Differential inhibition of BmRelish1-dependent transcription in lepidopteran cells by bracovirus ankyrin-repeat proteins

Single domain von Willebrand factor type C "cytokines" and the regulation of the stress/immune response in insects

The single domain von Willebrand factor type C (SVWC) appears in small secreted peptides that are arthropod-specific and are produced following environmental stress or pathogen exposure. Most research has focused on proteins with SVWC domain that are induced after virus infection and are hypothesized to function as "cytokines" to regulate the innate immune response. The expansion of SVWC genes in insect species indicates that many other functions remain to be discovered. Research in shrimp has elucidated the adaptability of Vago-like peptides in the innate immune response against bacteria, fungi and viruses after activation by Jak-STAT and/or Toll/Imd pathways in which they can act as pathogen-recognition receptors or cytokine-like signaling molecules. SVWC factors also appear in scorpion venoms and tick saliva, underlining their versatility to acquire new functions. This review discusses the discovery and function of SVWC peptides from insects to crustaceans and chelicerates and reveals the enormous gaps in knowledge that remain to be filled to understand this enigmatic group of secreted peptides.

The multifunctional polydnavirus TnBVANK1 protein: impact on host apoptotic pathway

Toxoneuron nigriceps (Hymenoptera, Braconidae) is an endophagous parasitoid of the larval stages of the tobacco budworm, Heliothis virescens (Lepidoptera, Noctuidae). The bracovirus associated with this wasp (TnBV) is currently being studied. Several genes expressed in parasitised host larvae have been isolated and their possible roles partly elucidated. TnBVank1 encodes an ankyrin motif protein similar to insect and mammalian IκB, an inhibitor of the transcription nuclear factor κB (NF-κB). Here we show that, when TnBVank1 was stably expressed in polyclonal Drosophila S2 cells, apoptosis is induced. Furthermore, we observed the same effects in haemocytes of H. virescens larvae, after TnBVank1 in vivo transient transfection, and in haemocytes of parasitised larvae. Coimmunoprecipitation experiments showed that TnBVANK1 binds to ALG-2 interacting protein X (Alix/AIP1), an interactor of apoptosis-linked gene protein 2 (ALG-2). Using double-immunofluorescence labeling, we observed the potential colocalization of TnBVANK1 and Alix proteins in the cytoplasm of polyclonal S2 cells. When Alix was silenced by RNA interference, TnBVANK1 was no longer able to cause apoptosis in both S2 cells and H. virescens haemocytes. Collectively, these results indicate that TnBVANK1 induces apoptosis by interacting with Alix, suggesting a role of TnBVANK1 in the suppression of host immune response observed after parasitisation by T. nigriceps.

An endoparasitoid wasp influences host DNA methylation

Parasitism by endoparasitoid wasps changes the expression of various host genes, and alters host immune and developmental processes. However, it is not clearly understood how parasitism changes host gene expression in a whole genome scale. This study focused on an epigenetic control of Cotesia plutellae, an endoparasitoid wasp, against its host, Plutella xylostella. Two DNA methyltransferases (DNMT-1 and DNMT-2) are encoded in the genome of P. xylostella. In addition, methyl-binding domain proteins (MBDs) and DNA demethylation factor, ten-eleven translation protein (TET) are encoded. DNA methylation of P. xylostella genomic DNA was confirmed by restriction digestion with Gla I specific to 5-methylcytosine. DNA methylation intensity in parasitized (P) larvae was decreased compared to that in nonparasitized (NP) larvae, especially at late parasitic stage, at which expression levels of both DNMT-1 and DNMT-2 were also decreased. DNA demethylation of P. xylostella was confirmed in both NP and P larvae by restriction digestion with PvuRts1I recognizing 5-hydroxymethyl cytosine. Parasitism also suppressed expression levels of TET and MBDs. Treatment of 5-aza-2′-deoxycytidine (AZA) reduced DNA methylation intensity of NP larvae, causing suppression of hemocyte-spreading behavior and delay of immature development. RNA interference of DNMT-1 or DNMT-2 mimicked the adverse effects of AZA.

Polydnaviruses: Nature's Genetic Engineers

Virus-host associations are usually viewed as parasitic, but several studies in recent years have reported examples of viruses that benefit host organisms. The Polydnaviridae are of particular interest because these viruses are all obligate mutualists of insects called parasitoid wasps. Parasitoids develop during their immature stages by feeding inside the body of other insects, which serve as their hosts. Polydnaviruses are vertically transmitted as proviruses through the germ line of wasps but also function as gene delivery vectors that wasps rely upon to genetically manipulate the hosts they parasitize. Here we review the evolutionary origin of polydnaviruses, the organization and function of their genomes, and some of their roles in parasitism.

Venom of Parasitoid Pteromalus puparum Impairs Host Humoral Antimicrobial Activity by Decreasing Host Cecropin and Lysozyme Gene Expression

Insect host/parasitoid interactions are co-evolved systems in which host defenses are balanced by parasitoid mechanisms to disable or hide from host immune effectors. Here, we report that Pteromalus puparum venom impairs the antimicrobial activity of its host Pieris rapae. Inhibition zone results showed that bead injection induced the antimicrobial activity of the host hemolymph but that venom inhibited it. The cDNAs encoding cecropin and lysozyme were screened. Relative quantitative PCR results indicated that all of the microorganisms and bead injections up-regulated the transcript levels of the two genes but that venom down-regulated them. At 8 h post bead challenge, there was a peak in the transcript level of the cecropin gene, whereas the peak of lysozyme gene occurred at 24 h. The transcripts levels of the two genes were higher in the granulocytes and fat body than in other tissues. RNA interference decreased the transcript levels of the two genes and the antimicrobial activity of the pupal hemolymph. Venom injections similarly silenced the expression of the two genes during the first 8 h post-treatment in time- and dose-dependent manners, after which the silence effects abated. Additionally, recombinant cecropin and lysozyme had no significant effect on the emergence rate of pupae that were parasitized by P. puparum females. These findings suggest one mechanism of impairing host antimicrobial activity by parasitoid venom.

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.

Polydnaviruses: From discovery to current insights

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

Modulation of the transcriptional response of innate immune and RNAi genes upon exposure to dsRNA and LPS in silkmoth-derived Bm5 cells overexpressing BmToll9-1 receptor

Injection or feeding of dsRNA is commonly used to induce specific gene silencing by RNAi in insects but very little research has been carried out to investigate non-specific effects on gene expression of dsRNA as pathogen-associated molecular pattern (PAMP). This study focuses on the potential role of the BmToll9-1 receptor to modulate the transcriptional response of innate immune and RNAi genes to dsRNA and lipopolysaccharide (LPS), which was used for comparison. To study this role, we took advantage of the silkmoth-derived Bm5 cell line, which does not express BmToll9-1 endogenously, and engineered a transformed cell line that permanently expresses BmToll9-1. Quantitative mRNA expression studies showed that BmToll9-1 can significantly alter the transcriptional response to dsRNA and LPS: (1) BmToll9-1 promotes the transcriptional response of Dicer2, encoding a key component of the RNAi machinery, and, to a lesser extent, that of transcription factors in the Jak-STAT and Toll pathways; and (2) BmToll9-1 represses the transcriptional induction of the IMD and Jak-STAT pathway genes, as well as the antimicrobial peptide (AMP) effector genes, by LPS. Thus, BmToll9-1 was identified as a modulator of innate immune and RNAi machinery gene expression that could be related to its preferential expression in the larval gut, the major barrier of pathogen entry. While BmToll9-1 was found to modulate RNAi-related gene expression, a reporter-based RNAi assay established no evidence for a direct interaction of BmToll9-1 with the intracellular RNAi machinery.

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

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

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