In vitro studies on the infection and replication of porcine circovirus type 2 in cells of the porcine immune system

Molecular basis for the different PCV2 susceptibility of T-lymphoblasts in Landrace and Piétrain pigs

Clinically, Landrace pigs are more susceptible to porcine circovirus-associated diseases (PCVADs) than Piétrain pigs. We previously found that porcine circovirus type 2 (PCV2) can infect T-lymphoblasts. The present study examined the replication kinetics of six PCV2 strains in the lymphoblasts of Landrace and Piétrain pigs. The results showed that T-lymphoblasts from Landrace pigs are much more susceptible to PCV2 infection than those from Piétrain pigs. In addition, PCV2 replication was strain-dependent. PCV2 binding to T-lymphoblasts was partially mediated by chondroitin sulfate (CS) and dermatan sulfate (DS). Phosphacan, an effective internalization mediator in monocytes that contains several CS chains, was also demonstrated to be involved in PCV2 internalization. Viral binding and internalization were not different between the two breeds, however, the subsequent step, the disassembly was. Although inhibition of serine proteases blocked PCV2 replication in both Landrace and Piétrain pigs, this only occurred at a neutral pH in Piétrain pigs, whereas this occurred also at a low pH in Landrace. This suggested that more proteases can cleave PCV2 in Landrace lymphoblasts than in Piétrain lymphoblasts, explaining the better replication. Through co-localization studies of viral particles with endo-lysosomal markers, and quantitative analysis of organelle sizes during viral internalization, it was observed that PCV2 may exhibit a higher propensity for viral escape from late endosomes in Landrace pigs (smaller) compared to Piétrain pigs. These results provide new understandings of the different PCV2 susceptibility in Landrace and Piétrain pigs.

Immune Molecules' mRNA Expression in Porcine Alveolar Macrophages Co-Infected with Porcine Reproductive and Respiratory Syndrome Virus and Porcine Circovirus Type 2

Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus 2 (PCV2) are economically important pathogens in swine, and pigs with dual infections of PCV2 and PRRSV consistently have more severe clinical symptoms and interstitial pneumonia. However, the synergistic pathogenesis mechanism induced by PRRSV and PCV2 co-infection has not yet been illuminated. Therefore, the aim of this study was to characterize the kinetic changes of immune regulatory molecules, inflammatory factors and immune checkpoint molecules in porcine alveolar macrophages (PAMs) in individuals infected or co-infected with PRRSV and/or PCV2. The experiment was divided into six groups: a negative control group (mock, no infected virus), a group infected with PCV2 alone (PCV2), a group infected with PRRSV alone (PRRSV), a PCV2-PRRSV co-infected group (PCV2-PRRSV inoculated with PCV2, followed by PRRSV 12 h later), a PRRSV-PCV2 co-infected group (PRRSV-PCV2 inoculated with PRRSV, followed by PCV2 12 h later) and a PCV2 + PRRSV co-infected group (PCV2 + PRRSV, inoculated with PCV2 and PRRSV at the same time). Then, PAM samples from the different infection groups and the mock group were collected at 6, 12, 24, 36 and 48 h post-infection (hpi) to detect the viral loads of PCV2 and PRRSV and the relative quantification of immune regulatory molecules, inflammatory factors and immune checkpoint molecules. The results indicated that PCV2 and PRRSV co-infection, regardless of the order of infection, had no effect on promoting PCV2 replication, while PRRSV and PCV2 co-infection was able to promote PRRSV replication. The immune regulatory molecules (IFN-α and IFN-γ) were significantly down-regulated, while inflammatory factors (TNF-α, IL-1β, IL-10 and TGF-β) and immune checkpoint molecules (PD-1, LAG-3, CTLA-4 and TIM-3) were significantly up-regulated in the PRRSV and PCV2 co-infection groups, especially in PAMs with PCV2 inoculation first followed by PRRSV. The dynamic changes in the aforementioned immune molecules were associated with a high viral load, immunosuppression and cell exhaustion, which may explain, at least partially, the underlying mechanism of the enhanced pulmonary lesions by dual infection with PCV2 and PRRSV in PAMs.

PCV2 Uptake by Porcine Monocytes Is Strain-Dependent and Is Associated with Amino Acid Characteristics on the Capsid Surface

Porcine circovirus type 2 (PCV2) is associated with several economically important diseases that are described as PCV2-associated diseases (PCVADs). PCV2 is replicating in lymphoblasts, and PCV2 particles are taken up by monocytes without effective replication or complete degradation. Glycosaminoglycans (GAGs) have been demonstrated to be important receptors for PCV2 binding and entry in T-lymphocytes and continuous cell lines. The objective of this study was to determine whether differences exist in viral uptake and outcome among six PCV2 strains from different disease outbreaks in primary porcine monocytes: Stoon-1010 (PCV2a; PMWS), 1121 (PCV2a; abortion), 1147 (PCV2b; PDNS), 09V448 (PCV2d-1; PCVAD with high viral load in lymphoid tissues [PCVADhigh]), DE222-13 (PCV2d-2; PCVADhigh), and 19V245 (PCV2d-2; PCVADhigh). The uptake of PCV2 in peripheral blood monocytes was different among the PCV2 strains. A large number of PCV2 particles were found in the monocytes for Stoon-1010, DE222-13, and 19V245, while a low number was found for 1121, 1147, and 09V448. Competition with, and removal of GAGs on the cell surface, demonstrated an important role of chondroitin sulfate (CS) and dermatan sulfate (DS) in PCV2 entry into monocytes. The mapping of positively/negatively charged amino acids exposed on the surface of PCV2 capsids revealed that their number and distribution could have an impact on the binding of the capsids to GAGs, and the internalization into monocytes. Based on the distribution of positively charged amino acids on PCV2 capsids, phosphacan was hypothesized, and further demonstrated, as an effective candidate to mediate virus attachment to, and internalization in, monocytes. IMPORTANCE PCV2 is present on almost every pig farm in the world and is associated with a high number of diseases (PCV2-associated diseases [PCVADs]). It causes severe economic losses. Although vaccination is successfully applied in the field, there are still a lot of unanswered questions on the pathogenesis of PCV2 infections. This article reports on the uptake difference of various PCV2 strains by peripheral blood monocytes, and reveals the mechanism of the strong viral uptake ability of monocytes of Piétrain pigs. We further demonstrated that: (i) GAGs mediate the uptake of PCV2 particles by monocytes, (ii) positively charged three-wings-windmill-like amino acid patterns on the capsid outer surface are activating PCV2 uptake, and (iii) phosphacan is one of the potential candidates for PCV2 internalization. These results provide new insights into the mechanisms involved in PCVAD and contribute to a better understanding of PCV2 evolution. This may lead to the development of resistant pigs.

Evaluation of the antiviral effect of four plant polysaccharides against duck circovirus

Recently, outbreaks of duck circovirus (DuCV) are frequently occurring worldwide due to secondary infections caused by post infection-induced immunosuppression. Due to a lack of preventive drugs and vaccines, the waterfowl industry losses are ever increasing. In this study, we extracted Astragalus polysaccharides (APS), pine pollen polysaccharides (PPPS), Aloe vera polysaccharides (AVE), and Ficus carica polysaccharides (FCPS) from Astragalus, pine pollen, aloe, and F. carica leaves, respectively. We randomly divided 150 one-day-old Cherry Valley ducks into five groups, which were inoculated with the DuCV solution and orally administered APS, PPPS, AVE, FCPS, and phosphate buffer saline (PBS), respectively. We collected the duck immune organs and serum samples at 8, 16, 24, 32, 40, and 48 days post-infection (dpi). Using clinical symptom analysis, molecular biology experiments, and serological experiments, we proved that plant polysaccharides could (a) improve the duck immunity, (b) reduce the viral load, and (c) mitigate DuCV-induced damage to immune organs, with both APS and PPPS having significant effects. Moreover, we detected viral load and cytokines within the first 8 dpi. Since the body's innate immunity could inhibit viral replication within five days of virus infection, 1-5 dpi was the best treatment time. Among the four polysaccharides showing in vitro anti-apoptotic activity, APS and PPPS significantly inhibited the DuCV infection-induced apoptosis of peripheral blood lymphocytes. Overall, since our findings show APS and PPPS having significant anti-DuCV effects both in vivo and in vitro, they can be promising candidates for preventing DuCV infection in ducks.

In Vitro Analysis of TGF-β Signaling Modulation of Porcine Alveolar Macrophages in Porcine Circovirus Type 2b Infection

Porcine circovirus 2 (PCV2) has been recognized as an immunosuppressive pathogen. However, the crosstalk between this virus and its host cells in related signaling pathways remains poorly understood. In this study, the expression profiles of 84 genes involved in transforming growth factor-beta (TGF-β) signaling pathway were probed in PCV2b-infected primary porcine alveolar macrophages (PAMs) by using an RT² profiler PCR array system. The protein expression levels of cytokines involved in the TGF-β signaling pathway were determined with a RayBiotech fluorescent Quantibody^® porcine cytokine array system. Results showed that 48, 30, and 42 genes were differentially expressed at 1, 24, and 48 h after infection, respectively. A large number of genes analyzed by a co-expression network and implicated in transcriptional regulation and apoptosis were differentially expressed in PCV2b-infected PAMs. Among these genes, TGF-β, interleukin-10, CCAAT/enhancer-binding protein beta (C/EBPB), growth arrest, and DNA-damage-inducible 45 beta (GADD45B), and BCL2 were upregulated. By contrast, SMAD family member 1 (smad1) and smad3 were downregulated. These results suggested that the TGF-β signaling pathway was repressed in PAMs at the early onset of PCV2 infection. The inhibited apoptosis was indicated by the upregulated C/EBPB, GADD45B, and BCL2, and by the downregulated smad1 and smad3, which possibly increased the duration of PCV2 replication-permissive conditions and caused a persistent infection. Our study may provide insights into the underlying antiviral functional changes in the immune system of PCV2-infected pigs.

Host immune response to infection with porcine circoviruses

Porcine circovirus type 2 (PCV2), which serves as a major causative agent of PCV2-associated diseases and causes severe loss to the pig industry worldwide, can dysregulate the immune response and induce immunosuppression in PCV2-infected pigs. Similar to PCV2, porcine circovirus type 3 (PCV3), a newly identified swine circovirus which might be closely associated with porcine dermatitis and nephropathy syndrome, reproductive disorder, and multisystemic inflammatory responses, also interferes with host immune defense. Interaction between host immune system and PCVs is considered to be a crucial determinant of pathogenicity in pigs. Here, we sought to briefly discuss the current knowledge regarding the interaction of porcine circovirus type 2 and/or 3 with host immune cells and immune responses to better depict the viral immunomodulatory capacity, pathogenic mechanisms, and the future research direction in host immune responses to infection with PCV2 and PCV3.

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.

Analysis of genetic variation of porcine circovirus type 2 within pig populations in central China

In order to investigate the genetic diversity of porcine circovirus type 2 (PCV2), 284 clinical tissue samples were collected from different pig farms in central China from 2015 to 2017. A total of 162 tissue samples (162/284, 57.04%) were positive for PCV2 by PCR, and subsequently, the complete genome of 36 of these PCV2 samples was cloned and sequenced. The sequencing results showed that 37 complete PCV2 sequences were obtained from 36 PCV2-positive clinical samples. These PCV2 strains were relatively conserved and extremely homologous to the representative classical PCV2 strains. Of these, 20 PCV2 strains belonged to genotype PCV2d, 14 belonged to PCV2b, and three others belonged to PCV2a. Coinfection with PCV2b and PCV2d was identified in one sample (DF-2). These results show that PCV2d may be gradually replacing PCV2b as the predominant PCV2 genotype in central China, and that other genotypes also exist in individual regions. The results of this study will aid in our understanding of the molecular epidemiology of PCV2.

Systemic immune response of gnotobiotic mice infected with porcine circovirus type 2 after administration of Lactobacillus reuteri L26 Biocenol™

In our previous study we confirmed an antiviral activity of probiotic Lactobacillus reuteri L26 which was mediated by stimulation of local intestinal immunity. The aim of this paper was to evaluate the influence of L. reuteri L26 on the systemic immune response in gnotobiotic mice infected with porcine circovirus type 2 (PCV2). A total of 30 germ-free mice were divided into 3 groups and animals in noninfected and infected control groups (NC and IC; n=10) received sterile de Man-Rogosa-Sharpe broth for 7 days and animals in experimental group L+PCV (n=10) were inoculated with L. reuteri L26. Subsequently, mice in L+PCV and IC groups were infected with PCV2; however, mice in the control group received virus cultivation medium (mock). The results showed an increase of percentage of cytotoxic cells (CD8+ and CD49b+CD8-) and oxidative burst of phagocytes, up-regulation of the gene expression of RANTES, granulocyte-macrophage colony-stimulating factor, interferon-γ and immunoglobulin A in blood above all in the later phase of infection (14 dpi) in L+PCV group accompanied by higher load of PCV2 in the serum. These findings indicate that L. reuteri L26 has a potential to induce systemic immune reaction, but in gnotobiotic mice immune stimulation can increase virus replication.

Breed Differences in PCV2 Uptake and Disintegration in Porcine Monocytes

Porcine circovirus type 2 (PCV2) is associated with various diseases which are designated as PCV2-associated diseases (PCVADs). Their severity varies among breeds. In the diseased pigs, virus is present in monocytes, without replication or full degradation. PCV2 entry and viral outcome in primary porcine monocytes and the role of monocytes in PCV2 genetic susceptibility have not been studied. Here, virus uptake and trafficking were analyzed and compared among purebreds Piétrain, Landrace and Large White and hybrid Piétrain × Topigs20. Viral capsids were rapidly internalized into monocytes, followed by a slow disintegration to a residual level. PCV2 uptake was decreased by chlorpromazine, cytochalasin D and dynasore. The internalized capsids followed the endosomal trafficking pathway, ending up in lysosomes. PCV2 genome was nicked by lysosomal DNase II in vitro, but persisted in monocytes in vivo. Monocytes from purebred Piétrain and the hybrid showed a higher level of PCV2 uptake and disintegration, compared to those from Landrace and Large White. In conclusion, PCV2 entry occurs via clathrin-mediated endocytosis. After entry, viral capsids are partially disintegrated, while viral genomes largely escape from the pathway to avoid degradation. The degree of PCV2 uptake and disintegration differ among pig breeds.

Porcine circovirus type 2 increases interleukin-1beta and interleukin-10 production via the MyD88-NF-kappa B signaling pathway in porcine alveolar macrophages in vitro

Porcine alveolar macrophages (PAMs) represent the first line of defense in the porcine lung after infection with porcine circovirus type 2 (PCV2) via the respiratory tract. However, PCV2 infection impairs the microbicidal capability of PAMs and alters cytokine production and/or secretion. At present, the reason for the imbalance of cytokines has not been fully elucidated, and the regulatory mechanisms involved are unclear. In this study, we investigated the expression levels and regulation of interleukin-1beta (IL-1β) and IL-10 in PAMs following incubation with PCV2 in vitro. Levels of IL-1β and IL-10 increased in PAM supernatants, and the distribution of nuclear factor kappa B (NF-κB) p65staining in nucleus, expression of MyD88 and p-IκB in cytoplasm, and DNA-binding activity of NF-κB increased after incubation with PCV2, while p65 expression in PAM cytoplasm decreased. However, when PAMs were co-incubated with PCV2 and small interfering RNA targeting MyD88, those effects were reversed. Additionally, mRNA expression levels of Toll-like receptors (TLR)-2, -3, -4, -7, -8, and -9 increased when PAMs were incubated with PCV2. These results show that PCV2 induces increased IL-1β and IL-10 production in PAMs, and these changes in expression are related to the TLR-MyD88-NF-κB signaling pathway.

Porcine circovirus type 2 capsid protein induces unfolded protein response with subsequent activation of apoptosis

Porcine circovirus type 2 (PCV2) has recently been reported to elicit the unfolded protein response (UPR) via activation of the PERK/eIF2α (RNA-activated protein kinase-like endoplasmic reticulum (ER) kinase/eukaryotic initiation factor 2α) pathway. This study attempted to examine which viral protein might be involved in inducing UPR and whether this cellular event would lead to apoptosis of the cells expressing the viral protein. By transient expression, we found that both replicase (Rep) and capsid (Cap) proteins of PCV2 could induce ER stress as shown by increased phosphorylation of PERK with subsequent activation of the eIF2α-ATF4 (activating transcription factor 4)-CHOP (CCAAT/enhancer-binding protein homologous protein) axis. Cap expression, but not Rep, significantly reduced anti-apoptotic B-cell lymphoma-2 (Bcl-2) and increased caspase-3 cleavage, possibly due to increased expression of CHOP. Since knockdown of PERK by RNA interference clearly reduced Cap-induced CHOP expression, caspase-3 cleavage, and apoptotic cell death possibly by partially rescuing Bcl-2 expression, we propose that there is connection between Cap-induced UPR and apoptosis via the PERK/eIF2α/ATF4/CHOP/Bcl-2 pathway. This study, together with our earlier studies, provides insight into the mechanisms underlying PCV2 pathogenesis.

Influence of Lactobacillus reuteri L26 Biocenol™ on immune response against porcine circovirus type 2 infection in germ-free mice

Probiotic bacteria are frequently used for prevention of bacterial infections of the gastrointestinal tract, but there are only limited studies on their efficacy against viral gut infections in animals. The aim of this study was to investigate the effect of probiotic Lactobacillus reuteri L26 Biocenol^TM on the innate and adaptive immune responses in germ-free Balb/c mice, experimentally infected by porcine circovirus type 2 (PCV2), which confers immunosuppressive effect. A total of 30 six-week-old female mice were divided into 3 groups and animals in experimental group LPCV (n=10) were inoculated with L. reuteri L26, animals in the control group (C; n=10) and experimental group PCV (n=10) received sterile De Man-Rogosa-Sharpe broth for 7 days. Subsequently, mice from both experimental groups were infected with PCV2; however, mice in the control group received virus cultivation medium (mock). Virus load in faeces, ileum and mesenteric lymph nodes (MLN); as well as gene expression of selected cytokines, immunoglobulin A (IgA) and polymeric Ig receptor (PIgR) in the ileum, and percentage of CD8+, CD19+ and CD49b+CD8- cells in the MLN were evaluated. Our results showed that L. reuteri significantly decreased the amount of PCV2 in faeces and in the ileum, and up-regulated the gene expression of chemokines, interferon (IFN)-γ, IgA and PIgR in the ileum. Increased IFN-γ mRNA level was accompanied by higher proportion of natural killer cells and up-regulated IgA and PIgR gene expressions were in accordance with significantly higher percentage of CD19+ lymphocytes in the MLN. These findings indicate that probiotic L. reuteri has an antiviral effect on PCV2 in the intestine which is mediated by stimulation of local gut immune response.

Effect of porcine circovirus type 2 (PCV2) on the function of splenic CD11c+ dendritic cells in mice

Porcine circovirus-associated disease (PCVAD) caused by porcine circovirus type 2 (PCV2) is an important disease in the global pig industry. Dendritic cells (DCs) are the primary immune cells capable of initiating adaptive immune responses as well as major target cells of PCV2. To determine whether PCV2 affects the immune functions of DCs, we evaluated the expression of endocytosis and co-stimulatory molecules on DCs (CD11c⁺) from PCV2-infected mouse spleen by flow cytometry (FCM). We also analyzed the main cytokines secreted by DCs (CD11c⁺) and activation of CD4⁺ and CD8⁺ T cells by DCs (CD11c⁺) through measurement of cytokine secretion, using ELISA. Compared with control mice, PCV2 did not affect the endocytic activity of DCs but it significantly enhanced TNF-α secretion and markedly decreased IFN-α secretion. Subsets of CD40⁺, MHCII⁺ CD40⁺ and CD137L⁺ CD86⁺ DCs did not increase obviously, but MHCII⁺ CD40^- and CD137L^- CD80⁺/CD86⁺ DCs increased significantly in PCV2-infected mouse spleen. Under the stimulation of DCs from PCV2-infected mouse, secretion of IFN-γ by CD4⁺ and CD8⁺ T cells and of IL-12 by CD8⁺ T cells was significantly lower than in control mice, while secretion of IL-4 by CD4⁺ T cells was remarkably higher. These results indicate that PCV2 modulates cytokine secretion and co-stimulatory molecule expression of DCs, and alters activation of CD4⁺ and CD8⁺ T cells by DCs. The immunomodulatory effects of PCV2 on DCs might be related to the host's immune dysfunction and persistent infection with this virus.

Depletion of follicular dendritic cells in tonsils collected from PMWS-affected pigs

Post-weaning multisystemic wasting syndrome (PMWS) is a relevant, worldwide disease caused by porcine circovirus type 2 (PCV2). Microscopically, PMWS is mainly characterized by lymphocytic depletion, macrophage infiltration and syncytia in lymphoid tissues. Some data suggest that follicular dendritic cells (FDCs) could be infected by PCV2, thus likely playing a role in the pathogenesis of PMWS. The present paper aims at assessing, qualitatively and quantitatively, the FDCs' network in the soft palate tonsils of clinically healthy and PMWS-affected pigs. Consecutive tissue sections were tested by immunohistochemistry to detect PCV2, FDCs and macrophages. FDCs and PCV2 antigens were quantitatively assessed by means of the Image J software and results submitted to statistical analysis. Our data demonstrated that FDCs are significantly reduced in PMWS-affected pigs compared with healthy pigs and that FDCs' depletion should be considered among microscopic features of PMWS. It is reasonable to hypothesize that depletion of FDCs further compromises the immune response and enhances the occurrence and the severity of secondary infections, which are relevant for the clinical manifestation of PMWS.

Experimental Reproduction of Postweaning Multisystemic Wasting Syndrome in Pigs by Dual Infection with Mycoplasma hyopneumoniae and Porcine Circovirus Type 2

The objectives of this study were to investigate the interactions between Mycoplasma hyopneumoniae and porcine circovirus type 2 (PCV2) and to establish a model for studying the pathogenesis of and testing intervention strategies for the control of PCV2-associated porcine respiratory disease complex (PRDC). Sixty-seven pigs were randomly assigned to four groups. Group 1 ( n = 17) pigs served as controls, group 2 ( n = 17) pigs were inoculated with M. hyopneumoniae, group 3 ( n = 17) pigs were dual infected with M. hyopneumoniae and PCV2, and group 4 ( n = 16) pigs were inoculated with PCV2. Pigs were inoculated intratracheally with M. hyopneumoniae at 4 weeks of age followed by intranasal inoculation with PCV2 at 6 weeks of age. Dual-infected pigs had moderate dyspnea, lethargy, and reduced weight gain. The overall severity of macroscopic lung lesions, PCV2-associated microscopic lesions in lung and lymphoid tissues, and the amount of PCV2-antigen associated with these lesions were significantly ( P < 0.05) higher in dual-infected pigs compared with all other groups. Four of 17 (23.5%) dual-infected pigs had decreased growth rate and severe lymphoid depletion and granulomatous lymphadenitis associated with high amounts of PCV2-antigen consistent with postweaning multisystemic wasting syndrome (PMWS). PCV2-antigen in lung tissue was most often associated with M. hyopneumoniae-induced peribronchial lymphoid hyperplasia, suggesting that this is an important site for PCV2 replication in the lung. This study indicates that M. hyopneumoniae potentiates the severity of PCV2-associated lung and lymphoid lesions, increases the amount and prolongs the presence of PCV2-antigen, and increases the incidence of PMWS in pigs.

Matrine displayed antiviral activity in porcine alveolar macrophages co-infected by porcine reproductive and respiratory syndrome virus and porcine circovirus type 2

The co-infection of porcine reproductive respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) is quite common in clinical settings and no effective treatment to the co-infection is available. In this study, we established the porcine alveolar macrophages (PAM) cells model co-infected with PRRSV/PCV2 with modification in vitro, and investigated the antiviral activity of Matrine on this cell model and further evaluated the effect of Matrine on virus-induced TLR3,4/NF-κB/TNF-α pathway. The results demonstrated PAM cells inoculated with PRRSV followed by PCV2 2 h later enhanced PRRSV and PCV2 replications. Matrine treatment suppressed both PRRSV and PCV2 infection at 12 h post infection. Furthermore, PRRSV/PCV2 co- infection induced IκBα degradation and phosphorylation as well as the translocation of NF-κB from the cytoplasm to the nucleus indicating that PRRSV/PCV2 co-infection induced NF-κB activation. Matrine treatment significantly down-regulated the expression of TLR3, TLR4 and TNF-α although it, to some extent, suppressed p-IκBα expression, suggesting that TLR3,4/NF-κB/TNF-α pathway play an important role of Matrine in combating PRRSV/PCV2 co-infection. It is concluded that Matrine possesses activity against PRRSV/PCV2 co-infection in vitro and suppression of the TLR3,4/NF-κB/TNF-α pathway as an important underlying molecular mechanism. These findings warrant Matrine to be further explored for its antiviral activity in clinical settings.

The expression level of gC1qR is down regulated at the early time of infection with porcine circovirus of type 2 (PCV-2) and gC1qR interacts differently with the Cap proteins of porcine circoviruses

Porcine circoviruses (PCV) are small, non-enveloped single-stranded DNA-viruses. Porcine circovirus type 2 (PCV-2) is the causal agent of post-weaning multisystemic wasting syndrome (PMWS) whereas porcine circovirus of type 1 (PCV-1) is non- pathogenic. gC1qR is a membrane-located receptor of the complement protein subunit C1q and interacts with PCV capsid proteins. The mechanisms associated with the triggering of PMWS are not well known and gC1qR may have a role in the life cycle and eventually in the pathogenicity of PCV. The objectives of this study were to determine the level of expression of gC1qR during early PCV-2 infection, to determine the region of PCV-2 capsid protein (Cap) required for the interaction with gC1qR and to evaluate the interaction of gC1qR with Cap proteins of different PCV strains. The results indicate that gC1qR transcripts are downregulated in the tonsils and the tracheo-bronchial lymph nodes of piglets infected by PCV-2 at the early time of the infection. The N-terminal amino acids (a.a. 1-59) of PCV-2b Cap, an arginine rich region, are involved in the interaction with gC1qR. Porcine gC1qR interacts with Cap proteins of two pathogenic viral strains, PCV-2a and PCV-2b, while interaction has been observed with only one Cap protein of two investigated strains of PCV-1. The amino acids 30 and 49 of PCV-1Cap, solely, were not responsible of the difference of interaction observed. We have also shown that gC1qR interacts strongly with PCV-2Caps and PCV-1 GER Cap. This result suggests that the different interaction of gC1qR with PCV Cap proteins may have an impact on the pathogenicity of the PCV.

Porcine Circovirus 2 Deploys PERK Pathway and GRP78 for Its Enhanced Replication in PK-15 Cells

Porcine circovirus type 2 (PCV2) infection induces autophagy and apoptosis. These cellular responses could be connected with endoplasmic reticulum (ER) stress. It remains unknown if PCV2 induces ER stress and if autophagy or apoptosis is primary to PCV2 infection or secondary responses following ER stress. Here, we demonstrate that PCV2 triggered unfolded protein response (UPR) in PK-15 cells by activating the PERK/eIF2α pathway without concomitant activation of IRE1 or ATF6. Since ATF4 and CHOP were induced later than PERK/eIF2α, it is clear that persistent PCV2 infection could lead to selective activation of PERK via the PERK-eIF2α-ATF4-CHOP axis. Therefore, PERK activation could be part of the pro-apoptotic signaling via induced expression of CHOP by PCV2. Since PERK inhibition by GSK2606414 or RNA silencing or suppression of eIF2α dephosphorylation by salubrinal limited viral replication, we suppose that PCV2 deploys UPR to enhance its replication. Over-expression of GRP78 or treatment with tauroursodeoxycholic acid could enhance viral capsid expression and/or viral titers, indicating that these chaperones, endogenous or exogenous, could help correct folding of viral proteins. Our findings provide the first evidence that ER stress plays a role in the pathogenesis of PCV2 infection probably as part of autophagic and apoptotic responses.

Immunity Elicited by an Experimental Vaccine Based on Recombinant Flagellin-Porcine Circovirus Type 2 Cap Fusion Protein in Piglets

In a recent study, we reported that a recombinant protein from fusion expression of flagellin to porcine circovirus type 2 (PCV2) Cap induced robust humoral and cell-mediated immunity that afforded full protection for PCV2 infection using BALB/c mice. Here, we further evaluated the immunogenicity and protection of the recombinant protein using specific pathogen free (SPF) pigs. Twenty-five 3-week-old piglets without passively acquired immunity were divided into 5 groups. All piglets except negative controls were challenged with a virulent PCV2 at 21 days after booster vaccination and necropsied at 21 days post-challenge. Vaccination of piglets with the recombinant protein without adjuvant induced strong humoral and cellular immune responses as observed by high levels of PCV2-specific IgG antibodies and neutralizing antibodies, as well as frequencies of PCV2-specific IFN-γ-secreting cells that conferred good protection against PCV2 challenge, with significant reduced PCV2 viremia, mild lesions, low PCV2 antigen-positive cells, as well as improved body weight gain, comparable to piglets vaccinated with a commercial PCV2 subunit vaccine. These results further demonstrated that the recombinant flagellin-Cap fusion protein is capable of inducing solid protective humoral and cellular immunity when administered to pigs, thereby becoming an effective PCV2 vaccine candidate for control of PCV2 infection.

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.

Development of real-time PCR assay for detection of porcine circovirus-like virus P1 in domestic pigs in China

Background

The porcine circovirus-like agent P1 is a newly discovered DNA virus with a single-stranded circular genome that is highly homologous to that of porcine circovirus type 2. P1 infection can cause symptoms resembling postweaning multisystemic wasting syndrome. This study aims to develop a rapid, sensitive and specific method to detect P1.

Results

A pair of primers was designed and used to amplify a 119 bp DNA fragment to generate a recombinant plasmid which was served as the standard. A SYBR I qPCR protocol was established using the P1 recombinant plasmid standard and the sensitivity, specificity and stability of this method was analyzed. The results demonstrate a strong correlation with P1 recombinant plasmid titers when virus DNA copy numbers fall in between 10⁰ ~ 10⁹ copies/μL. This method doesn’t detect pseudo rabies, porcine parvovirus or porcine reproductive and respiratory syndrome virus; moreover it can distinguish porcine circovirus type 2 from P1 by melting temperature analysis. Coefficient of variation for each batch of reaction is less than 5 %. The serum virus titers of P1 positive in this study were measured by this protocol to be 10³ to 10⁷ copies/mL.

Conclusions

The established qPCR is sensitive, specific, and reliable, which could be a useful tool when applied to quantification of P1 in a variety of samples from infected pigs.

Identification of Putative ORF5 Protein of Porcine Circovirus Type 2 and Functional Analysis of GFP-Fused ORF5 Protein

Porcine circovirus type 2 (PCV2) is the essential infectious agent responsible for causing porcine circovirus-associated diseases in pigs. To date, eleven RNAs and five viral proteins of PCV2 have been detected. Here, we identified a novel viral gene within the PCV2 genome, termed ORF5, that exists at both the transcriptional and translational level during productive infection of PCV2 in porcine alveolar macrophages 3D4/2 (PAMs). Northern blot analysis was used to demonstrate that the ORF5 gene measures 180 bp in length and overlaps completely with ORF1 when read in the same direction. Site-directed mutagenesis was used to show that the ORF5 protein is not essential for PCV2 replication. To investigate the biological functions of the novel protein, we constructed a recombinant eukaryotic expression plasmid capable of expressing PCV2 ORF5. The results show that the GFP-tagged PCV2 ORF5 protein localizes to the endoplasmic reticulum (ER), is degraded via the proteasome, inhibits PAM growth and prolongs the S-phase of the cell cycle. Further studies show that the GFP-tagged PCV2 ORF5 protein induces ER stress and activates NF-κB, which was further confirmed by a significant upregulation in IL-6, IL-8 and COX-2 expression. In addition, five cellular proteins (GPNMB, CYP1A1, YWHAB, ZNF511 and SRSF3) were found to interact with ORF5 via yeast two-hybrid assay. These findings provide novel information on the identification and functional analysis of the PCV2 ORF5 protein and are likely to be of benefit in elucidating the molecular mechanisms of PCV2 pathogenicity. However, additional experiments are needed to validate the expression and function of the ORF5 protein during PCV2 infection in vitro before any definitive conclusion can be drawn.

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.

Activation of the TLR/MyD88/NF-κB signal pathway contributes to changes in IL-4 and IL-12 production in piglet lymphocytes infected with porcine circovirus type 2 in vitro

Porcine circovirus type 2 (PCV2) causes immunosuppression in pigs. One causative factor is an imbalance in cytokine levels in the blood and lymphoid tissues. Many studies have reported changes in cytokine production, but the regulatory mechanisms involved have not yet been elucidated. In this study, we investigated alteration and regulation of IL-4 and IL-12 production in lymphocytes following incubation with PCV2 in vitro. The levels of IL-4 decreased and levels of IL-12 increased in lymphocyte supernatants, and the DNA-binding activity of NF-κB and the expression of p65 in the nucleus and p-IκB in the cytoplasm of lymphocytes increased after incubation with PCV2. However, these effects were reversed when lymphocytes were coincubated with PCV2 and the NF-κB inhibitor BAY11-7082. In addition, the expression of MyD88 protein increased and the expression of mRNA for the toll-like receptors (TLRs) TLR2, TLR3, TLR4 and TLR9 was upregulated when lymphocytes were incubated with PCV2. However, no change was seen in TLR7 and TLR8 mRNA expression. In conclusion, this study showed that PCV2 induced a decrease in IL-4 and an increase in IL-12 production in lymphocytes, and these changes were regulated by the TLR-MyD88-NF-κB signal pathway.

Global gene expression profiling of myeloid immune cell subsets in response to in vitro challenge with porcine circovirus 2b

Compelling evidence suggests that the early interaction between porcine circovirus 2 (PCV-2) and the innate immune system is the key event in the pathogenesis of Post-Weaning Multisystemic Wasting Syndrome (PMWS). Furthermore, PCV2 has been detected in bone-marrow samples, potentially enabling an easy spread and reservoir for the virus. To assess the gene-expression differences induced by an in-vitro PCV2b infection in different three different myeloid innate immune cell subsets generated from the same animal, we used the Agilent Porcine Gene Expression Microarray (V2). Alveolar macrophages (AMØs), monocyte-derived dendritic cells (MoDCs) and bone-marrow cells (BMCs) were generated from each animal, and challenged with a UK-isolate of a PCV2 genotype b-strain at a MOI of 0.5. Remarkably, analysis showed a highly distinct and cell-type dependent response to PCV2b challenge. Overall, MoDCs showed the most marked response to PCV2b challenge in vitro and revealed a key role for TNF in the interaction with PCV2b, whereas only few genes were affected in BMCs and AMØs. These observations were further supported by an enrichment of genes in the downstream NF-κB Signalling pathway as well as an up regulation of genes with pro-apoptotic functions post-challenge. PCV2b challenge increases the expression of a large number of immune-related and pro-apoptotic genes mainly in MoDC, which possibly explain the increased inflammation, granulomatous inflammation and lymphocyte depletion seen in PMWS-affected pigs.

Synergistic effects of sequential infection with highly pathogenic porcine reproductive and respiratory syndrome virus and porcine circovirus type 2

Background

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of porcine reproductive and respiratory syndrome (PRRS) and porcine circovirus type 2 (PCV2) is associated with postweaning multisystemic wasting syndrome (PMWS) in pigs. Coinfection with highly pathogenic PRRSV (HP-PRRSV) and PCV2 in the field has recently become extensive in some Asian countries. A synergistic pathogenicity between PRRSV and PCV2 infections has previously been reported. However, the consequences of the sequential infection of pigs with these two viruses are unknown.

Methods

Thirty 35-day-old piglets were randomly divided into six groups (n = 5 each): HP-PRRSV/PCV2 (group 1, inoculated with HP-PRRSV, then inoculated with PCV2 one week later), PCV2/HP-PRRSV (group 2, inoculated with PCV2, then inoculated with HP-PRRSV one week later), HP-PRRSV+PCV2 (group 3, inoculated with HP-PRRSV and PCV2 concurrently), HP-PRRSV (group 4, inoculated with HP-PRRSV), PCV2 (group 5, inoculated with PCV2), and the control (group 6, uninfected). This experiment lasted 28 days. Clinical symptoms and rectal temperatures were recorded each day after inoculation, body weight was recorded weekly, and serum samples were obtained for viral nucleic acid quantification and antibody titration. Variations in CD3⁺, CD4⁺ CD8^–, CD3⁺, CD4^–, and CD8⁺ cells, natural killer (NK) cells, and mononuclear cells were determined by flow cytometry. The serum concentrations of interferon γ (IFN-γ), tumor necrosis factor α (TNF-α), interleukin 10 (IL-10), and macrophage granulocyte-colony stimulating factor (GM-CSF) were determined. Pathological changes in different tissues from the experimentally infected pigs were recorded.

Results

The piglets in group 1 had the highest viral loads, the lowest antibody titers, the most-severe clinical signs, and the highest mortality (3/5, 60%; the mortality in the other groups was 0%), and interstitial pneumonia was more severe in this group compare to the other HP-PRRSV infected groups. The serum levels of IFN-γ, TNF-α, IL-10, and GM-CSF varied (increased or decreased) most widely in group 1, as did each immunocyte subgroup.

Conclusions

HP-PRRSV infection followed by PCV2 infection enhanced the replication of both viruses in the experimental piglets and led to more-severe clinical signs and lesions, indicating greater synergistic effects during the sequential infection of piglets with HP-PRRSV and then PCV2.

Transcription analysis of the porcine alveolar macrophage response to porcine circovirus type 2

BACKGROUND

Porcine circovirus type 2 (PCV2) is the causal agent of postweaning multisystemic wasting syndrome (PMWS), which has severely impacted the swine industry worldwide. PCV2 triggers a weak and atypical innate immune response, but the key genes and mechanisms by which the virus interferes with host innate immunity have not yet been elucidated. In this study, genes that control the response of primary porcine alveolar macrophages (PAMs), the main target of PCV2, were profiled in vitro.

RESULTS

PAMs were successfully infected by PCV2-WH strain, as evidenced quantitative real-time polymerase chain reaction (qPCR) and immunofluorescence assay (IFA) results. Infection-related differential gene expression was investigated using pig microarrays from the US Pig Genome Coordination Program and validated by real-time PCR and enzyme-linked immunosorbent assay (ELISA). Microarray analysis at 24 and 48 hours post-infection (HPI) revealed 266 and 175 unique genes, respectively, that were differentially expressed (false discovery rate <0.05; fold-change >2). Only six genes were differentially expressed between 24 and 48 HPI. The up-regulated genes were principally related to immune response, cytokine activity, locomotion, regulation of cell proliferation, apoptosis, cell growth arrest, and antigen procession and presentation. The down-regulated genes were mainly involved in terpenoid biosynthesis, carbohydrate metabolism, translation, proteasome degradation, signal transducer activity, and ribosomal proteins, which were representative of the reduced vital activity of PCV2-infected cells.

CONCLUSIONS

PCV2 infection of PAMs causes up-regulation of genes related to inflammation, indicating that PCV2 may induce systematic inflammation. PCV2 persistently induced cytokines, mainly through the Toll-like receptor (TLR) 1 and TLR9 pathways, which may promote high levels of cytokine secretion. PCV2 may prevent apoptosis in PAMs by up-regulating SERPINB9 expression, possibly to lengthen the duration of PCV2 replication-permissive conditions. The observed gene expression profile may provide insights into the underlying immunological response and pathological changes that occur in pigs following PCV2 infection.

Cellular pathogenesis of porcine circovirus type 2 infection

Porcine circovirus associated disease (PCVAD) and the associated histological lesions are thought to appear due to an increase in the amount of porcine circovirus type 2 (PCV2) present in an infected animal. However, examination of the cellular and molecular pathogenesis of PCVAD is complicated by the lack of a consistent cell culture model that replicates the animal phenotypes of persistent, asymptomatic infection, and acute, pathological disease typified by lymphocyte depletion. The porcine fetal retina cell line, VR1BL, shows a high permissiveness to PCV2 infection, 40 times higher than the alternative PK15 culture model, allowing for high titer viral production, with PCV2b growth higher than PCV2a growth. Cytopathic effect due to apoptosis is observed after challenge with high amounts of PCV2, but at low levels, infection is maintained in passaged cells. Thus, VR1BL cells may be used as a model system to examine both acute viral pathogenesis and cellular innate defense, as well as persistent PCV2 infection.

Porcine circovirus type 2 (PCV2): pathogenesis and interaction with the immune system

Porcine circovirus type 2 (PCV2) is the primary causative agent of porcine circovirus-associated disease (PCVAD). The virus preferentially targets the lymphoid tissues, which leads to lymphoid depletion and immunosuppression in pigs. The disease is exacerbated by immunostimulation or concurrent infections with other pathogens. PCV2 resides in certain immune cells, such as macrophage and dendritic cells, and modulates their functions. Upregulation of IL-10 and proinflammatory cytokines in infected pigs may contribute to pathogenesis. Pig genetics influence host susceptibility to PCV2, but the viral genetic determinants for virulence remain unknown. PCV2 DNA and proteins interact with various cellular genes that control immune responses to regulate virus replication and pathogenesis. Both neutralizing antibodies and cell-mediated immunity are important immunological correlates of protection. Despite the availability of effective vaccines, variant strains of PCV2 continue to emerge. Although tremendous progress has been made toward understanding PCV2 pathogenesis and immune interactions, many important questions remain.

A cucumber mosaic virus based expression system for the production of porcine circovirus specific vaccines

Potential porcine circovirus type 2 (PCV2) capsid protein epitopes, suitable for expression on the surface of cucumber mosaic virus (CMV) particles were determined by a thorough analysis of the predicted PCV capsid protein structure. The ab initio protein structure prediction was carried out with fold recognition and threading methods. The putative PCV epitopes were selected on the basis of PCV virion models and integrated into the plant virus coat protein, after amino acid position 131. The recombinants were tested for infectivity and stability on different Nicotiana species and stable recombinant virus particles were purified. The particles were tested for their ability to bind to PCV induced porcine antibodies and used for specific antibody induction in mice and pigs. The results showed that PCV epitopes expressed on the CMV surface were recognized by the porcine antibodies and they were also able to induce PCV specific antibody response. Challenge experiment with PCV2 carried out in immunized pigs showed partial protection against the infection. Based on these results it was concluded that specific antiviral vaccine production for the given pathogen was feasible, offering an inexpensive way for the mass production of such vaccines.

The effect of infection order of porcine circovirus type 2 and porcine reproductive and respiratory syndrome virus on dually infected swine alveolar macrophages

Background

Concurrent infection with porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV) is known as one of the major causes for porcine respiratory disease complex (PRDC). Dual infection with PCV2 and PRRSV is consistently to have more severe clinical presentations and pulmonary lesions than infection with PCV2 alone or PRRSV alone. However, it is not known if dual infections with PCV2 and PRRSV in different infection order may lead to different clinical symptoms in the host. To mimic the possible field conditions, swine alveolar macrophages (AMs) were inoculated with PCV2 and PRRSV in vitro simultaneously or with one virus 18 h earlier than the other. The cell viability, cytopathic effects, antigen-containing rates, phagocytotic and microbial killing capabilities, cytokine profiles (IL-8, TNF-α, and IFN-α) and FasL transcripts were determined, analyzed, and compared to prove the hypothesis.

Results

A marked reduction in PRRSV antigen-containing rate, cytopathic effect, and TNF-α expression level was revealed in AMs inoculated with PCV2 and PRRSV simultaneously and in AMs inoculated with PCV2 first then PRRSV 18 h later, but not in AMs inoculated with PRRSV first then PCV2 18 h later. Transient decrease in phagocytosis but constant reduction in microbicidal capability in AMs in the group inoculated with PCV2 alone and constant decrease in phagocytosis and microbicidal capability in AMs in all PRRSV-inoculated groups were noted. The levels of IL-8, TNF-α, IFN-α, and FasL transcripts in AMs in all groups with dual inoculation of PCV2 and PRRSV were significantly increased regardless of the infection orders as compared with infection by PCV2 alone or PRRSV alone.

Conclusions

Swine AMs infected with PCV2 first then PRRSV later or infected with PCV2 and PRRSV simultaneously displayed marked reduction in PRRSV antigen-containing rate, cytopathic effect, and TNF-α expression level. The different inoculation orders of PCV2 and PRRSV in AMs leading to different results in viral antigen positivity, cytopathology, and cytokine profile may explain, at least partially, the underlying mechanism of the enhanced pulmonary lesions in PRDC exerted by dual infection with PCV2 and PRRSV and the variable clinical manifestations of PRDC-affected pigs in the field.

Functional analysis of the interferon-stimulated response element of porcine circovirus type 2 and its role during viral replication in vitro and in vivo

Background

Porcine circovirus type 2 (PCV2) is associated with post-weaning multi-systemic wasting syndrome (PMWS) in young weaned pigs. Immune stimulation was found to activate the replication of PCV2 and exacerbate the clinical outcome of the infection. Proper amount of interferon-α (IFN-α) is able to enhance PCV2 infection and production in Porcine kidney-15 (PK-15) cells when administered after inoculation.

Methods

In the present study, luciferase reporter assays, construction of mutant viruses, Analysis the replication efficiency and the response to IFN-α treatment in PK-15 cells and animal experiments were carried out to analyze the function of interferon-stimulated response element (ISRE) of PCV2 and its role during viral replication in vitro and in vivo.

Results

A functional viral ISRE sequence, 5′-CTGAAAACGAAAGA-3′, was identified in Rep gene promoter (Prep) of PCV2. PCV2 Prep is composed of two mini promoters, the proximal one span the sequence +1 to -106, containing an ISRE while the distal mini promoter is composed of three tandem GC box like sites locate at -85 to -194. It was demonstrated that viral ISRE is necessary for porcine IFN-α initiated luciferase expression enhancement and it plays an important role in affecting the replication efficiency of PCV2 in vivo and in vitro.

Conclusions

These findings provide a theoretical basis for the Phenomenon of immunostimulation is able to enhance PCV2 infection, and improve the understanding of the complicated mechanisms involved in the host and pathogen interactions of PCV2.

Effects of DNA extraction method on Porcine circovirus-2 real-time polymerase chain reaction quantification in swine lymph node samples

Quantitative real-time polymerase chain reaction (PCR) has become an important tool for Porcine circovirus-2 (PCV-2) research and diagnosis. However, significant differences in detection limit and quantification data, among laboratories and quantitative real-time PCR methods, have been demonstrated. New efforts are required for providing more accurate and comparable results. The current study is an evaluation of the effects of DNA extraction procedures on PCV-2 quantification in lymph node samples. Differences, greater than 1 log(10) copies/g, were shown among PCV-2 loads detected after different extraction procedures. The work highlighted the critical role of the DNA extraction method in PCV-2 quantification by quantitative real-time PCR. This important aspect should be evaluated when comparing data from different laboratories or different studies. The PCV-2 quantification data should not be considered comparable before demonstrating the equivalence of the DNA extraction methods performed.

ORF1 but not ORF2 dependent differences are important for in vitro replication of PCV2 in porcine alveolar macrophages singularly or coinfected with PRRSV

The objective of this study was to investigate cytokine expression and in vitro replication of porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV) in pulmonary alveolar macrophages (PAMs) emphasizing PCV2 open-reading frame (ORF) origin (PCV2a or PCV2b) and PRRSV strain. Chimeric PCV2 viruses composed of different combinations of ORF1 and ORF2 of PCV2a or PCV2b (chimera PCV2a-2b and chimera PCV2b-2a) were constructed and five different PRRSV isolates were utilized: Type 1 (SD 01-08) or type 2 (NC16845b, VR-2332, MN-184, JA-142). PAMs were infected singularly or with combinations of PCV2b, PCV2a, chimera PCV2a-2b, and chimera PCV2b-2a, and one of the five PRRSV isolates. Real-time PCR was used to test PAMs (PCV2 mRNA) and supernatants (PRRSV RNA, PCV2 DNA, PCV2 mRNA) harvested at 24, 48, 72 and 96h post inoculation (hpi). Levels of IFN-γ, TNF-α and IL-10 were determined by quantitative ELISAs. PCV2 replication in PAMs was limited to groups inoculated with PCV2 strains containing ORF1 of PCV2a (PCV2a, chimera PCV2a-2b). Furthermore, in supernatants, PCV2 mRNA was only detected in groups coinfected with PRRSV regardless of strain at 48hpi supporting an enhancing effect of PRRSV on PCV2 infection. Changes in cytokine levels were minimal and associated with PRRSV strain for TNF-α. In summary, in vitro differences in PCV2 replication in PAMs inoculated with different PCV2-PRRSV combinations were independent of PCV2 ORF2 origin with minimal effects of concurrent PRRSV infection perhaps indicating that PCV2-specific changes in ORF1 may be more important than those in ORF2.

Immunology of porcine circovirus type 2 (PCV2)

The emergence of porcine circovirus type 2 (PCV2) associated diseases and particularly postweaning multisystemic wasting syndrome (PMWS) was a shock for the swine industry and formulated a considerable challenge for researchers in the area of viral immunology in swine. The unique features of PMWS of which emaciation and lymphoid depletion were the most evident indicated a deep involvement of the immune system of the pig in the pathogenesis of this condition and indicated that PCV2 was a singular pathogen. Also, the multifactorial nature of the disease complicated the understanding of PMWS pathogenesis. Nowadays, it is known that PCV2 deeply affects the functionality of the immune system of the pig but also the industry has been able to produce efficacious vaccines. In the present paper some of the most relevant immunological features of PMWS and of PCV2 infection in general will be reviewed.

Effects of porcine circovirus type 2 on expression of mRNA associated with endogenous antigen processing and presentation in pulmonary alveolar macrophages and circulating T lymphocytes in piglets

Pulmonary alveolar macrophages (PAMs) are the major target cells of porcine circovirus type 2 (PCV2). This study examined cellular immunity and expression of molecules associated with endogenous antigen processing and presentation in porcine PAMs for 28 days following infection with PCV2. The numbers of total T cells and T lymphocyte subpopulations were measured by flow cytometry (FCM). The expression of mRNA of large multifunctional peptidase 7 (LMP7), ubiquitin-specific protease (UBP), heat shock proteins 70 and 90 (HSP70 and HSP90), major histocompatibility class I (MHC-I), β(2)-microglobulin, glucose-regulated protein 94 (GRP94), calnexin and calreticulin of PAMs from PCV2 infected and control pigs were determined by real-time reverse transcriptase PCR. The absolute numbers of total T cells, T helper (Th) cells, cytotoxic T cells and γδ T cells, but not memory/activated Th cells, decreased following PCV2 infection. There was decreased expression of LMP7 mRNA at 3days postinfection (DPI), whereas expression of UBP and calreticulin mRNA was increased at 3 DPI, expression of HSP90 and β(2)-microglobulin mRNA was increased at 3 and 7 DPI and expression of GRP94 mRNA was increased at 14 DPI. PAMs from PCV2-infected piglets had lower surface expression of CD80/CD86 at 7 and 14 DPI and MHC-II at 7 DPI. These findings suggest that there are alterations in cellular immune function and in the endogenous antigen presentation capacity of PAMs in PCV2-infected piglets.

Development of an in situ assay for simultaneous detection of the genomic and replicative form of PCV2 using padlock probes and rolling circle amplification

Background

In this study we utilized padlock probes and rolling circle amplification as a mean to detect and study the replication of porcine circovirus type 2 (PCV2) in cultured cells and in infected tissue. Porcine circovirus type 2 is a single-stranded circular DNA virus associated with several severe diseases, porcine circovirus diseases (PCVD) in pigs, such as postweaning multisystemic wasting syndrome. The exact reason and mechanisms behind the trigger of PCV2 replication that is associated with these diseases is not well-known. The virus replicates with rolling circle replication and thus also exists as a double-stranded replicative form.

Results

By applying padlock probes and rolling circle amplification we could not only visualise the viral genome but also discriminate between the genomic and the replicative strand in situ. The genomic strand existed in higher numbers than the replicative strand. The virus accumulated in certain nuclei but also spread into the cytoplasm of cells in the surrounding tissue. In cultured cells the average number of signals increased with time after infection.

Conclusions

We have developed a method for detection of both strands of PCV2 in situ that can be useful for studies of replication and in situ detection of PCV2 as well as of DNA viruses in general.

Cytokine induction by immunostimulatory DNA in porcine PBMC is impaired by a hairpin forming sequence motif from the genome of Porcine Circovirus type 2 (PCV2)

Porcine Circovirus type 2 (PCV2) can cause postweaning multisystemic wasting syndrome (PMWS) in young pigs with severe immunosuppression as a major characteristic of the disease complex. Despite the dramatic involvement of the immune system, the interaction between PCV2 and the host is until date not well understood. The DNA genome of PCV2 contains sequences that in synthetic form (oligodeoxyribonucleotides; ODNs) can act immunomodulatory on porcine peripheral blood mononuclear cells (poPBMCs) in vitro. One such sequence (ODN PCV2/1) acts inhibitory on interferon (IFN)-α production induced by immunostimulatory DNA but not that induced by RNA, and the inhibitory activity is dependent on secondary structure formation. In the present study, the characteristic of ODN PCV2/1 was examined further by altering the nucleotide sequence to disrupt hairpin structure formation but still enable multimer structures through G-tetrads. This modification resulted in loss of IFN-α-inhibitory activity of the ODN and thus indicated the importance of hairpin structures. In addition, ODN PCV2/1 was compared to another inhibitory ODN (IRS 869) previously used in human and murine cells. In contrast to ODN PCV2/1, ODN IRS 869 did not inhibit IFN-α production induced by class A ODN 2216 but was a more efficient inhibitor of IFN-α production induced by plasmid DNA than ODN PCV2/1. In cultures induced by the RNA stimulator Poly I:C, however, a strong synergistic IFN-α stimulatory effect was seen in combination with ODN IRS 869. These results indicate that ODN PCV2/1 and ODN IRS 869 function through separate mechanisms to affect cytokine production by immune cells. The effect of ODN PCV2/1 was studied further by monitoring the expression of mRNA for IFN-α, IL-12p40, IL-10, IL-6, IFN-γ, IL-1β, TGF-β, and TNF-α in cultures of poPBMC stimulated with ODN 2216 or Poly I:C. Results from qPCR analyses showed that ODN PCV2/1 clearly inhibited the expression of IFN-α, IL-12p40, IL-10 and IL-6 when induced by ODN 2216, but did not seem to affect any of the cytokines examined when induced by Poly I:C. Initial studies using confocal microscopy and fluorochrome labelled ODNs indicate that ODN 2216 and ODN PCV2/1 co-localize in subpopulations of poPBMC.

Porcine circovirus type 2 DNA influences cytoskeleton rearrangements in plasmacytoid and monocyte-derived dendritic cells

Functional disruption of dendritic cells (DC) is an important strategy for viral pathogens to evade host defences. In this context, porcine circovirus type 2 (PCV2), a single-stranded DNA virus, impairs plasmacytoid DC (pDC) and conventional DC activation by certain viruses or Toll-like receptor (TLR) ligands. This inhibitory capacity is associated with the viral DNA, but the impairment does not affect all signalling cascades; TLR7 ligation by small chemical molecules will still induce interleukin-6 (IL-6) and tumour necrosis factor-α secretion, but not interferon-α or IL-12. In this study, the molecular mechanisms by which silencing occurs were investigated. PP2, a potent inhibitor of the Lyn and Hck kinases, produced a similar profile to the PCV2 DNA interference with cytokine secretion by pDC, efficiently inhibiting cell activation induced through TLR9, but not TLR7, ligation. Confocal microscopy and cytometry analysis strongly suggested that PCV2 DNA impairs actin polymerization and endocytosis in pDC and monocyte-derived DC, respectively. Altogether, this study delineates for the first time particular molecular mechanisms involved in PCV2 interference with DC danger recognition, which may be responsible for the virus-induced immunosuppression observed in infected pigs.

Porcine circovirus type 2 (PCV2) induces cell proliferation, fusion, and chemokine expression in swine monocytic cells in vitro

Granulomatous lymphadenitis is one of the pathognomonic lesions in post-weaning multisystemic wasting syndrome (PMWS)-affected pigs. This unique lesion has not been reported in direct association with viral infection in pigs. The objective of the present study was to evaluate whether porcine circovirus type 2 (PCV2) alone is able to induce functional modulation in porcine monocytic cells in vitro to elucidate its possible role in the development of granulomatous inflammation. It was found that the proliferation activity of blood monocytes (Mo) and monocyte-derived macrophages (MDM) was significantly enhanced by PCV2. During monocyte-macrophage differentiation, the PCV2 antigen-containing rate and formation of multinucleated giant cells (MGC) were significantly increased in MDM when compared to those in Mo. The MDM-derived MGC displayed a significantly higher PCV2 antigen-containing rate than did the mono-nucleated MDM. Supernatants from PCV2-inoculated MDM at 24 h post-inoculation induced an increased tendency of chemotactic activity for blood Mo. At the same inoculation time period, levels of mRNA expression of the monocytic chemokines, monocyte chemoattractant protein-1 and macrophage inflammatory protein-1, also significantly increased in PCV2-inoculated MDM. The results suggest that PCV2 alone may induce cell proliferation, fusion, and chemokine expression in swine monocytic cells. Thus, PCV2 itself may play a significant role in the induction of granulomatous inflammation in PMWS-affected pigs.