Immunological-based assays for specific detection of shrimp viruses

Biochemical and structural characterization of a recombinant fibrinogen-related lectin from Penaeus monodon

Fibrinogen-related lectins are carbohydrate-binding proteins of the innate immune system that recognize glycan structures on microbial surfaces. These innate immune lectins are crucial for invertebrates as they do not rely on adaptive immunity for pathogen clearance. Here, we characterize a recombinant fibrinogen-related lectin PmFREP from the black tiger shrimp Penaeus monodon expressed in the Trichoplusia ni insect cell. Electron microscopy and cross-linking experiments revealed that PmFREP is a disulfide-linked dimer of pentamers distinct from other fibrinogen-related lectins. The full-length protein binds N-acetyl sugars in a Ca²⁺ ion-independent manner. PmFREP recognized and agglutinated Pseudomonas aeruginosa. Weak binding was detected with other bacteria, including Vibrio parahaemolyticus, but no agglutination activity was observed. The biologically active PmFREP will not only be a crucial tool to elucidate the innate immune signaling in P. monodon and other economically important species, but will also aid in detection and prevention of shrimp bacterial infectious diseases.

Plasmolipin, PmPLP1, from Penaeus monodon is a potential receptor for yellow head virus infection

Plasmolipin has been characterized as a cell entry receptor for mouse endogenous retrovirus. In black tiger shrimp, two isoforms of plasmolipin genes, PmPLP1 and PmPLP2, have been identified from the Penaeus monodon EST database. The PmPLP1 is highly up-regulated in yellow head virus (YHV)-infected shrimp. Herein, the function of PmPLP1 is shown to be involved in YHV infection. The immunoblotting and immunolocalization showed that the PmPLP1 protein was highly expressed and located at the plasma membrane of gills from YHV-infected shrimp. Moreover, the PmPLP1 expressed in the Sf9 insect cells resided at the cell membrane rendering the cells more susceptible to YHV infection. Using the ELISA binding and mortality assays, the synthetic external loop of PmPLP1 was shown to bind the purified YHV and neutralize the virus resulting in the decrease in YHV infection. Our results suggested that the PmPLP1 was likely a receptor of YHV in shrimp.

Advances in the study of nodavirus

Nodaviruses are small bipartite RNA viruses which belong to the family of Nodaviridae. They are categorized into alpha-nodavirus, which infects insects, and beta-nodavirus, which infects fishes. Another distinct group of nodavirus infects shrimps and prawns, which has been proposed to be categorized as gamma-nodavirus. Our current review focuses mainly on recent studies performed on nodaviruses. Nodavirus can be transmitted vertically and horizontally. Recent outbreaks have been reported in China, Indonesia, Singapore and India, affecting the aquaculture industry. It also decreased mullet stock in the Caspian Sea. Histopathology and transmission electron microscopy (TEM) are used to examine the presence of nodaviruses in infected fishes and prawns. For classification, virus isolation followed by nucleotide sequencing are required. In contrast to partial sequence identification, profiling the whole transcriptome using next generation sequencing (NGS) offers a more comprehensive comparison and characterization of the virus. For rapid diagnosis of nodavirus, assays targeting the viral RNA based on reverse-transcription PCR (RT-PCR) such as microfluidic chips, reverse-transcription loop-mediated isothermal amplification (RT-LAMP) and RT-LAMP coupled with lateral flow dipstick (RT-LAMP-LFD) have been developed. Besides viral RNA detections, diagnosis based on immunological assays such as enzyme-linked immunosorbent assay (ELISA), immunodot and Western blotting have also been reported. In addition, immune responses of fish and prawn are also discussed. Overall, in fish, innate immunity, cellular type I interferon immunity and humoral immunity cooperatively prevent nodavirus infections, whereas prawns and shrimps adopt different immune mechanisms against nodavirus infections, through upregulation of superoxide anion, prophenoloxidase, superoxide dismutase (SOD), crustin, peroxinectin, anti-lipopolysaccharides and heat shock proteins (HSP). Potential vaccines for fishes and prawns based on inactivated viruses, recombinant proteins or DNA, either delivered through injection, oral feeding or immersion, are also discussed in detail. Lastly, a comprehensive review on nodavirus virus-like particles (VLPs) is presented. In recent years, studies on prawn nodavirus are mainly focused on Macrobrachium rosenbergii nodavirus (MrNV). Recombinant MrNV VLPs have been produced in prokaryotic and eukaryotic expression systems. Their roles as a nucleic acid delivery vehicle, a platform for vaccine development, a molecular tool for mechanism study and in solving the structures of MrNV are intensively discussed.

Nidoviruses of Fish and Crustaceans

Viruses with diverse virion architectures demarcated into four families in the order Nidovirales have been discovered in vertebrate mammalian and fish species, as well as in invertebrate crustacean and mosquito species. The order is unified by nidoviruses sharing intermediate (12.7 kb) to very long (31.7 kb) (+) ssRNA genomes, each possessing a long 5′-terminal gene encoding overlapping ORF1a and ORF1b reading frames that contain a diversity of functionally related enzymes and that are translated in toto using a −1 ribosomal frameshift mechanism, as well as by semiconserved strategies for transcribing a nested set of 3′-coterminal subgenomic mRNAs that translate the viral proteins. The nidovirus that is most important to an aquaculture species is yellow head virus (YHV), which causes disease in shrimp farmed throughout the Eastern Hemisphere and is classified in the genus Okavirus, family Roniviridae. Fathead minnow nidovirus, genus Bafinivirus, subfamily Torovirinae, family Coronaviridae, also causes disease in minnows grown for the baitfish industry in the United States. Virions similar in morphology to okaviruses and bafiniviruses have also been detected in several crab species. Of these, however, only Eriocheir sinensis ronivirus, which causes disease in the Chinese mitten crab, an important freshwater aquaculture species in China, has been shown to possess a ~22 kb ssRNA genome that supports its being a nidovirus, but its taxonomic classification awaits genome sequence analysis. This chapter provides an overview of the structure, replication and biology of these viruses with a particular focus on YHV disease characteristics, diagnostic methods and disease prevention strategies.

Rapid immunochromatographic test strip to detect swimming crab Portunus trituberculatus reovirus

Swimming crab reovirus (SCRV) is the causative agent of a serious disease with high mortality in cultured Portunus trituberculatus. A rapid immunochromatographic assay (ICA) was developed in a competitive assay format and optimized for the detection of SCRV. The gold probe-based ICA test comprised SCRV antigen and goat anti-chicken egg yolk antibody (IgY) sprayed onto a nitrocellulose membrane as the test line and control line, respectively. IgY-gold complexes were deposited onto the conjugate pad as detector reagents. The method showed high specificity with no cross-reactivity with other related aquatic pathogens. The detection limit of the ICA strip was 50 µg ml⁻¹. To evaluate the performance of the ICA test, the strip and an enzyme-linked immunosorbent assay (ELISA) were applied to the same samples (n = 90 crabs). The strip successfully detected SCRV in all of the artificially infected samples. Furthermore, the ICA strip and ELISA tests had high consistency (98.28%). The strip assay requires no instruments and has a detection time of less than 10 min. It is portable and easy to perform in the field. These results indicated that the developed strip could be a promising on-site tool for screening pooled crabs to confirm SCRV infection or disease outbreaks.

A Novel Detection Platform for Shrimp White Spot Syndrome Virus Using an ICP11-Dependent Immunomagnetic Reduction (IMR) Assay

Shrimp white spot disease (WSD), which is caused by white spot syndrome virus (WSSV), is one of the world’s most serious shrimp diseases. Our objective in this study was to use an immunomagnetic reduction (IMR) assay to develop a highly sensitive, automatic WSSV detection platform targeted against ICP11 (the most highly expressed WSSV protein). After characterizing the magnetic reagents (Fe₃O₄ magnetic nanoparticles coated with anti ICP11), the detection limit for ICP11 protein using IMR was approximately 2 x 10⁻³ ng/ml, and the linear dynamic range of the assay was 0.1~1 x 10⁶ ng/ml. In assays of ICP11 protein in pleopod protein lysates from healthy and WSSV-infected shrimp, IMR signals were successfully detected from shrimp with low WSSV genome copy numbers. We concluded that this IMR assay targeting ICP11 has potential for detecting the WSSV.