Porcine parvovirus capsid protein expressed in Escherichia coli self-assembles into virus-like particles with high immunogenicity in mice and guinea pigs

Design and Application of Biosafe Coronavirus Engineering Systems without Virulence

In the last twenty years, three deadly zoonotic coronaviruses (CoVs)-namely, severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV-2-have emerged. They are considered highly pathogenic for humans, particularly SARS-CoV-2, which caused the 2019 CoV disease pandemic (COVID-19), endangering the lives and health of people globally and causing unpredictable economic losses. Experiments on wild-type viruses require biosafety level 3 or 4 laboratories (BSL-3 or BSL-4), which significantly hinders basic virological research. Therefore, the development of various biosafe CoV systems without virulence is urgently needed to meet the requirements of different research fields, such as antiviral and vaccine evaluation. This review aimed to comprehensively summarize the biosafety of CoV engineering systems. These systems combine virological foundations with synthetic genomics techniques, enabling the development of efficient tools for attenuated or non-virulent vaccines, the screening of antiviral drugs, and the investigation of the pathogenic mechanisms of novel microorganisms.

Chimeric Porcine Parvovirus VP2 Virus-like Particles with Epitopes of South African Serotype 2 Foot-and-Mouth Disease Virus Elicits Specific Humoral and Cellular Responses in Mice

Southern Africa Territories 2 (SAT2) foot-and-mouth disease (FMD) has crossed long-standing regional boundaries in recent years and entered the Middle East. However, the existing vaccines offer poor cross-protection against the circulating strains in the field. Therefore, there is an urgent need for an alternative design approach for vaccines in anticipation of a pandemic of SAT2 Foot-and-mouth disease virus (FMDV). The porcine parvovirus (PPV) VP2 protein can embed exogenous epitopes into the four loops on its surface, assemble into virus-like particles (VLPs), and induce antibodies and cytokines to PPV and the exogenous epitope. In this study, chimeric porcine parvovirus VP2 VLPs (chimeric PPV-SAT2-VLPs) expressing the T-and/or B-cell epitopes of the structural protein VP1 of FMDV SAT2 were produced using the recombinant pFastBac™ Dual vector of baculoviruses in Sf9 and HF cells We used the Bac-to-Bac system to construct the recombinant baculoviruses. The VP2-VLP--SAT2 chimeras displayed chimeric T-cell epitope (amino acids 21-40 of VP1) and/or the B-cell epitope (amino acids 135-174) of SAT FMDV VP1 by substitution of the corresponding regions at the N terminus (amino acids 2-23) and/or loop 2 and/or loop 4 of the PPV VP2 protein, respectively. In mice, the chimeric PPV-SAT2-VLPs induced specific antibodies against PPV and the VP1 protein of SAT2 FMDV. The VP2-VLP-SAT2 chimeras induced specific antibodies to PPV and the VP1 protein specific epitopes of FMDV SAT2. In this study, as a proof-of-concept, successfully generated chimeric PPV-VP2 VLPs expressing epitopes of the structural protein VP1 of FMDV SAT2 that has a potential to prevent FMDV SAT2 and PPV infection in pigs.

Construction, expression and assemble of EMCV VLPs and their potency evaluation

Encephalomycarditis virus (EMCV) is an essential pathogen with a broad host range and causes enormous economic losses to the pig industry worldwide. Here, we constructed and assembled the EMCV virus-like particles (VLPs) in vitro and verified high efficiency of virus protection. Results showed that the proteins auto-assembled into VLPs successfully in vitro. The animal experiments revealed that high-titer antibody production is triggered by VLPs. Meanwhile, the mice challenged with EMCV were obviously protected. The protection rate of group VLPs with the adjuvant was 75%, while that of the VLPs group was 62.5% compared to the control. These findings indicate that recombinant EMCV VLPs have a remarkable anti-EMCV effect and could be a new vaccine candidate for the control of EMCV infection.

Virus-like Particle Vaccines and Platforms for Vaccine Development

Virus-like particles (VLPs) have gained a lot of interest within the past two decades. The use of VLP-based vaccines to protect against three infectious agents-hepatitis B virus, human papillomavirus, and hepatitis E virus-has been approved; they are very efficacious and offer long-lasting immune responses. Besides these, VLPs from other viral infectious agents (that infect humans, animals, plants, and bacteria) are under development. These VLPs, especially those from human and animal viruses, serve as stand-alone vaccines to protect against viruses from which the VLPs were derived. Additionally, VLPs, including those derived from plant and bacterial viruses, serve as platforms upon which to display foreign peptide antigens from other infectious agents or metabolic diseases such as cancer, i.e., they can be used to develop chimeric VLPs. The goal of chimeric VLPs is to enhance the immunogenicity of foreign peptides displayed on VLPs and not necessarily the platforms. This review provides a summary of VLP vaccines for human and veterinary use that have been approved and those that are under development. Furthermore, this review summarizes chimeric VLP vaccines that have been developed and tested in pre-clinical studies. Finally, the review concludes with a snapshot of the advantages of VLP-based vaccines such as hybrid/mosaic VLPs over conventional vaccine approaches such as live-attenuated and inactivated vaccines.

Development of virus-like particles-based vaccines against coronaviruses

Severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and the current severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are the most impactful coronaviruses in human history, especially the latter, which brings revolutionary changes to human vaccinology. Due to its high infectivity, the virus spreads rapidly throughout the world and was declared a pandemic in March 2020. A vaccine would normally take more than 10 years to be developed. As such, there is no vaccine available for SARS-CoV and MERS-CoV. Currently, 10 vaccines have been approved for emergency use by World Health Organization (WHO) against SARS-CoV-2. Virus-like particle (VLP)s are nanoparticles resembling the native virus but devoid of the viral genome. Due to their self-adjuvanting properties, VLPs have been explored extensively for vaccine development. However, none of the approved vaccines against SARS-CoV-2 was based on VLP and only 4% of the vaccine candidates in clinical trials were based on VLPs. In the current review, we focused on discussing the major advances in the development of VLP-based vaccine candidates against the SARS-CoV, MERS-CoV, and SARS-CoV-2, including those in clinical and pre-clinical studies, to give a comprehensive overview of the VLP-based vaccines against the coronaviruses.

Detection and Molecular Characterization of Porcine Parvovirus 7 in Eastern Inner Mongolia Autonomous Region, China

Porcine parvoviruses (PPV) and porcine circoviruses type 2 (PCV2) are widespread in the pig population. Recently, it was suggested that PPV7 may stimulate PCV2 and PCV3 replication. The present study aimed to make detection and molecular characterization of PPV7 for the first time in eastern Inner Mongolia Autonomous Region, China. Twenty-seven of ninety-four samples (28.72%) and five in eight pig farms were PPV7 positive. Further detection showed that the co-infection rate of PPV7 and PCV2 was 20.21% (19/94), and 9.59% (9/94) for PPV7 and PCV3. In addition, the positive rate of PPV7 in PCV2 positive samples was higher than that in PCV2 negative samples, supporting that PCV2 could act as a co-factor for PPV7 infection. In total, four PPV7 strains were sequenced and designated as NM-14, NM-19, NM-4, and NM-40. The amplified genome sequence of NM-14 and NM-40 were 3,999nt in length, while NM-19 and NM-4 were 3,996nt with a three nucleotides deletion at 3,097–3,099, resulting in an amino acid deletion in the Cap protein. Phylogenetic analysis based on the capsid amino acid (aa) sequences showed that 52 PPV7 strains were divided into two clades, and the four PPV7 strains in this study were all clustered in clade 1. The genome and capsid amino acid sequence of the four PPV7 strains identified in this study shared 80.0–96.9% and 85.9–100% similarity with that of 48 PPV7 reference strains selected in NCBI. Simplot analysis revealed that NM-19 and NM-4 strains were probably produced by recombination of two PPV7 strains from China. The amino acid sequence alignment analysis of capsid revealed that the four PPV7 strains detected in Inner Mongolia had multiple amino acid mutations in the 6 B cell linear epitopes compared with the reference strains, suggesting that the four PPV7 strains may have different characteristics in receptor binding and immunogenicity. In summary, this paper reported the PPV7 infection and molecular characterization in the eastern of Inner Mongolia Autonomous Region for the first time, which is helpful to understand the molecular epidemic characteristics of PPV7.

Design of live-attenuated animal vaccines based on pseudorabies virus platform

Pseudorabies virus (PRV) is a double-stranded DNA virus with a genome approximating 150 kb in size. PRV contains many non-essential genes that can be replaced with genes encoding heterogenous antigens without affecting viral propagation. With the ability to induce cellular, humoral and mucosal immune responses in the host, PRV is considered to be an ideal and potential live vector for generation of animal vaccines. In this review, we summarize the advances in attenuated recombinant PRVs and design of PRV-based live vaccines as well as the challenge of vaccine application.

The first report of porcine parvovirus 7 (PPV7) in Colombia demonstrates the presence of variants associated with modifications at the level of the VP2-capsid protein

There are a wide variety of porcine parvoviruses (PPVs) referred to as PPV1 to PPV7. The latter was discovered in 2016 and later reported in some countries in America, Asia, and Europe. PPV7 as a pathogenic agent or coinfection with other pathogens causing disease has not yet been determined. In the present study, we report the identification of PPV7 for the first time in Colombia, where it was found retrospectively since 2015 in 40% of the provinces that make up the country (13/32), and the virus was ratified for 2018 in 4/5 provinces evaluated. Additionally, partial sequencing (nucleotides 380 to 4000) was performed of four Colombian strains completely covering the VP2 and NS1 viral genes. A sequence identity greater than 99% was found when comparing them with reference strains from the USA and China. In three of the four Colombian strains, an insertion of 15 nucleotides (five amino acids) was found in the PPV7-VP2 capsid protein (540–5554 nt; 180–184 aa). Based on this insertion, the VP2 phylogenetic analysis exhibited two well-differentiated evolutionarily related groups. To evaluate the impact of this insertion on the structure of the PPV7-VP2 capsid protein, the secondary structure of two different Colombian strains was predicted, and it was determined that the insertion is located in the coil region and not involved in significant changes in the structure of the protein. The 3D structure of the PPV7-VP2 capsid protein was determined by threading and homology modeling, and it was shown that the insertion did not imply a change in the shape of the protein. Additionally, it was determined that the insertion is not involved in suppressing a potential B cell epitope, although the increase in length of the epitope could affect the interaction with molecules that allow a specific immune response.

A Novel Recombinant Virus-Like Particles Displaying B and T Cell Epitopes of Japanese Encephalitis Virus Offers Protective Immunity in Mice and Guinea Pigs

Virus-like particles (VLPs) are non-replicative vectors for the delivery of heterologous epitopes and are considered one of the most potent inducers of cellular and humoral immune responses in mice and guinea pigs. In the present study, VLP-JEVe was constructed by the insertion of six Japanese encephalitis virus (JEV) envelope protein epitopes into different surface loop regions of PPV VP2 by the substitution of specific amino acid sequences without altering the assembly of the virus; subsequently, the protective efficacy of this VLP-JEVe was evaluated against JEV challenge in mice and guinea pigs. Mice immunized with the VLP-JEVe antigen developed high titers of neutralizing antibodies and 100% protection against lethal JEV challenge. The neutralizing and hemagglutination inhibition (HI) antibody responses were also induced in guinea pigs vaccinated with VLP-JEVe. In addition, immunization with VLP-JEVe in mice induced effective neutralizing antibodies and protective immunity against PPV (porcine parvovirus) challenge in guinea pigs. These studies suggest that VLP-JEVe produced as described here could be a potential candidate for vaccine development.

Self-assembled raccoon dog parvovirus VP2 protein confers immunity against RDPV disease in raccoon dogs: in vitro and in vivo studies

BACKGROUND

Raccoon dog parvovirus (RDPV) causes acute infectious diseases in raccoon dogs and may cause death in severe cases. The current treatment strategy relies on the extensive usage of classical inactivated vaccine which is marred by large doses, short immunization cycles and safety concerns.

METHODS

The present study aimed at optimization of RDPV VP2 gene, subcloning the gene into plasmid pET30a, and its subsequent transfer to Escherichia coli with trigger factor 16 for co-expression. The protein thus expressed was purified with ammonium sulfate precipitation, hydrophobic chromatography, and endotoxin extraction procedures. VLPs were examined by transmission electron microscopy, dynamic light scattering, and the efficacy of VLPs vaccine was tested in vivo.

RESULTS

Results indicated that RDPV VP2 protein could be expressed soluble. Transmission electron microscopy and dynamic light scattering results indicated that RDPV VP2 self-assembled into VLPs. Hemagglutination inhibition antibody titers elicited by Al(OH)3 adjuvanted RDPV VLPs were comparable with RDPV inactivated vaccines, and the viral loads in the blood of the struck raccoon dogs were greatly reduced. Hematoxylin and eosin and Immunohistochemical results indicated that RDPV VLPs vaccine could protect raccoon dogs against RDPV infections.

CONCLUSIONS

These results suggest that RDPV VLPs can become a potential vaccine candidate for RDPV therapy.

Investigation of Kluyveromyces marxianus as a novel host for large-scale production of porcine parvovirus virus-like particles

Background

Porcine Parvovirus (PPV) is a Parvovirinae virus that can cause embryonic and fetal loss and death and mummification in affected fetal pigs. Unlike conventional vaccines, virus-like particles (VLPs) inherit the natural structure of their authentic virions and highly immunostimulatory that can induce strong humoral immune and T cell responses with no risk of pathogenicity. The production of PPV VLPs is still a challenge based on traditional expression platforms due to their low yields and high culture costs. Kluyveromyces marxianus is a safe and fast-growing eukaryote that can get high biomass with low-cost cultures. In this study, we investigated the expression and downstream processes of PPV VLPs in K. marxianus, and the potential for effective stand-alone vaccines.

Results

After optimization according to the codon bias of K. marxianus, the VP2 protein from Kresse strain was highly expressed. In a 5 L fermentator, the yield of PPV VLPs reached 2.5 g/L, quantified by HPLC, using a defined mineral medium after 48 h fermentation. Two strategies were established to purify intracellular PPV VLPs: (i) Using the cation exchange chromatography coupled with Sephacryl® S-500 HR chromatography to purify VLPs from the supernatants of pH adjusted cell lysates. (ii) Using anion exchange chromatography followed by cross-flow diafiltration to recover the VLPs precipitated in pH adjusted cell lysates. The purity of PPV VLPs reached about 95%, and total recovery was more than 60%. Vaccination of mice with the purified PPV VLPs induced high titers of specific IgG antibodies in sera, and showed hemagglutination inhibitions on both swine and guinea pig erythrocytes. Spleen lymphocyte proliferation and cytokines detection suggested the PPV VLPs produced by K. marxianus provoked the cellular immune and humoral immunity responses in mice.

Conclusions

This is the highest production of recombinant PPV VLPs achieved to date. The superiorities, Generally Recognized As Safe (GRAS), high production, short lead time, and low cost, make K. marxianus a greatly competitive platform for bioproduction of PPV VLPs vaccine.

Capsid assembly is regulated by amino acid residues asparagine 47 and 48 in the VP2 protein of porcine parvovirus

Porcine parvovirus (PPV) is a major cause of reproductive failure in swine and has caused substantial losses throughout the world. Viral protein 2 (VP2) of PPV is a major structural protein that can self-assemble into virus-like particles (VLP) with hemagglutination (HA) activity. In order to identify the essential residues involved in the mechanism of capsid assembly and to further understand the function of HA, we analyzed a series of deletion mutants and site-directed mutations within the N-terminal of VP2 using the Escherichia coli system. Our results showed that deletion of the first 47 amino acids from the N-terminal of the VP2 protein did not affect capsid assembly, and further truncation to residue 48 Asparagine (Asn, N) caused detrimental effects. Site-directed mutagenesis experiments demonstrated that residue 47Asn reduced the assembly efficiency of PPV VLP, while residue 48Asn destroyed the stability, hemagglutination, and self-assembly characteristics of the PPV VP2 protein. Results from native PAGE inferred that macromolecular polymers were critical intermediates of the VP2 protein during the capsid assembly process. Site-directed mutation at 48Asn did not affect the ability of monomers to form into oligomers, but destroyed the ability of oligomers to assemble into macromolecular particles, influencing both capsid assembly and HA activity. Our findings provide valuable information on the mechanisms of PPV capsid assembly and the possibility of chimeric VLP vaccine development by replacing the first 47 amino acids at the N-terminal of the VP2 protein.

Identification of a dominant linear epitope on the VP2 capsid protein of porcine parvovirus and characterization of two monoclonal antibodies with neutralizing abilities

Porcine parvovirus (PPV) is a major cause of reproductive failure in swine, and has caused huge losses throughout the world. The structural viral protein VP2, which is able to self-assemble into empty capsids, known as virus-like particles (VLPs), is crucial to induce PPV-specific neutralizing antibodies and protective immunity. In this study, twelve monoclonal antibodies (mAbs) against PPV were generated. The mAbs were characterized by indirect enzyme-linked immunosorbent assay (ELISA), western blotting (WB) and virus neutralization (VN) assay. Two mAbs were defined to be able to neutralize the standard PPV 7909 strain. Subsequently, peptide scanning was applied to identify linear epitopes. The peptide, ⁸⁹ESGVAGQMV⁹⁷ was defined as a precise linear epitope. Results from structural analysis showed that the epitope was exposed on the virion surface. Multiple sequence alignment analysis indicated that peptide ⁸⁹ESGVAGQMV⁹⁷ was not completely conserved, with a higher amino acid mutation rate at ⁹¹G, ⁹²V and ⁹³A position. Alanine-scanning mutagenesis further revealed that residues ⁸⁹E, ⁹⁰S, ⁹¹G, ⁹²V and ⁹⁴G were the core sites involved in antibody recognition. These findings may facilitate further understanding the function of the VP2 protein and development of diagnostic tools.

PCV cap proteins fused with calreticulin expressed into polymers in Escherichia coli with high immunogenicity in mice

BACKGROUND

Porcine circovirus type 2 (PCV2) is the main causative agent of porcine circovirus diseases (PCVDs) which causes huge yearly economic losses in the swine industry. Capsid protein (Cap) is the major structural protein of PCV2 that can induce a protective immune response. Therefore, developing a novel and safe subunit vaccine against PCV2 infection is needed.

RESULTS

In this study, the Cap gene was bound to the truncated calreticulin (CRT) (120-250 aa/120-308 aa) at the N/C terminal, and then the CRT-Cap fusion genes were expressed in Escherichia coli (E.coli). The size-exclusion chromatography and dynamic light scattering (DLS) data showed that the purified recombinant CRT-Cap fusion protein (rP5F) existed in the form of polymers. Immunization with rP5F stimulated high levels of PCV2 specific antibody and neutralization antibody in mice, which were almost identical to those induced by the commercial subunit and inactivated vaccines. The lymphocyte proliferation and cytokine secretion were also detected in rP5F immunized mice. According to the results of PCV2-challenge experiment, the virus loads significantly decreased in mice immunized with rP5F. The data obtained in the current study revealed that rP5F had the potential to be a subunit vaccine candidate against PCV2 in the future.

CONCLUSIONS

We have successfully expressed Cap-CRT fusion proteins in E.coli and optimized rP5F could form into immunogenic polymers. Mice immunized with rP5F efficiently induced humoral and part of cellular immune responses and decreased the virus content against PCV2-challenge, which suggested that rF5P could be a potential subunit vaccine candidate.

The immunogenicity of the virus-like particles derived from the VP2 protein of porcine parvovirus

Porcine parvovirus (PPV) is a major cause of the syndrome of sow reproductive failure that can cause economic losses. In this study, we developed a subunit vaccine against porcine parvovirus (PPV), composed of virus-like particles (VLPs) derived from a prokaryotic system, and evaluated its potential against PPV infection. The soluble recombinant VP2 protein was expressed in E. coli Transetta(DE3) cells using a pCold II prokaryotic expression vector at a low temperature of 15 °C. After expression and purification, the recombinant VP2 protein was successfully assembled into VLPs with a similar shape of PPV viron and also hemagglutination activity. PPV VLPs formulated in a water-in-oil-in-water adjuvant evoked high hemagglutination inhibition antibody and neutralization antibody titres in both guinea pigs, used as reference model, and target species, pigs. Immunization with VLPs vaccine stimulated high hemagglutination inhibition antibody and neutralization antibody responses in guinea pigs, used as reference, and target species, weaned pigs, and primiparous gilts. PPV VLPs from E. coli yielded complete fetal protection against PPV infection in primiparous gilts immunized with a single-dose vaccine. PPV VLPs inhibited the replication and spread of PPV in primiparous gilts, which was confirmed by the detection of PPV DNA and infectious PPV in nasal and rectal swabs of challenged sows. These results suggest that VLPs-based PPV vaccine is a promising PPV vaccine candidate.

Porcine Parvovirus 7: Evolutionary Dynamics and Identification of Epitopes toward Vaccine Design

Porcine parvovirus 7 (PPV7) belonging to the genus Chapparvovirus in the family Parvoviridae, has been identified in the USA, Sweden, Poland, China, South Korea and Brazil. Our objective was to determine the phylogeny, estimate the time of origin and evolutionary dynamics of PPV7, and use computer-based immune-informatics to assess potential epitopes of its Cap, the main antigenic viral protein, for vaccines or serology. Regarding evolutionary dynamics, PPV7 had 2 major clades, both of which possibly had a common ancestor in 2004. Furthermore, PPV7 strains from China were the most likely ancestral strains. The nucleotide substitution rates of NS1 and Cap genes were 8.01 × 10⁻⁴ and 2.19 × 10⁻³ per site per year, respectively, which were higher than those reported for PPV1-4. The antigenic profiles of PPV7 Cap were revealed and there were indications that PPV7 used antigenic shift to escape from the host’s immune surveillance. Linear B cell epitopes and CD8 T cell epitopes of Cap with good antigenic potential were identified in silico; these conserved B cell epitopes may be candidates for the PPV7 vaccine or for the development of serological diagnostic methods.

Ferulic acid isolated from propolis inhibits porcine parvovirus replication potentially through Bid-mediate apoptosis

Propolis from honeybee hives, which is a traditional Chinese medicine, is widely used in veterinary clinics. Many compounds have been identified and isolated from propolis. Ferulic acid (FA), one of the propolis components, previous studies have proven that it has antiviral effects. To study the mechanism of FA antiviral effects, experiments such as immunofluorescence, quantitative real-time PCR and immunoblotting were introduced. In porcine kidney (PK-15) cells, PPV infection induced the expression of the proapoptotic genes Bid, Bad, Bim and Bak, disrupted mitochondrial membrane potential, promoted mitochondria-mediated, caspase-dependent apoptotic signaling and induced apoptosis. Furthermore, the infected PK-15 cells had increased intracellular reactive oxygen species (ROS) generation. FA treatment, however, reversed these effects and increased cell viability. FA treatment also significantly decreased the PPV-induced expression of Bid, Cyt-c and Apaf-1, suggesting that ROS were involved in the activation of the mitochondria-mediated apoptosis pathway. This in vitro study showed that the antiviral activity of FA was probably associated with inhibiting the replication of PPV by blocking proapoptotic factors such as Bid, Bcl-2 and Mcl-1, and attenuating the mitochondria-mediated response by inhibiting the activation of the Bid-related signaling pathway. Pharmacological inhibitors inhibited PPV-induced apoptosis by blocking Bid, and also suppressed the expression of Caspase family proteins in ppv-induced apoptosis. Taken together, our results suggested that PPV induced PK-15 cell apoptosis via activation of Bid and Bid-related signaling pathways and that the mitochondria act as the mediators of these pathways. FA effectively and extensively attenuated this PPV action, and thus is a potential antiviral agent against PPV.

Ferulic Acid Protects against Porcine Parvovirus Infection-Induced Apoptosis by Suppressing the Nuclear Factor-κB Inflammasome Axis and Toll-Like Receptor 4 via Nonstructural Protein 1

Background

Porcine parvovirus (PPV) infection-induced apoptosis was recently identified as an important pathological factor in PPV-induced placental tissue damage, resulting in reproduction failure. In the present study, we demonstrate the possible involvement of toll-like receptor (TLR) 4 and nuclear factor (NF)-κB inflammasome activation in PPV infection-induced apoptosis and the protective potential of ferulic acid (FA). PPV infection significantly activated the expression levels of TLR4, NF-κB, MyD88, and interleukin (IL)-6. However, FA ameliorated the pathological process, prevented histological alterations, and inhibited the apoptosis rate in porcine kidney (PK-15) cells infected with PPV.

Results

FA inhibited PPV infection-induced inflammasome activation as shown by decreases in the expression of NF-κB, MyD88, and IL-6. FA also downregulated nonstructural (NS) 1 protein expression in infected PK-15 cells.

Conclusions

FA downregulated NS1 and TLR4 signaling, prevented the overproduction of reactive oxygen species, and suppressed the NF-κB inflammasome axis to inhibit PPV-induced apoptosis in PK-15 cells.

Water-soluble N-2-Hydroxypropyl trimethyl ammonium chloride chitosan enhanced the immunogenicity of inactivated porcine parvovirus vaccine vaccination on sows against porcine parvovirus infection

Porcine parvovirus (PPV) is one of the most common and important virus causes of infectious infertility in swine throughout the world. Inactivated PPV vaccine is broadly used, however, there is no appropriate immunomodulatory adjuvant for enhancing present vaccines and developing new ones. Therefore, in this study, the water-soluble N-2-Hydroxypropyl trimethyl ammonium chloride chitosan (N-2-HACC) was synthesized, the adjuvant potential of chitosan derivative was evaluated in inactivated PPV vaccine. Twenty adult healthy sows were assigned to four groups and vaccinated with synthesized PPV/N-2-HACC, commercial inactivated vaccine, N-2-HACC adjuvant and PBS. After insemination, all sows were challenged with the homologous PPV-H strain. In vivo immunization showed that sows immunized with the PPV/N-2-HACC induced more long-lasting HI antibodies and strong immune responses. Importantly, immunization of PPV/N-2-HACC significantly protected sows from homologous PPV-H strain infection. However, immunization of PPV/N-2-HACC didn't change the level of IL-2, IL-4 and IFN-γ and the production of CD4+, CD8 + T lymphocyte. The results indicated that PPV/N-2-HACC protect PPV infection mainly through enhancing the humoral immunity rather than cellular immunity. In addition, the mummified fetuses were observed from the control groups, but neither of the two vaccine groups. In conclusion, these results suggest that N-2-HACC can be exploited as an effective adjuvant for vaccine development, and the PPV/N-2-HACC are potent immunization candidates against PPV infection.

Large-scale manufacture of VP2 VLP vaccine against porcine parvovirus in Escherichia coli with high-density fermentation

Porcine parvovirus (PPV) virus-like particles (VLPs) are a potential vaccine candidate for the prevention of parvovirus-induced reproductive failure in pregnant sows. Currently, the Escherichia coli (E. coli) expression system is the most cost-efficient to express recombinant proteins. To overcome the limitations of protein misfolding and to prepare soluble highly bioactive antigen and high yields of protein, we optimized the PPV-VP2 gene, subcloned it into pET24a, pET26b, pET28a, and pET30a, and transformed it into E. coli BL21(DE3)-Tf16 competent cells. The pET28a plasmid was selected for further manipulations because it expressed high levels of the bioactive PPV-VP2 protein. Under optimal high-density fermenting conditions in a 70-L fermenter, the total yield of wet weight E. coli cells was 124.86 g/L, and PPV-VP2 protein was 2.5 g/L. After large-scale purification with Triton X-114 two-phase extraction as well as activated carbon powder adsorption, hemagglutination (HA) titers in the purified PPV-VP2 protein reached 2¹⁹ and endotoxin was reduced to 2500 EU/mL. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) results indicated that the purified PPV-VP2 protein self-assembled into VLPs. Immunogenicity assays in guinea pigs and pigs indicated that the ISA-201 VG adjuvanted PPV-VP2 VLP vaccine elicited hemagglutination inhibition (HI) and virus neutralization (VN) antibody titers comparable with PPV commercial inactivated vaccines, whereas viral loads in the spleen and liver of challenged guinea pigs were significantly reduced. In conclusion, our study provides a method for producing the PPV-VLP vaccine against PPV infection in E. coli and may offer a novel strategy for the soluble expression of other vaccine antigens.

Two novel protein chips for the detection of antibodies against porcine parvovirus

Background

PPV is one of the most important pathogens causing porcine reproductive disorder. It has been shown in clinical cases to be a commonly mixed infection with other important swine diseases which can aggravate the severity of the disease and bring serious economic losses to the pig industry. Serological methods, such as hemagglutination inhibition assays (HAI), serum neutralization (SN), and the modified direct complement-fixation (MDCF) test were utilized earlier, whereas the enzyme-linked immunosorbent assay (ELISA) is the most frequently applied assay to detect PPV-specific antibodies.

Results

We establish the visible protein chip and the cyanine dye 3 (Cy3)-labeled protein chip to detect the clinical serum from pigs. In this study, the recombinant protein VP2 of PPV was expressed in E.coli, purified with nickel magnetic beads, and then printed onto epoxy-coated glass slides for preparation of the protein chip. After a series of experiments, the conditions of antigen protein concentration, incubation time of primary antibody or secondary antibody, and optimal serum dilution fold were optimized, resulting in a successful visible protein chip and Cy3-labeled protein chip. The results showed that the positive serum, diluted up to 6000-fold, can be detected by the visible protein chip, and the positive serum, diluted up to 12,800-fold, can be detected by the Cy3-labeled protein chip, suggesting the high sensitivity of these protein chips. Moreover, the positive detection ratio, sensitivity, and specificity of these two kinds of protein chips were higher than those of commercial ELISA antibody detection kits.

Conclusion

Overall, these two protein chips can be used to rapidly diagnose clinical samples with high throughput.

Characterization of a recombinant pseudorabies virus expressing porcine parvovirus VP2 protein and porcine IL-6

Background

Porcine parvovirus (PPV) and pseudorabies virus (PRV) are the important etiological agents of swine infectious diseases, resulting in huge economic losses to the Chinese swine industry. Interleukin-6 (IL-6) has the roles to support host immune response to infections as a pleiotropic cytokine. It is essential to construct a live attenuated vaccine-based recombinant PRV that expresses PPV VP2 protein and porcine IL-6 for prevention and control of PRV and PPV.

Methods

The recombinant plasmid, pGVP2-IL6, was constructed by porcine IL-6 gene substituting for EGFP gene of the PRV transfer plasmid pGVP2-EGFP containing VP2 gene of PPV. Plasmid pGVP2-IL6 was transfected into swine testicle cells pre-infected with the virus rPRV-VP2-EGFP strain through homologous recombination and plaque purification to generate a recombinant virus rPRV-VP2-IL6. The recombinant PRV was further identified by PCR and DNA sequencing, and the expression of the VP2 protein and porcine IL-6 was analyzed by reverse transcription-PCR (RT-PCR) and Western blot. The virus titer was calculated according to Reed and Muench method. The immunogenicity of the recombinant virus was preliminarily evaluated in mice by intramuscular administration twice with the rPRV-VP2-IL6 at 4-week intervals.

Results

A recombinant virus rPRV-VP2-IL6 was successfully constructed and confirmed in this study. The properties of rPRV-VP2-IL6 were similar to the parental virus HB98 in terms of growth curve, morphogenesis and virus plaque sizes, and rPRV-VP2-IL6 was proliferated in different cell types. It induced specific antibodies against PPV as well as a strong increase of PPV-specific lymphocyte proliferation responses in mice immunized with rPRV-VP2-IL6, and provided partial protection against the virulent PPV challenge. rPRV-VP2-IL6 also induced a high level of neutralizing antibodies against PRV, and significantly reduced the mortality rate of (1 of 10) following virulent PRV challenge compared with the control (10 of 10).

Conclusions

The recombinant rPRV-VP2-IL6 might be a potential candidate vaccine against PRV and PPV infections in pigs.

Nanobody‑horseradish peroxidase and -EGFP fusions as reagents to detect porcine parvovirus in the immunoassays

Background

Antibodies are an important reagent to determine the specificity and accuracy of diagnostic immunoassays for various diseases. However, traditional antibodies have several shortcomings due to their limited abundance, difficulty in permanent storage, and required use of a secondary antibody. Nanobodies, which are derived from single-chain camelid antibodies, can circumvent many of these limitations and, thus, appear to be a promising substitute. In the presented study, a sandwich ELISA-like immunoassay and direct fluorescent assay with high sensitivity, good specificity, and easy operation were the first time to develop for detecting porcine parvovirus (PPV). After screening PPV viral particles 2 (VP2) specific nanobodies, horseradish peroxidase (HRP) and enhanced green fluorescent protein (EGFP) fusions were derived from the nanobodies by recombinant technology. Finally, using the nanobody-HRP and -EGFP fusions as probes, the developed immunoassays demonstrate specific, sensitive, and rapid detection of PPV.

Results

In the study, five PPV-VP2 specific nanobodies screened from an immunised Bactrian camel were successfully expressed with the bacterial system and purified with a Ni–NTA column. Based on the reporter-nanobody platform, HRP and EGFP fusions were separately produced by transfection of HEK293T cells. A sandwich ELISA-like assay for detecting PPV in the samples was firstly developed using PPV-VP2-Nb19 as the capture antibody and PPV-VP2-Nb56-HRP fusions as the detection antibody. The assay showed 92.1% agreement with real-time PCR and can be universally used to surveil PPV infection in the pig flock. In addition, a direct fluorescent assay using PPV-VP2-Nb12-EGFP fusion as a probe was developed to detect PPV in ST cells. The assay showed 81.5% agreement with real-time PCR and can be used in laboratory tests.

Conclusions

For the first time, five PPV-VP2 specific nanobody-HRP and -EGFP fusions were produced as reagents for developing immunoassays. A sandwich ELISA-like immunoassay using PPV-VP2-Nb19 as the capture antibody and PPV-VP2-Nb56-HRP fusion as the detection antibody was the first time to develop for detecting PPV in different samples. Results showed that the immunoassay can be universally used to surveil PPV infection in pig flock. A direct fluorescent assay using PPV-VP2-Nb12-EGFP as a probe was also developed to detect PPV in ST cells. The two developed immunoassays eliminate the use of commercial secondary antibodies and shorten detection time. Meanwhile, both assays display great developmental prospect for further commercial production and application.

A novel subunit vaccine based on the viral protein 2 of porcine parvovirus: safety profile in bred pigs at different stages of the reproduction cycle and in offspring

Porcine parvovirus 1 (PPV1) viral protein (VP) 2 is the primary antigen responsible for inducing specific protective immunity, so it is a desirable target for development of recombinant subunit vaccines to prevent PPV1 disease. The objective of this study was to evaluate repeated doses of a novel VP2-based PPV1 subunit vaccine, namely ReproCyc® ParvoFLEX, for safety in bred pigs and in offspring under experimental settings. Therefore, the investigation of safety at all breeding stages was evaluated in four independent studies involving: pre-breeding gilts (study A), breeding-age gilts and boars (study B), early and late gestating sows and offspring (study C) and lactating sows and offspring (study D). In all four studies, animals were free from PPV1 based on serology and PCR prior to inclusion. All studies comprised one or two vaccinated groups that received the PPV1 subunit vaccine and a negative control group. Thus, safety was established due to the lack of significant differences between the vaccinated groups and the corresponding unvaccinated (negative control) groups. Gilts, sows and boars were evaluated for local and systemic reactions after vaccination as well as for reproductive performance. The survival rate and average daily weight gain (ADWG) from birth to weaning in offspring was evaluated in studies C and D. Additionally, serology was determined in studies A, C and D. The vaccine was shown to be safe with no relevant significant differences between vaccinated and unvaccinated groups in any experiment. Therefore, repeated doses of ReproCyc® ParvoFLEX were safe in target animals at different stages of the reproductive cycle and in offspring, placing this vaccine as a suitable candidate for mass vaccination programs in breeding herds.

A novel porcine parvovirus DNA-launched infectious clone carrying stable double labels as an effective genetic platform

Porcine parvovirus (PPV) is one of the major pathogens causing reproductive failure of swine. However, its specific pathogenesis has not been fully elucidated. Infectious clone is a powerful tool for further studying the pathogenic mechanism of PPV. In the present study, a PPV infectious clone was constructed, and the clone carries His-tag and Flag-tag double-genetic marker at the end of the ns1 gene 3' terminal and vp1 gene 5' terminal, respectively. The PPV DNA fragment F1 (1-182) in 5' end and the other PPV DNA fragment F2 (4788-5074) in 3' end were synthesized and assembled to the lower copy plasmid to construct pKQLL(F1 + F2), while the PPV DNA genome as a template to amplify carrying tags sequence PPV middle DNA fragment F3 and F4 by introducing Flag and His tags sequence in primers. Subsequently, the fused fragment F3/F4 were cloned into the Stu I/Sna B I sites of pKQLL(F1 + F2) plasmid to assemble the complete full-length PPV DNA recombinant plasmids, named as pD-PPV. The pD-PPV was transfected into PK-15 cells to gain rescued PPV virus, designed as D-PPV. Moreover, D-PPV showed similar replicate capability and pathogenicity comparing to the wild-type parental PPV through in vitro and in vivo studies, and the double labels can effectively indicate the expression and localization of viral proteins. Finally, the rescued D-PPV was found to be a convenient tool for antiviral drug screening. These data indicated that the newly established reverse genetic system for PPV would be a useful tool for further studying the pathogenesis mechanisms of PPV, developing labeled vaccine and screening antiviral drug.

Protective humoral immunity in guinea pigs induced by PCV2 virus-like particles displaying the B cell linear epitope (228QQITDA233) of PPV1

Although PCV2 infections generally cause mild disease in pigs, concurrent co-infections with other pathogens can damage the immune system and cause more severe diseases, collectively termed porcine circovirus associated diseases (PCVAD). Involvement of porcine parvovirus (PPV, a common cause of reproductive failure in naïve dams) in PCVAD caused by PCV2, has been reported. As this co-infection can be difficult to eliminate, there is a critical need to develop an effective vaccine to protect against PPV or synergistic effects of PCV2 and PPV under field conditions. In this study, we designed chimeric PCV2 virus-like particles (cVLPs) displaying a B-cell epitope derived from PPV1 structural protein around the surface of the 2-fold axes of PCV2 VLPs, based on 3D-structure analysis of the PCV2 capsid. The cVLPs were successfully prepared, verified by transmission electron microscopy and chromatography, with robust antibody titers against PCV2 and PPV1 produced in mice and guinea pigs. In addition, in guinea pigs challenged with 10⁶ TCID₅₀ PCV2, cVLPs conferred more effective immune protection (based on viral load) than a commercial PCV2 vaccine. Finally, antibody responses and immune protection against PPV were also evaluated. In guinea pigs vaccinated with cVLPs, although PPV antibodies detected by a hemagglutination inhibition (HI) assay appeared later after vaccination in the PCV2 cVLPs group than in the commercial PPV vaccine group, there were fewer PPV genomic DNA copies in the PCV2 cVLPs group than in a PBS group. In conclusion, guinea pigs vaccinated with cVLPs developed effective protective immunity against PCV2 challenge, with some protective immunity against PPV. This study provided valuable research data to pursue molecular design of chimeric epitopes PCV2 VLPs.

Porcine Parvovirus Infection Causes Pig Placenta Tissue Damage Involving Nonstructural Protein 1 (NS1)-Induced Intrinsic ROS/Mitochondria-Mediated Apoptosis

Porcine parvovirus (PPV) is an important pathogen causing reproductive failure in pigs. PPV-induced cell apoptosis has been recently identified as being involved in PPV-induced placental tissue damages resulting in reproductive failure. However, the molecular mechanism was not fully elucidated. Here we demonstrate that PPV nonstructural protein 1 (NS1) can induce host cell apoptosis and death, thereby indicating the NS1 may play a crucial role in PPV-induced placental tissue damages and reproductive failure. We have found that NS1-induced apoptosis was significantly inhibited by caspase 9 inhibitor, but not caspase 8 inhibitor, and transfection of NS1 gene into PK-15 cells significantly inhibited mitochondria-associated antiapoptotic molecules Bcl-2 and Mcl-1 expressions and enhanced proapoptotic molecules Bax, P21, and P53 expressions, suggesting that NS1-induced apoptosis is mainly through the mitochondria-mediated intrinsic apoptosis pathway. We also found that both PPV infection and NS1 vector transfection could cause host DNA damage resulting in cell cycle arrest at the G1 and G2 phases, trigger mitochondrial ROS accumulation resulting in mitochondria damage, and therefore, induce the host cell apoptosis. This study provides a molecular basis for elucidating PPV-induced cell apoptosis and reproductive failure.

Trigger factor assisted self-assembly of canine parvovirus VP2 protein into virus-like particles in Escherichia coli with high immunogenicity

Canine parvovirus (CPV) has been considered to be an important pathogen, which can cause acute infectious disease in canids. Although current vaccines are effective in preventing CPV infection, safety problems still remain unsolved. In this study, a subunit vaccine against CPV based on virus-like particles (VLPs) with good safety and immunogenicity is reported. Soluble CPV VP2 protein was produced by co-expression of chaperone trigger factor (Tf16) in Escherichia coli (E.coli), and assembled into CPV VLPs which could be affected by NaCl and pH. At 250 mM NaCl pH 8.0, the VLPs co-expressed with Tf16 had similar size (25 nm) and shape with the authentic virus capsid under the transmission electron microscopy (TEM), which is also in accordance with the dynamic light scattering (DLS) data. Immunization with these particles could induce high-titer hemagglutination inhibition (1:12288) and neutralizing antibodies (1:6144) in guinea pigs. Splenic cells of them could secrete IFN-γ and IL-4 after stimulation by CPV. Thus, the VLPs produced by the new approach with high yield and immunogenicity could be a potential candidate for CPV vaccine.

Glutaraldehyde inactivation of enveloped DNA viruses in the preparation of haemoglobin-based oxygen carriers

Glutaraldehyde (GA), used medically as a disinfectant and as a crosslinker for haemoglobin (Hb)-based oxygen carriers (HBOCs), was investigated for its ability to inactivate viruses during the preparation of these artificial blood substitutes. Porcine parvovirus (PPV; a non-enveloped DNA virus) and porcine pseudorabies virus (PRV; an enveloped DNA virus) were used as the virus indicators. Upon treatment with 0.1 mM GA, the titer of PRV decreased from 9.62 log₁₀ to 2.62 log₁₀ within 0.5 h, whereas that of PPV decreased from 7.00 log₁₀ to 2.30 log₁₀ in 5 h. Following treatment with 1.0 mM GA, the titer of PRV decreased from 11.00 log₁₀ to 1.97 log₁₀ within 0.5 h, whereas that of PPV decreased from 7.50 log₁₀ to 3.43 log₁₀ in 4.5 h. During the polymerization of Hb with GA, the GA concentration decreased to 1.0 and 0.1 mM within 30 and 50 min, respectively, at a GA:Hb molar ratio of 10:1, whereas at a GA:Hb molar ratio of 30:1, GA decreased to those same concentrations in 1.5 and 2.5 h, respectively. This rapid decrease in GA concentration during its polymerization with Hb indicates that GA must be added into the Hb solution in a short time in order to get as high a initial concentration as possible. In this study, the GA can only inactivate PRV effectively, given that a longer time (4.5 h) was required for it to inactivate the PPV titer. This study therefore demonstrates that GA inactivates the enveloped DNA virus only during the preparation of HBOCs.