Genetic variation of porcine circovirus type 2 (PCV2) and its relevance to vaccination, pathogenesis and diagnosis

Phylogenetic Analysis and Serological Investigation of Porcine Circovirus Indicates Frequent Infection with Various Subtypes

Porcine circoviruses (PCVs) are notorious for triggering severe diseases in pigs and causing serious economic losses to the swine industry. In the present study, we undertook a comprehensive approach for the investigation of PCV prevalence, including the phylogenetic analysis of obtained PCV sequences, the determination of major circulating genotypes and serological screening based on different recombinant Cap proteins with specific immunoreactivity. Epidemiological surveillance data indicate that PCV2d and PCV3a are widely distributed in Southwest China, while PCV4 has only sporadic circulation. Meanwhile, serological investigations showed high PCV2 antibody positivity in collected serum samples (>50%), followed by PCV4 (nearly 50%) and PCV3 (30-35%). The analysis supports different circulation patterns of PCV2, PCV3 and PCV4 and illustrates the PCV2/PCV3 genetic evolution characteristics on a nationwide basis. Taken together, our findings add up to the current understanding of PCV epidemiology and provide new tools and insight for PCV antiviral intervention.

Revisiting Porcine Circovirus Infection: Recent Insights and Its Significance in the Piggery Sector

Porcine circovirus (PCV), a member of the Circoviridae family within the genus Circovirus, poses a significant economic risk to the global swine industry. PCV2, which has nine identified genotypes (a-i), has emerged as the predominant genotype worldwide, particularly PCV2d. PCV2 has been commonly found in both domestic pigs and wild boars, and sporadically in non-porcine animals. The virus spreads among swine populations through horizontal and vertical transmission routes. Despite the availability of commercial vaccines for controlling porcine circovirus infections and associated diseases, the continuous genotypic shifts from a to b, and subsequently from b to d, have maintained PCV2 as a significant pathogen with substantial economic implications. This review aims to provide an updated understanding of the biology, genetic variation, distribution, and preventive strategies concerning porcine circoviruses and their associated diseases in swine.

Virome of wild rats (Rattus norvegicus) captured far from pig farms in Jiangsu province of China reveals novel porcine circovirus type 2d (PCV2d) sequences

BACKGROUND

Porcine circovirus type 2 (PCV2) has caused great economic losses in the global pig industry. There have been published records of wild rats acting as the reservoirs of PCV2 (only PCV2a and PCV2b), but almost all of which were related to the PCV2-infected swine herds.

RESULTS

In this study, we carried out the detection, amplification, and characterization of novel PCV2 strains in wild rats that were captured far from pig farms. Nested PCR assay demonstrated that the kidney, heart, lung, liver, pancreas, and large and small intestines of rats were screened positive for PCV2. We subsequently sequenced two full genomes of PCV2 in positive sample pools, designated as js2021-Rt001 and js2021-Rt002. Genome sequence analysis indicated that they had the highest similarity to nucleotide sequences of porcine-origin PCV2 isolates in Vietnam. Phylogenetically, js2021-Rt001 and js2021-Rt002 were a part of the PCV2d genotype cluster, which is a predominant genotype circulating worldwide in recent years. The antibody recognition regions, immunodominant decoy epitope, and heparin sulfate binding motif of the two complete genome sequences coincided with those previously reported.

CONCLUSIONS

Our research reported the genomic characterization of two novel PCV2 strains (js2021-Rt001 and js2021-Rt002) and provided the first supported evidence that PCV2d could naturally infect wild rats in China. However, whether the newly identified strains have potential for circulating in nature in vertical and horizontal transmission or inter-species jumping between rats and pigs needs further research.

Carboxyl-Terminal Decoy Epitopes in the Capsid Protein of Porcine Circovirus Type 2 Are Immunogenicity-Enhancers That Elicit Predominantly Specific Antibodies in Non-Vaccinated Pigs

In the context of the carboxyl-terminus (C-terminus) of the capsid protein of porcine circovirus type 2a (PCV2a) and PCV2a vaccines, this study aimed to explore its unrevealing cryptic epitope and its relation to PCV2-infected herd immunity. To discover the C-terminus of the capsid protein of PCV2a, monoclonal antibodies (mAbs) were generated in this work. Two mAbs bound the two minimal linear epitopes (₂₂₉PPLKP₂₃₃ and ₂₂₈DPPLNP₂₃₃ (or ₂₂₉PPLNP₂₃₃)), which were located at the C-terminus of the capsid proteins of PCV2a and PCV2b, respectively. One mAb bound to the minimal linear epitope (₂₂₀QFREFNLK₂₂₇, peptide P82), but it neither bound the virus-like particle (VLP) of PCV2a nor produced positive staining in PCV2a-infected cells by immunofluorescence assay. Further, the residues 220-227 were not accessible on the surface of the VLP on the three-dimensional model, but the residues 228-231 extend toward the VLP exterior. Immunoassays were conducted in this study to screen anti-viral peptide-specific IgGs, which could differentiate vaccinated pigs from non-vaccinated ones. The data show two ₂₂₀QFREFNLKDPPLKP₂₃₃-containing peptides had a significantly higher binding reactivity with sera from PCV2-infected pigs in the control group than with sera from the VLP-vaccine group, particularly seen in sera from swine aged 15 weeks to 24 weeks. However, the peptide P82 had not this phenomenon in that test. This study confirmed that C-terminal epitopes play an important role in PCV2-induced decoy of swine humoral immunity.

Survey of porcine circovirus type 2 and parvovirus in swine breeding herds of Colombia

Background

One of the consequences of the presentation of reproductive failures in sows is the economic losses in production because it alters the estimated values of the volume of production, decreasing the productivity of the farm. Porcine circovirosis by porcine circovirus 2 (PCV2) has been associated with reproductive disorders, and porcine parvovirus (PVP) is one of the pathological agents most related to the presentation of reproductive failure in pigs. In Colombia, there are reports of the presence of PCV2 through molecular techniques, and PVP through serum tests; however, in the department of Tolima, the prevalence of these two viruses is unknown.

Objective

In this study, the aim was to establish a report of the prevalence of viruses in five municipalities of the department of Tolima‐Colombia.

Methods

Blood samples from 150 breeding sows of five municipalities in Tolima, Colombia, were obtained. Quantitative polymerase chain reaction (qPCR) was used to detect the PCV2 and PVP virus in the blood samples followed by PCR and sequencing of 16 PCR products of the amplification of the cap gene of PCV2. A phylogenetic tree was constructed to identify the genotype of the PCV2 virus.

Results

The presence of PCV2d in sows was detected in 135 samples (90%), as well as the identification of PVP in 2.6% of the samples. In addition, the phylogenetic analysis showed that 16 isolates were the PCV2d2 genotype.

Conclusion

PCV2d and PVP were found to coinfect the females, and the identification of variability in regions in the predicted amino acid sequence of the PCV2 capsid may be associated with virus pathogenicity.

Phylodynamic analysis of current Porcine circovirus 4 sequences: Does the porcine circoviruses evolutionary history repeat itself?

Four porcine circoviruses (PCVs) have been discovered over time and seem to share a common history, particularly for PCV-2 and -3. Despite being reported as apparently new viruses, rapidly emerging as a threat for the worldwide swine industry, they were then proven to have been circulating and coexisting with domestic pigs undetected for decades, without causing relevant health issues. A similar scenario could be true for the most recently identified PCV-4. However, its detection in Asia only and the limited genetic variability could suggest a truly recent origin. To investigate which of the above-mentioned scenarios is more plausible, a phylodynamic analysis was performed on all available PCV-4 sequences for which adequate metadata were available to reconstruct the viral history and evolution. Obtained results suggest an ancient origin, at least decades ago, followed by a prolonged low-level circulation and a moderate increase in viral population size after the second half of the XX century, in parallel with a progressive rise in pig population and farming intensification. A relevant local geographical clustering was also highlighted. The reason behind such low spreading capacity and limited geographical distribution compared to other circoviruses is currently obscure and will require dedicated studies, involving a more extensive sampling and sequencing activity.

Live attenuated Salmonella enterica serovar Choleraesuis vector delivering a virus-like particles induces a protective immune response against porcine circovirus type 2 in mice

The virus-like particles (VLPs) of porcine circovirus type 2 (PCV2) is an attractive vaccine candidate that retains the natural conformation of the virion but lacks the viral genome to replicate, thus balancing safety and immunogenicity. However, the assembly of VLPs requires cumbersome subsequent processes, hindering the development of related vaccines. In addition, as a subunit antigen, VLPs are defective in inducing cellular and mucosal immune responses. In this study, the capsid (Cap) protein of PCV2 was synthesized and self-assembled into VLPs in the recombinant attenuated S. Choleraesuis vector, rSC0016(pS-Cap). Furthermore, rSC0016(pS-Cap) induced a Cap-specific Th1-dominant immune response, mucosal immune responses, and neutralizing antibodies against PCV2. Finally, the virus genome copies in mice immunized with the rSC0016(pS-Cap) were significantly lower than those of the empty vector control group after challenge with PCV2. In conclusion, our study demonstrates the potential of using S. Choleraesuis vectors to delivery VLPs, providing new ideas for the development of PCV2 vaccines.

Molecular Epidemic Characteristics and Genetic Evolution of Porcine Circovirus Type 2 (PCV2) in Swine Herds of Shanghai, China

Porcine Circovirus 2 (PCV2) is a crucial swine pathogen and considered a primary causative agent of porcine circovirus-associated diseases (PCVADs), posing a serious economic threat to the swine industry across globe. The world’s biggest agricultural conglomerates have teamed up to create giant commercial pig farms across Shanghai due to the proximity of this region to more affluent lean-pork markets. Since its discovery, PCV2 has displayed extraordinary genetic diversity, and its genome is swiftly evolving through a series of mutations and recombinations. However, limited information on epidemiology, molecular characteristics, vaccine cross-protection, and the co-infection rate of PCV2 with other lethal swine diseases can adversely impact the pig production in the region. To investigate the molecular epidemic characteristics and genetic evolution of PCV2, pigs with doubtful symptoms of PCVADs were sampled from various commercial pig farms with a history of PWMS and/or PDNS across Shanghai from 2014 to 2018. Our results revealed the coexistence of multiple PCV2 genotypes (PCV2b, PCV2e, and PCV2d) among Shanghai pig herds and dominance of PCV2d among them. We also found critical amino acid substitutions in epitope regions of important capsid proteins in PCV2 isolates involved in viral replication and host immune escape. Spotted mutations may favor the prevalence and survival of various PCV2 genotypes despite availability of commercial vaccines. This study also provides insight into the co-infection status of PCV2 with major lethal swine viral diseases such as PPV and PPRSV. Collectively, these investigations will contribute to understanding the molecular epidemiology and evolution of PCV2 across the region.

Detection and genetic characteristics of porcine circovirus type 2 and 3 in Henan province of China

PCV2 is one of the most economically important viral agents in swine worldwide. Recently, PCV3 has been frequently reported, and the co-infection of PCV2 and PCV3 is common in China. In order to explore the distribution, epidemiology and genetic diversity of PCV2 and PCV3, a total of 1,760 clinical tissue samples were randomly collected from 18 different regions in Henan province of China from October 2018 to September 2019 and screened for the presence of PCV2 and PCV3 by a duplex real-time PCR assay. The results showed that the positive rates of PCV2 and PCV3 were 72.90% and 5.17% respectively, and the co-infection rate of the two viruses was 3.64%. PCV2 and PCV3 are prevalent all year round. The prevalence of PCV2 in diseased pigs was 83.98%, higher than that in slaughterhouse pigs, while the prevalence of PCV3 in diseased pigs was 2.16%, slightly lower than that in slaughterhouse pigs. Furthermore, the complete genomes of 14 PCV2 and 3 PCV3 strains were obtained, among which 1 belonged to PCV2a, 5 belonged to PCV2b and 8 belonged to PCV2d. A new variant strain (XX2) might escape the host immune system. The phylogenetic analysis of PCV3 showed high nucleotide identity (>98%) between sequences obtained in this study and reference sequences. The results of this study might enrich the epidemiological data of PCV2 and PCV3 in Henan province and provide reference information for the comprehensive prevention and control of PCVAD.

Simultaneous detection and genetic characterization of porcine circovirus 2 and 4 in Henan province of China

Porcine circovirus 4 (PCV4) was identified as a novel porcine circovirus in China in 2019. To investigate the prevalence and genetic characteristics of PCV2 and PCV4, 133 clinical samples (103 tissue samples and 30 serum samples) were collected from 30 different pig farms in Henan province of China, and a SYBR Green I-based duplex quantitative real-time polymerase chain reaction assay was established to detect PCV2 and PCV4 genomes simultaneously. The complete genome sequences of 20 PCV2 and 6 PCV4 strains from 19 and 6 clinical samples respectively were sequenced and analyzed. The results showed the detection limits of this assay were 80.2 copies/μL for PCV2 and 58.6 copies/μL for PCV4. The detection results of clinical samples revealed the PCV2 positive rate was 63.16% (84/133), the PCV4 positive rate was 33.33% (45/133), and the PCV2 and PCV4 co-infection positive rate was 21.05% (28/133). Among 20 PCV2 strains, 6 belonged to PCV2a, 6 belonged to PCV2b and 8 belonged to PCV2d. Co-infection with JZ1 (PCV2b) and JZ2 (PCV2d) strains was identified in one sample (JZ-1). Eleven putative recombination events were found through the recombination analysis, suggesting that the new PCV2 variant strains had circulated in Henan province, which contributes to our understanding of evolutionary characteristics of PCV2 in China. The possible genotypes of PCV4 strains were determined based on genomic sequences of 6 PCV4 strains in this study and 29 PCV4 reference strains available at GenBank. According to three different phylogenetic trees (ORF1, ORF2 and complete genome), all 35 PCV4 strains were clustered into two major genotypes (PCV4a and PCV4b), and 6 PCV4 strains in this study belonged to PCV4a. Additionally, the functional regions of PCV4 strains were predicted by comparison with other circoviruses, which are conducive to the further study of the biological functions of PCV4 genome.

The Isolated in Utero Environment Is Conducive to the Emergence of RNA and DNA Virus Variants

The host’s immune status may affect virus evolution. Little is known about how developing fetal and placental immune milieus affect virus heterogeneity. This knowledge will help us better understand intra-host virus evolution and how new virus variants emerge. The goal of our study was to find out whether the isolated in utero environment—an environment with specialized placental immunity and developing fetal immunity—supports the emergence of RNA and DNA virus variants. We used well-established porcine models for isolated Zika virus (RNA virus) and porcine circovirus 2 (DNA virus) fetal infections. We found that the isolated in utero environment was conducive to the emergence of RNA and DNA virus variants. Next-generation sequencing of nearly whole virus genomes and validated bioinformatics pipelines identified both unique and convergent single nucleotide variations in virus genomes isolated from different fetuses. Zika virus and PCV2 in utero evolution also resulted in single nucleotide variations previously reported in the human and porcine field samples. These findings should encourage further studies on virus evolution in placenta and fetuses, to better understand how virus variants emerge and how in utero viral evolution affects congenital virus transmission and pathogenicity.

Prevalence and genetic analysis of porcine circovirus type 2 (PCV2) and type 3 (PCV3) between 2018 and 2020 in central China

Porcine circovirus type 2 (PCV2) is the causative agent of porcine circovirus-associated diseases (PCVAD), causing substantial economic losses to the swine industry worldwide. PCV3, as a recently discovered virus, is associated with porcine dermatitis, nephropathy syndrome, reproductive failure, congenital tremors, and other clinical symptoms. To further investigate the epidemic profile and genetic diversity of the two viruses, a total of 198 samples from swine at various growth stages suspected for PCVAD on 55 different pig farms between 2018 and 2020 were analyzed for presence of PCV2 and PCV3 by using a multiplex real-time PCR assay. Among the 198 samples, 113 (57.07%) and 72 (36.36%) were positive for PCV2 and PCV3 respectively, and 39 (19.7%) were positive for PCV2 and PCV3 co-infection. Subsequently, whole genome sequences of 34 PCV2 and 19 PCV3 strains were obtained from 30 and 19 clinical samples, respectively. Of these, 8 PCV2 strains belonged to PCV2a, 10 belonged to PCV2b and 16 belonged to PCV2d, indicating PCV2d was the predominant PCV2 genotype circulating in central China. Furthermore, co-infection of different PCV2 genotype strains was identified in three samples (JZ-4, KF-2 and JY-1), and a cross-recombination was found in the ORF2 region of the sequenced 13 PCV2d strains whose putative parental strains were LN6/1999 (MF278777) and MEX/41238/2014 (KT795287) strains. The phylogenetic analysis of PCV3 showed high nucleotide identity (>98%) among sequences obtained in this study and reference sequences. These data will aid our understanding of the molecular epidemiology and evolution of PCV2 and PCV3.

Porcine circovirus 2 capsid protein produced in N. benthamiana forms virus-like particles that elicit production of virus-neutralizing antibodies in guinea pigs

Porcine circovirus type 2 (PCV2) is a non-enveloped, icosahedral virus of the Circoviridae family, with a small, circular, single-stranded DNA genome. PCV2 infections cause substantial economic losses in the pig industry worldwide. Currently, commercially produced PCV2 vaccines are expensive, whereas plant-based expression systems can produce recombinant proteins at low cost for use as vaccines. In this study, recombinant PCV2 capsid protein (rCap) was transiently expressed in Nicotiana benthamiana and purified by metal affinity chromatography, with a yield of 102 mg from 1 kg plant leaves. Electron microscopy confirmed that purified rCap self-assembled into virus-like particles (VLPs) at neutral pH. It was shown to provoke a strong immune response in guinea pigs. The results indicate that plant systems can enable production of large amounts of proteins to serve as candidates for subunit vaccines.

Emergence of porcine circovirus 2g (PCV2g) and evidence for recombination between genotypes 2g, 2b and 2d among field isolates from non-vaccinated pigs in Mizoram, India

The molecular genetics of fourteen Porcine Circovirus 2 (PCV2) isolates from non-vaccinated pigs that died of porcine circovirus associated disease (PCVAD) between 2012 and 2019 in the Mizoram state of North East India, was studied. The PCVAD in these pigs, that had shown characteristic clinical signs and lesions associated with post-weaning multi-systemic wasting syndrome and reproductive failure was confirmed with detection of PCV2 DNA in the tissue samples. Complete viral genomes of these fourteen field isolates were sequenced following in house developed overlapping PCR. The multiple sequence alignment of viral capsid proteins or the open reading frame 2 (ORF2) sequences showed highly conserved residues known for antibody recognition and genotype specificity, however, variations were noticed in the amino acid residues previously known as important for in vitro replication of PCV2. The phylogenetic analyses based on the complete genome sequences enabled identification of genotype PCV2g (9/14, 64.29%) for the first time in India along with genotypes PCV2d (3/14, 21.43%) and PCV2b (2/14, 14.29%). Further, recombination analyses showed evidence for recombination between the genotypes 2b, 2g and 2d. This is the first report on the prevalence of genotype PCV2g and natural inter-genotypic (2g-2b, 2g-2d and 2d-2g) recombinants in India. The findings indicate a non-vaccine driven, natural genotypic shift and signify the need for routine PCV2 surveillance and genotyping. Our analyses also provide a solid ground for future studies to understand the consequences of multiple PCV2 genotypes within a pig population with respect to vaccination, diagnostics and emergence of new genotypes.

The prevalence of porcine circovirus type 2e (PCV2e) in Korean slaughter pig lymph nodes when compared with other PCV2 genotypes

The objective of this study was to determine the prevalence of porcine circovirus type 2e (PCV2e) over other PCV2 genotypes (a, b, c and d) from the lymph nodes of 1,550 randomly selected slaughter pigs. Samples were obtained at a rate of five samples per farm from 310 farms between January 2018 and May 2020. Of the 1,550 lymph node samples, PCV2 DNA was detected in 762 (49.20%) samples. Among the 762 PCV2 DNA-positive samples, a single PCV2 genotype was detected in 744 samples, while multiple PCV2 genotypes were only detected in 18 samples. Of the 744 single infection cases, PCV2d was the most prevalent with 709 cases, followed by PCV2b (15 cases), PCV2a (14 cases) and PCV2e (6 cases). Of the 18 multiple infection cases, PCV2a+PCV2d was the most prevalent (7 cases) followed by PCV2b+PCV2d (3 cases), PCV2b+PCV2e (3 cases), PCV2a+PCV2b+PCV2d (3 cases) and PCV2a+PCV2b (2 cases). No PCV2c was detected in any of the single or multiple infection cases. The results of prevalence identified PCV2d as the current dominant genotype, while the newly emerging PCV2e maintained the lowest prevalence among the evaluated swine farms.

Porcine circovirus genotypes and their copathogens in pigs with respiratory disease in southern provinces of Vietnam

This study was conducted to investigate the genetic diversity of porcine circovirus type 2 (PCV2) and its coinfecting pathogens in pigs with respiratory disease in Vietnam. Samples from 127 clinical cases were obtained from different southern provinces of Vietnam from January 2018 to January 2020 for PCR and sequence analysis. The infection rate of PCV2 was 78.8%, and the major pathogens found in coinfections with PCV2 were porcine reproductive and respiratory syndrome virus, Mycoplasma hyopneumoniae, and Haemophilus parasuis. Forty-three PCV2-positive clinical samples were selected for amplification and sequencing of the ORF2 region. Phylogenetic analysis of PCV2 ORF2 showed that five of the sequences belonged to PCV2b (11.6%) and 38 belonged to PCV2d (88.4%), indicating that PCV2d strains were predominant in southern provinces of Vietnam. Alignment of the predicted amino acid sequences of the PCV2 capsid protein revealed polymorphic sites in the antibody recognition regions. This study demonstrates the prevalence of the PCV2d genotype in southern Vietnam and presents a comprehensive overview of the coinfecting pathogens associated with PCV2 in young pigs with respiratory disease.

Protective immunity in swine induced by Porcine Circovirus 2b inactivated vaccines with different antigen payload

Porcine Circovirus 2 (PCV2) vaccines are poorly standardized in terms of antigen payload and correlates of protection. Therefore, twenty, 45-day old piglets were divided into four groups of 5 animals each and vaccinated with 800 / 266 / 88 / 0 nanograms, respectively, of an inactivated PCV2b strain formulated in the same oil adjuvant. Twenty-six days later, all the pigs were challenged intranasally with the homologous PCV2b strain. No clinical signs were observed in the pigs under study. Viremia was observed after challenge in all the control pigs, as well as in 3 pigs of the 266 and 88-ng groups (one and two, respectively). No pigs of the 800-ng group developed viremia. On the basis of post challenge viremia, the PCV2b vaccine under study had a titer of 11 Protective Doses (PD) 50 %, and 1 PD₅₀ amounted to 74 ng of PCV2b Ag. Neutralizing and ELISA Ab titers showed no obvious correlation with protection in the single animals, even though the 800-ng group developed a significantly higher mean Ab response. All the pigs with a PCV2-specific, IFN-gamma response at 3 weeks after vaccination in whole blood samples were protected against viremia. In lymphoid tissues (mainly tonsils and ileum) the presence of sparse reactive histiocytes and multinucleated giant cells was the only PCV2-associated feature and, by immunohistochemistry, only 3 out of 20 subjects showed a low viral load.

Genotyping of PCV3 based on reassembled viral gene sequences

Porcine circovirus type 3 (PCV3) has been reported in many countries such as USA, China, Korea and many European countries during 2015–2018. The six PCV3 strains named IH, SJ, N5, N10, N13 and N62 were detected out of 220 samples by PCR methods while the prevalence our study was conducted in 2017 to 2018. The six detected strains were hard to genotype with reference viruses due to their diverse phylogenetic relationship. PCV3 capsid, ORF3 and replicase protein coding genes were reassembled at the nucleotide sequence level, then 16 new reassembled PCV3 sequences were generated. Based on the maximum likelihood mapping analysis of 303 PCV3 sequences a model with a combination of replicase, ORF3 and capsid protein coding genes was selected as the most appropriate target for genotyping, which provided the best support for the clade classification into three genotypes and several subtypes (genotype 1, genotype 2; subtype: a and b, genotype 3; subtype a, b, c, d, e, f, g, h). This study, the IH_Korea_2017 and N62_Korea_2018 strains belong to genogroup 3 (subtype a) the SJ_Korea_2017 strain genogroup 3 (subtype g) and the N5, N10, N13 Korea_ 2018 strains genogroup 3 (subtype f), respectively. In conclusion, this study may provide insights to classification of PCV3 genotypes around the world.

Deletion of a decoy epitope in porcine circovirus 2 (PCV2) capsid protein affects the protective immune response in mice

The swine pathogen porcine circovirus type 2 (PCV2) causes significant economic damage worldwide. The PCV2 capsid (CP) residues 169-STIDYFQPNNKR-180 have been identified as a decoy epitope that diverts the host immune response away from protective epitopes. However, the decoy epitope may include important linear or conformational protective epitopes against PCV2. In this study, we used the baculovirus system to express recombinant complete CP (1-233) and mutant CP (Δ169-180), in which the decoy epitope was deleted, and evaluated the immune response to these in mice. Immunization with mutant CP (Δ169-180) protein, which formed very low level of virus-like particles (VLPs), elicited significantly lower levels of PCV2 CP-specific IgG antibodies and a slightly lower neutralizing activity than immunization with the complete CP (1-233) protein. This finding suggests that the complete CP is important for efficient VLP assembly and induction of PCV2-specific IgG antibodies and neutralizing antibodies in mice. This study may provide useful information for next-generation vaccine design for PCV2 control.

Genetic Diversity of Porcine Circovirus Isolated from Korean Wild Boars

In Korea, three genotypes of porcine circovirus type 2 (PCV2a, PCV2b, and PCV2d) have been identified on domestic pig farms, while two genotypes (PCV2a and PCV2b) have been identified in wild boar populations. Here, we investigated genotype diversity and genotypic shift in 91 PCV2 isolates from 1340 wild boars captured in South Korea between 2013 and 2017. Phylogenetic analyses based on the complete ORF2 showed that the 91 PCV2 strains were detected as four genotypes by qPCR screening assay: PCV2a (2.2%, 2/91), PCV2b (16.5%, 15/91), PCV2d (80.2%, 73/91), and PCV2h (1.1%, 1/91). Only one intergenotype recombinant event was detected between PCV2 ORF2 in wild boars (PCV2b) and domestic pigs (PCV2a). Amino acid positions 86–89 within ORF2, which distinguishes the different genotypes, were conserved in all PCV2 genotypes isolated from South Korean wild boars, including TNKI in PCV2a/PCV2h, SNPR in PCV2b, and SNPL in PCV2d. The estimated nucleotide substitution rates in the ORF2 region of viruses from South Korean wild boars and domestic pigs were 5.8145 × 10⁻⁴ and 4.5838 × 10⁻⁴ substitutions per site per year (s/s/y), respectively. The times to the most recent common ancestor (tMRCA) for South Korean domestic pig PCV2 were 1937 (PCV2a), 1972 (PCV2b), 1999 (PCV2d-1), and 2000 (PCV2d-2). By contrast, the tMRCA for South Korean wild boar PCV2b and PCV2d were 1989 and 2001, respectively. Thus, the PCV2d genotype is prevalent among South Korean wild boars and domestic pigs.

The Carboxyl Terminus of the Porcine Circovirus Type 2 Capsid Protein Is Critical to Virus-Like Particle Assembly, Cell Entry, and Propagation

The capsid protein (Cap) is the sole structural protein and the main antigen of porcine circovirus type 2 (PCV2). Structural loops of the Cap play crucial roles in viral genome packaging, capsid assembly, and virus-host interactions. Although the molecular mechanisms are yet unknown, the carboxyl terminus (CT) of the PCV2 Cap is known to play critical roles in the evolution, pathogenesis, and proliferation of this virus. In this study, we investigated functions of CT. Removal of this loop leads to abrogation of the in vitro Cap self-assembly into virus-like particles (VLPs). Likewise, the mutated virus resists rescue from PK15 cell culture. A conserved PXXP motif in the CT is dispensable for VLP assembly and subsequent cell entry. However, its removal leads to the subsequent failure of virus rescued from PK15 cells. Furthermore, substituting either the PCV1 counterpart or an AXXA for the PXXP motif still supports virus rescue from cell culture but results in a dramatic decrease in viral titers compared with wild type. In particular, a strictly conserved residue (²²⁷K) in the CT is essential for VLP entry into PK15 cells, and its mutation to alanine greatly attenuates cell entry of the VLPs, supporting a mechanism for the failure to rescue a mutated PCV2 infectious DNA clone (K227A) from PK15 cell culture. These results suggest the CT of the PCV2 Cap plays critical roles in virus assembly, viral-host cell interaction(s), and virus propagation in vitro IMPORTANCE The carboxyl terminus (CT) of porcine circovirus type 2 (PCV2) capsid protein (Cap) was previously reported to be associated with immunorecognition, alterations of viral titer in swine sera, and pathogenicity. However, the molecular mechanisms underlying these effects remain unknown. In this study, roles of the critical residues and motifs of the CT are investigated with respect to virus-like particle (VLP) assembly, cell entry, and viral proliferation. The results revealed that the positively charged ²²⁷K of the CT is essential for both cell entry of PCV2 VLPs and virus proliferation. Our findings, therefore, suggest that the CT should be considered one of the key epitopes, recognized by neutralizing antibodies, for vaccine design and a target for drug development to prevent PCV2-associated diseases (PCVADs). Furthermore, it is important to respect the function of ²²⁷K for its role in cell entry if using either PCV2 VLPs for nanoscale DNA/drug cell delivery or using PCV2 VLPs to display a variety of foreign epitopes for immunization.

Phylogenetic analysis of porcine circovirus type 2 (PCV2) between 2015 and 2018 in Henan Province, China

Background

Porcine circovirus type 2 (PCV2) is the pathogen of porcine circovirus associated diseases (PCVAD) and one of the main pathogens in the global pig industry, which has brought huge economic losses to the pig industry. In recent years, there has been limited research on the prevalence of PCV2 in Henan Province. This study investigated the genotype and evolution of PCV2 in this area.

Results

We collected 117 clinical samples from different regions of Henan Province from 2015 to 2018. Here, we found that the PCV2 infection rate of PCV2 was 62.4%. Thirty-seven positive clinical samples were selected to amplify the complete genome of PCV2 and were sequenced. Based on the phylogenetic analysis of PCV2 ORF2 and complete genome, it was found that the 37 newly detected strains belonged to PCV2a (3 of 37), PCV2b (21 of 37) and PCV2d (13 of 37), indicating the predominant prevalence of PCV2b and PCV2d strains. In addition, we compared the amino acid sequences and found several amino acid mutation sites among different genotypes. Furthermore, the results of selective pressure analysis showed that there were 5 positive selection sites.

Conclusions

This study indicated the genetic diversity, molecular epidemiology and evolution of PCV2 genotypes in Henan Province during 2015–2018.

Phylogeographic and genetic characterization of porcine circovirus type 2 in Taiwan from 2001-2017

Porcine circovirus type 2 (PCV2) is an important pathogen that causes significant economic losses in the swine industry worldwide. Five major PCV2 genotypes have been identified, including PCV2a, PCV2b, PCV2c, PCV2d, and PCV2e. To investigate the prevalence and phylodynamics of the different PCV2 genotypes in Taiwan, 214 PCV2 ORF2 sequences from Taiwan and other countries were analyzed. Genotypic differences were observed among PCV2a, 2b, and 2d at amino acid position 89 in ORF2, with isoleucine (I), arginine (R), and leucine (L), respectively. Similar to other countries, a genotypic shift was also observed in Taiwan, where the predominant genotype shifted from PCV2b to 2d after 2010. The estimated nucleotide substitution rate of Taiwanese strains in the ORF2 region was 8.467 × 10⁻⁴ substitutions per site per year. This rapid evolution rate of PCV2 may lead to the genotypic shift observed in Taiwan. The times to the most recent common ancestor (TMRCA) for PCV2a, -2b, and -2d-2 was dated to 1970, 1992 and 2004, respectively. Thus, the PCV2a, -2b, and -2d genotypes were already present in Taiwan before the introduction of the PCV2 vaccine.

Genetic analysis of porcine circovirus type 2 (PCV2) strains between 2002 and 2016 reveals PCV2 mutant predominating in porcine population in Guangxi, China

Background

Porcine circovirus 2-associated disease (PCVAD) is acknowledged as one of the most economically important diseases for the swine industry worldwide. The aim of this study was to characterize and determine the genetic diversity of PCV2 in the porcine population of Guangxi, China.

Methods

The full length genome and open reading frame 2 (ORF2) of 95 PCV2 strains collected from the tissues and sera of pigs that had either died as a result of PCVAD or did not exhibit disease symptoms were analyzed.

Results

The results of multiple sequence alignments showed that there is considerable diversity among the PCV2 ORF2 sequences. Phylogenetic analyses based on the complete genome showed that current PCV2 strains in this study could be divided into PCV2a (1/95), PCV2b (39/95), PCV2d (43/95), PCV2e (10/95) and PCV2h (2/95). Among the 5 sub-genotypes, PCV2b was dominant in the porcine population from 2002 to 2008. The newly identified sub-genotype, PCV2d, was seen from 2003 and has increased every year. PCV2b and PCV2d formed two predominant genetic groups circulating in southern China between 2009 and 2013 and the sub-genotype PCV2d has become the dominant virus in China since 2014.

Conclusions

This study reveals the complex genetic diversity of PCV2 and improves our understanding regarding the epidemiological trends of PCV2 sub-genotypes in China.

Electronic supplementary material

The online version of this article (10.1186/s12917-019-1859-z) contains supplementary material, which is available to authorized users.

Longitudinal comparison of the humoral immune response and viral load of Porcine Circovirus Type 2 in pigs with different vaccination schemes under field conditions

Background: Porcine Circovirus type 2 (PCV2) infections are distributed worldwide and cause Porcine Circovirus Associated Disease (PCVAD). To minimize the impact of PCV2 infection on swine health and production, different vaccination schemes have been used since 2006. However, the association between vaccination schemes, virus load and disease under field conditions are not completely understood. Therefore, the objective of this study was to compare the effect of two different PCV2 vaccination schemes on the humoral response and PCV2 load in pigs after weaning under field conditions.

Methods: Two commercial pig farms (Farm A and B), endemically infected with PCV2, which were using two different PCV2 subunit vaccinations schemes for sow, gilts and piglets, were selected. We designed a longitudinal study and measured IgG levels by ELISA and virus load by quantitative PCR in pigs after weaning. Forty 3-week old piglets were randomly selected at weaning and followed for 20 weeks. IgG levels and virus loads were compared within and between farms and considered statistically different if the non-parametric Wilcoxon-test p value was lower than 0.05.

Results: We found that low virus loads were maintained in pigs from both farms regardless of the vaccination scheme used (p>0.05). However, there was significant difference in the mean IgG levels observed over time (p<0.05) while there were no significant differences in viral loads. This suggests that different humoral immune response is not associated with different virus loads observed over time.

Conclusions: These results are important because they can help to prevent PCV2 infections using different vaccination schemes to minimize the effect of PCVAD on swine health and production.

Porcine MKRN1 Modulates the Replication and Pathogenesis of Porcine Circovirus Type 2 by Inducing Capsid Protein Ubiquitination and Degradation

Porcine circovirus type 2 (PCV2) capsid protein (Cap) is a unique structure protein that plays pivotal roles in the process of viral replication and pathogenesis. Herein, we characterized a putative porcine Makorin RING finger protein 1 (pMKRN1) variant, an N-terminal-truncated variant of putative full-size porcine MKRN1 which has a unique expression pattern resulting from the porcine mkrn1 gene and which interacts with PCV2 Cap. A domain mapping assay showed that the C terminus of pMKRN1 and fragments (amino acids 108 to 198) of Cap are required for this interaction. PCV2 transiently upregulated pMKRN1 in PK-15 cells, but persistent viral infection downregulated pMKRN1 in major pathological tissues of PCV2-infected piglets. Overexpression of pMKRN1 significantly inhibited the generation of progeny PCV2 via ubiquitination and degradation of Cap, whereas knockout of pMKRN1 blocked Cap degradation and promoted progeny virus replication. pMKRN1 specifically targeted PCV2 Cap lysine residues 164, 179, and 191 to induce polyubiquitination and subsequent degradation. Mutation of either of the three lysine residues in the Cap protein or mutation of the histidine at residue 243 within the RING finger domain of pMKRN1 abrogated the E3 ligase activity of pMKRN1, rendering cells incapable of inducing Cap ubiquitination and degradation. Consistent with this finding, a Cap ubiquitination-deficient PCV2 strain showed enhanced virus replication and produced severe histological lesions in the lung and lymph node tissues compared with wild-type PCV2. Taken together, the results presented here suggest that PCV2 downregulates the pMKRN1 variant to avoid pMKRN1-mediated Cap ubiquitination and degradation, thus promoting viral replication and pathogenesis in its targeted tissues.IMPORTANCE Porcine circovirus type 2 is the pathogen to which pigs are the most susceptible, causing immense economic losses in the global swine industry, but whether host cells have developed some strategies to prevent viral replication is still unclear. Here, we found that porcine MKRN1 (pMKRN1) was upregulated in the early stage of PCV2 infection and mediated the polyubiquitination and degradation of Cap protein to block PCV2 replication, yet persistent PCV2 infection downregulated pMKRN1 levels to avoid degradation, promoting viral replication and pathogenesis in its targeted tissues. These data present new insight into the molecular mechanisms underlying the antiviral effects of pMKRN1 E3 ligase during PCV2 infection and also suggest potential new control measures for PCV2 outbreaks.

Identification of two novel CRESS DNA viruses associated with an Avipoxvirus lesion of a blue-and-gray Tanager (Thraupis episcopus)

The discovery of circular rep-encoding single stranded (CRESS) DNA viruses has increased spectacularly over the past decade. They represent the smallest animal viruses known worldwide infecting a wide variety of invertebrates and vertebrates in different natural and human-made environments. The extremely low similarity of nucleotide and protein sequences among different CRESS DNA genomes has challenged their classification. Moreover, the existence of capsid proteins (Cp) remains difficult to demonstrate which is crucial to understand the structural properties of these viruses. Here we describe two unclassified CRESS DNA viruses isolated from a cutaneous lesion, caused by a strain of Avipoxvirus, from a blue-and-gray tanager (Thraupis episcopus) in Southern Ecuador. Both viruses present replication-associated proteins (Rep) and one to two open reading frames (ORF), one of which represents a putative Cp. The two new Rep are long proteins characterized by the existence of the several highly conserved amino acid residues characteristic of rolling circle replication. Within the putative Cp we detected intrinsically disordered regions (IDR), potential protein and DNA binding regions, and nuclear localization signals (NLS), providing further evidence of presumed Cp. Despite being found on the same host lesion, both viruses show low similarity between each other (<60%) and other known CRESS DNA viruses. Furthermore, we analyze the evolutionary relationships within the CRESS DNA diversity. Additional sampling is needed to explore the possible pathogenic effects, prevalence and diversity (both phylogenetical and structural) of these viruses in wild bird populations.

The level of decoy epitope in PCV2 vaccine affects the neutralizing activity of sera in the immunized animals

Viral pathogens have evolved a wide range of tactics to evade host immune responses and thus propagate effectively. One efficient tactic is to divert host immune responses toward an immunodominant decoy epitope and to induce non-neutralizing antibodies toward this epitope. Therefore, it is expected that the amount of decoy epitope in a subunit vaccine can affect the level of neutralizing antibody in an immunized animal. In this study, we tested this hypothesis by generating an antibody specific to the decoy epitope on the capsid protein of porcine circovirus type 2 (PCV2). Using this antibody, we found that two commercial vaccines contained statistically different amounts of the decoy epitope. The vaccine with lower levels of decoy epitope induced a significantly higher level of neutralizing antibody after immunization. This antibody can be used as an analytical tool to monitor the quality of a vaccine from batch to batch.

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We generated a novel antibody specific to an immunodominant decoy epitope of PCV2.

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Using this novel antibody, we measured levels of decoy epitope in PCV2 vaccine.

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Decoy epitope in PCV2 vaccine affected the neutralizing antibody titer induction.

Phylogenetic and genetic variation analyses of porcine circovirus type 2 isolated from China

Porcine circovirus type 2 (PCV2) is a causative agent of PCV2-associated disease, which is a growing problem in the swine industry worldwide. High nucleotide substitution occurs in the capsid (Cap) gene of PCV2, which allows the continuous evolution and the emergence of novel PCV2 strains. In this study, we sequenced 24 Chinese PCV2 strains collected from healthy and diseased pigs between 2013 and 2015. Analyses of the genome, Cap and phylogeny classified the 24 Chinese PCV2 strains as PCV-2a (four of 24), PCV-2b (five of 24) and PCV-2d (15 of 24). All strains shared 89.5%-100% and 87.2%-100% identities with the nucleotide and amino acid (aa) sequences of Cap, respectively. Selection pressure analysis showed that five sites at the epitope regions in Cap were under positive selection. Further analysis by Jameson-Wolf antigenic index indicated that aa substitutions occurring at the epitope regions contributed to the antigenic alterations of the different PCV2 strains. High genetic variation and genotype shift to PCV2d occurred in recent years, and different genotypes coexisted in Chinese pig herds. The data provide evidence for the increased genetic diversity and insights into the molecular epidemiology of PCV2.

Exploitation of stable nanostructures based on the mouse polyomavirus for development of a recombinant vaccine against porcine circovirus 2

The aim of this study was to develop a suitable vaccine antigen against porcine circovirus 2 (PCV2), the causative agent of post-weaning multi-systemic wasting syndrome, which causes significant economic losses in swine breeding. Chimeric antigens containing PCV2b Cap protein sequences based on the mouse polyomavirus (MPyV) nanostructures were developed. First, universal vectors for baculovirus-directed production of chimeric MPyV VLPs or pentamers of the major capsid protein, VP1, were designed for their exploitation as vaccines against other pathogens. Various strategies were employed based on: A) exposure of selected immunogenic epitopes on the surface of MPyV VLPs by insertion into a surface loop of the VP1 protein, B) insertion of foreign protein molecules inside the VLPs, or C) fusion of a foreign protein or its part with the C-terminus of VP1 protein, to form giant pentamers of a chimeric protein. We evaluated these strategies by developing a recombinant vaccine against porcine circovirus 2. All candidate vaccines induced the production of antibodies against the capsid protein of porcine circovirus after immunization of mice. The candidate vaccine, Var C, based on fusion of mouse polyomavirus and porcine circovirus capsid proteins, could induce the production of antibodies with the highest PCV2 neutralizing capacity. Its ability to induce the production of neutralization antibodies was verified after immunization of pigs. The advantage of this vaccine, apart from its efficient production in insect cells and easy purification, is that it represents a DIVA (differentiating infected from vaccinated animals) vaccine, which also induces an immune response against the mouse polyoma VP1 protein and is thus able to distinguish between vaccinated and naturally infected animals.

Nanoparticle orientationally displayed antigen epitopes improve neutralizing antibody level in a model of porcine circovirus type 2

Recent advancements in biotechnology have enabled the rapid identification and subsequent expression of pathogenic microbial major antigens that induce protective immune responses. However, subunit vaccines have not been successfully commercialized mainly due to the lack of sufficient levels of neutralizing antibodies (NAs). High levels of NA rely on the efficient recognition and cross-linking of multiple neutralizing epitopes with B-cell receptors (BCRs). Nanoparticles are able to display coupled antigenic arrays at high density and provide multiple binding molecular scenarios with BCRs. The high-resolution antigenic structure makes it possible to accurately display stable neutralizing epitopes. Therefore, the development of a nanovaccine that orientationally displays neutralizing epitopes is a feasible strategy. To address this hypothesis, the capsid (Cap) protein of porcine circovirus type 2 as model antigen was conjugated to gold nanoparticles (AuNPs) through direct reaction of the mercapto group of the unique cysteines with AuNPs, rendering Cap-AuNPs to have neutralizing epitopes on outer surface and an immunodominant epitope buried within the inner surface. In vitro studies showed that AuNPs promoted the phagocytosis of Cap protein and NA levels were significantly improved, meanwhile antibody levels against the immunodominant epitope was significantly reduced. In mouse studies, Cap-AuNP-immunized mice displayed a high production of interleukin (IL)-4, IL-10, and interferon-γ, suggesting that Cap-AuNPs can effectively activate CD4⁺ and CD8⁺ T cells and balance Th1 and Th2 cellular responses. This study presents a new vaccine design strategy based on antigen structure, where nanoparticles are coupled to antigens in well-ordered arrays and orientationally display neutralizing epitopes to enhance NA levels.

[Occurrence of genotypes of porcine circovirus (PCV2) in pig farms using different vaccination strategies against PCV2]

OBJECTIVE

Since 2004/2005 a worldwide shift of the detection rate of porcine circovirus (PCV) has been observed from PCV2a towards PCV2b. Currently commercially available vaccines are based on genotype PCV2a. The study was conducted as a pilot study to evaluate the occurrence of PCV2a and PCV2b in farms with different vaccination strategies against PCV2.

MATERIAL AND METHODS

For this purpose a total of 405 piglets originating from nine farms (three farms with sow vaccination [SI], piglet vaccination [FI] and no vaccination [NI] against PCV2, respectively) were enrolled and followed from day 3 of life until slaughter. Serum of the piglets was examined for PCV2-DNA by quantitative PCR, genotype differentiating duplex PCR, and after sequencing of the total genome, PCV2 isolates were phylogenetically assigned. The evaluation included the data from 383 animals.

RESULTS

In eight farms PCV2 could be detected (1x PCV2a; 6x PCV2b; 1x PCV2a and PCV2b). PCV2b was found in SI-, NI- and FI-farms, whereas PCV2a was only detected in SI- and NI-farms. A proportion of 55.4% was PCV2-positive at least once during the entire study period (FI: 7.8%, SI: 65.4%, NI: 93.7%). Of these samples 4.7% were PCV2a-, 92.2% PCV2b- and 2.4% PCV2a- and PCV2b-positive. The mean content of PCV2-DNA in the serum of PCV2b positive animals was significantly higher than from PCV2a positive animals. PCV2 isolates were identified as PCV2b-1A (5/9 farms), PCV2b-1B (1/9 farms) und PCV2a-2D (2/9 farms).

CONCLUSION AND CLINICAL RELEVANCE

The increased detection rate of PCV2b in comparison to PCV2a could be confirmed. The present study gives hint that the vaccination of piglets using PCV2a-based vaccines may lead to a further shift of the detection rate from PCV2a to PCV2b. To assess the clinical relevance of this observation, extensive comparative studies should be taken into account, which also evaluate the efficacy of PCV2a-based vaccines in PCV2a- and PCV2b-positive farms.

Versatile carboxyl-terminus of capsid protein of porcine circovirus type 2 were recognized by monoclonal antibodies with pluripotency of binding

We designed the peptide (C3) mimetic carboxyl-terminus (Cterminus) of capsid protein of porcine circovirus type 2b (PCV2b-1A/1B) inducing humoral immunity and generating hybridomas. The positive reactivity of the mAbs to PCV2 capsid protein was demonstrated by Western blot assay. Those mAbs also showed positive signals on PCV2b infected swine lymphocytes by indirect immunofluorescence staining. The mAb 1H3 bound to three minimal linear epitopes (P62, DPPLNP; P67, DPPLNPK; P73, LKDPPLKP), which was located at Cterminus of the capsid protein of PCV2b-1A/1B, PCV2b-1C, and PCV2a-2A respectively. The mAbs 3B2 bound to only one minimal linear epitopes (P59, KDPPLNP). The mAbs 6B8 bound to two minimal linear epitopes (P59 and P67). Our data demonstrate the core motif (P62) within the P59 could be recognized by mAbs (3B2 and 6B8) in the free status by liquid phase blocking immunoassay (LPBI) but not be recognized by these mAbs in the fixed form on the plate by indirect ELISA (iELISA). However, the P73 could be recognized by mAb 1H3 by iELISA but no inhibition of the interactive binding of C3 and mAb 1H3 by LPBI. This study also indicated that IgM mAbs and defective Ig mAb have broad binding, moderate specificity and low affinity. This study confirm that mAbs have pluripotency of binding. It might be a phenomenon of antibody response to Cterminus of capsid protein of PCV2b.

Evidence of natural co-infection with PCV2b subtypes in vivo

Porcine circovirus type 2 (PCV2) is the causative pathogen of porcine circovirus-associated diseases (PCVAD). This virus evolves mostly through point mutations and genome recombination between different PCV2 genotypes (e.g. PCV2a and PCV2b), as has been confirmed in swine herds. In the current work, the complete PCV2 genome sequences of 69 clones derived from various tissues (lymph node, spleen and lung,) of an infected individual, were subjected to phylogenetic and alignment analyses. The results not only demonstrate co-infection with distinct PCV2b subtypes (e.g. 1B and 1C) in the same animal, but also highlight another mechanism of evolution - diverse point mutations acquired during immune evasion by this virus.

In silico analyses of antigenicity and surface structure variation of an emerging porcine circovirus genotype 2b mutant, prevalent in southern China from 2013 to 2015

Porcine circovirus type 2 (PCV2) is the pivotal pathogen causing porcine circovirus-associated diseases. In this study, 62 PCV2 isolates were identified from seven farms in southern China from 2013 to 2015 and phylogenetic trees were reconstructed based on whole-genome sequences or the cap gene. In this investigation, PCV2b was the main genotype in circulation throughout these farms. Furthermore, an emerging mutant (PCV2b-1C), isolated from PCV2-vaccinated farms, was the predominant strain prevalent on these farms. In addition, we isolated a new cluster that may represent evolution of the virus through recombination of PCV2b-1A/1B and PCV2b-1C. Finally, we discuss evidence that antigenicity and surface structure variation of the capsid resulted from mutation of the C-terminal loop (Loop CT) of the PCV2b-1C Cap in silico.

Porcine circovirus type 2 induces type I interferon production via MyD88-IKKα-IRFs signaling rather than NF-κB in porcine alveolar macrophages in vitro

Type I interferon (IFN-I) plays important roles in host antiviral responses. The interferon regulatory factor (IRF) and NF-κB transcription factors are thought to be important in the processes of viral secretion and triggering of interferon production. Recently, studies have shown that porcine circovirus type 2 (PCV2) can induce IFN-I production in vivo and in vitro, but the mechanisms underlying the production of PAMs infected with PCV2 remains unknown. Treatment of these cells with BAY11-7082, an inhibitor of NF-κB activation, allowed us to study the secretion of IFN-α and IFN-β in PAMs infected with PCV2. We found that IFN-α expression was induced following virus infection of PAMs. Notably, even after inhibitor treatment of PAMs infected with PCV2, secretion of IFN-α was significantly higher (P<0.05) compared with the PCV2 infection alone group. Our findings suggest that NF-κB plays a minor role in PCV2-induced type I interferon responses. To further characterize the signaling pathway that drives IFN-I expression in PAMs in response to PCV2, we used siRNA to silence the expression of Myeloid differentiation factor 88 (MyD88) and study the role of MyD88-IKKα-IRF signaling in IFN-I production in PAMs induced by PCV2. Our findings show that PCV2 induced IFN-α mRNA transcription, which is associated with the activities of MyD88, IRF7, and IRF3. Thus, PCV2 can induce IFN-I transcription via the MyD88-IKKα-IRF signaling axis.

Evaluation of the use of non-pathogenic porcine circovirus type 1 as a vaccine delivery virus vector to express antigenic epitopes of porcine reproductive and respiratory syndrome virus

We previously demonstrated that the C-terminus of the capsid gene of porcine circovirus type 2 (PCV2) is an immune reactive epitope displayed on the surface of virions. Insertion of foreign epitope tags in the C-terminus produced infectious virions that elicited humoral immune responses against both PCV2 capsid and the inserted epitope tags, whereas mutation in the N terminus impaired viral replication. Since the non-pathogenic porcine circovirus type 1 (PCV1) shares similar genomic organization and significant sequence identity with pathogenic PCV2, in this study we evaluated whether PCV1 can serve as a vaccine delivery virus vector. Four different antigenic determinants of porcine reproductive and respiratory syndrome virus (PRRSV) were inserted in the C-terminus of the PCV1 capsid gene, the infectivity and immunogenicity of the resulting viruses are determined. We showed that an insertion of 12 (PRRSV-GP2 epitope II, PRRSV-GP3 epitope I, and PRRSV-GP5 epitope I), and 14 (PRRSV-GP5 epitope IV) amino acid residues did not affect PCV1 replication. We successfully rescued and characterized four chimeric PCV1 viruses expressing PRRSV linear antigenic determinants (GP2 epitope II: aa 40-51, ASPSHVGWWSFA; GP3 epitope I: aa 61-72, QAAAEAYEPGRS; GP5 epitope I: aa 35-46, SSSNLQLIYNLT; and GP5 epitope IV: aa 187-200, TPVTRVSAEQWGRP). We demonstrated that all chimeric viruses were stable and infectious in vitro and three chimeric viruses were infectious in vivo. An immunogenicity study in pigs revealed that PCV1-VR2385EPI chimeric viruses elicited neutralizing antibodies against PRRSV-VR2385. The results have important implications for further evaluating PCV1 as a potential vaccine delivery vector.

Multiplex serology for common viral infections in feral pigs (Sus scrofa) in Hawaii between 2007 and 2010

Multiplex serology was performed for the detection of total immunoglobulin (Ig) and IgM antibodies against porcine circovirus type 2 (PCV2), porcine reproductive and respiratory syndrome virus (PRRSV), and swine influenza virus (SIV) antigens in feral swine (Sus scrofa). Serum samples were collected from the islands of Oahu (292 pigs) and Hawaii (52 pigs) between 2007 and 2010. The highest antibody prevalence was to PCV2 (63%), followed by SIV (7.8%) and PRRSV (5.8%). Antigen-specific IgM was detected at a much lower prevalence. PCR amplification and sequence analysis of PCV2 in three IgM-positive samples identified PCV2b as the only genotype. While the prevalence of PCV2 and PRRSV remained similar between 2007 and 2010, the percentage of SIV-positive samples on Oahu increased from 2% to 19%. Our results demonstrate the utility of multiplex serology for pathogen surveillance in feral pig populations.

Revisiting the taxonomical classification of Porcine Circovirus type 2 (PCV2): still a real challenge

BACKGROUND

PCV2 has emerged as one of the most devastating viral infections of swine farming, causing a relevant economic impact due to direct losses and control strategies expenses. Epidemiological and experimental studies have evidenced that genetic diversity is potentially affecting the virulence of PVC2. The growing number of PCV2 complete genomes and partial sequences available at GenBank questioned the accepted PCV2 classification.

METHODS

Nine hundred seventy five PCV2 complete genomes and 1,270 ORF2 sequences available from GenBank were subjected to recombination, PASC and phylogenetic analyses and results were used for comparison with previous classification scheme.

RESULTS

The outcome of these analyses favors the recognition of four genotypes on the basis of ORF2 sequences, namely PCV2a, PCV2b, PCV2c and PCV2d-mPCV2b. To deal with the difficulty of founding an unambiguous classification and accounting the impossibility to define a p-distance cut-off, a set of reference sequences that could be used in further phylogenetic studies for PCV2 genotyping was established. Being aware that extensive phylogenetic analyses are time-consuming and often impracticable during routine diagnostic activity, ORF2 nucleotide positions adequately conserved in the reference sequences were identified and reported to allow a quick genotype differentiation.

CONCLUSIONS

Globally, the present work provides an updated scenario of PCV2 genotypes distribution and, based on the limits of the previous classification criteria, proposes new rapid and effective schemes for differentiating the four defined PCV2 genotypes.

Expression of antigenic epitopes of porcine reproductive and respiratory syndrome virus (PRRSV) in a modified live-attenuated porcine circovirus type 2 (PCV2) vaccine virus (PCV1-2a) as a potential bivalent vaccine against both PCV2 and PRRSV

Co-infection of pigs in the field with porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV) is common and poses a major concern in effective control of PCV2 and PRRSV. We previously demonstrated that insertion of foreign epitope tags in the C-terminus of PCV2 ORF2 produced infectious virions that elicited humoral immune responses against both PCV2 capsid and inserted epitope tags. In this study, we aimed to determine whether the non-pathogenic chimeric virus PCV1-2a, which is the basis for the licensed PCV2 vaccine Fostera PCV, can express PRRSV antigenic epitopes, thus generating dual immunity as a potential bivalent vaccine against both PCV2 and PPRSV. Four different linear B-cell antigenic epitopes of PRRSV were inserted into the C-terminus of the capsid gene of the PCV1-2a vaccine virus. We showed that insertion of 12 (PRRSV-GP2 epitope II, PRRSV-GP3 epitope I, and PRRSV-GP5 epitope I), and 14 (PRRSV-GP5 epitope IV) amino acid residues did not impair the replication of the resulting PCV1-2a-PRRSVEPI chimeric viruses in vitro. The four chimeric PCV1-2a viruses expressing PRRSV B-cell linear epitopes were successfully rescued and characterized. An immunogenicity study in pigs revealed that two of the four chimeric viruses, PCV1-2a-PRRSVEPIGP3IG and PCV1-2a-PRRSVEPIEPIGP5IV, elicited neutralizing antibodies against PRRSV VR2385 as well as PCV2 (strains PCV2a, PCV2b, and mPCV2b). The results have important implications for exploring the potential use of PCV1-2a vaccine virus as a live virus vector to develop bivalent MLVs against both PCV2 and PRRSV.

Immune responses of piglets immunized by a recombinant plasmid containing porcine circovirus type 2 and porcine interleukin-18 genes

In this study, two recombinant plasmids containing the ORF2 gene of porcine circovirus type 2 (PCV2) with or without porcine interleukin-18 (IL-18) were constructed and evaluated for their ability to protect piglets against PCV2 challenge. Transient expression of the plasmids in PK-15 cells could be detected using Western blot. Piglets were given two intramuscular immunizations 3 weeks apart and were challenged with a virulent Wuzhi strain of PCV2 at 42 days after the initial immunization. All animals vaccinated with pBudCE4.1-ORF2 or with pBudCE4.1-ORF2/IL18 developed PCV2-specific antibody and T-lymphocyte proliferative responses. The levels of T-lymphocyte proliferation in piglets immunized with pBudCE4.1-ORF2/IL18 were significantly higher than in those immunized with pBudCE4.1-ORF2, and pBudCE4.1-ORF2/IL18 stimulated a significantly increased production of IFN-γ and IL-2. Furthermore, PCV2 challenge experiments showed that the DNA vaccine-immunized groups can partially prevent PCV2 viremia and significantly reduce the amount of PCV2 virus in the lymphoid tissues, and the piglets immunized by pBudCE4.1-ORF2/IL18 exhibit a marked inhibition of PCV2 replication compared to the pBudCE4.1-ORF2 group. These data demonstrate that the plasmid pBudCE4.1-ORF2/IL18 may be an effective approach for increasing PCV2 DNA vaccine immunogenicity.

Porcine circovirus: a historical perspective

Porcine circoviruses (PCVs) belong to the genus Circovirus and the family Circoviridae, and they are the smallest known viruses that replicate autonomously in mammalian cells. They are nonenveloped, and they have characteristic single-stranded, negative-sense, circular DNA. Two types of divergent PCVs are recognized: PCV1 and PCV2. About 20 years ago, PCV2 began to emerge as a major pathogen of swine around the world, leading to burgeoning knowledge about the virus and porcine circovirus-associated diseases. However, much of the history of its discovery, including the controversy related to its importance, is not recorded. This review examines current issues related to the biology of PCV2 in the context of the original studies related to determining its causal association with disease and to the evolving understanding of the complex pathogenesis of PCV2 infections.

A novel subunit vaccine co-expressing GM-CSF and PCV2b Cap protein enhances protective immunity against porcine circovirus type 2 in piglets

Porcine circovirus type 2 (PCV2) causes porcine circovirus-associated disease. Capsid (Cap) protein of PCV2 is the principal immunogenic protein that induces neutralizing antibodies and protective immunity. GM-CSF is an immune adjuvant that enhances responses to vaccines. In this study, recombinant baculoviruses Ac-Cap and Ac-Cap-GM-CSF expressing the Cap protein alone and co-expressing the Cap protein and porcine GM-CSF, respectively, were constructed successfully. The target proteins were analyzed by western blotting and IFA. Further, these proteins were confirmed by electron microscopy, which showed that Cap proteins could self-assemble into virus-like particles having diameters of 17-25nm. Animal experiments showed that pigs immunized with Cap-GM-CSF subunit vaccine showed significantly higher levels of PCV2-specific antibodies and neutralizing antibodies than pigs immunized with the Cap subunit vaccine and a commercial vaccine (Ingelvac CircoFLEX; P<0.05). After PCV2 wild strain challenged, Pigs receiving the Cap-GM-CSF subunit vaccine showed significantly higher average daily weight gain after wild-type PCV2 challenge than pigs receiving the other three vaccines (P<0.05). None of PCV2 DNA was detected in all immunized animals, except control animals immunized with phosphate-buffered saline. These results indicated that GM-CSF was a powerful immunoadjuvant for PCV2 subunit vaccines because it enhanced humoral immune response and improved immune protection against PCV2 infection in pigs. Thus, the novel Cap-GM-CSF subunit vaccine has the potential to be used as an effective and safe vaccine candidate against PCV2 infection.

Biology and Diseases of Swine

Swine are used in biomedical research as models for biomedical research and for teaching. This chapter covers normative biology and behavior along with common and emerging swine diseases. Xenotransplantation is discussed along with similarities and differences of swine immunology.

Porcine circovirus type 2 affects the serum profile of amino acids and intestinal expression of amino acid transporters in mice

PCV2 is highly pathogenic, however, its effect on the serum amino acids profile is unknown.

Complete Genome Sequence of Porcine Circovirus Type 2 Strain HB-MC1

Porcine circovirus type 2 (PCV2) is the primary causative agent of porcine circovirus-associated disease (PCVAD). Here, we report the complete genome sequence of PCV2 strain HB-MC1, which belongs to PCV2d.

The comparative profile of lymphoid cells and the T and B cell spectratype of germ-free piglets infected with viruses SIV, PRRSV or PCV2

Lymphocyte subsets isolated from germ-free piglets experimentally infected with swine influenza virus (SIV), porcine reproductive and respiratory syndrome virus (PRRSV) or porcine circovirus type 2 (PCV2) were studied and the profile of these subsets among these three infections was monitored. Germ-free piglets were used since their response could be directly correlated to the viral infection. Because SIV infections are resolved even by colostrum-deprived neonates whereas PRRSV and PCV2 infections are not, SIV was used as a benchmark for an effectively resolved viral infection. PRRSV caused a large increase in the proportion of lymphocytes at the site of infection and rapid differentiation of B cells leading to a high level of Ig-producing cells but a severe reduction in CD2^—CD21⁺ primed B cells. Unlike SIV and PCV2, PRRSV also caused an increase in terminally differentiated subset of CD2⁺CD8α⁺ γδ cells and polyclonal expansion of major Vβ families suggesting that non-specific helper T cells drive swift B cell activation. Distinct from infections with SIV and PRRSV, PCV2 infection led to the: (a) prevalence of MHC-II⁺ T cytotoxic cells, (b) restriction of the T helper compartment in the respiratory tract, (c) generation of a high proportion of FoxP3⁺ T cells in the blood and (d) selective expansion of IgA and IgE suggesting this virus elicits a mucosal immune response. Our findings suggest that PRRSV and PCV2 may negatively modulate the host immune system by different mechanisms which may explain their persistence.