Field tests of Poly(I:C) immunization with nervous necrosis virus (NNV) in sevenband grouper, Epinephelus septemfasciatus (Thunberg)

Fundamentals of Fish Vaccination

Fish health management has become a critical component of disease control and is invaluable for improved harvests and sustainable aquaculture. Vaccination is generally accepted as the most effective prophylactic measure for fish disease prevention, on environmental, social, and economic grounds. Although the historical approach for developing fish vaccines was based on the principle of Louis Pasteur's "isolate, inactivate and inject," but their weak immunogenicity and low efficacies in many cases, have shifted the focus of fish vaccine development from traditional to next-generation technologies. However, before any fish vaccine can be successfully commercialized, several hurdles need to be overcome regarding the production cost, immunogenicity, effectiveness, mode of administration, environmental safety, and associated regulatory concerns. In this context, the chapter summarises the basic aspects of fish vaccination such as type of vaccine, modalities of vaccine delivery, the immunological basis of fish immunization as well as different challenges associated with the development process and future opportunities.

Feeding with poly(I:C) induced long-term immune responses against bacterial infection in turbot (Scophthalmus maximus)

Poly(I:C) is a kind of chemosynthetic double-stranded RNA (dsRNA) analogue which could act as TLR3 agonist and induce IFN production. It is widely applied in anti-virus treatment and immunoregulation, as well as vaccine adjuvant in farmed animals. However, whether poly(I:C) could activate innate immune response to defense against bacterial infection remains unclear. In this study, we established a feeding trial model with different dose of poly(I:C) in turbot larvae, then challenged with Edwardsiella piscicida after 3–7 weeks resting period. The results show that feeding turbot with poly(I:C) exhibited a stronger inflammatory response and antioxidant stress ability, and significantly elevated the survival rate within the decreased bacterial loads. Importantly, the bacterial infection-induced white feces in hindgut of turbot were significantly alleviated after poly(I:C) feeding, and this administration induced protection could last for about 7 weeks. Taken together, these findings indicate that feeding turbot with poly(I:C) could enhance a long-term intestinal mucosal immunity in response to bacterial infection, suggesting that poly(I:C) might be a promising immunostimulant in aquaculture.

Poly (I:C)-Potentiated Vaccination Enhances T Cell Response in Olive Flounder (Paralichthys olivaceus) Providing Protection against Viral Hemorrhagic Septicemia Virus (VHSV)

Viral hemorrhagic septicemia (VHS), caused by viral hemorrhagic septicemia virus (VHSV), is a viral disease affecting teleosts, and is the major cause of virus-related deaths in olive flounder (Paralichthys olivaceus). Research has focused on ways to control VHS, and recently, the use of polyinosinic-polycytidylic acid poly (I:C)-potentiated vaccination has been investigated, whereby fish are injected with poly (I:C) and then with live pathogenic virus, resulting in a significant decrease in VHSV-related mortality. T cell responses were investigated in the present study after vaccinating olive flounder with poly (I:C)-potentiated vaccination to understand the ability of poly (I:C) to induce T cell immunity. Stimulation of T cell responses with the poly (I:C)-potentiated vaccination was confirmed by examining levels of CD3+ T cells, CD4-1+ T cells and CD4-2+ T cells. Higher levels of CD4-2+ T cells were found in vaccinated fish than CD4-1+ T cells, believed to result from a synergistic effect between poly (I:C) administration and pathogenic VHSV immunization. More importantly, the role of CD4-2+ T cells in the antiviral response was clearly evident. The results of this study suggest that the outstanding protection obtained with the poly (I:C)-potentiated vaccination is due to the robust immune response initiated by the CD4-2+ T cells.

Efficacy of live NNV immersion vaccine immunized at low temperature in sevenband grouper, Epinephelus septemfasciatus

The objective of this study was to investigate safety and efficacy using a low-temperature immunization protocol with NNV in sevenband grouper, Epinephelus septemfasciatus. Further, NNV specific antibody post immunization and intramuscularly challenge was also evaluated. Immunization at low temperature resulted in a low titer virus infection in brain tissues without any clinical symptoms of infection such as sluggish behavior and/or spinning, rotating swimming being observed, and no mortality was observed. Post challenge, NNV titer NNV giving an RPS of 100 %, increased in brain tissues of naïve (non-immunized) sevenband grouper NNV giving an RPS of 100 %, with a cumulative mortality of 100 % at 25 days post-infection. No mortality or disease symptoms NNV giving an RPS of 100 %, as NNV giving and of 100 %, observed in the groups immunized at low temperature with live NNV giving an RPS of 100 %. NNV giving an RPS of 100 %. NNV specific antibody was not detected in live NNV vaccinated sevenband grouper. This is the first study that confirms that field-scale NNV immersion vaccine can protect sevenband grouper against lethal infection with NNV at natural seawater temperature under the gradually increased from 14.3-24.8 °C.

Altered conformational structures of nervous necrosis virus surface protrusions and free coat proteins after incubation at moderate-low temperatures

Nervous necrosis virus (NNV) is a pathogenic fish virus belonging to family Nodaviridae. The objective of this study was to analyze stabilities of NNV surface protrusion and free coat protein (CP) conformational structures by analyzing changes of NNV infectivity and antigenicity after incubation at moderate-low temperatures. When cultured NNV suspension was incubated at 45 °C, its infectivity declined gradually but its antigenicity maintained. In contrast, both infectivity and antigenicity of purified NNV declined after incubation at 45 °C. After heat-treatment, surface protrusions of NNV particles disappeared completely, although viral particle structures maintained. Therefore, the reduction in NNV infectivity appeared to specifically occur as a result of heat-denaturation of virus surface protrusions. The loss of NNV infectivity in the presence of fetal bovine serum (FBS) was delayed compared to virus heated in the absence of FBS, demonstrating that FBS could function as a stabilizer for conformational structures of NNV surface protrusions. Moreover, the stabilizing function of FBS changed depending on salt concentration. Continued maintenance of antigenicity for heated cultured NNV suspension containing free-CPs may suggest that conformational structures corresponding to protrusion-domain of free-CP are more heat-stable than those of surface protrusions on NNV particles.

Lack of nervous necrosis virus (NNV) neutralizing antibodies in convalescent sevenband grouper Hyporthodus septemfasciatus after NNV infection

Viral nervous necrosis (VNN) is caused by nervous necrosis viruses (NNVs) belonging to genus Betanodavirus (Nodaviridae). It is one of the most serious diseases in aquaculture industry worldwide. In the present study, the kinetics of NNV-infectivity and NNV-specific antibodies in convalescent sevenband grouper Hyporthodus septemfasciatus after NNV infection was determined. When fish were infected with NNV at 17.5 °C, and reared for 84 days at natural seawater temperature (increasing rate: approximately 0.1 °C/day), NNV infectivity peaked on day 14 with 10^7.80 TCID₅₀/g at the highest, and declined to below the detection limit. When convalescent fish were reared at 27 °C, and re-infected with NNV at 10^4.3 or 10^6.3 TCID₅₀/fish, no mortality was observed although NNV multiplied up to 10^8.80 and 10^7.80 TCID₅₀/g at the highest, respectively, suggesting NNV-specific immune response. It also revealed that convalescent fish were re-infected by NNV although NNV multiplication was strongly regulated. Interestingly, NNV-specific antibodies were detectable in 20% and ≥80% of convalescent fish before and after re-infection with NNV, respectively. However, no NNV-neutralizing activity was detected before and after re-infection in almost all of the convalescent fish. Therefore, NNV-neutralizing antibodies might not be necessary for the protection of convalescent fish against NNV re-infection after previous NNV infection.

Poly (I:C) and imiquimod induced immune responses and their effects on the survival of olive flounder (Paralichthys olivaceus) from viral haemorrhagic septicaemia

The stimulation of immune genes by polyinosinic:polycytidylic acid (poly (I:C)) and imiquimod in olive flounder (Paralichthys olivaceus) and their role in control of viral haemorrhagic septicaemia virus (VHSV) infection were examined. Poly (I:C) (100 μg/fish) treated olive flounder had very low mortality (5%) post VHSV infection, while the imiquimod treated group had 65% and 85% mortality at a dose of 100 μg/fish and 50 μg/fish, respectively. Though the imiquimod treated group had high mortality, it was lower than the untreated group, which had 90% mortality. In vivo experiments were conducted to determine effect of the two ligands on immune modulation in the head kidney of olive flounder. Poly (I:C) activated the immune genes (TLR-3, TLR-7, MDA-5, LGP-2, IRF-3, IRF-7, IL-1β type I IFN and Mx) very early, within 1 d post stimulation, faster and stronger than imiquimod. Though Mx levels were enhanced by imiquimod, the host was still susceptible to VHSV. The poly (I:C) treated group had a high immune response at the time of infection and 1 dpi, though it decreased at later stages. The imiquimod treated group and the unstimulated group had a higher immune response to VHSV compared to the poly (I:C) treated group. The nucleoprotein copies of VHSV were very low in the poly (I:C) treated group but interestingly, were high in both untreated and imiquimod treated fish. Thus, host survival from a viral infection does not only depend on the quantity of immune response but also the time of response. Although imiquimod enhanced immune gene expression in olive flounder, a delayed response could be the reason for high mortality to VHS compared with poly (I:C), which induced the immune system effectively and efficiently to protect the host.

Protective immunity against rock bream iridovirus (RBIV) infection and TLR3-mediated type I interferon signaling pathway in rock bream (Oplegnathus fasciatus) following poly (I:C) administration

In this study, we evaluated the potential of poly (I:C) to induce antiviral status for protecting rock bream from RBIV infection. Rock bream injected with poly (I:C) at 2 days before infection (1.1 × 10⁴) at 20 °C had significantly higher protection with RPS 13.4% and 33.4% at 100 and 200 μg/fish, respectively, through 100 days post infection (dpi). The addition of boost immunization with poly (I:C) at before/post infection at 20 °C clearly enhanced the level of protection showing 33.4% and 60.0% at 100 and 200 μg/fish, respectively. To investigate the development of a protective immune response, rock bream were re-infected with RBIV (1.1 × 10⁷) at 200 dpi. While 100% of the previously unexposed fish died, 100% of the previously infected fish survived. Poly (I:C) induced TLR3 and Mx responses were observed at several sampling time points in the spleen, kidney and blood. Moreover, significantly high expression levels of IRF3 (2.9- and 3.1-fold at 1 d and 2 days post administration (dpa), respectively), ISG15 and PKR expression (5.4- and 10.2-fold at 2 dpa, respectively) were observed in the blood, but the expression levels were low in the spleen and kidney after poly (I:C) administration. Our results showed the induction of antiviral immune responses and indicate the possibility of developing long term preventive measures against RBIV using poly (I:C).

Purification of nervous necrosis virus (NNV) particles by anion-exchange chromatography

Nervous necrosis virus (NNV) belongs to genus Betanodavirus (family Nodaviridae). It is highly pathogenic to various marine fishes. In the present study, cultured NNV suspension was placed in dialysis tube at molecular weight cut off (MWCO) of 10⁶ and dialyzed against Dulbecco's phosphate buffered saline (D-PBS), 15mM Tris-HCl (pH 8.0), or deionized water (DIW) for 14days followed by anion-exchange chromatography. Infectivity titers of NNV suspensions were stable during dialyses. However, the antigenicity of NNV suspension was decreased to 2.5% by D-PBS dialysis, 11.8% by Tris-HCl dialysis, and 56.2% by DIW dialysis. Anion-exchange chromatograms revealed a total of four peaks (P300, P400, P600 and P700) for NNV suspension after D-PBS dialysis. Additional two peaks (P800 and P-OH) were detected in the NNV suspension after Tris-HCl or DIW dialysis. The substance from the P700 peak had the highest NNV-infectivity. Peak P700 commonly shared by the NNV suspensions after dialysis against the three different buffers. After Tris-HCl dialysis, no other protein except NNV coat protein (CP) at M_r 41,000 was detected from P700. However, after D-PBS dialysis, the P700 peak also contained P600 antigens. Therefore, the P700 peak after Tris-HCl dialysis represented the peak of highly purified NNV particles.

Dialysis buffer with different ionic strength affects the antigenicity of cultured nervous necrosis virus (NNV) suspensions

Nervous necrosis virus (NNV) belongs to the genus Betanodavirus (Nodaviridae). It is highly pathogenic to various marine fishes. Here, we investigated the antigenicity changes of cultured NNV suspensions during 14days of dialyses using a dialysis tube at 1.4×10(4) molecular weight cut off (MWCO) in three different buffers (Dulbecco's phosphate buffered saline (D-PBS), 15mM Tris-HCl (pH 8.0), and deionized water (DIW)). Total NNV antigen titers of cultured NNV suspension varied depending on different dialysis buffers. For example, total NNV antigen titer during D-PBS dialysis was increased once but then decreased. During Tris-HCl dialysis, it was relatively stable. During dialysis in DIW, total NNV antigen titer was increased gradually. These antigenicity changes in NNV suspension might be due to changes in the aggregation state of NNV particles and/or coat proteins (CPs). ELISA values of NNV suspension changed due to changing aggregates state of NNV antigens. NNV particles in suspension were aggregated at a certain level. These aggregates were progressive after D-PBS dialysis, but regressive after Tris-HCl dialysis. The purified NNV particles self-aggregated after dialysis in D-PBS or in Tris-HCl containing 600mM NaCl, but not after dialysis in Tris-HCl or DIW. Quantitative analysis is merited to determine NNV antigens in the highly purified NNV particles suspended in buffer at low salt condition.

Understanding the interaction between Betanodavirus and its host for the development of prophylactic measures for viral encephalopathy and retinopathy

Over the last three decades, the causative agent of viral encephalopathy and retinopathy (VER) disease has become a serious problem of marine finfish aquaculture, and more recently the disease has also been associated with farmed freshwater fish. The virus has been classified as a Betanodavirus within the family Nodaviridae, and the fact that Betanodaviruses are known to affect more than 120 different farmed and wild fish and invertebrate species, highlights the risk that Betanodaviruses pose to global aquaculture production. Betanodaviruses have been clustered into four genotypes, based on the RNA sequence of the T4 variable region of their capsid protein, and are named after the fish species from which they were first derived i.e. Striped Jack nervous necrosis virus (SJNNV), Tiger puffer nervous necrosis virus (TPNNV), Barfin flounder nervous necrosis virus (BFNNV) and Red-spotted grouper nervous necrosis virus (RGNNV), while an additional genotype turbot betanodavirus strain (TNV) has also been proposed. However, these genotypes tend to be associated with a particular water temperature range rather than being species-specific. Larvae and juvenile fish are especially susceptible to VER, with up to 100% mortality resulting in these age groups during disease episodes, with vertical transmission of the virus increasing the disease problem in smaller fish. A number of vaccine preparations have been tested in the laboratory and in the field e.g. inactivated virus, recombinant proteins, virus-like particles and DNA based vaccines, and their efficacy, based on relative percentage survival, has ranged from medium to high levels of protection to little or no protection. Ultimately a combination of effective prophylactic measures, including vaccination, is needed to control VER, and should also target larvae and broodstock stages of production to help the industry deal with the problem of vertical transmission. As yet there are no commercial vaccines for VER and the aquaculture industry eagerly awaits such a product. In this review we provide an overview on the current state of knowledge of the disease, the pathogen, and interactions between betanodavirus and its host, to provide a greater understanding of the multiple factors involved in the disease process. Such knowledge is needed to develop effective methods for controlling VER in the field, to protect the various aquaculture species farmed globally from the different Betanodavirus genotypes to which they are susceptible.

Role of the IFN I system against the VHSV infection in juvenile Senegalese sole (Solea senegalensis)

Senegalese sole is susceptible to marine VHSV isolates but is not affected by freshwater isolates, which may indicate differences regarding virus-host immune system interaction. IFN I induces an antiviral state in fish, stimulating the expression of genes encoding antiviral proteins (ISG). In this study, the stimulation of the Senegalese sole IFN I by VHSV infections has been evaluated by the relative quantification of the transcription of several ISG (Mx, Isg15 and Pkr) after inoculation with marine (pathogenic) and freshwater (non-pathogenic) VHSV isolates. Compared to marine VHSV, lower levels of RNA of the freshwater VHSV induced transcription of ISG to similar levels, with the Isg15 showing the highest fold induction. The protective role of the IFN I system was evaluated in poly I:C-inoculated animals subsequently challenged with VHSV isolates. The cumulative mortality caused by the marine isolate in the control group was 68%, whereas in the poly I:C-stimulated group was 5%. The freshwater VHSV isolate did not cause any mortality. Furthermore, viral RNA fold change and viral titers were lower in animals from the poly I:C + VHSV groups than in the controls. The implication of the IFN I system in the protection observed was confirmed by the transcription of the ISG in animals from the poly I:C + VHSV groups. However, the marine VHSV isolate exerts a negative effect on the ISG transcription at 3 and 6 h post-inoculation (hpi), which is not observed for the freshwater isolate. This difference might be partly responsible for the virulence shown by the marine isolate.

Comparison of the efficacy of Poly(I:C) immunization with live vaccine and formalin-killed vaccine against viral hemorrhagic septicemia virus (VHSV) in olive flounder (Paralichthys olivaceus)

Viral hemorrhagic septicemia (VHS) in olive flounder, Paralichthys olivaceus, causes significant economic loss for the flounder aquaculture industry in Korea. In this study, the immunogenicity of Poly(I:C) immunization with a live vaccine against the VHS virus (VHSV) was compared with that of a formalin-treated vaccine in the olive flounder. In vaccine trial I, fish pre-injected with Poly(I:C) were highly protected from VHSV infection 2 d later (survival rate: 96%) and the surviving fish (Poly(I:C)-VHSV group) showed a 100% survival rate against VHSV re-challenge. Mortality in fish pre-injected with diethylpyrocarbonate-treated water followed by injection with formalin-treated VHSV was only 2% (1 of 50 fish), whereas survivors (DEPC-FT VHSV group) showed an 80% survival rate. In vaccine trial II, 100% survival was observed in all Poly(I:C) vaccination groups-Poly(I:C)-VHSV 6, Poly(I:C)-VHSV 5, and Poly(I:C)-VHSV 4. In contrast, the survival rates of the groups administered the formalin-treated VHSV at a dose of 10(6), 10(5), and 10(4) TCID50 100 μL(-1) fish(-1) (DEPC-FT VHSV 6, DEPC-FT VHSV 5, and DEPC-FT VHSV 4) were only 8%, 12%, and 12%, respectively. The differences in the survival rates of the formalin-treated vaccine groups in trial I and trial II were attributed to the difference in the formalin-treatment period: the formalin-treated VHSV administered in trial I was not completely inactivated and worked as a live vaccine, which explains the 80% survival rate against VHSV challenge. Specific antibodies against VHSV were detected in sera from all vaccinated survivors, except the DEPC-VHSV 4 group. Furthermore, the specific antibody titers of fish vaccinated with the live and dead VHSV vaccines were similar, but the protective effects of the live and dead vaccines varied considerably. Our findings show that Poly(I:C) immunization with the live vaccine offers better protection than the formalin-treated vaccine against VHS in olive flounder and revealed that antibody levels are not a reliable indicator of the protective effect of the vaccine against the pathogen. In the future, elements of T cell immunity may be used as a means of evaluating the protective efficacy of a vaccine against VHSV instead of ELISA.

Use of Poly(I:C) Stabilized with Chitosan As a Vaccine-Adjuvant Against Viral Hemorrhagic Septicemia Virus Infection in Zebrafish

There is an urgent need for more efficient viral vaccines in finfish aquaculture worldwide. Here, we report the use of poly(I:C) stabilized with chitosan as an adjuvant for development of better finfish vaccines. The adjuvant was co-injected with inactivated viral hemorrhagic septicemia virus (VHSV) (CSpIC+iV vaccine) in adult zebrafish and its efficiency in protection against VHSV infection was compared to a live, attenuated VHS virus vaccine (aV). Both free and stabilized poly(I:C) were strong inducers of an antiviral state, measured by transcriptional activation of the genes of viral sensors: toll-like receptors, interferons, and interferon-stimulated genes, such as MXa within 48 h after injection. Both the CSpIC+iV and the aV formulations provided a significant protection against VHSV-induced mortality. However, when plasma from survivors was tested for neutralizing antibodies in an in vitro protection assay, we could not demonstrate any protective effect. On the contrary, plasma from aV vaccinated fish enhanced cytopathic effects, indicating that antibody-dependent entry may play a role in this system. Our results show that poly(I:C) is a promising candidate as an adjuvant for fish vaccination against viral pathogens, and that the zebrafish is a promising model for aquaculture-relevant vaccination studies.

Development of a stringent ELISA protocol to evaluate anti-viral hemorrhagic septicemia virus-specific antibodies in olive flounder Paralichthys olivaceus with improved specificity

Olive flounder were vaccinated with polyinosinic:polycytidylic acid [Poly (I:C)] to prevent viral hemorrhagic septicemia (VHS). Vaccine efficacy was verified by detection of anti- VHS virus (VHSV) antibodies using enzyme-linked immunosorbent assay (ELISA). In the study, ELISA absorbance values of the negative control group [Poly (I:C)-MEM10] were saturated when an ELISA protocol, that includes pretreatment of the fish sera with 5% skim milk, was used. However, the saturated OD values in the negative control did not correlate with a specific immune response against VHSV, because the group showed low survival rate (only 10%) following the VHSV challenge. Also, OD values of Poly (I:C)- VHSV group were high, and the group showed high survival rate (97.5%) against VHSV challenge test. It was suggested that the high OD values were possibly due to the presence of anti-fetal bovine serum (FBS) cross-reactivity. To compensate this, we subtracted the absorbance of infectious hematopoietic necrosis (IHNV)-Ag plates from those of the VHSV-Ag plates. However, the average value for the Poly (I:C)-VHSV group (0.167) was lower than expected even though high survival rate. We used an advanced ELISA system to pre-treat fish sera with 5% skim milk and two novirhabdoviruses as capture antigens as well as 50% FBS. The corrected absorbance values for pre-treated fish sera from the negative control Poly (I:C)-MEM10 and experimental Poly (I:C)-VHSV groups averaged 0.033 and 0.579, respectively. The specific VHSV antibody response of the vaccinated group was assessed using fish sera pretreated with skim milk and FBS and by calculating the corrected absorbance values from ELISA with two novirhabdovirus capture antigens.

Poly(I:C) induces antiviral immune responses in Japanese flounder (Paralichthys olivaceus) that require TLR3 and MDA5 and is negatively regulated by Myd88

Polyinosinic:polycytidylic acid (poly(I:C)) is a ligand of toll-like receptor (TLR) 3 that has been used as an immunostimulant in humans and mice against viral diseases based on its ability to enhance innate and adapt immunity. Antiviral effect of poly(I:C) has also been observed in teleost, however, the underling mechanism is not clear. In this study, we investigated the potential and signaling mechanism of poly(I:C) as an antiviral agent in a model of Japanese flounder (Paralichthys olivaceus) infected with megalocytivirus. We found that poly(I:C) exhibited strong antiviral activity and enhanced activation of head kidney macrophages and peripheral blood leukocytes. In vivo studies showed that (i) TLR3 as well as MDA5 knockdown reduced poly(I:C)-mediated immune response and antiviral activity to significant extents; (ii) when Myd88 was overexpressed in flounder, poly(I:C)-mediated antiviral activity was significantly decreased; (iii) when Myd88 was inactivated, the antiviral effect of poly(I:C) was significantly increased. Cellular study showed that (i) the NF-κB activity induced by poly(I:C) was upregulated in Myd88-overexpressing cells and unaffected in Myd88-inactivated cells; (ii) Myd88 overexpression inhibited and upregulated the expression of poly(I:C)-induced antiviral genes and inflammatory genes respectively; (iii) Myd88 inactivation enhanced the expression of the antiviral genes induced by poly(I:C). Taken together, these results indicate that poly(I:C) is an immunostimulant with antiviral potential, and that the immune response of poly(I:C) requires TLR3 and MDA5 and is negatively regulated by Myd88 in a manner not involving NK-κB. These results provide insights to the working mechanism of poly(I:C), TLR3, and Myd88 in fish.

VHSV G glycoprotein major determinants implicated in triggering the host type I IFN antiviral response as DNA vaccine molecular adjuvants

We have recently identified the two major determinants of the glycoprotein G of the viral hemorrhagic septicaemia rhabdovirus (gpGVHSV), peptides p31 and p33 implicated in triggering the host type I IFN antiviral response associated to these rhabdoviral antigens. With the aim to investigate the properties of these viral glycoprotein regions as DNA molecular adjuvants, their corresponding cDNA sequences were cloned into a plasmid (pMCV1.4) flanked by the signal peptide and transmembrane sequences of gpGVHSV. In addition, a plasmid construct encoding both sequences p31 and p33 (pMCV1.4-p31+p33) was also designed. In vitro transitory cell transfection assays showed that these VHSV gpG regions were able to induce the expression of type I IFN stimulated genes as well as to confer resistance to the infection with a different fish rhabdovirus, the spring viremia of carp virus (SVCV). In vivo, zebrafish intramuscular injection of only 1μg of the construct pMCV1.4-p31+p33 conferred fish protection against SVCV lethal challenge up to 45 days post-immunization. Moreover, pMCV1.4-p31+p33 construct was assayed for molecular adjuvantcity's for a DNA vaccine against SVCV based in the surface antigen of this virus (pAE6-GSVCV). The results showed that the co-injection of the SVCV DNA vaccine and the molecular adjuvant allowed (i) a ten-fold reduction in the dose of pAE6-Gsvcv without compromising its efficacy (ii) an increase in the duration of protection, and (iii) an increase in the survival rate. To our knowledge, this is the first report in which specific IFN-inducing regions from a viral gpG are used to design more-efficient and cost-effective viral vaccines, as well as to improve our knowledge on how to stimulate the innate immune system.

Cell Culture Medium Inhibits Antigen Binding Used in an ELISA for Detection of Antibodies against Nervous Necrosis Virus

Abstract We investigated the optimum dilution of nervous necrosis virus (NNV) for use as antigens to detect antibodies by an enzyme-linked immunosorbent assay (ELISA) in the Sevenband Grouper Epinephelus septemfasciatus. The ELISA values for a standardized suspension of antigens diluted with L-15 medium containing 1% fetal bovine serum decreased gradually with the dilution of the antigens, whereas those for the antigens diluted with distilled water (DW) initially increased with the dilution of the antigens, peaked at a 320-fold dilution, and then decreased thereafter. Additional studies revealed that binding of NNV antigens to ELISA wells was inhibited by fetal bovine serum and other substances in the L-15 medium. Sera obtained from Sevenband Grouper vaccinated with live NNV vaccine and survivors from natural NNV-infection were subjected to antibody detection by ELISA. All of the sera were positive by ELISA when the standardized suspension was diluted 320-fold, whereas sera from five out of the six survivors and two out of the six vaccinated fish were negative or weakly positive by ELISA using NNV antigens diluted 10-fold. We therefore concluded that cultured NNV solutions prepared in cell culture media may need to be diluted with distilled water for use in ELISA. Received January 28, 2014; accepted April 16, 2014.

Adjuvants and immunostimulants in fish vaccines: current knowledge and future perspectives

Vaccination is the most adequate method to control infectious diseases that threaten the aquaculture industry worldwide. Unfortunately, vaccines are usually not able to confer protection on their own; especially those vaccines based on recombinant antigens or inactivated pathogens. Therefore, the use of adjuvants or immunostimulants is often necessary to increase the vaccine efficacy. Traditional adjuvants such as mineral oils are routinely used in different commercial bacterial vaccines available for fish; however, important side effects may occur with this type of adjuvants. A search for alternative molecules or certain combinations of them as adjuvants is desirable in order to increase animal welfare without reducing protection levels. Especially, combinations that may target specific cell responses and thus a specific pathogen, with no or minor side effects, should be explored. Despite this, the oil adjuvants currently used are quite friendlier with respect to side effects compared with the oil adjuvants previously used. The great lack of fish antiviral vaccines also evidences the importance of identifying optimal combinations of a vaccination strategy with the use of a targeting adjuvant, especially for the promising fish antiviral DNA vaccines. In this review, we summarise previous studies performed with both traditional adjuvants as well as the most promising new generation adjuvants such as ligands for Toll receptors or different cytokines, focussing mostly on their protective efficacies, and also on what is known concerning their effects on the fish immune system when delivered in vivo.