Comparison of the protective efficacy of recombinant pseudorabies viruses against pseudorabies and classical swine fever in pigs; influence of different promoters on gene expression and on protection

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.

Safety and immunogenicity of a gE/gI/TK gene-deleted pseudorabies virus variant expressing the E2 protein of classical swine fever virus in pigs

Classical swine fever (CSF) and pseudorabies (PR) are both major infectious diseases of pigs, causing enormous economic losses to the swine industry in many countries. A marker vaccine that enables differentiation of infected from vaccinated animals (DIVA) is highly desirable for control and eradication of these two diseases in endemic areas. Since late 2011, PR outbreaks have been frequently reported in many Bartha-K61-vaccinated pig farms in China. It has been demonstrated that a pseudorabies virus (PRV) variant with altered antigenicity and increased pathogenicity was responsible for the outbreaks. Previously, we showed that rPRVTJ-delgE/gI/TK, a gE/gI/TK-deleted PRV variant, was safe for susceptible animals and provided a complete protection against lethal PRV variant challenge, indicating that rPRVTJ-delgE/gI/TK can be used as an attractive vaccine vector. To develop a safe bivalent vaccine against CSF and PR, we generated a recombinant virus rPRVTJ-delgE/gI/TK-E2 expressing the E2 protein of classical swine fever virus (CSFV) based on rPRVTJ-delgE/gI/TK and evaluated its safety and immunogenicity in pigs. The results indicated that pigs (n=5) immunized with rPRVTJ-delgE/gI/TK-E2 of different doses did not exhibit clinical signs or viral shedding following immunization, the immunized pigs produced anti-PRV or anti-CSFV neutralizing antibodies and the pigs immunized with 10(6) or 10(5) TCID50 rPRVTJ-delgE/gI/TK-E2 were completely protected against the lethal challenge with either CSFV Shimen strain or variant PRV TJ strain. These findings suggest that rPRVTJ-delgE/gI/TK-E2 is a promising bivalent DIVA vaccine candidate against CSFV and PRV coinfections.

Construction and immune efficacy of recombinant pseudorabies virus expressing PrM-E proteins of Japanese encephalitis virus genotype І

Background

Japanese encephalitis (JE) is an arboviral disease with high case fatality rates and neurologic or psychiatric sequelae among survivors in Asia, western Pacific countries and northern Australia. Japanese encephalitis virus (JEV) is the cause of JE and the emergence of genotype І (GI) JEV has displaced genotype III (GIII) as the dominant strains circulating in some Asian regions. The currently available JE vaccines are safe and effective in preventing this disease, but they are developed based on the GIII JEV strains.

Methods

The recombinant virus PRV TK⁻/gE⁻/PrM-E⁺ which expressed the premembrane (prM) and envelope (E) proteins of JEV SX09S-01 strain (genotype I, GI) was constructed by homologous recombination between the genome of PRV TK⁻/gE⁻/LacZ⁺ digested with EcoRI and plasmid pIE-CAG-PrM-E-BGH. Expression of JEV PrM and E proteins was analyzed by Western blot analysis. Immune efficacy of PRV TK⁻/gE⁻/PrM-E⁺ was further evaluated in mouse model.

Results

A recombinant pseudorabies virus (PRV TK⁻/gE⁻/PrM-E⁺) was successfully constructed. Mice experiments showed that PRV TK⁻/gE⁻/PrM-E⁺ could induce a high level of ELISA antibodies against PRV and JEV, as well as high titer of PRV neutralizing antibodies. After challenge with 1 × 10⁷ PFU virulent JEV SX09S-01 strain, the time of death was delayed and the survival rate was improved in PRV TK⁻/gE⁻/PrM-E⁺ vaccinated mice.

Conclusions

PRV TK⁻/gE⁻/PrM-E⁺ is a potential vaccine candidate against PRV and JEV GI infection in the future.

Classical swine fever in pigs: recent developments and future perspectives

Classical swine fever (CSF) is one of the most devastating epizootic diseases of pigs, causing high morbidity and mortality worldwide. The diversity of clinical signs and similarity in disease manifestations to other diseases make CSF difficult to diagnose with certainty. The disease is further complicated by the presence of a number of different strains belonging to three phylogenetic groups. Advanced diagnostic techniques allow detection of antigens or antibodies in clinical samples, leading to implementation of proper and effective control programs. Polymerase chain reaction (PCR)-based methods, including portable real-time PCR, provide diagnosis in a few hours with precision and accuracy, even at the point of care. The disease is controlled by following a stamping out policy in countries where vaccination is not practiced, whereas immunization with live attenuated vaccines containing the 'C' strain is effectively used to control the disease in endemic countries. To overcome the problem of differentiation of infected from vaccinated animals, different types of marker vaccines, with variable degrees of efficacy, along with companion diagnostic assays have been developed and may be useful in controlling and even eradicating the disease in the foreseeable future. The present review aims to provide an overview and status of CSF as a whole with special reference to swine husbandry in India.

An overview of live attenuated recombinant pseudorabies viruses for use as novel vaccines

Pseudorabies virus (PRV) is a double-stranded, DNA-based swine virus with a genome approximating 150 kb in size. PRV has many nonessential genes which can be replaced with genes encoding heterologous antigens but without deleterious effects on virus propagation. Recombinant PRVs expressing both native and foreign antigens are able to stimulate immune responses. In this paper, we review the current status of live attenuated recombinant PRVs and live PRV-based vector vaccines with potential for controlling viral infections in animals.

Vectored vaccines to protect against PRRSV

PRRSV is the causative agent of the most important infectious disease affecting swine herds worldwide, producing great economic losses. Commercially available vaccines are only partially effective in protection against PRRSV. Moreover, modified live vaccines may allow virus shedding, and could revert generating virulent phenotypes. Therefore, new efficient vaccines are required. Vaccines based on recombinant virus genomes (virus vectored vaccines) against PRRSV could represent a safe alternative for the generation of modified live vaccines. In this paper, current vectored vaccines to protect against PRRSV are revised, including those based on pseudorabies virus, poxvirus, adenovirus, and virus replicons. Special attention has been provided to the use of transmissible gastroenteritis virus (TGEV) as vector for the expression of PRRSV antigens. This vector has the capability of expressing high levels of heterologous genes, is a potent interferon-α inducer, and presents antigens in mucosal surfaces, eliciting both secretory and systemic immunity. A TGEV derived vector (rTGEV) was generated, expressing PRRSV wild type or modified GP5 and M proteins, described as the main inducers of neutralizing antibodies and cellular immune response, respectively. Protection experiments showed that vaccinated animals developed a faster and stronger humoral immune response than the non-vaccinated ones. Partial protection in challenged animals was observed, as vaccinated pigs showed decreased lung damage when compared with the non-vaccinated ones. Nevertheless, the level of neutralizing antibodies was low, what may explain the limited protection observed. Several strategies are proposed to improve current rTGEV vectors expressing PRRSV antigens.

Induction of protective immunity in swine by immunization with live attenuated recombinant pseudorabies virus expressing the capsid precursor encoding regions of foot-and-mouth disease virus

Foot-and-mouth disease (FMD) causes morbidity to livestock and serious economic consequences to its associated industry and therefore it is necessary to develop a safe and efficient vaccine to prevent or control this disease. A recombinant live attenuated virus vaccine, designated PRV-P1, was generated by insertion of an expression cassette containing CMV promoter, FMDV P1 gene and SV 40 poly-A into the gG gene region of a live attenuated pseudorabies virus vaccine strain (TK-/gG-/LacZ+). To determine the induction of protective immunity, 16 FMDV and PRV seronegative white swine were randomly divided into four groups and immunized intramuscularly. The parental virus (TK-/gG-/LacZ+) was injected into three pigs, the recombinant virus PRV-P1 into five pigs and commercial FMD-inactivated vaccine into five pigs, with PBS (negative control) into three pigs. All animals were immunized again 4 weeks later to boost the immune response and challenged with virulent type O FMDV O/ES/2001 strain 4 weeks after the second immunization. Results showed PRV-P1 vaccinated pigs induced high-level neutralizing antibody response to both FMDV and PRV, and strong CTL response against FMD antigen activation. Three of five pigs were completely protected against challenge with FMDV, one pig minimally protected and the other one had increased protection but not complete. However, one pig vaccinated with commercial FMD vaccine developed constant pyrexia. Average levels of antibodies against non-structural 3ABC proteins were significantly lower and efficacy on inhibition of FMDV replication was much increased in swine vaccinated with PRV-P1 than those immunized with commercial FMD vaccine after FMDV challenge. Our results showed that the recombinant PRV-P1 can induce not only humoral and cell-mediated immune responses but also partial protection against FMDV challenge, making it a good candidate for future development of the FMD vaccine.

Novel marker vaccines against classical swine fever

Classical swine fever (CSF) is one of the most devastating epizootic diseases of pigs worldwide. For eradication and control purposes, CSF vaccination is an important tool, and efficacious and safe attenuated vaccines have been available for many decades (for example, the C-strain vaccines). In addition to administering them parenterally, live attenuated vaccines are also administered orally for the control and eradication of CSF in wild boar populations. However, antibodies against live attenuated vaccines do not allow to differentiate infected from vaccinated animals (DIVA principle) and the mechanism responsible for attenuation is not known. Only a few years ago the first DIVA vaccines based on baculovirus-expressed E2 glycoprotein have been put on the market [Hulst MM, Westra DF, Wensvoort G, Moormann RJ. Glycoprotein E1 of hog cholera virus expressed in insect cells protects swine from hog cholera. J Virol 1993;67(9):5435-42]. However, these subunit E2 marker vaccines are less efficient and more than one parenteral application is necessary. Furthermore, oral vaccination is not possible. Taking these disadvantages into account, the development of novel CSF vaccines has been focussed on five different strategies, mainly based on genetically engineered constructs: (1) immunogenic CSFV peptides, (2) DNA vaccines, (3) viral vectors expressing CSFV proteins, (4) chimeric pestiviruses, and (5) trans-complemented deleted CSFV genomes (replicons).

Marker vaccine strategies and candidate CSFV marker vaccines

Classical swine fever (CSF) is an economically important highly contagious disease of swine worldwide. Classical swine fever virus (CSFV) is its etiological agent, and the only natural hosts are domestic pigs and wild boars. Although field CSFV strains vary in the virulence, they all result in serious losses in pig industry. Highly virulent field strains generally cause acute disease and high mortality; moderately virulent field strains raise subacute or chronic infections; postnatal infection by low virulent field strains produces subclinical infection and mortality in the new-born piglets. CSFV can cross the placental barrier, and this transplacental transmission usually results in mortality of fetuses and birth of congenitally infected pigs with a late-onset disease and death. Two main strategies to control CSF epidemic are systematic prophylactic vaccination with live attenuated vaccines (such as C-strain) and non-vaccination stamping-out policy. But neither of them is satisfying enough. Marker vaccine and companion serological diagnostic test is thought to be a promising strategy for future control and eradication of CSF. During the past 15 years, various candidate marker vaccines were constructed and evaluated in the animal experiments, including recombinant chimeric vaccines, recombinant deletion vaccines, DNA vaccines, subunit vaccines and peptide vaccines. Among them, two subunit vaccines entered the large scale marker vaccine trial of EU in 1999. Although they failed to fulfil all the demands of the Scientific Veterinary Committee, they successfully induced solid immunity against CSFV in the vaccinated pigs. It can be expected that new potent marker vaccines might be commercially available and used in systematic prophylactic vaccination campaign or emergency vaccination in the next 15 years. Here, we summarized current strategies and candidate CSFV marker vaccines. These strategies and methods are also helpful for the development of new-generation vaccines against other diseases.

Immunogenicity and protective efficacy of recombinant pseudorabies virus expressing the two major membrane-associated proteins of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) infection still remains today as the most significant health threat to swine and poses a challenge to current vaccination strategies. To develop a new generation of vaccine against PRRSV, a live attenuated pseudorabies virus (PRV) was used as vaccine vector to express the two major membrane-associated proteins (GP5 or M) of PRRSV in various forms. Four PRV recombinants, rPRV-GP5 (expressing native GP5), rPRV-GP5m (expressing GP5m, a modified GP5), rPRV-GP5-M (co-expressing GP5 and M proteins), rPRV-GP5m-M (co-expressing GP5m and M proteins) were generated. Mouse immunized with all these recombinants developed comparable PRV-specific humoral immune responses and provided complete protection against a lethal PRV challenge. However, the highest level of PRRSV-specific neutralizing antibodies and lymphocyte proliferative responses was observed in mice immunized with rPRV-GP5m-M. The immunogenicity and protective efficiency of rPRV-GP5m-M were further evaluated in the piglets. Compared to commercial PRRSV killed vaccine, detectable PRRSV-specific neutralizing antibody and higher lymphocyte proliferative responses could be developed in piglets immunized with rPRV-GP5m-M before virus challenge. Furthermore, more efficient protection against a PRRSV challenge was obtained in piglets immunized with rPRV-GP5m-M, as showed by the balanced body-temperature fluctuation, shorter-term viremia, lower proportion of virus load in nasal and oropharyngeal scrapings and tissues, and milder lung lesions. These data indicate that the recombinant rPRV-GP5m-M is a promising candidate bivalent vaccine against both PRV and PRRSV infection.

A recombinant pseudorabies virus encoding the HA gene from H3N2 subtype swine influenza virus protects mice from virulent challenge

The hemagglutinin (HA) gene of A/Swine/Inner Mogolian/547/2001 (H3N2) swine influenza virus (SIV) was recombined into the genome of pseudorabies virus (PRV) Bartha-K61 vaccine strain, generating a recombinant PRV expressing the HA gene, designated as rPRV-HA. One group of 15 mice was inoculated intranasally (i.n.) with 10(5.0) PFU of rPRV-HA, and another two control groups of mice (15 mice per group) were mock-inoculated or inoculated with Bartha-K61. Mice inoculated with rPRV-HA developed hemagglutination inhibition antibodies 3 weeks post-inoculation. Twenty-eight days post-inoculation, all mice were challenged i.n. with 10(5.0) TCID50 of A/Swine/Heilongjiang/74/2000 (H3N2). No challenge virus was isolated from vaccinated mice, and mild pathological lesions were observed only in lungs following challenge. The results demonstrate that the recombinant rPRV-HA expressing the HA gene from H3N2 SIV can protect mice from heterologous virulent challenge, and may represent a candidate vaccine against SIV.

Protective immunity induced by a recombinant pseudorabies virus expressing the GP5 of porcine reproductive and respiratory syndrome virus in piglets

Pseudorabies virus (PRV) has been developed as a vaccine vector for expressing foreign immunogens. Porcine reproductive and respiratory syndrome (PRRS), caused by porcine reproductive and respiratory syndrome virus (PRRSV), continues to be a major problem to the pork industry worldwide. Many vaccine strategies have been developed to control the disease but most of them turn out to be unsuccessful. The objective of this research was to explore the feasibility of PRV-based vector vaccine in protection against PRRSV. A live attenuated vaccine-based PRV recombinant expressing the envelope protein GP5 of PRRSV was generated using recombinant DNA techniques. The Bartha-K61-derived recombinant virus, named rPRV-GP5, was shown to express PRRSV GP5 efficiently. Sixteen healthy piglets were assigned to one of four groups (one to four, four pigs per group). Animals in Groups 1 and 2 were each inoculated intramuscularly and intranasally with 10(7.0) PFU of rPRV-GP5 and its parent Bartha-K61, respectively; Group 3 were vaccinated intramuscularly with one-dose of PRRS inactivated vaccine; Group 4 was served as non-vaccinated control. One month later, all animals were all challenged with 10(6.5) TCID(50) of virulent PRRSV CH-1a. All animals in Groups 1 and 3 remained clinically healthy before and after challenge, with only a short period of fever (no more than 41 degrees C and 3 days), mild and gradually improving lung and kidney lesions, and short-term viremia (2 and 3 week, respectively) in spite of no detectable anti-PRRSV antibody before challenge. On the other hand, all animals in the other two groups showed evident clinical signs with higher temperatures (more than 41 degrees C) after challenge, and severe lung, kidney and spleen lesions and extended viremia (4 weeks). The results indicate that the rPRV-GP5 is safe for vaccinates and able to confer significant protection against clinical disease and reduce pathogenic lesions induced by PRRSV challenge in vaccinated pigs.

An approach to a FMD vaccine based on genetic engineered attenuated pseudorabies virus: one experiment using VP1 gene alone generates an antibody responds on FMD and pseudorabies in swine

Foot-and-mouth disease (FMD) and pseudorabies (PR) are two important infectious diseases in swine. An attenuated pseudorabies virus (PRV) has been successfully used as a gene delivery vector for the development of live-viral vaccines. In this study, a recombinant PRV-VP1 virus was constructed by fusioning the VP1 gene of FMD virus in frame to the N-terminal sequence of the gG gene of PRV. To test the protective immunity, 15 FMDV sero-negative white swine were divided into three groups and immunized with the recombinant PRV-VP1 virus, commercial FMD vaccine and vector virus (TK(-)/gG(-)/LacZ(+)), respectively, and challenged intramuscularly with 20 minimal infecting doses (MID) of virulent type O FMDV 4 weeks after booster immunization. Swine vaccinated with PRV-VP1 acquired antibodies against both FMDV and PRV, however, anti-FMDV antibodies were much lower than those vaccinated with the commercial FMD vaccine. Our results suggested that the recombinant PRV-VP1 virus, which only expressed FMDV VP1 gene controlled by PRV gG promoter, could not protect swine from the challenge of 20 MID type O FMDV, but could delay and reduce the clinical symptoms of FMD.

Identification of T-cell epitopes in the structural and non-structural proteins of classical swine fever virus

To identify new T-cell epitopes of classical swine fever virus (CSFV), 573 overlapping, synthetic pentadecapeptides spanning 82% of the CSFV (strain Glentorf) genome sequence were synthesized and screened. In proliferation assays, 26 peptides distributed throughout the CSFV viral protein sequences were able to induce specific T-cell responses in PBMCs from a CSFV-Glentorf-infected d/d haplotype pig. Of these 26 peptides, 18 were also recognized by PBMCs from a CSFV-Alfort/187-infected d/d haplotype pig. In further experiments, it could be shown that peptide 290 (KHKVRNEVMVHWFDD), which corresponds to amino acid residues 1446-1460 of the CSFV non-structural protein NS2-3 could induce interferon-gamma secretion after secondary in vitro restimulation. The major histocompatibility complex (MHC) restriction for stimulation of T-cells by this pentadecapeptide was identified as being mainly MHC class II and partially MHC class I. In cytolytic assays, CSFV-specific cytotoxic T-lymphocytes (CTLs) were able to lyse peptide 290-loaded target cells. These findings indicate the existence of a CSFV-specific helper T-cell epitope and a CTL epitope in this peptide.

Chimeric (marker) C-strain viruses induce clinical protection against virulent classical swine fever virus (CSFV) and reduce transmission of CSFV between vaccinated pigs

Two live recombinant vaccines (Flc9 and Flc11) against classical swine fever (CSF) were evaluated for their capacity to reduce transmission of virulent CSF virus (CSFV) among vaccinated pigs. In Flc9 the 5' terminal half of the E2 gene of the C-strain, a CSFV vaccine strain, was exchanged with the homologous gene of the bovine viral diarrhoea virus (BVDV) strain 5250, the E(rns) gene was exchanged likewise in the chimeric Flc11 virus. Both recombinant vaccines induce an antibody response in pigs that can be distinguished from that induced after a wild-type CSFV infection. Four experiments were performed to estimate the reproduction ratio R after different vaccination-challenge intervals. Each group consisted of ten pigs [specified pathogen free (SPF) pigs or conventional pigs] that were vaccinated once, intramuscularly, either with Flc9 or Flc11 virus or that were not vaccinated. Vaccinated and susceptible pigs were challenged intranasally with the virulent CSFV strain Brescia or Behring, 1, 2 or 4 weeks after vaccination. Whether contact-pigs became infected was determined using a CSFV specific E2 (Flc9) or E(rns) (FLc11) antibody ELISA. In the unvaccinated control groups, virus secretion started from day 2 to 4 after inoculation and all contact pigs became infected. Contact pigs became infected in the group of pigs (SPF or conventional) vaccinated once with Flc9 virus and challenged 1-, 2- or 4-weeks later. The estimates of the R in the groups challenged at 1-, 2- and 4-weeks after vaccination were 0.38, 0 and 0.75, respectively. Contact infected pigs were not detected (R=0) in any of the groups of pigs, vaccinated with Flc11, only SPF pigs were used. In order to achieve a statistical significance of R within the vaccinated groups each of the experiments has to be repeated at least once. The R of pigs vaccinated with Flc11 virus and challenged at 1- or 2-weeks after vaccination was however significantly lower that the reproduction ratio of the unvaccinated groups (P=0.013). The R of pigs vaccinated with Flc9 virus and challenged at 1 (conventional pigs) or 2 weeks (SPF pigs) after vaccination was significantly lower that the reproduction ratio of the unvaccinated groups (P=0.013). In conclusion, both chimeric viruses Flc9 and Flc11 provided good clinical protection against a challenge with virulent CSFV at 1 or 2 weeks after vaccination. Further experiments should be carried out to study more aspects of the efficacy of these recombinant viruses before they can be used as a marker vaccine under field circumstances.

Introduction to classical swine fever: virus, disease and control policy

Classical swine fever virus is a spherical enveloped particle of about 40-60 nm in diameter with a single stranded RNA genome of about 12,300 bases with positive polarity, classified as a pestivirus within the family Flaviviridae. Natural hosts are domestic and wild pigs. The virus causes one of the most severe diseases in pigs world wide with grave economic consequences. The clinical picture of classical swine fever is variable, depending on the age of the affected animals and viral virulence. The virus is well characterised and reliable laboratory diagnostic procedures are available. In many parts of the world live attenuated vaccines are being used as a safe and efficient prophylactic tool. However, in EU Member States and several other countries vaccination is prohibited and CSF is controlled by a strict stamping out policy. In order to overcome the disadvantages of conventional vaccination inactivated marker vaccines have been developed that enable the distinction between vaccinated and infected animals. Whether these vaccines will be accepted as an additional tool in the framework of the stamping out policy is not yet decided.

Recombinant viral vector vaccines for the veterinary use

Recently, genetically engineering using recombinant DNA techniques has been applied to design new viral vaccines in order to reduce some problems which present viral vaccines have. Up to now, many viruses have been investigated for development of recombinant attenuated vaccines or live viral vectors for delivery of foreign immunogenic antigens. In this review, we introduced three kind of viruses; herpesviruses, vaccinia viruses, and adenoviruses, which have best widely been studied as recombinant vaccines or delivery vaccines for the veterinary use.