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

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.

Cellular adaptive immune response against porcine circovirus type 2 in subclinically infected pigs

Background

Porcine circovirus type 2 (PCV2) is a dominant causative agent of postweaning multisystemic wasting syndrome (PMWS), a multifactorial disease complex with putative immunosuppressive characteristics. Little is known about adaptive PCV2-specific immune responses in infected pigs. Therefore, the T and B cell responses following PCV2 infection in 3-week old SPF piglets infected with PCV2 or PCV2 plus porcine parvovirus (PPV) were studied.

Results

All animals were asymptomatically infected. At 7 days post infection (d p.i.), B lymphocyte and T lymphocyte numbers decreased in the dual infected, but not in the single infected piglets. At this time point a transient PCV2 viraemia was noted in the PCV2 infected groups. Antibodies against the infecting virus were detectable at day 24-28 p.i. for anti-PCV2 antibodies and at day 10 p.i. for anti-PPV antibodies, with no apparent influence of PCV2 on the early PPV antibody development. In the animals infected with PPV alone, IFN-γ secreting cells (SC) that were not specific for PCV2 were detected by ELISPOT assay at day 7 p.i. Interestingly, this response was absent in the PCV2/PPV dual infected animals. PCV2-specific IFN-γ SC were observed in the PCV2/PPV infected group at 7 d p.i. and in the PCV2 single infected group at 21 d p.i. A reduction in the numbers of IFN-γ SC was observed following anti-CD4 and anti-CD8 antibody treatment, suggesting roles for both CD4⁺ and CD8⁺ T cells in the response against PCV2 infection. This was supported by an observed increase in the percentage of IFN-γ positive CD8^hi cytotoxic T cells as well as IFN-γ positive CD8^-/low helper T cells after PCV2 in vitro re-stimulation.

Conclusions

Infection of weaned SPF piglets with PCV2 alone or combined with PPV does not induce disease and in both cases a relatively slow anti-PCV2 antibody response and weak T lymphocyte responses were found. Knowledge on such immunological characteristics is important for both PCV2 pathogenesis and vaccination.

[Endemic viral diseases: a serious economic problem in the Japanese pig industry]

As of February 2009, the Japanese pig industry included 6,890 farms housing a total of 9,899,000 pigs, and produces approximately half of the pig meat consumed in the Japanese domestic market. Although the number of pigs has not substantially changed over the past 20 years, the number of farms has decreased by 86%, indicating the rapid progression of scale expansion in Japan. Against this background, two emerging viral diseases first noted in the 1990s, porcine reproductive and respiratory syndrome (PRRS) and porcine circovirus associated diseases (PCVAD), are now endemic in many farms and causing serious economic losses. This review provides a brief overview of clinical aspects of these two endemic viral diseases and describes the current status of control efforts.

Ultrastructural Findings in Lymph Nodes from Pigs Suffering from Naturally Occurring Postweaning Multisystemic Wasting Syndrome

The aims of this study were to evaluate ultrastructural lesions in lymph nodes from postweaning multisystemic wasting syndrome (PMWS)-affected pigs and to correlate these alterations with detection of viral-like particles (VLPs). Samples of lymph nodes were taken from 4 PMWS-affected pigs and 2 healthy animals and processed by transmission electron microscopy. Significant ultrastructural alterations were only noted in PMWS-affected pigs, mainly in histiocytes and rarely in other cell types. Histiocytes showed severe swelling and proliferation of mitochondria, and proliferation and dilation of rough endoplasmic reticulum and Golgi complex. Infected histiocytes contained large numbers of intracytoplasmic inclusion (ICI) bodies with VLPs; some histiocytes also had intranuclear inclusions (INIs). Small inclusions were surrounded by double membrane, with a granular appearance or containing paracrystalline arrays; icosahedral VLPs were 8–17 nm in diameter. Large ICIs were double-membrane bounded or not and contained VLPs usually forming paracrystalline arrays. ICIs were often found next to mitochondria with severe swelling, and also inside them. INIs were not surrounded by membranes and contained virions of 10–13 nm diameter. Lymphocyte depletion was a striking finding of lymph nodes from PMWS-affected pigs. The inclusion bodies containing VLPs referred to in the present study should be classified as viral factories, suggesting that viral replication is probably a frequent event in macrophages, in which mitochondria might play a role.

Porcine myelomonocytic markers and cell populations

This review focuses in what is currently known about swine myeloid markers, the expression and function of these receptors in the biology of porcine myelomonocytic cells, the regulation of their expression along the different developmental stages of these cells and their utility to investigate the heterogeneity of monocyte and macrophage populations. Although the number of monoclonal antibodies recognizing surface antigens expressed on either swine granulocytes or monocytes is low compared with those available for human or mouse, they have contributed significantly to study the members of myeloid lineages in this species, allowing to discriminate different maturation stages of these cells in bone marrow and to reveal the heterogeneity of blood monocytes and tissue macrophages. Porcine myeloid cells share many similarities with humans, highlighting the relevance of the pig as a biomedical model.

Detection of porcine circovirus 2 in mammary and other tissues from experimentally infected sows

The aim of this study was to determine whether porcine circovirus 2 (PCV2) may infect the mammary gland of sows and be shed in the milk. Six pregnant sows were inoculated intranasally with PCV2 three weeks before their expected farrowing date and two further sows acted as uninfected controls. The animals remained clinically healthy and farrowed normally. Milk samples were collected from all sows on the first, second and third days of lactation. PCV2 DNA was detected in the milk of infected sows from day 1 of lactation but not in the milk of uninfected controls. PCV2 antigen and DNA were detected in the mammary gland and other tissues by immunohistochemistry and in-situ hybridization, respectively. Simultaneous detection of viral protein and DNA provided molecular evidence of PCV2 infection and replication within these tissues.

Development of cell-mediated immunity to porcine circovirus type 2 (PCV2) in caesarean-derived, colostrum-deprived piglets

The interaction between porcine circovirus type 2 (PCV2) and the pig immune system has been suggested to be a determinant event for the pathogenesis of postweaning multisystemic wasting syndrome (PMWS). To gain insight into the host immune mechanisms developed upon PCV2 infection, early innate and adaptive immune responses were examined in 1-week-old, caesarean-derived, colostrum-deprived piglets using a subclinical infection model of PCV2 in combination with lipopolysaccharide (LPS) as a potential immunostimulation factor. The use of LPS did not show any significant effect on the course of PCV2 infection, nor did in the evolution of the immunological parameters evaluated. Ex vivo responses were detected as early as 1 day post-infection (PI) and consisted of an elevation of the plasmatic levels of interleukin (IL)-8 in PCV2-inoculated pigs followed by an increase on plasmatic IFN-α at day 5 PI. Regarding IL-10, only one PCV2-inoculated pig was positive (day 7 PI); this pig was the only one in which viremia persisted until the end of the study. In vitro cytokine determination showed that, regardless of the treatment administrated to the pigs, an IL-10 release was observed when peripheral blood mononuclear cells (PBMC) cultures were stimulated with PCV2. Seroconvertion to PCV2 measured by an immunoperoxidase monolayer assay (IPMA) occurred between 7 and 14 days PI, whereas neutralizing antibodies (NA) did not appear until day 29 PI. PCV2 DNA was first detected in serum at day 7 PI, reaching the peak of viremia between days 14 and 21 PI, followed by a drop in viral load that was found coincident with the appearance of PCV2-specific IFN-γ-secreting cells (PCV2-IFN-γ-SC) and NA. Results from the present work suggest that viral clearance might be mediated by the development of PCV2-IFN-γ-SC in contribution to the PCV2-specific NA.

Porcine circovirus 2 inclusion bodies in pulmonary and renal epithelial cells

Porcine circovirus 2 (PCV2) is the cause of postweaning multisystemic wasting syndrome (PMWS). The most common lesions of PMWS are lymphohistiocytic to granulomatous lymphadenitis, interstitial pneumonia and interstitial nephritis, with intracytoplasmic amphophilic botryoid inclusion bodies in macrophages. In addition to these typical changes, intracytoplasmic botryoid inclusion bodies were observed in bronchial, bronchial glandular, and renal tubular epithelium of several pigs from 4 different farms in Western and Eastern Canada. PCV2 inclusion bodies were demonstrated to be located in the cytoplasm of epithelial cells by immunohistochemical staining for PCV2 and cytokeratin antigens and by ultrastructural demonstration of viral particles in the inclusion bodies within renal tubular epithelium.

Porcine circovirus 2 infection of epithelial cells is clathrin-, caveolae- and dynamin-independent, actin and Rho-GTPase-mediated, and enhanced by cholesterol depletion

Epithelial cells are the major in vivo target cells for porcine circovirus type 2 (PCV2). Although these cells are used for most studies of PCV2 gene expression and, little is known on PCV2 entry, attachment and internalization, in epithelial cells. PCV2 attachment to epithelial cells occurred rapidly and in a time-dependent manner. In contrast to attachment, internalization was slow. Immunofluorescent stainings revealed that during internalization, PCV2 co-localized with clathrin, but not caveolin. Blocking clathrin-mediated endocytosis increased instead of decreased the number of PCV2-infected cells by threefold, suggesting that it does not represent the main internalization pathway leading to a full replication. Further analysis with different inhibitors revealed that also macropinocytosis, dynamin-dependent internalization and membrane cholesterol play no role in PCV2 entry that leads to infection. Inhibition of small GTPases with Clostridium difficile toxin B reduced the number of PCV2-infected PK-15, SK and STs to 63+/-25%, 47+/-21% and 14+/-6%, respectively. Finally, inhibiting actin polymerization also blocked PCV2 infection, showing the need for actin during PCV2 infection. Together, these data indicate that a dynamin- and cholesterol-independent, but actin- and small GTPase-dependent pathway, allows PCV2 internalization in epithelial cells that leads to infection and that clathrin-mediated PCV2 internalization in epithelial cells is not followed by a full replication.

Porcine circoviruses: a minuscule yet mammoth paradox

Porcine circovirus type 2 (PCV2) is the primary causative agent for porcine circovirus-associated disease (PCVAD). PCVAD has been the cause of considerable economic losses to the pork industry worldwide. The disease is primarily characterized by wasting, enlarged lymph nodes, jaundice and weight loss in affected weanling pigs. Several other complex syndromes involving reproductive failure, enteritis, pneumonia and necrotizing dermatitis have also been associated with PCV2 infection. Lymphoid depletion, which is the hallmark lesion of PCVAD, predisposes the host to immunosuppression. Disease progression is further complicated by co-infections with other bacterial and viral pathogens. Despite the availability of effective vaccines for the last 2 years, newly emerging strains of the virus have been reported to cause more severe outbreaks in parts of the USA and Canada. While knowledge of the biology and pathogenesis of PCV2 has progressed considerably over the last 12 years since the disease was recognized, many questions still remain to be answered.

Protective immunity against porcine circovirus 2 by vaccination with ORF2-based DNA and subunit vaccines in mice

The protective immune response against porcine circovirus 2 (PCV2) infection in mice was characterized using flow cytometric analysis (FCM), assays of antibody (of different IgG isotypes) and viraemia, and histopathological examination. An open reading frame 2 plasmid (pORF2) and the capsid protein (Cap) of PCV2 were used as DNA and subunit vaccines, respectively. In FCM analysis, although pORF2 and Cap alone showed comparable efficacy in eliciting lymphoproliferative responses and Cap-specific CD4+ T cells, pORF2 was superior to the Cap protein in triggering CD8+ T cells. A virus neutralization assay showed that pORF2 evoked stronger recall virus-neutralizing (VN) antibody responses than the Cap protein on PCV2 challenge. Correspondingly, VN antibody kinetics coincided with those of Cap-specific IgG2a, but not with the kinetics of IgG and IgG1. Following virus challenge, real-time PCR and histopathological analysis confirmed that only low viral DNA loads and mild microscopic lesions appeared in pORF2-immunized mice. These findings indicate that CD8+ T cells and VN antibody responses correlating mainly with Cap-specific IgG2a play crucial roles in protecting against PCV2 infection, and that the protective immunity induced by the pORF2 plasmid is superior to that induced by the PCV2 Cap protein.

Development and validation of a SYBR green real-time PCR for the quantification of porcine circovirus type 2 in serum, buffy coat, feces, and multiple tissues

The emergence of multiple genotypes of PCV2, as demonstrated by phylogenetic analysis of whole genome or capsid sequences, makes it necessary to have quantitative diagnostic assays that perform equally well on all strains. The objectives of this study were to develop and validate a novel real-time polymerase chain reaction (PCR) assay targeting the highly conserved rep gene (ORF1) and investigate the effects of diagnostic specimen choice on its performance. The assay was tested in naturally infected conventional pigs, experimentally infected gnotobiotic pigs, and plasmid-spiked negative serum, lung tissue, and feces and found to have a linear detection range of 2.2x10(3) to 2.2x10(10) copies of PCV2 per mL. The assay successfully detected and quantified PCV2 DNA in serum, buffy coat, feces, and multiple lymphoid (bronchial, mesenteric, and superficial inguinal lymph nodes; thymus; tonsil; ileal Peyer's patches; and spleen), and non-lymphoid (myocardium; lung; kidney; liver; and gluteal muscle) tissues from naturally infected pigs. Across all tissues and sera of naturally infected pigs, the mean PCV2 concentration was 3.0logs higher in wasting versus non-wasting pigs. PCV2 concentration measured by tissue culture and immunohistochemical staining in homogenized liver samples of experimentally infected gnotobiotic pigs were compared to the concentrations estimated by quantitative PCR. Similar trends were noted with increasing PCV2 concentration detected in subclinically infected to severely PMWS-affected pigs across all assays. Our diagnostic assay was developed with a conserved target sequence, and performed efficiently in quantification of PCV2 in a variety of tissues from naturally and experimentally infected pigs.

Evidence of shedding of porcine circovirus type 2 in milk from experimentally infected sows

Detection of porcine circovirus type 2 (PCV2) was to evaluate the milk from experimentally infected sows using polymerase chain reaction (PCR) and virus isolation. Six pregnant sows were inoculated intranasally with PCV2 at 93 days of gestation, and milk samples were collected from all sows at 1, 3, 6, 9, 12, 15, 18, 21, 24, and 27 days of lactation. PCV2 was detected in milk as early as day 1 of lactation in all six sows. Thereafter, all infected sows remained positive by PCR for PCV2 in milk until 27 days of lactation. In addition, PCV2 itself was isolated from milk collected from a virus-infected sows. These results suggest that PCV2 may be shed in milk following infection of pregnant sows by the virus.

Domains involved in calcineurin phosphatase inhibition and nuclear localisation in the African swine fever virus A238L protein

The African swine fever virus A238L protein inhibits calcineurin phosphatase activity and activation of NF-kappaB and p300 co-activator. An 82 amino acid domain containing residues 157 to 238 at the C-terminus of A238L was expressed in E. coli and purified. This purified A238L fragment acted as a potent inhibitor of calcineurin phosphatase in vitro with an IC50 of approximately 70 nM. Two putative nuclear localisation signals were identified between residues 80 to 86 (NLS-1) and between residues 203 to 207 overlapping with the N-terminus of the calcineurin docking motif (NLS-2). Mutation of these motifs independently did not reduce nuclear localisation compared to the wild type A238L protein, whereas mutation of both motifs significantly reduced nuclear localisation of A238L. Mutation of the calcineurin docking motif resulted in a dramatic increase in the nuclear localisation of A238L provided an intact NLS was present. We propose that binding of calcineurin to A238L masks NLS-2 contributing to the cytoplasmic retention of A238L.

Porcine circovirus type 2 associated disease: update on current terminology, clinical manifestations, pathogenesis, diagnosis, and intervention strategies

Porcine circovirus type 2 (PCV2)-associated disease (PCVAD) continues to be an important differential diagnosis on pig farms in the United States and worldwide. Case trend analyses indicate that the incidence of PCVAD is on the rise in the United States. Accurate diagnosis is important in order to implement appropriate intervention strategies. PCVAD can manifest as a systemic disease, as part of the respiratory disease complex, as an enteric disease, as porcine dermatitis and nephropathy syndrome, or as reproductive problems. PCVAD may be only a sporadic individual animal diagnosis; however, PCVAD may also manifest as a severe herd problem accelerated and enhanced by concurrent virus or bacterial infections. This article is intended to discuss the most common disease manifestations, pathogenesis, diagnostic approaches, and intervention strategies associated with PCVAD in North America.

Enhanced replication of porcine circovirus type 2 (PCV2) in a homogeneous subpopulation of PK15 cell line

Post-weaning multi-systemic wasting syndrome (PMWS) has emerged as a major disease that poses a significant threat to the economics of global swine industry. Porcine circovirus type 2 (PCV2) is the causal agent of PMWS in pigs. Currently, the prevention of PCV2 infection based on vaccines is limited, and the available vaccines are either killed viral vaccines or recombinant protein based vaccines and not cost effective. The PK-15 cells, which is widely used for PCV2 propagation, is not efficient and heterogeneous in terms of permissivity to viral infection. In order to acquire a homogeneous porcine kidney cell line that can reliably produce PCV2 in high titers, cell clones that show high- (PK15-C1) or low-permissive (PK15-A2) phenotype to PCV2 infection were derived from heterogeneous PK15 parent cells by limiting dilution and cell cloning. Maximum virus titers in PK15-C1, PK15-A2 and PK15 parent cells were 10(8), 10(2) and 10(5) tissue culture infective dose 50 (TCID 50)/ml, respectively. The viral proteins of PCV2 were produced and accumulated faster in PK15-C1 cells than those in PK15 parent cells. These results indicate that PK15-C1 cell clone is more permissive to PCV2 infection than PK15 parent cells and thus will be useful for PCV2 replication in vitro, as well as, vaccines, diagnostic and research applications on PCV2.

A new method to identify cell types that support porcine circovirus type 2 replication in formalin-fixed, paraffin-embedded swine tissues

Porcine circovirus type 2 (PCV2) is detected in high amounts within the characteristic microscopic lesions of postweaning multisystemic wasting syndrome (PMWS) affected pigs. In spite of recent advances on disease pathogenesis, the precise cell types that support viral replication are still a major issue of scientific discussion. In this study, a new methodology to detect cell types that support PCV2 replication was designed. For this purpose, two in situ hybridisation (ISH) methods were developed and applied on tissues of PMWS naturally affected pigs using two probes designed from the ORF1 sequence of the virus. While the complementary probe (CP) detected ssDNA, ORF1 mRNA and replicative form (RF) of PCV2, the RF probe (RFP) exclusively hybridised with the RF of the virus, thus, only labelling cells where PCV2 replication is taking place. Both probes demonstrated to be specific and equally sensitive by an in vitro Southern blot hybridisation assay. ISH labelling with the CP was extensive in lymphoid tissues and of variable amount in other non-lymphoid tissues. With this probe, mainly macrophage-like cells were labelled but also other cell types such as hepatocytes and other epithelial cells. Tissues in which RFP labelling was found more frequently were lung, inguinal and mesenteric lymph nodes, tonsil and liver. Labelling with the RFP was always nuclear, and found in the same cell types as with the CP, although in a relatively low proportion of them; labelling of macrophage-like cells was infrequent. Therefore, the results indicate that at least a certain proportion of macrophages may support PCV2 replication, but main cells where PCV2 replicates are of epithelial/endothelial origin. In summary, the present study permitted the study of cell types that support PCV2 replication by the use of ISH on formalin-fixed, paraffin-embedded tissues of PMWS affected pigs.

Detection of PCV2 DNA by SYBR Green I-based quantitative PCR

We developed an assay for the detection and quantitation of porcine circovirus type 2 (PCV2) with the SYBR Green I-based real-time PCR. The real-time PCR provides a broad dynamic range, detecting from 10(3) to 10(11) copies of DNA per reaction. No cross-reactions were found in specimens containing PCV1. Because of the high sensitivity and specificity of the assay with a relatively rapid and simple procedure, real-time PCR can be used as a routine assay for the clinical diagnosis of PCV2 infection. In this study we applied real-time PCR assay to 80 clinical samples, collected from 40 pigs with postweaning multisystemic wasting syndrome (PMWS) and 40 healthy pigs in comparison with conventional PCR assay. In 56 of 80 samples, PCV2 DNA was detected by conventional PCR assay. All samples positive for PCV2 DNA in conventional PCR assay were also positive in real-time assay, and 12 of 24 samples that tested negative for PCV2 DNA in the conventional assay were tested positive in real-time PCR assay. Real-time PCR assay increased the number of samples in which PCV2 was detected by 15%. It is, therefore, considered to be a useful tool for the detection of PCV2.

Porcine circovirus diseases

Porcine circovirus type 2 (PCV2) is a member of the familyCircoviridae, a recently established virus family composed of small, non-enveloped viruses, with a circular, single-stranded DNA genome. PCV2, which is found all over the world in the domestic pig and probably the wild boar, has been recently associated with a number of disease syndromes, which have been collectively named porcine circovirus diseases (PCVD). Postweaning multisystemic wasting syndrome (PMWS), porcine dermatitis and nephropathy syndrome (PDNS) and reproductive disorders are the most relevant ones. Among them, only PMWS is considered to have a severe impact on domestic swine production. PMWS mainly affects nursery and/or fattening pigs; wasting is considered the most representative clinical sign in this disease. Diagnosis of this disease is confirmed by histopathological examination of lymphoid tissues and detection of a moderate to high amount of PCV2 in damaged tissues. Since PMWS is considered a multifactorial disease in which other factors in addition to PCV2 are needed in most cases to trigger the clinical disease, effective control measures have focused on the understanding of the co-factors involved in individual farms and the control or elimination of these triggers. PDNS, an immuno-complex disease characterized by fibrino-necrotizing glomerulonephritis and systemic necrotizing vasculitis, has been linked to PCV2, but a definitive proof of this association is still lacking. PCV2-associated reproductive disease seems to occur very sporadically under field conditions, but it has been characterized by late-term abortions and stillbirths, extensive fibrosing and/or necrotizing myocarditis in fetuses and the presence of moderate to high amounts of PCV2 in these lesions. Taking into account that scientific information on PCV2 and its associated diseases has been markedly expanded in the last 8 years, the objective of this review is to summarize the current state of knowledge of the most relevant aspects of PCV2 biology and PCVD.

Structure-dependent modulation of alpha interferon production by porcine circovirus 2 oligodeoxyribonucleotide and CpG DNAs in porcine peripheral blood mononuclear cells

DNA sequences containing CpG motifs are recognized as immunomodulators in several species. Phosphodiester oligodeoxyribonucleotides (ODNs) representing sequences from the genome of porcine circovirus type 2 (PCV2) have been identified as potent inducers (ODN PCV2/5) or inhibitors (ODN PCV2/1) of alpha interferon (IFN-alpha) production by porcine peripheral blood mononuclear cells (poPBMCs) in vitro. In this study, the IFN-alpha-inducing or -inhibitory activities of specific phosphodiester ODNs were demonstrated to be dependent on their ability to form secondary structures. When a poly(G) sequence was added to a stimulatory self-complementary ODN, high levels of IFN-alpha were elicited, and the induction was not dependent on pretreatment with the transfecting agent Lipofectin. In addition, the IFN-alpha-inducing ODN required the presence of an intact CpG dinucleotide, whereas the inhibitory activity of ODN PCV2/1 was not affected by methylation or removal of the central CpG dinucleotide. Of particular significance, the IFN-alpha inhibition elicited by ODN PCV2/1 was only effective against induction stimulated by DNA control inducers and not RNA control inducers, indicating activity directed to TLR9 signaling. The PCV2 genome as a whole was demonstrated to induce IFN-alpha in cultures of poPBMCs, and the presence of immune modulatory sequences within the genome of PCV2 may, therefore, have implications with regard to the immune evasion mechanisms utilized by PCV2.

Porcine circovirus type 2 replicase binds the capsid protein and an intermediate filament-like protein

Porcine circovirus type 2 (PCV2) is an important porcine pathogen that establishes persistent subclinical infections but may, on activation, contribute to the development of post-weaning multisystemic wasting syndrome (PMWS). This disease is characterized by weight loss, respiratory or digestive disorders and enlarged lymph nodes with lymphocyte depletion. The molecular mechanisms behind the development of the disease are completely unknown. In order to clarify functions of the different viral proteins and, if possible, to connect these new findings to molecular mechanisms behind the pathogenesis or the viral life cycle, a bacterial two-hybrid screening of a porcine expression library from PK-15A cells was conducted. Using viral proteins corresponding to ORFs 1, 2, 3 and 4 as bait, a number of interactions were identified and two of them were chosen for further characterization. GST pull-down assays confirmed that viral replicase (Rep) interacted with an intermediate filament protein, similar to human syncoilin, and with the transcriptional regulator c-myc. Furthermore, interactions of the viral proteins to each other revealed an interaction between PCV2 Rep and the capsid (Cap) protein and Cap to itself.

Porcine circovirus type 2 (PCV2) distribution and replication in tissues and immune cells in early infected pigs

Replication of porcine circovirus type 2 (PCV2) in pigs, as measured by spliced capsid mRNA (Cap mRNA) and viral DNA, was investigated following experimental infection. Peripheral blood mononuclear cells (PBMCs), and tissue from bronchial lymph nodes (BLN), inguinal lymph nodes (ILN), tonsils, lungs, liver, kidneys, spleen and thymus from infected pigs on different days post-infection (DPI) were assessed. PCV2 replication differed dramatically between tissues from the same infected pig. The virus actively replicated in most tested tissues at 14DPI in association with increased PCV2 associated lesions and PCV2 antigen levels, although no clinical signs correlated with PCV2 associated disease were observed in infected pigs during the course of the study. The PCV2 Cap mRNA was detected only at 13DPI in PBMCs from infected pigs, suggesting replication of the virus in circulating blood is transient and not a major site for PCV2 replication in vivo. Evaluation of the Cap mRNA and viral DNA synthesis in T and B lymphocyte and monocyte populations from PBMCs and BLN at various intervals post-inoculation revealed replication of PCV2 in all cell subpopulations; however, viral replication in B lymphocytes was greater than observed in mononuclear cells isolated from BLN at 14DPI indicating that B lymphocytes may be an important cell population for PCV2 replication. These findings further our understanding of the cell types permissive for PCV2 replication and the pathogenesis of PCV2 infection in vivo.

Cytokine and C-reactive protein profiles induced by porcine circovirus type 2 experimental infection in 3-week-old piglets

The purpose of this study was to determine serum profiles of cytokines at a protein level and Creactive protein (CRP) during the development of postweaning multisystemic wasting syndrome (PMWS) in experimentally inoculated pigs. Levels of serum IFN-alpha, IL-6, IL-10, and CRP were examined for a 35-day period in 10 piglets experimentally infected with PCV2 at 3 weeks of age. Four of the infected piglets developed severe PMWS at 14 to 21 days post-infection (d.p.i.) and died prior to termination of the experiment. The remaining six PCV2-infected piglets experienced transient fever, but did not display overt clinical signs of PMWS and were considered as subclinically infected. A bioassay was used to detect IL-6 and ELISAs were used to detect IFN-alpha, IL-10, and CRP. There were no significant differences in cytokine or CRP expression from 0 to 7 d.p.i. between the PMWS-affected and the subclinically infected piglets. Levels of IL-10 and CRP were elevated from 10 and 14 d.p.i. respectively in the PMWS-affected piglets compared to the subclinically infected piglets. There were no significant differences in IFN-alpha and IL-6 expression between the PMWS-affected piglets and the subclinically infected piglets. The present study shows that elevated levels of serum CRP and IL-10 were associated with PCV2-infected piglets that subsequently developed severe PMWS. This may help to provide further insight into the immunoaetiogenesis of this syndrome.

The capsid protein of beak and feather disease virus binds to the viral DNA and is responsible for transporting the replication-associated protein into the nucleus

Circoviruses lack an autonomous DNA polymerase and are dependent on the replication machinery of the host cell for de novo DNA synthesis. Accordingly, the viral DNA needs to cross both the plasma membrane and the nuclear envelope before replication can occur. Here we report on the subcellular distribution of the beak and feather disease virus (BFDV) capsid protein (CP) and replication-associated protein (Rep) expressed via recombinant baculoviruses in an insect cell system and test the hypothesis that the CP is responsible for transporting the viral genome, as well as Rep, across the nuclear envelope. The intracellular localization of the BFDV CP was found to be directed by three partially overlapping bipartite nuclear localization signals (NLSs) situated between residues 16 and 56 at the N terminus of the protein. Moreover, a DNA binding region was also mapped to the N terminus of the protein and falls within the region containing the three putative NLSs. The ability of CP to bind DNA, coupled with the karyophilic nature of this protein, strongly suggests that it may be responsible for nuclear targeting of the viral genome. Interestingly, whereas Rep expressed on its own in insect cells is restricted to the cytoplasm, coexpression with CP alters the subcellular localization of Rep to the nucleus, strongly suggesting that an interaction with CP facilitates movement of Rep into the nucleus.

Immunopathological effects of porcine circovirus type 2 (PCV2) on swine alveolar macrophages by in vitro inoculation

Porcine circovirus type 2 (PCV2) is the primary causative agent of postweaning multisystemic wasting syndrome (PMWS), a multifactorial disease, in pigs. Monocyte/macrophage lineage cells, including alveolar macrophages (AMs), are the major target cells for PCV2. Swine AMs are essential for the pulmonary defense system against various pathogens. Concurrent infection of lung with opportunistic pathogens in pigs suffered from PMWS is speculated as a feature of immunosuppression. The present study was conducted to characterize the effects of PCV2 inoculation on swine AMs in the in vitro system. The parameters selected for evaluation included PCV2 antigen- and nucleic acid-containing rate, viability, TUNEL-positive rate, phagocytosis, microbicidal capability, and capacity for production of reactive oxygen species (superoxide anion, O2-, and hydrogen peroxide, H2O2), cytokines, and chemokines. High intracytoplasmic PCV2 antigen- and nucleic acid-containing rate, absence of intranuclear signals for PCV2 antigen and nucleic acid, and lack of noticeable cell death were seen in PCV2-inoculated AMs. The PCV2-inoculated AMs displayed a transient as well as persistent reduction in the up-take and destruction of Candida albicans, respectively, accompanied by decrease in the production of O2- and H2O2. In PCV2-inoculated AMs, the levels of tumor necrosis-alpha (TNF-alpha) and interleukin-8 (IL-8) were significantly increased; the mRNA expression levels of alveolar macrophage-derived neutrophil chemotactic factors-II (AMCF-II), granulocyte colony-stimulating factor (G-CSF), monocyte chemotactic protein-1 (MCP-1), and IL-8 were strongly up-regulated. The reduced phagocytosis and microbicidal capability in conjunction with decreased production of reactive oxygen species in PCV2-inoculated AMs suggest that PCV2-containing AMs may favor the survival and spread of PCV2. It is speculated that the functional alterations observed in PCV2-containing AMs may be potentially harmful to the lung tissue and local pulmonary defense system, especially in those PCV2-infected pigs conditioned by various PMWS development-dependent co-factors.

Porcine dermatitis and nephropathy syndrome (PDNS) is associated with a systemic cytokine expression profile indicative of proinflammation and a Th1 bias

Porcine dermatitis and nephropathy syndrome (PDNS) is broadly discussed as a porcine circovirus type 2 (PCV2)-associated disease, although PCV2, in contrast to postweaning multisystemic wasting syndrome (PMWS), has to date not been proven to be the aetiologic agent. In order to better understand the complex immunopathology of PDNS, the systemic cytokine expression profiles of (i) five pigs suffering from PDNS, (ii) five animals suffering from naturally acquired PMWS and (iii) five controls were investigated at mRNA and protein levels by means of multiplex real-time RT-PCR and flow cytometric intracellular cytokine detection, respectively. IL-1alpha, IL-6 and IFN-gamma mRNA expressions were found to be elevated in PDNS pigs. At the protein level, an increased capacity of peripheral blood mononuclear cells to produce IL-2, IL-4, IL-6, IL-12, TNF-alpha and IFN-gamma was evident. Hematological investigations revealed a hypochromic anemia while basophils and monocytes were relatively and neutrophils absolutely increased in PDNS pigs. PCV2 antibody levels did not differ significantly between PDNS and PMWS affected animals. Taken results together, the cytokine profile of the PDNS affected animals together with hematological data pointed towards a proinflammatory condition supporting a Th1 bias. Cytokine data of PMWS affected animals exhibited only minor non-significant differences when compared to controls, only IL-10 was significantly decreased at the mRNA level.

Reduction of porcine reproductive and respiratory syndrome virus (PRRSV) infection in swine alveolar macrophages by porcine circovirus 2 (PCV2)-induced interferon-alpha

Two common viral pathogens of swine, namely, porcine circovirus type 2 (PCV2) and porcine reproductive and respiratory syndrome virus (PRRSV), were investigated in regard to their effects on monolayer cultures of swine alveolar macrophages (AMs). The purpose was to identify selected cellular changes and responses potentially associated with the clinical reactions of pigs infected with either or both of these viruses. Measurements included the (1) absolute and relative numbers of infected, viable, and apoptotic cells; (2) distribution of viral antigens; (3) levels of interferon-alpha (IFN-α) and tumor necrosis factor-alpha (TNF-α) produced and their association with the extent of virus-induced cytopathology. Four groups of AMs were studied, including mock-infected, PCV2 alone-infected (PCV2-A), PRRSV alone-infected (PRRSV-A), and PCV2 and PRRSV dually infected (PCV2/PRRSV) groups. The AMs of PCV2-A group had high antigen-containing rate without cell death. There was a marked increase in cell death and apoptosis in PRRSV-A group. However, a lower PRRSV-induced infectious rate, cell death, and apoptosis were seen in PCV2/PRRSV group. High levels of IFN-α production were detected in PCV2-infected groups, but not in mock-infected and PRRSV-A groups. The PRRSV-induced cytopathic effect (CPE) on MARC-145 cells or swine AMs was markedly reduced by pre-incubation of the cells with UV-treated or non-UV-treated supernatants of PCV2-infected AMs. In addition, the reduction in CPE was abolished when the supernatants of PCV2-infected AMs were pre-treated with a mouse anti-recombinant porcine IFN-α antibody. The results suggest that swine AMs were an important reservoir of PCV2; PCV2 infection reduced PRRSV infection and PRRSV-associated CPE in PCV2/PRRSV AMs; the reduction of PRRSV infection in AMs was mediated by IFN-α generated by PCV2 infection. The reduced PRRSV-associated CPE in AMs and increased pro-inflammatory cytokine production may lead to a more severe pneumonic lesion in those dually infected pigs.

Subset-dependent modulation of dendritic cell activity by circovirus type 2

Viral interactions with dendritic cells (DCs) have important consequences for immune defence function. Certain single-stranded DNA viruses that associate with a number of species, including humans and pigs, exhibit interesting characteristics in this context. Porcine circovirus type 2 (PCV2) can persist within myeloid DCs in the absence of virus replication. Internalization was observed with both conventional blood DCs and plasmacytoid DCs [natural interferon-producing cells (NIPCs)], as well as DC precursors. This PCV2-DC interaction neither induced nor inhibited DC differentiation. The maturation of myeloid DCs induced by a cocktail of interferon-alpha/tumour necrosis factor-alpha (IFN-alpha/TNF-alpha), and the ability to process and present antigen to T lymphocytes, remained intact in the presence of PCV2. The virus was clearly internalized by the DCs, a process noted with both mature and immature cells. This suggested a non-macropinocytic uptake, confirmed by an insensitivity to wortmannin but sensitivity to cytochalasin D, chlorpromazine and bafilomycin. Nevertheless, PCV2 was immunomodulatory, being effected through the reaction of NIPC to danger signals. When NIPCs responded to the CpG-oligonucleotide (CpG-ODN), their costimulatory function which induces myeloid DC maturation was clearly impaired by the presence of PCV2. This was caused by a PCV2-induced inhibition of the IFN-alpha and TNF-alpha normally produced following interaction with CpG-ODN. Thus, the immunomodulatory activity of PCV2 is mediated through the disruption of NIPC function. This would impair the maturation of associated myeloid DC and have major implications for the efficient recognition of viral and bacterial danger signals, favouring the establishment of infections additional to that of PCV2.

Effects of IFN-α on the Inflammatory Response of Swine Leukocytes to Bacterial Endotoxin

Because low-dose interferon-alpha (IFN-alpha) treatment had proved effective in several models of chronic inflammation and autoimmune disease, a possible role of IFN-alpha in modulating the response of swine leukocytes to bacterial endotoxin was investigated in this study. Exposure of swine peripheral blood mononuclear cells (PBMC) to low concentrations of human IFN-alpha caused a strong, dose-dependent decrease in CD14 expression, the lowest level being observed at 5 U/ml IFN-alpha. This result was confirmed if PBMC were later exposed to purified lipopolysaccharide (LPS). A 10-fold lower IFN-alpha concentration (0.5 U/ml) caused the largest reduction of tumor necrosis factor-alpha (TNF-alpha) accumulation in the medium of pulmonary alveolar macrophages (PAM), stimulated with bacterial LPS. At 0.5 U/ml, the expression of the TNF-alpha gene in PAM was also strongly reduced, as opposed to cells pretreated with 50 U/ml IFN-alpha. In contrast, expression of the interleukin-1beta (IL- 1beta) gene was stimulated and that of the IL-6 gene was not significantly affected at both IFN-alpha concentrations. Results point to an important role of IFN-alpha in control of the inflammatory response to bacterial endotoxin in pigs.

Effect of granulocyte-macrophage colony-stimulating factor on post-weaning multisystemic wasting syndrome in porcine circovirus type-2-transfected piglets

Post-weaning multisystemic wasting syndrome (PMWS) is a complex disease syndrome in swine, affecting nursery and fattening pigs. Although ongoing evidence suggests that porcine circovirus type-2 (PCV2) is the causal agent of PMWS, the host immune system appears to have a crucial role in the PMWS pathogenesis of PCV2-affected pigs. Owing to difficulties in producing a biologically pure form of PCV2 devoid of the other viral agents commonly present in swine tissues, we decided to use a tandem-cloned PCV2 DNA providing highly pure grade reagent in order to monitor the virulence of PCV2 alone or with an immunostimulating co-factor, granulocyte-macrophage colony-stimulating factor (GM-CSF). A single intramuscular injection of tandem-cloned PCV2 DNA into 5-week-old piglets produced plasmid to viral genome progeny and infectious particles as early as 8 days post-injection in all the organs tested (the lung, the tonsil and the inguinal, mesenteric, bronchial and upper-right axial lymph nodes). The initial plasmid load was not detected with the help of primers designed to specifically detect the acceptor plasmid, thus confirming the replication of the viral genome. Despite the presence of a high level of PCV2 genome copies in the lymphoid organs--the tonsil and the lung--and the presence of infectious particles, no detectable clinical manifestations or pathological lesions were observed in the transfected pigs over the period of observation, regardless of whether they had been co-injected with plasmid containing GM-CSF DNA or had received plasmid containing PCV2 DNA alone. GM-CSF encoding DNA injection had no significant effect on viral replication or on the production of viral particles and appearance of the disease.

Assessing the risk potential of porcine circoviruses for xenotransplantation: consensus primer-PCR-based search for a human circovirus

BACKGROUND

An important issue with respect to virus safety in xenotransplantation is the search for human analogues of porcine viruses, because transmission of a porcine virus followed by recombination with a related human virus may lead to a new emerging virus of unknown pathogenicity, host range and virulence. In case of circoviruses, two types of porcine circovirus (PCV1 and PCV2) are described, but the existence of an analogous human circovirus has not yet been investigated.

METHODS

This study describes the analysis of human samples with a consensus primer-PCR approach designed to amplify conserved regions from the rep gene of circoviruses from the genus Circovirus. DNA from human sera, lymph nodes, blood and urine was extracted and investigated with this method that has led previously to the identification of a new avian circovirus.

RESULTS

By screening 1101 samples (there of 168 from immunocompromised patients), no evidence for the existence of a human circovirus related to the genus Circovirus was obtained.

CONCLUSIONS

This result renders the existence of a human circovirus related to the porcine circoviruses more unlikely, nevertheless the presence of such a virus cannot be ruled out.

Features of cell degeneration and death in hepatic failure and systemic lymphoid depletion characteristic of porcine circovirus-2-associated postweaning multisystemic wasting disease

Tissue section replicates from lymphoid tissues and livers of gnotobiotic swine were examined by immunohistochemistry for the colocalization of porcine circovirus-2 (PCV-2) nucleocapsid and terminal deoxynucleotidyl transferase (TdT)-mediated incorporation of biotinylated nucleotides (UTP) onto the 3'-exposed hydroxyl groups (nick end labeling) nuclear deoxyribonucleic acid (TUNEL), a marker for apoptosis. Single- and dually stained replicates from uninfected controls, subclinically affected PCV-2-infected gnotobiotic pigs, PCV-2-infected piglets immunosuppressed with cyclosporine (Cys), and PCV-2-infected piglets with post-weaning multisystemic wasting syndrome (PMWS) were evaluated. Thymuses were used as positive controls for apoptosis absent PCV-2, tissue sections from dogs given hyperthermic stress were examined as positive controls for induced TUNEL. Tissues from heat-stressed dogs contained TUNEL-positive cell nuclei in both lymphoid tissues and liver, TUNEL was greatest shortly after the delivery of the hyperthermic insult. In uninfected control and subclinically affected PCV-2-infected gnotobiotic pigs, rare hepatocytes and lymphoid cells were TUNEL positive, the frequency of these was similar to that seen in uninfected controls. In PMWS-affected and Cys-treated PCV-2 piglets, the only consistent strongly positive TUNEL signal was contained within the cytoplasm of virus-positive phagocytic mononuclear cells. In phagocytes, some PCV-2 inclusions were TUNEL positive. Collectively, these data indicate that apoptosis is not the primary mechanism of lymphoid depletion and hepatocyte loss in PMWS. Apoptosis associated with systemic viral diseases may be attributable to pyrexia rather than direct or indirect effects of viruses on target cells.

A chimeric porcine circovirus (PCV) with the immunogenic capsid gene of the pathogenic PCV type 2 (PCV2) cloned into the genomic backbone of the nonpathogenic PCV1 induces protective immunity against PCV2 infection in pigs

Porcine circovirus type 2 (PCV2) is associated with postweaning multisystemic wasting syndrome in pigs, whereas PCV1 is nonpathogenic. We previously demonstrated that a chimeric PCV1-2 virus (with the immunogenic capsid gene of PCV2 cloned into the backbone of PCV1) induces an antibody response to the PCV2 capsid protein and is attenuated in pigs. Here, we report that the attenuated chimeric PCV1-2 induces protective immunity to wild-type PCV2 challenge in pigs. A total of 48 specific-pathogen-free piglets were randomly and equally assigned to four groups of 12 pigs each. Pigs in group 1 were vaccinated by intramuscular injection with 200 microg of the chimeric PCV1-2 infectious DNA clone. Pigs in group 2 were vaccinated by intralymphoid injection with 200 microg of a chimeric PCV1-2 infectious DNA clone. Pigs in group 3 were vaccinated by intramuscular injection with 10(3.5) 50% tissue culture infective doses (TCID(50)) of the chimeric PCV1-2 live virus. Pigs in group 4 were not vaccinated and served as controls. By 42 days postvaccination (DPV), the majority of pigs had seroconverted to PCV2 capsid antibody. At 42 DPV, all pigs were challenged intranasally and intramuscularly with 2 x 10(4.5) TCID(50) of a wild-type pathogenic PCV2 virus. By 21 days postchallenge (DPC), 9 out of the 12 group 4 pigs were viremic for PCV2. Vaccinated animals in groups 1 to 3 had no detectable PCV2 viremia after challenge. At 21 DPC the lymph nodes in the nonvaccinated pigs were larger (P < 0.05) than those of vaccinated pigs. The PCV2 genomic copy loads in lymph nodes were reduced (P < 0.0001) in vaccinated pigs. Moderate amounts of PCV2 antigen were detected in most lymphoid tissues of nonvaccinated pigs but in only 1 of 36 vaccinated pigs. Mild-to-severe lymphoid depletion and histiocytic replacement were detected in lymphoid tissues in the majority of nonvaccinated group 4 pigs but in only a few vaccinated group 1 to 3 pigs. The data from this study indicated that when given intramuscularly in pigs, the attenuated chimeric PCV1-2 live virus, as well as the chimeric PCV1-2 infectious DNA clone, induces protective immunity against PCV2 infection and could potentially serve as an effective vaccine.

Systemic cytokine profile in feeder pigs suffering from natural postweaning multisystemic wasting syndrome (PMWS) as determined by semiquantitative RT–PCR and flow cytometric intracellular cytokine detection

Postweaning multisystemic wasting syndrome (PMWS) is an economically important disease in pigs caused by porcine circovirus type 2 (PCV2). Development of this disease is presumably associated with an impairment of the immune system. We, therefore, investigated the systemic expression of relevant cytokines (IL-1, IL-2, IL-4, IL-6, IL-8, IL-10, IL-12p40, TNF-alpha, IFN-gamma) and IL-2Ralpha at mRNA (semiquantitative RT-PCR) and at protein level (flow cytometric intracellular cytokine detection after short-time stimulation of peripheral blood mononuclear cells) in 10 feeder pigs aged 14 weeks suffering from natural PMWS and in 10 clinically healthy pen-mates. Hematological examination revealed a significant (p < 0.001) relative lymphopenia in the diseased animals when compared to reference pigs. IL-1alpha and IL-10 mRNA levels were notably increased in the affected pigs, whereas IL-2 and IL-2Ralpha (CD25) mRNA levels tended to be down-regulated. IL-8, TNF-alpha and IFN-gamma mRNA expressions appeared to be slightly increased. Intracellular cytokine levels as measured by flow cytometry revealed an increase of IL-1beta, IL-2, and IL-6, whereas IL-12 and TNF-alpha expressions were not affected. IFN-gamma was slightly decreased in the diseased animals. In conclusion, despite the assumption, that the cellular immune response to PMWS as a virus-induced disease should be characterized by either a Th1 driven cytokine profile or a cytokine profile indicative of T cell immunosuppression, our results did not support that hypothesis. Nevertheless, data from intracellular cytokine detection suggest an even increased percentage of the remaining lymphocytes capable to produce IL-2 upon in vitro stimulation, which is in contrast to the slightly diminished IL-2 mRNA levels reflecting the in vivo situation at least at the mRNA level.

Dendritic cells harbor infectious porcine circovirus type 2 in the absence of apparent cell modulation or replication of the virus

Dendritic cells (DCs) play crucial roles in innate and adaptive immune responses, rendering them critical targets for virus infections. Porcine circovirus type 2 (PCV2) is associated with the development of postweaning multisystemic wasting syndrome (PMWS) in piglets. We demonstrate here that 80 to 90% of monocyte-derived and bone marrow-derived DCs interact with PCV2 similar to the early stages of an infection. There was no evidence for virus replication, but the virus did persist in DCs without loss of infectivity nor the induction of cell death. This could reflect an abortive infection, but there was no evidence of virus uncoating-the infectivity remained intact for at least 5 days. Alternatively, the results may reflect DC endocytosis of antigenic material. However, there was no modulation of DC surface major histocompatibility complex class I and class II, CD80/86, CD25, CD16, or CD14. Furthermore, infected DC did not transmit virus to syngeneic T lymphocytes, even when the latter were activated. Such coculture did not induce PCV2 replication or death of the lymphocytes or DCs. These results demonstrate that PCV2 can persist in DCs in the absence of virus replication or degradation. Such a silent virus infection presents a novel mechanism of not only immune evasion but also escaping the DC degradation pathway. Because of their migratory capacity, infection of DCs thus provides a potent vehicle for transport of the virus throughout the host without the need for replication. In addition, the lymphopenia seen in PMWS is not a direct effect of the virus on lymphocytes but would require additional events, as proposed by others.