Characterization of gene deletion mutants of Cyprinid herpesvirus 3 (koi herpesvirus) lacking the immunogenic envelope glycoproteins pORF25, pORF65, pORF148 and pORF149

Generation and Characterization of ORF55/ORF57-Deleted Recombinant Cyprinid herpesvirus 2 Mutants with Chimeric Capsid Protein Gene of Grouper Nervous Necrosis Virus

Cyprinid herpesvirus 2 (CyHV-2) is a pathogen that causes significant losses to the global aquaculture industry due to mass mortality in crucian carp and goldfish. This study demonstrates that the ORF55/ORF57 deletion mutants CyHV-2-Δ55-CP and CyHV-2-Δ57-CP obtained through homologous recombination replicate effectively within the caudal fin of Carassius auratus gibelio (GiCF) cells and exhibit morphologies similar to the CyHV-2 wild-type strain. Both mutants demonstrated a decrease in virulence, with CyHV-2-Δ57-CP exhibiting a more significant reduction. This serves as a reference for the subsequent development of recombinant attenuated vaccines against CyHV-2. Additionally, both mutants expressed the inserted RGNNV-CP (capsid protein of Redspotted grouper nervous necrosis virus) fusion protein gene, and inoculation with CyHV-2-Δ57-CP-infected GiCF cell lysates elicited an antibody response in the grouper. These results indicate that, while ORF55 and ORF57 genes of CyHV-2 are not required for viral replication in vitro, they do play a role in virulence in vivo. Additionally, expression of foreign protein in CyHV-2 suggests that the fully attenuated mutant of CyHV-2 could potentially function as a viral vector for developing subunit vaccines or multivalent recombinant attenuated vaccines.

Development of an attenuated vaccine against Koi Herpesvirus Disease (KHVD) suitable for oral administration and immersion

Since the end of the1990ies, Cyprinid herpesvirus 3 (also known as koi herpesvirus, KHV) has caused mass mortality events of koi and common carp all over the globe. This induced a high economic impact, since the KHV disease cannot be cured up to now, but only prevented by vaccination. Unfortunately, there is only one commercial vaccine available which is not approved in most countries. Therefore, there is an urgent need for new, safe and available vaccines. In this study, a live attenuated vaccine virus was generated by cell culture passages of virulent KHV, and shown to protect carp or koi after immersion or oral application against wild type challenge. An advantage of boost immunization was demonstrated, especially after oral application. Vaccination induced no or mild clinical signs and protecting antibodies have been measured. Additionally, the vaccine virus allowed differentiation of infected from vaccinated animals (DIVA) by PCR. The attenuation of the newly generated vaccine was tracked down to a partial deletion of open reading frame 150. This was confirmed by the generation of engineered ORF150 deletion mutants of wild-type KHV which exhibited a similar attenuation in vivo.

Development of a double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) for the detection of KHV

Koi herpesvirus disease (KHVD) caused by the koi herpesvirus (KHV) is difficult to diagnose in live fish, presenting a challenge to the koi industry. The enzyme-linked immunosorbent assay (ELISA) method cannot be widely used to detect KHV because few commercial anti-KHV antibody exists. Here, we developed an anti-ORF132 polyclonal antibody and confirmed its reactivity via indirect immunofluorescence assay and Western blotting. A double-antibody sandwich ELISA (DAS-ELISA) was established to detect KHV, monoclonal antibody 1B71B4 against ORF92 was used as the capture antibody, and the detection antibody was the polyclonal antibody against the truncated ORF132. The lowest limit was 1.56 ng/ml KHV. Furthermore, the DAS-ELISA reacted with KHV isolates, while no cross-reactions occurred with carp oedema virus, spring viraemia of carp virus, frog virus 3 and grass carp reovirus. Two hundred koi serum samples from Guangdong, China, were used in the DAS-ELISA test, and the positive rate of the koi sera was 13%. The clinical sensitivity and specificity of the DAS-ELISA relative to the traditional PCR method were 66.7% and 97.6%, respectively. Our findings may be useful for diagnosing and preventing KHVD in koi and common carp.

Oral Probiotic Vaccine Expressing Koi Herpesvirus (KHV) ORF81 Protein Delivered by Chitosan-Alginate Capsules Is a Promising Strategy for Mass Oral Vaccination of Carps against KHV Infection

Koi herpesvirus (KHV) is highly contagious and lethal to cyprinid fish, causing significant economic losses to the carp aquaculture industry, particularly to koi carp breeders. Vaccines delivered through intramuscular needle injection or gene gun are not suitable for mass vaccination of carp. So, the development of cost-effective oral vaccines that are easily applicable at a farm level is highly desirable. In this study, we utilized chitosan-alginate capsules as an oral delivery system for a live probiotic (Lactobacillus rhamnosus) vaccine, pYG-KHV-ORF81/LR CIQ249, expressing KHV ORF81 protein. The tolerance of the encapsulated recombinant Lactobacillus to various digestive environments and the ability of the probiotic strain to colonize the intestine of carp was tested. The immunogenicity and the protective efficacy of the encapsulated probiotic vaccine was evaluated by determining IgM levels, lymphocyte proliferation, expression of immune-related genes, and viral challenge to vaccinated fish. It was clear that the chitosan-alginate capsules protected the probiotic vaccine effectively against extreme digestive environments, and a significant level (P < 0.01) of antigen-specific IgM with KHV-neutralizing activity was detected, which provided a protection rate of ca. 85% for koi carp against KHV challenge. The strategy of using chitosan-alginate capsules to deliver probiotic vaccines is easily applicable for mass oral vaccination of fish.IMPORTANCE An oral probiotic vaccine, pYG-KHV-ORF81/LR CIQ249, encapsulated by chitosan-alginate capsules as an oral delivery system was developed for koi carp against koi herpesvirus (KHV) infection. This encapsulated probiotic vaccine can be protected from various digestive environments and maintain effectively high viability, showing a good tolerance to digestive environments. This encapsulated probiotic vaccine has a good immunogenicity in koi carp via oral vaccination, and a significant level of antigen-specific IgM was effectively induced after oral vaccination, displaying effective KHV-neutralizing activity. This encapsulated probiotic vaccine can provide effective protection for koi carp against KHV challenge, which is handling-stress free for the fish, cost effective, and suitable for the mass oral vaccination of koi carp at a farm level, suggesting a promising vaccine strategy for fish.

Oral vaccination using Artemia coated with recombinant Saccharomyces cerevisiae expressing cyprinid herpesvirus-3 envelope antigen induces protective immunity in common carp (Cyprinus carpio var. Jian) larvae

Cyprinid herpesvirus 3 (CyHV-3) is the etiological agent of koi herpersvirus disease (KHVD), which causes serious economic losses in global common carp and ornamental koi carp production of larvae as well as adult type fish. To control KHVD, vaccines against CyHV-3 utilizing different immunization routes have been developed, among them, oral vaccination is the most desirable method to prevent fish diseases occurring at the early larval stage. Here, we developed an oral subunit vaccine through the Saccharomyces cerevisiae cell surface display of CyHV-3 envelope protein pORF65, then, the recombinant yeast fed to Artemia which served as bio-encapsulation vector by subsequently feeding the common carp (Cyprinus carpio var. Jian) larvae. The fluorescent observation showed that the Artemia and S. cerevisiae could deliver intact antigen to the hindgut of carp larvae suggesting the possibility of the vector for oral immunization. On this basis, after three immunizations at a week interval, the oral vaccine induced high level of specific anti-pORF65 antibody. Meanwhile, a significant difference of immune-related genes expression occurred including cxca, IL-1β, IFN-a1, lysozyme, IgM and CD8α between vaccined group and blank control group. In addition, 30% of relative percent survival of carp larvae after immunization was obtained post the animal infection assay, offered an certain immune protection. Our results indicated that the oral pORF65 subunit vaccine bioencapsulated in Artemia induced the activation of immune response and high level of antibodies, which could be served as an oral vaccine candidate for the prevention of CyHV-3 infection.

Protective immunity against CyHV-3 infection via different prime-boost vaccination regimens using CyHV-3 ORF131-based DNA/protein subunit vaccines in carp Cyprinus carpio var. Jian

Cyprinid Herpesvirus 3 (CyHV-3), also known as Koi Herpesvirus (KHV), causes Koi Herpesvirus Disease (KHVD) which leads to serious economic losses worldwide. To exploit DNA/subunit vaccine candidates, CyHV-3 ORF131 gene and cDNA was cloned and analyzed in the present study. Major B cell epitopes of deduced CyHV-3 pORF131 was also predicted. Then the complete CDS of CyHV-3 ORF131 was inserted into pEGFP-N1 vector and a modified pYD1/EBY100 system to construct the DNA and subunit vaccine, respectively. Subsequently, carp were immunized with homologous and heterologous prime-boost regimens relying on the constructed DNA and oral subunit vaccines. Then the protective immunity generated from different vaccines and regimens as well as the capacity of yeast (Saccharomyces cerevisiae) as an oral vaccine vehicle was evaluated. Our study confirmed that CyHV-3 ORF131 gene consisted of 2 introns and 3 exons encoding a 428 amino acids peptide. Further analysis indicated that four fragments of CyHV-3 pORF131 contained the major B cell epitopes (Cys20~Val140, Ser169~Tyr245, Thr258~Pro390, Phe414~Gln428), which could be linked and expressed in E. coli (BL21) as a truncated pORF131. The expression of full-length CyHV-3 pORF131 by pEGFP-N1 and yeast surface display was verified by In vitro assays before vaccination. Immunization of carp with CyHV-3 ORF131 DNA and subunit vaccines could evoke the activation of immune-related genes such as CXCa, CXCR1, IL-1β, TNF-α, INF-a1, Mx-1, IgM, IgT1 and production of specific serum IgM measured by ELISA. RPS (relative percent of survival) ranging from 53.33% to 66.67% was acquired post challenge test. Moreover, flow cytometry analysis illustrated the delivery of surface-displayed CyHV-3 pORF131 to midgut after oral gavage. Thus, our findings suggest that CyHV-3 ORF131 can serve as DNA/subunit vaccines candidate and the yeast as an ideal oral vaccine vehicle.

Current knowledge and future prospects of vaccines against cyprinid herpesvirus 3 (CyHV-3)

Aquaculture is one of the world's most important and fastest growing food production sectors, with an average annual growth of 5.8% during the period 2001-2016. Common carp (Cyprinus carpio) is one of the main aquatic species produced for human consumption and is the world's third most produced finfish. Koi carp, on the other hand, are grown as a popular ornamental fish. In the late 1990s, both of these sectors were threatened by the emergence of a deadly disease caused by cyprinid herpesvirus 3 (CyHV-3; initially called koi herpesvirus or KHV). Since then, several research groups have focused their work on developing methods to fight this disease. Despite increasing knowledge about the pathobiology of this virus, there are currently no efficient and cost-effective therapeutic methods available to fight this disease. Facing the lack of efficient treatments, safe and efficacious prophylactic methods such as the use of vaccines represent the most promising approach to the control of this virus. The common carp production sector is not a heavily industrialized production sector and the fish produced have low individual value. Therefore, development of vaccine methods adapted to mass vaccination are more suitable. Multiple vaccine candidates against CyHV-3 have been developed and studied, including DNA, bacterial vector, inactivated, conventional attenuated and recombinant attenuated vaccines. However, there is currently only one vaccine commercially available in limited regions. The present review aims to summarize and evaluate the knowledge acquired from the study of these vaccines against CyHV-3 and provide discussion on future prospects.