Porcine circovirus type 2 (PCV2) infection decreases the efficacy of an attenuated classical swine fever virus (CSFV) vaccine

PRRSV infection inhibits CSFV C-strain replication via GSDMD-mediated pyroptosis

Classical swine fever virus (CSFV) and porcine productive and respiratory syndrome virus (PRRSV) both are significant infectious pathogens in pigs and pose great threats to the healthy development of the pig industry. PRRSV infection often reduces the antibody level of the CSFV attenuated vaccine and even leads to immune failure. In order to elucidate the potential mechanism of CSFV proliferation inhibition by PRRSV and screen out drugs that enhance the vaccine immune effect, we conducted experiments in the PAM39 cell line that can simultaneously support both PRRSV and CSFV infection. The results showed that PRRSV infection could induce gasdermin D (GSDMD) cleavage, promote cell pyroptosis, increase IL-1β secretion, and then inhibit CSFV replication. However, Astragalus polysaccharide treatment could reverse this phenomenon. The results elucidate the molecular mechanism of CSFV vaccine immune failure caused by PRRSV co-infection from the perspective of pyroptosis and provide a scientific basis for the prevention and control of clinical co-infection diseases.

Evaluation of Immune Status of Pigs against Classical Swine Fever for Three Years after the Initiation of Vaccination in Gifu Prefecture, Japan

In 2018, classical swine fever (CSF) reemerged in Gifu Prefecture, Japan, after 26 years of absence, and vaccination of domestic pigs using a live attenuated vaccine was initiated in 2019. Because the vaccine efficacy in piglets is influenced by the maternal antibody levels, vaccination should be administered at the optimal age by assuming the antibody level in sows. In this study, the shift in the antibody titer distribution in sows due to the initiation of vaccination to naïve herds and its influence on the vaccine-induced immunity rate in fattening pigs were investigated for 3 years. The results indicated that higher antibody titers were induced in first-generation sows after vaccine initiation because they were immunologically naïve, but the distribution of antibody titers shifted to lower levels along with their replacement with second-generation sows. The average vaccination age of fattening pigs became earlier year by year, and the vaccine-induced antibody rate was almost ≥80%. Based on the estimation of the optimal age for vaccination, it was found that vaccination at a younger age may reduce the risk of CSF infection. Taken together, the risk of CSF outbreaks can be reduced by administering vaccines at the optimal age based on the sequential monitoring of the sow's immune status.

Infection of PRRSV inhibits CSFV C-strain replication by inducing macrophages polarization to M1

It is a common sense that porcine reproductive and respiratory syndrome virus (PRRSV) infection could cause immune failure of classical swine fever (CSF) vaccine, and porcine alveolar macrophages (PAMs) are the target cells of both. To elucidate the role of macrophage polarization in PRRSV infection induced CSF vaccine failure, an immortal porcine alveolar macrophage line PAM39 cell line was used to investigate the effect of PRRSV or/and CSFV C-strain (CSFV-C) infection on macrophage polarization in vitro. Interestingly, PRRSV single infection or PRRSV co-infection with CSFV-C promoted PAM39 cells to M1, while CSFV-C single infection induced PAM39 cells to M2. After the construction of M1 and M2 PAM39 cells polarization models, M1 polarized PAM39 cells were found to inhibit the replication of CSFV-C, and Chinese medicine such as matrine, ginsenosides and astragalus polysaccharides could alleviate the polarization of PAM39 cells and the replication of CSFV-C. Furthermore, interferon (IFN)-γ and lipopolysaccharide (LPS) co-stimulation induced NF-κB activation while matrine treatment blocked M1 polarization-induced NF-κB pathway activation. These findings provided a theoretical basis for designing a new strategy to improve the immune effect of CSFV-C based on porcine alveolar macrophage polarization subtypes.

PRRSV inhibited the proliferation of CSFV by inducing IL-1β maturation via NLRP3 inflammasome activation

PRRSV and CSFV are both common infectious pathogens in porcine populations, posing significant threats to the healthy development of the porcine industry. Vaccine immunization is the main way to prevent and control these two diseases. Increasing studies have demonstrated that there is an interaction between PRRSV co-infection and CSFV vaccine immune failure. To investigate the effect of PRRSV infection on CSFV proliferation and its molecular mechanism, the proliferation dynamics of PRRSV/CSFV, the NLRP3 inflammasome components, and IL-1β expression levels were detected in PRRSV/CSFV alone- or co-infection. Subsequently, the relationship between inflammasome activation, IL-1β expression, and CSFV proliferation was analyzed through the construction of an inflammasome activation model, specific siRNA interference, and specific inhibitor treatment. The results showed that CSFV infection had a poor regulatory effect on NLRP3 inflammasome activation and IL-1β maturation, but PRRSV and CSFV co-infection could significantly up-regulate the expression of NLRP3 and ASC, induce Caspase-1 activation, and promote IL-1β maturation. It was further determined that NLRP3 inflammasome components played important roles in IL-1β maturation and inhibiting CSFV proliferation by PRRSV. Additional experiments indicated that PRRSV replication is essential for NLRP3 inflammasome activation, IL-1β maturation, and CSFV proliferation inhibition. More importantly, NLRP3 inflammasome activation is regulated by the TLR4-MyD88-NF-κB pathways. In conclusion, PRRSV infection induced IL-1β maturation by activating the NLRP3 inflammasome through the TLR4-MyD88-NF-κB pathways and then inhibited the proliferation of CSFV. These data further improved the theoretical basis for PRRSV inducing inflammatory factors and leading to the failure of CSFV immunization.

A prospective CSFV-PCV2 bivalent vaccine effectively protects against classical swine fever virus and porcine circovirus type 2 dual challenge and prevents horizontal transmission

Classical swine fever virus (CSFV) infection leading to CSF outbreaks is among the most devastating swine diseases in the pig industry. Porcine circovirus type 2 (PCV2) infection, resulting in porcine circovirus-associated disease (PCVAD), is also a highly contagious disease affecting pig health worldwide. To prevent and control disease occurrence, multiple-vaccine immunization is necessary in contaminated areas or countries. In this study, a novel CSFV-PCV2 bivalent vaccine was constructed and demonstrated to be capable of eliciting humoral and cellular immune responses against CSFV and PCV2, respectively. Moreover, a CSFV-PCV2 dual-challenge trial was conducted on specific-pathogen-free (SPF) pigs to evaluate vaccine efficacy. All of the vaccinated pigs survived and showed no clinical signs of infection throughout the experimental period. In contrast, placebo-vaccinated pigs exhibited severe clinical signs of infection and steeply increased viremia levels of CSFV and PCV2 after virus challenge. Additionally, neither clinical signs nor viral detections were noted in the sentinel pigs when cohabitated with vaccinated-challenged pigs at three days post-inoculation of CSFV, indicating that the CSFV-PCV2 bivalent vaccine completely prevents horizontal transmission of CSFV. Furthermore, conventional pigs were utilized to evaluate the application of the CSFV-PCV2 bivalent vaccine in field farms. An adequate CSFV antibody response and a significant decrease in PCV2 viral load in the peripheral lymph nodes were observed in immunized conventional pigs, suggesting its potential for clinical application. Overall, this study demonstrated that the CSFV-PCV2 bivalent vaccine effectively elicited protective immune responses and the ability to prevent horizontal transmission, which could be a prospective strategy for controlling both CSF and PCVAD in commercial herds.

Production and immunogenicity of a deoxyribonucleic acid Alphavirus vaccine expressing classical swine fever virus E2-Erns protein and porcine Circovirus Cap-Rep protein

Classical swine fever virus (CSFV) and porcine Circovirus type 2 (PCV2) are economically pivotal infectious disease viruses of swine. Alphaviral RNA replicon plasmids have been used as an important vector for constructing nucleic acid vaccines. Here, we aimed to construct a recombinant alphaviral plasmid vaccine pSCA1-E2-Erns-Cap-Rep for the prevention and control of CSFV and PCV2. Our results showed that the recombinant alphaviral plasmid vaccine pSCA1-E2-Erns-Cap-Rep was successfully constructed. The vaccine encoding E2 and Erns of CSFV, Cap, and Rep of PCV2 can induce E2, Erns, Cap, and Rep protein expression. ELISA analysis showed that mice-immunized pSCA1-E2-Erns-Cap-Rep plasmid vaccine produced higher anti-CSFV- and anti-PCV2-specific antibodies with dose- and time-dependent manners. Furthermore, neutralizing assays were measured using IF and ELISA methods. The results showed the production of neutralizing antibodies could neutralize CSFV (up to 2¹⁰.¹³) and PCV2 (2⁸.⁶) effectively, which exhibited the immune efficacy of the pSCA1-E2-Erns-Cap-Rep plasmid vaccine. Taken together, this pSCA1-E2-Erns-Cp-Rep plasmid vaccine could be considered a novel candidate vaccine against CSFV and PCV2.

The gC1qR Binding Site Mutant PCV2 Is a Potential Vaccine Strain That Does Not Impair Memory CD4+ T-Cell Generation by Vaccines

PCV2 has been reported to reduce the protective effects of various vaccines on immunized pigs. Our previous studies showed that the interaction of Cap and host protein gC1qR mediated the PCV2 infection-induced suppression of immune response. Thus, we wondered whether the gC1qR binding site mutant PCV2RmA could be a vaccine strain and whether this mutant PCV2RmA impairs other vaccines. Herein, we showed that PCV2 infection reduced the classic swine fever virus (CSFV) vaccine-induced generation of memory CD4⁺ T cells through the interaction of Cap with gC1qR. PCV2RmA can effectively induce the production of PCV2-specific antibodies, neutralizing antibodies, and peripheral blood lymphocyte proliferation in piglets at the same levels as the commercial inactivated PCV2 vaccine. The PCV2RmA-induced anti-PCV2 immune responses could eliminate the serum virus and would not lead to pathological lesions like wild-type PCV2. Moreover, compared to the commercial inactivated PCV2 vaccine, PCV2RmA is capable of inducing more durable protective immunity against PCV2 that induced production of PCV2-specific antibodies and neutralizing antibodies for a longer time via stronger induction of memory CD4⁺ T cells. Importantly, PCV2RmA infection did not impair the CSFV vaccine-induced generation of memory CD4⁺ T cells. Collectively, our findings showed that PCV2 infection impairs memory CD4⁺ T-cell generation to affect vaccination and provide evidence for the use of PCV2RmA as an efficient vaccine to prevent PCV2 infection. IMPORTANCE PCV2 is one of the costliest pathogens in pigs worldwide. Usage of PCV2 vaccines can prevent the PCV2 infection-induced clinical syndromes but not the viral spread. Our previous work found that PCV2 infection suppresses the host type I interferon innate immune response and CD4⁺ T-cell-mediated Th1 immune response through the interaction of Cap with host gC1qR. Here, we showed that the gC1qR binding site mutant PCV2RmA could effectively induce anti-PCV2 immunity and provide more durable protective immunity against wild-type PCV2 infection in pigs. PCV2RmA would not impair the generation of memory CD4⁺ T cells induced by classic swine fever virus (CSFV) vaccines as wild-type PCV2 did. Therefore, PCV2RmA can serve as a potential vaccine strain to better protect pigs against PCV2 infection.

Astragalus polysaccharide alleviated the inhibition of CSFV C-strain replication caused by PRRSV via the TLRs/NF‑κB/TNF-α pathways

It is a common phenomenon that PRRSV infection can interfere with the protective efficacy of the CSFV vaccine in clinical settings, and no effective treatment is available. In our previous study, we found that PRRSV infection could inhibit the replication of CSFV-C by promoting the high expression of inflammatory cytokines. In order to further investigate whether Chinese medicine could alleviate the inhibition effect, the PAM39 cells model, which was co-infected with PRRSV and CSFV-C, was established. The effects of Chinese medicine on this co-infection model, as well as the effect of astragalus polysaccharide on the TLRs/NF-κB/TNF-α pathways, were investigated. Our results demonstrated that PAM39 cells inoculated with different pathogenic PRRSV significantly inhibited the replication of CSFV-C and up-regulated the major inflammatory mediators, including TNF-α. For the following studies, 50 µM of astragalus polysaccharide was selected from six kinds of representative Chinese medicine based on their cytotoxicity, viral titers, and inflammatory mediators. Further experiments indicated that astragalus polysaccharide could alleviate the inhibition of CSFV-C replication in the co-infection group with no influence on cell viability. In addition, astragalus polysaccharide treatment clearly reduced P65 phosphorylation and down-regulated the expression of TLR7, TLR9, and TNF-α in co-infection group, implying that the TLRs/NF-κB/TNF-α pathways may play an important role in astragalus polysaccharide's anti-inflammatory response. In conclusion, astragalus polysaccharide treatment alleviated PRRSV-mediated inhibition of CSFV-C replication via the TLRs/NF-κB/TNF-α pathways, and the molecular mechanism of PRRSV co-infection leading to the failure of CSFV vaccine immunization was partially elucidated, providing a scientific basis for effective CSF prevention and control in pig farms.

A Triple Gene-Deleted Pseudorabies Virus-Vectored Subunit PCV2b and CSFV Vaccine Protects Pigs against PCV2b Challenge and Induces Serum Neutralizing Antibody Response against CSFV

Porcine circovirus type 2 (PCV2) is endemic worldwide. PCV2 causes immunosuppressive infection. Co-infection of pigs with other swine viruses, such as pseudorabies virus (PRV) and classical swine fever virus (CSFV), have fatal outcomes, causing the swine industry significant economic losses in many if not all pig-producing countries. Currently available inactivated/modified-live/vectored vaccines against PCV2/CSFV/PRV have safety and efficacy limitations. To address these shortcomings, we have constructed a triple gene (thymidine kinase, glycoprotein E [gE], and gG)-deleted (PRVtmv) vaccine vector expressing chimeric PCV2b-capsid, CSFV-E2, and chimeric E^rns-fused with bovine granulocytic monocyte-colony stimulating factor (E^rns-GM-CSF), designated as PRVtmv+, a trivalent vaccine. Here we compared this vaccine’s immunogenicity and protective efficacy in pigs against wild-type PCV2b challenge with that of the inactivated Zoetis Fostera Gold PCV commercial vaccine. The live PRVtmv+ prototype trivalent subunit vaccine is safe and highly attenuated in pigs. Based on PCV2b-specific neutralizing antibody titers, viremia, viral load in lymphoid tissues, fecal-virus shedding, and leukocyte/lymphocyte count, the PRVtmv+ yielded better protection for vaccinated pigs than the commercial vaccine after the PCV2b challenge. Additionally, the PRVtmv+ vaccinated pigs generated low to moderate levels of CSFV-specific neutralizing antibodies.

Molecular detection and seroprevalence of classical swine fever virus from 2016 to 2018 in pigs of Mizoram, India

Classical swine fever (CSF) is a fatal endemic disease of pig population of North eastern India in particular and India in general. Present study revealed molecular detection of CSFV and seroprevalence of the disease in pig population of Mizoram, India during 2016–2018. Serum samples from apparently healthy, unvaccinated pigs were collected in collaboration with the State Animal Husbandry and Veterinary Department, Mizoram and a total of 594 serum samples from 7 districts were subjected to detection of CSFV specific antibodies by indirect ELISA. A total of 206 (34.68%) serum samples were positive for CSFV antibodies by ELISA. District wise, Saiha district showed highest seroprevalence of the disease followed by Kolasib and Serchhip. Apart from this, during the same time period, CSFV suspected samples received in the Department of Veterinary Microbiology consisting of 269 serum samples, 10 whole blood and 83 tissue samples obtained from 8 districts of Mizoram were subjected to detection of NS5b and E2 mRNA transcripts by nRT-PCR of which a total of 42 (11.60%) samples including serum (5.58%), tissues (27.71%) and whole blood (40%) were positive for the NS5b and E2 mRNA transcripts, specific for CSFV. District wise analysis revealed that Aizawl has the highest percentage of positive samples of CSFV followed by Saiha and Lawngtlai district.

Assessment of the control measures of the category A diseases of Animal Health Law: Classical Swine Fever

EFSA received a mandate from the European Commission to assess the effectiveness of some of the control measures against diseases included in the Category A list according to Regulation (EU) 2016/429 on transmissible animal diseases ('Animal Health Law'). This opinion belongs to a series of opinions where these control measures will be assessed, with this opinion covering the assessment of control measures for Classical swine fever (CSF). In this opinion, EFSA and the AHAW Panel of experts review the effectiveness of: (i) clinical and laboratory sampling procedures, (ii) monitoring period and (iii) the minimum radii of the protection and surveillance zones, and the minimum length of time the measures should be applied in these zones. The general methodology used for this series of opinions has been published elsewhere; nonetheless, details of the model used for answering these questions are presented in this opinion as well as the transmission kernels used for the assessment of the minimum radius of the protection and surveillance zones. Several scenarios for which these control measures had to be assessed were designed and agreed prior to the start of the assessment. Here, several recommendations are given on how to increase the effectiveness of some of the sampling procedures. Based on the average length of the period between virus introduction and the reporting of a CSF suspicion, the monitoring period was assessed as non-effective. In a similar way, it was recommended that the length of the measures in the protection and surveillance zones were increased from 15 to 25 days in the protection zone and from 30 to 40 days in the surveillance zone. Finally, the analysis of existing Kernels for CSF suggested that the radius of the protection and the surveillance zones comprise 99% of the infections from an affected establishment if transmission occurred. Recommendations provided for each of the scenarios assessed aim to support the European Commission in the drafting of further pieces of legislation, as well as for plausible ad hoc requests in relation to CSF.

Sero-epidemiology of porcine parvovirus, circovirus, and classical swine fever virus infections in India

Reproductive problems in swine caused by porcine viruses pose a serious threat to the pig industry in developing countries like India. For evaluating the true extent of porcine infections, a total of 1308 representative sera samples were collected from 92 different pig farms covering 8 North-Eastern states and Punjab state of Northern India during a period of 2 years (2011-2013). Sera samples were tested for the presence of antibodies against porcine parvovirus (PPV), porcine circovirus-2 (PCV-2), and classical swine fever virus (CSFV) using commercial enzyme-linked immunosorbent assay (ELISA) kits. In the North-Eastern states, the seroprevalence of CSFV in non-vaccinated animals was 6.30% and that of PCV2 and PPV was 6.28% and 1.24%, respectively. In Punjab, the seroprevalence of CSFV in non-vaccinated animals was 44.44% and seroprevalence of PCV-2 and PPV was 34.07% and 39.10%, respectively. Detection of antibodies against more than one virus revealed that 4.66% animals had co-infection with PCV-2 and PPV, 1.75% with CSF and PPV, 1.98% with CSF and PCV-2, and 1.75% with all the three viruses. The receiver operator characteristics (ROC) curve analysis depicted that piglet mortality, parvovirus, and CSFV were the most important parameters with an AUC value of 0.997, 0.897, and 0.973, respectively. Incidence of single or co-infection with different viruses showed that the occurrence of single infection was significantly more prevalent than co-infection. This study provides useful information to set up future epidemiologic, flock management, and public animal health policies for the prevention and control of PCV-2, PPV, and CSF in India.

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.

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.

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.

The impact of porcine circovirus associated diseases on live attenuated classical swine fever vaccine in field farm applications

Porcine circovirus associated diseases (PCVADs) are among the most important diseases affecting the worldwide swine industry. Vaccination against porcine circovirus type 2 (PCV2) infection has been utilized for disease control and effectively reduces clinical signs of PCVADs. To evaluate the efficacy of the PCV2 vaccine in field farms, we conducted a trial using conventional pigs immunized with the subunit PCV2 vaccine followed by PCV2 challenge. Immunized pigs demonstrated lower serum viral loads, less viral antigen staining in lymph nodes, and higher average daily weight gain, confirming the protective efficacy of the vaccine. However, low levels of PCV2 infection were still detected in vaccinated pigs after challenge, suggesting that the PCV2 vaccine was unable to eradicate the virus, which could lead to asymptomatic PCV2 subclinical infection (PCV2-SI) in pig farms. Additionally, PCV2 infection is a risk factor for impaired pig immune response development during the weaning to growth stages, which is a crucial period to receive vaccines against classical swine fever (CSF). Therefore, the impact of PCV2-SI or PCV2-systemic disease (PCV2-SD) on live attenuated CSF vaccine was investigated. After PCV2 challenge, there was no difference in levels of classical swine fever virus (CSFV) neutralizing antibodies (NA) between pigs with PCV2-SD and PCV2-SI, suggesting that the efficacy of CSF vaccine was compromised. Moreover, results of long-term monitoring of CSFV NA titers in PCV2-SI pigs with minimized interference by maternally-derived antibodies suggested that serum PCV2 viral loads greater than 10² copies/mL may compromise the efficacy of CSF vaccine. Overall, a conventional pig model was established to demonstrate the impaired efficacy of the subunit PCV2 vaccine and its impact on the CSF vaccine in vaccination-challenge trials. Additionally, the impaired efficacy of the PCV2 vaccine resulted in increased PCV2-SI, eventually leading to compromised the live attenuated CSF vaccine induced NA response in field farm applications.

Commercial E2 subunit vaccine provides full protection to pigs against lethal challenge with 4 strains of classical swine fever virus genotype 2

Classical swine fever (CSF) still threatens the swine industry in China, with genotype 2 isolates of CSFV dominating the epizootics. In 2018 the first E2 subunit marker vaccine against CSFV (Tian Wen Jing, TWJ-E2®), containing a baculovirus-expressed E2 glycoprotein of a genotype 1.1 vaccine strain, was officially licensed in China and commercialized. To evaluate the cross-protective efficacy of TWJ-E2 against different virulent genotype 2 Chinese field isolates (2.1b, 2.1c, 2.1 h, and 2.2), 4-week-old pigs were immunized with the TWJ-E2 vaccine according to the manufacturer's instructions and then challenged with genotype 2 strains. A group vaccinated with the conventional C-strain vaccine was included for comparison. TWJ-E2 vaccinated pigs developed higher levels of E2 and neutralizing antibodies than those receiving the commercial C-strain vaccine. All TWJ-E2 and C-strain vaccinated pigs survived challenge without development of fever, clinical signs or pathological lesions. In contrast, all unvaccinated control pigs displayed severe CSF disease with 40-100% mortalities by 24 days post challenge. None of the TWJ-E2 and C-strain vaccinated pigs developed viremia, viral shedding from tonsils, E^rns protein in the sera, or viral RNA loads in different tissues after challenge, all of which were detected in the challenged unvaccinated controls. We conclude that vaccination of young pigs with TWJ-E2 provides complete immune protection against genotypically heterologous CSFVs and prevents viral shedding after challenge, with an efficacy at least comparable to that elicited by the conventional C-strain vaccine.

Co-Infection of Swine with Porcine Circovirus Type 2 and Other Swine Viruses

Porcine circovirus 2 (PCV2) is the etiological agent that causes porcine circovirus diseases and porcine circovirus-associated diseases (PCVD/PCVAD), which are present in every major swine-producing country in the world. PCV2 infections may downregulate the host immune system and enhance the infection and replication of other pathogens. However, the exact mechanisms of PCVD/PCVAD are currently unknown. To date, many studies have reported that several cofactors, such as other swine viruses or bacteria, vaccination failure, and stress or crowding, in combination with PCV2, lead to PCVD/PCVAD. Among these cofactors, co-infection of PCV2 with other viruses, such as porcine reproductive and respiratory syndrome virus, porcine parvovirus, swine influenza virus and classical swine fever virus have been widely studied for decades. In this review, we focus on the current state of knowledge regarding swine co-infection with different PCV2 genotypes or strains, as well as with PCV2 and other swine viruses.

Porcine circovirus 2 in the North Eastern region of India: Disease prevalence and genetic variation among the isolates from areas of intensive pig rearing

Porcine Circovirus type-2 (PCV-2) is considered as a major threat to the piggery sector in India. To ascertain the epidemiological status and infection level of PCV2, a pilot study was undertaken to find out the prevalence of PCV2 in swine population by ELISA and PCR in the interior and border areas of Meghalaya which includes the area where accessibility and medical aid is a rare phenomenon. A total of 249 serum samples were collected from October 2014 to February 2016 from three divisions of Meghalaya: Khasi, Jaintia and Garo Hills Divisions. The mean positivity of PCV-2 antibodies in suspected sera was 83.93% whereas 62.25% of the suspected samples respectively were found to contain PCV2 as detected by PCR. Additional 190 tissue samples were collected during necropsy from both symptomatic and asymptomatic animals following reported outbreak in this region, which indicated a mean positivity of 18.94% (36/190); out of which 13 samples were subjected to sequencing to find out the genetic diversity of PCV2 amongst the field isolates. Molecular characterization and phylogenetic analysis of PCV2 isolates based on cap gene depicted genetic diversity among the strains in pig population of Meghalaya as the isolates belonged to PCV2a, PCV2b-1c and PCV2d genotypes; identification of the PCV2d genotype is probably the first report from Meghalaya. Four isolates forming an outlier group in the phylogenetic tree were arising out of natural inter-genotypic recombination between PCV2a and PCV2b. PCV2 being immunosuppressive in nature impairs the host immune response increasing the susceptibility to other co-infections leading to disease severity and high mortality in pig population. This baseline data gives a brief epidemiological status of PCV2 infection and circulating PCV2 genotype in this region which will be useful in the formulation of control and eradication programs in remotes areas of Meghalaya where accessibility is less and vaccination is a rare practice.

Seroprevalence of selected viral pathogens in pigs reared in organized farms of Meghalaya from 2014 to 16

Aim:

A pilot study was carried out to find out the seroprevalence of Porcine circovirus 2 (PCV2), classical swine fever virus (CSFV), and Porcine respiratory and reproductive syndrome virus (PRRS) in pig population of Meghalaya.

Materials and Methods:

Serum samples were collected from piglets of 40–45 days age group, growers, and sows reared under organized and unorganized management in 11 districts of Meghalaya situated in the Khasi, Jaintia, and Garo hills divisions in the time period of 2014-2016 from apparently healthy and suspected pigs. Seroprevalence of PCV2, CSFV, and PRRS specific antibodies was detected by enzyme-linked immunosorbent assay (ELISA).

Results:

A total of 1899 serum samples were collected and screened using antibody ELISA kits specific for PCV2, CSFV, and PRRS. The highest antibody prevalence during the selected time periods was detected for PCV2 (80.8% in 2014, 79.1% in 2015, and 96.2% in 2016) followed by CSFV (76.4% in 2014, 66.09% in 2015, and 25.5% in 2016) and PRRS (2.8% in 2014, 2.7% in 2015, and 3.62% in 2016). The result indicates high seroprevalence for PCV2, which can be considered as an inducement factor due to the immunosuppressive nature of the virus, for animals being susceptible to other pathogens in farms where airborne transmission of PCV2 and postweaning multisystemic wasting syndrome among animals reared in close pens can be a major possibility.

Conclusions:

The data from this study indicates ubiquitous prevalence of PCV2 antibodies in the farm animals along with the endemic presence of swine fever and emergence of PRRS in an organized farm. There are few reports regarding PCV2 infections/outbreaks in pigs associated with reproductive failure from northern and southern part of India, but till date, there are no reports regarding concomitant infection of CSFV and PCV2 from India. Considerable high seropositivity of PCV2 indicates the need for high impact hygiene practice in farms, routine seromonitoring and implementation the vaccination program. To the author’s best knowledge, this is the first documented report on the seroprevalence of PCV2, CSFV, and PRRS from pig population of Meghalaya.

Secreted Expression of the Cap Gene of Porcine Circovirus Type 2 in Classical Swine Fever Virus C-Strain: Potential of C-Strain Used as a Vaccine Vector

Bivalent vaccines based on live attenuated viruses expressing a heterologous protein are an attractive strategy to address co-infections with various pathogens in the field. Considering the excellent efficacy and safety of the lapinized live attenuated vaccine C-strain (HCLV strain) of classical swine fever virus (CSFV), we proposed that C-strain has the potential as a viral vector for developing bivalent vaccines. To this end, we generated three recombinant viruses based on C-strain, one expressing the capsid (Cap) gene of porcine circovirus type 2 (PCV2) with the nuclear localization signal (NLS) (rHCLV-2ACap), and the other two expressing the PCV2 Cap gene without the NLS yet containing the signal peptide of the prolactin gene (rHCLV-pspCap) or that of the ubiquitin-specific peptidase gene (rHCLV-uspCap). All the recombinant viruses exhibited phenotypes similar to those of the parental virus and produced high-level anti-CSFV neutralizing antibodies (NAbs) in rabbits. Interestingly, rHCLV-uspCap and rHCLV-pspCap, but not rHCLV-2ACap, elicited detectable anti-Cap and -PCV2 NAbs in rabbits. Taken together, our data demonstrate that C-strain can be used as a viral vector to develop bivalent vaccines.

Cellular proteomic analysis of porcine circovirus type 2 and classical swine fever virus coinfection in porcine kidney-15 cells using isobaric tags for relative and absolute quantitation-coupled LC-MS/MS

Viral coinfection or superinfection in host has caused public health concern and huge economic losses of farming industry. The influence of viral coinfection on cellular protein abundance is essential for viral pathogenesis. Based on a coinfection model for porcine circovirus type 2 (PCV2) and classical swine fever virus (CSFV) developed previously by our laboratory, isobaric tags for relative and absolute quantitation (iTRAQ)-coupled LC-MS/MS proteomic profiling was performed to explore the host cell responses to PCV2-CSFV coinfection. Totally, 3932 proteins were identified in three independent mass spectrometry analyses. Compared with uninfected cells, 304 proteins increased (fold change >1.2) and 198 decreased (fold change <0.833) their abundance in PCV2-infected cells (p < 0.05), 60 and 61 were more and less abundant in CSFV-infected cells, and 196 and 158 were more and less abundant, respectively in cells coinfected with PCV2 and CSFV. Representative differentially abundant proteins were validated by quantitative real-time PCR, Western blotting and confocal laser scanning microscopy. Bioinformatic analyses confirmed the dominant role of PCV2, and indicated that mitochondrial dysfunction, nuclear factor erythroid 2-related factor 2 (Nrf2)-mediated oxidative stress response and apoptosis signaling pathways might be the specifical targets during PCV2-CSFV coinfection.

In Vitro Coinfection and Replication of Classical Swine Fever Virus and Porcine Circovirus Type 2 in PK15 Cells

Increasing clinical lines of evidence have shown the coinfection/superinfection of porcine circovirus type 2 (PCV2) and classical swine fever virus (CSFV). Here, we investigated whether PCV2 and CSFV could infect the same cell productively by constructing an in vitro coinfection model. Our results indicated that PCV2-free PK15 cells but not ST cells were more sensitive to PCV2, and the PK15 cell line could stably harbor replicating CSFV (PK15-CSFV cells) with a high infection rate. Confocal and super-resolution microscopic analysis showed that PCV2 and CSFV colocalized in the same PK15-CSFV cell, and the CSFV E2 protein translocated from the cytoplasm to the nucleus in PK15-CSFV cells infected with PCV2. Moreover, PCV2-CSFV dual-positive cells increased gradually in PK15-CSFV cells in a PCV2 dose-dependent manner. In PK15-CSFV cells, PCV2 replicated well, and the production of PCV2 progeny was not influenced by CSFV infection. However, CSFV reproduction decreased in a PCV2 dose-dependent manner. In addition, cellular apoptosis was not strengthened in PK15-CSFV cells infected with PCV2 in comparison with PCV2-infected PK15 cells. Moreover, using this coinfection model we further demonstrated PCV2-induced apoptosis might contribute to the impairment of CSFV HCLV strain replication in coinfected cells. Taken together, our results demonstrate for the first time the coinfection/superinfection of PCV2 and CSFV within the same cell, providing an in vitro model to facilitate further investigation of the underlying mechanism of CSFV and PCV2 coinfection.

Efficacy of a live attenuated vaccine in classical swine fever virus postnatally persistently infected pigs

Classical swine fever (CSF) causes major losses in pig farming, with various degrees of disease severity. Efficient live attenuated vaccines against classical swine fever virus (CSFV) are used routinely in endemic countries. However, despite intensive vaccination programs in these areas for more than 20 years, CSF has not been eradicated. Molecular epidemiology studies in these regions suggests that the virus circulating in the field has evolved under the positive selection pressure exerted by the immune response to the vaccine, leading to new attenuated viral variants. Recent work by our group demonstrated that a high proportion of persistently infected piglets can be generated by early postnatal infection with low and moderately virulent CSFV strains. Here, we studied the immune response to a hog cholera lapinised virus vaccine (HCLV), C-strain, in six-week-old persistently infected pigs following post-natal infection. CSFV-negative pigs were vaccinated as controls. The humoral and interferon gamma responses as well as the CSFV RNA loads were monitored for 21 days post-vaccination. No vaccine viral RNA was detected in the serum samples and tonsils from CSFV postnatally persistently infected pigs for 21 days post-vaccination. Furthermore, no E2-specific antibody response or neutralising antibody titres were shown in CSFV persistently infected vaccinated animals. Likewise, no of IFN-gamma producing cell response against CSFV or PHA was observed. To our knowledge, this is the first report demonstrating the absence of a response to vaccination in CSFV persistently infected pigs.

The application of a duplex reverse transcription real-time PCR for the surveillance of porcine reproductive and respiratory syndrome virus and porcine circovirus type 2

The porcine respiratory disease complex (PRDC) is the most common disease in commercial pork production worldwide. Porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV), the most important agents of PRDC, usually co-infect in the same pigs. In order to survey the prevalence of PCV2 and PRRSV in pigs of various ages, a duplex reverse transcription real-time PCR (DRT-rPCR) was developed and applied in the present study. The DRT-rPCR did not cross-react with 10 swine viruses other than PCV2 and PRRSV, with detection limits of 1 TCID50/ml for PCV2 and 6.3 TCID50/ml for PRRSV. Surveillance using DRT-rPCR together with serology revealed that in the five farms studied, pigs were most susceptible to PRRSV at 6-14 weeks of age, whereas susceptibility to PCV2 varied by the management system but was mostly at 10-14 weeks of age. Cross analysis of viral loads versus antibody titers revealed that PCV2 load was affected negatively by anti-PCV2 ORF2 antibody, which constituted the most important non-infectious factor affecting the development of PMWS. These results indicated that DRT-rPCR was developed and applied successfully to the surveillance of PCV2 and PRRSV in the field.

Porcine circovirus type 2 decreases the infection and replication of attenuated classical swine fever virus in porcine alveolar macrophages

Recently, it has been noted that porcine circovirus type 2 (PCV2) infection adversely affects the protective efficacy of Lapinized Philippines Coronel (LPC) vaccine, an attenuated strain of classical swine fever virus (CSFV), in pigs. In order to investigate the possible mechanisms of the PCV2-derived interference, an in vitro model was established to study the interaction of LPC virus (LPCV) and PCV2 in porcine alveolar macrophages (AMs). The results showed that PCV2 reduced the LPCV infection in AMs and the levels of PCV2-derived interference were dose-dependent. The PCV2-derived interference also reduced the replication level of LPCV in AMs. The full-length PCV2 DNA and its fragment DNA C9 CpG-ODN were involved in the reduction of LPCV infection in AMs, whereas UV-inactivated PCV2 was not. In addition, a moderate negative correlation between the LPCV antigen-containing rate and IFN-γ production was observed, and had a dose-dependent trend with the level of PCV2-inoculation. The results of the present study may partially explain how PCV2 infection interferes with the efficacy of LPC vaccine.

The challenges of classical swine fever control: modified live and E2 subunit vaccines

Classical swine fever (CSF) is an economically important, highly contagious disease of swine worldwide. CSF is caused by classical swine fever virus (CSFV), and domestic pigs and wild boars are its only natural hosts. The two main strategies used to control CSF epidemic are systematic prophylactic vaccination and a non-vaccination stamping-out policy. This review compares the protective efficacy of the routinely used modified live vaccine (MLV) and E2 subunit vaccines and summarizes the factors that influence the efficacy of the vaccines and the challenges that both vaccines face to CSF control. Although MLV provide earlier and more complete protection than E2 subunit vaccines, it has the drawback of not allowing differentiation between infected and vaccinated animals (DIVA). The marker vaccine of E2 protein with companion discriminatory test to detect antibodies against E(rns) allows DIVA and is a promising strategy for future control and eradication of CSF. Maternal derived antibody (MDA) is the critical factor in impairing the efficacy of both MLV and E2 subunit vaccines, so the well-designed vaccination programs of sows and piglets should be considered together. Because of the antigen variation among various genotypes of CSFV, antibodies raised by either MLV or subunit vaccine neutralize genotypically homologous strains better than heterologous ones. However, although this is not a major concern for MLV as the induced immune responses can protect pigs against the challenge of various genotypes of CSFVs, it is critical for E2 subunit vaccines. It is thus necessary to evaluate whether the E2 subunit vaccine can completely protect against the current prevalent strains in the field. An ideal new generation of vaccine should be able to maintain the high protective efficiency of MLV and overcome the problem of antigenic variations while allowing for DIVA.