Antilipopolysaccharide factor interferes with white spot syndrome virus replication in vitro and in vivo in the crayfish Pacifastacus leniusculus

Pathogenicity of Aeromonas veronii Isolated from Diseased Macrobrachium rosenbergii and Host Immune-Related Gene Expression Profiles

Aeromonas veronii is widespread in aquatic environments and is responsible for infecting various aquatic animals. In this study, a dominant strain was isolated from the hepatopancreas of diseased Macrobrachium rosenbergii and was named JDM1-1. According to its morphological, physiological, and biochemical characteristics and molecular identification, isolate JDM1-1 was identified as A. veronii. The results of artificial challenge showed isolate JDM1-1 had high pathogenicity to M. rosenbergii with an LD50 value of 8.35 × 105 CFU/mL during the challenge test. Histopathological analysis revealed severe damage in the hepatopancreas and gills of the diseased prawns, characterized by the enlargement of the hepatic tubule lumen and gaps between the tubules as well as clubbing and degeneration observed at the distal end of the gill filament. Eight virulence-related genes, namely aer, ompA, lip, tapA, hlyA, flgA, flgM, and flgN, were screened by PCR assay. In addition, virulence factor detection showed that the JDM1-1 isolate produced lipase, lecithinase, gelatinase, and hemolysin. Furthermore, the mRNA expression profiles of immune-related genes of M. rosenbergii following A. veronii infection, including ALF1, ALF2, Crustin, C-lectin, and Lysozyme, were assessed, and the results revealed a significant upregulation in the hepatopancreas and intestines at different hours post infection. This study demonstrates that A. veronii is a causative agent associated with massive die-offs of M. rosenbergii and contributes valuable insights into the pathogenesis and host defense mechanisms of A. veronii invasion.

Molecular Cloning, Characterization, and Expression of a Receptor for Activated Protein Kinase C1 (RACK1) Gene in Exopalaemon carinicauda Zoea Larvae under Aroclor 1254 Stress

The receptor for activated protein kinase C1 (RACK1) belongs to the typical WD repeat family, which is extremely conservative and important in multiple signal transduction pathways related to growth and development that coordinate the intracellular role of various life activities. As a novel protein with versatile functions, it was found in a variety of organisms. In a previous study, we identified the RACK1 sequence of white shrimp from transcriptome data. In this study, we employed specialized bioinformatics software to conduct an in-depth analysis of EcRACK1 and compare its amino acid sequence homology with other crustaceans. Furthermore, we investigated the expression patterns of RACK1 at different developmental stages and tissues, as well as at various time points after exposure to Aroclor 1245, aiming to elucidate its function and potential response towards Aroclor 1245 exposure. The length of EcRACK1 is 957 nucleotides, which encodes 318 amino acids. Moreover, there were seven typical WD repeats in EcRACK1, which have more than a 96% sequence identity with the RACK1 proteins of Penaeus. The results of tissue expression and spatiotemporal expression showed that it was significantly increased in the II and IV stages, but had a significant tissue specificity in the hepatopancreas, spermary, and muscle tissues of E. carinicauda, adult stage. Compared to the control, EcRACK1 was significantly induced in E. carinicauda zoea larvae exposed to Aroclor 1254 for 6, 10, 20, and 30 d (p < 0.05). These results suggested that EcRACK1 may play an important role in the larval development and environmental defense of E. carinicauda.

Correlative analysis of transcriptome and proteome in Penaeus vannamei reveals key signaling pathways are involved in IFN-like antiviral regulation mediated by interferon regulatory factor (PvIRF)

Interferon regulatory factors (IRFs) are crucial transcription factors that regulate interferon (IFN) induction in response to pathogen invasion. The regulatory mechanism of IRF has been well studied in vertebrates, but little has been known in arthropods. Therefore, in order to obtain new insights into the potential molecular mechanism of Peneaus vannamei IRF (PvIRF) in response to viral infection, comprehensive comparative analysis of the transcriptome and proteome profiles in shrimp infected with WSSV after knocking down PvIRF was conducted by using RNA sequencing (RNA-seq) and isobaric tags for relative and absolute quantification (iTRAQ). The sequence characterization, molecular functional evolution and 3D spatial structure of PvIRF were analyzed by using bioinformatics methods. PvIRF share the higher homology with different species in N-terminal end (containing DNA binding domain (DBD) including DNA sequence recognition sites and metal binding site) than that in C-terminal end. Within 4 IRF subfamilies of vertebrates, PvIRF had closer relationship with IRF1 subfamily. The DBD of PvIRF and C. gigas IRF1a were composed of α-helices and β-folds which was similar with the DBD structure of M. musculus IRF2. Interestingly, different from the five Tryptophan repeats highly homologous in the DBD of vertebrate IRF, the first and fifth tryptophans of PvIRF mutate to Phenylalanine and Leucine respectively, while the mutations were conserved among shrimp IRFs. RNAi knockdown of PvIRF gene by double-strand RNA could obviously promote the in vivo propagation of WSSV in shrimp and increase the mortality of WSSV-infected shrimp. It suggested that PvIRF was involved in inhibiting the replication of WSSV in shrimp. A total of 8787 transcripts and 2846 proteins were identified with significantly differential abundances in WSSV-infected shrimp after PvIRF knockdown, among which several immune-related members were identified and categorized into 10 groups according to their possible functions. Furthermore, the variation of expression profile from members of key signaling pathways involving JAK/STAT and Toll signaling pathway implied that they might participate IRF-mediated IFN-like regulation in shrimp. Correlative analyses indicated that 722 differentially expressed proteins (DEPs) shared the same expression profiles with their corresponding transcripts, including recognition-related proteins (CTLs and ITGs), chitin-binding proteins (peritrophin), and effectors (ALFs and SWD), while 401 DEPs with the opposite expression profiles across the two levels emphasized the critical role of post-transcriptional and post-translational modification. The results provide candidate signaling pathway including pivotal genes and proteins involved in the regulatory mechanism of interferon mediated by IRF on shrimp antiviral response. This is the first report in crustacean to explore the IFN-like antiviral regulation pathway mediated by IRF on the basis of transcriptome and proteomics correlative analysis, and will provide new ideas for further research on innate immune and defense mechanisms of crustacean.

A novel C-type lectin (SpccCTL) suppresses MCRV replication by binding viral protein and regulating antiviral peptides in Scylla paramamosain

Pattern recognition receptors (PRRs) play a crucial role in the recognition and activation of innate immune responses against invading microorganisms. This study characterizes a novel C-type lectin (CTL), SpccCTL. The cDNA sequence of SpccCTL has a full length of 1744 bp encoding a 338-amino acid protein. The predicted protein contains a signal peptide, a coiled-coil (CC) domain, and a CLECT domain. It shares more than 50 % similarity with a few CTLs with a CC domain in crustaceans. SpccCTL is highly expressed in gills and hemocytes and upregulated after MCRV challenge, suggesting that it may be involved in antiviral immunity. Recombinant SpccCTL (rSpccCTL) as well as two capsid proteins of MCRV (VP11 and VP12) were prepared. Pre-incubating MCRV virions with rSpccCTL significantly suppresses the proliferation of MCRV in mud crabs, compared with the control (treatment with GST protein), and the survival rate of mud crabs is also significantly decreased. Knockdown of SpccCTL significantly facilitates the proliferation of MCRV in mud crabs. These results reveal that SpccCTL plays an important role in antiviral immune response. GST pull-down assay result shows that rSpccCTL interacts specifically with VP11, but not to VP12. This result is further confirmed by a Co-IP assay. In addition, we found that silencing SpccCTL significantly inhibits the expression of four antimicrobial peptides (AMPs). Considering that these AMPs are members of anti-lipopolysaccharide factor family with potential antiviral activity, they are likely involved in immune defense against MCRV. Taken together, these findings clearly demonstrate that SpccCTL can recognize MCRV by binding viral capsid protein VP11 and regulate the expression of certain AMPs, suggesting that SpccCTL may function as a potential PRR playing an essential role in anti-MCRV immunity of mud crab. This study provides new insights into the antiviral immunity of crustaceans and the multifunctional characteristics of CTLs.

Quantification of proteomic profile changes in the hemolymph of crayfish during in vitro coagulation

Hemolymph is the circulatory fluid that fills the body cavity of crustaceans, analogous to blood in vertebrates. Hemolymph coagulation, similar to blood clotting in vertebrates, plays a crucial role in wound healing and innate immune responses. Despite extensive studies on the clotting process in crustaceans, no comparative quantitative analysis of the protein composition of non-clotted and clotted hemolymph in any decapod has been reported. In this study, we used label-free protein quantification with high-resolution mass spectrometry to identify the proteomic profile of hemolymph in crayfish and quantify significant changes in protein abundances between non-clotted and clotted hemolymph. Our analysis identified a total of two-hundred and nineteen proteins in both hemolymph groups. Furthermore, we discussed the potential functions of the top most high and low-abundant proteins in hemolymph proteomic profile. The quantity of most of the proteins was not significantly changed during coagulation between non-clotted and clotted hemolymph, which may indicate that clotting proteins are likely pre-synthesized, allowing for a swift coagulation response to injury. Four proteins still showed abundance differences (p < 0.05, fold change>2), including C-type lectin domain-containing proteins, Laminin A chain, Tropomyosin, and Reverse transcriptase domain-containing proteins. While the first three proteins were down-regulated, the last one was up-regulated. The down-regulation of structural and cytoskeletal proteins may affect the process of hemocyte degranulation needed for coagulation, while the up-regulation of an immune-related protein might be attributed to the phagocytosis ability of viable hemocytes during coagulation.

Immune responses and histopathological analyses of giant river prawn (Macrobrachium rosenbergii, De Man 1879) challenged with a sub-lethal dose of decapod iridescent virus 1 (DIV1) and chemical control investigation

Decapod iridescent virus 1 (DIV1) is a lethal virus that has a significant influence on the shrimp and prawn culture industries. The mechanism through which infected prawns respond to the DIV1 virus is currently unknown. Here, we examined in detail the clinical signs, histopathology, and humoral, cellular, and immune-related gene responses after a sub-lethal dose of DIV1 during the acute infection period of 0-120 hours post infection (hpi). Interestingly, at the end of the experiment, DIV1-infected prawns had black lesions on several external regions. The DIV1-infected prawns also exhibited few karyopyknotic nuclei in the gills and intestine tissues and exhibited increasing immunological responses, as revealed by significant increases in all examined parameters, including total hemocytes, phagocytosis, lysozyme, and overall bactericidal activity, from 6 to 48 hpi. In addition, between 72 and 120 hpi, all immune response activities of DIV1-infected prawn were impaired compared with those of normal prawns, indicating negative impacts on immunological parameters. A viral load analysis of various tissues by qPCR indicated that hemocytes were the dominant initial viral target tissues, followed by the gills and hepatopancreas. An expression analysis of crucial immune-related genes by qRT‒PCR revealed various expression patterns in response to DIV1 infection; in particular, fold changes in the relative expression of anti-lipopolysaccharide factors (ALFs), prophenoloxidase (proPO), lipopolysaccharide and β-1,3-glucan binding protein (LGBP) were observed. Additionally, five common chemicals, calcium hypochlorite [Ca(OCl)₂] at 16.25-130 ppm, hydrogen peroxide (H₂O₂) at 8.75-70 ppm, povidone iodine (PVP-I) at 3-24 ppm, benzalkonium chloride (BKC) at 20-160 ppm, and formalin at 25-200 ppm, had a significant effect on the killing of DIV1 particles in vitro within 24 h after exposure. These data will be helpful for determining the health status and immune defense mechanisms of giant river prawns during DIV1 infection periods. The study performed the first application of very common disinfectants, and the obtained information will be useful for implementing effective strategies to prevent and control DIV1 infection in both hatchery and grow-out ponds.

Review: Recent Applications of Gene Editing in Fish Species and Aquatic Medicine

Gene editing and gene silencing techniques have the potential to revolutionize our knowledge of biology and diseases of fish and other aquatic animals. By using such techniques, it is feasible to change the phenotype and modify cells, tissues and organs of animals in order to cure abnormalities and dysfunctions in the organisms. Gene editing is currently experimental in wide fields of aquaculture, including growth, controlled reproduction, sterility and disease resistance. Zink finger nucleases, TALENs and CRISPR/Cas9 targeted cleavage of the DNA induce favorable changes to site-specific locations. Moreover, gene silencing can be used to inhibit the translation of RNA, namely, to regulate gene expression. This methodology is widely used by researchers to investigate genes involved in different disorders. It is a promising tool in biotechnology and in medicine for investigating gene function and diseases. The production of food fish has increased markedly, making fish and seafood globally more popular. Consequently, the incidence of associated problems and disease outbreaks has also increased. A greater investment in new technologies is therefore needed to overcome such problems in this industry. To put it concisely, the modification of genomic DNA and gene silencing can comprehensively influence aquatic animal medicine in the future. On the ethical side, these precise genetic modifications make it more complicated to recognize genetically modified organisms in nature and can cause several side effects through created mutations. The aim of this review is to summarize the current state of applications of gene modifications and genome editing in fish medicine.

Comparative study of five anti-lipopolysaccharide factor genes in Litopenaeus vannamei

Anti-lipopolysaccharide factors (ALFs) are a family of common innate immune effectors in crustaceans, and they exhibit broad spectrum antimicrobial activity. In this study, we identified and characterized five novel ALF genes (designated as LvALF1-5) from the Pacific white shrimp (Litopenaeus vannamei) to investigate their potential immune functions. The amino acid sequence alignments showed that LvALFs contained two conserved cysteine residues, a hydrophobic N-terminal region, and the conserved signature sequence W(T/K)CPG(S)WT(A). They all shared high similarity with previously reported ALFs and were clearly novel members of the ALF family. The mRNA transcripts of LvALFs were most highly expressed in hemocytes and the hepatopancreas. After shrimp were stimulated with Vibrio parahaemolyticus or white spot syndrome virus, expression of the LvALFs was significantly induced in hemocytes and the hepatopancreas with various expression profiles. Recombinant proteins of LvALFs exhibited potent bacteriostatic activity in vitro. Together, these results suggest that LvALF1-5 participate in the immune response of Pacific white shrimp against invading pathogens.

Hallmarks of crustacean immune hemocytes at single-cell resolution

In invertebrates, hemocytes are the key factors in innate immunity. However, the types of invertebrate immune hemocytes are unclassified due to the limitation of morphological classification. To determine the immune hemocytes of crustaceans, the heterogeneity of hemocytes of shrimp Marsupenaeus japonicus and crayfish Procambarus clarkii, two representative crustacean species, were characterized in this study. The results of single-cell RNA sequencing indicated that shrimp and crayfish contained 11 and 12 types of hemocytes, respectively. Each of different types of hemocytes specifically expressed the potential marker genes. Based on the responses of shrimp and crayfish to the infection of white spot syndrome virus (WSSV) and the challenge of lipopolysaccharide (LPS), four types of immune hemocytes of crustaceans were classified, including semi-granular hemocytes involved in antimicrobial peptide production, granular hemocytes responsible for the production of antimicrobial peptides, hemocytes related to cell proliferation and hemocytes in immunity-activated state. Therefore, our study provided the first classification of crustacean hemocytes as well as of immune hemocytes of crustaceans at the single-cell resolution, which would be helpful to understand the innate immunity of invertebrates.

Protective Effects of Combined Utilization of Quercetin and Florfenicol on Acute Hepatopancreatic Necrosis Syndrome Infected Litopenaeus vannamei

This study aimed to determine the immunity, survival rate, and disease resistance of Litopenaeus vannamei treated using quercetin and florfenicol alone or in combination, after infection with acute hepatopancreatic necrosis syndrome caused by Vibrio parahaemolyticus (VPAHPND). After infection with VPAHPND, different types of feed were given to the shrimp for 5 days, including a control diet (drug-free), florfenicol only diet (15 mg/kg), quercetin only diet (400 mg/kg), a low-dose florfenicol/quercetin combined diet (200 mg/kg quercetin + 7.0 mg/kg florfenicol), a moderate-dose florfenicol/quercetin combined diet (400 mg/kg quercetin + 15 mg/kg florfenicol), and a high-dose florfenicol/quercetin combined diet (800 mg/kg quercetin + 30 mg/kg florfenicol). The cumulative mortality of shrimp was significantly reduced in the drug combination groups compared with either drug used alone (p < 0.05). The density of Vibrio was significantly lower and the immune parameters were significantly increased in the drug combination groups compared with either drug used alone (p < 0.05). Moreover, in the drug combination groups, the hepatopancreas tubules showed better integrity and structure compared with those when either drug was used alone. Therefore, compared with single drug treatment, the florfenicol and quercetin combination enhanced disease resistance, survival, and immune activity of VPAHPND-infected shrimp. When the combination treatment is used, the dosage of florfenicol can be reduced and a better therapeutic effect is obtained.

Taxifolin Inhibits WSSV Infection and Transmission by Increasing the Innate Immune Response in Litopenaeus vannamei

An outbreak of white spot syndrome virus (WSSV) can hit shrimp culture with a devastating blow, and there are no suitable measures to prevent infection with the virus. In this study, the activity of active molecules from Chinese herbs against WSSV was evaluated and screened. Taxifolin had the highest rate (84%) of inhibition of the WSSV infection. The viral infectivity and genome copy number were reduced by 41% when WSSV virion was pretreated with taxifolin prior to shrimp infection. A continuous exchange of taxifolin significantly reduced the mortality of shrimp infected with WSSV. Due to the WSSV virion infectivity being affected by taxifolin, the horizontal transmission of the virus was blocked with an inhibition rate of up to 30%, which would further reduce the cost of a viral outbreak. Additionally, the viral genome copy number was also reduced by up to 63% in shrimp preincubated in taxifolin for 8 h. There may be a connection to the enhancement of innate immunity in shrimp that resulted in a 15% reduction in mortality for taxifolin-fed shrimp after the WSSV challenge. After dietary supplementation with taxifolin, the resistance of larvae to WSSV was improved, indicating that taxifolin may be a potential immunostimulant for shrimp to prevent WSD. Therefore, the results indicate that taxifolin has application potential for blocking a WSSV outbreak and reducing the loss of shrimp culture.

Dietary glycerol monolaurate protects Cherax quadricarinatus against white spot syndrome virus infection

Glycerol monolaurate (GML), one of the medium-chain fatty acid esters, is often used as an emulsifier or preservative. Its biological functions include antibacterial and antiviral activities. In this study, we examined the effects of dietary GML on the resistance of the red claw crayfish to WSSV infection. Crayfish fed with 4 g/kg GML showed higher survival rate and lower WSSV copy numbers than the control after WSSV infection. A RT-qPCR analysis showed that GML supplementation enhanced the expression of immune-related genes, especially JAK and caspase. Our data indicate that GML affects the immune parameters of crayfish, including the total hemocyte counts and phenoloxidase, acid phosphatase, superoxide dismutase, lysozyme, and peroxidase activities. After treatment with GML, the apoptosis of hemocytes increased significantly in both WSSV-infected and uninfected crayfish. In summary, GML reduced the mortality of WSSV-infected crayfish, perhaps by modulating the innate immunity of the crayfish. Our study shows that GML can be used to induce the innate immunity and enhance the immune protection of the red claw crayfish against WSSV infection, either therapeutically or as a preventive measure.

Identification of three novel Spätzle genes in Eriocheir sinensis and their roles during white spot syndrome virus infection

Proteins of Spätzle family play an essential role in innate immunity in invertebrates by activating the Toll pathway to induce the expression of antimicrobial peptides. However, little is known about the function of Spätzle in in the immune response of the Chinese mitten crab. In the present study, three novel Spätzle genes (named as EsSpz1, EsSpz2, and EsSpz3) were identified from Eriocheir sinensis. The genome structure of EsSpz1 contains two exons and an intron. Three Spätzle proteins all contain a Pfam Spaetzle domain. In the evolution, EsSpz1-3 cluster with other Spätzle proteins from crustaceans. EsSpz1-3 were widely distributed in multiple immune tissues. The expression levels of EsSpz1-3 in the intestine were remarkably upregulated after white spot syndrome virus (WSSV) challenge. The knockdown of EsSpz1-3 remarkably decreased the expressions of crustins and anti-lipopolysaccharide factors during WSSV infection. Moreover, EsSpz1-3 silencing remarkably increased the expression of WSSV envelope protein VP28. These findings suggest that new-found EsSpz1-3 in E. sinensis could promote the synthesis of antimicrobial peptides and inhibit the expression of VP28 during WSSV infection. Our study indicates that EsSpz1-3 in E. sinensis may participate in the innate immune defenses against WSSV by inducing the expression of antimicrobial peptides. This study provides new knowledge for the function of Spätzle in the antiviral immune defense in crustacean.

Immune Response to Natural and Experimental Infection of Panulirus argus Virus 1 (PaV1) in Juveniles of Caribbean Spiny Lobster

Simple Summary

Experimental immunological challenges are widely used to corroborate the success of breeding programs for lines resistant to specific pathogens, to test the efficiency of new vaccines, and to improve immunity of cultured animals. The validation of experimental infection protocols is complex because it requires comparison with naturally infected organisms at different stages of the infection. The present study compares the immune response of lobsters under a natural process of viral infection (PaV1), versus the defense response of experimentally infected organisms. Innate immunity for infected lobsters was measured through cellular and plasmatic components. The results indicate that the immune response of organisms naturally or experimentally infected by PaV1 was similar, and provides the bases to corroborate that the immunological challenge was not exacerbated. Appropriate infection protocols can be useful for research aimed at increasing resistance to infectious diseases and reducing the use of antibiotics in aquaculture.

Abstract

Experimental infections have been used to better comprehend the immune system of organisms, and to probe for additives that generate greater resistance and help reduce antibiotic use in aquaculture. We compared the immune response of juveniles of the Caribbean spiny lobster, Panulirus argus, infected naturally with Panulirus argus virus 1 (PaV1) versus organisms infected experimentally, to determine the analogy between both infectious processes. The immunological response was measured by hemagglutination activity, hemocyte count, and total phenoloxidase activity in plasma and hemocytes in 211 individuals that were either naturally infected (110), or had been injected with viral inoculum and followed for six months (101). The samples were classified into the following four groups according to the severity of the infection: 0, uninfected; 1, lightly; 2, moderately; and 3, severely infected), which was determined on the basis of PCR and histological criteria. A permutational MANOVA showed that both the origin (natural and experimental), and the severity of the infection contributed significantly to explain the variation in the immune response of lobsters. The lack of significance of the interaction term indicated that the immunological response changed with the severity of the infection in a similar way, regardless of its origin. The results of the present study suggest that the experimental viral infection of PaV1 produces a defense response similar to the natural pathways of contagion, and provides the bases to validate an immunological challenge protocol for the first time in crustaceans. The discussion includes the perspective of the conceptual models of immune response within an ecological context.

Anti-lipopolysaccharide factors regulated by Stat, Dorsal, and Relish are involved in anti-WSSV innate immune defense in Macrobrachium nipponense

Anti-lipopolysaccharide factors (ALF) is an important antimicrobial peptide and critical effector molecule with a broad spectrum of antimicrobial activities in crustaceans. In addition to the previously reported five ALFs (MnALF1-5), another three ALFs [MnALF1, which is different from MnALF1 (ALF02818) that has been reported; MnALF6; and MnALF7] and an isoform of MnALF4 (MnALF4-isoform2) were newly identified from Macrobrachium nipponense in this study. MnALF6 has 134 amino acids and one single nucleotide polymorphism (SNP) in MnALF6 resulted in the change of 107th amino acid from E to D. Intron 1 retention produced longer transcript of MnALF6. The full length of MnALF7 has 691 bp with a 363 bp ORF encoding 120 amino acid protein. Three SNPs in MnALF2 resulted in the conversion of amino acids at positions 70, 73, and 91 from T⁷⁰I⁷³P⁹¹ to K⁷⁰L⁷³S⁹¹. The deletion of 13 bp in MnALF4 resulted in early termination of ORF, resulting in MnALF4-isoform2 with only 98 amino acids. The gDNAs of MnALF1, MnALF2, MnALF5, and MnALF6 contain three exons and two introns, while those of MnALF3 and MnALF7 contain three exons, one known intron, and one unknown intron. The MnALF1-7 in M. nipponense were widely distributed in multiple tissues. After white spot syndrome virus (WSSV) stimulation, the expression levels of MnALF1-7 changed. Knockdown of MnALF1-7 could evidently increase the expression of the envelope protein VP28 and the copy number of WSSV during viral infection. Further studies found that silencing of three transcription factors (Stat, Dorsal, and Relish) in M. nipponense significantly inhibit the synthesis of MnALF1-7 during the process of WSSV challenge. This study adds to the knowledge about the roles of ALFs in the innate immune responses to WSSV infection in M. nipponense.

Natural ingredient paeoniflorin could be a lead compound against white spot syndrome virus infection in Litopenaeus vannamei

White spot syndrome virus (WSSV) is an important pathogen causing high mortality in the shrimp industry in aquaculture, yet there is no treatment available to date. In order to find a treatment against WSSV infection, this study examined the anti-WSSV activity of eight natural compounds using shrimp larvae as a model. Among the eight compounds, paeoniflorin showed the most obvious anti-WSSV effect, with a maximum protection efficiency of WSSV-infected shrimp >60% at 100 μM. Furthermore, pretreatment and post-treatment experiments revealed that paeoniflorin could prevent and treat WSSV infection in shrimp. The antiviral activity of paeoniflorin in aquaculture water decreased rapidly with time, and the results showed that the stable anti-WSSV activity of paeoniflorin could only remain in water for 1 day. Thus, the dosing pattern of continuous medication changes was evaluated. Obviously, in the model of continuous change of paeoniflorin, WSSV copy numbers in the virus-treated shrimp group still progressively increased, while the virus content in WSSV_paeoniflorin -treated group continued to decrease. Interestingly, paeoniflorin inhibited horizontal transmission of WSSV to a certain extent. Notably, paeoniflorin significantly increased the expression of antimicrobial peptides of shrimp to resist WSSV. In conclusion, paeoniflorin has the potential to protect shrimp against WSSV.

Anti-lipopolysaccharide factor D from kuruma shrimp exhibits antiviral activity

Anti-lipopolysaccharide factors (ALFs) exhibit a potent antimicrobial activity against a broad range of bacteria, filamentous fungi, and viruses. In previous reports, seven groups of ALFs (groups A-G) were identified in penaeid shrimp. Among them, group D showed negative net charges and weak antimicrobial activity. Whether this group has antiviral function is not clear. In this study, the ALF sequences of penaeid shrimp were analyzed, and eight groups of ALF family (groups A-H) were identified. The four ALFs including MjALF-C2, MjALF-D1, MjALF-D2, and MjALF-E2 from kuruma shrimp Marsupenaeus japonicus were expressed recombinantly in Escherichia coli, and the antiviral activity was screened via injection of purified recombinant ALFs into shrimp following white spot syndrome virus (WSSV) infection. Results showed that the expression of Vp28 (WSSV envelope protein) decreased significantly in the MjALF-D2-injected shrimp only. Therefore, MjALF-D2 was chosen for further study. Expression pattern analysis showed that MjAlf-D2 was upregulated in shrimp challenged by WSSV. The WSSV replication was detected in RNA, genomic DNA, and protein levels using VP28 and Ie1 (immediate-early gene of WSSV) as indicators in MjALF-D2-injected shrimp following WSSV infection. Results showed that WSSV replication was significantly inhibited compared with that in the rTRX- or PBS-injected control groups. After knockdown of MjAlf-D2 in shrimp by RNA interference, the WSSV replication increased significantly in the shrimp. All these results suggested that MjALF-D2 has an antiviral function in shrimp immunity, and the recombinant ALF-D2 has a potential application for viral disease control in shrimp aquaculture.

Integrated Analysis of mRNA-Seq and MiRNA-Seq Reveals the Molecular Mechanism of the Intestinal Immune Response in Marsupenaeus japonicus Under Decapod Iridescent Virus 1 Infection

The intestine is not only an important digestive organ but also an important immune organ for shrimp; it plays a key role in maintaining homeostasis. Decapod iridescent virus 1 (DIV1) is a new type of shrimp-lethal virus that has received extensive attention in recent years. To date, most studies of the shrimp intestinal immune response under viral infections have relied on single omics analyses; there is a lack of systematic multi-omics research. In the current study, intestinal mRNA-seq and microRNA (miRNA)-seq analyses of Marsupenaeus japonicus under DIV1 infection were performed. A total of 1,976 differentially expressed genes (DEGs) and 32 differentially expressed miRNAs (DEMs) were identified. Among them, 21 DEMs were negatively correlated with 194 DEGs from a total of 223 correlations. Functional annotation analysis revealed that M. japonicus can regulate glycosaminoglycan biosynthesis (chondroitin sulfate, dermatan sulfate, and keratan sulfate), vitamin metabolism (retinol metabolism and ascorbate and aldarate metabolism), immune pathway activation (Toll and IMD signaling pathways, Wnt signaling pathway, IL-17 signaling pathway, and Hippo signaling pathway), immunity enzyme activity promotion (triose-phosphate isomerase), antimicrobial peptide (AMP) expression, reactive oxygen species (ROS) production, and cell apoptosis through miRNAs to participate in the host’s antiviral immune response, while DIV1 can influence Warburg effect-related pathways (pyruvate metabolism, glycolysis/gluconeogenesis, and citrate cycle), glycosphingolipid biosynthesis-related pathways (glycosphingolipid biosynthesis—globo and isoglobo series and glycosphingolipid biosynthesis—lacto and neolacto series), and the tight junction and adhesion junction of the intestinal mucosal epithelium through the host’s miRNAs and mRNA to promote its own invasion and replication. These results indicate that intestinal miRNAs play important roles in the shrimp immune response against DIV1 infection. This study provides a basis for further study of the shrimp intestinal antiviral immune response and for the formulation of effective new strategies for the prevention and treatment of DIV1 infection.

The potential of antimicrobial peptides isolated from freshwater crayfish species in new drug development: A review

The much-publicised increased resistance of pathogenic bacteria to conventional antibiotics has focused research effort on the characterization of new antimicrobial drugs. In this context, antimicrobial peptides (AMPs) extracted from animals are considered a promising alternative to conventional antibiotics. In recent years, freshwater crayfish species have emerged as an important source of bioactive compounds. In fact, these invertebrates rely on an innate immune system based on cellular responses and on the production of important effectors in the haemolymph, such as AMPs, which are produced and stored in granules in haemocytes and released after stimulation. These effectors are active against both Gram-positive and Gram-negative bacteria. In this review, we summarise the recent progress on AMPs isolated from the several species of freshwater crayfish and their prospects for future pharmaceutical applications to combat infectious agents.

Antiviral activity of esculin against white spot syndrome virus: A new starting point for prevention and control of white spot disease outbreaks in shrimp seedling culture

White spot syndrome virus (WSSV) is a pathogenic and threatening virus in shrimp culture for which there is no effective control strategy. Finding antiviral lead compounds for the development of anti-WSSV drugs is urgent and necessary; in this study, esculin from 12 monomeric compounds exhibited an excellent anti-WSSV activity. The results showed that esculin increased the survival rate of WSSV-infected shrimps by 59% and reduced the virus copy number in vivo over 90% at 100 μM. In the pre-treatment and post-treatment experiments, esculin could prevent and treat WSSV infection. Compared with the control group, the virus copy number decreased by 30% after 6 h of esculin pre-incubation with WSSV particles and inhibited horizontal transmission of WSSV to a certain extent. Considering that the antiviral activity of esculin was stable in the aquacultural water for 2 days, we evaluated the dosing pattern of continuous medication changes. Obviously, the survival rate of WSSV-infected shrimps was 0% at 108 h when no esculin exchange was made, while at 120 h the survival rate was over 40% at continuous medicine changes. In addition, esculin significantly increased the expression of antimicrobial peptides and thus improved the ability of shrimp to resist WSSV. Overall, our findings suggest that esculin has the potential to be developed into an anti-WSSV medicine.

Effects of dietary quercetin on the innate immune response and resistance to white spot syndrome virus in Procambarusclarkii

In recent years, the use of natural products with immune-stimulating and antimicrobial properties has attracted increasing attention in aquaculture researches. In our study, the effect of diet supplemented with quercetin, a flavonoid commonly found in some types of plants substance on the innate immune response and disease resistance in crayfish (Procambarus clarkii) against white spot syndrome virus (WSSV) is reported. It was found that dietary 40 mg/kg quercetin significantly reduced the mortality of crayfish and WSSV copy number after WSSV challenge. Dietary quercetin increased catalase (CAT), and lysozyme (LZM) activity in crayfish. Dietary quercetin increased the expression of NF-κB, anti-lipopolysaccharide factor (ALF) and toll-like receptor (TLR) genes in crayfish. The apoptosis rate of hemocyte was increased by quercetin supplement in crayfish. Our results suggest that dietary quercetin may affect the innate immunity of crayfish and protect crayfish from WSSV infection.

A crayfish ALF inhibits the proliferation of microbiota by binding to RPS4 and MscL of E. coli

Antimicrobial peptides (AMPs), most of which are small proteins, are necessary for innate immunity against pathogens. Anti-lipopolysaccharide factor (ALF) with a conserved lipopolysaccharide binding domain (LBD) can bind to lipopolysaccharide (LPS) and neutralize LPS activity. The antibacterial mechanism of ALF, especially its role in bacteria, needs to be further investigated. In this study, the antibacterial role of an anti-lipopolysaccharide factor (PcALF5) derived from Procambarus clarkii was analyzed. PcALF5 could inhibit the replication of the microbiota in vitro and enhance the bacterial clearance ability in crayfish in vivo. Far-western blot assay results indicated that PcALF5 bound to two proteins of E. coli (approximately 25 kDa and 15 kDa). Mass spectrometry (MS), far-western blot assay, and pull-down results showed that 30S ribosomal protein S4 (RPS4, 25 kD) interacted with PcALF5. Further studies revealed that another E. coli protein binding to PcALF5 could be the large mechanosensitive channel (MscL), which is reported to participate in the transport of peptides and antibiotics. Additional assays showed that PcALF5 inhibited protein synthesis and promoted the transcription of ribosomal component genes in E. coli. Overall, these results indicate that PcALF5 could transfer into E. coli by binding to MscL and inhibit protein synthesis by interacting with RPS4. This study reveals the mechanism underlying ALF involvement in the antibacterial immune response and provides a new reference for the research on antibacterial drugs.

Hepatopancreatic transcriptome analysis and humoral immune factor assays in red claw crayfish (Cherax quadricarinatus) provide insight into innate immunomodulation under Vibrio parahaemolyticus infection

Red claw crayfish (Cherax quadricarinatus) is an economically and nutritionally important specie. We aimed to assess the immunostimulatory response to C. quadricarinatus infection with Vibrio parahaemolyticus. After determining the LD50, we infected C. quadricarinatus and examined the differential expression profiles of hepatopancreas transcriptional genes, and observed the temporal changes of hepatopancreas pathological sections and serum immunoenzymatic activities at different time points to reveal the infection mechanism of V. parahaemolyticus and the immune detoxification mechanism of the organism. The results showed that V. parahaemolyticus infection with C. quadricarinatus caused hepatopancreas injury and the immune enzyme activity of the organism changed with time delay. Transcriptome analysis of 47,338 single genes obtained by RNA sequencing and de nove transcriptome assembly identified a total of 3678 differentially expressed genes (P < 0.05) in the expression profiles of susceptible and normal animals for comparative analysis, and 2516 differentially expressed genes (P < 0.05) in the expression profiles of asymptomatic (infection-resistant) and normal animals. GO and KEGG and analyses revealed immune-related pathways under V. parahaemolyticus infection, including Vibrio cholerae infection, phagosome, lysozyme, oxidative phosphorylation, antigen processing and presentation, apoptosis, and Toll-like receptor signaling, as well as significant differences in the expression patterns of related immune genes at different times (P < 0.05). These new experimental results reveal the molecular response of the hepatopancreas to V. parahaemolyticus infection and the corresponding adaptive mechanisms, thus further revealing the pathogenesis due to bacterial infection in the aquatic environment, and providing a reference for further understanding of microbial-host interactions in aquatic systems.

Single-cell RNA-seq analysis reveals penaeid shrimp hemocyte subpopulations and cell differentiation process

Crustacean aquaculture is expected to be a major source of fishery commodities in the near future. Hemocytes are key players of the immune system in shrimps; however, their classification, maturation, and differentiation are still under debate. To date, only discrete and inconsistent information on the classification of shrimp hemocytes has been reported, showing that the morphological characteristics are not sufficient to resolve their actual roles. Our present study using single-cell RNA sequencing revealed six types of hemocytes of Marsupenaeus japonicus based on their transcriptional profiles. We identified markers of each subpopulation and predicted the differentiation pathways involved in their maturation. We also predicted cell growth factors that might play crucial roles in hemocyte differentiation. Different immune roles among these subpopulations were suggested from the analysis of differentially expressed immune-related genes. These results provide a unified classification of shrimp hemocytes, which improves the understanding of its immune system.

Recent insights into anti-WSSV immunity in crayfish

White spot syndrome virus (WSSV) is currently the most severely viral pathogen for farmed crustaceans such as shrimp and crayfish, which has been causing huge economic losses for crustaceans farming worldwide every year. Unfortunately, study on the molecular mechanisms of WSSV has been restricted by the lack of crustacean cell lines for WSSV propagation as well as the incompletely annotated genomes for host species, resulting in limited elucidation for WSSV pathogenesis at present. In addition to the findings of anti-WSSV response in shrimp, some of novel cellular events involved in WSSV infection have been recently revealed in crayfish, including endocytosis and intracellular transport of WSSV, innate immune pathways in response to WSSV infection, and regulation of viral gene expression by host genes. Despite these advances, many fundamental gaps in WSSV pathogenesis are still remaining, for example, how WSSV genome enters into nucleus and how the progeny virions are fully assembled in the host cell nucleus. In this review, recent findings in WSSV infection mechanism and the antiviral immunity against WSSV in crayfish are summarized and discussed, which may provide us a better understanding of the WSSV pathogenesis as well as new ideas for the target design of antiviral drugs against WSSV in crustaceans farming.

Molecular and Functional Characterization of an Anti-lipopolysaccharide Factor Mm-ALF from Speckled Shrimp Metapenaeus monoceros

Anti-lipopolysaccharide factors (ALFs) are antimicrobial peptides of approximately 100 amino acid residues with a broad spectrum of antimicrobial activity. It is an amphipathic peptide with an N-terminal hydrophobic region and a lipopolysaccharide binding domain (LBD). In the present study, we report an isoform of the anti-lipopolysaccharide factor (Mm-ALF) from the speckled shrimp, Metapenaeus monoceros. A 359 bp cDNA encoded 119 amino acids, and the sequence showed 99.16% similarity to ALF from the shrimp Fenneropenaeus indicus. The mature peptide of 94 amino acids has a net charge of +8, molecular weight 10.62 kDa, and pI 10.11. The mature peptide Mm-ALF was recombinantly expressed in E. coli Rosetta-gami cells, and the peptide was isolated and purified. The rMm-ALF exhibited notable antibacterial activity against Gram-positive (Staphylococcus aureus and Bacillus cereus) and Gram-negative (Escherichia coli, Edwardsiella tarda, Aeromonas hydrophila, Pseudomonas aeruginosa, Vibrio parahaemolyticus, Vibrio harveyi, Vibrio alginolyticus, Vibrio proteolyticus, Vibrio cholerae and Vibrio fluvialis) bacteria.

Identification of two carcinin isoforms (MnCarc1 and MnCarc2) and their function in the antimicrobial immunity of Macrobrachium nipponense

Carcinin, a member of the crustin family, plays important roles in crustacean innate immunity. In this study, we identified two carcinin isoforms (MnCarc1 and MnCarc2) produced by alternative splicing from Macrobrachium nipponense. The full length of MnCarc1 and MnCarc2 cDNA are 1554 and 1495 bp with 687 and 609 bp open reading frame-encoding proteins that contain 228 and 202 amino acids, respectively. The genome of carcinin has nine exons and eight introns. MnCarc1 transcript contains all nine exons, whereas MnCarc2 only contains eight exons and lacks exon 4. MnCarc1 and MnCarc2 proteins contain a signal peptide, cysteine-rich regions, and a whey acidic protein domain. The phylogenetic tree shows that MnCarc1 and MnCarc2 are not grouped with other crustins and carcinins. MnCarc1 and MnCarc2 form a subgroup. MnCarc1 and MnCarc2 are widely distributed in various tissues. The expression of MnCarc1 and MnCarc2 were evidently upregulated at multiple time points in hemocytes and the intestine of M. nipponense after white spot syndrome virus, Vibrio parahaemolyticus, and Staphylococcus aureus challenges. Further studies showed that knockdown of MnDorsal or MnStat transcription factor could remarkably inhibit the upregulated expression of MnCarc1 and MnCarc2 caused by viral or bacterial challenges. In addition, recombinant MnCarc1 and MnCarc2 proteins could bind to various bacteria and polysaccharides and inhibit the growth of S. aureus and V. parahaemolyticus in vitro. This study indicated that carcinins from M. nipponense were involved in prawns innate immunity.

The feeding of dietary Codonopsis pilosula polysaccharide enhances the immune responses, the expression of immune-related genes and the growth performance of red swamp crayfish (Procambarus clarkii)

Polysaccharides have many functions in aquatic animals and are widely used as immunopotentiators. However, despite the emergence of serious diseases, few studies have explored the effects of Codonopsis pilosula polysaccharide (CPP) on crustaceans. We studied the effects of CPP on the growth performance, nonspecific immunity, antioxidant activity and disease resistance of red swamp crayfish (Procambarus clarkii). Healthy crayfish (5.80 ± 0.1 g) were fed diets supplemented with 0% (control), 0.05%, 0.1%, 0.15%, 0.20%, and 0.30% CPP for 8 weeks. At the end of the 8-week feeding trial, the optimal final body weight (FBW), weight gain (WG), specific growth rate (SGR), and feed conversion ratio (FCR) were observed in the crayfish fed the diets with 0.15% and 0.20% CPP, followed by those fed the diet with 0.30% CPP and then those fed the diet with 0.10% CPP, whereas the values of these parameters were obtained with the control crayfish (P < 0.05). The crayfish fed the diets with 0.15% and 0.20% CPP exhibited a significantly higher total hemocyte count (THC) and significantly increased phenoloxidase (PO), lysozyme (LZM), hemocyte (Hc), acid phosphatase (ACP) and alkaline phosphatase (AKP) compared with those belonging to the other groups (P < 0.05). The crayfish fed the diets with 0.15% and 0.2% CPP exhibited significantly higher total superoxide dismutase (T-SOD) and glutathione peroxidase (GPx) activities, a significantly increased total antioxidant capacity (T-AOC) and a significantly lower malondialdehyde (MDA) content compared with the other groups (P < 0.05), which indicated that antioxidant capacity was significantly induced by the CPP-supplemented diets. Significantly upregulated expression of immune-related genes (anti-lipopolysaccharide factors (alf), peroxiredoxin (prx5), cathepsin B (ctsb), mitochondrial manganese superoxide dismutase (mtMnsod), cyclophilin A (cypa), glutathione peroxidase (gpx), Toll-like receptor 3 (tlr3), and heat shock protein 70 (hsp70)) was detected in the crayfish fed the diets supplemented with 0.15% and 0.20% CPP diet compared with the levels observed in the control crayfish. These results showed that dietary CPP supplementation greatly improved the growth, immunity and antioxidant capacities of crayfish, and according to the observed results, 0.15%-0.2% is the recommended optimal level of CPP dietary supplementation for crayfish.

Pathogenicity of Aeromonas hydrophila causing mass mortalities of Procambarus clarkia and its induced host immune response

Outbreaks of mass mortalities among cultured Procambarus clarkia occurred in a commercial hatchery during the spring of 2019 in Jiangsu province of China. Here, we exploit the pathogenicity and immune response of Aeromonas hydrophila (GPC1-2), which was isolated from diseased P. clarkia. Crayfish challenged showed similar pathological signs to the naturally diseased P. clarkia, lethal dose 50% (LD₅₀) of the strain GPC1-2 to P. clarkia was 3.8 × 10⁶ CFU/mL. Detection of virulence-associated genes by PCR indicated that the strain GPC1-2 carried hlyA, aerA, alt, ast, act, aha, ahp, ahpA, and ahpB. Histopathological analysis of hepatopancreas revealed that the hepatic tubule lumen and the gap between the hepatic tubules became larger, and the brush border disappeared in the P. clarkia infected by GPC1-2. Quantitive real-time PCR (qRT-PCR) was undertaken to measure mRNA expression levels for six immune-related genes in P. clarkia after A. hydrophila infection. The expression level of proPO, NOS, ALF1, TLR2, PX, and AST were detected in hemolymph, hepatopancreas, gill and intestine tissues, and clear transcriptional activation of these genes were observed in the infected individuals. These results revealed pathogenicity of A. hydrophila and its activation of host immune response, which will provide a scientific reference for the breeding and disease prevention in P. clarkia culture.

Environmental concentrations of sulfamethoxazole increase crayfish Pacifastacus leniusculus susceptibility to White Spot Syndrome Virus

Antibiotics used for humans and livestock are emerging as pollutants in aquatic environments. However, little is known about their effect on aquatic organisms, especially in crustaceans. In the present study, the freshwater crayfish Pacifastacus leniusculus was exposed during 21 days to environmental concentrations of sulfamethoxazole (SMX) (100 ng/L and 1 μg/L). Subsequently, the crayfish susceptibility to infection was evaluated by using White Spot Syndrome Virus (WSSV) challenge, a well-known crustacean pathogen. The median survival time of the infected crayfish exposed to 100 ng/L SMX was one day, whereas the control and the group exposed to 1 μg/L SMX survived for two and three days, respectively. In order to elucidate the effect of SMX upon the crayfish immune response, new sets of crayfish were exposed to the same SMX treatments to evaluate mRNA levels of immune-related genes which are expressed and present in hemocytes and intestine, and to perform total and differential hemocyte counts. These results show a significant down-regulation of the antimicrobial peptide (AMP) Crustin 3 in hemocytes from the 100 ng/L SMX group, as well as a significant up-regulation of the AMP Crustin 1 in intestines from the 1 μg/L SMX group. Semigranular and total hemocyte cell number were observed to be significantly lower after exposure to 100 ng/L SMX in comparison with the control group. The present study demonstrates that environmentally relevant SMX concentrations in the water at 100 ng/L led to an increased WSSV susceptibility, that may have been caused by a reduction of circulating hemocytes. Nevertheless, SMX concentrations of 1 μg/L could marginally and for a few days have an immunostimulatory effect.

Molecular cloning and functional analysis of three STAT isoforms in red swamp crayfish Procambarus clarkii

The Janus kinase (JAK)/signal transducers and activators of transcription (STAT) signaling pathway is associated with the innate immune system and plays crucial roles in the mediation of immune response to viral infections. In this study, three STAT isoform cDNAs were cloned from the red swamp crayfish Procambarus clarkii, and they were designated as PcSTATa, PcSTATb, and PcSTATc. PcSTATa and PcSTATb were generated through the alternative splicing of the last exon, and PcSTATc was produced by intron retention. PcSTATa, PcSTATb, and PcSTATc contained 2382, 2337, and 2274 bp open reading frames encoding proteins with 793, 778, and 757 amino acid residues, respectively. Domain prediction analysis revealed that three isoforms of PcSTATs contain a STAT interaction domain, a STAT all-alpha domain, a STAT DNA binding domain, and a Src-homology 2 domain. The mRNA transcripts of three PcSTAT isoforms were detected in all examined tissues of male and female crayfish. The expression levels of the three PcSTAT isoforms in the hemocytes, gills, and intestines significantly changed after the white spot syndrome virus (WSSV) challenge. PcSTAT silencing by dsRNA interference could positively regulate the expression levels of three anti-lipopolysaccharide factors (PcALF1, PcALF2, and PcALF6) and two crustins (PcCrus1 and PcCrus2) and negatively regulate the expression levels of three ALFs (PcALF3, PcALF4, and PcALF5) and two crustins (PcCrus3 and PcCrus4). These results suggest that all three PcSTAT isoforms are involved in the host defense against WSSV infection.

Molecular cloning and expression analysis of two type II crustin genes in the oriental river prawn, Macrobrachium nipponense

Innate immunity is the primary defense of crustaceans against pathogens. Crustins, as antimicrobial peptides, are important to crustacean innate immunity. In this study, two kinds of Gly-rich crustin genes were cloned from Macrobrachium nipponense and were referred to as Mn-Gly-Cru1 and Mn-Gly-Cru2. These crustins belong to type II crustins with typical type II crustin structures. The full-length cDNA of Mn-Gly-Cru1 is 677 bp and contains a 576 bp open reading frame (ORF) encoding 191 amino acids. The full-length cDNA of Mn-Gly-Cru2 is 727 bp, with 573 bp ORF encoding 190 amino acids. The constructed phylogenetic tree indicated that Mn-Gly-Cru1 and Mn-Gly-Cru2 belong to the type IIa subfamily. RT-PCR analysis showed that Mn-Gly-Cru1 and Mn-Gly-Cru2 are widely distributed in various tissues. qRT-PCR results indicated that Mn-Gly-Cru1 is mainly expressed in the gills, whereas Mn-Gly-Cru2 is expressed at the highest level in hemocytes. The transcripts of Mn-Gly-Cru1 and Mn-Gly-Cru2 respond to bacterial or white spot syndrome virus (WSSV) stimuli. After injection of 48 h dsMnRelish, the expression of MnRelish, Mn-Gly-Cru1, and Mn-Gly-Cru2 were all inhibited. After WSSV, Vibrio parahaemolyticus, or Staphylococcus aureus challenge, MnRelish, Mn-Gly-Cru1, and Mn-Gly-Cru2 were all upregulated. However, the expression levels of MnRelish, Mn-Gly-Cru1, and Mn-Gly-Cru2 at 6 h bacteria or 36 h WSSV challenge were downregulated in Relish-silenced prawns when compared with the control (bacteria or WSSV challenge only, bacteria or WSSV challenge plus dsGFP injection). Results suggest that Mn-Gly-Cru1 and Mn-Gly-Cru2 play essential roles in M. nipponense innate immunity against bacteria or WSSV, and the expression levels of both genes are regulated by Relish transcriptional factor.

Research progress in innate immunity of freshwater crustaceans

Invertebrates lack adaptive immunity and innate immunity plays important roles in combating foreign invasive pathogens. Freshwater crustaceans, which are invertebrates, depend completely on their innate immune system. In recent years, many immune-related molecules in freshwater crustaceans, as well as their functions, have been identified. Three main immune signaling pathways, namely, Toll, immune deficiency (IMD), and Janus kinase-signal transducer activator of transcription (JAK/STAT) pathways, were found in freshwater crustaceans. A series of pattern recognition receptors (PRRs), including Toll receptors, lectins, lipopolysaccharide and β-1,3-glucan binding protein, scavenger receptors, Down syndrome cell adhesion molecules, and thioester-containing proteins, were reported. Prophenoloxidase activation system and antimicrobial peptide synthesis are two important immune effector systems. These components are involved in the innate immunity of freshwater crustaceans, and they function in the innate immune defense against invading pathogens. This review mainly summarizes innate immune signaling pathways, PRRs, and effector molecules in freshwater crustaceans.

The Anti-lipopolysaccharide Factors in Crustaceans

Anti-lipopolysaccharide factors (ALFs) are a type of antimicrobial peptide (AMP) which show broad-spectrum antimicrobial activity against Gram-positive bacteria, Gram-negative bacteria, fungi and viruses. In this chapter, we review the discovery and classification of this kind of antimicrobial peptide in crustaceans. The structure and function, as well as the mechanism of antibacterial and antiviral activities of ALFs will be summarized and discussed. We will then describe the expression and regulation of various ALF genes in different crustacean species. Finally, the application prospects of ALFs in drug development and disease-resistant genetic breeding will be pointed out and discussed. The review will also discuss several key questions such as the systematic classification and expression regulation of the ALF genes, as well as the future application of ALFs and ALF-derived peptides.

RPS27, a sORF-Encoded Polypeptide, Functions Antivirally by Activating the NF-κB Pathway and Interacting With Viral Envelope Proteins in Shrimp

A small open reading frame (smORF) or short open reading frame (sORF) encodes a polypeptide of <100 amino acids in eukaryotes (50 amino acids in prokaryotes). Studies have shown that several sORF-encoded peptides (SEPs) have important physiological functions in different organisms. Many ribosomal proteins belonging to SEPs play important roles in several cellular processes, such as DNA damage repair and apoptosis. Several studies have implicated SEPs in response to infection and innate immunity, but the mechanisms have been unclear for most of them. In this study, we identified a sORF-encoded ribosomal protein S27 (RPS27) in Marsupenaeus japonicus. The expression of MjRPS27 was significantly upregulated in shrimp infected with white spot syndrome virus (WSSV). After knockdown of MjRPS27 by RNA interference, WSSV replication increased significantly. Conversely, after MjRPS27 overexpression, WSSV replication decreased in shrimp and the survival rate of the shrimp increased significantly. These results suggested that MjRPS27 inhibited viral replication. Further study showed that, after MjRPS27 knockdown, the mRNA expression level of MjDorsal, MjRelish, and antimicrobial peptides (AMPs) decreased, and the nuclear translocation of MjDorsal and MjRelish into the nucleus also decreased. These findings indicated that MjRPS27 might activate the NF-κB pathway and regulate the expression of AMPs in shrimp after WSSV challenge, thereby inhibiting viral replication. We also found that MjRPS27 interacted with WSSV's envelope proteins, including VP19, VP24, and VP28, suggesting that MjRPS27 may inhibit WSSV proliferation by preventing virion assembly in shrimp. This study was the first to elucidate the function of the ribosomal protein MjRPS27 in the antiviral immunity of shrimp.

Glycogen phosphorylase of shrimp (Litopenaeus vannamei): Structure, expression and anti-WSSV function

Glycogen phosphorylase (GP, EC 2.4.1.1) catalyze the rate-limiting step in glycogenolysis in animals, forming glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond. Therefore, GP plays a crucial role in carbohydrate metabolism. In the present study, the full-length cDNA sequence of GP (LvGP) was cloned from shrimp, Litopenaeus vannamei. The obtained 3242-bp LvGP cDNA sequence included a 5'-terminal untranslated region (UTR) of 48 bp, an open reading frame (ORF) of 2559 bp encoding a polypeptide of 852 amino acids (aa) and a 3'-UTR of 635 bp. The predicted LvGP protein sequence contained a typical phosphorylase domain (113-829 aa) and shared 72%-97% identities with GP from other species. Phylogenetic analysis revealed that LvGP showed the closest relationship with GP from Marsupenaeus japonicus. Tissue expression profiles showed that LvGP existed in most examined tissues, with the most predominant expression in the brain, followed by the muscles and stomach. LvGP transcripts in hepatopancreas and hemocytes were up regulated after the WSSV challenge. Furthermore, the role of LvGP in shrimp defending against WSSV infection was investigated by RNA interference (RNAi). Our findings showed that WSSV proliferation and shrimp accumulative mortality increased significantly after LvGP RNAi (P < 0.01). The glycogen, glucose, and pyruvate content decreased in GP RNAi shrimp after WSSV injection, however, the lactate and ATP concentration enhanced. Moreover, lectin and anti-lipopolysaccharide factor2 (ALF2) were induced in LvGP silencing shrimp after WSSV infection, whereas the expression levels of crustin, ALF1 and ALF3 decreased. Our results suggested that the LvGP might play a crucial role in shrimp defending against WSSV infection by regulating metabolism and affecting the anti-infectious gene expression.

The Two NF-κB Pathways Regulating Bacterial and WSSV Infection of Shrimp

The outbreak of diseases ordinarily results from the disruption of the balance and harmony between hosts and pathogens. Devoid of adaptive immunity, shrimp rely largely on the innate immune system to protect themselves from pathogenic infection. Two nuclear factor-κB (NF-κB) pathways, the Toll and immune deficiency (IMD) pathways, are generally regarded as the major regulators of the immune response in shrimp, which have been extensively studied over the years. Bacterial infection can be recognized by Toll and IMD pathways, which activate two NF-κB transcription factors, Dorsal and Relish, respectively, to eventually lead to boosting the expression of various antimicrobial peptides (AMPs). In response to white-spot-syndrome-virus (WSSV) infection, these two pathways appear to be subverted and hijacked to favor viral survival. In this review, the recent progress in elucidating microbial recognition, signal transduction, and effector regulation within both shrimp Toll and IMD pathways will be discussed. We will also highlight and discuss the similarities and differences between shrimps and their Drosophila or mammalian counterparts. Understanding the interplay between pathogens and shrimp NF-κB pathways may provide new opportunities for disease-prevention strategies in the future.

Identification and molecular characterization of a novel anti-lipopolysaccharide factor (ALF) from red swamp crayfish, Procambarus clarkii

Anti-lipopolysaccharide factors are a group of small proteins with broad spectrum antiviral property and antibacterial activity. Herein, we obtained the genomic sequence of the Procambarus clarkii anti-lipopolysaccharide factor (PcALF) gene by using polymerase chain reaction to investigate its expression pattern in various tissues and in the immune tissues (Hepatopancreas) following exposure to pathogens. The deduced protein of PcALF was conserved; it displayed the signal peptides and putative lipo-polysaccharide binding domain, particularly the two conserved cysteine amino acid residues at both ends of the domain. The recombinant protein of PcALF was successfully expressed in Escherichia coli and rabbit anti-PcALF polyclonal antibodies were prepared. The qRT-PCR analysis showed unequal distribution of PcALF transcript in the examined tissues, however the transcript level was greatest in hepatopancreas. The challenge with peptidoglycan (PGN), lipo-polysaccharide (LPS) and Poly I:C significantly enhanced expression level of PcALF in hepatopancreas when compared with the PBS control. RNA interference of PcALF affected the mRNA expression levels of immune-related genes. Taken together, our data suggested that PcALF is an inducible protein and could play a key biological role in the innate immune defense of P. clarkii.

Hematopoietic tissue of Macrobrachium rosenbergii plays dual roles as a source of hemocyte hematopoiesis and as a defensive mechanism against Macrobrachium rosenbergii nodavirus infection

White tail disease caused by Macrobrachium rosenbergii nodavirus (MrNV) infection takes place only in nauplii, not adults, of M. rosenbergii prawn. Hemocyte homeostasis and immune-related functions derived from the hematopoietic tissue (Hpt) in adult prawn are presumed to play roles in resisting viral infection. To elucidate the role of the Hpt cell response to MrNV, a comparative transcriptome analysis was performed with MrNV-infected prawn at various time intervals. The results showed that there were 462 unigenes that were differentially expressed between mock and infected samples. BlastX sequence analysis revealed that two proteins, crustacean hematopoietic factor (CHF) and cell growth-regulating zinc finger protein (Lyar), are involved in hemocyte hematopoiesis and are up-regulated during MrNV infection. In fact, genes involved in cell growth regulation and immunity were highly expressed at 6 h and decreased within 24 h post-infection. Localization studies in the Hpt tissue revealed the presence of anti-lipopolysaccharide factor (ALF) and CHF mRNAs in Hpt cells. Considering these findings, we concluded that resistance to MrNV infection in adult prawn is due to an increase in humoral immune factors and the acceleration of hemocyte homeostasis by the dual roles of the Hpt organ in M. rosenbergii.

Evaluation of combination effects of Astragalus polysaccharides and florfenicol against acute hepatopancreatic necrosis disease-causing strain of Vibrio parahaemolyticus in Litopenaeus vannamei

The effects of oral administration of Astragalus polysaccharides (APS) and florfenicol (FFC), singly or in combination, on the survival performance, disease resistance, and immunity of Litopenaeus vannamei were investigated. After challenge with an AHPND-causing strain of Vibrio parahaemolyticus (VP_AHPND), shrimp were immediately fed a drug-free diet, diets containing only APS (200 mg·kg^-1) or FFC (15 mg·kg^-1), or diets containing low-dose (7.5 mg·kg^-1 FFC + 100 mg·kg^-1 APS), medium-dose (15 mg·kg^-1 FFC + 200 mg·kg^-1 APS), and high-dose (30 mg·kg^-1 FFC+400 mg·kg^-1 APS) drug combinations for 5 days. The cumulative shrimp mortality over 5 days after injection of VP_AHPND in the APS + FFC combination groups was significantly lower than that in the APS or FFC alone groups (p < 0.05). Immune parameters, including the total hemocyte counts (THCs), hemocyanin (HEM) concentration, antibacterial activity, activity levels of lysozyme (LZM), and levels of acid phosphatase (ACP), alkaline phosphatase (AKP), and phenoloxidase (PO) in cell-free hemolymph, and the expression levels of the immune-related genes anti-lipopolysaccharide factor (ALF), cathepsin B (catB), crustin, lectin (Lec), lysozyme (LZM), and Toll-like receptor (TLR) in hemocytes and hepatopancreas were determined in the shrimp. The values for these immune parameters in the drug combination groups were higher than those in the APS or FFC group (p < 0.05). Finally, in the histological examinations, the histological structural alignment and integrity of the hepatopancreatic tubules in the drug combination groups was better than that in the APS and FFC groups. Under the experimental conditions, dietary APS and FFC had a synergistic effect on immunity and disease resistance among shrimp after VP_AHPND infection.

WSSV-host interaction: Host response and immune evasion

As invertebrates, shrimps rely on multiple innate defense reactions, including humoral immunity and cellular immunity to recognize and eliminate various invaders, such as viruses. White spot syndrome virus (WSSV) causes the most prevalent and devastating viral disease in penaeid shrimps, which are the most widely cultured species in the coastal waters worldwide. In the last couple of decades, studies about WSSV implicate a dual role of the immune system in protecting shrimps against the infection; these studies also explore on the pathogenesis of WSSV infection. Herein, we review our current knowledge of the innate immune responses of shrimps to WSSV, as well as the molecular mechanisms used by this virus to evade host immune responses or actively subvert them for its own benefit. Deciphering the interactions between WSSV and the shrimp host is paramount to understanding the mechanisms that regulate the balance between immune-mediated protection and pathogenesis during viral infection and to the development of a safe and effective WSSV defensive strategy.

RNAi screening identifies a new Toll from shrimp Litopenaeus vannamei that restricts WSSV infection through activating Dorsal to induce antimicrobial peptides

The function of Toll pathway defense against bacterial infection has been well established in shrimp, however how this pathway responds to viral infection is still largely unknown. In this study, we report the Toll4-Dorsal-AMPs cascade restricts the white spot syndrome virus (WSSV) infection of shrimp. A total of nine Tolls from Litopenaeus vannamei namely Toll1-9 are identified, and RNAi screening in vivo reveals the Toll4 is important for shrimp to oppose WSSV infection. Knockdown of Toll4 results in elevated viral loads and renders shrimp more susceptible to WSSV. Furthermore, Toll4 could be a one of upstream pattern recognition receptor (PRR) to detect WSSV, and thereby leading to nuclear translocation and phosphorylation of Dorsal, the known NF-κB transcription factor of the canonical Toll pathway. More importantly, silencing of Toll4 and Dorsal contributes to impaired expression of a specific set of antimicrobial peptides (AMPs) such as anti-LPS-factor (ALF) and lysozyme (LYZ) family, which exert potent anti-WSSV activity. Two AMPs of ALF1 and LYZ1 as representatives are demonstrated to have the ability to interact with several WSSV structural proteins to inhibit viral infection. Taken together, we therefore identify that the Toll4-Dorsal pathway mediates strong resistance to WSSV infection by inducing some specific AMPs.

The TLR pathway mediated antiviral immune response is well identified in mammals, yet, Toll pathway governing this protection in invertebrates remains unknown. In the present study, we uncover that a shrimp Toll4 from a total of nine Tolls in L. vannamei confers resistance to WSSV thought inducing the NF-κB transcription factor Dorsal to inspire the production of some antimicrobial peptides (AMPs) with antiviral activity. The anti-LPS-factor (ALF) and lysozyme (LYZ) family are identified as the Toll4-Dorsal pathway targeted genes with the ability to interact with viral structural proteins in response to WSSV infection. These results suggest that the Toll receptor induces the expression of AMPs with antiviral activity could be a general antiviral mechanism in invertebrates and Toll pathway established antiviral defense could be conserved during evolution.

Cloning and Characterization of Two Toll Receptors (PcToll5 and PcToll6) in Response to White Spot Syndrome Virus in the Red Swamp Crayfish Procambarus clarkii

Toll/Toll-like receptors are key components in the innate immune responses of invertebrates. In this study, we identified two novel Toll receptors (PcToll5 and PcToll6) from the red swamp crayfish Procambarus clarkii. The complete cDNA sequence of PcToll5 is 4247 bp, encoding a 1293 amino acid polypeptide. The full-length 4688 bp PcToll6 encodes a putative protein of 1195 amino acids. Quantitative RT-PCR analysis indicated that PcToll5 and PcToll6 were constitutively expressed in all tissues studied. The highest expression levels of PcToll5 and PcToll6 were found in the intestine and gills, respectively, and were significantly upregulated from 24 to 48 h during white spot syndrome virus (WSSV) challenge. siRNA-mediated RNA interference results showed that PcToll5 and PcToll6 might regulate the expression of anti-lipopolysaccharide factors (PcALF2 and PcALF3) in vivo. Overexpression of PcToll5 and PcToll6 in Drosophila Schneider 2 (S2) cells activated the transcription of Drosophila antimicrobial peptides, including drosomycin (Drs), metchnikowin (Mtk), and attacin A (AttA), and shrimp Penaeidin-4 (Pen4). These findings provide significant information that PcToll5 and PcToll6 may contribute to host immune defense against WSSV in P. clarkii.

White spot syndrome virus (WSSV) suppresses penaeidin expression in Marsupenaeus japonicus hemocytes

Penaeidins are a unique family of antimicrobial peptides specific to penaeid shrimp and have been reported mainly function as anti-bacterial and anti-fungal. In order to investigate whether penaeidins could also respond to virus or not, we examined the effect of WSSV on MjPen-II (penaeidin in kuruma shrimp, Marsupenaeus japonicus) expression. In the control group, MjPen-II transcript level can be detected in almost all test tissues but was expressed most strongly in hemocytes. After WSSV infection, MjPen-II transcript level was significantly downregulated in hemocytes. Moreover, the proportion of MjPen-II+ hemocytes was not significantly different between non-infected and WSSV-infected shrimp, but the number of MjPen-II+ highly expressing hemocytes decreased after infection. In addition, MjPen-II was observed in the cytoplasm of granule-containing hemocytes. These results suggest that WSSV suppresses MjPen-II expression in hemocytes.

First characterization of an anti-lipopolysaccharide factor (ALF) from hydrothermal vent shrimp: Insights into the immune function of deep-sea crustacean ALF

Anti-lipopolysaccharide factor (ALF) is a type of antimicrobial peptides (AMPs) with a vital role in antimicrobial defense. Although a large amount of ALFs have been identified from neritic and fresh water crustacean species, no functional investigation of ALFs from deep-sea animals have been documented. In the present study, we characterized the immune function of an ALF molecule (named RspALF1) from the shrimp Rimicaris sp. residing in the deep-sea hydrothermal vent in Desmos, Manus Basin. RspALF1 shares 51.5%-62.4% overall sequence identities with known shrimp ALFs and contains the conserved LPS binding domain (LBD). Both recombinant RspALF1 (rRspALF1) and the LBD-derived peptide (ALF1P1) bound to the cell wall components of Gram-negative and Gram-positive bacteria and killed a wide range of bacteria, especially those from deep-sea hydrothermal field, by damaging bacterial cellular structures. The bactericidal activities of rRspALF1 and ALF1P1 were optimal and stably maintained from 4 °C to 37 °C, which is comparable to the ambient temperature range of the habitat of Rimicaris sp. In addition to bacteria, rRspALF1 and ALF1P1 also exhibited anti-fungal activity. rRspALF1 and ALF1P1 exhibited high killing efficiencies, which, in terms of MIC values, were ranged between 0.25 μM and 4 μM for bacteria and 4 μM-8 μM for fungi. When introduced in vivo, both rRspALF1 and ALF1P1 effectively inhibited bacterial infection in shrimp and reduced the dissemination of bacterial and viral pathogens in fish. Together, these results provide the first insight into the biological property of deep-sea ALF and indicate that RspALF1 very likely plays a significant role in immune defense by functioning as a highly effective antimicrobial with a broad target range.

A Prokineticin (PK)-like cytokine from Chinese mitten crab Eriocheir sinensis promotes the production of hemocytes via reactive oxygen species

Astakine is a cytokine-like factor containing a prokineticin domain, which directly participates in hematopoiesis and blood cell differentiation. In the present study, a novel Astakine gene was identified from Chinese mitten crab Eriocheir sinensis (designated as EsAst). The full-length cDNA of EsAst was of 1163 bp, consisting of a 5' untranslated region (UTR) of 120 bp, a 3' UTR of 656 bp, and an open reading frame (ORF) of 387 bp encoding a polypeptide of 128 amino acids. There were a signal peptide and a prokineticin domain with nine conserved cysteine residues in the deduced amino acid sequence of EsAst. EsAst shared higher similarity with Astakines from Penaeus monodon and Pacifastacus leniusculus, and it was closely clustered with the Astakine from shrimp P. monodon in the phylogenetic tree. The EsAst mRNA transcript was higher expressed in hemocytes and hepatopancreas. The relative expression level of EsAst in hemocytes was continuously increased from 1.5 to 48 h after Vibro anguillarum challenge compared that in the untreated control group. After Pichia pastoris GS115 challenge, the relative expression level of EsAst in hemocytes was also up-regulated. After rEsAst injection, ROS levels in HPT cells were also increased at 12 and 24 h, and the total hemocyte counts were also significantly increased at 6, 9, 12, and 24 h post rEsAst injection. The interference of EsAst expression with dsRNA injection could delay the recovery of hemocytes production post A. hydrophila stimulation. When mitochondrial complexes I was knock down by dsRNA, ROS levels were decreased and THCs were also decreased. Recovery of hemocyte production inducing by A. hydrophila stimulation and rEsAst injection were delayed with dsEsbc1 injection. When ROS levels were increased after RNAi of Lon protease, THCs were also increased. The expression levels of five genes (EsJNK, EsSTAT, EsPI3K, EsAKT1, EsP70S6K) involved in SAPK-JNK and mTOR signaling pathways were up-regulated at 12 and 24 h in rEsAst group and EsLon dsRNA group compared with that in EGFP dsRNA group, and were similar to the trend of ROS levels. These results collectively suggested that EsAst should be a novel Astakine to promote the production of hemocytes in a ROS-dependent way in E. sinensis.

The cyclin-dependent kinase 2 (CDK2) mediates hematopoiesis through G1-to-S transition in Chinese mitten crab Eriocheir sinensis

Cyclin-dependent kinases (CDKs), a family of cell cycle-related serine/threonine kinases, participate in various biological processes, and play crucial roles in the innate immunity. In the present study, a CDK2 (designed as EsCDK2) with a serine/threonine protein kinase catalytic domain was identified from Chinese mitten crab (Eriocheir sinensis). The full-length cDNA sequence of EsCDK2 was of 2405 bp with an open reading frame (ORF) of 909 bp. EsCDK2 shared 66%-81% sequence similarities with previously identified CDK2s. It was clustered with the CDK2 from Penaeus monodon in the invertebrate branch of the phylogenetic tree. The mRNA transcripts of EsCDK2 were highly expressed in hematopoietic tissue (HPT) and gonad, while lower in hemocytes, heart, gills, and muscle. EsCDK2 protein distributed in both cytoplasm and nucleus of HPT cells. The expression of EsCDK2 mRNA in HPT was significantly up-regulated and peaked at 3 h post stimulations with Aeromonas hydrophila (2.31-fold, p < 0.05) and Lipopolysaccharide (LPS) (2.02-fold, p < 0.05). After exsanguination, the total hemocyte counts (THC) decreased significantly to 0.42 × 10⁷/ml (0.39-fold, p < 0.05) at 0.5 h, then returned to a normal level at 6 h, while the mRNA expression of EsCDK2 in HPT cells was up-regulated at the early phase from 0.5 h to 6 h. After injection of EsCDK2-dsRNA, the mRNA expression level of EsCDK2 in HPT and THC both decreased to 0.53-fold (p < 0.01) and 0.78-fold (p < 0.05) at 24 h, respectively, and the percentage of new-born hemocytes in HPT also decreased significantly from 37.7% to 16.3% (0.43-fold, p < 0.01). After knocking down of EsCDK2, THC decreased dramatically at 6 h (0.65-fold, p < 0.01) post exsanguination, while returned normal at 6 h in PBS group. After interference of EsCDK2 mRNA expression, the percentage of G0-G1 phase cells significantly increased to 85.01% (1.26-fold, p < 0.01), while S phase and G2-M phase cells significantly decreased to 7.92% (0.46-fold, p < 0.01) and 7.07% (0.43-fold, p < 0.01) respectively, indicating that the cell cycle of HPT cells arrested at G1 phase. These results collectively demonstrated that EsCDK2 participated in the regeneration of hemocytes or hematopoiesis by regulating the transition from G1 to S phase in the cell cycle, and involves in the innate immune responses of E. sinensis.