain parasitoid virulence genes have evolved and how these changes potentially affect parasitoid-host interactions. The combination of phylogenetic data with comparative and functional genomics offers new avenues for understanding the evolution of biological diversity in this group of insects.

Parasitic suppression of feeding in the tobacco hornworm, Manduca sexta: parallels with feeding depression after an immune challenge

The parasitic wasp, Cotesia congregata, suppresses feeding in its host Manduca sexta. Feeding suppression in the host coincides with the emergence of the wasps through the host's cuticle. During wasp emergence, host hemocyte number declined, suggesting that the host mounts a wound/immune response against the exiting parasitoids and/or resulting tissue damage. Eliciting a different type of immune response by injecting heat-killed Serratia marcescens also resulted in a decline in feeding and a reduction in hemocyte number. Both the emerging wasps and the bacteria induced an increase in hemolymph octopamine concentration and a decrease in foregut peristalsis in M. sexta. The emerging parasitoids produced the largest changes. The source of the additional octopamine appeared to be the host in both cases. S. marcescens was found to contain no detectable amounts of octopamine. The parasitoids had insufficient octopamine to account for the amount found in host hemolymph and they did not secrete octopamine in vitro. One cause for the high concentration of octopamine in parasitized M. sexta was that octopamine was removed from the hemolymph approximately 23 times more slowly after the wasps emerged than prior to wasp emergence. The striking similarity between the effects of parasitoids and bacteria on M. sexta feeding, hemocyte number, hemolymph octopamine concentration, and foregut peristalsis supports the possibility that the immune/wound reaction induced by the emerging wasps could play a role in the suppression of host feeding. These results also support the hypothesis that M. sexta exhibit an immune-activated anorexia.

A novel venom peptide from an endoparasitoid wasp is required for expression of polydnavirus genes in host hemocytes

Maternal factors introduced into host insects by endoparasitoid wasps are usually essential for successful parasitism. This includes polydnaviruses (PDVs) that are produced in the reproductive organ of female hymenopteran endoparasitoids and are injected, together with venom proteins, into the host hemocoel at oviposition. Inside the host, PDVs enter various tissue cells and hemocytes where viral genes are expressed, leading to developmental and physiological alterations in the host, including the suppression of the host immune system. Although several studies have shown that some PDVs are only effective when accompanied by venom proteins, there is no report of an active venom ingredient(s) facilitating PDV infection and/or gene expression. In this study, we describe a novel peptide (Vn1.5) isolated from Cotesia rubecula venom that is required for the expression of C. rubecula bracoviruses (CrBVs) in host hemocytes (Pieris rapae), although it is not essential for CrBV entry into host cells. The peptide consists of 14 amino acids with a molecular mass of 1598 Da. In the absence of Vn1.5 or total venom proteins, CrBV genes are not expressed in host cells and did not cause inactivation of host hemocytes.

Negative regulation of prophenoloxidase (proPO) activation by a clip-domain serine proteinase homolog (SPH) from endoparasitoid venom

Most parasitic wasps inject maternal factors into the host hemocoel to suppress the host immune system and ensure successful development of their progeny. Melanization is one of the insect defence mechanisms against intruding pathogens or parasites. We previously isolated from the venom of Cotesia rubecula a 50 kDa protein that blocked melanization in the hemolymph of its host, Pieris rapae [Insect Biochem. Mol. Biol. 33 (2003) 1017]. This protein, designated Vn50, is a serine proteinase homolog (SPH) containing an amino-terminal clip domain. In this work, we demonstrated that recombinant Vn50 bound P. rapae hemolymph components that were recognized by antisera to Tenebrio molitor prophenoloxidase (proPO) and Manduca sexta proPO-activating proteinase (PAP). Vn50 is stable in the host hemolymph-it remained intact for at least 72 h after parasitization. Using M. sexta as a model system, we found that Vn50 efficiently down-regulated proPO activation mediated by M. sexta PAP-1, SPH-1, and SPH-2. Vn50 did not inhibit active phenoloxidase (PO) or PAP-1, but it significantly reduced the proteolysis of proPO. If recombinant Vn50 binds P. rapae proPO and PAP (as suggested by the antibody reactions), it is likely that the molecular interactions among M. sexta proPO, PAP-1, and SPHs were impaired by this venom protein. A similar strategy might be employed by C. rubecula to negatively impact the proPO activation reaction in its natural host.

A serine proteinase homolog venom protein from an endoparasitoid wasp inhibits melanization of the host hemolymph

Activation of prophenoloxidase (proPO) in insects is a defense mechanism against intruding microorganisms and parasites. Pattern recognition molecules induce activation of an enzymatic cascade involving serine proteinases, which leads to the conversion of proPO to active phenoloxidase (PO). Phenolic compounds produced by pPO-activation are toxic to invaders. Here, we describe the isolation of a venom protein from the parasitoid, Cotesia rubecula, injected into the host, Pieris rapae, which is homologous to serine proteinase homologs (SPH). The data presented here indicate that the protein interferes with the proteolytic cascade, which under normal circumstances leads to the activation of proPO and melanin formation.

Active suppression of D. melanogaster immune response by long gland products of the parasitic wasp Leptopilina boulardi

To develop inside their insect hosts, endoparasitoid wasps must either evade or overcome the host's immune system. Several ichneumonid and braconid wasps inject polydnaviruses that display well-studied immune suppressive effects. However, little is known about the strategies of immunoevasion used by other parasitoid families, such as figitid wasps. The present study provides experimental evidence, based on superparasitism and injection experiments, that the figitid species Leptopilina boulardi uses an active mechanism to suppress the Drosophila melanogaster host immune response, i.e. the encapsulation of the parasitoid eggs. The immune suppressive factors are localised in the long gland and reservoir of the female genital tractus, where virus-like particles (VLPs) have been observed. Parasitism experiments using a host tumorous strain indicate that these factors do not destroy host lamellocytes but that they impair the melanisation pathway. Interestingly, they are not susceptible to heating and are not depleted with prolonged oviposition experience, in contrast to observations reported for L. heterotoma, another figitid species. The mechanisms that prevent encapsulation of eggs from L. boulardi and L. heterotoma differ in several respects, suggesting that different physiological strategies of immunosuppression might be used by specialised and generalist parasitoids.

Comparative study of the strategies evolved by two parasitoids of the genus Asobara to avoid the immune response of the host, Drosophila melanogaster

Asobara tabida and Asobara citri are two braconid endoparasitoids of Drosophila melanogaster larvae. We studied and compared the strategies evolved by these two species to avoid the immune reaction of their host. A. tabida has no negative impact on host cellular defenses and its eggs avoid encapsulation by adhering to host tissues. At the opposite, we found that A. citri, whose eggs are devoid of adhesive properties, affects the host encapsulation abilities, hemolymph phenoloxidase activity and concentrations of circulating hemocytes. Some of these effects could directly rely on a severe disruption of the hematopoietic organ anterior lobes observed in parasitized larvae. This is the first report of the immune suppressive abilities of a parasitoid from the Asobara genus. Results are presented and discussed with respect to the strategies of virulence evolved by other parasitoids to counteract the D. melanogaster immune system.

The lipopolysaccharide and beta-1,3-glucan binding protein gene is upregulated in white spot virus-infected shrimp (Penaeus stylirostris)

Pattern recognition proteins such as lipopolysaccharide and beta-1,3-glucan binding protein (LGBP) play an important role in the innate immune response of crustaceans and insects. Random sequencing of cDNA clones from a hepatopancreas cDNA library of white spot virus (WSV)-infected shrimp provided a partial cDNA (PsEST-289) that showed similarity to the LGBP gene of crayfish and insects. Subsequently full-length cDNA was cloned by the 5'-RACE (rapid amplification of cDNA ends) technique and sequenced. The shrimp LGBP gene is 1,352 bases in length and is capable of encoding a polypeptide of 376 amino acids that showed significant similarity to homologous genes from crayfish, insects, earthworms, and sea urchins. Analysis of the shrimp LGBP deduced amino acid sequence identified conserved features of this gene family including a potential recognition motif for beta-(1-->3) linkage of polysaccharides and putative RGD cell adhesion sites. It is known that LGBP gene expression is upregulated in bacterial and fungal infection and that the binding of lipopolysaccharide and beta-1,3-glucan to LGBP activates the prophenoloxidase (proPO) cascade. The temporal expression of LGBP and proPO genes in healthy and WSV-challenged Penaeus stylirostris shrimp was measured by real-time quantitative reverse transcription-PCR, and we showed that LGBP gene expression in shrimp was upregulated as the WSV infection progressed. Interestingly, the proPO expression was upregulated initially after infection followed by a downregulation as the viral infection progressed. The downward trend in the expression of proPO coincided with the detection of WSV in the infected shrimp. Our data suggest that shrimp LGBP is an inducible acute-phase protein that may play a critical role in shrimp-WSV interaction and that the WSV infection regulates the activation and/or activity of the proPO cascade in a novel way.

Effects of the Diadromus pulchellus ascovirus, DpAV-4, on the hemocytic encapsulation response and capsule melanization of the leek-moth pupa, Acrolepiopsis assectella

DpAV-4 is a symbiotic ascovirus found in natural populations of the solitary endoparasitoid wasp Diadromus pulchellus. The female wasp injects this virus into the pupae of the leek-moth Acrolepiopsis assectella during oviposition. The ascovirus replicates in the pupal tissues and the consequent lysis of the cells occurs synchronously with egg hatching and the development of the wasp larva. We show here that encapsulation and capsule melanization were activated when minute nylon monofilaments were implanted into the hemocoel of non parasitized leek-moth pupae and that encapsulation and melanization were inhibited in pupae parasitized by D. pulchellus. When the pupae were infected by DpAV-4, melanization of the nylon monofilaments was abolished, but a capsule was still always formed. These results indicate that DpAV-4 is a free virus able to alter the defence system of the parasitized host so as to improve the development of the parasitoid wasp, D. pulchellus.

Effects of juvenile hormones and precocenes on the immune response of Spodoptera littoralis larvae to supernumerary larvae of the solitary parasitoid, Microplitis rufiventris Kok

The study represents one of the first direct tests of juvenile hormones (JHs) on the cellular immune response of a lepidopteran insect, Spodoptera littoralis (Boisd.), superparasitized by a solitary parasitoid, Microplitis rufiventris Kok. JHII-treated S. littoralis larvae exhibited a significant reduction in encapsulation response to the supernumerary M. rufiventris larvae at different developmental ages (0-4, 10-14 and 24-26 h) of supernumerary M. rufiventris embryos within 48 or 144 h after parasitization than either JHI-treated or control hosts. The reduction in encapsulation response was significantly greater in hosts which received higher JHII dose (5 &mgr;g) than those treated with lower one (1 &mgr;g).The progression of the encapsulation, culminating in melanization in the capsules occurred more frequently in precocene-treated hosts than either JH-treatment or control ones. Application of the anti-juvenile hormones (PI or PII) caused stronger encapsulation reaction to the injured supernumerary parasitoid larvae than JHI or JHII. In PI-treatments, the reaction was rapid and dramatic by lower (25 &mgr;g) dose treatments than higher ones (70 &mgr;g).The capsules formed around the supernumerary larvae of M. rufiventris showed signs of melanization within 48 h in PI-treated hosts. Melanization was more pronounced in either PI- or PII-treated hosts prior emergence of successful parasitoid larvae. Generally, the injured supernumerary larvae were seen within melanin-rich capsules in PI- and PII-treated hosts. Melanization was not observed in most capsules of JHI- and JHII-treated hosts. Additional physiological effects were observed: (1) in some cases, application of JHI to parasitized S. littoralis larvae inhibited subsequent emergence of its wasp, M. rufiventris, and (2) teratocytes (cells of wasp origin) from PI or PII-treated hosts were smaller in size than those observed in the JH-treated or control hosts.Finally, the dramatic increase in encapsulation responses of precocene-treated S. littoralis larvae of the supernumerary of M. rufiventris and definitely opposite reactions in typical hosts but JHII-treated hosts suggested that the cellular defense reaction may be under inhibitory hormonal control.The results of the study have implications for medically important insects such as mosquitoes and their interactions with parasites, as well as having significance for biocontrol and mass rearing programs using parasitoids.

Parasitism of Lacanobia oleracea (Lepidoptera) by the ectoparasitoid, Eulophus pennicornis, is associated with a reduction in host haemolymph phenoloxidase activity

When haemolymph from fifth instar Lacanobia oleracea was incubated in vitro, rapid melanization occurred. Similar levels of melanization occurred in haemolymph from larvae that had been experimentally injected with venom from the ectoparasitic wasp, Eulophus pennicornis. In contrast, haemolymph from larvae parasitized by this wasp melanized more slowly and less extensively. Phenoloxidase assays indicated that enzyme activity was present in haemocyte lysate supernatants, serum and plasma from L. oleracea and that on day 5 post-parasitization, fractions prepared from parasitized larvae had significantly less phenoloxidase activity than similar fractions from untreated or experimentally envenomated larvae. In addition, no PO activity was detectable in wasp venom, and the venom had no effect on L. oleracea plasma phenoloxidase activity in vitro. These results indicate that parasitism of L. oleracea by E. pennicornis suppresses host haemolymph phenoloxidase activity and that this suppression is not induced by adult wasp venom. The results are discussed with reference to the survival advantages of suppressing the activity of this host enzyme, and to the possible source(s) of putative suppressive factors.

Relationships between polydnavirus gene expression and host range of the parasitoid wasp Campoletis sonorensis

To evaluate the relationship between immune suppression and host range six lepidopteran species were parasitized by the ichneumonid parasitoid Campoletis sonorensis. Parasitism inhibited the growth of permissive hosts (Heliothis virescens, Helicoverpa zea, and Trichoplusia ni), whereas growth of semi-permissive (Spodoptera exigua, Agrotis ipsilon) and non-permissive hosts (Manduca sexta) was not significantly affected. The 29-36 kDa ovarian protein (OP), responsible for transient immunosuppression in the permissive host H. virescens, bound to and was endocytosed by hemocytes of permissive and non-permissive hosts. Expression of the cysteine-rich polydnavirus gene, VHv1.4, was detected in all the hosts, but declined only in semi- and non-permissive hosts at later times after parasitization. The VHv1.4 protein bound to hemocytes of permissive and semi-permissive hosts, but did not bind to hemocytes of the non-permissive host, M. sexta. Melanization of larval hemolymph was severely inhibited by parasitism in permissive hosts, but was unaffected in M. sexta. In the semi-permissive host, A. ipsilon, hemolymph melanization was transiently inhibited while viral genes were expressed. In conclusion, C. sonorensis OP transiently inhibits encapsulation in all hosts that were tested. The host range of C. sonorensis seems to be determined by whether or not the C. sonorensis ichnovirus (CsIV) is able to establish persistent infections of parasitized larvae to provide long-term suppression of host immunity.

Evidence for serine protease inhibitor activity in the ovarian calyx fluid of the endoparasitoid Venturia canescens

Endoparasitic wasps are able to develop inside permissive host insects due to their ability to overcome or evade the host's immune system. In the present study, we provide experimental evidence that ovarian calyx fluid of the ichneumonid endoparasitoid Venturia canescens has the potential to alter host haemocyte spreading and inhibit host haemolymph melanisation due to the presence of a putative serine protease inhibitor (serpin) activity. The existance of a serpin-like activity in the calyx fluid is also supported by experiments where the synthetic protease inhibitor p-APMSF had effects on cellular and cell-free immune reactions similar to ovarian calyx fluid. In addition, based on proteolytic digestion patterns of a wasp egg surface protein, we predict an Arg-specific trypsin-like protease activity in the host haemolymph which is possibly affected by calyx fluid components as well. Our data suggest that ovarian calyx fluid, deposited into the host together with the parasitoid egg, contains serpin activity which might transiently inactivate host defence reactions until other means of protection are established on the egg surface.

Alteration in FAD-glucose dehydrogenase activity and hemocyte behavior contribute to initial disruption of Manduca sexta immune response to Cotesia congregata parasitoids

Cotesia congregata and Manduca sexta were used as a model system to study the mechanism and effect of a polydnavirus (PDV). We hypothesized that (1) FAD-glucose dehydrogenase (GLD) (EC 1.1.99.10) hemolymph titer would increase in response to parasitism, (2) hemocyte targeting behavior would be altered by parasitism, and (3) changes observed in GLD activity and hemocyte behavior immediately post-parasitization would be due to the presence of PDV. GLD specific activity was measured at four time points early during parasitism using a spectrophotometric enzyme assay. Hemocyte behavior was measured using direct observations of hemocyte response to a foreign target in vitro. Results demonstrate that GLD increases immediately post-oviposition and post-injection of purified PDV, indicating that virions elicit nonself recognition. This increase relative to unparasitized controls also occurs in response to trioxsalen-UV inactivated virus, indicating that the initial disruption of the host immune response is not dependent upon viral transcription. Further, we demonstrate that plasmatocytes are actively spreading and aggregating but are not targeting nonself material in both parasitized and polydnavirus treatments. These results indicate that purified PDV is recognized as nonself and is triggering an immediate cellular immune response prior to viral transcription.

A possible mechanism for the physiological suppression of conspecific eggs and larvae following superparasitism by solitary endoparasitoids

Competition for possession of a host by internal solitary parasitoids has been attributed to physical combat and physiological suppression, but the mechanisms that result in what has been referred to as physiological suppression is poorly understood. Some insights are provided by the studies reported here using the solitary endoparasitoid, Campoletis sonorensis (Cameron). Embryos of C. sonorensis less than ten hours old rarely hatch in various artificial media, while embryos twenty hours or older generally hatch. These results suggest that young embryos in which the embryonic membranes have not yet formed are only able to develop in a narrow range of environments represented by the nonparasited hemolymph. In contrast, embryos in which the embryonic membranes are formed are able to develop in a wide range of environments represented by parasitized hemolymph which has been shown by a number of studies to change. These ideas were given support by studies reported here, where young and older eggs were incubated singly or paired. We suggest the general changes in the hemolymph of a parasitized host become unfavorable for the development of newly oviposited eggs.

Modulation of immune responses to parasitoids by polydnaviruses

Parasitoids are parasites that invariably kill their host. Polydnaviruses are injected by parasitoid wasps into the body cavity of their insect host and cause immunosuppression, allowing the parasitoid to develop in the absence of encapsulation. One of the targets of the polydnaviruses are the haemocytes of the host, which undergo significant changes in response to entry of the virus. In some systems, haemocyte apoptosis is induced, or haemocyte clumping may be seen; in others, the cells round up and fail to adhere to a substrate. Effects on haemocytes may be transitory or permanent (cell death). Various polydnavirus gene products have been identified that interfere with normal haemocyte function. Phenoloxidase activity also is inhibited during parasitism, and the effect is inducible by polydnavirus. In some systems, venom components may act synergistically with polydnavirus in mediating the virally-induced effects on the host immune system. Polydnaviruses are powerful influences on the host immune system, which serve to permit successful development of the parasitoid without triggering the host immune response.

Production of early expressed parasitism-specific proteins in alternate sphingid hosts of the braconid wasp Cotesia congregata

Parasitism of Manduca sexta larvae by the braconid wasp Cotesia congregata or injection of C. congregata polydnavirus (CcPDV) causes numerous alterations in host physiology, including developmental arrest, abrogation of host immunity, and the production of three abundant early expressed proteins (EP1, EP2, and EP3) that are secreted in large amounts into the host's hemolymph. Here we compare the levels of these proteins present in the hemolymph of three other sphingid species that vary in their compatibility for C. congregata. Hyles lineata was found to be permissive for C. congregata and EP1, EP2, and EP3 were present in larval hemolymph at levels comparable to those found in hemolymph from parasitized M. sexta larvae. By contrast, the lowest levels of EP proteins were found in hemolymph from parasitized Pachysphinx occidentalis larvae and this species was found to be completely refractory, since C. congregata eggs were invariably encapsulated. Parasitism of Sphinx vashti by C. congregata resulted in moderate levels of EP production. While the observed immune response was incomplete and some encapsulation of C. congregata eggs and/or larvae was observed, low numbers of S. vashti nevertheless were able to complete their development and emerge as adults. Thus, a correlation was established between host compatibility and induction of synthesis of the three parasitism-specific proteins, although the linkage between quantitative levels of EP production and the extent of encapsulation was variable.

Parasitism-induced Effects on Host Growth and Metabolic Efficiency in Tobacco Hornworm Larvae Parasitized by Cotesia congregata

Parasitism by the braconid wasp Cotesia congregata affects the growth of Manduca sexta larvae in a parasitoid 'dose-dependent' fashion. Following parasitization of fourth-instar larvae, more heavily parasitized larvae grew larger compared to those containing fewer parasitoids due to an increase in host dry weight. The differences in host mass appeared to arise after oviposition. A 'dose-dependent' enhancement of host dry weight would appear nutritionally beneficial for the parasitoids developing in more 'crowded' hosts. The efficiencies of conversion of ingested and digested food to body mass and the approximate digestibility of the diet ingested by the host caterpillar did not vary significantly with clutch size although parasitoids took slightly longer to develop in the more heavily parasitized hosts. Larval parasitoids developing in the presence of many competitors weighed up to 50% less than those developing in hosts with fewer endoparasitoids, although the weight of adult female parasitoids did not vary significantly with wasp clutch size. The maximum number of emerging wasps was 200 parasitoids, possibly representing the host's 'carrying capacity' for larvae parasitized in the fourth-instar. The ratio of emerging to non-emerging parasitoids decreased as parasitoid clutch size increased, with few or none emerging from very heavily parasitized hosts containing more than 400 parasitoids. Copyright 1997 Elsevier Science Ltd. All right reserved

Avoidance of encapsulation in the absence of VLP by a braconid parasitoid of Drosophila larvae: an ultrastructural study

The hymenopteran Asobara tabida Nees (Braconidae: Alysiinae) parasitizes the larval stages of Drosophila species. In D. melanogaster, 90% of A. tabida eggs avoid encapsulation and are found embedded within the tissues (digestive tube, fat body, tracheae, etc.) of their larval host. Electron microscopy was used to investigate if the embedment of the parasitic egg within the tissues of its host may prevent encapsulation. Study revealed that the outer layer of the A. tabida egg consists of a fibrous exochorion that exhibits adhesive properties within the tissues of D. melanogaster. Three hours after oviposition, any basement membrane surrounding the host tissues that is in contact with the parasitic egg is stuck to the exochorion. Attachment rapidly increases. Nine hours post oviposition, the A. tabida egg is almost totally surrounded by a sheath of host tissue, with only limited areas of the surface remaining uncovered and exposed to attack by the host hemocytes. It is therefore concluded that the adhesion of A. tabida eggs to the host tissues is likely to contribute to their protection from encapsulation during embryogenesis. Also, electron microscopy reveals that no virus-like particles occur in A. tabida, unlike any other braconid parasitoid studied. The absence of virus-like particles is discussed with regard to the importance of host regulation of the particles in other braconid species.

The prophenoloxidase activating system and associated proteins in invertebrates

In this review, we present arguments indicating that prophenoloxidase (proPO) activating system acts as a pattern recognition and defence system in invertebrate blood. Phenoloxidase (PO) activity has been found in the blood of many invertebrates. At least in arthropods, echinoderms and urochordates, the inactive pro-form, proPO has been found to be elicited by the microbial cell-wall components beta-1, 3-glucans, lipopolysaccharide and/or peptidoglycan. This activation seems to involve elicitor-binding proteins and serine protease(s). ProPO, the proPO-activating enzyme (ppA) and plasma elicitor-binding proteins, have been purified from some arthropods, and proPO and the beta-1, 3-glucan binding protein (beta GBP) have been cloned and sequenced from crayfish. Arthropod proPO has a molecular mass of 70-90 kDa and PO has a molecular mass of 60-70 kDa. The beta GBP also stimulates phagocytosis of fungal cells and, after reacting with beta-1, 3-glucan, blood-cell degranulation (and release of the proPO system). In addition, a cell-adhesion protein (of 70-100 kDa), apparently associated with the proPO system, has been purified from arthropods. This mediates blood-cell adhesion, degranulation, phagocytosis and encapsulation. The cell-adhesion protein and beta GBP bind to a common blood-cell membrane receptor. It would be interesting to see the sequences of more proPO system components and investigate whether the scheme for cellular communication and defence, involving the cell-adhesion protein, elicitor-binding proteins and the membrane receptor described in arthropods, applies to invertebrates in general.

Polydnaviruses: potent mediators of host insect immune dysfunction

Endoparasitic insects are used as biological control agents to kill many species of insect pest. One key to the success of parasitoids that develop in the hemocoel of their host is their ability to knock out the host's immune system, inducing a decline in the responsiveness of a variety of cellular and humoral components so that parasitoid eggs are not encapsulated. In many species parasitized by braconid and ichneumonid wasps, host immunosuppression appears to be mediated by polydnaviruses (PDVs) injected by the female parasitoid into the host hemocoel. The viruses exhibit a complex and intimate genetic relationship with the wasp, since viral sequences are integrated within the wasp's chromosomal DNA. Here Mark Lavine and Nancy Beckage summarize the current evidence for mechanisms of virally induced host immunosuppression in parasitized insects, as well as the roles of other factors including wasp ovarian proteins and venom components, in suppressing hemocyte-mediated and humoral immune responses. Interestingly, in some species, the PDV-induced host immunosuppression appears transitory, with older parasitoid larvae probably exploiting other mechanisms to protect themselves from the host's immune system during the final stages of parasitism. During the final stages of parasitism, the parasitoids likely exploit other mechanisms of immunoevasion via antigen masking, antigen mimicry, or production of active inhibitors of the hemocyte-mediated encapsulation response as well as inhibiting melanization.

Effects of parasitism by the braconid wasp Cotesia congregata on host hemolymph proteins of the tobacco hornworm, Manduca sexta

Parasitism by the braconid wasp Cotesia congregata causes major alterations in the hemolymph proteins of host tobacco hornworm larvae. Earlier studies showed that the total amount of hemolymph protein is reduced during parasitism, beginning almost immediately after the host is parasitized. Simultaneously, parasitism induces synthesis of large amounts of novel proteins that appear in the blood as early as 1-2 h post-parasitization. The present report confirms earlier studies describing the presence of novel proteins in last instar hosts, and also characterizes the effects of parasitism in altering the titers of several endogenous host hemolymph proteins normally produced by the fat body and other tissues. Analysis of hemolymph plasma using SDS-PAGE and densitometry, as well as immunodiffusion assays, showed that in terminal stage fifth instar host larvae, the titers of serpins and arylphorin were dramatically reduced relative to the levels of these proteins detected in nonparasitized gate II fifth instar larvae of the same age. The relative differences between parasitized and nonparasitized larvae increased with time following ecdysis to the fifth instar, so that the day 4 nonparasitized larvae had arylphorin titers of c. 30 mg/ml, whereas parasitized day 4 larvae with newly emerged wasps had only one sixth that amount of storage protein circulating in the hemolymph. Similarly, in nonparasitized larvae the hemolymph serpin concentration increased from c. 200 micrograms/ml (on day 0) to > 600 micrograms/ml (on day 4) in prewandering gate II larvae, but in parasitized larvae the hemolymph serpin concentration was maintained in the range of 100-200 micrograms per ml hemolymph until the pharate third instar parasites emerged from the host larva on day 4. In contrast, the level of hemolymph lipophorin was unaffected by parasitism, and lipophorin increased from c. 1.3 to > 3 mg/ml during the time interval between days 0 and 4 in both nonparasitized and parasitized larvae. Hemolymph titers of insecticyanin also were not significantly different in parasitized vs nonparasitized larvae, and in both types of larvae the concentration of this pigment decreased by c. 50% during the same time interval when lipophorin was increased significantly. Instead of causing a generalized inhibition of host hemolymph protein synthesis, parasitism causes a complex array of changes in the hemolymph protein profile of Manduca sexta, possibly via the mediation of hormonal modulators of host protein synthesis, or transcriptional or translational regulation of host gene expression by factors associated with the polydnavirus or molecules secreted by the parasites.(ABSTRACT TRUNCATED AT 400 WORDS)

The prophenoloxidase system and in vitro interaction of Trypanosoma rangeli with Rhodnius prolixus and Triatoma infestans haemolymph

The presence of the prophenoloxidase (proPO) system in the haemolymph of Rhodnius prolixus and Triatoma infestans and the role played by Trypanosoma rangeli in the in vitro activation of proPO were tested. Both R. prolixus and T. infestans whole blood preparations showed a very active ProPO system. The proPO cascade of the two insect species were differentially activated by microbial-derived extracts: laminarin was a better activator of T. infestans haemolymph than of R. prolixus blood, and lipopolysaccharides from Shigella flexneri or Pseudomonas aeroginosa caused significant proPO activation of T. infestans haemolymph but not of R. prolixus preparations. For the two insect species, neither T. rangeli from culture nor parasite lysates were able to trigger proPO activation. The presence of the parasite in R. prolixus haemolymph/laminarin assays, however, significantly reduced the level of proPO activation to that of spontaneous activating controls. The immobilization of T. rangeli in vitro in haemolymph preparations occurred in both insect species and was dependent on the proPO activation intensity. Our results suggest that the susceptibility of R. prolixus to T. rangeli haemocoel infection may be explained, at least in part, by the suppression of the insect immune defence system i.e., inhibition of proPO in the presence of this protozoan parasite.

Parasite-induced enhancement of hemolymph tyrosinase activity in a selected immune reactive strain of Drosophila melanogaster

Larval hemolymph tyrosinase activity in Drosophila melanogaster was detected with high performance liquid chromatography with electrochemical detection. The enzyme hydroxylated L-tyrosine, and oxidized the diphenol substrates L-dopa and dopamine. In larvae of a selected immune-reactive strain the rates of tyrosine hydroxylation, dopa oxidation, and dopamine oxidation were markedly increased during the early stages of melanotic encapsulation of the eggs of the parasitic wasp Leptopilina boulardi. Tyrosinase activity was not modified in parasitized larvae of a selected susceptible strain of D. melanogaster, in which hosts the parasitoids developed unmolested. During the same period of parasitization, the amount of free tyrosine in immune reactive larvae was approximately three times higher than in susceptible hosts. These data indicate that the tyrosinase system of the immune reactive strain is activated during parasitization, and this results in the synthesis of some precursors which ultimately produce a melanotic and sclerotic capsule around the eggs of the parasite. Based on known genetic information of the enzyme system in Drosophila, it appears that at least two genes may be involved in the activation process, one associated with the proenzyme for monophenol oxidase activity, and the second with the proenzyme for diphenol oxidase activity.