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

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.

Assessment of the Safety Profile of Chimeric Marker Vaccine against Classical Swine Fever: Reversion to Virulence Study

Chimeric marker vaccine candidates, vGPE-/PAPeV Erns and vGPE-/PhoPeV Erns, have been generated and their efficacy and capability to differentiate infected from vaccinated animals were confirmed in previous studies. The safety profile of the two chimeric marker vaccine candidates, particularly in the potential reversion to virulence, was evaluated. Each virus was administered to pigs with a dose equivalent to the vaccination dose, and pooled tonsil homogenates were subsequently inoculated into further pigs. Chimeric virus vGPE-/PAPeV Erns displayed the most substantial attenuation, achieving this within only two passages, whereas vGPE-/PhoPeV Erns was detectable until the third passage and disappeared entirely by the fourth passage. The vGPE- strain, assessed alongside, consistently exhibited stable virus recovery across each passage without any signs of increased virulence in pigs. In vitro assays revealed that the type I interferon-inducing capacity of vGPE-/PAPeV Erns was significantly higher than that of vGPE-/PhoPeV Erns and vGPE-. In conclusion, the safety profile of the two chimeric marker vaccine candidates was affirmed. Further research is essential to ensure the stability of their attenuation and safety in diverse pig populations.

Development of a dual immunochromatographic test strip to detect E2 and Erns antibodies against classical swine fever

Background

It is essential to consider a practical antibody test to successfully implement marker vaccines and validate vaccination efficacy against classical swine fever virus (CSFV). The test should include a serological antibody assay, combined with a tool for differentiating infected from vaccinated animals (DIVA). The immunochromatographic test strip (ICS) has been exclusively designed for detecting CSFV E2 antibodies while lacking in detecting Erns antibodies, which can be employed and satisfy DIVA strategy. This study developed a novel ICS for detecting CSFV E2/Erns dual-antibody. The effectiveness of ICS in evaluating the DIVA capability of two novel chimeric pestivirus vaccine candidates was assessed.

Methods

Recombinant E2 or Erns protein was transiently expressed in the plant benthamiana using Agrobacterium tumefaciens. ICS was subsequently assembled, and goat anti-rabbit IgG and recombinant CSFV E2 or Erns protein were plated onto the nitrocellulose membrane as control and test lines, respectively. The sensitivity and specificity of ICS were evaluated using sera with different neutralizing antibody titers or positive for antibodies against CSFV and other pestiviruses. The coincidence rates for detecting E2 and Erns antibodies between ICS and commercial enzyme-linked immunosorbent assay (ELISA) kits were also computed. ICS performance for DIVA capability was evaluated using sera from pigs vaccinated with conventional vaccine or chimeric vaccine candidates.

Results

E2 and Erns proteins were successfully expressed in N. benthamiana-produced recombinant proteins. ICS demonstrated high sensitivity in identifying CSFV E2 and Erns antibodies, even at the low neutralizing antibody titers. No cross-reactivity with antibodies from other pestiviruses was confirmed using ICS. There were high agreement rates of 93.0 and 96.5% between ICS and two commercial ELISA kits for E2 antibody testing. ICS also achieved strong coincidence rates of 92.9 and 89.3% with two ELISA kits for Erns antibody detection. ICS confirmed the absence of CSFV Erns-specific antibodies in sera from pigs vaccinated with chimeric vaccine candidates.

Conclusion

E2 and Erns proteins derived from the plant showed great potential and can be used to engineer a CSFV E2/Erns dual-antibody ICS. The ICS was also highly sensitive and specific for detecting CSFV E2 and Erns antibodies. Significantly, ICS can fulfill the DIVA concept by incorporating chimeric vaccine candidates.

Performance of a Differentiation of Infected from Vaccinated Animals (DIVA) Classical Swine Fever Virus (CSFV) Serum and Oral Fluid Erns Antibody AlphaLISA Assay

Classical swine fever virus (CSFV) is an OIE-listed disease that requires effective surveillance tools for its detection and control. The aim of this study was to develop and evaluate the diagnostic performance of a novel CSFV Erns IgG AlphaLISA for both serum and oral fluid specimens that would likewise be compatible with the use of CSFV E2 DIVA vaccines. Test performance was evaluated using a panel of well-characterized serum (n = 760) and individual (n = 528) or pen-based (n = 30) oral fluid samples from four groups of animals: (1) negative controls (n = 60 pigs); (2) inoculated with ALD strain wild-type CSFV (n = 30 pigs); (3) vaccinated with LOM strain live CSFV vaccine (n = 30 pigs); and (4) vaccinated with live CSFV marker vaccine on commercial farms (n = 120 pigs). At a cutoff of S/P ≥ 0.7, the aggregate estimated diagnostic sensitivities and specificities of the assay were, respectively, 97.4% (95% CI 95.9%, 98.3%) and 100% for serum and 95.4% (95% CI 92.9%, 97.0%) and 100% for oral fluid. The Erns IgG antibody AlphaLISA combined DIVA capability with solid diagnostic performance, rapid turnaround, ease of use, and compatibility with both serum and oral fluid specimens.

Host cell factors involved in classical swine fever virus entry

Classical swine fever virus (CSFV) is an ancient pathogen that continues to pose a threat to animal agriculture worldwide. The virus belongs to the genus Pestivirus and the family Flaviviridae. It causes a multisystemic disease that affects only pigs and is responsible for significant economic losses. CSFV infection is probably a multistep process that involves the proteins in the virus envelope and more than one receptor in the membrane of permissive cells. To date, the cellular receptors essential for CSFV entry and their detailed functions during this process remains unknown. All the viral envelope proteins Erns, E1 and E2 are involved in the entry process to some extent and the experimental approaches conducted until now have helped to unveil their contributions. This review aims to provide an overview of current knowledge on cellular molecules described to be involved in CSFV entry, including complement regulatory protein 46 (CD46), heparan sulphate (HS), Laminin receptor, Integrin ß3, Annexin II, MERKT and ADAM17. This knowledge would not only help to understand the molecular mechanisms involved in pestivirus infection, but also provide a rational basis for the development of nonvaccinal alternatives for CSFV control.

Generation and Efficacy of Two Chimeric Viruses Derived from GPE- Vaccine Strain as Classical Swine Fever Vaccine Candidates

A previous study proved that vGPE^- mainly maintains the properties of classical swine fever (CSF) virus, which is comparable to the GPE^- vaccine seed and is a potentially valuable backbone for developing a CSF marker vaccine. Chimeric viruses were constructed based on an infectious cDNA clone derived from the live attenuated GPE^- vaccine strain as novel CSF vaccine candidates that potentially meet the concept of differentiating infected from vaccinated animals (DIVA) by substituting the glycoprotein E^rns of the GPE^- vaccine strain with the corresponding region of non-CSF pestiviruses, either pronghorn antelope pestivirus (PAPeV) or Phocoena pestivirus (PhoPeV). High viral growth and genetic stability after serial passages of the chimeric viruses, namely vGPE^-/PAPeV E^rns and vGPE^-/PhoPeV E^rns, were confirmed in vitro. In vivo investigation revealed that two chimeric viruses had comparable immunogenicity and safety profiles to the vGPE^- vaccine strain. Vaccination at a dose of 10^4.0 TCID₅₀ with either vGPE^-/PAPeV E^rns or vGPE^-/PhoPeV E^rns conferred complete protection for pigs against the CSF virus challenge in the early stage of immunization. In conclusion, the characteristics of vGPE^-/PAPeV E^rns and vGPE^-/PhoPeV E^rns affirmed their properties, as the vGPE^- vaccine strain, positioning them as ideal candidates for future development of a CSF marker vaccine.

Construction and efficacy of a new live chimeric C-strain vaccine with DIVA characteristics against classical swine fever

To develop the new classical swine fever (CSF) vaccine candidate with differentiating infected vaccinated animals (DIVA) characteristics, a chimeric CSF virus (CSFV) was constructed based on an infectious cDNA clone of the CSF vaccine C-strain. The 5'- and 3'-untranslated regions (UTRs) and partial E2 region (residues 690-860) of the C-strain were substituted with the corresponding regions of bovine viral diarrhoea virus (BVDV) to construct the chimeric cDNA clone pC/bUTRs-tE2. The chimeric virus rC/bUTRs-tE2 was generated by several passages of pC/bUTRs-tE2-transfected PK15 cells. Stable growth and genetic properties of rC/bUTRs-tE2 were obtained after 30 serial passages. Compared to parental rC/bUTRs-tE2 (1^st passage), two residue mutations (M834K and M979K) located in E2 in rC/bUTRs-tE2 P30 were observed. Compared to the C-strain, rC/bUTRs-tE2 exhibited unchanged cell tropism and decreased plaque-forming ability. Substituting the C-strain UTRs with the BVDV UTRs resulted in significantly increased viral replication in PK15 cells. Compared to CSFV E^rns-positive and BVDV tE2-negative antibody responses induced by the CSF vaccine C-strain, immunization of rabbits and piglets with rC/bUTRs-tE2 resulted in serological profiles of CSFV E^rns- and BVDV tE2-positive antibodies, which are used to serologically discriminate pigs that are clinically infected and vaccinated. Vaccination of piglets with rC/bUTRs-tE2 conferred complete protection against lethal CSFV challenge. Our results suggest that rC/bUTRs-tE2 is a promising new CSF marker vaccine candidate.

Evaluation of an alternative method for determination of Protective Dose50 of Classical swine fever vaccines

Classical Swine Fever (CSF) is still one of the most economically important viral diseases of pigs. The disease is controlled by vaccination in the endemic countries. Hence, availability or supply of efficacious and potent vaccine in the field settings is of utmost importance. Currently, as per requirement of any Pharmacopoea, a CSF vaccine must contain 100 PD₅₀/dose which is determined by vaccinating pigs at 1/40^th and 1/160^th dilution of each dose followed by virulent challenge at 28 days post vaccination (dpv). Here, the control and the unprotected groups succumb to disease and need to be euthanized. Moreover, such challenge experiments are not feasible for each batch of the vaccine. In this communication, an alternate method of PD₅₀ dose calculation of live-attenuated CSF vaccines by measuring Serum Neutralizing Titre i.e Fluorescent Antibody Virus Neutralization (FAVN) titre of the vaccinated pigs at 28 dpv was established. This alternative method do not require the vaccinated pigs to be challenged. Serum samples, generated out of QC testing of eight batches of CSF vaccines in the laboratory, were tested and found that pigs having FAVN titre ≥10 were protected against challenge. Initially this test was optimized in serum samples of 12 animals and then validated with another 56 serum samples. It was found that the alternate method is 100% correlating with the challenge experiment. Thus, based on FAVN titre of the vaccinated animal serum, it can be predicted whether the pigs would or would not come through the challenge infection. Using the predicted status (protected/succumbed), PD50 can be calculated by applying Reed and Muench formula, hence alternate method can be used as routine QC test for potency of CSF vaccines. The newly developed assay was specific since no signal was observed in controls.

A Novel Blocking Enzyme-Linked Immunosorbent Assay Based on a Biotinylated Nanobody for the Rapid and Sensitive Clinical Detection of Classical Swine Fever Virus Antibodies

Monoclonal and polyclonal antibodies are mostly used for the development of traditional enzyme-linked immunosorbent assays (ELISAs), but the use of certain conventional antibodies may be limited by their low yield, the difficulty of their isolation, and their high cost. Heavy-chain antibodies derived from camelids with naturally missing light chains can overcome these deficiencies and are an excellent alternative to conventional antibodies. In this study, a nanobody (Nb)-AviTag fusion protein was constructed, and the feasibility of its use as a high-sensitivity probe in a blocking ELISA (bELISA) for classical swine fever virus (CSFV) was investigated. The CSFV E2 recombinant protein expressed by the CHO expression system exhibited good reactogenicity and immunogenicity and induced the production of high CSFV antibody levels in rabbits. Three different clones of Nbs were successfully isolated using a phage display system in alpaca, and an Nb1-AviTag fusion protein was successfully expressed using an Escherichia coli expression system. The purified Nb1-AviTag fusion protein was then biotinylated in vitro to obtain Nb1-biotin. A novel bELISA was developed for the detection of CSFV antibodies in clinical serum using Nb1-biotin as a probe. The cutoff value of bELISA was 32.18%, the sensitivity of bELISA was higher than that of the bELISA kit with IDEXX antibody, and the coincidence rate was 94.7%. A rapid, low-cost, highly sensitive and highly specific CSFV E2 antibody-based bELISA method was successfully established and can be used for the serological evaluation of CSFV E2 subunit vaccines and the ELISA-based diagnosis of CSFV infection. IMPORTANCE Currently, the epidemic situation of classical swine fever (CSF) is sporadic, and cases of atypical swine fever are on the rise in China. Therefore, it is necessary to accurately eliminate suspected cases by using highly sensitive and specific diagnostic techniques. In our study, a rapid, low-cost, highly sensitivity, highly reliable and reproducible, and highly specific classical swine fever virus (CSFV) E2 antibody-based blocking ELISA method was successfully established by using the phage display system and the Nb1-AviTag fusion expression platform. It provides a new technique for serological evaluation of CSFV vaccines and ELISA-based diagnosis of CSFV infection.

Optimized protocol for double vaccine immunization against classical swine fever and porcine reproductive and respiratory syndrome

BACKGROUND

Classical swine fever and porcine reproductive and respiratory syndrome have seriously affected the development of the swine breeding industry in China. Vaccine immunization remains the main way to prevent these infections. The aim of this study was to establish an optimized protocol for vaccine immunization against classical swine fever virus (CSFV) and porcine reproductive and respiratory syndrome virus (PRRSV).

METHODS

Blood samples were collected from the anterior vena cava of pigs after immunization, and blood indices, secreted levels of specific antibodies and neutralizing antibodies associated with humoral immunity, the proliferation capacity of T lymphocytes as a measure of cellular immunity, and secreted levels of IFN-γ and TNF-α were determined.

RESULTS

The results showed that simultaneous immunization against CSFV and PRRSV infections induced strong and specific humoral and T-cellular immune responses, high levels of cytokine IFN-γ secretion and delayed secretion of cytokine TNF-α. Moreover, significantly higher lymphocyte percentages and red blood cell and leukocyte counts were found in the group simultaneously immunized against CSFV and PRRSV. However, no statistically significant differences were observed in hemoglobin values, neutrophil counts, and median cell percentages among the S + PRRS, PRRS-S, and S-PRRS groups.

CONCLUSION

This study demonstrated that simultaneous immunization against CSFV and PRRSV had the advantages of inducing a rapid, enhanced, and long-lasting immune response. These findings provide a theoretical basis for the establishment of a reasonable and optimized vaccine immunization protocol against CSFV and PRRSV in combination with a variety of other vaccine inoculations.

Different Types of Vaccines against Pestiviral Infections: "Barriers" for "Pestis"

The genus Pestivirus of the family Flaviviridae mainly comprises classical swine fever virus (CSFV), bovine viral diarrhea virus 1 (BVDV-1), BVDV-2, border disease virus (BDV), and multiple new pestivirus species such as atypical porcine pestivirus (APPV), giraffe pestivirus, and antelope pestivirus. Pestiviruses cause infectious diseases, resulting in tremendous economic losses to animal husbandry. Different types of pestivirus vaccines have been developed to control and prevent these important animal diseases. In recent years, pestiviruses have shown great potential as viral vectors for developing multivalent vaccines. This review analyzes the advantages and disadvantages of various pestivirus vaccines, including live attenuated pestivirus strains, genetically engineered marker pestiviruses, and pestivirus-based multivalent vaccines. This review provides new insights into the development of novel vaccines against emerging pestiviruses, such as APPV and ovine pestivirus.

A modular and self-adjuvanted multivalent vaccine platform based on porcine circovirus virus-like nanoparticles

BACKGROUND

Virus-like particles (VLPs) are supramolecular structures composed of multiple protein subunits and resemble natural virus particles in structure and size, making them highly immunogenic materials for the development of next-generation subunit vaccines. The orderly and repetitive display of antigenic epitopes on particle surface allows efficient recognition and cross-link by B cell receptors (BCRs), thereby inducing higher levels of neutralizing antibodies and cellular immune responses than regular subunit vaccines. Here, we present a novel multiple antigen delivery system using SpyCatcher/Spytag strategy and self-assembled VLPs formed by porcine circovirus type 2 (PCV2) Cap, a widely used swine vaccine in solo.

RESULTS

Cap-SC, recombinant Cap with a truncated SpyCatcher polypeptide at its C-terminal, self-assembled into 26-nm VLPs. Based on isopeptide bonds formed between SpyCatcher and SpyTag, classical swine fever virus (CSFV) E2, the antigen of interest, was linked to SpyTag and readily surface-displayed on SpyCatcher decorated Cap-SC via in vitro covalent conjugation. E2-conjugated Cap VLPs (Cap-E2 NPs) could be preferentially captured by antigen presenting cells (APCs) and effectively stimulate APC maturation and cytokine production. In vivo studies confirmed that Cap-E2 NPs elicited an enhanced E2 specific IgG response, which was significantly higher than soluble E2, or the admixture of Cap VLPs and E2. Moreover, E2 displayed on the surface did not mask the immunodominant epitopes of Cap-SC VLPs, and Cap-E2 NPs induced Cap-specific antibody levels and neutralizing antibody levels comparable to native Cap VLPs.

CONCLUSION

These results demonstrate that this modularly assembled Cap-E2 NPs retains the immune potential of Cap VLP backbone, while the surface-displayed antigen significantly elevated E2-induced immune potency. This immune strategy provides distinctly improved efficacy than conventional vaccine combination. It can be further applied to the development of dual or multiple nanoparticle vaccines to prevent co-infection of PCV2 and other swine pathogens.

African Swine Fever Modified Live Vaccine Candidates: Transitioning from Discovery to Product Development through Harmonized Standards and Guidelines

The recent centennial anniversary of R.E. Montgomery's seminal published description of "a form of swine fever" disease transmitted from wild African pigs to European domestic pigs is a call to action to accelerate African Swine Fever (ASF) vaccine research and development. ASF modified live virus (MLV) first-generation gene deleted vaccine candidates currently offer the most promise to meet international and national guidelines and regulatory requirements for veterinary product licensure and market authorization. A major, rate-limiting impediment to the acceleration of current as well as future vaccine candidates into regulatory development is the absence of internationally harmonized standards for assessing vaccine purity, potency, safety, and efficacy. This review summarizes the asymmetrical landscape of peer-reviewed published literature on ASF MLV vaccine approaches and lead candidates, primarily studied to date in the research laboratory in proof-of-concept or early feasibility clinical safety and efficacy studies. Initial recommendations are offered toward eventual consensus of international harmonized guidelines and standards for ASF MLV vaccine purity, potency, safety, and efficacy. To help ensure the successful regulatory development and approval of ASF MLV first generation vaccines by national regulatory associated government agencies, the World Organisation for Animal Health (WOAH) establishment and publication of harmonized international guidelines is paramount.

Complete genome sequences of classical swine fever virus: Phylogenetic and evolutionary analyses

The classical swine fever virus (CSFV) outbreaks cause colossal losses of pigs and drastic economic impacts. The current phylogenetic CSFV groups were determined mainly based on the partial genome. Herein, 203 complete genomic sequences of CSFVs collected worldwide between 1998 and 2018 available on the GenBank database were retrieved for re-genotyping and recombination analysis. The maximum likelihood phylogenetic tree determined two main groups, GI and GII, with multiple sub-genotypes. The “strain 39” (GenBank ID: AF407339), previously identified as belonging to sub-genotypes 1.1 or 2.2 based on the partial sequences, is found to be genetically distinct and independent, forming a new lineage depicted as GI-2.2b. Ten potential natural recombination events were identified, seven of which were collected in China and found involved in the genetic diversity of CSFVs. Importantly, the vaccine strains and highly virulent strains were all involved in the recombination events, which would induce extra challenges to vaccine development. These findings alarm that attenuated vaccines should be applied with discretion and recommend using subunit vaccines in parallel with other preventive strategies for better management of CSFVs.

The FlagT4G Vaccine Confers a Strong and Regulated Immunity and Early Virological Protection against Classical Swine Fever

Control of classical swine fever virus (CSFV) in endemic countries relies on vaccination, mostly using vaccines that do not allow for differentiation of vaccinated from infected animals (DIVA). FlagT4G vaccine is a novel candidate that confers robust immunity and shows DIVA capabilities. The present study assessed the immune response elicited by FlagT4G and its capacity to protect pigs for a short time after vaccination. Five days after a single dose of FlagT4G vaccine, animals were challenged with a highly virulent CSFV strain. A strong, but regulated, interferon-α response was found after vaccination. Vaccinated animals showed clinical and virological protection against the challenge, in the absence of antibody response at 5 days post-vaccination. Upon challenge, a rapid rise in the titers of CSFV neutralizing antibodies and an increase in the IFN-γ producing cells were noticed in all vaccinated-challenged pigs. Meanwhile, unvaccinated pigs showed severe clinical signs and high viral replication, being euthanized before the end of the trial. These animals were unable to generate neutralizing antibodies and IFN-γ responses after the CSFV challenge. The results from the present study assert the fast and efficient protection by FlagT4G, a highly promising tool for CSFV control worldwide.

Taking a Promising Vaccine Candidate Further: Efficacy of ASFV-G-ΔMGF after Intramuscular Vaccination of Domestic Pigs and Oral Vaccination of Wild Boar

African swine fever (ASF) is a pandemic threat to the global pig industry and wild suids. A safe and efficacious vaccine could monumentally assist in disease eradication. In the past years, promising live attenuated vaccine candidates emerged in proof-of-concept experiments, among which was “ASFV-G-∆MGF”. In our study, we tested the vaccine candidate in three animal experiments intramuscularly in domestic pigs and orally in wild boar. Further, a macrophage-grown vaccine virus and a virus grown on permanent cells could be employed. Irrespective of the production system of the vaccine virus, a two-dose intramuscular immunization could induce close-to-sterile immunity with full clinical protection against challenge infection. After oral immunization, 50% of the vaccinees seroconverted and all responders were completely protected against subsequent challenge. All nonresponders developed ASF upon challenge with two acute lethal infections and two mild and transient courses. The latter results show a lower efficiency after oral administration that would have to be taken into consideration when designing vaccination-based control measures. Overall, our findings confirm that “ASFV-G-∆MGF” is a most promising vaccine candidate that could find its way into well-organized and controlled immunization campaigns. Further research is needed to characterize safety aspects and define possible improvements of oral efficiency.

Comparative efficacy evaluation of different CSF vaccines in pigs with CSF maternally derived antibodies

Classical swine fever (CSF) is a highly contagious and important swine disease in China. Sporadic outbreaks with mild clinical signs are still being reported despite massive vaccination with the CSF C-strain vaccine. One possible reason for vaccine failure could be interference from maternally derived antibodies (MDAs) during vaccination in the field. The aim of this study was to evaluate the efficacy of different CSF vaccines in the presence of MDAs and to assess the different vaccination schemes in the field. The results demonstrated that vaccination with a single dose of C-strain-PK vaccine protected pigs against severe clinical signs and significantly reduced viremia. The impact of MDAs was negligible. The interference was also mild during a prime and boost vaccination scheme using the C-strain-ST vaccine. In contrast, a significant influence of MDAs on the efficacy of the subunit E2 vaccine in a one-dose vaccination scheme was observed, with pigs showing severe clinical signs, CSF-associated death, typical pathological lesions and a high level of viremia after challenge, despite robust E2 antibody induction. A field vaccination and challenge study further confirmed the superior effectiveness of a single dose of C-strain-PK vaccine in the presence of MDAs in comparison to a routine prime and boost vaccination scheme applied in the field, with pigs having fever, chronic signs, significant viremia and shedding after challenge. Delaying the vaccination time from the age of 28 days to 45 days, when MDA was low, was beneficial for improving the clinical protection and immunity induced by vaccines. Altogether, the results presented here emphasize that a high-quality vaccine and a scientific design of the vaccination scheme based on serological surveillance are essential pillars to control and eliminate CSF in China.

Characterization of monoclonal antibodies that specifically differentiate field isolates from vaccine strains of classical swine fever virus

Classical swine fever virus (CSFV) is a major animal pathogen threatening the global pork industry. To date, numerous anti-CSFV monoclonal antibodies (mAbs) and their recognizing epitopes have been reported. However, few mAbs were systematically characterized for the capacity to differentiate field CSFV isolates from CSF vaccine strains, and the molecular basis associated with antigenic differences between vaccines and field isolates is still largely unknown. In the present study, recombinant CSFV structural glycoproteins E2 of both virulent and vaccine strains and Erns of vaccine strain were expressed using eukaryotic cells and murine mAbs generated against E2 and Erns. After serial screening and cloning of the hybridomas, the viral spectra of mAbs were respectively determined by indirect fluorescent antibody assay (IFA) using 108 CSFVs, followed by Western blot analysis using expressed glycoproteins of all CSFV sub-genotypes including vaccine strains. The antigenic structures recognized by these mAbs were characterized by epitope mapping using truncated, chimeric, and site-directed mutated E2 and Erns proteins. We have identified two vaccine-specific, one field isolate-specific, and two universal CSFV-specific mAbs and five novel conformational epitopes with critical amino acid (aa) motifs that are associated with these five mAbs: 213EPD215, 271RXGP274, and 37LXLNDG42 on E2 and 38CKGVP42, W81, and D100/V107 on Erns. Particularly, E213 of E2 is field isolate-specific, while N40 of E2 and D100/V107 of Erns are vaccine strain-specific. Results from our study further indicate that N40D of E2 mutation in field strains was likely produced under positive selection associated with long-term mass vaccination, leading to CSFV evasion of host immune response. Taking together, this study provides new insights into the antigenic structure of CSFV E2 and Erns and the differentiating mAbs will contribute to the development of a diagnostic strategy to differentiate C-strain vaccination from natural infection (DIVA) of CSFV in terms of elimination of CSF in China.

A Novel Competitive ELISA for Specifically Measuring and Differentiating Immune Responses to Classical Swine Fever C-Strain Vaccine in Pigs

Classical swine fever can be controlled effectively by vaccination with C-strain vaccine. In this study, we developed a novel competitive enzyme-linked immunosorbent assay (cELISA) based on a C-strain Erns specific monoclonal antibody (mAb 1504), aiming to serologically measure immune responses to C-strain vaccine in pigs, and finally to make the C-strain become a DIVA-compatible vaccine. The cELISA system was established based on the strategy that mAb 1504 will compete with the C-strain induced antibodies in the pig serum to bind the C-strain Erns protein. The cELISA was optimized and was further evaluated by testing different categories of pig sera. It can efficiently differentiate C-strain immunized from wild-type CSFV-infected pigs and lacks cross-reaction with other common swine viruses and viruses in genus Pestivirus such as Bovine viral diarrhea virus (BVDV). The C-strain antibody can be tested in pigs 7-14 days post vaccination with this cELISA. The sensitivity and specificity of the established cELISA were 100% (95% confidence interval: 95.60 to 100%) and 100% (95% confidence interval: 98.30 to 100%), respectively. This novel cELISA is a reliable tool for specifically measuring and differentiating immune responses to C-strain vaccine in pigs. By combining with the wild-type CSFV-specific infection tests, it can make the C-strain have DIVA capability.

Valosin-containing protein (VCP/p97) is responsible for the endocytotic trafficking of classical swine fever virus

Classical swine fever virus (CSFV), a member of the Flaviviridae enveloped RNA virus family, results in an epidemic disease that brings serious economic losses to the pig industry worldwide. Valosin-containing protein (VCP/p97), a multifunctional active protein in cells, is related to the life activities of many viruses. However, the role of VCP in CSFV infection remains unknown. In this study, it was first found that treatment of VCP inhibitors impaired CSFV propagation. Furthermore, overexpression or knockdown of VCP showed that it was essential for CSFV infection. Moreover, confocal microscopy and immunoprecipitation assay showed that VCP was recruited for intracellular transport from early endosomes to lysosomes. Importantly, knockdown of VCP prevented CSFV to release from early endosomes, suggesting that VCP is a key factor for CSFV trafficking. Taken together, our findings first demonstrate that the endocytosis of CSFV into PK-15 cells requires the participation of VCP, providing the alternative approach for the discovery of novel anti-flaviviridae drugs.

Search for additional tests for immunobiological evaluation of the candidate vaccines against African swine fever

The spread of African swine fever (ASF) in Eurasia has forced a return to the development of live vaccines based on naturally or experimentally attenuated strains of the virus including those resulting from genetic manipulations. This process includes evaluation of the immunomodulating properties of the vaccines. In this report we provide our assessment of two tests for immunobiological evaluation of a candidate live vaccine against ASF from the attenuated ASF virus (ASFV) strain KK-202: (i) investigation of the effect of the attenuated ASFV strain KK-202 on the protectiveness of the vaccine ASFV strain FK-32/135 and a vaccine against classical swine fever (CSF) from the strain LK-VNIIVViM; (ii) determination of the phagocytic activity of blood neutrophils in pigs inoculated with ASFV strains differing in virulence. A simultaneous or sequential inoculation of attenuated strain KK-202 (seroimmunotype II) and vaccine strain FK-32/135 (seroimmunotype IV) into pigs resulted in the loss of protection against the virulent strain France-32 (seroimmunotype IV). Following the simultaneous or sequential inoculations of the ASFV strain KK-202 and the CSF virus (CSFV) vaccine produced from the strain LK-VNIIVViM, the neutralizing antibody titers against the CSFV observed in the experimental groups (after vaccination and after the challenge infection with the virulent CSFV strain Shimen) were not different from those found in animals of the control group. The phagocytic activity of blood neutrophils was shown to increase from 30% in the norm to 50%-94% depending on the virulence of the ASFV strains inoculated into pigs. The results of this work demonstrate the ability of the attenuated ASFV strains to modulate the development of the cellular link of protective immunity without negative impact on the humoral immune response. The informative value of the described immunobiological tests in vivo and in vitro seems to be a more preferable alternative in comparison to the commonly used in vitro tests, which do not always correlate with the development of protection against ASF.

Designing a novel E2-IFN-γ fusion protein against CSFV by immunoinformatics and structural vaccinology approaches

Subunit vaccines with high purity and safety are gradually becoming a main trend in vaccinology. However, adjuvants such as interferon-gamma (IFN-γ) are required to enhance immune responses of subunit vaccines due to their poor immunogenicity. The conjugation of antigen with adjuvant can induce more potent immune responses compared to the mixture of antigen and adjuvant. At the same time, the selection of linker, indispensable in the construction of the stable and bioactive fusion proteins, is complicated and time-consuming. The development of immunoinformatics and structural vaccinology approaches provides a means to address the abovementioned problem. Therefore, in this study, a E2-IFN-γ fusion protein with an optimal linker (E2-R2-PIFN) was designed by bioinformatics approaches to improve the immunogenicity of the classical swine fever virus (CSFV) E2 subunit vaccine. Moreover, the E2-R2-PIFN fusion protein was expressed in HEK293T cells and the biological effects of IFN-γ in E2-R2-PIFN were confirmed in vitro via Western blotting. Here, an alternative method is utilized to simplify the design and validation of the antigen-adjuvant fusion protein, providing a potential subunit vaccine candidate against CSFV. KEY POINTS: • An effective and simple workflow of antigen-adjuvant fusion protein design and validation was established by immunoinformatics and structural vaccinology. • A novel E2-IFN-γ fusion protein with an optimal linker was designed as a potential CSFV vaccine. • The bioactivity of the newly designed fusion protein was preliminarily validated through in vitro experiments.

Bungowannah Pestivirus Chimeras as Novel Double Marker Vaccine Strategy against Bovine Viral Diarrhea Virus

Marker or DIVA (differentiation of infected from vaccinated animals) vaccines are beneficial tools for the eradication of animal diseases in regions with a high prevalence of the designated disease. Bovine viral diarrhea virus (BVDV)-1 (syn. Pestivirus A) is a flavivirus that infects predominantly cattle resulting in major economic losses. An increasing number of countries have implemented BVDV eradication programs that focus on the detection and removal of persistently infected cattle. No efficient marker or DIVA vaccine is yet commercially available to drive the eradication success, to prevent fetal infection and to allow serological monitoring of the BVDV status in vaccinated farms. Bungowannah virus (BuPV, species Pestivirus F), a related member of the genus Pestivirus with a restricted prevalence to a single pig farm complex in Australia, was chosen as the genetic backbone for a marker vaccine candidate. The glycoproteins E1 and E2 of BuPV were substituted by the heterologous E1 and E2, which are major immunogens, of the BVDV-1 strain CP7. In addition, the candidate vaccine was further attenuated by the introduction of a deletion within the N^pro protein coding sequence, a major type I interferon inhibitor. Immunization of cattle with the chimeric vaccine virus BuPV_ΔN^pro_E1E2 CP7 (modified live or inactivated) followed by a subsequent experimental challenge infection confirmed the safety of the prototype strain and provided a high level of clinical protection against BVDV-1. The serological discrimination of vaccinated cattle could be enabled by the combined detection of BVDV-1 E2- in the absence of both BVDV NS3- and BVDV E^rns-specific antibodies. The study demonstrates for the first time the generation and application of an efficient BVDV-1 modified double marker vaccine candidate that is based on the genetic background of BuPV accompanied by commercially available serological marker ELISA systems.

Characterisation and comparison of immune response mechanisms in an indigenous and a commercial pig breed after classical swine fever vaccination

The live attenuated classical swine fever (CSF) vaccine has been successfully used to prevent and control CSF outbreaks for 6 decades. However, the immune response mechanisms against the vaccine remain poorly understood. Moreover, very few reports exist regarding the breed differences in the response to CSF vaccine. In this study, we generated the peripheral blood mononuclear cell transcriptomes of indigenous Ghurrah and commercial Landrace pig breeds, before and 7 days after CSF vaccination. Subsequently, between and within-breed differential gene expression analyses were carried out. Results revealed large differences in pre-vaccination peripheral blood mononuclear cell transcriptome profiles of the two breeds, which were homogenised 7 days after vaccination. Before vaccination, gene set enrichment analysis showed that pathways related to antigen sensing and innate immune response were enriched in Ghurrah, while pathways related to adaptive immunity were enriched in Landrace. Ghurrah exhibited greater immunomodulation compared to Landrace following the vaccination. In Ghurrah, cell-cycle processes and T-cell response pathways were upregulated after vaccination. However, no pathways were upregulated in Landrace after vaccination. Pathways related to inflammation were downregulated in both the breeds after vaccination. Key regulators of inflammation such as IL1A, IL1B, NFKBIA and TNF genes were strongly downregulated in both the breeds after vaccination. Overall, our results have elucidated the mechanisms of host immune response against CSF vaccination in two distinct breeds and revealed common key genes instrumental in the global immune response to the vaccine.

Development and application of a high-sensitivity immunochromatographic test strip for detecting classical swine fever virus antibodies

Classical swine fever (CSF) is caused by classical swine fever virus (CSFV) and has led to huge economic losses in the pig industry worldwide. Although vaccination and other control measures have been carried out, it is essential to establish a rapid and valid method for CSF vaccination monitoring and clinical diagnosis. The CSFV E2 protein has been widely used as a major antigen for antibody detection. It is important to improve the affinity between the E2 protein and CSFV antibodies to improve the performance of the detection method. In this study, a recombinant E2 extracellular protein (amino acids 1-331) with a native homodimer conformation and high affinity for the anti-CSFV-E2 monoclonal antibody WH303 was expressed using a Bac-to-Bac baculovirus expression system. A novel immunochromatographic test strip based on the recombinant CSFV E2 protein was developed for CSFV antibody detection. The sensitivity of this strip for detecting CSFV standard-positive serum was 1:102400, 4 times higher than that of the previously developed CnC2 test strip. No cross-reactivity with antibodies of other swine viruses was observed. Detection of clinical swine serum samples (n = 813) demonstrated that the agreements of this E2 test strip with three commercial ELISA kits were 97.17% (790/813), 95.94% (780/813), and 93.73% (762/813), respectively. Our data indicate that a novel E2 test strip with enhanced sensitivity has been developed and can be applied for clinical sample detection, providing a new, powerful and simple approach for CSFV antibody monitoring. This article is protected by copyright. All rights reserved.

A novel linear epitope at the C-terminal region of the classical swine fever virus E2 protein elicits neutralizing activity

Classical swine fever virus (CSFV) is a member of the genus Pestivirus, which causes serious economic losses. The re-emergence of the disease in Japan in 2018 has increased awareness of CSFV. In this study, Balb/c mice were immunized with plant-derived E2 protein, and four monoclonal antibodies (mAbs) 4B11, 7B3, 11A5 and 6F3 were generated. Two of these mAbs, 4B11 and 7B3, effectively blocked CSFV infection of PK-15 cells. Both mAbs recognized a novel linear epitope, 256CLIGNTTVKVHASDER271. The neutralizing ability of anti-CSFV serum decreased 63%, when pre-incubated with the linear peptide at 200 μg/mL. Structural analysis showed that this linear epitope is present at the border of Domain C and Domain D on the surface of the E2 protein. Alignment of amino acid sequences showed that the epitope was conserved in different subgroups of CSFV but not in other members of the Pestivirus genus. Consistently with the analysis above, this epitope distinguished antibodies against CSFV from those against bovine viral diarrhea virus (BVDV). Our study provides an ideal candidate peptide for new vaccine design and differential diagnosis of CSFV. These findings will contribute to the control and eradication of classical swine fever.

Development of a Dendrimeric Peptide-Based Approach for the Differentiation of Animals Vaccinated with FlagT4G against Classical Swine Fever from Infected Pigs

Classical swine fever virus (CSFV) causes a viral disease of high epidemiological and economical significance that affects domestic and wild swine. Control of the disease in endemic countries is based on live-attenuated vaccines (LAVs) that induce an early protective immune response against highly virulent CSFV strains. The main disadvantage of these currently available LAVs is the lack of serological techniques to differentiate between vaccinated and infected animals (DIVA concept). Here, we describe the development of the FlagDIVA test, a serological diagnostic tool allowing for the differentiation between animals vaccinated with the FlagT4G candidate and those infected with CSFV field strains. The FlagDIVA test is a direct ELISA based on a dendrimeric peptide construct displaying a conserved epitope of CSFV structural protein E2. Although FlagDIVA detected anti-CSFV anti-bodies in infected animals, it did not recognize the antibody response of FlagT4G-vaccinated animals. Therefore, the FlagDIVA test constitutes a valuable accessory DIVA tool in implementing vaccination with the FlagT4G candidate.

Fatty Acid Synthase Is Involved in Classical Swine Fever Virus Replication by Interaction with NS4B

Classical swine fever virus (CSFV), a member of the genus Pestivirus of the family Flaviviridae, relies on host machinery to complete its life cycle. Previous studies have shown a close connection between virus infection and fatty acid biosynthesis, mainly regulated by fatty acid synthase (FASN). However, the molecular action of how FASN participates in CSFV replication remains to be elucidated. In this study, two chemical inhibitors of the fatty acid synthesis pathway [5-(tetradecyloxy)-2-furoic acid (TOFA) and tetrahydro-4-methylene-2R-octyl-5-oxo-3S-furancarboxylic acid (C75)] significantly impaired the late stage of viral propagation, suggesting CSFV replication required fatty acid synthesis. We next found that CSFV infection stimulated the expression of FASN, whereas knockdown of FASN inhibited CSFV replication. Furthermore, confocal microscopy showed that FASN participated in the formation of replication complex (RC), which was associated with the endoplasmic reticulum (ER). Interestingly, CSFV NS4B interacted with FASN and promoted overexpression of FASN, which is regulated by functional Rab18. Moreover, we found that FASN regulated the formation of lipid droplets (LDs) upon CSFV infection, promoting virus proliferation. Taken together, our work provides mechanistic insight into the role of FASN in the viral life of CSFV, and it highlights the potential antiviral target for the development of therapeutics against pestiviruses. IMPORTANCE Classical swine fever, caused by classical swine fever virus (CSFV), is one of the notifiable diseases by the World Organization for Animal Health (OIE) and causes significant financial losses to the pig industry globally. CSFV, like other (+)-strand RNA viruses, requires lipid and sterol biosynthesis for efficient replication. However, the role of lipid metabolism in CSFV replication remains unknown. Here, we found that fatty acid synthase (FASN) was involved in viral propagation. Moreover, FASN is recruited to CSFV replication sites in the endoplasmic reticulum (ER) and interacts with NS4B to regulate CSFV replication that requires Rab18. Furthermore, we speculated that lipid droplet (LD) biosynthesis, indirectly regulated by FASN, ultimately promotes CSFV replication. Our results highlight a critical role for de novo fatty acid synthesis in CSFV infection, which might help control this devastating virus.

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.

Transboundary Animal Diseases, an Overview of 17 Diseases with Potential for Global Spread and Serious Consequences

Animals provide food and other critical resources to most of the global population. As such, diseases of animals can cause dire consequences, especially disease with high rates of morbidity or mortality. Transboundary animal diseases (TADs) are highly contagious or transmissible, epidemic diseases, with the potential to spread rapidly across the globe and the potential to cause substantial socioeconomic and public health consequences. Transboundary animal diseases can threaten the global food supply, reduce the availability of non-food animal products, or cause the loss of human productivity or life. Further, TADs result in socioeconomic consequences from costs of control or preventative measures, and from trade restrictions. A greater understanding of the transmission, spread, and pathogenesis of these diseases is required. Further work is also needed to improve the efficacy and cost of both diagnostics and vaccines. This review aims to give a broad overview of 17 TADs, providing researchers and veterinarians with a current, succinct resource of salient details regarding these significant diseases. For each disease, we provide a synopsis of the disease and its status, species and geographic areas affected, a summary of in vitro or in vivo research models, and when available, information regarding prevention or treatment.

Genetically distinct pestiviruses pave the way to improved classical swine fever marker vaccine candidates based on the chimeric pestivirus concept

Classical swine fever (CSF) is one of the most important viral diseases of pigs. In many countries, the use of vaccines is restricted due to limitations of subunit vaccines with regard to efficacy and onset of protection as well as failure of live vaccines to differentiate infected from vaccinated animals (DIVA principle). Chimeric pestiviruses based on CSF virus (CSFV) and the related bovine viral diarrhea virus (BVDV) have been licensed as live marker vaccines in Europe and Asia, but cross-reactive antibodies can cause problems in DIVA application due to close antigenic relationship. To develop marker vaccine candidates with improved DIVA properties, three chimeric viruses were generated by replacing E^rns of CSFV Alfort-Tübingen with homologue proteins of only distantly related pestiviruses. The chimeric viruses “Ra”, “Pro”, and “RaPro” contained E^rns sequences of Norway rat and Pronghorn pestiviruses or a combination of both, respectively. In porcine cells, the “Pro” chimera replicated to high titers, while replication of the “Ra” chimera was limited. The “RaPro” chimera showed an intermediate phenotype. All vaccine candidates were attenuated in a vaccination/ challenge trial in pigs, but to different extents. Inoculation induced moderate to high levels of neutralizing antibodies that protected against infection with a genetically heterologous, highly virulent CSFV. Importantly, serum samples of vaccinated animals did not show any cross-reactivity in a CSFV E^rns antibody ELISA. In conclusion, the E^rns antigen from distantly related pestiviruses can provide a robust serological negative marker for a new generation of improved CSFV marker vaccines based on the chimeric pestivirus concept.

Assessing the Protective Dose of a Candidate DIVA Vaccine against Classical Swine Fever

Classical swine fever is a highly contagious and deadly disease in swine. The disease can be controlled effectively by vaccination with an attenuated virus known as the “Chinese” (C)-strain. A single vaccination with the C-strain provides complete protection against highly virulent isolates within days after vaccination, making it one of the most efficacious veterinary vaccines ever developed. A disadvantage of the C-strain is that vaccinated animals cannot be serologically differentiated from animals that are infected with wild-type Classical swine fever virus. Previously, a C-strain-based vaccine with a stable deletion in the E2 structural glycoprotein was developed, which allows for differentiation between infected and vaccinated animals (DIVA). The resulting vaccine, which we named C-DIVA, is compatible with a commercial E2 ELISA, modified to render it suitable as a DIVA test. In the present work, three groups of eight piglets were vaccinated with escalating doses of the C-DIVA vaccine and challenged two weeks after vaccination. One group of four unvaccinated piglets served as controls. Piglets were monitored for clinical signs until three weeks after challenge and blood samples were collected to monitor viremia, leukocyte and thrombocyte levels, and antibody responses. The presence of challenge virus RNA in oropharyngeal swabs was investigated to first gain insight into the potential of C-DIVA to prevent shedding. The results demonstrate that a single vaccination with 70 infectious virus particles of C-DIVA protects pigs from the highly virulent Brescia strain.

Research Progress and Challenges in Vaccine Development against Classical Swine Fever Virus

Classical swine fever (CSF), caused by CSF virus (CSFV), is one of the most devastating viral epizootic diseases of swine in many countries. To control the disease, highly efficacious and safe live attenuated vaccines have been used for decades. However, the main drawback of these conventional vaccines is the lack of differentiability of infected from vaccinated animals (DIVA concept). Advances in biotechnology and our detailed knowledge of multiple basic science disciplines have facilitated the development of effective and safer DIVA vaccines to control CSF. To date, two types of DIVA vaccines have been developed commercially, including the subunit vaccines based on CSFV envelope glycoprotein E2 and chimeric pestivirus vaccines based on infectious cDNA clones of CSFV or bovine viral diarrhea virus (BVDV). Although inoculation of these vaccines successfully induces solid immunity against CSFV, none of them could ideally meet all demands regarding to safety, efficacy, DIVA potential, and marketability. Due to the limitations of the available choices, researchers are still striving towards the development of more advanced DIVA vaccines against CSF. This review summarizes the present status of candidate CSFV vaccines that have been developed. The strategies and approaches revealed here may also be helpful for the development of new-generation vaccines against other diseases.

Novel chimeric E2CD154 subunit vaccine is safe and confers long lasting protection against classical swine fever virus

E2CD154 is a vaccine candidate against classical swine fever (CSF) based on a chimeric protein composed of the E2 glycoprotein fused to porcine CD154 antigen, and formulated in the oil adjuvant Montanide™ ISA 50 V2. This vaccine confers early protection in pigs and prevents vertical transmission in pregnant sows. The objectives of this study were to assess the safety of this immunogen in piglets, to compare several doses of antigen in the formulation, and to study the duration of the immunity provided by this vaccine for up to 9 months. Three trials were conducted by immunizing pigs with a two-dose regime of the vaccine. Challenge experiments were carried out with the highly pathogenic Margarita strain. No local or systemic adverse effects were documented, and neither macroscopic nor microscopic pathological findings were observed in the vaccinated animals. The three antigen doses explored were safe and induced CSF protective neutralizing antibodies. The dose of 50 μg was selected for further development because it provided the best clinical and virological protection. Finally, this protective immunity was sustained for at least 9 months. This study demonstrates that E2CD154 vaccine is safe; defines a vaccine dose of 50 μg antigen, and evidences the capacity of this vaccine to confer long term protection from CSFV infection for up to 9 months post- vaccination. These findings complement previous data on the evaluation of this vaccine candidate, and suggest that E2CD154 is a promising alternative to modified live vaccines in CSF endemic areas.

The ESCRT-I Subunit Tsg101 Plays Novel Dual Roles in Entry and Replication of Classical Swine Fever Virus

Classical swine fever (CSF), caused by classical swine fever virus (CSFV), is a highly contagious disease of swine with high morbidity and mortality that negatively affects the pig industry worldwide, in particular in China. Soon after the endocytosis of CSFV, the virus makes full use of the components of host cells to complete its life cycle. The endocytosis sorting complex required for transport (ESCRT) system is a central molecular machine for membrane protein sorting and scission in eukaryotic cells that plays an essential role in many physiological metabolic processes, including invasion and egress of envelope viruses. However, the molecular mechanism that ESCRT uses to regulate the replication of CSFV is unknown. In this study, we demonstrated that the ESCRT-I complex Tsg101 protein participates in clathrin-mediated endocytosis of CSFV and is also involved in CSFV trafficking. Tsg101 assists the virus in entering the host cell through the late endosome (Rab7 and Rab9) and finally reaching the lysosome (Lamp-1). Interestingly, Tsg101 is also involved in the viral replication process by interacting with nonstructural proteins 4B and 5B of CSFV. Finally, confocal microscopy showed that the replication complex of Tsg101 and double-stranded RNA (dsRNA) or NS4B and NS5B protein was close to the endoplasmic reticulum (ER), not the Golgi, in the cytoplasm. Collectively, our finding highlights that Tsg101 regulates the process of CSFV entry and replication, indicating that the ESCRT plays an important role in the life cycle of CSFV. Thus, ESCRT molecules could serve as therapeutic targets to combat CSFV infection.IMPORTANCE CSF, caused by CSFV, is a World Organization for Animal Health (OIE) notifiable disease and causes significant financial losses to the pig industry globally. The ESCRT machinery plays an important regulatory role in several members of the genera Flavivirus and Hepacivirus within the family Flaviviridae, such as hepatitis C virus, Japanese encephalitis virus, and dengue virus. Previous reports have shown that assembling and budding of these viruses require ESCRT. However, the role of ESCRT in Pestivirus infection remains to be elucidated. We determined the molecular mechanisms of the regulation of CSFV infection by the major subunit Tsg101 of ESCRT-I. Interestingly, Tsg101 plays an essential regulatory role in both clathrin-mediated endocytosis and genome replication of CSFV. Overall, the results of this study provide further insights into the molecular function of ESCRT-I complex protein Tsg101 during CSFV infection, which may serve as a molecular target for pestivirus inhibitors.

Generation and immunogenicity analysis of recombinant classical swine fever virus glycoprotein E2 and Erns expressed in baculovirus expression system

Classical swine fever (CSF) caused by the classical swine fever virus (CSFV) is a highly contagious swine disease resulting in large economical losses worldwide. The viral envelope glycoprotein E2 and E^rns are major targets for eliciting antibodies against CSFV in infected animals. In this report, the glycoprotein E2 and E^rns were expressed using the baculovirus system and their protective immunity in rabbits were tested. Twenty CSFV seronegative rabbits were randomly divided into five groups. Each rabbit was intramuscularly immunized with CSFV-E2, CSFV-E^rns, or their combination (CSFV-E2 + E^rns). Besides, a commercial CSFV vaccine (C-strain) and PBS were used as positive or negative controls, respectively. Four weeks after the second immunization, all the rabbits were challenged with 100 RID₅₀ of CSFV C-strain. High levels of CSFV E2-specific antibody, neutralizing antibody and cellular immune responses to CSFV were elicited in the rabbits inoculated with C-strain, CSFV-E2, and CSFV-E2 + E^rns. And the rabbits inoculated with the three vaccines received complete protection against CSFV C-strain. However, no neutralizing antibody was detected in the E^rns vaccinated rabbits and the rabbits exhibited fever typical of CSFV, suggesting the E^rns alone is not able to induce a protective immune response. Taken together, while the E^rns could not confer protection against CSFV, E2 and E2 + E^rns could not only elicit humoral and cell-mediated immune responses but also confer complete protection against CSFV C-strain in rabbits.

A Critical Review about Different Vaccines against Classical Swine Fever Virus and Their Repercussions in Endemic Regions

Classical swine fever (CSF) is, without any doubt, one of the most devasting viral infectious diseases affecting the members of Suidae family, which causes a severe impact on the global economy. The reemergence of CSF virus (CSFV) in several countries in America, Asia, and sporadic outbreaks in Europe, sheds light about the serious concern that a potential global reemergence of this disease represents. The negative aspects related with the application of mass stamping out policies, including elevated costs and ethical issues, point out vaccination as the main control measure against future outbreaks. Hence, it is imperative for the scientific community to continue with the active investigations for more effective vaccines against CSFV. The current review pursues to gather all the available information about the vaccines in use or under developing stages against CSFV. From the perspective concerning the evolutionary viral process, this review also discusses the current problematic in CSF-endemic countries.

Evaluation of classical swine fever E2 (CSF-E2) subunit vaccine efficacy in the prevention of virus transmission and impact of maternal derived antibody interference in field farm applications

Background

Classical swine fever (CSF) is one of the most devastating pig diseases that affect the swine industry worldwide. Besides stamping out policy for eradication, immunization with vaccines of live attenuated CSF or the CSF-E2 subunit is an efficacious measure of disease control. However, after decades of efforts, it is still hard to eliminate CSF from endemically affected regions and reemerging areas. Most of previous studies demonstrated the efficacy of different CSF vaccines in laboratories under high containment conditions, which may not represent the practical performance in field farms. The inadequate vaccine efficacy induced by unrestrained factors may lead to chronic or persistent CSF infection in animals that develop a major source for virus shedding among pig populations. In this study, a vaccination-challenge-cohabitation trial on specific-pathogen-free (SPF) pigs and long-term monitoring of conventional sows and their offspring were used to evaluate the efficacy and the impact of maternally derived antibody (MDA) interference on CSF vaccines in farm applications.

Results

The trials demonstrated higher neutralizing antibody (NA) titers with no clinical symptoms and significant pathological changes in the CSF-E2 subunit vaccine immunized group after CSFV challenge. Additionally, none of the sentinel pigs were infected during cohabitation indicating that the CSF-E2 subunit vaccine could provoke adequately acquired immunity to prevent horizontal transmission. In field farm applications, sows immunized with CSF-E2 subunit vaccine revealed an average of higher and consistent antibody level with significant reduction of CSF viral RNA detection via saliva monitoring in contrast to those of live attenuated CSF vaccine immunized sows possessing diverse antibody titer distributions and higher viral loads. Furthermore, early application of the CSF-E2 subunit vaccine in 3-week-old piglets illustrated no MDA interference on primary immunization and could elicit consistent and long-lasting adequate antibody response suggesting the flexibility of CSF-E2 subunit vaccine on vaccination program determination.

Conclusions

The CSF-E2 subunit vaccine demonstrated significant efficacy and no MDA interference for immunization in both pregnant sows and piglets. These advantages provide a novel approach to avoid possible virus shedding in sow population and MDA interference in piglets for control of CSF in field farm applications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40813-020-00188-6.

Porcine vaccines

The growth of large, intensive pig operations has made effective vaccination imperative in the swine industry. This growth in size has been accompanied by outbreaks of disease such as porcine respiratory and reproductive syndrome (PRRS) and porcine epidemic diarrhea. These have caused massive losses, and effective vaccines are not yet available. Older diseases such as pleuropneumonia, erysipelas, enzootic pneumonia, proliferative enteritis, pseudorabies, and colibacillosis still persist, and classical swine fever and influenza remain as ongoing threats. The large size of many operations has required the introduction of mass vaccination procedures such as vaccination through drinking water. The current African swine fever pandemic makes the development of an effective vaccine against this virus, a matter of urgent necessity.

Different clinical presentations of subgenotype 2.1 strain of classical swine fever infection in weaned piglets and adults, and long-term cross-protection conferred by a C-strain vaccine

Classical swine fever is an important swine disease in China, and sporadic outbreaks with mild clinical signs despite compulsory vaccination have raised questions about the virulence and pathogenicity of prevalent subgenotype 2.1 strains, and the ability of C-strain vaccines to cross-protect against them. To investigate this, three field isolates were evaluated in experimentally infected piglets and compared with the highly virulent reference Shimen strain. Clinical signs for the field strains ranged from mild to severe, and mortality ranged from 0 to 80 %. These data show differences in virulence among the subgenotype 2.1 field isolates and support the use of field strain GD191 as a genotype 2 challenge virus to assess efficacy of C-strain vaccines. In contrast to the historical genotype 1 strain, which caused acute infection with significant virus shedding in non-vaccinated animals, the subgenotype 2.1 GD191 strain produced different clinical manifestations in weaned piglets and adults. Adult pigs showed subclinical infection with viral shedding, whereas weaned piglets showed overt signs of infection. Efficacy of, and duration of immunity conferred by a C-strain vaccine were assessed using the reference Shimen strain and field isolate GD191 at 12 and 15 months after vaccination. A robust antibody response and sterilising protection were seen in all vaccinated animals and lasted up to 15 months post-vaccination. This study confirms that C-strain vaccines confer both clinical and virological protection against the historical genotype 1 Shimen strain and cross-protection against the prevalent genotype 2 field strain.

A Synthetic Modified Live Chimeric Marker Vaccine against BVDV-1 and BVDV-2

Bovine viral diarrhea virus (BVDV), a pestivirus which exists in the two distinct species BVDV-1 (syn. Pestivirus A) and BVDV-2 (syn. Pestivirus B), is the causative agent of one of the most widespread and economically important virus infections in cattle. For economic as well as for animal health reasons, an increasing number of national BVDV control programs were recently implemented. The main focus lies on the detection and removal of persistently infected cattle. The application of efficient marker or DIVA (differentiation of infected from vaccinated animals) vaccines would be beneficial for the eradication success in regions with a high BVDV prevalence to prevent fetal infection and it would allow serological monitoring of the BVDV status also in vaccinated farms. Therefore, a marker vaccine based on the cytopathic (cp) BVDV-1b strain CP7 was constructed as a synthetic backbone (BVDV-1b_synCP7). For serological discrimination of vaccinated from infected animals, the viral protein E^rns was substituted by the heterologous E^rns of Bungowannah virus (BuPV, species Pestivirus F). In addition, the vaccines were attenuated by a deletion within the type I interferon inhibitor N^pro protein encoding sequence. The BVDV-2 vaccine candidate is based on the genetic sequence of the glycoproteins E1 and E2 of BVDV-2 strain CS8644 (CS), which were introduced into the backbone of BVDV-1b_synCP7_ΔN^pro_E^rns Bungo in substitution of the homologous glycoproteins. Vaccine virus recovery resulted in infectious cytopathic virus chimera that grew to titers of up to 10⁶ TCID₅₀/mL. Both synthetic chimera BVDV-1b_synCP7_ΔN^pro_E^rns Bungo and BVDV-1b_synCP7_ΔN^pro_E^rns Bungo_E1E2 BVDV-2 CS were avirulent in cattle, provided a high level of protection in immunization and challenge experiments against both BVDV species and allowed differentiation of infected from vaccinated cattle. Our study presents the first report on an efficient BVDV-1 and -2 modified live marker vaccine candidate and the accompanying commercially available serological marker ELISA system.

Classical swine fever virus: the past, present and future

Classical swine fever (CSF) is among the most relevant viral epizootic diseases of swine. Due to its severe economic impact, CSF is notifiable to the world organisation for animal health. Strict control policies, including systematic stamping out of infected herds with and without vaccination, have permitted regional virus eradication. Nevertheless, CSF virus (CSFV) persists in certain areas of the world and has re-emerged regularly. This review summarizes the basic established knowledge in the field and provides a comprehensive and updated overview of the recent advances in fundamental CSFV research, diagnostics and vaccine development. It covers the latest discoveries on the genetic diversity of pestiviruses, with implications for taxonomy, the progress in understanding disease pathogenesis, immunity against acute and persistent infections, and the recent findings in virus-host interactions and virulence determinants. We also review the progress and pitfalls in the improvement of diagnostic tools and the challenges in the development of modern and efficacious marker vaccines compatible with serological tests for disease surveillance. Finally, we highlight the gaps that require research efforts in the future.

A single dose glycoprotein D-based subunit vaccine against pseudorabies virus infection

Pseudorabies Virus (PRV) is the causative agent of Pseudorabies (PR), also known as Aujeszky's Disease, one of the most important infectious diseases in swine, resulting in huge economic losses to the swine industry globally. The emergence of mutant PRV strains after 2011 resulted in a sharp decrease in the efficacy of available commercial vaccines. To develop a more effective vaccine that can prevent the spread of PRV, glycoprotein B (gB), glycoprotein C (gC) and glycoprotein D (gD) from recent PRV isolates were expressed in a baculovirus system and their protective efficacy was tested in mice and piglets. Neutralizing antibody titers (NAs) in mice vaccinated with gB, gC and gD peaked at 28 days after immunization and then slowly declined. NAs in the mice immunized with gD were remarkably higher than other groups. After a lethal challenge of 5 LD₅₀ with mutant PRV-HNLH strain, the survival rates of gB and gD were 100% and 87.5% respectively, which was significantly higher than gC group (50%). Piglets vaccinated with the gD and gB + D vaccines developed the highest NAs 7 days post immunization. No piglets in these two groups exhibited clinical symptoms, high body temperature or virus shedding following challenge with 10^6.6 TCID₅₀ with the mutant PRV-HNLH strain. Histopathology and immunohistochemistry showed remarkably reduced pathological damage and viral loads in gD and gB + D groups. Furthermore, the duration of the NAs induced by gD vaccine could maintain as long as four months after a single dose. The current study indicates that a gD-based vaccine could be developed for the efficient control of PRV.

Descriptive epidemiology of classical swine fever outbreaks in the period 2013-2018 in Colombia

Classical swine fever (CSF) is an infectious viral disease caused by an RNA virus belonging to the Pestivirus genus. A total of 134 outbreaks of CSF have occurred in the last seven years in the North of Colombia. The objective of this study was the characterization of the herds affected by CSF from 2013 to 2018. Most of the outbreaks (95%) occured in backyard piggeries. The principal causes of transmission of CSF were the introduction of infected pigs (38%), movements of people (37%) and unknown origin (13%). The epidemiological relationships with 15 affected farms explained 31 outbreaks. The overall attack and mortality rates were 39% and 32%, respectively. The main clinical signs were high fever (67%), incoordination of movements (54%), and prostration (52%). Seventy-three percent of the herds had not been vaccinated against CSF and 17% had been only partially vaccinated. A spatio-temporal analysis, using a Poisson regression model, revealed two clusters with high risk; the first and largest one from 2014 to 2016 had a relative risk (RR) of 13.4 and included part of the departments of Atlántico, Bolívar, Cesar, La Guajira, Norte de Santander, Magdalena and Sucre; and the second cluster (RR = 9.6 in 2016) included municipalities in the north of the department of Cordoba.

Serodynamic Analysis of the Piglets Born from Sows Vaccinated with Modified Live Vaccine or E2 Subunit Vaccine for Classical Swine Fever

Classical swine fever (CSF) caused by the CSF virus (CSFV) is one of the most important swine diseases, resulting in huge economic losses to the pig industry worldwide. Systematic vaccination is one of the most effective strategies for the prevention and control of this disease. Two main CSFV vaccines, the modified live vaccine (MLV) and the subunit E2 vaccine, are recommended. In Taiwan, CSF cases have not been reported since 2006, although systemic vaccination has been practiced for 70 years. Here, we examined the sero-dynamics of the piglets born from sows that received either the CSFV MLV or the E2 vaccine and investigated in the field the correlation between the porcine reproductive and respiratory syndrome virus (PRRSV) loads and levels of CSFV antibody. A total of 1398 serum samples from 42 PRRSV-positive farms were evaluated to determine the PRRSV loads by real-time PCR and to detect CSFV antibody levels by commercial ELISA. Upon comparing the two sow vaccination protocols (CSFV MLV vaccination at 4 weeks post-farrowing versus E2 vaccination at 4-5 weeks pre-farrowing), the lowest levels of CSFV antibody were found in piglets at 5-8 and 9-12 weeks of age for the MLV and E2 groups, respectively. Meanwhile, the appropriate time window for CSFV vaccination of offspring was at 5-8 and 9-12 weeks of age in the MLV and E2 groups, respectively. There was a very highly significant negative correlation between the PRRSV load and the level of CSFV antibody in the CSFV MLV vaccination group (P < 0.0001). The PRRSV detection rate in the pigs from the MLV group (27.78%) was significantly higher than that in pigs from the E2 group (21.32%) (P = 0.011). In addition, there was a significant difference (P = 0.019) in the PRRSV detection rate at 5-8 weeks of age between the MLV (42.15%) and E2 groups (29.79%). Our findings indicate that the vaccination of CSFV MLV in piglets during the PRRSV susceptibility period at 5-8 weeks of age may be overloading the piglet's immune system and should be a critical concern for industrial pork production in the field.

Identification of E2 with improved secretion and immunogenicity against CSFV in piglets

BACKGROUND

Outbreaks of Classical swine fever virus (CSFV) cause significant economic losses in the swine industry. Vaccination is the major method to prevent and control the disease. As live attenuated vaccines fail to elicit differentiable immunity between infected and vaccinated animals, subunit vaccine was considered as an alternative candidate to prevent and eradicate CSFV. Subunit vaccines present advantages in DIVA immunogenicity and safety. The technology was limited due to the low yield and the high cost with multiple and large doses. The native E2 signal peptide has not been well defined before. Here, the aim of this study is to develop a cost-effective and efficacious E2 vaccine candidate against CSFV with signal peptide and E2 sequence selection.

RESULTS

A novel CSFV E2 sequence (E2ZJ) was identified from an epidemic strain of Zhejiang for outstanding secretion in baculovirus and enhanced immunogenicity. E2 secretion induced with the selected signal peptide, SPZJ (SP23), increase at least 50% as compared to any other signal peptides tested. Besides, unique antigenic features were identified in E2ZJ. As indicated with immunized sera in IFA against CSFV infection, E2ZJ elicited CSFV antibodies at the earlier stage than other E2 types tested in mice. Moreover, higher level of neutralizing and CSFV antibodies against CSFV with E2ZJ was detected than other E2s with the same dosage at 28 dpi. Further, E2ZJ successfully elicited neutralizing immunity in piglets. A single dose of 5 μg of E2ZJ was sufficient to induce protective antibodies against CSFV in piglets and provided 100% protection against lethal virus challenge.

CONCLUSIONS

Our studies provide evidence that E2ZJ guided by a novel E2 signal peptide (SPZJ) was efficiently secreted and presented significantly improved immunogenicity than conventional E2 vaccines. Moreover, a single dose of 5 μg E2ZJ is efficacious against CSFV in piglets.

A neutralizing monoclonal antibody-based competitive ELISA for classical swine fever C-strain post-vaccination monitoring

Background

Virus neutralization test (VNT) is widely used for serological survey of classical swine fever (CSF) and efficacy evaluation of CSF vaccines. However, VNT is a time consuming procedure that requires cell culture and live virus manipulation. C-strain CSF vaccine is the most frequently used vaccine for CSF control and prevention. In this study, we presented a neutralizing monoclonal antibody (mAb) based competitive enzyme-linked immunosorbent assay (cELISA) with the emphasis on the replacement of VNT for C-strain post–vaccination monitoring.

Results

One monoclonal antibody (6B211) which has potent neutralizing activity against C-strain was generated. A novel cELISA was established and optimized based on the strategy that 6B211 can compete with C-strain induced neutralizing antibodies in pig serum to bind capture antigen C-strain E2. By testing C-strain VNT negative pig sera (n = 445) and C-strain VNT positive pig sera (n = 70), the 6B211 based cELISA showed 100% sensitivity (95% confidence interval: 94.87 to 100%) and 100% specificity (95% confidence interval: 100 to 100%). The C-strain antibody can be tested in pigs as early as 7 days post vaccination with the cELISA. By testing pig sera (n = 139) in parallel, the cELISA showed excellent agreement (Kappa = 0.957) with VNT. The inhibition rate of serum samples in the cELISA is highly correlated with their titers in VNT (r² = 0.903, p < 0.001). In addition, intra- and inter-assays of the cELISA exhibited acceptable repeatability with low coefficient of variations (CVs).

Conclusions

This novel cELISA demonstrated excellent agreement and high level correlation with VNT. It is a reliable tool for sero-monitoring of C-strain vaccination campaign because it is a rapid, simple, safe and cost effective assay that can be used to monitor vaccination-induced immune response at the population level.

Generation and Immunogenicity of a Recombinant Pseudorabies Virus Co-Expressing Classical Swine Fever Virus E2 Protein and Porcine Circovirus Type 2 Capsid Protein Based on Fosmid Library Platform

Pseudorabies (PR), classical swine fever (CSF), and porcine circovirus type 2 (PCV2)-associated disease (PCVAD) are economically important infectious diseases of pigs. Co-infections of these diseases often occur in the field, posing significant threat to the swine industry worldwide. gE/gI/TK-gene-deleted vaccines are safe and capable of providing full protection against PR. Classical swine fever virus (CSFV) E2 glycoprotein is mainly used in the development of CSF vaccines. PCV2 capsid (Cap) protein is the major antigen targeted for developing PCV2 subunit vaccines. Multivalent vaccines, and especially virus-vectored vaccines expressing foreign proteins, are attractive strategies to fight co-infections for various swine diseases. The gene-deleted pseudorabies virus (PRV) can be used to develop promising and economical multivalent live virus-vectored vaccines. Herein, we constructed a gE/gI/TK-gene-deleted PRV co-expressing E2 of CSFV and Cap of PCV2 by fosmid library platform established for PRV, and the expression of E2 and Cap proteins was confirmed using immunofluorescence assay and western blotting. The recombinant virus propagated in porcine kidney 15 (PK-15) cells for 20 passages was genetically stable. The evaluation results in rabbits and pigs demonstrate that rPRVTJ-delgE/gI/TK-E2-Cap elicited detectable anti-PRV antibodies, but not anti-PCV2 or anti-CSFV antibodies. These findings provide insights that rPRVTJ-delgE/gI/TK-E2-Cap needs to be optimally engineered as a promising trivalent vaccine candidate against PRV, PCV2 and CSFV co-infections in future.

Recombinant pseudorabies virus expressing E2 of classical swine fever virus (CSFV) protects against both virulent pseudorabies virus and CSFV

Both classical swine fever (CSF) and pseudorabies are highly contagious, economically significant diseases of swine in China. Although vaccination with the C-strain against classical swine fever virus (CSFV) is widely carried out and severe outbreaks of CSF seldom occur in China, CSF is sporadic in many pig herds and novel sub-subgenotypes of CSFV endlessly emerge. Thus, new measures are needed to eradicate CSFV from Chinese farms. The emergence of a pseudorabies virus (PRV) variant also posed a new challenge for the control of swine pseudorabies. Here, the recombinant PRV strain JS-2012-ΔgE/gI-E2 expressing E2 protein of CSFV was developed by inserting the E2 expression cassette into the intergenic region between the gG and gD genes of the gE/gI-deletion PRV variant strain JS-2012-ΔgE/gI. The recombinant virus was stable when passaged in vitro. A single vaccination of JS-2012-ΔgE/gI-E2 via intramuscular injection fully protected against lethal challenges of PRV and CSFV. Vaccination of piglets with the recombinant JS-2012-ΔgE/gI-E2 in the presence of high levels of maternally derived antibodies (Abs) to PRV can provide partial protection against lethal challenge of CSFV. Vaccination of the recombinant PRV JS-2012-ΔgE/gI-E2 strain did not induce the production of Abs to the gE protein of PRV or to the CSFV proteins other than E2. Thus, JS-2012-ΔgE/gI-E2 appears to be a promising recombinant marker vaccine candidate against PRV and CSFV for the control and eradication of the PRV variant and CSFV.