Fish reovirus GCReV-109 VP33 protein elicits protective immunity in rare minnows

Analysis of tissue tropism of GCRV-II infection in grass carp using a VP35 monoclonal antibody

Grass carp, one of the major freshwater aquaculture species in China, is susceptible to grass carp reovirus (GCRV). GCRV is a non-enveloped RNA virus and has a double-layered capsid, causing hemorrhagic disease and high mortalities in infected fish. However, the tropism of GCRV infection has not been investigated. In this study, monoclonal antibodies against recombinant VP35 protein were generated in mice and characterized. The antibodies exhibited specific binding to the N terminal region (1-155 aa) of the recombinant VP35 protein expressed in the HEK293 cells, and native VP35 protein in the GCRV-II infected CIK cells. Immunofluorescent staining revealed that viruses aggregated in the cytoplasm of infected cells. In vivo challenge experiments showed that high levels of GCRV-II viruses were present in the gills, intestine, spleen and liver, indicating that they are the major sites for virus infection. Our study showed that the VP35 antibodies generated in this study exhibited high specificity, and are valuable for the development of diagnostic tools for GCRV-II infection.

Hsp90 Regulates GCRV-II Proliferation by Interacting with VP35 as Its Receptor and Chaperone

The frequent outbreak of grass carp hemorrhagic disease caused by grass carp reovirus (GCRV), especially the mainly prevalent type II GCRV (GCRV-II), has seriously affected the grass carp culture in China. However, its pathogenic mechanism is still far from clear. In this study, the GCRV-II outer capsid protein VP35 was used as bait to capture interacting partners from Ctenopharyngon idellus kidney (CIK) cells, and heat shock protein 90 (Hsp90) was selected and confirmed interacting with VP35 through the C-terminal domain of Hsp90. Knockdown of Hsp90 or inhibition of Hsp90 activity suppressed GCRV-II proliferation, demonstrating that Hsp90 is an essential factor for GCRV-II proliferation. The confocal microscopy and flow cytometry showed that Hsp90 localized at both membrane and cytoplasm of CIK cells. The entry of GCRV-II into CIK cells was efficiently blocked by incubating the cells with Hsp90 antibody or by pretreating the virus with recombinant Hsp90 protein. Whereas overexpression of Hsp90 in CIK cells, grass carp ovary (GCO) cells, or 293T cells promoted GCRV-II entry, indicating that the membrane Hsp90 functions as a receptor of GCRV-II. Furthermore, Hsp90 interacted with clathrin and mediated GCRV-II entry into CIK cells through clathrin endocytosis pathway. In addition, we found that the cytoplasmic Hsp90 acted as a chaperone of VP35 because inhibition of Hsp90 activity enhanced VP35 polyubiquitination and degraded VP35 through the proteasome pathway. Collectively, our data suggest that Hsp90 functions both as a receptor for GCRV-II entry and a chaperone for the maturation of GCRV-II VP35, thus ensuring efficient proliferation of GCRV-II. IMPORTANCE Identification of viral receptors has always been the research hot spot in virus research field as receptor functions at the first stage of viral infection, which can be designed as efficient antiviral drug targets. GCRV-II, the causative agent of the grass carp epidemic hemorrhagic disease, has caused tremendous losses in grass carp culture in China. To date, the receptor of GCRV-II remains unknown. This study focused on identifying cellular receptor interacting with the GCRV-II outer capsid protein VP35, studying the effects of their interaction on GCRV-II proliferation, and revealing the underlying mechanisms. We demonstrated that Hsp90 acts both as a receptor of GCRV-II by interacting with VP35 and as a chaperone for the maturation of VP35, thus ensuring efficient proliferation of GCRV-II. Our data provide important insights into the role of Hsp90 in GCRV-II life cycle, which will help understand the mechanism of reovirus infection.

Grass Carp Reovirus VP35 Degrades MAVS Through the Autophagy Pathway to Inhibit Fish Interferon Production

Fish interferon (IFN) is a crucial cytokine for a host to resist external pathogens, conferring cells with antiviral capacity. Meanwhile, grass carp reovirus (GCRV) is a strong pathogen that causes high mortality in grass carp. Therefore, it is necessary to study the strategy used by GCRV to evade the cellular IFN response. In this study, we found that GCRV 35-kDa protein (VP35) inhibited the host IFN production by degrading mitochondrial antiviral signaling (MAVS) protein through the autophagy pathway. First, the overexpression of VP35 inhibited the IFN activation induced by polyinosinic-polycytidylic acid (poly I:C) and MAVS, and the expression of downstream IFN-stimulated genes (ISGs) was also decreased by using VP35 under the stimulation. Second, VP35 interacted with MAVS; the experiments of truncated mutants of MAVS demonstrated that the caspase recruitment domain (CARD) and proline-rich (PRO) domains of MAVS were not necessary for this binding. Then, MAVS was degraded by using VP35 in a dose-dependent manner, and 3-MA (the autophagy pathway inhibitor) significantly blocked the degradation, meaning that MAVS was degraded by using VP35 in the autophagy pathway. The result of MAVS degradation suggested that the antiviral capacity of MAVS was remarkably depressed when interrupted by VP35. Finally, in the host cells, VP35 reduced ifn transcription and made the cells vulnerable to virus infection. In conclusion, our results reveal that GCRV VP35 impairs the host IFN response by degrading MAVS through the autophagy pathway, supplying evidence of a fish virus immune evasion strategy.

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 Novel Subunit Vaccine Based on Outer Capsid Proteins of Grass Carp Reovirus (GCRV) Provides Protective Immunity against GCRV Infection in Rare Minnow (Gobiocypris rarus)

The grass carp hemorrhagic disease, caused by the grass carp reovirus (GCRV), has resulted in severe economic losses in the aquaculture industry in China. VP4 and VP35 are outer capsid proteins of GCRV and can induce an immune response in the host. Here, three recombinant baculoviruses, AcMNPV-VP35, AcMNPV-VP4, and AcMNPV-VP35-VP4, were generated to express recombinant VP4 and VP35 proteins from GCRV type II in insect cells by using the Bac-to-Bac baculovirus expression system to create a novel subunit vaccine. The expression of recombinant VP35, VP4, and VP35-VP4 proteins in Sf-9 cells were confirmed by Western blotting and immunofluorescence. Recombinant VP35, VP4, and VP35-VP4 were purified from baculovirus-infected cell lysates and injected intraperitoneally (3 μg/fish) into the model rare minnow, Gobiocypris rarus. After 21 days, the immunized fish were challenged with virulent GCRV. Liver, spleen, and kidney samples were collected at different time intervals to evaluate the protective efficacy of the subunit vaccines. The mRNA expression levels of some immune-related genes detected by using quantitative real-time PCR (qRT-PCR) were significantly upregulated in the liver, spleen, and kidney, with higher expression levels in the VP35-VP4 group. The nonvaccinated fish group showed 100% mortality, whereas the VP35-VP4, VP4, and VP35 groups exhibited 67%, 60%, and 33% survival, respectively. In conclusion, our results revealed that recombinant VP35 and VP4 can induce immunity and protect against GCRV infection, with their combined use providing the best effect. Therefore, VP35 and VP4 proteins can be used as a novel subunit vaccine against GCRV infection.

A developed subunit vaccine based on fiber protein VP56 of grass carp reovirus providing immune protection against grass carp hemorrhagic disease

Grass carp reovirus (GCRV) is the main viral pathogen that endangers grass carp seriously. Application of vaccine has been considered to be the most effective way to prevent virus infection. VP56 is a protein encoded by gene segment 7 of grass carp reovirus, and is predicted to share homology with fiber protein of mammalian reovirus (MRV). In our study, the immunogenicity of VP56 was evaluated by neutralization test. GCRV was incubated with mouse anti-VP56 antibody, and then was injected into grass carp. Results showed that disease progress and death occurrence was hindered in the experimental group compared with the control group. For further study, the recombinant VP56 protein (rVP56) expressed by pET-32a (+) vector was purified, and was used as subunit vaccine to immunize grass carp. After each fish (15 ± 1.5 g) was injected with 30 μg purified rVP56 intraperitoneally, the immune protective efficacy of recombinant VP56 protein was assessed by a series of immune parameters. The population of red blood cells in immunized fish increased significantly after 5 d post injection (dpi), and reached a peak with (2.98 ± 0.17) × 10⁹/ml at 7 dpi (p < 0.05). The numbers of white blood cells peaked with (8.42 ± 1.01) × 10⁷/ml at 7 dpi (p < 0.05). Additionally, the percentage of monocytes and neutrophils rose to a peak with (9.05 ± 0.92)% and (25.93 ± 2.60)% respectively at 5 dpi (p < 0.05 or p < 0.01), whereas lymphocytes reached the highest value of (85.81 ± 2.73) % at 14 dpi (p < 0.01). Serum antibody titer in the vaccinated fish increased significantly and reached a peak at 21 dpi (p < 0.01). The mRNA expression levels of type I interferon (IFN1), major histocompatibility complex class I (MHC I), Toll-like receptor 22 (TLR22), and immunoglobulin M (IgM) were significantly up-regulated in head kidney and spleen (p < 0.05 or p < 0.01). The GCRV challenge test showed that the relative survival rate in immunized group was 71%-75%. Collectively, the results indicated that rVP56 protein can induce immune protection in grass carp, and can be consider as a candidate vaccine against GCRV infection.

Aquatic animal viruses mediated immune evasion in their host

Viruses are important and lethal pathogens that hamper aquatic animals. The result of the battle between host and virus would determine the occurrence of diseases. The host will fight against virus infection with various responses such as innate immunity, adaptive immunity, apoptosis, and so on. On the other hand, the virus also develops numerous strategies such as immune evasion to antagonize host antiviral responses. Here, We review the research advances on virus mediated immune evasions to host responses containing interferon response, NF-κB signaling, apoptosis, and adaptive response, which are executed by viral genes, proteins, and miRNAs from different aquatic animal viruses including Alloherpesviridae, Iridoviridae, Nimaviridae, Birnaviridae, Reoviridae, and Rhabdoviridae. Thus, it will facilitate the understanding of aquatic animal virus mediated immune evasion and potentially benefit the development of novel antiviral applications.

Real-Time Dissecting the Entry and Intracellular Dynamics of Single Reovirus Particle

Reoviruses are non-enveloped viruses with wide host range, can cause serious infections in animals, plants and microorganism, e.g., aquareovirus, which is capable of causing serious haemorrhagic in aquatic animals. To date, the entry process of aquareovirus infection remains obscure. Real-time single-virus tracking are effective tools for exploring the details in viral infection process, which are crucial for understanding the pathogenic mechanism. Here, we used quantum dots-based single particle tracking technology combined with biochemical assays and ultrastructural observation to reveal unobservable infection steps and map dynamic interactions between a reovirus, Scophthalmus maximus reovirus (SMReV), and its host cell in real time. The results showed that the single membrane-bound reovirus particle can enter into the cell within several seconds through nascent clathrin-caoted pits, and most of the particles could internalize into cytoplasm within 30 min post-infection. The specific inhibitors analysis also showed that entry of SMREV depended on clathrin-mediated endocytosis rather than cavolin-mediated endocytosis. The motion analysis of internalized single particle indicated that the reovirus initially experienced slow and directed motion in the actin-enriched cell periphery, while it underwent relatively faster and directed movement toward the cell interior, suggesting that transport of SMReV was dependent on the cytoskeleton. Further, dual-labeling of virus and cytoskeleton and inhibitor analysis both demonstrated that transport of internalized SMReV was firstly dependent on actin filaments at the cell periphery, and then on microtubules toward the cell interior. Then visualization of SMReV trafficking in the endosomes revealed that the internalized reovirus particles were sorted from early endosomes to late endosomes, then part of them were delivered to lysosome. This study for the first time revealed the entry pathway, intracellular dynamic and the infection fate of fish reovirus in host cell in real time and in situ, which provided new insights into the infection mechanism of non-enveloped viruses.

Novel subunit vaccine based on grass carp reovirus VP35 protein provides protective immunity against grass carp hemorrhagic disease

The grass carp (Ctenopharyngodon idella) hemorrhagic disease, caused by grass carp reovirus (GCRV), is one of the most severe infectious diseases in aquaculture. Given that antiviral therapies are currently limitedly available, vaccination remains the most effective means for the prevention of viral diseases, such as GCRV. A reovirus strain, which was temporarily named GCRV-HN14, was recently isolated from grass carp in Henan province, China. The S11 gene fragment of GCRV-HN14 was speculated to encode viral structural protein VP35, which has no equivalent gene in other aquareviruses but has antigenic epitopes. In this study, the recombinant plasmid pET-32a-vp35 was constructed to express recombinant VP35 proteins in prokaryotic cells, which was used to create a novel subunit vaccine. The immune protection of recombinant VP35 protein was evaluated by a series of experiments in grass carp. Results showed that the number of white blood cells (WBC) in the peripheral blood increased significantly to 7.92 ± 0.72 × 10⁷/ml 5 days after vaccination (P < 0.05). The number of neutrophils and monocytes in WBC were significantly higher than those of the control 3 days after vaccination (P < 0.05) and maximally got to 12.22 ± 1.28% and 18.70 ± 1.78%, respectively. Owing to the significant increase in the number of lymphocytes (92.37 ± 2.10%; P < 0.01), the percentages of neutrophils and monocytes declined significantly (14 dpi; P < 0.01). Serum antibody levels induced by recombinant VP35 protein significantly increased 7 days post immunization and continued to increase until 5 weeks post vaccination. The mRNA expression levels of type I interferon (designated as IFN1), immunoglobulin M, Toll-like receptor 22 and major histocompatibility complex class I were up-regulated significantly in the head kidneys and spleens of immunized fish (P < 0.01). Grass carp immunized by recombinant VP35 protein showed that the relative percentage of survival was about 60% after it was challenged with GCRV. Overall, the results suggested that recombinant VP35 protein can induce immunity and protect grass carp against GCRV infection. Thus, it can be used as a subunit vaccine.

Lipid accumulation, oxidative stress and immune-related molecules affected by tributyltin exposure in muscle tissues of rare minnow (Gobiocypris rarus)

Tributyltin (TBT) is reported to induce adipogenesis in fish, which might affect nutritional qualities and health status. Muscle tissues account for the majority of body mass, and have been described as a major site of fat deposition and an immunologically active organ. Therefore, the present study aims to evaluate whether chronic exposures of TBT, at environmental concentrations of 1, 10 and 100 ng/L, affects lipid accumulation, oxidative stress and immune status in muscle tissues of rare minnow (Gobiocypris rarus). After 60 d of exposure, TBT increased contents of total lipid, total cholesterol, triglyceride and fatty acids in muscle tissues. Interestingly, TBT exposure disrupted fatty acid composition and increased contents of unsaturated fatty acids (such as eicosapentaenoic acid and docosahexaenoic acid) in muscle tissues, which might be a response to preserve membrane functions from TBT exposure. Meanwhile, the concentrations of hepatic fatty acid desaturase 2 (Δ6-desaturase) and stearoyl-CoA desaturase (Δ9-desaturase) were increased after TBT exposure, which might contribute the increase of unsaturated fatty acids. Furthermore, TBT increased muscle lipid peroxidation products, antioxidant enzymes (superoxide dismutase, catalase and glutathione peroxidase), and the expression of immune-related molecules (tumor necrosis factor alpha, interleukin 1 beta and nuclear factor kappa B) in muscle tissues. The disruption of TBT on the lipid accumulation, oxidative stress and immune-toxic effects in muscle tissues of fish might reduce nutritional qualities, and affect growth and health status, which might pose a constant and serious threat to fish and result in economic loss in aquaculture.