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

Protective efficiency and immune responses to single and booster doses of formalin-inactivated scale drop disease virus (SDDV) vaccine in Asian seabass (Lates calcarifer)

BACKGROUND

Scale drop disease virus (SDDV) threatens Asian seabass (Lates calcarifer) aquaculture production by causing scale drop disease (SDD) in Asian seabass. Research on the development of SDDV vaccines is missing an in-depth examination of long-term immunity and the immune reactions it provokes. This study investigated the long-term immune protection and responses elicited by an SDDV vaccine. The research evaluated the effectiveness of a formalin-inactivated SDDV vaccine (SDDV-FIV) using both prime and prime-booster vaccination strategies in Asian seabass. Three groups were used: control (unvaccinated), single-vaccination (prime only), and booster (prime and booster). SDDV-FIV was administered via intraperitoneal route, with a booster dose given 28 days post-initial vaccination.

RESULTS

The immune responses in vaccinated fish (single and booster groups) showed that SDDV-FIV triggered both SDDV-specific IgM and total IgM production. SDDV-specific IgM levels were evident until 28 days post-vaccination (dpv) in the single vaccination group, while an elevated antibody response was maintained in the booster group until 70 dpv. The expression of immune-related genes (dcst, mhc2a1, cd4, ighm, cd8, il8, ifng, and mx) in the head kidney and peripheral blood lymphocytes (PBLs) of vaccinated and challenged fish were significantly upregulated within 1-3 dpv and post-SDDV challenge. Fish were challenged with SDDV at 42 dpv (challenge 1) and 70 dpv (challenge 2). In the first challenge, the group that received booster vaccinations demonstrated notably higher survival rates than the control group (60% versus 20%, P < 0.05). However, in the second challenge, while there was an observable trend towards improved survival rates for the booster group compared to controls (42% versus 25%), these differences did not reach statistical significance (P > 0.05). These findings suggest that the SDDV-FIV vaccine effectively stimulates both humoral and cellular immune responses against SDDV. Booster vaccination enhances this response and improves survival rates up to 42 dpv.

CONCLUSIONS

This research provides valuable insights into the development of efficient SDDV vaccines and aids in advancing strategies for immune modulation to enhance disease management in the aquaculture of Asian seabass.

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.

Identification and analysis of differentially expressed microRNAs in gibel carp Carassius auratus gibelio responding to polyinosinic-polycytidylic acid (poly I:C) stimulation

Polyinosinic-polycytidylic acid (poly I:C) is a synthesized analogue of viral double-strand RNA and considered as a potential immunostimulant in aquaculture. MicroRNAs (miRNAs) have been reported to play important roles in the development of the immune system and in regulation of host antiviral responses. In our earlier study, it was found that poly I:C pre-treatment could stimulate the resistance against cyprinid herpesvirus 2 (CyHV-2) infection and enhance the antiviral immune response in gibel carp. To understand the role of miRNAs in regulating the host response to poly I:C treatment, we investigated the expression profiles of miRNAs in the head kidney of poly I:C-treated gibel carp with small RNA sequencing technology. When compared with the untreated group, a total of 24 differentially expressed miRNAs were identified in the poly I:C-stimulated fish, among which, 7 and 17 miRNAs were upregulated and downregulated, respectively. Analysis of target genes of these differentially expressed miRNAs found that most targeted mRNAs were involved in catalytic activity, peptidase activity and endopeptidase activity, and were enriched in the metabolic, protein processing in endoplasmic reticulum and oxidative phosphorylation signaling pathways, suggesting that poly I:C could alter the expression of metabolism-related miRNAs in the kidney of gibel carp. Besides, it was noted that some immune-related miRNAs, including inflammation-related miRNAs (miR-192 and miR-731) and interferon-related miRNAs (miR-194a and miR-122), were downregulated after poly I:C treatment. In summary, it was found that poly I:C could regulate the cellular levels of specific miRNAs involved in metabolism and immune responses in the head kidney of gibel carp, which may increase the capacity of the immune cells to fight against pathogens infection.

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.

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.

Toll-Like Receptors, Associated Biological Roles, and Signaling Networks in Non-Mammals

The innate immune system is the first line of defense against pathogens, which is initiated by the recognition of pathogen-associated molecular patterns (PAMPs) and endogenous damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs). Among all the PRRs identified, the toll-like receptors (TLRs) are the most ancient class, with the most extensive spectrum of pathogen recognition. Since the first discovery of Toll in Drosophila melanogaster, numerous TLRs have been identified across a wide range of invertebrate and vertebrate species. It seems that TLRs, the signaling pathways that they initiate, or related adaptor proteins are essentially conserved in a wide variety of organisms, from Porifera to mammals. Molecular structure analysis indicates that most TLR homologs share similar domain patterns and that some vital participants of TLR signaling co-evolved with TLRs themselves. However, functional specification and emergence of new signaling pathways, as well as adaptors, did occur during evolution. In addition, ambiguities and gaps in knowledge still exist regarding the TLR network, especially in lower organisms. Hence, a systematic review from the comparative angle regarding this tremendous signaling system and the scenario of evolutionary pattern across Animalia is needed. In the current review, we present overview and possible evolutionary patterns of TLRs in non-mammals, hoping that this will provide clues for further investigations in this field.

Innate immune responses against rock bream iridovirus (RBIV) infection in rock bream (Oplegnathus fasciatus) following poly (I:C) administration

Poly (I:C) showed promise as an immunoprotective agents in rock bream against rock bream iridovirus (RBIV) infection. In this study, we evaluated the time-dependent virus replication pattern and antiviral immune responses in RBIV-infected rock bream with and without poly (I:C) administration. In the poly (I:C)+virus-injected group, virus copy numbers were more than 18.9-, 24.0- and 479.2-fold lower than in the virus only injected group at 4 (4.73 × 10⁴ and 8.95 × 10⁵/μl, respectively), 7 (3.67 × 10⁵ and 8.81 × 10⁶/μl, respectively) and 10 days post infection (dpi) (1.26 × 10⁵ and 6.02 × 10⁷/μl, respectively). Moreover, significantly high expression levels of TLR3 (8.6- and 7.7-fold, at 4 and 7 dpi, respectively) and IL1β (3.6-fold at 2 dpi) were observed in the poly (I:C)+virus-injected group, but the expression levels were not significantly in the virus-injected group. However, IL8 and TNFα expression levels showed no statistical significance in both groups. Mx, ISG15 and PKR were significantly highly expressed from 4 to 10 dpi in the virus-injected group. Nevertheless, in the poly (I:C)+virus-injected group, Mx and ISG15 expression were significantly expressed from 2 dpi. In summary, poly (I:C) administration in rock bream induces TLR3, IL1β, Mx and ISG15-mediated immune responses, which could be a critical factor for inhibition of virus replication.

Gene expression regulation of the TLR9 and MyD88-dependent pathways in rock bream against rock bream iridovirus (RBIV) infection

Rock bream iridovirus (RBIV), which is a member of the Megalocytivirus genus, causes severe mass mortalities in rock bream in Korea. To date, the innate immune defense mechanisms of rock bream against RBIV is unclear. In this study, we assessed the expression levels of genes related to TLR9 and MyD88-dependent pathways in RBIV-infected rock bream in high, low or no mortality conditions. In the high mortality group (100% mortality at 15 days post infection (dpi)), high levels of TLR9 and MyD88 expressions (6.4- and 2.4-fold, respectively) were observed at 8 d and then reduced (0.6- and 0.1-fold, respectively) with heavy viral loads at 10 dpi (2.21 × 10⁷/μl). Moreover, TRAF6, IRF5, IL1β, IL8, IL12 and TNFα expression levels showed no statistical significance until 10 dpi. Conversely, in the low mortality group (28% expected mortality at 35 dpi), TLR9, MyD88 and TRAF6 expression levels were significantly higher than those in the control group at several sampling points until 30 dpi. Higher levels of IRF5, IL1β, IL8, IL12 and TNFα expression were also observed, however, these were not significantly different from those of the control group. In the no mortality group (0% mortality at 40 dpi), significantly higher levels of MyD88 (2 d, 4 d and 40 dpi), TRAF6 (2 dpi), IL1β (4 dpi) and IL8 (2 d and 4 dpi) expression were observed. In summary, RBIV-infected rock bream induces innate immune response, which could be a major contributing factor to effective fish control over viral transcription. MyD88, TRAF6, IL1β and IL8-related immune responses were activated in fish survivor condition (low or no mortality group). This is a critical factor for RBIV disease recovery; however, these immune responses did not efficiently respond in fish dead condition (high mortality group).

Rock bream iridovirus (RBIV) replication in rock bream (Oplegnathus fasciatus) exposed for different time periods to susceptible water temperatures

Rock bream iridovirus (RBIV) is a member of the Megalocytivirus genus that causes severe mortality to rock bream. Water temperature is known to affect the immune system and susceptibility of fish to RBIV infection. In this study, we evaluated the time dependent virus replication pattern and time required to completely eliminate virus from the rock bream body against RBIV infection at different water temperature conditions. The rock bream was exposed to the virus and held at 7 (group A1), 4 (group A2) and 2 days (group A3) at 23 °C before the water temperature was reduced to 17 °C. A total of 28% mortality was observed 24-35 days post infection (dpi) in only the 7 day exposure group at 23 °C. In all 23 °C exposure groups, virus replication peaked at 20 to 22 dpi (10⁶-10⁷/μl). In recovery stages (30-100 dpi), the virus copy number was gradually reduced, from 10⁶ to 10¹ with faster decreases in the shorter exposure period group at 23 °C. When the water temperature was increased in surviving fish from 17 to 26 °C at 70 dpi, they did not show any mortality or signs of disease and had low virus copy numbers (below 10²/μl). Thus, fish need at least 50 days from peaked RBIV levels (approximately 20-25 dpi) to inhibit the virus. This indicates that maintaining the fish at low water temperature (17 °C) for 70 days is sufficient to eradicate RBIV from fish body. Thus, RBIV could be eliminated slowly from the fish body and the virus may be completely eliminated under the threshold of causing mortality.

CpG ODN 1668 induce innate and adaptive immune responses in rock bream (Oplegnathus fasciatus) against rock bream iridovirus (RBIV) infection

Rock bream iridovirus (RBIV) causes severe mass mortalities in rock bream in Korea. CpG ODN 1668 showed promise as immunoprotective agents against RBIV infection in rock bream. In this study, we assessed innate/adaptive-related gene expression patterns in RBIV-infected rock bream with and without CpG ODN 1668 administration to determine important immune defense related factors that may affect fish survival. In the CpG ODN 1668+virus-injected group, virus copies were more than 7.4- to 790591-fold lower than in the virus-injected group at 4 d (8.79 × 10⁴ and 6.58 × 10⁵/μl, respectively), 7 d (5.30 × 10² and 2.29 × 10⁷/μl, respectively) and 10 dpi (7.79 × 10¹ and 6.16 × 10⁷/μl, respectively). Furthermore, in the CpG ODN 1668+virus-injected group, significantly higher levels of MyD88 (6 h, 1 d, 4 d and 7 dpi), IL1β (1 d, 2 d and 7 dpi) and perforin/granzyme (1 dpi) expression were observed, whereas these genes were not significantly expressed in the virus-injected group at that time points. Mx, ISG15 and PKR were significantly highly expressed at 4 d and 7 dpi and reduced when low viral loads at 10 dpi in the CpG ODN 1668+virus-injected group. Conversely, in the virus-injected group, Mx, ISG15 and PKR expression were significantly higher than the control group until 10 dpi. However, MHC class I, CD8, Fas, Fas ligand and caspases (3, 8 and 9) expression levels showed no statistically significant differences between virus- and CpG ODN 1668+virus-injected group. In summary, CpG ODN 1668 administration in fish induces innate immune response or cell death pathway, which could be a major contributing factor to effective fish control over viral transcription on 4 d to 10 dpi. Expression of MyD88, IL1β, perforin and granzyme-related immune gene response is critical factor for inhibition of RBIV replication.