Production and characterization of monoclonal antibodies directed against pseudorabies virus

Variations in glycoprotein B contribute to immunogenic difference between PRV variant JS-2012 and Bartha-K61

A newly emerged pseudorabies virus (PRV) variant has been identified in many Bartha-K61-vaccinated pig farms. This variant has caused great economic losses to the swine industry in China since 2011. Sequence analysis demonstrated that the gB gene of the emerging PRV variant JS-2012 had multiple variations compared with the vaccine strain Bartha-K61. In the study, a specific CRISPR/Cas9 system combined with homologous recombination was used to construct two recombinant viruses, BJB (Bartha-K61+JS-2012gB) and JBJ (JS-2012-ΔgE/gI+Bartha-K61gB), by interchanging the full-length gB genes between Bartha-K61 and JS-2012-ΔgE/gI. The two recombinant viruses showed similar characteristics in growth kinetics in vitro and similar pathogenicity in mice, as compared to their parental strains. Immunization of mice with inactivated BJB or JBJ followed by challenge of JS-2012 showed that BJB could increase protective efficacy to 80%, compared to only 40% protection by the parental Bartha-K61 strain. JBJ had a decreased protective efficacy of 65%, as compared to 90% protection by its parental JS-2012-ΔgE/gI strain. Exchange of the gB gene markedly altered the immunogenicity of the recombinant PRV. These data suggest that variations in gB might play an important role in the virulence of the reemergent PRV variant in China. Our results demonstrate the importance of gB in protective immunity and suggest that the recombinant virus BJB could be a promising vaccine candidate for eradication of the PRV variant.

Use of Staby(®) technology for development and production of DNA vaccines free of antibiotic resistance gene

The appearance of new viruses and the cost of developing certain vaccines require that new vaccination strategies now have to be developed. DNA vaccination seems to be a particularly promising method. For this application, plasmid DNA is injected into the subject (man or animal). This plasmid DNA encodes an antigen that will be expressed by the cells of the subject. In addition to the antigen, the plasmid also encodes a resistance to an antibiotic, which is used during the construction and production steps of the plasmid. However, regulatory agencies (FDA, USDA and EMA) recommend to avoid the use of antibiotics resistance genes. Delphi Genetics developed the Staby(®) technology to replace the antibiotic-resistance gene by a selection system that relies on two bacterial genes. These genes are small in size (approximately 200 to 300 bases each) and consequently encode two small proteins. They are naturally present in the genomes of bacteria and on plasmids. The technology is already used successfully for production of recombinant proteins to achieve higher yields and without the need of antibiotics. In the field of DNA vaccines, we have now the first data validating the innocuousness of this Staby(®) technology for eukaryotic cells and the feasibility of an industrial production of an antibiotic-free DNA vaccine. Moreover, as a proof of concept, mice have been successfully vaccinated with our antibiotic-free DNA vaccine against a deadly disease, pseudorabies (induced by Suid herpesvirus-1).

Induction of immune responses to glycoprotein gD of Aujeszky's disease virus with DNA immunization

In an attempt to produce a DNA vaccine to prevent Aujeszky's disease, the induction of immune responses against Aujeszky's disease virus (ADV) gD was investigated in mice. The plasmid was constructed by placing ADV gD gene downstream of murine cytomegalovirus immediate early promoter of expression vector pMYK, which was injected twice on the skin of mice by using a gene-gun. All mice showed neutralizing antibodies against ADV gD at 4 weeks after immunization. The induction of cytotoxic T lymphocytes and splenic natural killer cells was also observed at 6 weeks post immunization. These results indicate that ADV gD gene in the form of DNA vaccine may induce specific as well as non-specific immune responses in vivo.

A DNA vaccine coding for glycoprotein B of pseudorabies virus induces cell-mediated immunity in pigs and reduces virus excretion early after infection

Glycoproteins B (gB), gC and gD of pseudorabies virus (PRV) have been implicated as important antigens in protective immunity against PRV infection. As cell-mediated immunity plays a major role in this protective immunity, we determined the significance of these glycoproteins in the actual induction of cell-mediated immunity. We vaccinated pigs with plasmid DNA constructs coding for gB, gC or gD and challenged them with the virulent NIA-3 strain of pseudorabies virus. Vaccination with plasmid DNA coding for gB induced the strongest cell-mediated immune responses including cytotoxic T cell responses, whereas plasmid DNA coding for gD induced the strongest virus neutralising antibody responses. Interestingly, vaccination with gB-DNA reduced virus excretion early after challenge infection while vaccination with gC-DNA or gD-DNA did not.This is the first study to demonstrate that DNA vaccination induces cytotoxic T cell responses in pigs and that cell-mediated immunity induced by vaccination with gB-DNA is important for the reduction of virus excretion early after challenge infection.

Vaccination of pigs against pseudorabies virus with plasmid DNA encoding glycoprotein D

We analysed the ability of a plasmid carrying the gene encoding glycoprotein D (gD) of pseudorabies virus (PRV) to induce humoral and cell-mediated immune responses and assessed the protection provided by PRV-gD DNA vaccination against challenge infection with PRV. Immunization with plasmid PRV-gD induced neutralizing antibodies and lymphocyte proliferative responses both in mice and pigs. Moreover, when challenged with virulent PRV six weeks following the last immunization, PRV-gD DNA vaccinated pigs excreted virus for a significantly shorter period and showed less clinical symptoms than pigs vaccinated with a control plasmid. Thus, in the target animal, DNA vaccination with PRV-gD DNA induces protective immunity against challenge infection.

Vaccine-induced, pseudorabies virus-specific, extrathymic CD4+CD8+ memory T-helper cells in swine

Pseudorabies virus (PRV; suid herpesvirus 1) infection causes heavy economic losses in the pig industry. Therefore, vaccination with live attenuated viruses is practiced in many countries. This vaccination was demonstrated to induce extrathymic virus-specific memory CD4+CD8+ T lymphocytes. Due to their major histocompatibility complex (MHC) class II-restricted proliferation, it is generally believed that these T lymphocytes function as memory T-helper cells. To directly prove this hypothesis, 15-amino-acid, overlapping peptides of the viral glycoprotein gC were used for screening in proliferation assays with peripheral blood mononuclear cells of vaccinated d/d haplotype inbred pigs. In these experiments, two naturally processed T-cell epitopes (T1 and T2) which are MHC class II restricted were identified. It was shown that extrathymic CD4+CD8+ T cells are the T-lymphocyte subpopulation that responds to epitope T2. In addition, we were able to show that cytokine secretion can be induced in these T cells through recall with inactivated PRV and demonstrated that activated PRV-primed CD4+CD8+ T cells are able to induce PRV-specific immunoglobulin synthesis by PRV-primed, resting B cells. Taken together, these results demonstrate that the glycoprotein gC takes part in the priming of humoral anti-PRV memory responses. The experiments identified the first T-cell epitopes so far known to induce the generation of virus-specific CD4+CD8+ memory T lymphocytes and showed that CD4+CD8+ T cells are memory T-helper cells. Therefore, this study describes the generation of virus-specific CD4+CD8+ T cells, which is observed during vaccination, as a part of the potent humoral anti-PRV memory response induced by the vaccine.

Round table on control of Aujeszky's disease and vaccine development based on molecular biology

A summary is given on the 4 topics which were discussed during the round table and which represent current knowledge on the molecular biology of Aujeszky's disease (pseudorabies) virus. They include a review on 1. the genome and gene products of the virus; 2. the viral genes associated with virulence; 3. the immunological role of the viral gene products and 4. studies intended to compare the efficacy of several commercially available vaccines and to establish a possible correlation between antibodies against individual structural viral proteins and degree of protection. It was concluded that gI deleted vaccines appear to be the best choice for use in intensive vaccination programmes towards eradication of Aujeszky's disease virus. However, there remains a need for development of more potent vaccines which induce strong humoral and cell mediated immune responses and afford complete protection, virological protection included. It is often observed that live vaccine strains which are completely avirulent lose much capacity to replicate and spread within the vaccinated animal. It is, however, not excluded that a certain degree of dissemination may be needed to be fully efficacious. Loss of virulence may thus be accompanied by too much loss of immunogenicity. An improved genetic stability of live vaccine strains when they are obtained by genetic manipulation, possibly justifies a more widespread dissemination of the vaccine strain in the body compared to that with conventionally developed strains or compared to what is presently allowed.

Protection of mice and swine from pseudorabies virus conferred by vaccinia virus-based recombinants

Glycoproteins gp50, gII, and gIII of pseudorabies virus (PRV) were expressed either individually or in combination by vaccinia virus recombinants. In vitro analysis by immunoprecipitation and immunofluorescence demonstrated the expression of a gII protein of approximately 120 kDa that was proteolytically processed to the gIIb (67- to 74-kDa) and gIIc (58-kDa) mature protein species similar to those observed in PRV-infected cells. Additionally, the proper expression of the 90-kDa gIII and 50-kDa gp50 was observed. All three of these PRV-derived glycoproteins were detectable on the surface of vaccinia virus-PRV recombinant-infected cells. In vivo, mice were protected against a virulent PRV challenge after immunization with the PRV glycoprotein-expressing vaccinia virus recombinants. The coexpression of gII and gIII by a single vaccinia virus recombinant resulted in a significantly reduced vaccination dose required to protect mice against PRV challenge. Inoculation of piglets with the various vaccinia virus-PRV glycoprotein recombinants also resulted in protection against virulent PRV challenge as measured by weight gain. The simultaneous expression of gII and gp50 in swine resulted in a significantly enhanced level of protection as evaluated by weight evolution following challenge with live PRV.

Correlation between gI, gII, gIII, and gp 50 antibodies and virus excretion in vaccinated pigs infected with pseudorabies virus

Eleven groups of 8 pigs were vaccinated with different vaccinal strains of pseudorabies virus deleted or not for non-essential glycoproteins (gI, gX, gp 63), then were challenged 8-9 weeks later with a virulent strain. Antibodies against the major viral glycoproteins (gII, gIII, gp 50) were titrated at the day of challenge. Excretion of the challenge strain, growth performance of the pigs, and gI antibody responses from pigs vaccinated with a gI minus strain, were monitored after the challenge. The results demonstrated a strong association between gII and gIII antibody titres before the challenge, with the amount of the challenge strain excreted and clinical protection after the challenge. Furthermore, gI antibody titres after the challenge were negatively correlated with the level of gII and gIII antibodies at the day of challenge, the virus excretion and the clinical protection after the challenge. Implications of these results for vaccination of pigs and gI antibody screening for the infection are discussed.

Immunogenicity in Aujeszky's disease virus structural glycoprotein gVI (gp50) in swine

Aujeszky's disease virus (ADV) envelope glycoprotein gVI (gp50) was purified from virus-infected Vero cells by ion-exchange and immunoaffinity chromatography and its usefulness as a subunit vaccine was evaluated in active and passive immunization studies. Four-week-old piglets were immunized intramuscularly (IM) with purified gVI twice two weeks apart and challenged intranasally (IN) 10 days after the second immunization with 30 LD50 (10(8)PFU) of a virulent strain of ADV. Pigs, vaccinated with 100 micrograms of purified gVI, produced virus neutralizing antibodies and did not develop clinical signs after challenge exposure. The challenge virus was not isolated from nasal swabs and tonsils of gVI-vaccinated pigs, whereas non-vaccinated control pigs developed illness after challenge exposure with the same virulent ADV strain which was later recovered from their nasal swabs and tonsils. Pregnant sows vaccinated twice with purified gVI (IM) at a three week interval produced virus neutralizing antibodies in colostrum. Four-day-old sucking piglets born of vaccinated sows were passively protected by colostral antibodies against intranasal challenge with a lethal dose of virulent ADV. Sera from gVI-vaccinated pigs were distinguished from experimentally infected swine sera by their differential reactivity in enzyme-linked immunosorbent assay (ELISA) using four major viral glycoproteins (excluding gVI) as antigen purified by the use of lentil-lectin.

Evaluation of glycoprotein gII ISCOMs subunit vaccine for pseudorabies in pig

One of the pseudorabies virus glycoproteins, gII, was integrated into Quil A matrix and formed an antigenic presentation structure known as immunostimulating complexes (ISCOMs). The gII ISCOMs protected mice from a lethal infection of pseudorabies virus (PrV). Subsequently, the efficacy of gII ISCOMs as a vaccine was evaluated in pigs, a natural host species of PrV. Immunization of pigs with gII ISCOMs protected them from PrV-induced acute symptoms and death. The serum obtained from immunized pigs was clearly discriminated from that of infected pigs by immunoprecipitation. The data indicated that the ISCOM technique was useful in PrV subunit vaccine preparation and demonstrated the importance of glycoprotein gII as a component of PrV vaccine.

Marker vaccines, virus protein-specific antibody assays and the control of Aujeszky's disease

Vaccination of pigs is widely practised to control Aujeszky's disease (AD). Molecular biological research revealed that several conventionally attenuated virus vaccines harbour deletions in their genomes. The deleted genes are nonessential for virus replication and can be involved in the expression of virulence. These findings have prompted several groups to construct well-characterized deletion mutants of AD virus that do not express either glycoprotein gI, gX or gIII. These mutants have also been rendered thymidine kinase negative. Although data on vaccine efficacy and safety have been published, widely varying test conditions have made it impossible to identify the most efficacious deletion mutant vaccine(s). Vaccination enhances the amount of virus required for infection and reduces, but does not prevent, the shedding of virulent virus and the establishment of latency in pigs infected with virulent AD virus. Therefore, while a vaccination programme will reduce the circulation of virus in the field, it will not eliminate AD virus from pig populations. To eradicate AD, the ability to differentiate infected from vaccinated pigs is crucial. The use of marker vaccines enables us to identify infected pigs in vaccinated populations by detecting antibodies against the protein whose gene is deleted from vaccine strains. The antibody response to gI appears to persist for more than 2 years, and all of about 300 field strains tested so far express gI. The use of vaccines lacking gI in combination with an enzyme linked immunosorbent assay to detect antibodies to gI and culling of gI-seropositive pigs, may help to eradicate AD in countries where vaccination is widely practised.

Mapping and characterization of neutralizing epitopes of glycoproteins gIII and gp50 of the Indiana-Funkhauser strain of pseudorabies virus

Neutralizing monoclonal antibodies directed against pseudorabies glycoproteins gIII or gp50 were produced. Using these antibodies in competitive binding ELISAs, three overlapping epitopes for gp50 and two nonoverlapping epitopes for gIII were mapped.

Pseudorabies virus glycoprotein gIII is a major target antigen for murine and swine virus-specific cytotoxic T lymphocytes

Pseudorabies virus (PrV) is the etiological agent of Aujeszky's disease, a disease that causes heavy economic losses in the swine industry. A rational approach to the generation of an effective vaccine against this virus requires an understanding of the immune response induced by it and of the role of the various viral antigens in inducing such a response. We have constructed mutants of PrV [strain PrV (Ka)] that differ from each other only in expression of the viral nonessential glycoproteins gI, gp63, gX, and gIII (i.e., are otherwise isogenic). These mutants were used to ascertain the importance of each of the nonessential glycoproteins in eliciting a PrV-specific cytotoxic T-lymphocyte (CTL) response in mice and pigs. Immunization of DBA/2 mice and pigs with a thymidine kinase-deficient (TK-) mutant of PrV elicits the formation of cytotoxic cells that specifically lyse syngeneic infected target cells. These PrV-specific cytolytic cells have the phenotype of major histocompatibility complex class I antigen-restricted CTLs. The relative number of CTLs specific for glycoproteins gI, gp63, gX, and gIII induced in mice vaccinated with a TK- mutant of PrV was ascertained by comparing their levels of cytotoxicity against syngeneic cells infected with either wild-type virus or gI-/gp63-, gX-, or gIII- virus deletion mutants. The PrV-specific CLTs were significantly less effective in lysing gIII(-)-infected targets than in lysing gI-/gp63-, gX-, or wild-type-infected targets. The in vitro secondary CTL response of lymphocytes obtained from either mice or pigs 6 or more weeks after immunization with a TK- mutant of PrV was also tested. Lymphocytes obtained from these animals were cultured with different glycoprotein-deficient mutants of PrV, and their cytolytic activities against wild-type-infected targets were ascertained. The importance of each of the nonessential viral glycoproteins in eliciting CTLs was assessed from the effectiveness of each of the virus mutants to stimulate the secondary anti-PrV CTL response. Cultures of both murine or swine lymphocytes that had been stimulated with gIII- virus contained only approximately half as many lytic units as did those stimulated with either wild-type virus, a gX- virus mutant, or a gI-/gp63- virus mutant. Thus, a large proportion of the PrV-specific CTLs that are induced by immunization with PrV of both mice and pigs are directed against gIII. Furthermore, glycoproteins gI, gp63, and gX play at most a minor role in the CTL response of these animals to PrV.

Biological evaluation of glycoproteins mapping to two distinct mRNAs within the BamHI fragment 7 of pseudorabies virus: expression of the coding regions by vaccinia virus

Several glycoproteins from the unique short region of pseudorabies have been identified and characterized. The genes encoding at least four glycoproteins (gp50, gp63, gl, and gX) are located within the BamHI fragment 7 of pseudorabies. S1 nuclease mapping was used to determine that a 2.4-kb mRNA encompasses the coding region for gp50 and gp63 and probably represents a colinear transcript for these proteins. Using the same technique, a 2.8-kb mRNA was found to encode gl. No other mRNAs were found to be encoded on the opposite strand of DNA in this region. Various recombinant vaccinia vectors were made incorporating the coding regions for these two mRNAs. Pseudorabies recombinant vaccinia infected ST cells expressed glycoproteins that co-migrated with the authentic PRV glycoproteins upon polyacrylamide electrophoresis. Intracranial or intraperitoneal inoculation of mice with the recombinant viruses constructed to contain the mRNA coding regions resulted in various degrees of protection from a lethal challenge of pseudorabies virus.

Iscom of viral envelope proteins protects against Aujeszky's disease

An immunostimulating complex (iscom) containing the envelope proteins of pseudorabies virus (PRV) was prepared and its efficacy was evaluated in two experiments on sheep. In the first experiment, sheep were intramuscularly (i.m.) or intradermally (i.d.) vaccinated with PRV iscom doses varying between 1 and 81 micrograms. The vaccination was repeated on Day 21 and the animals were exposed to challenge infection by subcutaneous inoculation of 1000 TCID50 of the virulent Phylaxia strain on Day 35 after first vaccination. In the second experiment, sheep were i.m. vaccinated with single doses of iscom varying between 1 and 27 micrograms and challenge-infected on Day 14. It was found that: (1) the i.d. administration of PRV iscom has no advantage over i.m. administration (2); a single dose of greater than or equal to 3 micrograms of PRV iscom provided protection against the disease. In immunoblots, viral proteins of molecular masses 120, 109, 55, 53 and 32 kDa were detected with the sera obtained from iscom-vaccinated and subsequently challenge-infected sheep, but not with sera from sheep which were iscom-vaccinated only. The above findings indicated that: (1) by using iscom technology, potent subunit vaccines can be prepared to prevent Aujeszky's disease; (2) the selective incorporation of viral envelope proteins into iscoms gives the opportunity to discriminate between iscom-vaccinated and naturally infected animals.

Identification of the pseudorabies virus glycoprotein gp50 as a major target of neutralizing antibodies

A total of 108 monoclonal antibodies specific for pseudorabies virus (PRV) were isolated in a cellular fusion, using spleen cells from mice which had been immunized with a live strain (Kojnok strain). Twelve of them neutralized the Kojnok strain in vitro in the absence of complement, as well as 28 virulent strains of various geographical origin and isolated from various animal species. All of the 12 clones were specific for glycoprotein gp50. Eighteen other clones with no neutralizing activity were studied: 11 reacted with glycoprotein GIII, 3 with glycoprotein GII, 3 with the glycoprotein gp63 and 1 with the glycoprotein GI. Transfer to mice of ascitic fluids corresponding to clones reacting with gp50 and GIII showed that some of them provided the mice with the ability of resisting to virulent challenge. Thus it appears that glycoproteins gp50 and GIII are major immunogens of the virion.

Evaluation of pseudorabies virus glycoprotein gp50 as a vaccine for Aujeszky's disease in mice and swine: expression by vaccinia virus and Chinese hamster ovary cells

Pseudorabies virus (PRV) is an alphaherpesvirus which causes an economically important disease of swine. One of the PRV glycoproteins, gp50, was previously identified as the sequence homolog of herpes simplex virus glycoprotein gD (E.A. Petrovskis, J.G. Timmins, M.A. Armentrout, C.C. Marchioli, R.J. Yancey, Jr., and L.E. Post, J. Virol. 59:216-223, 1986). gp50 was evaluated as a PRV subunit vaccine candidate. gp50 protected mice from PRV-induced mortality either when delivered via infection with a recombinant vaccinia virus or when administered as a subunit vaccine produced in a eucaryotic cell line, Chinese hamster ovary (CHO) cells. In addition, gp50 synthesized in CHO cells protected pigs from lethal infection with PRV. This result demonstrates that a single viral glycoprotein could induce a protective immune response in the natural host of a herpesvirus infection.

DNA sequence of the gene for pseudorabies virus gp50, a glycoprotein without N-linked glycosylation

The DNA sequence was determined for a region of the pseudorabies virus (PRV) genome to which a mutation defining resistance to a monoclonal antibody has been mapped (M. W. Wathen and L. M. K. Wathen, J. Virol., 51:57-62, 1984). This sequence was found to contain an open reading frame that did not include an amino acid sequence directing N-linked glycosylation. This open reading frame was expressed in uninfected Chinese hamster ovary cells to produce the PRV glycoprotein gp50. When PRV-infected Vero cells were incubated in the presence of tunicamycin, the gp50 that was produced had an identical molecular weight to that produced in the absence of drug. When infected cells were incubated in the presence of monensin, the molecular weight of gp50 was reduced from 60,000 to 45,000, but was not sensitive to endo-beta-N-acetylglucosaminidase H. These observations led to the conclusion that gp50 does not contain N-linked carbohydrate, as predicted from the DNA sequence. A region of the amino acid sequence and the positions of the cysteine residues of PRV gp50 are homologous to glycoprotein D of herpes simplex virus.

Characterization and mapping of a nonessential pseudorabies virus glycoprotein

Antigenic variants of pseudorabies virus (PRV) containing mutations in a viral glycoprotein with a molecular weight of 82,000 (gIII) were isolated by selecting for resistance to a complement-dependent neutralizing monoclonal antibody (MCA82-2) directed against gIII. These mutants were completely resistant to neutralization with MCA82-2 in the presence of complement. Two mutants selected for further studies either did not express gIII or expressed an improperly processed form of the glycoprotein. The mutations were also associated with an altered plaque morphology (syncytium formation). The gIII gene was mapped by marker rescue of a gIII- mutant with cloned restriction enzyme fragments to the long unique region of the PRV genome between 0.376 and 0.383 map units. This corresponds to the map location of a glycoprotein described by Robbins et al. (J. Mol. Appl. Gen. 2:485-496, 1984). Since gIII is nonessential for viral replication in cell culture and has several other characteristics in common with the herpes simplex virus glycoprotein gC, gIII may represent the PRV equivalent to herpes simplex virus gC.