Quasi-immune response of Penaeus japonicus to penaeid rod-shaped DNA virus (PRDV)

Failed shrimp vaccination attempt with yellow head virus (YHV) attenuated in an immortal insect cell line

This short paper on yellow head virus Type-1 (YHV-1) of shrimp describes preliminary research on the potential for using YHV-1 attenuated in insect cells to protect shrimp against yellow head disease (YHD). YHV-1 can cause severe mortality in the cultivated shrimp Penaeus (Penaeus) monodon and Penaeus (Litopenaeus) vannamei.  No practical vaccination has been reported. The C6/36 mosquito cell cultures inoculated with YHV-1 become positive by PCR and by immunocytochemistry (immunopositive) for up to 30 split-cell passages. Shrimp injected with homogenates from low-passage cultures die from typical YHV-1 disease while shrimp injected with homogenates from high passage cultures do not, even though they become PCR positive and immunopositive for YHV-1. This suggested that viral attenuation had occurred during insect-cell passaging, and it opened the possibility of using homogenates from high-passage insect cultures as a vaccine against YHV-1. To test this hypothesis, homogenates from 30th-passage, YHV-positive cultures were injected into shrimp followed by challenge with virulent YHV-1. Controls were injected with homogenate from 30th-passage, naive (normal stock) insect-cell cultures. No shrimp mortality occurred following injection of either homogenate, but shrimp injected with the YHV-1 homogenate became both RT-PCR positive and immunopositive. Upon challenge 10 days later with YHV-1, mortality in shrimp injected with naive insect-cell homogenate was 100% within 7 days post-challenge while 100% mortality in the YHV-1 homogenate group did not occur until day 9 post-challenge. Kaplan-Meier log-rank survival analysis revealed that survival curves for the two groups were significantly different (p < 0.001). The cause of delay in mortality may be worthy of further investigation.

Oral administration of Synechococcus sp. PCC7942 trans-vp19 and trans-vp (19+28) genes improve the immune and antioxidant capacity in Procambarus clarkii under white spot syndrome virus stress

This study aimed to evaluate antioxidant capacity and protection from white spot syndrome virus (WSSV) challenge of Procambarus clarkii fed trans-vp19 and trans-vp (19 + 28) genes of Synechococcus sp. PCC7942 (Syn7942). P. clarkii were fed transgenic cyanobacteria continuously for 7 days, and then infected with WSSV after 12 h starvation. The daily mortality in each group was measured for 10 days and hepatopancreas and muscle of P. clarkii were examined for enzymes phenoloxidase (PO) activity, catalase (CAT) activity, glutathione peroxidase (GSH-px) activity, and malondialdehyde (MDA) concentration after immunization and viral challenge at different times. Compared with the WSSV-infected crayfish in positive control group (challenge and no vaccination) and wild type group (challenge, feeding wild-type Syn7942), vp19 group (challenge, feeding Syn7942 trans-vp19 gene) and vp (19 + 28) group [challenge, feeding Syn7942 trans-vp (19 + 28) genes] significantly improved the survival rate from 0% to 60% and 56.7%, respectively. Consistently, significantly greater PO, CAT, and GSH-px activity and significantly lower MDA concentration in the vp19 and vp (19 + 28) groups compared to the control group. These results demonstrate that the trans-vp19 and trans-vp (19 + 28) gene of Syn7942 significantly facilitated the immune and antioxidant capacity of crayfish. Therefore, the trans-vp19 and trans-vp (19 + 28) genes of Syn7942 could provide protection for crayfish as an anti-WSSV oral medication.

Viral Coinfections

In nature, viral coinfection is as widespread as viral infection alone. Viral coinfections often cause altered viral pathogenicity, disrupted host defense, and mixed-up clinical symptoms, all of which result in more difficult diagnosis and treatment of a disease. There are three major virus-virus interactions in coinfection cases: viral interference, viral synergy, and viral noninterference. We analyzed virus-virus interactions in both aspects of viruses and hosts and elucidated their possible mechanisms. Finally, we summarized the protocol of viral coinfection studies and key points in the process of virus separation and purification.

Immunostimulation of shrimp through oral administration of silkworm pupae expressing VP15 against WSSV

White spot syndrome virus (WSSV) is one of the most concerning pathogens in penaeid shrimp and can cause severe loss in shrimp aquaculture worldwide. Among the WSSV structural proteins, VP15, a DNA-binding protein located in the WSSV nucleocapsid, is an antiviral protein candidate to protect kuruma shrimp (Marsupenaeus japonicus) from WSSV infection. We identified that the truncated VP15, VP15_(26-57), is responsible for the protective effect against the WSSV. This study attempts to develop an immunizing agent against WSSV using silkworm pupa as a delivery vector through oral administration. The VP15, VP15_(26-57), and SR11 peptide derived from VP15_(26-57) were expressed in silkworm pupae. Oral administration of feed mixed with the powdered pupae that expressed VP15-derived constructs enhanced the survivability of kuruma shrimp with an overall relative percent survival (RPS) higher than 70%. There is no death for the group receiving pupa/VP15_(26-57), and the RPS is 100%. In addition, we also investigated the relative mRNA expression levels of immune-related genes by qPCR at different time points. Our results indicate that the oral administration of pupa/VP15-derived products could provide a high protective effect against WSSV and be a practical approach for controlling WSSV in aquaculture.

Strategy for understanding the biological defense mechanism involved in immune priming in kuruma shrimp

Since the 1970s, individuals that survive a specific infectious disease among crustaceans reportedly develop resistance to the given virulence factors. Quasi-immune response is a similar phenomenon of acquired resistance against white spot syndrome virus, also found in kuruma shrimp. This phenomenon, resembling immunological memory, is collectively called immune priming and recently attracts increasing attention. In this study, I review, along with recent findings, past attempts to immunize shrimp by administration of the pathogen itself or recombinant proteins of viral constituent factors. Moreover, I aimed at investigating the diversity of pattern recognition receptors in kuruma shrimp from the currently available information that allows for a better understanding of immune priming. This review would potentially help to elucidate the underlying mechanisms of immune priming in the future.

Oral administration of Saccharomyces cerevisiae displaying VP28-VP24 confers protection against white spot syndrome virus in shrimp

White spot syndrome virus (WSSV) is the major pathogen that leads to severe mortalities in cultured shrimp worldwide. The envelope proteins VP28 and VP24 of WSSV are considered potential vaccine candidate antigens. In this study, we utilized a Saccharomyces cerevisiae (S. cerevisiae) surface display system to demonstrate the feasibility of this platform for developing a vaccine candidate against WSSV. EBY100/pYD1-VP28-VP24 was generated, and the fusion protein VP28-VP24 was present on the surface of S. cerevisiae. Penaeus vannamei (P. vannamei) was used as an animal model. Oral administration of EBY100/pYD1-VP28-VP24 could induce significant activities of immune-related enzymes such as superoxide dismutase (SOD) and phenoloxidase (PO). Importantly, WSSV challenge indicated that oral administration of EBY100/pYD1-VP28-VP24 could confer 100% protection with a corresponding decrease in the viral load. The collective results strongly highlight the potential of a S. cerevisiae-based oral vaccine as an efficient control strategy for combating WSSV infection in shrimp aquaculture.

Recombinant viral proteins delivered orally through inactivated bacterial cells induce protection in Macrobrachium rosenbergii (de Man) against White Tail Disease

White tail disease (WTD) is a disease of Macrobrachium rosenbergii caused by Macrobrachium rosenbergii nodavirus (MrNV) and extra small virus (XSV) with the potential to devastate the aquaculture industry. The present study aimed to explore the possible protection of M. rosenbergii against the disease by oral administration of bacterially expressed recombinant capsid proteins of MrNV and XSV. Juvenile M. rosenbergii were fed with the feed coated with inactivated bacteria encapsulated expressed recombinant viral proteins either individually or in combination for 7 days. Challenge studies using WTD causing agents were carried out after 3 (group I), 10 (group II) and 20 (group III) days post-feeding of viral proteins. Recombinant capsid protein of MrNV showed better protection when compared to other treatments with relative per cent survival of 62.5% (group I), 57.9% (group II) and 39.5% (group III). Treatment controls of groups I, II and III showed 100%, 95% and 95% mortality, respectively. The study demonstrates that oral administration of recombinant capsid proteins of MrNV and XSV provides effective protection against WTD in freshwater prawn.

Research progress on viral accommodation 2009 to 2019

The viral accommodation hypothesis for crustaceans and insects was first proposed in 1998/2001, stimulated by observations that shrimp and insects or insect cell lines can coexist with both DNA or RNA viruses without showing any signs of disease (i.e., they tolerate, single to multiple, persistent infections, sometimes for a lifetime). A review of tests of the hypothesis up to 2007 was previously published in DCI. This was followed by a major revision in 2009 when the elusive memory element required by the hypothesis was proposed to reside in non-retroviral fragments of extant viruses, now called endogenous viral elements (EVE) that are autonomously inserted into the host genome as cDNA copied from viral mRNA. Here, progress in research on viral accommodation in crustaceans and insects over the decade following 2009 is reviewed. It culminates with a discussion of exiting research results from insects in 2019 that prove the existence of specific, adaptive and heritable immunity, at least in mosquitoes. It remains to be determined whether the same mechanisms also govern EVE acquisition and its protective RNA production in shrimp. The wide-ranging consequences of the revealed mechanisms for viral disease control in economic crustaceans and insects is discussed.

Enhanced Immune Protection of Mud Crab Scylla paramamosain in Response to the Secondary Challenge by Vibrio parahaemolyticus

“Immune priming” plays a vital part in the immune system of invertebrates, protecting against recurrent infections by pathogens, and can provide some ideas for the prevention and treatment of invertebrate diseases. Many invertebrates have been demonstrated recently to have immune priming, but the relevant mechanisms are not known. Expression of immune system–related genes in the hemocytes and hepatopancreas of the mud crab (Scylla paramamosain) before and after repeated stimulation with Vibrio parahaemolyticus were analyzed by real-time fluorescence quantitative polymerase chain reaction. Some molecules that may participate in the immune priming of S. paramamosain were screened out, and their possible roles in immune priming were interpreted. Crabs injected first with heat-killed V. parahaemolyticus (HkVp group) or physiologic (0.9%) saline (PS group) were rechallenged at 168 h with live V. parahaemolyticus (HkVp+Vp group and PS+Vp group, respectively). The log-rank test shows a significant difference in survival rate between the HkVp+Vp group and the other groups after the ICH (p < 0.05). Expression of genes involved in the toll-like receptor (TLR) signaling pathway and some antimicrobial peptide genes were detected. By, respectively, comparing gene quantification at different time points in hemocytes and the hepatopancreas, the molecules that may play a part in the early stage of the immune priming of S. paramamosain in the hemocytes are found to be down syndrome cell adhesion molecule (Dscam), Hyastatin, Cactus, Arasin, antilipopolysaccharide factor 3 (ALF3), ALF4, ALF5, and ALF6 as well as later acting molecules, such as Crustin, Dorsal, Pelle, and myeloid differentiation factor 88 (MyD88). The molecules that functioned throughout the entire period are TLR and Spaetzle. In the hepatopancreas, the molecules that may play a part in the early stages of immune priming are Dscam, Hyastatin, Arasin, ALF6, Pelle, Spaetzle, Dorsal and, in the later stage, ALF4. The molecules that functioned throughout the entire period are TLR, Crustin, Cactus, MyD88, ALF3, and ALF5. In summary, the immune function of S. paramamosain is enhanced after it receives the same repetitive stimulation by V. parahaemolyticus, indicating immune priming in S. paramamosain. Our study enriches research on immune priming in invertebrates and lays the foundation for further studies revealing the molecular mechanism of immune priming in crabs.

Molecular, histopathologic and electron microscopic analysis of white spot syndrome virus in wild shrimp (Fenneropenaeus indicus) in the coastal waters of Iran

So far, there have been no studies on the distribution of viral white spot syndrome in wild Indian white shrimp (Fenneropenaeus indicus) brooders at Iranian capture sites. This study was conducted to investigate the presence of white spot syndrome virus (WSSV) in wild Indian white shrimps in Iran, using PCR, histopathologic, and electron microscopic surveys. The samples were collected within two seasons (autumn and spring) and from two provinces (six capture sites), from the major hatcheries providing spawners. Eight hundred thirty-three samples were collected and analyzed first by PCR, after which the positive samples were examined using histological tests, and if inclusion bodies were observed, electron microscopy was also used. White spot syndrome virus was detected only at the capture sites in Sistan and Baluchistan Province, where the mean infection rate was significantly higher in the spring (8.7%) than in the autumn (2.03%). At the Chabahar, Pasabandar, and Govater capture sites, the mean infection rate was significantly higher (4.9%, 2.1%, and 9.2%, respectively), than in Hormozgan Province. The results showed that there was no significant difference in infection rate between the two different sizes and sexes of shrimps (P < 0.05). Phylogeny analysis revealed a close relationship between the viruses from this study and those in other Asian countries, including China, India, Bangladesh, Thailand, Taiwan, and South Korea. It is possible that the virus has spread across the Indian Ocean to other countries. Therefore, the spawners in this study, particularly those collected during the spring and those from capture sites in Sistan and Baluchistan Province, were found to be more susceptible to WSSV infection, and the virus might have been transmitted vertically from WSSV-infected brooders to post-larvae.

A new insight to biomarkers related to resistance in survived-white spot syndrome virus challenged giant tiger shrimp, Penaeus monodon

The emergence of diseases such as white spot disease has become a threat to Penaeus monodon cultivation. Although there have been a few studies utilizing RNA-Seq, the cellular processes of host-virus interaction in this species remain mostly anonymous. In the present study, P. monodon was challenged with WSSV by intramuscular injection and survived for 12 days. The effect of the host gene expression by WSSV infection in the haemocytes, hepatopancreas and muscle of P. monodon was studied using Illumina HiSeq 2000. The RNA-Seq of cDNA libraries was developed from surviving WSSV-challenged shrimp as well as from normal healthy shrimp as control. A comparison of the transcriptome data of the two groups showed 2,644 host genes to be significantly up-regulated and 2,194 genes significantly down-regulated as a result of the infection with WSSV. Among the differentially expressed genes, our study discovered HMGB, TNFSF and c-Jun in P. monodon as new potential candidate genes for further investigation for the development of potential disease resistance markers. Our study also provided significant data on the differential expression of genes in the survived WSSV infected P. monodon that will help to improve understanding of host-virus interactions in this species.

Selective expression of a "correct cloud" of Dscam in crayfish survivors after second exposure to the same pathogen

Arthropod hypervariable Dscam (Down syndrome cell adhesion molecule) may be involved in adaptive-like immune characteristics, namely immune priming, enabling the host to "learn" and "remember" pathogens previously encountered in arthropods. However, expression of Dscam in immune-primed arthropods after a second challenge has apparently not been confirmed. Herein, working with Dscam of Australian freshwater crayfish (Cherax quadricarinatus, i.e. CqDscam), we further investigated whether immune priming is mediated by "clouds" of appropriate (or "correct") CqDscam isoforms. In crayfish that survived a first WSSV challenge (immune priming), long-lasting CqDscam expression remained higher after a second WSSV challenge. Selective CqDscam isoforms were also induced after both challenges. Based on pathogen binding assays, these WSSV-induced CqDscam isoforms had a higher WSSV binding ability, perhaps mainly mediated by Ig3-spliced variants. We therefore hypothesized that in these crayfish survivors, an unknown selection process was generating a "correct cloud" of CqDscam against a previously encountered pathogen.

Quantitative analysis of the dose-response of white spot syndrome virus in shrimp

White spot syndrome virus (WSSV) is an important cause of mortality and economic losses in shrimp farming. Although WSSV-induced mortality is virus dose dependent and WSSV infection does not necessarily lead to mortality, the relationships between virus-particle dose, infection and mortality have not been analysed quantitatively. Here, we explored WSSV dose-response by a combination of experiments, modelling and meta-analysis. We performed dose-response experiments in Penaeus vannamei postlarvae, recorded host mortality and detected WSSV infection. When we fitted infection models to these data, two models-differing in whether they incorporated heterogeneous host susceptibility to the virus or not-were supported for two independent experiments. To determine the generality of these results, we reanalysed published data sets and then performed a meta-analysis. We found that WSSV dose-response kinetics is indeed variable over experiments. We could not clearly identify which specific infection model has the most support by meta-analysis, but we argue that these results also are most concordant with a model incorporating varying levels of heterogeneous host susceptibility to WSSV. We have identified suitable models for analysing WSSV dose-response, which can elucidate the most basic virus-host interactions and help to avoid underestimating WSSV infection at low virus doses.

Virological and Immunological Outcomes of Coinfections

Coinfections involving viruses are being recognized to influence the disease pattern that occurs relative to that with single infection. Classically, we usually think of a clinical syndrome as the consequence of infection by a single virus that is isolated from clinical specimens. However, this biased laboratory approach omits detection of additional agents that could be contributing to the clinical outcome, including novel agents not usually considered pathogens. The presence of an additional agent may also interfere with the targeted isolation of a known virus. Viral interference, a phenomenon where one virus competitively suppresses replication of other coinfecting viruses, is the most common outcome of viral coinfections. In addition, coinfections can modulate virus virulence and cell death, thereby altering disease severity and epidemiology. Immunity to primary virus infection can also modulate immune responses to subsequent secondary infections. In this review, various virological mechanisms that determine viral persistence/exclusion during coinfections are discussed, and insights into the isolation/detection of multiple viruses are provided. We also discuss features of heterologous infections that impact the pattern of immune responsiveness that develops.

Baculovirus Surface Display of Immunogenic Proteins for Vaccine Development

Vaccination is an efficient way to prevent the occurrence of many infectious diseases in humans. To date, several viral vectors have been utilized for the generation of vaccines. Among them, baculovirus-categorized as a nonhuman viral vector-has been used in wider applications. Its versatile features, like large cloning capacity, nonreplicative nature in mammalian cells, and broad tissue tropism, hold it at an excellent position among vaccine vectors. In addition to ease and safety during swift production, recent key improvements to existing baculovirus vectors (such as inclusion of hybrid promoters, immunostimulatory elements, etc.) have led to significant improvements in immunogenicity and efficacy of surface-displayed antigens. Furthermore, some promising preclinical results have been reported that mirror the scope and practicality of baculovirus as a vaccine vector for human applications in the near future. Herein, this review provides an overview of the induced immune responses by baculovirus surface-displayed vaccines against influenza and other infectious diseases in animal models, and highlights the strategies applied to enhance the protective immune responses against the displayed antigens.

Circulatory white spot syndrome virus in South-West region of Bangladesh from 2014 to 2017: molecular characterization and genetic variation

White Spot Syndrome Virus (WSSV), the etiological agent of White Spot Disease (WSD) is a major impediment for shrimp aquaculture in the worldwide. A critical threshold level of WSSV load in infected shrimp is an important trait for disease manifestation and WSSV transmission in cultured shrimp and subsequently make outbreaks. The present study investigated 120 naturally infected cultured shrimp samples by SYBR Green based qPCR assay for WSD diagnosis and quantification of WSSV load. Among them, 94 samples resulted a variable count of WSSV load ranging from 2.1 × 10⁸ to 2.64 × 10¹⁴ copies/g of shrimp tissue. The severity of WSSV infection was assessed based on the established critical threshold load of WSSV in shrimp tissue. Compared to the established critical threshold value of WSSV load in shrimp tissue, our findings showed the horrifying scenario of the severity of WSSV infection in cultured shrimps of Bangladesh that was found to be above the critical limit to initiate an outbreak in the Bangladeshi shrimp aquaculture industry. The latest phylogenetic pattern was altered from the former monophyletic history among WSSVs of Bangladesh due to a variation at 500th nucleotide of VP28 coding gene. Viruses characterized from recent outbreaks in 2015 and 2017 displayed amino acid substitution at position 167 (G→E) on the surface of VP28 protein which has demonstrated the probable replacement of indigenous virus pool. Therefore, it is imperative to take initiative for the management and prevention of WSSV outbreak to sustain shrimp aquaculture in South-West region of Bangladesh.

Virulence and genotypes of white spot syndrome virus infecting Pacific white shrimp Litopenaeus vannamei in north-western Mexico

White spot syndrome virus (WSSV) has caused substantial global economic impact on aquaculture, and it has been determined that strains can vary in virulence. In this study, the effect of viral load was evaluated by infecting Litopenaeus vannamei with 10-fold serial dilution of tissue infected with strain WSSV Mx-H, and the virulence of four WSSV strains from north-western Mexico was assessed along with their variable number of tandem repeat (VNTR) genotypes in ORF75, ORF94 and ORF125. The LD₅₀ of the Mx-H strain was a dilution dose of 10^-7.5 ; the mortality titre was 10^9.2 LD₅₀ per gram. In shrimp injected with 10^2.5 to 10^6.5 LD₅₀ , no significant virulence differences were evident. Using mortality data, the four WSSV strains grouped into three virulence levels. The Mx-F strain (intermediate virulence) and the Mx-C strain (high virulence) showed more genetic differences than those observed between the Mx-G (low-virulence) and Mx-H (high-virulence) strains, in ORF94 and ORF125. The application of high-viral-load inocula proved useful in determining the different virulence phenotypes of the WSSV strains from the Eastern Pacific.

Antimicrobial Compounds from Eukaryotic Microalgae against Human Pathogens and Diseases in Aquaculture

The search for novel compounds of marine origin has increased in the last decades for their application in various areas such as pharmaceutical, human or animal nutrition, cosmetics or bioenergy. In this context of blue technology development, microalgae are of particular interest due to their immense biodiversity and their relatively simple growth needs. In this review, we discuss about the promising use of microalgae and microalgal compounds as sources of natural antibiotics against human pathogens but also about their potential to limit microbial infections in aquaculture. An alternative to conventional antibiotics is needed as the microbial resistance to these drugs is increasing in humans and animals. Furthermore, using natural antibiotics for livestock could meet the consumer demand to avoid chemicals in food, would support a sustainable aquaculture and present the advantage of being environmentally friendly. Using natural and renewable microalgal compounds is still in its early days, but considering the important research development and rapid improvement in culture, extraction and purification processes, the valorization of microalgae will surely extend in the future.

Comparative proteomic analysis of Litopenaeus vannamei gills after vaccination with two WSSV structural proteins

White spot syndrome virus (WSSV) is one of the most devastating viral pathogens of cultured shrimp worldwide. Recently published papers show the ability of WSSV structural protein VP28 to vaccinate shrimp and raise protection against the virus. This study attempted to identify the joining proteins of the aforementioned shrimp quasi-immune response by proteomic analysis. The other envelope protein, VP36B, was used as the non-protective subunit vaccine control. Shrimp were intramuscularly injected with rVPs or PBS on day 1 and day 4 and then on day 7 their gill tissues were sampled. The two-dimensional electrophoresis (2-DE) patterns of gill proteins between vaccinated and PBS groups were compared and 20 differentially expressed proteins identified by mass spectrometry, some of which were validated in gill and hemocyte tissues using real-time quantitative RT-PCR. Many of identified proteins and their expression levels also linked with the shrimp response during WSSV infection. The list of up-regulated protein spots found exclusively in rVP28-vaccinated shrimp include calreticulin and heat shock protein 70 with chaperone properties, ubiquitin, and others. The two serine proteases, chymotrypsin and trypsin, were significantly increased in shrimp of both vaccinated groups compared to PBS controls. The information presented here should be useful for gaining insight into invertebrate immunity.

Immune responses of whiteleg shrimp, Litopenaeus vannamei (Boone, 1931), to bacterially expressed dsRNA specific to VP28 gene of white spot syndrome virus

In this study, dsRNA specific to VP28 gene of white spot syndrome virus (WSSV) of shrimp was synthesized in Escherichia coli in large scale and studied the immune response of shrimp to dsRNA-VP28. The haematological parameters such as clotting time and total haemocytes counts, and immunological parameters such as prophenoloxidase (proPO), superoxide dismutase (SOD), superoxide anion (SOA) and malondialdehyde content, as well as the mRNA expression of ten immune-related genes were examined to estimate the effect of dsRNA-VP28 on the innate immunity of Litopenaeus vannamei. The activities of proPO, SOA and SOD significantly increased in haemocyte after dsRNA-VP28 treatment, whereas MDA content did not change significantly. Among the ten immune-related genes examined, only the mRNA expression of proPO, cMnSOD, haemocyanin, crustin, BGBP, lipopolysaccharides (LPs), lectin and lysozyme in haemocytes, gill and hepatopancreas of L. vannamei, was significantly upregulated at 12 h after dsRNA-VP28 treatment, while no significant expression changes were observed in Toll receptor and tumour receptor genes. The increase of proPO and SOD activities, and SOA level and mRNA expression level of proPO, cMnSOD, haemocyanin, crustin, BGBP, LPs, lectin and lysozyme after dsRNA-VP28 stimulation indicate that these immune-related genes were involved in dsRNA-VP28-induced innate immunity in shrimp.

Reprint of "evolution of specific immunity in shrimp - a vaccination perspective against white spot syndrome virus"

Invertebrates lack true adaptive immunity and it solely depends on the primitive immunity called innate immunity. However, various innate immune molecules and mechanisms are identified in shrimp that plays potential role against invading bacterial, fungal and viral pathogens. Perceiving the shrimp innate immune mechanisms will contribute in developing effective vaccine strategies against major shrimp pathogens. Hence this review intends to explore the innate immune molecules of shrimp with suitable experimental evidences together with the evolution of "specific immune priming" of invertebrates. In addition, we have emphasized on the development of an effective vaccine strategy against major shrimp pathogen, white spot syndrome virus (WSSV). The baculovirus displayed rVP28 (Bac-VP28), a major envelope protein of WSSV was utilized to study its vaccine efficacy by oral route. A significant advantage of this baculovirus expression cassette is the use of WSSV-immediate early 1 (ie1) promoter that derived the abundant expression of rVP28 protein at the early stage of the infection in insect cell. The orally vaccinated shrimp with Bac-VP28 transduced successfully in the shrimp cells as well as provided highest survival rate. In support to our vaccine efficacy we analysed Pattern Recognition Proteins (PRPs) β-1,3 glucan lipopolysaccharides (LGBP) and STAT gene profiles in the experimental shrimp. Indeed, the vaccination of shrimp with Bac-VP28 demonstrated some degree of specificity with enhanced survival rate when compared to control vaccination with Bac-wt. Hence it is presumed that the concept of "specific immune priming" in relevant to shrimp immunity is possible but may not be common to all shrimp pathogens.

Reprint of "review of Dscam-mediated immunity in shrimp and other arthropods"

Although true adaptive immunity is only found in vertebrates, there is increasing evidence that shrimp and other arthropods exhibit immune specificity and immune memory. The invertebrate immune response is now called "innate immunity with specificity" or "immune priming", and its underlying mechanisms are still unclear. However, while vertebrate antibodies have no invertebrate homolog, the Down syndrome cell adhesion molecule (Dscam), which is a hypervariable protein created by alternative splicing, can function as a pathogen-specific recognizing molecule in arthropods. Here we review our current understanding of the Dscam-mediated immune responses in arthropods, especially in shrimp, and show that Dscam may be involved in both general innate immunity and the pathogen-specific immune response.

Evolution of specific immunity in shrimp - a vaccination perspective against white spot syndrome virus

Invertebrates lack true adaptive immunity and it solely depends on the primitive immunity called innate immunity. However, various innate immune molecules and mechanisms are identified in shrimp that plays potential role against invading bacterial, fungal and viral pathogens. Perceiving the shrimp innate immune mechanisms will contribute in developing effective vaccine strategies against major shrimp pathogens. Hence this review intends to explore the innate immune molecules of shrimp with suitable experimental evidences together with the evolution of "specific immune priming" of invertebrates. In addition, we have emphasized on the development of an effective vaccine strategy against major shrimp pathogen, white spot syndrome virus (WSSV). The baculovirus displayed rVP28 (Bac-VP28), a major envelope protein of WSSV was utilized to study its vaccine efficacy by oral route. A significant advantage of this baculovirus expression cassette is the use of WSSV-immediate early 1 (ie1) promoter that derived the abundant expression of rVP28 protein at the early stage of the infection in insect cell. The orally vaccinated shrimp with Bac-VP28 transduced successfully in the shrimp cells as well as provided highest survival rate. In support to our vaccine efficacy we analysed Pattern Recognition Proteins (PRPs) β-1,3 glucan lipopolysaccharides (LGBP) and STAT gene profiles in the experimental shrimp. Indeed, the vaccination of shrimp with Bac-VP28 demonstrated some degree of specificity with enhanced survival rate when compared to control vaccination with Bac-wt. Hence it is presumed that the concept of "specific immune priming" in relevant to shrimp immunity is possible but may not be common to all shrimp pathogens.

Review of Dscam-mediated immunity in shrimp and other arthropods

Although true adaptive immunity is only found in vertebrates, there is increasing evidence that shrimp and other arthropods exhibit immune specificity and immune memory. The invertebrate immune response is now called "innate immunity with specificity" or "immune priming", and its underlying mechanisms are still unclear. However, while vertebrate antibodies have no invertebrate homolog, the Down syndrome cell adhesion molecule (Dscam), which is a hypervariable protein created by alternative splicing, can function as a pathogen-specific recognizing molecule in arthropods. Here we review our current understanding of the Dscam-mediated immune responses in arthropods, especially in shrimp, and show that Dscam may be involved in both general innate immunity and the pathogen-specific immune response.

WSSV-induced crayfish Dscam shows durable immune behavior

One of the major gaps in our understanding of arthropod specific immune priming concerns the mechanism[s] by which the observed long-term (>2 weeks) protective effects might be mediated. Hypervariable Dscam (Down syndrome cell adhesion molecule) might support arthropod innate immunity with specificity for more extended periods. We show here that, in the relatively long-lived arthropod Cherax quadricarinatus, CqDscam does not behave like a typical, immediately-acting, short-lived innate immune factor: CqDscam was not induced within hours after challenge with a lethal virus, but instead was only up-regulated after 2-5 days. This initial response faded within ∼ 2 weeks, but another maximum was reached ∼ 1 month later. At around 2 months after the initial challenge, the virus-induced CqDscam bound to the virus virion and acted to neutralize the virus However, although CqDscam helped crayfish to survive during persistent infection, it nevertheless failed to provide any enhanced protection against a subsequent WSSV challenge. Thus, CqDscam is capable of supporting extended anti-virus immune memory in arthropods. Also, during a persistent virus infection, the balance of "immune firepower" in crayfish appears to be altered such that the general immune factors become depleted while CqDscam becomes relatively predominant.

Horizontal transmission dynamics of White spot syndrome virus by cohabitation trials in juvenile Penaeus monodon and P. vannamei

White spot syndrome virus (WSSV), a rod-shaped double-stranded DNA virus, is an infectious agent causing fatal disease in shrimp farming around the globe. Within shrimp populations WSSV is transmitted very fast, however, the modes and dynamics of transmission of this virus are not well understood. In the current study the dynamics of disease transmission of WSSV were investigated in small, closed populations of Penaeus monodon and Penaeus vannamei. Pair cohabitation experiments using PCR as a readout for virus infection were used to estimate transmission parameters for WSSV in these two species. The mortality rate of contact-infected shrimp in P. monodon was higher than the rate in P. vannamei. The transmission rate parameters for WSSV were not different between the two species. The relative contribution of direct and indirect transmission rates of WSSV differed between the two species. For P. vannamei the direct contact transmission rate of WSSV was significantly lower than the indirect environmental transmission rate, but for P. monodon, the opposite was found. The reproduction ratio R0 for WSSV for these two species of shrimp was estimated to be above one: 2.07 (95%CI 1.53, 2.79) for P. monodon and 1.51 (95%CI 1.12, 2.03) for P. vannamei. The difference in R0 between the two species is due to a lower host mortality and hence a longer infectious period of WSSV in P. monodon.

Protein profiling in the gut of Penaeus monodon gavaged with oral WSSV-vaccines and live white spot syndrome virus

White spot syndrome virus (WSSV) is a pathogen that causes considerable mortality of the farmed shrimp, Penaeus monodon. Candidate 'vaccines', WSSV envelope protein VP28 and formalin-inactivated WSSV, can provide short-lived protection against the virus. In this study, P. monodon was orally intubated with the aforementioned vaccine candidates, and protein expression in the gut of immunised shrimps was profiled. The alterations in protein profiles in shrimps infected orally with live-WSSV were also examined. Seventeen of the identified proteins in the vaccine and WSSV-intubated shrimps varied significantly compared to those in the control shrimps. These proteins, classified under exoskeletal, cytoskeletal, immune-related, intracellular organelle part, intracellular calcium-binding or energy metabolism, are thought to directly or indirectly affect shrimp's immunity. The changes in the expression levels of crustacyanin, serine proteases, myosin light chain, and ER protein 57 observed in orally vaccinated shrimp may probably be linked to immunoprotective responses. On the other hand, altered expression of proteins linked to exoskeleton, calcium regulation and energy metabolism in WSSV-intubated shrimps is likely to symbolise disturbances in calcium homeostasis and energy metabolism.

Protection of shrimp Penaeus monodon from WSSV infection using antisense constructs

White spot syndrome caused by white spot syndrome virus (WSSV) is one of the most threatening diseases of shrimp culture industry. Previous studies have successfully demonstrated the use of DNA- and RNA-based vaccines to protect WSSV infection in shrimp. In the present study, we have explored the protective efficacy of antisense constructs directed against WSSV proteins, VP24, and VP28, thymidylate synthase (TS), and ribonucleotide reductase-2 (RR2) under the control of endogenous shrimp histone-3 (H3) or penaedin (Pn) promoter. Several antisense constructs were generated by inserting VP24 (pH3-VP24, pPn-VP24), VP28 (pH3-VP28, pPn-VP28), TS (pH3-TS, pPn-TS), and RR2 (pH3-RR2) in antisense orientation. These constructs were tested for their protective potential in WSSV infected cell cultures, and their effect on reduction of the viral load was assessed. A robust reduction in WSSV copy number was observed upon transfection of antisense constructs in hemocyte cultures derived from Penaeus monodon and Scylla serrata. When tested in vivo, antisense constructs offered a strong protection in WSSV challenged P. monodon. Constructs expressing antisense VP24 and VP28 provided the best protection (up to 90 % survivability) with a corresponding decrease in the viral load. Our work demonstrates that shrimp treated with antisense constructs present an efficient control strategy for combating WSSV infection in shrimp aquaculture.

Protection of Penaeus monodon against white spot syndrome virus by inactivated vaccine with herbal immunostimulants

To improve the immune response in tiger shrimp Penaeus monodon against WSSV infection, juveniles (350 ± 10 mg) were vaccinated with formalin-inactivated WSSV and fed with herbal immunostimulants. The methanolic extracts of herbal immunostimulants such as Acalypha indica, Cynodon dactylon, Picrorrhiza kurrooa, Withania somnifera and Zingiber officinalis were incorporated in formulated diets at different concentrations; 250 (ED(1)), 500 (ED(2)), 1000 (ED(3)) and 2000 (ED(4)) mg kg(-1) of feed and fed for 60 days after vaccination. After 30 and 60 days intervals of feeding, the shrimps were challenged with WSSV, which were isolated and propagated from the infected crustaceans. The shrimps fed with control diets (C(1)) succumbed to death within 5 days after WSSV challenge, when no vaccination and immunostimulations were given. The other control groups (C(2) and C(3)) had slight improvements in all parameters including survival. The percentage survival was significantly (P < 0.05) increased to 30, 50 and 60% in the ED(2), ED(3) and ED(4) diets respectively after 60 days challenging. The better haematological, biochemical and immunological parameters were also found in the herbal extracts supplemented diets fed vaccinated shrimps. The present study revealed that the combined effect of immunostimulation and vaccination helped to boost the immune system against WSSV infection and hence this application can be adopted for shrimp culture.

TAT-mediated oral subunit vaccine against white spot syndrome virus in crayfish

White spot syndrome virus is a highly pathogenic virus that infects crayfish and other crustaceans. VP28 is one of its major envelope proteins, and plays a crucial role in viral infection. Cell-penetrating peptides are short peptides that facilitate cellular uptake of various molecular cargoes, and one well known example is TAT peptide from HIV-1 TAT protein. In this study, recombinant plasmids were constructed and transformed into Escherichia coli strain BL21 (DE3) to express TAT-VP28, VP28, TAT-VP28-EGFP and VP28-EGFP fusion proteins. Enzyme-linked immunosorbent assay (ELISA) and flow cytometry methods were used to confirm that TAT fusion proteins can translocate from the intestine to the hemolymph of the crayfish Cambarus clarkii. After immunization, activities of phenoloxidase and superoxide dismutase were analyzed, and it was found that rTAT-VP28 produced the most pronounced increase in both C. clarkii were vaccinated by oral administration of rTAT-VP28 and rVP28 for 7 and 14 days, and rTAT-VP28 resulted in the highest relative percent survival (RPS) (63.3% at 7 days, and 67.8% at 14 days), compared with rVP28 (44.4% at 7 days, and 53.6% at 14 days) following challenge with WSSV after the last day of feeding. This study reports the use of TAT-derived peptide as an oral delivery method of a subunit vaccine against WSSV in C. clarkii.

Oral vaccination of baculovirus-expressed VP28 displays enhanced protection against White Spot Syndrome Virus in Penaeus monodon

White Spot Syndrome Virus (WSSV) is an infectious pathogen of shrimp and other crustaceans, and neither effective vaccines nor adequate treatments are currently available. WSSV is an enveloped dsDNA virus, and one of its major envelope proteins, VP28, plays a pivotal role in WSSV infection. In an attempt to develop a vaccine against WSSV, we inserted the VP28 gene into a baculovirus vector tailored to express VP28 on the baculovirus surface under the WSSV ie1 promoter (Bac-VP28). The Bac-VP28 incorporated abundant quantity (65.3 µg/ml) of VP28. Shrimp were treated by oral and immersion vaccination with either Bac-VP28 or wild-type baculovirus (Bac-wt). The treatment was followed by challenge with WSSV after 3 and 15 days. Bac-VP28 vaccinated shrimp showed significantly higher survival rates (oral: 81.7% and 76.7%; immersion: 75% and 68.4%) than Bac-wt or non-treated shrimp (100% mortality). To verify the protective effects of Bac-VP28, we examined in vivo expression of VP28 by immunohistochemistry and quantified the WSSV copy number by qPCR. In addition to that, we quantified the expression levels shrimp genes LGBP and STAT by real-time RT-PCR from the samples obtained from Bac-VP28 vaccinated shrimp at different duration of vaccine regime. Our findings indicate that oral vaccination of shrimp with Bac-VP28 is an attractive preventative measure against WSSV infection that can be used in the field.

Surface-displayed VP28 on Bacillus subtilis spores induce protection against white spot syndrome virus in crayfish by oral administration

AIM

Surface-displayed heterologous antigens on Bacillus subtilis spores can induce the vertebrate animals tested to generate local and systematic immune response through oral immunization. Here, the protection potential of the recombinant spores displaying the VP28 protein of white spot syndrome virus (WSSV) was investigated in the invertebrate crayfish (Cambarus clarkii).

METHODS AND RESULTS

The VP28 protein was successfully displayed on the surfaces of B. subtilis spores using CotB or CotC as a fusion partner. Crayfish were administrated orally by feeding the feed pellets coated with B. subtilis spores for 7 days and immediately followed by WSSV challenge. Oral administration of either spores expressing CotB-VP28 or CotC-VP28 resulted in significantly higher relative survival rates of 37.9 and 44.8% compared with the crayfish orally administrated with the spores nonexpressing VP28 (10.3% relative survival rate). When challenges were separately conducted at 7 and 21 days after oral administration, the relative survival rates increased to 46.4 and 50% at 7 days post-oral administration, but decreased to 30 and 33.3% at 21 days after oral administration.

CONCLUSION

These evidences indicate that the surface-displayed VP28 on B. subtilis spore could induce protection of crayfish against WSSV via oral administration.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report to use the spore surface display system to deliver orally a heterologous antigen in an aquatic invertebrate animal, crayfish. The results presented here suggest that the spore-displayed VP28 might be suitable for an oral booster vaccine on prevention of WSSV infection in shrimp farming.

Shrimp molecular responses to viral pathogens

From almost negligible amounts in 1970, the quantity of cultivated shrimp (~3 million metric tons in 2007) has risen to approach that of the capture fishery and it constitutes a vital source of export income for many countries. Despite this success, viral diseases along the way have caused billions of dollars of losses for shrimp farmers. Desire to reduce the losses to white spot syndrome virus in particular, has stimulated much research since 2000 on the shrimp response to viral pathogens at the molecular level. The objective of the work is to develop novel, practical methods for improved disease control. This review covers the background and limitations of the current work, baseline studies and studies on humoral responses, on binding between shrimp and viral structural proteins and on intracellular responses. It also includes discussion of several important phenomena (i.e., the quasi immune response, viral co-infections, viral sequences in the shrimp genome and persistent viral infections) for which little or no molecular information is currently available, but is much needed.

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.

Class B scavenger receptor, Croquemort from kuruma shrimp Marsupenaeus japonicus: Molecular cloning and characterization

The scavenger receptor, Croquemort is a member of the CD36 superfamily comprising transmembrane proteins involved in the recognition of polyanionic ligands. Various researchers have proved that members of the CD36 superfamily are involved in immunity and developmental processes. In the present study, we report a cDNA encoding the kuruma shrimp, Marsupenaeus japonicus Croquemort scavenger receptor (MjSCRBQ) obtained from a cDNA library of lymphoid organ by RACE amplification. The full-length cDNA of 2098 bp consists an open reading frame of 1596 nucleotides that translates into a 532-amino acid putative protein, with a 5' untranslated region of 323 bp and 3' UTR of 153 bp. The MjSCRBQ is constitutively expressed in gills, heart, hemolymph, hepatopancreas, intestine, lymphoid organ, muscle, nerve, and stomach and at high levels in the brain. Expression analysis in lymphoid organs of shrimp infected with white spot syndrome virus (WSSV) revealed high levels of MjSCRBQ 72 and 120 h post-infection. The MjSCRBQ contains putative functional domains including transmembrane domains and a CD36 domain. Multiple alignments of MjSCRBQ amino acid sequences showed significant identity with Drosophila melanogaster SCRBQ (31%), Salmo salar SCRBQ (29%), Homo sapiens SCRBQ (28%) and Rattus norvegicus SCRBQ (30%). In a phylogenetic analysis, MjSCRBQ was identified in the invertebrate scavenger receptor cluster. This is the first report in crustaceans of the identification and characterization of a Croquemort scavenging receptor.

Mud crab susceptibility to disease from white spot syndrome virus is species-dependent

Background

Based on a report for one species (Scylla serrata), it is widely believed that mud crabs are relatively resistant to disease caused by white spot syndrome virus (WSSV). We tested this hypothesis by determining the degree of susceptibility in two species of mud crabs, Scylla olivacea and Scylla paramamosain, both of which were identified by mitochondrial 16 S ribosomal gene analysis. We compared single-dose and serial-dose WSSV challenges on S. olivacea and S. paramamosain.

Findings

In a preliminary test using S. olivacea alone, a dose of 1 × 10⁶ WSSV copies/g gave 100% mortality within 7 days. In a subsequent test, 17 S. olivacea and 13 S. paramamosain were divided into test and control groups for challenge with WSSV at 5 incremental, biweekly doses starting from 1 × 10⁴ and ending at 5 × 10⁶ copies/g. For 11 S. olivacea challenged, 3 specimens died at doses between 1 × 10⁵ and 5 × 10⁵ copies/g and none died for 2 weeks after the subsequent dose (1 × 10⁶ copies/g) that was lethal within 7 days in the preliminary test. However, after the final challenge on day 56 (5 × 10⁶ copies/g), the remaining 7 of 11 S. olivacea (63.64%) died within 2 weeks. There was no mortality in the buffer-injected control crabs. For 9 S. paramamosain challenged in the same way, 5 (55.56%) died after challenge doses between 1 × 10⁴ and 5 × 10⁵ copies/g, and none died for 2 weeks after the challenge dose of 1 × 10⁶ copies/g. After the final challenge (5 × 10⁶ copies/g) on day 56, no S. paramamosain died during 2 weeks after the challenge, and 2 of 9 WSSV-infected S. paramamosain (22.22%) remained alive together with the control crabs until the end of the test on day 106. Viral loads in these survivors were low when compared to those in the moribund crabs.

Conclusions

S. olivacea and S. paramamosain show wide variation in response to challenge with WSSV. S. olivacea and S. paramamosain are susceptible to white spot disease, and S. olivacea is more susceptible than S. paramamosain. Based on our single-challenge and serial challenge results, and on previous published work showing that S. serrata is relatively unaffected by WSSV infection, we propose that susceptibility to white spot disease in the genus Scylla is species-dependent and may also be dose-history dependent. In practical terms for shrimp farmers, it means that S. olivacea and S. paramamosain may pose less threat as WSSV carriers than S. serrata. For crab farmers, our results suggest that rearing of S. serrata would be a better choice than S. paramamosain or S. olivacea in terms of avoiding losses from seasonal outbreaks of white spot disease.

White spot syndrome virus: an overview on an emergent concern

Viruses are ubiquitous and extremely abundant in the marine environment. One of such marine viruses, the white spot syndrome virus (WSSV), has emerged globally as one of the most prevalent, widespread and lethal for shrimp populations. However, at present there is no treatment available to interfere with the unrestrained occurrence and spread of the disease. The recent progress in molecular biology techniques has made it possible to obtain information on the factors, mechanisms and strategies used by this virus to infect and replicate in susceptible host cells. Yet, further research is still required to fully understand the basic nature of WSSV, its exact life cycle and mode of infection. This information will expand our knowledge and may contribute to developing effective prophylactic or therapeutic measures. This review provides a state-of-the-art overview of the topic, and emphasizes the current progress and future direction for the development of WSSV control strategies.

Viral disease emergence in shrimp aquaculture: origins, impact and the effectiveness of health management strategies

Shrimp aquaculture has grown rapidly over several decades to become a major global industry that serves the increasing consumer demand for seafood and has contributed significantly to socio-economic development in many poor coastal communities. However, the ecological disturbances and changes in patterns of trade associated with the development of shrimp farming have presented many of the pre-conditions for the emergence and spread of disease. Shrimp are displaced from their natural environments, provided artificial or alternative feeds, stocked in high density, exposed to stress through changes in water quality and are transported nationally and internationally, either live or as frozen product. These practices have provided opportunities for increased pathogenicity of existing infections, exposure to new pathogens, and the rapid transmission and transboundary spread of disease. Not surprisingly, a succession of new viral diseases has devastated the production and livelihoods of farmers and their sustaining communities. This review examines the major viral pathogens of farmed shrimp, the likely reasons for their emergence and spread, and the consequences for the structure and operation of the shrimp farming industry. In addition, this review discusses the health management strategies that have been introduced to combat the major pathogens and the reasons that disease continues to have an impact, particularly on poor, small-holder farmers in Asia.

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.

Oral administration of bacterially expressed VP28dsRNA to protect Penaeus monodon from white spot syndrome virus

We explored the possibility of protecting Penaeus monodon against white spot syndrome virus (WSSV) infection via interference RNA technology by oral administration of bacterially expressed WSSV VP28dsRNA. Shrimp were given dsRNA orally via two methods. In the first method, pellet feed was coated with inactivated bacteria containing overexpressed dsRNA of the WSSV VP28 gene, and in the second method, pellet feed was coated with VP28dsRNA-chitosan complex nanoparticles. The treated shrimp were orally challenged with WSSV by feeding WSSV-infected tissue. The experiment was conducted for 30 days. The dsRNA-treated shrimp challenged with WSSV showed higher survival compared to control shrimp. Sixty-eight percent survival was observed in shrimp fed with feed coated with inactivated bacteria containing dsRNA of the WSSV VP28 gene whereas 37% survival was observed in shrimp fed with VP28dsRNA-chitosan complex nanoparticle-coated feed. The WSSV caused 100% mortality in shrimp fed with pellet feed coated with inactivated bacteria with empty LITMUS38i vector. At the end of the experiment, the tissue samples prepared from randomly selected shrimp that survived were analyzed via reverse transcriptase-polymerase chain reaction and Western blot analysis for WSSV. The samples were negative for WSSV. Based on the present data and the advantages of dsRNA, we believe that oral administration of crude extract of bacterially expressed VP28dsRNA is a potential therapeutic agent against WSSV infection of shrimp.

Knocking down caspase-3 by RNAi reduces mortality in Pacific white shrimp Penaeus (Litopenaeus) vannamei challenged with a low dose of white-spot syndrome virus

Apoptosis has long been observed in viral target organs of white-spot syndrome virus (WSSV)-infected shrimp and whether the phenomenon helps the shrimp to survive the infection or is a factor leading to mortality is still controversial. If the shrimp mortality is a result of triggered apoptosis, then inactivation of caspase-3, a key protein in the induction of apoptosis, should improve shrimp survival upon challenge with WSSV. To test this prediction, we identified and characterized a caspase-3 homologue (cap-3) from the Pacific white shrimp Penaeus (Litopenaeus) vannamei and used this information to silence cap-3 expression by RNA interference prior to WSSV challenge. After confirming the efficacy of cap-3 silencing, its effects on mortality at high and low doses of WSSV were evaluated. In a high-dose WSSV challenge, cap-3 silencing had no significant effect on WSSV-induced mortality, except for a delay in mean time to death. However, at a low-dose WSSV challenge, cap-3 silencing correlated with a lower level of cumulative mortality, relative to silencing of a control gene, suggesting that apoptosis may exacerbate rather than decrease mortality in WSSV-challenged shrimp.

A review on the morphology, molecular characterization, morphogenesis and pathogenesis of white spot syndrome virus

Since it first appeared in 1992, white spot syndrome virus (WSSV) has become the most threatening infectious agent in shrimp aquaculture. Within a decade, this pathogen has spread to all the main shrimp farming areas and has caused enormous economic losses amounting to more than seven billion US dollars. At present, biosecurity methods used to exclude pathogens in shrimp farms include disinfecting ponds and water, preventing the entrance of animals that may carry infectious agents and stocking ponds with specific pathogen-free post-larvae. The combination of these practices increases biosecurity in shrimp farming facilities and may contribute to reduce the risk of a WSSV outbreak. Although several control methods have shown some efficacy against WSSV under experimental conditions, no therapeutic products or strategies are available to effectively control WSSV in the field. Furthermore, differences in virulence and clinical outcome of WSSV infections have been reported. The sequencing and characterization of different strains of WSSV has begun to determine aspects of its biology, virulence and pathogenesis. Knowledge on these aspects is critical for developing effective control methods. The aim of this review is to present an update of the knowledge generated so far on different aspects of WSSV organization, morphogenesis, pathology and pathogenesis.

Protection of Procambarus clarkii against white spot syndrome virus using recombinant oral vaccine expressed in Pichia pastoris

The potential for oral vaccination of crayfish against white spot syndrome virus was investigated. The envelope proteins VP19 and VP28 were expressed in yeast (Pichia pastoris). The expressed proteins were used as oral vaccines in different forms viz., in whole culture form, whole culture sonicated form, whole culture centrifuged supernatant form, and cell residue form. The recombinant proteins were mixed with food pellets and fed to crayfish for 25 days. The vaccinated groups were divided into two even groups and challenged on the 3rd and 21st day of post vaccination. Among different vaccine groups the relative percent survival (RPS) values of sonicated form and supernatant form vaccines were found the best and met the criterion (>RPS 60%) of effective vaccine even after 21st day of post vaccination. Development of vaccine by using recombinant proteins VP19 and VP28 in yeast as expression vector was feasible with significant effects.

Shrimp vaccination trials with the VP292 protein of white spot syndrome virus

AIMS

Construction of a recombinant vector that expresses VP292 protein of white spot syndrome virus (WSSV) and to exploit the possibility of obtaining the vaccine conferring protection against WSSV infection in shrimps.

METHODS AND RESULTS

VP292 protein of WSSV was amplified from WSSV genomic DNA by PCR. The target 814 bp amplified product specific for VP292 protein was inserted in to pQE30 expression vector. The recombinant plasmid of VP292 protein was transformed and expressed in Escherichia coli under induction of isopropyl-1-1-thio-beta-D-galactoside (IPTG) and the immunoreactivity of the fusion protein was detected by Western blot. Shrimp were vaccinated by intramuscular injection of the purified protein VP292 of WSSV and challenged for 0-30 days. Vaccination trial experiments show that two injections with recombinant VP292 (rVP292) protein induced a higher resistance, with 52% relative percentage survival value, in the shrimp at the 30th day postvaccination.

CONCLUSIONS

The expression system of protein VP292 of WSSV with a high efficiency has been successfully constructed. Vaccination trials show significant resistance in the shrimp vaccinated twice with recombinant VP292.

SIGNIFICANCE AND IMPACT OF THE STUDY

Results of this study prosper the development of WSSV protein vaccine against WSSV infection in shrimps.

Markedly reduced severity of Dengue virus infection in mosquito cell cultures persistently infected with Aedes albopictus densovirus (AalDNV)

AalDNV-infected C6/36 cells serially passaged for over 10 weeks showed a decline in percentage of anti-AalDNV-positive cells (APC) from an initial 92% to approximately 20%. Cultures of persistent APC were indistinguishable from uninfected cultures by direct microscopy but most stained cells from early APC passages had enlarged nuclei with eosinophilic inclusions, while late APC passages had few and naive cells none. Super challenge of persistent APC cultures did not increase percentage APC and supernatants from persistent APC cultures gave low APC (40%) in naive C6/36 cell cultures. When challenged with dengue virus serotype 2 (DEN-2), naive C6/36 cells showed severe cytopathic effects (CPE) and high mortality within 4 days, as did early passage APC cultures. Remarkably, DEN-2 infections in persistent APC cultures were much less severe, being characterized by reduced DEN-2 infection percentage, retarded DEN-2 virion production, no CPE and no significant mortality. Reasons for rapid reduction in APC and resistance to superinfection upon serial passage remain unproven but may relate to production of AalDNV-defective interfering particles (DIP) by molecular mechanisms still open to speculation. More difficult to explain is cross-protection against DEN-2-induced mortality seen in persistent APC cultures. However, by comparison to work on shrimp viruses, we speculate that this may involve blockage of viral-triggered apoptosis. The phenomena described raise questions regarding the potential for persistent infections by unknown viruses to confound experimental results with insect cell lines.

Protection of Penaeus monodon against white spot syndrome virus using a WSSV subunit vaccine

Although invertebrates lack a true adaptive immune response, the potential to vaccinate Penaeus monodon shrimp against white spot syndrome virus (WSSV) using the WSSV envelope proteins VP19 and VP28 was evaluated. Both structural WSSV proteins were N-terminally fused to the maltose binding protein (MBP) and purified after expression in bacteria. Shrimp were vaccinated by intramuscular injection of the purified WSSV proteins and challenged 2 and 25 days after vaccination to assess the onset and duration of protection. As controls, purified MBP- and mock-vaccinated shrimp were included. VP19-vaccinated shrimp showed a significantly better survival (p<0.05) as compared to the MBP-vaccinated control shrimp with a relative percent survival (RPS) of 33% and 57% at 2 and 25 days after vaccination, respectively. Also, the groups vaccinated with VP28 and a mixture of VP19 and VP28 showed a significantly better survival when challenged two days after vaccination (RPS of 44% and 33%, respectively), but not after 25 days. These results show that protection can be generated in shrimp against WSSV using its structural proteins as a subunit vaccine. This suggests that the shrimp immune system is able to specifically recognize and react to proteins. This study further shows that vaccination of shrimp may be possible despite the absence of a true adaptive immune system, opening the way to new strategies to control viral diseases in shrimp and other crustaceans.