Expression and active testing of VP7 from GCRV (Grass carp reovirus) fused with cholera toxin B subunit in rice calli

Escherichia coli heat-labile enterotoxin B subunit as an adjuvant of mucosal immune combined with GCRV-II VP6 triggers innate immunity and enhances adaptive immune responses following oral vaccination of grass carp (Ctenopharyngodon idella)

The grass carp reovirus (GCRV) is the most major pathogen that has threatened the grass carp (Ctenopharyngodon idella) industry of China for years. Though the oral vaccine has many advantages, the current vaccines still do not provide complete protection. Therefor the exploration of new preventive strategies is urgently needed. In this study, heat-labile enterotoxin B subunit of Escherichia coli (LTB) was combined with VP6 from GCRV type II (GCRV-II) via Lactococcus lactis expression system to form a potent oral vaccine and determines if fusion of LTB to the protective vaccine antigen can enhance protection in the fish. The expression of recombinant protein was confirmed by Western-blotting and enzyme-linked immunosorbent assay. The rare minnow was set as the model for the evaluation of the experiment administrated orally. The immune response including the antibody titer and the immune-related gene expression, and the protective efficacy which included the virus loaded and the relative protection, were thoroughly investigated after the trial. The results indicated that LTB can significantly elicit a higher neutralizing antibody responses and enhanced T-cell priming, activities and proliferation in mononuclear cells from intestine, spleen and kidney tissues when compared to the VP6 vaccine alone. Moreover, the combined adjuvant can significantly up-regulate type I interferon signaling in different immune organs, especially the mucosa associated lymphoidtissue which could not be induced by VP6 along, result in the contribution of the improvement in adaptive immune responses of the fish. In addition, challenge study showed that LTB combined VP6 could greatly improve the relative percent survival of the fish during the virus infection. These results highlight that LTB has the potential value to be a mucosal adjuvant of the fish, approaching for improving the efficacy of vaccination against GCRV-II, which does elicit both non-specific and specific immune responses.

Recombinant surface display vaccine enhances the immersion immune effect against grass carp reovirus in grass carp (Ctenopharyngodon idella)

Grass carp (Ctenopharyngodon idella) is subject to a hemorrhagic disease caused by grass carp reovirus (GCRV), which can lead to mass mortality in grass carp culture, causing significant economic loss. Vaccination is the most promising strategy for the prevention of infectious diseases. Immersion vaccination is considered the most effective disease prevention method for juvenile fish because it can be implemented on many fish at once and administered without causing stress. However, immune responses by immersion vaccination are markedly less robust due to the skin barrier and insufficient antigen uptake. The display of heterologous proteins on the cell surface has been explored as a delivery system for viral antigens in veterinary and human vaccine studies. To improve the efficacy of the immersion vaccine, the major capsid protein (VP7) of GCRV was co-displayed with Aeromonas hydrophila outer membrane protein a (OmpA) and major adhesion protein (Mah) on the outer membrane surface of nonpathogenic Escherichia coli BL21 using the anchoring motif of ice-nucleation protein (Inp). The immune responses and protection efficiency against GCRV infection via both the injection and immersion routes were evaluated. The results indicated that the activities of anti-oxidant enzymes (ACP, AKP, SOD and T-AOC), as well as the expression of immune-related genes (TNF-α, IL-1β, MHCI and IgM) and specific VP7 antibody levels, were strongly increased in the grass carp from 7 to 21 days post-injection inoculation in a dose dependent manner. The cumulative mortality rates of injection-vaccinated groups were much lower than those of the control group after the GCRV challenge, and the relative percent survival (RPS) was greater than 80 %. Vitally, the surface co-display of vp7-Mah protein conferred marked protection to grass carp against GCRV infection after immersion administration (RPS >50 %); this was consistent with the production of high level of specific serum antibodies, non-specific immune responses, and the expression of immune-related genes. Moreover, the invasion analysis further showed that surface co-display of the vp7-Mah protein indeed significantly improved the invasion of E. coli BL21 (DE3) in vitro. Altogether, this study demonstrated that surface display GCRV core antigen vaccine system accompanied by invasion component from aquatic pathogenic microorganism is an effective prophylactic against GCRV viral diseases via the immersion administration approach.

Development of an indirect ELISA for detection of the adaptive immune response of black carp (Mylopharyngodon piceus)

Black carp (Mylopharyngodon piceus) is an important fishery resource and the main breeding target in China. Due to the lack of an assay of immunoglobulin M (IgM) antibodies in black carp, there is no effective method to evaluate adaptive immune response, which limits immunological studies and vaccine development. The present study used mAbs (monoclonal antibodies) against serum IgM of grass carp as capture antibodies. The results of Western blot analysis indicated that these antibodies had strong affinity and specificity to IgM heavy chain in black carp serum and were used to detect the antibody titer, optimize the conditions, perform a sensitivity test, and develop an indirect ELISA (enzyme-linked immunosorbent assay) to detect specific IgM antibodies in the serum. This detection method has good specificity and is effective only for grass carp (Ctenopharyngodon idella) and black carp and not for crucian carp (Carassius aumtus), silver carp (Hypophthalmichthys molitrix), bighead carp (Hypophthalmichthys nobilis), mandarin fish (Siniperca chuatsi), black bream (Megalobrama skolkovii), or yellow catfish (Pseudobagrus fulvidraco). The lowest antigen detection level was 0.05 μg/ml. The error of experimental repetition in the same sample was 1.61-4.61%. The levels of specific IgM in black carp serum were steadily increased after immunization, peaked on day 28, and then slowly decreased. Indirect ELISA can be applied to detect the changes in specific antibodies in black carp serum. Moreover, indirect ELISA provides a convenient and reliable serological detection method for immunological research and evaluation of immune effects of a vaccine in black carp.

The heat-labile toxin B subunit of E. coli fused with VP6 from GCRV (Grass carp reovirus) was expressed and folded into an active protein in rice calli

Grass carp reovirus (GCRV) is one of the most serious pathogens threatening grass carp (Ctenopharyngon idellus) production in China. VP6 could be suitable for developing vaccine for the control of GCRV. Transgenic plants are an attractive bioreactor for their safety and ability to make economical vaccines. The B subunit of Escherichia coli heat-labile enterotoxin (LTB) fused to VP6 (LTB-VP6) was transformed into rice calli by Agrobacterium tumefaciens-mediated gene transformation. Transgenic rice calli was confirmed by PCR analysis separately. The copy numbers of LTB-VP6 inserted into the rice genome are between 1 and 2. The expression level of LTB-VP6 in rice calli was 0.0005-0.0019%, an average of 0.0011% of the TSP(total soluble proteins). LTB-VP6 was folded and assembled into a pentameric form of approximately 305 kDa capable of binding monosialoganglioside (GM1). The suitable concentration of LTB-VP6 in TSP was 0.4 μg/μl. LTB-VP6 is stable and highly active at room temperature. LTB-VP6 binding to GM1 is affected with different affinities under different temperatures. LTB-VP6 had a strong binding affinity at 25 °C and pH 8.4. Our results showed that LTB-VP6 is capable of forming an active pentameric form protein. It provides an ideal alternative to plant-based vaccines against GCRV in aquaculture.

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.

Optimization of the efficacy of a SWCNTs-based subunit vaccine against infectious spleen and kidney necrosis virus in mandarin fish

Infectious spleen and kidney necrosis virus (ISKNV) cause a high mortality disease which brings substantial economic losses to the mandarin fish culture industry in China. This study was aimed at optimizing the efficacy of a SWCNTs-based immersion subunit vaccine (SWCNTs-M-MCP) which as a promising vaccine against ISKNV. Mandarin fish were vaccinated by immersion, then we designed an orthogonal experiment to optimize different parameters affecting vaccination such as immune duration of bath immunization, immune dose, and fish density when immunized. Our results showed that the highest relative percent survival (86.7%) was found in the group 6 with 8 h of immune duration, 20 mg/L of immune dose, and 8 fish per liter of fish density. And other immune responses (serum antibody production, enzyme activities, and immune-related genes expression) also demonstrated similar results. In addition, the expression of IRF-I in group 6 (8 h, 20 mg/L, 8 fish per liter) was significant extents, and about 16-folds increases were obtained than the control group at 21 d post-vaccination. And the highest specific antibody response was significantly increased (more than 4-folds) than control group which was found in group 6. The optimum immune duration, immune dose, and fish density of SWCNTs-M-MCP were 8 h, 20 mg/L, 8 fish per liter, respectively. Importantly, our results also showed that immune duration had the greatest effect on the immune response of our vaccine, followed by immune dose. The study reported herein provides a helpful reference for the effective use of vaccine in fish farming industry.

B subunit of cholera toxin fused with VP7 from GCRV (grass carp reovirus) was expressed in E. coli and folds into an active protein

Grass carp reovirus (GCRV) is one of the most serious pathogens threatening grass carp (Ctenopharyngodon idella) production and results in high mortality in China. To obtain a genetically engineered oral vaccine against GCRV, the cholera toxin B subunit (CTB) of Vibrio cholerae was fused to VP7 (CTB-VP7) and transformed into BL21(DE3) for expression. SDS-PAGE and Western blotting showed that the purified rCTB-VP7 fusion protein (rCTB-VP7) was approximately 49.0 kDa. The monomeric nature of rCTB-VP7 through multistage purification showed a binding affinity for GM1, a receptor for biologically active CTB. rCTB-VP7 is not vulnerable to disassembly by SDS but is vulnerable to disassembly by 2-mercaptoethanol. rCTB-VP7 is stable and highly active at room temperature. The binding affinity experiment between rCTB-VP7 and GM1 also confirms the effects of acid and alkalinity in solution on the structure of rCTB-VP7. rCTB-VP7 bound to GM1 with different affinities under different temperatures and pH values. Prokaryotic expression of rCTB-VP7 was characterized by high expression and easy purification and had a strong binding force with GM1 at 37 °C and pH 7.4. Our results suggest that rCTB-VP7 has the potential as an oral vaccine for protection against GCRV in aquaculture.