Characterization of OmpK, GAPDH and their fusion OmpK-GAPDH derived from Vibrio harveyi outer membrane proteins: their immunoprotective ability against vibriosis in large yellow croaker (Pseudosciaena crocea)

Polylactic-Co-glycolic Acid Polymer-Based Nano-Encapsulation Using Recombinant Maltoporin of Aeromonas hydrophila as Potential Vaccine Candidate

Aquaculture production has been incurring economic losses due to infectious diseases by opportunistic pathogens like Aeromonas hydrophila, a bacterial agent that commonly affects warm water aquacultured fish. Developing an effective vaccine with an appropriate delivery system can elicit an immune response that would be a useful disease management strategy through prevention. The most practical method of administration would be the oral delivery of vaccine developed through nano-biotechnology. In this study, the gene encoding an outer membrane protein, maltoporin, of A. hydrophila, was identified, sequenced, and studied using bioinformatics tools to examine its potential as a vaccine candidate. Using a double emulsion method, the molecule was cloned, over-expressed, and encapsulated in a biodegradable polymer polylactic-co-glycolic acid (PLGA). The immunogenicity of maltoporin was identified through in silico analysis and thus taken up for nanovaccine preparation. The encapsulation efficiency of maltoporin was 63%, with an in vitro release of 55% protein in 48 h. The particle size and morphology of the encapsulated protein exhibited properties that could induce stability and function as an effective carrier system to deliver the antigen to the site and trigger immune response. Results show promise that the PLGA-mediated delivery system could be a potential carrier in developing a fish vaccine via oral administration. They provide insight for developing nanovaccine, since sustained in vitro release and biocompatibility were observed. There is further scope to study the immune response and examine the protective immunity induced by the nanoparticle-encapsulated maltoporin by oral delivery to fish.

Functional study of Cygb in the immune response to Vibrio harveyi disease in yellow drum (Nibea albiflora)

Cytoglobin (Cygb) is a 21-kDa heme-protein that belongs to the globin superfamily and is expressed in vertebrate tissues. It can participate in the oxidative stress response in organisms through the porphyrin ring. Previous studies have shown that this protein, also known as YdCygb, has potential immune abilities in the infection of Vibrio harveyi in yellow drum (Nibea albiflora). In this study, we report the role of Cygb in the immune response of teleost fish for the first time. Quantitative RT-PCR analysis indicated that YdCygb was highly expressed in the liver and intestine of yellow drum, and its expression can be upregulated by pathogenic attack. The cellular distribution of YdCygb-EGFP proteins was observed in cell membrane, cytoplasm, and nucleus in the kidney cells of N. albiflora. Furthermore, a comparative transcriptome analysis between the YdCygb overexpression group and control vector group identified 28 differentially expressed genes (DEGs). The analysis showed that ANPEP, CLDN5, ORM1/2, SERPINC1 and HPN and ITGAM might play important regulatory roles to Cygb in fish. Notably, using GST-pull down technology, we identified 3-phosphoglyceraldehyde dehydrogenase and intermediate filament protein as direct interactors with YdCygb, playing a role against V. harveyi. The molecular and functional characterization of YdCygb provides better understanding of the genetic basis of disease resistance traits in yellow drum and sheds new light on the functioning of Cygb and its potential regulatory signaling pathway as well.

Oral vaccination of fish against vibriosis using spore-display technology

Oral vaccines are highly demanded by the aquaculture sector, to allow mass delivery of antigens without using the expensive and labor-intensive injectable vaccines. These later require individual handling of fish, provoking stress-related mortalities.

One possible strategy to create injection-free vaccine delivery vehicles is the use of bacterial spores, extremely resistant structures with wide biotechnological applications, including as probiotics, display systems, or adjuvants. Bacterial spores, in particular those of Bacillus subtilis, have been shown to behave as mucosal vaccine adjuvants in mice models. However, such technology has not been extensively explored against fish bacterial disease.

In this study, we used a laboratory strain of B. subtilis, for which a variety of genetic manipulation tools are available, to display at its spores surface either a Vibrio antigenic protein, OmpK, or the green fluorescence protein, GFP. When previously vaccinated by immersion with the OmpK- carrying spores, zebrafish survival upon a bacterial challenge with V. anguillarum and V. parahaemolyticus, increased up to 50 - 90% depending on the pathogen targeted. Further, we were able to detect anti-GFP-antibodies in the serum of European seabass juveniles fed diets containing the GFP-carrying spores and anti-V. anguillarum antibodies in the serum of European seabass juveniles fed the OmpK-carrying spores containing diet. More important, seabass survival was increased from 60 to 86% when previously orally vaccinated with in-feed OmpK- carrying spores. Our results indicate that B. subtilis spores can effectively be used as antigen-carriers for oral vaccine delivery in fish.

Effect of Ferredoxin Receptor FusA on the Virulence Mechanism of Pseudomonas plecoglossicida

Pseudomonas plecoglossicida is an aerobic Gram-negative bacterium, which is the pathogen of “Visceral white spot disease” in large yellow croaker. P. plecoglossicida is a temperature-dependent bacterial pathogen in fish, which not only reduces the yield of large yellow croaker but also causes continuous transmission of the disease, seriously endangering the healthy development of fisheries. In this study, a mutant strain of fusA was constructed using homologous recombination technology. The results showed that knockout of P. plecoglossicida fusA significantly affected the ability of growth, adhesion, and biofilm formation. Temperature, pH, H₂O₂, heavy metals, and the iron-chelating agent were used to treat the wild type of P. plecoglossicida; the results showed that the expression of fusA was significantly reduced at 4°C, 12°C, and 37°C. The expression of fusA was significantly increased at pH 4 and 5. Cu²⁺ has a significant inducing effect on the expression of fusA, but Pb²⁺ has no obvious effect; the expression of fusA was significantly upregulated under different concentrations of H₂O₂. The expression of the fusA gene was significantly upregulated in the 0.5~4-μmol/l iron-chelating agent. The expression level of the fusA gene was significantly upregulated after the logarithmic phase. It was suggested that fusA included in the TBDR family not only was involved in the transport of ferredoxin but also played important roles in the pathogenicity and environment adaptation of P. plecoglossicida.

Comparative immunogenicity of outer membrane protein K and whole-cell antigens of Vibrio parahaemolyticus for diagnosis

The immunogenicity of soluble outer membrane protein K (OmpK)- small ubiquitin-like modifier, OmpK inclusion bodies, formalin, and heat-killed Vibrio parahaemolyticus cells were prepared and studied in a mouse model. The results of whole-cell ELISA and Western blot (WB) revealed that the serum against soluble OmpK and OmpK inclusion bodies reacted only with homologous V. parahaemolyticus. Furthermore, recombinant OmpK proteins were not recognized by the serum against whole-cell V. parahaemolyticus antigens. Unexpectedly, the serum against formalin and heat-killed V. parahaemolyticus reacted broadly with homologous (an immunization strain) and heterologous (non-immunization strains) V. parahaemolyticus and Vibrio species. The WB results revealed that the serum against the two V. parahaemolyticus whole-cell antigens primarily reacted with proteins that were approximately 100, 70, 36, 28, and 22 kDa in the cell lysates from different Vibrio strains, rather than the recombinant OmpK. The 70 and 28 kDa proteins exhibited specificity to Vibrio species, while the 22 kDa protein was more specific to V. parahaemolyticus. This study showed the limitation of recombinant OmpK to prepare diagnostic antibodies and revealed several specific Omps of Vibrio sp. and V. parahaemolyticus that were promising in diagnosis and vaccine development.

Characterization and structure of glyceraldehyde-3-phosphate dehydrogenase type 1 from Escherichia coli

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key enzyme in the glycolytic pathway that catalyzes the conversion of D-glyceraldehyde 3-phosphate to 1,3-diphosphoglycerate. Here, the full-length GAPDH type 1 from Escherichia coli (EcGAPDH1) was cloned and overexpressed, and the protein was purified. Biochemical analyses found that the optimum reaction temperature and pH of EcGAPDH1 were 55°C and 10.0, respectively. The protein has a certain amount of thermostability. Crystals of EcGAPDH1 were obtained using the sitting-drop vapor-diffusion technique and X-ray diffraction data were collected to 1.88 Å resolution. Characterization of the crystals showed that they belonged to space group P41212, with unit-cell parameters a = b = 89.651, c = 341.007 Å, α = β = γ = 90°. The structure of EcGAPDH1 contains four subunits, each of which includes an N-terminal NAD+-binding domain and a C-terminal catalytic domain. Analysis of the NAD+-bound form showed some differences between the structures of EcGAPDH1 and human GAPDH. As EcGAPDH1 shares 100% identity with GAPDH from Shigella sonnei, its structure may help in finding a drug for the treatment of shigellosis.

Outer membrane protein A (OmpA) is a potential virulence factor of Vibrio alginolyticus strains isolated from diseased fish

Vibrio alginolyticus is one of the most serious causative agents of diseases in cultured marine fish and shellfish. However, the characteristics of virulence factors in pathogenic V. alginolyticus are poorly known. To gain insight into fish diseases caused by V. alginolyticus, we carried out two-dimensional gel electrophoresis (2-DE) combined with MALDI-TOF mass spectrometry to identify uniquely expressed proteins in the disease-causing V. alginolyticus. V. alginolyticus strains were isolated from marine environments and diseased fish obtained from southern Thailand. We identified seven unique proteins in the disease-causing V. alginolyticus strain. Among those, the outer membrane protein A (OmpA) had the strongest expression. Therefore, the function of this protein was further analysed. To investigate the role of OmpA protein, an in-frame deletion mutant of ompA was constructed using the homologous recombination method. Although the ompA mutant V. alginolyticus strain (ΔompA) grew normally, the mutant exhibited a significant defect in the swarming ability and the biofilm formation. Furthermore, Galleria mellonella larvae injected with the mutant bacteria had a significantly greater survival percentage than those injected with the wild-type strain, demonstrating that OmpA protein is required for the pathogenicity of V. alginolyticus. Together, this study suggests a potential target for vaccine development against pathogenic V. alginolyticus strain.

Protective Immunity against Vibrio harveyi in Grouper Induced by Single Vaccination with Poly (Lactide-co-glycolide) Microparticles Releasing Pleurocidin Peptide and Recombinant Glyceraldehyde-3-phosphate Dehydrogenase

The peptide adjuvant, pleurocidin (PLE), and the Vibrio harveyi antigen, recombinant glyceraldehyde-3-phosphate dehydrogenase (rGAPDH) protein, were encapsulated with poly (lactide-co-glycolide) (PLG) polymers in our previous study to produce PLG-encapsulated PLE plus rGAPDH microparticles (PLG-PLE/rGAPDH MPs) that sustained stable release of both PLE and rGAPDH as well as, after two-time vaccination with MPs, generated long-term protective immunity against V. harveyi in grouper. Stable controlled-release of PLE plus rGAPDH from PLG-PLE/rGAPDH MPs is an attractive feature for developing an effective single-dose vaccine. In the present study, therefore, we aim to evaluate whether single administration with PLG-PLE/rGAPDH MPs in grouper would result in protective immunity against V. harveyi. Peritoneal vaccination of grouper with one dose of PLG-PLE/rGAPDH MPs raised serum titers over a long 12-week period. Moreover, twelve weeks after vaccination, significant lymphocyte proliferation and maximum TNF-α production were found in grouper immunized with a single dose of PLG-PLE/rGAPDH MPs. More importantly, immune responses elicited by single vaccination with PLG-PLE/rGAPDH MPs protected 80% of fish against a lethal peritoneal challenge of the highly virulent V. harveyi (Vh MML-1). In conclusion, our data truly reveal the feasibility of the development of a single-dose vaccine against V. harveyi based on PLG-PLE/rGAPDH MPs.

Anti-Idiotype Vaccine Provides Protective Immunity Against Vibrio Harveyi in Grouper (Epinephelus Coioides)

Since anti-idiotype antibodies (anti-Id Abs) can display internal images similar to the epitopes of the original antigens, we aimed to produce an effective vaccine based on anti-Id Abs to protect grouper from Vibrio harveyi. Anti-Id IgG showing V. harveyi-like internal images was produced from rabbits immunized with the Id portion of grouper anti-V. harveyi antibodies and its Fab portion, anti-Id IgG (Fab), was then prepared to use as the anti-Id vaccine. The resulting anti-Id IgG (Fab) was intraperitoneally injected twice at a 21-day interval into grouper to evaluate its ability to induce effective anti-V. harveyi immunity and protection, in comparison with inactivated V. harveyi bacteria. We found that administration of grouper with anti-Id IgG (Fab) resulted in enhanced V. harveyi-specific serum titers, as well as lymphocyte proliferation. In addition, three weeks after boosting, 90% (18/20) of fish immunized with anti-Id IgG (Fab) survived at least 28 days after a lethal challenge of the heterologous, virulent strain of V. harveyi. The capability of this anti-Id IgG (Fab) to imitate the epitopes of V. harveyi antigens and effectively induce protective immunity would be advantageous for its application in developing an efficacious vaccine against V. harveyi for future farm use in fish.

Enhanced immune responses and effectiveness of refined outer membrane protein vaccines against Vibrio harveyi in orange-spotted grouper (Epinephelus coioides)

Vibriosis is a severe infection occurring in many commercially important marine fish species. In this study, vaccines containing Vibrio harveyi recombinant outer membrane protein K (rOmpK), outer membrane protein U (rOmpU) and rOmpK-OmpU fusion protein in addition to the metabolizable Montanide^TM ISA 763 A VG adjuvant were developed and evaluated in the orange-spotted grouper. The results indicate that recombinant V. harveyi protein-based vaccines resulted in a remarkably higher expression of IL-1β and IL-8 at 24 hr, and greater antibody production, as early as 2 weeks postimmunization. Notably, enhanced immune responses and significant protective efficacy against V. harveyi infections were observed in the fusion protein vaccine-injected fishes with relative per cent survival value of 81.8%. Additionally, the rOmpK-OmpU antisera presented a high bactericidal effect on not only V. harveyi, but also Vibrio parahaermolyticus and Vibrio alginolyticus. Our results demonstrated that the fusion protein rOmpK-OmpU was an effective vaccine candidate that exhibited potentially great versatility for controlling vibrio infections.

Identification of protective protein antigens for vaccination against Streptococcus dysgalactiae in cobia (Rachycentron canadum)

Streptococcus dysgalactiae is considered a causative agent of severe infection and economic loss for the cobia industry in Taiwan. In this study, protective antigens of this pathogenic bacterium were identified and screened in cobia (Rachycentron canadum). Outer surface proteins (OMPs) of this pathogen were extracted using mutanolysin digestion. Immunogenic targets were detected by western blot and then subjected to peptide sequencing using NanoLC-MS/MS. Two surface proteins, namely phosphoenolpyruvate protein phosphotransferase (PtsA) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), showed strong reactions with cobia antisera against S. dysgalactiae. Recombinant proteins were produced in Escherichia coli cells and their protective efficacies were investigated in cobia. Fish immunised with recombinant proteins, rPtsA + ISA (ISA 763 AVG) and rGAPDH + ISA, elicited higher levels of specific antibody responses against the recombinant proteins and had high levels of lysozyme activity. Notably, vaccinated fish were protected from lethal challenge with relative percentage of survival (RPS) values for rPtsA + ISA and rGAPDH + ISA groups being 91.67% and 83.33%, while 0% RPS value was found in both ISA injected and control groups. The results presented in the study demonstrate that the GAPDH and PtsA are promising vaccine candidates for preventing S. dysgalactiae disease in cobia.

Effectiveness of formalin-killed vaccines containing CpG oligodeoxynucleotide 1668 adjuvants against Vibrio harveyi in orange-spotted grouper

Vibrio harveyi is a major bacterial pathogen that causes serious vibriosis in cultured groupers, leading to massive deaths. In this study, we evaluated the immune responses and protective efficacy of vaccines containing V. harveyi formalin-killed cells (FKC) formulated with CpG ODN 1668-enriched plasmids (p30CpG and p60CpG) in the orange-spotted grouper. Results indicated that antibody titres were remarkably increased in vaccinated fish 2 weeks post-immunisation. Expression level of major histocompatibility complex (MHC) class II, CD 8, and toll-like receptor 9 was significantly upregulated in the spleen of fish immunised with CpG ODN 1668-adjuvanted vaccines, as recorded at 6 weeks after immunisation. Additionally, the FKC + p60CpG-vaccinated fish displayed greater mRNA levels of MHC I and tumor necrosis factor-alpha. Of note, the relative percent survival after V. harveyi challenge was significantly higher in FKC + p60CpG-vaccinated fish (96.2%) than in FKC + p30CpG-vaccinated (79.8%) and FKC-vaccinated fish (59.9%). These results demonstrate that the FKC + CpG ODN 1668 vaccines are promising candidates that could enhance both innate and adaptive immune responses, conferred remarkable protection, and CpG ODN 1668 is a potential adjuvant for vaccines against V. harveyi.

A formalin-inactivated vaccine provides good protection against Vibrio harveyi infection in orange-spotted grouper (Epinephelus coioides)

Vibrio harveyi is one of the most common threats to farmed grouper, so considerable efforts are in practice to control the pathogen. This study presents a highly effective vaccine against V. harveyi in the orange-spotted grouper with the help of a single intraperitoneal immunization. The vaccine candidate was in form of whole, formalin-inactivated V. harveyi cells combined with a metabolizable ISA763 AVG adjuvant. Our results indicated that the vaccine triggered a remarkably higher expression level of interleukin (IL)-1β, IL-6, IL-8, and IL-10 in the groupers' kidneys and spleens, as recorded after 1 and 3 days of immunization. Antibody titers were significantly elevated in the vaccinated fish. A pivotal observation was that the vaccine highly protected the grouper from a homologous V. harveyi strain challenge with relative percentage survival values of 100% and 91.7% at 6 and 12 weeks post-immunization, respectively. Vaccinated fish also demonstrated strong cross-protection against a heterologous bacterial isolate challenge. Therefore, the inactivated V. harveyi vaccine is a promising candidate that could stimulate good immune responses and confer remarkable protection in farmed groupers.

GapA, a potential vaccine candidate antigen against Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus)

Streptococcosis due to the bacterium Streptococcus agalactiae (S. agalactiae) has resulted in enormous economic losses in aquaculture worldwide, especially in the tilapia culture industry. Previously, there were limited vaccines that could be employed against streptococcosis in tilapia. This study aimed to develop a vaccine candidate using the glyceraldehyde-phosphate dehydrogenase protein (GapA) of S. agalactiae encoded by the gapA gene. Tilapia were intraperitoneally injected with PBS, PBS + Freund's adjuvant, PBS + Montanide's adjuvant, GapA + Freund's adjuvant, GapA + Montanide's adjuvant, killed S. agalactiae whole cells (WC)+Freund's adjuvant, or killed S. agalactiae whole cells (WC)+ Montanide's adjuvant. They were then challenged with S. agalactiae, and the relative percentage survival (RPS) was monitored 14 days after the challenge. The highest RPSs were observed in the WC groups, with 76.7% in WC + Freund's adjuvant and 74.4% in WC + Montanide's adjuvant groups; these were followed by the GapA groups, with 63.3% in GapA + Freund's adjuvant and 45.6% in GapA + Montanide's adjuvant groups. The RPS of the PBS group was 0%, and those of PBS + Freund's adjuvant and PBS + Montanide's adjuvant groups were 6.7% and 3.3%, respectively. Additionally, the IgM antibody responses elicited in GapA groups and WC groups were significantly higher than those in PBS groups. Furthermore, the expressions of cytokine (IL-1β and TNF-α) mRNAs in the GapA groups and WC groups were significantly higher than those in the PBS groups. Taken together, these results reveal that the GapA protein is a promising vaccine candidate that could be used to prevent streptococcosis in tilapia.

Construction of glutathione peroxidase (GPx) DNA vaccine and its protective efficiency on the orange-spotted grouper (Epinephelus coioides) challenged with Vibrio harveyi

The main aims of this study were to construct glutathione peroxidase (GPx) DNA vaccine of Vibrio harveyi ZJ0603 and to investigate its immune protective efficiency as a vaccine candidate on the orange-spotted grouper (Epinephelus coioides) treated with V. harveyi. Base on the cloning of ZJ0603 GPx gene, a DNA vaccine, named as pcDNA-GPx, was constructed by inserting GPx gene into pcDNA3.1 (+) plasmid. Orange-spotted groupers were immunized with the pcDNA-GPx plasmid by injection intramuscularly. The relative percent of survival (RPS) of fish vaccinated with the DNA vaccine against pathogenic V. harveyi infection was 77.5%. The expression of DNA vaccine was analyzed in the tissues of orange-spotted grouper by PCR and RT-PCR. The results indicated that pcDNA-GPx distributed and expressed in the head kidney, liver, spleen, gill and injected muscle at 7 and 28 days after vaccination. Significant specific antibody responses were also detected in the vaccinated orange-spotted groupers by indirect ELISA method. In a conclusion, DNA vaccine pcDNA-GPx showed an effective immune protection to the orange-spotted grouper treated with V. harveyi. The GPx can be used as a candidate DNA vaccine for the control of vibriosis.

Glyceradehyde-3-phosphate dehydrogenase as a suitable vaccine candidate for protection against bacterial and parasitic diseases

The enzyme glyceraldehyde-3-P-dehydrogenase (GAPDH) has been identified as having other properties in addition to its key role in glycolysis. The ability of GAPDH to bind to numerous extracellular matrices, modulation of host-immune responses, a role in virulence and surface location has prompted numerous investigators to postulate that GAPDH may be a good vaccine candidate for protection against numerous pathogens. Although immune responses against GAPDH have been described for many microorganisms, vaccines containing GAPDH have been successfully tested in few cases including those against the trematode-Schistosoma mansoni, the helminth-Enchinococcus multilocularis; the nematode filaria- Litomosoides sigmodontis; fish pathogens such as Aeromonas spp., Vibrio spp., Edwarsiella spp., and Streptococcus iniae; and environmental streptococci, namely, Streptococcus uberis and Streptococcus dysgalactiae. Before GAPDH-based vaccines are considered viable options for protection against numerous pathogens, we need to take into account the homology between the host and pathogen GAPDH proteins to prevent potential autoimmune reactions, thus protective GAPDH epitopes unique to the pathogen protein must be identified.

Phosphoglycerate kinase enhanced immunity of the whole cell of Streptococcus agalactiae in tilapia, Oreochromis niloticus

Streptococcus agalactiae is a Gram-positive bacterium and a severe aquaculture pathogen that can infect a wide range of warmwater fish species. The outer-surface proteins in bacterial pathogens play an important role in pathogenesis. We evaluated the immunogenicity of two of the identified surface proteins namely phosphoglycerate kinase (PGK) and ornithine carbamoyl-transferase (OCT). PGK and OCT were over-expressed and purified from Escherichia coli and used as the subunit vaccines in tilapia. Tilapia immunized with the S. agalactiae modified bacteria vaccine (whole cell preparations with recombinant PGK and OCT proteins) individually were tested for the efficacy. OCT and PGK combined with WC had a higher survival rate. A high-level protection and significant specific antibody responses against S. agalactiae challenge was observed upon the vaccinated tilapia with the purified PGK protein and S. agalactiae whole cells. The specific antibody titer against S. agalactiae antigen suggested that increased antibody titers were correlated with post-challenge survival rate. Il-1β expression profile was higher in PGK + WC-treated group. Tnf-α expression in the PGK + WC group was significantly increased. Taken together, our results suggested the combinations of recombinant protein and whole cell may elicit immune responses that reach greater protection than that of individual S. agalactiae components.

Cross-protective effect of a novel multi-antigen-chimeric vaccine against Streptococcus and Staphylococcus aureus infection in mice

Staphylococcal and streptococcal species are the most common pathogens that cause bovine mastitis. Induction of a broad-spectrum protective immunity against staphylococci and streptococci by combining multiple antigens into a single vaccine is highlighted. To develop a universal vaccine candidate, a GapC1-tIsdB-TRAP (GIT) construct was generated. The GIT contained the truncated GapC from Streptococcus dysgalactiae, and truncated IsdB and full-length TRAP from Staphylococcus aureus. The humoral and cellular immune responses elicited by GIT were evaluated in mice. Antibody levels against GIT displayed a consistent tendency with antibody levels against GapC, IsdB and TRAP. The level of IFN-γ was higher in the GIT group than in the IsdB group (P<0.05), and the level of IL-4 was higher in the GIT group than in the GapC or TRAP groups (P<0.05). The GIT group showed an improved protection against Streptococcus in comparison with GapC group. A significant difference in S. aureus challenge test was detected between the GIT group and the IsdB or TRAP groups (P<0.05) in per cent survival of mice, and a synergistic immunoprotection against S. aureus or S. dysgalactiae was produced in the GIT group. These results suggested that the GIT would be a promising common vaccine candidate against S. aureus and Streptococcus.

Pleurocidin Peptide Enhances Grouper Anti-Vibrio harveyi Immunity Elicited by Poly(lactide-co-glycolide)-Encapsulated Recombinant Glyceraldehyde-3-phosphate Dehydrogenase

Outer membrane proteins, such as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), are considered immunodominant antigens for eliciting protective immunity against Vibrio harveyi, the main etiological agent of vibriosis in fish. Cationic antimicrobial peptides (AMPs), such as pleurocidin (PLE), play important roles in activating and recruiting immune cells, thereby contributing to subsequent innate and adaptive immune responses. In the present study, we aimed to use PLE peptide as a potent adjuvant to improve the immunogenicity of V. harveyi recombinant GAPDH (rGAPDH). In order to prepare a controlled-release vaccine, PLE peptide and rGAPDH protein were simultaneously encapsulated into polymeric microparticles made from the biodegradable poly(lactide-co-glycolide) (PLG) polymer. The resulting PLG-encapsulated PLE plus rGAPDH (PLG-PLE/rGAPDH) microparticles, 3.21–6.27 μm in diameter, showed 72%–83% entrapment efficiency and durably released both PLE and rGAPDH for a long 30-day period. Following peritoneal immunization in grouper (Epinephelus coioides), PLG-PLE/rGAPDH microparticles resulted in significantly higher (p < 0.05, nested design) long-lasting GAPDH-specific immunity (serum titers and lymphocyte proliferation) than PLG-encapsulated rGAPDH (PLG-rGAPDH) microparticles. After an experimental challenge of V. harveyi, PLG-PLE/rGAPDH microparticles conferred a high survival rate (85%), which was significantly higher (p < 0.05, chi-square test) than that induced by PLG-rGAPDH microparticles (67%). In conclusion, PLE peptide exhibits an efficacious adjuvant effect to elicit not only improved immunity, but also enhanced protection against V. harveyi in grouper induced by rGAPDH protein encapsulated in PLG microparticles.

Visceral granulomas in farmed large yellow croaker, Larimichthys crocea (Richardson), caused by a bacterial pathogen, Pseudomonas plecoglossicida

An enzootic disease characterized by granulomas in internal organs occurred in cage-farmed large yellow croaker, Larimichthys crocea (Richardson), in April and November 2010, in Ningbo, Zhejiang Province. One bacterial strain, named XSDHY-P, was isolated from the diseased fish and identified by biochemical characterization, fatty acid methyl ester (FAME) analysis and multilocus sequence analysis (MLSA). According to the results obtained from the biochemical tests, FAME analysis and phylogenetic analysis derived from 16S ribosomal RNA, gyrB, oprF, oprI, oprL and rpoD gene sequencing, the bacterial isolate, XSDHY-P, was identified as Pseudomonas plecoglossicida. Moreover, lethal dose, 50% trials were carried out to demonstrate the virulence of XSDHY-P in large yellow croaker when administered at 2.13 9 105 colony-forming units per fish. Visceral granulomas were found in the experimentally infected fish as well as in the naturally infected fish, indicating that P. plecoglossicida is another bacterial pathogen that causes granulomatosis in L. crocea.

Evaluating the protective efficacy of antigen combinations against Photobacterium damselae ssp. piscicida infections in cobia, Rachycentron canadum L

Cobia, Rachycentron canadum L., is a very important aquatic fish that faces the risk of infection with the bacterial pathogen Photobacterium damselae ssp. piscicida, and there are few protective approaches available that use multiple antigens. In the present study, potent bivalent antigens from P. damselae ssp. piscicida showed more efficient protection than did single antigens used in isolation. In preparations of three antigens that included recombinant heat shock protein 60 (rHSP60), recombinant α-enolase (rENOLASE) and recombinant glyceraldehyde-3-phosphate dehydrogenase (rGAPDH), we analysed the doses that elicited the best immune responses and found that this occurred at a total of 30 μg of antigen per fish. Subsequently, vaccination of fish with rHSP60, rENOLASE and rGAPDH achieved 46.9, 52 and 25% relative per cent survival (RPS), respectively. In addition, bivalent subunit vaccines--combination I (rHSP60 + rENOLASE), combination II (rENOLASE + rGAPDH) and combination III (rHSP60 + rGAPDH)--were administered and the RPS in these groups (65.6, 64.0 and 48.4%, respectively), was higher than that achieved with single-antigen administration. Finally, in combination IV, the trivalent vaccine rHSP60 + rENOLASE + rGAPDH, the RPS was 1.6%. Taken together, our results suggest that combinations of two antigens may achieve a better efficiency than monovalent or trivalent antigens, and this may provide new insights into pathogen prevention strategies.

Potential use of chitosan nanoparticles for oral delivery of DNA vaccine in black seabream Acanthopagrus schlegelii Bleeker to protect from Vibrio parahaemolyticus

To develop an effective and easy-to-administer vaccine against vibriosis of fish, the chitosan nanoparticles-loaded DNA vaccine against Vibrio parahaemolyticus was studied. A DNA vaccine was constructed using the outer membrane protein K (ompK) gene of V. parahaemolyticus strain (OS4) and pEGFP-N2 , a eukaryotic expression vector, and the construct was named pEGFP-N2 -OMPK (pDNA). The pDNA was encapsulated in chitosan particles (chitosan/pDNA). The effective diameter, mean diameter and polydispersity of the particles were 284.4 nm, 218.9 nm and 0.160, respectively. Scanning electron microscopy showed that the particles are dispersed as individual nanoparticles with spherical shape of around 200 nm and are homogeneously distributed. Encapsulation efficiency and loading percentage of nanoparticles were 91.5% and 2.08%, respectively. Reverse transcription-polymerase chain reaction (RT-PCR) showed that RNA-containing information of the ompK gene existed in mid-intestine, liver, kidney and muscle 3 weeks after oral administration in black seabream Acanthopagrus schlegelii Bleeker. Expression of the reporter gene, green fluorescent protein (GFP), was observed in the above-mentioned tissues by fluorescence microscopy. Expression of the ompK gene within 3 weeks evoked an immune response. Black seabream was protected from V. parahaemolyticus (OS4), with 72.3% relative percentage survival (RPS) 3 weeks post-vaccination with chitosan/pDNA. The direct agglutination test indicated that oral administration with chitosan/pDNA induced an antibody immune response in fish against V. parahaemolyticus (OS4). Data obtained, here and in other related studies, suggest that chitosan nanoparticles are promising carriers for an oral pDNA vaccine.

Immunological study of the outer membrane proteins of Vibrio harveyi: insights that link immunoprotectivity to interference with bacterial infection

Vibrio harveyi is a bacterial pathogen that affects marine vertebrates and invertebrates. In this study, we identified 13 outer membrane proteins (OMPs) from a pathogenic V. harveyi strain and analyzed their immunological properties. In vivo immunogenicity analysis showed that antibodies specific to recombinant proteins of the 13 OMPs were detected in the antiserum of V. harveyi-infected rat. When used as subunit vaccines to immunize Japanese flounder (Paralichthys olivaceus), all OMPs were able to elicit specific serum antibody production in the vaccinated fish; however, only two OMPs (OMP173 and OMP214) induced high levels (>70%) of relative percent survival. Pre-incubation of V. harveyi with the antisera of protective OMPs significantly impaired bacterial infectivity against peripheral blood leukocytes (PBL), whereas the antisera of non-protective OMPs had no apparent effect on infection. OMP173 antibodies could bind whole V. harveyi cells and exhibit bactericidal effect in a complement-dependent manner. Passive immunization showed that fish received OMP173 antiserum before being infected with V. harveyi exhibited significantly reduced mortality rate and lower bacterial loads in liver, spleen, and kidney. Finally, treatment of FG cells with OMP173 prior to V. harveyi infection protected the cells from bacterial invasion to a significant extent. Take together, these results indicate that two of the examined OMPs induce protective immunity through production of specific antibodies that block bacterial invasion, and that one OMP is likely to be involved in host cell interaction during the infection process. Thus, the immunoprotectivity of the OMPs is probably associated with functional participations of the OMPs in bacterial infection.

Protection against Vibrio alginolyticus in crimson snapper Lutjanus erythropterus immunized with a DNA vaccine containing the ompW gene

The outer membrane proteins of Vibrio alginolyticus play an important role in the virulence of the bacterium and are potential candidates for vaccine development. In the present study, the ompW gene was cloned, expressed and purified. A DNA vaccine was constructed by inserting the ompW gene into a pcDNA plasmid. Crimson snapper Lutjanus erythropterus (Bloch) were injected intramuscularly with the recombinant plasmid pcDNA-ompW. The expression of the DNA vaccine was detected in gill, head kidney, heart, liver, spleen and injection site muscle of crimson snapper by RT-PCR 7 and 28 d post-vaccination. The ELISA results demonstrated that the DNA vaccine produced an observable antibody response in all sera of the vaccinated fish. In addition, crimson snapper immunized with the DNA vaccine showed a relative percentage survival (RPS) of 92.53%, indicating effective protection against V. alginolyticus infection.

Cloning, expression of Vibrio alginolyticus outer membrane protein-OmpU gene and its potential application as vaccine in crimson snapper, Lutjanus erythropterus Bloch

The outer membrane proteins of the marine aquatic animal pathogen, Vibrio alginolyticus, play an important role in the virulence of the bacterium and are potential candidates for vaccine development. In this study, the gene encoding an outer membrane protein-OmpU was cloned and expressed in Escherichia coli. Polyclonal antibodies were raised in rabbits against the purified recombinant OmpU, and the reaction of the antibody was confirmed by Western blotting using the isolated OmpU and the recombinant OmpU of V. alginolyticus. To analyze the immunogenicity of the recombinant OmpU, crimson snapper, Lutjanus erythropterus Bloch, were immunized by intraperitoneal injection, and antibody response was assessed by the enzyme-linked immunosorbent assay (ELISA). The results demonstrated that the recombinant OmpU produced an observable antibody response in all sera of the vaccinated fish. The vaccinated fish were challenged by virulent V. alginolyticus and observed to have high resistance to infection. These results indicate that the recombinant OmpU is an effective vaccine candidate against V. alginolyticus in L. erythropterus.

Identification of Omp38 by immunoproteomic analysis and evaluation as a potential vaccine antigen against Aeromonas hydrophila in Chinese breams

Aeromonas hydrophila is a fish pathogen causing systemic infections in aquatic environments, and determining its antigenic proteins is important for vaccine development to reduce economic losses in aquaculture worldwide. Here, an immunoproteomic approach was used to identify immunogenic outer membrane proteins (OMPs) of the Chinese vaccine strain J-1 using convalescent sera from Chinese breams. Seven unique immunogenic proteins were identified by two-dimensional (2-D) electrophoresis and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-TOF-MS). One protein of interest, Omp38, was expressed, and its immunogenicity and protective efficacy were evaluated in Chinese breams. The two groups of fish immunized with the inactivated vaccine and recombinant Omp38 protein showed significant serum IgM antibody levels after vaccination, compared with the fish injected with PBS buffer. In addition, the superoxide dismutase (SOD) activity, lysozyme (LSZ) activity and phagocytosis activity of head kidney lymphocytes of immunized groups were significantly higher than those of the control. The fish receiving inactivated vaccine and recombinant Omp38 protein developed a protective response to a live A. hydrophila challenge 45 days post-immunization, as demonstrated by increased survival of vaccinated fish over the control and by decreased histological alterations in vaccinated fish. Furthermore, protective effect was better in Omp38 group than in the inactivated vaccine group. These results suggest that the recombinant Omp38 protein could effectively stimulate both specific and non-specific immune responses and protect against A. hydrophila infection. Therefore, Omp38 may be developed as a potential vaccine candidate against A. hydrophila infection.

Development and efficacy of an attenuated Vibrio harveyi vaccine candidate with cross protectivity against Vibrio alginolyticus

Vibrio harveyi is a Gram-negative bacterial pathogen that can infect a wide range of marine animals. In previous studies, we have reported a virulent V. harveyi strain, T4D. In the present study, an attenuated mutant of T4D, T4DM, was obtained by selection of rifampicin resistance. Compared to the wild type, T4DM was different in whole-cell protein profile and much slower in growth rate when cultured in stress conditions caused by iron depletion. Virulence analysis showed that compared to T4D, T4DM exhibited a dramatically increased median lethal dose, impaired tissue dissemination capacity, defective hemolytic activity, and significantly reduced resistance against the killing effect of host serum. To examine the potential of T4DM as a live attenuated vaccine, Japanese flounder (Paralichthys olivaceus) were vaccinated with T4DM via intraperitoneal injection or immersion. The results showed that at one and two months post-vaccination, fish administered with T4DM via both approaches, in particular that of immersion, were effectively protected against not only V. harveyi but also Vibrio alginolyticus, another important fish pathogen. Microbiological analysis showed that following immersion vaccination, T4DM was recovered from the internal organs of the vaccinated fish in a time-dependent manner within the first 6 days post-vaccination. Serum antibodies against V. harveyi and V. alginolyticus were detected in T4DM-vaccinated fish, and, compared to serum from control fish, serum from T4DM-vaccinated fish was significantly enhanced in bactericidal activity. These results indicate that T4DM is an attenuated strain with residual infectivity and that T4DM can induce effective cross-species protection against both V. harveyi and V. alginolyticus when used as a live immersion vaccine.

Secreted glyceraldehyde-3-phosphate dehydrogenase as a broad spectrum vaccine candidate against microbial infection in aquaculture

AIMS

Subcellullar localizations and cross-immunities of GAPDHs from six common pathogenic bacteria in aquaculture were investigated.

METHODS AND RESULTS

Subcellullar localizations of GAPDHs of Edwardsiella tarda EIB202, Edwardsiella ictaluri ATCC33202, Aeromonas hydrophila LSA34, Vibrio anguillarum MVM425, Vibrio alginolyticus EPGS020401 and Vibrio harveyi VIB647 were analysed with Western blotting, indirect immunofluorescence and flow cytometry examinations. Immunoprotections of different recombinant GAPDHs against these pathogens were investigated with zebrafish model. Western blotting of subcellular extractions showed that all GAPDHs were secreted into extracellular medium and periplasmic space. In addition, GAPDHs were demonstrated to distribute in the outer membranes except MVM425 and VIB647. And, GAPDHs were confirmed to be present on the surface of these bacteria with indirect immunofluorescence and flow cytometry examinations. The remarkable cross-protective immunities of these recombinant GAPDHs were induced in zebrafish, and the relative protective survivals were almost over 60%.

CONCLUSIONS

Localizations of GAPDHs from these pathogenic bacteria were similar to many other causative agents. And, GAPDHs could be important protective antigens and give remarkable cross-immunity against different pathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

Recombinant GAPDH could be designed as a broad spectrum vaccine candidate against multiple microbial infections in aquaculture.

Enhancement of protective immunity in European eel (Anguilla anguilla) against Aeromonas hydrophila and Aeromonas sobria by a recombinant Aeromonas outer membrane protein

To develop a vaccine, which can simultaneously prevent the diseases caused by various pathogenic bacteria in fish, we try to find a conserved outer membrane protein (OMP) antigen from different bacterial pathogens. In this study, an OMP fragment of 747 bp (named as Omp-G), which was highly conserved in seven Aeromonas OMP sequences from the NCBI database, was amplified by PCR from one Aeromonas sobria strain (B10) and two Aeromonas hydrophila strains (B27 and B33) with the designed specific primers. The sequence was cloned into pGEX-2T (6 × His-tag) vector, expressed in Escherichia coli system, and then the recombinant protein (named as rOmp-G) was purified with nickel chelating affinity chromatography. The purified rOmp-G showed a good immunogenicity in rabbits and well-conserved characteristics in these three pathogens by enzyme-linked immunosorbed assay. Furthermore, the rOmp-G also showed good immunogenicity in eels (Anguilla anguilla) for eliciting significantly increased specific antibodies (P < 0.01), and providing higher protection efficiencies (P < 0.05) after the pathogens challenge. The values of the relative percent survival in eels were 70% and 50% for two A. hydrophila strain challenge, and 75% for A. sobria strain challenge. This is the first report of a potential vaccination in eels that simultaneously provide protectiveness against different Aeromonas pathogens with a conserved partial OMP.

Loop-mediated isothermal amplification method for rapid detection of Vibrio alginolyticus, the causative agent of vibriosis in mariculture fish

AIMS

The purpose of this study was to develop a loop-mediated isothermal amplification (LAMP) method for the rapid, sensitive and simple detection of Vibrio alginolyticus in mariculture fish.

METHODS AND RESULTS

LAMP primers were designed by targeting the gyrB gene. With Bst DNA polymerase, the target DNA can be clearly amplified for 60 min at 64 degrees C in a simple water bath. The detection sensitivity of the LAMP assay for the detection of V. alginolyticus is about 3.7 x 10(2) CFU ml(-1) (3.7 CFU per reaction). LAMP products could be judged with agar gel or naked eye after the addition of SYBR Green I. There were no cross-reactions with other bacterial strains indicating a high specificity of the LAMP. The LAMP method was applied to detect V. alginolyticus-infected fish tissues effectively.

CONCLUSIONS

The LAMP established in this study is a simple, sensitive, specific, inexpensive and rapid protocol for the detection of V. alginolyticus.

SIGNIFICANCE AND IMPACT OF THE STUDY

This LAMP method provides an important diagnostic tool for the detection of V. alginolyticus infection both in the laboratory and field.

The outer membrane protein VhOmp of Vibrio harveyi: pore-forming properties in black lipid membranes

Vibrio harveyi is known to cause fatal vibriosis in marine animals. Here, an outer membrane protein from V. harveyi, namely, VhOmp, was isolated and functionally characterized in terms of pore-forming contact with artificial lipid membranes. The native VhOmp exists as a trimer of a molecular weight similar to that of the porin OmpF from Escherichia coli. Reconstitution of VhOmp into black lipid membranes demonstrated its ability to form ion channels. The average pore conductance of VhOmp was revealed to be about 0.9 and 2 nS in 0.2 and 1 M KCl, respectively. Within transmembrane potentials of +/-100 mV, VhOmp pores behaved as ohmic conduits, and their conductance scaled linearly with voltage. Nonlinear plots of the pore conductance versus symmetrical salt concentrations at either side of the protein-incorporating membrane suggested the influence of interior channel functionalities on the passage of charged species. In the presence of Omp-specific polyclonal antibodies, the pore-forming property of VhOmp was modulated so that the usual step-like current increments were replaced by random transitory current fluctuations. VhOmp exhibited a strong biological activity by causing hemolysis of human red blood cells, indicating that VhOmp may act as a crucial determinant during bacterial infection to animal host cells.

An outer membrane protein, OmpK, is an effective vaccine candidate for Vibrio harveyi in Orange-spotted grouper (Epinephelus coioides)

The outer membrane proteins of the fish pathogen, Vibrio harveyi, have a role in interaction between bacterium and host and are potential candidates for vaccine development. In this study, the gene encoding an outer membrane protein, OmpK, which serves as the receptor for broad-host-range vibriophage KVP40 in V. harveyi, was isolated and characterized. Then the OmpK gene coding for mature peptide was subcloned into prokaryotic expression vector pBV220 and transformed into Escherichia coli DH5 alpha strain. After temperature induction, a recombinant protein was detected about 28 kDa in molecular weight and accounted for 24.8% of total proteins of whole cell as estimated by SDS-PAGE and scanning analysis of gel image. Polyclonal antibodies were raised in rabbits against the purified protein and the reaction of the antibody was confirmed by western blotting using the purified protein and crude extract of V. harveyi. Orange-spotted groupers (Epinephelus coioides) vaccinated with recombinant OmpK produced specific antibodies, and were highly resistant to infection by virulent V. harveyi. These results indicate that the OmpK is an effective vaccine candidate against V. harveyi in Orange-spotted groupers.