Immunological responses and protection in Chinese giant salamander Andrias davidianus immunized with inactivated iridovirus

Characterization of immunoglobulin loci in the gigantic genome of Ambystoma mexicanum

Background

The axolotl, Ambystoma mexicanum is a unique biological model for complete tissue regeneration. Is a neotenic endangered species and is highly susceptible to environmental stress, including infectious disease. In contrast to other amphibians, the axolotl is particularly vulnerable to certain viral infections. Like other salamanders, the axolotl genome is one of the largest (32 Gb) and the impact of genome size on Ig loci architecture is unknown. To better understand the immune response in axolotl, we aimed to characterize the immunoglobulin loci of A. mexicanum and compare it with other model vertebrates.

Methods

The most recently published genome sequence of A. mexicanum (V6) was used for alignment-based annotation and manual curation using previously described axolotl Ig sequences or reference sequences from other vertebrates. Gene models were further curated using A. mexicanum spleen RNA-seq data. Human, Xenopus tropicalis, Danio rerio (zebrafish), and eight tetrapod reference genomes were used for comparison.

Results

Canonical A. mexicanum heavy chain (IGH), lambda (IGL), sigma (IGS), and the putative surrogate light chain (SLC) loci were identified. No kappa locus was found. More than half of the IGHV genes and the IGHF gene are pseudogenes and there is no clan I IGHV genes. Although the IGH locus size is proportional to genome size, we found local size restriction in the IGHM gene and the V gene intergenic distances. In addition, there were V genes with abnormally large V-intron sizes, which correlated with loss of gene functionality.

Conclusion

The A. mexicanum immunoglobulin loci share the same general genome architecture as most studied tetrapods. Consistent with its large genome, Ig loci are larger; however, local size restrictions indicate evolutionary constraints likely to be imposed by high transcriptional demand of certain Ig genes, as well as the V(D)J recombination over very long genomic distance ranges. The A. mexicanum has undergone an extensive process of Ig gene loss which partially explains a reduced potential repertoire diversity that may contribute to its impaired antibody response.

Current status and development prospects of aquatic vaccines

Diseases are a significant impediment to aquaculture's sustainable and healthy growth. The aquaculture industry is suffering significant financial losses as a result of the worsening water quality and increasing frequency of aquatic disease outbreaks caused by the expansion of aquaculture. Drug control, immunoprophylaxis, ecologically integrated control, etc. are the principal control strategies for fish infections. For a long time, the prevention and control of aquatic diseases have mainly relied on the use of various antibiotics and chemical drugs. However, long-term use of chemical inputs not only increases pathogenic bacteria resistance but also damages the fish and aquaculture environments, resulting in drug residues in aquatic products, severely impeding the development of the aquaculture industry. The development and use of aquatic vaccines are the safest and most effective ways to prevent aquatic animal diseases and preserve the health and sustainability of aquaculture. To give references for the development and implementation of aquatic vaccines, this study reviews the development history, types, inoculation techniques, mechanisms of action, development prospects, and challenges encountered with aquatic vaccines.

The Immune System and the Antiviral Responses in Chinese Giant Salamander, Andrias davidianus

The Chinese giant salamander, belonging to an ancient amphibian lineage, is the largest amphibian existing in the world, and is also an important animal for artificial cultivation in China. However, some aspects of the innate and adaptive immune system of the Chinese giant salamander are still unknown. The Chinese giant salamander iridovirus (GSIV), a member of the Ranavirus genus (family Iridoviridae), is a prominent pathogen causing high mortality and severe economic losses in Chinese giant salamander aquaculture. As a serious threat to amphibians worldwide, the etiology of ranaviruses has been mainly studied in model organisms, such as the Ambystoma tigrinum and Xenopus. Nevertheless, the immunity to ranavirus in Chinese giant salamander is distinct from other amphibians and less known. We review the unique immune system and antiviral responses of the Chinese giant salamander, in order to establish effective management of virus disease in Chinese giant salamander artificial cultivation.

Oral Vaccination of Grass Carp (Ctenopharyngodon idella) with Baculovirus-Expressed Grass Carp Reovirus (GCRV) Proteins Induces Protective Immunity against GCRV Infection

The grass carp reovirus (GCRV) causes severe hemorrhagic disease with high mortality and leads to serious economic losses in the grass carp (Ctenopharyngodon idella) industry in China. Oral vaccine has been proven to be an effective method to provide protection against fish viruses. In this study, a recombinant baculovirus BmNPV-VP35-VP4 was generated to express VP35 and VP4 proteins from GCRV type Ⅱ via Bac-to-Bac baculovirus expression system. The expression of recombinant VP35-VP4 protein (rVP35-VP4) in Bombyx mori embryo cells (BmE) and silkworm pupae was confirmed by Western blotting and immunofluorescence assay (IFA) after infection with BmNPV-VP35-VP4. To vaccinate the grass carp by oral route, the silkworm pupae expressing the rVP35-VP4 proteins were converted into a powder after freeze-drying, added to artificial feed at 5% and fed to grass carp (18 ± 1.5 g) for six weeks, and the immune response and protective efficacy in grass carp after oral vaccination trial was thoroughly investigated. This included blood cell counting and classification, serum antibody titer detection, immune-related gene expression and the relative percent survival rate in immunized grass carp. The results of blood cell counts show that the number of white blood cells in the peripheral blood of immunized grass carp increased significantly from 14 to 28 days post-immunization (dpi). The differential leukocyte count of neutrophils and monocytes were significantly higher than those in the control group at 14 dpi. Additionally, the number of lymphocytes increased significantly and reached a peak at 28 dpi. The serum antibody levels were significantly increased at Day 14 and continued until 42 days post-vaccination. The mRNA expression levels of immune-related genes (IFN-1, TLR22, IL-1β, MHC I, Mx and IgM) were significantly upregulated in liver, spleen, kidney and hindgut after immunization. Four weeks post-immunization, fish were challenged with virulent GCRV by intraperitoneal injection. The results of this challenge study show that orally immunized group exhibited a survival rate of 60% and relative percent survival (RPS) of 56%, whereas the control group had a survival rate of 13% and RPS of 4%. Taken together, our results demonstrate that the silkworm pupae powder containing baculovirus-expressed VP35-VP4 proteins could induce both non-specific and specific immune responses and protect grass carp against GCRV infection, suggesting it could be used as an oral vaccine.

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.

Protective Immunity Induced by DNA Vaccination against Ranavirus Infection in Chinese Giant Salamander Andrias davidianus

Andrias davidianus ranavirus (ADRV) is an emerging viral pathogen that causes severe systemic hemorrhagic disease in Chinese giant salamanders. There is an urgent need for developing an effective vaccine against this fatal disease. In this study, DNA vaccines containing the ADRV 2L gene (pcDNA-2L) and the 58L gene (pcDNA-58L) were respectively constructed, and their immune protective effects were evaluated in Chinese giant salamanders. In vitro and in vivo expression of the vaccine plasmids were confirmed in transfected cells and muscle tissues of vaccinated Chinese giant salamanders by using immunoblot analysis or RT-PCR. Following ADRV challenge, the Chinese giant salamanders vaccinated with pcDNA-2L showed a relative percent survival (RPS) of 66.7%, which was significant higher than that in Chinese giant salamanders immunized with pcDNA-58L (RPS of 3.3%). Moreover, the specific antibody against ADRV was detected in Chinese giant salamanders vaccinated with pcDNA-2L at 14 and 21 days post-vaccination by indirect enzyme-linked immunosorbent assay (ELISA). Transcriptional analysis revealed that the expression levels of immune-related genes including type I interferon (IFN), myxovirus resistance (Mx), major histocompatibility complex class IA (MHCIA), and immunoglobulin M (IgM) were strongly up-regulated after vaccination with pcDNA-2L. Furthermore, vaccination with pcDNA-2L significantly suppressed the virus replication, which was seen by a low viral load in the spleen of Chinese giant salamander survivals after ADRV challenge. These results indicated that pcDNA-2L could induce a significant innate immune response and an adaptive immune response involving both humoral and cell-mediated immunity that conferred effective protection against ADRV infection, and might be a potential vaccine candidate for controlling ADRV disease in Chinese giant salamanders.

Vaccination with Recombinant Baculovirus Expressing Ranavirus Major Capsid Protein Induces Protective Immunity in Chinese Giant Salamander, Andrias davidianus

The Chinese giant salamander iridovirus (CGSIV), belonging to the genus Ranavirus in the family Iridoviridae, is the causative agent of an emerging infectious disease causing high mortality of more than 90% and economic losses in Chinese giant salamanders in China. In this study, a recombinant baculovirus-based vaccine expressing the CGSIV major capsid protein (MCP) was developed and its protective immunity in Chinese giant salamanders was evaluated. The recombinant Autographacalifornica nucleopolyhedrosis virus (AcNPV), expressing CGSIV MCP, designated as AcNPV-MCP, was generated with the highest titers of 1 × 10⁸ plaque forming units/mL (PFU/mL) and confirmed by Western blot and indirect immunofluorescence (IIF) assays. Western blot analysis revealed that the expressed MCP reacted with mouse anti-MCP monoclonal antibodies at the band of about 53 kDa. The results of IIF indicated that the MCP was expressed in the infected Spodoptera frugiperda 9 (Sf9) cells with the recombinant baculovirus, and the Chinese giant salamander muscle cells also transduced with the AcNPV-MCP. Immunization with the recombinant baculovirus of AcNPV-MCP elicited robust specific humoral immune responses detected by ELISA and neutralization assays and potent cellular immune responses in Chinese giant salamanders. Importantly, the effective immunization conferred highly protective immunity for Chinese giant salamanders against CGSIV challenge and produced a relative percent of survival rate of 84%. Thus, the recombinant baculovirus expressing CGSIV MCP can induce significant immune responses involving both humoral and cell-mediated immunity in Chinese giant salamanders and might represent a potential baculovirus based vaccine candidate for Chinese giant salamanders against CGSIV.

In vitro antiviral efficacy of moroxydine hydrochloride and ribavirin against grass carp reovirus and giant salamander iridovirus

Moroxydine hydrochloride (Mor) and ribavirin (Rib) have been reported to exhibit multi-antiviral activities against DNA and RNA viruses, but their antiviral activities and pharmacologies have seldom been studied in aquaculture. This paper has selected 3 aquatic viruses including a double-stranded RNA virus (grass carp reovirus, GCRV), a single-stranded RNA virus (spring viraemia of carp virus, SVCV) and a DNA virus (giant salamander iridovirus, GSIV) for antiviral testing. The results showed that Mor and Rib can effectively control the infection of GCRV and GSIV in respective host cells. Further study was undertaken to explore the antivirus efficiencies and pharmacological mechanisms of Mor and Rib on GCRV and GSIV in vitro. Briefly, compounds showed over 50% protective effects at 15.9 µg ml-1 except for the group of GSIV-infected epithelioma papulosum cyprinid (EPC) cells treated with Mor. Moreover, Mor and Rib blocked the virus-induced cytopathic effects and apoptosis in host cells to keep the normal cellular structure. The expression of VP1 (GCRV) and major capsid protein (MCP; GSIV) gene was also significantly inhibited in the virus-infected cells when treated with Mor and Rib. Cytotoxicity assay verified the 2 compounds had no toxic effects on grass carp ovary (GCO) cells and EPC cells at ≤96 µg ml-1. In conclusion, these results indicated that exposing GCRV-infected GCO cells and GSIV-infected EPC cells to Mor and Rib could elicit significant antiviral responses, and the 2 compounds have been shown to be promising agents for viral control in the aquaculture industry.

Immune response and protection in gibel carp, Carassius gibelio, after vaccination with β-propiolactone inactivated cyprinid herpesvirus 2

Herpesviral haematopoietic necrosis (HVHN) of gibel carp (Carassius gibelio) is a newly emerged infectious disease caused by cyprinid herpesvirus 2 (CyHV-2) and has caused huge economic losses in aquaculture operations. Currently, no effective methods are available for the control of the disease. In this study, β-propiolactone inactivated cyprinid herpesvirus 2 (CyHV-2) vaccine was prepared, and the immune response and protection in cultured gibel carp after vaccination was thoroughly investigated. This included blood cell counting and classification, phagocytic activity, lysozyme and superoxide dismutase activity, neutralizing antibody titration, immune gene expression analysis, and determination of the relative percent survival in vaccinated gibel carp. The results of blood cell counts indicated that the numbers of the red and white blood cells in the peripheral blood of immunized gibel carp increased significantly at day 4 and day 7 after vaccination (p < 0.01). The differential leukocyte count of neutrophils and monocytes were significantly different compared to the control group at day 4 and 7 and the percentage of lymphocytes reached a peak at day 21. The phagocytic percentage and phagocytic index peaked at day 4 post-vaccination. The lysozyme activity and superoxide dismutase activity were significantly increased compared to the control group (p < 0.01). The serum neutralizing antibody titer peaked (203.03 ± 13.44) at day 21. The qPCR analysis revealed that the expression of the immune genes interlukin 11 and complement component C3 were significantly up-regulated in the immunized group. The challenge test demonstrated that the immunized group had a relative survival rate of 71.4%. These results indicate that the inactivated CyHV-2 vaccine induced both non-specific and specific anti-viral immune responses that resulted in significant protection against HVHN disease and mortality in gibel carp.

Cloning, sequence analysis and expression profiles of Toll-like receptor 7 from Chinese giant salamander Andrias davidianus

The Chinese giant salamander, Andrias davidianus, is the largest extant amphibian species in the world, which is of significance due to its specific position in the evolutionary history of vertebrates. Currently, limited information about the innate immune system of this animal is known. In this study, the toll-like receptor 7 (TLR7), designated CgsTLR7, was cloned from Chinese giant salamander, A. davidianus. The full-length cDNA of CgsTLR7 is 3747 bp, with an open reading frame of 3150 bp, encoding 1049 amino acids. The TLR family motifs, including the leucine-rich repeat (LRR) and Toll/interleukin (IL)-1 receptor (TIR) domain are conserved in CgsTLR7, which includes 19 LRRs and a TIR domain. The predicted amino acid sequence of CgsTLR7 has 71%, 65%, 63% and 55% identity with turtle, chicken, human and fugu TLR7 homologues, respectively. Phylogenetic analysis showed that CgsTLR7 is closest to that of frog TLR7 among the examined species. Quantitative real-time PCR analysis revealed broad expression of CgsTLR7 in tissues from apparently healthy Chinese giant salamanders with the highest expression in the liver and the lowest expression in the intestine. The mRNA expression was up-regulated and reached a peak level in the kidney, liver and spleen at 12 h, 24 h and 48 h after infecting the animals with the giant salamander iridovirus (GSIV), respectively. These results suggest that CgsTLR7 has a conserved gene structure and might play an important role in immune regulation against viral infections in the Chinese giant salamander.

Identification of type I IFN in Chinese giant salamander (Andrias davidianus) and the response to an iridovirus infection

The type I IFNs play a major role in the first line of defense against virus infections. In this study, the type I IFN gene designated gsIFN was identified and characterized in the Chinese giant salamander (Andrias davidianus). The genomic DNA of gsIFN contains 5 exons and 4 introns and has a total length of 5622 bp. The full-length cDNA sequence of gsIFN is 1113 bp and encodes a putative protein of 186 amino acids that has a 43% identity to type I IFN of Xenopus tropicalis. The deduced amino acid sequence has the C-terminal CAWE motif, that is mostly conserved in the higher vertebrate type I IFNs. Real-time fluorescence quantitative RT-PCR analysis revealed broad expression of gsIFN in vivo and the highest level expression in blood, kidney and spleen. Additionally, the expression of gsIFN at the mRNA level was significantly induced in peripheral blood leucocytes after stimulation with poly I:C and after infection with the Chinese giant salamander iridovirus (GSIV). A plasmid expressing gsIFN was constructed and transfected into the Chinese giant salamander muscle cell line. Expression of the IFN-inducible gene Mx was up-regulated in the gsIFN-overexpressing cells after GSIV infection. The virus load and titer were significantly reduced compared with that in control cells. Additionally, a lower level of virus major capsid protein synthesis was confirmed by immunofluorescence assay compared to the control cells. These results suggest that the gsIFN gene plays an important role in the antiviral innate immune response.