DNA vaccine encoding myristoylated membrane protein (MMP) of rock bream iridovirus (RBIV) induces protective immunity in rock bream (Oplegnathus fasciatus)

Fish Iridoviridae: infection, vaccination and immune response

Each year, due to climate change, an increasing number of new pathogens are being discovered and studied, leading to an increase in the number of known diseases affecting various fish species in different regions of the world. Viruses from the family Iridoviridae, which consist of the genera Megalocytivirus, Lymphocystivirus, and Ranavirus, cause epizootic outbreaks in farmed and wild, marine, and freshwater fish species (including ornamental fish). Diseases caused by fish viruses of the family Iridoviridae have a significant economic impact, especially in the aquaculture sector. Consequently, vaccines have been developed in recent decades, and their administration methods have improved. To date, various types of vaccines are available to control and prevent Iridoviridae infections in fish populations. Notably, two vaccines, specifically targeting Red Sea bream iridoviral disease and iridoviruses (formalin-killed vaccine and AQUAVAC® IridoV, respectively), are commercially available. In addition to exploring these themes, this review examines the immune responses in fish following viral infections or vaccination procedures. In general, the evasion mechanisms observed in iridovirus infections are characterised by a systemic absence of inflammatory responses and a reduction in the expression of genes associated with the adaptive immune response. Finally, this review also explores prophylactic procedure trends in fish vaccination strategies, focusing on future advances in the field.

Evaluation of Formalin-Inactivated Vaccine Efficacy against Red Seabream Iridovirus (RSIV) in Laboratory and Field Conditions

Red seabream iridovirus (RSIV) is a major cause of marine fish mortality in Korea, with no effective vaccine available since its first occurrence in the 1990s. This study evaluated the efficacy of a formalin-killed vaccine against RSIV in rock bream under laboratory and field conditions. For the field trial, a total of 103,200 rock bream from two commercial marine cage-cultured farms in Southern Korea were vaccinated. Farm A vaccinated 31,100 fish in July 2020 and monitored them for 18 weeks, while farm B vaccinated 30,700 fish in August 2020 and monitored them for 12 weeks. At farm A, where there was no RSIV infection, the vaccine efficacy was assessed in the lab, showing a relative percentage of survival (RPS) ranging from 40% to 80%. At farm B, where natural RSIV infections occurred, cumulative mortality rates were 36.43% in the vaccinated group and 80.32% in the control group, resulting in an RPS of 54.67%. The RSIV-infectious status and neutralizing antibody titers in serum mirrored the cumulative mortality results. This study demonstrates that the formalin-killed vaccine effectively prevents RSIV in cage-cultured rock bream under both laboratory and field conditions.

Abnormalities in red blood cell production and pathogenesis of anemia in the progression of rock bream iridovirus (RBIV)

Rock bream iridovirus (RBIV), belonging to Megalocytivirus, causes severe mortality in rock bream. Almost all deaths associated with RBIV are accompanied by splenic enlargement and anemia. Although red blood cells (RBCs) are involved in the immune response against viral infections, their involvement in rock bream has not yet been studied in terms of the immune response against RBIV. In this study, the viral replication patterns, blood characteristics and anemia-related factors were evaluated in rock bream post RBIV infection. The virus-infected RBCs of rock bream demonstrated similarities in the expression levels of hemoglobins (HGB) (α and β), cytokine-dependent hematopoietic cell linker (CLNK) and hematopoietic transcription factor GATA (GATA), with significantly decreasing levels from 4 days post infection (dpi) to 17 (dpi), when the viral replication was at its peak. This suggests that the expression of blood-related genes is inadequate for HGB synthesis and RBC production, thereby causing anemia leading to death. Moreover, the levels of complete blood cell count (CBC) indicators, such as RBCs, HGB and hematocrit (HCT), significantly decreased from 10 to 17 dpi. This phenomenon suggests that blood-related gene expression and/or RBC-, HGB- and HCT-related levels are critical factors in RBIV-induced anemia and disease progression. These results highlight the significance of blood-mediated immune responses against RBIV infection in rock bream. Understanding blood-related gene levels to identify blood-related immune response interactions in rock bream will be useful for development of future strategies in controlling RBIV diseases in rock bream.

Megalocytivirus and Other Members of the Family Iridoviridae in Finfish: A Review of the Etiology, Epidemiology, Diagnosis, Prevention and Control

Aquaculture has expanded to become the fastest growing food-producing sector in the world. However, its expansion has come under threat due to an increase in diseases caused by pathogens such as iridoviruses commonly found in aquatic environments used for fish farming. Of the seven members belonging to the family Iridoviridae, the three genera causing diseases in fish comprise ranaviruses, lymphocystiviruses and megalocytiviruses. These three genera are serious impediments to the expansion of global aquaculture because of their tropism for a wide range of farmed-fish species in which they cause high mortality. As economic losses caused by these iridoviruses in aquaculture continue to rise, the urgent need for effective control strategies increases. As a consequence, these viruses have attracted a lot of research interest in recent years. The functional role of some of the genes that form the structure of iridoviruses has not been elucidated. There is a lack of information on the predisposing factors leading to iridovirus infections in fish, an absence of information on the risk factors leading to disease outbreaks, and a lack of data on the chemical and physical properties of iridoviruses needed for the implementation of biosecurity control measures. Thus, the synopsis put forth herein provides an update of knowledge gathered from studies carried out so far aimed at addressing the aforesaid informational gaps. In summary, this review provides an update on the etiology of different iridoviruses infecting finfish and epidemiological factors leading to the occurrence of disease outbreaks. In addition, the review provides an update on the cell lines developed for virus isolation and culture, the diagnostic tools used for virus detection and characterization, the current advances in vaccine development and the use of biosecurity in the control of iridoviruses in aquaculture. Overall, we envision that the information put forth in this review will contribute to developing effective control strategies against iridovirus infections in aquaculture.

A New Cell Line Derived from the Caudal Fin of the Dwarf Gourami (Trichogaster lalius) and Its Susceptibility to Fish Viruses

The detection of megalocytiviruses, especially the infectious spleen and kidney necrosis virus (ISKNV), in ornamental fish has increased with the rapid growth of the ornamental fish industry. In this study, dwarf gourami fin (DGF) cells derived from the caudal fin of the dwarf gourami (Trichogaster lalius), which is highly susceptible to red sea bream iridovirus (RSIV) and ISKNV, were established and characterized. The DGF cells were grown at temperatures ranging from 25 °C to 30 °C in Leibovitz's L-15 medium supplemented with 15% fetal bovine serum and were subcultured for more than 100 passages, predominantly with epithelial-like cells. DGF cells had a diploid chromosome number of 2n = 44. Although the initial purpose of this study was to establish a cell line for the causative agents of red sea bream iridoviral disease (RSIV and ISKNV), DGF cells were also susceptible to rhabdoviruses (viral hemorrhagic septicemia virus, hirame rhabdovirus, and spring viraemia of carp virus), exhibiting a significant cytopathic effect characterized by cell rounding and lysis. Additionally, viral replication and virion morphology were confirmed using virus-specific conventional polymerase chain reaction and transmission electron microscopy. Furthermore, both RSIV and ISKNV were replicated at high concentrations in DGF cells compared to other cell lines. Notably, the DGF cells maintained a monolayer during ISKNV infection, indicating the possibility of persistent infection. Thus, DGF can be used for viral diagnosis and may play a critical role in advancing our understanding of ISKNV pathogenesis.

DNA vaccine dual-expressing viral hemorrhagic septicemia virus glycoprotein and C-C motif chemokine ligand 19 induces the expression of immune-related genes in zebrafish (Danio rerio)

Glycoprotein (G protein)-based DNA vaccines are effective in protecting aquaculture fish from rhabdoviruses but the degree of immune response they elicit depends on plasmid concentration and antigen cassette. Here, we developed a DNA vaccine using the viral hemorrhagic septicemia virus G (VG) gene and chemokine (C-C motif) ligand 19 (CCL19)a.2 regulated by the CMV promoter as the molecular adjuvant. After transfection of the prepared plasmid (pVG + CCL19) into epithelioma papulosum cyprini cells, mRNA expression was confirmed through quantitative real-time polymerase chain reaction. The vaccine was intramuscularly injected into zebrafish (Danio rerio), and 28 days after immunization, viral hemorrhagic septicemia virus (10⁵ TCID₅₀/10 µl/fish) was intraperitoneally injected. A survival rate of 68% was observed in the pVG + CCL19 group but this was not significantly different from the survival rate of fish treated with pVG alone, that is, without the adjuvant. However, the expression of interferon- and cytokine-related genes in the spleen and kidney tissues of zebrafish was significantly increased (p < 0.05) on days 1, 3, 7, and 14 after immunization. Thus, CCL19a.2 induced an initial immune response as a molecular adjuvant, which may provide initial protection against virus infection before vaccination-induced antibody formation. This study provides insights on the functions of CCL19a.2 adjuvant in DNA vaccines.

Saponin and chitosan-based oral vaccine against viral haemorrhagic septicaemia virus (VHSV) provides protective immunity in olive flounder (Paralichthys olivaceus)

Production losses of olive flounder (Paralichthys olivaceus) have increased owing to viral haemorrhagic septicaemia virus (VHSV) infection. In this study, we determined safe concentrations of orally administered saponin and chitosan by analysing serum enzyme (AST/ALT) levels as biochemical markers of hepatic injury. Furthermore, we demonstrated the efficacy, duration of protection, and safety of saponin and chitosan-based vaccines with inactivated VHSV (IV). Oral administration of saponin, chitosan, and their combination did not induce fish mortality at all tested concentrations (0.29, 1.45, and 2.9 mg/g of fish body weight/day) 10 days after administration. However, AST level was high at a dose >0.29 mg/g of fish body weight/day. Both saponin and chitosan were found to be safe and acceptable for vaccination studies at a dose of 0.29 mg/g of fish body weight/day. Administration of IV alone did not induce protection at 2 and 4 weeks post vaccination (wpv). Olive flounders administered saponin + IV and chitosan + IV vaccines had higher immunity against VHSV with relative percentage survival (RPS) of 12.5-7.5% and 0-20.1%, respectively; however, additional immunisation with combination of saponin + chitosan + IV clearly enhanced the protection with RPS values of 10-15%, 26.7%, 42.9%, and 37.5% at 4, 8, 12, and 20 wpv, respectively. Although the RPS value of oral immunisation was not comparable to that of injectable vaccines, the manufacturing process is simple and oral administration causes less stress to juvenile fish. To investigate the development of a protective immune response, olive flounder were re-challenged with VHSV (10^7.8 TCID₅₀/fish) at 70 days postinfection; 100% of the previously unexposed fish died, whereas 80-100% of the previously immunised fish survived. Our results showed the possibility of developing preventive measures against VHSV using saponin and chitosan-based oral vaccines with inactivated virus.

Efficacy of saponin-based inactivated rock bream iridovirus (RBIV) vaccine in rock bream (Oplegnathus fasciatus)

Rock bream iridovirus (RBIV) causes severe mortality in rock bream (Oplegnathus fasciatus) for last two decades. In view of this constant threat of RBIV to the rock bream industry, we conducted the present study with the aim to develop a safe and efficient remedial measure against the virus. In this study, we evaluated the safety and potentiality of squalene, aluminium hydroxide and saponin adjuvants, singly or in combinations, which can be used for developing an efficient inactivated (IV) vaccine to protect rock bream from RBIV infection. The evaluation results demonstrated that saponin (Sa) has the required potential in enacting the antiviral immune response in the host and in providing protection against virus mediated lethality, without causing any adverted side-effects. The study further, showed that a single primary dose of Sa-adjuvanted IV vaccine can confer moderate protections in short (60.04% relative percent mortality (RPS) at 4 wpv) and medium (53.38% RPS at 8 wpv) term post RBIV challenge; whereas, the same vaccine when administered in a prime-boost strategy, it resulted enhanced 93.34% RPS post virus challenge at 4 and 8 wpv. The moderate to high survivability demonstrated by the Sa-adjuvanted IV vaccine, was substantiated by the significant (p < 0.05) upregulation of IL-1β, Mx and PKR gene transcript. All surviving fish from the Sa-adjuvanted IV vaccine groups were strongly protected from re-infection with RBIV (1.1 × 10⁷) at 70 days post infection (dpi). In conclusion, it can be inferred that, Sa-adjuvanted IV RBIV vaccine can be an efficient control measure to protect the rock bream aquaculture industry against the lethal RBIV virus.

Two bicistronic DNA vaccines against Vibrio anguillarum and the immune effects on flounder Paralichthys olivaceus

Chemokines are cytokines that can promote the activation and migration of immune cells, and increase the recognition of antigen by antigen-presenting cells (APC). Previous studies showed that a DNA vaccine can induce humoral and cellular immune responses of flounder after immunization. To explore the improvement of chemokines on the efficiency of OmpK vaccine, two bicistronic DNA candidate vaccines were constructed and the immune responses they induced in the flounder were investigated by reverse transcription polymerase chain reaction (RT-PCR), indirect immunofluorescent assay (IFA), H&E staining, flow cytometry (FCM), and quantificational real-time polymerase chain reaction (qRT-PCR). pBudCE4.1 plasmid as an expression vector, bicistronic DNA vaccines encoding OmpK gene and CC-motif ligand 4 gene (p-OmpK-CCL4), or Ompk gene and CC-motif ligand 19 gene (p-OmpK-CCL19) were successfully constructed. The results showed that two bicistronic DNA vaccines expressed Ompk protein of Vibrio anguillarum and CCL4/CCL19 proteins of flounder both in vitro and in vivo. After immunization, a large number of leucocytes in muscle were recruited at the injection site in treatment groups. The constructed vaccines induced significant increases in CD4-1⁺ and CD4-2⁺ T lymphocytes, and sIgM⁺ B lymphocytes in peripheral blood, spleen, and head kidney. The percentage of T lymphocytes peaked on the 14^th post-vaccination day whereas that of B lymphocytes peaked in the 6^th post-vaccination week. Moreover, the expression profiles of 10 immune-related genes increased in muscles around the injection site, spleen, and head kidney. After the challenge, p-OmpK-CCL4 and p-OmpK-CCL19 conferred a relative percentage survival (RPS) of 74.1% and 63.3%, respectively, higher than p-OmpK alone (40.8%). In conclusion, both CCL4 and CCL19 can improve the protection of p-OmpK via evoking local immune response and then humoral and cellular immunity. CCL4 and CCL19 will be potential molecular adjuvants for use in DNA vaccines.

Proteome profile of spleen in rock bream (Oplegnathus fasciatus) naturally infected with rock bream iridovirus (RBIV)

BACKGROUND

Rock bream iridovirus (RBIV) is one of the most dangerous pathogens that causes the highest mortality in the aquaculture of rock bream (Oplegnathus fasciatus). Even though RBIV infection leads to huge economic loss, proteome studies on RBIV-infected rock bream have not been conducted to provide information about the differential protein expression pattern by the host protection system.

OBJECTIVE

The purpose of this study was to investigate the protein expression patterns in spleens of rock bream olive after infection by RBIV or mixed infection by RBIV and bacteria.

METHODS

Depending on the infection intensity and sampling time point, fish were divided into five groups: uninfected healthy fish at week 0 as the control (0C), heavily infected fish at week 0 (0H), heavily mixed RBIV and bacterial infected fish at week 0 (0MH), uninfected healthy fish at week 3 (3C), and lightly infected fish at week 3 (3L). Proteins were extracted from the spleens of infected rock bream. We used 2-DE analysis with LC-MS/MS to investigate proteome changes in infected rock bream.

RESULTS

The results of the LC-MS/MS analyses showed different protein expression profiles after infection. Proteins related to oxygen transport and energy generation, such as hemoglobin, beta-globin, and ATP synthase, were mostly expressed in the infected spleen. Whereas proteins involved in structure and cell movement, such as tubulin, myosin, actin binding proteins, and intermediate filament proteins, were down-regulated in the infected spleens. The protein expression profiles between infection by RBIV and mixed infection by RBIV and bacteria showed similar patterns.

CONCLUSIONS

Our results indicated that infection by RBIV or mixed infection by RBIV and bacteria triggered energy generation and oxygen-transport, but cell migration and constructional changes in the spleen were extremely decreased.

Co-Expression Network Analysis of Spleen Transcriptome in Rock Bream (Oplegnathus fasciatus) Naturally Infected with Rock Bream Iridovirus (RBIV)

Rock bream iridovirus (RBIV) is a notorious agent that causes high mortality in aquaculture of rock bream (Oplegnathus fasciatus). Despite severity of this virus, no transcriptomic studies on RBIV-infected rock bream that can provide fundamental information on protective mechanism against the virus have been reported so far. This study aimed to investigate physiological mechanisms between host and RBIV through transcriptomic changes in the spleen based on RNA-seq. Depending on infection intensity and sampling time point, fish were divided into five groups: uninfected healthy fish at week 0 as control (0C), heavy infected fish at week 0 (0H), heavy mixed RBIV and bacterial infected fish at week 0 (0MH), uninfected healthy fish at week 3 (3C), and light infected fish at week 3 (3L). We explored clusters from 35,861 genes with Fragments Per Kilo-base of exon per Million mapped fragments (FPKM) values of 0.01 or more through signed co-expression network analysis using WGCNA package. Nine of 22 modules were highly correlated with viral infection (|gene significance (GS) vs. module membership (MM) |> 0.5, p-value < 0.05). Expression patterns in selected modules were divided into two: heavy infected (0H and 0MH) and control and light-infected groups (0C, 3C, and 3L). In functional analysis, genes in two positive modules (5448 unigenes) were enriched in cell cycle, DNA replication, transcription, and translation, and increased glycolysis activity. Seven negative modules (3517 unigenes) built in this study showed significant decreases in the expression of genes in lymphocyte-mediated immune system, antigen presentation, and platelet activation, whereas there was significant increased expression of endogenous apoptosis-related genes. These changes lead to RBIV proliferation and failure of host defense, and suggests the importance of blood cells such as thrombocytes and B cells in rock bream in RBIV infection. Interestingly, a hub gene, pre-mRNA processing factor 19 (PRPF19) showing high connectivity (kME), and expression of this gene using qRT-PCR was increased in rock bream blood cells shortly after RBIV was added. It might be a potential biomarker for diagnosis and vaccine studies in rock bream against RBIV. This transcriptome approach and our findings provide new insight into the understanding of global rock bream-RBIV interactions including immune and pathogenesis mechanisms.

Carbon nanotube-based DNA vaccine against koi herpesvirus given by intramuscular injection

Koi herpesvirus (KHV) also named Cyprinid Herpesvirus 3 (CyHV-3) is one of the most threatening pathogens affecting common carp production as well as the valued ornamental koi carp. The current commercial vaccines available are costly and potentially cause severe stress caused by live virus. KHV ORF149 gene has been proved encoding one of the main immunogenic proteins for KHV. In this study, we coupled a plasmid expression vector for ORF149 to single walled carbon nanotubes (SWCNTs) for an anti-KHV vaccine. The vaccine conferred an 81.9% protection against intraperitoneal challenge with KHV. Importantly, SWCNTs as a promising vehicle can enhanced the protective effects 33.9% over that of the naked DNA vaccine at the same dose. The protection was longer and serum antibody production, enzyme activities and immune-related gene expression were all induced in fish vaccinated with the nanotube-DNA vaccine compared with the DNA alone. Thereby, this study demonstrates that the ORF149 DNA vaccine loaded onto SWCNTs as a novel vaccine might provide an effective method of coping with KHV disease using intra-muscular vaccination.

A voting mechanism-based linear epitope prediction system for the host-specific Iridoviridae family

Background

The Iridoviridae family is categorized into five genera and clustered into two subfamilies: Alphairidovirinae includes Lymphocystivirus, Ranavirus (GIV), and Megalocystivirus (TGIV), which infect vertebrate hosts and Betairidovirinae includes Iridovirus and Chloriridovirus, which infect invertebrate hosts. Clustered Iridoviridae subfamilies possess host-specific characteristics, which can be considered as exclusive features for in-silico prediction of effective epitopes for vaccine development. A voting mechanism-based linear epitope (LE) prediction system was applied to identify and endorse LE candidates with a minimum length requirement for each clustered subfamily

Results

The experimental results showed that four conserved epitopes among the Iridovirideae family, one exclusive epitope for invertebrate subfamily and two exclusive epitopes for vertebrate family were predicted. These predicted LE candidates were further validated by ELISA assays for evaluating the strength of antigenicity and cross antigenicity. The conserved LEs for Iridoviridae family reflected high antigenicity responses for the two subfamilies, while exclusive LEs reflected high antigenicity responses only for the host-specific subfamily

Conclusions

Host-specific characteristics are important features and constraints for effective epitope prediction. Our proposed voting mechanism based system provides a novel approach for in silico LE prediction prior to vaccine development, and it is especially powerful for analyzing antigen sequences with exclusive features between two clustered groups.

IFN Signaling in Inflammation and Viral Infections: New Insights from Fish Models

The overarching structure of the type I interferon (IFN) system is conserved across vertebrates. However, the variable numbers of whole genome duplication events during fish evolution offer opportunities for the expansion, diversification, and new functionalization of the genes that are involved in antiviral immunity. In this review, we examine how fish models provide new insights about the implication of virus-driven inflammation in immunity and hematopoiesis. Mechanisms that have been discovered in fish, such as the strong adjuvant effect of type I IFN that is used with DNA vaccination, constitute good models to understand how virus-induced inflammatory mechanisms can interfere with adaptive responses. We also comment on new discoveries regarding the role of pathogen-induced inflammation in the development and guidance of hematopoietic stem cells in zebrafish. These findings raise issues about the potential interferences of viral infections with the establishment of the immune system. Finally, the recent development of genome editing provides new opportunities to dissect the roles of the key players involved in the antiviral response in fish, hence enhancing the power of comparative approaches.

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.

The Megalocytivirus RBIV Induces Apoptosis and MHC Class I Presentation in Rock Bream (Oplegnathus fasciatus) Red Blood Cells

Rock bream iridovirus (RBIV) causes severe mass mortality in Korean rock bream (Oplegnathus fasciatus) populations. To date, immune defense mechanisms of rock bream against RBIV are unclear. While red blood cells (RBCs) are known to be involved in the immune response against viral infections, the participation of rock bream RBCs in the immune response against RBIV has not been studied yet. In this study, we examined induction of the immune response in rock bream RBCs after RBIV infection. Each fish was injected with RBIV, and virus copy number in RBCs gradually increased from 4 days post-infection (dpi), peaking at 10 dpi. A total of 318 proteins were significantly regulated in RBCs from RBIV-infected individuals, 183 proteins were upregulated and 135 proteins were downregulated. Differentially upregulated proteins included those involved in cellular amino acid metabolic processes, cellular detoxification, snRNP assembly, and the spliceosome. Remarkably, the MHC class I-related protein pathway was upregulated during RBIV infection. Simultaneously, the regulation of apoptosis-related proteins, including caspase-6 (CASP6), caspase-9 (CASP9), Fas cell surface death receptor (FAS), desmoplakin (DSP), and p21 (RAC1)-activated kinase 2 (PAK2) changed with RBIV infection. Interestingly, the expression of genes within the ISG15 antiviral mechanism-related pathway, including filamin B (FLNB), interferon regulatory factor 3 (IRF3), nucleoporin 35 (NUP35), tripartite motif-containing 25 (TRIM25), and karyopherin subunit alpha 3 (KPNA3) were downregulated in RBCs from RBIV-infected individuals. Overall, these findings contribute to the understanding of RBIV pathogenesis and host interaction.

Intramuscular administration of a DNA vaccine encoding OmpK antigen induces humoral and cellular immune responses in flounder (Paralichthys olivaceus) and improves protection against Vibrio anguillarum

Outer membrane protein K (OmpK) is an immunogenic protein that could act as subunit vaccine candidate for Vibrio anguillarum. In this study, a DNA vaccine encoding the OmpK gene of V. anguillarum was constructed and confirmed to express OmpK in vitro and in vivo. To evaluate the potential of pcDNA3.1-OmpK (pOmpK) as vaccine candidate, the humoral and cellular immune responses, and protective effects were analyzed in flounder model. The results showed that the transcription and translation of OmpK gene occurred in both transfected hirame natural embryo (HINAE) cells and injected fish muscles, indicating the functionality of pOmpK to express OmpK. Fish immunized with pOmpK showed significant increase of surface IgM positive (sIgM⁺), CD4-1⁺, CD4-2⁺ lymphocytes and production of specific anti-V. anguillarum or anti-rOmpK antibodies, which indicate the activation of humoral and cellular immune responses after vaccination. Moreover, a relative percent survival (RPS) rate of 50.00% against V. anguillarum infection was obtained for flounder immunized with pOmpK. In conclusion, this study indicates that pOmpK is able to induce humoral and cellular immune responses and can be used as a DNA vaccine candidate.