Inhibition of white spot syndrome virus in Litopenaeus vannamei shrimp by sequence-specific siRNA

Experimental Infection Models and Their Usefulness for White Spot Syndrome Virus (WSSV) Research in Shrimp

White spot syndrome virus (WSSV) is marked as one of the most economically devastating pathogens in shrimp aquaculture worldwide. Infection of cultured shrimp can lead to mass mortality (up to 100%). Although progress has been made, our understanding of WSSV's infection process and the virus-host-environment interaction is far from complete. This in turn hinders the development of effective mitigation strategies against WSSV. Infection models occupy a crucial first step in the research flow that tries to elucidate the infectious disease process to develop new antiviral treatments. Moreover, since the establishment of continuous shrimp cell lines is a work in progress, the development and use of standardized in vivo infection models that reflect the host-pathogen interaction in shrimp is a necessity. This review critically examines key aspects of in vivo WSSV infection model development that are often overlooked, such as standardization, (post)larval quality, inoculum type and choice of inoculation procedure, housing conditions, and shrimp welfare considerations. Furthermore, the usefulness of experimental infection models for different lines of WSSV research will be discussed with the aim to aid researchers when choosing a suitable model for their research needs.

Natural component geniposide enhances survival rate of crayfish Procambarus clarkii infected with white spot syndrome virus

White Spot Disease (WSD), caused by white spot syndrome virus (WSSV), is an acute and highly lethal viral disease of shrimp. Currently, there are no commercially available drugs to control WSD. It is urgent and necessary to find anti-WSSV drugs. Natural compounds are an important source of antiviral drug discovery. In this study, the anti-WSSV activity of natural compound geniposide (GP) was investigated in crayfish Procambarus clarkii. Results showed that GP had a concentration-dependent inhibitory effect on WSSV replication in crayfish at 24 h, and highest inhibition was more than 98%. In addition, GP significantly inhibited the expression of WSSV immediate-early gene ie1, early gene DNApol, late gene VP28. The mortality of WSSV-infected crayfish in control groups was 100%, while it reduced by 70.0% when treated with 50 mg/kg GP. Co-incubation, pre-treatment and post-treatment experiments showed that GP could prevent and treat WSSV infection in crayfish by significantly inhibiting WSSV multiplication. Mechanistically, the syntheses of WSSV structural proteins VP19, VP24, VP26 and VP28 were significantly inhibited by GP in S2 cells. Furthermore, GP could also suppress WSSV replication by blocking the expression of antiviral immunity-related factor STAT to reduce ie1 transcription. Moreover, GP possessed anti-inflammatory and anti-oxidative activity in crayfish. Overall, GP has the potential to be developed as a preventive or therapeutic agent against WSSV infection.

siRNA-Mediated MrIAG Silencing Induces Sex Reversal in Macrobrachium rosenbergii

The insulin-like androgenic gland (IAG) gene is well known in male crustacean, and it is a key regulator in male sexual differentiation and maintaining the male sexual characteristic. The neo-female can be produced by silencing the MrIAG (Macrobrachium rosenbergii Insulin-like Androgenic Gland) in male Macrobrachium rosenbergii. This is the first time to use siRNA approach to silenced MrIAG in male M. rosenbergii. In the current study, the optimal injection dosage to achieve sex reversal is 0.5 μg/g body weight. After MrIAG silencing, the expression level of Dmrt11e, Dmrt99b, MRPINK, Mrr, Sxl1, and Sxl2 decreased significantly. As their long-term silencing effect of MrIAG, the dsRNA and siRNA approaches produce three and two individual neo-females, respectively. The neo-female has a wider brood chamber, ovipositing setae, and ovigerous setae, which is resembled normal female. After a long-term silencing with siRNA, most of the germ cells were arrested in spermatocytes stage, but the spermatocytes in control can further developed into spermatozoon. The seminiferous tubules are loosely arranged and the spermatocytes are more than spermatozoon in the 0.5 μg/g body weight treatment dose. This current study suggests a new path to obtain neo-females through siRNA silencing.

Effects of environmental stress on shrimp innate immunity and white spot syndrome virus infection

The shrimp aquaculture industry is plagued by disease. Due to the lack of deep understanding of the relationship between innate immune mechanism and environmental adaptation mechanism, it is difficult to prevent and control the diseases of shrimp. The shrimp innate immune system has received much recent attention, and the functions of the humoral immune response and the cellular immune response have been preliminarily characterized. The role of environmental stress in shrimp disease has also been investigated recently, attempting to clarify the interactions among the innate immune response, the environmental stress response, and disease. Both the innate immune response and the environmental stress response have a complex relationship with shrimp diseases. Although these systems are important safeguards, allowing shrimp to adapt to adverse environments and resist infection, some pathogens, such as white spot syndrome virus, hijack these host systems. As shrimp lack an adaptive immune system, immunization therapy cannot be used to prevent and control shrimp disease. However, shrimp diseases can be controlled using ecological techniques. These techniques, which are based on the innate immune response and the environmental stress response, significantly reduce the impact of shrimp diseases. The object of this review is to summarize the recent research on shrimp environmental adaptation mechanisms, innate immune response mechanisms, and the relationship between these systems. We also suggest some directions for future research.

Comparative genomic analysis of innate immunity reveals novel and conserved components in crustacean food crop species

BACKGROUND

Growing global demands for crustacean food crop species have driven large investments in aquaculture research worldwide. However, large-scale production is susceptible to pathogen-mediated destruction particularly in developing economies. Thus, a thorough understanding of the immune system components of food crop species is imperative for research to combat pathogens.

RESULTS

Through a comparative genomics approach utilising extant data from 55 species, we describe the innate immune system of the class Malacostraca, which includes all food crop species. We identify 7407 malacostracan genes from 39 gene families implicated in different aspects of host defence and demonstrate dynamic evolution of innate immunity components within this group. Malacostracans have achieved flexibility in recognising infectious agents through divergent evolution and expansion of pathogen recognition receptors genes. Antiviral RNAi, Toll and JAK-STAT signal transduction pathways have remained conserved within Malacostraca, although the Imd pathway appears to lack several key components. Immune effectors such as the antimicrobial peptides (AMPs) have unique evolutionary profiles, with many malacostracan AMPs not found in other arthropods. Lastly, we describe four putative novel immune gene families, potentially representing important evolutionary novelties of the malacostracan immune system.

CONCLUSION

Our analyses across the broader Malacostraca have allowed us to not only draw analogies with other arthropods but also to identify evolutionary novelties in immune modulation components and form strong hypotheses as to when key pathways have evolved or diverged. This will serve as a key resource for future immunology research in crustacean food crops.

Effect of silver nanoparticles on the metabolic rate, hematological response, and survival of juvenile white shrimp Litopenaeus vannamei

White spot syndrome virus (WSSV) is highly lethal and contagious in shrimps; its outbreaks causes an economic crisis for aquaculture. Several attempts have been made to treat this disease; however, to date, there is no effective cure. Because of their antimicrobial activities, silver nanoparticles (AgNPs) are the most studied nanomaterial. Although the antiviral properties of AgNPs have been studied, their antiviral effect against viral infection in aquaculture has not been reported. The AgNPs tested herein are coated with polyvinylpyrrolidone (PVP) and possess multiple international certifications for their use in veterinary and human applications. The aim of this work was to evaluate the survival rate of juvenile white shrimps (Litopenaeus vannamei) after the intramuscular administration of AgNPs. For this, different concentrations of metallic AgNPs and PVP alone were injected into the organisms. After 96 h of administration, shrimp survival was more than 90% for all treatments. The oxygen consumption routine rate and total hemocyte count remained unaltered after AgNP injection, reflecting no stress caused. We evaluated whether AgNPs had an antiviral effect in shrimps infected with WSSV. The results revealed that the survival rate of WSSV-infected shrimps after AgNP administration was 80%, whereas the survival rate of untreated organisms was only 10% 96 h after infection. These results open up the possibility to explore the potential use of AgNPs as antiviral agents for the treatment of diseases in aquaculture organisms, particularly the WSSV in shrimp culture.

A transcriptome study on Macrobrachium rosenbergii hepatopancreas experimentally challenged with white spot syndrome virus (WSSV)

The world production of shrimp such as the Malaysian giant freshwater prawn, Macrobrachium rosenbergii is seriously affected by the white spot syndrome virus (WSSV). There is an urgent need to understand the host pathogen interaction between M. rosenbergii and WSSV which will be able to provide a solution in controlling the spread of this infectious disease and lastly save the aquaculture industry. Now, using Next Generation Sequencing (NGS), we will be able to capture the response of the M. rosenbergii to the pathogen and have a better understanding of the host defence mechanism. Two cDNA libraries, one of WSSV-challenged M. rosenbergii and a normal control one, were sequenced using the Illumina HiSeq™ 2000 platform. After de novo assembly and clustering of the unigenes from both libraries, 63,584 standard unigenes were generated with a mean size of 698bp and an N50 of 1137bp. We successfully annotated 35.31% of all unigenes by using BLASTX program (E-value <10-5) against NCBI non-redundant (Nr), Swiss-Prot, Kyoto Encyclopedia of Genes and Genome pathway (KEGG) and Orthologous Groups of proteins (COG) databases. Gene Ontology (GO) assessment was conducted using BLAST2GO software. Differentially expressed genes (DEGs) by using the FPKM method showed 8443 host genes were significantly up-regulated whereas 5973 genes were significantly down-regulated. The differentially expressed immune related genes were grouped into 15 animal immune functions. The present study showed that WSSV infection has a significant impact on the transcriptome profile of M. rosenbergii's hepatopancreas, and further enhanced the knowledge of this host-virus interaction. Furthermore, the high number of transcripts generated in this study will provide a platform for future genomic research on freshwater prawns.

VP24 Is a Chitin-Binding Protein Involved in White Spot Syndrome Virus Infection

Oral ingestion is the major route of infection for the white spot syndrome virus (WSSV). However, the mechanism by which virus particles in the digestive tract invade host cells is unknown. In the present study, we demonstrate that WSSV virions can bind to chitin through one of the major envelope proteins (VP24). Mutagenesis analysis indicated that amino acids (aa) 186 to 200 in the C terminus of VP24 were required for chitin binding. Moreover, the P-VP24186-200 peptide derived from the VP24 chitin binding region significantly inhibited the VP24-chitin interaction and the WSSV-chitin interaction, implying that VP24 participates in WSSV binding to chitin. Oral inoculation experiments showed that P-VP24186-200 treatment reduced the number of virus particles remaining in the digestive tract during the early stage of infection and greatly hindered WSSV proliferation in shrimp. These data indicate that binding of WSSV to chitin through the viral envelope protein VP24 is essential for WSSV per os infection and provide new ideas for preventing WSSV infection in shrimp farms.

IMPORTANCE

In this study, we show that WSSV can bind to chitin through the envelope protein VP24. The chitin-binding domain of VP24 maps to amino acids 186 to 200 in the C terminus. Binding of WSSV to chitin through the viral envelope protein VP24 is essential for WSSV per os infection. These findings not only extend our knowledge of WSSV infection but also provide new insights into strategies to prevent WSSV infection in shrimp farms.

RNA interference technology used for the study of aquatic virus infections

Aquaculture is one of the most important economic activities in Asia and is presently the fastest growing sector of food production in the world. Explosive increases in global fish farming have been accompanied by an increase in viral diseases. Viral infections are responsible for huge economic losses in fish farming, and control of these viral diseases in aquaculture remains a serious challenge. Recent advances in biotechnology have had a significant impact on disease reduction in aquaculture. RNAi is one of the most important technological breakthroughs in modern biology, allowing us to directly observe the effects of the loss of specific genes in living systems. RNA interference technology has emerged as a powerful tool for manipulating gene expression in the laboratory. This technology represents a new therapeutic approach for treating aquatic diseases, including viral infections. RNAi technology is based on a naturally occurring post-transcriptional gene silencing process mediated by the formation of dsRNA. RNAi has been proven widely effective for gene knockdown in mammalian cultured cells, but its utility in fish remains unexplored. This review aims to highlight the RNAi technology that has made significant contributions toward the improvement of aquatic animal health and will also summarize the current status and future strategies concerning the therapeutic applications of RNAi to combat viral disease in aquacultured organisms.

Construction and application of a protein interaction map for white spot syndrome virus (WSSV)

White spot syndrome virus (WSSV) is currently the most serious global threat for cultured shrimp production. Although its large, double-stranded DNA genome has been completely characterized, most putative protein functions remain obscure. To provide more informative knowledge about this virus, a proteomic-scale network of WSSV-WSSV protein interactions was carried out using a comprehensive yeast two-hybrid analysis. An array of yeast transformants containing each WSSV open reading frame fused with GAL4 DNA binding domain and GAL4 activation domain was constructed yielding 187 bait and 182 prey constructs, respectively. On screening of ∼28,000 pairwise combinations, 710 interactions were obtained from 143 baits. An independent coimmunoprecipitation assay (co-IP) was performed to validate the selected protein interaction pairs identified from the yeast two-hybrid approach. The program Cytoscape was employed to create a WSSV protein-protein interaction (PPI) network. The topology of the WSSV PPI network was based on the Barabási-Albert model and consisted of a scale-free network that resembled other established viral protein interaction networks. Using the RNA interference approach, knocking down either of two candidate hub proteins gave shrimp more protection against WSSV than knocking down a nonhub gene. The WSSV protein interaction map established in this study provides novel guidance for further studies on shrimp viral pathogenesis, host-viral protein interaction and potential targets for therapeutic and preventative antiviral strategies in shrimp aquaculture.

Nucleic acid-induced antiviral immunity in shrimp

Vertebrates detect viral infection predominantly by sensing viral nucleic acids to produce type I interferon (IFN). In invertebrates, it has been believed that the IFN system is absent and RNA interference is a sequence-specific antiviral pathway. In this study, we found that injection of nucleic acid mimics poly(I:C), poly(C:G), CL097, poly C and CpG-DNA, afforded shrimp antiviral immunity, which is similar to the vertebrate IFN system. Using suppression subtractive hybridization (SSH) method, 480 expression sequence tags were identified to be involved in the poly(I:C)-induced antiviral immunity of the model crustacean Litopenaeus vannamei, and 41% of them were new genes. In the SSH libraries, several IFN system-related genes such as dsRNA-dependent protein kinase PKR, Toll-like receptor 3 (TLR3) and IFNγ-inducible protein 30 were identified. L. vannamei IKKε, whose vertebrate homologs are central regulators of the IFN-producing pathway, could significantly activate IFN reporter genes in HEK293T cells. In crustacean databases, many genes homologous to genes of the vertebrate IFN response, such as IRFs, PKR, ADAR (adenosine deaminase, RNA-specific) and other interferon-stimulated genes (ISGs) were discovered. These results suggest that shrimp may possess nucleic acid-induced antiviral immunity.

Transcriptome analysis on Chinese shrimp Fenneropenaeus chinensis during WSSV acute infection

Previous studies have discovered a lot of immune-related genes responding to white spot syndrome virus (WSSV) infection in crustacean. However, little information is available in relation to underlying mechanisms of host responses during the WSSV acute infection stage in naturally infected shrimp. In this study, we employed next-generation sequencing and bioinformatic techniques to observe the transcriptome differences of the shrimp between latent infection stage and acute infection stage. A total of 64,188,426 Illumina reads, including 31,685,758 reads from the latent infection group and 32,502,668 reads from the acute infection group, were generated and assembled into 46,676 unigenes (mean length: 676 bp; range: 200-15,094 bp). Approximately 24,000 peptides were predicted and classified based on homology searches, gene ontology, clusters of orthologous groups of proteins, and biological pathway mapping. Among which, 805 differentially expressed genes were identified and categorized into 11 groups based on their possible function. Genes in the Toll and IMD pathways, the Ras-activated endocytosis process, the RNA interference pathway, anti-lipopolysaccharide factors and many other genes, were found to be activated in shrimp from latent infection stage to acute infection stage. The anti-bacterially proPO-activating cascade was firstly uncovered to be probably participated in antiviral process. These genes contain not only members playing function in host defense against WSSV, but also genes utilized by WSSV for its rapid proliferation. In addition, the transcriptome data provides detail information for identifying novel genes in absence of the genome database of shrimp.

Activation of an immune response in Litopenaeus vannamei by oral immunization with phagocytosis activating protein (PAP) DNA

The phagocytosis activating protein (PAP) gene has been reported to stimulate the phagocytic activity of shrimp hemocytes and to protect shrimp from several pathogens. In this study oral administration of the chitosan-PAP gene to shrimp was investigated for its ability to induce immunity. The PAP gene was cooperated into a phMGFP plasmid, named PAP-phMGFP. Chitosan-PAP-phMGFP nanoparticles were formed by mixing a low molecular weight chitosan (50 kDa) and a high molecular weight chitosan (150 kDa) with various ratios of PAP-phMGFP. The optimal ratio of chitosan PAP-phMGFP nanoparticles was first determined by transfection into Chinese Hamster Ovary (CHO) cells before being used for oral immunization in shrimp. The chitosan-PAP-phMGFP nanoparticles at a ratio of 2:1 with the low molecular weight chitosan were optimum for transfecting the CHO cells. The shrimp were then fed with 25, 50, 100 and 150 μg/shrimp/day of chitosan-PAP-phMGFP (2:1) nanoparticles then challenged by the white spot syndrome virus (WSSV). Shrimp fed with 50 μg of chitosan-PAP-phMGFP nanoparticles per day for 7 consecutive days, produced the highest relative percent survival (RPS) (94.45 ± 9.86%). The presence of PAP-phMGFP was detected in every shrimp tissue including the hemolymph, lymphoid organ, heart, hepatopancreas, intestine and muscle. The folds increase of the PAP gene expression increased significantly together with an increase of the phagocytic activity in the immunized shrimp. The stability of the PAP-phMGFP in the immunized shrimp hemolymph was detected by determination of the expression of the GFP at various days after immunization ceased. GFP expression was detected until the 15th day but not at the 30th day after immunization ceased. A quantitative analysis of the WSSV copies in shrimp heart tissue was significantly reduced in the immunized shrimp. In addition, chitosan-PAP-phMGFP nanoparticles protected shrimp against WSSV, Yellow head virus (YHV) and Vibrio harveyi with RPS values of 83.34 ± 7.86%, 55.56 ± 15.72% and 53.91 ± 5.52%, respectively. This study therefore confirms the role of the PAP gene in shrimp immunity and may lead to the development of a way to prevent microbial diseases of shrimp at an industrial level by appropriate feeding of a chitosan/DNA complex.

Exploring RNAi as a therapeutic strategy for controlling disease in aquaculture

Aquatic animal diseases are one of the most significant constraints to the development and management of aquaculture worldwide. As a result, measures to combat diseases of fish and shellfish have assumed a high priority in many aquaculture-producing countries. RNA interference (RNAi), a natural mechanism for post-transcriptional silencing of homologous genes by double-stranded RNA (dsRNA), has emerged as a powerful tool not only to investigate the function of specific genes, but also to suppress infection or replication of many pathogens that cause severe economic losses in aquaculture. However, despite the enormous potential as a novel therapeutical approach, many obstacles must still be overcome before RNAi therapy finds practical application in aquaculture, largely due to the potential for off-target effects and the difficulties in providing safe and effective delivery of RNAi molecules in vivo. In the present review, we discuss the current knowledge of RNAi as an experimental tool, as well as the concerns and challenges ahead for the application of such technology to combat infectious disease of farmed aquatic animals.

Comparative efficacy of double-stranded RNAs targeting WSSV structural and nonstructural genes in controlling viral multiplication in Penaeus monodon

RNA interference (RNAi) is a potential strategy to control shrimp viral diseases, including the white spot disease caused by White Spot Syndrome Virus (WSSV). Selection of genes for targeting is an important criterion. We have compared the efficacy of dsRNAs targeting structural (vp28 and vp281) and nonstructural genes (rr1 and dnapol) of WSSV in controlling viral multiplication in Penaeus monodon. Targeting the rr1 and vp28 genes provided better protection (93.3% and 90% survival respectively) compared to vp281 and dnapol in experimentally infected shrimp. Temporal transcriptional analysis of the corresponding genes and PCR-based diagnosis of WSSV in samples collected at different time points in the experiment supported this observation, thereby indicating that targeting a combination of rr1 and vp28 would be effective in limiting WSSV multiplication.

Protection of shrimp against white spot syndrome virus (WSSV) with β-1,3-D-glucan-encapsulated vp28-siRNA particles

White spot syndrome virus (WSSV) is a major shrimp viral pathogen responsible for large economic losses to shrimp aquaculture all over the world. The RNAi mediated by siRNA contributes a new strategy to control this viral disease. However, the efficient approach to deliver the siRNA into shrimp remains to be addressed. In this investigation, an antiviral vp28-siRNA was encapsulated in β-1,3-D-glucan, and then the β-1,3-D-glucan-encapsulated vp28-siRNA particles (GeRPs) were delivered into Marsupenaeus japonicus shrimp. The results showed that the vp28-siRNA in GeRPs could be released in hemocytes of shrimp. It was found that the GeRPs containing the vp28-siRNA inhibited the replication of WSSV in vivo, which presented a better antiviral activity than the non-encapsulated vp28-siRNA. Further evidence indicated that the mortality of WSSV-infected shrimp was significantly delayed by the GeRPs containing vp28-siRNA. Therefore, our study presented that the glucan-encapsulated siRNA might represent a novel potential therapeutic or preventive approach to control the shrimp disease.

Uncovering the mechanisms of shrimp innate immune response by RNA interference

Because of the importance of shrimp in world aquaculture, there is much interest in understanding their immune system in order to improve their resistance to pathogenic microorganisms. An effective tool in studying genes involved in the immune response in shrimp is RNA interference (RNAi). RNAi, first recognized as an antiviral response against RNA viruses, is a cellular mechanism that is triggered by double-stranded RNAs and results in the degradation of homologous genes. In this review, we describe the current studies of genes in shrimp that employed RNAi technology to elucidate or confirm their functions. We also review the potential of RNAi to elicit antiviral response in shrimp.

Shrimp Pm-fortilin inhibits the expression of early and late genes of white spot syndrome virus (WSSV) in an insect cell model

Fortilin plays an important role in anti-apoptotic mechanisms and cell proliferation in many eukaryotic organisms. This work confirmed previous reports that Sf9 can support the replication of white spot syndrome virus (WSSV) genomic material by using immunohistochemistry with a specific antibody to detect the immediate early gene 1 (ie1) and by amplification of WSSV DNA and mRNA products. Using this insect-cell model system, we show that overexpression of Pm-fortilin in Sf9 cells inhibited the expression of WSSV early genes and late genes (WSSV-DNA polymerase, VP15 and VP28) but not an immediate early gene ie1. This is the first time that an insect cell line has been used to demonstrate interaction between a shrimp gene and genes of a shrimp virus.

Double-stranded RNA against white spot syndrome virus (WSSV) vp28 or vp26 reduced susceptibility of Litopenaeus vannamei to WSSV, and survivors exhibited decreased susceptibility in subsequent re-infections

The antiviral effect of vp28 or vp26 double-stranded (ds) RNA upon single or consecutive white spot syndrome virus (WSSV) intramuscular challenges with a high infectious dose was evaluated. The vp28 dsRNA showed the highest protection both in single (LT(50)=145h at 10d and 98h at 20d post treatment [dpt]) or consecutive (LT(50)=765h) WSSV challenges compared to vp26 dsRNA (LT(50)=126h at 10 d and 57h at 20dpt vs. consecutive challenge LT(50)=751h). Single WSSV challenges showed that animals treated with vp28 or vp26 dsRNA gradually lost the antiviral effect as virus challenge occurred at 10dpt (cumulative mortality 63% vs. 80%, respectively) or 20dpt (87% vs. 100%, respectively). In contrast, animals treated with vp28 or vp26 dsRNA and consecutively challenged with WSSV showed and extended lower susceptibility to WSSV. All dead animals were WSSV-positive by one-step PCR, whereas all surviving shrimp from single or continuous challenges were WSSV-negative as determined by reverse transcription (RT)-PCR. In conclusion, shrimp treated with a single administration of vp28 or vp26 dsRNA and consecutively challenged with WSSV showed a stronger and longer antiviral response than shrimp exposed once to WSSV at 10 or 20dpt.

White spot syndrome virus Orf514 encodes a bona fide DNA polymerase

White spot syndrome virus (WSSV) is the causative agent of white spot syndrome, one of the most devastating diseases in shrimp aquaculture. The genome of WSSV includes a gene that encodes a putative family B DNA polymerase (ORF514), which is 16% identical in amino acid sequence to the Herpes virus 1 DNA polymerase. The aim of this work was to demonstrate the activity of the WSSV ORF514-encoded protein as a DNA polymerase and hence a putative antiviral target. A 3.5 kbp fragment encoding the conserved polymerase and exonuclease domains of ORF514 was overexpressed in bacteria. The recombinant protein showed polymerase activity but with very low level of processivity. Molecular modeling of the catalytic protein core encoded in ORF514 revealed a canonical polymerase fold. Amino acid sequence alignments of ORF514 indicate the presence of a putative PIP box, suggesting that the encoded putative DNA polymerase may use a host processivity factor for optimal activity. We postulate that WSSV ORF514 encodes a bona fide DNA polymerase that requires accessory proteins for activity and maybe target for drugs or compounds that inhibit viral DNA replication.

Non-specific activation of antiviral immunity and induction of RNA interference may engage the same pathway in the Pacific white leg shrimp Litopenaeus vannamei

Many questions remain unanswered regarding RNAi-based mechanisms and dsRNA-induced antiviral immune responses in penaeid shrimp. In this study, we report the characterization in the white leg shrimp Litopenaeus vannamei of RNAi pathway associated proteins Lv-Ago 1 and Lv-Ago 2, two members of the Argonaute family of proteins, as well as Lv-sid 1, the first shrimp homologue of Sid-1, a membrane channel-forming protein implicated in the cellular import of dsRNA. To decipher their functional implication in RNAi-related phenomena, we monitored their relative expression following stimulation by specific and non-specific RNA duplexes of diverse length. The findings show that the length of small RNA duplexes plays a critical role in the activation of both RNAi-related and innate antiviral responses. They also suggest that these two mechanisms of antiviral response may activate the same pathway, requiring Lv-Sid 1 and Lv-Ago 2 induction.

Inhibition of Newcastle disease virus replication by RNA interference targeting the matrix protein gene in chicken embryo fibroblasts

Newcastle disease (ND) is an infectious viral disease of birds caused by the Newcastle disease virus (NDV), also known as avian paramyxovirus type 1 (AMPV-1), which leads to severe economic losses in the poultry industry worldwide. In this study, the application of RNA interference (RNAi) for inhibiting the replication of NDV in cell culture by targeting the viral matrix protein gene (M) is described. Two M-specific shRNA-expressing plasmid constructs, named pS(M641) and pS(M827), were evaluated for antiviral activity against the NDV strain NA-1 by cytopathic effects (CPE), virus titration and real-time RT-PCR. After 36h of infection, both pS(M641) and pS(M827) reduced virus titers by 79.4- and 31.6-fold, respectively, and they down-regulated mRNA expression levels of the matrix protein gene M by 94.6% and 84.8%, respectively, in chicken embryo fibroblast (CEF) cells, while only pS(M641) significantly decreased CPE, compared to the control group. These results indicated that the M gene 641 and 827 sites represent potential antiviral therapy targets, and RNAi targeting of the M gene could not only represent an effective treatment in Newcastle disease but also aid as a method for studying the replication of NDV.

The source of self: genetic parasites and the origin of adaptive immunity

Stable colonization of the host by viruses (genetic parasites) can alter the systems of host identity and provide immunity against related viruses. To attain the needed stability, some viruses of prokaryotes (P1 phage) use a strategy called an addiction module. The linked protective and destructive gene functions of an addiction module insures both virus persistence but will also destroy cells that interrupt this module and thereby prevent infection by competitors. Previously, I have generalized this concept to also include persistent and lytic states of virus infection, which can be considered as a virus addiction module. Such states often involve defective viruses. In this report, I examine the origin of the adaptive immune system from the perspective of a virus addiction module. The likely role of both endogenous and exogenous retroviruses, DNA viruses, and their defective elements is considered in the origin of all the basal components of adaptive immunity (T-cell receptor, RAG-mediated gene rearrangement, clonal lymphocyte proliferation, antigen surface presentation, apoptosis, and education of immune cells). It is concluded that colonization by viruses and their defectives provides a more coherent explanation for the origin of adaptive immunity.

Application of nucleic-acid-based therapeutics for viral infections in shrimp aquaculture

Viral infections are one of the major reasons for the huge economic losses in shrimp farming. The control of viral diseases in shrimp remains a serious challenge for the shrimp aquacultural industry, with major pathogens, such as the white spot syndrome virus, yellow head virus, Taura syndrome virus, hepatopancreatic parvovirus, and baculoviruses, being geographically widespread. In the absence of a true adaptive immune response system in invertebrates such as shrimp, one of the alternative and more specific approaches to counteract viral infections in shrimp could be the use of molecular-based gene transfer technologies, such as RNA interference (RNAi). The RNAi mechanism is initiated by double-stranded RNAs (dsRNAs), which are fragmented into shorter 21-23 nucleotides of short interfering RNAs (siRNAs) by a type III endonuclease, the Dicer. RNAi, which is mediated by small interfering RNA (siRNA), results in the sequence-specific post-transcriptional silencing of a target gene. This gene-silencing mechanism is universally conserved and is a well-known phenomenon that exists in many eukaryotes, including invertebrates. It has been recently extended to shrimp as an important potential tool in viral disease prevention. RNAi technology shows considerable promise as a therapeutic approach and efficient strategy for shrimp virus control in the aquaculture industry. Further progress in understanding the mechanism of siRNAs at the molecular level, as well as strategies to achieve their tightly regulated, stable, prolonged and tissue-specific expression in an effective manner, will definitely revolutionize therapeutic approaches for counteracting viral diseases of shrimp. In the present review, the recent development and potential use of RNAi in combating shrimp viral infections is discussed.

Oral delivery of DNA vaccine encoding VP28 against white spot syndrome virus in crayfish by attenuated Salmonella typhimurium

Protective immune responses in shrimp induced by DNA vaccines against white spot syndrome virus (WSSV) with intramuscular injection have been reported in recent reports. In this study, we investigated the utilities of attenuated Salmonella enterica serovar Typhimurium (Salmonella typhimurium) as a bactofection vehicle for the oral delivery of a DNA vaccine plasmid to crayfish (Cambarus clarkii). The DNA vaccine plasmid pcDNA3.1-VP28, encoding viral envelope protein VP28, was transformed to an attenuated S. typhimurium strain SV4089 and the resulting recombinant bacteria named SV/pcDNA3.1-VP28 were used to orally immunize crayfish with coated feed. Successful delivery of the DNA vaccine plasmid was shown by the isolation of recombinant bacteria SV/pcDNA3.1-VP28 from the vaccinated crayfish. The distribution analysis of plasmid pcDNA3.1-VP28 in different tissues revealed the effective release of DNA vaccine plasmid into crayfish. RT-PCR and immunoflurescence results confirmed the expression of protein VP28 in the vaccinated crayfish. Challenge experiments with WSSV at 7, 15, 25 days post-vaccination demonstrated significant protection in immunized crayfish with relative survival rate 83.3%, 66.7% and 56.7%, respectively. Studies on stability and safety of SV/pcDNA3.1-VP28 showed the recombinant bacteria could exist in crayfish at least 7 days but not more than 10 days and without any observable harm to the host. Our study here demonstrates, for the first time, the ability of attenuated Salmonella as a live vector to orally deliver a DNA vaccine against WSSV into the arthropod crayfish and provides a new way to design more practical strategies for the control of WSSV and other invertebrate pathogens.