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

Outer membrane proteins and vesicles as promising vaccine candidates against Vibrio spp. infections

Indiscriminate use of antibiotics to treat bacterial infections has brought unmanageable antibiotic-resistant strains into existence. Vibrio spp. represents one such gram-negative enteric pathogenic group with more than 100 species, infecting humans and fish. The Vibrio spp. is demarcated into two groups, one that causes cholera and the other producing non-cholera or vibriosis infections. People who encounter contaminated water are at risk, but young children and pregnant women are the most vulnerable. Though controllable, Vibrio infection still necessitates the development of preventative measures, such as vaccinations, that can lessen the severity of the infection and reduce reliance on antibiotic use. With emerging multi-drug resistant strains, efforts are needed to develop newer vaccines, such as subunit-based or outer membrane vesicle-based. Thus, this review strives to bring together available information about Vibrio spp. outer membrane proteins and vesicles, encompassing their structure, function, and immunoprotective role.

Transcriptomic analysis of Vibrio alginolyticus challenged by Rhizoma coptidis reveals mechanisms of virulence genes

Rhizoma coptidis, a Chinese herbal medicine widely used to treat various bacterial infections, has the potential to develop antibiotic substitutes to overcome the drug resistance of Vibrio alginolyticus. To study the inhibitory effect of R. coptidis on V. alginolyticus, we sequenced the transcriptomes of three groups of samples of wild-type V. alginolyticus (CK) and V. alginolyticus, which were stressed by 5 mg/mL R. coptidis for 2 h (RC_2 h) and 4 h (RC_4 h). CK was compared with RC_2 h and RC_4 h, respectively, and a total of 1565 differentially expressed genes (DEGs) (988 up-regulated and 577 down-regulated) and 1737 DEGs (1152 up-regulated and 585 down-regulated) were identified. Comparing RC_2 h with RC_4 h, 156 DEGs (114 up-regulated and 42 down-regulated) were identified. The ability of biofilm formation and motility of V. alginolyticus altered upon with different concentrations of R. coptidis. Interestingly, relative expression patterns of virulence genes appeared statistically significantly varied, upon different concentrations of R. coptidis extract. DEGs were annotated to the Gene Ontology (GO) database for function enrichment analysis and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, the results showed that the main enriched pathways, was those related to the virulence of V. alginolyticus. This study provides a new perspective for understanding the complex pathogenic mechanism of V. alginolyticus. R. coptidis could potnetially be used as alternative or complimnetary to antibiotics to treat infections after further research.

In silico designing and characterization of outer membrane protein K (OmpK) from Vibrio anguillarum and its expression in Nicotiana tabacum for the development of a plant-based vaccine against fish vibriosis

Vibriosis is caused by Vibrio anguillarum in various species of aquaculture. A novel, secure, and stable vaccine is needed to eradicate vibriosis. Here, for reverse vaccinology and plant-based expression, the outer membrane protein K (OmpK) of V. anguillarum was chosen due to its conserved nature in all Vibrio species. OmpK, an ideal vaccine candidate against vibriosis, demonstrated immunogenic, non-allergic, and non-toxic behavior by using various bioinformatics tools. Docking showed the interaction of the OmpK model with TLR-5. In comparison to costly platforms, plants can be used as alternative and economic bio-factories to produce vaccine antigens. We expressed OmpK antigen in Nicotiana tabacum using Agrobacterium-mediated transformation. The expression vector was constructed using Gateway® cloning. Transgene integration was verified by polymerase chain reaction (PCR), and the copy number via qRT-PCR, which showed two copies of transgenes. Western blotting detected monomeric form of OmpK protein. The total soluble protein (TSP) fraction of OmpK was equivalent to 0.38% as detected by ELISA. Mice and fish were immunized with plant-derived OmpK antigen, which showed a significantly high level of anti-OmpK antibodies. The present study is the first report of OmpK antigen expression in higher plants for the potential use as vaccine in aquaculture against vibriosis, which could provide protection against multiple Vibrio species due to the conserved nature OmpK antigen.

Secreted Extracellular Products of Flavobacterium covae as Potential Immunogenic Factors for Protection against Columnaris Disease in Channel Catfish (Ictalurus punctatus)

Columnaris disease caused by Flavobacterium covae leads to substantial economic losses in commercially important fish species worldwide. The US channel catfish (Ictalurus punctatus) industry is particularly vulnerable to this disease. Therefore, there is an urgent need to develop a vaccine to reduce the economic losses caused by this disease. Secreted extracellular products (SEPs) are considered to be essential bacterial virulence factors that often provide immunogenicity and protection. The current study sought to identify the main SEPs of F. covae and to evaluate their potential to provide protection in channel catfish against columnaris disease. SDS-PAGE analysis of SEPs revealed five protein bands with molecular weights ranging from 13 to 99 kDa. Mass spectrometry analysis showed that these SEPs were hypothetical protein (AWN65_11950), zinc-dependent metalloprotease (AWN65_10205), DNA/RNA endonuclease G (AWN65_02330), outer membrane protein beta-barrel domain (AWN65_12620), and chondroitin-sulfate-ABC endolyase/exolyase (AWN65_08505). Catfish fingerlings were vaccinated with SEPs, SEPs emulsified with mineral oil adjuvant, or heat-inactivated SEPs, or they were sham-immunized through intraperitoneal (IP) injection. After 21 days, an F. covae challenge showed 58.77% and 46.17% survival in the catfish vaccinated with the SEPs and the SEPs emulsified with adjuvant compared to the sham-vaccinated control (100% mortality within 120 h post-infection). However, the heat-inactivated SEPs failed to provide significant protection (23.15% survival). In conclusion, although SEPs contain potentially important immunogenic proteins, further work is needed to optimize their use for long-lasting protection against columnaris disease in fish. These results are significant given the economic impact of columnaris disease on fish farming worldwide.

Vibrio harveyi co-infected with Cryptocaryon irritans to orange-spotted groupers Epinephelus coioides

The orange-spotted grouper (Epinephelus coioides) is a high economic value aquacultural fish in China, however, it often suffers from the outbreak of parasitic ciliate Cryptocaryon irritans as well as bacterium Vibrio harveyi which bring great loss in grouper farming. In the present study, we established a high dose C. irritans local-infected model which caused the mortality of groupers which showed low vitality and histopathological analysis demonstrated inflammatory response and degeneration in infected skin, gill and liver. In addition, gene expression of inflammatory cytokines was detected to assist the estimate of inflammatory response. Furthermore, we also found that the activity of Na⁺/K⁺ ATPase in gill was decreased in groupers infected C. irritans and the concentration of Na⁺/Cl^- in blood were varied. Base on the morbidity symptom occurring in noninfected organs, we hypothesized that the result of morbidity and mortality were due to secondary bacterial infection post parasitism of C. irritans. Moreover, four strains of bacteria were isolated from the infected site skin and liver of local-infected groupers which were identified as V. harveyi in accordance of phenotypic traits, biochemical characterization and molecular analysis of 16S rDNA genes, housekeeping genes (gyrB and cpn60) and species-specific gene Vhhp2. Regression tests of injecting the isolated strain V. harveyi has showed high pathogenicity to groupers. In conclusion, these findings provide the evidence of coinfections with C. irritans and V. harveyi in orange-spotted grouper.

Functional characterization of a grouper nklysin with antibacterial and antiviral activity

Natural killer lysin (Nklysin) is a small molecule antimicrobial peptide produced by natural killer cells and T lymphocytes and widely expressed in vertebrates. Homologues of Nklysin have been found in several fish, but only several of biological activity was identified. In this study, we characterized a Nklysin from grouper (Epinephelus coioides), and explored its expression pattern and biological function in bacterial infection. We also investigated the role of Nklysin in viral replication and maturation. The nklysin gene of grouper encodes a 169 amino acid, sharing 92.90% identity to H. septemfasciatus NKlysin protein, containing a saposin B domain and six well-conserved cysteine residues that necessary for antimicrobial activity by forming three intrachain disulfide bonds. Analysis of qRT-PCR revealed that nklysin gene widely expressed in all tested tissues with the higher expressions in spleen. After bacterial challenge, the nklysin gene expression significantly varied in different tissues. In addition, a large-scale of the recombinant Nklysin protein was secreted in Pichia pastoris strain GS115. The MIC assay showed that the Nklysin protein directly inhibited growth of several pathogens, including Proteus mirabilis, Bacillus subtilis, Salmonella typhi, Escherichia coli, Shigella sonnei and Streptococcus agalactiae. Further analysis showed the Nklysin protein over-expression might prevent viral genes transcriptions and replication in FHM cells. Our findings suggested that the Nklysin of grouper might be a potential agent for antibacterial and antiviral infection in the future.

Molecular characterization and expression analysis of interleukin-15 (IL-15) genes in orange-spotted grouper (Epinephelus coioides) in response to Vibrio harveyi challenge

As a member of the γc family, interleukin 15 plays an important function in the immune response. In this study, we cloned an IL15 from Epinephelus coioides (named Ec-IL15). The open reading frame of Ec-IL15 is 528 bp, encoding 175 amino acids. Sequence alignment analysis showed that EcIL-15 has a conserved Pfam: IL15 domain and four cysteine residues. Subcellular localization studies have shown that Ec-IL15 is distributed in whole cells. In healthy groupers, Ec-IL15 was expressed in all 11 tissues tested and the highest in liver. After ConA, PHA, LPS and poly I:C stimulation, Ec-IL15 expression of HKLs was significantly upregulated. After V. harveyi infection, the expression of Ec-IL15 in 9 tissues was significantly upregulated and peaked within 48 h. In addition, recombinant Ec-IL15 protein can not only stimulate HKLs proliferation and cytokine expression, but also has the potential as an immune enhancer.

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.

Construction and immune efficacy of recombinant Lactobacillus casei expressing OmpAI of Aeromonas veronii C5-I as molecular adjuvant

Despite advancements in diagnosis and control, Aeromonas infections are considered the leading cause of economic aquaculture loss. In this study, to enhance DNA vaccine efficacy against Aeromonas infections, a fused DNA fragment (1504 bp) of the OmpAI gene from Aeromonas veronii (A. veronii) combined with the C5-I gene from the common carp was generated with splicing by overlapping PCR (SOE-PCR) and expressed in Lactobacillus casei strain CC16. Protein C5-I served as a molecular adjuvant for the antigen OmpAI. Two types of fusion antigens were developed (anchored and secretory). Generally, anchored-type antigens are more effective in inducing immune responses in fish than secretory antigens. Western blot analysis showed that the bands of both antigens were present at 58 kDa. After oral immunization, both DNA vaccines enhanced the serum levels of AKP, ACP, SOD and LZM in immunized carp; the genes IL-10, IL-1β, TNF-α, and IFN-γ in the heart, liver, spleen, head kidney, and intestinal tract were upregulated; and a stronger phagocytic response was triggered in immunized fish. In addition, common carp administered the fused antigens were more protected from Aeromonas challenge (60-73.3% protection). Recombinant Lactobacillus bacteria expressing the fused protein showed a greater propensity for colonization in the intestinal tract in immunized fish than in controls. Here, we provide a promising approach to improve DNA vaccine immunogenicity for protecting common carp from A. veronii infections.

The Alteration of Intestinal Microbiota Profile and Immune Response in Epinephelus coioides during Pathogen Infection

Epinephelus coioides, or grouper, is a high economic value fish species that plays an important role in the aquaculture industry in Asia. However, both viral and bacterial diseases have threatened grouper for many years, especially nervous necrosis virus, grouper iridovirus and Vibrio harveyi, which have caused a bottleneck in the grouper industry. Currently, intestinal microbiota can provide novel insights into the pathogenesis-related factors involved in pathogen infection. Hence, we investigated the comparison of intestinal microbiota communities in control group and pathogen-infected grouper through high-throughput sequencing of the 16S rRNA gene. Our results showed that microbial diversity was decreased, whereas microbial richness was increased during pathogen infection. The individuals in each group were distributed distinctly on the PLSDA diagram, especially the GIV group. Proteobacteria and Firmicutes were the most abundant bacterial phyla in all groups. Interestingly, beneficial genera, Faecalibacterium and Bifidobacterium, predominated in the intestines of the control group. In contrast, the intestines of pathogen-infected grouper had higher levels of harmful genera such as Sphingomonas, Atopostipes, Staphylococcus and Acinetobacter. Additionally, we investigated the expression levels of innate and adaptive immune-related genes after viral and bacterial infection. The results revealed that immunoglobulin T and proinflammatory cytokine levels in the intestine increased after pathogen infection. Through these unique bacterial compositions in diseased and uninfected fish, we could establish a novel therapeutic approach and bacterial marker for preventing and controlling these diseases.

Application of Outer Membrane Protein-Based Vaccines Against Major Bacterial Fish Pathogens in India

Aquaculture is one of the fastest-growing food-producing sectors in the world. However, its growth is hampered by various disease problems due to infectious microorganisms, including Gram-negative bacteria in finfish aquaculture. Disease control in aquaculture by use of antibiotics is not recommended as it leads to antibiotic residues in the final product, selection, and spread of antibiotic resistance in the environment. Therefore, focus is on disease prevention by vaccination. All Gram-negative bacteria possess surface-associated outer membrane proteins (OMPs), some of which have long been recognized as potential vaccine candidates. OMPs are essential for maintaining the integrity and selective permeability of the bacterial membrane and play a key role in adaptive responses of bacteria such as solute and ion uptake, iron acquisition, antimicrobial resistance, serum resistance, and bile salt resistance and some adhesins have virulence attributes. Antigenic diversity among bacterial strains even within the same bacterial species has constrained vaccine developments, but OMPs that are conserved across serotypes could be used as potential candidates in vaccine development, and several studies have demonstrated their efficacy and potential as vaccine candidates. In this review, we will look into the application of OMPs for the design of vaccines based on recombinant proteins, subunit vaccines, chimeric proteins, and DNA vaccines as new-generation vaccine candidates for major bacterial pathogens of fish for sustainable aquaculture.

Recombinant outer membrane protein C of Aeromonas salmonicida subsp. masoucida, a potential vaccine candidate for rainbow trout (Oncorhynchus mykiss)

Aeromonas salmonicida subsp. masoucida (ASM) is an important bacterial pathogen of salmonid fish, which can cause huge economic losses to the fish farming industry. In order to screen effective vaccine candidate proteins, four outer membrane proteins of ASM, including OmpA, OmpC, OmpK and OmpW, were selected and recombinantly expressed in Escherichia coli. The result of western blotting showed that these four recombinant proteins could be recognized by rainbow trout anti-ASM antibodies. The immune protective effects of the four rOMPs were also investigated, and the relative percentage survival (RPS) of rOmpA, rOmpC, rOmpK and rOmpW were 71.1%, 81.6%, 55.3% and 42.1%, respectively. The RPS of rOmpC was significantly higher than the other three rOMPs, so the immune responses of rainbow trout induced by rOmpC were further investigated. The results showed that vaccination with rOmpC could significantly induced the production of specific serum antibodies and proliferation of sIg + lymphocytes in peripheral blood. Meanwhile, RT-qPCR analysis showed that rOmpC could significantly enhance the expression of the MHC-II, TCR, CD4, CD8, IL-8 and IgM genes compared with the BSA immunized group. These results demonstrated that rOmpC could induce strong humoral immune response in rainbow trout and provided effective immune protection against ASM challenge, which indicated that OmpC is a promising vaccine candidate against Aeromonas salmonicida infection.

Construction and analysis of the immune effect of Vibrio harveyi subunit vaccine and DNA vaccine encoding TssJ antigen

Vibrio harveyi, a severe pathogen infects different kinds of sea animals, causes huge economic loss in aquaculture industry. In order to control the Vibriosis disease caused mainly by V. harveyi and other Vibrio spp., the best solution lies in developing corresponding efficient vaccines. In this study, we have cloned and analysed a putative antigen TssJ from the T6SS of V. harveyi, which has the potential as a vaccine against infection. The sequence analysis and western blotting experiments indicated that TssJ anchored in outer membrane and there were several antigenic determinants existed on its extracellular region. Two forms of universal vaccines, subunit vaccine and DNA vaccine, were developed based on TssJ and applied in Trachinotus ovatus. The results showed that both of the two vaccines could generate a moderate protection in fish against V. harveyi. The relative percentage survival (RPS) of subunit vaccine and DNA vaccine were 52.39% and 69.11%, respectively. Immunological analysis showed both subunit vaccine and DNA vaccine enhanced acid phosphatase, alkaline phosphatase, superoxide dismutase, and lysozyme activities. Specific serum antibodies against TssJ in the fish vaccinated with subunit vaccine was much higher than that in the DNA vaccine group. Several immune-related genes, i.e., IL10, C3, MHC Iα, MHC IIα, and IgM, were induced both by the two forms of vaccines. TNFα and Mx were only upregulated in the DNA vaccine group. However, the induction levels of these genes induced by DNA vaccine were higher than subunit vaccine. All these findings suggested that TssJ from V. harveyi had a potential application value in vaccine industry.

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.

Vesicles From Vibrio cholerae Contain AT-Rich DNA and Shorter mRNAs That Do Not Correlate With Their Protein Products

Extracellular vesicles secreted by Gram-negative bacteria have proven to be important in bacterial defense, communication and host–pathogen relationships. They resemble smaller versions of the bacterial mother cell, with similar contents of proteins, LPS, DNA, and RNA. Vesicles can elicit a protective immune response in a range of hosts, and as vaccine candidates, it is of interest to properly characterize their cargo. Genetic sequencing data is already available for vesicles from several bacterial strains, but it is not yet clear how the genetic makeup of vesicles differ from that of their parent cells, and which properties may characterize enriched genetic material. The present study provides evidence for DNA inside vesicles from Vibrio cholerae O395, and key characteristics of their genetic and proteomic content are compared to that of whole cells. DNA analysis reveals enrichment of fragments containing ToxR binding sites, as well as a positive correlation between AT-content and enrichment. Some mRNAs were highly enriched in the vesicle fraction, such as membrane protein genes ompV, ompK, and ompU, DNA-binding protein genes hupA, hupB, ihfB, fis, and ssb, and a negative correlation was found between mRNA enrichment and transcript length, suggesting mRNA inclusion in vesicles may be a size-dependent process. Certain non-coding and functional RNAs were found to be enriched, such as VrrA, GcvB, tmRNA, RNase P, CsrB2, and CsrB3. Mass spectrometry revealed enrichment of outer membrane proteins, known virulence factors, phage components, flagella and extracellular proteins in the vesicle fraction, and a low, negative correlation was found between transcript-, and protein enrichment. This result opposes the hypothesis that a significant degree of protein translation occurs in vesicles after budding. The abundance of viral-, and flagellar proteins in the vesicle fraction underlines the importance of purification during vesicle isolation.

Identification of outer membrane protein TolC as the major adhesin and potential vaccine candidate for Vibrio harveyi in hybrid grouper, Epinephelus fuscoguttatus (♀) × E. lanceolatus (♂)

Vibrio harveyi is a serious pathogen of scale drop and muscle necrosis disease in marine commercial fishes. Adhesion to and colonization of the host cells surfaces is the first and crucial step for pathogenic bacterial infection, which is usually mediated by outer membrane proteins (Omps). The objectives of this study were to identify the major adhesin in Omps that plays the essential role in adhesion of V. harveyi to the host cells, and to assess the potential of this adhesin as a vaccine candidate for V. harveyi infection. We observed that pathogenic V. harveyi adhered to the surface of grouper embryonic cells (GEM cells) and induced apoptosis of them. Native Omps were extracted from nine different V. harveyi strains, and five common Omp bands were isolated by SDS-PAGE analysis. Western blot analysis and an anti-native Omp antibodies blocking assay indicated that one strong and several weak immunoreactivity Omps bands presence. Next, a total of five Omps, including TolC, Agg (Agglutination protein), Omp47, Fla (Flagellin), and OmpW, were identified and their encoding genes were cloned, characterized, and expressed in E. coli. The purified recombinant TolC could competitively inhibit the invasion of V. harveyi to GEM cells in vitro, and anti-TolC antibody also could significantly block the adhesion of V. harveyi to GEM cells. When used to immunize hybrid groupers, the recombinant TolC could confer significant protection to fish against experimental V. harveyi challenge. These data suggested that outer membrane protein TolC functions as a major adhesin in V. harveyi and could be a potential vaccine candidate for V. harveyi infection.

Molecular Characterization, Phylogenetic, Expression, and Protective Immunity Analysis of OmpF, a Promising Candidate Immunogen Against Yersinia ruckeri Infection in Channel Catfish

Outer membrane porins, as the major components of Gram-negative bacterial membrane proteins, have been proven to be involved in interactions with the host immune system and potent protective antigen candidates against bacterial infection in fish. Outer membrane porin F (OmpF) is one of the major porins of Yersinia ruckeri (Y. ruckeri), the causative agent of enteric red mouth disease of salmonid and non-salmonid fish. In the present study, the molecular characterization and phylogenetic analysis of OmpF gene was studied, heterogenous expression, immunogenicity and protective immunity of OmpF were systemically evaluated as a subunit vaccine for channel catfish against Y. ruckeri infection. The results showed that OmpF gene was highly conserved among 15 known Yersinia species based on the analysis of conserved motifs, sequences alignment and phylogenetic tree, and was subjected to negative/purifying selection with global dN/dS ratios value of 0.649 throughout the evolution. Besides, OmpF was also identified to have immunogenicity by western blotting and was verified to be located on the surface of Y. ruckeri using cell surface staining and indirect immunofluorescence assays. Moreover, recombinant OmpF (rtOmpF) as a subunit vaccine was injected with commercial adjuvant ISA763, significantly enhanced the immune response by increasing serum antibody levels, lysozyme activity, complement C3 activity, total protein content, SOD activity, immune-related genes expression in the head kidney and spleen, and survival percent of channel catfish against Y. ruckeri infection. Thus, our present results not only enriched the information of molecular characterization and phylogenetics of OmpF, but also demonstrated that OmpF holds promise to be used as a potential antigen against Y. ruckeri infection 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.

Evaluation of bivalent vaccines candidates among VAA, OmpK and OmpR from Vibrio anguillarum in flounder (Paralichthys olivaceus)

Outer membrane protein (Omp) K, OmpR and VAA have been identified with good immunogenicity from Vibrio anguillarum, and their recombinant proteins showed variable relative percent survival (RPS) in previous study. In order to develop effective bivalent vaccine candidates, recombinant (r) VAA + rOmpK (AK), rVAA + rOmpR (AR), rOmpK + rOmpR (KR) among VAA, OmpK and OmpR, or formalin-killed cells (FKC) of V. anguillarum were immunized in flounder, respectively. Results revealed that AK, AR, KR and FKC could induce the proliferation of surface membrane immunoglobulin-positive B lymphocytes or CD3⁺ T lymphocytes in peripheral blood lymphocytes, and significantly enhance the total antibodies, specific antibodies and immune-related gene than those of control group. AK, AR, KR or FKC showed RPS of 74.92%, 78.49%, 82.09% and 56.99%, respectively. These results indicated that three bivalent vaccines AK, AR and KR could induce strong cellular and humoral immunity, and had high protection against V. anguillarum infection in flounders.

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.

The outer membrane proteins of Stenotrophomonas maltophilia are potential vaccine candidates for channel catfish (Ictalurus punctatus)

Channel catfish (Ictalurus punctatus) is an important agricultural fish that has been plagued by Stenotrophomonas maltophilia (S. maltophilia) infections in recent years, some of them severe. The outer membrane proteins (OMPs) of S. maltophilia are one of the most immunogenic and highly conserved candidates for vaccine development in aquaculture. The present study investigated OMPs of S. maltophilia as vaccine on immune response and disease resistance against S. maltophilia of channel catfish and investigated the enhancement effect of natural adjuvants Propolis (Pro), FIG polysaccharide (Fcps), Glycyrrhizine (Gly) to OMPs of S. maltophilia for further study. The results indicated that channel catfish injected intraperitoneally with OMPs showed better immune response including leukocytes phagocytosis activity, serum bactericidal activity, complement C3, IgM level and an increasement of resistance against S. maltophilia compared to the control group. Moreover, Pro, Fcps and Gly could enhance the immune response of OMPs. The relative percent of survival (RPS) was 73.33%, 66.67%, 63.33%, 60%, 0% in fish injected OMPs + Pro, OMPs + Fcps, OMPs + Gly, OMPs and 0.65% normal saline, respectively. These results suggested that OMPs used as vaccine could induce and stimulate immune response and enhance disease resistance in channel catfish, especially for Pro as immunoenhancer. Results revealed that OMPs were an effective vaccine against S. maltophilia in channel catfish.

Identification of novel vaccine candidates against Acinetobacter baumannii using reverse vaccinology

Acinetobacter baumannii (Ab) is a global emerging bacterium causing nosocomial infections such as pneumonia, meningitis, bacteremia and soft tissue infections especially in intensive care units. Since Ab is resistant to almost all conventional antibiotics, it is now one of the 6 top-priorities of the dangerous microorganisms listed by the Infectious Disease Society of America. The development of vaccine is one of the most promising and cost-effective strategies to prevent infections. In this study, we identified potential protective vaccine candidates using reverse vaccinology. We have analyzed 14 on-line available Ab genome sequences and found 2752 homologous core genes. Using information obtained from immuno-proteomic experiments, published proteomic information and the bioinformatics PSORTb v3.0 software to predict the location of extracellular and/or outer membrane proteins, 77 genes were identified and selected for further studies. After excluding those antigens have been used as vaccine candidates reported by the in silico search-engines of PubMed and Google Scholar, 13 proteins could potentially be vaccine candidates. We have selected and cloned the genes of 3 antigens that were further expressed and purified. These antigens were found to be highly immunogenic and conferred partial protection (60%) in a pneumonia animal model. The strategy described in the present study incorporates the advantages of reverse vaccinology, bioinformatics and immuno-proteomic platform technologies and is easy to perform to identify novel immunogens for multi-component vaccines development.

Identification of DLD, by immunoproteomic analysis and evaluation as a potential vaccine antigen against three Vibrio species in Epinephelus coioides

Vibrio spp. represent a serious threat to the culture of Epinephelus coioides (Orange-spotted Grouper) in Southeast Asia. In this study we used two-dimensional electrophoresis (2-DE) and Western blotting to identify common immunogenic proteins of Vibrio alginolyticus, Vibrio harveyi and Vibrio parahaemolyticus. Membranes were probed with orange-spotted grouper anti-V. alginolyticus sera and accordingly 60, 58 and 48 immunogenic protein spots were detected. By matching analysis for the three Western blotting membranes, 6 cross immunogenic spots for the three Vibrio species were identified. They were Outer membrane protein W (OmpW), dihydrolipoamide dehydrogenase (DLD), succinate dehydrogenase flavoprotein subunit(SDHA), elongation factor Ts(Ts), peptide ABC transporter periplasmic peptide-binding protein and phosphoenolpyruvate carboxykinase(PEPCK). One of the proteins, DLD, was used to evaluate the cross protective function for E. coioides with a bacterial immunization and challenge method. The relative percent survival rate of E. coioides against V. alginolyticus, V. harveyi and V. parahaemolyticus was 90%, 86% and 80%, respectively. This work may provide potential cross protective vaccine candidate antigens for three Vibrio species, and DLD may be considered as an effective cross-protective immunogen against three Vibrio species.

Protein transduction domain of transactivating transcriptional activator fused to outer membrane protein K of Vibrio parahaemolyticus to vaccinate marbled eels (Anguilla marmorata) confers protection against mortality caused by V. parahaemolyticus

Although immersion and oral vaccination are the most practical methods for fish farmers, their applications are very limited due to low immune stimulation effect. We used the protein transduction domain (PTD) of transactivating transcriptional factor (TAT) derived from HIV TAT protein to increase the delivery efficiency of aquatic protein vaccines. Vibrio parahaemolyticus outer membrane protein K (ompK), a reported vaccine candidate for the prevention of V. parahaemolyticus infection, was fused with TAT-PTD expressed in Escherichia coli. We found that PTD-ompK fusion protein effectively penetrated into marbled eel bodies. Analysis of ompK antibody titres demonstrated that immersion vaccination with PTD-ompK was superior to ompK alone and induced robust immune stimulation in marbled eels. Both active and passive protection analyses against immersive challenge with V. parahaemolyticus strains demonstrated that marbled eels immunized with PTD-ompK survived significantly longer than those immunized with ompK alone. Our results indicated that TAT-PTD could be served as is an efficient delivery system for aquatic immersion vaccinations against various infectious diseases commonly seen in aquatic farm industry.

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.

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.

VscO, a putative T3SS chaperone escort of Vibrio alginolyticus, contributes to virulence in fish and is a target for vaccine development

Type III secretion system (T3SS) in Vibrio alginolyticus is essential for its pathogenesis. VscO's homologous proteins FliJ, InvI and YscO have been suggested to be putative chaperone escorts although its function in V. alginolyticus is unclear. To investigate the physiological role of VscO, a mutant strain of V. alginolyticus with an in-frame deletion of the vscO gene was constructed in the present study. One finding was that the mRNA expression levels of SycD, VopB and VopD proteins decreased in the ΔvscO mutant. In addition, the ΔvscO mutant showed an attenuated swarming ability and a ten-fold decrease in the virulence to fish. However, the ΔvscO mutant showed no difference in the biofilm formation and ECPase activity. Complementation of the mutant strain with the vscO gene could restore the phenotypes of the wild-type strain. Finally, the recombinant VscO protein caused a high antibody titer and an effective protection against lethal challenge with the wild-type strain V. alginolyticus. These results indicated that VscO protein has a specific role in the pathogenesis of V. alginolyticus and it may be a candidate antigen for development of a subunit vaccine against vibriosis.

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.

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.

Expression and immunogenicity analysis of accessory colonization factor A from Vibrio alginolyticus strain HY9901

The accessory colonization factor A (ACFA) of Vibrio alginolyticus plays an important role in the efficient colonization of the bacterium and is potential candidates for vaccine development. In present study, the acfA gene was cloned, expressed and purified. Western blot analysis revealed protein recognition with the native ACFA in different V. alginolyticus strains. To analyze the immunogenicity of the recombinant ACFA, Lutjanus erythropterus Bloch were immunized by intraperitoneal injection, and the results demonstrated that the recombinant ACFA produced an observable antibody response in all sera of the vaccinated fish. The differential expressions of RAG1 gene in various tissues of L. erythropterus were analyzed by fluorescent quantitative real-time PCR, and the results showed the RAG1 mRNA expression was significantly up-regulated in thymus, head kidney and spleen tissue. Furthermore, the protective property of recombinant ACFA was evaluated through challenge with six heterogeneous virulent V. alginolyticus strains, and the immunohistochemical analysis in different tissues after challenge with V. alginolyticus. The results showed L. erythropterus vaccinated with recombinant ACFA were more tolerant of the infection by virulent V. alginolyticus strains. The data indicate that the recombinant ACFA could provide heterologous protection for the different virulent V. alginolyticus strains.

Draft genome sequence of the fish pathogen Vibrio harveyi strain ZJ0603

Vibrio harveyi is an important pathogen that causes vibriosis in various aquatic organisms. Here, we announce the draft genome sequence of V. harveyi strain ZJ0603, which was isolated from diseased Orange-spotted grouper (Epinephelus coioides) in Guangdong, China.

Immune responses and protective efficacy of a novel DNA vaccine encoding outer membrane protein of avian Pasteurella multocida

Avian Pasteurella multocida is a causative agent of fowl cholera. Two proteins OmpH and OmpA are the major immunogenic antigens of avian P. multocida, which play an important role in inducing immune responses that confer resistance against infections. In the present study, we used pcDNA3.1(+) as a vector and constructed DNA vaccines with the genes encoding the two antigens mentioned above. These DNA vaccines include monovalent (pcDNA-OMPH, pOMPH and pcDNA-OMPA, pOMPA), divalent combination (pcDNA-OMPH+pcDNA-OMPA, pOMPH+pOMPA) and fusion of two gene vaccines (pcDNA-OMPH/OMPA, pOMPHA). The immune responses to these DNA vaccines were evaluated by serum antibody titers, lymphocyte proliferation assay and titers of a cytokines, IFN-γ. The protective efficacy after challenging with a virulent avian P. multocida strain, CVCC474, was evaluated by survival rate. A significant increase in serum antibody levels was observed in chickens vaccinated with divalent combination and fusion DNA vaccines. Additionally, the lymphocyte proliferation (SI value) and the levels of IFN-γ were both higher in chickens immunized with divalent combination and fusion DNA vaccines than in those vaccinated with monovalent DNA vaccines (P<0.05). Furthermore, the protection provided by divalent combination and fusion DNA vaccines was superior to that provided by monovalent DNA vaccines after challenging with the avian P. multocida strain CVCC474. And the protective efficacy in chickens immunized three times with the fusion DNA vaccine was equivalent to the protective efficacy in chickens vaccinated once with the attenuated live vaccine. This suggests that divalent combination and fusion DNA vaccines represent a promising approach for the prevention of fowl cholera.

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.

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.

Immunoproteomic analysis and identification of novel immunogenic proteins from Vibrio harveyi

AIMS

The main aim of this study was to screen novel immunogenic proteins of Vibrio harveyi, which could be vaccine candidates.

METHODS AND RESULTS

  Whole-cell proteins of V. harveyi, strain Li01 and Huang01, were first separated by isoelectric focusing, followed by 2D-PAGE, respectively. Immunogenic proteins were identified by Western blotting, using Epinephelus coioides antisera against V. harveyi strain Li01. Western blot analyses revealed 16 shared immunogenic protein spots in both strains. All of the immunogenic proteins were successfully identified and corresponded to 15 proteins. None of these proteins have been previously reported as immunogenic for V. harveyi. Of the 15 proteins, 11 are specific immunoreactive proteins and four are nonspecific immunoreactive proteins. Furthermore, outer membrane protein N (spot 2) and oligopeptide ATP-binding cassette (ABC) transporter (spot 3) were used as immunogens to immunize E. coioides for investigation of their protective abilities and activities. The E. coioides immunized with OmpN has abilities to fight against infections caused by V. harveyi Li01 and Huang01. However, vaccination with oligopeptide ABC transporter induces low protective immune response in fish.

CONCLUSIONS

  Eleven novel specific antigens were found, and OmpN could potentially be used as vaccine candidate for the development of novel vaccine against V. harveyi.

SIGNIFICANCE AND IMPACT OF THE STUDY

These data show that immunoproteomics methods can be successfully applied in identifying immunogenic proteins of V. harveyi, which helps to search for the protective antigens in future.

Evaluation of the vaccine potential of a cytotoxic protease and a protective immunogen from a pathogenic Vibrio harveyi strain

Vibrio harveyi is an important aquaculture pathogen that can infect a number of fish species and marine invertebrates. A putative protease, Vhp1, was identified from a pathogenic V. harveyi strain isolated from diseased fish as a protein with secretion capacity. Vhp1 is 530 amino acids in length and shares high sequence identities with several extracellular serine proteases of the Vibrio species. In silico analysis identified a protease domain in Vhp1, which is preceded by a subtilisin-N domain and followed by a bacterial pre-peptidase C-terminal domain. Purified recombinant protein corresponding to the protease domain of Vhp1 exhibited apparent proteolytic activity that was relatively heat-stable and reached maximum at pH 8.0 and 50 degrees C. The activity of purified recombinant Vhp1 protease was enhanced by Ca(2+) and inhibited by Mn(2+) and ethylenedinitrilotetraacetic acid. Cytotoxicity analyses indicated that recombinant Vhp1 protease was toxic to cultured Japanese flounder cells and could cause complete cell lysis. Immunoprotective analysis using Japanese flounder as an animal model showed that purified recombinant Vhp1 in the form of a denatured and proteolytically inactive protein was an effective subunit vaccine. To improve the vaccine potential of Vhp1, an Escherichia coli strain that expresses and secrets a cytotoxically impaired Vhp1 was constructed, which, when used as a live vaccine, afforded a high level of protection upon the vaccinated fish against lethal V. harveyi challenge. Taken together, these results demonstrate that Vhp1 is a cytotoxic protease and an effective vaccine candidate against V. harveyi infection.

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.