Porcine circoviruses--small but powerful

Iron‑sulfur clusters in viral proteins: Exploring their elusive nature, roles and new avenues for targeting infections

Viruses have evolved complex mechanisms to exploit host factors for replication and assembly. In response, host cells have developed strategies to block viruses, engaging in a continuous co-evolutionary battle. This dynamic interaction often revolves around the competition for essential resources necessary for both host cell and virus replication. Notably, iron, required for the biosynthesis of several cofactors, including iron‑sulfur (FeS) clusters, represents a critical element in the ongoing competition for resources between infectious agents and host. Although several recent studies have identified FeS cofactors at the core of virus replication machineries, our understanding of their specific roles and the cellular processes responsible for their incorporation into viral proteins remains limited. This review aims to consolidate our current knowledge of viral components that have been characterized as FeS proteins and elucidate how viruses harness these versatile cofactors to their benefit. Its objective is also to propose that viruses may depend on incorporation of FeS cofactors more extensively than is currently known. This has the potential to revolutionize our understanding of viral replication, thereby carrying significant implications for the development of strategies to target infections.

Comparison of immune effects of porcine circovirus type 2d (PCV2d) capsid protein expressed by Escherichia coli and baculovirus-insect cells

Porcine circovirus type 2 (PCV2) is an important pathogen harmful to global pig production, which causes immunosuppression and serious economic losses. PCV2 capsid (Cap) protein expressed by E. coli or baculovirus-insect cells are often used in preparation of PCV2 subunit vaccines, but the latter is expensive to produce. It is therefore crucial to comparison of the immune effects of Cap protein expressed by the above two expression systems for reducing the production cost and guaranteeing PCV2 vaccine quality. In this study, the PCV2d-Cap protein lacking nuclear localization signal (NLS), designated as E. coli-Cap and Bac-Cap, was expressed by E. coli and baculovirus-Spodoptera frugiperda Sf9 (Bac-Sf9) cells, respectively. The expressed Cap proteins could self-assemble into virus-like particles (VLPs), but the Bac-Cap-assembled VLPs were more regular. The two system-expressed Cap proteins induced similar specific IgG responses in mice, but the neutralizing antibody levels of Bac-Cap-immunized mice was higher than those of E. coli-Cap. After PCV2 challenge, IL-10 in Bac-Cap immunized mice decreased significantly than that in E. coli-Cap. The lesions and PCV2 antigen positive cells in tissues of mice immunized with E. coli-Cap and Bac-Cap were significantly reduced, and Bac-Cap appeared mild lesions and fewer PCV2 antigen-positive cells compared with E. coli-Cap immunized mice. The study indicated that Cap proteins expressed by E. coli and Bac-Sf9 cells could induce specific protective immunity, but the latter induced more effective immunity, which provides valuable information for the research and development of PCV2 vaccine.

Crystal structure of the ATPase domain of porcine circovirus type 2 Rep protein

PCV2 belongs to the genus Circovirus in the family Circoviridae, whose genome is replicated by rolling circle replication (RCR). PCV2 Rep is a multifunctional enzyme that performs essential functions at multiple stages of viral replication. Rep is responsible for nicking and ligating single-stranded DNA and unwinding double-stranded DNA (dsDNA). However, the structure and function of the Rep are still poorly understood, which significantly impedes viral replication research. This study successfully resolved the structure of the PCV2 Rep ATPase domain (PRAD) using X-ray crystallography. Homologous structure search revealed that Rep belonged to the superfamily 3 (SF3) helicase, and multiple conserved residues were identified during sequence alignment with SF3 family members. Simultaneously, a hexameric PRAD model was generated for analysing characteristic structures and sites. Mutation of the conserved site and measurement of its activity showed that the hallmark motifs of the SF3 family influenced helicase activity by affecting ATPase activity and β-hairpin just caused the loss of helicase activity. The structural and functional analyses of the PRAD provide valuable insights for future research on PCV2 replication and antiviral strategies.

Dynamics and Conformations of a Full-Length CRESS-DNA Replicase

Circular Rep-encoding single-stranded DNA (CRESS-DNA) viruses encode for a Replicase (Rep) that is essential for viral replication. Rep is a helicase with three domains: an endonuclease, an oligomeric, and an ATPase domain (ED, OD, and AD). Our recent cryo-EM structure of the porcine circovirus 2 (PCV2) Rep provided the first structure of a CRESS-DNA Rep. The structure visualized the ED to be highly mobile, Rep to form a homo-hexamer, bound ssDNA and nucleotides, and the AD to adopt a staircase arrangement around the ssDNA. We proposed a hand-over-hand mechanism by the ADs for ssDNA translocation. The hand-over-hand mechanism requires extensive movement of the AD. Here, we scrutinize this mechanism using all-atom Molecular Dynamics (MD) simulation of Rep in three states: (1) Rep bound to ssDNA and ADP, (2) Rep bound to ssDNA, and (3) Rep by itself. Each of the 700 nsec simulations converges within 200 nsec and provides important insight into the dynamics of Rep, the dynamics of Rep in the presence of these biomolecules, and the importance of ssDNA and ADP in driving the AD to adopt the staircase arrangement around the ssDNA. To the best of our knowledge, this is the first example of an all-atom MD simulation of a CRESS-DNA Rep. This study sets the basis of further MD studies aimed at obtaining a chemical understanding of how Rep uses nucleotide binding and hydrolysis to translocate ssDNA.

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.

Rapid and Easy-Read Porcine Circovirus Type 4 Detection with CRISPR-Cas13a-Based Lateral Flow Strip

First identified as a new circovirus in Hunan Province in China in 2019, porcine circovirus (PCV4) is now widely detected in other Chinese provinces and South Korea. In recent years, the virus has threatened pig health and operations in the pig industry. Hence, early PCV4 detection and regular surveillance are required to control the spread of infection and prevent collateral damage to the industry. Due to PCV4 being difficult to isolate in vitro, molecular detection methods, such as conventional PCR and real-time PCR, and serological assays are currently the main methods used for the detection of PCV4 infection. However, they are time-consuming, labor-intensive, and complex and require professional personnel. To facilitate rapid pen-side PCV4 diagnoses, we used clustered regularly interspaced short palindromic repeats (CRISPR) and Cas13a technology to develop a quick testing kit. Five recombinase-aided amplification (RPA) primer sets were designed based on the conserved PCV4-Cap gene nucleotide region, which were used to determine several key lateral flow strip (LFD) characteristics (sensitivity, specificity, and accuracy). The results showed that the RPA-Cas13a-LFD reaction could detect PCV4 within 1.5 h in genomic DNA harboring a minimum of a single copy. Furthermore, the assay showed good specificity and absence of cross-reactivity with PCV2, PCV3, or other porcine viruses. When we tested 15 clinical samples, a high accuracy was also recorded. Therefore, we successfully developed a detection assay that was simple, fast, accurate, and suitable for on-site PCV4 testing.

miR-214-5p/C1QTNF1 axis enhances PCV2 replication through promoting autophagy by targeting AKT/mTOR signaling pathway

Porcine circovirus type 2 (PCV2) is the causative agent of PCV2-associated disease, which causes a relevant economic impact on the global swine industry. Accumulating data have indicated host microRNAs play essential roles in numerous virus replication of pigs, while their roles in PCV2 replication remain unclear. Herein, we demonstrated that PCV2 infection downregulated the expression of miR-214-5p in PK15 cells, and miR-214-5p promoted PCV2 replication. C1q/tumor necrosis factor-related protein 1 (C1QTNF1) was then identified as a target gene of miR-214-5p, and C1QTNF1 suppressed PCV2 replication. Interestingly, miR-214-5p/C1QTNF1 axis negatively regulated AKT/mTOR signaling, and then enhanced PCV2 replication through promoting autophagy in PK15 cells. Collectively, our findings provide insight into the mechanism of PCV2 replication and highlight miR-214-5p and C1QTNF1 as potential novel targets for the treatment of PCV2 infection.

Reekeekee- and roodoodooviruses, two different Microviridae clades constituted by the smallest DNA phages

Small circular single-stranded DNA viruses of the Microviridae family are both prevalent and diverse in all ecosystems. They usually harbor a genome between 4.3 and 6.3 kb, with a microvirus recently isolated from a marine Alphaproteobacteria being the smallest known genome of a DNA phage (4.248 kb). A subfamily, Amoyvirinae, has been proposed to classify this virus and other related small Alphaproteobacteria-infecting phages. Here, we report the discovery, in meta-omics data sets from various aquatic ecosystems, of sixteen complete microvirus genomes significantly smaller (2.991-3.692 kb) than known ones. Phylogenetic analysis reveals that these sixteen genomes represent two related, yet distinct and diverse, novel groups of microviruses-amoyviruses being their closest known relatives. We propose that these small microviruses are members of two tentatively named subfamilies Reekeekeevirinae and Roodoodoovirinae. As known microvirus genomes encode many overlapping and overprinted genes that are not identified by gene prediction software, we developed a new methodology to identify all genes based on protein conservation, amino acid composition, and selection pressure estimations. Surprisingly, only four to five genes could be identified per genome, with the number of overprinted genes lower than that in phiX174. These small genomes thus tend to have both a lower number of genes and a shorter length for each gene, leaving no place for variable gene regions that could harbor overprinted genes. Even more surprisingly, these two Microviridae groups had specific and different gene content, and major differences in their conserved protein sequences, highlighting that these two related groups of small genome microviruses use very different strategies to fulfill their lifecycle with such a small number of genes. The discovery of these genomes and the detailed prediction and annotation of their genome content expand our understanding of ssDNA phages in nature and are further evidence that these viruses have explored a wide range of possibilities during their long evolution.

First Identification and Phylogenetic Analysis of Porcine Circovirus Type 4 in Fur Animals in Hebei, China

A novel circovirus called porcine circovirus type 4 (PCV4) was recently detected in pigs suffering from severe clinical diseases in Hunan province, China. There are few reports on the origin and evolution of PCV4, although some researchers have conducted epidemiological investigations of PCV4 and found that PCV4 is widespread in pigs. Based on the previous study, we detected PCV2 in farmed foxes and raccoon dogs with reproductive failure. To explore whether the PCV4 genome also exists in fur animals, we detected 137 cases admitted from fur animal farms in Hebei China between 2015 and 2020, which were characterized by inappetence, lethargy, depression, abortion, and sterility. The overall infection rate of PCV4 was 23.36% (32/137), including 20.37% (22/108) for raccoon dogs, 18.75% (3/16) for foxes, and 53.85% (7/13) for minks. Finally, five raccoon dog-origin PCV4 strains and one fox-origin PCV4 strain were sequenced in our study, whose nucleotide identities with other representative PCV4 strains varied from 96.5% to 100%. Phylogenetic analysis based on the complete genomes of PCV4 strains indicated a close relationship with those of PCV4 strains identified from pigs. To our knowledge, this is the first study to detect PCV4 in fur animals. Interestingly, we also identified PCV4 in a mixed farm (feeding pigs and raccoon dogs at the same time). In summary, our findings extend the understanding of the molecular epidemiology of PCV4 and provide new evidence for its cross-species transmission.

Molecular Epidemiology and Genetic Characterization of PCV2 Circulating in Wild Boars in Southwestern Ethiopia

Porcine circovirus type 2 (PCV2) is one of the most relevant infectious agents affecting domestic pigs. Recently, a surprising PCV2 genetic heterogenicity has been reported in Africa. Nevertheless, the knowledge of the epidemiology of PCV2 in African countries, in both domestic and wild species, is limited and sparse. Having this in mind, in the present study, the PCV2 circulation and its molecular epidemiology in Southwestern Ethiopia have been investigated by collecting 64 samples from domestic pigs, wild boars, and warthogs. PCV2 genome presence was detected and quantified using qPCR and ORF2 sequencing was attempted on positive samples. Ten samples, 8 wild boars, 1 domestic pig, and 1 warthog, tested PCV2 positive. Complete ORF2 sequences were obtained from 5 wild boars; 4 of those were classified as PCV2d and 1 as PCV2b. Both PCV2b and PCV2d were related to strains of Asian origin, most commonly from China. The role of this country in the exportation of PCV2 strains in Ethiopia, and Africa in general, might be supported by the crescent economic relationship between the two continents. The obtained evidence also testifies to the inadequacy and/or poor application of biosecurity measures separating wild and domestic animals. Further, extensive and systematic studies should be performed to more deeply characterize the molecular epidemiology of PCV2 in this region, in order to improve our understanding of these ecological niches in the evolution and dispersal of PCV2.

Porcine Circovirus Type 2 Vaccines: Commercial Application and Research Advances

Porcine circovirus type 2 (PCV2) infection can lead to porcine circovirus-associated disease (PCVAD), causing great economic losses to the global swine industry. Conventional vaccination programs are a major measure in the prevention and control of this disease. Currently, there are 5 commercially available PCV2 vaccines in the international market and 10 kinds commercially available PCV2 vaccines in the Chinese market that confer good efficacy against this virus by alleviating clinicopathological manifestations and enhancing growth performance in pigs. In addition, diverse experimental PCV2 vaccines with protective efficiency have been developed, including attenuated chimeric, nucleic acid, subunit, multivalent, and viral-vectored vaccines. These experimental vaccines have been shown to be relatively effective in improving the efficiency of pig production and simplifying prevention procedures. Adjuvants can be used to promote vaccines with higher protective immunity. Herein, we review the application of multiple commercial vaccines over the years and research advances in experimental vaccines, which provide the possibility for the development of superior vaccines to successfully prevent and control PCV2 infection in the future.

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.

Rapid detection of porcine circovirus type 2 by a red latex microsphere immunochromatographic strip

Abstract

To establish a rapid and specific antigen detection method for porcine circovirus type 2 (PCV2), monoclonal antibodies (mAbs) were produced against the PCV2 epidemic strains and a red latex microsphere immunochromatographic strip was established. A total of eight anti-PCV2b and four anti-PCV2d mAbs were produced, and seven mAbs were confirmed to react with PCV2a, PCV2b, and PCV2d strains using an immunoperoxidase monolayer assay. The results of micro-neutralization tests showed that the mAbs 2C8, 9H4, 10G7, 7B9, and 7C7 had good neutralizing activity, whereas the neutralizing activity of the mAbs 4B3, 4C9, 6H9, and 7E2 was lower than 50%. Three mAbs, 4B3, 7C7, and 9H4, and PCV2 pAb were selected for the establishment of a red latex microsphere immunochromatographic strip, and the combination of mAb 7C7 labeled with red latex microspheres and mAb 9H4 exhibited the greatest detection ability. The immunochromatographic strip had minimum detection limits of 10^2.5 TCID₅₀/0.1 ml, 10^0.7 TCID₅₀/0.1 ml, and 10^1.5 TCID₅₀/0.1 ml for PCV2a/CL, PCV2b/MDJ, and PCV2d/LNHC, respectively. Furthermore, no cross-reactivity was found for African swine fever virus, classical swine fever virus, porcine respiratory and reproductive syndrome virus, porcine parvovirus, porcine pseudorabies virus, porcine circovirus type 1, transmissible gastroenteritis virus, porcine epidemic diarrhea virus, porcine rotavirus, or porcine deltacoronavirus using the immunochromatographic strip. Using PCR as a reference standard, the detection sensitivity, specificity, and overall coincidence rate of the immunochromatographic strip were 81.13%, 100%, and 90.00%. Additionally, the detection ability of the immunochromatographic strip was correlated with that of virus titration. The immunochromatographic strip was used to detect 183 clinical disease samples, and the average positive detection rate was 22.95%. In summary, this method has good sensitivity and specificity and is simple, convenient, and quick to operate. It has high application value for on-site diagnosis of PCV2 and virus quantification.

Key points

• A red latex microsphere immunochromatographic strip for PCV2 detection was developed.

• The method was not only simple to operate, but also takes less time.

• The method had good sensitivity and specificity.

Cap Is the Protease of the Porcine Circovirus 2

Circoviruses are the smallest single-stranded DNA viruses that infect mammalian species, avian species, fish, and insects. The infections of circoviruses are known to be associated with a series of fatal diseases, but the protease of circovirus still remains unknown. In this research, we identified viral capsid protein (Cap) as the protease of porcine circovirus type 2 (PCV2), to our knowledge the first circoviruses protease to be reported. First, we found that the expression of host proteins is affected due to PCV2 infection in the porcine kidney (PK-15) cells. Then, by proteomic analysis, 253 host proteins that were down regulated were identified due to direct or indirect effects of PCV2. Further, Cap expression, but not other ORFs of PCV2, significantly reduced both JMJD6 (bifunctional arginine demethylase and lysyl-hydroxylase) and CCT5 (the chaperonin containing TCP1 subunit 5) in PK-15 cells. Finally, the results in vitro hydrolysis assays demonstrated that Cap could directly degraded either JMJD6 or CCT5 with different catalytic efficiency. In summary, our study expands repertoire of PCV2 Cap and promotes the development of inhibitors toward the anti-PCV2.

The Astounding World of Glycans from Giant Viruses

Viruses are a heterogeneous ensemble of entities, all sharing the need for a suitable host to replicate. They are extremely diverse, varying in morphology, size, nature, and complexity of their genomic content. Typically, viruses use host-encoded glycosyltransferases and glycosidases to add and remove sugar residues from their glycoproteins. Thus, the structure of the glycans on the viral proteins have, to date, typically been considered to mimick those of the host. However, the more recently discovered large and giant viruses differ from this paradigm. At least some of these viruses code for an (almost) autonomous glycosylation pathway. These viral genes include those that encode the production of activated sugars, glycosyltransferases, and other enzymes able to manipulate sugars at various levels. This review focuses on large and giant viruses that produce carbohydrate-processing enzymes. A brief description of those harboring these features at the genomic level will be discussed, followed by the achievements reached with regard to the elucidation of the glycan structures, the activity of the proteins able to manipulate sugars, and the organic synthesis of some of these virus-encoded glycans. During this progression, we will also comment on many of the challenging questions on this subject that remain to be addressed.

The health benefits of selenium in food animals: a review

Selenium is an essential trace mineral important for the maintenance of homeostasis in animals and humans. It evinces a strong antioxidant, anti-inflammatory and potential antimicrobial capacity. Selenium biological function is primarily achieved by its presence in selenoproteins as a form of selenocysteine. Selenium deficiency may result in an array of health disorders, affecting many organs and systems; to prevent this, dietary supplementation, mainly in the forms of organic (i.e., selenomethionine and selenocysteine) inorganic (i.e., selenate and selenite) sources is used. In pigs as well as other food animals, dietary selenium supplementation has been used for improving growth performance, immune function, and meat quality. A substantial body of knowledge demonstrates that dietary selenium supplementation is positively associated with overall animal health especially due to its immunomodulatory activity and protection from oxidative damage. Selenium also possesses potential antiviral activity and this is achieved by protecting immune cells against oxidative damage and decreasing viral replication. In this review we endeavor to combine established and novel knowledge on the beneficial effects of dietary selenium supplementation, its antioxidant and immunomodulatory actions, and the putative antimicrobial effect thereof. Furthermore, our review demonstrates the gaps in knowledge pertaining to the use of selenium as an antiviral, underscoring the need for further in vivo and in vitro studies, particularly in pigs.

Isolation and characterization of SGF3, a novel Microviridae phage infecting Shigella flexneri

In the context of widespread bacterial contamination and the endless emergence of antibiotic-resistant bacteria, more effective ways to control pathogen infection are urgently needed. Phages become potential bactericidal agents due to their bactericidal specificity and not easy resistance to bacteria. But an important factor limiting its development is the lack of phage species. Therefore, the isolation of more new phages and studying their biological and genomic characteristics is of great significance for subsequent applications. So, in this study, SGF3, a Microviridae phage, which has shown lytic activity against Shigella flexneri, was isolated, purified, and characterized. Morphological and phylogenetic analyses identified it as a phiX174 species belonging to the Microviridae family. The latent period of phage SGF3 was 20 min, with an average burst size of approximately 7.1. Host spectrum experiments indicated its strong host specificity. Furthermore, the biofilm removal efficiency was increased by 20%-25% when SGF3 was coupled with other phages. In conclusion, the phage SGF3 found in this study was a lytic phage belonging to the Microviral family, and could be added as an auxiliary material in the phage cocktail. Studies of its characteristics and bactericidal properties had enriched the germplasm resources of microphages, provided more potential material in fighting against emerging and existing multidrug-resistant bacteria.

Characterization of a dynamic self-replicating mammalian expression vector based on the circular ssDNA genome of beak and feather disease virus

In vivo nucleic expression technologies using DNA or mRNA offer several advantages for recombinant gene expression. Their inherent ability to generate natively expressed recombinant proteins and antigens allows these technologies to mimic foreign gene expression without infection. Furthermore, foreign nucleic acid fragments have an inherent ability to act as natural immune adjuvants and stimulate innate pathogen- and DNA damage-associated receptors that are responsible for activating pathogen-associated molecular pattern (PAMP) and DNA damage-associated molecular pattern (DAMP) signalling pathways. This makes nucleic-acid-based expression technologies attractive for a wide range of vaccine and oncolytic immunotherapeutic uses. Recently, RNA vaccines have demonstrated their efficacy in generating strong humoral and cellular immune responses for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). DNA vaccines, which are more stable and easier to manufacture, generate similar immune responses to RNA, but typically exhibit lower immunogenicity. Here we report on a novel method of constructing self-amplifying DNA expression vectors that have the potential to amplify and enhance gene/antigen expression at a cellular level by increasing per cell gene copy numbers, boost genomic adjuvating effects and mitigate through replication many of the problems faced by non-replicating vectors such as degradation, methylation and gene silencing. These vectors employ a viral origin rolling circle replication cycle in mammalian host cells that amplifies the vector and gene of interest (GOI) copy number, maintaining themselves as nuclear episomes. We show that these vectors maintain persistently elevated GOI expression levels at the cellular level and induce morphological cellular alterations synonymous with increased cellular stress.

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.

Comparison of Serum and Dietary Selenium Levels in Participants with a Positive History of Recurrent Herpes Lesions and Healthy Individuals

Aim

In this study, we aimed to compare the level of serum and dietary selenium in participants with a positive history of recurrent herpes labial lesions and healthy controls.

Materials and Methods

This cross-sectional study, conducted during 2020-2021, evaluated the selenium serum level of 40 participants with a positive history of recurrent herpes labial lesions who had referred to Motahhari Laboratory in Shiraz, compared with 38 healthy controls. The selenium level of the serum was assessed by an absorption device, Atomic Graphite Furnace Model FS-240-AAS, made by a US Company. Independent T-test was used to compare the selenium level of males and females. In order to assess the mean age value and gender distribution between the two evaluated groups, the independent T-test and chi-square test were used, respectively. The serum selenium level was compered between both control and test groups.

Results

The level of serum selenium was not statistically correlated with its dietary level in group 1 (participants with recurrent herpes labialis, P value = 0.18) and group 2 (healthy controls, P value = 0.6). The serum selenium level was compared between groups 1 and 2, which was significantly higher in healthy controls (P value < 0.0001). In contrast, dietary selenium level was not significantly different between patients with a history of herpes labialis and healthy controls (P value = 0.48). The level of serum selenium was not statistically correlated with its dietary level in group 1 (P value = 0.18) and group 2 (P value = 0.6).

Conclusion

Patients with recurrent herpes labialis had lower serum selenium level as compared to the healthy controls.

Nuclear Cytoskeleton in Virus Infection

The nuclear lamina is the main component of the nuclear cytoskeleton that maintains the integrity of the nucleus. However, it represents a natural barrier for viruses replicating in the cell nucleus. The lamina blocks viruses from being trafficked to the nucleus for replication, but it also impedes the nuclear egress of the progeny of viral particles. Thus, viruses have evolved mechanisms to overcome this obstacle. Large viruses induce the assembly of multiprotein complexes that are anchored to the inner nuclear membrane. Important components of these complexes are the viral and cellular kinases phosphorylating the lamina and promoting its disaggregation, therefore allowing virus egress. Small viruses also use cellular kinases to induce lamina phosphorylation and the subsequent disruption in order to facilitate the import of viral particles during the early stages of infection or during their nuclear egress. Another component of the nuclear cytoskeleton, nuclear actin, is exploited by viruses for the intranuclear movement of their particles from the replication sites to the nuclear periphery. This study focuses on exploitation of the nuclear cytoskeleton by viruses, although this is just the beginning for many viruses, and promises to reveal the mechanisms and dynamic of physiological and pathological processes in the nucleus.

Genetic association of polymorphisms in porcine RGS16 with porcine circovirus viral load in naturally infected Yorkshire pigs

Regulator of G protein signaling 16 (RGS16) is known to be associated with porcine circovirus type 2 (PCV2). PCV2 associated disease (PCVAD) is a serious problem in the swine industry. The representative symptoms of PCVAD are high viral titer proliferation and decreased average daily gain. In this study, we identified single nucleotide polymorphisms (SNPs) in the RGS16 region, including the upstream region. Of the 22 identified SNPs, rs332913874, rs326071195, and rs318298586 were genotyped in 142 Yorkshire pigs. These SNPs were significantly associated with the PCV2 viral load. Moreover, the haplotype combination was also related to the PCV2 viral load. The haplotype and diplotype analysis also had a significant difference with the PCV2 viral load. Taken together, our results suggest that RGS16 SNPs considerably affect the PCV2 viral load.

Anti-inflammatory activity of arctigenin against PCV2 infection in a mouse model

Arctigenin (ACT) is a novel anti-inflammatory lignan extracted from Arctium lappa L, a herb commonly used in traditional Chinese herbal medicine. In this study, we investigated the molecular mechanism whereby ACT inhibits PCV2 infection-induced proinflammatory cytokine production in vitro and in vivo. We observed that in PCV2 infection+ACT treated PK-15 cells, proinflammatory cytokine production was significantly reduced, compared to the PCV2-infected cells. The transfection and luciferase reporter assay confirmed that ACT suppressed NF-κB signalling pathway activation following PCV2 infection in PK-15 cells. Furthermore, western blotting demonstrated that ACT suppressed the NF-κB signal pathway in PCV2 infection-stimulated PK-15 cells by inhibiting the translocation of p65 from the cytoplasm to the nucleus and IκBα phosphorylation. BALB/c mice were used as a model to evaluate the anti-inflammatory effect of ACT in vivo. We found that the BALB/c mice inoculated with PCV2 infection + ACT treated showed a significant reduction of proinflammatory cytokine production in serum, lung and spleen tissue, compared to the PCV2-infected mice. Western blotting confirmed that ACT suppressed the NF-κB signal pathway in PCV2-infected mice by inhibiting the translocation of p65 from the cytoplasm to the nucleus and IκBα phosphorylation in lung tissue. Our studies first demonstrate that ACT inhibits PCV2 infection-induced proinflammatory cytokine production by suppressing the phosphorylation and nuclear translocation of NF-κB in vitro and in vivo. These results will help further develop ACT as a Traditional Chinese herbal medicine remedy in the treatment of porcine circovirus-associated diseases.

Porcine circovirus-2 in Africa: Identification of continent-specific clusters and evidence of independent viral introductions from Europe, North America and Asia

Porcine circovirus-2 (PCV-2) is associated with several disease syndromes in domestic pigs that have a significant impact on global pig production and health. Currently, little is known about the status of PCV-2 in Africa. In this study, a total of 408 archived DNA samples collected from pigs in Burkina Faso, Cameroon, Cape Verde, Ethiopia, the Democratic Republic of the Congo, Mozambique, Nigeria, Senegal, Tanzania and Zambia between 2000 and 2018 were screened by PCR for the presence of PCV-2. Positive amplicons of the gene encoding the viral capsid protein (ORF2) were sequenced to determine the genotypes circulating in each country. Four of the nine currently known genotypes of PCV-2 were identified (i.e. PCV-2a, PCV-2b, PCV-2d and PCV-2 g) with more than one genotype being identified in Burkina Faso, Ethiopia, Nigeria, Mozambique, Senegal and Zambia. Additionally, a phylogeographic analysis which included 38 additional ORF2 gene sequences of PCV-2s previously identified in Mozambique, Namibia and South Africa from 2014 to 2016 and 2019 to 2020 and available in public databases, demonstrated the existence of several African-specific clusters and estimated the approximate time of introduction of PCV-2s into Africa from other continents. This is the first in-depth study of PCV-2 in Africa and it has important implications for pig production at both the small-holder and commercial farm level on the continent.

Mitochondrial Localization Signal of Porcine Circovirus Type 2 Capsid Protein Plays a Critical Role in Cap-Induced Apoptosis

Porcine circovirus 2 (PCV2), considered one of the most globally important porcine pathogens, causes postweaning multisystemic wasting syndrome (PMWS). This virus is localized in the mitochondria in pigs with PMWS. Here, we identified, for the first time, a mitochondrial localization signal (MLS) in the PCV2 capsid protein (Cap) at the N-terminus. PK-15 cells showed colocalization of the MLS-EGFP fusion protein with mitochondria. Since the PCV2 Cap also contained a nuclear localization signal (NLS) that mediated entry into the nucleus, we inferred that the subcellular localization of the PCV2 Cap is inherently complex and dependent on the viral life cycle. Furthermore, we also determined that deletion of the MLS attenuated Cap-induced apoptosis. More importantly, the MLS was essential for PCV2 replication, as absence of the MLS resulted in failure of virus rescue from cells infected with infectious clone DNA. In conclusion, the MLS of the PCV2 Cap plays critical roles in Cap-induced apoptosis, and MLS deletion of Cap is lethal for virus rescue.

The Oryx Antelope (Oryx gazella): An Unexpected Host for Porcine Circovirus-2 (PCV-2)

For several years after its discovery, Porcine circovirus 2 (PCV-2) represented a major threat to the swine industry through economic losses due to the associated clinical syndromes, decreased production performances in both symptomatic and asymptomatic animals and disease management costs. Widespread vaccination administration has largely reduced the impact of this infection and represents the most effective control measure. The efficacy of vaccination is threatened by the emergence of novel (or uncommon) PCV-2 genotypes. In addition to domestic pigs, PCV-2 has been detected in several other species, a fact which could have an impact on new variant emergence and maintenance. Considering this, the present study assessed the distribution of the minor PCV-2c genotype in non-Suidae ungulates in Namibia. Red hartebeests (Alcelaphus buselaphus caama) (n = 44), kudus (Tragelaphus strepsiceros) (n = 10) and oryxes (Oryx gazella) (n = 54), whose mediastinal lymph nodes were sampled after slaughtering during the period 2019–2021, were included in the study. Two oryxes (3.7%; 95% CI = 0.45–12.75%) were PCV-2-positive by PCR. Complete genome sequence was obtained for the two samples identifying them as PCV-2c genotype. The sequences were identical and shared a high percentage of identity (~99.9%) with those recently obtained from warthogs living in the same area. The present study confirms the presence of the PCV-2c genotype (previously considered extinct) in Namibian wild animal populations and demonstrates greater than expected PCV-2 host plasticity. Because of the role these niches can have in the maintenance and evolution of minor PCV-2 genotypes, more extensive and dedicated studies should be performed to prepare authorities to promptly react to potential emerging threats from these viruses.

PCV2 targets cGAS to inhibit type I interferon induction to promote other DNA virus infection

Viruses use diverse strategies to impair the antiviral immunity of host in order to promote infection and pathogenesis. Herein, we found that PCV2 infection promotes the infection of DNA viruses through inhibiting IFN-β induction in vivo and in vitro. In the early phase of infection, PCV2 promotes the phosphorylation of cGAS at S278 via activation of PI3K/Akt signaling, which directly silences the catalytic activity of cGAS. Subsequently, phosphorylation of cGAS at S278 can facilitate the K48-linked poly-ubiquitination of cGAS at K389, which can been served as a signal for recognizing by the ubiquitin-binding domain of histone deacetylase 6 (HDAC6), to promote the translocation of K48-ubiquitinated-cGAS from cytosol to autolysosome depending on the deacetylase activity of HDAC6, thereby eventually resulting in a markedly increased cGAS degradation in PCV2 infection-induced autophagic cells relative to Earle’s Balanced Salt Solution (EBSS)-induced autophagic cells (a typical starving autophagy). Importantly, we found that PCV2 Cap and its binding protein gC1qR act as predominant regulators to promote porcine cGAS phosphorylation and HDAC6 activation through mediating PI3K/AKT signaling and PKCδ signaling activation. Based on this finding, gC1qR-binding activity deficient PCV2 mutant (PCV2RmA) indeed shows a weakened inhibitory effect on IFN-β induction and a weaker boost effect for other DNA viruses infection compared to wild-type PCV2. Collectively, our findings illuminate a systematic regulation mechanism by which porcine circovirus counteracts the cGAS-STING signaling pathway to inhibit the type I interferon induction and promote DNA virus infection, and identify gC1qR as an important regulator for the immunosuppression induced by PCV2.

PCV2 is well known for its ability to induce immunosuppression in pigs. However, how PCV2 infection interferes cGAS-STING signaling is still poorly understood. Herein, we demonstrate that PCV2 infection can phosphorylate porcine cGAS via gC1qR-mediated PI3K/AKT signaling to silence the catalytic activity of cGAS, while activates PKCδ signaling to promote histone deacetylase 6 (HDAC6) activation depending on the assistance of gC1qR. Subsequently, phosphorylation of cGAS facilitates the poly-ubiquitination of cGAS, then ubiquitinated-cGAS proteins are recruited and transported to autolysosome by activated HDAC6 depending on its deacetylase activity and ubiquitin-binding function, thereby eventually resulting in the autophagic degradation of cGAS in PCV2-infected cells. This study reveals that PCV2 can inhibit the activation of cGAS signaling pathway through two different mechanisms at different stages of infection and clarifies the internal relationship and cooperation model between these two mechanisms.

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.

Truncated Diphtheria Toxin DT390 Enhances the Humoral Immunogenicity of Porcine Circovirus Type 2 Capsid Antigen in Mice

Porcine circovirus type 2 (PCV2) is the causative agent of PCV-associated disease, which harms the swine industry worldwide. Open reading frame 2 of PCV2 encodes the principal immunogenic capsid (Cap) protein, which induces neutralizing antibodies and protective immunity. Cap has been developed as a subunit vaccine against PCV2 infection, although its use is hindered by low immunogenicity. Here, we hypothesized that the truncated diphtheria toxin DT390 might enhance the immunogenicity of Cap. To verify this hypothesis, we fused Cap with DT390, which was expressed using the unique diphtheria toxin-resistant Pichia pastoris expression system. We assessed the immunogenicity of DT390-Cap using BALB/c mice. DT390-Cap induced significantly higher Cap-specific and neutralizing antibodies than Cap alone with or without the ISA201 adjuvant. DT390-Cap with ISA201 adjuvant induced production of more Cap-specific antibodies and neutralizing antibodies than Ingelvac CircoFLEX (positive control). DT390-Cap induced slightly higher Th2-associated interleukin-4 production than Cap alone but did not affect Th1-associated interferon-γ production. The protection study demonstrated that DT390-Cap induced more effective protective immunity than Cap alone, when challenged with PCV2. The viral loads in the lungs, liver, and thymus in mice immunized using DT390-Cap were significantly lower than in those immunized with the corresponding Cap with or without the ISA201 adjuvant. Taken together, the engineered DT390 effectively enhanced the immunogenicity and protective immunity of Cap in mice. Thus, DT390-Cap is a promising novel vaccine candidate against PCV2 infection.

Scaling relation between genome length and particle size of viruses provides insights into viral life history

In terms of genome and particle sizes, viruses exhibit great diversity. With the discovery of several nucleocytoplasmic large DNA viruses (NCLDVs) and jumbo phages, the relationship between particle and genome sizes has emerged as an important criterion for understanding virus evolution. We use allometric scaling of capsid volume with the genome length of different groups of viruses to shed light on its relationship with virus life history. The allometric exponents for icosahedral dsDNA bacteriophages and NCDLVs were found to be 1 and 2, respectively, indicating that with increasing capsid size DNA packaging density remains the same in bacteriophages but decreases for NCLDVs. We argue that the exponents are largely shaped by their entry mechanism and capsid mechanical stability. We further show that these allometric size parameters are also intricately linked to the relative energy costs of translation and replication in viruses and can have further implications on viral life history.

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Capsid and genome size allometric exponent gives insights into viral life history

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The allometric exponent of NCLDVs is almost twice that of bacteriophages

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The exponent is largely shaped by the viral entry mechanism and capsid stability

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The relaxed genome size constraint allows large viruses to evolve greater autonomy

Applicability of Raman spectroscopy on porcine parvovirus and porcine circovirus type 2 detection

Porcine parvovirus (PPV) is one of the major infectious causes of reproductive failure of swine. This disease is characterized by embryonic and fetal infection and death, responsible for important economic losses. PPV is also implicated as a trigger in the development of post-weaning multisystemic wasting syndrome (PMWS) caused by Porcine circovirus type 2 (PCV2). Their detection is PCR-based, which is quite sensitive and specific, but laborious, costly and time-demanding. Therefore, this study aimed to assess Raman spectroscopy (RS) as a diagnostic tool for PPV and PCV2 due to its label-free properties and unique ability to search and identify molecular fingerprints. Briefly, swine testis (ST) cells were inoculated with PPV or PCV2 and in vitro cultured (37 °C, 5% CO₂) for four days. Fixed cells were then submitted to RS investigation using a 633 nm laser. A total of 225 spectra centered at 1300 cm^-1 was obtained for each sample (5 spectra/cell; 15 cells/replicate; 3 replicates) of PPV-, PCV2-infected and uninfected (control) ST cells. Clear statistical discrimination between samples from both virus-infected cells was achieved with a Principal Component - Linear Discriminant Analysis (PCA-LDA) model, reaching sensitivity rates from 95.55% to 97.77%, respectively to PCV2- and PPV-infected cells. These results were then submitted to a Leave-One-Out (LOO) validation algorithm resulting in 99.97% of accuracy. Extensive band assignment was analyzed and compiled for better understanding of PPV and PCV2 virus-cell interaction, demonstrating that specific protein, lipids and DNA/RNA bands are the most important assignments related to discrimination of virus-infected from uninfected cells. In conclusion, these results represent promising bases for RS application on PCV2 and PPV detection for future diagnostic applications.

Apoptosis Triggered by ORF3 Proteins of the Circoviridae Family

Apoptosis, a form of the programmed cell death, is an indispensable defense mechanism regulating cellular homeostasis and is triggered by multiple stimuli. Because of the regulation of apoptosis in cellular homeostasis, viral proteins with apoptotic activity are particular foci of on antitumor therapy. One representative viral protein is the open reading frame 3 (ORF3) protein, also named as apoptin in the Circoviridae chicken anemia virus (CAV), and has the ability to induce tumor-specific apoptosis. Proteins encoded by ORF3 in other circovirus species, such as porcine circovirus (PCV) and duck circovirus (DuCV), have also been reported to induce apoptosis, with subtle differences in apoptotic activity based on cell types. This article is aimed at reviewing the latest research advancements in understanding ORF3 protein-mediated apoptosis mechanisms of Circoviridae from three perspectives: subcellular localization, interactions with host proteins, and participation in multiple apoptotic signaling pathways, providing a scientific basis for circovirus pathogenesis and a reference on its potential anticancer function.

Detection and genetic characterization of porcine circovirus 4 (PCV4) in Guangxi, China

Porcine circovirus type 4 (PCV4), a novel circovirus, was identified in pigs with serious symptoms, including porcine dermatitis and nephropathy syndrome (PDNS)-like signs, in China in 2019. This study investigated the prevalence and genome diversity of PCV4 in pigs from Guangxi Province, China, between 2015 and 2019. Thirteen of 257 (5.1%) samples were positive for PCV4, 9 of these (69.2%) PCV4-positive samples were coinfected with PCV2 or PCV3, and one PCV4-positive sample was coinfected with both PCV2 and PCV3. Three complete PCV4 genomes shared 36.9-73.8% nucleotide similarity with other representative circovirus genomes. Phylogenetic analysis indicated that PCV4 was most closely related to bat-associated circovirus and mink circovirus. In summary, this is the first epidemiological investigation and evolutionary analysis of PCV4 in Guangxi Province, China, and the results provide insight into the molecular epidemiology of PCV4.

Structural Perspectives of Beak and Feather Disease Virus and Porcine Circovirus Proteins

Circoviruses represent a rapidly expanding group of viruses that infect both vertebrate and invertebrate hosts. Members are responsible for diseases of veterinary and economic importance, including postweaning multisystemic wasting syndrome in pigs, and beak and feather disease (BFD) in birds. These viruses are associated with lymphoid depletion and immunosuppressive conditions in infected animals leading to systemic illness. Circoviruses are small nonenveloped DNA viruses containing a single-stranded circular genome, encoding two major proteins: the capsid-associated protein (Cap), comprising the entirety of the viral capsid, and the replication-associated protein (Rep). Cap is the only protein component of the virion and plays crucial roles throughout the virus replication cycle, including viral attachment, cell entry, genome uncoating, and packaging of newly formed viral particles. Rep mediates recognition of replication origin motifs in the viral genome sequence and is responsible for endonuclease activity enabling nicking of the circular DNA and initiation of rolling-circle replication (RCR). Porcine circovirus 2 (PCV2) was the first circovirus capsid structure to be solved at atomic resolution using X-ray crystallography. The structure revealed an assembly comprising 60 monomeric subunits to form virus-like particles. Each Cap monomer harbors a canonical viral jelly roll domain composed of two, four-stranded antiparallel β-sheets. Crystal structures of two distinct macromolecular assemblies from BFD virus Cap were also resolved at high resolution. In these structures, the exposure of the N-terminal arginine-rich motif, responsible for DNA binding and nuclear localization is reversed. Additional structural investigations have also elucidated a PCV2 type-specific neutralizing epitope, and interaction between the PCV2 capsid and polymers such as heparin. In this review, we provide a snapshot of the structural and functional aspects of circovirus proteins.

Conformational Dynamics of Nonenveloped Circovirus Capsid to the Host Cell Receptor

Circovirus, comprising one capsid protein, is the smallest nonenveloped virus and induces lymphopenia. Circovirus can be used to explore the cell adhesion mechanism of nonenveloped viruses. We developed a single-molecule fluorescence resonance energy transfer (smFRET) assay to directly visualize the capsid's conformational feature. The capsid underwent reversible dynamic transformation between three conformations. The cell surface receptor heparan sulfate (HS) altered the dynamic equilibrium of the capsid to the high-FRET state, revealing the HS-binding region. Neutralizing antibodies restricted capsid transition to a low-FRET state, masking the HS-binding domain. The lack of positively charged amino acids in the HS-binding site reduced cell surface affinity and attenuated virus infectivity via conformational changes. These intrinsic characteristics of the capsid suggested that conformational dynamics is critical for the structural changes occurring upon cell surface receptor binding, supporting a dynamics-based mechanism of receptor binding.

Establishment of an SYBR Green-based real-time PCR assay for porcine circovirus type 4 detection

Porcine circovirus 4 (PCV4) is a novel circovirus first discovered in China in April 2019. Here, we established an SYBR Green I-based real-time PCR for quantitative detection of PCV4. A pair of specific primers was designed based on the conserved region of Cap of PCV4. The standard curve of the established real-time PCR. assay showed a good linear relationship. The sensitivity of the established real-time PCR was 100 times greater than that of conventional PCR, and the detection limit of the assay was 3 × 10¹ copies. There was no cross-reactivity with other swine DNA viruses, showing good specificity. The intra-group variation coefficient was 0.37-0.78 %, and the inter-group variation coefficient was 0.57-0.94%, indicating that the assay has good repeatability. Moreover, the analysis of clinical samples showed that the positive detection rate of PCV4 was 10.71% (18/168), while that of conventional PCR was 8.93% (15/168). Interestingly, co-infection with PCV2 or PCV3, or both PCV2 and PCV3, was also detected. In conclusion, the established SYBR Green I-based real-time PCR may be a cost-effective and rapid method for PCV4 clinical diagnosis.

Simultaneous detection of duck circovirus and novel goose parvovirus via SYBR green I-based duplex real-time polymerase chain reaction analysis

Beak atrophy and dwarfism syndrome (BADS) is commonly caused by co-infection with duck circovirus (DuCV) and novel goose parvovirus (NGPV). Therefore, concurrent detection of both viruses is important for monitoring and limiting BADS, although such a diagnostic test has not been reported. In this study, we developed a duplex, SYBR Green I-based real-time polymerase chain reaction (PCR) assay to enable the simultaneous detection of DuCV and NGPV. The assay readily distinguished between the two viruses, based on their different melting temperatures (Tm), where the Tm for DuCV was 80 °C and that for NGPV was 84.5 °C. Other non-target duck viruses that were tested did not show melting peaks. The detection limit of the duplex assay was 10¹ copies/μL for both viruses. This method exhibited high repeatability and reproducibility, and both the inter-assay and intra-assay variation coefficients were <1.6%. Thirty-one fecal samples were collected for clinical testing using real-time PCR analysis, and the results were confirmed using sequencing. The rate of co-infection was 6.5%, which was consistent with the sequencing results. This duplex real-time PCR assay offers advantages over other tests, such as rapid, sensitive, specific, and reliable detection of both viruses in a single sample, which enables the quantitative detection of DuCV and NGPV in clinical samples. Using this test may be instrumental in reducing the incidence of BADS and the associated economic losses in the duck and goose industries.

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SYBR Green based PCR to simultaneously detect duck circovirus and goose parvovirus.

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The assay had specificity, sensitivity and reproducibility.

PCV2 inhibits the Wnt signalling pathway in vivo and in vitro

Porcine circovirus type 2 (PCV2) is an important pathogen of the current pig industry. The Wnt signalling pathway plays an important role in the growth of young animals. In this study, we mainly elucidated the relationship between PCV2 and the Wnt signalling pathway. In an in vivo experiment in mice, we demonstrated the downregulatory effects of PCV2 infection on expression levels of downstream components of the Wnt signalling pathway. Weight loss in mice was reversed by activating the Wnt signalling pathway, and the body weight was still significantly higher than that in mice infected with PCV2. We detected levels of growth hormone (GH) in the liver and sera, which showed that GH was also downregulated in mice challenged with PCV2. Lithium chloride, the activator of Wnt signalling, upregulated GH, albeit to a significantly lesser degree than that in corresponding non-stimulated mock mice. In vitro studies showed that PCV2 infection downregulated protein expression of β-catenin and mRNA expression of matrix metallopeptidase-2 (Mmp2), downregulated protein expression of β-catenin in the cytoplasm and nucleus, and reduced the activity of the TCF/LEF promoter, demonstrating that PCV2 inhibited activation of the Wnt signalling pathway in vitro. Finally, we found that Rep protein of PCV2 might be responsible for the inhibitory effect.

PCV2 Induces Reactive Oxygen Species To Promote Nucleocytoplasmic Translocation of the Viral DNA Binding Protein HMGB1 To Enhance Its Replication

Porcine circovirus type 2 (PCV2) is an important swine pathogen that causes significant economic losses to the pig industry. PCV2 interacts with host cellular factors to regulate its replication. High-mobility-group box 1 (HMGB1) protein, a major nonhistone protein in the nucleus, was recently discovered to participate in viral infections. Here, we demonstrate that nuclear HMGB1 negatively regulated PCV2 replication as shown by overexpression of HMGB1 or blockage of its nucleocytoplasmic translocation with ethyl pyruvate. The B box domain was essential in restricting PCV2 replication. Nuclear HMGB1 restricted PCV2 replication by sequestering the viral genome via binding to the Ori region. However, PCV2 infection induced translocation of HMGB1 from cell nuclei to the cytoplasmic compartment. Elevation of reactive oxygen species (ROS) induced by PCV2 infection was closely associated with cytosolic translocation of nuclear HMGB1. Treatment of PCV2-infected cells with ethyl pyruvate or N-acetylcysteine downregulated PCV2-induced ROS production, suppressed nucleocytoplasmic HMGB1 translocation, and decreased PCV2 replication. Collectively, these findings offer new insight into the mechanism of the PCV2 evasion strategy: PCV2 manages to escape restriction of its replication by nuclear HMGB1 by inducing ROS to trigger the nuclear-to-cytoplasmic translocation of HMGB1.IMPORTANCE Porcine circovirus type 2 (PCV2) is a small DNA virus that depends heavily on host cells for its infection. This study reports the close relationship between subcellular localization of host high-mobility-group box 1 (HMGB1) protein and viral replication during PCV2 infection. Restriction of PCV2 replication by nuclear HMGB1 is the early step of host defense at the host-pathogen interface. PCV2 then upregulates host reactive oxygen species (ROS) to prevent sequestration of its genome by expelling nuclear HMGB1 into the cytosol. It will be interesting to study if a similar evasion strategy is employed by other circoviruses such as beak and feather disease virus, recently discovered PCV3, and geminiviruses in plants. This study also provides insight into the justification and pharmacological basis of antioxidants as an adjunct therapy in PCV2 infection or possibly other diseases caused by the viruses that deploy the ROS-HMGB1 interaction favoring their replication.

PCV2 co-infection does not impact PRRSV MLV1 safety but enhances virulence of a PRRSV MLV1-like strain in infected SPF pigs

Co-infection by a type 1 modified live vaccine-like strain (MLV1-like) of porcine reproductive and respiratory syndrome virus (PRRSV) and a type 2 porcine circovirus (PCV2) was identified on a French pig farm with post-weaning multisystemic wasting syndrome (PMWS). An in vivo experiment was set up to characterize the virulence level of the MLV1-like strain compared with the parental MLV1 strain, and to assess the impact of PCV2 co-infection on the pathogenicity of both PRRSV strains. Six groups of six pigs each were inoculated only with either one of the two PRRSV strains or with PCV2, or co-inoculated with PCV2 and MLV1 or PCV2 and MLV1-like strains. Six contact pigs were added to each inoculated group to assess viral transmission. The animals were monitored daily for 35 days post-inoculation for clinical symptoms. Blood and nasal swabs were sampled twice a week, and tissue samples were collected during necropsy for viral quantification. Compared to MLV1-infected pigs, animals infected with the MLV1-like strain had increased PRRSV viremia and nasal shedding, a higher viral load in the tonsils, and lymph node hypertrophy at microscopic level. PCV2 co-infection did not influence clinical, virologic or transmission parameters for MLV1, but co-infected MLV1-like/PCV2 pigs had the most severe lung lesions, the highest viremia in contact animals and the highest transmission rate. Our study demonstrated that the MLV1 strain tested was safe when co-inoculated with PCV2 in piglets. However, co-infection by the MLV1-like strain and PCV2 resulted in increased virulence compared with that due to a single infection.

Comparing the efficiency of different Escherichia coli strains in producing recombinant capsid protein of porcine circovirus type 2

The present study was aimed at comparing different E. coli strains in expressing the capsid protein of Porcine Circovirus 2 (PCV2). Full length capsid protein could be expressed only in Rosetta-gami 2 (DE3) pLysS strain using pET32b (+) vector. This confirmed that only those strains which possess tRNAs for rare codons can express the full length capsid protein. Purification of full length capsid protein could not be achieved even after several attempts using native and denaturing conditions. Subsequently, an attempt was made for expression of N-terminal truncated capsid protein using the same expression system. Truncated capsid protein was successfully expressed, purified and characterized by western blotting. The truncated capsid protein was also shown to be efficacious in testing serum samples using an optimized indirect ELISA, wherein a diagnostic sensitivity of 88.89% and specificity of 90.82% was obtained as compared to commercially available GreenSpring® porcine circovirus (PCV2) ELISA test kit. Thus, the expressed truncated capsid protein appears to be a promising diagnostic agent for PCV2. The comparative analysis suggests that cluster of arginine residues at N-terminal of capsid protein not only affects its expression in some E. coli strains but also its purification by Ni-NTA chromatography, when expressed as a histidine tagged fusion protein.

Antiviral Effect of Epigallocatechin Gallate via Impairing Porcine Circovirus Type 2 Attachment to Host Cell Receptor

The green tea catechin epigallocatechin gallate (EGCG) exhibits antiviral activity against various viruses. Whether EGCG also inhibits the infectivity of circovirus remains unclear. In this study, we demonstrated the antiviral effect of EGCG on porcine circovirus type 2 (PCV2). EGCG targets PCV2 virions directly and blocks the attachment of virions to host cells. The microscale thermophoresis assay showed EGCG could interact with PCV2 capsid protein in vitro with considerable affinity (Kd = 98.03 ± 4.76 μM), thereby interfering with the binding of the capsid to the cell surface receptor heparan sulfate. The molecular docking analysis of capsid–EGCG interaction identified the key amino acids which formed the binding pocket accommodating EGCG. Amino acids ARG51, ASP70, ARG73 and ASP78 of capsid were found to be critical for maintaining the binding, and the arginine residues were also essential for the electrostatic interaction with heparan sulfate. The rescued mutant viruses also confirm the importance of the key amino acids of the capsid to the antiviral effect of EGCG. Our findings suggest that catechins could act as anti-infective agents against circovirus invasion, as well as provide the basic information for the development and synthesis of structure-based anti-circovirus drugs.

Porcine Circovirus Type 2 Induces Single Immunoglobulin Interleukin-1 Related Receptor (SIGIRR) Downregulation to Promote Interleukin-1β Upregulation in Porcine Alveolar Macrophage

Multisystemic inflammation in pigs affected by porcine circovirus type 2 (PCV2) indicates the disordered expression of inflammatory cytokines. However, the PCV2-induced expression profile of inflammation cytokines and its regulating mechanism remain poorly understood. In this study, inflammatory cytokines and receptors in porcine alveolar macrophages (PAMs) after PCV2 infection were profiled in vitro by an RT² Profiler^TM PCR array assay. The regulatory mechanism of interleukin-1β (IL-1β) expression was investigated. Results showed that 49 of 84 inflammation cytokines and receptors were differentially expressed (p < 0.05, absolute fold change ≥2) in PAMs at different stages post-PCV2 infection. Moreover, the overexpression of single-immunoglobulin interleukin-1 related receptor (SIGIRR) or the blocking of NF-κB activation by its inhibitor markedly decreased IL-1β secretion. This finding suggested that PCV2-induced overexpression of IL-1β was associated with the downregulation of SIGIRR and the activation of NF-κB. Furthermore, the excessive activity of NF-κB in SIGIRR-knockout PAMs cell line, indicating that SIGIRR negatively regulated IL-1β production by inhibiting the activation of NF-κB. Overall, PCV2-induced downregulation of SIGIRR induction of NF-κB activation is a critical process in enhancing IL-1β production in PAMs. This study may provide insights into the underlying inflammatory response that occurs in pigs following PCV2 infection.

Selenium, Selenoproteins and Viral Infection

Reactive oxygen species (ROS) are frequently produced during viral infections. Generation of these ROS can be both beneficial and detrimental for many cellular functions. When overwhelming the antioxidant defense system, the excess of ROS induces oxidative stress. Viral infections lead to diseases characterized by a broad spectrum of clinical symptoms, with oxidative stress being one of their hallmarks. In many cases, ROS can, in turn, enhance viral replication leading to an amplification loop. Another important parameter for viral replication and pathogenicity is the nutritional status of the host. Viral infection simultaneously increases the demand for micronutrients and causes their loss, which leads to a deficiency that can be compensated by micronutrient supplementation. Among the nutrients implicated in viral infection, selenium (Se) has an important role in antioxidant defense, redox signaling and redox homeostasis. Most of biological activities of selenium is performed through its incorporation as a rare amino acid selenocysteine in the essential family of selenoproteins. Selenium deficiency, which is the main regulator of selenoprotein expression, has been associated with the pathogenicity of several viruses. In addition, several selenoprotein members, including glutathione peroxidases (GPX), thioredoxin reductases (TXNRD) seemed important in different models of viral replication. Finally, the formal identification of viral selenoproteins in the genome of molluscum contagiosum and fowlpox viruses demonstrated the importance of selenoproteins in viral cycle.

Immunogenicity of plant-produced porcine circovirus-like particles in mice

Porcine circovirus type 2 (PCV-2) is the main causative agent associated with a group of diseases collectively known as porcine circovirus-associated disease (PCAD). There is a significant economic strain on the global swine industry due to PCAD and the production of commercial PCV-2 vaccines is expensive. Plant expression systems are increasingly regarded as a viable technology to produce recombinant proteins for use as pharmaceutical agents and vaccines. However, successful production and purification of PCV-2 capsid protein (CP) from plants is an essential first step towards the goal of a plant-produced PCV-2 vaccine candidate. In this study, the PCV-2 CP was transiently expressed in Nicotiana benthamiana plants via agroinfiltration and PCV-2 CP was successfully purified using sucrose gradient ultracentrifugation. The CP self-assembled into virus-like particles (VLPs) resembling native virions and up to 6.5 mg of VLPs could be purified from 1 kg of leaf wet weight. Mice immunized with the plant-produced PCV-2 VLPs elicited specific antibody responses to PCV-2 CP. This is the first report describing the expression of PCV-2 CP in plants, the confirmation of its assembly into VLPs and the demonstration of their use to elicit a strong immune response in a mammalian model.

Transmission of Porcine Circovirus 3 (PCV3) by Xenotransplantation of Pig Hearts into Baboons

Porcine circovirus 3 (PCV3) is a newly described member of the virus family Circoviridae. PCV3 is highly distributed among pigs and wild boars worldwide. A sudden introduction of PCV3 was recently observed in a herd of triple genetically modified pigs generated for xenotransplantation. These animals were used as donor pigs for orthotopic heart transplantation into baboons. In four cases, PCV3-positive hearts were transplanted, and transmission of PCV3 to the recipient was observed. PCV3 was found in all organs of the recipient baboons and a higher virus load was found in animals with a longer survival time of the transplant, indicating replication of the virus. This is the first report showing trans-species transmission of PCV3 to baboons by transplantation of a heart from a PCV3-positive donor pig. Sequence analysis showed that PCV3a and PCV3b were present in the infected pigs and were transmitted. Experiments to infect human 293 cells with PCV3 failed.

Porcine HMGCR Inhibits Porcine Circovirus Type 2 Infection by Directly Interacting with the Viral Proteins

Porcine circovirus type 2 (PCV2) is the etiological agent of porcine circovirus diseases and porcine circovirus-associated diseases (PCVDs/PCVADs). However, the pathogenesis of PCV2 is not fully understood. We previously found that 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) is negatively associated with PCV2 infection in vitro and in vivo. HMGCR inhibits the early stages of PCV2 infection, while PCV2 infection induces the phosphorylation of HMGCR to inactivate the protein. In this study, we investigated the possibility that adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK), and protein phosphatase 2 (PP2A) participate in HMGCR-mediated inhibition of PCV2 infection and the interaction of porcine HMGCR with PCV2 proteins. The results showed that AMPK activity fluctuated in cells during the early stage of PCV2 infection, while PP2A had little effect on PCV2 infection and HMGCR activity. Furthermore, PCV2 infection may enhance or maintain the level of phosphorylated HMGCR by directly interacting with the protein in PK-15 cells. These findings may provide a better understanding of PCV2 pathogenesis, and HMGCR may be a novel PCV2 antiviral target.

Recent progress on porcine circovirus type 3

Porcine circovirus 3 (PCV3) is a newly identified virus that belongs to the genus Circovirus in the family Circoviridae. Since the first identification of PCV3 in domestic swine in 2016 in the USA, exciting progress on PCV3 has emphasized the importance of the virus. The aim of this review is to present recent advances in the molecular characteristics, epidemiology, and pathogenesis of PCV3. The virus spreads widely throughout almost all tissues of pig and wild boar in various countries, with a gradual increase of the infection. PCV3 is a pathogen associated with porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs, reproductive failure, and cardiac and multiorgan inflammation. Furthermore, PCV3 has been detected in other animals and ticks, suggesting that PCV3 possesses cross-species transmission abilities and has an unexpectedly broad distribution and circulation in the wild, where these animals may serve as potential reservoirs for PCV3 and pose a threat to the swine industry or even to humans. Moreover, several detection methods, which can specifically detect PCV3 or differentiate PCV3 from the other viruses, are also reviewed. The present review provides updated knowledge on PCV3-related research. Identification of the prevailing strain of PCV3 and its reservoirs is essential for researchers to understand PCV3 infections and PCV3-related diseases.