Protein tyrosine phosphatases encoded in Cotesia plutellae bracovirus: sequence analysis, expression profile, and a possible biological role in host immunosuppression

Isolation, complete characterization and phylogeography of the first bacteriophage against Vibrio neocaledonicus, which encodes a pyruvate phosphate dikinase and represents a novel viral family

Vibrio are widely distributed in aquatic environments and are major pathogens commonly found in aquaculture environments, playing a significant role in human production activities and maintaining ecological stability. Here, a novel phage, vB_VneS_J26, which infects Vibrio neocaledonicus, was isolated from coastal seawater in Qingdao, China. Transmission electron microscopy revealed that vB_VneS_J26 exhibits siphovirus morphotype, with a linear double-stranded DNA genome of 82,477 bp in length and G+C content of 45.11 mol%, encoding 122 putative ORFs. Three auxiliary metabolic genes related to carbon metabolism and host cell redox processes were identified, including a pyruvate phosphate dikinase, which catalyses the reversible conversion between phosphoenolpyruvate and pyruvate and is rarely detected in viruses. Whole-genome phylogenetic and comparative genomic analyses suggested that vB_VneS_J26 represents a potential novel viral family, comprising six isolated vibriophages, proposed as Modirecodeviridae. Phylogeographic analysis indicated that Modirecodeviridae is primarily distributed in epipelagic and mesopelagic zones of the Arctic and temperate tropical oceans.

Persistent expression of Cotesia plutellae bracovirus genes in parasitized host, Plutella xylostella

Cotesia plutellae (= vestalis) bracovirus (CpBV) is symbiotic to an endoparasitoid wasp, C. plutellae, and plays crucial roles in parasitism against the diamondback moth, Plutella xylostella. CpBV virion genome consists of 35 circular DNAs encoding 157 putative open reading frames (ORFs). This study re-annotated 157 ORFs with update genome database and analyzed their gene expressions at early and late parasitic stages. Re-annotation has established 15 different viral gene families, to which 83 ORFs are assigned with remaining 74 hypothetical genes. Among 157 ORFs, 147 genes were expressed at early or late parasitic stages, among which 141 genes were expressed in both parasitic stages, indicating persistent nature of gene expression. Relative frequencies of different viral circles present in the ovarian lumen did not explain the expression variation of the viral ORFs. Furthermore, expression level of each viral gene was varied during parasitism along with host development. Highly up-regulated CpBV genes at early parasitic stage included BEN (BANP, E₅R and NAC₁), ELP (EP1-like protein), IkB (inhibitor kB), P494 (protein 494 kDa) family genes, while those at late stage were mostly hypothetical genes. Along with the viral gene expression, 362 host genes exhibited more than two fold changes in expression levels at early parasitic stage compared to nonparasitized host. At late stage, more number (1,858) of host genes was regulated. These results suggest that persistent expression of most CpBV genes may be necessary to regulate host physiological processes during C. plutellae parasitism.

Transgenic Expression of a Viral Cystatin Gene CpBV-CST1 in Tobacco Confers Insect Resistance

A viral gene, CpBV-CST1, was identified from a polydnavirus Cotesia plutellae bracovirus (CpBV). Its protein product was significantly toxic to lepidopteran insects. This study generated a transgenic tobacco plant expressing CpBV-CST1 Expression of transgene CpBV-CST1 was confirmed in T1 generation (second generation after transgenesis) in both mRNA and protein levels. Young larvae of Spodoptera exigua (Hübner) suffered high mortalities after feeding on transgenic tobacco. All 10 T1 transgenic tobacco plants had no significant variation in speed-to-kill. In order to further explore insect resistance of these transgenic tobaccos, bioassays were performed by assessing antixenosis and antibiosis. S. exigua larvae significantly avoided T1 plants in a choice test. Larvae fed with T1 plant exhibited significant decrease in protease activity in the midgut due to consuming CpBV-CST1 protein produced by the transgenic plant. Furthermore, the transgenic tobacco exhibited similar insect resistance to other tobacco-infesting insects, including a leaf-feeding insect, Helicoverpa assulta, and a sap-feeding insect, Myzus persicae These results demonstrate that a viral cystatin gene can be used to develop insect-resistant transgenic plant, suggesting a prospective possibility of expanding the current transgenic approach to high-valued crops.

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

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

Characterization of a protein tyrosine phosphatase as a host factor promoting baculovirus replication in silkworm, Bombyx mori

The relevance of protein tyrosine phosphatase (PTP) to host-pathogen interaction is highlighted in mammalian studies, whereas less is known in insects. Here we presented the categorization of the PTP complement of silkworm and characterized their homologous relationship with human and fruit fly PTPs. Among the 36 PTP genes, ptp-h, which was proposed to be the origin of baculovirus ptp belongs to atypical VH1-like dual-specific PTP subset and encodes a catalytic active protein. The maximum expression level of Bmptp-h was at 5th instar and in fat body. Bombyx mori nucleopolyhedrovirus (BmNPV) infection potently induced its expression in silkworm larvae and in BmE cells. Knock-down of Bmptp-h by RNA interference significantly inhibited viral replication, and over-expression enhanced viral replication as determined by viral DNA abundance and BmNPV-GFP positive cells. These results suggest that BmPTP-h might be one of the host factors that is beneficial to baculovirus infection by promoting viral replication.

Selectivity of a translation-inhibitory factor, CpBV15β, in host mRNAs and subsequent alterations in host development and immunity

An endoparasitoid wasp, Cotesia plutellae, parasitizes young larvae of the diamondback moth, Plutella xylostella. Its symbiotic virus, C. plutellae bracovirus (CpBV), has been shown to play a crucial role in inducing physiological changes in the parasitized host. A viral gene, CpBV15β, exhibits a specific translational control against host mRNAs by sequestering a eukaryotic translation initiation factor, eIF4A. Inhibitory target mRNAs have high thermal stability (>≈9 kcal/mol) of their secondary structures in 5'UTR. To determine the specificity of translational control in terms of 5'UTR complexity, this study screened target/nontarget mRNAs of CpBV15β using a proteomics approach through an in vivo transient expression technique. A proteomics analysis of host plasma proteins showed that 12.9% (23/178) spots disappeared along with the expression of CpBV15β. A total of ten spots were chosen, in which five spots ('target') were disappeared by expression of CpBV15β and the other five ('nontarget') were insensitive to expression of CpBV15β, and further analyzed by a tandem mass spectroscopy. The predicted genes of target spots had much greater complexity (-12.3 to -25.2 kcal/mol) of their 5'UTR in terms of thermal stability compared to those (-3.70 to -9.00 kcal/mol) of nontarget spots. 5'UTRs of one target gene (arginine kinase:Px-AK) and one nontarget gene (imaginal disc growth factor:Px-IDGF) were cloned and used for in vitro translation (IVT) assay using rabbit reticulocyte lysate. IVT assay clearly showed that mRNA of Px-IDGF was translated in the presence of CpBV15β, but mRNA of Px-AK was not. Physiological significance of these two genes was compared in immune and development processes of P. xylostella by specific RNA interference (RNAi). Under these RNAi conditions, suppression of Px-AK exhibited much more significant adverse effects on larval immunity and larva-to-pupa metamorphosis compared to the effect of suppression of Px-IDGF. These results support the hypothesis that 5'UTR complexity is a molecular motif to discriminate host mRNAs by CpBV15β for its host translational control and suggest that this discrimination would be required for altering host physiology to accomplish a successful parasitism of the wasp host, C. plutellae.

Evolutionary mechanisms driving the evolution of a large polydnavirus gene family coding for protein tyrosine phosphatases

Background

Gene duplications have been proposed to be the main mechanism involved in genome evolution and in acquisition of new functions. Polydnaviruses (PDVs), symbiotic viruses associated with parasitoid wasps, are ideal model systems to study mechanisms of gene duplications given that PDV genomes consist of virulence genes organized into multigene families. In these systems the viral genome is integrated in a wasp chromosome as a provirus and virus particles containing circular double-stranded DNA are injected into the parasitoids’ hosts and are essential for parasitism success. The viral virulence factors, organized in gene families, are required collectively to induce host immune suppression and developmental arrest. The gene family which encodes protein tyrosine phosphatases (PTPs) has undergone spectacular expansion in several PDV genomes with up to 42 genes.

Results

Here, we present strong indications that PTP gene family expansion occurred via classical mechanisms: by duplication of large segments of the chromosomally integrated form of the virus sequences (segmental duplication), by tandem duplications within this form and by dispersed duplications. We also propose a novel duplication mechanism specific to PDVs that involves viral circle reintegration into the wasp genome. The PTP copies produced were shown to undergo conservative evolution along with episodes of adaptive evolution. In particular recently produced copies have undergone positive selection in sites most likely involved in defining substrate selectivity.

Conclusion

The results provide evidence about the dynamic nature of polydnavirus proviral genomes. Classical and PDV-specific duplication mechanisms have been involved in the production of new gene copies. Selection pressures associated with antagonistic interactions with parasitized hosts have shaped these genes used to manipulate lepidopteran physiology with evidence for positive selection involved in adaptation to host targets.

In vivo transient expression for the functional analysis of polydnaviral genes

Transient expression of a foreign gene in an organism is useful to determine its physiological function. This study introduces an efficient expression technique in the insect system using a recombinant eukaryotic expression vector. A recombinant construct expressing an enhanced green fluorescence protein (EGFP) gene under an immediately early promoter was injected into the larval hemocoel of Spodoptera exigua along with a cell transfection reagent. The expression of EGFP occurred earlier, and persisted for longer period with increasing injection dose. However, there was significant variation in expression efficiency among different cell transfection reagents. In addition, the transfection efficiency measured by RT-PCR varied among tissues with high expression of EGFP in hemocytes and fat body, but not in epidermis, gut, and nerve tissues. Two functional genes (CpBV15α and CpBV15β) derived from a polydnavirus were inserted into the eukaryotic expression vector and injected into S. exigua larvae. Expression levels in hemocytes and fat body were measured by RT-PCR and immunofluorescence assay. Both mRNAs and proteins were detected in the two tissues, in which expression signals depended on the amount of injected DNA. These immunosuppressive factors significantly inhibited hemocyte behavior, such as hemocyte-spreading, nodule formation, and phagocytosis. These results demonstrate the use of in vivo transient expression of polydnaviral genes for direct analysis of biological function in the host insect.

A novel polydnaviral gene family, BEN, and its immunosuppressive function in larvae of Plutella xylostella parasitized by Cotesia plutellae

A full genome sequence of the episomal form of Cotesia plutellae bracovirus (CpBV) suggests 11 BEN family genes. This study analyzed their expression and physiological function in the viral host, Plutella xylostella. All 11 BEN family genes were expressed during entire parasitization period of P. xylostella larvae. In addition, these BEN family genes were expressed in fat body, gut, epidermis, and hemocytes in final larval instar of parasitized P. xylostella. The 11 BEN family genes were transiently expressed in nonparasitized larvae by injection of each viral segment containing its corresponding BEN family gene. The transient expression of BEN family genes significantly suppressed hemocyte nodule formation in response to bacterial challenge. Subsequent injection of double-stranded RNA specific to each BEN family gene suppressed the expression of the BEN family gene and rescued the immunosuppression. These results indicate that 11 BEN family genes are expressed in larvae parasitized by C. plutellae and play crucial role in inducing immunosuppression. Homologous BEN family genes were found in other bracoviral genomes. We propose BEN domain-containing genes as a new functional gene family in polydnaviruses.

Host translational control of a polydnavirus, Cotesia plutellae bracovirus, by sequestering host eIF4A to prevent formation of a translation initiation complex

Host translational control is a viral strategy to exploit host cellular resources. Parasitization by some endoparasitoids containing polydnaviruses inhibits the synthesis of specific host proteins at post-transcriptional level. Two host translation inhibitory factors (HTIFs) have been proposed in Cotesia plutellae bracovirus (CpBV). Parasitization by C. plutellae inhibited storage protein 1 (SP1) synthesis of Plutella xylostella at post-transcriptional level. One HTIF, CpBV15β, inhibited the translation of SP1 mRNA in an in vitro translation assay using rabbit reticulocyte lysate, but did not inhibit its own mRNA. To further analyse the discrimination of target and nontarget mRNAs of the inhibitory effect of HTIF, 5' untranslated regions (UTRs) of SP1 and CpBV15β mRNA were reciprocally exchanged. In the presence of HTIFs, the chimeric CpBV15β mRNA that contained SP1 5' UTR was not translated, whereas the chimeric SP1 mRNA that contained CpBV15β 5' UTR was translated. There was a difference in the 5' UTR secondary structures between target (SP1) and nontarget (CpBV15α and CpBV15β) mRNAs in terms of thermal stability. Different mutant 5' UTRs of SP1 mRNA were prepared by point mutations to modify their secondary structures. The constructs containing 5' UTRs of high thermal stability in their secondary structures were inhibited by HTIF, but those of low thermal stability were not. Immunoprecipitation with CpBV15β antibody coprecipitated eIF4A, which would be required for unwinding the secondary structure of the 5' UTR. These results indicate that the viral HTIF discriminates between host mRNAs according to their dependency on eIF4A to form a functional initiation complex for translation.

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.

Exogenous JH and ecdysteroid applications alter initiation of polydnaviral replication in an endoparasitoid wasp, Cotesia plutellae (Braconidae: Hymenoptera)

Polydnaviruses are a group of double-stranded DNA viruses and are symbiotically associated with some ichneumonoid wasps. As proviruses, the replication of polydnaviruses occurs in the female reproductive organ at the pupal stage. This study analyzed the effects of two developmental hormones, juvenile hormone (JH) and ecdysteroid, on the viral replication of Cotesia plutellae bracovirus (CpBV). All 23 CpBV segments identified contained a conserved excision/rejoining site ('AGCTTT') from their proviral segments. Using quantitative real-time PCR based on this excision/rejoining site marker, initiation of CpBV replication was determined to have occurred on day 4 on the pupal stage. Pyriproxyfen, a JH agonist, significantly inhibited adult emergence of C. plutellae, whereas RH5992, an ecdysteroid agonist, had no inhibitory effect. Although RH5992 had no effect dose on adult development, it significantly accelerated viral replication. The results of immunoblotting assays against viral coat proteins support the effects of the hormone agonists on viral replication.

Transient expression of specific Cotesia plutellae bracoviral segments induces prolonged larval development of the diamondback moth, Plutella xylostella

A polydnavirus, Cotesia plutellae bracovirus (CpBV), possesses a segmented and dispersed genome that is located on chromosome(s) of its symbiotic endoparasitic wasp, C. plutellae. When the host wasp parasitizes larvae of the diamondback moth, Plutella xylostella, at least 27 viral genome segments are delivered to the parasitized host along with the wasp egg. The parasitized P. xylostella exhibits significant immunosuppression and a prolonged larval development. Parasitized larvae take about 2 days longer than nonparasitized larvae to develop until the wandering stage of the final larval instar, and die after egress of the full grown wasp larvae. Developmental analysis using juvenile hormone and ecdysteroid analogs suggests that altering endocrine signals could induce the retardation of larval developmental rate in P. xylostella. In this study we used a transient expression technique to micro-inject individual CpBV genome segments, and tested their ability to induce delayed larval development of P. xylostella. We demonstrated that a CpBV segment was able to express its own encoded genes when it was injected into nonparasitized larvae, in which the expression patterns of the segment genes were similar to those in the larvae parasitized by C. plutellae. Twenty three CpBV genome segments were individually cloned and injected into the second instar larvae of P. xylostella and their effects assessed by measuring the time taken for host development to the cocooning stage. Three CpBV genome segments markedly interfered with the host larval development. When the putative genes of these segments were analyzed, it was found that they did not share any common genes. Among these segments able to delay host development, segment S27 was predicted to encode seven protein tyrosine phosphatases (CpBV-PTPs), some of which were mutated by insertional inactivation with transposons, while other encoded gene expressions were unaffected. The mutant segments were unable to induce prolonged larval development of P. xylostella. These results suggest that CpBV can induce prolonged larval development of P. xylostella, and that at least some CpBV-PTPs may contribute to the parasitic role probably by altering titers of developmental hormones.

A SERI technique reveals an immunosuppressive activity of a serine-rich protein encoded in Cotesia plutellae bracovirus

Polydnavirus genome is segmented and dispersed on host wasp chromosome. After replication, the segments form double- stranded circular DNAs and embedded in viral coat proteins. These viral particles are delivered into a parasitized host along with parasitoid eggs. A serine-rich protein (SRP) is predicted in a polydnavirus, Cotesia plutellae bracovirus (CpBV), genome in its segment no. 33 (CpBV-S33), creating CpBVSRP1. This study explored its expression and physiological function in the diamondback moth, Plutella xylostella, larvae parasitized by C. plutellae. CpBV-SRP1 encodes 122 amino acids with 26 serines and several predicted phosphorylation sites. It is persistently expressed in all tested tissues of parasitized P. xylostella including hemocyte, fat body, and gut. Its physiological function was analyzed by injecting CpBV-S33 and inducing its expression in nonparasitized P. xylostella by a technique called SERI (segment expression and RNA interference). The expression of CpBV-SRP1 significantly impaired the spreading behavior and total cell count of hemocytes of treated larvae. Subsequent RNA interference of CpBV-SRP1 rescued the immunosuppressive response. This study reports the persistent expression of CpBV-SRP1 in a parasitized host and its parasitic role in suppressing the host immune response by altering hemocyte behavior and survival.

Translation inhibitory factors encoded in Cotesia plutellae bracovirus require the 5'-UTR of a host mRNA target

Physiological processes of the diamondback moth, Plutella xylostella, larvae parasitized by Cotesia plutellae are altered by several parasitic factors including a polydnavirus, C. plutellae bracovirus (CpBV). Two homologous genes, CpBV15alpha and CpBV15beta, have been proposed as host translation inhibitory factors (HTIFs). This study analyzed their effects on host gene expression at a post-transcriptional level. A proteomic approach using two dimensional electrophoresis revealed that the parasitization resulted in 24.0% (60/250 spots) reduction of gene expression compared to nonparasitized control. It also indicated that the transient expression of CpBV15alpha or CpBV15beta in nonparasitized larvae resulted in 26.0% (65/240 spots) or 28.0% (70/240 spots) reduction, respectively. Seven spots that were not detected in the transiently expressed samples were further analyzed by a tandem mass spectrometry. These proteins were predicted to be associated with host cell signaling and metabolism. To investigate translation inhibitory effects of CpBV15alpha and CpBV15beta, capped mRNA of a storage protein 1 (SP1) of P. xylostella, a common inhibitory target of both HTIFs, was prepared by in vitro transcription and translated in vitro in the presence or absence of recombinant HTIFs prepared from Sf9 cells by recombinant baculoviruses. Translation of SP1 mRNA containing 5'-untranslated region (5'-UTR) was inhibited by both HTIFs. However, translation of SP1 mRNA without 5'-UTR was insensitive to the exposure of both HTIFs. Both HTIFs inhibited the host gene translation in a dose-dependent manner. In addition, these two factors showed cooperative inhibition. This study suggests that CpBV15alpha and CpBV15beta inhibit host mRNAs directly by acting on translation machinery, in which 5'-UTR of target mRNAs would be required for the inhibitory action.

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.

Tranosema rostrale ichnovirus repeat element genes display distinct transcriptional patterns in caterpillar and wasp hosts

The endoparasitic wasp Tranosema rostrale transmits an ichnovirus to its lepidopteran host, Choristoneura fumiferana, during parasitization. As shown for other ichnoviruses, the segmented dsDNA genome of the T. rostrale ichnovirus (TrIV) features several multi-gene families, including the repeat element (rep) family, whose products display no known similarity to non-ichnovirus proteins, except for a homologue encoded by the genome of the Helicoverpa armigera granulovirus; their functions remain unknown. This study applied linear regression of efficiency analysis to real-time PCR quantification of transcript abundance for all 17 TrIV rep open reading frames (ORFs) in parasitized and virus-injected C. fumiferana larvae, as well as in T. rostrale ovaries and head-thorax complexes. Although transcripts were detected for most rep ORFs in infected caterpillars, two of them clearly outnumbered the others in whole larvae, with a tendency for levels to drop over time after infection. The genome segments bearing the three most highly expressed rep genes in parasitized caterpillars were present in higher proportions than other rep-bearing genome segments in TrIV DNA, suggesting a possible role for gene dosage in the regulation of transcription level. TrIV rep genes also showed important differences in the relative abundance of their transcripts in specific tissues (cuticular epithelium, the fat body, haemocytes and the midgut), implying tissue-specific roles for individual members of this gene family. Significantly, no rep transcripts were detected in T. rostrale head-thorax complexes, whereas some were abundant in ovaries. There, the transcription pattern was completely different from that observed in infected caterpillars, suggesting that some rep genes have wasp-specific functions.

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.

Immunoevasive property of a polydnaviral product, CpBV-lectin, protects the parasitoid egg from hemocytic encapsulation of Plutella xylostella (Lepidoptera: Yponomeutidae)

Immunosuppression is the main pathological symptom of the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae), parasitized by an endoparasitoid wasp, Cotesia plutellae (vestalis, Hymenoptera: Braconidae). C. plutellae bracovirus (CpBV), which is a symbiotic virus of C. plutellae, has been known to be the main parasitic factor in the host-parasitoid interaction. CpBV-lectin, encoded in the viral genome and expressed in P. xylostella during early parasitization stage, was suspected to play a role in immunoevasion of defense response. Here we expressed CpBV-lectin in Sf9 cells using a recombinant baculovirus for subsequent functional assays. The recombinant CpBV-lectin exhibited hemagglutination against vertebrate erythrocytes. Its hemagglutinating activity increased with calcium, but inhibited by adding EDTA, indicating its C-type lectin property. CpBV-lectin showed specific carbohydrate-binding affinity against N-acetyl glucosamine and N-acetyl neuraminic acid. The role of this CpBV-lectin in immunosuppression was analyzed by exposing hemocytes of nonparasitized P. xylostella to rat erythrocytes or FITC-labeled bacteria pretreated with recombinant CpBV-lectin, which resulted in significant reduction in adhesion or phagocytosis, respectively. The immunosuppressive activity of CpBV-lectin was further analyzed under in vitro encapsulation response of hemocytes against parasitoid eggs collected at 1- or 24-h post-parasitization. Hemocytic encapsulation was observed against 1-h eggs but not against 24-h eggs. When the 1-h eggs were pretreated with the recombinant CpBV-lectin, encapsulation response was completely inhibited, where CpBV-lectin bound to the parasitoid eggs, but not to hemocytes. These results suggest that CpBV-lectin interferes with hemocyte recognition by masking hemocyte-binding sites on the parasitoid eggs.

A viral histone H4 encoded by Cotesia plutellae bracovirus inhibits haemocyte-spreading behaviour of the diamondback moth, Plutella xylostella

Histone H4 is highly conserved and forms a central-core nucleosome with H3 in eukaryotic chromatin. Its covalent modification at the protruding N-terminal region from the nucleosomal core can change the chromatin conformation in order to regulate gene expression. A viral H4 was found in the genome of Cotesia plutellae bracovirus (CpBV). The obligate host of the virus is an endoparasitoid wasp, C. plutellae, which parasitizes the diamondback moth, Plutella xylostella, and interrupts host development and immune reactions. CpBV has been regarded as a major source for interrupting the physiological processes during parasitization. CpBV H4 shows high sequence identity with the amino acid sequence of P. xylostella H4 except for an extended N-terminal region (38 aa). This extended N-terminal CpBV H4 contains nine lysine residues. CpBV H4 was expressed in P. xylostella parasitized by C. plutellae. Western blot analysis using a wide-spectrum H4 antibody showed two H4s in parasitized P. xylostella. In parasitized haemocytes, CpBV H4 was detected predominantly in the nucleus and was highly acetylated. The effect of CpBV H4 on haemocytes was analysed by transient expression using a eukaryotic expression vector, which was injected into non-parasitized P. xylostella. Expression of CpBV H4 was confirmed in the transfected P. xylostella by RT-PCR and immunofluorescence assays. Haemocytes of the transfected larvae lost their spreading ability on an extracellular matrix. Inhibition of the cellular immune response by transient expression was reversed by RNA interference using dsRNA of CpBV H4. These results suggest that CpBV H4 plays a critical role in suppressing host immune responses during parasitization.

Two homologous parasitism-specific proteins encoded in Cotesia plutellae bracovirus and their expression profiles in parasitized Plutella xylostella

A wasp, Cotesia plutellae, parasitizes the diamondback moth, Plutella xylostella, and interrupts host physiology for wasp survival and development. Identification of parasitism-specific factors would be helpful to understand the host-parasitoid interaction. This study focused on identification of a 15-kDa protein found only in plasma of the parasitized P. xylostella. Degenerate primers were designed after N-terminal amino acid sequencing of the parasitism-specific protein and used to clone the corresponding gene from the parasitized P. xylostella by a nested reverse transcriptase-polymerase chain reaction (RT-PCR). Two homologous genes were cloned and identified as "CpBV15alpha" and "CpBV15beta," respectively, due to the identical size (158 amino acid residues) of the predicted open reading frames, in which they shared amino acid sequences in both terminal regions, but varied in internal sequences. Southern hybridization analysis indicated that both genes were located on C. plutellae bracovirus genome. Real-time quantitative RT-PCR revealed that both genes were mostly expressed at the late parasitization period, which was further confirmed by an immunoblotting assay using CpBV15 antibody. A recombinant CpBV15 protein was produced from Sf9 cells via a baculovirus expression system. The purified CpBV15 protein could enter hemocytes of P. xylostella and were localized in the cytosol. Along with the sequence similarities of CpBV15s with eukaryotic initiation factors, their putative biological role has been discussed in terms of the host translation inhibitory factor.

Comparison of three methods of parasitoid polydnavirus genomic DNA isolation to facilitate polydnavirus genomic sequencing

A major long-term goal of polydnavirus (PDV) genome research is to identify novel virally encoded molecules that may serve as biopesticides to target insect pests that threaten agriculture and human health. As PDV viral replication in cell culture in vitro has not yet been achieved, several thousands of wasps must be dissected to yield enough viral DNA from the adult ovaries to carry out PDV genomic sequencing. This study compares three methods of PDV genomic DNA isolation for the PDV of Cotesia flavipes, which parasitizes the sugarcane borer, Diatraea saccharalis, preparatory to sequencing the C. flavipes bracovirus genome. Two of these protocols incorporate phenol-chloroform DNA extraction steps in the procedure and the third protocol uses a modified Qiagen DNA kit method to extract viral DNA. The latter method proved significantly less time-consuming and more cost-effective. Efforts are currently underway to bioengineer insect pathogenic viruses with PDV genes, so that their gene products will enhance baculovirus virulence for agricultural insect pests, either via suppression of the immune system of the host or by PDV-mediated induction of its developmental arrest. Sequencing a growing number of complete PDV genomes will enhance those efforts, which will be facilitated by the study reported here.

Transient expression of an EP1-like gene encoded in Cotesia plutellae bracovirus suppresses the hemocyte population in the diamondback moth, Plutella xylostella

A genome project has been launched and aims to sequence total genome of Cotesia plutellae bracovirus (CpBV). This on-going research has identified seven EP1-like (ELP) genes in the CpBV genome. A group of ELP genes has been speculated as an immunosuppressant encoded in Cotesia-associated bracoviruses. This study analyzed gene expression of these seven CpBV-ELPs in the parasitized diamondback moth, Plutella xylostella. Of these, six CpBV-ELPs were expressed in P. xylostella parasitized by C. plutellae. However, their expression levels varied in different tissues and parasitization stages. Especially, CpBV-ELP1 showed a persistent and ubiquitous expression pattern in both reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescence assays. When nonparasitized P. xylostella was transfected with a recombinant CpBV-ELP1 in a eukaryotic expression vector, CpBV-ELP1 was expressed for at least 3 days and the proteins were detectable in the cytoplasm of hemocytes. The transfected larvae showed significant reduction in total hemocyte numbers, compared with larvae injected with the cloning vector alone. Co-transfection with double-stranded RNA could knock down the expression of CpBV-ELP1 and prevented the reduction of the hemocyte population. This study demonstrates that CpBV-ELP1 plays a physiological role in suppressing host immune response presumably by its hemolytic activity during C. plutellae parasitization.

A viral lectin encoded in Cotesia plutellae bracovirus and its immunosuppressive effect on host hemocytes

An endoparasitoid wasp, Cotesia plutellae, induces immunosuppression of the host diamondback moth, Plutella xylostella. To identify an immunosuppressive factor, the parasitized hemolymph of P. xylostella was separated into plasma and hemocyte fractions. When nonparasitized hemocytes were overlaid with parasitized plasma, they showed significant reduction in bacterial binding efficacy. Here, we considered a viral lectin previously known in other Cotesia species as a humoral immunosuppressive candidate in C. plutellae parasitization. Based on consensus regions of the viral lectins, the corresponding lectin gene was cloned from P. xylostella parasitized by C. plutellae. Its cDNA is 674 bp long and encodes 157 amino acid residues containing a signal peptide (15 residues) and one carbohydrate recognition domain. Open reading frame is divided by one intron (156 bp) in its genomic DNA. Amino acid sequence shares 80% homology with that of C. ruficrus bracovirus lectin and is classified into C-type lectin. Southern hybridization analysis indicated that the cloned lectin gene was located at C. plutellae bracovirus (CpBV) genome. Both real-time quantitative RT-PCR and immunoblotting assays indicated that CpBV-lectin showed early expression during the parasitization. A recombinant CpBV-lectin was expressed in a bacterial system and the purified protein significantly inhibited the association between bacteria and hemocytes of nonparasitized P. xylostella. In the parasitized P. xylostella, CpBV-lectin was detected on the surface of parasitoid eggs after 24 h parasitization by its specific immunostaining. The 24 h old eggs were not encapsulated in vitro by hemocytes of P. xylostella, compared to newly laid parasitoid eggs showing no CpBV-lectin detectable and easily encapsulated. These results support an existence of a polydnaviral lectin family among Cotesia-associated bracovirus and propose its immunosuppressive function.

A putative protein translation inhibitory factor encoded by Cotesia plutellae bracovirus suppresses host hemocyte-spreading behavior

An endoparasitoid, Cotesia plutellae (Hymenoptera: Braconidae), possesses a mutualistic bracovirus (CpBV), which plays significant roles in the parasitized host, Plutella xylostella (Lepidoptera: Plutellidae). CpBV15beta, a viral gene encoded by CpBV, is expressed at early and late parasitization periods, suggesting that it functions to manipulate the physiology of the parasitized host. This paper reports a physiological function of CpBV15beta as an immunosuppressive agent. The effect of CpBV15beta on cellular immunity was analyzed by assessing hemocyte-spreading behavior. Parasitization by C. plutellae caused altered behavior of hemocytes of P. xylostella, in which the hemocytes were not able to attach and spread on glass slides. CpBV15beta was expressed in Sf9 cells using a baculovirus expression system and purified from the culture media. When hemocytes of nonparasitized P. xylostella were incubated with purified CpBV15beta protein, spreading behavior was impaired in a dose-dependent manner at low micro-molar range. This inhibitory effect of CpBV15beta could also be demonstrated on hemocytes of a non-natural host, Spodoptera exigua. CpBV15beta protein significantly inhibited F-actin growth of hemocytes in response to an insect cytokine. Similarly, cycloheximide, a eukaryotic translation inhibitor, strongly inhibited the spreading behavior and F-actin growth of P. xylostella hemocytes. Under in vitro condition, hemocytes of nonparasitized P. xylostella released proteins into the surrounding medium. Upon incubation of hemocytes with either CpBV15beta or cycloheximide, their ability to release protein molecules was markedly inhibited. This study suggests that CpBV15beta suppresses hemocyte behavior by inhibiting protein translation.

Transient expression of protein tyrosine phosphatases encoded in Cotesia plutellae bracovirus inhibits insect cellular immune responses

Several immunosuppressive factors are associated with parasitism of an endoparasitoid wasp, Cotesia plutellae, on the diamondback moth, Plutella xylostella. C. plutellae bracovirus (CpBV) encodes a large number of putative protein tyrosine phosphatases (PTPs), which may play a role in inhibiting host cellular immunity. To address this inhibitory hypothesis of CpBV-PTPs, we performed transient expression of individual CpBV-PTPs in hemocytes of the beet armyworm, Spodoptera exigua, and analyzed their cellular immune responses. Two different forms of CpBV-PTPs were chosen and cloned into a eukaryotic expression vector under the control of the p10 promoter of baculovirus: one with the normal cysteine active site (CpBV-PTP1) and the other with a mutated active site (CpBV-PTP5). The hemocytes transfected with CpBV-PTP1 significantly increased in PTP activity compared to control hemocytes, but those with CpBV-PTP5 exhibited a significant decrease in the PTP activity. All transfected hemocytes exhibited a significant reduction in both cell spreading and encapsulation activities compared to control hemocytes. Co-transfection of CpBV-PTP1 together with its double-stranded RNA reduced the messenger RNA (mRNA) level of CpBV-PTP1 and resulted in recovery of both hemocyte behaviors. This is the first report demonstrating that the polydnaviral PTPs can manipulate PTP activity of the hemocytes to interrupt cellular immune responses.