A Semliki Forest virus replicon vectored DNA vaccine expressing the E2 glycoprotein of classical swine fever virus protects pigs from lethal challenge

Development of a Ferritin-Based Nanoparticle Vaccine against Classical Swine Fever

The occurrence of classical swine fever (CSF) poses a significant threat to the global swine industry. Developing an effective and safe vaccine is crucial for preventing and controlling CSF. Here, we constructed self-assembled ferritin nanoparticles fused with the classical swine fever virus (CSFV) E2 protein and a derived B cell epitope (Fe-E2B) using a baculovirus expression system (BVES), demonstrating enhanced immunogenicity. Furthermore, we provide a detailed evaluation of the immunological efficacy of the FeE2B in rabbits. The results showed that robust and sustained antibody responses were detected in rabbits immunized with the Fe-E2B nanoparticle vaccine, comparable to those elicited by commercially available vaccines. Additionally, we demonstrated that the vaccine effectively activated crucial immune factors IFN-γ and IL-4 in vivo, increasing their levels by 1.41-fold and 1.39-fold, respectively. Immunization with Fe-E2B enabled rabbits to avoid viremia and stereotypic fever after CSFV challenge. In conclusion, this study highlights the potential of ferritin nanoparticles as antigen-presenting carriers to induce robust immune responses, proposing a candidate vaccine strategy for the prevention and control of CSF.

Efficiency Comparison of a Novel E2 Subunit Vaccine and a Classic C-Strain Vaccine against Classical Swine Fever

Classical swine fever (CSF) is one of the most important viral diseases in swine, causing severe economic losses in the swine industry. In China, CSF is one of the key diseases that needs to be controlled; the government has implemented control measures, and vaccination with C-strain vaccines (C-vacs) has been compulsory since the 1950s. C-vacs do not allow the differentiation of field virus-infected and vaccinated animals (DIVA). In 2012, China proposed a goal of eradicating CSF. Additionally, a baculovirus-expressed E2 subunit vaccine (E2-vac) was licensed in 2018. However, the C-vac and E2-vac characteristics have not been compared. Here, we demonstrate that both the C-vac and E2-vac provide complete protection against CSF in pigs. The E2-vac allows DIVA, and the E2 antibody responses of stimulated pigs are developed earlier and are stronger than the C-vac antibody responses. Therefore, the E2-vac is a new candidate licensed vaccine to completely eradicate CSF on pig farms.

Path to Success and Future Impact of Nucleic Acid Vaccines: DNA and mRNA

The rapid development of mRNA vaccines for COVID-19 has both astonished the world and raised concerns about their safety, perhaps because many people do not realize the decades’ long efforts for nucleic acid vaccines, both mRNA and DNA vaccines, including the licensure of several veterinary DNA vaccines. This manuscript traces the milestones for nucleic acid vaccine research and development (R&D), with a focus on the immune and safety issues they both raised and answered. The characteristics of the two entities are compared, demonstrating the similarities and differences between them, the advantages and disadvantages, which might lead toward using one or the other technology for different indications. In addition, as the SARS-CoV-2 pandemic has once again highlighted the importance of One Health, that is, the interactions between animal and human pathogens, focus will also be given to how DNA vaccine utilization and studies both in large domestic animals and in wildlife pave the way for more integrated approaches for vaccines to respond quickly to, and prevent, the global impacts of emerging diseases.

The Key Role of Nucleic Acid Vaccines for One Health

The ongoing SARS-CoV-2 pandemic has highlighted both the importance of One Health, i.e., the interactions and transmission of pathogens between animals and humans, and the potential power of gene-based vaccines, specifically nucleic acid vaccines. This review will highlight key aspects of the development of plasmid DNA Nucleic Acid (NA) vaccines, which have been licensed for several veterinary uses, and tested for a number of human diseases, and will explain how an understanding of their immunological and real-world attributes are important for their efficacy, and how they helped pave the way for mRNA vaccines. The review highlights how combining efforts for vaccine development for both animals and humans is crucial for advancing new technologies and for combatting emerging diseases.

The E2 glycoprotein is necessary but not sufficient for the adaptation of classical swine fever virus lapinized vaccine C-strain to the rabbit

Classical swine fever virus (CSFV) C-strain was developed through hundreds of passages of a highly virulent CSFV in rabbits. To investigate the molecular basis for the adaptation of C-strain to the rabbit (ACR), a panel of chimeric viruses with the exchange of glycoproteins E^rns, E1, and/or E2 between C-strain and the highly virulent Shimen strain and a number of mutant viruses with different amino acid substitutions in E2 protein were generated and evaluated in rabbits. Our results demonstrate that Shimen-based chimeras expressing E^rns-E1-E2, E^rns-E2 or E1-E2 but not E^rns-E1, E^rns, E1, or E2 of C-strain can replicate in rabbits, indicating that E2 in combination with either E^rns or E1 confers the ACR. Notably, E2 and the amino acids P108 and T109 in Domain I of E2 are critical in ACR. Collectively, our data indicate that E2 is crucial in mediating the ACR, which requires synergistic contribution of E^rns or E1.

Complex Virus-Host Interactions Involved in the Regulation of Classical Swine Fever Virus Replication: A Minireview

Classical swine fever (CSF), caused by classical swine fever virus (CSFV), is one of the most devastating epizootic diseases of pigs in many countries. Viruses are small intracellular parasites and thus rely on the cellular factors for replication. Fundamental aspects of CSFV-host interactions have been well described, such as factors contributing to viral attachment, modulation of genomic replication and translation, antagonism of innate immunity, and inhibition of cell apoptosis. However, those host factors that participate in the viral entry, assembly, and release largely remain to be elucidated. In this review, we summarize recent progress in the virus-host interactions involved in the life cycle of CSFV and analyze the potential mechanisms of viral entry, assembly, and release. We conclude with future perspectives and highlight areas that require further understanding.

Classical swine fever vaccines-State-of-the-art

Due to its impact on animal health and pig industry, classical swine fever (CSF) is still one of the most important viral diseases of pigs. To control the disease, safe and highly efficacious live attenuated vaccines exist for decades. These vaccines have usually outstanding efficacy and safety but lack differentiability of infected from vaccinated animals (DIVA or marker strategy). In contrast, the first generation of E2 subunit marker vaccines shows constraints in efficacy, application, and production. To overcome these limitations, new generations of marker vaccines are developed. A wide range of approaches have been tried including recombinant vaccines, recombinant inactivated vaccines or subunit vaccines, vector vaccines, and DNA/RNA vaccines. During the last years, especially attenuated deletion vaccines or chimeric constructs have shown potential. At present, especially two new constructs have been intensively tested, the adenovirus-delivered, Semliki Forest virus replicon-vectored marker vaccine candidate "rAdV-SFV-E2" and the pestivirus chimera "CP7_E2alf". The later was recently licensed by the European Medicines Agency. Under field conditions, all marker vaccines have to be accompanied by a potent test system. Particularly this point shows still weaknesses and it is important to embed vaccination in a well-established vaccination strategy and a suitable diagnostic workflow. In summary, conventional vaccines are a standard in terms of efficacy. However, only vaccines with DIVA will allow improved eradication strategies e.g. also under emergency vaccination conditions in free regions. To answer this demand, new generations of marker vaccines have been developed and add now to the tool box of CSF control.

Veterinary Replicon Vaccines

Vaccination is essential in livestock farming and in companion animal ownership. Nucleic acid vaccines based on DNA or RNA provide an elegant alternative to those classical veterinary vaccines that have performed suboptimally. Recent advances in terms of rational design, safety, and efficacy have strengthened the position of nucleic acid vaccines in veterinary vaccinology. The present review focuses on replicon vaccines designed for veterinary use. Replicon vaccines are self-amplifying viral RNA sequences that, in addition to the sequence encoding the antigen of interest, contain all elements necessary for RNA replication. Vaccination results in high levels of in situ antigen expression and induction of potent immune responses. Both positive- and negative-stranded viruses have been used to construct replicons, and they can be delivered as RNA, DNA, or viral replicon particles. An introduction to the biology and the construction of different viral replicon vectors is given, and examples of veterinary replicon vaccine applications are discussed.

Co-expression of the C-terminal domain of Yersinia enterocolitica invasin enhances the efficacy of classical swine-fever-vectored vaccine based on human adenovirus

The use of adenovirus vector-based vaccines is a promising approach for generating antigen-specific immune responses. Improving vaccine potency is necessary in other approaches to address their inadequate protection for the majority of infectious diseases. This study is the first to reconstruct a recombinant replication-defective human adenovirus co-expressing E2 and invasin C-terminal (InvC) glycoproteins (rAd-E2-InvC). rAd-E2-InvC with 2 x 10(6) TCID50 was intramuscularly administered two times to CSFV-free pigs at 14 day intervals. No adverse clinical reactions were observed in any of the pigs after the vaccination. The CSFV E2-specific antibody titer was significantly higher in the rAd-E2-InvC group than that in the rAdV-E2 group as measured by NPLA and blocking ELISA. Pigs immunized with rAd-E2-InvC were completely protected against lethal challenge. Neither CSFV RNA nor pathological changes were detected in the tissues after CSFV challenge. These results demonstrate that rAd-E2-InvC could be an alternative to the existing CSF vaccine. Moreover, InvC that acts as an adjuvant could enhance the immunogenicity of rAdV-E2 and induce high CSFV E2-specific antibody titer and protection level.

Generation and Efficacy Evaluation of a Recombinant Pseudorabies Virus Variant Expressing the E2 Protein of Classical Swine Fever Virus in Pigs

Classical swine fever (CSF) is an economically important infectious disease of pigs caused by classical swine fever virus (CSFV). Pseudorabies (PR), which is caused by pseudorabies virus (PRV), is another important infectious disease of pigs and other animals. Coinfections of pigs with PRV and CSFV occur occasionally in the field. The modified live vaccine Bartha-K61 strain has played an important role in the control of PR in many countries, including China. Since late 2011, however, increasing PR outbreaks caused by an emerging PRV variant have been reported in Bartha-K61-vaccinated swine populations on many farms in China. Previously, we generated a gE/gI-deleted PRV (rPRVTJ-delgE) based on this PRV variant, which was shown to be safe and can provide rapid and complete protection against lethal challenge with the PRV variant in pigs. Here, we generated a new recombinant PRV variant expressing the E2 gene of CSFV (rPRVTJ-delgE/gI-E2) and evaluated its immunogenicity and efficacy in pigs. The results showed that rPRVTJ-delgE/gI-E2 was safe for pigs, induced detectable anti-PRV and anti-CSFV neutralizing antibodies, and provided complete protection against the lethal challenge with either the PRV TJ strain or the CSFV Shimen strain. The data indicate that rPRVTJ-delgE/gI-E2 is a promising candidate bivalent vaccine against PRV and CSFV coinfections.

Thioredoxin 2 Is a Novel E2-Interacting Protein That Inhibits the Replication of Classical Swine Fever Virus

The E2 protein of classical swine fever virus (CSFV) is an envelope glycoprotein that is involved in virus attachment and entry. To date, the E2-interacting cellular proteins and their involvement in viral replication have been poorly documented. In this study, thioredoxin 2 (Trx2) was identified to be a novel E2-interacting partner using yeast two-hybrid screening from a porcine macrophage cDNA library. Trx2 is a mitochondrion-associated protein that participates in diverse cellular events. The Trx2-E2 interaction was further confirmed by glutathione S-transferase (GST) pulldown, in situ proximity ligation, and laser confocal assays. The thioredoxin domain of Trx2 and the asparagine at position 37 (N37) in the E2 protein were shown to be critical for the interaction. Silencing of the Trx2 expression in PK-15 cells by small interfering RNAs significantly promotes CSFV replication, and conversely, overexpression of Trx2 markedly inhibits viral replication of the wild-type (wt) CSFV and to a greater extent that of the CSFV N37D mutant, which is defective in binding Trx2. The wt CSFV but not the CSFV N37D mutant was shown to reduce the Trx2 protein expression in PK-15 cells. Furthermore, we demonstrated that Trx2 increases nuclear factor kappa B (NF-κB) promoter activity by promoting the nuclear translocation of the p65 subunit of NF-κB. Notably, activation of the NF-κB signaling pathway induced by tumor necrosis factor alpha (TNF-α) significantly inhibits CSFV replication in PK-15 cells, whereas blocking the NF-κB activation in Trx2-overexpressing cells no longer suppresses CSFV replication. Taken together, our findings reveal that Trx2 inhibits CSFV replication via the NF-κB signaling pathway.

IMPORTANCE

Thioredoxin 2 (Trx2) is a mitochondrion-associated protein that participates in diverse cellular events, such as antioxidative and antiapoptotic processes and the modulation of transcription factors. However, little is known about the involvement of Trx2 in viral replication. Here, we investigated, for the first time, the role of Trx2 in the replication of classical swine fever virus (CSFV), a devastating pestivirus of pigs. By knockdown and overexpression, we showed that Trx2 negatively regulates CSFV replication. Notably, we demonstrated that Trx2 inhibits CSFV replication by promoting the nuclear translocation of the p65 subunit of NF-κB, a key regulator of the host's innate immunity and inflammatory response. Our findings reveal a novel role of Trx2 in the host's antiviral response and provide new insights into the complex mechanisms by which CSFV interacts with the host cell.

Classical swine fever in China: a minireview

Classical swine fever (CSF), caused by Classical swine fever virus (CSFV), is an OIE-listed, highly contagious, often fatal disease of swine worldwide. Currently, the disease is controlled by prophylactic vaccination in China and many other countries using the modified live vaccines derived from C-strain, which was developed in China in the mid-1950s. This minireview summarizes the epidemiology, diagnostic assays, control and challenges of CSF in China. Though CSF is essentially under control, complete eradication of CSF in China remains a challenging task and needs long-term, joint efforts of stakeholders.

A novel alphavirus replicon-vectored vaccine delivered by adenovirus induces sterile immunity against classical swine fever

Low efficacy of gene-based vaccines due to inefficient gene delivery and expression has been major bottleneck of their applications. Efforts have been made to improve the efficacy, such as gene gun and electroporation, but the strategies are difficult to put into practical use. In this study, we developed and evaluated an adenovirus-delivered, alphavirus replicon-vectored vaccine (chimeric vector-based vaccine) expressing the E2 gene of classical swine fever virus (CSFV) (rAdV-SFV-E2). Rabbits immunized with rAdV-SFV-E2 developed CSFV-specific antibodies as early as 9 days and as long as 189 days and completely protected from challenge with C-strain. Pigs immunized with rAdV-SFV-E2 (n=5) developed robust humoral and cell-mediated responses to CSFV and were completely protected from subsequent lethal CSFV infection clinically and virologically. The level of immunity and protection induced by rAdV-SFV-E2 was comparable to that provided by the currently used live attenuated vaccine, C-strain. In contrast, both the conventional alphavirus replicon-vectored vaccine pSFV1CS-E2 and conventional adenovirus-vectored vaccine rAdV-E2 provided incomplete protection. The chimeric vector-based vaccine represents the first gene-based vaccine that is able to confer sterile immunity and complete protection against CSFV. The new-concept vaccination strategy may also be valuable in vaccine development against other pathogens.

Enhanced immunity against classical swine fever in pigs induced by prime-boost immunization using an alphavirus replicon-vectored DNA vaccine and a recombinant adenovirus

Classical swine fever (CSF) - caused by the classical swine fever virus (CSFV) - is a fatal disease of pigs that is responsible for extensive losses to the swine industry worldwide. We had demonstrated previously that a prime-boost vaccination strategy using an alphavirus (Semliki Forest virus, SFV) replicon-vectored DNA vaccine (pSFV1CS-E2) and a recombinant adenovirus (rAdV-E2) expressing the E2 glycoprotein of CSFV induced enhanced immune responses in a mouse model. In this study, we evaluated further the efficacy of the heterologous prime-boost immunization approach in pigs, the natural host of CSFV. The results showed that the pigs (n=5) receiving pSFV1CS-E2/rAdV-E2 heterologous prime-boost immunization developed significantly higher titers of CSFV-specific neutralizing antibodies and comparable CD4(+) and CD8(+) T-cell proliferation, compared to the pigs receiving double immunizations with rAdV-E2 alone. When challenged with virulent CSFV Shimen strain, the pigs of the heterologous prime-boost group did not show clinical symptoms or viremia, which were observed in one of the 5 pigs immunized with rAdV-E2 alone and all the 5 control pigs immunized with an empty adenovirus. The results demonstrate that the heterologous DNA prime and recombinant adenovirus boost strategy can induce solid protective immunity.

Construction and cellular immune response induction of HA-based alphavirus replicon vaccines against human-avian influenza (H5N1)

Several approaches are being taken worldwide to develop vaccines against H5N1 viruses; most of them, however, pose both practical and immunological challenges. One potential strategy for improving the immunogenicity of vaccines involves the use of alphavirus replicons and VP22, a herpes simplex type 1 (HSV-1) protein. In this study, we analysed the antigenic peptides and homogeneity of the HA sequences (human isolates of the H5N1 subtype, from 1997 to 2003) and explored a novel alphavirus replicon system of VP22 fused with HA, to assess whether the immunogenicity of an HA-based replicon vaccine could be induced and augmented via fusion with VP22. Further, replicon particles expressing VP22, and enhanced green fluorescent protein (EGFP) were individually used as controls. Cellular immune responses in mice immunised with replicons were evaluated by identifying specific intracellular cytokine production with flow cytometry (FCM). Animal-based experimentation indicated that both the IL-4 expression of CD4(+) T cells and the IFN-gamma expression of CD8(+) T cells were significantly increased in mice immunised with VPR-HA and VPR-VP22/HA. A dose titration effect vis-à-vis both IL-4 expression and IFN-gamma expression were observed in VPR-HA- and VPR-VP22/HA-vaccinated mice. Our results revealed that both VPR-VP22/HA and VPR-HA replicon particles presented a promising approach for developing vaccines against human-avian influenza, and VP22 could enhance the immunogenicity of the HA antigens to which it is fused.

A sindbis virus replicon-based DNA vaccine encoding the rabies virus glycoprotein elicits immune responses and complete protection in mice from lethal challenge

A sindbis virus replicon-based DNA vaccine encoding rabies virus glycoprotein (G) was developed by subcloning rabies G gene into a sindbis virus replicon-based vaccine vector (pAlpha). The self-amplification of RNA transcripts and translation efficiency of rabies G was analyzed in pAlpha-Rab-G-transfected mammalian cells using RT-PCR, SDS-PAGE and Western blot analysis. The transfected cells also showed induction of apoptosis which is an important event in the enhancement of immune responses. Further, immune responses induced with replicon-based rabies DNA vaccine (pAlpha-Rab-G) was compared with conventional rabies DNA vaccine and commercial cell culture vaccine (Rabipur) in intramuscularly injected mice. The mice immunized with replicon-based rabies DNA vaccine induced humoral and cell mediated immune responses better than conventional rabies DNA vaccine however, comparable to Rabipur vaccine. On challenge with rabies virus CVS strain, replicon-based rabies DNA vaccine conferred complete protection similar to Rabipur. These results demonstrate that replicon-based rabies DNA vaccine is effective in inducing both humoral and cellular immune responses and can be considered as effective vaccine against rabies.

Assessment of the cell-mediated immunity induced by alphavirus replicon-vectored DNA vaccines against classical swine fever in a mouse model

We have previously shown that an alphavirus replicon-vectored DNA vaccine (pSFV1CS-E2) encoding the E2 glycoprotein of classical swine fever virus (CSFV) completely protected the immunized pigs from lethal challenge. These animals developed only low or moderate level viral-specific antibody titers before challenge, implying that cell-mediated immunity (CMI) probably played an important role in the protective immunity against CSFV conferred by the DNA vaccine. In this study, the CMI induced by pSFV1CS-E2 and its derivative pSFV1CS-E2-UL49 encoding a fusion protein of CSFV E2 and pseudorabies virus (PRV) VP22 was evaluated in a mouse model by lymphoproliferation assays based on CFSE or WST-8, intracellular cytokine staining, and cytokine ELISA. The results showed that both vaccines induced CSFV-specific lymphoproliferative responses and cytokine production, and pSFV1CS-E2-UL49 induced stronger lymphoproliferative responses and higher cytokine levels than pSFV1CS-E2. These findings suggest that the alphavirus replicon-delivered DNA vaccines are capable of inducing CMI, and PRV VP22 is able to enhance the immunogenicity of the co-delivered antigen.

Therapeutic and prophylactic applications of alphavirus vectors

Alphavirus vectors are high-level, transient expression vectors for therapeutic and prophylactic use. These positive-stranded RNA vectors, derived from Semliki Forest virus, Sindbis virus and Venezuelan equine encephalitis virus, multiply and are expressed in the cytoplasm of most vertebrate cells, including human cells. Part of the genome encoding the structural protein genes, which is amplified during a normal infection, is replaced by a transgene. Three types of vector have been developed: virus-like particles, layered DNA-RNA vectors and replication-competent vectors. Virus-like particles contain replicon RNA that is defective since it contains a cloned gene in place of the structural protein genes, and thus are able to undergo only one cycle of expression. They are produced by transfection of vector RNA, and helper RNAs encoding the structural proteins. Layered DNA-RNA vectors express the Semliki Forest virus replicon from a cDNA copy via a cytomegalovirus promoter. Replication-competent vectors contain a transgene in addition to the structural protein genes. Alphavirus vectors are used for three main applications: vaccine construction, therapy of central nervous system disease, and cancer therapy.

DNA vaccines and their applications in veterinary practice: current perspectives

Inoculation of plasmid DNA, encoding an immunogenic protein gene of an infectious agent, stands out as a novel approach for developing new generation vaccines for prevention of infectious diseases of animals. The potential of DNA vaccines to act in presence of maternal antibodies, its stability and cost effectiveness and the non-requirement of cold chain have heightened the prospects. Even though great strides have been made in nucleic acid vaccination, still there are many areas that need further research for its wholesome practical implementation. Major areas of concern are vaccine delivery, designing of suitable vectors and cytotoxic T cell responses. Also, the induction of immune responses by DNA vaccines is inconclusive due to the lack of knowledge regarding the concentration of the protein expressed in vivo. Alternative delivery systems having higher transfection efficiency and the use of cytokines, as immunomodulators, needs to be further explored. Recently, efforts are being made to modulate and prolong the active life of dendritic cells, in order to make antigen presentation a more efficacious one. For combating diseases like acquired immunodeficiency syndrome (AIDS), influenza, malaria and tuberculosis in humans; and foot and mouth disease, Aujesky’s disease, swine fever, rabies, canine distemper and brucellosis in animals, DNA vaccine clinical trials are underway. This review highlights the salient features of DNA vaccines, and measures to enhance their efficacy so as to devise an effective and novel vaccination strategy against animal diseases.

Protection of pigs from lethal challenge by a DNA vaccine based on an alphavirus replicon expressing the E2 glycoprotein of classical swine fever virus

In a previous study, it has been shown that a Semliki Forest virus (SFV) replicon vectored DNA vaccine (pSFV1CS-E2) expressing the E2 glycoprotein of classical swine fever virus (CSFV) conferred full protection for pigs immunized three times with 600 microg of the vaccine. This study was designed to evaluate further the efficacy of the vaccine with lower dosage and fewer inoculations. Pigs were immunized twice with 100 microg of pSFV1CS-E2 (n=5) or control plasmid pSFV1CS (n=3), respectively, and challenged with virulent Shimen strain 6 weeks following the booster immunization. Pigs immunized with pSFV1CS-E2 developed high titers of specific neutralizing antibodies against CSFV after the booster, and the antibody titers increased rapidly upon challenge. The immunized animals showed no clinical symptoms except short-term fever and low-level viremia, whereas, the control pigs immunized with the control plasmid produced no detectable antibody prior to challenge, and showed obvious clinical signs following challenge, and two pigs died of illness. All control animals developed extended viremia as detected by nested RT-PCR and real-time RT-PCR. Severe pathologic lesions typical of CSFV infection were observed at necropsy. It is concluded that the alphavirus replicon-vectored DNA-based vaccine can be a potential marker vaccine against CSFV.