Adaptive immune response to Edwardsiella tarda infection in ginbuna crucian carp, Carassius auratus langsdorfii

NITR12+ NK Cells Release Perforin to Mediate IgMhi B Cell Killing in Turbot (Scophthalmus maximus)

B lymphocytes engaged in humoral immunity play a critical role in combating pathogenic infections; however, the mechanisms of NK cells in regulating the responses of B cells remain largely unknown. In the present study, we established an Edwardsiella piscicida infection model in turbot (Scophthalmus maximus) and found that the production of IgM was decreased. Meanwhile, through establishing the head kidney-derived lymphocyte infection model, we revealed that the impairment of IgMhi B cells was associated with bacterial infection-induced perforin production. Interestingly, we reveal that perforin production in NK cells is tightly regulated by an inhibitory novel immune-type receptor, NITR12. Moreover, we confirm that inhibiting NITR12 can result in elevated perforin production, engaging the impairment of IgMhi B cells. Taken together, these findings demonstrate an innovative strategy of NK cells in mediating B lymphocyte killing in turbot and suggest that relieving NK cells through NITR12 might be the target for the development of efficacious vaccines.

Adjuvant effect of allogeneic blood in vaccines against edwardsiellosis in ginbuna crucian carp Carassius auratus langsdorfii

Edwardsiella tarda (E. tarda), an intracellular pathogen, has caused severe economic losses in aquaculture. Effective vaccine development for E. tarda prevention is urgently needed. A previous study indicates that cell-mediated immunity (CMI) might play an important role in E. tarda infection. We believe that the involvement of allograft rejection and CMI has now been well documented in mammals and some fishes. However, there is still little research on the application of blood allograft rejection in vaccine development. In the current study, we investigate the immune response and vaccine effect in fish vaccinated with allogeneic blood + formalin-killed cells vaccine (FKC), allogeneic blood + phosphate-buffered saline (PBS), PBS + FKC and PBS + PBS. In the challenge test, the relative percentage survival (RPS) of the allogeneic + FKC, the allogeneic blood + PBS and the PBS + FKC group was 61.46, 35.41, and 30.63 % respectively. The up-regulated expression of Th1-related genes IFN-γ 1, IFN-γ 1rel2, IL-12p35 and T-bet suggests the protection is via CMI induction. Only in the allogeneic + FKC group, gene expression of IFN-γ 1, IL-12p35 and T-bet is significantly higher, indicating synergy between the two substances. Furthermore, among the fish injected with the allogeneic blood cells, syngeneic blood cells and PBS group, only in the fish of the allogenic blood cells injection group, did expression of IFN-γ 1, IFN-γ 2 and IFN-γ rel2 gene expression significantly increased. The results indicate that the rejection was induced by allogeneic components. Thus, our findings might provide essential information and insights into vaccine development in aquaculture.

Exploring the roles of phytobiotics in relieving the impacts of Edwardsiella tarda infection on fish: a mini-review

Edwardsiellosis caused by Edwardsiella tarda resulted in significant economic losses in aquaculture operations worldwide. This disease could infect a wide range of hosts, including freshwater, brackish water, and marine aquatic animals. Currently, antibiotics and vaccines are being used as prophylactic agents to overcome Edwardsiellosis in aquaculture. However, application of antibiotics has led to antibiotic resistance among pathogenic bacteria, and the antibiotic residues pose a threat to public health. Meanwhile, the use of vaccines to combat Edwardsiellosis requires intensive labor work and high costs. Thus, phytobiotics were attempted to be used as antimicrobial agents to minimize the impact of Edwardsiellosis in aquaculture. These phytobiotics may also provide farmers with new options to manage aquaculture species' health. The impact of Edwardsiellosis in aquaculture worldwide was elaborated on and highlighted in this review study, as well as the recent application of phytobiotics in aquaculture and the status of vaccines to combat Edwardsiellosis. This review also focuses on the potential of phytobiotics in improving aquatic animal growth performance, enhancing immune system function, and stimulating disease resistance.

Characterization of CD3γ/δ gene and its immune response in Qihe crucian carp Carassius auratus after challenged by Aeromonas veronii and Poly(I:C)

CD3γ/δ found in non-mammalian vertebrates is a CD3 homolog with structural characteristics similar to both mammalian CD3γ and CD3δ, and plays important roles in T cell recognization and immune response in fish. In this study, the full-length of CD3γ/δ from Qihe crucian carp (named CaCD3γ/δ) was cloned and characterized, then the expression response profiles and potential immune functions was explored after Aeromonas veronii and Poly(I:C) challenge. The results showed that the full-length of CaCD3γ/δ was 819 bp including a 5'-UTR of 141 bp, a 3'-UTR of 168 bp, and an ORF of 510 bp encoding a putative 169-aa protein with an estimated MW of 18.71 kD and a theoretical pI of 8.77. The protein sequence of CaCD3γ/δ contained a Leu-Leu and a CXXXC motif in the extracellular domain, and an ITAM and a Leu-Ile motif in the cytoplasm, and a residue of Asn in the transmembrane. CaCD3γ/δ was constitutively expressed in the spleen, liver, gill, and blood of Qihe crucian carp. After the carp were challenged with Poly(I:C) and Aeromonas veronii, the mRNA expression levels of CaCD3γ/δ were significantly changed in the spleen, head kidney, intestine and gill, according to the results of qPCR. However, compared with A. veronii, Poly(I:C) challenge can rapidly induce the CaCD3γ/δ expression levels in head kidney, intestine and spleen, which suggested CaCD3γ/δ may be differentially modulated by different pathogens. Moreover, the results of immunohistochemical analysis showed that the CaCD3γ/δ⁺ secreted cells in the spleen and gills of Qihe crucian were increased after challenged with Poly(I:C), as well as the spleen challenged with A. veronii, but at different levels. Combined with the fact that vascular congestion, necrosis of parenchymal cells, and inflammatory cells including lymphocytes infiltration were also observed in the gill and spleen of Qihe crucian carp treated with A. veronii and Poly(I:C) revealed by pathological analysis, it was predicted that CaCD3γ/δ⁺ T lymphocytes may participated in the immune response against pathogens. This study will contribute to understand the important role of CaCD3γ/δ⁺ T lymphocytes in the immune response of Qihe crucian carp, and provide new insights for the prevention and treatment of the diseases of Qihe crucian carp.

Characterization of the pathogenesis and immune response to a highly virulent Edwardsiella tarda strain responsible for mass mortality in the hybrid snakehead (Channa maculate ♀ × Channa argus ♂)

Edwardsiella tarda is reported as the causative agent of the systemic disease Edwardsiellosis in fish, which lead to huge economic losses in aquaculture. The pathogenicity and immune response to a highly virulent E. tarda isolate responsible for mass mortality in hybrid snakehead were performed. After species identification, morphology and virulence gene detection of Edwardsiella isolated from hybrid snakehead, the pathogenicity of the strain and histopathological changes in infected fish were analyzed. The infected fish exhibited typical acute hemorrhagic symptoms and enlarged internal organs. Histopathology revealed that the liver, spleen, kidney and intestinal tissues of diseased fish exhibited marked inflammatory with vacuolar degeneration and cell necrosis. Subsequently, humoral immune factors such as superoxide dismutase, lysozyme and acid phosphatase activities were detected as serum indicators, and real-time quantitative PCR was used to investigate immune-related genes (STAT1, HSP70, IgM, IL-6, IL-8, TRAF2, CD40, HLA-DMA and LCK) expression patterns in liver, spleen and head kidney. The results showed that these enzyme activity indicators and immune-related gene expression were significantly activated compared with healthy fish. These data provide insight into the pathogenic mechanisms and host immune responses of E. tarda, which could be useful for the future prevention and treatment of Edwardsiellosis in fish.

Why Does Piscirickettsia salmonis Break the Immunological Paradigm in Farmed Salmon? Biological Context to Understand the Relative Control of Piscirickettsiosis

Piscirickettsiosis (SRS) has been the most important infectious disease in Chilean salmon farming since the 1980s. It was one of the first to be described, and to date, it continues to be the main infectious cause of mortality. How can we better understand the epidemiological situation of SRS? The catch-all answer is that the Chilean salmon farming industry must fight year after year against a multifactorial disease, and apparently only the environment in Chile seems to favor the presence and persistence of Piscirickettsia salmonis. This is a fastidious, facultative intracellular bacterium that replicates in the host’s own immune cells and antigen-presenting cells and evades the adaptive cell-mediated immune response, which is why the existing vaccines are not effective in controlling it. Therefore, the Chilean salmon farming industry uses a lot of antibiotics—to control SRS—because otherwise, fish health and welfare would be significantly impaired, and a significantly higher volume of biomass would be lost per year. How can the ever-present risk of negative consequences of antibiotic use in salmon farming be balanced with the productive and economic viability of an animal production industry, as well as with the care of the aquatic environment and public health and with the sustainability of the industry? The answer that is easy, but no less true, is that we must know the enemy and how it interacts with its host. Much knowledge has been generated using this line of inquiry, however it remains insufficient. Considering the state-of-the-art summarized in this review, it can be stated that, from the point of view of fish immunology and vaccinology, we are quite far from reaching an effective and long-term solution for the control of SRS. For this reason, the aim of this critical review is to comprehensively discuss the current knowledge on the interaction between the bacteria and the host to promote the generation of more and better measures for the prevention and control of SRS.

Vitamin D3 protects turbot (Scophthalmus maximus L.) from bacterial infection

Accumulating evidence supports that vitamin D₃ (VD₃) possesses immunomodulatory properties besides its classical actions in calcium and bone homeostasis. In this study, juvenile turbots were fed with the diets containing 0 IU/kg VD₃ or the optimum dose of 400 IU/kg VD₃ for 8 weeks. To investigate the effects of VD₃ on anti-infectious immunity in fish, 10⁷ CFU Edwardsiella tarda was injected intraperitoneally to each juvenile turbot after the feeding trial. Our results showed that the mortality of infected turbots with dietary VD₃ was much lower than that in VD₃ deficient group, and the supplementation of dietary VD₃ significantly reduced the bacterial load in the spleen of infected turbots. Further analysis demonstrated that the production of reactive oxygen species (ROS) in haemocytes and lysozyme activity in serum was elevated, and the responses of T cells and B cells were modulated in VD₃-supplemented turbots. Moreover, the inflammation was significantly exacerbated in the infected turbots fed with 0 IU/kg VD₃ compared to the fish fed with 400 IU/kg VD₃. In addition, the head kidney macrophages (HKMs) in turbots were isolated and incubated with VD₃in vitro, the results showed that VD₃ significantly promoted the bactericidal activity in HKMs. In conclusion, our study has shown clear evidence that VD₃ positively regulates the innate and adaptive immunity in fish, which is beneficial to the defense in fish against pathogen infection.

Cytokines Induced by Edwardsiella tarda: Profile and Role in Antibacterial Immunity

Edwardsiella tarda is a Gram-negative bacterial pathogen with a broad range of hosts, including fish and mammals. In the present study, we used an advanced antibody array technology to identify the expression pattern of cytokines induced by E. tarda in a mouse infection model. In total, 31 and 24 differentially expressed cytokines (DECs) were identified in the plasma at 6 h and 24 h post-infection (hpi), respectively. The DECs were markedly enriched in the Gene Ontology (GO) terms associated with cell migration and response to chemokine and in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with immunity, diseases, and infection. Ten key DECs, including IL6 and TNF-α, were found to form extensive protein-protein interaction networks. IL6 was demonstrated to inhibit E. tarda infection and be required for E. tarda-induced inflammatory response. TNF-α also exerted an inhibitory effect on E. tarda infection, and knockdown of fish (Japanese flounder) TNF-α promoted E. tarda invasion in host cells. Together, the results of this study revealed a comprehensive profile of cytokines induced by E. tarda, thus adding new insights into the role of cytokine-associated immunity against bacterial infection and also providing the potential plasma biomarkers of E. tarda infection for future studies.

Host lactosylceramide enhances Edwardsiella tarda infection

Edwardsiella tarda is a Gram-negative bacterium causing economic damage in aquaculture. The interaction of E. tarda with microdomains is an important step in the invasion, but the target molecules in microdomains remain undefined. Here, we found that intraperitoneal injection of E. tarda altered splenic glycosphingolipid patterns in the model host medaka (Oryzias latipes) accompanied by alteration of glycosphingolipid metabolism-related gene expressions, suggesting that glycosphingolipid levels are involved in E. tarda infection. To ascertain the significance of glycosphingolipids in the infection, fish cell lines, DIT29 cells with a high amount of lactosylceramide (LacCer) and glucosylceramide (GlcCer), and GAKS cells with a low amount of these lipids, were treated with methyl-β-cyclodextrin to disrupt the microdomain. E. tarda infection was suppressed in DIT29 cells, but not in GAKS cells, suggesting the involvement of microdomain LacCer and GlcCer in the infection. DL-threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol, an inhibitor of glycosphingolipid-synthesis, attenuated the infection in DIT29 cells, while Neu3-overexpressing GAKS cells, which accumulated LacCer, enhanced the infection. E. tarda possessed binding ability towards LacCer, but not GlcCer, and LacCer preincubation declined the infection towards fish cells, possibly due to the masking of binding sites. The present study suggests that LacCer may be a positive regulator of E. tarda invasion.

Atlantic Salmon Pre-smolt Survivors of Renibacterium salmoninarum Infection Show Inhibited Cell-Mediated Adaptive Immune Response and a Higher Risk of Death During the Late Stage of Infection at Lower Water Temperatures

Bacterial kidney disease (BKD) is widespread in many areas of the world and can cause substantial economic losses for the salmon aquaculture industry. The objective of this study was to investigate the pathophysiological response and gene expression profiles related to the immune response at different water temperatures and to identify the best immunopathological biomarkers to define a phenotype of resistance to BKD. The abundance of msa transcripts of R. salmoninarum in the head kidney was significantly higher in infected fish at 11°C. R. salmoninarum induced significantly more severe kidney lesions, anemia and impaired renal function at 11°C. In addition, the expression pattern of the genes related to humoral and cell-mediated immune responses in infected fish at 11 and 15°C was very similar, although R. salmoninarum induced a significantly greater downregulation of the adaptive immune response genes at the lower water temperature. These results could be due to a suppressed host response directly related to the lowest water temperature and/or associated with a delayed host response related to the lowest water temperature. Although no significant differences in survival rate were observed, fish infected at the lowest temperature showed a higher probability of death and delayed the mortality curve during the late stage of infection (35 days after infection). Thirty-three immunopathological biomarkers were identified for potential use in the search for a resistance phenotype for BKD, and eight were genes related specifically to the adaptive cell-mediated immune response.

Teleost cytotoxic T cells

Cell-mediated cytotoxicity is one of the major mechanisms by which vertebrates control intracellular pathogens. Two cell types are the main players in this immune response, natural killer (NK) cells and cytotoxic T lymphocytes (CTL). While NK cells recognize altered target cells in a relatively unspecific manner CTLs use their T cell receptor to identify pathogen-specific peptides that are presented by major histocompatibility (MHC) class I molecules on the surface of infected cells. However, several other signals are needed to regulate cell-mediated cytotoxicity involving a complex network of cytokine- and ligand-receptor interactions. Since the first description of MHC class I molecules in teleosts during the early 90s of the last century a remarkable amount of information on teleost immune responses has been published. The corresponding studies describe teleost cells and molecules that are involved in CTL responses of higher vertebrates. These studies are backed by functional investigations on the killing activity of CTLs in a few teleost species. The present knowledge on teleost CTLs still leaves considerable room for further investigations on the mechanisms by which CTLs act. Nevertheless the information on teleost CTLs and their regulation might already be useful for the control of fish diseases by designing efficient vaccines against such diseases where CTL responses are known to be decisive for the elimination of the corresponding pathogen. This review summarizes the present knowledge on CTL regulation and functions in teleosts. In a special chapter, the role of CTLs in vaccination is discussed.

Desialylation by Edwardsiella tarda is the initial step in the regulation of its invasiveness

Edwardsiella tarda is a gram-negative bacterium causing significant economic losses to aquaculture. E. tarda possesses NanA sialidase which removes sialic acids from α2–3 sialo-glycoprotein of host cells. However, the relationship between NanA sialidase activity and E. tarda invasiveness remains poorly understood. Furthermore, the pathway of sialic acid metabolism in E. tarda remains to be elucidated. We studied sialidase activity in several E. tarda strains and found that the pathogenic strains exhibited higher sialidase activity and greater up-regulation of the NanA mRNA level than non-pathogenic strain. Pathogenic strains also showed higher rates of infection in GAKS cells, and the infection was drastically suppressed by sialidase inhibitor. Additionally, NanA gene overexpression significantly increased infection and treatment of E. tarda with free sialic acid enhanced the rate of infection in GAKS cells. Sialic acid treatment enhanced mRNA levels of two N-acetylneuraminate lyases and one N-acetylneuraminate cytidylyltransferase. E. tarda uses sialic acid as a carbon source for growth via N-acetylneuraminate lyases. The strains with high N-acetylneuraminate cytidylyltransferase level showed greater sialylation of the lipopolysaccharides and glycoproteins. Our study establishes the significance of desialylation by E. tarda sialidase in the regulation of its invasiveness.

Global profiling and characterization of Japanese flounder (Paralichthys olivaceus) kidney microRNAs regulated by Edwardsiella tarda infection in a time-dependent fashion

Japanese flounder (Paralichthys olivaceus) is an important economic fish species farmed in China and other countries. It is susceptible to infection by Edwardsiella tarda, a severe fish pathogen with a broad host range. In this study, we employed high-throughput deep sequencing technology to identify, in a global scale, flounder kidney microRNAs (miRNAs) induced by E. tarda at different stages of infection. Differentially expressed miRNAs (DEmiRNAs) and mRNAs (DEmRNAs) exhibiting significantly altered expression levels before and after E. tarda infection were examined. A total of 96 DEmiRNAs were identified, for which 2779 target genes were predicted. Eighty-seven miRNA-mRNA pairs, involving 29 DEmiRNAs and 86 DEmRNAs, showed negative correlations in their expression patterns. GO and KEGG enrichment analysis revealed that the putative target genes of the DEmiRNAs were associated with diverse biological processes, cellular components, and molecular functions. One of the DEmiRNAs, pol-miR-182-5p, was demonstrated to regulate sphingosine-1-phosphate receptor 1 (PoS1PR1) negatively in a manner that depended on the specific interaction between the seed sequence of pol-miR-182-5p and the 3'-UTR of PoS1PR1. Overexpression of pol-miR-182-5p in flounder cells promoted apoptosis and inhibited cellular viability. Knockdown of PoS1PR1 in flounder enhanced E. tarda invasion and dissemination in fish tissues. These results provide new insights into miRNA-mediated anti-bacterial immunity in flounder.

A DNA Vaccine Encoding the VAA Gene of Vibrio anguillarum Induces a Protective Immune Response in Flounder

Vibrio anguillarum is a pathogenic bacterium that infects flounder resulting in significant losses in the aquaculture industry. The VAA protein previously identified in flounder is associated with a role in immune protection within these fish. In the present study, a recombinant DNA plasmid encoding the VAA gene of V. anguillarum was constructed and its potential as a DNA vaccine, to prevent the infection of V. anguillarum in flounder fish, investigated. We verified the expression of the VAA protein both in vitro in cell lines and in vivo in flounder fish. The protective effects of pcDNA3.1-VAA (pVAA) were analyzed by determination of the percentage of sIgM⁺, CD4-1⁺, CD4-2⁺, CD8β⁺ lymphocytes, and the production of VAA-specific antibodies in flounder following their immunization with the DNA vaccine. Histopathological changes in immune related tissues, bacterial load, and relative percentage survival rates of flounder post-challenge with V. anguillarum, were all investigated to assess the efficacy of the pVAA DNA vaccine candidate. Fish intramuscularly immunized with pVAA showed a significant increase in CD4-1⁺, CD4-2⁺, and CD8β⁺ T lymphocytes at days 9, 11, and 14 post-vaccination, reaching peak T-cell levels at days 11 or 14 post-immunization. The percentage of sIgM⁺ lymphocytes reached peak levels at weeks 4–5 post-immunization. Specific anti-V. anguillarum or anti-rVAA antibodies were induced in inoculated fish at days 28–35 post-immunization. The liver of vaccinated flounder exhibited only slight histopathological changes compared with a significant pathology observed in control immunized fish. Additionally, a lower bacterial burden in the liver, spleen, and kidney were observed in pVAA protected fish in response to bacterial challenge, compared with pcDNA3.1 vector control injected fish. Moreover, the pVAA vaccine confers a relative percentage survival of 50.00% following V. anguillarum infection. In summary, this is the first study indicating an initial induction of the T lymphocyte response, followed by B lymphocyte induction of specific antibodies as a result of DNA immunization of flounder. This signifies the important potential of pVAA as a DNA vaccine candidate for the control of V. anguillarum infection.

Correlates of protective immunity for fish vaccines

Vaccination is one of the most effective disease control strategies that has contributed to the significant reduction of disease outbreaks and antibiotics usage in salmonid aquaculture. To date, licensing of fish vaccines is to a limited extent based on in vitro correlates of protection, as done for many mammalian vaccines. This is because the immunological mechanisms of vaccine protection have not been clearly elucidated for most fish vaccines. Herein, we provide an overview of the different steps required to establish correlates of protective immunity required to serve as benchmarks in optimizing vaccine production in aquaculture. We highlight the importance of optimizing challenge models needed to generate consistent results used during vaccine development as a basis for establishing immune correlates of protection. Data generated this far shows that antibodies are potentially the most reliable correlates of protective immunity for fish vaccines. Our findings also show that antigen dose can be optimized to serve as a correlate of protection for fish vaccines. Further, there is need to establish signatures of T-cell protective immunity when antibodies fail to serve as proxies of immune protection, particularly for vaccines against intracellular pathogens. We can anticipate that documentation of efficacy for future vaccines in aquaculture, particularly batch testing will be based on in vitro correlates of protective immunity.

Intracellular Bacterial Infections: A Challenge for Developing Cellular Mediated Immunity Vaccines for Farmed Fish

Aquaculture is one of the most rapidly expanding farming systems in the world. Its rapid expansion has brought with it several pathogens infecting different fish species. As a result, there has been a corresponding expansion in vaccine development to cope with the increasing number of infectious diseases in aquaculture. The success of vaccine development for bacterial diseases in aquaculture is largely attributed to empirical vaccine designs based on inactivation of whole cell (WCI) bacteria vaccines. However, an upcoming challenge in vaccine design is the increase of intracellular bacterial pathogens that are not responsive to WCI vaccines. Intracellular bacterial vaccines evoke cellular mediated immune (CMI) responses that “kill” and eliminate infected cells, unlike WCI vaccines that induce humoral immune responses whose protective mechanism is neutralization of extracellular replicating pathogens by antibodies. In this synopsis, I provide an overview of the intracellular bacterial pathogens infecting different fish species in aquaculture, outlining their mechanisms of invasion, replication, and survival intracellularly based on existing data. I also bring into perspective the current state of CMI understanding in fish together with its potential application in vaccine development. Further, I highlight the immunological pitfalls that have derailed our ability to produce protective vaccines against intracellular pathogens for finfish. Overall, the synopsis put forth herein advocates for a shift in vaccine design to include CMI-based vaccines against intracellular pathogens currently adversely affecting the aquaculture industry.

The development of cellular immune defence in marine medaka Oryzias melastigma

Environmentally induced alterations of the immune system during sensitive developmental stages may manifest as abnormalities in immune organ configuration and/or immune cell differentiation. These not only render the early life stages more vulnerable to pathogens, but may also affect the adult immune competence. Knowledge of these sensitive periods in fish would provide an important prognostic/diagnostic tool for aquatic risk assessment of immunotoxicants. The marine medaka Oryzias melastigma is an emerging seawater fish model for immunotoxicology. Here, the presence and onset of four potentially sensitive periods during the development of innate and adaptive cellular immune defence were revealed in O. melastigma: 1.) initiation of phagocyte differentiation, 2.) migration and expansion of lymphoid progenitor cells, 3.) colonization of immune organs through lymphocyte progenitors and 4.) establishment of immune competence in the thymus. By using an established bacterial resistance assay for O. melastigma, larval immune competence (from newly hatched 1dph to 14dph) was found concomitantly increased with advanced thymus development and the presence of mature T-lymphocytes. A comparison between the marine O. melastigma and the freshwater counterpart Oryzias latipes disclosed a disparity in the T-lymphocyte maturation pattern, resulting in differences in the length of T-lymphocyte maturation. The results shed light on a potential difference between seawater and freshwater medaka in their sensitivity to environmental immunotoxicants. Further, medaka immune system development was compared and contrasted to economically important fish. The present study has provided a strong scientific basis for advanced investigation of critical windows for immune system development in fish.

Adjuvant effect of recombinant interleukin-12 in the Nocardiosis formalin-killed vaccine of the amberjack Seriola dumerili

Nocardiosis causes serious economic damage in the fish farming of Japanese yellowtail fish. Nocardia seriolae identified as pathogenic bacterium is an intracellular-pathogen. In general, induction of cell-mediated immunity (CMI) is effective in infection defense against intracellular-pathogen. However, the conventional formalin-killed N. seriolae (FKC) vaccine induces humoral immunity. Interleukin-12 (IL-12) is Th1 type heterodimeric cytokine and induces cell differentiation in mammals. Our previous study showed that recombinant amberjack IL-12 has a role in CMI induction in vitro and could be a possible CMI inducing adjuvant. However, its adjuvant effect of fish IL-12 was not studied. In the present study, six types of amberjack recombinant IL-12 (rIL-12) were mixed and injected into amberjack with FKC. Firstly, we analyzed Th1- and Th2- related gene expression and monitored Th1/Th2 status followed by investigation of antibody titer. As a result, Th1-type immunity was induced in FKC + rIL-12 vaccinated fish. Secondly, we checked Th1/Th2 status of vaccinated fish after 10 days of N. seriolae infection using the expression of related genes. High T-bet/GATA-3 ratio was observed in FKC + rIL-12 vaccinated fish, suggesting that Th1 cells possesing antigen memory were induced against N. seriolae infection. Finally, the survival rate in challenge test showed that 88% of FKC + rIL-12 vaccinated fish was survived at 34 days after N. seriolae injection whereas PBS (control) and FKC only were exterminated. These result suggest that i) rIL-12 is viable CMI inducible adjuvant and ii) production of Th1 cells having antigen memory resulting from activation of IL-12 signaling pathway is important for defense against N. seriolae infection.

Molecular characterization of a T cell co-stimulatory receptor, CD28 of rock bream (Oplegnathus fasciatus): Transcriptional expression during bacterial and viral stimulation

CD28 is a co-stimulatory receptor that provides a critical second signal alongside T cell receptors for the activation of naive T cells. We characterized the CD28 gene of rock bream, which has a deduced amino acid sequence of 221 residues with an extracellular Ig-superfamily V domain, transmembrane region, and cytoplasmic tail. The conservation in domain structures and other motifs shows that it is highly likely that RbCD28 is a homologue of mammalian CD28 and may have related co-stimulatory functions. RbCD28 is constitutively expressed in most tissues that were analysed, with a relatively higher expression in teleost lymphoid organs, such as spleens, gills, trunk kidneys and skin. Unlike human CD28, RbCD28 is highly expressed in skin and gill-associated lymphoid organs. Although gills showed constitutive expression of RbCD28 in control animals, after a pathogen challenge, induction of CD28 was low, particularly in RSIV and E. tarda infection. Whereas induction of RbCD28 was observed in kidney during E. tarda and S. iniae infection, downregulation was observed during RSIV infection. In the case of the liver, E. tarda caused an initial upregulation of RbCD28. RbCD28 activation of T cells in the spleen was limited to S. iniae infection. Activation of RbCD28 observed in lymphoid organs during infection of various pathogens shows its key role as a co-stimulatory receptor of T cells.

Development and application of monoclonal antibodies against IgM of black rockfish Sebastes schlegeli

Monoclonal antibodies (mAbs) against black rockfish Sebastes schlegeli serum immunoglobulin M (IgM) were developed, which showed a specific reaction with the heavy chain of S. schlegeli IgM in Western blotting and with surface IgM positive (sIgM⁺ ) lymphocytes in indirect immunofluorescence. mAb 2A6 was employed to investigate the antibody and sIgM⁺ lymphocyte responses of S. schlegeli injected with inactivated Edwardsiella tarda, by ELISA and flow cytometry. Compared with controls, the level of specific antibodies and the percentage of sIgM⁺ lymphocytes both increased in the immunized fish and simultaneously reached their peaks at day 35 after immunization.

Characterization of CD3+ T lymphocytes of Japanese flounder (Paralichthys olivaceus) and its response after immunization with formalin-inactivated Edwardsiella tarda

The CD3 complex is an important cell surface marker of T lymphocytes and essential for T lymphocytes activation in higher vertebrates. In the present work, the CD3ε of Japanese flounder (Paralichthys olivaceus) was recombinantly expressed in E. coli BL21 (DE3) and used as an immunogen to produce mouse anti-rCD3ε polyclonal antibodies, which could specifically recognize a 20 kDa protein in the membrane proteins of peripheral blood lymphocytes (PBL) of Japanese flounder by co-immunoprecipitation assay. Mass spectrometric analysis showed the 20 kDa protein was the native CD3ε of Japanese flounder. Both the flow cytometric analysis and double immunofluorescence assay (DIFA) showed that the CD3⁺ T lymphocytes could be identified specifically by the mouse anti-rCD3ε polyclonal antibodies, which didn't cross-react with the sIgM⁺ lymphocytes. Immunohistochemistry showed that CD3⁺ T lymphocytes could be detected in gill, skin, stomach, intestine, spleen, liver, head-kidney and mid-kidney. Flow cytometric analysis showed the percentages of CD3⁺ T lymphocytes in the PBL, spleen lymphocytes (SL) and head-kidney lymphocytes (HKL) of Japanese flounder increased rapidly after immunization with formalin-inactivated Edwardsiella tarda, and reached their peak levels at 5th day with 12.6%, 9.7% and 8.7%, respectively, and then decreased gradually. These results suggested that CD3⁺ T lymphocytes play important roles in mucosal and cell-mediated immunity, and the results would deepen our understanding on the roles of teleost T lymphocytes in the immune response.

Naringenin suppresses Edwardsiella tarda infection in GAKS cells by NanA sialidase inhibition

Edwardsiella tarda (E. tarda) is a gram-negative bacterium, which causes Edwardsiellosis in aquaculture. Previous studies indicate that E. tarda NanA sialidase plays crucial roles in infection through the desialylation of glycoproteins in fish cells. On the other hand, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid, classic sialidase inhibitor, negatively regulates E. tarda infection of goldfish scale GAKS cells. Here, to development the suppression model of E. tarda infection for aquaculture application, the possibility of NanA inhibitory activities in citrus phytochemicals was evaluated as citrus extracts have widely been used as a supplement in fish diets for the improvement of meat quality. Some flavanones such as naringenin, hesperetin, hesperidin and naringin showed sialidase inhibitory activity toward recombinant NanA in vitro. Among them, naringenin showed the most potent inhibitory activity and its inhibitory pattern was non-competitive. Naringenin significantly suppressed E. tarda infection in GAKS cells at 200 and 400 μM without bactericidal effect on E. tarda. On the other hand, naringin, glycosylation form of naringenin, showed slight suppression of E. tarda infection toward GAKS cells, suggesting the glycosides on flavanone could be important for NanA inhibition. Fluorescence microscopy analysis verified that number of invading E. tarda in GAKS cells was declined by naringenin treatment. The present study exhibited the possibility of naringenin as an effective ingredient in fish diet for the inhibition of E. tarda infection.

The protection effect of LEAP-2 on the mudskipper (Boleophthalmus pectinirostris) against Edwardsiella tarda infection is associated with its immunomodulatory activity on monocytes/macrophages

Liver-expressed antimicrobial peptide 2 (LEAP-2) is a cationic peptide that plays an important role in the host's innate immune system. However, the mechanism by which LEAP-2 modulates/regulates the host defense against pathogens remains largely unknown. In this study, we identified a cDNA sequence encoding LEAP-2 homolog (BpLEAP-2) in the mudskipper, Boleophthalmus pectinirostris. Sequence analysis revealed that BpLEAP-2 belonged to the fish LEAP-2A cluster and that it was closely related to ayu LEAP-2. BpLEAP-2 mRNA was detected in a wide range of tissues, with the highest level of transcripts found in the liver. Upon infection with Edwardsiella tarda, BpLEAP-2 mRNA expression was significantly increased in the liver, kidney, spleen, and gill, but decreased in the intestine. Chemically synthesized BpLEAP-2 mature peptide did not exhibit antibacterial activity against E. tarda in vitro. Intraperitoneal injection of BpLEAP-2 (1.0 or 10.0 μg/g) resulted in significantly improved survival rate and reduced tissue bacterial load in E. tarda-infected mudskippers. In E. tarda-infected fish, BpLEAP-2 (0.1, 1.0, or 10.0 μg/g) eliminated E. tarda-induced tissue mRNA expression of BpTNF-α and BpIL-1β. In monocytes/macrophages (MO/MФ), BpLEAP-2 (1.0 or 10.0 μg/ml) induced chemotaxis, enhanced respiratory burst, and inhibited E. tarda-induced mRNA expression of BpTNF-α and BpIL-1β. At a concentration of 10.0 μg/ml, BpLEAP-2 also significantly enhanced the bacterial killing efficiency of MO/MФ. No significant effect was seen in the phagocytic activity of MO/MФ upon treatment with BpLEAP-2. Our study provides evidence, for the first time, that LEAP-2 exhibited immunomodulatory effects on immune cells, and protected the host from pathogenic infections independent of direct bacterial killing function.

Essential oils to control ichthyophthiriasis in pacu, Piaractus mesopotamicus (Holmberg): special emphasis on treatment with Melaleuca alternifolia

In vitro effect of the Melaleuca alternifolia, Lavandula angustifolia and Mentha piperita essential oils (EOs) against Ichthyophthirius multifiliis and in vivo effect of M. alternifolia for treating ichthyophthiriasis in one of the most important South American fish, Piaractus mesopotamicus (Holmberg), were evaluated. The in vitro test consisted of three EOs, each at concentrations of 57 μL L(-1) , 114 μL L (-1) , 227 μL L(-1) and 455 μL L (-1) , which were assessed once an hour for 4 h in microtitre plates (96 wells). The in vitro results demonstrated that all tested EOs showed a cytotoxic effect against I. multifiliis compared to control groups (P < 0.05). The in vivo treatment for white spot disease was performed in a bath for 2 h day(-1) for 5 days using the M. alternifolia EO (50 μL L (-1) ). In this study, 53.33% of the fish severely infected by I. multifiliis survived after the treatment with M. alternifolia (50 μL L (-1) ) and the parasitological analysis has shown an efficacy of nearly 100% in the skin and gills, while all the fish in the control group died. Furthermore, the potential positive effect of M. alternifolia EO against two emergent opportunistic bacteria in South America Edwardsiella tarda and Citrobacter freundii was discussed.

Identification and functional characterization of multiple interleukin 12 in amberjack (Seriola dumerili)

Interleukin (IL) -12 is a heterodimeric cytokine mainly produced by monocytes, macrophages, and dendritic cells in mammals. IL-12p70 composed of IL-12p35 and IL-12p40, is known to play a crucial role in promoting cell-mediated immunity (CMI) through Th1 differentiation and IFN-γ production. Although two types of IL-12p35 (p35a, p35b) and three types of IL-12p40 (p40a, p40b and p40c) have been identified in several fish species, the knowledge on functional characteristics of teleost IL-12 is still limited. In the present study, we cloned two types of IL-12p35 and three types of IL-12p40 genes in amberjack and yellowtail, and analyzed their expressions in response to stimulation with Nocardia seriolae in amberjack. As a result, four types of IL-12 (IL-12p35a, p35b, p40a and p40b) and IFN-γ mRNA were increased by live-N. seriolae stimulation but not by formalin-killed N. seriolae, suggesting that four types of IL-12 (p35, p35b, p40a and p40c) participate in promoting CMI. Subsequently, we produced six types of recombinant IL-12p70 (rIL12p70) protein in insect cells. Head kidney leukocytes were cultured with formalin-killed N. seriolae and six types of rIL-12p70 to elucidate the role of amberjack IL-12p70 in induction of CMI. After stimulation, IFN-γ expression was elevated whereas IL-10 expression was suppressed in Head kidney leukocytes stimulated with four types of rIL-12 (p40a/p35a, p40c/p35a, p40a/p35b, p40a/p35b). On the other hand, two types of rIL-12 (p40b/p35a, p40b/p35b) only elicited down regulation of IL-10 expression. These results indicate that all amberjack IL-12p70 isoforms are involved in Th1 -differentiation and promotion of CMI with different manners. Fish IL-12 has a potential for the promising vaccine adjuvant.

Recombinant sialidase NanA (rNanA) cleaves α2-3 linked sialic acid of host cell surface N-linked glycoprotein to promote Edwardsiella tarda infection

Edwardsiella tarda is one of the major pathogenic bacteria affecting both marine and freshwater fish species. Sialidase NanA expressed endogenously in E. tarda is glycosidase removing sialic acids from glycoconjugates. Recently, the relationship of NanA sialidase activity to E. tarda infection has been reported, however, the mechanism with which sialidase NanA aids the pathogenicity of E. tarda remained unclear. Here, we comprehensively determined the biochemical properties of NanA towards various substrates in vitro to provide novel insights on the potential NanA target molecule at the host cell. GAKS cell pretreated with recombinant NanA showed increased susceptibility to E. tarda infection. Moreover, sialidase inhibitor treated E. tarda showed a significantly reduced ability to infect GAKS cells. These results indicate that NanA-induced desialylation of cell surface glycoconjugates is essential for the initial step of E. tarda infection. Among the natural substrates, NanA exhibited the highest activity towards 3-sialyllactose, α2-3 linked sialic acid carrying sialoglycoconjugates. Supporting this finding, intact GAKS cell membrane exposed to recombinant NanA showed changes of glycoconjugates only in α2-3 sialo-linked glycoproteins, but not in glycolipids and α2-6 sialo-linked glycoproteins. Lectin staining of cell surface glycoprotein provided further evidence that α2-3 sialo-linkage of the N-linked glycoproteins was the most plausible target of NanA sialidase. To confirm the significance of α2-3 sialo-linkage desialylation for E. tarda infection, HeLa cells which possessed lower amount of α2-3 sialo-linkage glycoprotein were used for infection experiment along with GAKS cells. As a result, infection of HeLa cells by E. tarda was significantly reduced when compared to GAKS cells. Furthermore, E. tarda infection was significantly inhibited by mannose pretreatment suggesting that the bacterium potentially recognizes and binds to mannose or mannose containing chains following desialylation. Together, these results suggest that E. tarda may employ endogenous NanA to desialylate α2-3 glycoproteins on host cells, thus revealing one of the potential binding molecules during infection.

T Cells in Fish

Cartilaginous and bony fish are the most primitive vertebrates with a thymus, and possess T cells equivalent to those in mammals. There are a number of studies in fish demonstrating that the thymus is the essential organ for development of T lymphocytes from early thymocyte progenitors to functionally competent T cells. A high number of T cells in the intestine and gills has been reported in several fish species. Involvement of CD4⁺ and CD8α⁺ T cells in allograft rejection and graft-versus-host reaction (GVHR) has been demonstrated using monoclonal antibodies. Conservation of CD4⁺ helper T cell functions among teleost fishes has been suggested in a number studies employing mixed leukocyte culture (MLC) and hapten/carrier effect. Alloantigen- and virus-specific cytotoxicity has also been demonstrated in ginbuna and rainbow trout. Furthermore, the important role of cell-mediated immunity rather than humoral immunity has been reported in the protection against intracellular bacterial infection. Recently, the direct antibacterial activity of CD8α⁺, CD4⁺ T-cells and sIgM⁺ cells in fish has been reported. In this review, we summarize the recent progress in T cell research focusing on the tissue distribution and function of fish T cells.

Comparative analysis of adaptive immune response after vaccine trials using live attenuated and formalin-killed cells of Edwardsiella tarda in ginbuna crucian carp (Carassius auratus langsdorfii)

Edwardsiella tarda is an intracellular pathogen that causes edwardsiellosis in fish. Although vaccine trials with formalin-killed cells (FKC) have been reported, the vaccinations failed in protect against E. tarda infection. On the other hand, a live attenuated vaccine strategy is effective against edwardsiellosis; however, the mechanism underlying its effectiveness in fish is unclear. In the present study, we compared the adaptive immune responses in fish vaccinated with FKCs and live attenuated vaccines to elucidate the induction of adaptive immune responses following vaccination. After challenge with E. tarda, live cell (LC)-vaccinated fish showed high survival rates, high IFN-g and T-bet gene expression levels, and increased cytotoxic T lymphocytes (CTLs). In contrast, all FKC-vaccinated fish died following E. tarda infection. In addition, FKC vaccination induced high IL-4/13A and IL-10 expression levels and increased antibody titers, whereas Th1-like responses were suppressed. These results indicate that LC vaccination contributes to protection against E. tarda infection by inducing cell-mediated immunity (CMI). Thus our study findings could contribute to the development a vaccine that induces CMI against edwardsiellosis.

Role of intestinal inflammation in predisposition of Edwardsiella tarda infection in zebrafish (Danio rerio)

Edwardsiella tarda, an enteric opportunistic pathogen, is associated with acute to chronic edwardsiellosis in cultured fish, resulting in heavy losses in aquaculture. To date, the pathogenesis of E. tarda has been extensively studied and a great deal of vaccine candidates have been attempted. However, the research on the predisposition of E. tarda infection is poorly reported. In this study, the effects of intestinal inflammation on E. tarda infection were investigated using a zebrafish model that influenced by perturbation of intestinal microbiota. Featured symptoms of edwardsiellosis were observed in intestinal inflammatory zebrafish compared with healthy fish. Higher bacterial numbers were detected in both mucosal tissues (intestine, skin and gills) and lymphoid tissues (liver, spleen and kidney) of inflammatory zebrafish while the bacterial loads in healthy zebrafish appeared to be relatively lower by 10-100 folds. Moreover, significant up-regulation of IL-1β, TNF-α and iNOS was noticed in multiple tissues of zebrafish with intestinal inflammation between 6 and 72 h post infection. However, only moderate elevation was observed in the gills and liver of healthy fish. Furthermore, the expression of genes involved in neutrophil recruitment (mpx, IL-8 and LECT2) and antimicrobial response (β-defensin and hepcidin) showed notable up-regulation in the intestine of inflammatory zebrafish. These results demonstrate that fish with intestinal inflammation is more susceptible to E. tarda and the antimicrobial response during E. tarda infection might inhibit the growth of intestinal microbiota. Our results suggest that maintaining good management to avoid intestinal inflammation is a feasible prevention measure against edwardsiellosis.

Role of CD4(+) and CD8α(+) T cells in protective immunity against Edwardsiella tarda infection of ginbuna crucian carp, Carassius auratus langsdorfii

Edwardsiella tarda is an intracellular pathogen that causes edwardsiellosis in fish. Our previous study suggests that cell-mediated immunity (CMI) plays an essential role in protection against E. tarda infection. In the present study, we adoptively transferred T-cell subsets sensitized with E. tarda to isogenic naïve ginbuna crucian carp to determination the T-cell subsets involved in protecting fish from E. tarda infection. Recipients of CD4(+) and CD8α(+) cells acquired significant resistance to infection with E. tarda 8 days after sensitization, indicating that helper T cells and cytotoxic T lymphocytes plays crucial roles in protective immunity to E. tarda. Moreover, transfer of sensitized CD8α(+) cells up-regulated the expression of genes encoding interferon-γ (IFN-γ) and perforin, suggesting that protective immunity to E. tarda involves cell-mediated cytotoxicity and interferon-γ-mediated induction of CMI. The results establish that CMI plays a crucial role in immunity against E. tarda. These findings provide novel insights into understanding the role of CMI to intracellular pathogens of fish.